
Physicists investigate why matter and antimatter are not mirror images - dschuetz
https://www.economist.com/science-and-technology/2018/09/22/physicists-investigate-why-matter-and-antimatter-are-not-mirror-images
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losvedir
Interesting. I was just talking with a friend about that this weekend. He's
working on another experiment, the Deep Underground Neutrino Experiment (DUNE)
which is investigating this question, among others. They're shooting a
neutrino beam from Fermi Lab in Chicago to a detector deep underground in
South Dakota, allowing them to measure several characteristic parameters of
these neutrinos. One of them, delta_cp, will give insight into the question of
the matter-antimatter imbalance[0].

As just a lowly software developer, it's quite amazing to me to see what
cutting edge physicists are up to. The engineering scale of these experiments
is mind boggling.

[0]
[https://en.wikipedia.org/wiki/CP_violation#CP_violation_and_...](https://en.wikipedia.org/wiki/CP_violation#CP_violation_and_the_matter%E2%80%93antimatter_imbalance)

~~~
nkrisc
Since you mentioned it I'll plug it (simply because I think it's cool):
FermiLab has a lot of public events they host as well as various interesting
lectures [http://news.fnal.gov/newsroom/public-
events/](http://news.fnal.gov/newsroom/public-events/)

It's in Batavia, about an hour's drive outside Chicago.

~~~
breatheoften
Are there any good podcasts offering behind the scenes perspectives on
experiments at this level?

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stephengillie
Article doesn't match title. They're studying gravity's pull on antimatter.

~~~
gumby
Actually it does. The article intro starts with the fundamental question (why
is there an apparent imbalance in the amount of matter/antimatter), then says
that one experiment is this gravity one. It returns at the end to the point
that the standard model doesn't predict a difference, but then again the
standard model doesn't predict the mismatch either.

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Aardwolf
Is it possible that if they were, they would annihilate each other, so you
couldn't live in an area where both are present, and so we are necessarily
born in a place where there's much more of only one type? The antimatter might
be outside of the observable universe, safely away from interacting with us
for the foreseeable future.

~~~
stephengillie
There was a short story about an antimatter alien who visited Earth. It shook
hands with a human and both were annihilated.

~~~
Aardwolf
Wouldn't it already be annihilated by air particles though :)

~~~
stephengillie
It was very early science fiction. And yes it would.

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muthdra
Wouldn't it already be annihilated by photons from the sun?

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madrocketsci
Do we really _know_ for sure that there is more matter than antimatter in the
universe?

All we collect from distant galaxies is light, which, as far as I know,
interacts with antimatter in the exact same way as with matter. Do we see any
gamma-ray producing zones in intergalactic space that might form the
boundaries between "matter-zones" and "antimatter-zones"?

~~~
wlesieutre
You're spot on with the gamma producing zones. We don't see any of those, and
that's why we don't think any galaxies are made of antimatter.

[https://www.scientificamerican.com/article/how-do-we-know-
th...](https://www.scientificamerican.com/article/how-do-we-know-that-dista/)

~~~
phkahler
I thought there were signs of gamma radiation surrounding galaxies. Perhaps
the gas filled void between galaxies is antimatter while the galaxies are
matter. Perhaps antimatter is repelled by both types and regular matter is
attracted to both types. This might lead to a gaseous cloud pushing things
apart and clumps of regular matter. It would also allow some configurations to
accelerate and violate conservation of momentum, so it doesn't seem likely.

To me there are more questions than "does antimatter fall up?" There are 4
interactions to test.

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realize
I’ve always wondered about this, perhaps someone who understands this subject
could explain.

Say matter and antimatter were created in roughly equal proportions then some
collided to create energy. Couldn’t then this energy coalesce back to regular
matter through the mass-energy equivalence? E=mc^2? Repeat this a bit and
you’d end up with more matter than antimatter.

As I said, this is so simple there must be an easy argument against it but
I’ve never heard the idea addressed.

~~~
MrEldritch
realize,

It's not _obviously_ impossible, but in fact this is never observed - no
particle is _ever_ created without a corresponding antiparticle. (The
corresponding laws are "lepton number conservation" and "baryon number
conservation" \- basically, the total number of electrons[1] minus the total
number of anti-electrons[2] remains constant, and so does the total number of
quarks minus antiquarks. All of the interactions of the Standard Model respect
these.[3][4]

There are various beyond-standard-model theories that allow breaking baryon
and lepton number conservation _individually_ , with the combined number of
(baryons - leptons) being a conserved quantity instead; but they also almost
all predict that protons should be slightly unstable (because being able to go
from _[Exotic Mystery Particle]_ to baryons + leptons means you should also be
able to go from baryons (like the proton) to _[Exotic Mystery Particle]_ and
leptons) but we've looked _really really hard_ for evidence of extremely rare
proton decays and have yet to find any.

[1](plus muons, and tauons, and the three corresponding flavors of neutrinos)

[2](plus anti-muons, and anti-tauons, and anti-neutrinos)

[3]Even the observed violations of matter-antimatter asymmetry ("CP
violation") still respect these conserved quantities; they just involve things
like anti-kaons decaying slightly but measurably faster than kaons.

[4]On the other hand, there's no particular reason to expect _gravity_ to
respect these; For instance, we think black holes can consume matter, and then
convert it to energy in the form of Hawking radiation as they slowly decay,
without having to bother with eating an equal quantity of antimatter. But
honestly we're just guessing on that front.

~~~
raattgift
> For instance, we think black holes can consume matter, and then convert it
> to energy in the form of Hawking radiation as they slowly decay, without
> having to bother with eating an equal quantity of antimatter.

> we're just guessing

Black holes (BHs) are not a very realistic candidate for solving baryon
asymmetry. Where's the antimatter outside the horizon of a modern (as in after
structure formation) astrophysical BH? If it's not there, it can't fall in.
This is really hard to work around even for early direct-collapse super-
massive BHs; hierarchical growth is already essentially ruled out. Worse, how
do you keep signatures of annihilations out of the region near the BHs,
including the accretion material and any jets?

Or are you expecting primordial BHs to couple differently to baryons and their
antis? How do you suppress that difference in the weak field limit, or more
generally after first light? (And in either case, how do you make sure that
virtually all of the antimatter is locked up in BHs?) Essentially you keep
coming back to having the stress-energy already significantly (really, almost
entirely) segregated into particles and their antis, around the time of
gravitational collapse, or you depart dramatically from General Relativity in
a regime in which it is already supported by evidence.

Finally, where are you hiding all these black holes, whenever they formed? If
only BHs break baryon symmetry, the contribution to \Omega implies a lot of
lensing. (Speculating in the direction of a dust of tiny remnants or the like
is also hard work, and usually involves beyond-the-standard-model new physics
anyway, although there is a small literature that involves operators like
\partial_{\mu}F(R)J^{\mu}, where J^{\mu} is the baryon or lepton current, and
R is the curvature scalar or the Riemann tensor
(R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}) or a more complex term, and afaik
none of these model-builders take backreaction into account yet.)

~~~
ajross
> Black holes (BHs) are not a very realistic candidate for solving baryon
> asymmetry.

I'm reasonably certain grandparent wasn't proposing this as a theory, just
using it as a simple gedankenexperiment to show that gravity isn't inherently
respectful of charge conservation.

~~~
MrEldritch
Bingo.

Well, not quite - black holes _would_ be respective of charge conservation! A
black hole only has three properties, in our current understanding of general
relativity - but "electrical charge" is one of those properties.

But there's nothing that stops it, say, eating protons and spitting out
positrons later.

~~~
madrocketsci
Do you mind if I ask you some questions about this?

I always have trouble picturing how, dynamically, a charge inside an event
horizon is supposed to be able to propagate an electric field outside the
event horizon of a black-hole. (retarded vector potential travels from a
charge along light-paths to another point in space-time. There isn't any way
for the influence of a point charge to get out?)

Perhaps some other related questions too: Charge and current density is a
4-vector in SR, which transforms along with all the other 4-vectors (momentum-
energy 4-vector, etc). In a situation where the effective mass of an object
_reversibly_ lowered to the event-horizon (slowly moved relative to the event
horizon with small velocity) goes to zero (all the mass energy ends up
somewhere else) - wouldn't the effective charge density from a non-infalling
external observer's perspective also be going to zero?

If we're just drawing a box around a black-hole and declaring that charge is
conserved, we would have as much/little reason to declare any other
conservation also holds?

~~~
raattgift
These are good questions! You might consider taking them to a forum where
you'll get a more rigorous answer though. :D

Fully classically, the field lines point to the sources; they get "stuck" to
the horizon as the source crosses. To a naive outside distant observer for
whom the horizon subtends a small angle of the sky, so does each source. When
thinking about collapsing charged matter forming a new black hole, substitute
a spherically symmetric shell and shrink its area, while keeping the charge
and mass constant and uniformly distributed on the shell -- the electric field
and gravitational field outside the shell then both follow gauss-laws, so even
without a horizon, observers outside the shell at a large distance (such that
the shell looks virtually pointlike) cannot get the full information about the
shell using only local measurements, including whether the shell is inside or
outside a horizon.

Semiclassically, one can use virtual photons which aren't as restricted as
real matter, especially in that the black hole horizon is not necessarily a
trapping surface for them. Typically one sets up the black hole as a
background that has already determined the relevant quantum fields, and then
introduces a test particle onto that background. If the test particle radiates
a photon, the black hole will only react once the photon enters the horizon;
unless and until that happens, the background is kept constant. (Hawking
introduces negative energy particles in his formalism precisely to keep the
background always constant.) Changing the background is tricky, but never
involves real particles crossing from the interior of the horizon to the
exterior.

> we would have as much/little reason to declare any other conservation also
> holds

Sure, no-hair as a _theorem_ (rather than as a principle) only tells you that
given classical vacuum, Maxwell's electromagnetism (in tensor form), and an
eternal black hole metric, spacetime and all its contents are totally
determined everywhere by a small handful of parameters. As a principle it
suggests that perturbing that setup (e.g. by adding a source outside the
horizon) does not lead to wildly inaccurate results.

I'm sorry that I don't understand what it is that you're asking in your
second-last paragraph. There is a body of literature on black hole "mining"
(it's a common thought-experiment when trying to distinguish between general
relativity an alternative theory, especially a quantum one) that maybe touches
on what you're curious about.

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amelius
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)?

~~~
MrEldritch
>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.

~~~
amelius
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).

~~~
MrEldritch
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.)

~~~
antidesitter
This is probably referring to
[https://en.wikipedia.org/wiki/Kaluza–Klein_theory](https://en.wikipedia.org/wiki/Kaluza–Klein_theory).

