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Comment by amelius | Hacker News Reader

Couldn't open the article. Do they mean literally mirror images (like geometrically mirrored in a plane)?

This makes me wonder, in chemistry you have the concept of chirality. What would happen if you'd literally mirror a non-chiral molecule (down to the quark level)?

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>Do they mean literally mirror images

Almost! To be more specific, the laws of physics are CP-symmetric[1] - if you take any interaction involving matter and/or antimatter, and you swap all the positively charged particles with their negatively charged antiparticles and vice versa, and then you invert the coordinate system (swapping left-handed and right-handed chirality, aka "literally mirror image") it should now behave exactly the same.

Basically, antimatter is matter with the charges reversed and the handedness mirrored.

[1]techically it's only CPT symmetric - you're only guaranteed perfect symmetry under simultaneous reversal of charge, parity (handedness), and time. But almost all processes (anything mediated by gravity, electromagnetism, or - as far as we can tell - the strong nuclear force) are perfectly CP-symmetric; As far as we can tell, CP symmetry violation only shows up in rare scenarios involving the weak nuclear force, where some particles can take slightly longer to decay than their antiparticles.

Thanks!

This makes me wonder, since we've obviously never actually mirrored any particle, is there any chance that charge will (ultimately) also turn out to be a geometric concept? (By analogy to magnetism, where changing the orientation of the current flips the polarity of the magnetic effect).

Depending on what you mean by "geometric", maybe - there's been attempts at constructing theories where the electromagnetic field is also, like gravity, a product of spacetime geometry; these generally involve adding extra dimensions to give spacetime enough freedom to hold those behaviors, and then curling up the extra dimensions small enough to explain why we hadn't noticed them yet.

(String theory is one of the distant descendants of these attempts.)

This is probably referring to https://en.wikipedia.org/wiki/Kaluza–Klein_theory.

More in as: all properties (except for gravitational) are opposed to the other variant; e.g. charge, spin, parity but not mass.

see the wiki entry for some information: https://en.wikipedia.org/wiki/Antimatter

There is a concept of chiral symmetry [0] in QCD---the symmetry whereby left and right handed quarks can be phase rotated independently. It is explicitly broken by quark masses, but the dominant effect is actually the dynamical breaking by the QCD vacuum [1].

However, as far as anyone knows, the chirality at the quark level has nothing to do with the chiral symmetry breaking apparent in eg. the organic molecules of life whereby one enantiomer is used and not the other.

[0] https://en.wikipedia.org/wiki/Chirality_(physics)#Chiral_sym...

[1] https://en.wikipedia.org/wiki/Chiral_symmetry_breaking

The entire article addresses the question of whether antimatter "falls-up" or "falls-down". In other words, does gravity act in the normal way on antimatter, or is its action inverted?

That would mean that matter and anti-matter should generally slowly repel (due to the (anti-)gravity), but upon forceful collision they ultimately annihilate each other. That would mean that matter just touching anti-matter should not lead to annihilation of both.

Perhaps you can view this archived copy instead: http://archive.is/nEAjt

Sorry, I've posted an empty submission first, w/o the URL. This one above should work now, or this one: https://www.economist.com/science-and-technology/2018/09/22/...

Mea Culpa

They want to test whether antimatter falls up in a gravitational field.

Gravitational mass is not meant to be affected by charge / parity conjugation - so anti-matter `should` behave the same way in a gravitational field.

The article ends with:

“Few really believe that antimatter falls up, because so much existing theory predicts it will not. But in 1887 existing theory predicted that the apparent speed of light would vary with the speed at which the observer was moving. When Albert Michelson and Edward Morley showed that it does not, they tore up the rule book of physics. It could happen again.”

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