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In science-fiction, what are referred to as "laser guns" are usually just plasma weapons that travel slower than the speed of light. But I want to have actual laser - not plasma - weapons in my universe, but I don't want them to travel at the speed of light. Is that possible?

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  • $\begingroup$ I think this is pretty far from what you want, but slow light is a thing. We've slowed light down to 17m/s. $\endgroup$
    – Cort Ammon
    Commented Aug 20, 2021 at 4:40
  • $\begingroup$ I'm not sure where you get the idea that a laser is a plasma weapon, or where the idea that plasma can be made to travel at the speed of light. For the sake of clarity, you should do at least a minimum of research so that you can then ask the question that you mean to. $\endgroup$ Commented Aug 20, 2021 at 4:50
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    $\begingroup$ @RogueAnt I never said that all lasers are plasma or that plasma travels at the speed of light - I just said that in Sci-Fi, when someone says "laser gun", they usually mean plasma weapons that aren't related to lasers and don't travel at the speed of light. $\endgroup$ Commented Aug 20, 2021 at 4:54
  • $\begingroup$ I'm pretty sure with the reality-check tag, you'll be wanting to define your terms. Laser usually means laser after all. You should also be aware that it's usual to wait 24 too 48 hours before awarding the accepted answer tick as it discourages our international audience from contributing other (perhaps better) answers. $\endgroup$ Commented Aug 20, 2021 at 5:02
  • $\begingroup$ @RogueAnt Oh. Okay then. $\endgroup$ Commented Aug 20, 2021 at 5:52

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Yes, but not in a way you will like.

A laser is electromagnetic radiation which never has mass and will only travel at lightspeed. While we have slowed down light, it requires a medium or material to do it*. So unless that medium or material is inbetween you and your target all the time you are out of luck.

However, what if you change this around? Rather than fire a beam of light, you fire a carrier that holds the light? Say you use a way to stop the light using a metamaterial, and at impact this light is released? It would have the added benefit of being potentially bullet-shaped and capable, but all the lightbeams and pulsed flashes you expect are gone.

*from @CortAmmon 's comment: https://en.wikipedia.org/wiki/Slow_light

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  • $\begingroup$ Yeah yeah shoot a spool of super "conductive" fibre optics, isn't it denying the laser here as useful energy carrier, eh. $\endgroup$
    – MolbOrg
    Commented Aug 20, 2021 at 13:49
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Lasers are not plasma weapons.

Plasma is a "soup" of free electrons and ionized atomic nuclei. Lasers are coherent beams of photons.

Photons travel always at the velocity of light in the medium they propagate in ($c$ in vacuum, $c/n$ in a medium other than vacuum, with $n$ being its refractive index). End of the story.

N.B. Since $n$ is always bigger than 1, in any medium other than vacuum the photons travel slower than the speed of light in vacuum.

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  • $\begingroup$ That doesn't really answer my question... $\endgroup$ Commented Aug 20, 2021 at 3:54
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    $\begingroup$ @The-Anonymous, it does. A Laser will always travel at the speed of light. If it were to be "slow" it would not be a laser, it would be a laser-adjacent weapon that shoots handwavium beams $\endgroup$
    – Joshua
    Commented Aug 20, 2021 at 4:10
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    $\begingroup$ The short answer to the question is "no" $\endgroup$
    – Henry Shao
    Commented Aug 20, 2021 at 4:49
  • $\begingroup$ "n is always bigger than 1" Strictly not true. Phase velocity can exceed c. By definition n = c/v. ... v > c ⇒ n < 1. $\endgroup$ Commented Aug 22, 2021 at 3:43
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If you mean lasers going slower than $c$, as in the speed of light in a vacuum, then your only choice is shooting them in a medium other than the vacuum of space. Light is slower when travelling through a gas medium such as an atmosphere. However, the difference in speed between a vacuum and an atmosphere such as ours is practically negligible.

You would need a very special medium such as a Bose-Einstein condensate to really slow down light (its speed falls to 17m/s in such a medium).

Notice that in any case slowing down a laser will not allow you to see it coming.

As an alternative, consider laser powered rockets. Basically you shoot a rocket, but instead of the rocket burning fuel in order to accelerate, you push it with lasers. This allows them to go arbitrarily much further and faster, and depending on the power you spend they might hit the target as a red or white hot stream of molten metal that from a distance might look like a laser beam as seen in some videogames.

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  • $\begingroup$ If the speed of a laser is 17m/s, why won't it be visible? $\endgroup$ Commented Aug 20, 2021 at 13:51
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    $\begingroup$ @The-Anonymous laser not being visible has nothing to do with speed. The things you see are photons bouncing from an object into your eyes. You don't see an apple, you see photons that bounced from an apple. A laser is a beam of photons. It's literally not possible for other photons to bounce of of those and reach your eyes, which would be necessary for you to see the laser. $\endgroup$
    – Davor
    Commented Aug 20, 2021 at 14:10
  • $\begingroup$ Huh... Yeah, that makes sense. $\endgroup$ Commented Aug 20, 2021 at 14:54
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    $\begingroup$ Upvoted even though, technically, even light moving through jello is traveling at "the speed of light" Even if that's slower than the speed of light in a vacuum. $\endgroup$
    – ShadoCat
    Commented Aug 20, 2021 at 21:58
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    $\begingroup$ @The Square-Cube Law: To be accurate, you aren't seeing the laser beam itself in smoke, you are seeing some of the photons that were in it but got scattered out of the beam by the smoke particles. No different than your car's headlights on a clear night, vs fog or snow. $\endgroup$
    – jamesqf
    Commented Aug 21, 2021 at 0:28
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Not an actual laser, no. Laser is light, and by definition, the speed of any light is the speed of light. In vacuo that is ~300.000 km/s or so.

But you can cheat. Your "laser" might be some mixture of (waves hands quickly) light ions in an electromagnetically bound self-contained matrix, sort of a Kugelblitz. The energy contents of the "package" is several orders of magnitude beyond that of a simple laser, which explains why it is preferred to the latter, even though its speed is lower and avoiding a hit is possible. You could call this a Kugelblitz device, a Rydberg projector, or a "soliton gun".

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    $\begingroup$ “I always call it a goober.” - Spider-Man $\endgroup$
    – Joe Bloggs
    Commented Aug 22, 2021 at 12:38
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Don't shoot straight.

Suppose I want to go get some Jolly Ranchers. The bodega that has all the flavors is 1 mile away. My Chevy Nova only goes 100 miles per hour. But I want it to take all day to get there.

I take a circuitous route. I set off in the opposite direction and go through town, laying on the horn, taking in the sights.

So too your laser. You can have it get there slowly even at the speed of light if you send your light somewhere else, reflect it, reflect it, curve it, reflect it etc.

If you are not talking about a banked shot from the Death star but little flying lasers of the "pew pew" variety, you could create extradimensional reflectors. Extra dimensions do not violate physics. If your beam is bouncing off reflectors in adjacent third dimensions that could slow it down; from your standpoint it is going in a straight line but it is actually going much, much farther when you count the extradimensional excursions.

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  • $\begingroup$ Yeah! That makes sense. But why can't the same thing be done for a death star shot? $\endgroup$ Commented Aug 20, 2021 at 13:48
  • $\begingroup$ @The-Anonymous OK you talked me into it. Death star shot too! $\endgroup$
    – Willk
    Commented Aug 20, 2021 at 18:17
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As the other answerers have already pointed out, lasers are by definition light, and light by definition always travels through space at the speed of light in whatever medium it's travelling through. So you have two options: either it must travel through some medium with a lower speed of light, or it must travel through more space than just the Euclidean straight-line distance from the start to the end of its trajectory.

Other answers have already proposed either changing the medium or changing the trajectory to not be a straight line. I think there is only one more possibility: warp space around the laser beam, so that the straight line trajectory covers more spatial distance than it would in a flat spacetime. An Alcubierre warp drive is a hypothetical device which warps space in order to achieve "faster-than-light" travel for something which cannot travel through space faster than light; imagine something like this in reverse, which warps space to achieve "slower-than-light" travel for something which cannot travel through space slower than light.

Now, why anyone would go to all this trouble in order to make their weapons slower is beyond me... but if you can invent a plausible reason for it in your story, go for it.

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If the goal is to be able to watch the beam of light travel like in movies, that's not going to work.

However, one possibility is that pulling the trigger only begins the process of sending the beam - like a camera flash, there's a charge-up followed by a discharge a little bit later. The charge-up can't happen in advance because it leaks away too quickly to be practical, though.

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Realistically no, a electromagnetic wave will only propagate at the speed of light in that material.(air) It can change very slightly with things like temperature and pressure but not to the extreme levels needed for it to be perceptible to human senses.

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You haven't included a hard science tag, so, sure! Just have a gun that alters the local speed of light so that lasers go slower than the speed of light. Perhaps they could store up light to make a more powerful laser.

You control the physical laws of your universe, you can them do whatever you want. It may not be physically possible in our universe, but in a sci fi universe it can be.

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