From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: Laser weapons
Organization: University of Toronto Physics/Astronomy/CITA
Date: Thu, 3 Sep 1992 15:04:53 GMT

In article <rwallace.715515667@unix1.tcd.ie> rwallace@unix1.tcd.ie (russell wallace) writes:
>In <BtyGuo.FBz@helios.physics.utoronto.ca> neufeld@helios.physics.utoronto.ca (Christopher Neufeld) writes:
>
>>In article <rwallace.715424570@unix1.tcd.ie> rwallace@unix1.tcd.ie (russell wallace) writes:
>>>In <1992Sep1.213233.7293@spss.com> markrose@spss.com (Mark Rosenfelder) writes:
>>>
>>>>I'd like to throw some laser weapons into an sf story, but I find I don't
>>>>know enough about lasers.  
>>>
>>>>Some questions:
>>>>Are these things even practical as hand weapons?  
>>>>How much energy do they use?
>>>>Do they make a noise when fired?
>>>>Does the frequency matter?  I.e., would it make more sense to generate
>>>>  ultraviolet or gamma rays rather than visible light?
>>>
>>>Basically, lasers as we know them today are inherently useless as
>>>weapons - they don't put out enough power.
>>>
>>   Well, pocket HeNe's don't put out much power, but even fifteen years
>>ago a tabletop laser could put out a beam, a small fragment of which
>>reflected from a cracked lens could burn through clothes and burn the
>>skin underneath.
>>[...] For instance, the forty foot CO2 laser
>>here at the Toronto Science Centre [...] can immediately
>>ignite wood placed at the focus of an IR lens. [...]
>>but I would say that the focus of that laser is probably
>>about as healthy a place to be as the flame from an oxyacetylene torch.
>
>So, a laser forty foot long, doubtless weighing many tons and consuming
>God knows how much power, still is no use as a weapon;
>
   As a hand weapon, clearly not. I would not say that it is 'inherently
useless' as a weapon. It's impractical at present. I'm beginning to like
the oxyacetylene example, since that lens has a finite focal length you
can't make a good weapon of that particular 25 year old laser at ranges
more than a metre or so from the aperature. Think of it as a bulky, power
hungry welding torch. There are circumstances under which I would
consider a welding torch to be an unfriendly object.

>>>To get enough power from an energy weapon, you want to tap it directly
>>>from a nuclear reaction.
>>>
>>   Well, that's hardly necessary. You can do significant damage to a
>>person by dropping just a kilojoule of energy onto a small spot over a
>>short time interval. I believe flashlight batteries deliver more than
>>that much energy over their lifetimes.
>
>Not really; to raise the temperature of 1 litre of water (the main
>constituent of the human body) by 10 degrees K (hardly enough to kill
>someone), takes 42,000 joules of energy.
>
   That's not what's going to happen. If I dump a kilojoule of energy
onto a half-centimetre diameter spot in a time scale short compared to
the thermal diffusion times in the body (ie. in about a second or less)
there will be localized heating there enough to do serious damage. You'll
have flash vaporisation of the surface, and a shock wave moving through
the person from the target site. More damage than the bullet from a small
sidearm. You've seen laser scalpels in television footage, they don't
work by heating up litres of the person in ten degree steps.

>(in fact, I would say that no laser we can built *at all*, never
>mind as a hand weapon, could burn a hole through ceramic and person in
>the fraction of a second of time available).
>
   Antiarmour lasers exist now as laboratory curiosities. They deliver
energy in multiple short pulses to give the plasma layer time to
dissipate. They don't so much burn through something as blast through it.
They are not portable.

   On the whole I agree, lasers are not practical at present as
battlefield weapons, and are unlikely to be practical in any but special
applications for the foreseeable future. In cases where the time of
flight of the projectile is important, or where the target has an active
defense, such as evading or shooting at the incoming projectile, then
lasers have some advantages. Somebody might even be able to think of a
case where a slow thermal loading of a target is strategically useful,
heating up a bunker or a tank to drive people out of it and into the open
even if you can't burn through it. As antipersonnel weapons they seem
unlikely precisely because there are more efficient ways to kill somebody
with a given amount of energy than by shining a light at them.
   This is not to say that there don't exist now lasers which would make
good (multiton) weapons, but rather that almost anything they can do now
can be better handled by more conventional technology at lower cost,
lower bulk, higher efficiency, higher durability, and higher reliability.

>Russell Wallace    rwallace@unix1.tcd.ie


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