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? image 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? We need to make a few reasonable assumptions. Let’s assume that the laser gun fires a pulse with as much energy as a bullet has kinetic energy, KE. The energy, E, of light is related to its momentum, p, by E=pc, where c is the speed of light. This gives a momentum of $$ E=KE=pc $$ $$ p=\frac{KE}{c}$$ What is the kinetic energy of a bullet? A little searching reveals that a .22 bullet is \~2.5g and fires with a muzzle velocity of \~330m/s. Kinetic energy is given by KE=1/2mv^2, where m is mass and v velocity. So, the momentum of a laser pulse with equal energy would be $$ p=\frac{mv^2}{2c} $$ $$ p=\frac{.0025kg*(330m/s)^2}{2*3\cdot 10^8m/s} $$ $$ p=4.5\cdot10^{-7}kg \cdot m/s $$ For comparison, the momentum (p=mv) of a .22 bullet is .83 kgm/s. The momentum of a laser gun is 2 million times less than the momentum of a .22. But is momentum all we should consider? I suspect the ‘kick’ we feel on the recoil is directly related to the force that the gun exerts on the holder. This means that instead of momentum we need to consider impulse, momentum per time. We estimate the time it takes to fire a .22 is \~.1s, so the force delivered 8.3 N. Let’s estimate the time it takes a laser gun to fire. Unfortunately, not having a laser gun to fire (feel free to send me one, dear readers), we’re more or less going to have to guess at the firing time. Most movies with laser guns show pulses of light (which, incidentally would move so fast we wouldn’t see them) on the order of a meter or two long. Given the speed of light, this would give a firing time of \~30 nanoseconds. This would give a force delivered of 15 N. This is close to what we estimated or a .22. So, if movies are to be believed (and really, why wouldn’t we believe them?), it seems like laser guns may well have recoil. Note: It is worth questioning if we need the same energy for a laser as for a bullet. That could certainly change our estimate. Maybe we’ll return to this question again.*

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By Jesse