Anti-matter = repulsive gravity?

[PM 2Ring]

Oh, hmm, yeah this might be fine.
But clearly you want to define the stuff that exists in your average atom as "regular," no?

Well, yeah. OTOH, adopting the convention that electrons are antimatter kinda solves the puzzle of why antimatter is comparatively rare.

Seriously though, it doesn't really solve anything, it just transforms the description of the underlying problem into a more complicated description.

[mfb]

I think they wanted to define it so all positive charge is 'regular' matter.

Well, that cannot work, as up-type-quarks and down-type-quarks have different charge. A \Delta(1232)- consists of 3 down-quarks and therefore must be matter, but it has a negative charge.

>> OTOH, adopting the convention that electrons are antimatter kinda solves the puzzle of why antimatter is comparatively rare.
That is the point with B-L-conservation.

Actually, I think that it could be useful to flip the convention for leptons. The baryon number then becomes a "matter number", [new] leptons (like the positron) get 1 and [new] antileptons (like the electron) get -1.

[DrZiro]

Regardless of whether or not it would be a better convention, my point was that if it's possible to define electrons as antimatter, that refutes the hypothesis that antimatter undergoes repulsive gravity. Provided that we can actually measure the gravity on electrons. Which I still don't know.

[yurell]

We can measure the gravity on electrons (I've done it myself), but saying "your argument is wrong if I redefine the subject of your argument!" isn't exactly constructive.

[mfb]

Well, you just would have to reformulate it:
"Baryonic antimatter and leptonic matter could have repulsive gravity"

But as we know, this would be really bad for GR and we have no sign of such a repulsive gravity yet. The trapped neutral antihydrogen at CERN might give interesting results in the future.

[Carlington]

Am I understanding this right? Are we discussing whether we should define positively-charged leptons (positrons, up quarks, etc.) as matter, and negatively-charged leptons (electrons, down quarks) as anti-matter? Isn't that effectively just flipping what we define as matter and anti-matter backwards? If we're going to follow that path, what do we do with our neutral leptons (neutrinos).
Matter of fact, what do we do with our neutrinos now? Are they defined as matter, anti-matter, or do we just leave them in the "ill-defined" basket for the time being?
[offtopic]

Spoiler:

In a similar vein, why is it that while quarks and leptons both have two main "families", the quarks are split into positively-charged and negatively-charged families, but the leptons are split into negative and neutral? Actually, throughout the process of typing this post and frantically researching elementary particles on Wikipedia, I'm slowly coming to realise why neutrinos cause so many problems. Hmm.

[/offtopic]

[SU3SU2U1]

Matter of fact, what do we do with our neutrinos now? Are they defined as matter, anti-matter, or do we just leave them in the "ill-defined" basket for the time being?

We have observed no right-helicity neutrinos thus far. This means that neutrinos may be majorana particles (which is a fancy way of saying a neutrino is its own anti-particle).

In a similar vein, why is it that while quarks and leptons both have two main "families", the quarks are split into positively-charged and negatively-charged families, but the leptons are split into negative and neutral?

There is no obvious reason things have the charges they do, but proponents of grand unified theories say its because everything fits into a larger theory.

Just like the current "quark" is actually a field with three component "colors," some have suggested you can put several components of the standard model together as fewer fields with different symmetries. You can fit the standard model's many fields into two components in an SU(5) symmetry group (the 5 dimensional and 10 dimensional representations).

Even more elegant- the entire standard model fits perfectly into the 16 dimensional representation of the SO(10) symmetry group, with room left over for a right-helicity sterile neutrino.

[erik542]

I'm pretty sure that even classically, things with negative mass is bad news. Taking Newton's laws, two particles with equal but oppositely charged masses will shoot off to infinity together.

[thoughtfully]

But it's Ok when two oppositely-charged particles do it?

[mfb]

The point is that they will stay at the same distance and continue to accelerate in one direction, if one has negative (inertial) mass. This does not happen without negative masses, regardless of the charge.