The complete intuitive explanation for the twin's 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.

[Xylot]

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

[couldbeahuman]

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

[tripleaskillz]

As always, a great way to teach advanced physics

[PortalUser]

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.

[SamiAhmad]

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

[Viperclarkx]

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!

[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.

[boblobgobstopper]

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

[one]

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

[georgwagner]

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

[A_GoogIe_User]

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

[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

[tempname]

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?

[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 👌

[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!

[BobzBlue]

There wouldn't be an apparent time change of the destination relative to the sister because the speed of light from the destination would be effectively halved. 7.5 year delay from twin one to twin two (7.5 years in the past), but when the sister has accelerated it would double the time for light to reach her to 15 years. With the jump in time of the 2nd twin going to 7.5 years in the future (7.5-15=-7.5) the sister would still be looking at twin two 7.5 years in the past, same as how it looked before she accelerated.🤔

[nirinarabeson]

You are the best spacetime YouTuber in all reference frames Mahesh

[obscurity]

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

[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.