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What I understand about the twin paradox is that if a person stays at rest with something lets say the earth and a traveller moves with a great velocity with respect to the firsy person, then the time of traveller becomes very much smaller than the time of person who stays back, which led to the conclusion that one would age very much and one would not.

But if you see from the frame of the traveller, the person on rest on earth seems to be moving very fast, and according to this the person on earth should be younger than the traveller.

So, who is actually going to be younger?

One of my friends had asked me this question, and I replied that from the frame of the traveller if you measure the velocity of earth bound person you will have to take the same velocity in negative and you will get the same result. Even though he accepted my answer; I am myself not satisfied with it.

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closed as off-topic by John Rennie, jinawee, DavePhD, Emilio Pisanty, Kyle Kanos May 14 at 13:02

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This paradox has a well-known explanation/solution. Have you tried researching online or in your textbook? –  BMS May 14 at 1:38
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Not a paradox. At some point they must compare clocks while in the same inertial frame. One of them experiences accelerations and one doesn't. So they have two quite different experiences and it is easy to tell which one accelerated and that is the one which "went faster" than the other. Plus, your SR time calculations don't account for acceleration. If they never meet, then they can't compare clocks. So it is a non-starter as far as a reasonable question to ask. –  C. Towne Springer May 14 at 2:23
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This question shows insufficient effort. Searching this site for twin paradox finds many questions that deal with this issue. Googling finds many, many more. –  John Rennie May 14 at 8:44

1 Answer 1

Your description of the twin paradox is not correct.

For two inertial observers in relative motion, both observers find the other's clock to run slow (without contradiction). The situation is symmetric and the question "which one is actually slow?" is meaningless since the two twins can only directly compare their clocks at one event - the event that they are together.

For the twin paradox, one of the twins is not inertial - one twin must reverse course to return to the other twin (where they can directly compare their clocks at a second event) while the other twin remains inertial.

The situation is not symmetric and, in fact, the twin that reverses course objectively ages less than the twin that does not.

This is a well known consequence of the fact that, between any two events, the greatest elapsed time is along a "straight" world line through the two events.

Since the worldline of the twin that reverses course is curved or "kinked", the elapsed time along that world line is less than the elapsed time along the worldline of the inertial twin.

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