Thread: Time dialation question

I think we've all read the standard example where an astronaut leaves earth travelling at nearly the speed of light for a year, then comes back and is many years in the future.

However, I'm having difficulty understanding one specific part of this story.

From what I understand, it's perfectly OK from the point of view of the astronaut to say that the *earth* is moving away from him at nearly the speed of light. So wouldn't that imply that from his POV, it is the *earth* that is time-traveling into the future?

So from the earths POV, the astronaut is traveling into the future, but from the astronaut's POV the earth is traveling into the future. So what determines which point of view "wins" when the earth and the astronaut come back together?

Although I need to admit that I dont 100% understand the math behind all of this, but the key is that one of the observers needs to do a U turn and return back to earth. Earth is the stationary frame of reference. But the spaceship needs to accelerate ( back in the direction of the earth ) so it is the one that actually changes frames. The key is what is doing the acceleration?
This paradox is referred to as the twins paradox and is explained in much more detail at:
Twin paradox - Wikipedia, the free encyclopedia

Originally Posted by David Sharp

I think we've all read the standard example where an astronaut leaves earth travelling at nearly the speed of light for a year, then comes back and is many years in the future.

However, I'm having difficulty understanding one specific part of this story.

From what I understand, it's perfectly OK from the point of view of the astronaut to say that the *earth* is moving away from him at nearly the speed of light. So wouldn't that imply that from his POV, it is the *earth* that is time-traveling into the future?

So from the earths POV, the astronaut is traveling into the future, but from the astronaut's POV the earth is traveling into the future. So what determines which point of view "wins" when the earth and the astronaut come back together?

The thing is that even though from the astronaut's point of view his and the earth's acceleration are relative, they are moving through space at different speeds. Since the astronaut is proportionally moving through more space in the three spatial dimensions than earth is, he is proportionately moving through less of the time dimension than earth. It's the same concept that explains why photons do not experience time.

Check out Brian Green's The Elegant Universe for a brilliant explanation

What happens is that the more an object approaches the speed of light, the more that time slows down for that object, but to the rest of the universe, time passes normally.

So naturally I would age slower at a faster speed.

"slower" relative to different inertial frames. There is no absolute frame of reference so this statement needs to be restated.

Originally Posted by tom

Although I need to admit that I dont 100% understand the math behind all of this, but the key is that one of the observers needs to do a U turn and return back to earth. Earth is the stationary frame of reference. But the spaceship needs to accelerate ( back in the direction of the earth ) so it is the one that actually changes frames. The key is what is doing the acceleration?

Yes. The velocity is relative; whether it is the astronaut who is moving, or the earth, is relative; it depends on your frame of reference. But acceleration here is not relative - in any inertial reference frame, the astronaut is accelerating, the earth is not.

A situation that might illustrate the relativity of velocity is that of being on a train. If the ride is sufficiently smooth, you can't "feel" the motion of the train. Drinks can be served, and they remain in the glass exactly as they should. But when the train accelerates, watch what happens to the drink.

it is "right now" everywhere in the universe....right now
getting from point A to point B is where the apparent dialation comes into effect

Originally Posted by BowTieBear

it is "right now" everywhere in the universe....right now

From our viewpoint!

Yes, relativity of simultaneity. What is happening "right now" depends on the frame.