Ray Guns! (18)

1 Name: Less Anonymous 2005-12-15 05:07 ID:rUlk+8SN

I'm a sci-fi writer, and I usually try to write my stuff so that someone with knowledge of physics will not feel a compulsion to scoff at any part of it. I also do a lot of action stuff.

So, to those of you who are good with physics, I ask you, what sort of weapons are practical for futuristic soldiers? I usually depict them as having armor which effectively negates any harm that would be caused by modern day firearms (absorbs all the kinetic energy so the soldier doesn't have to). So a directed energy weapon seems better.

I thought about magnetic weapons (gauss rifles), but then realized the recoil would be too intense for a handheld weapon, so I ruled out that possibility. Here's types of weapons I've thought of so far (for my purposes, assume each of these has as much power as they need):

Lasers: Do lots of damage from 'shearing' effects, but I don't think they'd work well in a humid environment or in the rain. Would a perfectly reflective surface deflect a laser beam regardless of power?

Plasma: Often used in sci-fi, but I read it would disperse quickly in an atmosphere. Some kind of containment field would be needed to get it to the target, right?

Ion: Shooting a stream of ions... what would this do?

'Particle Beams': Often used in anime (or so I'm told, don't watch much of it myself) to describe a weapon without actually explaining what it shoots. But I picture something like a handheld particle accelerator, shooting a bunch of particles at something. I imagine this could transfer a lot of kinetic energy to the target if the particles moved fast enough.

Masers: Microwaving your enemies is mean, but I imagine it would be effective.

What do you guys think?

2 Name: !WAHa.06x36 2005-12-15 14:26 ID:Heaven

Gah! I posted a long reply to this, and it disappeared! What the hell?

3 Name: Mad Scientist 2005-12-15 14:34 ID:/horEHie

>magnetic weapons [...] recoil would be too intense for a handheld weapon

Not necessarily. The damage a bullet does depends mainly on its kinetic energy (0.5*m*v^2), recoil on its momentum (m*v). If you keep the same recoil, kinetic energy scales lineary with muzzle velocity. Railguns with with up to 15000 m/s have been demonstrated, a modern rifle has around 900 m/s. So a with same recoil a railgun could be up to 16 times more powerfull.

>Lasers: Do lots of damage from 'shearing' effects[..]

Are you referring to mechanical shearing? This is simply be the same effect as with explosives, as laser can rapidly vaporize material.
Humidity/rain is an issue but not to the point of ineffectiveness (assuming a high powered laser in the kW range, and ranges up to several km).

>Would a perfectly reflective surface deflect a laser beam regardless of power?

Yes. But bar some "mirror field" you aren't going to get anywhere near 100% reflectiveness under field conditions.(not that 100% is achivable in reality in the first place...). The usual 90%-reflective-household mirror stuff is no obstacle for a proper laser beam.

>Plasma [...] Ion [...] Particle Beams

Those are basically the same: high velocity charged particles. They work by heating up the target area; due to their nature as dissociated electrical charges they also can fry unprotected electronic devices.
Plasma torches do exist and are used to cut and weld steel but their plasma jet disperses after a few centimeters. As you wrote they are limited by atmospheric friction (cooling down/dispersal)


Useless if your target is inside a Faraday cage. A simple metal mesh as you can (or at least should :)) see it in the front of a microwave oven will suffice.

A few thoughts in general:
-If high powered lasers were comonly used on a battlefield people probably would be blind long before dead. I have yet to see any kind of SF where proper laser safety was practised.
-Power (especially portable) is usually ignored but a major problem. There is no high density electrical storage on the horizon and the common heat->electricity conversion is rather inefficient. So if you care about details cook up some force field insulated capacitor that can store a high electrical charge directly (plus if it gets damaged it will evaporate in an enormous lightning arc)

4 Name: !WAHa.06x36 2005-12-15 15:32 ID:SuJEGHj+

> If you keep the same recoil, kinetic energy scales lineary with muzzle velocity.

This is true, and high-velocity projectiles would also penetrate armour better. However, they also penetrate flesh better, to the point where they just go through and don't do much damage. Still unpleasant, though.

> The usual 90%-reflective-household mirror stuff is no obstacle for a proper laser beam.

On a battlefield, it would matter, though. However, mirrors get dirty, and degrade as they are damaged. Just blast away until you've burned away the mirror.

Internally, lasers use special layered optics that are tuned to be nearly perfectly reflective, but only in a narrow range of frequencies.

> As you wrote they are limited by atmospheric friction...

That, and as the particles are charged, they repel each other and the beam disperses. Higher beam energies minimize this, but the effect still exists. The atmosphere might be the bigger problem, though.

I guess that theoretically they could just blast their way through the atmosphere, heating it until a line of near vaccuum is created. That'd take quite a bit of energy, though.

5 Name: !WAHa.06x36 2005-12-15 15:35 ID:SuJEGHj+

Regarding power sources, here's something pretty hilarious:


It's a design proposal for a laser rifle that would use 740 grams of polonium-210 as a power source. This has some problems, such as:

  • Polonium-210 is ridiculously radioactive. It will even degrade any materials close to it, just from nuclear reactions with the high-energy alpha particles it emits.
  • The rifle would generate about 100kW of heat at all times, including when not in use.
  • That energy is used to heat gas inside a closed container to a temperature of 2200 kelvins, producing a pressure of 270 atmospheres.

6 Name: Mad Scientist 2005-12-15 17:55 ID:/horEHie

>On a battlefield, it would matter, though [...] Just blast away until you've burned away the mirror.

With a high energy laser (kW range) the reflective surface should be burnt away instantly (even the laser rifle you posted below would vaporize a 0.9 reflective alumininum layer in a single pulse and still have about 25% of its energy left. That is ignoring both beam dispersal and assuming all of the beam gets reflected). And realistically the "mirror" will be more of the brushed metal variety and be covered at least with dust.
Anti laser armour would need some material with high heat conductivity (ideally a room temperature supraconductor. We are talking about SF after all) that easily evaporates to disperse the beam.

>Internally, lasers use special layered optics[...]

And even those wouldn't last without cooling.

>the particles are charged, they repel each other

They are also parallel currents and attract each other via their magnetic fields. I have to admit though, so far I have been to lazy to properly calculate these two effects.

The link is interesting. Also:

>The rifle would generate about 100kW of heat

This is an SF-related pet peeve of mine: waste heat.
Half of all the supertanks, starships, and pocket-sized-guns-that-can-evaporate-a-hill probably would melt alone due to their internal conversion inefficiencies.

7 Name: !WAHa.06x36 2005-12-15 21:11 ID:Heaven

> They are also parallel currents and attract each other via their magnetic fields.

Not in a co-moving reference frame, so I'm guessing the repulsion wins out in every case except maybe at light speed. I haven't done the maths either, I'm just talking out my ass.

8 Name: Mad Scientist 2005-12-16 10:31 ID:/horEHie

>Not in a co-moving reference frame[...]

Ah, you're right. Below is a more naive approach:

Two equal charges q moving parallel with speed v in distance r:
F(magnetic)=q*v*B and B=mu*q*v/(4*pi*r^2)

F(magnetic)/F(electric) = v^2*mu*epsilon = v^2/c^2

9 Name: Mad Scientist 2005-12-16 10:45 ID:/horEHie

Addendum to 8:
The question would be if one could treat an ion beam similar to a collection of wires and apply Ampere's law. A wire's magnetic field is ~1/r, the field of a point source (as above) ~1/r^2.

10 Name: !WAHa.06x36 2005-12-16 15:19 ID:Heaven

> v^2/c^2

Ah, just what I wanted to see.

11 Name: Less Anonymous 2005-12-18 03:23 ID:rUlk+8SN

Yeah, I thought lasers sounded the most practical. Like I said, though, I've considered describing a plasma weapon that would create a containment field all the way to the target and send plasma down it (I think that's how the plasma weapons work in the Halo video games). Or one that fires a small projectile that's basically a containment field generator... which would shut down upon impacting something.

I'm not worrying about how these weapons generate power or cool themselves yet. Such things are fairly easy to explain by making up exotic materials.

>>4 is correct about the over-penetration issue. The ease with which it would pass through a human being (or alien being) would limit its effectiveness and make it dangerous to use anywhere near innocent civilians.

12 Name: Mad Scientist 2005-12-18 14:44 ID:kKot5kqJ

>I've considered describing a plasma weapon that would create a containment field all the way to the target

This is where the "scoffing by people with a knowledge of physics" part comes into play. All too often the term field is used to denote something that simply acts like a invisible container or wall, but nothing like the macroscopically observable everday (force) fields. (And, yes, I am aware that those "walls and containers" satisfy the basic definition of "field")

So if you say "containment field":
- What property of matter does it affect? (charge, mass, ..)
- If you can project it from your weapon to the target, why can't you use it to interact with the target directly?
- How is the field influenced by matter? If it can't pentrate denser matter, raindrops and foliage will pose a problem. (On a sidenote, David Drake occasionally mentioned this as a drawback of his powerguns)

>Or one that fires a small projectile that's basically a containment field generator

If the projectile can carry enough energy to generate your field why not use this energy as an substitute explosive? If one can manufacture a generator that small, your tech level might be advanced enough for bullets containing metallic hydrogen or a nuclear isomer (both of which can be used as powerful explosives).

>[Power and cooling] Such things are fairly easy to explain by making up exotic materials.

They are easy to handwave using technobabble, like everything else.
But high performance materials only go so far (insulation). In the end you have to dump all your used energy.
So with high powered direct energy weapons your soldiers will at least be a shining beacon in the IR spectrum.

>[...] make it dangerous to use anywhere near innocent civilians.

In really don't see a way around that.
Projectiles will go through their target (that's an issue even today and the only resort are bullets that can be stopped by armoured vest), lasers will blind bystanders, plasma has IR, UV, and gamma radiation as sideeffects (depending on power this might lead to vision loss and/or burns) and stray rounds are prone to start fires.

I'd be interested what kind of armour you have in mind. As long as it's not some fully enclosing powersuit, smart ammunition might an alternative.

13 Name: Less Anonymous 2005-12-19 00:07 ID:rUlk+8SN


I haven't really thought much about the nature of such a field. I personally prefer the method I came up with of using a projectile that produces a containment field.

The issue with using a solid projectile to cause damage is that it would have to be effective against both armored and unarmored opponents. Given the force needed to penetrate the armor I describe in my stories, this is extremely problematic. A slug that could penetrate such armor would not be very effective against unarmored targets.

Plasma will work on either one. So will lasers.

I don't really use technobabble a lot. If its use seems the only solution, I just don't go into too much detail. As for cooling, most of the 'hot' parts on these weapons just don't conduct heat very well. I won't go into detail because I started this thread to talk about the effectiveness of the weapons, not how to cool them.

As for the overpenetration problem, I just meant that it would be really bad if you're using projectiles that go through brick like it was drywall.

The armor is a 'powersuit', as you called it. Self-contained, highly damage resistant, powered armor. It's like a second skin for the wearer, and controlled via a neural link to the onboard computers. Semi-rigid metallic plates over flexible material which acts like artificial muscle as well as protecting the wearer, which is itself covering an envionmental protection suit.

14 Name: Mad Scientist 2006-01-17 06:17 ID:1GrRF5CP

I saw that article. It has to be a joke.

1, there isn't 740 grams of purified polonium 210 on Earth. There might not be 740 grams of polonium 210 in the Solar System.

2, the radiation emitted by its power source would kill a soldier carrying it in a matter of days, sooner if a bullet or shell fragment strikes the casing and lets the polonium leak out.

If by some bizarre and implausible means you got a hundred thousand such rifles--I don't know where they could come from, maybe from aliens? maybe from the replicators on the Enterprise? this is just for the sake of argument--the single most effective military use for them would be to give them to your enemies. They'd be a hell of a lot more dangerous to the soldiers using them than to the people they were trying to kill with them.

15 Name: !WAHa.06x36 2006-01-17 14:38 ID:SuJEGHj+


If you look up the webpage of the people who made the proposal, it's kind of hard to decide if they're serious or just some weird kind of crackpots.

There is definitely not 740 grams of polonium 210 in the solar system, since it has a half-life measured in months. The only way to get it is to manufacture it. This is done all the time, but nowhere near enough to power even a single rifle. This is "just a matter of engineering", of course.

The radiation probably wouldn't kill the soldier - it's all alpha, and those are trivial to shield. Neither would the radiation kill the soldier if a bullet struck the casing - the massive explosion caused by the 270 atmospheres of pressure would kill him. The rest of the squad might die from the radiation spread from it, though.

16 Name: Mad Scientist 2006-01-29 16:07 ID:1GrRF5CP

Higher velocity projectiles are more destructive in soft tissue due to hydrostatic shock, especially as the velocity at impact approaches the speed of sound in water.

This is already a known bug and/or feature of conventional firearms. Assuming full metal jacket type military ammunition, even a bullet from a pistol, having only a small fraction of the kinetic energy of a rifle bullet, will usually go all the way through a human being.

All alpha and beta radiation is accompanied by gamma radiation as the particles in the nucleus give up part of the binding energy and shift to a more stable energy state.

Polonium 210 emits alpha radiation at 5000 times the rate of radium, gram for gram. Every alpha particle is accompanied by gamma rays. And while I do not know if the particular frequency of gamma rays emitted by polonium 210 is especially nasty to biological systems (some radioisotopes are toxic well beyond what you'd expect just from counting emissions; strontium 90 and cobalt 60 come immediately to mind), but the sheer volume of gamma ray emission surely makes up for it.

17 Name: !WAHa.06x36 2006-01-30 15:01 ID:A1OupT79

> All alpha and beta radiation is accompanied by gamma radiation as the particles in the nucleus give up part of the binding energy and shift to a more stable energy state.

Except if the decay goes straight to the ground state of the daughter nucleus, as is the case for Polonium 210.

However, there is a very weak branch (0.00121%) that does not go to the ground state, and does cause secondary gammas at 803 keV. So no, every alpha particle isn't accompanied by gamma rays, but every one in 100000 is. Normally you could disregard this, but as you point out, with intensities this ridiculously high, and the efficient shielding of the alpha particles from the main branch, it could very well become significant. I haven't bothered to do the maths, though.
(Reference: http://ie.lbl.gov/toi/nuclide.asp?iZA=840210)

18 Name: !WAHa.06x36 2006-01-30 15:05 ID:Heaven

Also, that page says it's "Naturally occurring", so I guess it's part of some decay chain. So disregard what I said about how much of it exists. There probably exists quite a bit of it at any given time.

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