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Logistics of Lasers as Weapons in SF

  1. Jan 20, 2014 #1
    Hello. I just joined this wonderful community, so excuse me if I'm doing something wrong. I'm new to using forums.

    Anyway, I had a question. SF usually features 'phasers' as weapons, but from what I hear it doesn't sound too realistic. I thought maybe lasers would be better weapons, say if you could get pistol sized lasers that could seriously harm another individual. So I was wondering what the logistics of such a weapon would be from the sciences point of view?

    Like how powerful would the laser have to be? How practical would it be to use in medium sized distances, like the same distances that you usually see people using phasers on Star Trek? Can lasers even theoretically penetrate the skin in a decent amount of time and seriously harm your insides? Would something like a tin can or other metal be useful in reflecting the laser away and reducing its harm? Anything else I haven't even thought of to ask?

    I greatly appreciate any help you could give. Again, if I'm doing something wrong, just let me know how I can do the right thing. :D
     
  2. jcsd
  3. Jan 20, 2014 #2

    Curious3141

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    Depends on whether you want realism (more toward the hard sci-fi end of the spectrum).

    A realistic laser beam would be:

    a) invisible (even with an output in the visible spectrum) in the absence of scattering by dust or reflection.

    b) soundless unless it causes something to combust/explode

    c) *completely* impossible to dodge once the "trigger" has been pulled. The only theoretical possibilities for light beam avoidance are if the manoeuvre is initiated before the beam is activated or if the aim is wrong.

    I've not mentioned the practical considerations of power output vs size, but sci-fi will involve some hand-waving, so I guess you can handwave away that part - hypothesise a futuristic "magic" crystal of small form factor that can output a collimated, coherent beam of megawattage in the visible spectrum. I guess the power requirements can also be handwaved away in some fashion.
     
  4. Jan 21, 2014 #3
    How much power would be required to ionize air to make it light even if it is completely dustless, and make a hearable noise because of heat expanding?
    Or is it completely impossible?
     
  5. Jan 21, 2014 #4
    Well, since this is the Sci-Fi writing section lets work backwards in order to keep everyone happy.

    Duelling pistoliers 10 paces apart (er... 14.8 meters apart.) And we want to see a nice, visible, beam of death from the pistol reach this far.

    So to keep things simple; the beam is comprised of a 1L cylinder of air, of length 14.8m and diameter 0.93cm. (I'm mixing the orders of units just so it's more relatable.)

    Considering only Nitrogen;
    There's 0.0718 mol of Nitrogen in that 1 L of air and the first ionisation energy of Nitrogen is 1402.3 kJ/mol, which means that it'll take ~ 100kJ (27.78 W h) which according to Wolphram Alpha is 1.3x the typical battery energy content of an alkaline long-life D battery.

    So, as long as your pistol can instantaneously dump the entire energy content of an Energizer bunny along this path of air (just the Nitrogen in our case) you might just singe your opponents nose, but top marks for showmanship and we don't really want to kill anyone. ;)
     
    Last edited by a moderator: Jan 21, 2014
  6. Jan 21, 2014 #5
    "So, as long as your pistol can instantaneously dump the entire energy content of an Energizer bunny along this path of air (just the Nitrogen in our case) you might just singe your opponents nose"

    Sorry if i misunderstand something, but 100 KJ is really only enough for a scratch, or does it make a bigger wound, just dont ionize air, as only a tiny fraction of it swallowed?
    (So realistically, you could only see dust with a visible light beam.)
     
  7. Jan 21, 2014 #6

    Ibix

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    Korok's calculation is the amount of energy needed to light up the air between you and your target, with nothing left at the end. You'd want pistol bullet energies on top of that to do damage.

    Rule of Cool aside, why do you want your personal weapons to emit visible beams? You're wasting energy on air, which isn't likely to shoot back, and thereby cutting your range. You ruin your nightvision, and you're giving away your position and asking for (probably automated) counter-battery fire.

    Invisible lasers mounted with laser sights would make more sense.
     
  8. Jan 21, 2014 #7
    " You ruin your nightvision, and you're giving away your position and asking for (probably automated) counter-battery fire."

    Or illuminate the target area with it, help correcting fire (like tracker bullets), if they have some automated counter battery fire, they can track even an invisible beam based of the minor scaterring caused by heat bubbles, dust.

    Otherwise ok i understood what Korok wrote, that is reasonable, just i assumed previously that such strong beam can be similar to lightning.
     
  9. Jan 21, 2014 #8
    You caught me :) I didn't address the lethality aspect of the first question but the "How much power would be required to ionize air to make it light..." of your question. In fact, re-reading it, I didn't even address it in terms of Power output.

    P = E/t so we're talking about 100 Kilowatts

    I think what I was trying to suggest was that sci-fi loves using the colourful, and frankly far more interesting looking, trope of visible beams and bolts because it's aesthetically pleasing/arresting. But ultimately it's a case of intentionally using large amounts of energy (in the right form) to create the visual aspect of your weapon which could have gone into its lethality instead. Which in itself is quite interesting in terms of story-telling; the weapon's manufacturer could be putting a bit of pizzazz in their work as an optional extra for the discerning, style concious soldier of fortune.

    Here's an article about a 100kW laser (http://www.engadget.com/2010/12/12/northrop-grummans-100-kilowatt-laser-fired-for-six-hours-straig/) which I'm sure would burn a hole through your dueller in seconds but not nessesarily do it with visual finesse. Not to mention be pretty damn cheeky to come to a duel where one guy has a pistol and the other guy drags a 10 tonne laser behind him.
     
    Last edited by a moderator: Jan 21, 2014
  10. Jan 21, 2014 #9

    Ibix

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    I suggested the laser pointer (or even your weapon on low power) to help correct fire. You don't need something like tracer for a laser weapon - the laser pointer follows the exact trajectory that your weapon laser follows, unlike with bullets which drop and get blown around by wind. And it seems to me that there's a bit of difference between spotting a bit of scatter off atmospheric dust and spotting a bright ionisation trail (can I call it a "ravening beam of energy"? E.E. "Doc" Smith's energy weapons were always ravening).

    Anyway. I'm no soldier and it's your universe.
     
  11. Jan 22, 2014 #10
    Wow, thanks for all the replies you guys! These are really fascinating to read.

    So there are two questions that no one answered yet. If a laser could be used as a deadly weapon, is it theoretically possible to find a metal reflective enough to give the opponent some protection? I'm thinking the metal armor would reflect much of the laser yet still absorb some, so it would eventually melt/grow a laser sized hole but still offer the wearer some time to react.

    Also, would such a deadly laser pierce right through and leave a hole through the muscle, for example, or burst the person on fire? Ditto for the metal, would it heat and melt the metal around the contact point of the laser first or shoot right through?
     
  12. Jan 22, 2014 #11
    Depending on the strength, I'm sure a laser could punch right through something. With today's technology, most industrial strength lasers would most likely set their target on fire if they're some distance away.
     
  13. Jan 22, 2014 #12
    "the laser pointer follows the exact trajectory that your weapon laser follows"

    If the target is a flyer drone for example, you wont see the tracer spot on the sky.

    Otherwise i just asked about ionisation.
    Well visible light beams that scatter and reflect on dust not good for stealth attack, good for intimidation (suppress fire), illuminate a dark area... of course like flashlight (IG flashlights :D), they are only really cool looking during night.

    Otherwise, visibility assets aside, what are the advantages of IR lasers? Are they swallowed less? Earth's atmosphere is transparent to visible light, but swallows a certain amount of IR (greenhouse effect) or how is it?
     
  14. Jan 22, 2014 #13
    Industrial lasers cut by heating the material - then blasting it with compressed gases, generally. Their "target" isn't moving, and is not more than an inch or so from the "weapon", so it's not really all that equivalent.

    For a laser to do lethal damage to a target, it would need to be able to dump a huge amount of energy into the target - generally heat. It would have to do it rapidly, or the target would jump into cover before the laser could do more than sting. Assuming you could heat the target quickly enough, it would either burn them, or cause their blood to turn to steam (BOOM!) under their skin.

    Another possibility - The Thranx (insectoid race) used a beam weapon that would ionize air to make it conductive enough to deliver an electrical charge to a target. I'm not sure if this would work IRL, but it's more realistic than blasters and phasers.
     
  15. May 31, 2015 #14
    I have a few notes about laser sidearms here.

    You only need about one kilojoule worth of energy in each laser bolt to drill a hole 4 centimeters in diameter and 30 centimeters deep through the unfortunate person you are shooting at. If you pulse the beam.

    The laser will need a large lens or focusing mirror, to bring the spot size down to the required 1 millimeter. Dr. Luke Cambell figures a standard laser sidearm would have a lens about 2 centimeters in diameter and a focusing range of about 50 meters.

    A lithium-polymer battery has an energy density of 1.08 kJ/cm3. So you are looking at about one cubic centimeter of battery per shot in the "magazine", before the laser has to be recharged. This will shrink as battery technology improves.
     
  16. Jun 7, 2015 #15
    Isn't there a significant risk of blooming if one tries to pulse the beam?
     
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