# How confident can we be that the speed of light in a medium is constant?

I have recently found this article http://news.harvard.edu/gazette/1999/02.18/light.html it tells that physicists have been able to slow the speed of light. Is this hokum? If not how is it possible to use light as a measure of distance? Surely doing this would effect the waveform of light rendering it impossible to tell how far it has travelled or how long it has been travelling.

• Well the speed of light in vacuum was defined a constant back in 1983. In media, it can be reduced from this defined value. Dec 31, 2014 at 17:43
• if it can be reduced then how can it be defined as a constant? Dec 31, 2014 at 17:44
• Light can interact with optical media and the resulting electromagnetic wave/atomic excitations will travel at a slower speed. That's strictly speaking not "light", though. It's much better called pseudo-particles or coupled states. Dec 31, 2014 at 17:46
• Kyle Kanos used his words wisely when he said "light in vacuum" with emphasis in italics. Those pesky attributes matter a lot. Dec 31, 2014 at 17:47
• Speed of light in a material is not the same as the speed of light in vacuum. What we naively want to think of as individual particles lose a bit of their identity when there are lots of "particles" about. Photons turn into phonons, and then back again, over and over. Photons themselves however always travel at the "speed of light". A better name for "speed of light" is perhaps "speed of a massless particle", or even better, "the one speed that all local observers will agree is the same." That's just too verbose, so physicists use "speed of light" instead. Dec 31, 2014 at 17:57

That article is about slowing the speed of light in a physical medium (note the mention of the 'exotic medium' in the third paragraph), a common and well-known phenomenon (even ordinary transparent mediums like water or glass will slow light waves traveling through them to some extent), the newsworthy aspect was just the huge degree of slowing they were able to achieve. From what I understand, in quantum theory that basic explanation is that the photons are being repeatedly absorbed and re-emitted as they travel through the medium (not by individual particles of the medium but rather by collective vibrational modes known as 'phonons', see the discussion here), which slows them down even though they still travel at the speed of c between absorptions.

• Is it phonons or polaritons? I already forgot my solid state physics classification of "-ons"! Dec 31, 2014 at 17:49
• @CuriousOne - The FAQ discussion I linked to called them phonons, but it's possible the author got it wrong, I haven't studied solid state physics myself. Dec 31, 2014 at 17:52
• I hate solid state physics, especially since I had to learn it using Kittel's book...aaaaargh! Dec 31, 2014 at 17:54
• @CuriousOne the correct term would be exciton-polariton here, but usually just abbreviated as polariton as you said yourself. I know...quasiparticles..a nightmare, ey? :) Happy new year by the way! Jan 1, 2015 at 0:32
• @Phonon: There you go! So many -ons, so little interest in learning any of them. Happy new year to you and everyone! Jan 1, 2015 at 0:38

Actually The velocity of light in a Vacuum is now used to define the length of one meter.

Yes the velocity of light in different mediums is different, and when light passes from one medium to another, its velocity changes. At the interface, there is both reflection and refraction of light, and a change in velocity, however slight.

This is why light is specified as a velocity " in a vacuum "

If I remember right V = 299, 792, 458 meters per second " in a vacuum "

The 1925 Michelson Experiment came up with a slightly lower number, but his experiment was from mountain top to mountain top in California thru a smoke filled valley air. The light needed to make 4224 complete round trips across the valley and back to the source every second to synchronize with a spinning glass mirror that had 16 facettes on a circle.

This means the one way distance in a vacuum would need to be 299,792,458 / 8448 = 35,486.79664 meters of travel distance apart. if the light had traveled in a vacuum.

The actual distance was less, so the velocity of light in smoky air was less. I will look up my answer to this distance.

Michelson did have a slight error in his calculated distance because for the thing to work the distance and the RPM's of the machine needed to both be exact numbers.

I looked up my calculations for the actual distances traveled by the light and found the following.

Actual one way distance was 35,424.42968 meters x 8448 = 299,265,581.9 m/sec in smoky atmosphere. This is 99.824253% of the speed of light in a Vacuum.

If you look up Michelson's 1925 experiment on the web, you will see that he made a tiny distance error in the last 10 cm of the internal set up of his contraption.

I had thought this method could be utilized to measure atmospheric density between two points. If the atmospheric density changes, then the light velocity will change, and the cyclical resonance distance will change.

• There's a proposed edit of this answer which looks like it was supplied by OP but possibly when he/she wasn't logged in. Jan 1, 2015 at 20:05

There are two things usually called, the speed of light.

The first thing $c=\sqrt{\frac{1}{\epsilon_0\mu_0}}$, a number that is related to the electromagnetic properties of the classical vacuum, so you can measure $\epsilon_0$ by doing electric measurements, and measure $\mu_0$ by doing magnetic measurements, and then compute $c$ from those experimental results.

Since any inertial frame could do those same measurements, they would compute the same $c=\sqrt{\frac{1}{\epsilon_0\mu_0}}$, so the number $c$ doesn't change. But it also turns out that electromagnetic waves (e.g. light) can travel through the classical vacuum, and they travel at speed $c$, so we call it the speed of light (in a vacuum). Since every inertial frame also sees the same laws, every inertial frame sees the same fact that electromagnetic waves (e.g. light) can travel through the classical vacuum, and they travel at speed $c$. (Also the speed $c$ of the wave is not related to the speed of the source.) These facts are all sometimes what is meant by the speed of light (in a vacuum) is constant. That $c$ doesn't depend on your inertial frame, that the speed of the wave doesn't depend on the source, and that the speed of the wave doesn't depend on the inertial frame. OK, so the speed $c$ doesn't ever change, but it is not the speed of light, it is only the speed of light in a vacuum.

So, the second thing that is often called the speed of light, is the speed of an electromagnetic wave in a medium (air, glass, water, something exotic, etc.). This is not constant, if fact there are frames (relative to the medium) where the speed of an electromagnetic wave in that medium is zero.

I wanted to make this a comment, but I don't have enough points yet. I remember studying this back in the 2nd semester of E&M (relativistic) and like most things in physics, it's hard to explain what's really going on in English.

The model we used to explain the "slowing" of light in a medium was to model each of the atoms as a harmonic oscillator (like a spring attached to fixed point) and when the light - previously traveling in a vacuum - hit the medium, it turned from pure vacuum vibrations into springy electromagnetic atom vibrations. "Plasmons" i think is the correct name for these. If the medium is opaque to the incident light's wavelength, surface plasmons are generated.