Antimatter, Negative Mass, & Gravity: Unveiling the Mysteries

[joychandra]

In many websites I read that antimatter has antigravity properties. Does negative mass exist. Does antimatter react differently with gravity.

 

[diazona]

That's false, antimatter reacts to gravity the same way normal matter does.

 

[belliott4488]

"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.

 

[Pengwuino]

In many websites I read that antimatter has antigravity properties. Does negative mass exist. Does antimatter react differently with gravity.

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.

 

[Bob S]

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.

 

[Pengwuino]

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.

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.

 

[Bob S]

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.

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).

 

[maverick_starstrider]

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.

 

[diazona]

Electromagnetic forces are so large relative to gravitational forces that no gravity experiments are possible on charged anti-matter.

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?

 

[maverick_starstrider]

I don't know if it would be. Which makes me question if it's actually been done.

 

[Bob S]

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?

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.

 

[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.

Anyone else remember hearing about this? Did it ever happen?

 

[Phrak]

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.

 

[Bob S]

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:
http://capp.iit.edu/hep/pbar/
These are very large files.

 

[maverick_starstrider]

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

 

[Bob S]

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

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?

 

[maverick_starstrider]

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.

 

[Phrak]

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.

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.

 

[maverick_starstrider]

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

 

[Bob S]

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

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..

 

[Phrak]

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..

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.

 

[Bob S]

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.

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.

 

[Phrak]

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.

Same. But equal?

 

[Nick89]

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...

 

[Vanadium 50]

In many websites I read that antimatter has antigravity properties.

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

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.

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.

 

[belliott4488]

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.

That's a good point. If one wanted to postulate that particles and their antimatter partners somehow had different gravitational properties, then the particles that are self-conjugate in the Standard model would suddenly have to have different particle/antiparticle states, one with each of the allegedly different gravitational properties. It's not clear to me how that could make any sense, e.g. an "antiphoton" would have different gravitational interactions than a photon (??).

 

[Bob S]

From Bob S:
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.

Same. But equal?

Based on the fact that the proton and anti-proton have equal but opposite Q/pc = Q/βγMc² (same orbit in Tevatron), and the fact that they have the same β and γ (exact same revolution frequency in Tevatron), their masses are the same.
α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω

 

[LURCH]

In many websites I read that antimatter has antigravity properties. Does negative mass exist. Does antimatter react differently with gravity.

Antimatter is predicted to have the same response to gravity as matter. However, as has been stated, testing this would require making a large enough mass of antimater to measure gravitational effects.

Negative mass is a different matter. Negative energy would have negative mass, and is predicted to be gravitationaly repulsive. I have a link to a site, but I rather suspect the site is a voilation of copyright. There was an article in the January 2000 issue of SciAm.

 

[Bob S]

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..

The Mossbauer Effect experiment at Harvard is a beautiful experiment, and it showed that photons from iron-57 gravitate. It certainly implies that antimatter also would gravitate. But photons do not have quarks, or "mass" meaning E² - (pc)² =0.

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.

.
protons are uud, neutrons are udd, I believe that anti-protons are u-bar,u-bar,d-bar, anti-neutrons are u-bar,d-bar,d-bar.

 

[ExactlySolved]

This thread has made me think about the gravitational effect of antimatter from the point of view of the dirac sea, and to my surprise this line of thinking would predict that the 'holes' will repel matter gravitationally (because the holes represent an abscence of mass, which means less gravitational attraction in the direction of the hole). Does anyone know how to resolve this non-standard prediction within the dirac sea picture, or do we just say that the gravitational effect of antimatter is beyond the limits of the dirac sea picture?

 

[George Jones]

Isn't it a hole in a sea of negative-energy states, so the hole represents absence of negative-energy?

 

[ExactlySolved]

The sea of negative energy states is normally packed full of electrons, and this vacuum state has no net gravitational attraction, since the sea of electrons is homogeneous and isotropic. Now if we remove one of these electrons and send it some where far away, then all that is left is a hole where that electron used to be. Because of the hole the sea is no longer isotropic or homogeneous, and so if you are an ordinary matter particle looking at the hole then you have an infinite line of mass behind you, and an infinite - 1 line of electrons in front of you, therefore you will be pulled backwards, away from the hole.

Mathematically, I had to think carefully because normally a single point does not contribute to an integral, but in this case the mass of the hole, if we imagine it as being an empty site on a lattice with all other sites occupied, is less than the mass on neighboring sites by a factor of infinity, and so a delta function is appropriate which renders allows the integral to be effected in a finite way by the subtraction of a single point.

 

[Vanadium 50]

protons are uud, neutrons are udd, I believe that anti-protons are u-bar,u-bar,d-bar, anti-neutrons are u-bar,d-bar,d-bar.

That's a very superficial view. Those are the valence quarks. The proton is also composed of gluons, and sea quark-antiquark pairs. In fact, the parts other than the valence quarks carry the majority of the momentum of a proton.

I maintain my previous position - a different pull on matter and antimatter would yield a composition dependent force. Which is not observed.

 

[George Jones]

I know almost nothing about this, so I'll probably find myself in the sea over my head, but here goes ... :uhh:

The sea of negative energy states is normally packed full of electrons,

So isn't it a sea of negative energy electrons?

and this vacuum state has no net gravitational attraction, since the sea of electrons is homogeneous and isotropic.

This is true if the sea were all the electrons were positive energy, or if the see were all negative energy electrons.

Now if we remove one of these electrons and send it some where far away, then all that is left is a hole where that electron used to be.

Where a negative energy electron used to be? Thus, if a normal positive energy electron drops into the hole, it falls to a lower energy (in this case, negative) and emits radiation; pait annihilation.

Because of the hole the sea is no longer isotropic or homogeneous, and so if you are an ordinary matter particle looking at the hole then you have an infinite line of mass behind you, and an infinite - 1 line of electrons in front of you, therefore you will be pulled backwards, away from the hole.

Shouldn't "infinite line of mass behind you" be "infinite line of negative energy behind you", etc., so a normal electron is pulled (gravitationally) towards the hole?

 

[ExactlySolved]

It is correct that the electrons in the dirac sea have negative energy, so this together with what GR says about the gravitational effect of negative energy would resolve the contradiction, thanks George.

It looks like what I said would be true for a large homogeneous mass of ordinary matter with a hole in it, the hole could be treated as a gravitational repeller, my mistake was extending this picture too far.