But a^2 + b^2 = c^2 is only true in a flat 2 dimensional (Ecludian) space. Whether it’s true of reality is not a given, since it’s not inherently true that what we perceive as flat is actually flat. As far as we’ve measured Pythogoras’s theorem, it’s pretty accurate, but space could be warped slightly and we’d just not have measurements accurate enough over large enough distances to tell. (Cue the astrophysicists who’ll come up with some evidence to prove me wrong).
“It’s just a model.”
- Knight of the Holy Grail
All this physics stuff is just the best model physicists have been able to come up with which matches all our observations. It describes what the universe does, not necessarily why it does it.
Well, we’ve measured on scales comparable to the size of the observable Universe and it still holds, but you’re right, it could be that the scale of curvature is just larger yet than that. It should be noted that space is definitely curved on small scales (like the scale of the Solar System); it’s just on the large scales that it’s flat (or very close to it).
Why is there any maximum speed at all (or technically, speed unreachable from below by particles with positive mass, or whatever it is)? All answers will end up coming down to: the laws of physics happen to be that way (at least, experiments indicate so). You can derive it from various assumptions about the laws of physics, but at rock bottom, there will still be something non-tautological in the laws ensuring this. Obviously, it’s possible to imagine a universe with different laws of physics which has no maximum speed; we just happen not to live in one like that. C’est la vie.
Why is the maximum speed as large as it is? Well, I think Chronos’s post #22 discussed this best, but just to reiterate what everyone’s been saying in perhaps a different way: if every length in the world were notionally doubled, so that the speed was notionally twice as fast, would we notice any difference? No, all the length ratios and so on which we could calculate would come out the same as before, and since all our measurements are ultimately in terms of these ratios, everything would look exactly the same to us. So there’s no question of why c is as large as it is, in itself; rather, the only meaningful questions are “Why does [whatever dimensionless value you calculate from c and other physical quantities] come out to the particular value it comes out to?”. I suppose if you were to posit a specific question of that form, we could try to tackle it, but I’m actually not sure that’s part of what you were asking about.
I think Mijin came closest to what the OP seemed to ask, but I hope this is more useful and tidy still.
There is a very important constant of nature, c, that has units of speed. It is the lowest unobtainable relative speed between things with rest mass, and the highest obtainable speed for information. It is also the square root of the proportionality constant between the mass and energy versions of the same underlying stuff.
Light is not unusual; several energetic influences, that can carry energy and information over a distance, travel at this speed, including light but also radio, xrays, and all other kinds of electromagnetic information, and changes in electric or magnetic fields (which is actually the same phenomenon), and changes in gravitational fields, and - I guess, though this is going out on a limb - changes in the force fields for the two nuclear forces (which are tiny over large enough distances to care anyway). That is, all these things travel at c when nothing interferes with them. They do different things when they are interacting with something along the way.
Much of this got said elsewhere, but I don’t think it was clear enough that there was this answer to the question about what makes light so special.
Allow me to state a bit differently what’s already been said.
I’ve underlined your error. Yes, you measure the objects separating at 1.2c. But that’s in YOUR frame of reference (perspective), not that of one of the objects. And neither object is moving > c RELATIVE TO YOU, i.e., from your frame of reference.
In fact, from the perspective of one of the objects, the other’s speed is always less than c. That’s what relativity states.
I see now that bonzer’s post #21, which is quite good, made the same point; sorry I missed it before.
Velocity is distance divided by time. Distance is a measure of space. What is not immediately apparent is that Space is not constant in the presence of mass, and that mass increases with velocity. Since light has 0 mass, it can reach a velocity that is unbound by it’s mass, and can travel over the smallest possible distance in the smallest possible amount of time. d/t = c. We use meters and seconds, and it comes out to c = 299,792,458 meters per second. It only comes out that way for mass-less particles. If you use furlongs, and fortnights, it comes out different numbers, but the same speed. If you are measuring masses in space, then space itself is different, or perhaps time is different, so the numbers always turn out to be less for other moving masses, but exactly the same for mass-less particles (Light), independent of the relative direction of travel. Space itself can apparently expand faster than C, but if it does, you cannot observe it.
At least that is how I understand it.
Tris
Say, for example, that the universe is simply a computer simulation of a universe and we’re all just very high level constructs built upon self-replicating patterns in Conway’s Game of Life. Since the rules for how the game operates has it that a cell is affected by its bounding cells, information can propagate at no more than 1 cell per round.
Now supposing that a “quark” in our understanding of physics is one of these cells. Billions of quarks make up each atom and billions of atoms make up life in the human frame. No matter how effective a method of transport we can create, we are still inherently limited to one cell’s width per round.
In the real universe, the size of a “cell” and the duration of a “round” are constants. Whether information can propagate at a rate of no more than 1 cell per round–like the GoL–or greater, I don’t know. But it’s probably a decent bet that it is an effect of this sort that establishes the limit.
Because the length of a meter is defined as 1/299,792,458th of the distance that light travels in a second. 
Yes for the strong nuclear force (which is mediated by massless gluons), but no for the weak nuclear force (which is mediated by massive W and Z particles). The speed at which weak nuclear effects propagate will depend on the energy involved in the process.
Nitpick: An atom contains less than a thousand quarks, and can be as few as three. And somewhat more than a nitpick, but the interpretation of the Universe as a cellular automaton doesn’t really gain you anything, and is really only popular among Eric Weisstein.
Ah, thank you for the correction. I really shouldn’t offer, as answers, things I already think are uncertain. I make enough mistakes even when I don’t think that.
But, a nitpick on your nitpick - the interpretation of the Universe as a cellular automaton must be pretty popular with somebody who prints really big books and distributes one copy each to shopping mall bookstores, where they demonstrate gravity. Did you hear that somebody invented an enormously powerful new computer, and switched it on, and asked it if it could figure out the meaning of existence? The machine hummed and got hot and made the lights dim for a few minutes, and then answered that it would be able to figure out the meaning of existence, but it would take a long time, and it would have to invent Mathematica first.
Can you give an example of an explanation that you would find satisfactory? The standard explanation is that Einstein (or maybe it was Lorentz or Poincare) postulated that the speed of light was a constant, and the mathematical consequences agreed with a lot of “strange” observations that didn’t seem to follow from the old theory. But I don’t think that is going to satisfy you.
It’s an analogy not an answer. Something that someone can easily visualize and feel happy about like it makes sense.
(And I wasn’t aware that anyone else was using or had popularized the analogy. I came up with it independently.)
Since from a photon’s point of view it is everywhere always, could the universe be “drawn” by just a single photon? If so, nothing could happen until the photon stopped by to draw it. Therefore, everything would be constrained by the speed of that photon.
I have no idea if this is true, but it seems to make sense.
First of all, a photon isn’t everywhere always; some places it isn’t ever. Second, what makes you think that it’s meaningful to talk of the Universe being “drawn”, by a photon or otherwise?
Science doesn’t answer “why”.
Please go down the hall to the “Metaphysics” wing of the building…
"It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we say about Nature. " – Niels Bohr
Exactly. When we meet intelligent aliens, they may have different - but equally valid - ways of describing their observations of the universe. That is, the observations and predictions would be the same as ours but their explanations of “why” could be entirely different but equally valid.
Think of a TV show being broadcast. Say an episode of Who Wants to be a Millionaire. That broadcast flies off into space at light speed.
I leave earth in my spaceship and engage my FTL engines and catch up to the broadcast. Thing is I will catch up with the end of the show first and will see the show in reverse order. People will be giving the answers before they are asked the question.
If you object to it being a TV broadcast consider the same will be true as I view the earth. I will be outpacing light leaving earth but catching up to light that already left. As a result I will see the earth moving backwards in time.
Effects would precede causes.
Er, I don’t understand your description, Whack-a-Mole. Why would the show be viewed in reverse order? And why would that be a problem?