Does light accelerate to C or does it start off at C? Does the fact that light has a limit to it’s speed indicate anything about light? Such as does it have some kind of mass? Or is it passing through something that is keeping it’s speed constant?
IANAP, but it’s my understanding that a lot of confusion is the result of calling it the “speed of light,” and that it really should be called the “speed of causality.” Once you understand the speed of causality, the speed of an electromagnetic wave in a vacuum will make sense. (Or at least it’s supposed to make sense.)
The speed of light is always C. It cannot go slower or faster (so no acceleration).
As we measure it that speed can change in a different medium (e.g. vacuum vs water vs glass etc) but the speed of light is a constant.
So, it starts off at C and stays there.
I won’t claim to understand this well, so pending someone who does who can give a good intuitive picture, I’ll just link this for now…
I guess it needs to be said the speed of light in a given medium is always C in that medium. That speed can differ in different mediums.
Does light have a deceleration curve when it enters a new medium? I do not think so. Isn’t that a reason for something like Cherenkov Radiation?
I don’t think this is definitionally correct. The value of c is constant. We don’t say that c changes according to medium.
Ok…how do we reconcile light moving at C and measure its speed differently in different mediums? (really asking)
c is a fundamental constant. Light travels at c in vacuum, so the constant is loosely referred to as the speed of light. But when propagating in some medium, light does not travel at c.
In other words, c is not defined as “the speed of light in whatever medium is under consideration”.
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And since the OP asked about mass, c is the speed one would expect of anything massless. Gravitons, for instance, are also believed to be massless, and so gravity also propagates at c. And neutrinos aren’t quite massless, but they’re really, really close, so they travel at very close to c.
When light slows down when passing through a medium, on the micro level that’s still light traveling at c in between interactions, but frequently interacting and getting briefly absorbed and re-emitted.
I don’t really understand the term “emitted” when light is emitted there doesn’t seem to be any kind of propulsion involved. I just can’t seem to grasp why C is so consistent.
Here is a nice explanation of the “slowing” of light through a material (no math!).
This is a different phenomenon that the laboratory experiments that slow light to a few miles an hour and even “stop” it. That is quantum mechanical in nature and involves a phenomena called electromagnetically induced transparency (EIT). In this scenario, the photons are being “stored” in atoms and released with delay, but with the same quantum properties.
We should remember here that if the speed of light is constant it must be the other variables that change.
Speed = distance/time
So, if speed is constant (no acceleration or deceleration), distance and/or time become variable (and both do).
Another way of looking at the question is to ask in spacetime, not just space. In spacetime, everything is travelling at c all the time. Indeed it is the only speed anything can ever travel at. What differentiates things is what part of your travel is in space, and what part is in time. When something accelerates to a different speed in space, it changes the proportion of its travel in time with travel in the space dimensions. We mostly perceive the world as travelling through time at the speed of causality. Time passes at one second per second. If you want to cause an event 10 seconds in the future, it will take 10 seconds minimum for your action to take effect.
Light never travels in time, it only ever travels in space (same as any other massless thing.) It doesn’t accelerate from the point of emission, it is always only ever travelling in space, and thus from any viewpoint, travelling in space at the speed of causality.
This.
There’s a limiting speed at which information can travel, called “c”. Information can cause actions, so causality cannot travel any faster than c, also. There are “things” that can travel faster than c, such as the spot where a laser beam hits a wall when you spin the laser around. But that “thing” can’t convey information. There’s nothing you can do to that spot of light when it goes past you that will change how it looks further along the wall. Likewise, you can squirt water out of a hose at some speed, and swing the hose around so that the spot where it hits the ground travels, but the water hitting the ground isn’t going any faster, just the location, the intersection of trajectory and ground.
Now, because of the relativity of Einstein and Lorentz, people moving at different speeds will observe different things (after they take account of the travel time of the light they’re looking with of course). A “thing” moving faster than c will, according to people moving differently, do some odd stuff like be in three places at once, or get to the destination before it leaves the origin. As the spot where the water hits the ground can do. But we’re talking about a “thing” that can’t convey information from one location to another.
So, all of this is about the speed c. And many things move at c, at least when they’re not interacting with anything such as a medium they have to travel through. All electromagnetic radiation does this, so, light, infrared, ultraviolet, X-rays, gamma rays, radio, and for that matter changes in electric or magnetic fields. Gravity waves also do this.
Light traveling through glass moves more slowly. But, it might be fair to stop calling it light while it’s in the glass, as it now has to involve moving electrons around. That’s why the factor by which the propagation slows in a medium is roughly proportional to the product of its density and its dielectric constant.
Thus, “speed of light” isn’t a great way of naming c. And, c doesn’t change in glass or any other medium, even though the speed at which light propagates does.
This c is something much deeper than just a speed. It’s also the square root of energy over mass and thus sets the rate at which mass is consumed in for example a nuclear bomb explosion. It’s possible to use high school math and derive this (I have the notes on how to do it, though I don’t remember how to do it any longer).
I am, by the way, a physicist (though a pretty mediocre one).
e = m c2
Only… we’ve never actually measured the speed of light, at least according to this guy:
This video does a good job of illustrating how it works. I think this plugs into what @Francis_Vaughan was talking about in post #14 above.
Just adding my concurrence that, as already said, this is the key concept.
Another way of looking at it is that time dilation as expressed in special relativity involves the crucial factor (1 - \frac{v^2}{c^2}) . This tells us that photons experience zero time, and from a photon’s frame of reference, it arrives at its destination instantaneously.
The concepts of causality, simultaneity, and time dilation at c all tell us the same thing: that the speed of light is in a sense infinite, in that it travels as fast as the most basic laws governing spacetime allow. If you think about it from the photon’s reference frame, you cannot arrive anywhere faster than instantaneously.
Does that mean a photon is everywhere at once? Or that the universe has no size since it can travel from one end to the other in no time?
I don’t mean to be crazy about this but, if I turn on a light in my home, are those photons everywhere in the universe?
Better still, has this post affected the whole universe? 
No, from your reference frame, the photon is only travelling at c, so it will take (for example) almost 4.4 years to get to Alpha Centauri. It’s just that if you dig a little deeper into the underlying physics, it’s clearly impossible (and indeed meaningless) to speak of any speed faster than c. It’s a very fundamental constant of the universe. The fact that it happens to be a rather specific number expressed in MPH is purely a happenstance arising from our system of measurement.