# How constant light?

• Selnex

#### Selnex

It has been suggested by a notable scientist that the speed of light is constant. However does this mean that the speed of light is the same regardless of what medium it is traveling through? I am nearly certain I have heard that the speed of light is not the same in water as it is in air. More importantly perhaps, there was that experiment done to attempt to prove that ether in space does not exist and I think that experiment was dependent on the conceptual notion that light would travel at a different rate in ether as it would in a vaccum.

Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD

When it is said that the speed of light is constant, it is meant that it will be measured as the same regardless of who is doing the observing or what speed they are moving relative to each other.

Two observers are moving with respeect to each other. A beam of light passes both of them. Each will measure that light beam as traveling at the same speed with respect to themselves.

The speed of light may also vary within the Heisenberg uncertainty principle. Since the vacuum contains fleeting virtual particles, fundamentally the speed of light is constant there only in the average over quanta.

Accelerating influences like inflation and vacuum energy/quintessence may cause variations in light speed in vacuo.

Originally posted by nbo10
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD

basically that's just a restatement of the principle of inertia as applied to light. there probably isn't a real vacuum in space so light speed is never constant.

Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.
HUGE misconception here. Matter, so most mediums (media?) is almost entirely empty space. Light travels at C through these empty spaces. But when it hits an atom, it is absorbed (or reflected). When absorbed, there is a delay before it is re-emitted (if it is re-emitted). This delay is responsible for the APPARENT speed of light being less than C when traveling through a medium. In actuality, whenever light exists as light it is ALWAYS traveling at C.

Note - The coordinate (i.e. non-local) speed of light varies in the presence of a gravitational field.

Pete

Hi,
No Russ. I have to disagree with you. Though your explanation is partially acceptable, it is not relevant in this case.
When one is talking about the constancy of the speed of Light, he is talking about the ultimate speed of propagation of information by any means. That is the heighest attainable speed. In vacuum light travels with a speed of c, which cannot be attained by any material particle. Whereas, when light is traveling through some medium, it is traveling with a lesser speed, but still the speed limit for information propagation is c, and not the speed of light in that medium. Hence, it is possible for a material particle to have speed greater than the speed of light in the medium. The phenomenon of Cherenkov radiation is a good example of it.
A better explanation of the original querry can be given as:
Einstein's theory of relativity does not directly say anything about the speed of light. It does say that there is an ultimate limit to the speed by which information can be propagated. (i.e. the only invariant speed in the spacetime transformation is finite, unlike infinite in Newtonian relativity.)
It is an experimental fact that the speed of light in vacuum does not depend upon the frame of referrance ...so it must be invariant and hence the symbol "c" (which just denotes invariant speed of the transformation) can be successfully identified with the speed of light in vacuum.
This is nothing to do with the fact that light travels with different speeds in different media, the laws of spacetime transformation does not change with the media and hence the value of "c". It is just a coincidence that "c" and speed of light in vacuum are the same.

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It's not a coincidence; Special Relativity quite clearly mandates that a particle travels at light speed if and only if its rest mass is zero. Since light has zero rest mass, it travels at light speed always, not just in a vacuum.

I agree with Russ, the "slowing" of light in materials is an apparent effect. We measure when light enters the object and when light comes out of the object with no reason to believe we're observing the exact same photon.

Hurkyl

A while ago there were some researchers who suggested that C (max) may have changed over the life of the universe.

Russ makes the matter seem to be composted of billiard balls which swallow upt and spit of BB's which are the photons. I've never pictured matter that way. The atom is not like a biiliard ball. Instead picture matter made of of a smeared out cloud out electrons over a lattice of ions.

EM wise (i.e. for a light*wave*) inside matter there is a preferred frame of referance and in that frame of referance the permiability of matter "u" and the permitivity of matter "e" is differnt than empty space. The wave velocity in matter (for a homogeneous and and istotropic medium) is v = 1/sqrt(ue). For matter v < c.

The constancy of light applies to vacuum.

Pmb

Hurkly - What's your point? You're just quoting someone who says the same thing as someone above. That was answered by an undergrad - it's just basically repeating the opinion mentioned above.

It's not very meaningful to say that a photon is emitted by an atom and reaborbed by the atom its already attached to, especially since the wave function of the electrons spread over the surrounding atoms.

However I'm of the opinion that neither view is an exact explanation.

Pete

A while ago there were some researchers who suggested that C (max) may have changed over the life of the universe.

I have heard about that, too and been wondering how rapidly this proposed change is believed to be occurring?

Lurch

I don't know, sorry.

No Russ. I have to disagree with you. Though your explanation is partially acceptable, it is not relevant in this case.
This may just be a question of semantics, but I don't think so. The problem is that if you fire one photon at a time through a medium, you will NOT get a constant C. Depending on the path each photon takes and the number of times it is absorbed and re-emitted, its apparent speed will be different. Thats why the apparent C through a medium is actually an AVERAGE apparent C.

Russ makes the matter seem to be composted of billiard balls which swallow upt and spit of BB's which are the photons. I've never pictured matter that way. The atom is not like a biiliard ball. Instead picture matter made of of a smeared out cloud out electrons over a lattice of ions.
Electrons are tiny and located far away from the nucleus of the atom (compared to their size). Thats a lot of empty space. The billiard ball analogy doesn't fit because billiard balls are solid. Atoms are mostly empty space. Also, you are thinking about solids. Gases are mediums too. I read somewhere that on its trip through the atmosphere a photon of blue light is only scattered about once before getting to the ground. Thats a long way to travel in a medium without hitting something.

russ said
Electrons are tiny and located far away from the nucleus of the atom (compared to their size). Thats a lot of empty space. The billiard ball analogy doesn't fit because billiard balls are solid. Atoms are mostly empty space.
That anaglogy is flawed. A photon does not move around inside an atom. A photon does not move around inbetween electrons either.

Do you know what is meant by the term "electron cloud"?

Pete

Originally posted by nbo10
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD
Look at that!
Is it because the vacuum is all alike?
Would you say that the velocity of light is also constant is same kind of waters?

I had came across last night the Ewald-Oseen Extinction Theorem, which I suppose is your "more complete answer".

When a light wave hits an opaque surface, it interacts with surface molecules causing them to vibrate which generates an electromagnetic wave that fully cancels the primary wave inside the opaque surface.

For a transparent medium, the cancellation is not total and the primary (precursur) wave indeed reaches the other side of the medium at the full speed of c, although extremely weak because the interaction with the molecules of the medium scatter the energy around... eventually the energy gets across and the amplitude of the wave on the far side matches that on the near side, taking a time consistent with a wave of velocity c / n (n = index of refraction).

The catching and releasing model would then be a good approximation to what actually happens.

Hurkyl

basically that's just a restatement of the principle of inertia as applied to light. there probably isn't a real vacuum in space so light speed is never constant.
No, it's not the principle of intertia. The speed of light is referred to as c because it is indepedent of the movement of your particular frame of reference. If you yourself are moving at .5 c relative to somebody on earth, light will still appear to be moving at c relative to YOU as well as on EARTH! That is what is unusual about lightspeed, and is where all the time dilation and length contraction effects originate.

Do you know what is meant by the term "electron cloud"?
Yes, the electron cloud is the probability function of where around the atom you find electrons.
That anaglogy is flawed. A photon does not move around inside an atom. A photon does not move around inbetween electrons either.
You seem to be implying that the electron cloud is an impenetrable barrier to the passage of light and that any light entering the cloud gets captured. It is not. In fact, the electron cloud doesn't even always surround the entire atom. Sometimes light travels right through an atom without being absorbed and sometimes it doesn't. This often depends on the wavelength. Thats what transparency is.