# Light sails

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Hey. Ive seen recently in the news about ligh sails for space crafts. apparently they work by reflecting photons of light backwards and some momentum transfered from the photon to the sail.

However, I thought that photons where massless and therefore had no momentum.

Anyone know how these work?

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they are massless, but they do carry momentum.

waves do not require mass to carry momentum, the momentum they carry is proportional to their frequency. there is a quantum mechanical explanation for this that i'm not too familiar with, but it is there.

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It is a consequence of the "mass-shell constraint" that all physical particles obey

$m^{2}c^{4}= E^{2}-p^{2}c^{2}$

where $m$ is the mass, $E$ is the energy, $p^{2}$ is the three momentum dot itself and $c$ is the speed of light. For massless particles like the photon we have

$E = |p|c$ .

So the magnitude of the momenta is proportional to the energy.

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surely this would not be very affective though... light is reflected all the time and it doesnt cause the mirrors to move at all

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surely this would not be very affective though... light is reflected all the time and it doesnt cause the mirrors to move at all

It is a tiny effect, but in space where there is little resistance such sail could work quite well.

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surely this would not be very affective though... light is reflected all the time and it doesnt cause the mirrors to move at all

True, it's a small effect — you can manipulate atoms with light, but it requires more power to manipulate macroscopic objects. Very roughly speaking, 1.21 GW would allow you to levitate a 1 kg object, or give it a 1g acceleration.

http://blag.xkcd.com/2008/02/15/the-laser-elevator/

But the draw is that the power source does not need to be launched with the ship.

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But the draw is that the power source does not need to be launched with the ship.

And that's a huge draw! The great majority of the space shuttle's mass at launch is the fuel needed just to get it into orbit. To go anywhere else, the great majority of the mass of what's left would have to be fuel also. Newton's third law is not very forgiving, and having something to "react against" that you didn't have to "act" on in the first place would be incredibly valuable, and is probably the only way craft will ever make really long trips in any kind of a useful timeframe.

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Hey thanks to everyone for the replies!!

With that said, wouldn't the solar wind (the stream of partciles from the sun) have more of a pushing effect on the sails rather than the light?

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Perhaps the solar wind would help with the sails as well, but remember that charged particles get bent by magnetic fields, and the sun does also have a magnetic field.

Some sails are intended to use charged particles, and instead of being a flat sail they are more a web shape, and they have a strong electric charge to them. This way they save on weight.

A solar sail would not work very well far from the sun, but some have suggested using one regardless for interstellar travel and shining a giant spotlight on it from the solar system.

Actual tests with solar sails have so far had quite a bit of trouble. In any case they can't be used to lift off. The propulsion I'd like to see is a nuclear lightbulb. They could both lift off and travel reasonably fast in our own solar system.

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