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Negative mass

  1. Jun 10, 2009 #1
    In many websites I read that antimatter has antigravity properties. Does negative mass exist. Does antimatter react differently with gravity.
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  3. Jun 10, 2009 #2


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    That's false, antimatter reacts to gravity the same way normal matter does.
  4. Jun 10, 2009 #3
    "antimatter" is kind of a bad name - it's still matter in most senses of the word. What makes it "anti" is that certain types of charges, like electric charge, are opposite for a particle and its antiparticle partner. The positron is just an electron with opposite electric charge - in all other ways they're identical. Some particles have no charge at all, so their antimatter partners are exactly the same as they are.
  5. Jun 10, 2009 #4


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    No on all counts. Antimatter does not have any "anti-gravity" properties, whatever that means. Negative mass does not exist and antimatter acts like regular matter in a gravitational field.
  6. Jun 10, 2009 #5
    Cern (in Switzerland) has been making neutral anti-hydrogen atoms, but there is no difinitive experiment on whether they have positive or negative gravity with respect to ordinary matter. Fermilab (near Chicago) has been producing antiprotons since 1985, and they behave just as expected; same mass as protons, opposite charge, and annihilates very fast unless they are contained in relativistic beams at very-very high vacuum.. Antiprotons have positive mass, the same as protons. They are very expensive to produce (guess 10 million antiprotons per $???). The neutral pion is its own antiparticle, and it has a very short half life, so all we can study are its decay products.
  7. Jun 10, 2009 #6


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    This statement doesn't even make sense. For one it's contradictory. Anti-hydrogen is an anti-proton. Why do you believe that anti-matter doesn't behave like normal matter in a gravitational field? Mass is not something that changes between matter and anti-matter.
  8. Jun 10, 2009 #7
    An anti-proton has a charge of about -1.6 x 10-19 Coulombs. An anti-hydrogen atom has a positron attached (charge = +1.6 x 10-19 Coulombs), in an (anti-)Bohr orbit, so it has no net charge. I agree that mass does not change between matter and anti-matter, but would you be willing to say that charge does not change either? Electromagnetic forces are so large relative to gravitational forces that no gravity experiments are possible on charged anti-matter. As for how antimatter behaves in a gravitational field of normal stellar matter, we simply do not know. CERN would like to do that experiment.

    PS We do know that whenever anti-matter annihilates with matter, it produces an equal amount of matter and antimatter, so this in itself does not prove that antimatter has positive mass. But matter - anti-matter annihilation at rest produces 2 x the rest mass energy of one particle (e.g., electron or proton) so we can say that matter and anti-matter have the same "rest mass" (m0c2).
    Last edited: Jun 10, 2009
  9. Jun 10, 2009 #8
    I'd be inclined to say that even perturbation effects in charge distributions of a neutral atom would far dominate any gravitational effects. But then again I could be wrong since GR, etc, isn't really my bag.
  10. Jun 10, 2009 #9


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    It would not be hard in principle to test the response of an antielectron or antiproton in the Earth's gravitational field, for example... has that really never been done?
  11. Jun 10, 2009 #10
    I don't know if it would be. Which makes me question if it's actually been done.
  12. Jun 10, 2009 #11
    It would be very hard. At Fermilab, the anti-protons have a total mass E of about 9 times the rest mass m0c2, and its position is controlled by both electric and magnetic fields. Checking the gravitational force on charged anti-matter would have to be done in an extremely high vacuum, and without any electric fields. A charged particle can create an image charge (of opposite sign) on any grounded metal surface which will attract it to the surface. How do you maintain an extremely high vacuum without an enclosure, except in outer space? I have looked at singly-charged ion charges with microscopes (like the Michelson Oil Drop Experiment) but we would need anti-oil drops to do it.
  13. Jun 10, 2009 #12
    Years ago (late 80's, maybe early 90's) I recall reading about plans for an experiment in which someone was going to create a vacuum chamber many meters tall (I don't recall how many, but I remember picturing it as being something like 20 m) and inject antiprotons upward at the bottom so that they could shoot ballistically to the top and then fall back down again. The idea, of course, was to verify that they would take the same time as for protons, thus confirming that their gravitational interactions were the same.

    Anyone else remember hearing about this? Did it ever happen?
  14. Jun 10, 2009 #13
    As electric charge, the labels off pairs of particles as matter and antimatter, such as e+ and e-, are artificial. Physics is invariant under an arbitrary relabeling of any set of particle-antiparticle pairs.

    If all references in the world to an anti-S quark were replaced with an S quark, and all references to an S quark were replaced with an anti-S quark, statements in physics would remain the same--true statements about them would still true statements.
    All the equations of particle interaction would remain true as the original.
  15. Jun 10, 2009 #14
    Quote from belliott4488:
    Years ago (late 80's, maybe early 90's) I recall reading about plans for an experiment in which someone was going to create a vacuum chamber many meters tall (I don't recall how many, but I remember picturing it as being something like 20 m) and inject antiprotons upward at the bottom so that they could shoot ballistically to the top and then fall back down again. The idea, of course, was to verify that they would take the same time as for protons, thus confirming that their gravitational interactions were the same."

    I am not aware of such an experiment. Neutrons (a neutral particle) with several hundred micro-electron-volt energy will rise only several feet (more or less, I forget) before falling back down. Charged antiprotons would be much more difficult. For those who are interested in details of proposed anti-gravity (or anti-matter gravity) experiments, read the articles in the second document under "some useful documents" in this article:
    These are very large files.
    Last edited: Jun 10, 2009
  16. Jun 10, 2009 #15
    Well yes, in theory, but that's the whole point that we're talking about. It's always been taken as a given that antiparticles behave identically to particles, its just never, actually, been checked.

    P.S. In regards to belliot's comment, that seems like a very doable experiment which suggests to me that it may not be accurate enough given the cloud of doubt over the issue
  17. Jun 10, 2009 #16
    Let's calculate the velocity of a 100 micro-eV antiproton, and decide if it is low enough.

    So (1/2) Mv2 = 0.1 milli-eV
    Mv2 = 0.2 meV
    But Mv2 = Mc2(v/c)2 = 0.2 meV
    Or (v/c)2 = 0.2 meV/938 x 106 eV = 2.1 x 10-13
    Or v = 140 meters per sec (still much too fast).
    How do you cool anti-protons to 100 micro-eV, and keep them away from electric fields that might upset the experiment?
  18. Jun 10, 2009 #17
    My "Well yes, in theory" was in regards to Phrak's comment. Not yours Bob S. I guess we were just posting at the same time so your post got in ahead of mine.
  19. Jun 10, 2009 #18
    Yeah. In theory. If a particle and its antiparticle were found experimentally to fall at different rates, this would be a nice development. Anything to throw a shoe into the works.
  20. Jun 10, 2009 #19
    Hey, as a person who deals pretty much entirely with large non-relativistic quantum systems my intuition would say they're identical. I'm just saying it's probably true that no one's checked and no harm in that
  21. Jun 10, 2009 #20
    Certainly CPT (charge parity time) reversal invariance is the same. Based on Fermilab and CERN experiments, the (absolute)* masses are the same (based on magnetic field strengths to deflect antiprotons).
    recall E2 = (Mc2)2 + (pc)2

    Magnet systems measure momentum pc

    pc = +/- sqrt(E2 -(Mc2)2)
    Is pc > 0 or < 0 in this equation?

    Is the fact that Fermilab has to change the polarity of their magnets to bend and store antiprotons sufficient proof that pc > 0? Then is M > 0? So let's forget about negative masses and think about +/- gravity..
  22. Jun 10, 2009 #21
    I don't follow. But you're on to something. The equivalence principle says you don't change the polarity to hold protons with negative mass. You have to keep them from falling into the center of curvature by pushing them outward.
  23. Jun 10, 2009 #22
    Right now (I just checked) the Fermilab Tevatron is running at 980.2 GeV (980 x 109 eV), and the protons (about 7 x 1012) are going in one direction (around a 4-mile-circumference ring), and the antiprotons (about 1.6 x 1012) are going in the other. This proves that the anti-proton has either the same sign mass and opposite sign charge, or vice-versa. The fact that an anti-proton with a positron makes anti-hydrogen shows that they have the opposite sign charge to a positron and a proton. So they have the same sign mass.
  24. Jun 10, 2009 #23
    Same. But equal?
  25. Jun 11, 2009 #24
    If the same EM fields are acting on both the protons and antiprotons to keep them going in circles, wouldn't that imply that their masses must be exactly the same? I can imagine that even the slightest difference in mass would make either the protons or antiprotons fly out of the centre of the tube in a matter of nanoseconds in that speed...
  26. Jun 11, 2009 #25

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    Which websites? This is not a true statement, so I would question the reliability of those sites.

    I strongly disagree. If anti-matter gravitated differently, photons must also (at least if you want energy to be conserved) as they are self-conjugate. But Pound and Rebka showed that photons gravitate exactly like ordinary matter.

    Additionally, if antimatter gravitated differently, the sea antiquarks in nuclei would gravitate differently and you'd see a composition dependent force of gravity. Eotvos and others have failed to find such a thing to exquisite precision.
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