# Gravity-Why not repulsive?

I was thinking about gravity when I thought that gravity only attracts.Why doesn't gravity repel?Do you think there is some unidentified example where gravity repels?

mathman
"Why" is a question that is almost impossible to answer about any fundamental physicial phenomenum. Similar questions, c=300,000 km/sec (approx), like charges repel - unlike charges attract, fine structure constant=1/137 (approx), etc.

I was thinking about gravity when I thought that gravity only attracts.Why doesn't gravity repel?Do you think there is some unidentified example where gravity repels?

Gravity can repel if the pressure is negative

Is the OP asking "why, unlike other forces, is the force of gravity universally attractive?"

If so, isn't this wrong in itself, since three of the four fundamental forces are universally attractive, and only one either repulsive or attractive depending likeness or lack of it respectively (since electromagnetism is one phenomenon)?

And then, is this a relativity question at all?

Finally, the last poster doesn't seem to understand what naturally causes pressure. I assume he is talking about the phenomenon where a hot air balloon rises, which is due to a pressure effect. There is no negative pressure in the balloon (in fact the pressure is equal) nor is the phenomenon observed "gravitational repulsion" (it is merely the tendancy for heavier things to sink, displacing the lighter balloon).

cheers,

neopolitan

Is the OP asking "why, unlike other forces, is the force of gravity universally attractive?"

If so, isn't this wrong in itself, since three of the four fundamental forces are universally attractive, and only one either repulsive or attractive depending likeness or lack of it respectively (since electromagnetism is one phenomenon)?

And then, is this a relativity question at all?

Finally, the last poster doesn't seem to understand what naturally causes pressure. I assume he is talking about the phenomenon where a hot air balloon rises, which is due to a pressure effect. There is no negative pressure in the balloon (in fact the pressure is equal) nor is the phenomenon observed "gravitational repulsion" (it is merely the tendancy for heavier things to sink, displacing the lighter balloon).

cheers,

neopolitan

You seem to misunderstand what negative pressure is. There is nothing unusual about "negative pressure". In physics this is called tension. Ordinary positive pressure pushes outwards. Negative pressure simply pulls inwards.
By including pressure in the source for gravity, Einstein opened the possibility that a system with very large negative pressure could have zero or even negative gravitational mass that would repel instead of attract

Last edited:
One answer, though probably not the one you're looking for, is because the graviton is a spin-2 particle. The QFT book by Zee has an interesting discussion about this.

Gravity can repel if the pressure is negative

Which pressure?

Bose's name is crossed out, that means he is no longer signed up to the forums. The "negative pressure" will remain a mystery.

In General Relativity the curvature of spacetime around a massive object determines the trajectory of a particle so that it follows a geodesic path largely determined by the particles velocity and independly of the mass of the particle (unless the particle's mass is so great compared to that of the massive body that it significantly distorts the spacetime curvature itself. For example a normal particle, a photon (with zero rest mass) and an anti particle (with negative mass??) all fall towards the massive body. I would be interested to know if General Relativity predicts that two anti-particles attract or repel each other gravitationally. If anti particles repel each other this would imply that anti particles can never clump together and form a massive anti-body or anti black hole, so maybe that is not correct because I have read on a number of occasions that black holes and even stars could be made entirely of antimatter. Is that true?

cristo
Staff Emeritus
Anti particles do not have negative mass!!!

Anti particles do not have negative mass!!!

I was not sure. That was why I put two question marks ;) Thanks for the answer :)

Astronuc
Staff Emeritus
Gravity can repel if the pressure is negative
Nonsense!

Gravity can repel if the pressure is negative.

Nonsense!

Is it not true that tension in the stress energy tensor counts as negative energy and effectively reduces the inertial mass of the system?

If that is true then application of the equivalence priciple suggests that if tension reduces the inertial mass of a system then it also reduces the gravitational mass of the system. No?

malawi_glenn
Homework Helper
Nonsense!

http://supernova.lbl.gov/~evlinder/turner.pdf

Anyway, I have no clue what he meant, just a paper I found. I never heard of repulsive Gravity in my Cosmolgy courses..

EDIT: Now I saw that Bose referred to a book from Cambridge Uni Press.

http://supernova.lbl.gov/~evlinder/turner.pdf

Anyway, I have no clue what he meant, just a paper I found. I never heard of repulsive Gravity in my Cosmolgy courses..

I assume you have heard of dark energy, the accelerating expansion of the universe. Does General Relativity have explanation for these observations? Is not the possibility of a non-zero cosmological constant in effect introducing the concept of a form of negative gravity?

Garth
Gold Member
Bose said:
Gravity can repel if the pressure is negative
Nonsense!
Actually it is not nonsense, this is precisely what Dark Energy is all about, i.e. DE is a form of energy with negative pressure that is able to make the universe accelerate in its expansion due to the action of a repulsive gravitational force.

However, you do have to have a great deal of pressure for its gravity to become repulsive., the Dark Energy requires an equation of state of: $p \leqslant -\frac{1}{3} \rho c^2$.

The most popular suggestion for DE has an equation of state $p = - \rho c^2$.

Garth

Last edited:
Astronuc
Staff Emeritus
Since the poster did not mention DE nor the 2nd Friedman equation, I took the comment in context of matter as we observe it here on earth, and was reflecting on another thread that was implying anti-gravity was somehow related to anti-matter.

Turner's paper makes the point - ". . . dark energy has the following defining properties: (1) it emits no light; (2) it has large, negative pressure, pX ~ $\rho$X; and (3) it is approximately homogeneous (more precisely, does not cluster significantly with matter on scales at least as large as clusters of galaxies)."

But the thing is that we don't really understand dark energy yet. That's just one of a few interpretations.

I personally think that instead of DE being an exotic form of negative pressure energy, instead it simply represents the failure of GR. The thing is that GR is a local, i.e. geometric theory, but on cosmological scales you need a global, i.e. topological theory. It's certainly possible that there might be a less exotic interpretation of DE using a topological description of relativity, and there are certainly people out there exploring that angle.

But the point is that saying that DE represents negative pressure is arguably in the range of debatable speculation.

turbo
Gold Member
In General Relativity the curvature of spacetime around a massive object determines the trajectory of a particle so that it follows a geodesic path largely determined by the particles velocity and independly of the mass of the particle (unless the particle's mass is so great compared to that of the massive body that it significantly distorts the spacetime curvature itself. For example a normal particle, a photon (with zero rest mass) and an anti particle (with negative mass??) all fall towards the massive body. I would be interested to know if General Relativity predicts that two anti-particles attract or repel each other gravitationally. If anti particles repel each other this would imply that anti particles can never clump together and form a massive anti-body or anti black hole, so maybe that is not correct because I have read on a number of occasions that black holes and even stars could be made entirely of antimatter. Is that true?
Google on "Cern" "athena" "antihydrogen" for some pretty exciting stuff. They intend to make cold neutral antihydrogen and experiment with it. One of the tests is to see if the gravitational infall rate of antihydrogen is identical to that of hydrogen (Testing the weak equivalence principle). Also, CPT invariance requires that the masses of hydrogen and antihydrogen be identical and that they should be spectrally identical - more interesting tests to be made.

http://alpha.web.cern.ch/alpha/hbar.html [Broken]

Last edited by a moderator:
Since the poster did not mention DE nor the 2nd Friedman equation, I took the comment in context of matter as we observe it here on earth, and was reflecting on another thread that was implying anti-gravity was somehow related to anti-matter.

Turner's paper makes the point - ". . . dark energy has the following defining properties: (1) it emits no light; (2) it has large, negative pressure, pX ~ $\rho$X; and (3) it is approximately homogeneous (more precisely, does not cluster significantly with matter on scales at least as large as clusters of galaxies)."

So you agree that the statement by Bose that "Gravity can repel if the pressure is negative" is not nonsense after all (in the cosmological scale)?

It should be noted that the OP asked "Do you think there is some unidentified example where gravity repels?" Giving an unqualified "No" answer, when you are actually privately restricting the conditions to those found on Earth on a small scale is obscuring the truth somewhat.

Last edited:
So you agree that the statement by Bose that "Gravity can repel if the pressure is negative" is not nonsense after all (in the cosmological scale)?

It should be noted that the OP asked "Do you think there is some unidentified example where gravity repels?" Giving an unqualified "No" answer, when you are actually privately restricting the conditions to those found on Earth on a small scale is obscuring the truth somewhat.
I agree that the truth is obscurbed by simply saying "No" or "Nonsense" especially since there is at least several examples of "antigravity" postulated to exist (or did exist) in nature. One example is found in the Inflationary Universe Model where the universe expanded at an extremely accelerated rate. This is an example of "antigravity." I recall that this could have resulted by either a postive cosmological expansion or tension (negative pressure) by some bizzare field (scalar field?). Then there is the example of a vacuum domain wall which is an wall made of matter which has tension in it and is created by some sort of bizzare field as well. Then there is a cosmic string which has a positive engergy density and tension. The result is no gravitational attractipon at all since these effects (due to energy vs due to tension) cancel out.

Pete

Astronuc
Staff Emeritus
So you agree that the statement by Bose that "Gravity can repel if the pressure is negative" is not nonsense after all (in the cosmological scale)?

It should be noted that the OP asked "Do you think there is some unidentified example where gravity repels?" Giving an unqualified "No" answer, when you are actually privately restricting the conditions to those found on Earth on a small scale is obscuring the truth somewhat.
Giving an unqualifed example is incorrect. I can give plenty examples of repulsion/attraction with respect to electrical fields, magnetic fields, mechanical force which involve tension (positive stress) or compression (negative stress) which do not invoke gravity or anti-gravity. So in general to say strictly "gravity repels if pressure is negative", one would be incorrect.

malawi_glenn
Homework Helper
Guys, drop it :-)

cristo
Staff Emeritus