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Name: Clarence L.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 9/12/2004


Question:
If you have a laser in space (assuming no friction at all), and you turn it on, would the laser move backward?


Replies:
Clarence,

I will take a stab at this one. If I understand your question, if you fire a laser in space (with no friction) will there be a recoil effect, or will the laser device move opposite the laser beam?

I would say no since the laser is emitting light in the form of photons which have no mass. F=ma with m=0 gives F=0.

Bob Hartwell


Dear Clarence,

Yes! The light emitted by the laser has energy and momentum. A force is needed to push the light in the forward direction. Then, by Newton's 2nd Law, (For every Action, there is an equal and opposite reaction), the light must exert a reaction force on the laser.

The force, however, is not very large. A 1 watt output laser, which is more powerful than you would find in student laboratories, emits 1 joule of light per second. By a calculation using a little relativity and mechanics, this would exert a force of 3.3 E-9 N = 0,0000000033 N. = 1.2E-8 oz = 0.000000012 oz. After one year, the laser will have accelerated to a speed of about 0.1 m/s or about 4 inches/second. Not at all like the Starship Enterprise!

Best, Dick Plano


Absolutely. But it is weak.

A 1-billion-watt laser is a nice size, a bit more power than our biggest electrical power plants. By E=mc2, (c =3x10^8 meter/second), it would emit light weighing 10^-8 kilograms/second. Because that mass is thrown at 3x10^8 m/sec, by F=mv, the thrust is 3 Newtons, or 0.3 kilogram-weight, or 0.7 pound-force. Not much thrust for a giant nuclear power-plant. Accelerates itself somewhat slowly.

A solar sail which catches sunlight of equal wattage and reflects it backwards, would generate twice the thrust. One unit of thrust for stopping the light, and another for starting it backwards.

The laser only starts the light. And it pays the full energy price of generating the photon. A mirror doesn't have to.

Spacecraft propulsion using on-board lasers would only be worth doing once one has a top-notch energy source, like anti-matter with near-100% conversion efficiency, and the need to accelerate to near light-speed, and no need to do it very fast.

Jim Swenson



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