# Light Speed in Vacuum: Does It Reach Cosmic Limit?

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• Sundown444
In summary: I wasn't sure what you were asking.In summary, the speed of light is theoretically maximum 299,792,458 meters per second in a vacuum but in reality it does not go that fast due to the presence of matter.
Sundown444
As we all know, light itself is the fastest thing in the universe, and the cosmic speed limit, basically. That said, it is said to go at 299,792,458 meters per second in a vacuum. That is pretty fast. However, I have a question I want to ask...

Does light really go that fast in a vacuum?

I ask because from what I know, the vacuum of space is not that empty, as while there is little matter still, the vacuum has some space gas and dust in it, making the vacuum of space not a perfect vacuum. Further more, gas and dust in space can be any of the three states of matter, and in this case, that possibly includes the gaseous state and the solid state. From what I know, in either states of matter, as well as liquids, light tends to go at a slower speed than its speed in a vacuum. So I am thinking the speed of light would not go as fast as it should in the vacuum of space. Maybe close to it, but still, since the vacuum of outer space is considered a partial vacuum.

So I guess what I mean to ask is if light needs a perfect vacuum to go its full speed, but I am not sure on this altogether and that is why I am asking. Can someone please fill me in here on this?

I believe you are correct. Light should travel somewhat slower in space than the theoretical maximum. But air has a refractive index of 1.000293, and outer space has, at least, a 13 order of magnitude difference in density compared to sea level (106 to 10-3particles per cm3 vs 1019). That should put the refractive index of space much closer to one than even air, in which light already travels 99.97% of c. If I remember correctly, the refractive index of the interstellar medium is something on the order of 1+10-10.

The refractive index changes with frequency, an effect known as dispersion, and while small this dispersion can be seen in various radio frequencies and used to map the density changes of the ISM.

Nik_2213, vanhees71, Klystron and 3 others
Drakkith said:
I believe you are correct. Light should travel somewhat slower in space than the theoretical maximum. But air has a refractive index of 1.000293, and outer space has, at least, a 13 order of magnitude difference in density compared to sea level (106 to 10-3particles per cm3 vs 1019). That should put the refractive index of space much closer to one than even air, in which light already travels 99.97% of c. If I remember correctly, the refractive index of the interstellar medium is something on the order of 1+10-10.

The refractive index changes with frequency, an effect known as dispersion, and while small this dispersion can be seen in various radio frequencies and used to map the density changes of the ISM.
Thanks for the input, Drakkith. With how close the speed you said, the 99.97 percent of c one; if I am not mistaking it to be the speed of light in a partial vacuum; it makes sense to round it to the speed of light in a perfect vacuum.

Come to think of it, breaking the speed of light in a vacuum could be possible, in a partial vacuum that is. With a high enough force, 99.975 of c, or if not, a similar number, could be exceeded in a partial vacuum. (Have some particle accelerators already achieved exceeding that speed in some way?)

However, the speed of light in a perfect vacuum, as we all know, unfortunately can't be exceeded, or even be reached for that matter. Still, I don't think light itself could reach its perfect vacuum speed as long as there is at least some matter to be around in such a vacuum.

vanhees71
Sundown444 said:
With a high enough force, 99.975 of c, or if not, a similar number, could be exceeded in a partial vacuum.
It is indeed possible (but much easier to do in a medium with higher refractive index). Google for “Cherenkov radiation” for more.

vanhees71
Nugatory said:
It is indeed possible (but much easier to do in a medium with higher refractive index). Google for “Cherenkov radiation” for more.
Oh, I know about Cherenkov radiation. I guess I will still look it up for more information, but I knew what Cherenkov radiation was for a while now. Thanks though.

Made me wonder if this could be used to measure the one-way speed of light in a refractive medium - I applied Grinkle's Law of Googling which loosely summarized (the full derivation is tedious and really adds little insight) states that if I can't find something I just thought about on the Web, its because I don't know the right search words, and finally found this -

The approach described is not what I naively had in mind (I was pondering parallel slow/fast signal paths, I don't believe that approach holds any water and I was Googling to try and confirm that) but these more clever folks do claim to have put an error bound on the one-way speed of light in a refractive medium, if I am reading the conclusion properly.

Although the 'c-speed' is unaffected, gravity lensing may take light on a less direct path, apparently slowing it...

Maybe a way to ask this question is: if light is passing through absolute vacuum ... with one hydrogen atom in it ... that doesn't absorb the light ... what does that atom do to slow down the light, by how much, and with what dependence on distance? (Last time I remember one of those discussions I remember it went into models of the electromagnetic field as a curvature of space - M theory maybe?, but I don't know if that was sound let alone do I have any understanding of it)

Sundown444
Grinkle said:
Made me wonder if this could be used to measure the one-way speed of light in a refractive medium
It cannot.

Mike S. said:
if light is passing through absolute vacuum ... with one hydrogen atom in it
If the mean free path exceeds the wavelength, you can't use the continuum approximation.

If the mean free path exceeds the wavelength, you can't use the continuum approximation.
Fair enough. Also feel free to break "light" down to "a photon" if you prefer. But the approximation has to break down for us to see the phenomenon behind it, doesn't it?

It cannot.

I don't doubt that. If you have time to glance at the paper I linked, I'd be interested in knowing if I mis-read and overstated the authors claims or if the paper is overstating the conclusions.

One thing that is relevant to the discussion is the extinction theorem. Basically, for a given medium and frequency there is a characteristic length scale. Much less than this distance you can assume that the wave propagates at c. Much more than this distance you can assume that the wave propagated at c/n where n is the index of refraction. Some measurements of the speed of light are subject to criticism on the basis of the extinction theorem, but many are not.

Sundown444

## 1. What is the speed of light in a vacuum?

The speed of light in a vacuum is approximately 299,792,458 meters per second, or about 186,282 miles per second.

## 2. Does light always travel at the same speed in a vacuum?

Yes, according to the theory of relativity, the speed of light in a vacuum is constant and does not change, regardless of the observer's perspective.

## 3. Is the speed of light in a vacuum the fastest possible speed?

Yes, the speed of light in a vacuum is considered to be the fastest possible speed in the universe. Nothing can travel faster than the speed of light.

## 4. Can anything slow down or stop light in a vacuum?

No, light in a vacuum does not slow down or stop. It continues to travel at a constant speed until it encounters an obstacle or is absorbed by matter.

## 5. What is the significance of the speed of light in a vacuum?

The speed of light in a vacuum is a fundamental constant in physics and plays a crucial role in our understanding of the universe. It is used in various equations and theories, including Einstein's theory of relativity.

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