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Negative Mass?

  1. Dec 8, 2011 #1
    I was thinking, can an object have negative mass? If an object with positive mass needs infinite energy to go at the speed of light, would an object with negative mass need infinitely negative energy (if negative mass were possible, this would be possible too right?) If an object had negative mass, could it go faster than the speed of light?

    I know that dark matter is supposed to have negative pressure, how does that work? If something can have negative pressure, could it have negative mass?

  2. jcsd
  3. Dec 8, 2011 #2
    No object can have negative mass. That would violate the principle of least action.
  4. Dec 8, 2011 #3
    Okay thanks. Then how can an object have negative pressure?
  5. Dec 8, 2011 #4


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    Perhaps you're thinking of dark energy, rather than dark matter?

    Negative pressure occurs in the cosmological constant - positive vacuum energy implies negative pressure, and drives expansion of empty space.
  6. Dec 8, 2011 #5
    Hmm... I thought it was dark matter but now that I look back into the book i learned about it from, i guess it is dark energy. I do get those mixed up a lot.

    I think i understand now. Thanks for clearing those things up for me.
  7. Dec 9, 2011 #6
    negative mass is meaning less..........................
  8. Dec 9, 2011 #7


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    Actually, electrons in the solutions of the Dirac equation corresponding to negative energy also have negative mass. The interpretation of Dirac was that all these states are filled in the vacuum and we only notice if one is missing. This missing electron then behaves as a particle with positive mass and positive charge - the positron.
    What you are probably having in mind, i.e. particles moving with superluminal speed is known as tachyons. These hypothetical particles would have imaginary mass. However they violate causality and are therefore not considered to be valid particles.
  9. Dec 9, 2011 #8
    I read in a physics book that negative matter is hypothesized to exist, but there is no evidence towards its existence. The book said there would be many strange things, such as force. Since F=ma, a -m would mean a negative would mean a -a, meaning a force acting on a body would cause it to accelerate in the opposite direction as the applied force. Since energy is K=1/2mv^2, I would assume a negative mass would imply negative energy. Again, there's no evidence that it exists.
  10. Dec 9, 2011 #9
    Ah, you guys mustn't be up-to-date on your Robert Forward reading! GR doesn't care if mass is positive or negative to be mathematically consistent. Negative mass does, however, violate energy conditions as I think Acut pointed out. But these principles aren't necessary for GR. So it has driven some scientists to actually work out just how negative mass would behave.

    It was Forward who pointed out some of the most bizarre characteristics of negative mass. Like Freespader said, if you pushed on negative mass, it would push back! It would also be gravitationally attracted to positive mass...but positive mass would be repelled from negative mass! So if you took a ball and negative mass and a ball of positive mass and attached them with a rod...they would whiz off toward the positive mass ball. This wouldn't even violate conservation of momentum, because the negative mass would have negative momentum. Ugh, makes the brain hurt!

    Forward proposed (not seriously, but just as a consequence of the above logic) a spaceship drive using such a system. It could achieve arbitrarily large accelerations. BUT, it still wouldn't move faster than the speed of light. The whole point of Forward's analysis was to stay consistent with GR since it is GR that "allows" negative mass in the first place.

    Negative mass, or at least negative energy (which would be equivalent) would be needed for a traversable wormhole. To hold open the throat of the wormhole you would need "stuff" surrounding it that would counteract the incredible forces trying to close it back up. Since the throat would want to collapse and thus pull on a 'ring' of negative mass, the negative mass would respond by trying to open the throat further. If you could tune the thing, you would have a stable, traversable wormhole.

    Wacky. And most likely entirely hypothetical. But fun to think about!
  11. Dec 10, 2011 #10
    Wouldn't negative mass be repelled by gravity and from regular matter? If gravity is a force pulling down, then the negative mass would accelerate in the opposite direction - up.
  12. Dec 12, 2011 #11
    It's like a double negative...my words weren't very clear in the above (I shouldn't have said the negative mass was attracted, rather that it moved toward the positive mass).

    Let's say you have a positive mass that produces a gravitational field. Then you drop in a negative test mass. The negative mass feels a force pushing AWAY from the positive mass...BUT, the negative mass always responds by moving TOWARD the force acting on it. So while it is officially repelled, it moved towards a positive mass.

    Then if you switch perspectives and say you have a negative field producing mass and drop in a positive test mass. The test mass is repelled and responds like regular matter, moving away from the negative mass.

    Tie the two together and viola, propellant-less propulsion! :)
  13. Dec 12, 2011 #12
    OK, I've got you now. I don't know if physicists know or not, but what would happen if they collided? Would the negative mass and positive mass destroy each other, like antimatter, but with no energy (E=mc^2+-mc^2)? I was thinking, if you have negative mass collide with regular mass, the normal force would push it out (causing it to move in more), which would spiral out of control. However, would the negative matter pushing on the matter create a negative force, in which case there would be a positive force counteracting it, and thus pushing it out, like a normal normal force?
  14. Dec 13, 2011 #13


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    Most of these effects of particles with at least "effective" negative mass have been studied in semiconductors. Electrons at the upper edge of the valence band behave typically as if they had negative mass. E.g. you can observe how they decelerate in an electric field which would otherwise accelerate electrons in states with positive mass.
    And no, particles with positive and negative mass won't anihilate like matter and anti-matter but simply scatter from each other.
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