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RobC
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Why does light move at the speed it does, why not half the speed or double?
RobC said:Why does light move at the speed it does, why not half the speed or double?
RobC said:why does it not travel at 150000 m/sec or 600000 m/sec ? Why 300000 m/sec ?
It has as much significance as the conversion between miles and kilometers. In some places in everday life (mainly the US) you use miles, elsewhere you use kilometers, and if you want to convert between the two you need this conversion factor. The conversion factor is completely arbitrary and based on the definition of kilometers and miles.RobC said:I have a feeling that there is something that we are missing and discovering why light travels at the speed it does has some fundamental significance,
Because the BIPM committee met and voted to define the meter that way. There is no physical reason, it was just an arbitrary decision by a committee.RobC said:What I am saying is that why does it not travel at 150000 m/sec or 600000 m/sec ? Why 300000 m/sec ?
RobC said:I have a feeling that there is something that we are missing and discovering why light travels at the speed it does has some fundamental significance,
That is the question answered. If you stick to one particular set of units then the speed of light is what it is and not a different value because you have chosen those units and are sticking to them.pixel said:I think he is asking why, if we stick to one particular set of units, the speed of light is what it is and not a different value.
Dale said:That is the question answered. If you stick to one particular set of units then the speed of light is what it is and not a different value because you have chosen those units and are sticking to them.
Yes - and that point is reached when you get to the value of one the unitless constants that PeterDonis mentioned in post #4.pixel said:So the speed of light in vacuum is determined by these properties of vacuum. You could of course then ask why these properties are what they are, etc. and will reach a point where you just have to accept that that's the way the universe works and "why" questions can't always be answered.
But "as fast as it is and not faster or slower" refers to the value that the speed has. The only property it has is that it is invariant. "Faster or slower" is not a property but a value, and hence is just a question of units.pixel said:but I don't think it's so much about the value of the speed of light as the property of speed that it has. In other words, what makes light as fast as it is and not faster or slower.
Those are also artifacts of the SI system of units and are set arbitrarily by a vote of the BIPM committee. Those constants don't even exist under other unit systems.pixel said:The speed of an em wave is given by 1 divided by the square root of (με) where μ is the permeability and ε is the permittivity of the medium. So the speed of light in vacuum is determined by these properties of vacuum.
Dale said:For example, you might want to know why the speed of light is 27 million times faster than the airspeed velocity of an unladen swallow. That is a dimensionless ratio and if you dig down enough, that will depend on the dimensionless fine structure constant, not the dimensionful speed of light.
Again, that is actually a question about the fine structure constant. The dimensionful constants, like c, factor out.pixel said:Suppose I want to know why the speed of light is not, say, 30 million times faster than the airspeed velocity of an unladen swallow instead of 27?
Dale said:"Faster or slower" is not a property but a value, and hence is just a question of units.
Depends on the frame of reference.pixel said:A pitcher throws a ball and it takes a certain amount of time to reach the catcher. The next pitch arrives in less time. Is it not faster?
"certain amount of time"pixel said:A pitcher throws a ball and it takes a certain amount of time to reach the catcher. The next pitch arrives in less time. Is it not faster? Has anyone mentioned units?
Dale said:"certain amount of time"
Using any units is not the same as not using units.pixel said:Yes, and using any units for that time.
The ratio between those two speeds is dimensionless because the units cancel. As with the question about the ratio of the speed of light to the speed of an unladen swallow posed above, if you grind through the physics (this will involve some serious biomechanics and biochemistry because a pitcher's arm is a very complex system, unsuited for thought experiments) you will eventually find that that ratio is what it is because of the value of one or more of the fundamental unitless constants of nature.pixel said:A pitcher throws a ball and it takes a certain amount of time to reach the catcher. The next pitch arrives in less time. Is it not faster? Has anyone mentioned units?
pixel said:I think he is asking why, if we stick to one particular set of units, the speed of light is what it is and not a different value.
In other words, why is light as fast as it is. Of course, there's no answer to that.
RobC said:Why does light move at the speed it does, why not half the speed or double?
PeterDonis said:A question that's more about an actual physical constant is a question about a dimensionless number, such as the fine structure constant--that's the one that is most closely related to the speed at which we observe light to move. But if you ask why the fine structure constant has the value it has, the answer is that we don't know: we don't have a theory that predicts that it should have a particular value, we just measure the value it actually has. (This measurement does not depend on our choice of units, since the fine structure constant is dimensionless.) So that's probably the best answer to the underlying question you are asking.
Mass is the modulus of the energy-momentum four vector, which is tangent to the particle's worldline. If the mass is non-zero then the particle's worldline is timelike. If it is zero then the particle's worldline is null.nitsuj said:Why must a mass-less particle travel at c...what about the relationship of momentum, mass and energy means a photon must "go" c?
Does anything about that relationship imply that light cannot go less than c, or faster?
k, just checking that light moves at the perfect speed!Ibix said:Mass is the modulus of the energy-momentum four vector, which is tangent to the particle's worldline. If the mass is non-zero then the particle's worldline is timelike. If it is zero then the particle's worldline is null.
So "it has zero mass" is, in relativity, just another way of saying "it travels at the speed of light".
I agree. On the simple side, if the speed of light is not infinite, then it must have some speed,c. But that leads to the question of why the speed is c. A more complicated answer is that c is related to other constants of electromagnetism, μ and ε, as @pixel pointed out in post #12. They may not explain the constant c but they all go together in a coherent system. So the value of c does not stand alone, it is intimately related to the strength of fundamental electromagnetic forces.checksix said:Perhaps the answer that the OP was looking for is that the reason light travels at the speed it does, is because the universe is apparently constructed with a speed limit. Light, being massless, can travel faster than anything else, but it cannot travel faster than the speed limit. Physicists call this speed limit "C" ("celeritas" = Latin for "swiftness"). The actual units (meters per second, miles per hour, etc) are a human construct, but the underlying speed "C" is part of the structure of our universe.
Those are also just a question of units, set by arbitrary convention, with no other content. They don't even exist in some unit systems.FactChecker said:A more complicated answer is that c is related to other constants of electromagnetism, μ and ε,
This is the fine structure constant, at least in any physically meaningful sense.FactChecker said:it is intimately related to the strength of fundamental electromagnetic forces.
Many questions about magnitude are dependent on units. That does not make them all meaningless questions.Dale said:Those are also just a question of units, set by arbitrary convention, with no other content. They don't even exist in some unit systems.
FactChecker said:Many questions about magnitude are dependent on units. That does not make them all meaningless questions.
I don't think that the OP was asking why c was the magic digits 299792458. That is clearly dependent on units. He was asking why it is 299792458 m/s = 186282 mi/sec = etc. etc. etc. and not slower or faster. In any space-time coordinate system, the speed of light is determined and it is proper to ask if there is more logic connected to it than simply measuring it. Just saying that it depends on units doesn't tell him anything he didn't already know..PeterDonis said:No, it just means they don't always mean what the people asking them think they mean. The people asking them often think they are asking a question about physics, when they are really asking a question about human conventions like choices of units.
Because: http://www.bipm.org/en/publications/si-brochure/metre.htmlFactChecker said:He was asking why it is 299792458 m/s = 186282 mi/sec = etc. etc. etc. and not slower or faster.
I will not debate the concept of slower or faster with you. If someone asked you why a bicycle was slower than a 2000 horsepower drag racer, would you say it is because of units?Dale said: