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No, not really. c shows up in lots of equations but it is always the speed of light. It’s not just a random number that happens to corespond to the speed of light in a vacuum. Do you have any reason to suspect that if the speed of light in a vacuum were to change, the value of c in all these other equations would remain the same?
:dubious:
Do you have a cite for this? Some objects appear to have the sum of their recesional and transverse velocities greater than c, but this is actually an illusion caused by not doing the math properly. What does the redshift of an object with a recesional velocity greater than c look like?
No, c is an invariant fundamental constant and the speed of light in a vacuum is only one of the values it represents. The fact that it is always identfied with this value is more from how the value of c was derived and how we view the physical universe. Relativty does not need light to travel at c (to avoid the confusion
that c should always be identified as the speed of light in a vaccum some prefer the name Einstein’s constant).
If we wished to alter some parameters so that the speed of light in a vacuum is different from the value it is now, we could either alter the value of c or make the photon into a massive particle.
Yes I know you can have redshifts greater than 1 that do not correspond to superluminal recesion velocites. z > 3 for bodies recding from us at superluminal speed, here is a paper by Davis and Lineweaver on the subject:
re: redshift and superliminal recession velocities
MC, would I be correct in inferring that the equations used in Special Relativity are actually describing motion through a spatial coordinate system that is assumed to be in non-expanding space? e.i., SR uses definitions of displacement and velocity that don’t account spatial expansion across extremely large distances. e.i., SR works fine in local space, but shouldn’t applied to situations involving distances that are so large that spatial expansion has a significant impact on the actual distance between the observer and the light source?
Special Relativity assumes a globally flat coordinate system, which is inconsistent with expanding space. But the more powerful (and more complicated) General Relativity has no problem with it.
And asterion, it depends on who’s counting. In principle, if I’m going from here to some other star, I can get there in as short a time as I like… by my measure of time. From the point of view of someone waiting on Earth, it’ll take me longer than it takes light. If I’m going .99c, then yes, it’ll only take me a little bit longer than light, from the point of view of someone on Earth, and it’ll take me a considerably shorter time than that, by my clock. But if I shine some light around in my ship, I still measure it as going 3*10[sup]8[/sup]m/s relative to me. So I wouldn’t say “for all practical purposes”, but I might say “for some practical purposes”.
People are going to flame me for this im sure, but I have studied a bit on the subject, and have drawn my own conclusion. The speed of light is just that… a SPEED. At the present technological level that man is at today, and with the present theories and experiments, it is said that we can never reach this speed. I disagree. I believe we will not only reach it, but surpass it many times. Why? Simple. Because of past evidence. Just about 100 years ago, the first motor car was built. Today there is a space shuttle. From the first motor car to the space shuttle in 100 years. Now in the whole scheme of time, 100 years is NOTHING… literally NOTHING. We are talking about a planet that will be here for BILLIONS of years. If I would have produced the idea of the space shuttle 100 years ago, every human on earth would have called me crazy. I believe that several million years from now, we will be traveling to other galaxies easily. Just an opinion! Peace!
-Brad
Typhoon This isn’t the forum for opinions. That’s IMHO. C isn’t just a speed, it helps define the properties of the universe.
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This is a tremendous and unwarranted leap here. C is indeed a key variable in defining the properties of the universe. But you’re assuming that c, the speed of light in a vacuum, can somehow be uncoupled from c, the universal constant.
If, hypothetically, there were a region of space where c was twice what it is now, there is no reason to believe that you couldn’t simply plug this new value for c into the existing equations, for this region of space. In other words, c is a “fundamental constant” but if you could change the properties of the vacuum and, therefore, change the value of c, you’d probably also be changing other “properties” of space as well in which c manifests as a constant.
All this is, of course, highly speculative. Yet it is precisely these seemingly trivial loose ends which drive the great pardigm shifts in science.
If you could travel at the speed of light, you could walk to the front of the spacecraft, turn around, and NOT see yourself until you returned to your seat.
People often compare it to the supposedly unbreakable sound barrier. Sure, 100 years ago people found the very idea of us ever building anything that goes faster than sound ridiculous, but since then we’ve done it many times.
However, the sound barrier was an engineering hurdle, not a physical one. Back then they should have suggested that conceiveably if we had the technology we could go faster than sound, but the very idea of us ever building something like that was laughable.
Light speed, however, is a speed limit that we are sure (based on current knowledge) can’t be passed. It isn’t a matter of building a spaceship powerful enough. The closer you get to light speed, the more energy you use (and when you get really close we’re talking RIDICULOUS amounts of energy) until to reach light speed you need infinite energy. Of course, since the universe itself doesn’t have infinite energy, we can’t build anything like that. Even for the most advanced race, it’s theoretically as well as physically impossible.
Maybe some day someone will come up with a way to bend the laws of physics or discover a loop-hole (or possibly a worm-hole). But until then, the light speed limit is just something we’re going to have to deal with.
Yes it is one of the orginal axioms of special relativty that light travels at c but the point I’m making is that it needn’t be formulated so that light travels at c (and by that I don’t mean alter the alue of c, just have c and the speed of light in a vacuum set at different speeds) the speed c equally corresponds to the speed of a gluon in a vacuum. It may seem like a minor point, but the implication that relativity is somehow dependent on the properties of light rather thna vice versa is incorrect.
Oh, that’s easy. Just give the photon a mass. Any massless particle must travel at exactly c, and any massive particle must travel at less than c. At the moment, we’re pretty sure that the photon is massless. At least, we can put an upper bound on the mass of the photon, and that bound is pretty darn low (less than 2*10[sup]-16[/sup] eV, as of July 2000… I don’t know if that’s been refined since then). It would be surprising to find that the photon has a mass of, say, 10[sup]-20[/sup] eV, but it would not be earthshaking. And if we did find that, then the “speed of light” would depend on the energy of the light and would therefore no longer be a fundamental constant, but c would still be a fundamental constant.
If this sounds outlandish, then consider the neutrino. As of a decade or two ago, it was generally assumed that the neutrino was massless, since the upper bounds on its mass were so much lower than anything else we knew of. Therefore, it was assumed that neutrinos travelled at c. In that time frame, one might perfectly well have referred to c as “the speed of neutrinos”; the only reasons that the term “speed of light” was preferred are historical. But the “speed of neutrinos” most certainly can be uncoupled from c, and it has been, since we now know that the neutrino does, indeed, have mass.
Incidentally, MC, a minor nitpick in your post. You can’t set the speed of something to any speed other than c. If a particle doesn’t travel at exactly c, then the speed at which it does travel will depend on its energy (or, equivalently, on the reference frame in which you observe it).
But Scharnhorst, of course, did not assume massive photons to reach his result.
I think that part of the problem here is the short hand assumption that the speed of light in a vacuum is a property of light. It’s not – it’s a property of the vacuum. Once again, there is no a priori reason to believe that our hypothetical region of empty space with a different value for the speed of light in a vacuum would not also have a correspondingly different value of c in, say, the equation E=mc[sup]2[/sup]
It’s quite true that lots of stuff seems to propagate at c. But, as MC and Chronos have pointed out, c pops up in lots of other places, too. This strongly suggests that c is a manifestation of some subtle property of the universe rather than merely being a cosmic speed limit.
The values of fundamental constants aren’t set in stone, Apart from the anthropic principle, there is no reason for them to have any particular value. They were fixed at some point in the very early universe by a process on which we can only dimly speculate. (Indeed, one intriguing – though hardly mainstream – interpretation of inflation is a sudden drop in the value of c rather than physical superluminal expansion.) Scharnhorst’s calculations offer a tantalizing glimpse behind the veil.
What confuses me is; while we have innumerable explanations and examples as to why you can’t accelerate anything to the speed of light, yet light itself, as photons, does travel at the speed of light. So, how do photons (or anything) get there?
I don’t presume that photons start off slower than c and get accelerated. I"m just wondering how anything is created on the other side of this insurmounatble barrier. (I.e. I light a match and all of a sudden there’s light that wasn’t there before.)
