5 ms·

> massless spin-2 field >> To get antigravity, you need to find a way to have negative energy without it making the vacuum unstable. > Agreed. And as far as w

> massless spin-2 field >> 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. Which part are you agreeing to? Spin-2 essentially means that like charges attract and unlike charges repel. Matter with the opposite gravitational charge from all the matter (incl. antimatter) in the universe we know about could have segregated in the early universe. Opposite-gravitational-charge matter (incl. antimatter) is unobserved, but not in-principle unobservable. Vacuum instability doesn't arise since m_i stays the same as m_g changes sign under gravitational charge changes.

philipov 8y ago
1) They meant there's no way to get negative energy particles which could be the source of antigravity. That would destabilize the vacuum because it would allow you to keep lowering the energy without bound. 2) Intuitively if antimatter is regular matter travelling backwards in time, then at the start of time, wouldn't it all go the other way? Kind of like one huge quantum fluctuation.
- pdonis 8y ago
  > if antimatter is regular matter travelling backwards in time It isn't. More precisely, while the mathematical trick that this phrase refers to (which also, btw, requires reversing all charges and flipping parity) works as far as making calculations simpler while still getting the right answer, it doesn't correspond to anything physical.
pdonis 8y ago
> Spin-2 essentially means that like charges attract and unlike charges repel. No, that's spin-1. Spin-2 means there is no such thing as "unlike charges"; the interaction is always attractive.
- raattgift 8y ago
  For charge product q1q2 > 0 : even integer spin attractive q1q2 > 0 : odd integer spin repulsive Jumping up classically, for a vector field like charges repel. For a (symmetric) rank-2 tensor field like charges attract. The weak equivalence principle only requires m_i = m_g up to the sign of the latter. It's the energy conditions which constrain that, notably the WEC component of the positive energy theorem.
  - pdonis 8y ago
    > For a (symmetric) rank-2 tensor field like charges attract. For a spin-2 field all charges attract. There is no such thing as "unlike charges". > The weak equivalence principle only requires m_i = m_g up to the sign of the latter. I don't know where you're getting that from. The WEP requires that the trajectory of a body moving solely under gravity depends solely on its initial position and velocity. That means m_i = m_g. It does not allow flipping the sign; that would mean there would be two possible trajectories for a given initial position and velocity, not one.
  - pdonis 8y ago
    > For charge product q1q2 > 0 : even integer spin attractive q1q2 > 0 : odd integer spin repulsive Yes, I see I misread your previous post; for spin-1 unlike charges attract and like charges repel. That I agree with.

philipov 8y ago

1) They meant there's no way to get negative energy particles which could be the source of antigravity. That would destabilize the vacuum because it would allow you to keep lowering the energy without bound. 2) Intuitively if antimatter is regular matter travelling backwards in time, then at the start of time, wouldn't it all go the other way? Kind of like one huge quantum fluctuation.

pdonis 8y ago
> if antimatter is regular matter travelling backwards in time It isn't. More precisely, while the mathematical trick that this phrase refers to (which also, btw, requires reversing all charges and flipping parity) works as far as making calculations simpler while still getting the right answer, it doesn't correspond to anything physical.

pdonis 8y ago

> Spin-2 essentially means that like charges attract and unlike charges repel. No, that's spin-1. Spin-2 means there is no such thing as "unlike charges"; the interaction is always attractive.

raattgift 8y ago
For charge product q1q2 > 0 : even integer spin attractive q1q2 > 0 : odd integer spin repulsive Jumping up classically, for a vector field like charges repel. For a (symmetric) rank-2 tensor field like charges attract. The weak equivalence principle only requires m_i = m_g up to the sign of the latter. It's the energy conditions which constrain that, notably the WEC component of the positive energy theorem.
- pdonis 8y ago
  > For a (symmetric) rank-2 tensor field like charges attract. For a spin-2 field all charges attract. There is no such thing as "unlike charges". > The weak equivalence principle only requires m_i = m_g up to the sign of the latter. I don't know where you're getting that from. The WEP requires that the trajectory of a body moving solely under gravity depends solely on its initial position and velocity. That means m_i = m_g. It does not allow flipping the sign; that would mean there would be two possible trajectories for a given initial position and velocity, not one.
- pdonis 8y ago
  > For charge product q1q2 > 0 : even integer spin attractive q1q2 > 0 : odd integer spin repulsive Yes, I see I misread your previous post; for spin-1 unlike charges attract and like charges repel. That I agree with.