Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 01/12/2003 23:17:28
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We had a question on the show last night that I couldn't answer. Perhaps you lot can help. Here's the background :
Space scientists building a solar sail suggest that photons hitting it will impart to it their momentum / energy and push a spacecraft along.
The question is - given E=mc^2, how do photons travel at the speed of light unless they have no mass ? And how do they accelerate to light-sped in the first place ? I know that they can also be viewed as a wave, but if the solar sail idea is to work, then they must also ahve a mass presumably ?
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Photons might not have significant mass, but they can still impart energy due to a collision. Temperatures near absolute zero are obtained using streams of photons (i.e. lasers) to strike a cloud of atoms. There's some quantum physics involved, but essentially, the Doppler effect makes it so that only atoms that are travelling against the direction of the velocity vector of the photon are affected by it. The net effect is gradual loss of molecular motion, thus cooling the atom cloud. Really cool stuff.
It's commonly accepted by physicists that photons have no "real" mass in the classical sense, but they have relativistic mass as per the Einstein equation. You can't use traditional mechanics with relativistic mass, so when talking about photons imparting momentum, you're really talking about them imparting energy which in turn is converted to work presumably by the photoelectric affect in the case of these solar sails. (or alternatively, by the energy-momentum theorem which relates energy put into a system to the momentum imparted on the system and vice versa)
If you want to know more about photon mass and all the relativity involved, go to this site:
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html
It will probably make your head hurt, I know it did mine.
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I remember studying this very problem, but I don't remember the specifics. I do remember that the solar sail is better if it's a reflector because bouncing the photons gives twice the impulse of just absorbing them. It is a better practical solution because there is less heat buildup.
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John
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If the photons have inelestic collisions and bounce off, isn't most of the energy staying with the photons? You'd need an awful lot of surface area and an awful lot of photons to net any significant energy. Also, if they ARE leaving some energy behind, would there be a shift in the photons' wavelengths? Say you're bouncing UV off of this sail, would the UV lose enough energy to re-emit as visible light? Sounds like a cool physics problem to me...given a photon density in a region of space and the speed of the photons, calculate the surface area needed to generate 1 million Joules of energy per minute.
What about photoelectric cells? Would they be more efficient than simply collecting fractions of photon kinetic energy? I've heard that photoelectric cells have gotten quite good lately.
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They're still only 40 percent effecftive unfortunately (I think)
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40% efficiency is quite good in the world of energy production. Your car is only 30% efficient in the conversion of chemical fuel to motive force. The rest is lost as heat, noise, and friction.
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The answer to your problem is simple- photons have no mass. In addition, photons can only travel at the speed of light, so do not need to accelerate (they are emitted at the speed of light). While they have no mass, it can be proved that photons have a momentum- and it is this momentum which impacts the solar sails- which has to be very large indeed!
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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So what happens when light is "slowed down" by passing through a different medium ? When it exits that medium presumably it speeds up again and hence has to accelerate doesn't it ?
chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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I was wondering the same as Chris, and also, how can something have momentum if it has no mass ? momentum = mass x velocity. If m is zero then how can it have momentum ?!
G
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Check out this site for explanation of photon "mass":
http://www.usatoday.com/weather/resources/basics/wonderquest/photonmass.htm
Regarding photon speed- photons always travel at the speed of light- but light is "slowed down" in anything other than a vacuum not due to the photons travelling at a reduced speed, but due to collisions with other particles meaning that the path of light is no longer straight and so, it takes longer to reach its destination. As you can then see, the photons don't need to accelerate as they always travel at c but maybe not in a straight line though . Light and therefore photons have to travel at a constant speed, as that is what the special theory of relativity is based on (assuming of course, that it is correct and that the speed of light is not slowing down as the universe gets older)
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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Oh- and here is how a photon's momentum is calculated:
http://www.tau.ac.il/~lab3/MOSSBAUER/Recoil.html
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe