RE: Laser as Reactionless Propulsion

Billy Brown (
Tue, 20 Apr 1999 15:48:15 -0500

Ross A. Finlayson wrote:
> Does shooting a laser from a laser generator not generate opposite force
> of the beam output?

Lasers have recoil, just like anything else - it is very small, because the beam has very little mass, but it is measurable.

> Do photons from a laser upon impact of a reflective surface cause an
> inertial reaction, no matter how infinitesimal?

Yes, it does. These two forces are exactly equal, so your idea:

> If lasers have no "kick" upon firing, and photons from a coherent laser
> do cause an inertial reaction, then lasers could be used as reactionless
> propulsion.

unfortunately won't work. You can use a stationary laser to push a moving object, but you can't use the laser/mirror pair to move itself. Darn, another great idea shot down by those nasty conservation laws!

> As a craft propelled by such moved along, it's trail would be a beam of
> coherent light flying at the speed of light away from the engine.

As an interesting note, you CAN get thrust by simply firing the laser out the back of your spaceship. Unfortunately, we currently can't build lasers that move enough energy to make this practical - the thrust would be far too low for even the most modest of needs.

Now, if we could make very powerful, compact, lightweight lasers, and power them with a nuclear energy source, we would have a very efficient means of propulsion (and this is not a new idea, so neither of us can lay claim to it).

Billy Brown, MCSE+I