Anti-Gravity from Matter-AntiMatter Repulsion

  • Thread starter sanman
  • Start date
745
18
There's a theory being proposed that the reason for the expansion of the universe is because of the mutual repulsion of matter and anti-matter, in what is essentially anti-gravity:

http://www.universetoday.com/84934/antigravity-could-replace-dark-energy-as-cause-of-universes-expansion/

So just as a thought experiment - what if this was true? What if matter and anti-matter repel each other in what amounts to an anti-gravitational force?

Would there be any way to usefully harness this for propulsion purposes?

Could you make some kind of gravitational catapult from matter-antimatter repulsion?
Unlike EM force, whose influence falls off very quickly beyond short distances, your gravitational catapult would sustain its accelerative effect on you over a vast span of distance.

If your rocket is made of matter, and somehow has an equal mass of anti-matter attached to it, then is it effectively "gravitationally neutral"? (ie. like "buoyancy neutral")
Could you hover near the event-horizon of a black hole without too much effort, like in the Disney movie?

So just as conventional objects made of matter will fall downwards towards Earth, then would objects made of anti-matter "fall upwards"? (ie. away from our matter-based planet)

And just as another hypothetical speculation, what if this means that there are large islands of anti-matter waiting for us outside our galaxy? I know it sounds pie-in-the-sky to talk about traveling outside our galaxy, but how could we safely do so if we're in danger of running into some anti-matter and getting anihilated by it?

Also, if there are entire galaxies and solar systems out there composed of anti-matter, then could these potentially generate "anti-life" (ie. life composed of anti-matter)???
How do you communicate with life made of anti-matter?

These days we have BECs and atom lasers, which are supposed to be able to measure gravitational effects with great sensitivity while self-shielding from charge effects. I'd read that recently for the first time, a BEC had been briefly created from anti-hydrogen.

Since hydrogen and anti-hydrogen are each overall charge-neutral, that could help in focusing measurement on the gravitational behavior of anti-matter wrt to matter.
(This sounds like a job for some topnotch national lab to attempt such an experiment, using antimatter, BECs and high vacuum.)

If BECs and atom lasers already allow us to manipulate apparent mass properties using wave mechanics, and if we can apply these principles to matter-antimatter repulsive interactions, then couldn't similarly manipulate gravity/spacetime?
 
308
0
Matter and anti-matter still attract each other gravitationally, so that's no to most of your questions.
 

JesseM

Science Advisor
8,492
12
Matter and anti-matter still attract each other gravitationally, so that's no to most of your questions.
The article suggests it might be more complicated than this if we apply the CPT symmetry of quantum field theory to gravity, I'd like to see what other physicists think of his argument though:
Massimo Villata, a scientist from the Observatory of Turin in Italy, began the study with two major assumptions. First, he posited that both matter and antimatter have positive mass and energy density. Traditionally, the gravitational influence of a particle is determined solely by its mass. A positive mass value indicates that the particle will attract other particles gravitationally. Under Villata’s assumption, this applies to antiparticles as well. So under the influence of gravity, particles attract other particles and antiparticles attract other antiparticles. But what kind of force occurs between particles and antiparticles?

To resolve this question, Villata needed to institute the second assumption – that general relativity is CPT invariant. This means that the laws governing an ordinary matter particle in an ordinary field in spacetime can be applied equally well to scenarios in which charge (electric charge and internal quantum numbers), parity (spatial coordinates) and time are reversed, as they are for antimatter. When you reverse the equations of general relativity in charge, parity and time for either the particle or the field the particle is traveling in, the result is a change of sign in the gravity term, making it negative instead of positive and implying so-called antigravity between the two.
 

Jonathan Scott

Gold Member
2,255
959
Experiments related to the principle of equivalence already show that all known forms of energy have the same sign for gravitational purposes.

Also, particles such as the neutral pion and the photon are their own antiparticles. How would they behave?
 
745
18
Experiments related to the principle of equivalence already show that all known forms of energy have the same sign for gravitational purposes.

Also, particles such as the neutral pion and the photon are their own antiparticles. How would they behave?
Maybe they would be gravitationally neutral to one another (ie. no gravitational attraction or repulsion)
 
188
0
sanman said:
... I'd read that recently for the first time, a BEC had been briefly created from anti-hydrogen
Is this really true? I thought the temperature of the anti-hydrogen was too high for the BEC transition to take place. But sure, if a BEC of photons can be made, anti-hydrogen doesn't seem impossible, but if you have a reference I would be very interested.

From the http://cerncourier.com/cws/article/cern/30577" [Broken]

"... Back down to the sober reality here at CERN, we would be happy just to demonstrate trapping of antihydrogen in principle. This means initially trapping just a few anti-atoms – not making a BEC or antihydrogen ice. ..."
 
Last edited by a moderator:
24,392
6,029
Maybe they would be gravitationally neutral to one another (ie. no gravitational attraction or repulsion)
That can't be right, because photons are also their own antiparticles, and we know that photons "fall" in the Sun's gravitational field; they are attracted to the Sun.

On consideration, that observation alone may be enough to scuttle the whole idea of antiparticles gravitationally repelling particles.
 
24,392
6,029
The article suggests it might be more complicated than this if we apply the CPT symmetry of quantum field theory to gravity, I'd like to see what other physicists think of his argument though:
I thought it was already known that gravity being a spin-two field quantum mechanically was enough to show that it has to be always attractive? I seem to remember reading something like that in discussions of the research done on quantizing GR in the 1960's (among other places, I remember seeing something about it in the Feynman Lectures on Gravitation--unfortunately I don't have my copy handy to check).
 
I thought it was already known that gravity being a spin-two field quantum mechanically was enough to show that it has to be always attractive? I seem to remember reading something like that in discussions of the research done on quantizing GR in the 1960's (among other places, I remember seeing something about it in the Feynman Lectures on Gravitation--unfortunately I don't have my copy handy to check).
I think this is somewhat reversed. The spin 2 classification for the graviton in low energy effective field theories of gravity is a result of assuming gravity is attractive only, not the other way around.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
22,603
4,881
Maybe they would be gravitationally neutral to one another (ie. no gravitational attraction or repulsion)
Impossible. Light has the same gravitational attraction as matter to better than 2%.
 
308
0
When you reverse the equations of general relativity in charge, parity and time for either the particle or the field the particle is traveling in, the result is a change of sign in the gravity term, making it negative instead of positive and implying so-called antigravity between the two.
I don't get this. If GR is CPT invariant, why does that term change sign?
 
244
1
Heh, fancy seeing you here, Sanman, was just coming over to see what others thought.


The first thing that came to mind for me is the messed up way a gravity well would interact with a region with opposite curvature. My thoughts jumped immediately to the old Penrose diagrams of rotating black holes connecting universes and anti-universes.

penrose_kerr.gif



Additionally, isn't P-symmetry just a reversal of one coordinate, (x, y, z) -> (-x, y, z), rather than (x, y, z) -> (-x, -y, -z)?
 
24,392
6,029
I think this is somewhat reversed. The spin 2 classification for the graviton in low energy effective field theories of gravity is a result of assuming gravity is attractive only, not the other way around.
Hmm...I thought spin-2 for the graviton was a consequence of the fact that gravity requires a tensor theory; neither a scalar (spin-0) nor a vector (spin-1) theory can account for the low-energy phenomena. Again, I don't have references handy so I may be misremembering.
 
745
18
That can't be right, because photons are also their own antiparticles, and we know that photons "fall" in the Sun's gravitational field; they are attracted to the Sun.

On consideration, that observation alone may be enough to scuttle the whole idea of antiparticles gravitationally repelling particles.
What I meant was that photons would be gravitationally neutral wrt each other, and not to matter. This was in reply to the comment that photons are their own anti-particles.

Impossible. Light has the same gravitational attraction as matter to better than 2%.
Are you saying that photons are gravitationally attracted to each other?
Okay, then maybe photons are attracted equally to matter and to antimatter. But that doesn't prove that matter and antimatter are attracted to each other.

It seems to me that the best way to answer this question is thru experimental measurement.
Unfortunately, charge interaction dominates between matter and antimatter at short distances. Perhaps the use of BECs and atom lasers at significant distances could then definitively show whether anti-particles attract or repel each other.
 
244
1
Anti-hydrogen is neutral.
 
745
18
Yes, it's charge-neutral, which is what would diminish any charge interaction between it and hydrogen across a distance. This would allow any gravitational interaction to stand out more.
 
Hmm...I thought spin-2 for the graviton was a consequence of the fact that gravity requires a tensor theory; neither a scalar (spin-0) nor a vector (spin-1) theory can account for the low-energy phenomena. Again, I don't have references handy so I may be misremembering.
Hmm, good point. I guess it depends on where you start from.

Cheers
 
24,392
6,029
What I meant was that photons would be gravitationally neutral wrt each other, and not to matter. This was in reply to the comment that photons are their own anti-particles.
But how does the gravity know that the interaction is between two photons instead of a photon and matter? Gravity couples to energy, and photon energy "looks just like" matter energy. (In fact, since atoms are held together by electromagnetic forces, a significant portion of the energy in matter is really energy of virtual photons anyway.)
 
745
18
But how does the gravity know that the interaction is between two photons instead of a photon and matter? Gravity couples to energy, and photon energy "looks just like" matter energy. (In fact, since atoms are held together by electromagnetic forces, a significant portion of the energy in matter is really energy of virtual photons anyway.)
Okay, how does a positive charge know it's interacting with a negative charge at a distance?

If charge can know, why can't gravity know? Whatever is at the root of why antimatter has opposite charge sign to normal matter, may help gravity to distinguish what it is coupling with.

It's still worth measuring how anti-matter behaves in a gravitational field. Our own Earth produces plenty of gravity, which should help us find out which way anti-matter will fall.
 
24,392
6,029
Okay, how does a positive charge know it's interacting with a negative charge at a distance?

If charge can know, why can't gravity know?
Because there are two kinds of charges (opposite signs), but only one kind of energy.

It's still worth measuring how anti-matter behaves in a gravitational field. Our own Earth produces plenty of gravity, which should help us find out which way anti-matter will fall.
I agree completely that this is worth measuring. Unfortunately, the Earth's gravity is really weak, comparatively speaking, even though the Earth is a large object, so it's not as easy to measure as it might seem.
 
308
0
I'd still like to see where exactly in GR charge makes an appearance. Last I heard gravity didn't couple to the internal symmetry groups of the particles.

That CPT thing is non-sense. If you can show me where that "gravity term" is proportional to charge, fine.

In fact you don't even need to go to GR. Show me where charge appears in Newton's Law? Or is that wrong too now because one guy said so on a random website?
 
Last edited:
244
1
Okay, how does a positive charge know it's interacting with a negative charge at a distance?
If you want to have an easier to imagine picture, try this: the positive charge emits virtual photons and the negative charge accepts virtual photons.

[PLAIN]http://www.kirksville.k12.mo.us/khs/Teacher_Web/alternative/electricfield.gif [Broken]


Masses don't exchange virtual gravitons, they interact with the geometry of their local spacetime, and other masses interact with each other through their influence on local geometry.


Figure out how to draw one of these for gravity, in particular one for gravity with charges:
1197-3.JPG
 
Last edited by a moderator:
24,392
6,029
If you want to have an easier to imagine picture, try this: the positive charge emits virtual photons and the negative charge accepts virtual photons.
I think it's a little more complicated than that. The Usenet Physics FAQ has a good page on virtual particles and how they mediate forces:

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Also, since photons are their own antiparticles, there is no real difference between "emitting" and "accepting" one. With real photons, ones we actually observe, we can finesse this by decreeing that the earlier event is the "emission" and the later one is the "reception" of the photon, and since real photons go at the speed of light, which event is first is the same in all reference frames. But virtual photons don't have to move at the speed of light--they can go faster, or slower, and if they go faster, there is no frame-invariant way to say which event is first. That's why it's normally just said that virtual particles are "exchanged".

Masses don't exchange virtual gravitons, they interact with the geometry of their local spacetime, and other masses interact with each other through their influence on local geometry.
But from a quantum standpoint, the local spacetime geometry has to be quantized too, and when you try to do that, at least in the "obvious" way, you do get masses interacting by exchanging virtual gravitons. (I realize that is not a complete picture and there are a lot of issues in this area, which is why we still don't have a good quantum theory of gravity. But even in the current candidate quantum gravity theories, such as string theory, you still have virtual gravitons being exchanged--in string theory, they are among the simplest string states.) The difference is that, while there are positive and negative electrical charges, there is only one kind of "gravitational charge", since that is just energy, and the energy of gravitating objects is always positive--more precisely, the stress-energy tensor always satisfies what is called the "weak energy condition". (I think that's the right one--but experts, please correct me if I've misstated it.)

There is one exception to that generalization: "dark energy", which is energy associated with a cosmological constant, or, equivalently, associated with what we normally think of as "empty space". The stress-energy tensor of dark energy violates the weak energy condition, which is why dark energy can cause the universe's expansion to accelerate (a kind of "gravitational repulsion"). But dark energy has nothing to do with antimatter, and the special properties of its stress-energy tensor can't be possessed by the stress-energy tensor of any "normal" kind of substance, whether it's matter or antimatter (or light or anything else).
 
I'd still like to see where exactly in GR charge makes an appearance. Last I heard gravity didn't couple to the internal symmetry groups of the particles.

That CPT thing is non-sense. If you can show me where that "gravity term" is proportional to charge, fine.

In fact you don't even need to go to GR. Show me where charge appears in Newton's Law? Or is that wrong too now because one guy said so on a random website?
I think in theories of gravity we think of "charge". In the traditional theories (newton) where gravity is a force we think of the "charge" as being mass in analogy to EM. This paper is equating this "charge" not to mass but to part of the energy/momentum 4 vector.
 

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top