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How Dense Can You Get?

  1. Dec 18, 2007 #1
    Hello everybody!

    I have a question that's been bugging me for some time now, considering density and mass and gravity and so on.

    I'm writing this short story for a local magazine (Yes, it's Sci-fi :approve:) and I came upon the idea that you could make what I call a "Gravity bomb."

    A weapon that would use gravity to destroy... well, stuff.

    And here is how I pictured it.
    It would look like a typical rocket-type missile, but instead of being filled with gunpowder and exploding on impact it would - after some kind of chemical reaction within or something - I don't know :confused: - acquire such a big mass that it would have such a powerful gravity field that it would suck in everything in it's surrounding, ripping trees out of the ground, tearing down buildings, lifting cars and people in the air and towards itself.

    And after some time, let's say 30 sec, it would destabilise and explode, blowing away everything that it had pulled toward itself and thus causing more damage.
    (I'm evil, I know)

    I thought of a dying star, but kinda in reverse (and a lot lot smaller :smile:)... First having this small object or something with a lot of density that would then destabilise and explode.

    I'm currently going for this:
    The missile would have something inside like materials that would, upon impact with an object/ground, react with each other and have a far greater mass than they had before - and I'm talking HUGE mass - thus creating this BIG gravitational field.
    The new material would be highly unstable and would destabilise fast, whether in contact with the air or in another way, causing it to explode.

    Now, the big question o:)!

    Would anything like this be even remotely possible?
    Could you, if you had the technology, make two or more (hypothetical) materials react in such a way, that the resulting material's structure would arrange itself in a way that there would be almost no space between it's particles or atoms, resulting in high density and therefore a very strong gravitational pull.

    And could it be possible that anything as small as a soccer ball would have all these properties?
    Would you do it using nanotechnology?
    Just a chemical reaction?
    What do you think would be the (easiest) way to accomplish this?
    Or am I going about this completely the wrong way and should just go and lock myself in the closet for talking complete nonsense?

    So come on great minds, help me out here and offer me a your opinion/new idea/anything!

    I mean, sure, I could just make stuff up to explain this, but I would actually like a little science behind it.
  2. jcsd
  3. Dec 18, 2007 #2
    I don't know much about gravity, but why would it destabilize or explode? To the best of my knowledge black holes don't regularly explode, and wouldn't this just be a mini black hole? I'd think it'd suck everything in around it and just keep growing denser and more massive until it ran out of matter in its area of the universe to suck in and would eventually evaporate or float off into a different solar system to suck in other stars and planets.
  4. Dec 18, 2007 #3


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    Where would this mass come from???? There can't be any more mass than it started with!!!
  5. Dec 18, 2007 #4
    Calm down, man :smile:
    You see, that's why I need you folks to point out such things :smile:

    Ok, so you definitely can't get more mass than you put in, right?
    Can we somehow get around this problem?
    How would I explain a sudden gravity field that wasn't there before, do you know what I mean?
    Last edited: Dec 18, 2007
  6. Dec 18, 2007 #5
    It would. In fact, anything within its event horizon (which is basically the point of no return - once within it, nothing can escape) would be dragged into it, adding more mass and more density to the black hole. Eventually, the planet itself would join it (the "p" in the equation below is a rho, the symbol for density, not a lower case p):

    p = m/v

    If more matter joins the blsck hole, then its volume must increase so density wouldn't increase, right? No, because the black hole still remains the size of a particle - the density is so high that gravity is so strong that it even crushes the atoms (density is mass per unit volume).
    Last edited: Dec 18, 2007
  7. Dec 18, 2007 #6
    So there is no way in hell, that you could make a gravity field so strong but not get a black hole?
  8. Dec 18, 2007 #7
    I'm nowhere experienced enough to give you a definite answer, but I guess so. There must be some kind of minimum density that black holes must have for them to be...black holes.
  9. Dec 18, 2007 #8
    Damn it...
    Ok, putting mass and density aside...
    Could you produce such a gravity field in another way?
  10. Dec 18, 2007 #9
    yeah I don't know much about black holes to be of help much either but..

    at what point do stars implode under their own weight to become blackholes? maybe if you knew the answer to that it might help...
  11. Dec 18, 2007 #10


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    Sure you could. There's lots of things between regular matter and black holes. The most obvious is neutronium.

    The trouble is, gravity is directly equivalent to the amount of mass - and that alone. Where does the mass come from?
  12. Dec 19, 2007 #11
    Yes, i've heard about the "Schwarzschild radius".
    The earth has a schwarzschild radius of 9mm, meaning that if you would compress all of the world's mass into a "ball" which had a radius of less than 9mm it would turn into a black hole.

    So everything has the potential of being a black hole, you just have to compress it to a right size.

    Neutronium... A matter composed primarily of neutrons.
    That sound's good.
    But you're right, It still doesn't solve the problem of the magically appearing mass :rolleyes:
  13. Dec 19, 2007 #12


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    Relativity predicts gravitomagnetism. Though it predicts something very small, look up the work by the ESA on it, it's far larger than thought under certain circumstances:


    Only a matter of time, methinks.
  14. Dec 19, 2007 #13
    A gravity bomb? That would be awesome. As to its construction, that is the poser.
  15. Dec 19, 2007 #14


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    Of course, if the payload of your missile contained one end of a wormhole... you'd have some interesting choices about where to put the other end.
  16. Dec 19, 2007 #15
    So, dst, let me ask you if I got this right.
    I probably didn't, but here I go.

    So, what this device is doing is using gravitomagnetism by spinning a superconductor very very fast and so generating a gravitational field?

    And what's the difference between gravitoELECTRIC and gravitoMAGNETIC field?
    (It's on the "sketch" of the device in the text)
  17. Dec 19, 2007 #16


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    It's a fair bit stranger than that. Look up things like "Lense-Thirring Effect" or "Frame dragging". Something rotating fast or massively will simply "drag" space & time around it. I don't know whether you could call that a "magnetic" field in the classical sense, because it's quite different. The effect is tiny, but with a superconductor it appears to be much larger (still ludicrously small though).

    As for the idea of "gravitoelectric" and "gravitomagnetic" fields, that would be an analogy to Maxwell's equations and hence the idea of one inducing the other.

    Here's more on that: http://arxiv.org/ftp/gr-qc/papers/0107/0107012.pdf
    Last edited: Dec 19, 2007
  18. Dec 19, 2007 #17
    Here would be the main problem I would see: where does the mass come from? If it comes from the surroundings on impact, then the damage would be much greater from the act of gaining mass then from the act of a gravitational pull.

    In addition, if you are aiming to damage the area through a black hole, the act of compressing the object into a black hole would probably need the use of a high energy explosion around the mass, and would likely detroy the entire earth (and more) before making the mass into a black hole.
  19. Dec 19, 2007 #18
    Another problem with this is the act of keeping the superconductor spinning really fast. Although superconductors are void of friction, air resistance is a problem. For one, if this indeed could cause damage, you would have to keep it from spinning fast until impact, at which time it would start spinning (that is huge amounts of angular acceleration. Just huge). Aside from that, a vacuum would need to be maintained constantly in order to combat air resistance, and due to the speed that the superconductor would be spinning, it would have to be an absolutely perfect PERFECT vacuum, which would require a huge missile as well as a clean environment which...just cannot survive any use as a bomb.
  20. Dec 19, 2007 #19
    Ok, no black holes, that's clear now. :cool:
    But let's, just for the fun of it, say that you would be able to produce these perfect conditions. A completely air-tight chamber with perfect PERFECT vacuum.
    And let's say, that you could have two (or maybe even more) of these chambers each spinning in the other direction.

    Do you know what I mean?

    You would have a chamber that was spinning in one way with the superconductor inside (also spinning), and another air-tight chamber around that one which would be turning in the opposite direction.

    Would this increase the speed of the conductor and dragging of space & time even more, or would they just cancel each other out?

    (By the way, thanks a lot guys. This is really helping me understand these things a lot lot better than I had before.
    So thanks for the effort :blushing:)
  21. Dec 19, 2007 #20


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    You might break someone's nail with those 0.000000000x newtons of force it outputs.
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