Special Relativity: This Is Why You Misunderstand It

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Does time really slow down when you move? What about gravitational fields? What's the resolution to the twin paradox and what's up with Newton's bucket. In this video, I tell you how it really works.
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00:00 Intro
00:34 Space+Time = Spacetime
04:30 Proper Time
07:59 Time Dilation
10:21 The Twin Paradox
13:59 Newton's Bucket
15:25 Time Slows Down Near Black Holes
19:30 Learn More on Brilliant
#physics

[@MrVasile]

I always learn something from Dr. Sabine. Today one of the things I learned is that in Germany they teach the Pythagoras Theorem in kindergarten!

[@angellestat2730]

I would not be surprise if she is the one who learn that when she was in kindergarten :)

[@KwieKokosnuss]

From her view the math and physics of your school time are kindergarten basics. ;-)

[@garysevenofnine]

I came here to write this and saw you beat me to it. 😆

[@janami-dharmam]

but I now have no questions that answers; I am confused!

[@jmaniere]

Otherwise Sabine's "special" humor ...

[@etienne8576]

“as we learned in kindergarten” 😂 That’s why you are so clever Sabine. I wish I attended the same kindergarten!😂

[@TheReaverOfDarkness]

I think your teaching style is great. I always understand things way better after watching your videos than anywhere else. And part of that is because on some of these topics, you're one of the few sources who actually seems to understand the subject in the first place.

[@w0tch]

Yeah exactly, the sad truth is that she is among the few who actually deeply understand these stuff

[@geoffreymilward3293]

What I particularly liked was your distinction between proper time and co-ordinate time. Thank you. It's really helped me understand geodesics in GR in a much more geometric way.

[@stylis666]

Indeed. That was always confusing to me and this was like having all the puzzle pieces that still didn't make a picture just shoved in place in one move by Sabine and boom: picture :D And I was like, ooooooooh, well that explains a lot :D Somehow instead of being all abstract and shizzle Sabine ties it to the actual universe making it all far more practical and real and way easier to understand.

[@iurlc]

Are you sure you understand really what she is telling with this diagram and 45° light for speed of light and the hyperbolic curves? @3:40 45° for speed of light is an error. When something move with the speed of light, the time stand still. There is no choose for an other angle than 0° to the right in the diagram. Think about the light clock which explain special relativity. If the one you look at is standing still relative to you, the light in his light clock jumps up and down. When he accelerate, you will see the light beam of his clock get diagonal and the clock ticks slower. If he gets (which is not real possible) speed of light, the light beam will move horizontal with him and the clock stand still. No ageing!!! The explanation of the twin paradox is not correct. Make the twin paradox with 3 twins symmetrically! One is moving to the left, one is moving to the right and on remains on earth. Due to the complete symmetrically situation, the moving twins must see each other younger! And the one remain on earth will see both a little younger. Why should it not possible to see each other younger? There is an issue with seeing the other older. Imagine they are real single egg twins (same gender, same look like). If one of the twins see after the journey the other twin older, than he can see into the future. He will see e.g. if he is 20 years after the other return from flight, how he will look like when he is e.g. 40 years like his returning twin. And this should not possible. If you draw your diagram correct, you will see, that there is no possibility, that they can come to the same time coordinate. So there is always a time difference between them, and if you draw the diagrams for both you will see that the other one is always back in time - so the other one is always younger. Her acceleration story is also wrong. Imagine an endless train station and an endless train. In each cabin of the train is a light clock you can see also from outside. And at the train station in same distance like the cabins are also light clocks. When standing still all light clocks are synchronized (have the same number of counts on the displays). Both the one on train station and the one in the train see the same time. Now the train accelerate. The one in the train see train light clock with normal speed, but with increasing speed the light clocks at the train station ticks slower. The one at the train station see clock from the train station with normal speed but the clock in the train ticks slower. And when the train decelerate the one in the train see the clocks at the train station become faster until they have the normal speed when the train stops. And the one on the train station see the clocks in the train become faster until they have the normal speed when the train stops. But for the one in the train the clocks at the station have less ticks count. For the one at the station the clocks in the train have less ticks count. There is no difference caused by acceleration, if the acceleration is 1g. And at the way back the same happens. So both see each other younger!!!! And even when you take in account length contraction. There is now way, that in this example the clock from one ticks faster then from the other one. And finally - if you think the traveling one see the earth one older - what does it mean? It means, that from the view of the traveling one (his clocks tick from his view normal) that the clocks from the earth one have speed up. But which formula on physics show speeding up clocks? If traveling speed e.g. is 0.5c and as long you accelerate with values man did not die, you can neglect the influence of acceleration cause by general relativity. I like her videos - but this one was simply retelling wrong stories, which are based on a lot of tiny wrong assumptions - starting with the wrong diagram, where traveling with speed of light is 45° (or other) instead of 0° to the right in her example.

[@mcwolfbeast]

Definitely a great, condensed description that is a lot less confusing and allowed me to grasp a few fundamental principles that eluded me so far because they were never explained to me in relation to each other. Thank you, Sabine!

[@jovetj]

Ha! I see what you did there...

[@stoyanfurdzhev]

You deserved your like

[@ix12]

@@jovetj what did s/he do there? :) I feel like I'm missing smth :)

[@jovetj]

@@ix12 Raving about the "condensed" description of things very small (e.g. light)

[@alonamaloh]

I must have read the same incomprehensible books 30 years ago. I ended up becoming a mathematician and I never formally studied Relativity. This is far, far better. Thank you!

[@hyperduality2838]

GRAVITATION is equivalent or dual (isomorphic) to ACCELERATION -- Einstein's happiest thought, the principle of equivalence or duality! Space is dual to time -- Einstein. Time dilation is dual to length contraction -- Einstein, special relativity. The Twin paradox is duality -- which twin is actually moving? Questions are dual to answers. Points are dual to lines -- the principle of duality in geometry. Space, length, distance is defined by two dual points -- space duality. The future is dual to the past -- time duality. We predict (syntropy) the future and remember the past. Absolute time (Galileo) is dual to relative time (Einstein) -- time duality. My absolute time is your relative time and your absolute time is my relative time -- time duality. If time is dual them space must be dual. Time duality is dual to space duality. Duality within duality! "Always two there are" -- Yoda.

[@bsadewitz]

It is astonishingly simple, yet profoundly counterintuitive. I actually had already understood most of what was in this video. To do this, (I think!) you just consider time as another dimension. It doesn't even matter whether it actually is or not (whatever that even means). I thought of it like this: a helicopter has three degrees of freedom it can move in--the three dimensions. Now, just presume that everything actually moves at the speed of light. Add a fourth dimension. Now, think of the helicopter: any motion in one of the three spatial dimensions is motion that is not happening any of the other dimensions--including the temporal dimension. Now, its speed in three dimensions is the sum of its speed moving in each of the spatial dimensions. Now, as everything moves at the speed of light, its motion in the fourth dimension is the speed of light minus however fast it is moving in the fourth dimension. Light, therefore, does not move in the fourth dimension at all, and the faster you go in the three spatial dimensions, the slower you will go in the fourth dimension. Gravity is the curvature of space-time itself, and this changes the "distance" one must travel to get between two points in a given amount of time. That is gravitational time dilation. Of course, without the math this is just a fanciful narrative. It does not even get us to a testable hypothesis. If I could do that, I'd be a physicist. ;-)

[@bsadewitz]

@@hyperduality2838 Equivalence is not duality. The two concepts are orthogonal.

[@hyperduality2838]

@@bsadewitz Orthogonality, perpendicularity = duality. Duality:- two equivalent descriptions of the same thing -- Leonard Susskind, physicist. Are you saying Einstein & Susskind are idiots? Equivalence, similarity = duality! Wave are equivalent or dual to particles -- quantum duality. In physics you learn to generalize everything -- abstraction.

[@bsadewitz]

@@hyperduality2838 I didn't even call you an idiot, so why would I be calling Suskind an idiot? What I'm saying is that the equivalence principle simply is not what you seem to be implying it is (?) It refers to the equivalence of gravitational and inertial mass. Look, I've been down this road before. William James wrote in his treatise on the subjective effects of nitrous oxide: "There is a reconciliation ! Reconciliation ^conciliation ! By God, how that hurts ! By God, how it does n t hurt ! Reconciliation of two extremes. By George, nothing but othmg ! That sounds like nonsense, but it is pure 0#sense ! Thought deeper than speech ! Medical school; divinity school, school! SCHOOL! Oh my God, oh God, oh God ! The most coherent and articulate sentence which came was this : - There are no differences but differences of degree between different degrees of difference and no difference." This reminds me of what you're saying now. James concluded that, "the togetherness of things in a com mon world, the law of sharing, of which I have said so much, may, when perceived, engender a very powerful emotion ; that Hegel was so unusually susceptible to this emotion throughout his life that its gratification became his supreme end, and made him tolerably unscrupulous as to the means he employed; that indifferentism is the true outcome of every view of the world which makes infinity and continuity to be its essence, and that pessimistic or optimistic attitudes pertain to the mere accidental subjectivity of the moment; finally, that the identification of contradictories, so far from being the self-developing process which Hegel supposes, is really a self-consuming process, pass ing from the less to the more abstract, and terminating either in a laugh at the ultimate nothingness, or in a mood of vertiginous amazement at a meaningless infinity." ia802305.us.archive.org/31/items/thewilltobelieve00jameuoft/thewilltobelieve00jameuoft.pdf

[@daviddavis-vanatta1017]

Dear Dr. Hossenfelder: First, I really like your YT videos. They take some fresh looks at things that I've read before, and even if one of them doesn't answer all my questions and struggles with a topic (some of this actually IS difficult stuff!), each one adds to my understanding, gives some new perspectives, helps raise a different set of questions, etc. So, thank you for these lesson videos. Somewhat incredible to me how the Twin Paradox has produced a century of discussion and debate, and to this day without universal agreement. There are presentations of it that say, as clearly, simply, forcefully, and every bit as certainly that the explanation, or resolution, of the (apparent) paradox requires NO treatment of acceleration whatsoever to explain the real effect of differential aging. Those explanations often require accepting and using the (special) relativistic phenomenon of length contraction, and its effects. One thing is sure, and something not always made clear, while acceleration may (or, to me, may not) be required to resolve the paradox, General Relativity has no place in describing this paradox, and absolutely is not needed for a full resolution of it. One can indeed "totally describe" acceleration within the conceptual theory and the mathematics of special relativity, alone. But acceleration as the correct resolution seems to be not so categorical. Some accept and use it, successfully, and others disagree that the required acceleration is the correct explanation. These latter treatments, the ones that are focused on acceleration as the causal explanation, also seem to use time dilation effects to explain the resolution of the apparent aging paradox. While those that eschew acceleration as the central, causal explanation tend to use length contraction effects. Given that time dilation and length contraction are really one in the same, two sides of one coin, "mirror" effects of each other, etc., I suspect that both resolutions, that is, both those claiming that the causal effects for differential aging are inherent in the acceleration periods, and those who equally say that no treatment whatsoever of the acceleration periods is required to resolve the paradox, are probably both accurate in their thinking and equally in full accord with SR. As wonderful as this lesson is to consider, excellent things in it to absorb, I don't think I can see how it is that some treatment of the three acceleration periods on this journey, provides the sought-after answer here, that is, it is the accelerations that provides the conceptually correct explanation for the differential aging effects found in this experiment that resolve the apparent paradox of the differential aging. For a good write-up that takes the position that “the acceleration incurred by the traveling twin is incidental and the paradox can be fully resolved” without it, see Scientific American, “A Matter of Time,” March, 2012, by Ronald C. Lasky (Dartmouth). A final important fact to note: this apparent paradox is not just theoretical. Its basic tenets have been very well-tested in any number of experimental settings, including ones involving precise clocks and co-relative travel. Without exception, these have confirmed many times, using various setups, that the seemingly paradoxical differential aging effect is both altogether real, all the experiments end with the two clocks no longer having the same time, and that they differ precisely as special relativity says they should.

[@LowellBoggs]

Thank you so much for this video. This is my second watch and you explained several concepts that other videos have muddied up. I'll watch again in a few weeks.

[@enforced9550]

I have never came across the concept of hyperbolic distance we use on the space time diagram before, and that it gives us the proper time passed, absolutely stunning and well explained!

[@zekicay]

The most interesting thing to me with this proper time is that all particles moving at the speed of light experience no time - their proper time is always zero.

[@franklittle8124]

@@zekicay Yes. A photon is everywhere on its path through the universe at once, from its perspective.

[@Tore_Lund]

@@franklittle8124 -or formulated differently: the Universe has no length and no time passes from the photon is created until it interacts with an electron at the other side of the Universe zero distance away.

[@SandipChitale]

I think the pronounced hyperbolic appearance is because of the choice of 45-degree convention Sabine talks about. If the true proportions were uses between time and space axis the hyperbolas will still be asymptotically hyperbolic but for all practical purposes straight lines near x axis. That will also clarify as to why the photon travelling at the speed of light goes to infinity in no time and thus does not experience it as the elapsed time for its journey is zero. And for same reason photon does not experience the distance as the space gets shortened to zero in its direction of travel.

[@stoyanfurdzhev]

@@zekicay Great!

[@CleverNeologism]

The "proper" in "proper time" does not mean "correct" or "appropriate"... in this case, "proper" takes on a slightly archaic meaning in English: "belonging to itself". In other words, "proper time" literally means "the time belonging to the object in question", or "it's own time", and is exactly that.

[@kylelochlann5053]

Correct. You get a thumbs up. But keep in mind that this eigen-time is physical time.

[@jonathanborrelli2749]

You said: 1. Acceleration solves twin paradox 2. Gravity is not a force, so it not accelerates you So if the twin only uses the force of gravity of a star to turn around and to go back to his twin he did not accelerate to come back. So there is either a paradox or something wrong about what you said

[@FengXingFengXing]

Remember change DIRECTION is also acceleration, velocity is vector.

[@ChlorieHCl]

If you consider gravity then the one in freefall (using only the gravity effect to return) is not accelerating, but the one staying on the planet *is accelerating* constantly by the ground they're standing on (since that's the only force acting upon them).

[@FengXingFengXing]

Ships clock will run slower when in star's gravity field, same effect if use rocket engine for turn.

[@johnt.inscrutable1545]

This was so entertaining and elucidating as well. Sadly, what it elucidated for me was how little I fully understand these ideas. Though intuitively they seem to makes sense. Thank you, JTI

[@richsadowsky8580]

"Moves just left or right, like American politics maybe..." Sabine, I love the humor you inject into your explainer videos. Long time subscriber here

[@fannyalbi9040]

“no absolute rest” even in cemetery 😢

[@franklittle8124]

Except that it is more like moves between center and off-the-scale right - with the center governments never quite reversing the damage done by the other.

[@alex.pozgaj]

Wonderful! I saw 1000 videos about these topics already, and I thought to have quite a solid understanding of it sll, but this video really made it all fall into place! "Acceleration is not relative" is so important, but I don't think I ever heard it before.

[@HowardS185]

ditto here, and I have a BS and MS in physics!

[@snack711]

what i dont understand is how you define absolute acceleration without gravity. but maybe im missing the point.

[@HowardS185]

@@snack711 If you were in an area of free space, far from any sources of gravity, and someone turned on your rocket engine, you would feel the acceleration. This would happen regardless of your initial motion.

[@cazcam2000]

@@HowardS185 That's because of "Inertia". Inertia opposes a change in motion.

[@TheHesseJames]

@@c4pt4ina69 It is relative. You could indeed put it like that. You also could say that when on Earth, sitting at your desk you are constantly accelerating at 1 g. It would be equal to sitting in a spaceship accelerating at 1 g.

[@Dismythed]

This was the best video I have seen on the subject. So many people get caught up on the "move away from earth" diagram that they miss that it is about acceleration, not direction. Even slowing down is acceleration and the direction of movement does not matter is what so many science communicators fail to understand or fail to explain to their audience. Thanks, Sabine.

[@stewiesaidthat]

What most people don't understand is that acceleration and deceleration forces DECREASE lifespan. That's why motors and engines are run at a constant speed. It's why passengers/cargo is strapped to the vehicle's frame. So that the vehicle endures the forces, not the passengers. Only at constant velocity, 0 gravity, does lifespan increase. This is all well documented in engineering. Why is physics still stuck in the 18th century?

[@robertgoss4842]

Dr. Hossenfelder: Your videos are among the best learning tools I have ever come across. I'm no physicist, but I think I have a pretty good layman's understanding of the theory of special relativity (far less so of the general theory). This video greatly helped me further grasp the special theory. Thank you for your many careful, and comprehensible, programs

[@ChuckHenebry]

Those books you and I read 30 years ago were written in a style designed to maximize the reader's sense of wonder, because wonder (and a dash of incomprehension) is what sold books. What I love about your videos, Sabine, is that you never stoop to that cheap rhetorical trick. You're here to give us understanding, not gobbledygook.

[@SirBlot]

Maybe I had a better book than ppl on here.

[@croozerdog]

These days it's articles like "The universe isn't real" People flip out, not their fault really, these articles don't care to explain what "real" means in science and that it isn't the same as a philosophical or social real haha

[@Nat-oj2uc]

Huh what a nonsense. There was no conspiracy or any agenda. First of all maybe you're not smart enough to understand those books. Secondly being taught simplified explanation so you could think you got it isn't the same as actually getting it. And the most important thing is those books were written many years ago and there was not enough time to simplify it to your liking. You think she would be able to explain it that way if there was no previous knowledge she built her understanding on?

[@eonasjohn]

Thank you for the video.

[@ryancraig2795]

I never got as far as relativistic physics in university, but about 30 years ago I read Roger Penrose's "The Emperor's New Mind" which did a pretty good job of explaining these concepts, and more. This was a great refresher - very easy to follow.

[@kwimms]

Yeah, I read that story too.

[@stylis666]

That is so cool! I think that Dr. Penrose is a very insightful and kind man. And I think that he and I would both agree that Sabine is the better physicist because she is as brilliant but has a giant head start because of the work of people like Penrose. He had a lot less to work with, taking him a lot more time to get where Sabine pretty much started from. But anyway, that's not why I commented. Do you know what the title of that book is referring too?

[@hieronymusholt-xj2rg]

Should be an endash: -

[@BillGreenAZ]

I read that book 30 years ago and it's one of my favorite books that I've read in the past 30 years. I should read it again.

[@kwimms]

Douglas Hofstadter had some great books around the same time dealing with more computing science rather than the laymanistic fairy tale put out by Penrose and his 'pal' Hawking. Sabine is an actor, btw. Pop!

[@GrimmSpector]

Excellent, I agree it's one of the hardest concepts I have explaining to folks, and sometimes I do it poorly. Have to hang on to the link to this one!

[@brilliantcosmicspeck7956]

Love your descriptions Sabine. You explain things very well. I have been sending your videos to my mom who, for some reason, is convinced she cannot understand science. Your videos are proving her notion wrong and she is getting interested in science. We now have something additional to talk about. Thank you Sabine !!

[@stoyanfurdzhev]

I bet your mom is a very religious person.

[@ewanholmes4559]

I mean my mum is similar and she's not religious at all

[@lucashouse9117]

@@stoyanfurdzhev most older people are just not interested in learning new things. Religion doesn't have much to do with it.

[@andoletube]

@@stoyanfurdzhev stupid comment

[@robertnull]

Cool story, though I bet special relativity will still prove to be too hard a topic to digest for her :)

[@jandobbelsteen8953]

Nice video! That reminds me: in 2005, the Dutch astrophysicist Vincent Icke published a simple book: "Niks relatief, De speciale relativiteitstheorie zonder formules", which translates to "Nothing relative, Special Relativity without Formulas". It takes him only 70 pages to completely explain in layman's terms what it is all about. The nice thing about the book is, that if you flip it upside down and start reading from the back, you will have a book "Nothing Relative, Special Relativity WITH Formulas". This side only takes 63 pages. I'm sorry, I think it's never been translated into other languages than Dutch, but if you're German, you might be able to read it and understand it (according to a German colleague, you might have to read out loud).

[@saimon174666]

I've seen so many explanations about twin paradox, but this is hands down the best, thanks!

[@stylis666]

And the simplest. She just tells you what happens and why we see what we see. The graphs show me exactly why I didn't understand any of it when other people explained it. And Sabine is like, Bob accelerates and that's what matters. And I was like, ooooooh, well that could've save me 50 hours of lectures that made no sense to me.

[@MrCmon113]

@@stylis666 That "acceleration" is absolute should be extremely alarming. That means acceleration isn't just the second time derivative of position. It's fundamental. And it looks exactly like gravity. Why?

[@LesCish]

I'd never seen an explanation of the twin paradox that made sense to me. Until now. Thank you!

[@BradleyLayton]

Agreed, I've been puzzling over that twin's paradox for years. I'll have to go back and do the vector calculus and look at how the answer comes out of the acceleration.

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@jamesduncan6729]

Sabine's videos contain the most accurate and concise summations of these kinds of topics and I absolutely love it ❤️

[@eonasjohn]

Yes.

[@chriskennedy2846]

Many of Sabine’s science videos are very informative and provide a great public service to non-scientists looking to better understand confusing topics such as: quantum theory or nuclear fusion. Unfortunately, we will not be able to add this relativity video to her list of shining examples. It is riddled with erroneous statements and also relies on a convoluted explanation that will appear to be complete gobbledygook to a non-scientist seeking a better understanding of relativity and the twin paradox. She even contends that you don’t need gravity or general relativity to solve the twin paradox. Einstein’s official explanation for the twin paradox was published in 1918 (titled: Dialog about objections against the theory of relativity) and broke down comparative time dilation events between stationary and traveling twins into 5 steps: 1) initial acceleration away from Earth 2) constant (inertial) velocity away from Earth after acceleration to desired speed 3) slowing, turning around and reaccelerating to head back toward Earth 4) constant (inertial) velocity toward Earth 5) slowing down and coming to a complete stop back on Earth. Contrary to what Sabine claims in this video, Einstein explained that step 3) was responsible for resolving the paradox because a “gravitational field” appeared due to the fact that the traveler experienced an acceleration that was equivalent to gravity. (This is known as the Principle of Equivalence and was published by Einstein in 1911.) This was mentioned by Sabine with her elevator example toward the end of this video as she (correctly) showed that a person in an elevator being pulled up (by the elevator) would feel their feet pressing against the floor and it would be indistinguishable from being in a motionless elevator on the surface of the Earth. Einstein contends that since at the step 3) turnaround, the traveling twin would be in a lower position of the induced gravitational field, the traveler would experience an additional slowing of clock time that the Earth twin would not experience and this would explain how the traveling twin would have aged less than the Earth twin upon reunification at the end of the Journey. If you are not familiar with Einstein’s 1911 Equivalence paper, please seek it out and read it or find Richard Feynman’s explanation using light pulses in The Feynman Lecture on Physics series (which you can find for free online). Once you have a good grasp of how the elevator explanation justifies the difference in clock times by comparing the frequency shift of light signals (or time intervals of light pulses) you will see how this doesn’t apply to Einstein’s 1918 example. This turned out to be a big problem for Einstein. There is nothing (to my knowledge) in the historical record about the reason for the physics community’s endorsement of the Einstein 1918 explanation while Einstein was alive and then later, the quiet shift toward replacing that resolution with the “space-time” diagram example which is now everywhere, including in this video. It’s possible that enough smart scientists spotted the error that Einstein made in the attempted resolution of his own theory and replaced it with the “space-time” diagram. By the way - the official resolution used by most spacetime diagrams these days is to completely remove acceleration from the example by having triplets instead of twins. The triplet traveling away from Earth simply transfers her clock time to the third triplet traveling the same velocity (from farther out) toward Earth. The exact moment when their paths cross - the outward triplet will transfer her clock time to the Earthbound triplet and continue the simulated non-accelerating journey. I will give the community credit. Although this explanation is also a complete work of fiction and is science at its almost worst, at least (unlike Einstein 1918) it is self-consistent, so the community will get as much mileage out of this explanation until more people realize that actual time dilation experienced in the GPS clock system (which is a very real phenomenon that I do not dispute) shows that neither Einstein 1918 or space-time 2023 have no basis in reality.

[@YbisZX]

I don't think so. If "acceleration causes time-dialation", you may think that total dialation of time depends on the periods of acceleration. But it is wrong. Bob can accelerate in the same short periods (hours) and travels for years with constant velocity - resulting time dialation will depend on time and speed of the main part of the journey. Changing the frame of reference (acceleration) may be the cause of the start of the time-dialation, but amount of dialation depends on total path.

[@NoNameAtAll2]

please add uncertainty values

[@SheanHoxie]

It’s cause she gets rid of the gobbledygook 😂

[@stevecreighton3352]

Sabine, you explain things with a clarity that only someone who completely understands the subject can do.

[@onetruekeeper]

I don't think she undestands the mistakes in Special Relativity Theory. She has her degree so why should she care whether or not us uneducated people understand it ?

[@davidbarnett.2313]

Ah yes! Like all learning, you have to work the concepts until you understand them. If this discussion does not make intuitive sense, we have Brilliant to help you in your quest. Just a few US dollars please.

[@Watchyn_Yarwood]

So very true.

[@w0tch]

Thanks so much for clarifying this, so much videos I watched that just confused me before this 😅 I would love an explanation if there is any on what actually happens during acceleration that makes proper time run slower.

[@Brandon-rc9vp]

Best quick and clean explanation on KZbin for twin paradox, great job Sabine!

[@markpmar0356]

I would say that this explanation of the twins paradox, hyperbolic curves in spacetime, and the difference between co-ordinate time and proper time is the best I've seen so far. I can understand the difference between "e equals m c squared" and "e equals gamma m c squared" more fully now and you've done this with fewer words and fewer diagrams as well. Even the math portion is accessible. So, thumbs-up from this viewer. The key, finally, is to more fully grasp the difference between acceleration and the gravitational effect.

[@luudest]

11:10 thanks for the info about the difference between speed and velocity. Always thought it is the same.

[@KaiHenningsen]

Simplistically, Velocity is like speed combined with direction. It's the difference between "you're driving at X km/h (or mph)" and "you're driving from A-city to B-city at X km/h (or mph)".

[@davidpnewton]

Then you've always misunderstood badly. Speed is a scalar quantity. It has only magnitude. Velocity is a vector quantity. It has both magnitude and direction.

[@luudest]

@@davidpnewton this!

[@erinm9445]

@@davidpnewton This response is rude. Clearly they get it now, and don't need it re-explained. Your response and editorializing adds nothing.

[@stoyanfurdzhev]

No, your are a fool all the same.

[@user-uj9cc5ch5p]

Always give me something to thing to think about Sabine. I really enjoy your videos. Mr. X

[@dexter8705]

Thank you Sabine this qas a great video, nobody teaches inertial and non inertial frames better than you do. Love ya work

[@yesthatsam]

It’s clearly the best way to explain it I’ve encountered in over 30 years of interest on the subject. Especially because you presented both special and general relativities, and used each time the same examples (paradoxes, spring, …), that helped so much. I hope there is an award for physics vulgarisation because this video would win it hands down. Thank you very much for this exceptional clarity.

[@hyperduality2838]

GRAVITATION is equivalent or dual (isomorphic) to ACCELERATION -- Einstein's happiest thought, the principle of equivalence or duality! Space is dual to time -- Einstein. Time dilation is dual to length contraction -- Einstein, special relativity. The Twin paradox is duality -- which twin is actually moving? Questions are dual to answers. Points are dual to lines -- the principle of duality in geometry. Space, length, distance is defined by two dual points -- space duality. The future is dual to the past -- time duality. We predict (syntropy) the future and remember the past. Absolute time (Galileo) is dual to relative time (Einstein) -- time duality. My absolute time is your relative time and your absolute time is my relative time -- time duality. If time is dual them space must be dual. Time duality is dual to space duality. Duality within duality! "Always two there are" -- Yoda.

[@berniv7375]

There is not an award for the vulgarisation of physics I am glad to say but there should be an award for the popularisation of physics.

[@RWin-fp5jn]

Agree, but is also exceptionally wrong. Imagine 100 years of stagnation and confusion amongst top physicists on what they all agree is rather 'basic' theory. You should also watch her colleague Don Lincoln's (Fermilab) explanation of SR on youtube contradicting directly what Sabine claims. It is speed and not acceleration that matters. And he is right! The issue here is that Einstein claims speed is not absolute. But it is in the case of SR! So even Einstein got it wrong. Notice Albert speaks of time 'dilation' (since when do we use a term like that in physics?) and 'length' contraction, as if an object becomes detached from spacetime and shrinks in length all by it selve? Bitte Bitte Herr Einstein und Fraulein Sabine; let's call it for what it is; Speed contracts frontal spacetime. Period! If you speed you have less frontal space (hence appear shrunk) and less frontal time (hence your clock ticks slower). Speed impacts the grid itself!. Goes for macro object (this is what SR is all about) and it goes for the tiny unaligned subatomic particles making top restmass, given the appearance of radial ST contraction around restmass. GR describes that geometry wise. I say hats off to academia to for keeping us in the dark for 100 years, but it is time we start teaching the obvious truth now. Sabine can do way better then this!

[@richardhussong7232]

@@RWin-fp5jn - Lincoln and Hassenfelder do not contradict one another in any way. Because special relativity is quite counter-intuitive, you cannot expect to understand it by thinking about it in everyday terms. The only way to truly understand it is to do the math. Fortunately, special relativity in one spatial dimension can mostly be done using only high-school algebra, so it is reasonably accessible to most people. The thing to keep in mind is this: if you can't correctly solve the problems in a relativity textbook, then you don't understand relativity at all!

[@RWin-fp5jn]

@@richardhussong7232 Dicky, before adding a comment, I suggest you actually look up youtube and see Don Lincoln explain the twin paradox of special relativity. He sums it up; Anyone who suggests that acceleration determines which twin experiences time dilation does NOT understand special relativity. So if Sabine says it does, then both are NOT in agreement (unless `woke' speech determines contradiction = agreement). But Don is right, if even for the wrong explanation. It is about speed, because the speeding twin physically CHANGES its frontal grid, whilst the static twin does not. This is also the deeper reason behind gravity. It is no wonder we failed to understand gravity for 100 years. Even Einstein got it horribly wrong as far as the fundament under SR is concerned.

[@auseryt]

This makes so many videos about space time and general relativity SO MUCH clearer! Especially the part between minute 8 and 10.

[@pluto9000]

Why is there 3 hyperbaly?

[@auseryt]

@@pluto9000 you mean the three hyperbolic lines? Along such a line all events are at the same proper time according to the length metric of space time.

[@auseryt]

@Balungi Francis can you add anything useful or is spam your only way to communicate?

[@stoyanfurdzhev]

Otherwise everything is cristal clear.

[@johnjohnson7070]

Yes. This was absolutely much clearer - and understandable - than "those" books. Thanks for that.

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@paulmccaffrey2985]

Mind Blown! I've been told many times that time dilation was as a result of acceleration, but it was never explained to me. Now I get it! Thank you Dr. Sabine.

[@schmetterling4477]

Acceleration is required. There is no way to leave and to later come back without changing velocity, hence acceleration. It's just not the correct explanation for the size of the effect.

[@erikafein4353]

@@schmetterling4477 Acceleration is not required if the world is a hypersphere and Bob makes the round trip :)

[@schmetterling4477]

@@erikafein4353 That's one big if. ;-)

[@itfrombit]

Wow, what a great video! A frustrating one too, because everytime I believe I know something, you come around with another video and I am reminded of the simple fact that I got it wrong - until now. And I have been thinking about these things for several decades. Sabine is the best science communicator in existence - period. Not making a topic more complicated than necessary, but - most importantly - not making it any easier. I salute you!

[@treeinthewood]

Things I learned from this video: 1. Einstein obliterated the Minkowski bobblehead industry; 2. Physicists' humor doesn't exceed 45°; 3. Newton was concerned with buckets; 4. Flat earthers aren't entirely wrong; 5. Climbing a mountain can accelerate aging. Taking a selfie near a black hole can promote longevity, though.

[@richardneifeld7797]

Books and videos are only as insightful as the clarity of knowledge and thinking of the presenter. This on is spot on. Thank you for distinguishing gravity and acceleration!

[@sdsa007]

very good… i learned a lot! Thank you for your insight! Very good specific, concise refutations of popular ideas that are completely wrong!

[@LP6_yt]

Excellent video. There have been a number of poor/inaccurate explanations of this by some otherwise decent KZbinrs in recent years. This one hits the mark.

[@stoyanfurdzhev]

A like from Sabine.

[@Taomantom]

I had to digest the last section twice. That was an exceptionally well stated point of view. And my universal understanding of space/time acceleration is enhanced. The working in on the curvature influence helped! Thank you for all you do. Missed you Wednesday.

[@MarvinMonroe]

Digest twice? That means you had to swallow it again after you had already eaten and digested it. Really gross

[@morenofranco9235]

Sabine. All things are relative. My relatives are things. My relatives took all my things. And I love your work.

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@lingarajpatnaik6514]

Amazing and brave! I thought I noticed SR misrepresented in text and popular books. After looking at this video I shall have to revisit my books and rework. Great teacher! A gift!

[@jaim3_mm]

As always that was an amazing experience, thanks for your work Sabine

[@carcharhinus_555]

Oh my god, this was *so much* better than most of the books! It's a comprehensible explanation for anybody with a bit of understanding of maths. Ok, it requires the "leap of faith" in the maths in the beginning, but it answers a lot of questions answers to which are surprisingly hard to find, on which others used hundreds of pages and didn't come close. Immediately bookmarked for the next time someone is curious. Honestly: this is *really* good - or at least it applies very well to my type 🙂

[@DrWhom]

too bad

[@BruteZ7957]

Same

[@fkeyvan]

Her explanation is incorrect. But she says with a lot of confidence

[@BruteZ7957]

@@fkeyvan how so

[@fkeyvan]

@@BruteZ7957 because she says that Alice is at rest because there are no forces on her. And that Alice’s proper time is therefore not the longest. But that’s only true from Alice’s frame. Not from Bob’s. From Bob’s rest frame Alice is moving. According to Bob , Alice moves away turns around and returns. Therefore according to Bob , Alice has the longer proper time and Bob measures Alice’s clock ticking less. I think the mistake Sabine makes is considering all motion from Alice’s frame only. In other words she considers Alice’s frame as absolute rest

[@morenofranco9235]

Kerist! Sabine! This sounds like learning calculus at my father's desk. He was fun about everything EXCEPT math. That is when the beating and ear twisting started. I NEVER failed math at school. Failure=Pain. And today, 60 years later... I UNDERSTAND EXACTLY, what relationship you are describing. I love you.

[@samirsaha2163]

Simple and beautiful, Sabine! Many thanks!!

[@wardieleppan8443]

Love her videos! Just got her book Existential Physics just before Christmas. A great read. A few chapters I had to read twice because I’m a pea brain but can highly recommend it. Not to be missed!!

[@pumbaa667]

That was crystal clear ! One of the best explanation I saw on this topic.

[@samiehessi8163]

Thank you. You explained all the key points about this concept. I thought I've understood it until seeing this :)

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@urick4321]

Absolutely Fascinating! Back in the 90s I read "Einsteins Universe" by Nigel Calder, and it was interesting, but I came away a little confused. Your video has brought me closer to understanding relativity. Thank you! You are amazing!

[@RafaelDominiquini]

If Alice remains on Earth, always experiencing the same acceleration (10 m/s²), and Bob goes on a trip, but his ship always maintain the same acceleration (10 m/s²) for the entire trip, holding a speed close to the speed of light for the majority of the time, when Bobs meets Alice again, will be any time dilation between the two?

[@robertbutsch1802]

Bob has to return to Alice for them to meet again. This requires a change in velocity (because of a change in direction so that Bob can get back to where he left Alice) that Alice doesn’t experience meaning he experiences an acceleration that Alice does not. As Sabine points out, this asymmetric situation is responsible for the different proper times experienced by Bob and Alice.

[@RafaelDominiquini]

@@robertbutsch1802 But in my question the two are experiencing the same acceleration! Imagine that both have a accelerometer with them logging the values the entire time and they can compare the log after...

[@yziib3578]

I like your question. Because it equalise the acceleration, that is the acceleration is the same for both Alice and Bob and so there is no time dilation caused by acceleration. Many years ago I read a solution to the twin paradox that does not require the appeal to acceleration. The solution is to do with length contraction. From Alice perspective, it is the length of Bob space ship that is smaller. From Bob perspective it is the length of the journey from the Earth to the destination that is smaller. When they meet up after Bob completes his trip. From Alice point, Bob travelled distance is the total distance there and back. From Bob point, the distance is a fraction of the distance, seen by Alice, and so he has aged less than Alice.

[@RafaelDominiquini]

@@yziib3578 I don't understand your response. First you said there is no time dilation and after you said there are!

[@yziib3578]

@@RafaelDominiquini There is 2 causes of time dilation, one is acceleration, greater the acceleration the slower the time, two is relative velocity, the greater the velocity the slower the time. My reference to no time dilation was to do with Alice and Bob having the same acceleration, which means same slow down in time and since both experience this there is no time dilation caused by acceleration. My reference to time dilation was to do with Alice and Bob having different relative velocity.

[@imid-ltd]

Thank you for this remarkable clarification. I've been frustrated by assumptions about my personal motivations, but can see how an oversight in proper time can lead to the continuation of an error in coordinate time. So when an observer takes a judgmental position because the consequences of an oversight are severe, then it's corrected without apprehension whether we're aware of it or not.

[@poopcatapult2623]

So much clearer, thanks! I always understood the bucket argument as and argument for why Mach's strong principle is not a part of GRT. The situation of Newton turning the bucket should be symmetric to instead the whole universe turning relative to the bucket, but the math doesn't check out.

[@johnburke568]

There is a good Witten lecture on maximizing proper time on YT. This helped me orient more to try and understand his talk better. Thanks.

[@vasilyp]

Excellent way to explain time dilation! Thank you! and as always with the appropriate humor and attitude 😉

[@tomschmidt381]

I really like these videos about time. Casually time seems like such a simple concept yet the deeper one examinees it the more nonintuitive it becomes. I liked the way you explained frames of reference, I thought I understood it but your example crystalized it for me.

[@KaiHenningsen]

The real problem is that all these effects only become noticeable at significant speeds. Since we evolved without ever having interactions with similar speeds, our intuition about the world doesn't include these effects. It is much easier to learn about something that doesn't flat-out contradict our intuitions. Quantum mechanics has similar problems for the same reason. (And failing this kind of leap is what causes flat-earthers, for example.)

[@jasonmakuch9417]

Thank you so much for this excellent video Dr. Hossenfelder! You explained time dilation due to acceleration through spacetime and due to gravity, which is not a force but the absence of one, which is also the result of acceleration so well and so concisely! You made it very simple to understand! Far better than the books I've read! And I love your sense of humour, your hilarious jokes made me lol! Thank you so much for these great free videos, they are a great service to the public, you have become a professor to the people of the World! Please keep making your excellent videos! Take care! Much love! And to anyone reading this I wish you peace and love! World Peace and Unity!!! Science is Real! Science is the study of reality and of the truth! Everyone must learn the truth and stop believing in lies!!!

[@RWBHere]

Thanks Sabine. This is the best explanation I've heard since first learning about this subject, too many decades ago. I'll be sharing this video. The responses should be interesting.

[@natewatson1951]

I think this was a lot more helpful than the books that I read when I was younger, because at least where I live, mechanical physics and relativistic physics are taught completely separately and they usually just abstract away the Nuance of acceleration rather than velocity for the purposes of special and general relativity. By bridging them, you made this make a lot more sense.

[@hyperduality2838]

GRAVITATION is equivalent or dual (isomorphic) to ACCELERATION -- Einstein's happiest thought, the principle of equivalence or duality! Space is dual to time -- Einstein. Time dilation is dual to length contraction -- Einstein, special relativity. The Twin paradox is duality -- which twin is actually moving? Questions are dual to answers. Points are dual to lines -- the principle of duality in geometry. Space, length, distance is defined by two dual points -- space duality. The future is dual to the past -- time duality. We predict (syntropy) the future and remember the past. Absolute time (Galileo) is dual to relative time (Einstein) -- time duality. My absolute time is your relative time and your absolute time is my relative time -- time duality. If time is dual them space must be dual. Time duality is dual to space duality. Duality within duality! "Always two there are" -- Yoda.

[@alorcalderon279]

As good as it gets for this difficult subject. Thank you! I’ll still look for absolute rest no matter what the physics says about it.

[@pedromalafayabaptista3655]

Thank you for the enlightenments you provide, Dr.

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@RichardAuber]

Thank you, Dr. That was the clearest explanation yet and the humor and irony were charming.

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@mina_en_suiza]

I also had the "Einstein for beginners" book (the German version), but I had the same problem with it as with most popular science books: They went to great length to explain the easy parts very well, but once it gets complicated, they rush through it. This is the frustrating part of 99% of all popular science books: Great at explaining what you already know or can easily understand, but a total loss when getting to the real stuff. This video was actually great, but I don't know how helpful it would have been to my younger self. I can't judge, because it actually didn't tell me anything really new. When my kids are a bit older (and know English), they're gonna be my test subjects.

[@MmmhMarky]

In college, there was a science textbook that did the same thing. It went into detail like a few pages explaining simple stuffs, but when the hard stuffs appear, it rushed over it.

[@dennisbrown5313]

LOL - so true in all physic course, even higher level ones.

[@mina_en_suiza]

@@dennisbrown5313 In mathematics, we cleverly write: "The proof of the following steps is left as exercise to the reader."

[@renedekker9806]

Part of the reason for that might also be that it is easier to understand stuff you already know... 😉 That is, the point where the science book gets difficult differ between different people.

[@mina_en_suiza]

@@renedekker9806 Sure. However, I really noticed a repeating pattern: People putting much less effort in explaining the tough stuff well than the easy parts. I understand the reason: It's easy to explain what you yourself really and truly understand. It gets hard if you approach your own limits. That's one of the things, I really appreciate from Sabine: She never pretends that particle physics or general relativity are dead easy, but she makes it less intimidating.

[@tomtoups]

WOW! I've watched a lot of videos on this topic, but this is the only one that has explained it so clearly. I've learned so much. She can make a complex idea accessible without having to use grossly incomplete analogies and metaphors.

[@tomtoups]

@Savage samm thanks 👍

[@timesquare5473]

@Savage samm Lets not . 🤡

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@EventHorizon618]

Great video as always. You mention the negative sign in the Lorentzian Distance is there "because it works". I think it is possible to use Einstein's (that man again) train thought experiment to show that (ct)² - x² = (ct')² - x'² and therefore determine that (ct)² - x² is invariant for all observers.

[@gstlb]

“As we learned in kindergarten.” I have to quit watching these as I’m drinking my morning coffee 😂

[@thepeff]

Great analogy for path-dependence in four dimension, great video as always

[@gshort4707]

Fantastic summary of relativity, especially Newton's bucket. I hadn't heard of that one before, but for a long time I've wondered about the relativity of spinning objects. Great explanation, easy to understand. Thank you.

[@damianlukowski9996]

I'm not satisfied with the bucket. Is spin absolute or not?

[@gshort4707]

@@damianlukowski9996 when an object is spinning then each point in the object is constantly changing direction. A change of direction is an acceleration towards the new direction. Acceleration is absolute. Even in an otherwise empty universe with nothing else to compare with and no frames of reference you would still be able to measure acceleration. That is how I understand it.

[@damianlukowski9996]

@@gshort4707 How do you distinguish a spinning and non-spinning bucket in an empty universe? If both is possible, the universes must differ in more than the shape of water, because both shapes should have a cause.

[@stevejorgensen1105]

By far, the clearest explanation that I’ve encountered and the first one that I ever thought I had understood afterward.

[@svensven8994]

I'm really glad to see she pointed out that Special Relativity works in any reference frame, i.e., accelerating, rotating, anything, as long as the spacetime you're working in is flat. So many people only think it applies to inertial frames!

[@stewiesaidthat]

Tesla completely rejected the theory of relativity. He insisted that mass and energy were not equivalent and told the New York Times in 1935 that “Einstein's relativity work is a magnificent mathematical garb which fascinates, dazzles and makes people blind to the underlying errors.Sep 26, 2011 I agree with Tesla about the math making people blind to the errors of relativity. Time-dilation?.Where is that coming from? Everyone putting out these Twin Paradox videos is making the same error. Believing that acceleration in space equals less acceleration in time. Sure. You can take a clock, an instrument specially engineered to measure acceleration in space and hold it up as proof of time-dilation. But what is time-dilation? In biology, it's acceleration through the milestones of an organism's lifespan. The hatching date of a chicken is determined by the mass of the egg and the radiant heat the egg absorbs. F=ma. Market weight is reached based on the protein levels in the feed. Less protein, less acceleration (weight gain). How does gravity, inertial/non-inertial frames affect any of that? Plain an simple. It doesn't. Mechanicsl clocks measure acceleration in space. Biological clocks measure acceleration in time (change in the atomic structure of the mass). Moving about in space doesn't necessarily affect the atomic structure of the mass. In fact, if you study the engineering specs for the atomic clock, you will see that the cesium-133 atom is chilled to absolute zero to prevent a change in the atoms atomic structure when a force is applied. This notion that space and time are one frame belies the total ignorance of the scientific community. Want more proof that there is no time-dilation as predicted by relativity. An astronaut's heart rate is in accelerated state during lift-off. You don't need to be biology to understand that an accelerated heart rate equates to a shorter lifespan. Just ask a hummingbird. Then, there is the Breakthrough Starshot experiment. The solar sail is heating up with acceleration in space. The atoms of the solar sail are also being accelerated in time as it is being accelerated in space. How do you prevent the sail from being accelerated in time? Cryostasis. The same exact solution as employed in the atomic clock. No wonder people don't understand Special Relativity. Because it's fecal. It's made up mathematical nonsense that should have never made the light of day. But now you are stuck. In a hole so deep you can't extricate yourselves. What are you going to do? Apologize for your ignorance? You certainly can't be trusted with science after lying to the public for 100 years. The public is going to want answers for all of the taxpayers' dollars wasted on your religion. It can't be that hard to understand the difference between acceleration in space and acceleration in time. To understand that the clock is an instrument that measures acceleration in space alone. I don't know what claiming to have a PhD in physics constitutes. But if you don't understand the Laws of Physics, then you have a worthless degree in mathematical nonsense.

[@ytbit]

Fun fact: the precision of atomic clocks is such that when a time laboratory moves its clocks to another floor (or even to a different height in a rack) they must calibrate for the change in gravitation.

[@erinm9445]

Wow!

[@thomasherrin6798]

.....change in their acceleration!?!

[@franklittle8124]

Pretty impressive, I know that gravimeters used in geophysical field work can detect the change in gravity between a tabletop and floor, but to be able to detect the relativistic effect of that change is amazing.

[@MiccaPhone]

One could include acceleration sensors into each clock to self-re-calibtate each clock automatically. Wondering if this is already done.

[@fredflickinger643]

Thanks Sabine, although I learned that I had a good grasp on Special Relativity, I couldn't have explained it and connected all together so well!

[@hyperduality2838]

GRAVITATION is equivalent or dual (isomorphic) to ACCELERATION -- Einstein's happiest thought, the principle of equivalence or duality! Space is dual to time -- Einstein. Time dilation is dual to length contraction -- Einstein, special relativity. The Twin paradox is duality -- which twin is actually moving? Questions are dual to answers. Points are dual to lines -- the principle of duality in geometry. Space, length, distance is defined by two dual points -- space duality. The future is dual to the past -- time duality. We predict (syntropy) the future and remember the past. Absolute time (Galileo) is dual to relative time (Einstein) -- time duality. My absolute time is your relative time and your absolute time is my relative time -- time duality. If time is dual them space must be dual. Time duality is dual to space duality. Duality within duality! "Always two there are" -- Yoda.

[@croozerdog]

People like you who don't underestimate their viewers are a great thing to move on from after checking out easier channels. Thanks for teaching people without a PHD this way, seriously doing something good for the world. I struggled with non-scientific subjects at college, so I never got the chance to go to uni and actually learn intersting stuff like this. Our system of education makes it near impossible. People like you still give me actual education where the system failed.

[@schmetterling4477]

If you can't learn, then you simply can't learn. That is not a fault of the education system.

[@croozerdog]

@@schmetterling4477 Everyone can learn. My country locks education in several "levels". I failed in the subjects German and economics. The higher education I want to do does not have these. Other countries have different problems. You don't say to a USA citizen that they simply can't learn well if they don't have 150k to throw away lmao

[@schmetterling4477]

@@croozerdog And if you are feeling sorry for yourself, then your Mom didn't raise you right. ;-)

[@croozerdog]

@@schmetterling4477 so wanting stuff to be better is not being raised right? I don't feel sorry for myself, I feel sad for the problems in my country and want them to change. or did you mean that people who feel sorry for themselves were raised wrong? welp I guess fuck all depressed people and their parents

[@schmetterling4477]

@@croozerdog Stuff doesn't get better by wanting it. Stuff gets better by learning. NEXT!

[@pontuslandgren7997]

Awesome explanation of acceleration causing time dialation, never understood it before 😊

[@fsmvda]

You could have said that the reason for the 45 degrees being C is that is what you get when you set C = 1 by choosing time and distance units to get that and cancel it out of equations.

[@JackPullen-Paradox]

Or, you could determine that that would make a good graph of a cone, where a three dimensional cone would be (Δ x - h)² + (Δ y - k)² = (Δ z - m)²; so, a four dimensional cone would be (Δ x - h)² + (Δ y - k)² + (Δ z - m)² = (Δ t - n)². Or, eventually, Δ x² + Δ y² + Δ z² = c²Δ t². That leads to Δs² = -c²Δ t² + Δ x² + Δ y² + Δ z², because we want to include the interior as well as the surface of the cone. When² Δs² = 0, we are talking about the surface of the cone; when Δs² < 0 (Δs² is a notation convention, so it can be negative; it is really the differential), then we are talking about the inside of the cone; when Δs² > 0, then we are talking about the outside of the cone. All of these have physical meanings concerning the type of event. Light is represented as the surface of the cone; time-separated as the interior; space-separated as the exterior. The slope of the cone is c. If c = 1 then the slope corresponds to a 45 degree angle. So the 45 degree angle is a convention as you've indicated.

[@ffggddss]

As a grad student 50 yr ago, when MTW* was new, and Prof. Misner was teaching us relativity using that book, I'm glad to see both SR & GR being "properly" presented. Praise for Sabine, and for John Archibald Wheeler's vision of the topic! Mainly for recognizing that the use of a theory rests on top of a basic understanding of the underlying, important concepts. Which you have conveyed beautifully! Fred * MTW = _Gravitation,_ by Misner, Thorne, and Wheeler, IMHO a ground-breaking treatment of special & general relativity

[@bobjones7908]

Kip signed my copy around 1990, and he added "don't believe everything you read here". I think Taylor was the one who really emphasized you don't need GR for acceleration, but it is buried in MTW.

[@ffggddss]

@@bobjones7908 Edwin Taylor, of "Spacetime Physics" by Taylor & Wheeler, 1963/66? A friend of mine had that as a textbook in an undergrad course at U.Md. Anyway, I didn't think it was "buried" in MTW; I recall a bit of material about Rindler coordinates from there, e.g., and "how to outrun a light ray" (by constant acceleration, a, starting with a "lead" of c²/a), both in flat spacetime, i.e., SR. Which, BTW, demonstrates that in a constantly accelerating frame, there's a "horizon!"

[@daviddavis-vanatta1017]

100% agree about the excellent, superb job Taylor & Wheeler ("Spacetime Physics") does in explaining relativistic physics, at least to me. I struggled with my undergrad tests on it until I just gave up. Much later in life I found Taylor & Wheeler. Fabulous! If only I could have had that for my UG intro to relativity text, like @ffggddss's friend at U. Md. *sigh* But at least I eventually found that book! I'll go back to look at their treatment of the resolution to the Twin (seeming) Paradox. I think it's a bit different from the one shown here.

[@briancrowther3272]

DR Sabine, this is fascinating. i took physics at Sheffield uni 1976-79, along with geology and physical geography. I ended up after being a petroleum geologist, a science teacher at high school junior science, & pre uni physics and earth science, in Sydney Australia. I’m now 65 and what I’m trying to say is I have done this all my life. I found special relativity really hard to understand at uni and keep revisiting to understand it better and better. We didn’t get to general relativity on my course. One nice thing Eistien said that helped me feel better was we are not designed to see in 4 D and even he does not fully see it, it just knows the maths works. I think I am good at physics because I see how things move (I was tested in this and go off the bell curve for the general population in that trait, hence the geology as well) and then have just enough maths ability to back that up, even if I don't have great teachers (had 5 maths teachers in my last 2 years, ie the pre uni course, at school, and as a teacher I now realise that is like poison to learning). Like you when a kid at school my kids loved relativity in the New South Wales Course. The resources to teach it got better and better with the advent of the web. So, my understanding got better. Brian Cox's shows got me into trying to understand general relativity and his co-authored book, "Why e= m c squared?" is just fantastic for all this. In that book I realised like you say gravity is not a force. So cool. When my boss retired the school did not honour him after 40yrs science teaching so I did it by making a social do and raised money for a gift to present at the associated dinner. He got fly fishing wading boots. But I didn't tell him. I did a talk about entropy (based on Brian Cox's show) and what is happening in that regard, happening in the universe while we are at the dinner ie the arrow of time can only move forward so as to obey Maxwell’s 2nd law of thermodynamics (entropy always increases). Then I got a cuckoo clock and placed it in a cardboard box and made out this was the gift from the staff. I presented this to the boss as such calling it "an entropy machine". Then to the Principal of the school I informed him re general relativity that this machine does not work on the force of gravity it is a result of the earth curving space time and that time runs slower closer to the Earth's surface, this gives the acceleration which we feel as a force of gravity, ie the ground pushing up.. Thank you for this. It helps, I'll have to re watch it a few times and dovetail that with what’s locked away in my old brain cells, dredging that up to synthesise and once again attempt to understand it better. Being evolved to grab the low hanging fruit in 3D is a pain when attempting to see in 4D. I think that our atoms being organised in such a way to allow us to able to do this is the miracle. This is one of life’s pleasures and I feel so lucky to be in a position to do this. Despite a life time of doing physics, you have relieved me of one important misconception, relativity is for constant velocity and general for acceleration. Thankyou.

[@schmetterling4477]

So now that we have heard your irrelevant life story, is there anything of importance that you would like to contribute? ;-)

[@shorthand1121]

This was REALLY good ... I learned Special Relativity in high school but we never touched it in my physics undergrad except waving hands at it for high energy particle physics ... but I never got this kind of clarity on the time paradox and always figured it simply had to do with the energy expended in acceleration ... never considering that you didn't need to either leave or arrive back with 0 relative velocity. (We just always assume second-order continuity b/c that's the world we live in.)

[@ramonortiz7462]

Time is not a thing nor was time discovered!! Man invented time to SYNCHRONIZE from one moment to the next moment in the CONSTANT present!! Why don't they know what GRAVITY is ITSELF in ANY FUNDAMENTAL WAY??

[@pesilaratnayake162]

Great explanation, Sabine. Tied together a lot of loose ends in my understanding. I hadn't heard of Newton's Bucket, though, so that's a new concept for me. I guess it's another way to detect acceleration, although it seems more restrictive than the spring, since an acceleration would be necessary to have an even surface for the water in the first place. Maybe a fluid sphere, held in shape by pneumatic pistons that change their applied force as the fluid exerts a force on the sphere. Springs sound a lot simpler!

[@whatapk]

👆👆LET'S TALK 💬NOW. THANKS FOR WATCHING📺.

[@dubberj2847]

I will need to watch this a few more times to properly understand all of the concepts. However I would say this is one of the best and most concise explanations of Special relatively that I have watched. Especially the explanation of the mathematics and diagrams that describe Spacetime. Sabine makes the mathematics very simple to understand.

[@aupotter2584]

@@nemlehetkurvopica2454 On the contrary, I disagree w the explanation of Twin paradox based on Lorentz transformation alone, as velocity is relative and Sabine has clearly pointed out that both Alice and Bob will get older at the same time which makes no sense at all, though I still dunno why acceleration causes time dilation as told by Sabine, or earlier by one of my Physics professors, because I also think that acceleration is relative as well, just thinking of centripetal force and the centrifugal force relative to the rotating frame of reference...

[@Kycilak]

@@aupotter2584 Centrifugal force is exactly the consequence of acceleration being not relative. Let me demonstrate: Imagine you have 2 people - First and Second - in space and Second is on a carousel. When the carousel is not spinning, both are in a weightless state and at rest relative to each other. Now consider the carousel spinning. When looking from outside we can clearly see that carousel acts on Second in its seat because if it didn't second would not go round but instead would go straight. This force that acts on him that makes him go round is called centripetal force. From the view of Second the whole world is spinning, so you conclude that he may think that everything is rotating around. But he can feel the seat of the carousel acting on his bottom with the centripetal force. He may not see himself rotating (instead everything else is) but even though he should be at rest (according to his eyes) he knows there is a force acting upon him. So we see that both First and Second agree who is the one that the force acts upon and since acceleration is proportional to force they both agree Second is being accelerated. Hence acceleration is not relative.

[@aupotter2584]

​@@Kycilak You've just shown to me that centripetal and centrifugal forces are relative: the First sees the carousel exerting an inward centripetal force on the Second to maintain the circular motion, while the Second sees the opposite as experiencing an outward centrifugal force. The acceleration is hence relative, pointing inward when viewed from the First's inertial frame at rest, and outward when viewed from the Second's rotating frame of reference.

[@Kycilak]

@@aupotter2584 If the forces were relative as is speed for example, Second would have to see a force acting upon First. But that is not the case, both know Second is being acted upon. And even from Second's view the force is acting inward. The seat of the carousel accelerates him towards the center of rotation from the view of both.

[@aupotter2584]

@@Kycilak Good points, but carousel is something we used to play w so we always have 'mixed' feelings from both frames of reference... if we scale it up to our Earth, we won't recognize it spinning fast as viewed from outer space, but we do experience a weaker gravitational force due to the centrifugal force... moreover, rotating frame of reference is not an inertial frame, so we'll also observe a centripetal force acting on every star or galaxy appeared to be spinning around us.

[@nanow1990]

I am writing a paper but I wanted to point out interesting thought experiment: Time dilation is an emergent property of computational resources per particle in given region of space. More p density in fixed volume v the less resources are being allocated per 1 particle and thus slower atomic clock runs, similar to computer when you put more extensive settings in physics simulation you get less frames/iters per second. This explains every single effect in physics, physics are emergent properties of reality, they are an intelligent design around limitation.

[@stewiesaidthat]

Time is the result of acceleration. Acceleration comes from Force. What causes delayed seed emergence? Egg hatching? Physiological maturity in plants and animals? None of that has to do with the ticking of a clock or motion through space other than the Earth's rotational speed creating a 24 hour day/night cycle in which biological organisms have evolved to take advantage of the available energy. Take two identical seedlings. Plant one in the south and one in the north. Which one has the longest lifespan? You relativist would say the one in the south because its traveling faster in space. But the one in the north is growing in a higher gravity environment. So which is it? Trying to solve for X by using relativity just shows how ignorant and intellectually challenged people really are.

[@mnada72]

That is insightful explanation. Yet I have to watch it several times to feel comfort about it 😊. Thank you.

[@davidparker2173]

Didn't grasp much, but did acquire a sore brain, proving I did attend the subject. Amazing I am able to come to some grasp of these topics if I attend them enough. Provides good exercise if nothing else. Appreciate your humor.

[@vibaj16]

Vsauce's video "Which way is down?" provides a less in-depth, yet easier to understand explanation of some parts of relativity. It's the first video that made me finally understand how gravity isn't a force, and that freefall is actually not acceleration.

[@barryon8706]

Thank you, Sabine! And your wonderful staff, Alice and Bob. They don't get enough credit for the good they've done in physics and cryptography.

[@human_shaped]

Alice and Bob are probably due for an honorary joint Nobel prize by now.

[@pixelfairy]

Will bob ever get the message? Will Eve foil her plans again?

[@axle.australian.patriot]

IKR! Alice and Bob are going to start competing with Albert. "Yes, those guys again" lol

[@hyperduality2838]

GRAVITATION is equivalent or dual (isomorphic) to ACCELERATION -- Einstein's happiest thought, the principle of equivalence or duality! Space is dual to time -- Einstein. Time dilation is dual to length contraction -- Einstein, special relativity. The Twin paradox is duality -- which twin is actually moving? Questions are dual to answers. Points are dual to lines -- the principle of duality in geometry. Space, length, distance is defined by two dual points -- space duality. The future is dual to the past -- time duality. We predict (syntropy) the future and remember the past. Absolute time (Galileo) is dual to relative time (Einstein) -- time duality. My absolute time is your relative time and your absolute time is my relative time -- time duality. If time is dual them space must be dual. Time duality is dual to space duality. Duality within duality! "Always two there are" -- Yoda.

[@axle.australian.patriot]

@@hyperduality2838 So any thoughts on how we solve this paradox? Or like me, I am attempting to accept the paradox as part of the ultimate truth of the universe and write that into my view of the physics?

[@skwervin1]

I wish I had someone like you 40 years ago when I was learning physics in high school. Biology and chemistry I had no problems with but some parts of physics just melted my mind because my teacher just couldn't explain stuff and just told us to read the book. Back in those days we didn't have KZbin or the internet.. I honestly think I could have gone further in physics if I'd had a half decent teacher. It also didn't help that in those days, girls were not encouraged to do higher level sciences or maths but then again I did always like proving people wrong!

[@dancincoolkid]

Prospective teacher here, hoping to do exactly what you wished a high school physics teacher did for you! I remember taking physics in high school and being equally as perplexed, and then after a degree in college (and with much accompanying, self-taught, grueling hours) things finally made some sort of sense! Hope I can translate that knowledge to other physics hopefuls 🤞

[@CJ-gn8qm]

How strange? I was the exact opposite at school. Physics made total sense to me and chemistry was hieroglyphics! Not surprisingly I went into engineering! Though for my degree I did have to catch up on the chemistry!

[@SAMACAG]

Would You be so kind, and look at these two Clips? Common Sense The Needle Common Senes Red Pinhead We would love your feedback ... :)

[@SAMACAG]

Sabine has not done her homework. Please watch: Common Sense Einstein Super Star

[@mr01steam]

Fantastic explanation. How many mis-concepts this video has swept away? Great explanation.

[@McBobaa]

I can't really say this video made things any clearer for me, I think it only made it more confusing 😆 I was lost as soon as you started talking about proper time. I appreciate the effort though.

[@ed.puckett]

Thank you for making this so clear. As you usually do.

[@stoyanfurdzhev]

I like it.

[@dougpine4746]

You slide into the sponsor at the end like a pro lol. Space-time is more difficult to understand when a person is at the beginning of a lifetime as compared to nearer the end.

[@vinceturner3863]

Thanks Sabine, yes your explanations are better than a book.

[@anslicht4487]

Watching your videos from years back vs now (I binge on them repeatedly) it's interesting that your German accent with traces of British has become British with traces of German. Among all your other accomplishments, kudos on being ever more smoothly bilingual as well. I have none of your skills but do appreciate what you bring us (and so am a Patron).

[@PedroGondalles]

did you understand everything ?

[@brothermine2292]

It's disappointing that Sabine didn't present the Twins equation that provides the amount of time that passed for Bob's clock as a function of Bob's (constant non-accelerated) velocity, as observed by Alice during a time T measured on Alice's clock. The minus sign in the hyperbolic equation can be removed by simply adding the negative term to both sides of the equation, which produces a Pythagorean equation in 4-dimensional Minkowski spacetime, which shows that the square of Bob's rate of aging (relative to Alice) plus the square of Bob's velocity through 3D space (relative to Alice) equals the square of c (the speed of light through 3D space). In Minkowski 4D spacetime, the time axis is orthogonal to the three spatial axes. By the Pythagorean theorem for right triangles, the left side of the equation equals the square of the hypotenuse of the right triangle. Taking the square root of both sides shows the length of the hypotenuse is c. The hypotenuse is the 4D velocity that Bob is traveling through 4D spacetime, and the fact that it equals the constant c independent of Bob's velocity through 3D space is remarkable, and has an aroma of being fundamental. This Pythagorean equation leads to some well-known corollaries: (1) The faster something travels through 3D space, the more slowly it ages. (2) Anything traveling through 3D space at the speed of light c (such as light and gravity waves) doesn't age. A little-known corollary is that anything traveling through 3D space faster than c ages at an imaginary rate (the square root of a negative number)... but it's unclear whether aging at an imaginary rate is physically possible or even has physical meaning.

[@pluto9000]

TLDR

[@brothermine2292]

@@pluto9000 : It's much shorter than the video we just watched. I tried to be concise, but without sacrificing clarity.

[@Kenjuudo]

Great read. Don't mind the trolls.

[@marcelob.5300]

@@pluto9000 tsdc (too short, didn't care)

[@brothermine2292]

@@marcelob.5300 : ADHD

[@frankwijnans444]

There a very good reason why light is 45degrees: the factor c transforms seconds to meters and visa versa. And the system is constructed to ensure the light speed is constant.

[@alanoneill4058]

This is only about the second time I've been moved to comment online about anything. This is great!

[@dumbofliesvermont5684]

Thank you. Understanding the difference between proper time and coordinate time was an eye opener for me. I had always wondered how the traveler in the twin paradox could stay alive because "everything slows down." I expected chemical reaction rates to slow as well and so he would die due to lack of biological activity. Now I see that's laughable. Thank you again.

[@lawrencedoliveiro9104]

What she calls “proper time” is what I learned to call “interval” (Lorentzian distance). The “interval” between two points on the path of the same light beam is always zero.

[@MiccaPhone]

He would rather die from the high g-forces. With low g-forces the time dilation would be too small to be visible to human perception.

[@RagingGeekazoid]

Death is a biological process, and that slows down too.

[@Mr.Not_Sure]

Needs to note that acceleration Sabine is talking about -- is 4D acceleration.

[@roycater1]

Great video, thank you. You said the Minkowski definition of spacetime distance, with a negative time term, was accepted because it works, “it’s a keeper”. I think this issue is much deeper. Einstein (that guy again!) told us all observer’s views are equally valid. Our individual “reality” is composed of the events that have already impinged upon us. They are an unrepresentative time and distance textured version of our surroundings that may be further biased by our motion and by any distorting fields. This reality totally determines all physical outcomes for us, even though our personal version of reality is disputed by all other observers. I think the idea that all views are equally valid, is correct but incomplete. At every event in space and time there can only be one status of local spacetime. Local spacetime may be curved, it may have other fields, they may be changing and objects may be present. There is a unique universal reality comprised of all these local event realities. All observers’ realities are separated from this universal reality. Some might call this God’s view, I prefer to call it spacetime’s reality. You can visualise this universal reality with a space-time diagram where instant communication of events is possible. In spacetime’s reality a local region of flat spacetime is Euclidian and the distance between events can be calculated using 4D Pythagoras with a positive time term. When our unrepresentative view is mapped onto a space-time diagram we see a range of events along an incoming light path as simultaneous. Because the distance travelled along a light path is time multiplied by “c” and there is no time difference, there can be no difference of distance either. (Text books derive the spacetime interval with its negative time term from this start). Minkowski spacetime is a consequence of our distorted reality. Because we judge all theory on its match to our peculiar observations, all our theories are descriptions of our “myopic” reality and not of spacetime’s “all-seeing” universal reality. We can infer things about universal reality or observe what has happened nearby after some delay. This new understanding, with a bit more logic, also allows us to explain why acceleration is “absolute” and why the speed of light is anomalous. What do you think?

[@SuperMagnetizer]

I think light speed is dependent on field, rather than frame. And since field sources move, light speed is, as you say, anomalous.

[@roycater1]

would you like to discuss this @@SuperMagnetizer

[@phillipcoetzer8186]

This explanation got me pondering the whole day ... in my mind it always seemed that acceleration distorts space and velocity distorts time. And the way I understand gravity is like when you pull the plug in a bath of water ... the closer the water gets to the hole the faster it moves so that the amount of water displaced remains the same for any given radius. The idea of distortion does not sit well with my intuition but rather that a body seems to absorb spacetime (or the infamous Ether)somehow. So as we move through spacetime thanks to the earth holding us in place or we accelerate through empty space the same effect appears because the same thing is happening. But intuitively it goes to reason that this line of thought leads to the idea that one can accelerate through space indefinitely till surpassing the speed of light and that corresponds with the idea that one can stand on a planet indefinitely accelerated by its surface. The explanation you give seems to imply that once acceleration stops no matter the velocity of two bodies being compared... time moves equally for both. And that's what got me pondering.