# Twin paradox - which twin is older?

Hi everybody! :)
can anyone help me with this idea: in the twin paradox, a twin that travels at speed of light in space should be younger than the one that remains on earth. But why can it be other way? Can we say that the twin on the earth is traveling at speed of light compared to the twin in space, from his frame of reference? If we say that twin in a spacecraft is relatively still, than the twin on the earth is moving. If this experiment was really possible, how could we say which twin would be older?
Thanks :-)

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mathman
The age difference shows up when they try to get together. If one doesn't accelerate, but the other does (i.e. turns around), then the accelerating one ends up younger.

Thanks, I understand that, but I don't understand if they met, why would the twin that remained on earth been older? Why couldn't it be that all of us that remained on Earth were aging more slowly than the twin in spacecraft? thanks:)

There's no absolute concept of "younger" or "older" unless the two twins are in the same point of spacetime. In order to compare their ages, we have to bring them together. When that happens, the one who's been travelling at constant speed will turn out to be older than the one who's been changing directions.

JesseM
There's no absolute concept of "younger" or "older" unless the two twins are in the same point of spacetime. In order to compare their ages, we have to bring them together. When that happens, the one who's been travelling at constant speed will turn out to be older than the one who's been changing directions.
One consequence of this is that if the traveling twin continued traveling at constant speed, and someone strapped a giant rocket to the Earth so that it could be accelerated and catch up with the traveling twin, in this situation it would be the traveling twin who was older, not the Earth twin. As hamster143 says, whichever twin changes directions (or accelerates in any way) will end up being younger than the one who travels at constant velocity between their meetings. This is closely analogous to the fact that if you draw two paths on a 2D plane which cross at two different points, and one path is a straight line between the two points while the other bends, then the straight-line path will always have a shorter distance than the path that bends (because a straight line is the shortest distance between points in 2D space).

Thanks everybody:)

Fredrik
Staff Emeritus
Gold Member
There's been a lot of twin paradox threads lately. I'm quoting myself from one of them:
My standard answer is "Check out #3 and #142 (page 9) in this thread". Post #142 is just what Mentz and Ich are talking about. Post #3 contains a spacetime diagram that you should check out.

See also kev's argument and DrGreg's spacetime diagram here about why it doesn't make sense to say that it's the acceleration that causes the age difference.

There's been a lot of twin paradox threads lately. I'm quoting myself from one of them:
Maybe it is time the twin paradox had its own subforum in the Special and General Relativity forum and its own sticky.

Fredrik
Staff Emeritus
Gold Member
A sticky would be a good idea actually. I have felt that way for years.

A sticky would be a good idea actually. I have felt that way for years.

I'm adding my vote for sticky too!:)

Hi everybody! :)
can anyone help me with this idea: in the twin paradox, a twin that travels at speed of light in space should be younger than the one that remains on earth. But why can it be other way? Can we say that the twin on the earth is traveling at speed of light compared to the twin in space, from his frame of reference? If we say that twin in a spacecraft is relatively still, than the twin on the earth is moving. If this experiment was really possible, how could we say which twin would be older?
Thanks :-)
The twin that moves the greatest distance in the alotted time (which would be the same total time for both) would be the younger because motion in spacetime "subtracts" (not linearly but by a hyperbolic formula) from proper time and "youngifies" the mover. ALWAYS, ALWAYS, ALWAYS

Think of it this way - did you ever see a little boy who was always on the go and was 2 feet tall? He just never stood still long enough to grow.