Special Relativity: This Is Why You Misunderstand It

[@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 ...

[@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.

[@odiewan67]

German humor: it's not a laughing matter

[@Qichar]

I don't recommend visiting a black hole if you want longevity. From your perspective, you won't live any longer. It's just that everybody you ever knew will be gone by the time you return home.

[@Lagrange_Point_6]

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.

[@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 :)

[@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

[@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.

[@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.

[@HoratioHoodoo]

YES! So frustrated that I hadn't heard it stated this way before.

[@geoffreymilward3293]

@@iurlc in SR the light cone always has 45° angles in my frame of reference, in my coordinate time and space. In the rest frame of the photon there isn't a meaningful space time diagram, indeed photons can be said not to have a rest frame. When I taught SR I always emphasized always be clear which frame you are measuring the coordinates in.

[@iurlc]

@@geoffreymilward3293 This 45° is a big error. Imagine something else than a photon. e.g. small molecule. Imagine it is moving very close to the speed of light. No matter which scale you use for time and space. I can find a speed for this molecule, that with the selected scale of the diagram it will have an angle very close to the x-axis. So speed of light can't be 45°. With logical extension (higher speed than the molecule), it must be 0° (equal the x-axis).

[@sudheerbright]

Thanks

[@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

[@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)

[@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.

[@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.

[@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.

[@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?

[@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).

[@mentalitydesignvideo]

but if you read it out loud in Dutch people will think you are chocking and will perform a Heimlich maneuver, which really interferes with reading comprehension.

[@morenofranco9235]

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

[@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

[@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 😂

[@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

[@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?

[@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.

[@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.

[@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]

@@QuantumGravityResearch1 can you add anything useful or is spam your only way to communicate?

[@stoyanfurdzhev]

Otherwise everything is cristal clear.

[@TravisCotter]

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

[@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.

[@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!

[@octaviannkya9417]

She got it wrong tho, in the water bucket example the cause of concavity is centrifugal Force not acceleration

[@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.

[@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.

[@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!!

[@VelvetCondoms]

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.

[@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

[@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. ;-)

[@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.

[@pumbaa667]

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

[@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.

[@Company-59]

Dr. Sabine, usually I do enjoy your videos and the way you explain complex matters in relatively easy terms. But, this time, it was different. You know, I made my physics Abitur in special relativity and I got 15 points for it. The graph that you showed in the beginning was the result of what we talked about in school. It was not the beginning, the beginning was our teacher, explaining the twin paradox in very easy words. Everybody could understand it. I’m afraid, I could only understand your explanation because I already knew the results. But, perhaps some just feeble minded. Cheers!

[@schmetterling4477]

She is just not a good science communicator. She is not a good physicist, either. She is simply a good self-promoter. The Trump light of physics. ;-)

[@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!

[@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.

[@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.

[@stevejorgensen1105]

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

[@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

[@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?

[@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.

[@dexter8705]

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

[@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.

[@etienne8576]

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

[@kathrynhart1040]

hahah I just said the same

[@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.)

[@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!

[@thepeff]

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

[@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.

[@jaime_mm36]

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

[@Tom-sp3gy]

Sabine I think this is one of your best videos yet! Thanks for all the effort you are taking educating us all. You truly deserve more than just words of praise. I hope one day the physics community actually names an important science institution or department in your honor. At the very least, Germany should bestow on one of its finest daughters a medal of excellence.

[@vasilyp]

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

[@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.

[@mikehalpin1385]

Question: If acceleration slows time, and if I was to build a centrifuge that could create say 1000 G's, and I put an atomic clock in it for delta t my time, would that clock undergo a time dilatation relative to outside the centrifuge? Thanks for all your videos, Mike H.

[@Tore_Lund]

Yes that clock would tick slower in respect to an outside observer or a clock closer to the center of the disk.

[@justdev8965]

It appears I shouldn't have even passed kindergarten

[@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.

[@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.

[@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.

[@Raging.Geekazoid]

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

[@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!

[@pedromalafayabaptista3655]

Thank you for the enlightenments you provide, Dr.

[@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!

[@ed.puckett]

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

[@stoyanfurdzhev]

I like it.

[@marvinco33]

6:13 time stamp, I think you meant the other line? As the line above would be |VC|

[@toyfabrik2993]

Concerning the twin paradox: You pointed out that Bob's path through spacetime is different from Alice's path, because of accelerating forces acting on Bob, breaking the symmetry. But instead of de-accelerating and accelerating again in order to get back home, couldn't Bob just do some sort of "swing by" maneuver around celestial objects, thereby making a turn driven by gravity only (and following a straight path through spacetime all the way)? Would that make a difference? Because no spring or other acceleration-gauge could detect any forces acting on Bob that way. Would he still be younger when he returns?

[@stewiesaidthat]

The twin paradox arises because 'physicists' don't understand physics. Their knowledge of 'physics' is strictly limited to INORGANIC matter. They are trying to apply that knowledge to ORGANIC physics. It doesn't compute. The force that accelerates the hands of a clock is not the same force that accelerates a person in time - biological processes. What accelerates a seed out of the ground? Temperature. If you heat up a clock, it decelerates the hands of the clock. The permeability of the clock's circuitry changes with changes in temperature, causing electronic devices to run slower. You can not use a mechanical clock that measures motion to measure the biological processes that affect your acceleration in time. Time and space are two separate and disparate entities. Traveling faster in space does not equate to traveling slower in time. Acceleration/deceleration events increase stress and shorten lifespan. This all started because some layman, with no education, stuck his nose where it didn't belong. It's about time that 'physicists' stop worshipping false gods and none of his theories have been proven. And if the experiments are properly interpreted, thoroughly disproven. Stop wasting everyone's time on disproven theories.

[@toyfabrik2993]

@silverrahul , well well, but doing a swing-by means travelling in a straight line with constant velocity, as far as spacetime is concerned. So there would be no broken symmetry due to acceleration. How could the twin-paradox arise in that scenario?

[@toyfabrik2993]

@silverrahul , interesting reasoning. But i would have argued exactly the opposite way: If the swing-by is performed in such a way that the spacecraft's velocity is not significantly altered, then the time dilation would be relatively small, and the overall effect on the twin paradox would be at a MAXIMUM. The traveling twin would return home without having experienced any significant phases of deceleration and acceleration along its trip, having followed what spacetime consideres to be a straight line. So almost no age difference should show up, leaving us without any "paradox". Why is that false reasoning?

[@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 ?

[@Brandon-rc9vp]

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

[@SecretEyeSpot]

I watched this 4 times in a row, but i still feel like I have so much more to go to truly understand

[@helifynoe6956]

I gave up long ago making sense of what was discussed about special relativity(SR). Everyone was describing this relativistic phenomena, but NOT discuss the ABSOLUTE physical cause of the SR phenomena. So, I decided to discover the SR phenomena myself by starting from scratch in understanding motion. End result was me understanding the absolute cause of SR, and deriving the SR equations, along with deriving the Lorentz Transformation equations. All these derivations came from my single space-time-motion diagram, which is composed of a motion vector, which represents the motion of any object present within space-time, and tied to it is a length scalar, which represents the length of an object. My derivation method is the simplest method ever possible. Sadly, conformity rules. Thus new deriving methods such as mine, are ignored 100%.

[@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?

[@j-sonquarc1507]

You use a 45° angle for c, because the amount of change in space equals the amount of change in time. Correct me, if I'm wrong.

[@jeanf6295]

More precisely the ratio of space and time units is arbitrary, so you can choose any kind of angle. Do the same diagram in SI units and you would not be able to distinguish the space axis and the light trajectory.

[@eonasjohn]

Thank you for the video.

[@alanoneill4058]

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

[@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.

[@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

[@GrzegorzStyczen]

What a superb video! I still have one problem with 'gravity is not a force' that I don't understand, we can convert it to energy like electricity (using a waterfall for example). Same with how we use gravity to accelerate spacecraft using a slingshot maneuver, the spacecraft is freefalling so it should not be accelerating? Where's the energy coming from?

[@erinm9445]

I liked your questions so I looked them up! For the slingshot maneuver, the energy comes from whatever planet the spacecraft is being slingshot around. The planet will lose a little bit of kinetic energy (speed), and that energy is transferred to the spacecraft. Here is a stack enchange convo on the topic, the top answer with the cue ball explanation is excellent! I wasn't able to find a clear answer for the water wheel, but I'm guessing that it's related to acceleration coming from the earth and from the water wheel itself.

[@Tore_Lund]

@@erinm9445 I know I'm not answering the core of your question first, but energy is not an intrinsic property in the universe. It is an emergent property of Entropy, the tendency for things to reach the lowest energy level, spread out and mix and equalise. So the waterwheel is driven by fusion in the Sun, like most other energy sources on Earth. Gravity simply acts as a spring, a temporary store of energy in this example, it is the Sun that adds potential energy to the water by lifting it to the top of the mountain. To answer where the free falling spacecraft gets it kinetic energy from in special Relativity and not with Newton, when there is no force acting on it so it isn't accelerating, it is still a question of perspective, The Earth is accelerating up to it and that mass of Earth is transferring kinetic energy to the spacecraft when it crashes onto the surface of of the spacecraft, but until that moment, the spacecraft is at rest. Another help in visualisation is to think of space itself as the only thing feeling the force of Gravity, that earth is reeling space itself in and the spacecraft simply riding along as log on a river no force is acting upon it and it is not experiencing acceleration, the water around it has the same speed as the log. Where did that energy come from then?: Not from gravity, either the spacecraft used engines to get potential energy from altitude, a net zero energy gain, like the waterwheel originating from its fuel made with energy from the Sun as well. Or it came from a planet with lower gravity, so indeed a net energy gain. That energy would have come from the inhomogeneities in the Universe causing planets to be different which arose from the slight variations in density and temperatures in the early universe after the Big Bang. So that might be energy originating from the Big Bang itself, but then again, all "energy" originates from the inhomogeneous of the early Universe's ordered state which also caused stars to form . The crux of you question is that you consider "energy" a real tangible phenomenon, but in reality, it is more like a way of accounting for the detours Entropy takes on its way to the bottom, which human engineers have invented to build steam engines. It is really not a "real" property of nature.

[@GrzegorzStyczen]

@@erinm9445 Thanks for helping me understand this folks! If the slingshotted spaceship is actually accelerating, I would also guess (as a non-scientist) that the energy to accelerate the spacecraft comes from the planet's own acceleration, but without the things touching to transfer it rises a lot of questions. Relativity makes it even trickier (from the spaceship's point of view the planet is accelerating?). I have an alternative explanation: The spaceship is not accelerating but just covers more distance due to time moving slower in a heavy gravity zone. But then I don't understand why the spaceship keeps having this boosted velocity even when it comes out of the planet's influence. @Tore I must say it's difficult to simplify it all to entropy but I will definitely ponder about that interpretation, thank you

[@theodoresweger4948]

I love the quick comical sayings I have to be quick to catch them, you are my favorite Dr. Sabine. Thanks, you are awesome...

[@JayVal90]

Ok the distinction of coordinate time =/= proper time makes things super understandable. Also the fact that “path length” as measured by that hyperbolic distance formula is “perceived time on that path” and not some weird combination of the two helps. I’m curious what happens if you define c such that it’s at 90 degrees instead of 45 degrees in those spacetime diagrams. I know they’ll be less useful (maybe), but certain simplified things may make more sense.

[@Anonymous-df8it]

90 degrees would be covering infinite distance in zero time (relative to the observer)

[@nenhard]

Then you would have extra dt*dx component in formula for proper time

[@Anonymous-df8it]

@@nenhard ???

[@nenhard]

@@Anonymous-df8it Metric tensor in such coordinates would not be.diagonal.

[@dynapb]

I also wonder why they don't use 90 degrees for speed of light travel. To me it makes more sense on the graph because at the speed of light time stops so that would make it a horizontal line. Probably some math thing. Also if you could go faster than light the travel line would move to the negative quadrant and you would be going back in time. Makes more sense to me!

[@JamesMcCaughan]

The Ptolemaic system of the heavens was a mathematical construction that correctly tracked heavenly movements for 1400 years until Galileo discovered with his telescope that Venus had phases like that of the moon, which were not predicted by the system. This led, at first, to the replacement of the geocentric kinematical construction by a similar and simpler heliocentric construction of Copernicus, which gave phases to Venus. But in turn kinematical construction was replaced by dynamical nature with Newton’s law of gravity. There was a reason from nature, not the mathematics, why the sun should be at the centre of the solar system. With Newton the philosophy driving the physics switched from idealism, saving the appearances with prediction, to realism, explaining how nature behaved. Construction is like a scaffold enfolding a building. The scaffold touches the building, but the links holding the construction together do not reflect how the building is. Relativity is a construction. By starting with reference frames it is homocentric. It begins with the mind by choice of frames and not with nature; it remains a mental exercise by working with the links of the construction. Nature does not require a reference frame to do what it does; nature does not require Special Relativity. The twin paradox was solved by George Darwin using light Doppler shift. We have two ways of doing physics: Ideal physics or observer based physics and Real Physics or physics of the observed. The philosophy driving the physics determines the type of questions that can be put to nature and the way they can be answered. In answering life’s biggest questions through physics in Dr Hossenfelder’s latest book "Existential Physics", the philosophy driving her physics provides her answers. In her KZbin channel presentation "Special Relativity: This is why You Misunderstand It," Dr Hossenfelder leaves us with five takeaways: 1. Acceleration is absolute. 2. The reason that time slows down is acceleration 3. Time dilation is a real effect and has been measured. 4. Special Relativity does describe acceleration, but only in flat space-time. General Relativity describes gravity in curved space-time. 5. Gravity is not a force, which is why being at rest with a surface of gravitating body requires an acceleration and that too slows down time. The role of acceleration is the critical element in four of the points. The twin paradox is solved through the asymmetry of the experiences of the twins. Although the acceleration at turn around of the travelling twin is identical to the acceleration of the stationary twin as seen by the travelling twin, nevertheless the travelling twin experiences the forces from that acceleration, but the stationary twin does not. The result is that, according to Dr Hossenfelder, the stationary twin reads coordinate time and the travelling twin a shorter proper time. This prediction has been entirely a mental process involving acceleration with the construction of Special Relativity (item 4). It uncovers a “phases of Venus” situation for the construction: I regret to say that item 2 is false in nature. That acceleration does not affect clocks has been experimentally verified at CERN in J. Bailey et al, “Measurement of relativistic time dilation for positive and negative muons in a circular orbit”, Nature 268 (1977) pp 301-305. The acceleration of the muons in the circular orbit was 10^18g, but the dilation of decay times was that of Special Relativity alone. Item 5 falls as well, but for a different reason. If gravity is not a force, then there is collateral damage to Newton’s laws of motion. Dr Hossenfelder bases her claim on the fact that one does not feel the force of gravity in free fall, but that is the exception for that force alone. Consider: 1. A spring stretched between two hands. 2. The same spring stretched by the same amount pulling a mass with acceleration across a smooth surface. 3. The same spring and mass hanging vertically at rest in a gravitational field, stretched by the same amount again. In the first case the spring, obeying Hooke’s law, is extended by two equal and opposite forces, since the spring is at rest. In the second, the spring has to be extended to exert a force on the mass, but the spring requires two opposite forces at either end to be extended. These forces are not equal because the spring and mass have common acceleration. The force that the mass exerts on the spring is the real inertial force F identically= mg, where g is the acceleration. In the third, the spring is extended by the same amount as in the second, but spring and mass are at rest. The force that the mass exerts on the spring is its weight W= mg from Newton’s second law of motion. One can also write W identically= mg where g is now the strength of the gravitational field. The field is everywhere in the space surrounding the earth. Weight is defined as the force exerted by gravity on the mass. When one measures weight on a weighing machine, it is the normal force N from the machine platform that is measured, which yields the weight by keeping the mass at rest through W-N = 0 from Newton’s second law of motion. One is weightless when N disappears in free fall, but W continues. Exceptionally, one does not feel W in free fall as gravity applies to every last atom in the body, which does not produce internal strain, either as stretch if the external force pulls, or compression if the external force pushes. One feels the strain as real inertial forces. My apologies for spoiling your presentation, but it shows the power of accounting for observations through nature, where force is central, and not from mathematics of a theoretical construction. Lost in Math sums up the situation. "Real Physics vs Ideal Physics", near completion, has been developed over 26 years. My background is 50 years teaching undergraduate physics, mainly at the first and second year level, at the University of Sydney, finishing as a Senior Lecturer in 1999 and then Honorary Senior Lecturer till 2015. My research field was Cosmic Radiation (Extensive Air Showers using cloud chambers and image intensifiers) till 1991. For the sake of the future of physics we should talk or correspond. Yours sincerely, Dr James McCaughan

[@sanzidarefin]

Sabine is in a different mood today! It's like if she screams a little louder, "Gravity is not a force" will be a meme GIF and what not with her style of delivery! Then again, I understand why she is delivering in this way. I just understood my physics classes are all somewhat wrong! But at least the situation is not like chemistry where every atom model is wrong and every model has an exception! No wonder I am a fisheries graduate now. Because now, Only exceptional things are interesting!🤣🤣🤣🤣🤣🤣

[@Tore_Lund]

Chemistry is a mess because chemists insists it is a special field of science and not that it in reality is physics and can be deduced to much simpler rules.

[@sanzidarefin]

@@Tore_Lund thats why physicists have the dream of "theory of everything"

[@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? ;-)

[@jeremypeltier986]

But Sabine, Newton bucket paradox is still an ongoing paradox that even Einstein did not solve : why isn't acceleration relative? Why is it absolute ? Einstein was very inspired by Mach and its principle that the whole universe explains local physics. Einstein wanted to make a theory where inertia is also relative but did not succeed. I do not think it is that simple. Do you have a theory that fundamentally explains why acceleration is absolute ?

[@fuxpremier]

It's an observation, not a theory. The same way, Galileo observed uniform motion to be relative. It's something we observe, not something we can deduce out of the blue. Mach's idea would induce that inside a rotating bucket in empty space, the shape of water would stay flat. The idea that the shape of water depends on long range action from distant stars is just absurd.

[@jeremypeltier986]

@@fuxpremier why would it be absurd?

[@fuxpremier]

@@jeremypeltier986 It is indeed true that this principle did inspired Einstein, he's even the one who came up with the name "Mach's principle", but that doesn't make it correct at all. Einstein was looking for a theory that could explain gravitational attraction as an apparent acceleration, i.e. he was trying to prove gravitational forces were not external but inertial. As gravity is induced by the matter surrounding us, he saw Mach's principle as an inspiring idea. But Mach's principle is very different from Einstein's theory. Indeed, Mach did not quantify his theory, making it irrefutable, but above all, in his view, it is distant stars which cause inertia, not celestial bodies surrounding us. And above all it's supposed to cause all forms of inertia, not only the apparent inertia due to the curvature of space-time. It is not true to say we don't know what causes inertia. It is internal interactions within an isolated system. Massless particles don't have inertia, therefore they travel at the speed of light and don't experience space-time, the spacetime distance between the interaction that creates them and the one that annihilates them is always 0 (ds²=0). But all fundamental particles are massless, they gain their masses only by interacting with one another (fundamental particles gain their mass by interacting with the Higgs field, composite particles gain their masses by the interactions of their fundamental constituents together). Inertia, thus space-time, is an emergent property of interactions within a composite body. A good thought experiment is to consider a massless box full of photons bouncing at the surface of the box. When one try to apply a force to the box, it will show a resistance to acceleration which is proportional to the amount of energy carried by the photons (you can do the maths by applying conservation of energy and momentum to the box): the photons interact with one another by the medium of the box. Individually, they don't have inertia, but the box itself gets one. That shows that inertia is not due to the action of distant stars but on the contrary to the internal interactions between the fundamental constituents of the body. One doesn't need to postulate the existence of inertia but it's on the contrary an emergent property of interactions theories at large enough scales. As inertia is necessary for spacetime to exist, many researchers think that general relativity must also be an emergent theory at larger scale and is not a fundamental feature of the universe. But as long as we have not found a better theory, this is speculation. At least though, enough is known about the topic to fully reject Mach's principle.

[@jeremypeltier986]

@Fux Premier I really like your answer! Thank you. I understood Mach's principle about distant stars more like this : through their interactions with other bodies, bodies gain their inertia and the collective of all bodies in the universe contributes to the strengths of the local gravitationnal strength (G). I thought the distant stars idea was more of a poetic illustration than the real principle.

[@fuxpremier]

@@jeremypeltier986 Well, I would say that's the classical confusion about Mach's principle. There are dozens of formulations of it but they don't seem to be equivalent. I would blame Einstein for this because he's the one who made reference to it without actually applying it. Mach's idea was that the same way we consider distant stars to be fixed in heliocentric model, they are the only fix point that could give a reference for inertial reference frames (as he see it as a relative notion). If this "inertia interaction" was decreasing over distance, that would mean that massive or fast traveling objects would change inertia of closed bodies. The way I understand his idea is that distant stars can't spin fast (around the origin of the coordinates system) in an inertial reference frame because that would mean they would be traveling arbitrarily fast. Because of this argument, the inertial reference frame for the whole universe must somehow be consistent with very distant stars to be fixed points, therefore this hypothetical interaction can't be vanishing with distance.

[@omerkaya545]

0:22 The significance of the passage of time.

[@relativemotion2077]

Concerning Newton's buckets and Mach's principle, it's all very well to say "and acceleration is absolute..." but not everyone accepts this as a solution. If acceleration is absolute there must be some kind of field that determines what is and isn't inertial motion, this way acceleration will be absolute, relative to this field (this field is usually called "space-time"). But this is still a case of relative motion. Moreover, we can still follow Mach's hypothesis and consider whether this inertial field might be coupled to matter - which in fact it is (this gives rise to gravity); further, we might also consider whether the inertial field might be wholly defined by some integral over matter. This is Mach's principle proper, and it has not been disproven as of yet.

[@kylelochlann5053]

Acceleration is absolute (if we're discussing the 4-acceleration as the 3-acceleration is relative) and is any motion relative to the local gravitational field.

[@aupotter2584]

I agree w you, acceleration is by no means absolute and it's, like velcocity, is also relative, just thinking of centripetal force and the centrifugal force relative to the rotating frame of reference...

[@GerardP.Kuebler]

I am puzzled when you say gravity is not a force, since gravity does provide the force we call weight. Weight is what stretches the spring near the earth,s surface. The spring would also stretch when suspended in a vehicle going in a circle at constant speed. Also if the earth spins fast enough we would be weightless at the equator.

[@relativemotion2077]

@@GerardP.Kuebler I have not said that gravity is not a force. I regard it as a force.

[@3kinformaticamanutencaoeve91]

Ms. Sabine is such person and chanel that the algorithm needs to indicate to everybody.

[@jfvenne]

I have a hard time with the fact that gravity is not a force ... by definition F=MA . When falling , even if you don't feel acceleration, your body is still a mass, and your body is still accelerate, then by definition you are affected by a force (keep in mind that acceleration is absolute, like you said)

[@superbacedia0476]

That's Newtonian physics, which is wrong. It's taught in school because it gives the right answer in any situation your likely to come across unless you go into certain scientific fields. Gravity isn't a force, it's just you being accelerated, like how you feel a little heavier when an elevator starts going up, or when you are pushed back against the seat in a car. If it helps you can think of it as TIME pushing you against the earth. Remember, even if you think you are completely still, you are still moving through TIME. We live in 4D Spacetime, not 3D space moving through time separately.

[@superbacedia0476]

Sorry, my other comment goes off on a tangent. The actual answer to your question is that the F in F=MA is a different meaning of the term Force. That is force as it is used in reference to things being pushed around rather than a Force like Electromagnetism or Gravity (when people incorrectly refer to gravity as a force)

[@arandomguy777]

F=ma is wrong. Theres an extra term for space time curvature

[@umbertocaurla209]

Dear Sabine, I think you have made a great job trying to explain Relativity to those that don´t have a solid mathematical base. Your aproch seems to me just amaizing. I enjoyed a lot every time you said "Because gravity it´s not a force" 😅 Thank you very much for your effort each saturday!

[@MarkLenhart]

Scammer. Report them!

[@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.

[@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.

[@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.

[@marlanebozzi7002]

"As we learned in kindergarten" 😂

[@mskiptr]

Frankly, the single most confusing thing about understanding SR for me was the hidden assumption of Galilean transformation. After I've learned about Lorentz transformation (from minutephysics series about SR) it all suddenly made so much more sense. Even the popsci dumps of trivia and their inaccurate visualizations

[@-_Nuke_-]

Hello! Can you explain that hidden assumption of Galilean transformation further?

[@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.

[@mskiptr]

​@@-_Nuke_- Well, it basically boils down to this: (sorry for the wall of text) Let's imagine you're floating in space. No gravity, friction, etc. Now, you throw a rock to the left with a speed of 10m/s At the same time you shoot a bullet to the right. It goes 100m/s Under normal, Newtonian physics, if you wanted to figure out what speed does the bullet travel at, from the perspective of the rock, it would just be 110m/s - that is the (numerical) sum of their speeds. To look at it a bit more formally, from your perspective the rock has a velocity of -10m/s, you have the velocity of 0 and the bullet has the velocity of +100m/s. These can be treated as vectors and to transform this situation to the point of view of the bullet, you just subtract its velocity from everything. Thus, the rock moves at (-10m/s) - (+100m/s) = -110m/s, your velocity is 0 - (+100m/s) = -100m/s and the bullet itself has velocity of (+100m/s) - (+100m/s) = 0. But what is speed and velocity if not just change of position per unit of time? In relation to that, very similar calculations can be done for relative positions of our three objects at any given point in time. For example, after 5 seconds your position is at 0, rock is at -50 meters and the bullet is at +500 meters. But the positions relative to the bullet are -550m, -500m and 0, respectively. This whole situation can be neatly drawn using a spacetime diagram. And yes, we're still in Newtonian physics here. It's just that spacetime diagrams are a really simple concept if you're not doing special relativity, so I guess nobody needed them. (grawing with ascii is a bit hard, so I'll just give formulas you can plug into something like GeoGebra) If you plot this, you will see this entire 'world' - all it's future and past (though the latter is a bit broken). The y axis is measured in seconds and the x axis in dekameters. rock : y = x/-1 you : 0 = x bullet : y = x/10 Every point there represents some specific time and place. You can see all the places (and their times) the bullet will visit, as well as all the places (and their times) the rock will visit. Now, this specific spacetime diagram represents the world from _your_ perspective. However, we can easily obtain one that corresponds to the bullet's perspective. To do this, we just move every horizontal stripe left or right, such that it stays at the same y and all the distances between its points stay the same, but the point representing the bullet is located at x=0. This way, we obtain the following diagram: rock : y = x/-11 you : y = x/-10 bullet : 0 = x And that was essentially the Galilean transformation - a way to transform one perspective into another (or one spacetime diagram into another) that respects Newtonian physics and all the classical laws of relative motion. But, with special relativity comes Lorentz transformation. A different way of transforming perspectives (or spacetime diagrams) that doesn't have many obvious properties of the Galilean transformation. Like adding or subtracting velocities to get the new perspective, or events maintaining their y when you change the point of view. To go back to the example from beginning, the relative velocity between the rock and the bullet isn't 110m/s, because the simple addition 100m/s + 10m/s isn't the correct way in this case

[@-_Nuke_-]

@@mskiptr I think that in the time of Newton and Galileo they weren't thinking about causality hard enough. Of course I don't blame them, it's a really hard topic. But if you really think about causality you will arrive at a Minkowski spacetime. If light had the ability to travel instantly from point A to point B, like they used to think back then, then I can think of many causal paradoxes that arise from that. After some thinking I think it's easy to see that it's mandatory for light / and other mass less objects / to have some upper speed limit that they can't travel faster than it. Having that upper limit to be equal to 1 for clarity (the units after 1 are irrelevant... They could be anything) and giving it a name like L=1 we have successfully made L a (1) *constant* The next big leap would be to understand that this L is the speed limit to our Universe. We know that, the more mass sometimes has the more energy it needs for us to spend in order to move it. Reversely the less mass it has the least amount of energy it requires. Following that logic things with no mass are in constant motion. But at what speed? It should be a huge speed! One could put those things together, and theorize that the speed that any mass-less object will have will be that L speed. It will be exactly the speed limit of our Universe. Finally knowing that in Galilean relativity can only have a relative speed through space. Meaning that even if an object A isn't moving in space, it's speed might still be huge from the perspective of another object B that it's traveling at that huge speed and looking at A and thinking "I'm not traveling in space, he is, and at a huge speed too". What does that tell us about a speed limit? What do we get if we take Galilean relatively and we plug in an upper speed limit? Well, since we have a speed limit, it's a speed limit that EVERYONE should agree on. So L can't be a relative thing, it should be (2) *invariant* Invariant to who is doing the observation - it doesn't matter, since L will always be an upper speed limit from all points of view. So from (1) and (2) we have successfully reached the conclusion that L is a constant and invariant speed limit of the Universe. Simply thinking in terms of causality. Now, the really good part begins. We have a speed that it's constant and invariant for all observers, so the best way to examine what that means for us, is to make a clock that every "tic" happens at that speed of L. We see that once we try to observe the clock while being at rest relative to it VS observing that clock while the clock running away from us - since even light now has a speed limit, we will get different results! The more the clock runs away from us, the longer tics we will measure relative to measuring them by moving alongside with the clock. Therefore since the clock is measuring time, time itself cannot be absolute in that framework. We have arrived at Einstein's relativity!

[@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!

[@thamalupiyadigama1216]

Good work. Can you do a video about the paradox of an electric charge resting in a gravitational field? (Will it radiate? What is the relation to the Unruh effect?)

[@deinauge7894]

that sounds like a fun one :) I just checked the calculations and all I can say is: As an observer falling past a resting charge, you would see radiation coming from it. It's the strongest perpendicular to the gravitational field. I guess you knew that part... If we consider an accelerating observer (just sitting near the charge, so it is also resting in the gravitational field), the special relativistic calculation doesn't work. All I can say now is: Imagine a big charged steel ball sitting on a perfect insulator (to avoid quantum effects... make it as big and heavily charged as possible). Now if it radiates, it has to lose energy. There is no obvious source for it, so I conclude there is no radiation for an observer at rest relative to the charge. If we want to calculate that for a single particle we allways run into the problem that we have to bombard it with fields to keep it from falling. the big steel ball is the easier case.... And the problem(?) remains that all events should be the same in every reference frame. Effects from the field as seen by the free falling observer have to be compensated by other effects as seen by the sitting observer.

[@brothermine2292]

By ”resting” I assume you mean accelerating upward so it doesn't fall and appears motionless.

[@thamalupiyadigama1216]

@Brother Mine Yes. The charge is accelerating ("upward") with repect to the freely falling frame. But the observer is also in the same frame as the charge. Hence, the charge is in rest for the observer.

[@thamalupiyadigama1216]

@dein auge I'd like to add some few points to show why this is an interesting problem. 1. Uniform gravitational field is flat spacetime viewed by an accelerating observer. 2. Free falling charge will not radiate as it is rest in its own frame which is an inertial frame. 3. The charge in rest wrt the observer is accelerating wrt the freely falling frame so it should radiate. 4. But, if the observer observes the radiation is debatable. If so, living on the earth surface we should observe such radiation. 5. You have to use Rindler's coordinates to analyse what observer will observe. 6. Rohlich and some others' calculations suggest there's a event horizon preventing the observer observing the radiation. 7. As the Rindler coordinates and Kruskal coordinates of a black hole is mathematically related we can discuss analog results in blackhole physics. 8. As far as I know this is not fully resolved and I have read that there are some interesting consequences when we applies QFT to the problem.

[@el-vado]

I think attributing relativity theory to Albert Einstein is a stretch. All work was done by Henri Poincare and Hendrik Lorentz. Including relativity principle itself, the concept of space-time with pseudo-euclidean metrics, four-vector formalism, and group invariance of Maxwell equations. You think it is an over-statement? Why then we use terms like "Lorentz transformation" and "Poincare group"? Why didn't Einstein receive a Nobel prize for the "Einstein's" relativity?

[@olasek7972]

BS

[@BobbieGWhiz]

So it’s a big conspiracy and injustice that only the few enlightened scholars such as yourself understand?

[@el-vado]

Just trying to give proper credit where it truly belongs. Unfortunately, the author of this clip keeps propagating the misleading popular narrative. You don't have to believe me, just put a little effort into getting yourself familiar with the story. I am only pointing out to the facts and naming conventions that should raise your suspicion. BTW, I have another good news for you: it wasn't Isaac Newton, who discovered the "Newton's" law of gravity ;) And no, it is not a conspiracy theory, but a well established fact. Pretty ugly fact, BTW. Less pretty than the well-known legend of the Newton's apple. Well, you always have a choice of taking either a blue or a red pill. Keep believing the pulp fiction or do some research on your own.

[@dr.bsphysicshelp8816]

@@el-vado Einstein received the Nobel Prize for his explanation of the photo-electric effect.

[@el-vado]

@@dr.bsphysicshelp8816 Exactly. Isn't it ironic? Ask everyone on the street what AE famous for and the answer will be "relativity theory". But the members of the Nobel committee for years consistently rejected his nominations for the relativity theory. At the end, they caved in and agreed to hand the prize but for a different paper. It was humiliating, AE did not even attend the ceremony.

[@SarahStarmer]

Thank you for this video. I have never met two physicists who both have the same interpretation of Special Relativity and I really like the fact that so many different internally consistent interpretations are possible.

[@stewiesaidthat]

Special Relativity is a fallacy, which is why there are many worlds interpretations of it.

[@Thomas-gk42]

​@@stewiesaidthat ooohh, QM.....failed

[@Thomas-gk42]

​@SAMAC AG but she's not an Einstein, she's a woman, a smart one

[@cinegraphics]

She's wrong. Time dilation does not come from acceleration. You can make the acceleration part as short as you wish and then continue with constant speed for a long time. The more time you travel the bigger the time dilation. So, the acceleration period influence can be made negligible, and time dilation will depend purely on the constant speed part of the trip. Hence, acceleration doesn't explain the twin paradox. There's only one thing that does explain it. Velocities aren't relative. They are absolute. Relativity is wrong.

[@mathematiknet]

@@cinegraphics I don't think so. If you draw a picture you will see, that the acceleration at the turning point skip more time on the t-axis.

[@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!

[@aleksandrnovakov2578]

What I would have liked to hear more are some exaples on how and why that specific way of measuring distances in space-time "just works". It's probably the very only point that's still unclear to me, but I'll go research it my self. Sabine is awesome as always. My mind is bending just like space-time 😂

[@amarissimus29]

Why does the hypotenuse equal the sum of the squares of the other sides? It was born that way. If the model does not fit the reality and does not make accurate predictions, it's no good. If it does, then 'just works' is perfectly acceptable. Don't go poking at it. That's the path to postmodernism and depression.

[@td866]

@@amarissimus29 I hope you're kidding. the mathematics of geometry explains the hypotenuse as well as the Lorentz transformation- it doesn't just work. The equation she shows isn't just a guess at a model but is derived from the geometry. I' think Sabine did a great job but since OP asked about how why the equation works, Brian Greene goes into more detail here kzbin.info/www/bejne/jne5Y5mbgKl-b6c, It doesn't "just work".

[@Nat-oj2uc]

@@td866 string theory is also derived but it is not realistic. You can't just rely on math you need to do a reality check

[@td866]

@@Nat-oj2uc Dumb reply about string theory and irrelevant to my reply. The guy I was replying to made a comment about the equations (Lorentz) not being something derived but fits with reality and not to worry about where it came from. That guys response was dumber than yours.

[@Nat-oj2uc]

@@td866 your reply is the dumbest and yes it's related. It should fit reality. Many theories are derived but only Einstein's theory fits the observations the best. Mathematical proof is not enough

[@winstonsolipsist1741]

I have some special relatives that I constantly misunderstand.

[@Ramz_914]

Good one😅

[@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.

[@SarahBerry-fg5yr]

Acceleration is a vector quantity. You can move around in a circle (or get turned around due to gravity-which isn’t a force but a deformation in spacetime) at a constant speed and you’re still continually accelerating. Acceleration isn’t a force and gravity isn’t a force, but acceleration does account for time dilation.

[@AliM-mh1bk]

Brilliant explanation. I have one question. Clocks are mechanical devices that are designed to keep coordinate time. Why, how to they slow down and speed up based on acceleration? Why does the mechanical mechanism slow down and speed up?

[@whirledpeas3477]

Very good question. Same as a heartbeat, a nail rusting, etc. It's just the way it is, though