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From Blasters to Phasers: How Close Are We to Laser Guns?

CJ Miozzi | 28 May 2014 20:05
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Recoil

Super Battle Droids

When a modern gun is fired, there's a strong kickback that, depending on the caliber of the round, can be powerful enough to injure an operator who isn't properly wielding the weapon. In fiction, blasters have distinct recoil, whereas phasers do not. Would a laser gun have recoil?

Recoil is the result of one of the fundamental laws of physics: the conservation of momentum. Before you pull the trigger, the total momentum of the bullet and firearm, taken as a system, is zero. After pulling the trigger, the total must remain zero, so whatever momentum is imparted to the bullet speeding towards your target must be matched by the recoil of the gun pushing back against you. Momentum is a function of mass and velocity, so given two objects with equal momentum, the one that is more massive will be traveling slower - that's why the gun doesn't kill you through recoil.

In actuality, we need to consider the momentum of both the bullet and the gases escaping the barrel created by the explosive combustion, and it turns out that the gases impart more recoil on the gun than the bullet itself. That's why suppressors reduce recoil significantly: they divert and trap the gases, releasing them slowly over a long period of time.

In the case of a laser, there is no explosion adding gases to the system. Further, the "projectile" being emitted is a stream of photons, which are massless. Lasers are powered by focusing the traveling direction of photons emitted from atoms. When we apply energy to an atom, it enters an "excited" state, and it returns to its normal state by releasing photons. A piece of red-hot metal glows because its atoms have been excited through heating and are releasing photons. While photons do exert some pressure - the concept of solar sails depends on this fact - that pressure is negligible for our purposes.

What about a particle-beam weapon? Again, it would not be powered by exploding gases. Particle accelerators use electromagnetic fields to send particles speeding along in a desired direction. There is some debate over whether or not this would produce appreciable recoil, but we'll err on the side of "no recoil" for the following reasons. Firstly, there are no gases or other matter added to the system, which we've described to be a significant source of recoil in modern guns. Secondly, atomic and subatomic particles are so minuscule that even when accelerated to near-light speeds, their momentum is negligible. And lastly, electromagnetic fields violate Newton's law of action and reaction and do not conventionally follow the principles of conservation of momentum, though an explanation as to why is beyond the scope of this article.

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