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If there is anti matter. Is there an anti gravity force if there is gravity?

by acd 8y ago

strstr 8y ago
No, but charges are inverted for particles vs antiparticles.
majewsky 8y ago
If you're thinking of "negative gravity", then particles exhibiting it fall under the "exotic matter" umbrella. We don't know what exotic matter actually is, although its existence is predicted by the current models on the expansion of the universe. Antimatter still has regular (i.e. positive) mass, so gravity works in the usual (attracting) way.
- zamalek 8y ago
  The specific type of exotic matter would be imaginary matter, that is matter with imaginary mass (a complex number) - tachyons. Negative matter, which is also exotic but has a negative real mass, behaves normally under gravity so long as the two bodies are sufficiently different in absolute mass.
GolDDranks 8y ago
Basically, antimatter has it's all "quantum numbers" inverted of it's normal counterpart, but it's energy content is not inverted. Strength of gravity is a measure of the energy content, so antimatter should have normal gravity. If it doesn't that's some exciting news!
philipov 8y ago
Antimatter is a consequence of quantum field theory, but all signs indicate gravity can not be successfully described as a quantum field. Gravity has properties such as having its interaction strength grow with the amount of charge (energy), which is very different from the forces described by quantum fields that have constant interaction strength. To get antigravity, you need to find a way to have negative energy without it making the vacuum unstable. It's most typical to describe gravity as dynamical spacetime, not a field existing in that spacetime. The closest people have come to a quantum theory of gravity has been to show that in certain toy universes gravity in a bulk space is the dual of strongly-coupled field interactions existing on the boundary of that space-- that is holography. Information-theoretic approaches have have also suggested that gravity emerges as the 'hydrodynamics of entanglement.' All this suggests gravity is not a fundamental force, but is far from a coherent real-world theory at this point. The closest to an "antigravity" in general relativity is the cosmological constant, thought to explain dark energy. Its tiny (10^-122) positive value behaves like a constant negative energy attributed to the vacuum itself, driving the exponential expansion of space.
- xhrpost 8y ago
  This is one of the aspects of theoretical physics that I find fascinating. Is there a good book on this that doesn't involve an insane amount of math? (ex. similar to BHoT)
  - semi-extrinsic 8y ago
    Zee's "Quantum Field Theory in a Nutshell" is fairly light on the math IIRC. That doesn't mean it's an easy read, though.
  - w1 8y ago
    The Order of Time spends some time qualitatively discussing quantum gravity, and the audiobook narration by Benedict Cumberbatch is really excellent.
- pdonis 8y ago
  > all signs indicate gravity can not be successfully described as a quantum field That's not quite correct. The quantum field theory of a massless spin-2 field was developed in the 1960s and 1970s and is perfectly well-defined. The problem with it then was that it is not renormalizable; but today basically all QFTs are viewed as effective field theories anyway, which means not being renormalizable is not a showstopper. The real issue is that we have no prospect of measuring any quantum aspects of gravity now or in the foreseeable future, so there is no way to test the QFT that has been developed against experiment. > Gravity has properties such as having its interaction strength grow with the amount of charge (energy), which is very different from the forces described by quantum fields that have constant interaction strength. This is not correct either. The coupling constant of the massless spin-2 field is just Newton's gravitational constant (modulo some constant factors that depend on your units). The fact that the total interaction in a specific case varies with energy is no different from the fact that the total EM force between two charged objects varies with the charge. Energy is the source for gravity just as charge is the source for EM; the source is not the same as the coupling constant. > To get antigravity, you need to find a way to have negative energy without it making the vacuum unstable. Agreed. And as far as we know that is not possible. > All this suggests gravity is not a fundamental force This is quite possible, yes, but even if it were true it would not necessarily make the spin-2 massless QFT I referred to above invalid; it would just make it an effective theory that emerges from whatever is the next level down. (Which, as I noted above, is basically how we view all QFTs today anyway, including the Standard Model of particle physics.) > The closest to an "antigravity" in general relativity is the cosmological constant, thought to explain dark energy. Its tiny (10^-122) positive value behaves like a constant negative energy attributed to the vacuum itself, driving the exponential expansion of space. No, a positive CC does not have negative energy. It has positive energy and negative pressure. The effective source in the Friedmann equation is the energy density plus three times the pressure, which for a positive CC is negative (because the negative pressure outweighs the positive energy density).

If there is anti matter. Is there an anti gravity force if there is gravity?

by acd 8y ago

If there is anti matter. Is there an anti gravity force if there is gravity?

strstr 8y ago

No, but charges are inverted for particles vs antiparticles.

majewsky 8y ago

If you're thinking of "negative gravity", then particles exhibiting it fall under the "exotic matter" umbrella. We don't know what exotic matter actually is, although its existence is predicted by the current models on the expansion of the universe. Antimatter still has regular (i.e. positive) mass, so gravity works in the usual (attracting) way.

zamalek 8y ago
The specific type of exotic matter would be imaginary matter, that is matter with imaginary mass (a complex number) - tachyons. Negative matter, which is also exotic but has a negative real mass, behaves normally under gravity so long as the two bodies are sufficiently different in absolute mass.

GolDDranks 8y ago

Basically, antimatter has it's all "quantum numbers" inverted of it's normal counterpart, but it's energy content is not inverted. Strength of gravity is a measure of the energy content, so antimatter should have normal gravity. If it doesn't that's some exciting news!

philipov 8y ago

Antimatter is a consequence of quantum field theory, but all signs indicate gravity can not be successfully described as a quantum field. Gravity has properties such as having its interaction strength grow with the amount of charge (energy), which is very different from the forces described by quantum fields that have constant interaction strength. To get antigravity, you need to find a way to have negative energy without it making the vacuum unstable. It's most typical to describe gravity as dynamical spacetime, not a field existing in that spacetime. The closest people have come to a quantum theory of gravity has been to show that in certain toy universes gravity in a bulk space is the dual of strongly-coupled field interactions existing on the boundary of that space-- that is holography. Information-theoretic approaches have have also suggested that gravity emerges as the 'hydrodynamics of entanglement.' All this suggests gravity is not a fundamental force, but is far from a coherent real-world theory at this point. The closest to an "antigravity" in general relativity is the cosmological constant, thought to explain dark energy. Its tiny (10^-122) positive value behaves like a constant negative energy attributed to the vacuum itself, driving the exponential expansion of space.

xhrpost 8y ago
This is one of the aspects of theoretical physics that I find fascinating. Is there a good book on this that doesn't involve an insane amount of math? (ex. similar to BHoT)
- semi-extrinsic 8y ago
  Zee's "Quantum Field Theory in a Nutshell" is fairly light on the math IIRC. That doesn't mean it's an easy read, though.
- w1 8y ago
  The Order of Time spends some time qualitatively discussing quantum gravity, and the audiobook narration by Benedict Cumberbatch is really excellent.
pdonis 8y ago
> all signs indicate gravity can not be successfully described as a quantum field That's not quite correct. The quantum field theory of a massless spin-2 field was developed in the 1960s and 1970s and is perfectly well-defined. The problem with it then was that it is not renormalizable; but today basically all QFTs are viewed as effective field theories anyway, which means not being renormalizable is not a showstopper. The real issue is that we have no prospect of measuring any quantum aspects of gravity now or in the foreseeable future, so there is no way to test the QFT that has been developed against experiment. > Gravity has properties such as having its interaction strength grow with the amount of charge (energy), which is very different from the forces described by quantum fields that have constant interaction strength. This is not correct either. The coupling constant of the massless spin-2 field is just Newton's gravitational constant (modulo some constant factors that depend on your units). The fact that the total interaction in a specific case varies with energy is no different from the fact that the total EM force between two charged objects varies with the charge. Energy is the source for gravity just as charge is the source for EM; the source is not the same as the coupling constant. > To get antigravity, you need to find a way to have negative energy without it making the vacuum unstable. Agreed. And as far as we know that is not possible. > All this suggests gravity is not a fundamental force This is quite possible, yes, but even if it were true it would not necessarily make the spin-2 massless QFT I referred to above invalid; it would just make it an effective theory that emerges from whatever is the next level down. (Which, as I noted above, is basically how we view all QFTs today anyway, including the Standard Model of particle physics.) > The closest to an "antigravity" in general relativity is the cosmological constant, thought to explain dark energy. Its tiny (10^-122) positive value behaves like a constant negative energy attributed to the vacuum itself, driving the exponential expansion of space. No, a positive CC does not have negative energy. It has positive energy and negative pressure. The effective source in the Friedmann equation is the energy density plus three times the pressure, which for a positive CC is negative (because the negative pressure outweighs the positive energy density).