If the Speed of Light changed, would we even notice it.

[WebFeet]

The most fundemental constant that we rely on in the Universe is the Speed of Light.

 

Many of our measurements are based on its invariant nature, those that aren't depend on a set scale of dimension.

We are well aware that dimensions can vary depending on the strength of the graviational field in which they exist. Time measured by an orbitting satelite will be different to that here on Earth. A metre ruler here on Earth would still measure 1 metre at the event horizon of a BlackHole, but would occupy less space - space being more compressed.

 

The amount of Mass in a metre rule, using e=mc^2, is inversely proportional to the speed of light.

If we used the metre ruler to determine a fixed length and measured how much time was taken for light to travel from end of it to the other, is this a valid way to measure it ?

After all, are we not essentially measuring the speed of light against itself.

 

If halfway along the ruler the speed of light decreased by a factor, then the mass would in turn increase and the elapsed time would show that along that metre rule, the speed of light is constant.

 

Hence my question.

 

If there were localised variations in the Speed of Light, given that our method of measurement is depedant on the Speed of Light, would we even notice it ?

[Will]

Wouldn't that be dependant on how extreme the difference was in speed?

[WebFeet]

Wouldn't that be dependant on how extreme the difference was in speed?

A variation could be from zero to whatever number you want to use, + or -.

So in this case size really doesn't matter.

 

The question is, if our method of measurement is in some way based on the speed of light, albeit through mass or any other association, would that not invalidate the method.

[Will]

it would. i don't know too much about this, but i'm assuming that we measure it based on the most commonly accuring instance. and what i mean by that is that we wouldn't use any speed of light as our constant, but we would use the instances that we see everyday or that are most common.

[Qfwfq]

Tricky matter. Essentially, velocity is a pure (or dimensionless) number and that of light is exactly 1. This is the point of view most appropriate to Special Relativity. A different choice means using different units for spacelike and timelike intervals and this boils down to a choice of space-time coordinates, which certainly has no effect if done globally. Doing it locally is a matter of differential geometry and is what General Relativity is all about. In GR, changing coordinates locally may or may not make a difference physically, the cases in which it does come across as changing the curvature of space, not as changing the velocity of light, unless of course you use a choice of units to this effect.

[WebFeet]

Tricky matter. Essentially, velocity is a pure (or dimensionless) number and that of light is exactly 1. This is the point of view most appropriate to Special Relativity. A different choice means using different units for spacelike and timelike intervals and this boils down to a choice of space-time coordinates, which certainly has no effect if done globally. Doing it locally is a matter of differential geometry and is what General Relativity is all about. In GR, changing coordinates locally may or may not make a difference physically, the cases in which it does come across as changing the curvature of space, not as changing the velocity of light, unless of course you use a choice of units to this effect.

 

Although I made mention of GR in my original post, the problem is not really to do with changing the curvature of space.

 

I'm sat here trying to think of a way of explaining it a little clearer.... it's not easy.

 

Essentially the rules of how our Universe works are bound to the constant Speed of Light. All methods of measurement are in some way related to this constant.

If the Speed of Light was not constant, then one of the things that would be effected is the methods we use for measurement.

As the Speed of Light varies, so everything that is derived from it would also be subject to the same variation, including the methods of measurement we use.

These variations would be syncronised between the speed of light and the method, resulting in a constant result.

 

An analogy would be if there was a constant that everything was blue and when looking for exceptions to this rule you had to look through a blue filter. The result being that everything would conform, there would be no exceptions.

 

I hope that makes sense, it sort of does to me.

[Will]

Well I'm still of the opinion that the measurement we use is of the most commonly occuring instance of the speed of light. The speed of light may change under certain conditions but for all practical purposes we measure the most frequently observed speed of light.

 

Now this is a problem because that means that all of our formulas are based on one instance of a variable. I don't know anything about these formulas, but whenever people have told me of formulas involving the speed of light to explain the laws of our universe, i ask them how it could be possible to use something like the speed of light (which is not the same everywhere).

 

I don't know if any of that made sense. It's late and I may not have phrased that properly, but I guess essentially what I'm saying is that I agree with WebFeet that using something like the speed of light in formulas poses a problem, since it is a value which is not constant. However my thoughts remain that when we apply the speed of light to our formulas we make it a constant by using the most frequently occuring instances of it (the speed of light, that is).

[WebFeet]

Well I'm still of the opinion that the measurement we use is of the most commonly occuring instance of the speed of light. The speed of light may change under certain conditions but for all practical purposes we measure the most frequently observed speed of light.

 

It's not the value that we use that I have issue with, but with how we measured it in the first place.

 

The Speed of Light in a vacuum is a constant. There is no variation as to its measurement, it always come out at exactly the value. Hence why it is known as the Universal Constant.

 

If every unit of measurement is some way relates back to the Speed of Light, then what we are saying is :-

Measure the Speed of Light with reference to... the Speed of Light. The answer will always be exactly the same.

[Qfwfq]

the problem is not really to do with changing the curvature of space.

It might not seem obvious to you, but it is. I hadn't misunderstood your point.

 

To better understand the link between locally changing the velocity of light and curvature, I guess you would need to fully understand differential geometry and GR.

[BlameTheEx]

We would have to propose characteristics of space that don't alter in proportion with C.

[WebFeet]

We would have to propose characteristics of space that don't alter in proportion with C.

Precisely.

Unforunately, all of our current references rely on C.

 

To better understand the link between locally changing the velocity of light and curvature, I guess you would need to fully understand differential geometry and GR.

You reference to GR is made on the assumption that C is constant. However all the measurements that you could make to verify the link between curvature and the speed of light would be made with reference to a unit, any unit, which has as its basis, the speed of light.

This should invalidate your results.

 

All experiments must be verified independantly.

Any experiment to do with measuring the Speed of Light cannot be verified independant of the Speed of Light.

 

edited : typos

[Qfwfq]

You reference to GR is made on the assumption that C is constant.

No it isn't. I was discussing local variations of c, which is what you are talking about.

 

However all the measurements that you could make to verify the link between curvature and the speed of light would be made with reference to a unit, any unit, which has as its basis, the speed of light.

The most natural choice is for velocity to be dimensionless. The canonical choice is also c = 1.

 

All experiments must be verified independantly.

Any experiment to do with measuring the Speed of Light cannot be verified independant of the Speed of Light.

Quite true for a global value. I was discussing the meaning of different local values.

 

At the most, it would equate to a warping of space time, it might not even equate to that. It's a scalar analogue to a gauge field. For a given physical space-time manifold you can make a gauge choice with some arbitrarity. Other choices would equate to a physical effect. A global choice won't equate to a physical effect.

 

 

We would have to propose characteristics of space that don't alter in proportion with C.

That is putting the horse before the cart. Ever since SR, c is a geometrical property of space-time, save for "odd" choices of coordinate frame.

[WebFeet]

No it isn't. I was discussing local variations of c, which is what you are talking about.

 

In mentioning GR, you are implying that C does not have local variations

It is a basic postulate of the theory of relativity that the speed of light is constant.

 

The most natural choice is for velocity to be dimensionless. The canonical choice is also c = 1.

 

If we were to use a value of c=1, the question then is how can we measure anything against it, seeing as how it is dimensionless.

The only way is a direct comparison. You fire off a photon in the same direction as the velocity you are comparing.

If the two travel at exactly the same velocity, then you can say that the object was travelling at the speed of light at the time of the test. Whether it would continue to travel at this velocity could only be determined by continuing the test.

 

All this tells you is that against your reference, the object was travelling at the same velocity. It tells you nothing about what actual vecocity your reference photon was travelling at.

 

This is the whole point of my question. As the properties of all matter are defined by the speed of light, how can we determined if its values changes when the propoerties of matter are directly related to the speed of light ?

 

I hope that makes sense. B)

 

 

Quite true for a global value. I was discussing the meaning of different local values.

These are no different from global values. After all global values are only the average taken over a series of local values.

 

At the most, it would equate to a warping of space time, it might not even equate to that. It's a scalar analogue to a gauge field. For a given physical space-time manifold you can make a gauge choice with some arbitrarity. Other choices would equate to a physical effect. A global choice won't equate to a physical effect.

That is putting the horse before the cart. Ever since SR, c is a geometrical property of space-time, save for "odd" choices of coordinate frame.

As mentioned previously, it is a basic postulate of the theory of relativity that the speed of light is constant.

If it isn't, and experimentally there is no way to validate it without using itself as a reference, then this does raise a question or two about Relativity.

 

The properties of an object would not only be relative to the frame of reference, but also to the speed of light in both frames.

 

It may mean :-

Time intervals and/or lengths change according to the speed of the system and the speed of light in that system relative to the speed of light in the observer's frame of reference

[Aki]

How exact/precise is really the speed of light? Can it be a little bit off?

[maddog]

How exact/precise is really the speed of light? Can it be a little bit off?

According to proponents of VSL it can be a little or a lot. However, one such person

was using this to replace Inflation Theory in Cosmology (João Miguello) who wrote

the book, "Faster than Light". In his theory C has like a phase transition, say like

water "freezing". This freezing state would slow C, the speed of light would slow

down by a fixed factor. During the inflation period this speed could have been

significantly faster. We are talking the first few seconds of the big bang. This is

so far only theory and no fact. In fact no evidence yet for either VSL or Inflation.

 

I can see in one respect that Webfeet's idea could work -- a restricted example.

Maybe not what he is thinking. It would work like this. We the universe to have

a mystical phase transition for the speed of C instantaneously to C/2 say. No one

would magically have recollection of what the measurement was before. Then

all future measurements would be against the new value C* = C/2. However,

were their memories in tact. Then people would detect the change and might

consider that space had contracted ot time had expanded. Either way it is not

likely people would consider the C itself had changed. What Qfwfq was saying

of the convention of C = 1 (since it being such a valid constant) that it can be

made dimensionless by proper factorization. I think you (Webfeet) missed that

point. It is a convention in Relativity and High Energy physics to do so. I myself

am still baffled by the what the point of all this is. B)

 

Maddog

[WebFeet]

What Qfwfq was saying of the convention of C = 1 (since it being such a valid constant) that it can be made dimensionless by proper factorization. I think you (Webfeet) missed that point. It is a convention in Relativity and High Energy physics to do so. I myself am still baffled by the what the point of all this is.

 

Maddog, how do you produce a valid value for the Speed of Light, before factorization, that does not rely on a method of measurement which is itself dependant on the speed of light ?

 

I'm saying to ignore factorization because you will have had to measure it in the first place before you can then apply factorization.

[paultrr]

The original VSL idea was to get around the need for inflation to explain the uniformity of the CMB itself. Certain variants on this tend to admixture the two since that time. One thing of note, using EM formula there is a solid reason the vacuum has the speed of C it does. But during inflation those conditions would be different. As such there is no actual way to know what the speed of light was in those first few brief fractions of a second. Best safest assumption is that C was different then. However, there is little evidence after that point about C changing, except that certain aspects of an accelerated expansion tend to imply that C may slow down after time, never speed up.

 

If such a phase transition happened in our present time with given vacuum conditions now there would be a signature of such within the CMB locally. You'd have regions that do not seem to relate or appeared not uniform.