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Accelerating the Speed of Light

Beady

Philosopher
Beady
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One of those things that enters your mind while you wait to fall asleep:

The speed of light is 186,000 miles per second in a vaccuum. It slows down when it enters another medium (if a vaccuum can be called a medium), such as a lens, a copper wire, whatever. The deceleration is essentially caused by friction with the medium, and the energy is bled off as heat and/or light (I know this is a highly-abbreviated explanation, but work with me).

What happens when it leaves such a medium and reenters a vaccuum? Does it speed back up? If so, where does the energy for the acceleration come from? And, since it takes more energy to accelerate than to maintain a constant speed, what happens to the extra energy needed for the acceleration when speed-in-a-vaccuum is reestablished?
 
I don't think there is energy loss when a photon slows down, since photons have no mass.

eta: That was probably wrong, believe what Tobias wrote below instead :)
 
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Beady said:
One of those things that enters your mind while you wait to fall asleep:

The speed of light is 186,000 miles per second in a vaccuum. It slows down when it enters another medium (if a vaccuum can be called a medium), such as a lens, a copper wire, whatever. The deceleration is essentially caused by friction with the medium, and the energy is bled off as heat and/or light (I know this is a highly-abbreviated explanation, but work with me).

What happens when it leaves such a medium and reenters a vaccuum? Does it speed back up? If so, where does the energy for the acceleration come from? And, since it takes more energy to accelerate than to maintain a constant speed, what happens to the extra energy needed for the acceleration when speed-in-a-vaccuum is reestablished?
Click to expand...
false.

The light doesn't slow down. The light simply has to take a tangled path. The individual photon/wave would still move at the speed of light, but it will have to move through a maze. And the path the individual photon will follow is quite long. So, overal the speed of light through the substance will be less than C, but only if you look at it from "point of entry to point of exit". If you follow the path of the photon as it goes through the matter it will always be traveling at c. So, once it gets out of the matter that is making it go slower, there is no longer any weird path to follow, and it will now continue moving at the speed c, but no longer having to travese a tangled path, but will instead move at the apparant speed of c(even though it always had the speed c).

Hope that explains it.

Imagine a car driving through a maze at a constant speed of 10km/h, and the maze is 10km long. if it takes 3 hours to get out does that mean that the car only drove at 3.1/3rd km/h, or does it just mean it had to drive an odd way that made the total distance traveled 30km/h.

:)
 
The heat in a wire comes from electrons hitting the atoms of the metal. No light is involved (unless it gets really hot).
 
Heat in an electircal wire has nothing to do with photons entering the wire -. The EMF causes electrons to move in the wire - the moving electrons will sometimes collide with the metal atoms and transfer some of their kinetic energy to them (in the form of vibrational energy I think). This energy is then lost as heat (IR radiation).

The original question was a good one I think - when I started doing my undergrad Physics degree, this was a question that bugged me, and took various forms, such as "would red light travelling in a medium therefore appear a different colour". The explanation given by Tobias is an excellent simplified explanation of what goes on - for a more detailed explanation I recommend the appropriate Chapter in Eugene Hecht's "Optics" which I think is a brilliant textbook. I do think that teaching the notion that light slows down in other media is a bit disingeneous and high school physics teachers should think the terminology through more carefully.
 
Beady said:
So, where does the heat in an electrical wire come from?
Click to expand...
OK - I'll bite ... electrical wires conduct electricity - the net movement of electrons from one end to the other driven by a difference in potential between the two ends ... generally the wire has a resistance to the movement of these electrons (lets not detour off towards superconductors here ;)) - the electrons have to bounce around a bit, sometimes bouncing other electrons out of an atom and replacing them - energy lost in all this bouncing is dissipated as heat and light ... the most obvious example of this is a ... :lightbulb

However, you were talking about photons of light - they have different properties to electrons ...

(ETA: whoops - others beat me to it ...)
 
MetalPig said:
Also, electrons go nowhere near the speed of light. More like a few cm per second, if I remember correctly.
Click to expand...

Drift velocity (the average of all electrons) in a copper wire carrying current is indeed very slow. The instantaneous velocity of individual electrons, however, can be much larger. In graphene (a single layer of graphite), for example, they can travel around 10^6 m/s, which is quite fast:
http://physicsweb.org/articles/news/9/11/6
That's much larger than typical materials, but the difference between drift velocity and individual electron velocity is often pretty large.
 
TobiasTheCommie said:
false.

The light doesn't slow down. The light simply has to take a tangled path. The individual photon/wave would still move at the speed of light, but it will have to move through a maze.
Click to expand...

Exactly right. When I describe this to people who haven't had QED, I call it a slalom.
 
Beady said:
So, where does the heat in an electrical wire come from?
Click to expand...

More specifically than the answers you have been given, for the same reason that relativity gives rise to electromagnetism, moving electrons in a band structure are more attracted to the protons and so move to lower energy levels. The energy is given off as heat.

Superconducting subverts this by being so cold that there aren't any lower energy states to go into, given a certain limited current.
 
I think the question still makes sense even in classical electromagnetism with a continuous medium into which the light is propagating. As such I don't think quantum mechanics is necessary for a satisfactory explanation. But it is cooler of course :)
 
Almo said:
How do you explain it to people who HAVE had QED?
Click to expand...

The amplitudes of the photons interact with the amplitudes of the electrons in such a way that the summation of histories is reinforced most for ideal, maximal probability paths that are not in a straight line.

The nice thing about this is that QED also explains why light goes in a straight line (or a geodesic) in a vacuum.
 
You're all wrong.

TIME goes slower in glass. Well known fact.
Didn't my neighbour's crystal therapist tell me about it on Sunday?
 
Maxwell's Demon said:
I do think that teaching the notion that light slows down in other media is a bit disingeneous and high school physics teachers should think the terminology through more carefully.
Click to expand...

I agree. Indeed, I find it easier to explain to kids the 'maze' concept of light moving through a medium, as it's easier to discuss a range of other factors on the topic of EMR.

The thing is, I didn't understand the concept myself when I first started to teach it (I have a biology background). It was only because I asked somebody to explain it to me that I happened on the analogy myself, and used it in class.

Athon
 
Soapy Sam said:
You're all wrong.

TIME goes slower in glass. Well known fact.
Didn't my neighbour's crystal therapist tell me about it on Sunday?
Click to expand...

Time goes much more slowly when you're listening to Philip Glass.
 
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