Science Forums 2

[CraigD]

Originally Posted by wade_b

Does a laser exhibit recoil?

Originally Posted by wade_b

The real question: is that recoil measurable in the laser itself, and does it follow the rules of Newtonian physics?

Yes.

The momentum of light, be it from a lightbulb, a laser, a microwave or radio antenna, etc, is easy to describe. It’s energy divided by the speed of light. Force = momentum/time and Power = energy/time, so the force of light is Power/c

A couple of examples:

• A typical laser pointer (I use one of these as a cat toy)
  • Power: <0.001 W
  • Force: 0.001 / c = about 3e-12 N
  • Mass: about 0.005 kg
  • Acceleration: Force / Mass = about 6e-10 m/s/s
  • Example consequence: Left turned on free-floating in space for about 53 years, if pointed the same direction, my cat-toy laser pointer would increase its velocity in the opposite direction by about 1 m/s, a slow walking speed. (It’s batteries don’t last nearly that long, I can attest )
• A weak old microwave oven (Like nearly every American, I have one of these)
  • 100 W (output power, not input)
  • Force: about 3e-6 N
  • Mass: about 10 kg
  • Acceleration: about 6e-7 m/s/s
  • Example consequence: If you cut its side off (my microwave emits photons from the side of its interior where the buttons are), it would accelerate itself to 1 m/s in about 19 days. However, when you throw in the necessary hardware to keep it supplied with power for 19 days, the system becomes much more massive

As best I can tell, what Shawyer’s proposing is similar to my microwave oven example, but without cutting the side open. This results in just a microwave oven, which will accelerate slightly in the opposite direction of any leaks in its oven compartment (I understand the window and door edges leak a little on most of them), an unintentional effect which I assume Shawyer’s system wouldn’t have, being sealed better than a microwave oven.

A more mundane example is a person standing on a sled on an frozen lake, propelling himself by pitching softballs. Shawyer’s proposes that you can propel the sled using just one softball that you pitch repeatedly to a friend also standing on the sled, if he has a catcher’s mitt that’s designed a special way.