Why Solutions to the Twin Paradox are WRONG

Do "solutions" to the twin paradox leave you confused or skeptical? You're not alone. An examination of three popular KZbin videos pinpointing where and why they get it wrong. Plus, Albert and Emmy get chatty.
Full Twin Paradox Playlist: • The Twin Paradox
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[@ScienceClicEN]

Great video, I agree that the paradox is all about the absoluteness of acceleration. I would tend to say however that acceleration *is* absolute in relativity (as opposed to Newtonian mechanics in which it is simply relative). To me that's precisely the answer to the paradox : acceleration is not relative in relativity, one of the spaceships accelerates more than the other, and hence comes back younger than the other twin. You can simply measure it with an accelerometer in each spaceship. And by integrating the acceleration measured by both accelerometers, you can tell which spaceship has accelerated the most, and hence which of the two twins will be younger. You are right though that the "solutions" usually don't tackle this subtelty which is in reality the whole core of the paradox : how do you know which of the two spaceships has truely accelerated ? This is answered through the notions of geodesics, and in my opinion this paradox is perfectly designed to show how spacetime in relativity has an intrinsic metric (the youngest twin is the one who has travelled the shortest path), as opposed to the purely abstract coordinate description of spacetime in Newtonian mechanics.

[@dialectphilosophy]

Merci beacoup d'avoir regardé! You're correct that the framework of relativity (both special and general) treats acceleration as absolute, and in SR this leads to the rotating planes of simultaneity solution. When we made this video we were somewhat under the impression that GR was supposed to "relativize" accelerated frames of reference in the same way SR relativized moving frames of reference, later we got this misconception cleared up. If you've watched our other videos, you can probably tell we're quite against the idea of absolute acceleration. As you point out, defining the concept via an accelerometer is certainly pretty solid stance from an empirical standpoint. But this doesn't resolve the issue that an accelerometer would still have to be calibrated in an inertial frame before it could be considered an accurate indicator of absolute movement. Additionally, since we feel that the true mystery of the twin paradox is posed by the question "what breaks the symmetry of the twins' motion?", even if you posit absolute acceleration as the answer to this question, it cannot be the only answer, since in curved spacetime one can travel geodesics which are not globally minimal, and therefore inertial twins can wind up being younger than their accelerating counterparts. This would leave us at two agents of asymmetry, one for flat spacetimes and one for curved spacetimes, which in our opinion seems somewhat awkward.

[@ScienceClicEN]

@@dialectphilosophy Thanks for the reply! I see, I'd still be tempted to argue that one twin would have to witness their accelerometer move over time (their acceleration has to change in order to come back, at least in a flat spacetime), while the other twin would see their accelerometer motionless from start to finish. So, regardless of how the accelerometers were calibrated, there seems to be one twin for whom things "changed over time", hence breaking the symmetry. But you could argue that it doesn't work if the twin is moving along a circle, on which the acceleration stays constant in magnitude... It's not easy, and even less so in curved spacetime, as you said! Still, that's frustrating haha! I feel like it should be possible to determine empirically which frames are inertial. Maybe by looking at the symmetries in the motion of objects ? By trying to find the frames that maximize the amount of symmetries ? In the sense that an "inertial frame" would be defined as the "most symmetric" frame, the one in which our description of motion is the simplest. But maybe that doesn't work, I'll think about it! Thanks for your videos, these are great topics that are rarely seen in sci-com!

[@dialectphilosophy]

@@ScienceClicEN Did you know Einstein wrote, in a 1918 paper, that "it is a great satisfaction for a consistently thinking man to understand that the concept of absolute movement -- which kinematically makes no sense anyway -- need not be introduced in physics?" For a while it appears he believed that, with the discovery of GR, he had achieved the doing-away-of absolute motion; basically it took someone inventing Newton-Cartan physics to change his mind lol. We didn't know all that back when we first made this video -- it sort of blew our minds to realize we weren't the first ones to get frustrated by all this! And you're absolutely right, even an uncalibrated accelerometer would still register jerk. And for a system already in a state of jerk, it would register a change in jerk, etc., all the way down the whole line of derivatives. So perhaps one could argue that, while an accelerometer doesn't enable you to differentiate a state of absolute motion from absolute rest, it would allow you, as you say, to define an absolute "change". Empirically defining the inertial frame is something we're very stuck on as well. At some level, there has to be a better operative definition than merely an accelerometer. Defining an inertial frame makes sense to us only if you already have a large collection of systems from which you can extract a sort of "average" -- your idea of maximizing symmetries is definitely intriguing! Anyhow, if you have any brilliant flashes of insight definitely let us know, because it has given us many a headache 😂 And thank you for the compliment!

[@sarahbell180]

​@@ScienceClicEN Oh, hey ScienceClic! Didn't expect to see you here, you are one of my favorite physics channel on this site thanks to your amazing conceptual and accurate explanations of various advanced physics concepts. (especially love your videos on quantum field theory, best I've seen so far!) So I figure it'd be cool to mention what I think! It's important to distinguish the notion of an inertial frame and what that really is in reality. Conceptually, an inertial observer is an observer undergoing its natural motion and a force is any outside interaction so as to perturb this inertial motion. Of course, it can be asked whether nature even works this way and is hence a metaphysical description. Now then, one asks 'what really is the 'natural' motion objects take and which forces act?' This is the whole question of building theories of physics and it is thus not going to be a question settled by measurement alone! The goal of physics is to build together an explanation on the nature of the universe based on these measurements. The fact that Einstein and Newton theories of gravitation are both theories that are consistent and able to explain the same generic results, like an accelerometer being 0 in free fall, tells you the question of discovering an inertial frame can not be settled by experiment alone without extra assumptions on the nature of your physical theory. And, that is a large part Dialect's point (in a later video on inertial frames), although I don't think it is an issue and simply the nature of physical theories and science in general. That this means you can not so simply deduce the nature of 'which twin is older?' without some extra knowledge is not of consequence since as a hypothetical question, this would be based in establishing a physics theory and scenario of which to deduce and hence we can be 'given' an inertial frame in the framework of our theory of relativity without needing to know forces. And, as a question of reality, then there is already something in the universe that probably says who is inertial and who is accelerating and so which twin is older and so that there is a consistent answer to the question, and is merely a matter experiment and knowledge to find who is accelerating and who is older which should be entirely expected since this is a question of epistemics now more than ontology. Relativity is about true absolute physics rather than what some observers 'think'. That said, one pictures an inertial frame as 'felt' forceless, meaning an accelerometer in that frame reads 0. So, we may formulate the idea of an operational inertial frame, a frame which reads 0 on an accelerometer. General relativity *postulates* that this operational inertial frame is equivalent to this metaphysical inertial frame. You can't test that aspect (you can test if a frame in free fall is 'operationally inertial' however, which is the equivalence principle). General relativity also harbors the advantage based on the idea that forces are local interactions, so all force contribution will be from locally defined sources (thus, I believe locality to be a strong factor in justifying the idea of a force).

[@RexGalilae]

Coincidentally, I decided to revisit the twin paradox after watching one of your videos on relativity when I stumbled upon this. Begs the question, scienceclic twin paradox video coming out when? ;) PS. Pretty sure Newton himself understood that acceleration is absolute. People find it easier to introduce "ghost" forces in accelerated frames of reference since it makes the math easier but this again goes to show how such frames have an absolute distinction from inertial frames

[@johnschultz4345]

Sorry, but actually THIS explainer is wrong. There really isn't a paradox and your anti-solution is incorrect. If we adopt your preferred example where the twins move away from each other but then eventually turn around and return back to their origin, then obviously there is an inertial frame that makes all of twins' movements entirely symmetrical. In this frame, BOTH twins do NOT maintain their inertial frames as they move. Certainly at their respective turn around points, they both deviate sharply from what had been their previously inertial movement. If there were free floating oranges in their rockets, then when they turned around those balls would have all splattered against their walls. You can draw the space time diagram and regardless of what INERTIAL frame you choose BOTH twins will have kinks in their world lines. Neither of them maintained inertial movement! If they experienced the same forces/accelerations in the origin's inertial frame, then they will have both aged the same when they both return to the origin, but less than if a triplet had remained at the origin. Now, you will argue, but all motion is relative! Can't we simply adopt the frame of reference of one twin (and then the other), claim the other twin is the one who is moving, and then come to contradicting conclusions? The answer is, no, you can't actually. It comes back to inertial frames. With the twins relatively moving away from one another and then coming back together again, then at least one of them (and possibly both of them) must have gone through some non-inertial motion, which people variously call a change in velocity, acceleration, forces, frame switching, etc. Furthermore, we CAN measure in an ABSOLUTE sense which of the twins experienced (more of) that non-inertial motion by having them carry accelerometers. If one of the twins' accelerometers stays at zero the whole time, then that twin is the one who maintained their inertial frame and the other must have deviated from theirs. But all of that is largely besides the point. The real answer to the Twin "Paradox" has to do with in which frame of reference you ultimately do your final clock/age comparison. If you declare the "stationary" twin as the frame of reference in which you will ultimately compare and that twin experiences no non-inertial motion (i.e. - their accelerometer constantly reads 0 throughout the scenario), then we know that clocks in that frame will run fastest in SR. Any clocks that are moving relative to that frame will run slower, no matter how they move. So long as the moving clock ultimately comes back into that same frame (i.e. - their relative velocity becomes zero) and the clocks are then compared in that frame, then the stationary one will have ticked the most.

[@gman8563]

An accelerometer doesn’t measure “absolute” acceleration, it just measures the acceleration of an instrument relative to the observe. Moreover, to work properly you would have to calibrate your accelerometer in an inertial frame prior to being able to use it, so your whole argument is rather rendered meaningless. You can’t even define what absolute acceleration would mean, that’s why Einstein called it nonsense

[@eleventhchimp]

@@gman8563 A accelerometer most certainly measures absolute acceleration. It does not have to measure the acceleration against any other object. I assume you agree that you can’t measure a distance on an object? You measure a distance between two objects. You have a distance to something. Speed is (simplified) the change in distance, and thus can only be measured (or even have a meaning) between two objects. For acceleration this is not true. You calibrate an accelerometer by turning it around. What is this mined quote you have that you have misenterpreted in a way to make you believe that Einstein agrees with your misconceptions?

[@lancebradshaw4829]

This has been said before in the comments, but I want to reiterate that acceleration is not relative. Alice can't say that she is sitting still while Bob is accelerating or vice-versa. Both of them will know that they are in an accelerating frame. It will produce a force that can be detected. That will let them know that neither of them are in an inertial frame. So special relativity is insufficient to give you an answer. General relativity must be added. If both of their clocks were synchronized before they started accelerating, both accelerated at the same rate in identical ships for the same duration and both turned around after the same amount of time, then the situation is perfectly symmetrical and Alice and Bob will both age at the same rate. The situation has to be asymmetrical in order for one person to age more quickly than the other.

[@dialectphilosophy]

Hey Lance, thanks for watching. We discuss (and debunk) some of those objections in our follow-up videos, check them out!

[@signorellil]

The "paradox" can be easily solved without GR even if we assume there's no acceleration (and BTW Special Relativity can deal with acceleration - you don't really need GR here). The path of the twins through spacetime is different even if they relation is symmetrical. That's it. No real problem

[@user-gn6jj8qh1w]

@@signorellil On theory, you're correct. But some people does not understand the concept of "path of the space time". Naturally people think time is independent from velocity or spatial coordinate. I think anybody explain this problem should show the actual calculation from both side. So at least most of the people does not confused by the explanation itself. But even after such explanation people still don't get your explanation. It's for the people who already understand the theory I guess. Hence it's inappropriate as the explanation for ordinary people.

[@BartvandenDonk]

Well more important is, NOT removing mass. It is fundamental when you talk about time dilation. When reaching the speed of light, you'll create your own gravity. Holding a straight direction will become impossible. I do not understand why I never see something about bending, spirals in combination with gravity.

[@fizzy4149]

@@dialectphilosophy -- Great videos. I watched them all (I think). But it would be helpful if you could somehow order them so the viewer knows what to watch and in what order. You can do like they do in the PBS spacetime videos where they'll say "Before you watch this video, you should watch these three videos," and then show the titles and thumbnails below.

[@Paradoxolog]

Acceleration isn't relative. You can't just confidently say you're at rest while the other plane is accelerating, while your favourite tea mug is flying backwards towards the wall.

[@dialectphilosophy]

While our modern theories of physics treat acceleration as "absolute", this concept is in-itself undefinable, and all our measurements of acceleration are still relative to other masses/objects. See our other videos for more detail!

[@Paradoxolog]

@@dialectphilosophy I've just seen it. Science does require some pre-existing knowledge otherwise experiments wouldn't be permitted in science and that's just not science. Your entire argument is resting on a similar argument to "Just because I've dropped this ball a million times and it fell to the floor, it doesn't mean it will fall to the floor the next time I let go". That is absolutely a reasonable thing to say but science does not claim it is 100% certain about anything, just that it has the best known model to explain real phenomena until the next best model comes along. You have not proposed any new model here to explain the observed phenomenom. All you are saying is that no argument can be proven sound in science because doing so would require pre-existing knowledge of the premises. This is trivial knowledge known by most scientists by the way.

[@gman8563]

@@Paradoxolog you clearly didn’t watch his other videos

[@felipec]

​@@dialectphilosophytry saying acceleration is relative with a cup of coffee on your lap while somebody accelerates the car.

[@cameronconrad5568]

This video is actually incorrect. It states that acceleration, and therefore inertia, is not absolute. But while time and length are relative, the laws of physics (which includes inertia) are absolute.

[@dialectphilosophy]

Absolute acceleration is an assumed postulate of our modern theories, not a derivable law. Check out our other videos for more on the topic!

[@robertmadeo7672]

@@dialectphilosophy An accelerometer can never tell you your true speed since you may start your acceleration while moving 0.5c toward the Leo constellation. The only thing that matters is your absolute speed using the fixed stars as your reference frame.

[@Mr.Not_Sure]

Your mistake is at 7:04. Not "all frames are equal", but "all inertial frames are equal".

[@mathboy8188]

I can appreciate this video's confused mistake - it's perfectly natural to imagine that the twins are in a perfectly symmetric situation, and that each twin should be able to declare that it's the other who's motion is changing, and that it's only thanks to reference to the stars or Earth or whatever that the twins aren't on the same footing. That was my first reaction when encountering this "paradox". But then I thought about it for a moment, and no, the twins are definitely not on the same footing. Identifying a frame of reference doesn't mean you've identified an _INERTIAL_ frame of reference. Twin B can make his frame the "he's at rest" frame, but it doesn't make it an _INERTIAL_ frame - and in fact it's not. If twin B is in an enclosed room in his ship, with no view outside and nothing inside but him freely floating, then twin B will experience being slammed into some wall of the room at about the trip's halfway point as his ship turns around, and so twin B know that outside his enclosed room, either (Einstein's Equivalence Principle): 1. a gravitational field has been encountered (the ship containing his room has landed on a planet), or 2. the ship containing his room is experiencing an acceleration (equivalent to the ship's velocity has changed, equivalent to his ship-frame of reference is not an inertial frame of reference). Meanwhile, twin A, who IS in an inertial frame, if kept in a view-less enclosed room, experiences no acceleration during his "trip" (being at rest, "moving" through time). Twin A doesn't fall into a wall of that room, but just floats the whole time.

[@trsomas]

Mistake in this video #2. The video considers a symmetrical version of the twin paradox in which both spaceships move away from each other, take a turn, move towards each other and come back to near each other. It says that this cannot be explained using acceleration because both are accelerating relative to each other. Correct answer is this can be and must be explained using proper acceleration. When the spaceships are moving at constant velocity relative to an inertial frame, then each observer will see that the other observer's clock is slower. When the spaceships are accelerating (proper acceleration), then according to General Theory of Relativity, each observer will see that the other observer's clock is faster. The net result is that, at the end of the journey they will agree that their clocks recorded equal time durations.

[@EduIreland]

Acceleration is not completely relative. If Alice accelarates, energy needs to be used against her inertia to cause such acceleration, and if the only other object in the scenario is Bob, the only options Alice has are to either push against Bob, which will cause him to accelarate in the opposite direction, or Alice can push something she has on her ship (usually fuel) in the opposite direction of her acceleration, in which case, she also loses mass. In both methods, the acceleration requires another element (Bob or fuel) to which Bob accelerates against. Also, the acceleration will only last while there is contact between the two parts, the moment that that contact ceases, so does the acceleration and now the two entities are moving away from each other at a constant velocity. Finally, in the given scenario, when Bob decides to turn around, they either need to interact with another element or spend some of their own mass in order to change their velocity, while Alice doesn't, which is then not symmetrical anymore.

[@brianwatson9687]

I can't believe how wrong you are about this. The "stay-at-home" twin is always in the same inertial frame. She knows this because Newton's 1st law is always obeyed. The "out-and-back" twin KNOWS he is not in a single inertial frame because when he turns around he can observe that the 1st law is violated, i.e., isolated objects in his frame accelerate. This is where the asymmetry comes from. It is fundamental to understanding the problem. Now if each twin moves apart, turns around and returns, the symmetry is there and each twin will have the same age when they meet (but less than someone who stayed on earth).

[@dialectphilosophy]

Your objection is not valid here, as acceleration itself is also a relative concept -- see our videos "Inertial Frames", "Newton vs. Mach", or "Why Relativity Doesn't Add Up" for more on this topic.

[@DANGJOS]

​@@dialectphilosophy Is who exchanged momentum with who also relative? The accelerating twin must exchange momentum with something (a piece of itself in the case of a rocket) in order to accelerate, while the twin at home does not.

[@AstralTraveler]

The whole 'solution' fails in case of a linear and constant motion of one twin - it's the velocity of motion itself that supposedly causes time dilation and time dilation leads to definitive effects - so in the end it would be possible for both twins to figure out which one of them is moving and what is the velocity in relation to 'stationary' twin. The whole concept of relative motion breaks apart...

[@brianwatson9687]

No, acceleration is not a relative concept. You just wrong about this.@@dialectphilosophy

[@zenastronomy]

​​@@dialectphilosophyacceleration is not relative. as far as i understand it. acceleration has never been relative. you are mixing up the inability of the accelerating observer to tell why he is accelerating. whether gravity or a rocket. but acceleration can always be felt. just now you sitting at home you are feeling acceleration. how by being pulled into the ground and not floating away into the air. you are experiencing acceleration. acceleration of gravity but nonetheless acceleration. you can feel it. you know it. if the ground wasn't underneath you, you'd be falling down. you can feel this tug on your body. you can do experiments to show this acceleration.

[@anatoliulmer1329]

Sorry but unfortunately that's wrong. Yes you can absolutely measure the acceleration through inertial forces. You can very well feel that force during e.g. take-off of an airplane while everyone at the ground will not. And yes you will also feel that without the planet around because that's due to the acceleration and not Earth's gravity. Actually the twin problem can also be explained without the acceleration from earth (or whatever fixed point) as long as one knows who's turning around and traveling back to the initial position and who's been at rest at that point. Also the symmetric attraction/repulsion you've been talking about in the end is Newton's mechanics and not relativity...

[@dialectphilosophy]

Measuring acceleration is not an "absolute" measurement -- it is a measurement made relative to the observer. Watch our other videos for more information on this topic, and learn why even Einstein thought absolute motion was nonsense!

[@anatoliulmer1329]

@@dialectphilosophyThe inertial forces are absolute, i.e. in your thought experiment the acceleration will be measurable only by spaceships actually accelerating and not by ships that do not accelerate (which is represented by the earth system in most other scenarios). What you're talking about is the symmetry coming from the lack of absolute inertial reference frames, i.e. special relativity. In general relativity, even if you (of course) still have no absolute reference frame, but you can measure (as described before by resulting effects like inertial forces, or simply by the lack of rocket fuel in your tank because you needed to use that for acceleration) who is accelerating. It's not about absolute motion but about relative acceleration. Please read more carefully before replying. The arrogant tone in which you're replying is the same as in the video, which just makes it more embarrassing when you're wrong. (which I can not yet make absolute statement about (yet))

[@jarnold1949]

You are mistaken. Acceleration is objectively different from being "at-rest." If you are in a spacecraft that is not accelerating, a ball will float freely inside. If your spacecraft accelerates, the ball will move to a wall opposite of the acceleration, and will press against it for as long as there is acceleration.

[@dialectphilosophy]

Watch our follow-up videos, we discuss those objections!

[@serektaibah4091]

acceleration may be different from at rest but its still relative .. an observer will always always say he is at rest and the others are accelerating while the rest seeing him accelerating .. this is why acceleration does not break the symetrie of the twin paradox .. infact ANY movement is relative if you look at something doing whatever motion infront of you .. that thing will see you do the exact same motion but in the opposite direction

[@apolloniuspergus9295]

Yes, but you could also say that the ball is accelerating, while you and the spaceship are still. Do you see the problem?

[@jarnold1949]

@@apolloniuspergus9295 No. If the spaceship is accelerating you and the ball both press against a surface opposite the acceleration. If it is no accelerating, you and the ball simply float. No problem at all.

[@apolloniuspergus9295]

@@jarnold1949 Who is accelerating? You and the ball or the spaceship?

[@runepedro]

There’s no way to pick a preferencial frame of reference between two objects in Uniform Linear Motion and that’s it. It has nothing to do with an accelerated object. In the scenario where one twin is left on Earth, that one is the one with zero acceleration. The other would notice it has accelerated and there’s nothing in special relativity which contradicts this.

[@mshparber]

I disagree. The person is experiencing force while accelerating. The one on the earth does not experience force

[@furia151]

but it is super easy to say who is accelerating. To accelerate you have to use energy. so it is absolute.

[@peterrobinherbert]

The person who made this video doesn't understand the Twin Paradox at all. Both Alice and Bob will agree on who is older if they both do the math correctly.

[@marcv2648]

The math doesn't include acceleration... so how exactly does that work?

[@SocratesAlexander]

Who is accelerating is obvious. The one who burns fuel is accelerating.

[@rhvc2734]

lol 🤣🤣

[@rxotmfrxotmf8208]

The statement that you do not know which twin is accelerating if you remove the background is wrong. We DO know which twin is accelerating. Einstein stated so himself: the frame that experiences a force is the frame that is accelerating (his famous example of an elevator accelerating at 9.81 m/s^2 is indistinguishable from a stationary one on the earth's surface), and the frame that experiences no force is inertial. So the twin that turns around first has to undergo negative and then positive acceleration, where a force is experienced during any acceleration. For that twin, time slows down. If both twins go in opposite directions, turn around, and return to their starting positions in exactly the same way, both their clocks will have slowed by the same amount relative to a clock placed at their starting (stationary/inertial) point, but both twins' clocks will still be synchronized with each other.

[@dialectphilosophy]

Hey thanks for watching -- the statement that "force" or "inertial frames" breaks the symmetry does not provide any better a solution to the paradox than saying that acceleration does, as it is essentially the same statement. See our follow up videos for more.

[@kato_dsrdr]

That doesn't make sense. The symmetry is still not breaking.. What if both of the twins fire their rockets then??

[@rxotmfrxotmf8208]

@@kato_dsrdr Symmetry breaking occurs only when the twins each undergo different accelerations. If both accelerate/decelerate in exactly the same way in different directions, their symmetry is preserved, but not their symmetry with their starting point because that point has remained stationary w.r.t. them, relatively speaking.

[@madnorbi]

Why is there a paradox in the simplified model? We could say Bob needs to do something to turn around, in order to turn it has to do something e.g. throw some mass in the non Alice direction. That breaks the simmetry.

[@user-ll2gi2dl2n]

It's only a paradox if one insists on motion being purely relative. In this case the assignment of motion is arbitrary, so which twin is older is up in the air. In reality only the traveling twin gains energy through acceleration, so only that twin's frame actually changes.

[@nooneinparticular3370]

Didn't Einstein have great respect for Noether, much like everyone else those days? I find that "Shush, the men are speaking" very inappropriate if so, as I'm pretty sure she was in his circle of friends, and might have even had his help emigrating (she was a Jewish German in the 30's), although I might be confusing her with Gödel on that detail.

[@noname-sg6qx]

I disagree with you on the part where acceleration is absolute, the thing is when ever you are accelerating like in a car or something, then you feel a force, now heres the thing with Bob and Alice, suppose its Bob who reversed the thrusters, to Bob it may look like Alice is coming back and to Alice it may look like Bob is coming back, but in fact Bob can confirm that he is the one who truly accelerated, since he is the one who felt the force, for Bob it may look like Alice is coming back, but Bob felt the force, and Alice didnt, so basically the acceleration of alice is fake

[@AdamAlbilya1]

Not necessarily, Bob can also fire the rockets and still claim he is stationary and does not feel a force. Think of a helicopter stays in mid air and a skydiver. The pilot accelerates upward, but will claim Bob's claim with regard to a skydiver, either if he dives away, i.e. pilot is above (analog to the twins separation part), or towards it, i.e. pilot is below (analog to twins reunion).

[@henriconfucius5559]

@@AdamAlbilya1 He feels a force, because he is the reference. And the rocket is pushing him.

[@AdamAlbilya1]

@@henriconfucius5559 Not in case of a gravity force from the opposite direction, nullifying all forces thus he will feel no acceleration, i.e. as if he's standing still (not accounting for the forces lost in favor of vibrations/heat etc. of course). Think of a hammingbird flapping its wings to stay still in mid-air against gravity or, as I mentioned, you inside a helicopter hovering in mid-air against gravity- your engines are on, but you feel no acceleration.

[@leeaxe]

​@@AdamAlbilya1 You and author of this video are wrong, you misunderstand the velocity and acceleration.

[@AdamAlbilya1]

@@leeaxe If acceleration was the only factor then the amount of time/distance you travel at fixed speed, after you've reached the high speed desired and stopped accelerating, wouldn't be a factor of how much you've traveled through time. But it is, isn't it?

[@jibeshbeura7572]

the one who accelerates must feel a psudo force where as the one who does not accelerate dont feel it.does this break symetry of the problem

[@JohnSmith-ch9sm]

100% agree

[@frankmuennemann9545]

I don't understand why this video hasn't been pulled down. It's claims are simply wrong.

[@riverchess-so7pr]

for the same reason that flat earth videos are not pulled down. Welcome to youtube.

[@Raptor302]

But don't you need to expend energy to accelerate? A change in direction at a constant speed also counts as an acceleration, but presumably you also need energy to change direction? It may not always be apparent to observers who is accelerating, but the universe is keeping track. Whoever is expending energy is the one accelerating.

[@pablovogel6986]

You are very wrong because acceleration is absolute, you can tell if you are accelerating without reference frame. Relativity, classical Galilean or Einstein is about INERTIAL REFERENCE FRAMES, wich are the ones NO ACCELERATING. Those are the ones that are indistinguishable and there are note INERTIAL preferred frame. Accelerating frames invalidates all results both in Galilean and Einstein

[@HowDoYouKnowThough]

A notion of absolute acceleration is only possible if you introduce a third observed mass into the system. That is his point.

[@dialectphilosophy]

Nothing tells us that acceleration is absolute -- it is an undefinable concept, which even Einstein himself admitted. Perhaps watching one of our numerous other videos on the subject before senselessly trolling another video and broadcasting your own ignorance.

[@ptayebati]

Unlike velocity, acceleration is not just a relative concept. In fact in your example of only two rockets being the only two objects in the universe you know you are accelerating when you turn the rockets on and your face starts deforming and your back pressed against your seat vs when you turn the rockets off. That is Einstein’s equivalence principle.

[@dialectphilosophy]

Parviz, check out our other videos, "Can You Feel Force", "Inertial Frames," or "Einstein, Gravity, and the Twin Paradox" if you want to learn more about why no definition of absolute acceleration can be given.

[@Matt-pq4tq]

This is still relative. You feel the force on your back because the rocket is accelerating relative to YOU, and has to push you along. Same with the rest of the rocket attached to the engine, it feels a force from the engine. For the purpose of the experiment we have to imagine that all parts of the rocket including the person all begin to accelerate on their own at the same time, thus not feeling any accelerating force from each other. And there would be no way to tell that you were accelerating.

[@odinata]

@@dialectphilosophy denying the definition of acceleration doesn't make the scientific definition of acceleration wrong

[@peterrobinherbert]

The title of this does not even make sense. The Twin "paradox" does not need a solution. It's not even a paradox, just something that is counter intuitive. It is perfectly consistent and has experimental confirmation.

[@marcv2648]

Which experiment was that?

[@andrewpope799]

The mistake this video makes is that it tries to apply concepts of special relativity to non-inertial reference frames (reference frames that accelerate). Once non-inertial reference frames start being used, special relativity is no longer accurate; you need to use general relativity instead, which nowhere near as simple as reference frames are in special relativity models. That is the "true paradox" in the twin paradox: trying to explain the behavior of an accelerating reference frame using special relativity, which doesn't include accelerating reference frames. In short, acceleration isn't relative in the same way that velocity is. Velocity can be interchanged between reference frames, but acceleration cannot. If I were falling toward the Earth, my acceleration would be about 9.8m/s^2, which is completely fine. However, if the Earth were accelerating toward me at 9.8m/s^2, then at the low speeds we're travelling at, the force causing the acceleration can be closely estimated by F=ma. Given the Earth's large mass, there isn't a force large enough to bring about such a large force. In any inertial reference frame, the Earth and I could be seen as accelerating towards each other, but my acceleration will always be much greater than the Earth's. Likewise, if one of the twins never turns his or her rocker on, then there is no force applied to the rocket, and it cannot be accelerating; instead, it is the other twin's reference frame that is accelerating. The twin paradox has been physically tested through comparing atomic clocks on airplanes with atomic clocks in labs, and the clocks on the planes have returned slightly behind those left in the lab. General relativity completely accounts for this difference in both the lab's inertial reference frame AND the plane's non-inertial reference frame.

[@user-gn6jj8qh1w]

In fact, you really don't need to take into account acceleration to solve this. You can solve this problem with special relativity only. Try to apply Lorentz transformation to time & space coordinate of the destination from the perspective of the other side of the twin. You'll see the position of the twins are actually symmetrical yet it doesn't cause any contradiction. But I also found that I really really didn't understand the concept of special relativity at all. Theory is super consistent but it's also so wired commonsensicality.

[@MattersChris]

7:13 Neither apple nor earth accelerate in your example. Free fall in gravity field is not an acceleration. Acceleration would happen if apple was on earth surface and couldn't free fall any more. You are mistaken about one important thing. In special relativity accelerating frames differ compared to inertial ones. Velocities are relative but acceleration is treated as ABSOLUTE.

[@dialectphilosophy]

Absolute acceleration cannot be defined, and is only a convenient schema, like absolute time or space once were. Please watch our other videos for further clarification on this.

[@MattersChris]

@@dialectphilosophy OK, let's try this way then. What about gravity? Can you define gravity or is it convenient schema as well?

[@kanizh]

@@dialectphilosophy Relativity tells that you could not measure your speed. So if you are on a rocket with no windows, you could not tell your speed. You need to measure it against other object, i.e. speed is relative to other objects. Meanwhile, acceleration is easy to measure with accelerometer. You have one in your mobile phone. Acceleration is absolute value.

[@balabuyew]

Acceleration can be measured. So, the question which twin accelerates has an easy answer.

[@GumbyTheGreen1]

Wow, this is completely wrong. Acceleration *IS* absolute because the force that causes it is absolute. We know from Newton's laws that things accelerate when and only when they're acted on by a force, and we know from relativity that causality is the same (not relative!) in every reference frame. So in both of the twins' frames, their acceleration away from each other is _caused by_ a force on just one of them (e.g., one of their rockets firing, which only that twin can feel), not just correlated with that force. And note that the traveling twin doesn't have to accelerate all the way back to the earth in order to age less than their sibling. For example, if they fly away from earth for 5 years, instead of turning around and flying back for 5 years, they could just continue away from the earth for another 5 years and then "stop" relative to the earth so that they're back in the same frame of reference. Then both of the twins' ages would be the same. It would just take longer for them to verify it with each other (since they'd have to send signals to each other from light years away), but they would certainly come to an agreement on their ages. "The whole point of relativity is that there are no preferred frames of motion". No, the point of *special* relativity is that there are no preferred *inertial* frames of motion. Accelerating frames are *not* inertial. And the point of general relativity is 1) that accelerating frames are equivalent to those that are stationary in a gravitational field and 2) that inertial frames in flat space are equivalent to those that are free falling in a gravitational field (curved space). Therefore, claiming that the question of who's inertial and who's accelerating in flat space is relative is like claiming that the question of who's free falling toward the earth and who's standing on the earth is relative. Would that make any sense? "Alice and Bob won't agree on who is older" after they've reunited? Um, what?? What if one of them ages 50 years (while the other only ages 1 year) and dies of old age right after they reunite? And/or what if the spaceships are equipped with stopwatches? Would there be any disagreement then? Of course not! The amount of time that any person, clock, or other system has been subject to is an absolute fact, not a relative one. A few minutes on the relevant Wikipedia page should've allowed you to realize most of this yourself before spreading questionable info (not that it gets everything right; e.g., it downplays the critical role of force and acceleration). en.m.wikipedia.org/wiki/Twin_paradox Note: An older version of this comment incorrectly said that transitioning through different reference frames and rotating planes of simultaneity were causally more important than acceleration.

[@dialectphilosophy]

We appreciate your passion, however the "switching frames" theory does not account for the asymmetry, 1) because inertial frames are circularly defined (see our video, "Do Inertial Frames Resolve the Twin Paradox?") and 2) because switching frames does not actually resolve the paradox, as demonstrated by cases of the paradox set in curved spacetime. (See our video, Even More Paradoxical: The Twin Paradox in Curved Spacetime.) Thanks for watching!

[@GumbyTheGreen1]

@@dialectphilosophy Inertial frames can be defined as those that are not acted on by any net forces (i.e., the particles that mediate those forces) in a flat spacetime (or that are falling in a gravitational field). Does that sound like a circular definition to you? If so, how? Well I didn't only mention the frame switching; I also mentioned the rotation of planes of simultaneity, which I'd wager also occurs in the versions of the paradox that take place in curved spacetime. We could also look at it in terms of the spacetime interval, which you kind of did in one of your videos. And note that I've updated my comment to put more emphasis on the role of acceleration since that's ultimately what causes everything else (as we know from general relativity and simple reasoning). But are you trying to find an even deeper root cause than that? Root causes are hard to come by - no matter how deep you go, it almost always seems like there's room to go deeper. So are you sure that there's any conceivable resolution to this paradox that would satisfy you? P.S. I've watched all your videos on the topic (some more than once to make sure I'm not missing something in them).

[@Newtspeare]

The Wikipedia article is written by somebody who doesn't understand relativity. If you read the talk page, you will see there are plenty of other editors pointing out that his 'specific example' is nonsense, but the author refuses to change it, saying it is based on a reference. You come across as a typical religious physicist who blindly accepts mainstream nonsense, and feels a religious duty to attack infidels.

[@GumbyTheGreen1]

@@Newtspeare I just read those sections of that Talk page. They aren't saying that the example is nonsense, just that they'd like a note to be added to it. And I don't see anything there that's relevant to what I'm saying. If I'm missing it, feel free to mention exactly what it is. Do you have an argument against some specific thing I've said? If so, go ahead and make it. No need for psychoanalysis, ad hominems, or any other forms of lazy tribalism. I couldn't care less about what's mainstream, just about what seems to be supported by the best evidence and arguments.

[@Newtspeare]

@@GumbyTheGreen1 I wasn't just referring to the current Talk Page, if you search the archives you will see people have been criticising the 'specific example' for years. Wikipedia's 'specific example' states that by accelerating to 4/5 of the speed of light, the travelling-twin reduces the distance to the star from 4 to 12/5 light-years. Nobody would believe such nonsense if they didn't blindly accept what they are told, but you recommended the page to Dialect presumably because you believe it to be true. It is also contradicted by the Wikipedia page on Bell's spaceship paradox.

[@Kuvina]

Hello dialect. I discovered your channel through your tensor video, and I think your videos are great! Although I will admit I'm slightly bothered by your accusatory tone in this video, I can see that you have clearly changed to a more respectful tone in your later videos, which I definitely appreciate. I have a bit of a question though. I know this is a long comment, but if you would read the whole thing, it would make my day, and I really do think it's important, so if you do choose to read it all the way through, thank you so much. The paradox that you introduce is one in which there is nothing in the universe other than the 2 observers, and the only thing you know is that they both see the other one accelerating. In my experience, this is not the same as how I have always heard the twin paradox. I've always heard it where you do have other objects in the universe (like the earth) to base motion around. This is also the way that most explanations introduce it. Something that you didn't mention is that a lot of these explanations also mention the fact that this version is not a true paradox. So your version is a paradox, and so it's unsolvable. The version most explanations explain is not a paradox, and so it is solvable. Because of this, I think this whole controversy is just a debate about semantics. Yes, acceleration is technically relative, but by adding other objects to the universe, you gain something to base acceleration off of, making it, for most intents and purposes, effectively absolute. In this way, acceleration (or change in inertial frames, whatever you want to call it) both does and doesn't solve the twin paradox. If you make acceleration absolute, then it does solve the popular version of the paradox (which isn't actually a paradox). If you acknowledge that acceleration is not absolute, then it demonstrates how your version of the paradox can't even be solved in the first place (making it a true paradox). For that reason, I think that solutions to the twin paradox are not "wrong" as you say, but they just don't solve the same thing that you consider to be the "only true" version of the twin paradox. In other words, the only way to actually solve the paradox is to make acceleration absolute, or to make something else absolute. In one of your later videos, you mentioned that it's not acceleration that determines time dilation, it's the curvature of space time. Yes that's true, but then doesn't that mean that spacetime itself is now absolute? The only way for it to be a paradox is if spacetime isn't absolute, like how you state in this video's introduction, in which case you don't have enough information to know what the curvature of spacetime looks like. In the case of flat spacetime (the way the paradox is almost always given), it would seem that the measured acceleration from an accelerometer does in fact give you enough information to resolve the paradox. So even though acceleration technically isn't relative, for all intents and purposes (at least in the context that the twin paradox is almost always given), it may as well be. That's why I don't think it's an issue to consider acceleration to be absolute, and use that to resolve the paradox. By stating how you have the earth as a reference, and that it's not really a paradox, these explanations are saying that their version of the paradox is solvable, which means that it's reasonable to treat acceleration as absolute. I think it's important to acknowledge that just because an explanation doesn't go as deep as possible, it doesn't mean it's wrong. Just because something is not perfectly accurate, it doesn't mean it's not a useful model. And by the looks of it, the vast majority of people are content with these explanations. Just like how classical phenomena can be explained by a gravitational force, even though gravity isn't a real force, and is instead just the curvature of spacetime. The notion of causation is purely semantic in nature. These videos are not meant to take a deep look into theoretical physics, they're meant to explain science concepts to lay people, and in that aspect, I think considering acceleration to be absolute is a perfectly reasonable thing to do. So now I just want to ask: why are you so passionate about hating these videos when they're so clearly just explaining a more popular version of the paradox to the general public, without needing to go so deep into it? In my opinion, a lot of them succeed in this goal, and there's no reason to fault them for it.

[@dialectphilosophy]

Hi Kuvina, thanks for watching, and we appreciate your interest. Certainly our tone is a little accusatory here, but have no doubt, it is FULLY warranted, and we are happy to explain why. First off, it should be noted that most of the popular twin paradox videos on KZbin are riddled with misconceptions and errors, and don't do a very good job of explaining science concepts to lay people at all. Ted Ed's popular video completely conflates time dilation with the doppler effect for instance; Science Asylum and Dr. Lincoln both wrongly try to make the case that you can construct the paradox without involving acceleration at all; Eugene Khurtoryanski mistakenly promotes the General Principle of Relativity, etc. To the newcomer just attempting to begin learning modern physics, all these errors can perpetuate some very wrong ideas about relativity, and do serious damage to their ability to progress in their understanding of modern physics, while to the layperson only interested in grasping the general concepts, such an explanation will come off with at best as vague and sightly confusing. Obviously no one is perfect, but given the number of viewers and level of professionalism at which these video promote themselves, the propagation of such misconceptions and lack of accountability on the part of their creators to acknowledge their roles in the continuations of misunderstandings deserves every ounce of our frustration and ire. As for the paradox, the problem stems from a lack of awareness on these video's parts on what their viewers are truly confused about in the first place. Sure, understanding the nature of rotating planes of simultaneity is difficult, but it's not the answer to the question most people are asking. The question most people are asking is: "what breaks the symmetry? What allows one observer to say their motion is more real than the other's?" And that question still has no answer (it can't be coordinative acceleration, and absolute acceleration is undefinable, see our later videos for more on this) so as long as the question that you are concerned with answering is that which regards the asymmetry of the twins' observations, then the paradox certainly still is a paradox. Frankly, if you think giving half-answers and side-stepping major philosophical and ontological issues to give a "digestible" or "popular" answer to a question that really doesn't an answer, rather than fully and responsibly acknowledging that our modern theories don't neatly wrap everything up. in a logically consistent bow, then we would reserve an equal amount of frustration and ire for yourself.

[@Kuvina]

Thanks for reading and replying. I completely agree that the popular explanations are filled with mistakes, and they should be held accountable for those mistakes. I do still think it's reasonable to postulate that acceleration is absolute, as long as you acknowledge that this is a simplification of reality, and it's impossible to empirically define an inertial frame. But I guess we'll just have to agree to disagree on that one.

[@petpaltea]

@@Kuvina I completely agree with you, acceleration is absolute because of equivalence principle. Like I said somewhere else in this or other video of his, he is positing special relativity rules in Newtonian ideal homogenous space (where time is absolute) and where speeds AND accelerations are relative. But in general relativity, where speed of light or maximum rate of causality is finite and absolute, the space and time are relative. We also have to think about space-time as one "object," four-dimensional manifold, where time dilations and length contractions also applies, and where acceleration is the same "disturbance" as gravity (again, because of equivalence principle).

[@philsipad]

Acceleration is absolutely not relative. The object that is accelerating is the one that turns matter into energy. If there are two identical rocket ships speeding away from each other, the one ends up with less fuel is the one that has undergone more accelaration.

[@WSFeuer]

you can't accelerate without first specifying what you are accelerating relative to... that's all he means by it

[@LAMilner]

Fuel dilation.

[@andyshreene6308]

Thanks for making this. I was always bothered by those other twin paradox videos too. I'm glad I'm not the only one.

[@shubhamshahi.]

Me too

[@odinata]

Being bothered by them doesn't make the science wrong

[@moti43]

Accelerate is absolute because you can measure it in your reference frame.

[@good4usoul]

I agree with you. Newton's 2nd Law, and Impulse-Momentum Theorem Rely on net force being equal to mass*acceleration, or Delta p/Delta t. That net force is not subject to opinion, and at 7:00 when it is said, "Each twin has an equal right to say that it is the other twin who accelerated" is simply not true. However, I do think that the idea promoted by the video is one of two incompatible descriptions of Relativity. One of these descriptions of Relativity begins with Mach's Principle, which is, in essence, the claim, that "Each twin has equal right to say that it is the other twin who accelerated". The other description of Relativity begins with a principle that the laws of physics are not dependent on the velocity of the observer.

[@Genie890]

Relativity says " one inertial frame is not preferred over the other."...you changed that statement and conveniently omitted the inertial frame part. The minute physics video is the correct explanation...

[@moti43]

you are wrong. It is important who is accelerating and who stand still

[@pdivos]

correct

[@dingdongsilver4783]

What if one is in orbit around the earth and one is firing their rockets to sit still a fixed distance from the earth? The one in orbit is falling while the one sitting still is actually having to accelerate to fight the distortion in spacetime caused by earth's gravity. I haven't done the math, but it seems like even if it mattered who was accelerating and who was sitting still, the argument fundamentally breaks down in that situation.

[@mikkel715]

@@dingdongsilver4783 The GPS satellites shows the reality of relativity. Especially because they are in a slightly elliptical orbit.

[@dingdongsilver4783]

@@mikkel715 ok but 5hat has nothing to do with the situation I just brought up, also it still leaves questions. If relativity is 100% accurate in its current form then you can calculate the movement from the satellite's point of view and the math says the earth's clock should be moving slower.

[@mikkel715]

@@dingdongsilver4783 For this "precise" math. GPS needs four satellites to triangle position, all with different velocities relative to each other. In theory, three satellites are enough, but an additional is needed for a shared time offset, since mobiles clocks are not atomic clocks, and so much too less precise. My first comment relates to that an elliptical orbit causes different velocities at different positions.

[@khimhernane9252]

I respectfully disagree An accelerated frame can be distinguished easily. It is absolute. Only velocity is relative in this case. It comes down to the two postulates of SR. Especially the first postulate. In an inertial frame all laws of physics is the same. Therefore, once in an accelerated frame the laws of physics won't hold. This why it is distinguishable. If in some way we observe that a law of physics does not hold in your frame it is an accelerated frame. So if im in a closed box in space without windows and I dropped one of my tooth to the ground if I observe it to just move horizontally away from me. I know a physics law is broken so I must be accelerating. If another person in a different box happens to observe this occur to me, he can also say that I accelerating. All im saying is that an accelerated frame can easily be determined.

[@dialectphilosophy]

Hey, thanks for your comment! Check out our other video, Can You Feel Force, if you'd like more clarity on the subject of accelerated frames -- but long story short, deducing that you are the one experiencing a force requires knowledge of whether your frame is inertial or not, which in an twins-in-empty-universe scenario cannot be established.

[@ajayvarghese4516]

@@dialectphilosophy does laws of physics hold in accelerating frame

[@andreassteinhauser9508]

Huh, really? My understanding is, that acceleration can be measured locally. So Alice could know, if she's accelerating or not. I claim, that there's no twin paradox, if both are accelerating in opposite directions and come back to the same location. The twin paradox happens, if one is for example not accelerating and the other is, and this difference can be measured.

[@AntiCitizenX]

Except acceleration IS an absolute. All observers in all inertial frames of reference will agree that the twin in the rocket ship is the one who accelerated. They can even measure their own subjective acceleration through use of an accelerometer. That's the asymmetry.

[@The_Canonical_Ensemble]

"Except acceleration IS an absolute" In relativity, 4-acceleration is absolute, acceleration is relative.

[@AntiCitizenX]

@@The_Canonical_Ensemble the hyperbolic rotation angle due to a change in inertial frame of reference is consistent across all observers.

[@The_Canonical_Ensemble]

@@AntiCitizenX Anybody converting between the same two coordinate systems will use the same transformation matrix. Whats your point?

[@AntiCitizenX]

@@The_Canonical_Ensemble It means that acceleration is not “relative.”

[@The_Canonical_Ensemble]

@@AntiCitizenX It's easy to show that the magnitude of acceleration is different for different inertial coordinate systems. For example, when an object is accelerating parallel to the relative velocity of the coordinate systems, the formula for transforming the acceleration from one to the other is: a' = a / (γ(1 - uv/c^2))^3 If the magnitude of acceleration was the same for for all inertial observers it would simply be a' = a. Hence, the magnitude of the acceleration is relative to the coordinate system. Now, it is true that in special relativity whether or not an object is accelerating at all will be the same for all inertial observers. However, this breaks down in general relativity or if you allow for non-inertial coordinate systems. For example, if an object is orbiting around a planet, then it is in an inertial reference frame but an observer using schwarzschild coordinates will calculate them to have an acceleration even though according to the object it is not accelerating.

[@ablebaker8664]

No... There is a difference. The acceleration produces a counterforce equivalent to gravity. Each observer has the reference of their own inertial acceleration to compare to their visually apparent acceleration away from each other. By comparing the rate they appear to be moving away relative to each other to their own G-force, each can determine independently of each other, which of them is accelerating, and at what rate. This is true if both accelerate, if either accelerates, and if they both accelerate at different or similar rates. If both accelerate away from each other and then return, there is no paradox. Their clocks BOTH slow by time dilation according to their own rate of acceleration and those rates of acceleration are independently discernible. When they reverse their course and return to each other, the age of each will be a matter of the time dilation each experienced, and that will have a measurable dependency on the G-force generated by the acceleration of each, independent of the other. If they experience equal g-force, their clocks will slow similarly and each will have the same net time dilation. If they experience different g-forces, each will be able to predict their aging difference on return by comparing their visually apparent acceleration to the g-force they experience. If one remains stationary while the other accelerates away, the stationary observers clock may appear to the accelerating observer to slow down, BUT if it's compared to the G-force generated by acceleration, the accelerating observer will be able to correct this and deduce that it is his own clock which appears to him to tick quite normally, that has slowed. If gravity wells are involved, then it becomes an n-body problem with the time dilation effects from each source of acceleration force accounted over the duration of the trip at whatever rate of change to that G-force that each experiences. What is important is not whether they are in relative motion. What is important is acceleration. An observer on a relatively stationary planet still experiences acceleration from planetary gravity, and so does the traveler leaving the gravity well on their trip away from the stationary observer, BUT the traveler has an additional acceleration which both observers can independently verify as unique to the traveler by the additional acceleration force experienced only by the traveler. Regardless of what other bodies become involved in the problem, it is the acceleration forces imposed on both observers that produce the temporal effect each experiences. The observer experiencing the highest acceleration (and g-forces) will experience the largest time dilation effect.

[@Wink-Wright]

Why can't acceleration be absolute when you have two ships? You can always measure the change in velocity of a ship to its pilot, which gives you an inertial frame to reference from. Dropping something within the cabin while accelerating also does this. As mentioned in every video you referenced, the dialation is clear based on the spacetime path taken. The spacetime diagrams illustrate this, but you've decided to ignore it. Weird video, its misinformation and the humor is sexist.

[@benradick1489]

I think acceleration is absolute...

[@kevinmcnamee6006]

I think we've all experienced the feeling of acceleration, so it should be pretty easy for Bob and Alice to figure out who is accelerating and who is not. Acceleration is not relative.

[@timhill9039]

As others have noted here, you CAN tell which of the two rockets accelerates. If you seal a human inside each rocket with no windows etc, you can detect which rocket undergoes acceleration entirely within your sealed room (using any number of simple methods, including just "feeling" the rocket accelerate). This breaks the supposed symmetry and resolves the paradox. If BOTH rockets were to accurate exactly the same amount, then BOTH turn around and return to the same point neither twin would be older than the other.

[@dialectphilosophy]

The acceleration you measure is merely "relative" to a pre-established inertial reference system; it cannot be used to define a non-inertial frame which would allow an observer to truly know they are accelerating. See our follow-up videos for more on this.

[@timhill9039]

@@dialectphilosophy Incorrect. If you are inside a sealed box you CAN measure acceleration if the box is accelerated by some means. By definition your reference inertial frame is your own local frame BEFORE the acceleration began (because in a sealed box you have no other frame), but nevertheless you know you were accelerated, and by how much. That means there IS a measurable difference between the two rockets that can be determined without looking out the window. Sure, looking out of the window, the VISUAL appearance is the same from both rockets (they recede from each other at any increasing rate), but there are experiments you can perform to determine which is accelerating (most obviously because you can feel it). This is what breaks the apparent visual symmetry, and is a result of the difference between special and general relativity. (What you CANNOT determine in your sealed box is if you accelerated or if you are in a gravitational field, which is why the twin paradox applies equally to accelerations and twins in different gravitational fields.)

[@milosvukovic2378]

What breaks the symmetry is the FORCE the rocket twin PERCEIVES upon firing of his rockets! The Earth-twin or other platform-twin DOESN'T perceive that force. Variables are "perceivables". Space, time, velocity are perceivables equaly as FORCE is. The Earth twin perceives space, time and velocity, and the rocket twin perceives all the three, and force in adition to them! So, the twin paradox outcome boils down to the question: "Who is subjected to force and in which measure/amount (time/path and intensity of action)?" The answer to it will determine who will be younger upon reunion. We neglect the Earth's gravity effect on time passage and the Earth has a "free fall" from the standpoint of the rocket-brother at the very moment he gets subjected to the propulsion (he also moves out of the zero reference frame determined by the center of total mass of the cosmos, breaks it whereas the earth remains in it)

[@davannaleah]

You have neglected the fact that, for the object to be accelerated, a force must be applied resulting in an expenditure of energy. The other obect may 'appear' to accelerate relative to the accelerating object but there is no force applied.

[@seneca983]

I don't see a problem with absolute acceleration. Acceleration *is* absolute in special relativity. We can also measure acceleration with an accelerometer and that measured acceleration is absolute (unlike, say, velocity or position). You have argued that accelerometers need to be calibrated and whether they're calibrated in an inertial or accelerating frame changes the result. However, as shown by the channel "Physics - problems and solutions" in a video titled "Is Acceleration Relative??? Dialect is WRONG!!!" you can, at least in principle, calibrate an accelerometer such that it can detect acceleration even if that acceleration is the same as it was calibrated in. I also know that you have seen that video since there's a pinned comment by you under that video.

[@romanempire3905]

Respectfully, I think you're wrong. We can measure acceleration and therefore both twins would agree who had a change in their acceleration/gravity. That's where the symmetry breaks.

[@dialectphilosophy]

Check out our other videos if you want answers to those questions -- or check out "The Twin Paradox in Curved Spacetime" if you want to learn why acceleration actually has nothing to do with the breaking of the symmetry whatsoever!

[@romanempire3905]

Dialect well, it's not the acceleration per se that breaks the symmetry but the change in reference frame. Both twins would agree that only one of them changed reference frames during the time they were separated. Check out Fermilab's video about this, he did 2 videos - one with a little math involved and one without math at all.

[@dialectphilosophy]

@@romanempire3905 Fermilab’s explanation is sadly incorrect and promotes an entirely false conception about the paradox, as changing references frames actually has nothing to do with the breaking the symmetry. Check out our videos “Do Inertial Frames Resolve the Twin Paradox?” and “The Twin Paradox in Curved Spacetime” to learn why.

[@romanempire3905]

Dialect I just watched your video on inertial reference frames. I think you completely missed the point because you moved quickly to talk about acceleration and forces. This shows you didn't understand the issue and the solution to the apparent paradox as was perfectly explained in the Fermilab video. In the Fermilab video he gives an example with no acceleration or forces whatsoever to show that on the most basic level what causes the break of symmetry is the changing of reference frames and not acceleration or forces. In that great video he explained very neatly that you can devise a thought experiment (which is all we do here anyway) in which ALL observers are inertial, meaning they don't change velocity, but they transfer INFORMATION between them so the information itself (the time elapsed in traveling from one point to another) is changing reference frames. I hope it helps and finally gives you a good and deep understanding of this amazing topic.

[@gman8563]

@@romanempire3905 The twin paradox REQUIRES that a clock/object return to an initial location, else you can’t even set-up the paradox. Fermilab doesn’t understand the problem any more than you do. In the example he gives, the observers just pass the information back and forth about what time a clock WOULD read if it WERE to travel along a return journey. They could theoretically pass back and forth any information they wanted to, like that corresponding to a clock that isn’t even on their ship, rendering the whole set-up pretty silly when you think about it. Additionally, you can recover the paradox simply by having a fourth observer passing earth in the opposite direction with the opposite velocity - he shares his information with earth and then bam, you’re right back at the paradox. Finally, in curved spacetime the inertial-twin can actually end up being the younger one instead of the accelerating one, rendering the whole “changing frames” hypothesis mute. Dialect explains this neartly in his later videos, maybe you should watch them before assuming you know more than other people ( especially those who’ve clearly studied the problem for years) and then go about promoting the wrong answer.

[@somnolentprometheus8466]

It's so silly. He claims that the twins have to somehow agree as if it's some arbitrary thing, but even without an earth, the rocket twin can do experiments inside his ship that prove just to them that they are definitely not in an inertial frame of reference for the entire trip, while, at the same time, if someone just on earth without reference to the rocket ship, were to make the same experiments, they would determine they are in an inertial frame of reference. That's the assymetry

[@oldjoec3710]

Right on, Prometheus. Both twins get slammed in the back by the respective accelerations. The original inertial frame of reference is still there, even if there's nothing in it. If they left a clock at that point, they would both find that they had aged less than the clock when they return. These two are just executing two different copies of the same twin paradox.

[@somnolentprometheus8466]

Exactly. Acceleration isn't even necessary. Imagine twin A, on earth. Twin B, was on its way out of earth already, but when he crosses paths with twin A, they throw a doll to space. Twin B makes it to andromeda, where his friend "Friend (TM)" was already on the inward journey to earth. Twin B tells "Friend" how much time his clock recorded. "Friend" records his own time from andromeda to earth. 0 acceleration at ALL points. Total time to andromeda is DIFFERENT from total time counted on earth. But if you trace a path from twin A starting position to end position, the doll they threw in space, which actually has a clock on it, would measure time normally. However, the path you trace following Twin B forces the doll's clock to have a discontinuity when he asks his "Friend" how fast the doll's clock ticks. That path necessarily breaks the doll's clock ticking, while twin A from earth always measures the clock ticking at the same rate. In fact, if twin B kept going, we would have complete symmetry in the situation, as they would both measure the constant doll clock ticking. Only when the path changes, does both DOLL CLOCK and EARTH CLOCK show a jump. How could it be, the doll was thrown in space when both twins were in the same bloody place. And they both initially found some clock rate (different for each one) that was constant. How can they produce such garbage videos, these Dialect people, and they can't even think of this argument within 20 seconds of thinking. No need for accelerations, or fine-tuning lasers to count time, or assuming we are all already accelerated, or whatever. It's all nonsense from people who haven't even opened a textbook.

[@ninajoyce9906]

100% agree

[@grawl69]

Thank you, you may be the first one on YT to formulate the problem clearly in this way. Comments here show that people still don't get what you are talking about. :) You talk about logical grounds, only hinting at the physical (Mach's) explanation. They talk about physics straight away, calling physical sensation during acceleration etc. Completely misunderstanding the point. Most people cannot accept that twin paradox (i.e. the meaning / cause of absolute acceleration) is a difficult and deep problem at the fundaments of physics. They think it has a simple solution, Einstein solved everything and others just can't get it. Even most physicist think so! This is so funny and sort of sad at the same time. I recommend 'The Most Fundamental Problem of Gravity is Solved' on Unzicker's Real Physics.

[@dekox]

But acceleration IS absolute! You can measure it and know which of the two ships is accelerating. Using an accelerometer you can measure your acceleration. And if your accelerometer reads nothing and a spaceship standing still 1 km away starts accelerating towards you, you know that it is accelerating and not you.

[@dialectphilosophy]

That's incorrect; an accelerometer only reads acceleration relative to the frame of calibration. Moreover, acceleration is mathematical and cannot be defined unless it is defined in relation to a specified coordinate system. See our videos "Newton vs. Mach" or "Why Relativity Doesn't Add Up" for more

[@MrCmon113]

​@@dialectphilosophyYou calibrate it at the start. Also you feel the rockets pushing you, which the other twin doesn't. Even if you didn't know about the outside world, you'd know for sure that you're either being accelerated or under gravitational influence and you'd come out being younger either way.

[@CT-pi2gl]

If the acceleration and direction change occurred via gravity assist from a planet, I don't believe the accelerometer would read anything but zero the whole time.

[@peterrobinherbert]

And the maker of this video doesn't understand relativity. There is something that can tell which twin is accelerating because as he is accelerating he will see light curve whereas the stay at home twin will not see the local light curve. It is not a symmetric situation. As I said earlier the twin paradox is not even a paradox, its just counter intuitive. It is perfectly consistent. The math works and it has been confirmed experimentally. It doesn't need a solution. There is no problem to solve.

[@mericegeericek1032]

The paradox isn't just about that. Instead of going on opposite directions, imagine the twin in the rocket travelling near the speed of light at a circular motion which's radius is bigger than our Milky way galaxy or any fixed state where Earth also has an acceleration relatively. Does math work same in that situation too by adding absolute acceleration which Earth also absolutely has.

[@mericegeericek1032]

All of the search is to understand why. Experiment totally proves the point but that also states that Gravity also is related strongly to relative motion

[@alejrandom6592]

Exactly!

[@LudusYT]

Sorry, but I believe that 1:33 and 1:49 are incorrect. Neither Alice nor Bob can say that they are stationary. They are both accelerating, and therefore not in inertial reference frames. I think that is where many students become confused with this. I haven't taken GR yet, but I assume that is where we get into more of the non-inertial reference frame stuff. You can't say that only one observer is accelerating in your example, because they both feel a force, which means (by Newton's second law) that they are both accelerating.

[@angel_machariel]

The narrator used clumsy description of the process, but luckily didn't use the word "accelerate". It's a huge problem that instantaneously reached speed still can be interpreted by the masses as "acceleration". So you'd be right. It's not an inertial frame of reference. It's better to use examples where objects were moving already, instead of beginning at speed = 0! That would eliminate all illusions of acceleration. But then you'll find even less people will understand it, even though the Lorentz Transformation is quite easy to use. But 99% doesn't even have that relative basic math skill either... So good luck explaining. But when you get there, just remember, all you need is the Lorentz Transforms to create the world like and simultaneity line for the traveler. Then notice how the traveler changes frame of reference INSTANTLY at half-way point. And that is the way to solve the paradox. You'll see.

[@trsomas]

When you talk about coordinate acceleration, they both are accelerating. When you talk about proper acceleration, then only the twin who goes into a spaceship and comes back is accelerating. Proper acceleration is acceleration relative to an inertial frame. We can also say that proper acceleration depends on total real force (no pseudo force). The twin in spaceship experiences real force, so only he has proper acceleration. kzbin.info/www/bejne/mpLaf3Zqaa1miJo

[@ANGRYpooCHUCKER]

Close but no cigar: you do NOT need GR to solve this "paradox." The key fact that most people miss is that the Earth twin and rocket twin both START in the SAME INERTIAL FRAME. At some point, the rocket twin leaves and then comes back, but the Earth twin is still in an inertial frame. Thus, the rocket twin is objectively doing his/her journey within two DIFFERENT inertial frames and thus you can't apply SR in the SAME WAY with him/her.

[@ulyssesmombo]

Hi! Great video but I didn’t agree with your reasoning. Yes, it’s not possible to determine which of two bodies is actually accelerating in space when considering relative velocities alone. From Newton's second law, acceleration can be measured through an unbalanced force. So, body A at rest (or constant velocity) will not experience a force, body B will and so is accelerating. Hence, A is continuously in an inertial frame, whereas B only is for part of the journey. Due to this distinction I would suggest that each frame preferentially has a different experience.

[@dialectphilosophy]

Thanks for watching! Check out our follow up videos if you'd like to see those ideas (and whether or not they can actually explain the broken symmetry) explored in detail.

[@briandavis9414]

You put a lot of time into this to be completely wrong. You started wrong with two accelerating ships, interpreted the perspectives incorrectly from Alice and Bob, and concluded incorrectly too. You mischaracterize the entire problem and arrive at a contrary conclusion. Misconceptions are rife and you should probably take down the video but you've got the views and interaction that gives you the cash to drive you forward so you'll leave it up.

[@dialectphilosophy]

Hi Brian, feel to check out our other videos if you want to learn about why Einstein agrees with us, or why your prior conceptions are ill-founded, or why the stifling of critical thinking and philosophical questioning by hive-minded trollers such as yourself only inhibits the advancement and progress of physics. Thank you!

[@danthewalsh]

I liked your video on the Christoffel symbols, but this video wrongly accuses other explanations on KZbin of being incorrect. There is absolutely an asymmetry between the two twins due to the difference in their acceleration, and this is what makes one twin age while the other does not. Special relativity does *not* state that proper acceleration is relative, it states that velocity is relative. There is absolutely an experimentally measurable difference between the twin who stays home in the inertial frame and the one who leaves on the trip, which can be measured by an accelerometer. When the acceleration occurs, it changes the lines of simultaneity of the accelerating twin such that the twin left behind will age very rapidly during the turnaround from the traveling twin’s point of view. Why do you object to this explanation?

[@dialectphilosophy]

Hey there, thanks for watching. Unfortunately, these other videos are indeed incorrect. An accelerometer only measures acceleration relative to a frame of calibration, and is therefore not a measure of absolute acceleration, a concept which cannot actually be defined and which even Einstein objected to. We discuss this in several other videos, most recently "Why the Theory of Relativity Doesn't Add Up." We recommend you check them out.

[@danthewalsh]

@@dialectphilosophy Even if the accelerometer was “calibrated” in an accelerating reference frame, this could be easily determined by performing two measurements, one of which has the sensor rotated by 180 degrees. By comparing the two measurements, it is easy to determine whether the accelerometer is biased in the plane of rotation. Regardless, the physical setup is asymmetric, too. The traveling twin really does have to boost its rockets to turn around, while the twin on Earth does not. The occupants of the ship can *feel* the acceleration, while the twin on Earth cannot. Even without appealing to force, it’s really quite easy to detect whether you are in an inertial reference frame: just set a mass down in front of you at rest (in your frame) and measure whether it moves once you release it. This is an accelerometer that requires no calibration besides the calibration of distances and clocks in your frame, which is something you already make use of in your videos. So no, acceleration is a well-defined concept, and this makes the traveling twin intrinsically different from the twin who stays home.

[@DANGJOS]

​@@dialectphilosophy I'm curious as to your answer to this point. You didn't respond.

[@GrouchierThanThou]

Acceleration is absolute in the sense that you can equate not accelerating to moving along a geodesic of the curved spacetime (aka free fall). The absolute acceleration is then just the deviation from free fall.

[@rsm3t]

If we're going to say you can only measure acceleration wrt distant stars, then we may as well just return to Mach's principle and be done with modern physics. But firing a rocket necessitates that you experience acceleration. Alice and Bob follow different paths from point A in spacetime to point B in spacetime. In the usual scenario, Alice's path is geodesic (modulo a slight deviation due to Earth's and Sol's gravitational fields), while Bob's is not. In your modified problem, neither Alice nor Bob is following a geodesic path. The clue to that is that they both have to fire rockets in order to reach point B. So both will have shorter path lengths in spacetime than the geodesic path from A to B. If they undergo equal but opposite acceleration profiles, then they will age by the same amount. I think this video will only confuse people.

[@spencerftn1]

In the beginning of the video you have a screenshot from Fermilab. But, you didn't address his video. Science Asylum has a video too. Both of them say thee time difference is because the "traveling twin" has 2 inertial reference frames, distinct from each other. And even though they point out the acceleration on MinutePhysics, Henry says that the time difference is from the traveling twin having two inertial reference frames.

[@RadicDotkey]

You're also wrong, even more than your examples. You claim the acceleration is arbitrary. Speed is, not the acceleration which is felt as a G force by anyone, anywhere in space.

[@ayushdeshmukh284]

@Angus Chandler That's the point. If both have accelerated exactly equally, both will end up with the same age. If A never turns on his engines, he will be older. If he accelerates less than B, he will also be older. As for reducing the objects to two points with nothing else in the universe, the experiment becomes impossible. Those objects cannot accelerate for any reason. (Other than gravity, according to which they aren't truly accelerating)

[@mikkel715]

@@ayushdeshmukh284 Was the first acceleration maybe actually a deceleration from an earlier acceleration? Maybe for only one of the objects..➡️➡️🔄⏯️🔂

[@nickandrew4650]

You say these other videos are all wrong, then claim that acceleration is not absolute? It is absolute; I don't need to look at fixed stars to tell whether I'm going faster or slower. In the Earth's gravitational field I am accelerating even when standing still on the ground. An object in freefall is not accelerating.

[@mementomori7160]

But that's the thing, when in free fall you don't feel any acceleration, because every part of your body is accelerated at the same time equally, but standing on the ground, the ground pushes on your feet which push more and more of your body, and that's why you can tell where the ground is. If there existed a constant force acting on every atom of your body, you wouldn't be able to feel it, but for the observer not affected by this force you'd seem accelerating, then how can he(the observer) say that you are affect by this force and they are not, if the problem is symmetrical? It does make sense And trust me, not long ago I thought that acceleration should be absolute, but now that I think about it(if what I said was unclear I might try to make it clearer next time) it really isn't Edit: I check their videos and they made one exactly about that

[@paulfrindle7144]

This is just wrong - in fact the previous 2 I just saw in this series are wrong too. These videos are just confusing people even more. Objects do not need to be physically moving with relation to each other to experience different time at all. It's about acceleration. So if you have 2 clocks and place one on the ground and another up Mt Everest, they will experience a constant recalibration in time - even though they have not moved in relation to each other. This is because of the time dilation caused by the acceleration of the earth's gravity - even when they are NOT moving in relation to each other.

[@JPMESrocks]

Which reference frame is accelerating is absolute and relies on the net forces acting on the reference frames therefore which frame is accelerating is not relative. I do wonder of the outcome of the paradox in a situation in which both frames accelerate

[@joepierson3859]

If both accelerated identically than they come back the same age.

[@wesjohnson6833]

Yet "firing" my rocket is a physical fact. And all agree on such events in relativity. Kinda the main point of the theory, not that everything is relative. Taking the stars and earth away isn't going to change that event. Relativity is an invariance theory. There is one solution, someone is younger. As for acceleration...did you get up this morning and think you were floating in space? Or did you understand you were still on earth? If so, then you detected acceleration.

[@paradisephoenix3862]

wes johnson while “firing” the rocket is a physical fact its consequences are not. If I fire the rocket to turn around I could say that when I did so you started moving towards me just as you can say that I’m turning around when I fire the rocket I can say you turned around when I fired the rocket which is why we shouldn’t consider them as ships but equal objects like two identical planets for example

[@wesjohnson6833]

@@paradisephoenix3862 Except one of the planets is using energy to change their momentum. And since both energy and momentum are conserved, we'll be able to tell which did what. As far as perception is concerned, there will always be a frame of reference where they both start out with equal motion so which one does the moving will be detectable also. The consequences are not physical? Something about an equal and opposite reaction??

[@EugeneKhutoryansky]

You are misrepresenting what I said in my video. You claim that I said that both Adam and Sarah will agree that it is Adam who accelerated. This is not the case. I stated that Adam will believe that he was standing still during the entire journey, including during the time when he fired his rockets.

[@dialectphilosophy]

Hi Eugene, thanks for leaving your comment. We really love your videos! But in this instance, your video makes the same mistake that other 'solutions' to the twin paradox make, and misses the point of the paradox entirely. It is true at the opening in your video you state that both Adam and Sarah each believe they are standing still. However, later you state that it is only from Adam's point of view that the gravitational field exists when he "fires his rockets". You give no explanation as to why Sarah sees no such symmetrical field on her end. Hence, what you have actually done without realizing it is attribute the appearance of the gravitational field to the "firing of the rockets". This is equivalent to assigning, a priori, an absolute acceleration to Adam's ship, i.e. you are saying, even if he believes he is standing still, the rest of the universe disagrees and sees him as accelerating. This is absolute acceleration. Hence, we have not misinterpreted what you have said; you have simply neglected to extend the implications of your own ideas to their logical endpoint. The twin paradox is really about trying to understand why we can assign this a priori, absolute acceleration to one reality over the other.

[@EugeneKhutoryansky]

@@dialectphilosophy No, I am not making the mistake that the other videos did. I explicitly stated that Adam believes that he is not accelerating, and that Adam will instead believe that he is standing still during the entire journey, including during the time that he is firing his rockets. The reason he is the only one of the two observers who believes that the gravitational field is present is precisely because this gravitational field is necessary for Adam to believe that he is standing still and not accelerating. Both Adam and Sarah will agree that the rockets are creating a force on Adam’s ship. However, from Adam’s point of view, the gravitational field exactly balances out the force from his rockets, causing the ship to stand still while the rockets are on. Adam will also believe that this gravitational field is causing the Earth and the rest of the Universe to accelerate towards him. So from Adam’s point of view, it is Sarah and the rest of the entire universe that is accelerating, while his ship never accelerates and remains at rest during the entire journey.

[@dialectphilosophy]

@@EugeneKhutoryansky Hey Eugene, you've again missed the point. A better question to ask is perhaps, why does Sarah not see a gravitational field like Adam does? The twin paradox is about symmetry. Take away the Earth and the universe and just leave Adam and Sarah, and everything Adam experiences Sarah should too (including the appearance of a time-dilating gravitational field). In your comment above you write, "the reason he is the only one of the two observers who believes that the gravitational field is present is precisely because this gravitational field is necessary for Adam to believe that he is standing still and not accelerating." This is tautological, since you have already decided, from a universal frame, that Adam is the one accelerating. If you had decided that Sarah was the one accelerating from the universal frame, i.e, firing her rockets, then SHE would need a gravitational field in order to believe she in not accelerating. YOU are the one making the decision whether it is Adam or Sarah firing their rockets. Nothing in your formalism tells you who is the one turning around/accelerating/firing their rockets and hence who is the one who should see the gravitational field -- you are assuming it based on a priori prejudices. This is where you make the same mistake as the other videos. You do not understand that you are not getting out any more information than the assumptions you are putting in. You are not getting any closer to explaining the paradox because you are not able to tell us why Sarah's perspective is more real than Adam's.

[@EugeneKhutoryansky]

@@dialectphilosophy You can take away the Earth, take away all the stars, and take away the rest of the entire Universe. Suppose that you just have Adam and Sarah each in their own individual space ship. Adam and Sarah will both agree that Adam fired his rockets, because they can both see the flames coming out from the back of his spaceship. They will also both agree that Sarah did not fire her rockets. For Adam to believe that he is standing still (and not accelerating) he needs to believe that there is a gravitational field exactly balancing the force from his rockets. For Sarah to believe that she is standing still, she needs to believe that there is no gravitational field, because her rockets are off and not exerting any forces on her spaceship. From Sarah’s point of view, Adam is accelerating towards her because of the force from his rockets. From Adam’s point of view, Sarah is accelerating towards him due to the gravitational field he thinks is present.

[@dialectphilosophy]

@@EugeneKhutoryansky Awesome -- you are very close to understanding the point we are making! It boils down to the assumptions you have made about "firing your rockets". We're going to try to be a little more precise with what we are saying, so we can best elucidate where our disagreement lies. Let's define our problem succinctly. We wish to know what breaks the symmetry of the twin paradox. Here is your argument, broken down into a step-by-step inference, as we understand it: 1) Sarah and Adam do not agree who is accelerating. (Acceleration is subjective.) 2) Sarah and Adam do agree that Adam fires his rockets. ('Firing rockets' is objective.) 3) Adam 'firing his rockets' is the cause for the appearance of a gravitational field. 4) The gravitational time contraction compensates for the kinematic time dilation, so that Adam and Sarah both agree on who has aged less. Now the question we are asking is: what is the objective, shared reality here between Adam and Sarah that is responsible for the breaking of the symmetry? Your video states it is the appearance of the gravitational field. However, as deduced in 3) the gravitation field appears ONLY because Adam 'fired his rockets'. Hence, according to your argument, it is actually 'firing the rockets' that breaks the symmetry of the situation. So the twin paradox is resolved by the firing rockets. This is certainly reasonable, because it is an asymmetry that exists within the scenario. But that was not what your video was arguing. And moreover, if you begin to examine it, it feels a little arbitrary. There are many ways to accelerate that do not involve rocket combustion. Are you saying the release of chemical energy on Adam's end is responsible for the asymmetry? Moreover, how certain could Sarah ever be that Adam is really firing his rockets? What if his ship is just on fire? What if we don't have access to the information about who really fired their rockets and who didn't? The statement "fired his rockets" is really just a statement about what the rest of the universe believes Adam is doing. The rest of the universe believes that he is turning around. Now, take away the rest of the universe, and suddenly the situation becomes a lot more confusing and paradoxical.

[@timl2k11]

Why wouldn’t all observers agree it is the space traveling twin that really accelerates? From earth, as the space traveling twin accelerates I would be able to see this. Everything in the ship would be affected by the acceleration (nothing would be freely floating anymore for the duration of the acceleration. The person in the ship would see this, so would the person on earth. The person in the ship looking at the person on earth would not see any change in acceleration of the person on earth. That is as they look at the person on earth as they reverse direction, the dropped apple would still accelerate to the ground at 1g. So yes acceleration is absolute. The person in the ship could do experiments that show he is accelerating and the person on earth could see the results of those experiments. The person on earth, while the rocket is accelerating could do experiments that show there is no change in his acceleration and the person on the rocket could see this. They would both agree who is accelerating. Is this wrong?

[@sistajoseph]

I am convinced you are mistaken. Acceleration is absolute. To every acceleration there is an equal and opposite deceleration. I think that alone should make it absolute. Think about it.

[@stewiesaidthat]

It's not so much that Acceleration is absolute -> nothing can travel faster than light because it becomes light. It's the fact that space and time are separate frames of reference. Relativists are taking the readout from a device that measures distances in space and applying those values to the time frame. Then, without any evidence, are asserting that time-dilation is occurring. This is what happens when they allow religion in schools and kids don't get the proper education.

[@Raptor302]

I would think so. And it should be possible to determine. Whoever is expending energy is the one accelerating.

[@alejrandom6592]

Your conclusion is right but your reasoning is not

[@user-ll2gi2dl2n]

Acceleration requires energy. The more a given mass accelerates the more energy it has. Whats the difference between the traveling twin and the non traveling twin? Energy. What distorts spacetime perception or energy?

[@donr7327]

Acceleration is absolute - an observer "feels" the CHANGE in velocity regardless of what he or any observer thinks the relative velocity is. That said, these solutions should compute the age differences using general relativity, not special. The effect would be much less. However there's still a paradox. Take 2 ships with identical flight paths apart and back together (same acceleration / deceleration etc.) Who's older? Intuition says they'll be the same age, but I don't know how to show that using SR.

[@santiagoerroalvarez7955]

If A and B have the same flight plan, but in opposite directions, they should come back to the same point. You could introduce an observer C that doesn't accelerate and is present at both events: when they fly away from each other and when they get back together. Because that observer doesn't feel any acceleration (and that, as you pointed out, is measurable) his metric is the usual Minkowski metric. You could draw a spacetime diagram in which both A and B fly away from C, travel a certain distance and come back. Both paths are obviously symmetric, but because we now have C's coordinates and metric, we can calculate all the relevant spacetime intervals and conclude that A and B are indeed the same age when they get back together. The video correctly states that all reference frames are equally valid, but that doesn't mean that their metrics are the same. Say you wanted to consider A's coordinate system, for instance. These coordinates must satisfy that x=0 for all points in A's trajectory. In a spacetime diagram that uses these coordinates, A would appear to be stationary, as his path would look as a straight line in the time direction. B would appear to fly away, accelerate, and fly back to A's position. However, the paradox is avoided when we consider the fact that, in these coordinates, the metric has changed. It is no longer the Minkowski metric, because A isn't really inertial. If you were to calculate the spacetime intervals using the new coordinates and the new metric, you would find that both A and B are the same age when they get back together. To be clear, I'm not sure right now how that metric would look like. But I am certain about the fact that metrics always change in the precise way to preserve the lengths of all spacetime intervals. Hope this helped. It's a little mathematical but, in the end, the only way to deepen our understanding and clear up these false paradoxes is to take a look at the math. And I say "false" because this paradox is not a sign that the theory is wrong, but rather that we aren't really understanding an aspect of the theory.

[@kevinharrison5019]

The assertion that because there is no absolute velocity, there is no absolute acceleration seems false. Because acceleration is a change in velocity, this "absolute velocity" is cancelled out. This clearly applies to newton, where any two "inertial" frames of reference will behave the same regardless of their relative velocities to one another. Velocity doesn't make any difference, but the change in velocity makes a great deal of distance. Can somebody weigh in on whether acceleration is arbitrary in the same way as velocity in the theory of relativity?

[@Andrey.Balandin]

Kevin, you are absolutely right, there is a way to distinguish whether you are accelerating or not and you can tell the whether it's you or someone else is accelerating. Dialect's claim that accelerating and non-accelerating (inertial) frames should be indistinguishable is false. In special relativity, you can't tell if you are are standing still or moving at a constant speed. In GR you can't tell if you are accelerating due to a force pushing you or if you are standing in a gravitational field on something fixed that prevents you from falling with the field. But you can't tell if you are free falling in a gravitational field or floating free or moving at a constant speed. One simple experiment you can run in all of those cases is bounce an apple in your hand. If it is floating next to you - you are either in free fall in gravity or there is no gravity and you are moving at a constant speed. If your apple falls back into your hand, you are in an accelerating frame, that is either resisting free fall with gravity or accelerating under an external force. You can also suspend a weight on a spring and measure the acceleration by spring extension. That said, all three videos are wrong in their explanation of the paradox because they rely on acceleration. But special relativity works without acceleration. The time dilation, length contraction and mass increase effect happens in motion at a constant speed. The real solution is in length contraction. As soon as you start moving, the distance you need to travel becomes shorter. You no longer have travel as far as seemed initially. And even though your time ticks slower and you think the same of others, you will find that you don't agree on how far you had to travel, and that's how you know who is travelling - the one who thinks the distance is shorter. The net effect is that less time passes for the one who is travelling and they end up younger. Acceleration does slow down the time passage even further which only exacerbates time dilation effect through GR which is in addition to time dilation of SR. So there is a way to tell who is moving (by comparing the distance measurements between observers) but not by any experiment that you can run inside the ship (your reference frame).

[@debtanupatra3385]

Acceleration in flat space time is absolute. If you keep a filled water glass in the spaceship the water will spill out of the glass while changing velocity. No matter which frame you look from. The spilling of water defines the non-inertialness. The water of the glass kept on earth frame wont spill out. (Even if you remove the gravity of earth). Because its inertial. (Not accelerating). Conclusion: the frame whose water of glass spills out, will be the one who ages slower. If both glass(of two accelerating spaceships) spills water then the symmetry is conserved, otherwise not!! There is no absolute rest frame. But your own frame is the "special" frame, not the stars situated far away from you!! (Stars are not ethers and no ether exists. 😂)

[@dialectphilosophy]

Absolute acceleration is impossible to define: check out our follow-up videos for more details!

[@ninajoyce9906]

@@dialectphilosophy i still dont fully understand, if there are forces acting on that object it is accelerating, how can there be perceived forces on another object, isnt that the whole point of inertial frames, Einstein's first postulate.

[@stewiesaidthat]

@@dialectphilosophy have any of you kids actually had a real job. Why don't you join the NAVY and apply for duty on an aircraft carrier. You will then see real acceleration and deceleration forces in action. Then you will truly understand that acceleration/deceleration forces INCREASE aging.

[@nauka9634]

This guy is so wrong about this. Close to lunacy.

[@Zorro33313]

7:44 - The answer is "reaction force". Neither bob nor Alice can claim they're at rest, if they experience *reaction force* that occurs when acceleration is applied to mass.

[@b.munster2830]

Exactly!

[@eleventhchimp]

In both special and general relativity, acceleration is absolute (the free falling object is the inertial one btw). Distance, position, speed and velocity are all spatial relationships between two objects and cannot be measured on a single object (because they are all between objects). None of them would have any meaning in a universe with only one object in it. This is not true for acceleration. If there was only one object in the universe, we would still be able to measure it’s acceleration. Actually, if there was only one object in the universe, it would not be able to accelerate. An acceleration requires a force. A force is always between two objects with mass and influences them equally, but in opposite directions. The total change in momentum in an isolated system is always 0. If you consider all three objects; alice, bob, and the mass that has to accelerate away from bob for him to turn back, the problem is no longer symmetric. If they both turn around and accelerate back it becomes symmetric again (now there are four objects), but in this case there will be no age difference between alice and bob. The paradox is apparent only, and requires a misunderstanding or obfuscation of special relativity.

[@dialectphilosophy]

While SR and GR treats acceleration as absolute, even Einstein thought absolute motion made no sense; this is particularly because there is no definition for "absolute" acceleration that does not circularly rely on some type of coordinative acceleration. We suggest you watch one of our many other other videos for more information on this subject.

[@eleventhchimp]

@@dialectphilosophy You can measure acceleration without any external reference, in complete isolation, using a simple mems accelerometer, an optical or coherent matter wave inertial sensor or in a number of other ways.

[@WalterBislin]

@@eleventhchimp Inertial Navigation Systems even rely on this fact.

[@Jim-tv2tk]

One twin gets crushed by G forces, the other doesn't. The frames are not the same. Both agree on who did the acceleration. Really confused by this video.

[@costel13112002]

acceleration is absolute, not relative to other frame of reference. If you were in an empty universe in a space ship and fire your rockets you would feel acceleration while the exhausted gas would not.

[@tulliusagrippa5752]

Wrong. Both you and they are wrong. The resolution has nothing whatever to do with acceleration. The answer to the paradox lies in the spacetime metric. The proper time lapse along different world lines that join the departure event to the reunion event is different in general. Proper time is the time recorded on the traveller’s on-board clock (your wrist watch, or your “ticker” I.e. heart, and so is directly related to the aging process of the traveller). If the proper time lapse is different for two travellers that part company and then meet again, then the travellers have aged by different amounts. Your problem is that you are trying to explain the paradox from a space based perspective. Relativity can only be properly understood in terms of spacetime. The paradox arises from the fact that the verbal description of the situation builds a symmetry in the wording that does not exist in the actual physics. There is in fact no paradox when the physics is analyzed - the person whose world line has the greatest proper time lapse ages most. For a clear non-technical description of the correct resolution, see Penrose’s discussion of the paradox in his Road to Reality.

[@AntiCitizenX]

The spacetime metric depends on the existence of an objective acceleration in order to reveal a disparity in proper time. The one IS the other.

[@cm5754]

The paradox is that both twins claim their other has a different time lapse. The issue in the paradox is how to solve the issue that there is a mathematical symmetry between the coordinate systems of the two twins. The answer, clearly, is that the mathematical symmetry in question is not respected by general relativity, because one twin "actually" accelerated and the other didn't.

[@imreolah6077]

In the Twin paradox, only one accelerates, not both, which aging slower. If both accelerate, they are aging identical. This is consistent with general relativity, the gravitation, or acceleration causing slower aging. This video with absolute and relative acceleration is bogus.

[@wiredvibe1678]

Did you watch the video? Whether one is accelerating or both is irrelevant.

[@hungles8624]

@@wiredvibe1678 it’s not because when one has to accelerate to change direction meaning they are in a non inertial frame of reference and that is the point in which they would agree that twin b is older

[@wiredvibe1678]

@@hungles8624 if you add another point of reference then you are no longer talking about the twin paradox, you are talking about the Triplett paradox. Sure the Triplett paradox is not a paradox, however, it is also not the twin paradox.

[@fruityoverlord9937]

@@wiredvibe1678 Even with just two reference points there is no symmetry. If one point accelerates away from the other, you ask who is accelerating? Well, you would know because only one of them feels actual forces. The only way you do get symmetry is if both accelerate away at the same rate, both will feel the same forces, have the same proper time, and age the same.

[@david_porthouse]

The twin “paradox” is merely the hyperbolic triangle inequality and if a “solution” to it is “wrong”, then you should be able to tell us what’s wrong with the elliptical triangle inequality.

[@doodeedah6409]

THANK YOU! Exactly! I don’t know a lot of physics, but I’m always dissatisfied with all solutions to the twin paradox I’ve always watched/read. They always introduce asymmetry to their solution, which exactly defeats the whole point of the paradox: they’re supposed to be twins with identical relative observations of each other. So many physics videos have unequivocally taught me that acceleration is absolute (which I never understand). But even if it’s true, why can’t both twins accelerate with their own rockets? After all, symmetry is the whole point of the paradox, hence “twins”. So I always wanted to challenge those solutions, but never knew to whom. I’m glad there’s finally a video that directly addresses the question I’ve always had and validates that I’m not crazy.

[@omarjoseffernandez2436]

Even in the absence of a reference point in an isolated system those two spheres would still be subject to time dilation as per lorenz transforms (relative to each other as long as their average velocity is not equal)

[@ksotar]

Sorry, but no. You stick to Special Relativity, but once you leave inertial frames with constant speeds, and start to use acceleration and gravity, you're in domain of General Relativity, which is whole another story. And apple falling to Earth is not equal Earth falling on apple, because a) gravitational force accelerates apple much more than Earth and b) from the apple POV not only Earth falls on it, but the whole Universe, that's kinda strange and gives a hint who's actually falling.

[@dialectphilosophy]

You're correct here in that our assertion about the apple and the earth was not entirely precise. What we intended to convey with it was more the spirit of general relativity, which was motivated by Einstein's belief that all motion was relative, though this belief did not pan out in the final theory itself. But the statement is still somewhat correct in GR, since the apple is not "falling" but on an inertial geodesic path.

[@oldjoec3710]

The accelerating participants in this thought experiment ABSOLUTELY know they are accelerating. They feel the force of acceleration, and they can see that there is no source of gravity. So, ithey have all the data they need to know that they are NOT in inertial frames of reference. The premise of your objection (and therefore your entire video) is wrong. The frames of reference are NOT interchangeable, as they would be if they were inertial.

[@zenglider2145]

I visualized the same scenario about 2 space ships departing in opposite direction and then each applying a force to reverse their directions and meeting again. I had no idea if either one is older upon completion, and now after watching your video, I still don't because you didn't answer the question. If you don't know the answer, what's the point in criticizing other explanations?

[@JanPBtest]

6:37 Acceleration _is_ absolute in _special_ relativity. This is a well-known "philosophical defect" of the theory This follows from one distinguishing feature of the special theory: its spacetime comes with an _a priori affine (linear) structure._ This is what intrinsically distinguishes accelerated motion: its worldlines are not _straight lines_ in the global affine structure sense. This "God-given" affine structure was something that bothered Einstein himself and was one of the reasons he sought to get rid of it (he succeeded in doing so in the general relativity theory). That's why what you say at 7:00 is not correct in the special theory in which there are in fact intrinsically "better" frames distinguished by the global vector space structure. As I said, Einstein found this issue somewhat unacceptable and sought to "fix" it. 7:43 That's why this is again incorrect. Assuming the special theory, both trajectories again have well-defined deviations from straight-line trajectories and the difference (if any) between those deviations results in the elapsed time difference upon the reunion. In the general theory the spacetime metric is generated solely by those two moving objects and again their elapsed times are well-defined numbers. The only way you could be correct is if this configuration (only two small bodies in the universe) had non-unique solutions for the spacetime metric yielding an ambiguous answer. I don't know this from the top of my head although the uniqueness theorems about the Schwarzschild solution (only one object in the universe) seem to indicate otherwise. So the bottom line in the general theory case is that even if there are only two objects in the universe, the situation is _not_ symmetric because the spacetime metric (the shape of spacetime itself) is coupled to the motion of the two rockets, so there is a third very important actor on this stage.

[@manog8713]

You strated well but did not get anywhere really. Hope you continue on this. people talk about TWINS paradox as if it is an exceptional situation in SR. Well, it isn't. All of SR is about this situation in an infinite forms. The main probelm is to preserve the symmetry of motion in Inertial frames and undrestand the notion of time and space consistently. The notion of time vs. clocks, space vs. rods and the connection between Mathematics and reality is not so clear.

[@whatshouldinamemyself481]

3:21 who else cracked up at the earth rolling away? like that's what I've always wanted to do

[@jok2000]

You need to read up on what an inertial reference frame is.

[@MrBendybruce]

I feel like I'm missing something here. Surely the twin that experienced acceleration, can claim that they actually felt that acceleration, whereas the other twin can claim that they did not?

[@DApple-sq1om]

You are not missing anything and are correct.

[@motorepoloh]

Are you really saying that proper acceleration is not absolute? If Alice is the travelling twin, she will need to turn back and in doing so, she would experience proper acceleration (i.e. her accelerometer would read a non-zero value). Bob would not experience proper acceleration (his accelerometer would read zero). Both Bob and Alice would agree on that. Also, are you saying that including Earth and fix stars would resolve an otherwise real paradox? You cannot be more wrong. As stated above, both twins would agree who underwent acceleration and who is older, regardles of the Earth, stars or empty space beign there! Here are a couple of correct resolutions to the twin paradox: math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_intro.html

[@alexandergraham6619]

He's right, he's saying that the version with the earth is correctly resolved, but his simpler version, without absolute acceleration, is not.

[@motorepoloh]

@@alexandergraham6619 If the two observers go apart and then come back together, at least someone had to experience proper acceleration and that is absolute. So yes he's wrong. I don't see how including or excluding the earth changes that (we are excluding gravitational effects of the earth anyway). Observer One is in his inertial frame at rest (you don't need to specify in regards to what) and observer Two is speeding away relative to him. When observer Two changes direction from +v to -v he changed the reference frame and experienced acceleration. You can say that from the perspective of observer Two, he can say he's at rest and observer One is speeding away. But observer Two knows that he fired the rockets to stop and than turn, he will experience proper acceleration, he would feel the force pushing him. That is absolute and both observers would agree on that. There is no paradox.

[@trsomas]

You are right. The travelling twin has a non-zero proper acceleration while taking a turn. kzbin.info/www/bejne/gIGmdX6qlpd_jKc

[@Raging.Geekazoid]

2:30 The perspectives aren't equally valid. Whoever fires the rockets on their spaceship can feel themselves being accelerated, i.e. pushed into their seat cushions. 6:40 Same thing: Except for gravity, you can feel acceleration. 7:00 I think you're overinterpreting GR. Free-falling frames are equivalent in GR, regardless of the gravitational field. But that doesn't include frames of objects that are accelerated by other forces. 7:15 The frame on Earth's surface isn't free-falling, so it's not "valid". 7:30 There's the vacuum. At least one of the objects is moving relative to the vacuum. 7:40 A scale with a 1kg object on it will tell you how much you're accelerating. 8:20. The stars are no more important than Earth. What's important is the spaceship's immediate surroundings, i.e. the vacuum. That's the only thing that can directly affect it. I like your references to Emmy Noether, but what about Lorentz? He has the answers you're looking for. Time dilation is caused by absolute motion through the vacuum, which DOES have a reference frame, even though we haven't detected it yet.

[@dialectphilosophy]

Hey there, we appreciate your annotations! However, you've confused some ideas. Your main issue is in regarding the vacuum as a type of ether. The vacuum of space does not possess that property, i.e. you can't measure motion relative to it. (Quantum foam as an ether was at one time proposed by Dirac, but it has not proved viable in any modern theory of physics thus far.) Your remarks about our over-interpretation of GR are technically correct, since GR only upholds General Covariance and not the General Principle of Relativity. However, our point in the video was more meant to illustrate the relative, coordinative nature of motion that was a strong inspiration for GR, and how the twin paradox resolution contradicts it. Lastly, your remark about "feeling force" is one we receive a lot -- so we made an entire video about it! Check out "Can You Feel Force", which debunks that common misconception, as well as explains why your "1kg object" scale/accelerometer argument also does not work.

[@GrouchierThanThou]

What this video fails to understand is that the resolution of the paradox is in general relativity where you have a curved spacetime. Two twins accelerating relative to each other in a curved spacetime will move differently with respect to the geodesics of that curved spacetime, which is what breaks the symmetry and explains why one twin is older than the other when they meet again: because their path through spacetime had a greater time-like component. In the very special case of a universe containing only the twins and nothing else the curvature of the spacetime is symmetric and therefore the twins will have the same age when they meet again. So no paradox there either. So yeah, acceleration is absolute in the sense that you can equate not accelerating to moving along a geodesic of the curved spacetime (aka free fall). The absolute acceleration is then just the deviation from free fall.

[@dialectphilosophy]

We actually treat the twin paradox in curved spacetime in one of our follow-up videos! Check it out!

[@mathboy8188]

You don't need GR. Simply note that the world line of one twin must be a straight in any inertial frame, while the world line of the other twin cannot be a straight line in any inertial frame.

[@j8acob1]

I don't understand how there can be any question as to who is accelerating. If you accelerate you can feel it, and if you accelerate too fast, you die. On the other hand if you're not accelerating you don't feel it and also live. Does this not break the symmetry?

[@dialectphilosophy]

Jacob, check out our other videos, "Can You Feel Force", "Inertial Frames," or "Einstein, Gravity, and the Twin Paradox" if you want to learn more about why no definition of absolute acceleration can be given.

[@joepierson3859]

correct, acceleration is absolute it can be measured with accelerometers

[@mikkel715]

And what when you decelerate?

[@j8acob1]

@@mikkel715 acceleration and deceleration are the same thing. A change of speed over time in a certain direction. Deceleration is just acceleration in the opposite direction. You can measure when you are accelerating, by how much and in what direction

[@mikkel715]

@@j8acob1 So when does time dilation increase and when does it decrease again?

[@wolphramjonny7751]

This video is wrong on many things. You can just learn math an calculate the proper time (what you measures in your own clock) that each twin has experienced on each situation: If one of them is the only one that accelerates, the proper time will be less for him. if both accelerate to depart and to return, they both aged the same. And acceleration is absolute in especial relativity: an observer can tell if he is accelerating or not (or at least believe that he is at rest but in the presence of a gravitational field, which we do not need to consider in special relativity).

[@sghrd]

I didn't get the point: I my opinion, also in absence of any other fixed object, if I am in an accelerating spaceship, I am sensing the acceleration, or a fictitious force, so the non accelerating twin, can say that the acceleration is mine. Maybe I am confusing something of your explanation.

[@trsomas]

When the solutions say that the twin in the spaceship accelerates, then they are talking about Proper acceleration. Proper acceleration is absolute. It does not depend on the frame of reference.

[@marcovillalobos5177]

Really good point, hope he sees it

[@johnblacksuperchemist2556]

QUESTION...........You act like in the video if there are 2 spaceships and nothing else in the universe and one ship accelerates there is NO WAY to test to know which ship is accelerating. THAT IS BS. All they have to do is hold a ball out and let go of the ball without throwing the ball. If the ball floats they are NOT accelerating. If the ball flies to one part of the ship they they KNOW they are accelerating. I do not get why you are saying it is impossible to know who is accelerating??????????????????????

[@wesbaumguardner8829]

He is talking about within the constructs of the theory itself. Einstein posits that there is no absolute reference frame. Without an absolute reference frame, it would be impossible for the ball to determine whether or not there is any acceleration or motion because there is nothing to act upon the ball to make it resist the acceleration. The whole theory is nonsensical if you ask me.

[@johnblacksuperchemist2556]

@@wesbaumguardner8829 ...............Of course it is nonsensical. It is like when a person tries to say the universe would not exist if people were not there to see it. Because in physics we say an "observer" is needed to collapse the wave function. But in physics an "observer" is NOT defined as a person. And so those people just sound like idiots. Just like the guy in the video. He obviously does not understand the definitions of some physics words and is building from those wrong definitions. He might as well be a flat earther.

[@Bizija123]

@@johnblacksuperchemist2556 It's a common myth that people think an 'observer' collapses the wave function. Quantum theory does not say that at all. An observer is not even needed to collapse the wavefunction. It is actually the MEASUREMENT of a quantum system that collapses the wavefunctions.

[@nicolo4844]

Misinformative video. Minute physics and physics girl videos are actually very good videos to understand the paradox (that only apparently is a paradox and infact it's how reality works) There is no need to involve earth or the "fixed stars"

[@Skellborn]

Wow. It's the first time i even understood, why it is called a paradox. The subtle reframing of the question makes all the difference. Thanks alot

[@CT-pi2gl]

I think perhaps the simplest answer to the paradox is that a comparison of the two observers' local elapsed times cannot be made, unless they area brought back together again. The details of how they are brought back together will determine their elapsed time comparison. But if they simply set off in opposite directions forever, the two viewpoints are symmetric.