[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.

[raattgift]

> A black hole only has three properties, in our current understanding of general relativity - but "electrical charge" is one of those properties

An isolated black hole, at a suitable coordinate time, has mass, electric charge, angular momentum (three components), linear momentum (three components) and spatial position (three components). Holding the BH at the spatial origin drops the last six.

Also, a slight wrinkle: this state is asymptotic -- at timescales less than light-crossing there can be substantial additional hair. At longer timescales, some configurations can persist on much longer than light-crossing scales -- one example is the magnetic field at the newly formed horizon of a isolated collapsed rotating magnetar.

"isolated" here can get tricky in practice as well.

But in the usual case, no, you can't look at a black hole and tell whether someone much earlier threw (classical picture) in one shell of matter of mass M vs two concentric shells of matter at 1/2 M each or three concentric shells of matter of 1/4 M, 1/2 M and 1/4 M (or 1/2 M, 1/4 M and 1/4 M, etc.). But what's this shells picture for non-negligible charge? (Switching to a dust doesn't help, fwiw).

More formally, the no-hair theorem says that in a stationary electrovacuum, a black hole solution takes on a specific form. That mostly means that we should be able to perturb a Kerr-Newman BH solution and get the right results for an astrophysical BH.

[MrEldritch]

I would argue that "position" and "linear momentum" are more properties of one's particular choice of inertial reference frame than properties of the black hole itself specifically. And yes, angular momentum has three scalar components, but it's also one single vector. "Mass, charge, and angular momentum" makes three properties.

>at timescales less than light-crossing there can be substantial additional hair.

Ahh, thank you - that clears up some misconceptions of mine that have always confused me, like "Wait, so if black holes have no hair, how can they wobble and ring-down and produce gravitational waves after a black hole merger?"

[raattgift]

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

Oh, I see what you mean, but it's not clearly because of symmetries breaking inside the horizon, rather than high-energy pairs taking energy from the background. Semiclassically: collapse an isolated star (here we depart from Hawking's formalism), and observe nothing but photons (with wavelengths comparable to the curvature radius) forming an atmosphere densest around ~ 3m \lt r \lessapprox 4m until m is very small, at which point you'll observe pair production in the atmosphere densest around ~ 4m. \lessapprox 4m is the back reaction mess of the inner atmosphere on chaotic and mostly plunging trajectories, and the highly dynamical part of the spacetime. [cf. Unruh https://link.aps.org/doi/10.1103/PhysRevD.15.365 nb 3rd paragraph, "It must be remembered that talk about particles is a very crude and metaphorical way of talking about the physics near the horizon of the black hole", and Giddings https://arxiv.org/abs/1511.08221 ] In the Hawking formalism the background is static, and that forces the use of an infalling negative energy; that's not a real symmetry being broken gravitationally, and so I'm wary about the idea of breaking baryon (and lepton) symmetry with black holes.

So really this is mostly a wordy objection to "spitting out".

[raattgift]

> gravity isn't inherently respectful of charge conservation

It is unless you reject minimal coupling of electromagnetism to curvature. Otherwise where would one insert the metric into the inhomogeneous Maxwell equation dH = J (in arbitrary local coordinates x^i, H = 1/2 H_{ij} dx^i \wedge dx^j) ?

[JBorrow]

There are other things that must be conserved - not just energy - in the current standard model of particle physics. For example, you have to conserve charge - and matter and antimatter have opposite charges.

In that case, say I have 100 electrons that I want to turn into positrons. If I was able to turn these into pure "energy", first I'd have to figure out how to give that "energy" charge (so it could conserve charge in that step), and then I would have to create some other particles to balance out the 100 positive charges that the positrons would give me, ending up with 300 particles.

The reason that we think that there's more matter than antimatter in the real universe is because of a thing called "Charge-Parity Violation" [1] that is the focus of a lot of current research.

[1] https://www.symmetrymagazine.org/article/charge-parity-viola...

[paulddraper]

Momentum is an example of another conserved quantity.

[MrEldritch]

Incidentally, fun fact - you already know that X, Y, and Z linear momentum are all conserved separately. In special relativity, mass-energy conservation gets folded into this as well - an object's mass-energy is just the component of linear momentum along the time axis! (and "rest mass" is its value in the reference frame where the object is otherwise stationary and moving only through time.)

[antidesitter]

This is conservation of 4-momentum, whose spatial components are ordinary momentum and whose temporal component is energy. By Noether’s theorem, conservation of 4-momentum is due to the invariance of physical laws under 4-translations (spatial and temporal).

[fahadkhan]

I am not a physicist. But obvious question is why would the energy not coalesce back to equal matter and anti matter?

[misnome]

This is a good question! In reality, for this process they do: see https://en.wikipedia.org/wiki/Pair_production. But ignoring this and JBorrows (correct) comments on other properties to be conserved, we can ask:

IF more matter is made (in some process) than anti-matter, then why? To explain this, we would need some sort of mechanism where matter was treated differently to antimatter in the laws of physics... and then you are back to the original question of looking for differences.

Non-gravitationally, the general case of antimatter behaving differently to matter is covered by the concept of https://en.wikipedia.org/wiki/CP_violation - we do know of cases where this is the case, but not strongly enough to explain what we observe in the universe - the article looks a little heavy but does have a section on the matter-antimatter imbalance that is a little easier to read.

[jcims]

I had a similar thought but then wondered if the question would be why there's a bias in which type of matter condenses out of the energy. All this stuff is way over my head but it's fun to think about.

[MrEldritch]

>if the question would be why there's a bias in which type of matter condenses out of the energy

Yes, it's exactly this! The fact that there's more matter than antimatter in the universe means that, somewhere, some process has to break the symmetry and be 'biased', treating antimatter differently than matter. (There are known processes that do this - certain specific meson decays that are ever-so-slightly biased towards decaying into matter rather than antimatter, other weird stuff like that - but the observed phenomena are nowhere near strong enough to explain the degree to which matter predominates in the observed universe.)

As far as we can tell, all interactions involving gravity, the electromagnetic field, and the strong nuclear interaction[1] are perfectly symmetric with respect to matter versus antimatter. The weak nuclear interaction (which is involved in a lot of processes that transform particles into other kinds of particles, like radioactive decay) does break it - that's what's up with those meson decays - but only a little bit, in very specific cases.

[1]It's actually a bit of a puzzle why the strong force doesn't violate this symmetry; according to our understanding of the math for the strong interaction, it totally could - there are terms that naturally appear in the equations that would lead to it. But, the values for all of those terms appear to be as near to zero as we can measure. There's no obvious reason this should be the case, especially because another force - the weak nuclear force - has no problem with slightly violating that symmetry.

[DenisM]

There doesn’t have to be a break in rules symmetry tho. Imagine that universe is a giant dipole of a magic field F, one end of it carries huge field +F the other -F. Next imagine, that presence of +F is likely to precipitate matter from energy, while -F is likely to yield antimatter. Thus a perfectly symmetric system of rules will produce area with only matter and only anti-matter. And blazing inferno in between the two zones.

We just need to find out what the F F is.

[HeadsUpHigh]

But we haven't observed any areas with antimatter only up to now.

[DenisM]

That was not my point, I only provided an example where asymmetry of laws is not strictly necessary to explain asymmetry of the outcome.

As to your point - the other side of the dipole could be beyond visible (to us) universe.

[andrepd]

>Couldn’t then this energy coalesce back to regular matter through the mass-energy equivalence? E=mc^2?

That's just not how that works.

[pc86]

[citation needed], not because you're wrong (you're not) but that's about as unhelpful of a response as you can make without just personally insulting the person.