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. To have 2, you would have to change the rod to some other rod with different length.

 

[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 $c\approx 3\cdot 10^8\frac{m}{s}$.

 

[weirdoguy]

Both definitions lead to

Yes, and people are asking why it has 3 and not other number.

 

[Ibix]

It is, but the point is it has 3 in front of it, and people are asking why not 2, or 47. To have 2, you would have to change the rod to some other rod with different length.

Or, as I said, vary $\alpha$, which would actually change $c$ in the sense you mean.

But that just leads to the question of why $\alpha$ has the value it has, to which the answer is "we don't know".

 

[A.T.]

if we stick to the definition of meter from years ago, as a length of a particular rod,

The original definition of a meter was based on the Earth's circumference. The later definitions roughly match it.

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

If the Earth had a different size, or if they had decided to take a different fraction of the Earth's circumference to be a meter, then the numerical value of c in m/s could have 2 or 47 in front.

 

[Sagittarius A-Star]

Yes, and people are asking why it has 3 and not other number.

That's because "1 meter" was defined this way.

For comparison:
The (dimensionless) relativistic speed of sound is
$\beta = v/c \approx 1{.}144 \times 10^{-6}$. It is independent of the definition of $c$.

 

[Averagesupernova]

But that just leads to the question of why α has the value it has, to which the answer is "we don't know".

I will state that alot of this is above my head but I have often wondered the same thing about the speed of light. Why is it what it is and not something else? I have watched threads that discuss this and am always disappointed to see the answer of "because the meter is the length that it is", or similar non answers. I came to the conclusion that it is what it is because we always measure it this way and we don't have a better answer. That at times has translated into: "because we always measure it this way and I don't feel comfortable saying we don't know why". It's nice to see someone say: "we don't know".

 

[Dale]

I think no one ever grasped what people are actually asking about

Really? You think we don’t grasp the question?

No, we understand what they are asking. The problem is just that people don’t like the answer, not that we don’t understand the question.

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⋅108ms and not e.g. 2⋅108ms?

That doesn’t change the answer a bit. It is that value because of the choice that we made in defining the units. In this case, because of the specific bar that was selected to define the meter. If we had selected a different bar then it could have been some other value.

People just don’t like the fact that the value of $c$ is unimportant. What is physically important is the value of $\alpha$. There is no misunderstanding of the question, just a reluctance to hear the answer.

 

[Dale]

I will state that alot of this is above my head but I have often wondered the same thing about the speed of light. Why is it what it is and not something else? I have watched threads that discuss this and am always disappointed to see the answer of "because the meter is the length that it is", or similar non answers. I came to the conclusion that it is what it is because we always measure it this way and we don't have a better answer. That at times has translated into: "because we always measure it this way and I don't feel comfortable saying we don't know why". It's nice to see someone say: "we don't know".

But we do know. We have the committee notes from the BIPM and all of the discussion. We have all of the information that we could possibly have about the human choices that led to $c$ having the value that it does in SI.

What we do not know is why $\alpha$ has the value that it has.

The value of $c$ is something that we decide by our choice of units. The value of $\alpha$ is something that nature decides.

You have come to a wrong conclusion. I don’t know why you are disappointed nor why you mistakenly believe it is a non answer, but the answer is correct. The value of $c$ is purely due to our choice of units.

The same cannot be said of $\alpha$.

 

[Sagittarius A-Star]

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.

I think this can be an answer to the question "Why does light move in a given timeframe along a certain number of atoms in the old meter prototype?".

This can be argued via the Bohr radius:

Undergrad Post in thread 'Isn't the speed of light tautologically 299792458 m/s?'

Jan 12, 2024

But if the current definition of the meter implies that the speed of light is constant

The definition of the meter via the old meter prototype in Paris had the same property.

This can be argued via the Bohr radius.

The length of the old meter prototype $L_1$ is a multiple (factor := $k$) of the Bohr radius $a_0$:
$L_1 = k * a_0 = k * \frac {\hbar}{m_ec\alpha}$.

Source:
https://en.wikipedia.org/wiki/Bohr_radius

=> $\ \ \ c = k * \frac {\hbar}{m_e L_1 \alpha}$

So, if $L_1$ is assumed to be "constant with time", then you will measure a constant $c$.

• Sagittarius A-Star

• 

 

[Dale]

For anyone who thinks that I am gaslighting or giving a non-answer I encourage you to actually go through the work and do the relevant calculations on your own. Here is where I described the process that I went through to learn this stuff:

https://www.physicsforums.com/showpost.php?p=2011753&postcount=55

https://www.physicsforums.com/showpost.php?p=2015734&postcount=68

 

[GRQFT]

The value of $c$ is purely due to our choice of units.

Just curious, was this considered to be true prior to 1905? That is, was c then interpreted simply as the propagation speed of light (an EM wave) in vacuum?

 

[weirdoguy]

Really? You think we don’t grasp the question?

Yes, and for me this:

In this case, because of the specific bar that was selected to define the meter. If we had selected a different bar then it could have been some other value.

proves what I wrote. I know that it could have been some other value for different bar. The issue is not selecting different bar, but why with this particular bar we get the number that we get and not some other. Why c is not 20m/s, with 1m being the 1m we all agree on? @Ibix on the other hand did grasp the issue.

Again, I'm not asking this question, I'm just bringing up the issue that I think exists, the issue of talking past each other between 'the knowledgable' and people who ask this question.

 

[Sagittarius A-Star]

Just curious, was this considered to be true prior to 1905? That is, was c then interpreted simply as the propagation speed of light (an EM wave) in vacuum?

Prior to 1905 it was considered that light waves propagate by vibrations of an ether, which fills space, and that light moves with $c$ only relative to the ether. Goal of the Michelson–Morley experiment was to measure the velocity of Earth relative to the ether while moving around the sun, but the experiment had a negative result, which can be explained by SR.

Source:
https://en.wikipedia.org/wiki/Aether_theories#Luminiferous_aether

 

[Matterwave]

And @Ibix on the other hand did grasp the issue.

Could you clarify what it was about @Ibix answer that brought you to this conclusion?

For example, @Dale already brought up the fine structure constant in posts #25 and #26. I again brought up constants with no current theoretical explanation in my answer in post #28.

Was it @Ibix illustration in discussing atomic distances that did it?

 

[GRQFT]

Prior to 1905 it was considered that light waves propagate by vibrations of an ether, which fills space, and that light moves with $c$ only relative to the ether. Goal of the Michelson–Morley experiment was to measure the velocity of Earth relative to the ether while moving around the sun, but the experiment had a negative result, which can be explained by SR.

Source:
https://en.wikipedia.org/wiki/Aether_theories#Luminiferous_aether

So, you think that Maxwell himself (who clearly was aware of Faraday's law and Maxwell's displacement [the curl of B equation]) thought that too? The reason I ask is that, as I understand it, the interaction of E & B through those two equations is what continuously regenerates and propagates an EM wave without the need for any medium at all.

 

[Sagittarius A-Star]

So, you think that Maxwell himself (who clearly was aware of Faraday's law and Maxwell's displacement [the curl of B equation]) thought that too?

Yes. Maxwell wrote a paper about the ether.

https://sites.pitt.edu/~jdnorton/teaching/Einstein_graduate/pdfs/Maxwell_ether.pdf

 

[Matterwave]

So, you think that Maxwell himself (who clearly was aware of Faraday's law and Maxwell's displacement [the curl of B equation]) thought that too? The reason I ask is that, as I understand it, the interaction of E & B through those two equations is what continuously regenerates and propagates an EM wave without the need for any medium at all.

Maxwell literally wrote the encyclopedia article on Ether: https://en.wikisource.org/wiki/Encyclopædia_Britannica,_Ninth_Edition/Ether_(2.)?hl=en-US

Yes, he was a firm believer in the Ether.