# Relative Speed

From what I understand, the speed of any object in the universe can only be measured relative to another object in the universe. . . .

If this is the case then how can we say time slows down for an object travelling at super high speed. Who is to say which object is moving at super high speed and which object is stationary?

Cheers,
davies65

Doc Al
Mentor
If this is the case then how can we say time slows down for an object travelling at super high speed.
Time slows down for a moving object only as measured by the "stationary" frame with respect to which it is moving. As far as the object itself is concerned, time is passing at the usual rate.

Who is to say which object is moving at super high speed and which object is stationary?
No one. Or better, any frame is just as good as any other. Speed is relative to whoever is measuring it and so is time dilation.

Dale
Mentor
2020 Award
From what I understand, the speed of any object in the universe can only be measured relative to another object in the universe. . . .
Actually, the "relative" in relativity refers to measurements relative to a reference frame, not relative to another object. You can certainly make a reference frame where a given object is at rest, but it isnt necessary. It is a subtle difference, but important.

In any inertial reference frame the moving clock is ticking slow.

1 person
Ok thanks. So if take off in my space ship and zoom around the universe at a velocity close to the speed of light then come back to earth will all of my school friends be older than me?

phinds
Gold Member
Ok thanks. So if take off in my space ship and zoom around the universe at a velocity close to the speed of light then come back to earth will all of my school friends be older than me?

More likely they'll all be long dead if you are traveling for any length of time at "close to the speed of light".

But why wouldnt I be the one who has aged more quickley? Who is moving fast and who is stationary?

But why wouldnt I be the one who has aged more quickley? Who is moving fast and who is stationary?

Because you are the one who accelerated. Your dead friends were not accelerated to a v close to c, but you were. Even though it looked like they were from your point of view, the only one who was actually accelerated was you.

Therefore, you don't age more quickly, they do.

ghwellsjr
Gold Member
But why wouldnt I be the one who has aged more quickley? Who is moving fast and who is stationary?
As DaleSpam said in #3, it's motion in an Inertial Reference Frame (IRF) that determines aging. You can do the calculation in any IRF but you will find that they all produce the same results. Your friends could remain at low speed in an IRF but you can't. Maybe you are at low speed for part of the time in a different IRF while your friends are at high speed, but you'll always spend enough time at an even higher speed that you end up aging less.

1 person
phinds