Faster than the speed of light

[Aki]

I found something interesting at this site: http://www.nuc.umr.edu/reactor/reactor.html (in the middle of the page, "The Blue Glow". The article says that "beta particles are emitted with such high kinetic energies that their velocities exceed the speed of light in water". How could that be possible? According to Einstein's E=mc^2, the mass would be infinite!

[Tormod]

The speed of light in vacuum is the currently known speed limit of light. However, in water light travels more slowly so there is no paradox here.

Tormod

[Uncle Martin]

Originally posted by: Aki
How could that be possible? According to Einstein's E=mc^2, the mass would be infinite!

The relative mass of any matter increases with velocity, which is another way of saying how much energy is required to increase it's velocity. It does not increase in size. I hear this misunderstanding often. This is why only massless particles can travel at c, for as matter approachs c, it's relative mass increases to infinity(the energy required to accelerate further )so it would require infinite energy to propel even one atom to c, which, if possible, would not increase in size to fill the universe. Rest mass and relative mass are two very different things.

[TeleMad]

Actually, the speed of light in water - and any other medium - is also c. It is the apparent speed of light that is reduced.

I just found this out about a week ago. I was thinking about light. I had no problem with the fact that light slows down when it enters a denser medium (such as leaving air and entering glass). It was when it returned to the less dense medium that puzzled me. How does light automatically speed up when it leaves a dense medium, such as leaving glass and returning to air....isn't this an acceleration without a force? After all, the glass doesn't spit the light out; and one can't say that the electric and magnetic fields somehow accelerate the light as that would have something accelerating itself. I asked at a different discussion form and the solution was simple: light doesn't slow down in the medium, so there is no acceleration when it exits.

Now, as to exactly why light appears to slow down in a medium, I was never able to get a definitive answer. One offered was that the photons keep getting absorbed by electrons, and then reradiated, causing delays (though the speed between atoms woudl still be c). I don't buy that for 2 reasons: photons are emitted in random directions and so photons would not be reradiated in the same direction that they were originally traveling. Second, photons of different wavelengths than those absorbed can be emitted. So electron-absorption followed by reemission would scatter the light and maybe even produce different colors - a red laser beam entering a prism would scatter in all directions (and maybe even change color)...that doesn't happen: the beam remains coherent and retains its original color.

The other explanation was that the light waves cause electrons in the medium to oscillate, and since electrons are electrically charged, their acceleration produces new electromagnetic waves, and these new EM waves interefere with the original waves. Sounds good. But I still don't know exactly how that would cause an apparent reduction in speed of light.

[Bo]

well i like your theory, but i cant completely agree with it, since your main argument that 'energy is created' when the light leaves the medium is not correct. The reason that light travels slower is because the frequency of the light rises a bit (depending on the medium). Since frequency~Energy this energy is temporarely stored in the frequency. The rate in which this frequencychange happens is due to the electromagnetic properties of the material, so i think actually your last point is a prove for the accepted theory (i dont really see how it could alter the apperent speed of the light)
I completely agree with your agrument as to why absorption/emission wouldnt do the trick. (another argument: absorption only happens for light of a medium-specific energy; yet this is not observed in the change of c in a medium)

Bo

[GAHD]

telemad,The photon enters one of the electron layers of an atom and is absorbed by an electron, converting this electron into a high-energy varient. This high energy electron jumps up a level in orbit, then continues on its orbital path to the point exactly opposite the point where it absorbed the photon. At this point the electron drops back to it's proper 'level' and emits the photon along the appropriate trajectory.

Think of it as shooting a ball of water with a pellet; there will be a wave of energy that goes from the point of entry around to the opposite end, where if another pellet is positioned, it will be sent on it's way.

That was a simplified version, you also have to take into account the alteration in trajectory caused by the material, also known as refraction.

EDIT: forgot to mention that the toal distance now traveled by that photon was increased by having to follow the 'shere' of the electron's path rather than the straight line it normally would have.

[GAHD]

Originally posted by: Bo
well i like your theory, but i cant completely agree with it, since your main argument that 'energy is created' when the light leaves the medium is not correct. The reason that light travels slower is because the frequency of the light rises a bit (depending on the medium). Since frequency~Energy this energy is temporarely stored in the frequency. The rate in which this frequencychange happens is due to the electromagnetic properties of the material, so i think actually your last point is a prove for the accepted theory (i dont really see how it could alter the apperent speed of the light)

I completely agree with your agrument as to why absorption/emission wouldnt do the trick. (another argument: absorption only happens for light of a medium-specific energy; yet this is not observed in the change of c in a medium)
Bo

An interesting idea, the main flaw I find in this is that we have yet to show that different frequencies of light travel at different speeds. If i'm wrong on this please show me some reserch into that area.

Edit with a further thought; if frequency=energy then the photons emited would HAVE to retain their frequency if the speed of light is constant; the energy of the frequency is in excess of that needed to accelerate the photon to C, indicating that once that energy is re-emitted it would have to be emitted with that same frequency in order for the electron to drop back into it's valence layer.

[Tormod]

Originally posted by: TeleMad
Actually, the speed of light in water - and any other medium - is also c. It is the apparent speed of light that is reduced.

I just found this out about a week ago. I was thinking about light. I had no problem with the fact that light slows down when it enters a denser medium (such as leaving air and entering glass). It was when it returned to the less dense medium that puzzled me. How does light automatically speed up when it leaves a dense medium, such as leaving glass and returning to air....isn't this an acceleration without a force? After all, the glass doesn't spit the light out; and one can't say that the electric and magnetic fields somehow accelerate the light as that would have something accelerating itself. I asked at a different discussion form and the solution was simple: light doesn't slow down in the medium, so there is no acceleration when it exits.

The *logic* of this sounds good. However, refraction only explains that light is slowed down when it passes through a medium. It does not explaing why it speeds up.

Here is a brief, popularised explanation of refraction for thos not familiar with the concept:
http://science.howstuffworks.com/light12.htm


Light does speed up instantly, wihtout accelleration. I think the main reason is that the speed of light in a vacuum (ie, when there is no longer any medium to pass through) the light wave is completely unbounded in space and is free to travel at it's speed limit. There is nothing to slow it down.

I think the argument that light cannot slow because it cannot accellerate again has a fallacy: How did the light wave achieve c in the first place? There are no catapults in the stars. Light moves at c when in a vacuum, and slower when in a medium.

It's like electricity: when you switch on the lights in your house, electricity flows through the wires *almost* instantly, only limited by the refraction caused by the conductors in the wires.

If c did not slow down in a medium, then there would be no need for superconductors!

Tormod

[GAHD]

another thought; frequency is the distance between waves, is not the amplitude of those waves(the 'height') actually what dertermins the energy they posess?

[lindagarrette]

Now, I'm confused as to what is doing the " speeding." The photon which I assume is composed of various frequencies of light waves, take a constant amount of time for us to detect its source but is it la trajectile. Don't all the waves get from start to finish at the same time regardless of length? What is left of a photon if a bunch of frequencies, like UV are filtered out? Is it smaller? or are there fewer of them?