[namaan (OP)]

So of course most of you know the answer to the typical why question better than I do: because that is how it must be for Maxwell's equations, that describe a fundamental reality of our universe and hold up to experimental results, to make any sense.

But what I mean to ask is what purpose does reference frame invariance of c serve in our universe? Or to put it another way, if our universe worked in a way that it agreed with the common sense of laymen like yours truly by having a constant speed of light, but where you can catch up to photons that just left you, why would you not want to live in such a universe?

[Soul Surfer]

The important feature of the sort of universe you might like is that the laws of cause and effect would be scrambled!   Fortunately we don't get what we wish for because we only see the effects that we would like and not the vast number of other nasty things that would happen.

[yor_on]

'c' is what it is, and what relativity builds on Nam

Tell me when you find out what it was about, I'm as intrigued as you there.

[namaan (OP)]

'c' is what it is, and what relativity builds on Nam

*chuckle* Well, I don't know how to chuckle, but you get the point.

Tell me when you find out what it was about, I'm as intrigued as you there.

Woah, that's a lot of expectation, but actually the way I unwittingly phrased the last sentence in the OP seemed, at least to me, to suggest a possible "answer". I'm inclined to agree with Soul Surfer's mention of a scrambling of the law of cause and effect. I remember reading someone's post in another topic that (from memory) when looking at a nearby galaxy, it is so red-shifted that it appears to be moving near the speed of light. And the poster further suggested that logic would dictate that our galaxy too may be traveling at near the speed of light. In this case the incredible and unimaginably fast speed of light suddenly becomes not so incredible. In fact we may be going so fast from an absolute reference frame, that if it weren't for reference frame invariance, then we might just be able to pass the light leaving the head lamps of our cars when we go faster than 88 miles per hour (pun intended). That would make for a rather messy universe indeed. [Edit: Ok, so I read into this and another source seems to say we are actually moving at about 1.3 million mph with the CBR as a reference frame; fast, but not quite near c. Let's just call the above a hypothetical situation.]

But actually to me there's a second equally important reason reference frame invariance is important. I see it as a way of separating the past, present and future in much the same way that entropy does. When photons leave your body, they carry with them your past. And the interesting bit is, while you can intersect with the past of other observers, you can never catch up with your past no matter how fast you go. Our past is always pulling away at the speed of light, no matter how fast we're moving or where we are in the universe.

So the thinking goes...

[yor_on]

Yep, I agree. From 'locality' you can use 'c' as a clock. Doing so will give you a 'past' a 'present' and a 'future'. Nota bene, 'c' as a time device this is, split down to ultimately Plank size. You only need to remember that 'time' is a strictly local phenomena, not a 'global'. Although 'locality' will rule and give you the same measurements relative your local clock, no matter where you are or how fast you go, and so create a 'universality' of constantly being the same 'local time' for us all, as proved if our local clocks share the same point in SpaceTime.

[Bored chemist]

You can calculate c from the properties of a vacuum.
Since a vacuum can't really have a frame of reference*, the properties of that vacuum must be invariant. So c must also be invariant.


* in that nothing can't "know" where it is or how fast it is moving.

[CliffordK]

You can calculate c from the properties of a vacuum.
Since a vacuum can't really have a frame of reference*, the properties of that vacuum must be invariant. So c must also be invariant.

* in that nothing can't "know" where it is or how fast it is moving.

Ummm...
Isn't Space a Vacuum?  Better than essentially all man-made vacuums.  Or, are you meaning a theoretical perfect vacuum that doesn't really exist?

Can't you have frames of reference?  Cosmic Microwave Background Radiation?  Planets, Stars, Galaxies, etc. 

Even in a vacuum bell, you can create an artificial frame of reference.

[Soul Surfer]

In electromagnetics Maxwells equations show that the vacuum has clearly measurable properties of electric field permittivity and electromagnetic permeability these define the velocity of electromagnetic radiation.

In quantum mechanics the vacuum contains a vast array of continually changing virtual particles that exist within the limits defined by the uncertainty principle and in effect give the vacuum these properties in just the same way that mass distorts space-time

[yor_on]

It's a interesting, and somehow strangely elegant observation BC. I need to think about how you mean there. Space is in one way definitly invariant, but then you have gravity and different observers descriptions of where any 'speccific patch of gravity' exist according to their rulers and clocks.