Why twin's paradox is NOT about acceleration?

Chapters:
00:00 What is the twin's paradox?
00:48 Why acceleration doesn't solve twin's paradox
2:24 Twin's paradox without acceleration (Earth's frame)
4:42 The traveling frame
7:13 My new website - floatheadphysics (ad)
8:48 Earth's frame again - with the flag
11:38 Travelling frame again - with the flag
13:30 The resolution!
14:45 Relativity of simultaneity
17:02 Isn't the root cause the acceleration?
18:20 What do they 'see'?

In this video, we'll intuitively resolve the twin's paradox. This version of the twin's paradox involves no acceleration. And no, you don't need equivalence principle, and you don't need general relativity to solve it. Twin's paradox can be completely solved using special theory of relativity and the correct usage of relativity of simultaneity.

Link to the website:

[GEOFERET]

As a 60 year old physicist, I can say that this is the best demonstration of the twin's paradox I have seen, and I saw the first one when I was 15. I also love your enthusiasm. Bravo!

[Vengeance-be]

School taught me how to do maths in physics like memorizing it... but mahesh taught me how to visualize it .. every inch of my understanding of physics has a contribution of him

[Murdee]

I just genuinely love how Mahesh “has a dialogue” with Einstein

[ShawnHCorey]

The twin's paradox has three velocities: the velocity of the Earth, the velocity of the outbound trip, and the velocity of the return trip. That means there are three inertial frames, one for each velocity. And in special relativity, an inertial frame is an observer. There are three observers but only two people, the twins. This is what confuses people.

Try drawing the space-time diagrams for each of the frames.

In the first one, the Earth is not moving and the spaceships are. Time for the outbound trip is slower than Earth time and time for the return trip is also slower. We have: Earth's time > outbound time + return time. This is the part everyone agrees on.

From the point of view of the outbound trip, the Earth is moving away from the ship, so its time is slower. But to return to Earth, the ship has to leave this frame faster than the Earth or it will never catch up to it. Time for the return trip is even slower than Earth's time. We get: slow Earth time > outbound regular time + even slower return trip.

And from the point of view of the return trip, we get a similar result: slow Earth time > even slower outbound time + return regular time.

I think the space-time diagrams make things a lot clearer.

[MichalPlichta]

You are just King of Physics... saw a lot sci channels none of them really explain in such clear way... you are a truly awesome educator!

[mweave]

Excellent explanation. Thanks again Mahesh. There are very few explanations of twins that even mention simultaneity, you have once again cracked the case by starting with intuition. I would love to help you with the animations, let me know if you would like some after effects help to really bring your message home. Keep it up!!

[kuji]

This is by far the best explanation for the twin paradox I’ve seen, and I have watched a lot of YouTube videos on this spanning close to a decade. This holds true for basically all of your videos. Thank you!!

[ikhlasulkamal]

Thank you sir for also explaining the photon part and the age disagreement, it answers my question in previous video about "seeing the future" related to andromeda paradox

[erinm]

This is such a fantastic video, well explained! It seems to me that the explanation for the "lost 30 years" is essentially the Andromeda Paradox. It would be interesting to do a video on the Andromeda Paradox too, and then link it back to the twin paradox explanation here.

There is an interesting technicality that a few people are pointing out in the comments, which I think is worth considering: that your example here does in fact include acceleration, but that acceleration itself has a deeper and more subtle meaning than what we are used to. Essentially, the meaning of acceleration *is* a change in reference frames. Therefore, to say that the change of reference frame is the cause, is equivalent to saying that acceleration is the cause. In the example here, none of the twins are accelerating, but the clock information does accelerate--first it was moving away from earth, then it was moving towards earth, and that is still a symmetry breaking and is, by definition, an acceleration. I like the idea that the argument over reference-frame-change vs acceleration is meaningless, because they are two ways of expressing the same thing.

[PADARM]

You did it. You don't leave room for any "but what if" and now my brain rest at last. The explanations with spacetime graphs never left me 100 percent satisfied, because for me there was still "symmetry" but with your explanation there is clearly and visually no symmetry.

[ToboGamers]

17:22 "Acceleration causes change of frame. Change of frame causes relativity of simultaneity. Relativity of simultaneity causes the resolution to the paradox." Based on definition of root cause, this would actually mean the acceleration is the root cause, not relativity of simultaneity. You are correct in saying that relativity of simultaneity is the correct explanation and not general relativity because the thought experience is done in flat space time.

To again ignore the role of acceleration in breaking the symmetry is still an incomplete solution. The explanation in the video shows it is the spaceship that is changing frames. This modification of the original paradox has already assumed it is the spaceship that is acceleraing. But if you do not consider acceleration and the fact that it's absolute, the people on spaceship can say it is in fact the people on Earth who are changing frames. Then you can construct an symmetrical situation with this scenario (i.e. the original eEarth is always moving to the left, then a second earth moves to the right during the change in reference frame, there is only 1 spaceship that is stationary and remains in the same frame, and the ropes are attached to the earths instead of the spaceships) and the math all works out to be the same and the paradox would still be there.

Ultimately, you can explain the discrepancy between the twins' age due to their relative movement in variety of ways, such as through relativity of simultaneity, change in frames, doppler, space time diagrams etc, but all these explanation are incomplete without incorporating acceleration to break the symmetry, i.e. it is the spaceship that is accelerating and not the Earth.

[robwilliams]

Great video. Simultaneity in relativity is often overlooked. Nice explanation of why you can't ignore it. And it was great to highlight that what you measure using scientific apparatus, clocks, rulers etc is not the same thing as what you a see using your eyes or cameras. Nice.

[haoyuanliu]

Can’t wait to hear you illustrating GENERAL RELATIVITY intuitively. Must be an even more mind-blowing journey！

[kfawell]

That was such a good experience. I'm simultaneously Happy, impressed, dazzled, and awakened. I understand others comparing this to other explanations, but I want to say this is the first time it's ever really been explained. Your approach of having a conversation and asking all those questions is so incredibly effective. Thank you so much. Special relativity is itself an amazing leap of imagination. And then on top of that solving the paradoxes are a bunch of other giant leaps.

[TechnooRam]

Bro this guy is succeeding fenyman in his teaching skills at this point 😅

[Jester]

If I understand correctly, on the returning spaceship they would calculate earth's clock to be running slow but if they looked out the window they would see it running fast! Earth's clock measures 5 years (from 35 to 40) while ship clock measures 10 (from 10 to 20). However, since the image they see at the red planet is from time 2.7 they will see the remaining 37.3 years of Earth's history sped up (and blue shifted) during their 10 year return trip.

[daemanuhr]

I liked your explanations in the first 18 minutes of the video, but I felt like you were missing a key part of the explanation that would make it more intuitive. But then I saw the final section starting at 18:20, and I was very happy to see that you covered the key part. Your explanation was great!

[sidduporandla]

That is how I thought you should explain in the first video.(the rope one)
But because of my SSC exams I don't want to use my brain in understanding this video. But this video looks great. The hardest part in understanding physics is using imagination. This video makes us to them imagine easier.

[sgiri]

Relativity is the very engaging topic possible only when mahesh sir turns around.Keep doing your good work sir.
First view sir. Thank you 🎉for your videos expecting this long time.

[arjun_ragafanatic]

Hi Mahesh! Excellent work and real mind-bender of a concept. If I may offer a suggestion, I think the point would have really been hammered if you showed the length contracted version of spaceship 1 from the frame of spaceship 2. The reason is that there is a third spatio-temporal simultanety event that appears to be violated (i.e. the simultaneity of the ends of the two ropes being at earth at the same time (at time 20). A visualization of this simultaenity in the three frames of reference would have made a lot of sense.