One can also work out that anything without mass must travel at this fastest possible speed c. If the speed of light were different, light would propagate at that speed instead.Ī more explicit answer is found in ZBoson's comment: Light propagates through space at the maximum possible speed.
In this follow up, the answer to your question is hinted at, but not directly answered: Because the phrase "faster than light," in our universe, is exactly equivalent to the phrase "straighter than straight," or "more horizontal than horizontal." It doesn't mean anything. That's why nothing in our universe can go faster than light. Either the arrow points straight to the right or it doesn't, and once it does, it can't be made to point any straighter.
It isn't even really meaningful to think about something as being "more horizontal than horizontal." Viewed in this light, the whole idea seems rather silly. There's no rotation you can apply to that arrow to make it point more in the horizontal direction. There's no way to go faster through space. So you've increased your velocity through space as far as it can go. See, what happens if you want to move through space as fast as possible? Well, obviously you rotate the arrow - your four-velocity - until it points straight along the horizontal axis. This also explains why the phrase "faster than light" has no meaning in our universe. To answer the second, see this quote from the post you referred to: I'm not particularly qualified to answer either, but I can read. Your first question is asking why light is the only thing that goes that fast while your second is asking why something can't go faster. Basically, extending the metaphor from RRC's post, we have a "universal speed limit". "Why is light the fastest thing in the universe?" and "How do we know that there isn't faster than light can?" are two very different questions. They are qualitatively different, and so I remind myself of that by thinking of motion and propagation as separate phenomena.īut the upshot is valid: Light propagates through the vacuum at the fastest possible speed because in the vacuum, there's nothing to slow it down. I just find it useful to consider the progress through space of massive things as being qualitatively different than the progress through space of massless things. The distinction between "motion" and "propagation?" In the context in which I'm speaking here, it's purely a semantic one. Change back to light-seconds and seconds, and c is once again sensibly and sanely equal to one. But that's just because you're working in a stupid system of units. If you use units other than the correct ones - for instance, if you decided to be stupid and use the meter and the second - you'd find that the numerical value of the speed of light is some oddball number. It's equal, in other words, to one, when you use appropriate units for distance and duration. In our universe, that speed of propagation is equal to one light-second per second. Instead, it just continues propagating as fast as it can, because nothing interferes with it. Well, in the vacuum, light doesn't interact with anything, so it's not delayed at all. Glass is denser than air, so light is delayed more frequently as it propagates through glass than when it propagates through air. In essence, every time light interacts with matter, it gets delayed slightly. Light that propagates through glass, for example, does so at a rate slower than the rate at which it propagates through air. The rate at which light propagates depends on what its propagating through. (I reiterate that what I'm describing right now is philosophy rather than physics, so please bear that in mind.) Instead, it propagates. Light, on the other hand, does not "move" in the same sense. Just take it, for the moment, as an axiom. Again, there are geometrical reasons for this that I won't bother to elaborate on right now. The speed of a thing that moves can never be measured, by anything else that moves, to be greater than c. Of course we know that "speed" is only meaningful when described relative to something, so that naturally raises the question of relative to what is that speed limited to less than c? The answer is relative to anything.
Things that move are limited, for a variety of reasons involving geometry that I won't bother going into right now, to speeds less than the speed of light. It's a subtle distinction, and one that's frankly not all that well grounded either in theory or observed fact, but it helps focus the mind. This becomes a bit clearer, at least to me, when you stop thinking of light as something that moves and start thinking of it instead as something that propagates.
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