You would perceive the answers before the questions, but that’s no more a problem than is rewinding a tape in your VCR. Relativity is not about the order in which events are merely perceived.
I took physics way back in college, but it seems to me that the various flavors of this answer are just not understanding the question correctly, and getting bogged down by the units.
So how about this: c could be expressed in units that are themselves fundamental, no? We could define, say, the “ped” to be the diameter of a proton (at rest, of course). And we could define the “tranistan” as one period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
And then c could be expressed as so many peds per tranistan, right? I don’t want to try to do the math to figure out that value, so let’s call that number X.
So now, the question can be expressed as: Why is c equal to X peds per tranistan?
We humans did not define the diameter of the proton, and we did not define the period of radiation of the caesium 133 atom. So is there a fundamental reason why X has the value that it does?
No, wait, you are only watching a representation of it all that runs backward. The bigger issue is that you would be seeing ALL of it after you had been a (potential) witness of it on earth. A related example that would put effect before cause would be if you could fly by the radio waves and receive and watch the broadcast, and then travel to earth and see them recording the broadcast you just watched. The effect of there being a broadcast for you to watch would precede the cause of them filming it for you. That’s the thing you aren’t going to be able to make happen.
If you had a faster-than-light device of some sort, though, you could make that happen.
OK, now we’re getting into the realm of dimensionless quantities, where the question is at least meaningful. There are about two dozen apparently independent fundamental dimensionless quantities known, and it would be possible to express the speed of light in peds per transistan as some mathematical combination of those. But that just shuffles the question off, to the question of why those two dozen fundamental dimensionless quantities have the values they do, and the answer to that is simply “we don’t know”. It may be that some of them can be derived mathematically in the same way that, say, pi is, or at least that some of them can be expressed in terms of others… But if so, we don’t yet know how.
Sounds like the Anthropic Principle unless I am misunderstanding.
Basically why are the physical constants of our universe the way they are? Do they have to be this way?
My understanding of it is that the constants could have been anything (loosely speaking)…most of which would preclude life forming (or even planets and stars). Our universe happens to have the right combo so we are here to ask the question but the bottom line is there could have been any number of universes and we just got “lucky” (which is to say had the numbers been otherwise we wouldn’t ever have existed to ask the question so not like we would have cared…not existing and all that).
Indeed I think the odds are spectacularly against the numbers being chosen to be just right for us to exist. So either posit an infinite stretch of universe creation and sooner or later one gets us (infinite monkey thing) or we won a staggeringly unlikely to win cosmic rolling of the dice lottery.
Sure, although…
In the same sense that the constants could have been different, just as well, the laws of physics themselves could have been different. So I don’t feel like this kind of reasoning does as much as might be hoped. Then again, I don’t think there’s necessarily terribly much that can be done, either. To the extent that the constants can be shown to have natural relations to each other from various discovered laws of physics, that simplifies things, and provides some sort of explanation, but there will still always be some non-tautological nub for which nothing more can be said than “Yup, that’s the way our world is. We can consistently imagine other ways a world could be, but that’s not the way ours is.”
It’s faster.
It might be germaine to the OP’s question to answer the following related question:
Why does it turn out that the square root of the proportionality constant between energy and mass is the same as the speed of all massless objects?
Germaine to what?
Why is the speed of light what it is?
Well, it is profoundly convenient that the velocity of light is extraordinarily high because:
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It enables radiocommunications throughout the world to be almost real time, so that we don’t have to wait to hear what a pilot says thousands of miles away.
It enables us to play tennis and do many other activities with real time feedback. -
It provides an extraordinary multiplier for solar energy coming to earth. Any significantly smaller number and earth would be a giant ice cube, devoid of life.
This same multiplier gives us an incredible amount of energy for our nuclear power plants.
Compare c with the speed of sound.
How fortuitous that sound is relatively slow. This enables us to have stereophonic hearing, and tell the direction from which a sound originates.
Anybody who thinks these values arose on their own, from nothing, has a religious faith in the quaintly impossible. They pin their hopes on nothing, from which they think they came, and to which they believe, or pretend to believe, they will ultimately go.
Orville, welcome to the boards.
Witnessing belongs in Great Debates.
The short answer is that the total energy of an object, according to Special Relativity, is equal to gammamc^2, where gamma = 1/sqrt(1 - (v/c)^2) (a factor that shows up often in relativity). For an object at rest, gamma = 1, so the total energy is just the energy associated with the mass. For an object moving at a slow speed (i.e., much less than c), the energy is a little higher, with the extra being approximately 1/2 m*v^2 (this is the Newtonian formula for the kinetic energy). For a massless object, if it’s to have any energy at all (and if it didn’t have energy, it wouldn’t exist), gamma must be infinite, which occurs when v = c.
A longer explanation would also go into why it is that in relativity the total energy is given by gammamc^2, but this will do for now.
Where won’t it be, ever? I think it’s useful to think of the universe being drawn in that it might help understand why we have causality. Nothing can happen until it is manifested. These manifestations have a mandatory delay, the speed of light.
Well, for instance, a photon that goes from the fluorescent light bulb above me to my desk won’t ever be in Des Moines, Iowa.
This always baffles me.
Pretend I am going for a ride on that photon. My understanding is, to me, the Universe is infinitely thin so there is zero distance between me and your desk and it takes zero time, again according to my watch, to traverse zero distance.
Yet to everyone else not in my reference frame I do travel some distance that does take some time to traverse.
Color me confused.
Right. At the very least it’s not a suffiicient answer as to why c need be the ultimate spped limit. This can be seen from the following illustration:
Suppose you had an instrument sensitive enough to pick up the sounds of human voices from people standing on the ground, even though it acts within your jet, which is miles away and quickly moving more miles away. If you wanted to capture the bits of converstion in reverse order, all that you would need is a speed greater than sound.
I think though, this is more an error in the way that people explain causality being broken.
FTL could break causality, and not merely in a “you might see a bunch of events, (that are all in the past) in the wrong order” way.
Basically as soon as you assert that time is relative you open the door to contradictions (were those two events simultaneous?, who’s clock is running slower? etc). However, as it happens, thanks to the speed limit before energy, information, whatever can be transmitted, these contradictions can never be exploited. Instead they “cancel out” in practice.
That’s my understanding anyway. It’s probably wrong 
Perhaps I should start a new thread but if anyone can take a shot at answering my post #74 above that’d be great.
One of those things I have never been able to get straight in my head and it bugs me. 
I wasn’t sure why you were confused. If you accept the basic premise of time dilation, then the case you mentioned is just an instance of that, so you shouldn’t be confused. What’s confusing you?
One thing, though, is that as someone noted above, it is arguable that photons do have a very small rest mass. (I don’t know if this is accepted by all physicists, some, or only a few, or just thought possible, or what.) If this is so, then it takes a tiny amount of time from the photon’s frame of reference for it to make trip, not no time at all. Would that un-confuse you? (If so, why?)
We cannot rule out the possibility that the photon has a mass, but if it does, it’s far, far smaller than that of any known particle mass. It’s a matter of taste whether you consider “very small but nonzero mass” or “no mass at all” to be the simpler explanation, per Occam.
To add to Chronos’s post…
It is worth noting that the photon is required to be massless in the standard model of particle physics. Giving a non-zero mass to the photon would require major changes to quantum electrodynamics.
That doesn’t mean it’s not an experimental question – everything is – but it would be somewhat unexpected given how well quantum electrodynamics seems to work (so far).
(Technobabble: Quantum electrodynamics is an abelian gauge theory, which means that electroweak symmetry breaking leaves QED’s U(1) part of the SU(2)xU(1) gauge symmetry unbroken, which means the associated gauge boson (the photon) remains uncoupled to the Higgs, which means the photon is massless.)