Question

It is said that if I had a twin and traveled somewhere far in space near the speed of light I would comeback younger than my twin. But according to relativity I can never tell if it is myself who is moving away from the planet or the planet who is moving away while I stay still. Then how does the universe know it is me who should b younger and not my twin on Earth?

Answer 1

The situation is not symmetrical since the travelling twin is changing direction.

Answer 2

What do you mean? The twin could be travelling in a straight line, not changing direction. And even then how does it make it any different whether the twin travels changing direction or not?

Answer 3

It's too long to explain today ;-)

If you have time, you can read this:
http://en.wikipedia.org/wiki/Twin_paradox

Answer 4

http://www.pbs.org/wgbh/nova/space/how-big-universe.html

Answer 5

In short, changing directions means acceleration, and acceleration means non-inertial reference frames, which means general relativity takes over.

Answer 6

To clarify this a little bit, the apparent paradox is that each twin experiences the same phenomenon (seeing the other twin move away and then return), but nature somehow decides which one to age faster.

The paradox is resolved, as mentioned above, by the recognition that the experiences of the two twins are *not* in fact symmetric, because one twin undergoes acceleration in order to return home while the other does not. The simple fact that the twins experience different phenomena over the course of this experiment is enough to resolve the paradox we thought we had.

If you want to know exactly *how* the acceleration of one twin plays a role in deciding which one is older upon their reunion, a simple space-time diagram would be sufficient. It's important to remember that the only reason this thought-experiment is troubling is that we tried to find the answer by exploiting a symmetry that does not exist. Had we used a space-time diagram to begin with, the answer would have been immediately obvious and everyone would be sleeping a bit easier.

Answer 7

For completeness, I've attached three space-time diagrams, each drawn in a different reference frame. The worldline of the "stationary twin" is green, and the worldline of the "traveling twin" is red.

Frame A is the reference frame of the stationary twin. Frame B is the reference frame of the traveling twin as he leaves his planet, and Frame C is his reference frame as he returns to his planet. Think carefully and make sure you understand and agree with them.

The time experienced by an individual is given by the path length of his worldline. Notice that regardless of which frame I choose, the traveling twin (red) will always have a longer worldline. I drew straight lines (i.e. constant velocity, except for the corners), but this would be true no matter how the traveling twin accelerated. Do you see why?

There are two "events" in question here - The first is when the twins separate, the second is when they reunite. These two events correspond to two points on a space-time diagram. Because he does not accelerate, the "stationary twin" moves between these two points in a straight line, while the "traveling twin" does not - his worldline will be curved or bent, and since the shortest distance between two points is a straight line, the unaccelerated twin will always have the shortest possible worldline and will therefore experience the smaller amount of time - i.e. he will age less, while the traveling twin will age more.

It's worth noting, by the way, that "the shortest distance between two points is a straight line" only makes sense in flat spacetime. If you introduce gravitation (i.e. General Relativity), the situation becomes much more complicated. But that's for another day.