From: rwallace@unix1.tcd.ie (russell wallace)
Subject: Re: Laser weapons
Organization: Trinity College, Dublin
Date: Thu, 3 Sep 1992 10:21:07 GMT

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. I know somebody to whom this happened. Ruined a
>perfectly good shirt and left some marks on his skin for a while. He
>didn't even feel it at first, and the tracery on his skin showed how he
>moved back and forth behind the table looking at the apparatus.
>   There are lasers which could make you severely uncomfortable nowadays,
>though the beam would have to sit on you for a large fraction of a second
>before you expect serious damage. For instance, the forty foot CO2 laser
>here at the Toronto Science Centre, built for Expo '67, can immediately
>ignite wood placed at the focus of an IR lens. Cinder blocks take a while
>to go through, 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; someone wearing a
Kevlar flak jacket with ceramic inset plates would be able to get out of
the way before the beam burned through their armor; even someone wearing
only light clothing would only suffer a painful burn. (Remember that the
target is not going to be obliging enough to stand still while you aim
the laser, that even if you can keep the laser trained on the target you
cannot keep the beam on the same *part* of the target, and that to kill
(as opposed to causing a painful, superficial burn), the beam must drill
deep into the target and hit a vital organ. Also, people are mostly
composed of water, which has a very high heat capacity; you'll need a
*lot* of energy to drill a hole in someone.) Compare the effectiveness
of this laser as a weapon, with a cheap, efficient and utterly deadly
assault rifle.

>>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. The target is not going to
stand still long enough to let you pump that amount of energy into him.
And if laser weapons are in common use, he will probably be wearing
Kevlar with ceramic inset plates, which will be *very* difficult to burn
through (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).

>>So the first practical energy weapon will probably be a fusion gun,
>>using a fusion reaction to produce energy, arranged so that the energy
>>is emitted from one end of the reactor. Whether the beam will primarily
>>consist of charged particles, infrared, visible light or X-rays I don't
>>know; it would depend on exactly how the device worked.
>>
>   David Drake has weapons of this description in some of his novels. I
>believe he describes them in _Target_. It's supposed to be an inertial
>confinement fusion reaction run with a hole in the reaction chamber. I'm
>not convinced that it would be a very effective weapon in an atmosphere,
>where we consider 'effective' to mean it does more damage to a target
>twenty metres away than it does to the wielder.

Good point: if the weapon produces a plasma flame, this is not going to
be an effective weapon at significant range in atmosphere (or indeed at
all). You really want it to generate a beam of light/infrared/X-rays or
whatever. Can any engineer types out there comment on whether this is
even theoretically possible?

>>The only other viable idea for an energy weapon I've heard of is the
>>X-ray laser powered by a nuclear bomb, suggested for the SDI project; a
>>weapon that requires a nuclear explosion at the firer's end probably
>>isn't what you're looking for though :-).
>>
>   That's the quick and easy laser gun. SDI did also consider visible
>light chemical lasers using a hydrogen-fluorine power source.

Hardly quick and easy; they still haven't managed to even prove that the
X-ray laser is possible, never mind actually build a working prototype.
And the visible light chemical laser doesn't put out enough power to be
of any use.

>>Russell Wallace, Trinity College, Dublin  rwallace@unix1.tcd.ie


>-- 
> Christopher Neufeld....Just a graduate student  | Confidence is the feeling
> neufeld@helios.physics.utoronto.ca    Ad astra  | you have before you
> utzoo.utoronto.ca!generic!cneufeld              | understand the situation.
> "Don't edit reality for the sake of simplicity" |
--
"To summarize the summary of the summary: people are a problem"
Russell Wallace, Trinity College, Dublin
rwallace@unix1.tcd.ie