matter-antimatter definition

[mathematic]

The usual defintion of matter and antimatter is as follows:

matter - quarks (protons, etc.), antimatter - antiquarks (anti-protons, etc.)
matter - leptons (electrons, etc.), antimatter (positrons, etc.)

Question: Are these definitions necessarily coupled? In other words, could electrons be called antimatter and positrons matter, while leaving the the definitions for protons,etc. as is?

[swansont]

There's a lot of physics where it wouldn't make a difference, similar to how + and - charge designations are arbitrary. There may be deeper reasons within particle physics (beyond my depth of study) where the distinction matters.

 

One reason why perhaps you couldn't hasn't been answered yet. We don't know why there is the matter/antimatter asymmetry we see in the universe. There may be some property or mechanism that will require we stick with our current classification.

[ajb]

Question: Are these definitions necessarily coupled? In other words, could electrons be called antimatter and positrons matter, while leaving the the definitions for protons,etc. as is?

Mathematically it comes out of separating your fields into positive and negative frequency parts, which we can do in a nice way on flat space-time. For the most part the difference is just a plus and minus sign which makes little difference in most places. We can treat matter and antimatter on equal footing with a little care, for the most part.

 

This is because of what we call CP-symmetry. C is charge conjugation which exchanges a particle with its antiparticle and P which inverts the spacial coordinates. This is the "mirror world" of antiparticles. The electromagnetic and strong interactions are (as far as we can tell) symmetric under CP-symmetry.

 

However, the weak interaction is known to violate CP-symmetry and so does not treat particles and antiparticles in quite the same way.

 

This is important for the matter antimatter imbalance we see today. In the early Universe there must have been some phase of CP-symmetry violation meaning more matter than antimatter survived the inevitable annihilation into photons.

 

Note that we also have T-symmetry which is inversion in time. Due to some fundamental theorems we always have CPT-symmetry.

[mathematic]

One of my speculations is that, based on the idea that the net totality of particles (no. matter minus no. antimatter) is preserved in any interaction, the net totality of the universe should be 0, since it started out that way. If the electrons were designated as antiparticles, it would be easier to preserve net totality. Otherwise the excess of matter would be have to be balanced by antineutrinos.

[Sensei]

One reason why perhaps you couldn't hasn't been answered yet. We don't know why there is the matter/antimatter asymmetry we see in the universe. There may be some property or mechanism that will require we stick with our current classification.

 

If there is no spectral lines difference between Hydrogen and Anti-hydrogen, and other nucleus and their anti-nucleus, we can't entirely rule out idea that some distant galaxies are not made of antimatter.

After all only photons is what reaches us from distant objects.

[swansont]

If there is no spectral lines difference between Hydrogen and Anti-hydrogen, and other nucleus and their anti-nucleus, we can't entirely rule out idea that some distant galaxies are not made of antimatter.

After all only photons is what reaches us from distant objects.

We should see annihilation radiation from when the matter was nearby. There is no reason why they would separate from each other.