Laser Launching

Lasers seem to be on my mind recently.  Just yesterday, the class I TA for (E&M for engineers) talked about the momentum carried by E&M waves.  This called to mind a discussion I had with a housemate a few weeks back.  He had heard somewhere that 'they' were thinking of launching satellites with lasers.  No way, I thought to myself.  Satellites are too heavy.  However, his question has been hovering around in my mind, so I've decided to try and answer it: can we use a laser to launch a satellite into orbit?

Laser Gun Recoil: Follow-up

Matt Springer over at Built on Facts has a very nice post following up on my earlier analysis of whether or not a laser gun would recoil.  In my analysis I came to the conclusion that the momentum delivered was much less than that of a conventional gun, but that the impulse, and hence the force delivered, was about the same.  However, that force was delivered over a ~30ns timescale, which left open the question of whether or not the wielder would be able to feel that recoil.

While I had thought to possibly return to this issue at some point, Matt has beat me to it.  This is fortuitous, because I wasn't sure quite where to start with the question, while he approaches it in a very sensible, clear manner.  The gist of his post is that he compares our response to short time scale forces to our ability to sense sound.  I won't go through his calculations, you can check those out for yourself, but he concludes that it is very unlikely that we could feel the recoil of our laser gun.

Case closed.  Though, like any good scientist, we may choose to reopen it later if more evidence comes to light.

Would a laser gun recoil?

Today I'd like to approach a question near and dear to many a geek heart: do laser guns have recoil?

Let's motivate our question a little bit.  I've wondered about this question since I saw star wars.  Though I'm no firearms expert, the recoil in guns must come from conservation of momentum principles. Momentum is conserved in a system. The gun starts with zero momentum. We fire, give the bullet momentum, and so to keep the system at zero momentum, the gun must gain equal and opposite momentum. That is, the gun will move backwards.

All of that was for conventional guns. Light carries momentum, so if we fire a pulse of light, we expect our laser gun to recoil. So yes, they do have recoil. Satisfied, dear readers? Neither am I. The question we really mean to ask is, does a laser gun have noticeable recoil?