Can light ever travel slower than vaccum speed?

[Drosera]

Hello all!

 

I understand that light travels at the speed of light *in a vaccum*. What puzzles me is why doesn't it travel at the same speed when not in vaccum. Is this because photons are absorbed and re-emitted by particles in non-vaccum areas? (the absorbtion and emition causing a perceived slowing in speed) Or are there other factors I'm not aware of?

 

Thx, and take care!

[half-death]

Hello!

As far as I know, absorption and re-emission is what explains refraction and thus, the speed of light do not vary in that case...

But, I do not know if it always, always constant...

[Justin]

I've often ranted about the famous Einstein equation, E=MC^2 and the amount of times that it's miss-quoted, as it relates to the maximum speed of light in 'free space'. A vacuum is not free space. There's no such thing as free space, you'd have to move beyond the effects of the Universe to get to free space and as magnetic and electric fields (not to mention gravitational fields) follow a 1/distance squared rule it tends towards no effect but never gets there. Even if you could remove yourself from the effects of the Universe, then the equipment that you used to measure the light speed would still screw you up, as it would pollute the pristine 'free space' that you'd found.

 

Having said that when light is not in free space, then the local magnetic/ electric fields will effect the photons as they're an electromagnetic phenomenon, causing the speed of light to vary, and for some reason this seems all ways to decrease the speed. May be if we understood more we could create a region of space where we could increase the limit, rather like the negative refraction materials that have now been developed?

 

Does this help at all?

[Drosera]

Hi Justin,

 

Your answer may indeed help.

 

So just to clarify, the speed of photons is never "the speed of light" because there are always electrical or magnetic fields that affect their speed.

 

AS I understood thing before your answer, light always traveled at its' maximum speed ©, but when affected by a force, such as gravity, the speed didn't change, but the wavelength did. But what you are saying is this isn't always true, right?

 

Thanks.

[Tormod]

So just to clarify, the speed of photons is never "the speed of light" because there are always electrical or magnetic fields that affect their speed.

 

This is purely academic tautology. The photon, which is the carrier of electromagnetic force, must travel at the speed of light because it *is* light.

 

c is simply the definition of the speed of light in a perfect vacuum - it doesn't matter if, theoretically, vacuum isn't empty space.

[cwes99_03]

Hello all!

 

I understand that light travels at the speed of light *in a vaccum*. What puzzles me is why doesn't it travel at the same speed when not in vaccum. Is this because photons are absorbed and re-emitted by particles in non-vaccum areas? (the absorbtion and emition causing a perceived slowing in speed) Or are there other factors I'm not aware of?

 

Thx, and take care!

 

As far as I remember, and I will look it up when i get some free time in an hour or two, this is the correct assumption.

 

Refraction is the slowing down of light. Everything that allows light past it refracts light a bit. There is an n value for the refraction of light by different gasses, glasses, liquids etc. The greater the n value, the slower light travels through the medium. The reason we get rainbows and prisms work, is that light of different frequency has a different value of n in the same medium. Thus red light travles through the prism more slowly than violet and it gets "bent" more.

At least this was one way it was described in my gen physics class. I haven't looked at this in a while and can't remember. What I do remember is a lab experiment I did with a prism to prove the equation for the value of n in a prism of a particular type of glass using a mercury arc lamp (because it gives three very well defined spectra in violet green yellow and orange frequencies of light.)

 

What's really interesting is when science says that in some mediums n values of less than 1 can exist. (BTW n=[v_medium]/c, thus the speed in the medium normalized by the "maximum speed of light" measured in a vaccum.) Yep that means light is breaking the light speed barrier. I never have looked further into this to understand what actually is going on.

[Erasmus00]

What's really interesting is when science says that in some mediums n values of less than 1 can exist. (BTW n=[v_medium]/c, thus the speed in the medium normalized by the "maximum speed of light" measured in a vaccum.) Yep that means light is breaking the light speed barrier. I never have looked further into this to understand what actually is going on.

 

Its actually the phase and not the group velocity of light that can exceed c. For a good visual tutorial, check this out. A nice visual explanation of the difference.

 

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

 

-Will

[CraigD]

I think the answer to the question “can light ever travel slower than the speed of light in a vacuum ©” hinges on the distinction between the terms “light” and “photon”.

 

“Light” certainly can, and does, travel slower than c in medium other than vacuum. Precisely how much slower (or, in atypical cases involving measurement of phase or group velocity rather than signal velocity, faster, or even negative!) is given by that medium’s refractive index, (n=v/c). Was this not the case, the refractive index, and refraction itself, which make such things as lenses and spectroscopic prisms possible, would not exist.

 

Expand on half-death’s post #2 (“…absorption and re-emission is what explains refraction…”), here’s my take on the actual behavior of a photon in a non-vacuum medium. Note that, for the sake of simplicity, I’m ignoring the effects of gravity predicted by General Relativity. In a mundane laboratory (eg: one on Earth), these effects are negligible). I’m also neglecting to describe particles’ behavior in quantum mechanical terms, because for a large number of interactions of many different-energied particles, quantum mechanical effects tend to average to what they would be given a classical (Newtonian) description.

1. photon P1 traverses a distance D1 between its starting point, and the first electron it encounters, E1, in time T1=D1/c
2. P1 is absorbed by E1, changing the radius of its orbit around its nucleus, by an amount given by the energy (frequency) of P1, in time Ta1
3. E1 returns to its original orbit, releasing a photon P2 with the same energy as P1, in time Tb1
4. steps 1-3 repeat for photon P2, electron E2, and photon P3, giving distance D2 and times T2, Ta2, and Tb2

Summed over many repetitions, then, the speed of light in the medium containing electrons En is

v = Sum(n=1 to m)(Dn/(Tn+Tan+Tbn)),

where m is the number encountered by a series of related photons passing through a given distance in the medium.

 

The summary of all this is that the speed of light in a specific medium (and, hence, that medium’s refractive index) depends on the average distance a photon travels in it before encountering an electron, and how long the encounter takes, on average. If 0 encounters occur, the resulting v = c, and the medium is effectively a vacuum.

 

Note that this explanation works for situations involving the speed of light for a single photon (in which complicating concepts like group and phase velocity have no relevance), and predicts v <= c . For many-photon ensembles, the explanation becomes much more complicated, and v < 0 or v > c become possible, as v is no longer related to the position of a single photon, but instead attributes of the many-photon ensemble.

 

Thus, (ignoring Special Relativity and Quantum Mechanics) “light” can travel slower than c, but photons cannot.