Can you tell who is moving through time? [closed]

Question

In relativity, there is no way to tell if you are moving through space. So, if you were inside of a box, there would be no way for you to tell if you were moving or not. However, can you know who is moving through time? Could you just turn on a flashlight while you are inside of a box and count the light travel time, in meters, going by?

And if you CAN tell who is moving through time, does this mean there is actually a preferred frame of reference?

Does the rule that there are no preferred frames of reference ONLY APPLY to movement through space, or does this rule ALSO APPLY to movement through time?

We know photons do not travel through time. So, could this be used as a reference frame?

Answer 1

However, can you know who is moving through time?

In the sense that this phrase has meaning, every massive object is moving through time at $c$.

Could you just turn on a flashlight while you are inside of a box and count the light travel time, in meters, going by?

Sure, just shine the flashlight on your wristwatch. Then multiply the time your wristwatch reads by $c$ to get the distance you have moved through time.

Answer 2

Special relativity has two main postulates which are true for any relativistic problem

• In any reference frame, light travels at the velocity $c$.
• Every reference frame is equally valid and no experimentation can be used to deduce if one is in motion or not.

These two postulates are fundamental and have been approved by experimentations. I think these are enough to answer your questions.

I would like to point out that the phrase "moving through time" is not understandable, are you talking about proper time - $\tau$? Or the specific time coordinate used by the observer in motion - $t$?

Answer 3

By definition every observer is moving through time. In fact, in his own reference frame, am observer only moves through time, and not space. This time is called his Proper Time.

If you are asking the more basic question of how we define time at a fundamental level, I think from a physics standpoint we would say time has to be defined by events. A number $t_1,~ t_2,~t_3$ etc can be assigned to a series of events like the ticking of a hand on a clock. Preferably events with a regular and well known duration, like cycles of a light clock, or frequency of radiation from an atom. If you were to take the minimalist view (a la Descartes) of a blind observer in a box, or even a brain in a vat, probably the only measure of time would be successive thoughts or mental processes of a conscious observer. A pure vacuum in an empty universe with no events to mark it, would not have a meaningful definition of "time" – at least this is my best understanding in the context of modern physics.