Relativity question?

Question

My dilemma relates to rocket man aging more slowly than his counterpart on Earth. What if our traveler is stationary, but the Earth moves away from him instead of him moving away from the Earth? (Ignore the effects of Earth leaving it's orbit, please) Is it the Earth folks that age more slowly, then? The relative motion between the traveler and the Earth is the same either way, and so why would it matter who's doing the moving?

If the answer is that it's not the relative velocity between the two that matters, but rather absolute velocity referenced to some theoretical motionless state, how could that state ever be determined? Also, if we imagined the whole universe was moving in some direction at high speed, and rocket man were traveling the opposite direction at the same speed, he would become motionless relative to this reference? So, instead of elongating in the direction of travel, he would shrink?!

Thanks for your help
Signed,
Relatively Confused

Answer 1

Phil

I think if I am not mistaken, it is not so much the direction but the speed. As we approach the speed of light in any direction we age less than our counterparts on earth. Also I do not think there is any shape to our universe. There are no observable edges. Everything is moving away from us (odd) and increasing its speed. (Even more odd) So to make a long story longer, we cannot be stationary. So theoretically if we could somehow figure out what direction we were moving we couldn't stand still. (I guess I agree on this point) So you may have hit upon something here. If we could figure out a motionless state maybe we will move back in time. That may take all of the energy of known universe to do so. That's odd, it is how the universe supposedly started.......

Our universe is a physical impossibility anyway. Our puny little brains can only figure out what we can see and THAT itself would be our best guess anyways.

Thanks for reading

Answer 2

You do not need to take account of acceleration at all to solve this one. You need to count reference frames.

The question is posed as though symmetric - as though there are just two reference frames involved. In fact, there are not - there are three.

There is the frame of the "stationary" twin, the frame of the twin travelling away and the frame of the twin returning. This is now clearly not symmetric. The two twins will age identically with respect to each other as the travelling twin departs, but there will be a difference introduced by turning around and coming back.

To see that this is not acceleration imagine doing the experiment with three clocks. The twin clock heads out and, at the truning point, meets another clock going in the other direction back to Earth. It passes the time it measures to this clock as they pass - this is a single event at a single point in spacetime so all observers will agree on it. The clock will read a different time to the one on Earth when it arrives.

Answer 3

Ah yes--as noted above, the twin paradox. It was addressed in two physics classes I had, and it's simply a rather crude illustration of time dilation. It's always anticipated and pointed out that (again, as noted above) it depends on acceleration. There is a special theory of relativity which is simplified to exclude acceleration, hence gravity. The general theory includes it. General relativity is a more advanced study, so knock yourself out if you want. I never learned it.

A better example than the twin paradox is a particle generated from collisions in the upper atmosphere. I forget which one it is. Although it travels at high speed toward the ground we know where it's made, we know how fast it flies, and we know how long it lives. What this means is that it should disappear before it reaches the ground, but it DOES reach the ground. From our point of view, it travels so fast that time has slowed down for it and it lives long enough to reach the ground, although it would never have that long a life if it were standing still. From its point of view, distances have shortened. So it can live its short life and make it to the ground because it's traveling a much shorter distance than we observe. It's traveling at a speed where special relativity (time dilation, foreshortened distance) applies and can be observed.

Answer 4

What about cases of immense gravity? In a situation like that you do not even need to move in the way we understand moving. So who moves or whether or not they are moving at all really does not matter. It is the distortion of space time that means anything. So if the earth is moving fast enough to be affected by a change in space time those people would age more slowly than a stationary astronaut who is not affected by a change in space time. Word up my brother, I am off for a spot of tea.

Answer 5

You are describing the famous "twin paradox" from special relativity.
http://en.wikipedia.org/wiki/Twin_paradox

The problem appears symmetrical at first. Doesn't everything that happens to one twin also happen to the other? How does physics 'choose' which twin ages more slowly?

There is one asymmetry. To find out who aged more, you have to make a comparison between the two twins, which means they must be in the same place at the same time. So, you have to apply an acceleration to one twin to reverse their trajectory and bring them to the location of the other twin. This acceleration is asymmetrical, and the twin that has the acceleration applied to it is the one that ages less.

If you accelerate both twins equally, they will both age the same amount. In this case, there is no paradox, because you get a symmetric outcome from symmetric inputs.

Answer 6

this the called the twin paradox. the solution is to have an absolute inertial frame of reference. one twin is moving relative from the frame of reference while the other is stationary.

Answer 7

This is a relatively humorous question, get it ahem ok on to the question. I would assume that the earth would in turn suffer the slower time but that's only if the theory of relativity is completely correct, the problem with theory is just that and this stays theory because there are too many unknown factors such as if the universe is actually expanding, and although Hubble tells us it does there is nothing decisive, the only true proof we have is clocks held by shuttle crew contrasted to ground command time.