Why is the speed of light what it is?

[Dale]

And the topic of the paper is clearly the speed of light

You really should read more than just the title. The topic is the speed of light but the actual content of the paper is as I described. It acknowledges that only changes in dimensionless constants are physically meaningful, specifically identifies the fine structure constant as the thing measured, and then simply by fiat attributes any measurable variation in the fine structure constant to a change in c.

The assumption of a variation has fascinating consequences. It explains directly the (otherwise not understood) cosmological inflation (said by the authors). And even better: The problem of Dark Energy,

You mean that it would explain those things if it (the fine structure constant) did vary. But it doesn’t vary so that isn’t the explanation.

We can be very sure that the question of Dark Energy will never be solved on the basis of present understanding, i.e. a constancy of c.

I am not very sure about that. I am more certain that believing a counterfactual assumption will not solve the question either.

We know that our measurement tools a cheating us (by dilation and contraction). So it makes sense to ask for the truth. - This was also an argument of Lorentz when he once discussed relativity with Einstein.

I know very well where this line of thought comes from. Please read the forum rules.

 

[Dale]

Or is it just what it is, with no known reason for that specific value?

The reason is well known. It has that specific value because the BIPM committee members voted for it to have that value.

 

[FactChecker]

The reason is well known. It has that specific value because the BIPM committee members voted for it to have that value.

I hope we are talking about the same thing. I am not asking about the units of measurement so much as the actual physical movement of the light. So it sounds like there is nothing in physics to set limits or a preference for the particular motion that light has now. So if the light always went twice as far per unit time as it does now (regardless of the units used), that would not have violated any fundamental law of physics as we know it.

 

[Dale]

I am not asking about the units of measurement so much as the actual physical movement of the light.

If you are not asking about the units then you are asking about dimensionless constants, specifically the fine structure constant.

So it sounds like there is nothing in physics to set limits or a preference for the particular motion that light has now.

Yes, there is nothing in known physics which determines the fine structure constant. It is considered to be fundamental.

 

[FactChecker]

An interesting video on observations to see if there is any variation of the fine structure constant.

 

[AgentSmith]

This is similar to asking why the value of Pi happens to be what it is.
It simply is a universal constant, something which is not disputed.
Arguably Pi, c, and other constants might differ in some interpretation of a 'multiverse', (not something I am keen on),
However. even if that was the case, it makes no difference to the Universe we exist in.

There is a difference between mathematical constants like Pi, and physical constants like c. The latter are experimental, while mathematical constants not.

 

[Nugatory]

There is a difference between mathematical constants like Pi, and physical constants like c. The latter are experimental, while mathematical constants not.

This statement is correct for dimensionless physical constants like the fine structure constant.
However, it's not correct for $c$; its value was chosen by a committee.

 

[FactChecker]

There is a difference between mathematical constants like Pi, and physical constants like c. The latter are experimental, while mathematical constants not.

Good point. But even $\pi$, if it is simply defined as $\pi = \frac {\text{circle circumference}} {\text{circle diameter}}$, can be experimental and depend on curvature.

 

[Gary_T2018]

It doesn't have to be.

You most certainly can slow it down by juggling either medium or temperature.

What you wanted to ask is the question:

Why at this moment there is no solid theory which supports the possibility of moving faster than the speed of light in vacuum?

 

[jbriggs444]

You most certainly can slow it down by juggling either medium or temperature.

You can slow down light. You can't change c. The named constant "c" denotes the speed of light in a vacuum. The numeric value of its speed depends on your choice of units. Our conventional choice of units is such that c has a defined value that is not experimentally determined.

 

[Gary_T2018]

You can slow down light. You can't change c. The named constant "c" denotes the speed of light in a vacuum. The numeric value of its speed depends on your choice of units. Our conventional choice of units is such that c has a defined value that is not experimentally determined.

You are right.

But "c" happens to be the fastest speed allowed by pretty much all our well-founded theory and observation. When people talk about the speed of light, they aren't talking about the speed of light, they are talking about "c" - why this particular value?

I think a cheat-ish way to answer that question is:

Clearly our universe has a limit on how fast information and particles can move - and that happens to be the value "c".

The question is in the same league as "why can't we see blow <insert certain particles here>" and "why is the universe <insert the size of the universe here>?"

There appears to be a limit somewhere.

 

[Vanadium 50]

its value was chosen by a committee.

This is a key point. One might as well ask why there are 12 inches in a foot.

I know you don't like this explanation; nevertheless it is true.

 

[FactChecker]

Why at this moment there is no solid theory which supports the possibility of moving faster than the speed of light in vacuum?

Because the logic of the Special theory of Relativity is based on distortions of space and time, it applies to everything that moves, not just light. There are two systems of geometry which are compatible with some simple assumptions: Galilean and Einsteinian. (see https://arxiv.org/pdf/physics/0302045.pdf ). When experiments showed that the speed of light in a vacuum was always measured the same, the Galilean theory was ruled out. That left only the Einsteinian theory, which includes formulas for adding velocities. With those formulas for adding velocities, nothing will be measured as moving faster than c. Light and all electromagnetic waves move at the maximum measureable velocity, c.

 

[suremarc]

I don't understand the discussion about changes in the speed of light being "physically meaningless". Suppose we switched back to prototype SI units, and that the speed of light suddenly shrank by a factor of 1000. We could measure the discrepancy in the speed of light by comparing the length of a light-second to the prototype meter stick. The speed of light would turn out to be only 300 km per second, post-shrink. Now the length of some man-made meter stick may not matter to a physicist, who is only interested in the formulation of fundamental laws, but it matters to humans, who happen to inhabit the spacetime on which those laws are formulated.

My understanding is that dimensionful quantities are independent of any system of units, even if they can only be expressed relative to those units. I compare it to vector spaces; while vectors can only expressed in a given basis, vectors themselves are basis independent.

 

[Nugatory]

We could measure the discrepancy in the speed of light by comparing the length of a light-second to the prototype meter stick.

You have an additional assumption here, namely that the length of that prototype meter stick is constant.

Without that assumption, the only physically significant thing we have is still a dimensionless ratio, namely the ratio of the distance that light travels in a specified amount of time (for now, we'll set aside the question of how we specify that time) to the length of the prototype meter stick.

However, your point that the length of that prototype meter stick does have some practical significance is well-taken. The number 299792458 was chosen because it leads to a definition of the meter that fits within the error bars of our ability to measure all then-existing meter sticks; that is, the definition was backwards-compatible.

 

[Dadface]

http://cds.cern.ch/record/618057/files/0305457.pdf
This may be a useful source of relevant information.

 

[OldYat47]

Look up the Lorentz transform. There are excellent articles that explain why the speed of light is constant for all observers even if their individual frames of reference are moving at different velocities.

 

[AgentSmith]

This statement is correct for dimensionless physical constants like the fine structure constant.
However, it's not correct for $c$; its value was chosen by a committee.

The committee did not pick a value out of thin air. It was ultimately based on experimental values.

 

[Nugatory]

The committee did not pick a value out of thin air. It was ultimately based on experimental values.

To be precise, it was chosen to fall within the error bars of the previous definition of the meter. There's a conceptual shift here, in that earlier experimental measurements of the speed of light are reinterpreted as measurements of the length of the meter.

 

[AgentSmith]

deleted OP

To be precise, it was chosen to fall within the error bars of the previous definition of the meter. There's a conceptual shift here, in that earlier experimental measurements of the speed of light are reinterpreted as measurements of the length of the meter.

I know that. But still the new definition had to fit within experimental results. (We could and have set it to 1, but there would be a ripple effect.)

 

[Dale]

But still the new definition had to fit within experimental results.

No, it didn’t. They chose to have it fit within the error range of previous experimental results, but they most certainly did not have to. They chose to keep it within previous experimental ranges for convenience, but if they had decided to be inconvenient then they could have chosen any arbitrary value whatsoever.

There is simply no way to get an exact value through experiment, and c is an exact value in SI units. It is purely the outcome of a committee.

 

[AgentSmith]

No, it didn’t. They chose to have it fit within the error range of previous experimental results, but they most certainly did not have to. They chose to keep it within previous experimental ranges for convenience, but if they had decided to be inconvenient then they could have chosen any arbitrary value whatsoever.

There is simply no way to get an exact value through experiment, and c is an exact value in SI units. It is purely the outcome of a committee.

No way to get an exact value through experiment. Glad you're here to tell me these things, professor. Of course they could have chosen any value whatsoever, but that would be pretty d**m stupid. Ya think there is a reason they chose not to be stupid?

 

[Dale]

No way to get an exact value through experiment. Glad you're here to tell me these things, professor.

Me too, thanks!

Of course they could have chosen any value whatsoever,

Good, you understand. The value of c in SI units is not experimental.

but that would be pretty d**m stupid. Ya think there is a reason they chose not to be stupid?

Sure, there were lots of great reasons to not be stupid. But anyone working with committees knows that they are fully capable of being stupid anyway.

 

[Vanadium 50]

Glad you're here to tell me these things, professor. Of course they could have chosen any value whatsoever, but that would be pretty d**m stupid. Ya think there is a reason they chose not to be stupid?

You do know that the 1799 definition of the meter was known to be incompatible with the 1798 definition? So historically there have been examples of "pretty d**m stupid".

 

[AgentSmith]

You do know that the 1799 definition of the meter was known to be incompatible with the 1798 definition? So historically there have been examples of "pretty d**m stupid".

We are now into the 21st century. Hopefully we have progressed a bit.

 

[phinds]

We are now into the 21st century. Hopefully we have progressed a bit.

Ah, you optimist you.