Is the speed of light motivated by other factors, or is it an axiom in itself?
It’ a fundamental constant that defines our universe, like the gravitational attraction constant (how force between two masses). It just is.
Sped of light in a vaccuum, that is.
One thing I have heard is it’s the motion of the shortest ‘quantum’ distance (plank length?) in the shortest ‘quantum’ time unit (plank time?).
I have never measured or timed either personally, so I can’t vouch for their accuracy nor their true existence.
There’s a fundamental speed built into the fabric of spacetime called c. This speed c shows up all over the place in relativity calculations, and would be significant even if there happened to not be anything that actually traveled at that speed. Basically, if space and time are aspects of the same thing, then it should be possible to measure space with time units, or vice-versa, and if you were to do that, then speeds would be dimensionless. c is the speed that is equal to 1, in such units.
Well, one of the results of relativity is that any particle that has zero mass must travel at exactly c. Relativity itself is silent on the question of whether any such particles exist, but to the best of our ability to measure, the photon seems to be such a particle. So light travels at c.
Doesn’t gravity effect light? That would imply that light speed is variable, depending on how much space-time is bent.
But light never effects gravity…
Hmmm.
Nope, the speed is still constant.
In cases where gravity is strong enough (say, a black hole), any change is explained by relativity as a contraction of space and/or dilation of time. In a simple form… distance = speed x time. If you divide either side of that equation by half, you have to divide the other side by half. It just so happens that the speed part is never the part divided in half - it’s always distance or time. (Which, when thinking of the speed of light is probably redundant, since time is a distance, just in a dimension we’re used to thinking of differently than the other three.)
I don’t follow.
If speed * time = distance, then (1/2)speed * time = (1/2)distance.
I just divided speed, which you state is never done, and the universe has not imploded (and Zuul ordering us to worship of Gozer).
Anyway, from our view point, that black hole has a measurable radius. But that light that got trapped will never reach my eye. It’s speed (from my viewpoint) has reached zero, no matter how much you wish to modify the time reference.
Gravity does affect light, but in such a way that (assuming you define things in a way that makes sense in curved spacetime) the speed is still the same. And light affects gravity, too, though in practice it’s hard to get enough energy in light for it to be relevant.
Also, from kanicbird:
This is basically true, but it’s rather putting the cart before the horse. It follows from what I said about measuring space and time in the same units: There’s some speculation that not just space and time, but everything, can be put into the same units. If that’s so, then any physical value (not just speeds) can be expressed as unitless, and for any kind of quantity, there’s some value of it that’s equal to 1. Now, we have good reason to believe that c is the speed that’s equal to 1, but not really for any other types of quantities. Many people guess that Newton’s constant (big G, that determines the strength of gravity) and Planck’s constant (hbar, relevant for quantum mechanics) might also be equal to 1, and if you assume this, then the units you end up using for everything are the Planck units. If the nature of the Universe is such that there is some fundamental indivisible distance or time, then the Planck length or time are as good a guess as any for what those fundamentals are, but nobody even knows that there is such a fundamental distance in the first place. And the Planck units are defined right from the start such that the Planck length divided by the Planck time is c, but that’s because of what we know about c, not the reason for it.
“Hard” sounds vague. Any examples of this in nature?
Note that the speed of light is only a constant in a vacuum, and light travels slower through different materials. It is possible for particles to travel faster in a medium than light travels in that same medium, and Cherenkov radiation is emitted when that happens.
You can write the equation that way. You can also write that 2+2=5. Neither is consistent with experimental results.
I admit, it’s a grossly oversimplified attempt to explain relativity. They key is to understand that both time and distance are relative concepts, and they change to preserve the speed of light as a constant. I don’t really have time for more, but Wikipedia has some good resources you might start with.
This is not true at all. The speed of light is not constant and is effected by other things. This was first proven by Jean Foucault in 1850. Jean Foucault proved that light is slowed down by transparent media such as water, air, and glass.
The value of c (which represents the speed of light) is the speed of light when in a vacuum.
Now to answer the initial question. Why is the speed of light the speed of light.
The answer to your literal question is that we call it the speed of light because that is the speed light travels in a vacuum. It’s like saying why do we call a screwdriver a screwdriver. The answer is that we call it because it performs the function of driving screws.
To the question I think you were asking “why is the speed of light 299,792,458 meters per second”. To that question we need to put on our some pretty heavy math hats, but in the simplest of terms light in the most basic form can be called one of 3 things.
- electromagnetic waves (this is the correct answer)
- protons (mostly accurate)
- massless matter (when discussed as it’s relationship to relativity and special relativity
The speed of light is best calculated with special relativity in mind, and it helps to understand what exactly it means when someone says massless matter. However, to expedite the process I will cut lots of edges off and sum it up like this.
There are two forces at work here. The first is time and this is a representation of the forces that came together to form time and the big bang. In this relationship we know that the closer to the speed of light an object with mass goes the slower in time it goes. So what happens when an object without mass goes faster and faster and faster. Well this is where the limit on speed comes in. Light being the best equipped known “thing” to go as fast as possible in our universe is able to go up to 299,792,458 meters per second.
Which is why the speed of light is so important to science. It represents the max speed an object without mass can travel, and based on this fundamental principle we have learned about how time is warped by various forces. Especially gravity.
None that I know of, but I wouldn’t rule out the possibility of observable gravitational effects from the giant magnetic lobes of supermassive galaxies. And come to think of it, I suppose that the gravitational effects of light were also relevant to the evolution of the very early Universe, though that would have been at a time earlier than we’re able to directly measure.
Nice responses thank you. So it seems like the speed of light is basically an “axiom of the universe” at this stage in our understanding.
More or less. The way the laws of our universe appear to work is that the specific speed Light travels at is, say “C”. But light travels at that particular speed because it cannot go any faster. That is the only real speed limit. If you remember that E=MC^2, you can see the relationship between mass and energy. If you accelerate a mass-bearing thing (particle, object, whatever) at C, it has infinite energy (and would require infinite energy to accelerate to that point).
an teh uniburs do asplode…
However, a massless can move without requiring any energy. It has no mass to accelerate.
I don’t think that’s right. Its derivable from the fact that physics is the same in any inertial reference frame.
Einstein took both statements as axioms, but the consistency of the speed of light is actually a consequence of Lorentz invariance.
The speed of light is determined by the magnetic and electrical characteristics of the medium. There is exactly one speed at which a changing electric field causes a changing magnetic field that recreates the electric field a little farther on…this is a description of a wave propagating through space.
You can make the light move slower by increasing either the magnetic or electrical properties of the material. Changing the magnetic character usually makes the material opaque, so that is not commonly done. Changing the electrical character can be done, and optical glass is a good example. Because the light travels slower in the glass, the direction of the wave is changed at the air/glass interfaces, allowing lenses and prisms to work.
If you make the medium highly conductive, you can short out the electrical part of the wave, and cause it to reverse direction…a mirror. It is no accident that very shiny metals tend to be excellent electrical conductors.
ETA: For the purposes of this post, vacuum should be considered a medium.
Okay, just so I’m sure I have this straight and can then start to piece the rest of this stuff together, what I’m taking away from this is:
c=the maximum speed of light which is always constant.
However, the actual speed that light travels can be less than c depending upon the medium through which it is traveling.
Therefore, “the speed of light” can be constant or relative depending on the context of the discussion.
Is that more or less it (at least that part of it)?
Well, I would like to point out that Science (capital S, indicating the body of folks and opinions on the subjects of how shit works) was just as sure about the wrong answers we had as little as a century or so ago. And equally sure about the wrong answers we had two centuries ago, when the universe was only a few hundred thousand kilometers in radius, and light was pretty much sloshing around instantaneously.
We keep getting better and better at measuring stuff, and because all the terms we have for how much of something we’ve got are derived from other measures of what we had, we keep having to redefine what the units are. Light moving through a vacuum is just about the first invariant thing we found to measure, so we use it as a ruler. It turned out the Earth wasn’t forty million meters in polar circumference. It also turned out that the Earth doesn’t turn on it’s axis anywhere nearly as regularly as would be convienient for the mathematics of science (small s science, the method of investigation)
So, we took our current best approximation for what a second was: the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom and we decided that is exactly one second. (And the Earth can turn turn turn as it wants from now on.) Then we measure: the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second and we called that a meter.
The reason is: space just is that way, and time just is that way. The numbers work out that way because we defined them to work out that way. But, by using the precision we have developed by defining units that way, we have repeatedly examined and found that light in a vacuum always moves at that speed. (you can distort the space it moves through so it appears to follow a curved path, but it still moves at C)
Setting C equal to 1 gives you an entirely different set of units. The new units will measure the same characteristics of the universe that the old ones do. If we were still using furlongs, and fortnights the universe would still be the same. The physics class tests would be a whole lot harder, though.
Tris
The posts in the thread show different interpretations of the OP. To be sure, note:
(1) There is a fundamental constant of the universe c. It has units of speed because humans chose to measure one of the spacetime dimensions in different units from the other three. Somewhat confusingly, c is often called “the speed of light”. I suspect that the OP is asking why c has the value it does (a perfectly valid, if not imminently answering, question). Some of the replies have taken this interpretation.
(2) Light travels at c in vacuum. In something other than vacuum, light travels either at c or at some slower speed, depending on how you choose to define “light”. In most applications, it is sensible to say that light slows down in a material. Some of the replies have taken the OP to be about why light itself travels at whatever speed it does in whatever situation.