Is the constancy of speed of light is an ultimate reality?

[Zakkie]

Over the history of Science nothing could prevail as ultimate. Theories have come,governed and lastly have vanished. The Einstein has based his entire thought on fact that speed of light in vacuum is constant and nothing can exceed this limit. Up to now it stands the tests and no objection to it has a sound justification.

Can this be regarded as an ultimate reality that is not going to be changed?

[Mechsmith]

No, The speed of light "to an observer" is subject to many variables. I regard it "c" as just another observer effect. If you are just getting into it you can google things like "red shift", Hubble Constant, Relativity, etc. to get you started.

For instance you will find out that the speed of light is related to the speed of time which is related to the speed of gravity which is related to mass which is related to density which is related to distance.

Probably why they called them "relativity theories"

[Atreyu]

I don't think that Mechsmith's reply was apropos. Of course he is right that the speed of light in a vacuum, c, is only an absolute and fixed constant for us, and that an entity with a different psychic apparatus might perceive the same phenomenon (light travelling in a vacuum) quite differently. But I think Zakkie was only asking if 'c' is an "ultimate reality" in the sense of trying to understand the Universe, and in this context the absolute nature of light, or anything else, is irrelevant. The only thing that matters is that 'c' is an absolute and fixed variable for human beings, and it is we who are trying to explain the Universe in an intelligent and comprehensive way. So for all practical purposes indeed 'c', the speed of light in a vacuum, is not going to change.

What will change is how this fact is integrated into various explanatory schemes, how important a variable it is in any scheme, etc. But the speed of light in a vacuum is not going to ever change, just as 2+2 will ever equal 5. Both are "facts", and it's not the "facts" that change over time, it is how mankind deals with those facts, how those facts are interpreted, analyzed, and integrated into various schemata....

[Mechsmith]

Hi Atreyu,

I occasionally try to point out that the value that we call "c" is dependent on other things. We have declared that value "c" which then we can use measure other things. As a consequence we can use it now to measure a kilometer. We can also use it to measure time. Now when we use it to measure time we must also measure a kilometer to make a useful measurement. Now it gets interesting. Take a flashlight and a meter stick to Jupiter. Uh-Oh. The light turns a bit bluish. ( the light is blue shifted due to the gravity field of Jupiter.) Now there are more waves in the length of the meter stick) Now we have to figure what has changed. What has changed has the length of the time interval. This is basically why we invented "c". "c" is the speed of light to an observer. This allows us to reconcile everything that we can see to understandable terms. Mostly used to reconcile time and distance. You also build black holes with it.

The speed of light "c"has been DECLARED a constant. It's probably not constant

[Sy Borg]

A number of physicists have proposed a VSL (variable speed of light), although it's not mainstream.

[Mechsmith]

Greta, I had recently run across an article showing that different wave lengths may travel at different speeds. This, if true, would change a few conceptions as to the nature of space and time.

[Sy Borg]

The observations so far have been that the different wavelengths travel at different speed through different media, but they travel at the same rate through a vacuum. However, the vacuum of space has been found to not be "nothing" but is full of potential energy. If space is not a true nothingness then perhaps there is enough substance to it to affect the speed of light, albeit at extremely subtle levels?

[Atreyu]

I occasionally try to point out that the value that we call "c" is dependent on other things. We have declared that value "c" which then we can use measure other things. As a consequence we can use it now to measure a kilometer. We can also use it to measure time. Now when we use it to measure time we must also measure a kilometer to make a useful measurement. Now it gets interesting. Take a flashlight and a meter stick to Jupiter. Uh-Oh. The light turns a bit bluish. ( the light is blue shifted due to the gravity field of Jupiter.) Now there are more waves in the length of the meter stick) Now we have to figure what has changed. What has changed has the length of the time interval. This is basically why we invented "c". "c" is the speed of light to an observer. This allows us to reconcile everything that we can see to understandable terms. Mostly used to reconcile time and distance. You also build black holes with it.

The speed of light "c"has been DECLARED a constant. It's probably not constant

I gotcha. Good points.

But all I was saying was that I don't believe we'll ever to be able to witness the speed of light in a vacuum at a speed other than 'c'. And this is because I'm assuming that we, the observers, are not going to change in a fundamental way.

So, since we are what we are, the theory that "c" is the absolute and fixed speed of light in a vacuum is not going to change....

[Surreptitious57]

Greta : the speed of light is variable but only outside vacuum. For example when it passes between any two mediums of different densities such as air and water. Water has a higher density than air. And so light will slow down when it enters it and vice versa. This is why refraction occurs. However in space it always travels at c since that is regarded as vacuum

[IllicitTranslocator]

So basically, the speed of light in a vacuum is a constant, but it can be slowed down; it's a little like a speed limit. My question is, is there a way/material that would slow down light enough that we could observe the light passing through little bits at a time, and whether or not, as you zoomed in, would it be a smooth transition, or would the transfer of light be somewhat "jumpy," because of the way light is transmitted through being absorbed and re-emitted as it passes through a thing. Also, how does the light actually take the slowest path? We are taught that its a little like a 2 wheeled lawn mower, moving from concrete to grass, and as it slows down on one wheel, the the path bends toward the normal line. However, light isnt at all like a lawn mower, so how does light know to take the shortest path etc.

[Rbwinn]

I have studied the constant speed of light in some detail. In working the problem the way Einstein did, there are a couple of things Einstein and scientists after his time have seemed to overlook, which can be seen in the equations used before Lorentz and Einstein and the equations used after. Before 1887 the equations used to describe relativity were the Galilean transformation equations with Sir Isaac Newton's absolute time interpretation. The Michelson-Morley experiment blew that out of the water because the speed of light measured c in a moving frame of reference the same as in a frame of reference at rest. So Lorentz proposed substitution of his equations for the Galilean transformation equations with a length contraction in the moving frame of reference, which was modified by Einstein in 1905 to include time dilation, resulting in Special Relativity. In Einstein's words, "As judged from this (moving) reference-body, the time which elapses between two strokes of the clock is not one second, but 1/sqrt(1-v^2/c^2) seconds, i.e. a somewhat larger time. As a consequence of its motion the clock goes more slowly than when at rest."
Taking this definition of time, it appeared to me that what Einstein had was an approximation based on seeing the speed between frames of reference the same from either frame of reference, whereas, reality would seem to indicate that an observer in each frame of reference would get a different speed from the other if the rates of their clocks were different. For example, in the Hafele-Keating experiment, if a clock was flown in an airplane in the direction of the rotation of earth, the clock on the airplane was slower than a clock on the ground, in accordance with the effects of special Special Relativity according to scientists. If a clock was flown opposite to the direction of earth's rotation, it was faster than a clock on earth because of the effects of gravitation in accordance to the equations of General Relativity. But in either case, how would an observer on the airplane perceive the speed of the airplane compared to an observer on the ground? So we come to a divergence of opinion. Scientists of today would say that they get exactly the same speed between frames of reference because that is what the equations they use show. Common sense, which Einstein said needed to be abandoned if relativity was to be understood, tells us that if a clock is slower on the airplane, the pilot using the time of that clock will get a faster speed for the airplane than an observer on the ground using a clock on the ground will get. Or for an airplane going the opposite direction, the pilot would get a slower speed for the airplane than an observer on the ground using a clock on the ground. The difference between scientists and their scientific time approach to the problem and the common sense approach I just described seems to be in the equations being used. Scientists are using the Lorentz equations and the equations of General Relativity which say that the speed must measure the same from either frame of reference. People using the common sense description are using the equations scientists threw away in 1887, the Galilean transformation equations. To show how the Galilean transformation equations work in this application, let us say that

x'=x-vt
y'=y
z'=z
t'=t

x is a coordinate in S, the frame of reference at rest, x' is the same coordinate in S', the frame of reference in motion. v is the speed of S' relative to S, t is the time of a clock in S.
But according to scientists we have a clock in S' that is slower or faster then a clock in S, depending on which way around the earth it is going. Since t' is already defined to be equal to t in our first set of equations, we cannot use t' to represent the time of the clock in S'. The axioms of algebra require us to use a different variable to represent the time of that clock, so we use (t2)'. Now we use the Galilean transformation equations again with the time of the clock in S'.

x'= x - (v2)(t2)'
y'=y
z'=z
(t2)' = (t2)

The last equation of the first set of Galilean transformation equations, t'=t, shows that the time of the clock in S is being used in both frames of reference in this set of Galilean transformation equations. The time of the slower or faster clock in S' is not even being considered. In like manner, (t2)' = (t2) in the second set of Galilean transformation equations indicates that the time of the clock in S' is being used in both frames of reference, and the time of the clock in S is not being considered at all. Consequently, we have different speeds between frames of reference, (v2) in the second set as compared to v in the first. My opinion is that these sets of Galilean transformation equations give a correct interpretation of relativity, as compared to the length contraction idea that Lorentz and Einstein used. But to show how the Lorentz equations relate to two sets of Galilean transformation equations and why they work for scientists, we do what Einstein did and set the speed of light equal to c according to the time of either clock. This means that light will be traveling at 186,000 miles per second as measured in either frame of reference. First of all we have to consider that we are really talking about velocity of light because a photon traveling in the -x direction in S has a velocity of -c. But we will just consider a photon traveling in the +x direction the way Einstein did for the purposes of this discussion. If x is the coordinate of the photon in S, then x=ct, and if x' is the coordinate of the photon in S', then x'=c(t2)' because the clock in S' is being used to measure the speed of the photon. So now we put these values into the Galilean transformation equations.

x' = x - vt
c(t2)' = ct - vt
(t2)' = (t -vt/c) = (t-vx/c^2), where x is the coordinate of the photon

Consequently, the Lorentz equations give the same ratio for the x' coordinate of the photon divided by the (t2)' coordinate using that Einstein obtained in the Lorentz equations using x' and t'.

x'/(t2)' = (x-vt)/(t-vx/c^2) = (x-vt)gamma/(t-vx/c^2)gamma

gamma = 1/sqrt(1-v^2/c^2)

By the same token the differential equations Einstein used for the Lorentz equations to derive the equation E=mc^2 are the same as are derived from this interpretation of the Galilean transformation equations.