The best explanation I have seen so far of the necessity of the speed of light is locality, that having no limit to the speed at which objects may influence other objects would essentially render location a meaningless concept, as e.g. photons could transmit force instantly over arbitrarily large distances.
Is this not simply countered by the existence of speed? For locality to exist, force carriers need to propagate at some speed, but MUST this necessarily be the same for all force carriers and an upper limit for all motion? Does this somehow break causality by potentially allowing an object to move faster than a force carrier it emits?
>>7723363
You seem to be the lost.
The memes are that way --> /s4s/
>>7723373
So you're just an utter moron then, yes? Yes.
>>7723363
There has to be a speed limit for things to make sense unless our understanding of the universe is very very wrong.
Does that speed have to be C? Maybe not but C is what it seems to be.
>>7723384
>There has to be a speed limit for things to make sense unless our understanding of the universe is very very wrong.
So people say. Why exactly do they say that? I'm looking for the reason that there must be an upper limit, the best one I've seen thus far is outlined in the OP, but it doesn't seem complete.
>>7723391
Let's pretend things can be arbitrarily fast.
I'm on a planet with you, you fall in a pit and die. I get in my ship and travel FTL to another planet. When I arrive I look back at where I left through a telescope, what do I see?
>>7723397
You'd be able to see your location as it looked x number of years ago, where x is the number of light-years between the two planets.
What's your point? That you'd be able to see yourself eventually if you waited around?
>>7723397
A computer monitor which URL says boards.4chan.org/sci/thread
>>7723363
>The best explanation I have seen so far of the necessity of the speed of light is locality, that having no limit to the speed at which objects may influence other objects would essentially render location a meaningless concept, as e.g. photons could transmit force instantly over arbitrarily large distances.
But that's totally irrelevant, as you noted.
The reason the speed of light is necessary is the concept of relativity - that the laws of physics are the same for all observers moving at a constant speed, regardless of the speed they're traveling. Einstein didn't invent this; it can be traced back to Galileo.
As it turns out, there are exactly three ways to construct a system of physics where this is true, by constructing coordinate transformations that let you convert from one observer's point of view to another. One of them has space and time being completely unrelated, which is the framework of classical physics; one of which causes matter to be unstable; and the third gets you every implication of special relativity including an inherent maximum speed.
As it turns out, the universe we live in is described by the third. If we lived in the universe described by the first, arbitrarily large speeds would be possible, and there would be no particular reason for lightspeed to be a universal maximum. However, the ways we observe electromagnetic radiation and very quickly moving objects behaving only make sense if the universe is described by the third.
>>7723403
Yes I would be able to see myself, and perhaps well before that depending on how much faster than light my ship is and how far I travel.
Anyhow if I'm there looking at myself before I left if my ship has an FTL communications device what's to stop me from messaging myself? Maybe to warn about your impending fall?
>>7723407
And if you want to know a little more about the details of these three system, and why the third necessarily implies a universal maximum speed, I can go into that.
>>7723407
My question is more along the lines of this: is a universe possible wherein all the laws of physics are identical to ours (in the familiar classical sense, let's ignore the quirks of general relativity for the time being) but there is no upper limit to speed?
In other words, is there something fundamental about the nature of our universe that requires an upper speed limit, or is it an apparently arbitrary constraint? I am not doubting the validity of relativity, but its necessity in a universe like ours.
>But that's totally irrelevant, as you noted.
Actually it's not, going to the basics of locality and causality is actually pretty insightful. It's something that I wouldn't have thought of; if your photons can instantly travel anywhere, all distances are effectively zero and location has no meaning. But that's only the case if they move infinitely fast, not if they have some lower finite speed, which is why I think the answer is incomplete.
>>7723408
> what's to stop me from messaging myself?
I think you might want to think that one through again, sport. You haven't gone backwards in time just because there's a lag in your signal.
>>7723425
In relativity FTL = backwards in time actually sport.
Read up on relativity of simultaneity. Light speed limit preserves causality.
>>7723433
Perhaps for the real universe, but not for the one in this thread. If light moves at c, but c is not a speed limit/constant in all reference frames, you can simply move ahead of it. That's the question, why does this speed need to be a limit. I'm not asking for evidence that it is the case, I believe you, I'm asking why it must be the case, of if it must be.
>>7723436
If it wasn't the case you'd have a universe where the laws of physics, or laws of nature if you prefer since I'm not talking about the laws we know necessarily I'm saying the real laws that govern how shit works, would be different from different frames of reference.
Could that exist? Sure. It'd be super weird though and not like our universe.
>>7723425
>My question is more along the lines of this: is a universe possible wherein all the laws of physics are identical to ours (in the familiar classical sense, let's ignore the quirks of general relativity for the time being) but there is no upper limit to speed?
No. Without the existence of an aether for light to travel in, Maxwell's equations (which are awfully classical indeed) only work if the universe obeys the Lorentz transforms, which in turn necessarily imply that there is *some* maximum speed.
Also, the mass-energy-momentum relationship underlies basically all of quantum mechanics, and you *need* quantum mechanics to explain familiar large-scale properties of our world like why hot iron glows red, then orange, then white as it is heated, rather than emitting infinite power in ultra-high-energy gamma rays. And the mass-energy-momentum relationship also relies on Minkowski space and the Lorentz transforms and thus inherently implies a maximum finite speed.
>>7723408
Just because you see photons sent long time in the past, does not mean the past is in the present.
As for speed limit, consider the following:
1. Assume light travels instantaneously
2. Build optical computer that is infinitely fast with light
3. Make computer process infinite loop infinitely fast
You would basically halt the universe, because in order for time to advance, events must happen at finite rates with respect to each other.
You can probably phrase this in terms of the halting problem...
>>7723447
FTL = time travel if your accepting relativity as true that's what I was getting at.
Basically between FTL, causality, and relativity you can only pick 2 otherwise they start to contradict one another. We seem to live in a universe where relativity and causality are true and FTL is false.
>>7723443
> Maxwell's equations (which are awfully classical indeed) only work if the universe obeys the Lorentz transforms, which in turn necessarily imply that there is *some* maximum speed.
>>7723443
Ok, so there are details about the universe that simply don't work without a maximum speed.
To give you background, I'm wondering about fine tuning and the anthropocentric effect (I might be using the wrong term there), basically the idea that we see the universe we see, which appears so finely tuned for life, because if it were not so there wouldn't be any of us in it to experience it's not-so-fine-tuned-ness.
I'm wondering if the speed of light, or a universal speed limit of another value, is one of these properties, or if life could have arisen in a universe without one. If it broke locality, or causation, I'd assume it were impossible. For finer details, like glowing metal or the detailed behaviour of electromagnetic systems, I don't know if that would impact life.
Essentially, I'd find it very weird if we lived in a universe with a speed limit and it were not necessary for life for that limit to exist, and I'm trying to establish if this is the case.
>>7723456
Anthropic principle, not anthropocentric effect.
>>7723456
A universe like ours but with no speed limit probably wouldn't have life or at least not intelligent life since there'd be no causality and how could intelligence come about in an environment like that?
You could have though a universe with arbitrarily fast speeds but an absolute frame of reference. At one time we thought this was how our universe worked and the aether was the theoretical absolute standard of rest. In such a universe though only measurements and observations made with respect to the aether could be correct. So in that universe the laws of physics could be different from different frames of reference.
>>7723456
>Essentially, I'd find it very weird if we lived in a universe with a speed limit and it were not necessary for life for that limit to exist, and I'm trying to establish if this is the case.
Well, as I mentioned, there's three ways to build a universe where relativity (in the sense of space not being absolute, not as in Einstein) holds true.
One of them underlies classical physics, one of them underlies modern physics, and in one of them life's probably impossible because matter should be unstable.
So, if we assume that purely-classical life is possible, and posit an aether, then the anthropic principle doesn't require the existence of a maximum speed. However, there's another wrinkle - the universe with a maximum speed is much *simpler.* For instance, relativity collapses Maxwell's separate electric and magnetic fields into a single phenomenon - the effects of the magnetic field can be derived directly from analyzing Coloumb's law when charges are moving relative to one another. And it unifies energy and momentum into a single four-vector - no longer two separately conserved quantities, but a single fundamental thing. And of course, you no longer need "space" and a mysteriously separate "time" but only one singular unified thing.
And it seems quite plausible to say that simpler universes are more "likely" - after all, it takes less information to specify them.
>>7723467
(And note that, incidentally, this has nothing at all to do with causality. The classical universe of #1 has absolute time which moves forwards at the same rate for all observers, so causality is preserved no matter how fast you're moving.)
>>7723467
Interesting. That's a fair point about simplicity.
>>7723476
Yea in a purely classical universe you have absolute time/space so speeds don't matter but in a relativistic one you need a speed limit to have causality.
>>7723363
Mass cannot travel faster than light because the legs of a right triangle cannot be longer than the hypotenuse.
E^2 = (mc^2)^2 + (pc^2)^2
>>7723478
go read about the anthropic principle. iirc unless I'm mixing up that name with another metaphysics argument it is very related to the simplicity argument.
I am not Leonard Susskind.
>why must there be an upper limit?
In the relativistic universe, velocities don't add normally (as I'm sure you already know). There is a gamma factor, which depends on the speed of light (an experimentally determined constant found in the Michelson-Morley experiments). As we know from classical mechanics, momentum p= m*v. We also know classically that Force is the derivative of momentum, F= [math] \frac{d}{dt} ( \gamma mv) [/math] (because the gamma factor times v is the actual velocity).
Taking the derivative gives [eqn] \gamma^3 ma= \frac{ma}{(1-\frac{v}{c}^2)^{ \frac{3}{2} } } [/eqn]
As you can see, as velocity approaches c, the denominator approaches 0, and so the Force required is infinite. This means if force remains constant, since mass remains the same, the acceleration must approach 0 as well, and thus we cannot have anything with mass travel at or faster than c.
>>7723363
physics makes shit super heavy to infinity
so you have to be massless and technically not in existence. weyl fermions have to obey c, but a psuedo particle may not have to.
>>7723497
But is this true in curved spacetime?
>>7723510
>Special relativity
>Acceleration
Pick one.
>>7723363
the speed of force carriers isn't necessary at all
in fact, you could imagine a physical system in which no force carriers are present. (This would be very boring)
Anyways, there are no necessities, the universe just is.
Quantum teleportation happens faster than light but it does not carry information.
>>7724182
If there were no force carriers, in what sense would there be anything, or any way to tell whether there was? It would be indistinguishable from nothing.
