# Time dilation sanity check

1. Dec 11, 2003

### Ziggity

If someone leaves on a spaceship from Earth at near the speed of light, the clock on the spaceship will appear be moving slowly when viewed from the earth.

When the guy on the spaceship looks at the clocks on Earth, they will appear to be moving fast, right? This makes sense since when the guy on the spaceship returns to Earth more time will have passed for the Earth than for him.

I just wanted to make sure since it seemed that some people in the "time dilation" thread were saying that the clocks on Earth will appear to be moving slowly when viewed from the spaceship. I might have been misinterpreting what people were saying.

Thanks,
Ziggity

2. Dec 12, 2003

### HallsofIvy

No, people have been saying exactly what you thought. Since speed is "relative", in the frame of the person on the spaceship, the person is not moving while the planet is moving at high speed. The person on the spaceship will see people on the planet moving slowly relative to himself just as people on the planet would see time moving slower for him.

That is applying special relativity while the spaceship is moving a constant speed relative to the planet. In order for the "twin paradox" to come into play, you have to have the person accelerate away from the planet initially, then accelerate to turn around and finally accelerate to land on the planet. All that acceleration takes you out of special relativity and into general relativity.

3. Dec 12, 2003