How to rule out that the speed of light was different in the past?

In summary, the constancy of the speed of light is a fundamental principle in modern physics, supported by current experimental evidence. There is no evidence to suggest that the speed of light was different in the past, and the idea is at odds with current scientific understanding. The fine structure constant, which determines the speed of electromagnetic wave propagation, is the relevant factor to look at for any changes over time. The speed of light is a defined constant, not a measurement, and is logically impossible to have been different in the past. Any changes in the speed of light would be cancelled out by changes in the electromagnetic field strength, making it impossible to detect any differences in the speed of light.
  • #36
Vanadium 50 said:
α is absolutely not the speed of light morphed into a dimensionless form.
Insofar as it is anything at all besides α, it is the charge of the electron morphed into a dimensionless form.

c is a factor that comes about because we historically measured time in seconds and length in meters. (And is equal to a dimensionless 1 in sane units). It's a conversion factor, like the dozen. No more, no less. It tells us about spacetime, not electromagnetism.
No, the charge of the electron is defined to be ##-e## in the SI. As detailed in #27 the ingredient of ##\alpha## that's not defined since 2019 by defining the units s, m, kg, and A, is the "permittivity of the vacuum", ##\epsilon_0## which is now to be measured. The same holds for the "permeability of the vacuum", ##\mu_0##, which now is no longer defined but has to be measured. In the SI before 2019 (since 1948 or so) ##\mu_0## was defined through the definition of the A via the force of two infinitelylong straight wires of negligible width: ##\mu_0^{(\text{old})}=4 \pi \cdot 10^{-7} \text{N} \cdot \text{A}^{-2}##. Now it's to be measured and the current value is ##μ_0^{(\text{new})} = 1.25663706212(19) \cdot 10^{-6} \text{N}\cdot \text{A}^{-2}##.
 
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  • #37
Nugatory said:
To be fair, his question is basically "How can we be so sure it hasn't changed if we can't measure it?"
Then I would argue that if you can't tell, you're free to use any units you want.
 
  • #38
Lluis Olle said:
Yes, but then do the permittivity and permeability ratio of vacuum (free space) could be different some billions of years ago?
I think there is a redundancy between the three "constants of nature" in
##c= \frac{1}{\sqrt{\epsilon_0 \mu_0}}##. Only two of them are needed. The third is only calculated from the other two and could be substituted in all physics books. I would regard ##\mu_0## as least important, because the magnetic field is only a Lorentz-transformed electric field and there exist no magnetic monopoles.
 
  • #39
It's a matter of definition, and within the SI ##c## is defined, and both ##\epsilon_0## and ##\mu_0## must be measured somehow. It's of course enough to measure one of them and then use the relation to ##c## to calculate the other.

Historically it was the other way around: the analogue of ##\epsilon_0## and ##\mu_0## was known in the 19th century from measuring the relation of the charge in electrostatic and magnetostatic units (Kohlrausch and Weber 1855, measuring the charge on a Leiden bottle by measuring forces on test charges (electrostatic measurement) and comparing it to the magnetic flux due to the current when discharging it (magnetostatic measurement)).

Then famously Maxwell discovered his equations of the electromagnetic field, i.e., he added the "displacement current" to the Ampere Law, as it was known from action-at-a-distance models (e.g., a la Neumann) at the time, and predicted the existence of electromagnetic waves with a phase velocity given by the said relation between electrostatic and magnetostatic units of charge, which is analogous to ##c=1/\sqrt{\epsilon_0 \mu_0}## when using SI units. The resulting value was pretty close to the then known speed of light, so that Maxwell could conjecture that light might be just electromagnetic waves.
 
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  • #40
Vanadium 50 said:
How do we know that a dozen was twelve in the past?
Its not translation-invariant, some locations around me it is 13 or 14, depending on whether bagels or donuts are involved
 
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  • #41
Vanadium 50 said:
I think it makes it worse and not better. It mixes the fundamental with the practical.

Vanadium 50 said:
Then I would argue that if you can't tell, you're free to use any units you want.
That was my point
 
  • #42
Lluis Olle said:
permeability ratio of vacuum
Is an artifact of our system of units. (This was clearer under the older definitions) Did you think it was a measured quantity and just happened to be 4π? Gosh, what are the chances of that!

The c that comes here is the same c in the Lorentz force law (and is 1 for suitable choice of velocity units).

Maybe the way to think about it is this way. Back when solving trig identities, the teacher said that [itex]\sin^2 \phi + \cos^2 \phi[/itex] is "just a fancy way of writing 1". In exactly the same way, 300,000 m/s is "just a fancy way of writing 1". Asking whether it was different in the past is the same as asking if the number 1 was different in the past.

Just as 1 meter to the left is the same as 1 meter up, 300,000 meters is the same as 1 second.
 
  • #43
Vanadium 50 said:
Asking whether it was different in the past is the same as asking if the number 1 was different in the past.
I think that observations from distant Galaxies (which is kind of looking into the past), don't rule out that c was different (compared to our local and current environment).
 
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  • #44
Lluis Olle said:
I think that observations from distant Galaxies (which is kind of looking into the past), don't rule out that c was different (compared to our local and current environment).
Why do you think that?
 
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  • #45
PeterDonis said:
Why do you think that?
Because is my understanding that the concept of the "uniformity" of c is a local concept, and could be not so "uniform" at the cosmological level. And for "local" I mean in the spacetime sense.
 
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  • #46
Lluis Olle said:
Because is my understanding that the concept of the "uniformity" of c is a local concept, and could be not so "uniform" at the cosmological level. And for "local" I mean in the spacetime sense.
It makes no sense to say that the local ##c## here is different from the local ##c## there. This, again, is where you need a change that affects the observed physical phenomena - like the spectrum of hydrogen.
 
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  • #47
Lluis Olle said:
my understanding that the concept of the "uniformity" of c is a local concept

Perhaps you are conflating locally flat spacetime with what you call uniformity of c.
 
  • #48
PeroK said:
It makes no sense to say that the local ##c## here is different from the local ##c## there. This, again, is where you need a change that affects the observed physical phenomena - like the spectrum of hydrogen.
As I said, "here" and "there" are spacetime concepts at the cosmological level in the context I'm talking. If I'm not wrong (that would be no surprise for me anyway), even Einstein considered that GR was "locally" correct, but...

And there's an open debate about the "redshift" of Quasars...
 
  • #49
Grinkle said:
Perhaps you are conflating locally flat spacetime with what you call uniformity of c.
Outside the "locality" environment - which I'm unable to say if it's 1 billion YL or whatever -, who knows? It's not obvious, and the scientific data about Quasars and Galaxies is an open debate.
 
  • #50
victorvmotti said:
So, assuming that other dimensionful constants involved in this particular ratio known as α were not different in the past, say in 2 billion years ago, we can refer to the data from the Oklo mine natural nuclear reactor. And it looks like we have experimental evidence here on our planet from at least 2 billion years ago, ruling out that the speed of light was different in the past!
Yes. However, if you are assuming that ##c## could vary then it is a little odd to assume that none of the other constants in ##\alpha## can vary. To me, that assumption is objectionable.

Since we are detecting a possible variation in ##\alpha## it is far better (in my opinion) to simply measure it and report any variation than to try to assert that such variation in ##\alpha## corresponds to a variation in ##c##.
 
  • #51
Dale said:
Since we are detecting a possible variation in ##\alpha## it is far better (in my opinion) to simply measure it and report any variation than to try to assert that such variation in ##\alpha## corresponds to a variation in ##c##.

So back again to your point earlier. It is impossible, logically, to know if the speed of light was different in the past or not!
 
  • #52
victorvmotti said:
So back again to your point earlier. It is impossible, logically, to know if the speed of light was different in the past or not!
Right. We can experimentally test for variations in ##\alpha##, and all of the physics are captured by that. Anything further that we try to say specifically about ##c## is just an assumption.

Since $$\alpha=\frac{e^2}{2 \epsilon_0 h c}$$ we can take a non-variation in ##\alpha## to mean that ##c## has doubled and ##h## has halved!
 
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  • #53
Dale said:
To me, that assumption is objectionable.
Well of course big claims demand axtraordinary evidence. But the Hubble redshift assertions do make a tempting target!
 
  • #54
Dale said:
little odd to assume that none of the other constants in can vary.
Especially because c is present in the definition of the fine structure constant in some systems of units and not others.

In MKSA, [itex]\alpha = e^2/2\epsilon_0 hc[/itex]. If it varies, my money is on the 2 changing. After all, LEP at CERN measured the number 3 experimentally and got 2.99 +/- 0.01.
 
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  • #55
Lluis Olle said:
Einstein considered that GR was "locally" correct, but...
Do you have a related link?

Lluis Olle said:
And there's an open debate about the "redshift" of Quasars...
If you refer to the "tired light" hypothesis, there exists evidence against it, for example
The tired light model does not predict the observed time dilation of high redshift supernova light curves. This time dilation is a consequence of the standard interpretation of the redshift: a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift z=1.
Source:
https://astro.ucla.edu/~wright/tiredlit.htm
 
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  • #56
Lluis Olle said:
is my understanding that the concept of the "uniformity" of c is a local concept
Where are you getting that understanding from? Please give a reference.
 
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  • #57
Lluis Olle said:
there's an open debate about the "redshift" of Quasars...
What open debate? Please give a reference.
 
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  • #58
Dale said:
Right. We can experimentally test for variations in ##\alpha##, and all of the physics are captured by that. Anything further that we try to say specifically about ##c## is just an assumption.

Since $$\alpha=\frac{e^2}{2 \epsilon_0 h c}$$ we can take a non-variation in ##\alpha## to mean that ##c## has doubled and ##h## has halved!
No! Since 2019 we can't do this, because ##c##, ##h##, and ##e## are fixed within the SI to define the units used to do measurements. What's not defined but must be measured is now ##\epsilon_0##! So using the new SI it's ##\epsilon_0## that may have changed with time. So far there's no hint at such a variation modulo the (high) accuracy in measuring spectral lines from far-distant objects.
 
  • #60
vanhees71 said:
No! Since 2019 we can't do this, because c, h, and e are fixed within the SI to define the units used to do measurements.
Sure we could. We could just use non-SI units.
 
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  • #61
E.g., we can just use the old SI. There we had ##\Delta \nu_{\text{CS}}## and ##c## as in the new SI, but the kg was still defined by the prototype in Paris, and the A was defined via setting ##\mu_0=4 \pi \cdot 10^{-7} \text{N} \cdot \text{A}^{-2}##. Thus also ##\epsilon_0=1/(\mu_0 c^2)## was defined. So using the old SI units a measured change of ##\alpha## would imply a change of ##h## or ##e## (or both).
 
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  • #62
vanhees71 said:
So using the old SI units a measured change of would imply a change of ħ orc (or both).
Or π! (Of course, under the new definitions, π is a measured quantity.)
 
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  • #63
Frabjous said:
Let’s say that we measured a change in α, would defining the fundamental constants still be the preferred method to define units?
Let's say that we defined the kilogram by an artifact, and noticed the mass of this artifact were changing over time. Would that be a good reason to redefine units?

Oh wait...that actually happened.
 
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  • #64
PeterDonis said:
Where are you getting that understanding from? Please give a reference.
For example, in this Wikipedia article (I'm not about VSL theories!) you can read:
Accepted classical theories of physics, and in particular general relativity, predict a constant speed of light in any local frame of reference and in some situations these predict apparent variations of the speed of light depending on frame of reference, but this article does not refer to this as a variable speed of light.
You can only define and measure the "speed of light in vacuum" locally. Let's say that I measure locally the speed of light to be 299 792 458 m/s, or that I fix that value and the way I measure that value locally. If I measure then the speed of a photon that comes from a distant Galaxy or Quasar, then locally I would measure that speed... but I can't tell what was the "local" speed then and there where the photon was produced.
 
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  • #65
Lluis Olle said:
Let's say that I measure locally the speed of light to be 299 792 458 m/s
That's no measurement of the speed of light. That's a measurement, if the scale on your ruler is accurate.
 
  • #66
Lluis Olle said:
You can only define and measure the "speed of light in vacuum" locally.
No, you can only directly measure the fine structure constant and things that might depend on it (like the speed of light in vacuum locally, if you are using units where that is a measured quantity instead of a defined one--note that in SI units it s defined, not measured). But that in no way means you cannot indirectly measure the fine structure constant and things that might depend on it at distant locations or in the past.
 
  • #67
Lluis Olle said:
in this Wikipedia article (I'm not about VSL theories!) you can read:
This article has nothing to do with the thread topic or the concern you raised.
 
  • #68
PeterDonis said:
No, you can only directly measure the fine structure constant and things that might depend on it (like the speed of light in vacuum locally, if you are using units where that is a measured quantity instead of a defined one--note that in SI units it s defined, not measured). But that in no way means you cannot indirectly measure the fine structure constant and things that might depend on it at distant locations or in the past.
Of course for each of the definition you need also the "mises en pratique" for the units. E.g., to realize the definition of the kg via the defined values Planck action ##h## (and also of the definitions of the s, the, m, and the A) with the Kibble balance what's accurately measured are quantities like the magnetic-flux quantum in superconductors (Josephson constant).

You find the corresponding brochures in English here:

https://www.bipm.org/en/publications/mises-en-pratique
 
  • #69
PeterDonis said:
No, you can only directly measure the fine structure constant and things that might depend on it (like the speed of light in vacuum locally, if you are using units where that is a measured quantity instead of a defined one--note that in SI units it s defined, not measured). But that in no way means you cannot indirectly measure the fine structure constant and things that might depend on it at distant locations or in the past.
But in the fine structure, there're other "constants" playing other than π and c.

For example, the Universe is expanding and seems the expansion is accelerating. Locally, lets say at every eventpoint of the worldline of a photon that comes from a distant Galaxy, its locally measured speed is c. But as space is expanding, then for a non-local observer the speed exceeds c if computed globally, but not measured locally.

What I don't understand (among other billion of things) is that my measuring 1 meter rod is not expanding itself, is the space outside the rod that's expanding - or I could not measure the expansion!
 
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  • #70
Your meter rod is held together by the electromagnetic force. How much stronger is the electromagnetic interaction compared to the gravitational one? How much do you think does thus "Hubble expansion" affect your meter stick used for the purpose of local (in time and space!) measurements?
 
<h2>1. How do scientists measure the speed of light?</h2><p>Scientists use a variety of methods to measure the speed of light, including using lasers, mirrors, and precise timing devices. One of the most accurate methods is the use of interferometry, which involves splitting a beam of light and measuring the interference patterns as it travels through different mediums.</p><h2>2. Can the speed of light change over time?</h2><p>According to the theory of relativity, the speed of light is considered to be a constant in a vacuum and cannot change. However, some theories suggest that the speed of light may have been different in the early universe or during the Big Bang.</p><h2>3. What evidence do we have that the speed of light has remained constant?</h2><p>One of the main pieces of evidence for the constancy of the speed of light is the fact that the laws of physics, including the speed of light, have been consistent and predictable throughout history. Additionally, experiments such as the Michelson-Morley experiment have consistently shown the speed of light to be constant.</p><h2>4. How can we rule out the possibility of a changing speed of light in the past?</h2><p>Scientists use a variety of methods, including astronomical observations and laboratory experiments, to test the constancy of the speed of light. These experiments have consistently shown that the speed of light remains constant, ruling out the possibility of it changing in the past.</p><h2>5. What implications would a changing speed of light have on our understanding of the universe?</h2><p>If the speed of light were found to have changed in the past, it would challenge our current understanding of the laws of physics and the formation of the universe. It could also have significant implications for our understanding of time and space, as well as the accuracy of many scientific theories and calculations.</p>

1. How do scientists measure the speed of light?

Scientists use a variety of methods to measure the speed of light, including using lasers, mirrors, and precise timing devices. One of the most accurate methods is the use of interferometry, which involves splitting a beam of light and measuring the interference patterns as it travels through different mediums.

2. Can the speed of light change over time?

According to the theory of relativity, the speed of light is considered to be a constant in a vacuum and cannot change. However, some theories suggest that the speed of light may have been different in the early universe or during the Big Bang.

3. What evidence do we have that the speed of light has remained constant?

One of the main pieces of evidence for the constancy of the speed of light is the fact that the laws of physics, including the speed of light, have been consistent and predictable throughout history. Additionally, experiments such as the Michelson-Morley experiment have consistently shown the speed of light to be constant.

4. How can we rule out the possibility of a changing speed of light in the past?

Scientists use a variety of methods, including astronomical observations and laboratory experiments, to test the constancy of the speed of light. These experiments have consistently shown that the speed of light remains constant, ruling out the possibility of it changing in the past.

5. What implications would a changing speed of light have on our understanding of the universe?

If the speed of light were found to have changed in the past, it would challenge our current understanding of the laws of physics and the formation of the universe. It could also have significant implications for our understanding of time and space, as well as the accuracy of many scientific theories and calculations.

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