In relativity, where does time pass the slowest?

Question

Almost every introductory article in relativity says that if I go in a spaceship and travel at speeds approaching the speed of light, on my return I will find that much more time has passed on earth. Now here's my problem. Since all movement is relative, you can't really say whether earth moved at those speeds in relation to me, or the other way round.So what determines that myself and the loyal earthling will agree afterwards, that time has definately passed slower in the spaceship?

Answer 1

>"you can't really say whether earth moved at those speeds in relation to me..."

Yes you can. Only the astronaut, and not the earthbound observer, felt the huge forces of the acceleration that were required to turn around. As a result, both agree that the astronaut will be younger. *Speed* is relative, but *acceleration* is not.

The earthbound observer says, "It's 10 light-years to Vega, and the rocket was moving at nearly c. Therefore, it takes 20 years for the round trip."

The astronaut says, "It's NOT 10 light-years to Vega, stupid! Earth and Vega are moving backwards at nearly c, and (because of length contraction) it's only 10 kilometers distance! Vega gets to me in less than a second so the round trip takes a second or two."

The astronaut also agrees that the earthbound observer is very old when he returns. You can explain this in various ways; the elegant way to expalin it is in Special Relativity, where it has to do with relativity of simultenaity, but that takes some time to explain.

The quickest way to explain it is in terms of General Relativity. When the astronaut fires his rockets, he observes that a huge gravitational field has arisen, because he gets pushed against the back wall of his spacecraft. He's at the bottom of this well, and Earth is at the top. Clocks at the top of gravitational wells run faster than clocks at the bottom, and we're talking light-years of separation here, so he perceives Earth's clock to run hugely fast as he turns around. As a result, he agrees that the Earth observer is 20 years old when he gets back.

Note that the answer to your question is that during the trip, Earth observes the rocket's clock running slower and the rocket observes Earth's clock running slower, and they are BOTH correct in their respective reference frames. The astronaut is younger when he returns because he accelerated, and not because his clock was "really" slower during the trip.

Answer 2

No-one knows and won't - until time passes to a stop.

But until then with the theory of relativity in mind,
Time passes the slowest when you are waiting for someone!

Now everyone knows that s a relative Fact!

Answer 3

The trick here is that it's not symmetric - the earthbound person doesn't accelerate relative to the rest of the universe, while the rocket ship guy *did* (once at launch, then a deaccelerate and accelerate at turn-around, and another de-accelerate when they get home).

Answer 4

speed is relative, yet no longer acceleration. the two observers be attentive to that it became the spacecraft that speeded up, because of the fact the spacecraft feels it yet no longer the earth observer. This breaks the symmetry.

Answer 5

Surely this depends on location. When you arrive back on earth the time that has past on earth since your departure will determine the length of time you were away. Perception of time and aging may not equate though.

Answer 6

That's probably where the "control" of your little experiment come into play. Obviously, perception will depend upon which way you are looking at it - it is after all only relative!

Answer 7

TIme moves the slowest between paychecks.