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According to: https://en.wikipedia.org/wiki/Cosmic_microwave_background the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space"

Also, https://en.wikipedia.org/wiki/Vacuum#Outer_space says "no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter"

And https://physics.stackexchange.com/tags/vacuum/info: "This rather theoretical requirement is never achieved in practice, because even if space does not contain any atoms / electrons / nucleons, it does contain a lot of photons and neutrinos. But we still call it a vacuum, as an approximation of the theoretical vacuum."

Then on https://en.wikipedia.org/wiki/Speed_of_light we have "The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second (approximately 300,000 km/s (186,000 mi/s)). It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second"

Given that, at least according to Wikipedia, in practice vacuum does not exist, and it's filled with radiation, is it actually possible to measure the speed of light in vacuum? Also, has anyone even observed light in vacuum, ever?

If not, what types of vacuums has light been measured in? Are there any records about this?

Additionally, if true vacuum doesn't even seem to exist and it's filled with radiation (CMB), can we really assume that light doesn't need a medium to propagate? Wouldn't it effectively be propagating through whatever is filling up the space? Would we ever be able to tell the difference?

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    $\begingroup$ Are you suggesting that electromagnetic radiation (light) propagates through electromagnetic radiation (CMB microwaves)? Doesn’t that seem problematic? $\endgroup$
    – G. Smith
    Commented Aug 16, 2019 at 0:20
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    $\begingroup$ Have you tried to estimate the effects? $\endgroup$
    – Qmechanic ♦
    Commented Aug 16, 2019 at 1:04
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    $\begingroup$ I've removed a comment that should have been an answer, and replies to it. Please use comments to improve the question, and use answers to answer the question. $\endgroup$
    – rob ♦
    Commented Aug 16, 2019 at 3:14
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    $\begingroup$ How would you measure an exact value? $\endgroup$
    – safesphere
    Commented Aug 16, 2019 at 5:40
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    $\begingroup$ If you interpreted it that way, then it would be your personal theory, not mainstream physics. This site does not allow personal theories. If it did, it would be inundated with nonsense. In mainstream physics, light is an EM wave, and electric waves do not travel through magnetic fields or vice versa. $\endgroup$
    – G. Smith
    Commented Aug 16, 2019 at 17:04

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Science is full of ideals in its wordings. This is one of them.

SI has fixed the speed of light in a vacuum to be 299,792,458 m/s. If there was indeed light propagating through a perfect vacuum, that would be its speed ... because we define it to be.

For practical purposes, however, we need to be able to design experiments with which to measure distances using this definition. We have done these sorts of experiment regularly in high vacuum, on par with or more extreme than the vacuum of interstellar space. When we look at the effect matter has in slowing the speed of light, we find that the difference between its speed in a perfect vacuum and an achievable vacuum is smaller than the measurement error on our experimental devices. Before we had fixed the speed of light to be a constant, we had measured it to within 1 m/s.

How much of an effect does it have? I'm having trouble finding sources to give a definitive answer, but based on the refractive index of hydrogen as a function of pressure, I would expect interstellar levels of hydrogen to slow light by a factor on the order of µm/s. It's very difficult to measure physical things to 8 or 9 digits, and µm/s is 15 digits away from the speed of light, so our measurements in a high vacuum are as usable as if they were in a perfect vacuum.

If, at some point in the future, we discover that this approach is flawed, we will amend it, as has been done several times before—the most recent amendment being fixing the kilogram as a function of several fundamental constants.

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    $\begingroup$ @NicoBrenner The meaning is to define the meter. Once you have defined a second (which is defined based on the superfine transitions of Cesium), you can define the meter by using a beam of light and the fact that we defined its velocity. As for the issue of the fact that we cannot remove all EM radiation, light exhibits superposition. If you have two light waves which cross, you can figure out what happens by taking each light wave individually and then summing the two effects. Each lightwave has the same effect as-if no other lightwave existed. $\endgroup$
    – Cort Ammon
    Commented Aug 16, 2019 at 3:43
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    $\begingroup$ One might be more pedantic, and say that light travels at 299,792,458 m/s in any arbitrary EM radiation environment, but because light exhibits superposition, it's much easier to just say that it travels that fast in a perfect vacuum, and the superposition laws of light show that it will travel exactly the same speed in any EM environment. Now this, itself, is a bit of a simplification. At really really really really extreme brightness levels of light, we actually do see some quantum interactions within light waves. But at the low end... $\endgroup$
    – Cort Ammon
    Commented Aug 16, 2019 at 3:45
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    $\begingroup$ ..., such as anything below 2.4GW in air, we can say that two light beams simply do not interact with eachother in any meaningful way (there are a handful of lasers in the entire world powerful enough to exhibit this effect). And the SI measurement takes that to its ultimate extreme -- a perfect vacuum. $\endgroup$
    – Cort Ammon
    Commented Aug 16, 2019 at 3:46
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    $\begingroup$ Given all of that, the goal of the SI system defining the speed of light, defining a second, and other things like that, is to permit you to recreate precisely the same system of measurement as was used on past experiments (to within a reasonable level of error). $\endgroup$
    – Cort Ammon
    Commented Aug 16, 2019 at 3:48
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    $\begingroup$ @NicoBrenner Yes, for as many units as we have been able to manage. However, just one year ago, we could say that the kilogram was an absolute measure. There was a lump of Platinum-Irridium alloy sitting in a French vault from which the kilogram was defined. There has been a great interest as of late to get away from such approaches, and to instead fix our unit systems to what appear to be constants of our universe. $\endgroup$
    – Cort Ammon
    Commented Aug 16, 2019 at 3:55
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The “few atoms of hydrogen per cubic meter” is a vacuum for visible light. The adjective “perfect” is both unnecessary and irrelevant in describing light propagating through a vacuum.

The wavelength of visible light is around 500 nm. So roughly $8 \ 10^{18}$ wavelengths fit in a cubic meter. A few hydrogen atoms are utterly irrelevant. Their presence cannot explain the wave behavior of light and from the perspective of such a phenomena the region is vacuum.

Since neutrinos don’t interact with light, their presence is also unimportant, regardless of their quantity. And photons are the same thing as light so it hardly makes sense to complain about the presence of photons when discussing the presence of light.

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    $\begingroup$ Shouldn’t you ask GlenlyAi about that statement? I completely disagree with it and certainly will not try to justify it. There is no known EM phenomenon that is not included in our current model of EM. Also, there is no circularity involved there. Not sure why you think there is. $\endgroup$
    – Dale
    Commented Aug 16, 2019 at 3:24
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    $\begingroup$ The meter depends on the speed of light. The speed of light does not depend on the meter. There is no circularity whatsoever. $\endgroup$
    – Dale
    Commented Aug 16, 2019 at 3:55
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    $\begingroup$ @NicoBrenner No, the speed of light is often expressed in meters per second. It doesn't depend on the definition of a meter in the slightest (it used to, but back then, the meter wasn't defined in terms of the light speed, so again, there was no circular dependency). You're mixing two different definitions of the speed of light and the meter. Pick one, and your confusion will disappear. $\endgroup$
    – Luaan
    Commented Aug 16, 2019 at 8:24
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    $\begingroup$ @NicoBrenner As per modern definitions, the speed of light in a vacuum is a fixed number in the units of m/s. If we measured the speed of light in a vacuum to be different than that number, the result would be a different length for the meter. The meter depends on the speed of light in a vacuum, but not vice versa. But in the end, even that wouldn't really happen, because the speed of light is not really the speed of light in a vacuum anyway - it has a much more general meaning, and light happens to be one of the things that propagates at the so-called speed of light. $\endgroup$
    – Luaan
    Commented Aug 16, 2019 at 8:27
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    $\begingroup$ @NicoBrenner if the speed of light depends on the definition of the meter then how did light propagate in the billions of years prior to the definition of the meter? Light propagated just fine prior to the BIPM using it to define the length of a meter. Furthermore, not only is your objection obviously wrong, it is completely irrelevant to my answer. In dimensionless terms there is on the order of 10^18 wavelengths of visible light in the volume occupied by a single hydrogen atom. The use of the meter in the answer is because it was used in the question, but any units could be used. $\endgroup$
    – Dale
    Commented Aug 16, 2019 at 11:07
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Trying to address your confusion about whether the definitions are circular... I think the root of your confusion may be in understanding what this means from the Wikipedia article you quoted:

Its exact value is 299,792,458 metres per second

You bolded the above sentence, but it is not really the important one. The important one was later in your quote:

a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second

I think you also may be confused about the difference between values and the physical quantities they represent.

So to determine what a meter is based on the above definition, you first need to measure the speed of light. Imagine you are able to make an experiment that measures how far light goes in 1 second. Perhaps it draws pencil marks on two walls very far apart. Say you run the experiment 10 times and look at the different results. The outputs from the different runs of the are not values like [299792458.1 m, 299792457.7 m, 299792458.3 m, ...]. Instead, the outputs are all called "299792458 m". Of course they do have measurement error, but the error is in the actual physical measurement (the locations of the pencil marks).

If someone wants to use this experiment to, say, manufacture a very accurate meter stick (maybe a "light-second stick" in this case), they would need to actually use their meter stick to measure the distance between the pencil marks on the wall, and use that to know whether their meter stick is too large or two small and adjust their manufacturing process based on that.

Does this give you any insight, or am I off base here about your confusion?

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  • $\begingroup$ Thank you Justin, I really appreciate your answer and explanation. What you are saying makes sense. However, I am not really confused. To define the concept of speed, you are using the concepts of time and distance. Which is fine if you want them to be purely conceptual, but the moment you try to pin them down to something "real", then everything becomes relative to whatever you arbitrarily choose as the base to measure everything else. $\endgroup$
    – Nico Brenner
    Commented Aug 16, 2019 at 19:21
  • $\begingroup$ To illustrate using your example, how would you time that 1 second in your experiment to determine the speed of light relative to the walls? Lets say you use some sort of watch. Now, how is that watch's time defined? Can you make the definition of the watch's time independent from other physical measures/units? $\endgroup$
    – Nico Brenner
    Commented Aug 16, 2019 at 19:25
  • $\begingroup$ @NicoBrenner - You don't actually need to define the concept of speed at all to carry out the imaginary experiment I proposed. You just need to measure how far light goes during 1 second. Distance is still real without meters. $\endgroup$
    – Justin
    Commented Aug 16, 2019 at 19:26
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    $\begingroup$ If you are going to include both current and previous definitions, they will be not only circular, but inconsistent. You need to choose whether you want to have a human/historical view of units or whether you want to have a self-consistent/scientific view of them. You can't really do both at the same time. The history of measurement may be important and good to know, but you can't make sense out of the new SI definitions unless you understand them as full redefinitions. $\endgroup$
    – Justin
    Commented Aug 16, 2019 at 20:46
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    $\begingroup$ Yes, that's a pretty good way of saying it. The units are a means of communication about the physical properties, but they are definitely not as real as the properties themselves. $\endgroup$
    – Justin
    Commented Aug 17, 2019 at 2:00
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the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space" [...]

Given that, at least according to Wikipedia, in practice vacuum does not exist, and it's filled with radiation, [...]

You seem to be assuming that a vacuum in which electromagnetic radiation (such as the CMB) is present is not a “true” vacuum. But if you define the vacuum in such a way, then we could never measure the speed of light in a vacuum, because as soon as you inject light into the vacuum it stops being a perfect vacuum. See the problem here? Your notion of what a vacuum is is a much too purist and restrictive one.

Additionally, if true vacuum doesn't even seem to exist and it's filled with radiation (CMB), can we really assume that light doesn't need a medium to propagate?

I think what you mean is that light can propagate in a vacuum (that is, that light doesn’t need matter to propagate), not that light doesn’t need a medium to propagate. As for a “medium”, the truth is we don’t really know what light is and how or why it propagates, so the statement about a medium belongs to the realms of philosophy or metaphysics, not physics. We do have excellent mathematical descriptions of light, the vacuum, and other physical phenomena. Our mathematical models say light propagates in a vacuum, and all existing experimental evidence confirms this to an overwhelming degree of certainty. That’s really all that physics has to say about this issue of propagation.

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  • $\begingroup$ You are right that a vacuum with light in it wouldn't be a true vacuum. However my point was, what happens when light is demonstrably the only thing in a vacuum? It seems like we've never seen that. $\endgroup$
    – Nico Brenner
    Commented Aug 16, 2019 at 3:36
  • $\begingroup$ @Nico I didn’t say that a vacuum with light in it wouldn’t be a true vacuum. It was you who said it, and I pointed out that such a definition makes your question unanswerable (Dale expressed a similar sentiment in the last sentence of his answer). As for whether it is actually a correct statement, I think that depends on the context, since vacuum is typically taken to mean either “space devoid of matter” (which allows the presence of photons) or “the lowest energy state of a quantum field” (which doesn’t). In the context of measuring the speed of light, I’m pretty sure CMB is irrelevant. $\endgroup$
    – GenlyAi
    Commented Aug 16, 2019 at 4:03
  • $\begingroup$ @NicoBrenner the part of your question that did make sense to me is the one questioning whether the presence of hydrogen atoms in interstellar space, rather than CMB, affects our measurement of the speed of light. Cort Ammon answered that nicely. $\endgroup$
    – GenlyAi
    Commented Aug 16, 2019 at 4:07
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There are many answers which point at different ways of understanding the question and convey different types of information about it. I am very thankful for everyone that has contributed to this discussion.

This answer is to condense the info contained in the other answers so far, which together I feel do answer the posted question.

Here's what I learned:

1) We have actually never observed light in "true vacuum", as "true vacuum" doesn't really exist (or it is ill-defined)

2) Given 1), we haven't really been able to measure the speed of light in vacuum. However, this doesn't seem to matter, as we have agreed to define the speed of light as a fixed quantity

3) There is no universally-accepted definition of vacuum, it means different things in different fields or contexts

4) Despite 1), 2), and 3), it doesn't really matter for the purposes of physics, because the models work anyway and the models are not reality, just useful mathematical tools to help us make predictions

5) There aren't really any absolute units of anything. Constants and units are abstract concepts defined through the mathematical models of physics and the values of the units are defined by agreement and governed by the SI. So for example, there is no problem with defining the speed of light as a function of the meter and at the same time defining the meter in terms of the speed of light

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    $\begingroup$ The speed of light is not “defined as a function of the meter”. It is a fundamental property of our universe and is not tied to a specific system of units. If there are intelligent aliens on some far away planet somewhere who have thought about physics, I assure you they understand very well what the notion of the speed of light means, and know its value in their own system of units, even if they never heard of a “meter”. $\endgroup$
    – GenlyAi
    Commented Aug 16, 2019 at 5:15
  • $\begingroup$ @GenlyAi I'm confused, are you saying the speed of light is not really a speed? In that case, what would it be instead? and how could it be used to then define the rest of the units? Or are you saying that speeds don't need to have units of distance/time? $\endgroup$
    – Nico Brenner
    Commented Aug 16, 2019 at 5:19
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    $\begingroup$ Here is an example of a fundamental property that can be expressed without reference to any particular unit of length: the Balmer series An intelligent species on another planet will at some point in its technological development discover that the multiple lines of the hydrogen spectrum are interconnected as described by the Balmer series. The Balmer series exists, independent of what units you happen to use to express the indidual lines of the hydrogen spectrum. The speed of light exists, independent of what unit you use to express it. $\endgroup$
    – Cleonis
    Commented Aug 16, 2019 at 7:50
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    $\begingroup$ @NicoBrenner no, I’m not saying any of these things. The speed of light is definitely a speed, and speed measures distance per unit of time. None of this contradicts what I said earlier. I think you’re fixating on the idea that in the triple of units: time, distance, speed, it is time and distance that are the more fundamental ones, and speed is a derived concept. But that’s not true: in fact, each two of those three can be used to define the third. So, you can define a unit of distance by fixing a unit of time (a second) and a unit of speed (the universe conveniently provides that one). $\endgroup$
    – GenlyAi
    Commented Aug 16, 2019 at 8:32

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