# Need a little help on special relativity.

1. Jul 29, 2007

### Hessian

Hi, I would like to clarify a few special relativity questions that i cannot quite grasp.

1. There are two people on earth, person number one person is on a spaceship and is ready to take off at near the speed of light. Person number two is standing near the spaceship and will watch person number one take off.
Now, the person watching the spaceship is (who is at rest in his frame of referance) watching a clock that is on the spaceship, as soon as the ship takes of at near the speed of light he will see the clock running alot slower than his own clock correct? But will he see the ship moving at near the speed of light? Or will he see the ship moving in synch with the slowed down clock, in other words will he see the ship moving very slow, or it may seem to him that the ship is stuck in the air?
And the same question vice versa. Person number one on the spaceship see's person number two's clock running incredibally fast, but will he also see person number two moving incredibally fast or will he still be moving at his normal speed and only person number two's clock (or time) is moving fast to him.
Maybe this wil explain to me why i never understood how the twins paradox was actually a paradox!

2. One person is living on a rogue planet moving at near the speed of light, But in his opinion he is at rest. So if he looks down at a clock on earth as he passes by, will he see it moving alot faster than his own clock or will it appear to be running the same as his own clock.

Thanks for your time, and i am sorry if these topics have already been done to death.

2. Jul 29, 2007

### JesseM

He won't see the velocity slow down as the ship accelerates...the v in the time dilation equation is the actual velocity you measure.
Fast? No, it doesn't work that way--there is no such thing as absolute speed in relativity, only relative speed, so since the twin on the ship measures the earth to be traveling at high speed relative to his own rest frame, the earth's clocks will be running slow in the ship's frame.
See above, the "paradox" has to do with the fact that in each observer's rest frame, it is the other observer's clock that's running slow. The key to resolving it is that this reciprocity only works when comparing inertial rest frames, but the twin that moves away and then returns to earth has to accelerate, so his inertial rest frame on the return journey is different than his inertial rest frame on the outward journey.
Again, all speed is relative, so you can't say the planet is "moving at near the speed of light" in absolute terms, only relative to something else (like the Earth).
Again, he'll measure it running slower, since in his frame he's at rest and the Earth is moving at near the speed of light. (Note that asking what he'll measure is different than asking what he'll see, because vision is affected by the Doppler effect as well as time dilation...what you measure is what happens once you correct for the fact that different signals from a moving object take a different amount of time to reach you because the object's distance from you is changing.)

3. Jul 30, 2007

### Hessian

Ok, thanks for clearing that up for me, I appreciate it. I think am beginning to understand.