What if Lightspeed were not constant.

[KennyC]

I know it's blasphemy, but what are the reasons (other than Einsteins assumption) that lightspeed must be constant. It's always bugged me that this one quantity is "nailed down" and it screws up everything else...time, space, mass...

 

What if it were not constant but actually did vary. Couldn't we look at something else being the "universal constant" and see what would happen with all the equations?

 

I'm sure there is more to it than I know, but someone please tell my why lightspeed has to be a constant.

 

KAC

[Ophiolite]

It doesn't have to be a constant. It's just that when you make it a constant lots of other things fall nicely into place, which didn't when you allowed its speed to be relative. So having a constant light speed works better for all kinds of physical theories than having a relative light speed.

[KennyC]

It doesn't have[/i'] to be a constant. It's just that when you make it a constant lots of other things fall nicely into place, which didn't when you allowed its speed to be relative. So having a constant light speed works better for all kinds of physical theories than having a relative light speed.

 

Oh, I understand that. That thing that I keep wondering though is what we might discover if we reworked all those equations holding something else constant, like space or time... I certainly don't have (or have lost) the mathematics necessary to do it.

 

KAC

[fermions]

anyway I've heard about the varying speed of light theory... it seemed to suggest that we don't need an inflation theory to explain why the universe is like what it's like at present... and also that energy is not conserved

has anyone heard about it?

[Gilded]

Hmm, a rather interesting topic. Wasn't there a thread where someone explained how lightspeed can have anything at all to do with for example, the energy that's in a 30g muffin? If not, how? :F

[Radical Edward]

The critical thing about the speed of light being constant postulate, is that it is only a spatially and temporally local postulate. i.e. the speed of light is the same for all observers, right here, right now. The simplest explanation for this I can give, is the way we derive the speed of light which we do from Maxwell's equations. It turns out that c = 1/(sqrt(epsilon*mu) where epsilon and mu are the permeability and permittivity of free space respectuvely. Both of these constants are invariant with the velocity of the observer, and so it follows that the speed of light is also invariant with the velocity of the observer. If Maxwell had lived a bit longer, odds are that he would have cracked (special) relativity before Einstein did, because much of his investigation was headed in that direction anyway.

 

None of this however stops the speed of light being different somewhere else, or at a different time. However there would be observable ramifications if this were the case. For example, the energy of a photon is closely linked to the wavelength, and thus the speed. since the wavelength is dependent on speed, changing the speed would alter the spectrum emitted by atoms. It would also alter other constants, such as the fine structure constant, which would impact on fusion and fission rates and so on. none of these things are observed and so it appears that c is not only the same for all observers at a particular spacetime location, but the same for all observers everywhere and everywhen.

[swansont]

c varying in time leads to energy not being conserved.

 

c not being the same for all observers means that Maxwells equations no longer satisfy the wave equation (as RE touched upon). Which means that your radio no longer sees an EM wave if you are moving with respect to the source.

 

Lots of weird stuff would be happening.

[KennyC]

anyway I've heard about the varying speed of light theory... it seemed to suggest that we don't need an inflation theory to explain why the universe is like what it's like at present... and also that energy is not conserved

has anyone heard about it?

 

Any references? Links?

 

KAC

[KennyC]

c varying in time leads to energy not being conserved.

 

c not being the same for all observers means that Maxwells equations no longer satisfy the wave equation (as RE touched upon). Which means that your radio no longer sees an EM wave if you are moving with respect to the source.

 

Lots of weird stuff would be happening.

 

Good thought, but what if something else was changing to compensate. (kind of like shrinkage and mass change at relativistic speeds that are invisible in the observers frame of reference). That's exactly the equations/results that would have to change to allow for it, it seems.

 

KAC

[ed84c]

C in object= C/Refraction Index

[Janus]

C in object= C/Refraction Index

 

No, c stands for the speed of light in vacuum. When you talk about the apparent speed of light due to refractive index, this can not be refered to as c.

[TrueHeart]

I know exactly why light speed has to be the same for all observers. Light (EM radiation) is the quintessential mediator of all that exists, so her unique speed is inviolable and expressly tantamount to an infinity. Note that infinity plus any number, minus any number, times any number or divided by any number, is still infinity. Hence, light speed will seem to be uniform among all observers. In the number line that is speed, c is infinity... Einstein said as much. Speeds in excess of c are 'undefined'. True, in the study of cosmology, as described by General Relativity, speeds in excess of c are defined -- but all that is a fudge... if you study the formulae of cosmology, there are a lot of expedient compromises employed to arrive at useful descriptors of the whole universe. It's a cheat so to speak; SR is the purist part of relativity.

 

Furthermore, if light were not the same to each observer... if light moved relative to some master cosmic space/aether, then where would that leave all the poor hapless folks who live on all those zillions of galaxies that are ever speeding away from us in every which direction we look, hmmm?? It would leave them up the creek without a paddle; their worlds would be ever distorted and uninhabitable; their surroundings would be all hot and blue-shifted in one direction, yet pale and red-shifted in the other. Not only people (aliens) but physics itself would come to a crashing halt, I think, if that were the case. We would no longer live in a wide wonderful world.

 

'Not to mention, if light did not behave as relativity describes it, then we would live in a world where objects could OUTRUN being seen. And worse yet, we would live in a world where objects could smash into you with devastating impact, before you would even feasibly have the opportunity to see them coming. That just ain't right!

 

edit: you were right, KennyC, it would be "blasphemy" indeed if light failed to behave in its known relativistic manner... good word!

[fermions]

http://arxiv.org/abs/astro-ph/0305457 <- one of the few links about VSL

anyway isn't there always news saying that light maybe travelling faster in the earlier times of the universe and it's been slowing down?

[KennyC]

http://arxiv.org/abs/astro-ph/0305457 <- one of the few links about VSL

anyway isn't there always news saying that light maybe travelling faster in the earlier times of the universe and it's been slowing down?

 

Thanks!

KAC

[[Tycho?]]

If you measure the speed of light in vacum, you always get the same value, no matter how you move.

[maverick88]

'']If you measure the speed of light in vacum, you always get the same value, no matter how you move.

 

But what is the reason for it?

[goodyhi11]

from my understanding is that when the light travels at a constant speed, when it come near by a big massive object, the light will be going toward the massive object which cause the light to curve.

[5614]

when light goes from one density medium to a different density medium it changes speed...

 

"it changes speed"... is this explained because of the point of observation or what?

[Giacomo525]

because in a vacuum there's nothing to slow it down

[5614]

what?

 

i asked how do you explain light "slowing down" when it enters a relatively more dense medium from what is was previously travelling through.

[Janus]

what?

 

i asked how do you explain light "slowing down" when it enters a relatively more dense medium from what is was previously travelling through.

 

Light is not actually "slowed down" as it passes through a medium' date=' it is "delayed".

 

As light travels through a medium the photons are constantly being absorbed and re-emitted by the atoms of the medium. There is a time lag between the absorption and emission(during this time the photons are stored as an increased energy state of the atom, and do not exist as photons), that leads to the apparent "slow down" of light. The photons still travel at c when between atoms.

 

A loose analogy would be a car that travels at a constant 60 mph while moving. On a freeway such a car takes 1 min to cover 1 mile. Then the same car enters a city and has stop lights to deal with. The car still travels at 60 mph between lights, but has to spend a part of its time waiting at lights. Thus it will take more than 1 min for the car to travel 1 mile. Its [i']apparent[/i] speed drops below 60 mph, even though it never travels slower than 60 mph.

[5614]

oh yeah... thanks for the whole post, but the 1st line made me remember, i thought i knew the answer and the word "delayed" just triggered my memory.

 

now this i dont think i know the answer to... WHY does this photon delay cause it to change direction?

or is that just due to the angle of photon emission?

[Giacomo525]

sorry 5614, I wasn't responding to you...

[[Tycho?]]

I know it's blasphemy' date=' but what are the reasons (other than Einsteins assumption) that lightspeed must be constant. It's always bugged me that this one quantity is "nailed down" and it screws up everything else...time, space, mass...

 

What if it were not constant but actually did vary. Couldn't we look at something else being the "universal constant" and see what would happen with all the equations?

 

I'm sure there is more to it than I know, but someone please tell my why lightspeed has to be a constant.

 

KAC[/quote']

 

Its not based on an assumption. If you actually get some apparatus, and measure the speed of light in vacum, its always c. It was that it was constantly measured this way which caused Eiensiten to come up with relativity, not relativity that assumed c was a constant.

[swansont]

'']Its not based on an assumption. If you actually get some apparatus, and measure the speed of light in vacum, its always c. It was that it was constantly measured this way which caused Eiensiten to come up with relativity, not relativity that assumed c was a constant.

 

Yes, but Einstein still had to postulate it. It already had to hold in E&M applications in order for Maxwell's equations to work, so it wasn't too much of a stretch. But it has some bizarre implications for the uninitiated.