# How speed changes time?

1. Dec 7, 2011

### Snip3r

Hi I m new here. i hv been thinking,searching abt this but could get no where
If i m moving in a rocket away from earth, people on earth will notice my clock ticking slower.But they are moving at the same velocity wrt me.So will i notice their clocks ticking slower?if not why?essentially i m having hard time trying to understand how speed changes time when there is no absolute rest. per Einstein if i m in space ship travelling at say 0.866C so when i travel for 10 yrs wrt earth my clock would only show 5 years..can i say earth is in motion wrt me and ticks only 2.5 yrs as my clock read 5 yrs?i m sure am missing something here can some one explain it more clearly?

thx

2. Dec 7, 2011

### ghwellsjr

Time dilation is always with respect to a stationary frame and is related to the speed that any observer/object/clock is moving in that frame. In a frame in which the earth is at rest, your clock will tick at one half the normal rate because you are the one that is moving and the earth's clocks will tick at the normal rate because it is not moving. In a frame in which you are at rest, your clock will tick normally and since the earth is moving, its clock will tick at one half the rate. You need to pick one frame in which to assign the speed of objects and then you can determine the time dilation for each of those objects. Just think what it would be like if someone said that you were traveling at 0.866c away from the earth and at the same time the earth was traveling 0.866c away from you and then they tried to say that the net speed difference was way more the 0.866c. That would be nonsense, wouldn't it? In the same way, when you assign time dilation to both objects, you get nonsense. So don't do that.