What Medium Does Light Travel Through in Space?

[bhobba]

I'm confused

My goof - wrong paper. Now fixed. The correct paper (by @Demystifier who often posts here) is:

http://thphys.irb.hr/wiki/main/images/2/2c/Casimir.pdf

Sorry
Bill

 

[bhobba]

Okay, I'll read what you referred to, but what about the Casimir effect, the Lamb shift, spontaneous emission, etc.? Surely these are real, yes?

For spontaneous emission, the issue is that, according to ordinary QM, the energy levels are stationary and remain the same. The solution is that, due to the electric field of electrons, they are coupled to the Quantum EM field permeating all space. This coupling means they are not stationary.

Thanks
Bill

 

[PeroK]

Don't try to tell me that SI units are a European conspiracy to try to confuse Americans!

 

[tech99]

Those are just artifacts of the SI units. They are not part of nature. They don’t even exist in some other unit systems.

Space is clearly physical, but not because of some unit specific constants. Space, or more precisely, spacetime is the geometry of physics. It does not need to be assigned any material properties to be physical.

Maxwell noticed that the "cgs" units of permittivity and permeability were related by a number equal to the speed of light. So that the electric force between two charges would be related to the magnetic force between the two charges when moving by a factor equal to c. This seems to me to be part of Nature.

 

[A.T.]

The speed of light (or more appropriately the speed of causality) may be an inherent limiting velocity of our 4D Minkowski spacetime, but it's also a specific value.

What do you mean by "specific value" here? Would the universe look any different to us, if that value was different? Or would everything that we use to measure be affected accordingly, resulting in observations indistinguishable from ours?

 

[Dale]

Maxwell noticed that the "cgs" units of permittivity and permeability were related by a number equal to the speed of light.

I am not sure which cgs units have that feature. In Gaussian units permittivity and permeability are both dimensionless and speed is not.

 

[GRQFT]

What do you mean by "specific value" here?

By specific I mean a particular numerical value.

Would the universe look any different to us, if that value was different?

Well, since the spacetime interval explicitly contains c (S^2 = c^2Δt^2 - Δx^2 - Δy^2 - Δz^2), I suspect that the the causal perception/description of relative events would change. But I'm not sure.

 

[Sagittarius A-Star]

By specific I mean a particular numerical value.

Well, since the spacetime interval explicitly contains c (S^2 = c^2Δt^2 - Δx^2 - Δy^2 - Δz^2), I suspect that the the causal perception/description of relative events would change. But I'm not sure.

A re-definition of "1 meter" (by assigning a different numerical value for $c$ in the SI-definition) would not change physics. But it will not be done, to avoid confusion.

 

[A.T.]

By specific I mean a particular numerical value.

The particular numerical value is just a matter of the unit system, as was already explained to you.

Well, since the spacetime interval explicitly contains c (S^2 = c^2Δt^2 - Δx^2 - Δy^2 - Δz^2),

In that equation c is the just the conversion factor between length and time units, again completely unit system related. In some unit systems c = 1.

 

[weirdoguy]

The particular numerical value is just a matter of the unit system, as was already explained to you.

Over the years here I've seen tons of threads about that, and I think no one ever grasped what people are actually asking about. Eventually all OPs are gaslight into oblivion, and stop asking. So the question is: if we stick to the definition of meter from years ago, as a length of a particular rod, and the definition of second that was used then, then why the value of c is (approximately) $3\cdot 10^8\frac{m}{s}$ and not e.g. $2\cdot 10^8\frac{m}{s}$? In other words, why there is this particular numerical relation between c, and some arbitrarily chosen length that we call 'meter'.

And none of you is answering this question Of course you can ask this question about any physical constant. For me this question is not answerable, but that's another story. I'm just baffled that everyone always misses the point.

 

[Ibix]

Over the years here I've seen tons of threads about that, and I think no one ever grasped what people are actually asking about. Eventually all OPs are gaslight into oblivion, and stop asking. So the question is: if we stick to the definition of meter from years ago, as a length of a particular rod, then why the value of c is (approximately) $3\cdot 10^8\frac{m}{s}$ and not e.g. $2\cdot 10^8\frac{m}{s}$? In other words, why there is this particular numerical relation between c, and some arbitrarily chosen length that we call 'meter'.

And none of you is answering this question

That particular rod is that particular length because you picked it. So the value is arbitrary.

However, if you keep the same rod and vary the fine structure constant, $\alpha$, you either change the size of atoms or vary light speed (depending on how you want to look at it) and you will get a different value. That would be a real physical change, not just a unit redefinition.

People do search for evidence of variation in $\alpha$ (any work reported in popular sources as 'looking for changes in $c$' is actually looking for changes in $\alpha$), but so far have come up empty. And why it has the value it has is not known, so the answer still boils down to "because that's the value it has". But at least you don't get into fights over metrology.

 

[A.T.]

why there is this particular relation between c, and some arbitrarily chosen length that we call 'meter'.

Because of the choice of length that we call 'meter'.

 

[weirdoguy]

So the value is arbitrary.

It is, but the point is it has 3 in front of it, and people are asking why not 2, or 47.

 

[Sagittarius A-Star]

So the question is: if we stick to the definition of meter from years ago, as a length of a particular rod, then why the value of c is (approximately) $3\cdot 10^8\frac{m}{s}$ and not e.g. $2\cdot 10^8\frac{m}{s}$?

It makes no difference if we take the old or new definition of "1 meter".
Both definitions lead to $c\approx 3\cdot 10^8\frac{m}{s}$.