Not inside of materials. The speed of light changes inside of materials, so lamdba no longer equals c/nu--it's now c/n*1/nu. Just as the permeablility can stay the same as the permittivity changes, the frequency can stay the same as the wavelength changes.

Worzel,

If you check the link I posted above you will see this:

"There is another interesting possibility for breaking the light-barrier by an extension of the Casimir effect. Light in normal empty space is " slowed" by interactions with the unseen waves or particles with which the quantum vacuum seethes. But within the energy-depleted region of a Casimir cavity, light should travel slightly faster because there are fewer obstacles. A few years ago, K. Scharnhorst of the Alexander von Humboldt University in Berlin published calculations4 showing that, under the right conditions, light can be induced to break the usual light-speed barrier. Under normal laboratory conditions this increase in speed is incredibly small, but future technology may afford ways of producing a much greater Casimir effect in which light can travel much faster. If so, it might be possible to surround a space vehicle with a " bubble" of highly energy-depleted vacuum, in which the spacecraft could travel at FTL velocities, carrying the bubble along with it. "

This experiment has been performed. Under these conditions light travels faster than "C" in a "normal" vacuum. So then, what is the velocity of light?

Tobin,

The wavelength and frequency of light in vacuum (whatever that really is) is directly related as the reciprocal. That will determine the amount of refraction when that light enters a transparent medium. Yes, the wavelength will be compressed depending on the refractive index of the medium. The frequency will not change since that would mean the light would change color.

Surroundimg a spaceship with a bubble of negatively charged energy in order to travel faster than light is a real possibility, explored in the so-called Alcubierre-Broeck drive concept;

unfortunately the details of this concept are a little difficult to achieve, as the spacecraft has to be shrunk to the effective size of an atom in order to fit inside the bubble.
Maybe one day, but not in the immediate future.

Quantum tunneling is a real FTL effect, like quantum entanglement; but these phenomena are not believed to carry usable information at faster than light speeds.

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One more thing. According to Einstein:

""the mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 × 10^20, the energy being measured in ergs, and the mass in grammes"

This is easily experimentally verified.

Eburacum45,

Quantum tunneling can carry usable information at FTL speed. It is subject to restrictions, but not those envisioned by Einstein.

Read more here:

http://www.physics.sfsu.edu/~ebierma...tCausality.pdf

Oh, you're talking about c, when you quoted me you seemed to be questioning my statement about vacuums.

Again, what you post is interesting. But the effect is very small and difficult to reproduce compared to the apparent slowing down of light through a medium. The Cassimir effect not withstanding do you agree with what I originally posted?

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If logic doesn't work, then surely it does.

There seems to be some possibility of faster than light transmisson over very small distances by quantum tunneling, or though vacuum containing negative energy; but the information seems to be subject to uncertainty, so needs a classical channel message as well to set up a comparison; this classical channel message is slower than light.

However, if any of these tiny effects do allow FTL information transfer at some time in the future, the door then opens to causality paradoxes; you can't have messages passing around in relativistic space time without eventually running into causality paradoxes.

This may or may not rule out any FTL information transfer altogether.

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1. That's good, but let's make that "rest mass". And taking up space isn't necessary. No experiment has ever found the electron to take up any space, except that "space" dictated by the Heisenberg Uncertainty principle.

2. magnetic fields, electric fields, electromagnetic fields, .... A field is an entity with the potential to cause a change in momentum with time (or equivalently, cause a spatial gradient in potential energy). Certainly, magnetic fields are easy to demonstrate.

3. That's right. Real photons can exert momentum, and virtual photons are the exchange particle of the Electromagnetic force (but I don't think that discussions of quantum field theory and virtual exchange particles are appropriate for 3rd graders).

Either your son's teacher or the science dept or, ... has their head in a fog about these things. This is clearly BAD Science, no question, based upon interconnected layers of misconceptions.

I heard more like 3 atoms per cubic meter... a near vaccum...

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Religion is a culture of faith; science is a culture of doubt.

Worzel,

If you mean do I agree with the absortion and re-emission statement, then, yes. The question is "what is a true vacuum?" The classical definition of a vacuum breaks down when quantum properties of matter and space are considered. As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".

TravisM,

Perhaps you are thinking of the density per cubic centimeter. Check here:

http://hypertextbook.com/facts/2000/DaWeiCai.shtml

Here is a site listing representative gas densities of the medium filling the Galaxy.

This one tabulates representative densities averaged over very large volumes of space. There is a lot of mostly empty space between galaxies and especially between galaxy clusters. Best estimate for this average is about 0.25 protons per cubic meter.

Fair enough. I was just giving the normal answer to the misconception about light only going at c in a vacuum. I really don't understand all this vacuum pressure, energy, etc. at all.

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If logic doesn't work, then surely it does.

Sorry, it wasn't Eddington, but J.B.S. Haldane!

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T. Anderson

There is the speed of light in a vacuum = 2.99792458e8 m/s, and then there is anomalous dispersion and quantum entanglement. For an explanation of the former, go here.

Maybe you should edit your original post. Even though we are all aware that we shouldn't believe everything we read, many people pick up bits like that

It appears that quote is attributed to a number of people including Eddington, Haldane, Clarke et al.

http://www.quotationspage.com/quote/27537.html

http://www.quoteshead.com/show_autho...r-Eddington/2/

Not proof he said it but it was something he would say.

Spaceman,

Please define "vacuum".

I'm sort of concerned that no one bothered to point out that this experiment is entirely fruitless. There's ALWAYS light shining into the glass, at least if there's a third grader near it. I assume this experiment is done during the day time, or in a room with, say, a light turned on. Or even if it's not, you can leave a glass of water on the counter in the middle of the day, and watch it not overflow for hours on end.

Did someone forget to inform this teacher that light is the thing that lets you see, and not just some magic beam that comes from a flashlight?

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Ok, I will try, though quantum field theory is not my bag; I am an astronomer.

Spacetime is filled with quantized "force fields": electric, magnetic, electromagnetic, etc. That is the vacuum as we understand it today. Light is a disturbance (or traveling wave) in the quantized electromagnetic field. When this disturbance interacts with say an "electron field" (not to be confused with an electric field), energy is exchanged in the form of quanta (photons, electrons). An electron is said to 'exist at a location' where its "electron field" (square of wave function) is a maximum. As long as the electromagnetic disturbance is passing through space that is essentially 'empty' of matter fields with maximum amplitudes, then light is said to be traveling through a vacuum and does so at speed c. I probably haven't explained this precisely right, but it's probably not too far off. I invite someone else closer to the "field" to spiff this up.

The expression "speed of light in a vacuum" has its origins in Maxwell's equations that predates quantum mechanics, and so what is meant by such a "vacuum" has changed since Maxwell.

I believe JBS Haldane used the qualifier "queerer" rather than "stranger". It is possible that Eddington was paraphrasing Haldane.

Some interesting comments here:

http://www.colorado.edu/philosophy/v...r/Briefs/c.pdf

"...there is no such thing as a perfect vacuum in the universe and so any
observed light beam must travel through a medium and will have an effective speed c/n, where n is the index of refraction of the medium. While this usually yields speeds less than c, some media, such as highly ionized plasma, can have an index of refraction less than 1 over a limited light frequency range. In that case, the effective light speed can exceed c."

Yes, JBSH used the word "queerer" (back before "queer" had its current connotation). I should have mentioned that in my original post.

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The project will not only cost more than you imagine, it will cost more than you can imagine.

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Well, yeah, if you want to get pedantic about it, he's right. Although for most photons (or disturbances in the electromagnetic field) traveling through the 'medium of space', this index of refraction is not consequential.
He's also gotta be careful by what he means by c/n > 1. There are also some recently discovered media with negative indeces of refraction (n &lt; 0). Does that mean that v_photon &lt; 0? I don't imagine so.

Edit: I take back this last comment. Go to this web page (and click on "What is Left-handed material?") where they note that these 'metamaterials' with negative indeces of refraction have the property that the energy in the light wave moves in the opposite direction of the wave propagation.

As the guy at the link points out a negative index of refraction implies a photon velocity >C in the medium, not &lt;0.

Victor Stenger mentions a case of n &lt; 1, not a negative index of refraction (n &lt; 0).
I've posted a new link on such materials in my previous posting (now edited). But I think we've moved pretty far afield from the original post.

So...
I'm the only one bothered by the fact that this teacher doesn't understand that the glass is bombarded by photons constantly, and not only when the flashlight is turned on?
Curious. Curious indeed...

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"Isn't that just...bread?"

You're right, this is pretty far. But on this site it says "Amazingly, it is possible to construct exotic materials having frequency regions where the sign of (graphic formula) must be negative...".

I think that can be taken as a negative index.

No, you're not the only one. I've already commented above that the teacher and/or the science dept have thoroughly confused themselves with misconceptions.

It's just that this topic has gone off on a tangent or two with some of Evan's inquiries. I and others contributed to the tangent, and everyone else probably lost interest. Apologies on my behalf. Hopefully, Boris got all he wanted to know.

Yeah, I saw comments. It's just really eating at me that someone could forget that the light they see by also counts as light. And hey, I don't really know what everyone else is talking about

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"Isn't that just...bread?"