The reason I mentioned the solar sail a few posts ago is that demonstrates how little momentum light carries - the solar sail needs to be quite huge to grab enough light from the sun to move, and even then, the force is very slight. The momentum of photons is simply so close to zero that you can ignore them in most calculations.

(There is a really cool idea I read a paper on last year, or maybe the year before, that bounces a laser beam between the ship and a ground station to try and make solar sails practical. Then the light reflects off your ship, you get double momentum from it (it is like catching it and throwing it again - you get a force when you catch it, and another force, the reaction force, when you throw it). If you can align your equipment to reflect it multiple times, you can get considerable improvement in efficiency. Where does the kinetic energy for your ship come from? As the photon bounces back and forth, you grab a bit of its energy for your ship. This limits the max efficiency you can theoretically get, but this still beats the hell out of the simple solar sail - the paper said the researchers got 1000x the momentum out of the same power beam in the lab. They could in theory do better, but this is still impressive. But I digress.)

Anywho, the momentum carried by the photon is proportional to the energy of the photon, which comes from Einstein's equations. You can see a brief explanation here: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relmom.html#c2

To give a force, X, to the bullet, you'd need a ravening death beam going backward of power Y if you wanted to explain away recoil by the field absorbing the momentum.

The relationship between them is Y ~= speed of light * X. For the M16's bullet, I calculated back on page 8 that it gets a force of about 3.4 kN.

To have the photons carry away the reaction force, you'd have a beam backward of over a terawatt!


Recall my earlier discussion on how much damage a laser weapon of about 2 kJ of power could do. It fired them off in ~1/100 of a second, meaning it had an output power of about 200 kilowatts. 2e5 watts. A terawatt is 1e12 watts.

Our massless recoil's beam is 5,000,000 more powerful than my killer laser pistol's beam!


This is obviously unreasonable. In the M16 example, the weapon got about two joules of kinetic energy in the kickback, 1000x less than the bullet got. With this example, the recoil takes about a gigajoule - one billion joules of energy, about a million times more than the bullet receives.

Since this energy cannot be created, it needs to come from the weapon's batteries. Now, what kind of infantry weapon, designed to put 2 kJ of energy into its bullet (about the same as the M16, again), has batteries able to put out 1 GJ / shot?

This result is absurd. When you saw a coilgun fire, did the people standing behind it instantly die from hard radiation exposure? Of course not - because in the coil gun, the gun simply gets the recoil, just like the M16.