Because (relativistic) notions of time are derived from our understanding of complicated maths, and not from our direct experience, would it not be more fundamental to propose that time is defined as that which causes/allows our subjective experience to change?

I am still struggling to understand the underlying issue here.

Time certainly exists. Our intuitive sense of a constant moving time was dispelled by special and general relativity so that we now see time as an element of the spacetime of our universe. So asking "Does time exist" is no more puzzling than asking "Does space exist". But time is different from space also. Time is the conjugate variable to energy. Position is the conjugate variable to momentum.

So at some level asking "what is time and why does it have the properties that it has" is a good question, but physics does not have the answers. Our universe started at the Big Bang, but what does it mean to approach that moment? Is that t=0 or does t=0 actually express a meaningless concept? Is time continuous or discrete? These are all fundamental questions, but no one has the answers. Until all of these concepts are unified in one theory that will probably also need to resolve the anomalies between gravity and the other three forces, it is just speculation.

I agree that the question is non-specific. My point is that, even if science confirms exactly how everything works by repeatable experiments and models, will it really explain what time is? The explanation is based upon science which is based upon our subjective experience. Or is it? Actually I don't think it is. If I die, everyone else will still "experience time". Does that mean that the existence of time is an objective truth? On the other hand, time has no meaning to me when I'm dead, and one could argue that time is subjective experience. Or is it that, in all of the above, you can replace "time" with "more or less anything" ?

The frame of reference of a photon doesn't experience time.

I think the bold is exactly right. Science may be able to derive a physical rationale for time in the same sense that there is one for sound, for example. That will not answer the questions about your perception of time and the significance of time after you are dead. But those may well be RGT questions, not SMP questions.

I guess that it's a hilarious conincidence that a photon isn't a living being and therefore doesn't experience things. Surely for anything to have coinscience/awareness it must have >0 rest mass, and therefore it must experience time...

If you are in a moving frame you experience time normally, it just seems as if everyone else is moving and experiences slowed time. So wouldn't the photon see time normally but would observe us not to experience time?

No, time stops in the photon's frame of reference. The photon experiences every point in the universe along its trajectory at the same instant. There is no concept of time in this reference, and that has nothing to do with the photon not having "consciousness".

But there is no such thing as the frame of reference of a photon (although we can see what you mean in some classical limit). It can have no observer and no coordinate system to register observations. You cannot perform measurements in it.

Actually, I am claiming something that disagrees with "a photon moves on a lightlike trajectory has nothing to with whether or not it has consciousness". I am claiming that it is impossible for a particle moving along a lightlike trajectory to have consciousness. This may not be true if technology advances by huge amounts and we create "animals" the size of strings or something. But assuming we can't do any crazy **** like that, the point I am making is that every conscious/aware being must move along a timelike trajectory and must therefore experiences time.

Good post.

If time doesn't exist, it's at least a very good assumption, or invention if you will. We are like ants though, having great problems with what meaning to put into the word "existence" in the first place.

How do you know it was after one second?

I am still not sure this is true. If you attached a massless clock to a photon it would appear to stop moving in our frame of reference, but the clock would still be running in the photon's frame. It would experience every point in the universe at the same instant because the contraction of space would bring all distances to zero, at least that seems to be the limit as v->c in special relativity. All of these discussions involve inertial frames so that should be ok, I think.

My underline issue was:

Is time part of the fundamental structure of the universe, 4-th dimension in which events occur in sequence, or did we "invent" time upon the 3 dimensions so we could compare changes in our 3 dimensional world and by that time is not itself measurable but only a concept that enables us to sequence and compare events?

I was stating that when someone says a second he is comparing one change in 3d space vs a benchmark change in 3d space.

Haha,don't worry it wasn't lost to me.

If time was a real dimension wouldn't time travel be possible (in the past)? I'm pretty sure entropy of space prevents that...