A photon thruster is an advanced spacecraft propulsion system. It works by ejecting photons - basically, by shining a light. This works because photons have momentum which is directly proportional to their energy.

Because the exhaust velocity of a photon thruster is as high as the laws of physics permit, that is, c, it has the best specific impulse possible. This comes at a price, however. The thrust produced by a photon thruster is calculated according to the following formula: F=P/c, where F is the thrust (force) in newtons, P is the total radiated power in watts and c is the speed of light in a vacuum. If you run the numbers, you'll discover that a photon thruster needs to put out a little over three hundred megawatts just to develop one newton of thrust. If we assume for illustration purposes that this thruster has zero mass (which is obviously impossible), then if you strapped it to a one kilogram mass, it would accelerate at only one meter per second per second. This is downright lousy!

On top of that, since it expends no propellant, it's hard to figure out how much delta-V a photon thruster equipped rocket actually has. The best way is to figure out how much power your fuel stores can generate, and for how long, then figure the thrust and plug the that into F=ma. Either way, the photon thruster, despite its awesome specific impulse, is a thoroughly impractical device.

There might be an exception to this, though - a solar sail can be thought of as a photon thruster where the source of photons is not onboard the spaceship being accelerated. These can be practical, despite their pathetic thrust, because they have very low mass and because a star is a very powerful photon source indeed. Also, solar sails get a tiny amount of thrust from other radiation that strikes them - this can only help, though its effect might be epsilon squared.

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