Particle beam interaction

[Ford Prefect]

I was recently watching the movie Char's Counterattack, in which one character uses a particle beam to intercept another particle beam. Thinking of that, I began to list off the various ways in which I have seen beam weapons interacting in fiction. Perhaps notable examples include two beams bending away from each other in Neon Genesis Evangelion, or one beam 'drilling' through another in a recent remake movie of Evangelion. It's even popped up in Star Wars, with a relatively recent novel featuring a sect of Jedi who use blasters and are known to shoot down blaster bolts.

I ended up thinking of the Large Hadron Collider, given that it is a device designed to ram particle beams into other particle beams, but my knowledge of that is basically trivia. I don't actually know what happens when particle beams collide. I'm wondering if anyone can help out here.

[Khaat]

I don't actually know what happens when particle beams collide.

I don't either, but it sounds like an 80's anime love song lyric.

WARNING: WHAT YOU READ BELOW IS NOT HARD SCIENCE

If the particle is just accelerated (probably a stream of particles), normal planetary magnetic fields would bleed it's energy or vector pretty quickly. The odds of hitting such a thing would be, well, astronomical. With attendant effects. Of course, if they are the same-charge particle, they'd simply repel one another with a near-miss (vector change again). I never understood the sci-fi fascination with sub-atomic-mass-driver weapons. Lasers, sure. Neutron beams? Proton beams? There has to be a more efficient way to deliver kinetic energy to your target!

If it's a "magnetic conduit/capsule" of contained plasma, I suppose there could be a diffusion spray at the point of convergence, if the conduits/capsules disrupted each other and dumped the contents. Fireworks. "Shooting down blaster-bolts" would fit this model.

[Starglider]

If the beams are even slightly diffuse, then they will pass through each other with no result. If they are perfectly focused, then they will be so narrow that it is effectively impossible for them to line up perfectly and interact. Note that it is currently impossible to make a 'perfectly focused' particle beam outside of an accelerator; charged particle beams self-diffuse and the only known method for making a neutral particle beam is to neutralise a charged particle beam, which unavoidably diffuses it slightly.

Of course soft sci-fi 'containment tubes' and 'plasma bolts' may play by different rules.

If the particle is just accelerated (probably a stream of particles), normal planetary magnetic fields would bleed it's energy or vector pretty quickly.

Um, no. Planetary magnetic fields are negligable and barely affect weaponised particle beams. At most you might have to include it as a correction in your targeting software.

I never understood the sci-fi fascination with sub-atomic-mass-driver weapons. Lasers, sure. Neutron beams? Proton beams? There has to be a more efficient way to deliver kinetic energy to your target!

Why? Particle accelerators are essentially mass drivers operating on a large number of very small masses. They're the only practical way to accelerate projectiles near-instantaneously to c-fractional velocities, i.e. deliver kinetic damage with a laser-like weapon. Sure you can mess about with conventional guns and missiles but then you're allowing the target to dodge and/or shoot down your projectiles. AFAIK there's no fundamental reason why the upper energy efficiency limit for lasers is any better than that for particle beams, and lasers have very poor penetration characteristics (but are easier to keep focused over long ranges).

[Ford Prefect]

If the beams are even slightly diffuse, then they will pass through each other with no result. If they are perfectly focused, then they will be so narrow that it is effectively impossible for them to line up perfectly and interact. Note that it is currently impossible to make a 'perfectly focused' particle beam outside of an accelerator; charged particle beams self-diffuse and the only known method for making a neutral particle beam is to neutralise a charged particle beam, which unavoidably diffuses it slightly.

Man, that is the single most boring answer ever. Thanks for your help, though.

[Aranfan]

If the beams are even slightly diffuse, then they will pass through each other with no result. If they are perfectly focused, then they will be so narrow that it is effectively impossible for them to line up perfectly and interact. Note that it is currently impossible to make a 'perfectly focused' particle beam outside of an accelerator; charged particle beams self-diffuse and the only known method for making a neutral particle beam is to neutralise a charged particle beam, which unavoidably diffuses it slightly.

Wouldn't there be like, induction effects and such to change the paths of the beams?

[Starglider]

If the beams are even slightly diffuse, then they will pass through each other with no result.

Wouldn't there be like, induction effects and such to change the paths of the beams?

Only for charged particle beams, and even then the effects will be practically insignificant because at c-fractional velocities the interaction time is so ridiculously brief. Perhaps if you could fire a focused beam directly back along the vector of an incoming focused beam it would make a difference, but I doubt this is possible under any practical circumstances.

[Sriad]

In any case, the relativistic speed you need with a particle beam make shooting bullets out of midair with bullets seem like shaking hands.

If you DO manage it somehow, you get a spray of hard radiation in all directions -like with a particle accelerator. But then again, outside a particle accelerator managing it almost requires handwavium physics, so you can have whatever effect you want. Just like 0/0.

[Winston Blake]

The problem is that particles in a real particle beam can't really interact much with each other, or the beam will tend to scatter itself. In science fiction, particle beams (or should I say 'generic energy beams') act more like 'jets of water' than beams of particles, typically 'splashing' when they hit each other, i.e. with lots of interaction between the constituent 'energy' or particles or whatever.

This is a very natural, intuitive assumption about how flowing streams of 'energy' should behave when they collide - since that's how everyday 'flowing substances' work - but unfortunately the reality doesn't match the intuition. The other 'intuitive' behaviour I can think of is for them to bounce off each other like collisions between everyday solid bodies.

The Evangelion example is rather unusual, since the collision is like neither everyday materials nor real particle beams. In fact IIRC the beams actually accelerate back up to their original speed after the 'pause', which is pretty bizarre. For example, what on Earth were the high-velocity (so-called) positrons in the rear of the beam doing while the 'head' slowed down to a few meters per second? If anything, that collision was more like an interaction between two living animals, plus some of the 'unstable repulsion' behaviour that kids are familiar with from playing with magnets.

[Ford Prefect]

The Evangelion example is rather unusual, since the collision is like neither everyday materials nor real particle beams.

Yeah, it's completely wacky (Gainax practically invented homing lasers, after all), but it looked pretty cool. Unlike all these boringly accurate responses. I mean, I wasn't really expecting anything enormously exciting like 'beams twisting around each other' or 'enormous explosion', but you'd think there'd be something.

In fact IIRC the beams actually accelerate back up to their original speed after the 'pause', which is pretty bizarre. For example, what on Earth were the high-velocity (so-called) positrons in the rear of the beam doing while the 'head' slowed down to a few meters per second?

I'm fairly sure it's possible just to wave it off as it being slow motion for the benefit of emphasis rather than the beam actually slowing.

[Covenant]

Basically, they're interacting in a way that makes sense for a Giant Magic Magnet, but particle beams are not giant magnet beams that can be bent around like a piece of wire. Even a charged particle beam doesn't behave like a giant magnet slinky, not in any sense that we're aware of currently. It's not that it's impossible, just really, really impractical. We could probably invent a way for it to work, but I think the levels of energy involved are inconsistant with the kind of firepower output we observe. Anime curvey laser beams are mostly there for the fun value, or at best they're just really goofy kinds of magnetically bound charged particle beams--sometimes trapped in containment tube magic fields.

This isn't to say that charged particles DO NOT make goofy wobbles and stuff, I mean, look at the aurora effects and other such things. It's just that these stuff you see if way, way too dense operating the way it behaves at extreme energy levels, and driven by a need for coolness. I mean, with a sufficently advanced magnetic device you could possibly bend particle beams into all sorts of shapes. Or glowy plasma. But none of these explain the weapon systems. I bet you could make glowy, spinning, wierdass light tubes from a sufficently advanced piece of desk art using funny cold plasmas and electric currents. But unless Neon Genesis Evangelion is making weapons out of neon lights, not likely front-line warfare stuff. Check out the pictures of the sun, for example, to see some really goofy behavior around those magnetic wells and geysers.

The 'drilling' particle beams make slightly more sense, but require densities of particles that are pretty astounding.

Regardless, are these realistic weapons? No. But you certainly can do goofy stuff with magnetics and charged particles.

[Ford Prefect]

We could probably invent a way for it to work, but I think the levels of energy involved are inconsistant with the kind of firepower output we observe.

It's pretty hilarious. Zone of the Enders had 'lasers' that could home in on enemy by bending spacetime. Despite how cool it looked, I couldn't help but muse how the amount of energy that must have gone into bending was probably much greater than the actual beams put out. I once had a friend say 'it's probably possible to make homing charged particle beams by manipulating electromagnetic fields' and I didn't want to make his idea sound completely terrible. As far as soft science fiction goes it's not especially offensive (and it does look rather elegant, beyond even the usual 'Itano Circus').

Though, on this train of thought, I imagine that as a defensive measure you could, perhaps, particle beams out of the way for an astonishing energy cost. And assuming you can somehow remotely play with magnetic fields in this way, would it be easier (or at all possible) to 'scatter' the particles that make up the beam.

The 'drilling' particle beams make slightly more sense, but require densities of particles that are pretty astounding.

Naturally, the scene I'm thinking of is kooky in its own way, it's pretty indicative of the habit in science fiction to depict such weapons as, as Winston puts it, 'generic energy beams'. Actually thinking aobut the array of science fiction media that I am familiar with (and it's quite a lot), this practice is almost universal, even in stuff which is considered 'hard' science fiction (Alastair Reynolds, I'm looking at you). They often are just jet of really bright water that melts shit.