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?

[Freethinker]

Originally posted by: 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?

 

The "height" is actually the density. There is a higher particle count in the "higher" parts of the wave and fewer in the lower parts. These waves are made up of oscillations in density count.

[Uncle Martin]

Density of what exactly?

[GAHD]

that's what I'm trying to figure uout. Waves are waves no matter how you count them; if you super impose different frequencies you start having those frequencies cancel each other out where the peaks are in opposition, and stack where the peaks coincide.

now aplitude generally indicates the actual energy of a wave; a high peak indicates high energy(amps), where a high frequency gerally just indicates penetration(voltage).

a high'particle count' doesn't seem to have a place in a single particle/wave.

In the case of light stronger light has a high amplitutde; high energy infra read and low energy infra red are only defined by the amplitude of the wave in question(the reason LASER beam power is indipendant of it's frequency). Then again in the case of LASERS the beam is coherant and of a relatively uniform frequency, and thus the waves stack more than they cancel. In that case I can see where a 'high particle count' could come into play.

So I'm still trying to puzzle out what Bo and TeleMad are trying to say, as frequency=energy doesn't quite compute.

[FrankM]

GAHD 07/09/2004 06:47 AM - frequency=energy doesn't quite compute.

 

Max Planck already said it,"the energy of a radiation is proportional to its frequency." Albert Einstein

latter confirmed this and coined a new term, the quantum of energy.

 

Energy of a photon = E = hv

 

E = energy v = frequency h = Planck's constant = 6.63 X 10-34 J.s

 

To sum up: the higher the frequency the more energy a photon has.

 

http://www.ucdsb.on.ca/tiss/stretton/chem1/elecon3.html

[GAHD]

"Photosynthesis is started when a plant absorbs light. It is the frequency of light that is important." from http://www.ucdsb.on.ca/tiss/stretton/chem1/elecon3.html

The 'higher frequencies' Do work better for plants to a degree; in several varients of plants 'red' light causes the strectching of intrnodes(the space in between the branch offshoots), where blue light causes the plant to become more 'bushy' and leafy. Higher frequency light is simply better at causing reactions ala photosynthisis to occur. A higher amplitude would cause the plant to grow faster, simly because it receives more 'energy'(discounting that too high an amplitude would burn or vaporise a plant). A similar effect is observed in your microwave, where a specific frequency causes sympathetic vibrations in water; hightening the frequency causes thease vibrations to break down, where heightening the amplitude increases the speed of the reation.

BY example; white dwarfs emit high amplitude, high frequncy waves which could literally vaporise matter. Yet these high frequencies would simply pass harmlessly through most matter were it not for the high amplitude.

[TeleMad]

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.

 

No, the spontaneous emission of photons by electrons dropping down energy levels is a random event. If the process were as deterministic as you claim, then we would not see absorption lines in spectra from distant stars when the light passes through clouds of gas: but we do.

 

GAHD: 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.

 

...

 

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.

 

I disagree. One of the reasons that a "leading edge" of a light pulse can leave certain media sooner than light could reach that point were it traveling through a vacuum (i.e., what people consider experiementh showing light traveling faster than c through certain media) is because a photon interacting with an electron on one side of the medium can "instantaneously" cause an effect on an electron in an atom at the opposite, far side of the medium. What I read explained it kind of as all of the atom's electron orbits basically forming a sort of single, unified electron orbit, where an effect anywhere in that orbit could "instantaneously" cause an effect anywhere in that orbit, even if that caused effect would appear to exceed c. According to what you said above, this couldn't happen...the effect (electrons) would have to travel spherically around each atom in the chain, increasing the distance traveled, making it go slower than c.

[TeleMad]

Tormod: 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.

 

No, refraction deals both with directions (light moving from air into dense media, where is "slows down", and also with light moving from a dense medium into air, where is "speeds up").

 

A light ray of wavelength 589 nm (produced by a sodium lamp) traveling through air is incident on a smooth, flat slab of crown glass at an angle of 30.0 [degrees] to the normal, as sketched in Figure 22.11. Find the angle of refraction, [theta sub 2].

 

..

 

[theta sub 2] = sin^-1(0.329) = 19.2 [degree]

 

We see that the ray is bent toward the normal, as expected.

 

Exercise. If the light ray moves from inside the glass toward the glass-air interface at an angle of 30.0 [degrees] to the normal, determine the angle of refraction.

 

Answer. 49.5 [degrees] away from the normal.

(College Physics: Fifth Edition, Raymond A Serway & Jerry S Faughn, Harourt College Publishers, 1999, p732)

 

and also...

 

A light beam traveling through a transparent medium of index of refraction n1 passes through a thick transparent slab with parallel faces and index of refraction n2 (Fig. 22.12). Show that the emerging beam is parallel to the incident beam.

 

Reasoning. To solve this problem, it is necessary to apply Snell's law twice, once at the upper surface and once at the lower surface. The two equations will be rlated because the angle of refraction at the upper surface equals the angle of incidence at the lower surface. The ray passing through the slab makes equal angles with the normals at the entry and exit points." (College Physics: Fifth Edition, Raymond A Serway & Jerry S Faughn, Harourt College Publishers, 1999, p733)

 

 

 

Besides, if you don't think that refraction occurs when light exits a dense medium, then what mysterious force bends light when it moves from a dense medium into air? And why does this mysterious force follow the mathematics of refraction?