7
$\begingroup$

Laser guns in science fiction are extremely common to the point of being a common cliche. According to atomic rockets lasers would be much more effective as pulsed lasers compared to continous-wave lasers. If the power source and construction of such a laser was hand waved would a laser like this be lethal if used against an unarmored human?

  • The lasers wavelength is green or 532 nanometers.
  • It has 500 terawatts of power with a pulse duration of several femtoseconds.
  • The target of this laser is a regular human test subject who shall be called Bob.
  • The laser can be fired multiple times in quick succession with no limit being found on the amount of times it can be fired.
$\endgroup$
3
  • $\begingroup$ Part 1: Your question as it stands is unanswerable. Okay, your pulse has a power of 500 terawatts, but the pulse lasts only several femtoseconds. How many such pulses does your laser fire in a second? What is your beam diameter? $\endgroup$
    – user73910
    Commented Jun 8 at 23:27
  • $\begingroup$ Part 2: A common marketing trick some laser companies use is to claim that a pulsed laser has a power of [Insert big number here]. Stated only in the fine print is a disclaimer saying that the pulse only lasts a short amount of time, and only a few are fired per second, so the amount of energy delivered to the target would actually be very low. Sure, a pulsed laser can claim a high number on a marketing pamphlet...but won't be melting any ICBMs or drones. $\endgroup$
    – user73910
    Commented Jun 8 at 23:31
  • 2
    $\begingroup$ Part 3: Some quick math. (500*10^12) J/S * (10^-15) S= 0.5J. That is how much energy your laser delivers in a single pulse. So again, what is your laser's beam diameter, and how many such pulses are fired per second? $\endgroup$
    – user73910
    Commented Jun 8 at 23:34

2 Answers 2

13
$\begingroup$

Pulsing that fast is mechanically similar to a continuous laser

Pulsing a laser helps, but if the pulse is too fast, it does not give the plasma time to get out of the way negating the whole point of the pulse. Pulsing a high energy laser 100ish times a second allows time for the plasma to expand and dissipate between shots so that you don't waste a lot of power being absorbed by the opaque plasma cloud. However, because each pulse is only 1mW and DARPA determined several years back that the minimum continuous power output of an anti-personnel laser would be about 1kW, you would need this weapon to pulse about 1 million times per second to reach the minimum power output over time for a militarily useful anti-personnel laser. Pulsing a laser so fast that the plasma can not meaningfully expand between shots like this would result in a similarly wasteful burn pattern as a continuous laser.

But high speed lasers DO make for very good electrolasers

Killing a human with a laser takes a lot of heat and energy. Messing with pulse rates and wave patterns can move this number up or down a little bit, but as it turns out, electrocution is a FAR more efficient way to kill a person requiring only 13.5 watts (0.135A at 100V using an AC current over 1 second is typically fatal). Also electrocution weapons have several distinct advantages over other weapons like thier ability to kill even if you don't hit a vital organ. You can also turn up the voltage and down the amperage to turn the same pulse into a non-lethal stun weapon more similar to a police taser, but with much better range, accuracy, and reusability.

Normally, the obstacle with electrocuting someone is the difficulty of spanning the distance between the weapon and the target because you need some kind of wire to transmit the electrical current to the target; however, one thing that short pulse, powerful lasers are really good at is ionizing a thin, well formed "wire" of air. When it comes to making an ionized plasma filament through the air, the point is to NOT give the plasma time to move out of the way; so, the shorter and more energetic your pulse, the narrower and more energy efficient you can make your plasma filament.

Depending on the range and reliability you are trying to give this weapon, you will need to fire a 7e13W beam for approximately 1e-17 to 4e-15 seconds which is well within your setting's tech limits which means you only need a laser with a total power draw of less than a watt to turn a taser into an electrolaser if you can pulse it fast and hard enough.

So instead of a 1000 watt laser, you could use your miniaturized fast pulse laser technology to make an equally deadly, but much more efficient electrolaser. With modern handheld lasers and tasers already able to exceed power outputs of 10 watts per kilogram, and a standard combat rifle coming in at 3.5kg, it is conceivable using technologies not much more advanced than we have today that you could make a 35 watt electro laser combat rifle capable of delivering a fatal or debilitating shock in just under half a second.

$\endgroup$
4
  • $\begingroup$ Re: the last paragraph -- I would assume that some spy agency has already built one. $\endgroup$
    – FlaStorm32
    Commented Jun 8 at 12:54
  • $\begingroup$ I think your first paragraph shows a confusion between pulse duration and period. Pulse duration is how long a given pulse lasts, which IRL can be between several picoseconds down to femtoseconds. Pulse period is the time between pulses. $\endgroup$
    – user73910
    Commented Jun 8 at 23:48
  • $\begingroup$ @user73910 Sorry, I did not explain this well, but I was going off the basis of how weak an individual pulse would be and how rapidly you'd have pulse your laser to get get up to a weapons grade laser. As L.Dutch pointed out, each pulse is only 1mJ; so, you'd have to pulse the laser a minimum of 1 million times a second to have enough power output to be an effective anti-personnel weapon. Because plasma takes several milliseconds to expand enough to get out the way, an ideal pulse rate for a weaponized pulse laser should be nor more than a few hundred pulses per second. $\endgroup$
    – Nosajimiki
    Commented Jun 9 at 1:54
  • 1
    $\begingroup$ @FlaStorm32 There is a USA based company that specializes in military grade electro lasers that publicly announced they were under contract to develop one a few years back, but the project has since become classified... so... there is a very good chance that such a weapon exists by now, but its details are not public knowledge. $\endgroup$
    – Nosajimiki
    Commented Jun 9 at 1:57
6
$\begingroup$

1 TeraWatt is $10^{12}$ W.

1 femtosecond is $10^{-15}$ s.

1 pulse of 1 TW laser will deliver 1 mJ of energy.

Boiling 1 gram of water requires 2.54 kJ, that is 2.54 million pulses of 1 TW laser, or 2.54 ns.

You can see that the amount of pulses in 1 ms shot would be enough to pierce a hole through a human body. Plus all the damage created by the shock waves produced by the expanding hot gases.

$\endgroup$
2
  • 1
    $\begingroup$ The hot gases are key, I think. Not much energy would be delivered to the target, sure, but if the OP is willing to turn down the wavelength of the laser into the UV spectrum, it would start ionizing hydrogen into plasma, breaking chemical bonds in H2O. If the laser is initially divided into, say, ten 100 GW lasers that are a small space apart and focused on the target, it’ll effectively be a plasma cannon. $\endgroup$ Commented Jun 7 at 14:27
  • $\begingroup$ How did you get the "... amount of pulses in 1ms shot..." part? I didn't see the duty cycle information needed (beyond "... in quick succession..." paired with "several femotoseonds"). Knowing that would let us get the average power, which would give us a first order effect to work with (e.g. your boiling water comparison). Some research indicates practical industrial/scientific fW lasers are in the 10mW to 1W range, which suggests boiling 1cc of water in... days. Obviously the military ones would be more powerful, but it shows that we're missing a key number. $\endgroup$
    – Cort Ammon
    Commented Jun 8 at 15:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .