Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 24/12/2009 04:05:30

Title: How does a solar sail work?
Post by: chris on 24/12/2009 04:05:30

Photons have no mass; this being the case, how does a solar sail work?

Title: How does a solar sail work?
Post by: Mazurka on 24/12/2009 11:20:58

The solar wind consists of charged particles (mainly hydrogen nuclei/protons and electrons) which could do some pushing although I vaugely recall (from days as a science fiction roleplay nut) that Drexler did some work on materials he claimed would be suitable as a solar sail based on photons. 

Persoanlly I am not entirely conviced (although that miught be that i am not a good enough physicist) and blame Crooks Radiometer for putting ideas into peoples heads...

   

Title: How does a solar sail work?
Post by: chris on 24/12/2009 11:41:22

Mmm, that's interesting; Friedrich Zander, who was a Russian scientist, proposed the idea of photon pressure in 1924. What I'm trying to understand, however, is how exactly "photon pressure" works? How does this massless "particle" give a surface a push?

chris

Title: How does a solar sail work?
Post by: JP on 24/12/2009 16:27:32

Conservation of momentum holds whether or not something has mass.  This is a very rough analogy, but it's kind of like shooting tiny bullets at the sail.  Each bullet will give the sail a little "push" due to conservation of momentum.  These pushes are each tiny, but since there's a lot of them and almost nothing in space to slow the sail down, it adds up to pretty high speed over time.

More mathematically, conservation of momentum says that the final momentum of the sail, p_s after the photon is absorbed has to be equal to the initial momentum of the photon p_ph, or:
p_s=p_ph,
and the photon's momentum is equal to its energy divided by the speed of light.  Most sails would be designed to reflect photons, in which case the sail's momentum would have to be twice that of the incident photon (since both the direction and the magnitude of the total momentum have to be the same before and after the process).

Title: How does a solar sail work?
Post by: chris on 25/12/2009 01:33:33

Thanks. But as momentum = mv, if m=0, how can there be any conservation of momentum?

Chris

Title: How does a solar sail work?
Post by: JP on 25/12/2009 03:29:22

p=mv is the classical formula for momentum.  Remember that photons are moving at the speed of light, so you have to use relativity.  In relativity, rest mass, m, momentum, p, energy, E, and the speed of light, c, are related through:

E²-p²c²=m²c⁴.

A photon has zero rest mass (m=0), so you still have
p=E/c.  For massive particles moving at low speeds, you can also show that the usual equation, p=mv is just the lowest-order approximation to the relativistic equation

p=mv/(1-v²/c²),

where v is the velocity of the particle.

Title: How does a solar sail work?
Post by: chris on 26/12/2009 10:57:47

Thank you so much for that incredibly clear explanation.

Chris

Title: How does a solar sail work?
Post by: yor_on on 27/12/2009 04:12:05

So is this a proof of the photons 'travel'?
As they need a 'speed' to have a momentum.

And how about their energy?

They have a electromagnetic energy that influences matter, but not other photons if I understands it right?

Either they travel and then the momentum is a product of their travel, which then seems to say that they should have one 'shape' and not exist in 'many paths' where we count on probability to say whether a photon might have passed us or not.

Or they don't travel and 'many paths' is a reasonable idea, which then leaves momentum to become something truly strange when it comes to photons.

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Why I say that you can't have both is due to that if both exist. 'probability of many simultaneous paths' and momentum, then it seems as those not realized paths also should contain a momentum, which then should be testable by f ex. a solar-sail or something similar. It should also mean that there is a continuous 'push' everywhere if so?

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Sorry wrote this rather late, but it still makes sense, to me :)
You can look at 'many paths' two ways, as a mathematical tool describing probabilities where the solution will be the actual path, or as a straight proposal which is true for all possible paths.

From the observers perspective the first may seem to make the most sense but as I understands it it doesn't make the same sense from the photons perspective.
http://www.sporys.info/PhysicsTheMysteryoftheTwoSlotExperiment.htm

And that one is confusing to me.
If one assume that a photon exist without 'matter', that is being 'mass less', then the momentum it has should, if it is what we call a 'force', need a 'source'.

In the photons case that 'source' doesn't have much to offer, its intrinsic 'energy' or/and speed. When it comes to matter you have both mass and speed, and both are easily proof able.

But to have a 'speed' normally is to have a path, and we have clear problems showing that path.
And if we can't prove the photons path what have we left? It's intrinsic 'energy' perhaps.

But that 'energy' seems very selective, normally I would expect electromagnetic energy to influence other electromagnetic energy but photons doesn't influence each other although they transmit it?

Title: How does a solar sail work?
Post by: Pmb on 27/12/2009 13:06:46

Quote from: chris on 24/12/2009 04:05:30

Photons have no mass; this being the case, how does a solar sail work?

Photons do have inertial mass. You're thinking of proper mass which is zero for a photon. The inertia mass, m, of a particle is the m in p = mv. Thus even though a photon has zero proper mass it still has momentum and its this momentum that is given to the sail that is how it exerts a force on it.