The triplet paradox!

We will finally resolve the twin's paradox with complete intuition!
Chapter:
00:00 Intro
00:45 Why you don't understand Twin's paradox
1:28 Einstein's clock synchronisation
2:30 What do the brothers see?
3:14 What does the sister see?
4:46 The first half of twin's paradox
5:30 The relativity of simultaneity
7:53 Relativity of simultaneity equation
10:47 Resolving the first half of twin's paradox
12:20 It's NOT the acceleration!
14:24 Who is accelerating?
16:38 Resolving twin's paradox (What the brothers see)
17:12 Resolving twin's paradox (What the sister sees)
19:31 Summary

[@mweave]

Mahesh, you have cracked the code. Most videos on these topics have the joyless tone of ‘I am the expert who understands this, allow me to demonstrate my understanding’ You totally reframe it as an intuition journey, back to the basic principles. I have always craved someone to explain these topics somehow. These smart people in the past didn’t just write down equations from scratch, they all started with an intuition like you are explaining. Love what you’re doing. Keep it up.

[@darrennew8211]

I like the @dialectphilosophy channel too. The effect isn't caused by acceleration, because you can do the same thing using gravity. You can have one brother hovering over the surface of the Earth at 9.8m/ss with the other person bouncing outwards and then falling back downwards entirely in freefall. Or you can do a gravity slingshot around a gravity source without any acceleration.

[@eshafto]

And a very surprising number of these experts get it wrong. Are they not experts? Are they just parroting something they heard and didn't really understand the whole picture? Are they simplifying for their audience to the point where their answer doesn't actually work? Next up: Hawking radiation and the Oberth effect. I don't know if anyone will ever overcome their education to the point where they can explain the Bernoulli principle, though.

[@darrennew8211]

@@eshafto I saw a great explanation of the Bernoulli principle that might still be wrong. Basically, as the neck of the horizontal tube narrows, the only atoms going into it are the ones that are going more horizontal than vertical, so the flow tends to be faster.

[@eshafto]

@@darrennew8211 and so they exert less pressure against the sides! Thank you! Is it right? No idea. But it’s the closest I’ve seen to an explanation that makes sense to me.

[@vedwards5027]

What if I were to tell you that the reason you don’t understand this is because it is wrong and not applicable to our reality. When you finally learn to critically analyze information, you will find that much of science that you just couldn’t understand is actually erroneous science and the correct response is to not understand it because it’s false.

[@Xylot]

You're not just a great teacher, you're a great storyteller too!

[@Mahesh_Shenoy]

Thank you for noticing. I am deliberately working on it!

[@paulg444]

only a great story teller can be a great teacher !!.. He is delivering !

[@opguy_219]

​@@Mahesh_Shenoysir i want to contact you , canu give me any of your social media😊

[@iurlc]

I like his videos. But here he made some mistakes. He introduced a third twin at an other place and argued @6:20 that he had not the same age. When the star ship starts it has no speed - so all have the same age. Later @18:00 he tells the the clock will jump - that is not correct - clocks are counting step by step and not jumping. Fact is that what all tell be a paradox is not a paradox. Both will see each other younger, because they are never at the same time. Think about two persons are far away and see each smaller the same here each see each other younger. To avoid all this silly staff with acceleration, deceleration and direction change. Make the thought experiment really symmetric. All three twins live on earth. One fly to the left and the other to they right. And when they come back both must see each other younger - the experiment is completely symmetric. And the one on earth must be seen from the other two twins younger - but not so much younger as the other flying twin was seen. An the one on earth will see them also a little younger.

[@PhucNguyen-vf1zt]

​@@iurlc cool man 😄 But I think we should also stop using 'human’. We should use ‘clock’, triplets 'atomic clocks’. That sounds much cooler and much more accurate. Using ‘human’ as an analogy is so confusing. though i dont know how atomic clocks work🤣

[@PortalUser2]

When you talked about the twin "jumping ahead by 7.5 years" after the change in reference frame, it would probably have been worth mentioning that if that twin was shooting a photon at you once per year, you would still have passed all 10 photons on the way to visit the twin (doppler shifted of course) - some might be confused by that. But that was good and probably the closest people will get to understanding it without space-time diagrams drawn in 2D. Thanks.

[@jorgerego1182]

So if the jump happens "instantaneously", would the twin receive 7 photons at the same time?

[@PortalUser2]

@@jorgerego1182 No, if looking at the photons hitting you it would look like it was all happening in fast forward. In other words, in your 5 years you would see 10 years of events happening at twice the rate. It is just that it looks like the first 7.5 years events occurred in the past, while the last 2.5 years occur on your way to visit the twin. So it looks like only 2.5 years has elapsed over your 5-years....But you still see the whole 10 years play back in fast-forward mode.

[@manojaggarwal2773]

@@PortalUser2 Actually closer to 18.5 years of events. At a 2:1 time dilation, she must be going about .85c. So the photons that reaches her when she passes the first twin are 8.5 years old. In the 10 years it takes her to make the trip, the twin will also age 10 years. So when she arrives at the 2nd twin, he will be 18.5 years older - all of which happened in 5 years of her time.

[@aaronbredon2948]

And if you do the calculations from an inertial reference frame moving like the first half of the journey, you also get the same final result. This is part of why you cannot have an objective reference frame, only inertial and accelerated reference frames. The whole “paradox” was caused by changing from one inertial reference frame to another without adjusting for the acceleration effects when following the moving twin.

[@manojaggarwal2773]

@@aaronbredon2948 you do get the same result. When the sister arrives at the 2nd twin, he will be 18.7 years older for both of them. This must be the case for BOTH reference frames. The sister will only age 5 years in both reference frames. Same result. But getting there is different. The sister is passing the first twin. They both see the age of the 2nd as 8.5 years younger than he actually is (the time light takes to travel from the 1st twin to the 2nd). However, the sister will think that the light only traveled 4.25 light years and thus infer he is actually only 4.25 years older. In order for the age to match when she gets there, she has to see the 2nd twin move faster to "catch-up," otherwise the ages wouldn't match and that would be a big problem. The same thing happens if the sister is perfectly still and it is the twins moving toward her. Thus relativity is preserved. The frames align. Again, time dilation and length contraction are not the only thing happening here.

[@tripleaskillz]

As always, a great way to teach advanced physics

[@Mahesh_Shenoy]

Thank yoU!!

[@sgiri2012]

Mahesh sir fans raise hands

[@shaurya9335]

🤚🏻

[@anupamshukla6357]

✋🏼

[@maahiyaraj3330]

✋️

[@vegitoblue7063]

✋🏻

[@kamatchinmay]

✋

[@couldbeahuman8079]

probably my current favorite youtuber, I always found this stuff fascinating and I’m so glad that I found someone who can explain these really complex topics in a digestible way while still keeping their mind breaking wonder

[@Viperclarkx01]

Man!-I am 36 and never did well in math or science when I was in school. I feel like I’m only now through your videos learning what I wanted to know when I was a teenager. Bless you and your fantastic work!

[@YogeshGandhi]

I am 42, and still breaking my head on relativity. Never able to understand it completely :)

[@chlojolo]

Relativity of simultaneity is really underdiscussed, underrated and underappreciated, maybe because it is so mind-bending. It should be obvious that if two observers don’t agree on how fast time passes, then they cannot possibly agree on the order in which all events occur. But somehow talk about time dilation often skips over this stuff. And then learners come away with the idea that time dilation is just an apparent change or an illusion or something. But no, the real order in which (some) events occur depends on your relative motion. And most relativity “paradoxes” are just failures to fully reckon with relativity of simultaneity. In some ways it’s more counterintuitive than QM weirdnesses. Anyway thanks for bringing attention to this topic.

[@wesjohnson6833]

@chlojolo It feels great to finally be able to say with understanding: "There is no definite order of events for events space like separated". ( Because there is always a legal frame of reference that define a different order.}' Have you used the relativity of simultaneity to explain length contraction yet?

[@SamiAhmad36]

The twin's paradox confused me for so much time. But, finally you made sense of it. You are incredible!

[@tmpqtyutmpqty4733]

How does that make sense? 17:20 When she leaves earth the far away tween jumps time. I'm sorry but that doesn't seem right even taking into account the bizarre nature of special relativity.

[@SamiAhmad36]

​@@tmpqtyutmpqty4733 In the frame of the girl in ship: When she is accelerating to the right, the signal from the central tower reaches the planet on the right earlier; and by the time the signal reaches Earth(on left) 7.5 years have gone on the right. So when the clock on earth begins, the clock on the right planet already reads 7.5 years. 5:30 Watch here

[@PADARM]

​@@tmpqtyutmpqty4733 because in that instant she changed her reference frame from rest to movement and that means the relativity of simultaneity comes into play.

[@wesjohnson6833]

@@PADARM And what do we call that when she changes from rest to movement? Oh, that's right...acceleration.

[@PADARM]

@@wesjohnson6833 Not necessarily. She can jump into a ship that is in constant, non-accelerating motion.

[@one6632]

You definitely deserve many awards because this exactly is the way of teaching I was looking for from long time... Hats off🎩

[@heck-r]

I watched and read multiple things regarding this in the past, but this is the first one that finally resolved it for me complately, and cleared up my confusion about the acceleration. The last thing made it clear that there is no global "moment", but there was still a bit of magic there even though I knew it was something with the reference. This was the completion of the whole thing for me, so thanks :) This also helped me get a bit closer to what I'm getting wrong about the non-observable part of the universe I think, although I'm still not entirely sure about that. My problem was generally that things that are far enough are "getting further" faster than light due to expansion (like space is getting generated between them / stretching). Since the "speed" of the escaping is the space itself becoming more instead of the actual speed (which is what allows the conceived speed to be faster than light), it seemed to me like it should be possible to reach things in the non-observable universe by speeding up enough, since that would cause the distance the shrink enough for the target to fall back into the observable / reachable distance, where the distance is not enough for expansion to be enough. However that should not be possible because that would allow a trip back as well, and the original observation point would determine the trip not to be possible / take infinite time, meaning that from the original point the arrival of the traveler is impossible, while for the traveller it is possible. With this video what I assume happens is that the non-observable stuff is actually non-observarble, because even though the distance technically shrinked, the relative moment for the target also synced in a way that expansion already happened, keeping it outside the observable stuff, and making observable-ness consistent. However this does not change the distance from the theoretical border of the observable universe, which would mean that the perceived distance change is not exactly a shrink, but rather like the effect of gravity, where it affects / shrinks stuff more the closer it is, and becomes less and less as you get farther. Although that maynot be a correct representation because the shrinking is consistent, it is just perceived as if it wasn't the case due to the corresponding expansion. It would be nice to get a feedback on this. Maybe even a new video idea :D

[@PADARM]

I have been watching videos and reading about General Relativity for more than 5 years and you have been the Only One who managed to explain the most complex concepts of GR. Bravo Mahesh!! 👏👏I think the key reason is the method of conversation that you have with Einstein 👌

[@bass2564]

Much agreed about the teaching method; this video is strictly within special relativity though.

[@PADARM]

@@bass2564 I know but he talks about acceleration as well

[@koifish528]

i love these videos. they make it so much easier to understand these difficult concepts

[@bensyversen]

Really great video, thank you for making this. I can appreciate the time and effort it must have taken you to make such a complex concept so intuitive and entertaining!

[@flexico64]

A phrase I've heard that connected a few puzzle pieces is that traveling a significant fraction of c is a "shortcut to the future." When the traveling twin accelerates, she "turns" in spacetime, so she is pointing towards the other planet farther in the future than what the homebound twin sees. So the twin/planet on the opposite side doesn't "jump time" so much as the traveling twin turns toward an instance of them farther into the future. I hope my phrasing is clear; my brain hurts a bit after putting that together! XD

[@PADARM]

That's a great way to look at it.

[@redwolfmedia2281]

Wow, that's really cool and explained well thank-you. But what if she starts to deccelerate.

[@flexico64]

@redwolfmedia2281 Basically, she turns the opposite direction, towards the reference frame she started on. If she turns enough, she will start heading back toward the planet where she started, but to an instance of it in the future.

[@zephsmith3499]

> "the traveling twin turns toward an instance of them farther into the future." I'm still not getting my head around that. Likely partly the imprecision of words vs math, or maybe there's some underlying concept I haven't yet grasped. But I don't understand what it means to "turn towards an instance... farther into the future". What kind of "turning" is that? Any help?

[@razorblade6219]

You have to think in 4 dimensions kinda but imagine a 2d chart with space in any direction being x and time being Y. As you move in X you lose some movement in Y but something not moving in X would be going full speed in Y. So if you were moving and turned back towards the other planet you’d reach it in less of your time due to having moved but the planet not having moved has more time than you now.

[@akashalampally712]

By watching your videos, my interest has reached a peak level in this topic. I have never seen a video like this. Thank you for making videos on such topics.

[@A_GoogIe_User]

Amazing, you are better then any of the other bigger science channels. You deserve a lot more subscribers.

[@ckpioo]

woah! you are such a great teacher I have seen many teachers who are also very excited while teaching but you! you are on a completely different plane of excitement and I love it so much, it really somehow changes so much while learning for the viewers, thank you so much!

[@mikehaddick4837]

Been really loving these videos! You explain such fascinating yet difficult topics in such fun and easy to digest ways, keep these up!

[@georgwagner4438]

You are literally the reason, tens of thousands of people now understand Einsteins Special Relativity Theorie, your Explainations are absolutely amazing!

[@Rationalific]

This is one of the hardest things I've ever tried to understand. This may be the best video I've seen on the subject (and I've seen a number of them). The thing is, while most videos explain it, they only explain it from one perspective or a few perspectives, but this video tries to explain it from many perspectives and helps me to understand. Especially the ending part talking about the changing of frames when coming to a stop, was helpful. I do actually think that this is why many look at the aspect of acceleration, and I think your point about one knowing if one is accelerating or not is helpful. Acceleration not being relative appears to be the key that solves the relativity of simultaneity. You feel it. There is a force acting on you. So it's not up for debate. That's what makes it kind of click for me. And I think that's why some look at it from that perspective... And the relativity of simultaneity can then be solved for all parties when one party feels a force and matches the velocity of the other object. Wow... Fascinating stuff.

[@darrennew8211]

I found the @dialectphilosophy channel incredible at explaining these advanced things too.

[@Rationalific]

@@darrennew8211 I'll check that out, too. Thanks!

[@darrennew8211]

@@Rationalific For example, he addresses the same situation in general relativity, where the twin doesn't need to accelerate to return to their starting position to compare clocks. Satellites orbiting Earth are experiencing time more slowly while not accelerating, and we're experiencing time faster while accelerating.

[@curiousburke]

Hands down the best explanation!! You said a video on the twin paradox was coming, and I thought you wouldn't be able to explain it to my satisfaction, but you did! After countless hours, and so many bogus explanations, the cloud has finally lifted!

[@-_Nuke_-]

I don't want to burst your bubble but that explanation was wrong. Unequivocally WRONG. And if you want i can explain why...

[@williamwalker39]

@@-_Nuke_- Please explain your idea.

[@DANGJOS]

​@@-_Nuke_- Yes, please explain what you're referring to.

[@-_Nuke_-]

@@DANGJOS I will explain but you should also watch the videos from a KZbin team called Dialect, that explain why most if not all KZbin expirations to the twins paradox are wrong. Starting with this video "Why solutions to the twin paradox are wrong" from Dialect. Where he explains why "rotating planes of simultaneity" just like they are being explained in the video is NOT the answer to the paradox. The solutions that most KZbin gives are called apriorism. That means that there is a apriori axiomatically given quality to the Earthbound twin that is NOT given to the spaceship twin. That apriori quality is that the presence of the Earth and the fixed stars, make the Earthbound twin somehow inertial simply by being on the Earth and appearing stationary in regards to the fixed stars. Where the spaceship twin simply by not being on the Earth and because it is firing its rockets we know beforehand (a priori) that it is the one who undergoes absolute acceleration. So given these obscure axioms. The paradox loses its meaning... The point of the paradox, isn't to give an answer to who is going to age more or less. The point is to investigate whether or not we can (without external knowledge) deduce which object is inertial and which isn't. But if we beforehand assign that the Earthbound twin is the one who is truly inertial because of being on the Earth and stationary relative to the fixed stars, we have already answered our own question... There is no point solving the twin paradox since we have already agreed, that the Earthbound twin never accelerates... The key point here is that BOTH twins in Mahkesh example AGREE that its the spaceship twin who is accelerating... But the paradox says, what happens when they DON'T agree... And that hasn't been answered.... If they don't agree then the length contraction that was shown in the video from the spaceship twin's perspective should now work the other way around, by the spaceship twin assuming that it's ITSELF who is at rest and the Earthbound twin the one accelerating... Einstein (the Man, not the theory) tried to do that in his own explanation which involved general relativity, by having the spaceship twin assume that he himself is not accelerating but his twin on the Earth is under a gravitational field which makes the Earthbound twin not inertial. So in this case Einstein DOESN'T a priori assign the Earth twin as inertial, since being on the Earth constitutes a constant and upward acceleration... Einstein's explanation though is still considered incomplete. As if we remove the Earth and the fixed stars and the rocket engines all together and we just have 2 featureless spheres, then we truly can't know the answer to the paradox using these methods anymore.

[@curiousburke]

@@-_Nuke_- Yes, I would like to hear your explanation.

[@ashishrout3964]

Stumbled upon your channel, now loving every bit of it

[@kantoros]

Amazing video, I think this is the first I've seen that gives me some sort of intuition for how all this works! Everyone else keeps using spacetime diagrams, and sending regular data packets and counting them and it always seemed so artificial, I never really understood *why* they would work like that. thank you so much!

[@nirinarabeson]

You are the best spacetime KZbinr in all reference frames Mahesh

[@tempname8263]

BUT! What happens if she suddenly stops right after reaching Earth? Will she see her twin go from 7.5 years back to 0 years all of a sudden? Time travel, anyone?

[@braddofner]

Wow! Where did this guy come from!? Great video, great charisma, good delivery, not condescending... and happily excited. I'm impressed by your passion and ability to explain something incredibly complex and difficult in a way anyone could understand easily. Hats off to you sir. I will enjoy seeing your other videos. May you find success and stay in health.

[@thisvideoisgreat4289]

This video is a success. I've watched lots of explanations, but only now it starts clicking in my head. Thanks.

[@karanarora2490]

For the past few days I was trying to understand this problem. I was reading the book 'Special relativity and classical field mechanics', and this part mentioned in detail, the author just mentioned it happens due to acceleration. The timing of the video is perfect according to my reference frame.

[@darrennew8211]

But it's not due to acceleration. You can use a gravity "slingshot" to come back to where you started while never accelerating. You can also have one twin hovering in a gravity field while another bounces up and falls back down. You can have one orbiting while the other hovers in the gravity field accelerating. It all gives different answers. It hasn't anything to do with acceleration. It has everything to do with how far someone went through 4-space.

[@fomalhaut86]

@@darrennew8211 ... "gravity slingshot" is acceleration ... just like any other change of velocity in time 🙃

[@darrennew8211]

@@fomalhaut86 No, it isn't. It's the space ship following the geodesic. It's only acceleration in Newtonian physics, which is exactly the problem we're talking about.

[@fomalhaut86]

@@darrennew8211 ❤🙆‍♂🙈🙏👍

[@wesjohnson6833]

@@darrennew8211 You keep writing this and spreading misinformation. This paradox is in flat space and only proper acceleration is considered. Hence, no slingshots.

[@punctepuncte2668]

Hi, Mahesh. Will you make videos about general relativity too? I really hope so. General relativity is something I never understood, but I am very confident you could make us understand. Congratulations for these amazing videos..

[@ytashu33]

Thank you! I had heard the term "frame hopping" as the "solution" for this paradox, but that always felt non-intuitive and artificial. The way you explained the difference (that if the girl twin accelerates, she clearly knows that it is her that is changing frames and therefore there is no paradox), finally made sense to me. Can't say that i fully grok all this, i have been trying to understand this paradox for years, but i can say that after watching your video, my understanding went from, say about 60% to maybe 85%!! Thank you for your simple and intuitive way of explaining things without using any Jargon. The great explainer, Richard Feynman, would be so proud of you, i am sure!!

[@obscurity3027]

This channel is one of my favorites on KZbin. Awesome content, brother!

[@PortalUser2]

Looking at the comments, for those confused: When travelling towards your twin (5 years on your clock) looking through your telescope it would "look" (via the photons) like everything was happening at a faster rate (in fast forward with blue shifted photons), so you would see all 10 years happening in your 5-years while you travelled there. It is just that in this frame of reference we would consider that 7.5 years of those photons were emitted "before you left", and the remaining 2.5 years of photons were emitted "while you travel". So on paper your twin's clock is moving at half the rate. But based on the photons they actually see, it looks like the twin's clock is moving faster! This isn't a contradiction, it is just hard to show without drawing it as a 2D space-time diagram. (corrected after @silverrahul's comment below)

[@silverrahul]

" _it would "look" (via the photons) like everything was happening at twice the rate_ " it wont be twice the rate. It will be about 3.75 times

[@PortalUser2]

@@silverrahul Thanks for the correction and of course you are right - there are the photons already on the way, plus the new photons emitted during the journey. I just wanted to make sure viewers were not confused by the video mistakenly thinking it would actually "look" like the clocks were ticking slower (in terms of the photons hitting the ship).

[@erinm9445]

The more I think through time dilation, the more it seems like the idea of nonlocal "now" is completely meaningless. Like, we need to account for it mathematically for calculation reasons, but does that math have any meaning for reality? If two people in the same place at the same time can disagree what is happening "now" many light years away, then does "now" have any nonlocal meaning? Or, again, is it just a mathematical accounting mechanism?

[@PortalUser2]

@@erinm9445 You could think of the accounting that if you imagine some small sinusoidal oscillation, it will *always* sweep through the same "area" of spacetime for every second that passes on it's clock. Or maybe, imagine that each full wavelength in the oscillation, the same "area" of spacetime would have been swept through. That would be true for everyone and from every frame of reference. I am sure the creator of the video will do more exciting follow ups soon to help in the understanding.

[@Yaaalala]

Hi, apologies for a long comment, but since it seems like you understand this deeply, please help me understand one thing, my mind lags when I think about it. Lets say we accelerate at the start of the journey to the same speed as said, so the time-dilatation factor would be the same (2), but the distance would not be 10 years journey from the planets frame, but a 1000. This means the “far away” brother would be long dead from my new accelerated reference frame in an instant? (jump in time 750 years)? But wait, here is the real question: What if I change my mind after just a few meters and decelerate back? In this case, the distance of the whole journey was not really taken, so it should not matter. Would this mean time “rewinds” to my reference frame, and he is alive again? Surely not, this is impossible. The brother on the close planet also should not age much from a few meters journey very near him even at that speed. So how is the other one dead? Shouldn’t he be the same age as the living twin on that close planet? I know I am missing something crucial here, but after watching the video multiple times, I have no idea how to resolve this. Thank you for your patience.

[@anupamshukla6357]

Lets get him to 100K as soon as possible ❤

[@jmunt]

I’ve watched a LOT of videos on relativity and time dilation, and I had gotten to a point of knowing the solution to this paradox mathematically and proved it to myself by charting out space time diagrams, but this is the first time I’ve ever understood it intuitively without needing to think about the math. Thanks so much!

[@DJ_Force]

I've heard this explained many many times for many many years. Never made sense until now Amazing! Definitely a successful video!

[@Sinzari]

The original twins paradox has the girl starting from the same frame of reference as the brother before leaving, so acceleration is actually the issue. This is a different paradox altogether and is naturally simpler because there is no acceleration.

[@thedeemon]

didn't watch the whole video? :)

[@wesjohnson6833]

Yes acceleration is the issue. Every video I've seen that denies acceleration adds to the original problem to hide that rather obvious outcome. He who accelerates more will be younger...wherever they meet up to compare.

[@juliavixen176]

​@wesjohnson6833 There are dozens of variations on this "Twin Paradox" setup. Ultimately, the answer for all of them is that whoever travels the longest spacetime interval will experience the least amount of proper time. Nothing else matters. By the way, in flat Minkowski spacetime, two objects in inertial motion can only be at the same location in time and space once... and then drift apart forever. (Like a V or X shape) To make a "round trip" and close the path, someone needs to "turn around" and that result can not be either of the two original inertial reference frames. It makes a triangle, and two sides of a triangle are always longer than the third side. One twin takes one side, and the other twin takes *two* sides.

[@jeremylakeman]

Acceleration skews the axis of simultaneity.

[@wesjohnson6833]

@@jeremylakeman Yes, the cause of the effect. This author wants to pretend the bullet killed the deer while the rifle had nothing to do with it.

[@MichaelFrith]

Acceleration is important for establishing an asymmetry between the two twins. The confusion results from using coordinated space and time rather than spacetime distance. It is the spacetime distance that defines the physical situation of each twin. You will find that the spacetime distance of the accelerated twin will be less than the non-accelerated twin. That is, the accelerated twin will have aged less.

[@juliavixen176]

The accelerated twin travels a _longer_ spacetime interval, which means that they experience a _shorter_ proper time... and thus younger. Yeah, it sounds backwards but It's that minus sign on the time coordinate that makes longer spacetime interval work out to shorter proper time.

[@deraadsfs9235]

From either twin's perspective, the other has accelerated. Absolute acceleration cannot be defined from relative velocities.

[@alexjohnward]

No. One twin pushed something away to accelerate, the other didn't.@@deraadsfs9235

[@cm5754]

@@deraadsfs9235 That is true if we just look at coordinates. But one of them really accelerated, and the other didn’t . There has to be an actual physical difference between them, apart from coordinate systems they might pick

[@Obiekt219R]

@@deraadsfs9235 it appears to each that the other has accelerated. But if they both have an accelerometer, the twin that turns around will show acceleration. The twin on the planet will not.

[@b.munster2830]

Wow, best and most complete explanation I’ve seen on KZbin, ever. Well done!

[@-_Nuke_-]

Well, let me introduce you to "Dialect". Google it, they have a channel here on KZbin. They have a lot of videos on the twin's paradox and after watching them you might understand what im going to say now: This "most complete" explanation that you just watched, is only 50% complete! So, really very far away from the "most complete"

[@ADeadlierSnake]

Ive been upset with myself for my entire adult life for being unable to understand relativity. Ive watched countless videos on it and had it "explained" to me by people who say they understand it, but ive never felt like i was making any progress in my understanding until i saw your videos. These are magnitudes better than any others out there. Thank you!

[@snaatanraina]

I was the one who asked about the electricity playlist in the previous video. Sir, I watched all those videos and it helped me a lot, but I had watched Derek Muller's (a PhD physicist) video where he claims that electrons carrying potential energy around a complete conducting loop transferring their energy to the load is all FALSE. He explains that how actually energy is transferred through electromagnetic fields and electricity flowing in one isolated conducting wire can cause electric current in the nearby isolated conducting wire. 1st video where he discusses the main question: kzbin.info/www/bejne/mHmsmZqulttsgrssi=yqd0b_RLQ66aEKq2 2nd video where he performs the real experiment: kzbin.info/www/bejne/pXrCiWWZgq2Bm5Isi=R44c_cECV0L3-t1O I had a hard time understanding these videos. Sir, what are your views on his claim and the videos.

[@Mahesh_Shenoy]

I too had a hard time understanding it. It will take me a while to be able form concrete shareable opinions. Right now, I am pretty shaky on it. And it’s in my backlog.

[@hugegamer5988]

This is the same as the acceleration vs reference frames debate here, Both are correct. For reference, go see what a lead PHD director at fermilab has to say: kzbin.info/www/bejne/rJ-qiHePm9yYiqs In your example you CANNOT have those field distributions without having a particle carrier, trying to separate the particle from its field is not reality, it’s like trying to say you can have a change in reference frame velocity without acceleration. Both explanations are correct, just from a different view. Source: have taken many undergraduate classes in electrical engineering and have designed many microwave range circuits pouring over transmission line design, impedance matching, antenna design, and hours upon hours on network analyzers making circuits play nice on smith charts.

[@three_mountaineers775]

Have you watched the ElectroBOOM videos on the topic? He gives an engineering spin on the topic (originally objecting to the claim but being convinced in the end). kzbin.info/www/bejne/b5nLlIiKbplqfKssi=2LHjMMzpuN7PZb2n I'm not sure if this was shown by Derek, but Mehdi (ElectroBOOM) shows on an oscilloscope the voltage across the light bulb in same setup but with an open circuit at the ends, revealing an even more bizarre behaviour. the light bulb will still turn on at half the voltage only until the EM wave discovers the break, at which point the voltage across the light bulb will then drop to zero. I.e. the lightbulb will blink. This behaviour actually explains how the setup does not violate information transfer being limited to the speed of light. In this setup, it is still impossible for the person operating the switch and lightbulb to see whether there is a break in the wire or not at faster than the speed of light. Before the EM wave can reach the break, the observed behaviour of the light bulb is exactly the same regardless of whether there is a break in the wire or not.

[@shrivatsa8604]

Hello sir, would you make a video on special relativity on rotating objects, time dilation on bodies that are rotating at an axis, lenth contraction and other stuff please?

[@romandotbsbd]

Finally! All the previous illustration of this paradox, were missing the details and intricacy which you presented in this video. Thanks man.

[@fortyofforty5257]

Mahesh, you are very enthusiastic and effective at communicating complex ideas. Plus, relativity is so counterintuitive compared to what we experience in our daily lives, it takes a lot of mental gymnastics to understand it.

[@Sol-En]

Wow, this is brilliant explanation !

[@ghegogago8297]

that's the most powerful video I've ever seen with all the other ones of this playlist. You are an absolute winner in terms of explaining things intuitively and I absolutely feel in debt with you for this ahah. But I have a question: at 15:04, when it comes to length contraction, if we connected the planets with a super-long piece of concrete, wouldn't we see this long (and now unique) piece of concrete contracting? So shouldn't its extremes also be closer to each other? In that case, how can she know that SHE is accelerating and not the planets? (Without the "feeling" of acceleration) THANK YOU SO MUCH I HOPED TO SEE THIS VIDEO FOR LONG!!

[@renedekker9806]

_"if we connected the planets with a super-long piece of concrete"_ - that Bell's Spaceship paradox, in that paradox the connection is imagined to be a string. He has promised to make a video about that, but in short: _"wouldn't we see this long (and now unique) piece of concrete contracting"_ - if the spaceships accelerate the same amount at the same time, then the string would break. To keep the string from breaking, they cannot accelerate the same amount. _"how can she know that SHE is accelerating"_ - when the string does not break, because the planets appear to accelerate at different rates (and she ignores the "feeling"), she would not necessarily know whether it is her or the planets that have proper acceleration. Apart from that, she can simply measure whether she is the one accelerating with an accelerometer (the "feeling").

[@keithdubose2150]

Best explanation, clear, without ego, and the radio tower in the middle helps ! Subscribed!

[@unknownnepali772]

Wow! I always knew relativity of simultaneity had to do something to this paradox but couldn't somehow process everything....best explanation on KZbin till date.hats off!!!

[@mortadasaleh7423]

Great Job, Mahesh 👍

[@Mahesh_Shenoy]

Thanks :)

[@silverrahul]

My only complaint is the number of times you keep saying "now he sees this"," now she sees that ". And like you mentioned at one point, you dont really mean "see" because of doppler effects etc . But, i absofreakinglutely guarantee you that 90% of your audience will go away from this video believing that he/she actually "sees" that. You should instead use phrases like " in her frame, now this happens" , " in his frame, now that happens "

[@Mahesh_Shenoy]

Agreed. I totally 'see' it now! 😂😩. I will make a pinned comment with FAQs soon. Thanks.

[@ankitraj9684]

I have been watching pbs space time and fermulab for so long yet they were never able to give me such a delicate balance of computation, theory and thought experiment…Kudos!

[@bjornragnarsson8692]

This was great! I love your enthusiasm and am loving your videos!

[@barmalini]

Thank you! Every time I watch your video, I feel I'm becoming smarter and a better person in general

[@Mahesh_Shenoy]

Wow, that's incredibly awesome to hear :)

[@zeesan9004]

But what if the rocket starts right from earth, exactly when the light pulse reach? At the moment isn't the age still same and doesn't the paradox still remain? edit: sorry, I only watched half of the video by the time I comented.

[@Scramasax.]

Wow, man... My brain just exploded and resurrected back again. This experiment is FIRE! U are awesome paradox teller)

[@jameslam5801]

It is an excellent explanation. If the twin brother far away is omitted in the argument and replaced with a synchronised clock, it may be even better since it will stick to the original description of the paradox. 😊

[@shiikamawp4428]

So interesting! You keep me more excited during the whole story more than almost all movies))

[@MrCook-if5wo]

Even though it only appears for a few frames in the video, I really appreciate the derivation of xv/c^2 you show. I've never seen how that came about without the use of spacetime diagrams and hyperbolic trig. It's so much more understandable the way you calculate it. Thanks.

[@laerteoliveira7923]

Finalmente alguém me explicou o paradoxo de forma correta e completa. Parabéns!

[@LowellBoggs]

Another outstanding video. I recently watched the video by dialect channel on this subject and thought that they skipped over the relativity of simultaneity to quickly for me to understand. Thank you for this clear presentation.

[@-_Nuke_-]

If you have watched Dialects videos, there is one that explains why solutions like these here, that involve "rotating planes of simultaneity" are still wrong. So Dialect actually debunks this "explanation". Better go back and rewatch them.

[@janmackovcak]

Thanks for the nice intuitive explanation, I didn’t get it during the lecture but now it makes way more sense:)

[@edwardsinke5798]

Very good work. You sir, are an inspirational person. Don’t stop making your videos!

[@pragya9245]

Sir these videos are super interesting, from someone who found physics super confusing and annoying i love it now!, all thanks to you

[@igorstajcic951]

Hi Mahesh Fantastic channel, one of the best out there. I’m having so much fun watching it especially the videos related to general relativity. Thanks a lot for that. Keep it going. Speaking of paradoxes….. Concepts of general relativity are so interesting, but very hard to get your mind around at the same time. I can understand them, but I cannot really grasp them. My brain just objects to such counter intuitive conclusions. When ever I think about it, it is so easy to create a paradox. All you need to do is to drive general relativity to the extreme and a paradox will appear. You explain some of the most famous ones, but there are other paradoxes that are not easily explained. There is infinite number of possible paradoxes. What do you say about the below one? That is my favorite. As far as we know, the universe is finite and ever expanding. It has aroung 93 bilion light years in diameter right now. With that in mind, I jump to my spaceship and I accelerate it to 87% of the speed of light. When I make the measurement from inside the ship, I find that the universes diameter has shrunk to about 46.5 billion light years. It is now twice as small by diameter as it was when I was at rest on Earth. There is length contraction factor of 2 due to the difference in speed between my ship and the rest of the universe, so it all makes sense. So far so good, I can understand it - kind of :D. Now, I step on the pedal and continue accelerating the ship towards the speed of light. Every subsequent measurement confirms that the universe's diameter is getting smaller and smaller as the ship approaches the speed of light and the length contraction factor increases. Inevitably, as the ship continues accelerating, the universe is going to become smaller then the ship at some point. How can that be and what would happen then? The ship flies out of the universe, what ever that means? Actually, I think, well before that happens, the universe should shrunk below Schwarzschild radius and collapse into a gigantic black hole crushing my ship and ending my reference frame forever. But you are still on Earth and from your perspective nothing bad has happened. The universe is still there and you can only see a ship flying almost at speed of light. From that point if view, I could decelerate the ship, come back to Earth, go and have a beer with you and tell you how I saw the universe collapsing into a black hole, crushing Earth, my ship and everything else and how we both died. Mahesh, how can that be?

[@LouisBarjon]

Oh my god !!!! I've seen like 10 youtube videos on the twin paradox and this is one of the best I've seen !!!

[@amantyagi8298]

@FloatHeadPhysics i love your videos brother, can you please any a question that have been troubling me since high school - so lets say there is a positive charge at a long distance hence its charge field will travel at the speed of light all over the space then, after a long time we uncover or charge a particle negatively near us then as soon as that particle is negatively charged it will experience a force of attraction due to that positively charged particle we placed firstly but you see the charge fields from our negatively charged particle will take some time to reach the positively charged particle hence it will not experience any force.... isn't it the violation of 3rd law of newton? please answer

[@user-qk4qy3tu5l]

Enjoyable and gives me @ chance of understanding. Fantastic.

[@tdahanayake]

I am medical doctor from Sri Lanka. I really appreciate your efforts. Believe that you'll reach million subs very soon.

[@tyson31415]

That was the most fun I've had getting that relativity-induced "brain stretching" feeling in a long time. Thank you for making this!

[@achtsieben87]

Thank you so much for your video, explanation is nearly clear now. Sister sees twin brother as younger than she, but as she decelerates on final approach to Earth her present moment jumps forward into the twins future.

[@yaelfeldman6965]

Years after first hearing about this Paradox, today is the first day I actually understood the explanation given. Thank you!!

[@ProWeltfrieden]

Love your energy. It was a bless watching this video!

[@effectingcause5484]

The relativity of simultaneity.... Mahesh I cannot believe how well you explain so many of these physics concepts that are usually so hard to grab on to

[@-_Nuke_-]

His explanation is wrong though.

[@effectingcause5484]

@@-_Nuke_- Ok you explain it then

[@jxb178]

You really do a great job on these videos!

[@sgiri2012]

Sir,Did you try any intro music for all of your KZbin videos ? Or any plans in future?

[@ToboGamers]

To say its not acceleration is oversimplification and in fact, even at 14:27, you partly touched on this when you explained why both twins agree that the twin sister is the one who is accelerating. Absolute acceleration is required to break the asymmetry. Otherwise if we ignore acceleration completely, we can introduce a second twin sister (Sis-B) who starts 10 light-years to the left of the moving twin sister (Sis-A) in the moving twin sister's frame, and this Sis-B moves with Sis-A when she moves, such that Sis-B always maintains 10 light-years distance from Sis A. From the point of view of Sis-A, she will observe the twin brothers are the ones that are accelerating to her and Sis-B is stationary to her. Then she can do the exact same calculations with relativity of simultaneity as done in the video (as the situation is 100% symmetrical to the situation in the video) and the paradox returns because they now disagree with the ages again. So relativity of simultaneity is only part of the explanation; the absoluteness of acceleration is still needed.

[@silverrahul]

it's the other way round. saying it is acceleration is an oversimplificaton. Acceleration is part of the answer, but it is not the answer

[@ToboGamers]

I never said acceleration is the only factor, although other videos do do so and they are indeed oversimplification as well especially when they state acceleration is the difference and leave it at that, without clarifying why acceleration is absolute and not relative. It's not "the other way around". The correct answer is that both need to be considered for the explanation to be complete, which the title of this video does not suggest.

[@MrSuperpaco]

Wonderfull video, as allways. When I was studing relativity at school and the twins paradox apeared, the teacher resolved it simply. He said that the way to solve that was thinking about who is really moving fast. The twins at the planets or the sister at the spaceship?. The teacher added... From the perspective of the twins in the planets, the ship is moving faster, ok. But from the perspective of her, she is not moving, but the twins, their plantets and the rest of the universe is moving, so we can conclude that time dilatation affects her and not to the whole universe. At that time I accepted his explanation, but later I realised that his explanation had no sense

[@Sluppie]

This actually helped me to understand, thanks. The fact that acceleration only distorts all of space for the accelerator really helped me to understand this problem in a new way.

[@jamesyoungerdds7901]

Great video, thank you! Took me a couple of minutes to figure out your t-shirt, but very clever. Love physics shirts, but hadn't seen that one before 😄

[@user-py4rx7iu2i]

Nice content bro... I understand it at the first time I saw it. The way you explain it is Awesome.

[@ziaulhasanhamim3931]

I was expecting this video. Thanks once again sir.

[@lucasbrelivet5238]

I see each inertial frame as a different universe where distant events have different simultaneity and things have different lengths and different durations. When you accelerate, you go to another inertial frame, i.e., a different universe, and when you decelerate, you come back to the previous universe. Also, since those universes intersect at different points in space time, two people accelerating simultaneously but at different places, ie leaving their universe at the same time, but different points in space, won't end up at the same time in their new universe.

[@arjun_ragafanatic]

Hi Mahesh! This is a great explanation of the triplet paradox! HOWEVER, this is NOT an explanation of the twins paradox. In the twins paradox, they both start at the same position and their clocks are synchronized in both their reference frames. Thus you cannot resolve the twins paradox without invoking general relativity. Even in your other video, you use two spaceships that sync in space in order to reframe the problem without acceleration. However in the twins paradox, there is no abrupt change of reference frame. In-fact general relativity is what reconciles the difference in timing between the two frames of reference

[@meetsys]

This was exactly what always made me confused. Thank you so much

[@hsaneener9292]

If i say im a twin on a bus traveling forward in motion And my twin is sitting nwxt to me beut in a chair that faces a different direction do you still call it forward motion? Or is like that example of the train that looks like an optical illusion going both directions?

[@IncoGnito-ji5du]

Time is different for every person in this paradox. Earth-twin has his own time, sis-twin has her own in her spaceship, alien-twin has his own on his planet. You need to chose whose timeframe you follow.

[@genshinsbizzareadventures]

Actually 4 time frames ,include yourself too

[@channel4me434]

That is why this video is completely wrong, it doesn't solve the paradox. He adds an extra twin brother in the same reference frame as the earth one to show that the traveller that from his respective, there is a time difference of 7½ year. But the same applies if you add twin sister that travels in another rocket 5 lightyears behind the first rocket. If you do the same math, the earth-brother sees the same time difference and so from his perspective, the rocket sisters are 10 years older than he.

[@chandramoulimukherjee6653]

I just done get one part. Why does accelerating jumps the other person by 7.5 years? Sure, we can understand who is accelerating since acceleration is absolute but how does that make the other person jump 7.5 years?

[@silverrahul]

Look up relativity of simultaneity> it is not the accelerating per se that makes the others time jump. Rather, it is the change in frame,

[@JohnDoe33408]

Great explanation. Also love your Key & Peele skits.

[@redtoxic8701]

I've always been told acceleration is the solution but I never knew how exactly the process plays out accounting for it. So thank you for sharing such an easily understandable and intuitive explanation!

[@darrennew8211]

But it's not, because you can do the same thing using gravity "slingshots" to return where you started. Or you can have one person hovering in a gravity well (accelerating) while the other is in orbit (not accelerating) and see the same thing.

[@thetruetri5106]

That makes things a lot clearer to me. Also explains how it is possible there is no time passing for a photon

[@StackingGains]

This question always confused me, and made me way less confident in my understanding of physics. Thank you for anawering this! I knew acceleration had something to do with it, but didn't see how it interacted with relativity. Seeing that it interacts on the relativity of simultaneity is really the key as you said, beyond knowing it had something to do with acceleration.

[@gokai9320]

This is all great. Now I’m curious to see what a tangential observer would see. It really helps paint the picture of how time and space interact.

[@rsm3t]

I find it simpler to think in terms of spacetime intervals calculated using the Minkowski metric, rather than doing all the simultaneity, dilation, and contraction calculations. Say A stays at a x=0, and waits 20 years for B (who starts from x=0 at t=0) to return. B turns around at C=(10, 5*sqrt(3)) (first coordinate is time in years, second is distance in light years, both measured in A's frame of reference). So B travels from (0,0) to (10,5*sqrt(3)). The Minkowski metric, ds^2 = dt^2 - dx^2, gives ds = sqrt(100-75) = 5 years, a timelike interval. A similar calculation from (10, 5*sqrt(3)) to (20, 0) gives 5 years for the return trip. Thus the combined timelike interval for B's path is 10 years. But A "traveled" from (0,0) to (20,0), a timelike interval of 20 years. And C went from (0, 5*sqrt(3)) to (20, 5*sqrt(3)), also 20 years. It's no different than two people traveling from city A to city B using two different routes, except for involving timelike intervals rather than spacelike intervals. We wouldn't say there's a paradox if one traveler reports traveling 100 miles while the other reports 200 miles; likewise, it's not paradoxical for travelers following different paths through spacetime between the same two events to report different timelike intervals. Because the metric has a minus sign, crossing a positive amount of space in a positive amount of time (as measured in A's coordinate system) reduces the timelike interval for the traveler.

[@rwd420]

Very good explanation, man! 👍

[@mayadhanve3508]

Hi Mahesh, u r just amazing teacher, who could tought in a lay man language enable to ease to understand the complex topics

[@gled9880]

Her acceleration affects her clock only. It has no effect upon the brothers'. *Her description* of the brothers' situations changes when she switches (analytically, not physically) to a different frame of reference, but nothing about their situations changes physically. To appreciate this, it might help to realise that we can dispense with the sister and her spaceship entirely without changing anything in the analysis of the brothers' situation. In the reference frame in which they are stationary, the photons arrive at both brothers simultaneously, then ten years pass. In the reference frame in which the planets are moving to the left, the photon arrives at the "distant" brother first. 7.5 years (as recorded on this brother's clock) pass, then the other photon reaches the other brother (who is at this point in time "here"). A further 2.5 years pass at which point the (originally)" distant" brother arrives "here". His clock has now been running for ten years. These are two different descriptions of the same physical events. Likewise we could dispense with the brothers entirely in our analysis of the sister's situation. We could choose one frame of reference - perhaps the one in which the planets are at rest - and conclude that 10 years elapse according to her clock, or we could break down the journey into any number of segments and analyse each one in an arbitrary frame of reference with the same result: 10 years will have elapsed according to her clock when she returns to the planet where she started.

[@silverrahul]

Your description makes understanding of the situation more difficult, not less. should be avoided by those new to special relativity and to twin paradox.

[@wesjohnson6833]

Well, said. Much clearer than instantaneous acceleration and discreet "time jumping".

[@Learner..]

Best video on twin paradox , I have ever seen 🥂. Btw sir can you tell the book name yu read ?

[@zsqduke]

Is this jumping of 7.5 years during acceleration the same as gravitational time dilation in general relativity? How to intuitively understand this in the framework of GR?

[@narfwhals7843]

It is not "the same" because there is no gravity here. Gravitational time dilation happens in a gravitational field, aka curved spacetime. But spacetime is flat here, so there is no gravity. This works just fine in the context of general relativity, because special relativity is just general relativity in the special case of flat spacetime. There is no issue. It is "equivalent" to standing on a planet with surface gravity of that same acceleration. But only locally, in a small region around the accelerated observer.

[@zephsmith3499]

Very cool, best (relatively) simple resolution I've ever seen. Q: what does this mean about distant (ie: red shifted) objects in an expanding universe? We already know that what we see is billions of years what existed billions of years ago due to speed of light delays, and is red shifted (time dilation/doppler shift). How do the factors you mention stack on top of this, if at all? The other part I'm pondering is that: if the far twin gets 7.5 years older from her perspective (and twice as close) as soon as she gains speed - what happens if she immediately decelerates to be at rest relative to both twins (but still close the the near twin)? Would the far twin get younger again from her perspective?

[@silverrahul]

" _what happens if she immediately decelerates to be at rest relative to both twins (but still close the the near twin)? Would the far twin get younger again from her perspective?_ " yes, if she decelerates , then in her frame, the far twin would once again be younger.

[@rajeevganesh7144]

And all these years I thought I understood relativity. Way to go bro 👏