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Without going into too much detail, let's say someone has a superpower which gives them "control" over actual, physical light: Photons, electromagnetic radiation, what have you. This power requires more effort to move more light around or make it do more unnatural things; it's easiest to use it to basically create very convincing visual illusions by (for example) making the light coming from a certain spot look the same way it would if it had reflected off of an object that isn't actually there. With a power like this, how difficult would the following kinds of things be to do?:

  • Push an object around. Photons are supposed to be capable of transferring momentum, so could a person with this kind of power concentrate a lot of photons in a small area and give them all momentum in a particular direction in order to hit an object and make it move? If so, how hard would it be to actually move something solid and "big", say even just baseball-sized, a noticeable amount?
  • Laser Cooling. Assuming that the person has the basic idea of how laser cooling actually works, just how precise of control would they need to have over light to actually cool down a volume of gas substantially (say, to use it as an attack by making it cold enough to hurt skin)? For that matter, how long does real laser cooling actually take to get something from room temperature to "cold"?
  • Set something on fire. If a magnifying lens can be used with sunlight to burn an ant, then a power like this seems like it should be able to concentrate light on a much larger scale and direct a pretty substantial amount of heat at a single point. Right?
  • Cut through an object or person. This seems likely enough to be doable since we use highly-concentrated lasers right now to cut through steel. Unless there's something I don't understand about how laser cutting works that would make it impractical in this case?
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    $\begingroup$ Is this power limited to manipulating existing light, or it allows its wielder to shoot ray beams like Cyclops? $\endgroup$ – Alexander Apr 6 '18 at 20:28
  • $\begingroup$ Mainly manipulating existing light, but including a small ability to "amplify" it to get a little more or less as might be needed for illusions to show the correct colors. $\endgroup$ – User6257 Apr 6 '18 at 20:45
  • $\begingroup$ I think we need some more limits on this power to provide excellent answers, especially with the science-based tag. Do they have control over virtual photons, and thus all magnetic properties and the things that keep molecules together? Can they alter electromagnetic fields and thus throw lightning bolts? As written right now, it sounds like you've given this person total control over one of the four fundamental forces of nature. Is that what you were imagining? $\endgroup$ – Dubukay Apr 6 '18 at 22:22
  • $\begingroup$ @User6257, this is a well developed first question, but for future reference, please note that multiple-question questions are frequently closed for being "too broad." You're asking four and I'm surprised someone hasn't logged a close vote (but it's a fun question, and we're a bit more forgiving for fun questions). Just to let you know. Please read through our tour and help center when you get a chance. Cheers. $\endgroup$ – JBH Apr 6 '18 at 22:49
  • $\begingroup$ @Dubukay I'll be honest and say I don't really know anything about virtual photons. The power is supposed to be emphatically "control over light" and meaningfully distinct from "control over electricity", so I imagine they wouldn't be able to do anything quite like that. I'm not necessarily looking for an "extremely hard science" answer but just kind of trying to read the overall plausibility of these things to figure out which one(s) I want to actually use. $\endgroup$ – User6257 Apr 7 '18 at 0:41
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If I am reading the question correctly it seems that they can only control existing light, not spontaneously generate light on their own.

This would then come down to how much light is in the immediate area for them to direct. Also this could look really weird depending how they did it, almost like a big shadow or dark spot appearing as they redirect the light in a different direction.

The sun puts out about 1300 Watts/m^2 on Earth. If they can gather the light from a moderate area, say 10 m^2, then they have about 13 kW of power to play with.

They would obviously be more powerful in daylight, or with other light sources around. To supplement this I would recommend that they carry flares or other fast sources of light, I can imagine they could do some really interesting things around say a room full of high power lasers.

  • Push an object around:

Not likely, the photon pressure from the sun is measure in micro Newtons per square meter. So even if they can put a million times the suns photons on a 1 m^2 surface they are only exerting around 1 newton of force (about 1/4 of a pound). This would likely move the object, but it would also unfortunately first incinerate the item with about 1300 Mega Watts of power.

They could potentially focus some of that power to move objects around by creating thermal air expansion (AKA wind) or small fires/explosions to move the items.

  • Laser Cooling:

This does work, with a bunch of caveats that make it effectively useless as a super power. Currently it only works for individuals atoms or at best molecules by changing the particle momentum. It really only works when the samples are isolate at near vacuum and have already been significantly cooled. Also it requires lasers to strike both sides of the particles, which really won't work on anything larger than a single particle as all the other atoms get in the way of the lasers. So unless they work at a lab where they are super cooling individual atoms, the power is not going to be of any use.

  • Set something on fire.

Totally doable. The Autoignition temperature of many burnable materials are in the 200-300 °C range. As any kid with a magnifying glass knows it only takes the sun light from a small area (the size of a magnifying glass lens) focused into a small area to start a fire. If they can make any illusions larger than a few square centimeters they can start a fire.

  • Cut through an object or person:

This is likely going to take some practice and effort, but is probably doable to some extent with the powers you describe. If they can make realistic phantom objects then they can manipulate the color (i.e. frequency of the light), as well as the direction that it is moving. If they have fine enough control of these they can make a light beam equivalent to a laser.

Laser cutters do exist, but they are usually only used to cut thin materials and they use a fair amount of power to do so. The amount of power increases with thickness because there is more material that must be vaporized by the laser. Vaporizing literally means that the material is rapidly melted and boiled into a gas that blows away.

Going back to the first answer it all revolves around how much area they can operate their powers on (basically how big an illusion they can make) you can then use that size to figure out how much solar power they have to play with. Most high power (cutting a few mm thick steel) laser cutters are in the 1-10 kilowatt range, while the low end (effectively wood burners) are in the >1 watt range.

Laser cutters won't however instantly sever limbs or cut through thick objects, they can however cause damage, severe burns, cut though thin items (< 1 cm), or cut through thicker objects if they remain stationary. So you could brand someone, maybe cut off a finger, or give them a really deadly burn, but are not likely to have enough power to sever limbs

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  • $\begingroup$ One more ability that ties into the "pushing things" and "setting them on fire" mode of operation would be to cause laser ignition of fusion pellets. Takes a lot of power, but precise control would make it pretty easy. Then you have your own little sun to make more light to play with. $\endgroup$ – Stephan Apr 7 '18 at 3:09
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All of these ideas seem to require the manipulation and concentration of a great deal of light, far more than a simple visual image would require. No idea how much, but orders of magnitude would not be unlikely. So I'd suspect these would require a great deal "more effort".

While it's certainly not impossible, such a wielder might want to emphasize visual attacks - blinding strobes and flashes, disorienting displays to confuse an opponent, and the like.

Dazzler got better at manipulating light as she got more expert, but I don't recall her ever getting to the point of lasers.

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Push something around

I'm drawing mostly on the math I previously did in this answer about a photonic engine.

Radiation pressure is $$P = \frac{I}{c}$$ where $P$ is pressure, $I$ is incident radiation, and $c$ is the speed of light, which you may recall is very large. Since both $P$ and $I$ are in terms of unit area, you can re-write the equation in terms of force and energy: $$F = \frac{E}{c}.$$ Here, $F$ is force in Newtons, and $E$ is energy in watts. So to put 1 N of force on something, which is not very much force at all, you have to hit it with 300 MW, which is basically a death ray.

So you can move something, but you will also vaporize it long before it gets very far.

Laser cooling

You could do this just fine, as long as you can confine and insulate the atoms that you are dealing with. Laser cooling won't work that quickly, and anything nearby will emit heat and re-heat whatever you are trying to cool down.

When scientists perform laser cooling in a laboratory (see Corbitt, et al., 2007), they do it to solid objects mounted on seismic isolators in an insulated vacuum chamber. They also used a Faraday isolator and half-wavelength passing mirror to achieve other optical effects. I doubt a person with powers could replicate these effects.

In a real life scenario, if you started to cool the gas in a room, you would have gas leaking in from adjacent rooms through any fitting that isn't gas-tight, you would have heat transferred from the walls by convection and radiation. Finally, air is made up of mutliple molecules. you will have the additional problem that the wavelengths that can laser cool oxygen would heat nitrogen and vica versa.

I don't think this is feasible.

Set something on fire / cut through someone

With enough energy, anything is possible. It depends on the level of your flagrant violation of conservation of energy.

If you have to eat all the energy you put out, then a 10,000 calorie diet (feasible, if you put your mind to it) would give you about 30 MJ of spare energy to use. The laser weapon system tested by the US Navy is only 33 kW, so our big eater could use 1000 seconds (about 16 minutes) of laser powerful enough damage drones and such if he had 100% efficient power conversion, using magic or whatever. This person would have to have nearly this good power conversion, the rest of the energy would be dissipated as waste heat, one way or another. If you were only 50% efficient, dropping ~15 kW of heat energy into your body would not be great. It would raise your body temperature by 1 K every ~15 seconds, which would kill you pretty fast.

The 33 kW will light flammable things on fire, easily, with a few seconds of application. If you want to really cut someone in half, a 1 MW laser would probably do the trick. A person eating as above would get about 30 seconds of man-bisecting laser power from all that food, again at near 100% efficiency.

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