Could antimatter be "negative matter"?

Question

This question is inspired by an answer on WorldBuilding.SE.

The question on worldbuilding is about the Alcubierre Drive, and the answer is a good one, pointing out that it requires "negative matter", which we have never observed and is hard to reconcile with current theory. However, in one of the comments, someone states

Antimatter could be negative matter, but we haven't produced enough of it to find out. (I.e it goes boom before we have time to see which way it falls.)

This doesn't square with my limited non-physicist understanding of antimatter & negative matter. Is this correct?

Answer 1

Good question. Here I'm going to talk about four types of matter, just to distinguish: matter, antimatter, dark matter, and negative matter.

Matter

The dictionary definition of matter is

physical substance in general, as distinct from mind and spirit; (in physics) that which occupies space and possesses rest mass, especially as distinct from energy.

This was a good definition before the 20th century, but now we know $E = mc^2$ is true, which really makes life confusing. As Wikipedia says, there isn't really a great definition for matter now, but we'll say an object's mass can be either the stuff that makes up all physical objects, like you and me, or what arises from motion and interaction energies.

Matter is, of course, made of atoms. An atom is composed of protons, neutrons, and electrons. Protons and neutrons are not fundamental; they are composed of quarks and gluons.

Antimatter

Antimatter is composed of the aptly named antiparticles. These antiparticles are positrons (as opposed to electrons), antiprotons, and antineutrons. These antiparticles have the same mass as normal particles, but opposite charges, as well as other properties such as lepton and baryon numbers. Collisions between matter and antimatter result in annihilation for both parties.

It is important to note antimatter has the same mass as normal matter in consideration of the other two "types" of matter.

Dark matter

We don't really know what dark matter is, honestly. We think it makes up around 27% of the mass and energy in the observable universe. We also think it is non-baryonic (as in, it isn't made up of protons and neutrons like normal matter). Some possible candidates for dark matter are hypothetical particles, like super-symmetric particles and axions. We think dark matter is different from negative matter.

Negative matter

Negative matter is matter that has an opposite sign to the mass of normal matter - like -2 grams, or -5 pounds. It would show incredibly strange properties and violate some energy conditions. It is used in speculative theories, such as the construction of warp drives and wormholes. The closest known representative of this very exotic idea is the pseudo-negative pressure density produced by the Casimir effect. Because the standard model doesn't explain gravity and general relativity doesn't explain the EM, strong, or weak forces, a quantum theory of gravity might be needed to really understand how negative matter works.

Looking at these definitions, I think it's pretty clear that antimatter and negative matter are definitely not the same, as antimatter has the same mass but opposite charge, while negative matter has opposite mass.

Hope this helps. I have linked to websites throughout this answer with more information.

Answer 2

It's highly implausible that antimatter has any "negative" properties, because antiness isn't a thing. The relationship between matter and antimatter is like mirror reflection (precisely, it's CPT). Protons and antiprotons are mirror images of each other; neither one is the mirror image. Antiprotons get the "anti" prefix merely because they're much less common than protons.

Some particles (photons, for example) are "mirror symmetric", hence antiparticles of themselves. We know that photons fall down (from the bending of starlight by the sun, for example). If antiparticles antigravitate then photons would have to fall up also, which isn't even self consistent.

In the Standard Model, all of the matter particles (fermions) are made of left-handed and right-handed parts coupled through the Higgs interaction. In SO(10) grand unification, all of the left-handed fermions are aspects of the same particle, and the right-handed fermions are their "mirror reflections" (CPT duals), so every matter particle is half "pro" and half "anti", making the idea of antimatter as a class of matter even more nonsensical. We don't know if SO(10) grand unification is correct, but the fact that it's possible to unify all of the fermions suggests that they probably are unified, even if not in exactly that way.

The equivalence principle and Einstein's rocket argument also shows that everything must gravitate in the same way. This even includes matter with a negative mass, if it exists. Even in Newtonian gravity negative-mass test particles fall, because the mass of the test particle cancels out of the equation.

So if we found that antiprotons fall up, it would mean that everything we think we understand about particles, that all seems to make sense, is profoundly wrong. I won't say it's impossible, but I'd bet on finding leprechauns before I'd bet on this.