Time travelling twin paradox, what do you see at turnaround?

Question

I understand there is no absolute present and the age of the twin on earth has to be adjusted when the spacecraft does the turnaround.

But what I want to know is what does the twin in the spacecraft see when looking through the large telescope pointing at earth.

Does he see things in rapid fast forward? Like the earth suddenly heats up for the passage of time to be accounted for?

Answer 1

Yes, the twins will indeed see the other's clocks moving in "fast-forward", when the spaceship is making its return trip. To see why, lets assume the ship is travelling at half the speed of light. Imagine the returning ship passing some "markers" in space at one and two light years distance from Earth. The ship will first pass the 2 ly marker. Two years later, he will pass the one ly marker. Finally, after two more years, he arrives finally at Earth. BUT this is NOT what the earthbound twin sees. The earthbound twin will see his passing the 2 ly marker two years after it actually happens, since light takes 2 years to get to Earth. He will see his twin pass the 1 ly marker only 1 year after he passed the first marker. And he will see his twin arrive at Earth one year after he passes the 1 ly marker. So the earthbound twin will see four years pass on the ship's clock in only 2 years time...

In answer to gp4rts. Yes, you are right, but I was not refering to time dilation, which is pretty small at only half the speed of light. The asker wanted to know what he would actually see. The time dilation effect which causes the moving clock to actually run slower is swamped by the illusionary "speedup" which happens when the ship is returning, so the clock will "appear" to run faster, as I said...

Answer 2

The time dilation equation is not affected by direction: the equation is

t = t0/√[1-(v^2/c^2)];

as you can see, it doesn't matter whether v is + or -, the observed time is affected the same way.

What does make a difference is the observer's viewpoint. Whoever "thinks" he is stationary, will observe a slowing time on the moving object. The person in the rocket observing earth will see time moving more slowly on earth. The observer on earth observing the rocket will see time moving more slowly on the rocket. That is the paradox.

Answer 3

I don't think it matters. Light from the Earth would still be passing you at the same speed whether you were moving toward it or away from it. What would change is the color, the blue-shift, red-shift effect.

TIme would be moving faster on Earth if you were at or near light speed but whether you were coming or going from it wouldn't have any factor on what you'd see, rather your speed.

http://en.wikipedia.org/wiki/Redshift