# Adding mass to light (In laser weaponry) to convert it to kinetic energy

The fictional tech:

Lets say I have a crystal, that when a light waves pass through it, imparts some of it's mass onto the light wave. - This is handwave and I'm cool with it.

The crystal is developed in a zero light environment and sealed in a cylinder for use in weaponry.

The idea is that if I fire a concentrated laser beam through said crystal, the beam will still be traveling at the speed of light, but now will apply kinetic force to whatever it hits.

In my rough story draft it vibrates, eventually disintegrating when it's "used up", and then replaced.

The problem:

While I really want to use this idea, I am worried about the reaction the process will have when the beam passes through the crystal. But I'm wondering if newton's 3rd law would apply, in which case the person firing the thing is doomed.

What will happen when the laser beam passes through the cylinder and gains mass?

• Hi! This article about light molecules isn't what you're talking about exactly, but might somehow give you some other ideas. Commented Dec 7, 2016 at 13:01
• @ThomBlairIII Interesting read. The Rydberg blockade means you can't really weaponize it though :(
– Lu22
Commented Dec 7, 2016 at 13:34
• IKR?! Darn Rydberg! Always ruining perfectly good weapons. I have warehouses FULL of weapons he's blocked plausifying. What am I supposed to do with them all??? >:( Commented Dec 7, 2016 at 13:54
• A similar type of Technology already exist look up solar sail Commented Dec 7, 2016 at 14:47
• @BryanMcClure That's partly what I based the kinetic laser on, only you would have to amplify the force applied to whatever you're hitting. Hence the handwavium crystal.
– Lu22
Commented Dec 8, 2016 at 4:41

Light has zero rest mass; that's not the same as it having zero mass. The mass of a photon can be derived from its energy using the good ol' $E = mc^2$, and the photon's energy depends on its frequency.

Photons always travel at the speed of light, regardless of their frequency or how fast you move while observing them. Since momentum depends on mass and velocity, and the velocity of light is fixed, you can also derive the momentum of a photon from its frequency.

This means that to increase the total momentum of a laser, there are two things you can do: increase the frequency of the photons or add more photons.

You might still largely have to hand wave after that though, because the amount of energy you need to add to get any serious momentum from light is so high, the kinetic effects would be largely irrelevant.

For example, you say the output is a purple beam. So lets say you are blueshifting down to violet and then adding photons to increase momentum. Violet is 380-450nm, so say 400nm. That means each photon has an energy of $5\times10^{-19}$J and a momentum of $1.7\times10^{-27}$ kg m/s. For comparison, per wikipedia a 5.56 NATO cartridge has a momentum of 3.8 kg m/s. If you multiply out the number of photons to get 3.8 kg m/s of momentum, your laser's energy will be 1.1GJ, which wolframalpha helpfully tells me is equivalent to detonating a quarter of a ton of TNT.

As for Newton's third law, well momentum must be conserved. However much momentum you add to the laser is equivalent to the recoil you will feel when you fire the weapon. So in the above example, we have a laser gun with about as much recoil as a normal M-16 that hits like a decently sized bomb. If that sounds good, great.

If not, I recommend hand-waving more. Whether or not your magic device handwaves conservation of momentum depends on how much kinetic energy you want to impart in the target. If it's smallish, just accept it as recoil, otherwise handwave it.

Keep in mind that just because your laser doesn't have much momentum of its own doesn't mean it can't throw things around. Hitting something with a lot of laser energy will cause rapid ablation - parts of the surface will heat up so fast that they explode outward in one direction, which also pushes the object back in the other direction, away from the laser. You don't have to worry about the laser's momentum here because the momentum of the object comes from the equal and opposite momentum of the surface being blown off. This would be a case of the photon energy being converted into kinetic energy in the target.

• Very detailed answer. I didn't thing the damage dealt would be so much. The idea was to have my lasers fire from a mech's palms, so I was banking on a half meter radius beam. This may be way too overpowered so I'll have to nerf it a little.
– Lu22
Commented Dec 7, 2016 at 14:47
• @Lu22 It's actually better to use a smaller beam. The damage from a laser comes from heating up an object, and the wider the beam the less energy per square meter of target surface area, so the less it heats up. If you pass the same amount of energy to a smaller area, that part will heat up more, faster and may explode because the material suddenly turns from solid to liquid or gas, and the gas is superheated and takes up more space. Commented Dec 7, 2016 at 15:02
• You have a point there.
– Lu22
Commented Dec 7, 2016 at 15:08

When light hits a surface is does actually transfer momentum (see light pressure) and, of course, energy (go out into the sun and test).

Mass is energy and energy is mass. The energy of a photon (which is the same as its mass, photons having no rest mass) is proportional to its frequency, or, if you prefer, inversely proportional with its wavelength. Adding mass to the photons is exactly equivalent to reducing their wavelength; what happens when the beam goes through the crystal is that the color of the beam will shift towards the blue end of the spectrum; see blueshift. Where the energy comes from is left unexplained, because magic works best when unexplained.

Newton's 3rd law has nothing to do with magic.

• That's very interesting. Coincidentally my kinetic beams are purple, so that fits. I'm a programmer so I'm clueless, does a shorter wavelength mean slower beam?
– Lu22
Commented Dec 7, 2016 at 12:38
• No. The speed of light is constant, does not depend on wavelength (but it does depend on the medium in which it propagates). Low-energy radio longwave photons and very high energy gamma-ray photons all travel at the speed of light. Commented Dec 7, 2016 at 12:45
• @Lu22 shorter wavelength has no effect on speed, only frequency will change accordingly. Commented Dec 7, 2016 at 12:45
• Also, although the answer is technically correct, it take 3 GW to produce 1 pound of force. At these levels energy transfer effects will far overshadow simple momentum transfer. Commented Dec 7, 2016 at 15:07
• Maybe the opponent has an equally magic armor which can easily dissipate the energy in the form of neutrinos or something. Commented Dec 7, 2016 at 15:42

# What you describe is impossible

Light cannot have a 'mass' imparted to it, other than the equivalent 'mass' it has by E = mc$^2$. Changing the mass of a photon, but keeping it at the same speed (speed of light) will violate conservation of momentum unless there is a reaction force in the crystal. But you are willing to hand-wave that. Great!

# Therefore it is fine for a story

So what is the problem? You just violated conservation of momentum (which can be considered a generalization of Newton's third law). Great. Keep on violating. If you're going to hand-wave a little, why not hand-wave a lot? Laser blaster crystals sound like lots of fun.