Understanding Time Dilation in Special Relativity

[timpeac]

Hi,

Just out of idle curiosity - why do we talk of "time dilation" in terms of special relativity when in fact it contracts which is the opposite? Is there a reason or is it just accepted usage?

 

[Nabeshin]

Hi,

Just out of idle curiosity - why do we talk of "time dilation" in terms of special relativity when in fact it contracts which is the opposite? Is there a reason or is it just accepted usage?

What do you mean it contracts? The specific word "dilation" refers to the lengthening of physical processes. i.e in my rest frame a strobe light flashes once per second, but if I put the same strobe light on a spaceship and shoot it off at relativistic velocities, I now see it flashing only once every two seconds. The period of flashing is dilated, not contracted.

 

[timpeac]

What do you mean it contracts? The specific word "dilation" refers to the lengthening of physical processes. i.e in my rest frame a strobe light flashes once per second, but if I put the same strobe light on a spaceship and shoot it off at relativistic velocities, I now see it flashing only once every two seconds. The period of flashing is dilated, not contracted.

Ah yes, I see. I think it's a case of you say tomahtoes I say tomaytoes -the way I was thinking of it is that the person left on Earth considers that for every one his seconds one on the ship his lasts 0.8 (or whatever) as long and so his has contracted. Yours is of course the same view but from the other perspective - and I suppose it is more valid like that since it is the ship that has undergone the acceleration and not him after all.

 

[Nabeshin]

Ah, I see where your confusion was coming from now. Yeah, we call it dilation because we define the time of physical processes as the time occurring in their rest frames. So whenever we observe these processes in a moving frame, they always appear to take longer. Just for clarification, the guy on the rocket ship (or whatever) sees the clocks on Earth run .8s for every one second he experiences. In that respect, the situation is completely symmetric.