Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Science fiction concepts

  1. Mar 17, 2006 #1
    In case the name didn't give it away, I like science fiction. A while ago (more than a year ago) I decided to look into space combat from a scientific standpoint. After spending huge amounts of time procrastinating from real work by looking up laser efficiencies, theoretical maximums for focusing particle beams, physical constants for a number of potential armor materials, and so on, I find that there are things about the physics that Google simply won't tell me. So I'm hoping some of you might be able to help me with these questions. (I'll probably have more, too - this is just the list I came up with now.)
    Some of these answers might be complicated - I think I can handle it, if you are all nice and use small words. I am a PhD student, but in zoology. I've had a brush or two with physics, science is no stranger, but my territory is the ecology of grasshoppers, not something even remotely related to the subjects I'm asking questions about.
    Anyway, thanks in advance for the help.

    1) How does one compute acceleration per meter of barrel length for a railgun or coil (gauss) gun (also called a mass driver)? Clearly, each additional meter of length gains you less acceleration, since it has less time to act on the projectile. Equally clearly, higher velocities give you a greater chance of hitting a maneuvering target. And, of course, at a certain point the barrel length just gets ridiculous, even for a zero-drag environment.
    Before someone points out the obvious, I know that railguns and gauss guns are really different beasts - I'm looking for an equation for each.

    2) What is a plasma mirror? Something I read suggested that a plasma mirror reflects any light below the mirror’s frequency. What does this mean? Is it true? If so, can a plasma mirror withstand intense blasts (weapons-grade pulsed lasers, for instance)? (And is a plasma mirror related to a "plasma window"?)

    3) How do you calculate the speed at which a gas or plasma expands? For instance, if you lose containment on your spacecraft’s fusion reactor, how do you calculate how rapidly the no longer contained plasma takes to expand?

    4) I assume there must be a way to take known physical properties of a material (density, possibly distance between nuclei and bond strength as well) and calculate how well it stops particle and EM radiation (of, obviously, known characteristics, since x-rays and hypervelocity charged particles have separate characteristics). How does one do that?

    5) To keep things all about the plasma, a question about beam spread. If you are busy letting plasma blast out of an electromagnetic bottle, either to provide propulsion, or to burn your enemies to cinders, how does one calculate beam spread? For a particle beam, I’d look at particle velocity from heat, time to target, and calculate how far off the beam axis a particle could wander from thermal issues in that time. But for a plasma beam, the thermal velocity is the (potentially insanely high) velocity of the “bolt” itself. A well-designed mag bottle should be coming as close to possible to forcing every last ionized particle to fly out along the beam axis, with no wandering.

    6) Is there any way to take a known set of shapes with known mass colliding at a known velocity and determine from that information the impulse of the collision?

    7) There’s a design out for a magnetic sail spacecraft that is supposed to be shoved around by a beam of plasma shot from elsewhere. I’m aware of the basic interactions of charged particles and magnetic fields, but in this case something is puzzling me. Does the sail stop the beam? Initially, I assumed the beam would “splash” against the sail, electrons going one way, ions going (slightly less readily) another. Then I did some calculations (which, I am sure, were awful for numerous reasons) that suggested that no reasonable field could exert the forces to cause that much charge separation, and that the charge separation would keep the beam together, and let it barrel on through the sail. So what really happens?
  2. jcsd
  3. Mar 18, 2006 #2


    User Avatar

    Staff: Mentor

    1. Have you taken high school physics yet? That's just the distance equation with f=ma in it: d=1/2 (f/m)t^2. Solve for time and plug back in to find velocity.
    2. Sounds like technobabble.
    3. Static pressure and Bernoulli's equation (convert static pressure to velocity pressure).
    4. Not sure - density is a key though.
    5. For a particle stream, there should be little or no dispersion.
    6. Mass vs spring constant - shape isn't all that important.
    7. Not sure.
  4. Mar 18, 2006 #3
    I aced two semesters of college physics.
    Ok, let me rephrase the question - what are the forces involved? For the gauss gun I'd need to know the force on a ferromagnet in a given magnetic field, and I can't find that anywhere.
    My bad on that - I should have phrased the question correctly the first time.

    Search "plasma mirror" on Google, and you'll get a bunch of technical hits, none of which, however, bother to explain what a plasma mirror IS, just how they are using it. The military appears to have ever designed a plasma antenna (same idea, slightly different use) for use in some air and naval craft.
    I've tried to vet concepts pretty thoroughly - I'm not going to ask about gravity lances, FTL drives, or force fields, because those are just silly. I have, however, run across concepts in the literature that people seem to just assume their readers understand.

    Thanks for the other info. I'll look up Bernoulli's equation tomorrow.
  5. Mar 18, 2006 #4
  6. Mar 18, 2006 #5


    User Avatar

    In a rail gun, current flows up on rail, through the armature which travels perpendicular to the rails, and down the other rail. The result is a magnetic field between the two rails (B = 2x(µ0/2π)x(I/r)) and an intercepting field by the armature. The rails repel one another (F = µ0I1I2L/2piR) and they both repel the armature (F = ILB). Since rails are both fixed the net result is a propulsive force on the armature, which will be accelerated forward by electromagnetic means (A=ILB/M).

    You probably read that here: http://sci-phys-plasma.caeds.eng.uml.edu/1997/04-97-15.htm [Broken]

    I don't think plasma mirrors exist, possibly under a different name. There is no such thing as weapons-grade pulsed lasers either, don't try and apply sci-fi too much to reality. :smile:

    Read this for your answer: http://en.wikipedia.org/wiki/Blackbody

    You're not going to be busy letting a plasma blast out of an electromagnetic bottle to vaporize your enemies are propel you past the speed of light, don't hurt your head too much. The electromagnetic bottle is called a Penning trap. Plasma beams I think would be very inefficient and uneconomical to use as weapons, either small-scale for infantry on an alien world, or large-scale as defense an interstellar merchant ship.

    I think you could look at a plasma beam the same way as you could a particle beam, because plasma is made of particles. Electrons and nuclei have just been disassociated. In a normal particle beam such as the one in your CRT television or computer monitor, it shoots out electrons.

    Are you thinking about antimatter here? Impulse is the change in momentum.

    The magnetic sail spacecraft is called a solar sail. It is not shoved around by a beam of plasma, it works from using something called radiation pressure to push it along.

    From what I hear, you may not be as proficient as you think. The sail doesn't stop absorb the light, it reflects as much of it as possible, and uses the reflection as a sort of thrust.
    Last edited by a moderator: May 2, 2017
  7. Mar 18, 2006 #6
    ovoleg - thanks for the link.

    Anyone know what it is for a gauss gun? I've gathered that they are actually better, but harder to build. That whole "no rails to explode outward and corrode with every shot" thing.
    Nice link. Currently, there seems to be some interest in using a system much like the one described in that link as an antenna that can be switched on, used, and then switched off to reduce an aircraft's radar signature.
    Um...what? Didn't you just give me a link about plasma mirrors??
    Now....I'm not SciFiGuy because I want to build it tomorrow, I'm just interested in what might make realistic sense in 50, 100, 200 years. Given that there is work being done on continuous beam lasers meant to burn down missiles (http://www.israeli-weapons.com/weapons/missile_systems/systems/THEL.html) [Broken], and that pulsed lasers enhance kill probabilities through "impulse kills" (a term used to describe a kill caused when a laser superheats a small patch of a target causing that small patch to explode outward and do shock damage, http://www.dtic.mil/ndia/smallarms/Moore.pdf [this link is rather optomistic, but describes the concept well]), there will be, at some point, pulsed weapons-grade lasers.
    And there's always the old designs for the "porcupine bomb" nuclear bomb pumped x-ray laser cluster (http://en.wikipedia.org/wiki/Laser_applications, which includes mentions of several laser weapon projects).
    Maybe I missed it, but it seems that's about radiating energy, not stopping radiation. My question is more about "how do you shield yourself from the radiation coming off nuclear reactors, solar flares, and hostile energy weapons?"
    Now, maybe radiating and absorbing radiation are linked nicely, but if so, I missed it, and I beg your indulgence.
    I should hope I wasn't planning on propelling myself past light speed....however, there are already design concepts for plasma rockets with perfectly sane operation paramters (http://www.space.com/businesstechnology/technology/vasimr_rocket_020807-1.html) [Broken].
    Why? I'll admit that I don't think they'll be anywhere near as efficient as kinetic weapons in planetary combat, and probably too short-ranged in space, but in a world of directed energy weapons, point defense is going to be nasty. There may be value in energy weapons, even ineffcient ones, simply because, unlike an anti-tank missile, they can't be shot down or intercepted.
    If you've got some calculations on the inefficiencies, I'd love to see them. I've been keeping reams of calculations in an Excel spreadsheet to compare various ideas given certain baselines.
    Well, that's my question - is it just a particle beam with way too many particles? The reason why I suspect it isn't is because of how the two systems fire. A particle beam, CRT, linear acceleration, syncotron, or other, accelerates the particles itself - it provides the energy of acceleration, without regard to the direction of thermal velocity for the particles already. (Take thermal velocity, multiply by the time to target [correctled relativistically, since we want the time the particles, not you, experience] and you have the radius of beam spread). A plasma beam uses the energy of the plasma to fire - the thermal velocity is what wings the plasma out the barrel/drive tube.
    Imagine both systems as tubes of bouncing ping-pong balls. A particle beam blasts air down the tube, hurling all the ping-pong balls out. Some of them may have been bouncing in a direction perpendicular to the tube axis - they will exit with that velocity intact, and move off the "beam" axis. A plasma beam just lets some of the balls bounce out. To do this, though, they have to have fairly specific velocities (direction-wise), or they won't make it out of the tube. I'd expect different beam spreads because of this.
    Maybe I'm wrong - but if you could explain away my logic, I'd be convinced I was.
    I'm thinking about kinetic weapons. I mean impulse. I'd like to be able to figure out what kind of projectiles fullerene armor (http://www.isracast.com/tech_news/091205_tech.htm) can deflect, and I need impulse to calculate the pressures involved.
    Um, no. I know what a solar sail is, and I mean a mag sail. It's described here: http://www.newscientist.com/article.ns?id=dn6543
    Last edited by a moderator: May 2, 2017
  8. Mar 18, 2006 #7
    Ok..since everyone seems to think that plasma mirrors are something I got out of a sci fi book, let me provide a link to technical article about using plasma mirrors to reflect high-intensity lasers.
    http://www.clf.rl.ac.uk/Reports/2001-2002/pdf/92.pdf [Broken]
    That's the phenomenon I want explained.
    Last edited by a moderator: May 2, 2017
  9. Mar 18, 2006 #8


    User Avatar

    With regards to the "magnetic sail". I've heard of people designing sails which are then pushed by a laser beam, but not plasma... where did you see this?
  10. Mar 18, 2006 #9
    Several places.
    I've already posted this link http://www.newscientist.com/article.ns?id=dn6543
    but maybe it isn't working for you.
    The bottom of this Wikipedia article discusses using a particle beam to push a mag sail.
    More on the plasma beam end of things.
    And one last link.
  11. Mar 18, 2006 #10


    User Avatar

    I haven't been able to find much on plasma antenna's either. I find a lot of companies selling them, but no body describing what they are, how they work, or how the look like.

    Oh I seem to've been mixed up, I thought only 71 results came up on Google, I look again and find 2.8x106.

    You've done some good research! :smile:

    No need for that, I think I misunderstood your question.

    Radiation coming from nuclear reactors is different from solar flares' radiations and electromagnetic radiation. You would need some sort of dense, highly reflecting material, because if it absorbed all the radiation it would probably end-up heating-up a bit. I don't know what to use. I can't think of anything available nowadays.

    Unless of course you were thinking of armoring for the spaceship, then you could probably used some very dense material like depleted uranium to absorb it all. Because you're out in space, excess heat can radiate off without worrying about somebody wearing it getting burned. Assuming you got a great thermal insulator such as aerogel.

    The article cited seems to talk about an ion-drive technique for spacecraft propulsion. This is the kind used by many of today's space probes, such as Deep Space 1 and 2. http://en.wikipedia.org/wiki/Deep_Space_2

    Cool links. I'll read them all. :biggrin: :wink:
    Last edited by a moderator: May 2, 2017
  12. Mar 18, 2006 #11
    Well, from what I can determine, plasma, under the right circumstances can act to transmit, reflect, or absorb radiation. A plasma antenna transmits radiation in a directed fashion, serving as a tight-beam radio antenna. From what I've read, the main advantages are that the direction of the antenna can be changed rapidly through altering current flow in the device, and that the antenna can be shut off, and disappear, to reduce the radar signature of the craft when not needed.
    Of course, what I really want to understand is how plasma changes from reflecting to absorbing to transmitting. Sounds like there's great potential to deal with incoming EM radiation, but without knowing the physics, it's hard to guess what might be realistic.
    Thanks! I've spent about a year with this as one of my hobbies. Obviously, though, some questions have me stumped, or I wouldn't be here asking about them!
    Yeah, I assume particle and EM radiation may be different beasts. I gather that particles "braking" through radiation shielding can dump a lot of EM energy, and that, if they "brake" fast, that energy will be, itself, EM radiation in the x-ray bands.
    Ooh...nice lead on the aerogel. I see possibilities (and not just particle shielding ones) there.
    I think fullerenes look like the best armor in terms of deflecting kinetic weapons and dealing with the thermal effects of energy weapons. However, what I really want are some calculations. One of the issues is with particle beams - unlike most lasers, they dump their energy across a path into the armor, not on the surface. This means that, for certain beams and armor thicknesses, much of the beam energy will strike PAST the armor layer. Some calculations on beam penetration would help me establish whether you need 1 km of armor (in which case, forget it, particle beams are just killers) or 3 cm. I expect that high-frequency lasers (x-ray, for instance) may also penetrate armor some distance without having to blow through it.
    I also suspect that neutronium would make nice rad shielding, but I'm trying to stay within reasonable realism limits.
    The coolest of them all is http://www.projectrho.com/rocket/index.html, which does what I'm trying to do (with, obviously, a different slant, or why would I repeat it?) in terms of trying to define the science behind sci-fi.
  13. Mar 18, 2006 #12


    User Avatar

    Staff: Mentor

  14. Mar 18, 2006 #13
    Well, I'll admit that grad student salaries don't put me in front of plasma TVs that often, so I don't know that much about them, but here's a quote from the article:
    So Agile Mirror is made of plasma (maybe the operational parts of plasma TVs are too - I don't know), and it's the plasma that is interacting with the microwave radar - reflecting it, I think. You've said that it does this by varying the opacity. Ok. How? What's the physics behind making a variable-opacity sometimes-reflective sheet of plasma?
  15. Mar 19, 2006 #14


    User Avatar
    Staff Emeritus
    Science Advisor

    I don't think the opacity is varying per se.

    I believe the situation is that a plasma will reflect waves of low frequency, and transmit waves of high frequency.

    The frequency at which this transition occurs (reflection vs transmission) depends on the characteristics of the plasma - essentially it's electron density. Thus our ionosphere reflects radio waves, but transmits light. A denser plasma will thus reflect light as well as radio waves, which is apparently what the laser people are taking advantage of.

    See for instance:


    I think the technical term for this frequency is "plasma frequency", but I'm really not positive.

    You might also try talking about this topic on rec.arts.sf.science (that's a usenet newsgroup).
    Last edited: Mar 19, 2006
  16. Mar 19, 2006 #15


    User Avatar

    Neutronium is something hard to find correct and comprehensive information about, but here is the best place I've run across. https://www.physicsforums.com/showthread.php?t=83095

    That would be here. Do you have a Google account? Anybody that needs one, I'll refer you.

    Plasma TVs work by electronically exiting inert gas molecules in a glass chamber. The phosphors light up in the right pattern, and it produces a picture. Sounds like the mirror thing.

    Does each pixel have its own chamber, or is it just a big one?
    Some raw data and formulae.
    http://www.oz.net/~coilgun/mark1/spread2.htm [Broken]
    http://www.oz.net/~coilgun/mark1/spread3.htm [Broken]
    http://mgc314.home.comcast.net/velocity.htm [Broken]
    Last edited by a moderator: May 2, 2017
  17. Mar 20, 2006 #16
    Thanks - that seems to match what else I'd read, which probably means that my dubious net source was actually correct.
    Looks like the data on the frequency here will let me backtrack to figuring out what that is in real (and engineering) terms.
    Ah yes...explodes and kills you with radiation. Sounds....fun. I rather suspected that something bad would happen when you dragged it away from that massive grav field.
    Incidentally, there's no way of actually generating gravity, right? Aside, of course, from bringing a lot of mass along with you.
    Nope. I don't.
    I think it's a different phenomenon - pervect's info matched a comment I'd seen earlier indicating that it really is reflecting the incoming like, not just phosphorescing.
    Ok - that helps. Some of these seem to be optimized for specific designs, though. Since no one's designed a superconducting quench gun model yet, that could be difficult to adjust for.
    Still, looks like these are the equations I need, once I sort through the specifics to find the generalities.
    Last edited by a moderator: May 2, 2017
  18. Mar 22, 2006 #17


    User Avatar

    Right now, it seems like doing that would be quite a feat.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook