I just have to say that I am honored that you remember slapping my thought down that long ago! 
Thanks for the link.
Now that that’s settled, I wonder if we can address the question raised by my misreading the thread title: How does gravy react with anti matter?
Deliciously.
Pretty spectacularly, one would think, with release of lots of energy and particle-antiparticle pairs.
If you want a delicious reaction you have to use anti-gravy.
So gravy anti-gravy ain’t wavy?
I’m more concerned with the reaction products of putting pesto on your antipasto.
I think there’s a sort of Pauli Spelling exclusion principle which allows a co-existence there.
Two vowels in the same position as long as they are not the same?
Pasta and antipasta on the same plate, though? Tricky…
Only when poured from a gravy boat.
With the caveat that particle accelerators are horribly inefficient. Yeah, you’ll get out all of the energy that you put in to those particular particles, but you’ll be putting orders of magnitude more energy into just heating up your equipment, and that energy you’re not getting back.
Is that what Philadelphians put on their antipasta?
Interesting that you still seem to have the same thought after all this time. Let’s check back in another 10 years!
Probably because I don’t understand well enough to understand why it can’t be true. It fits my naïve visualization still? I accept though that such is my recalcitrant ignorance. Mostly. 
To be fair, if you primarily hear just fanciful language about extra dimensions and woo-y antimatter in public-facing science literature, how can you be expected to make proper sense of it? Perhaps that’s where the SDMB can enter, though.
Recalling from the older discussion, one important take-away is that matter and antimatter really are just two sides of the same coin. It’s not “Matter is normal, and antimatter is mysterious and weird”. They are both normal and act normally and enter the mathematics of it all identically.
As an analogy that is remarkably close to the actual math in the real case of matter/antimatter…
The equation
\ \ \ x^2-a^2=0
has two solutions. You can have x=a and you can have x=-a. These solutions are 100% on equal footing. If I wanted to, I could say a is the “solution” and -a is the “anti-solution”, but those are just arbitrary words. They don’t affect anything.
To drive that point home, let’s define a new symbol y that is just the negative of x:
\ \ \ y=-x,
i.e. just a change of perspective. We could then talk about solutions of this equation in terms of y:
\ \ \ y^2-a^2=0.
This has solutions y=-a and y=a. Or the other way around. It doesn’t matter. And whether I talk about x or y is irrelevant to any mathematical conclusions, so long as I’m consistent in my definitions here and in anything I use x or y for downstream.
And, I could say “In the convention where y is our variable of interest, we’ll call a the solution and -a the anti-solution.” But that’s the opposite of the language assigned above in the original x viewpoint, since y=-x.
Matter and antimatter are exactly like this. Our most basic physical principles have a symmetry in them that means the very presence of a particle means the presence of it’s anti-particle, where applicable. This isn’t a thing tacked on late in the story. It’s very fundamental and falls out from the defining principles of relativity and quantum mechanics.
On the other side of the story are the potential extra dimensions. Hopefully the above conveys that it wouldn’t even make sense to label one or the other type of matter as the one that should “hide” in those extra dimensions. They are twins and obey the same core rules.
Antimatter seems exotic (and therefore perhaps able to do magical things) only because it’s not in one’s everyday experience. In the early universe, non-equilibrium processes were present that allowed every 10,000,000,000 matter particles and 10,000,000,000 antimatter particles to develop a slight asymmetry and become roughly 10,000,000,001 matter particles and 9,999,999,999 antimatter particles. When the dust settled, all that was left was the tiny amount of residual matter plus lots of the ambidextrous stuff like photons, and everything around us is made from that residual stuff – matter. But, with minor tweaks in the details here or there, antimatter could have been the winner and everything would have been just as normal, and we would have unknowingly labeled them the other way around.
Making antimatter feel more “common”…
Here are a few examples of down-to-earth antimatter situations to give a more practical sense that matter and antimatter are equal partners.
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Carbon-11 has too few neutrons to be radioactively stable. Carbon-14 has too many neutrons. They both want to decay toward stability, but that means opposite sorts of decays for them. 11C decays by emitting a positron (i.e., anti-electron) and 14C decays by emitting an electron. This is nice and tidy and symmetric and mundane, and one side involved an antiparticle.
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Gamma rays are produced in lots of ways. When a gamma ray smacks into something (assuming suitable energy), the most likely thing for the gamma ray to do is to go “poof” and become an electron/positron pair. That is, it makes a particle and an antiparticle, and these are on equal footing in this process. (The electron carries on its merry way. The positron tries to carry on its merry way, but it’s unfortunately moving through matter, so it annihilates with another electron in the material soon enough, producing new photons that carry on their merry way.)
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Cosmic rays are raining down on us all the time. At Earth’s surface, the cosmic ray flux is mostly muons and antimuons. So, you’re hit with antiparticles (and similar particles) all the time.
These are all examples of antimatter and matter both just hanging around being normal, though it’s hard to notice without special equipment. The main point is that antimatter isn’t a mysterious long-lost aspect of the universe that we get to twist to our theoretical whims. It’s just… normal stuff.
So my lay understanding of it first comes to a pause here.
I had understood that theoretically they should be exactly two sides of the same coin, so to speak, but that they are, in certain small yet observable ways, not. For reasons that are not quite understood.
Those differences are hypothesized to be the origin of matter’s excess in our observable universe.
So starting off there, is my understanding in the ball park?
My, apparently ignorant, imagining is not that one is “hidden” or “exotic” other than relative to interaction with the other. They are equally mundane or thrilling depending on your POV. The imagining is that each are, in such n-dimensional space, actually the exact anti versions of each other, if only each was able to fully observe the other, but one has some bits that exist oriented differentially in some of those dimensions to the other. And that some anti particles settle even more into different n-dimensional grooves relative to others. The small asymmetries relative to each other are the result only of what one can observe of the other (excepting of by gravity which can travel through the n-dimensional space …)
I do appreciate your trying to reduce my ignorance here and I will do my best to follow.
There are some very, very tiny differences between some particles and their antiparticles, but still nothing that says “this one is the weird one that should be hidden away in extra dimensions”.
As yet another example, there are some pairs of particles which are antiparticles of each other, such as the pi+ and the pi-, or the W+ and W-, for which we can’t even say which one is the “normal” particle and which is the “antiparticle”. They don’t typically occur in real form in most familiar conditions in nature, and when they are produced as real particles, it’s generally in pairs. And some particles, including the ordinary everyday familiar photon, are their own antiparticle.
And nothing in my imagining would suggest that?
Or would find anything remarkable that some specific matter antimatter pairs do not have bits that protrude differently in n-dimensional space.
Yes. In one small corner of the physics there lies some slight behavioral difference that allows for the massive but not-quite-complete annihilation in the early universe. So far we know of one such source of this sort of behavioral difference, but it’s too slight, so we’re looking for more. For that one insufficient source, though, it works like this:
In the Standard Model, there are different types of quarks. These quarks experience the strong interaction and electromagnetism and gravity all as the quarks that they are. The quarks also experience the so-called weak interaction, but when they do, there is a freedom to blur the lines between which quark is which, and nature has chosen to take advantage of that freedom. So, when the various quarks experience the weak force, they are slightly twisted up with one another in a quantum mechanical way. That twisting up is well-measured and can be encapsulated in a matrix of numbers.
And, when that set of numbers is simplified to its core essence for our matter/antimatter purposes, there is one “summary” number that tells us how much asymmetry there is. That number (called the Jarlskog invariant, for anyone that wants the name) can in principle take any value from -1 to 1. For quarks, the measured number is about +0.000003. So, there’s a little bit of matter/antimatter asymmetry here, but it’s small. (We don’t know why it’s so small.)
In this post, and in the older thread, the speculation seems to be that the missing antimatter isn’t really missing but just hiding; i.e., not speculatively billions of light-years away but speculatively right under our very noses. That’s the thing that crashes right out of the gate.
Your latest post seems to have evolved the speculation in a good way, though, which we can separately address. In particular, given that the above asymmetry (called the charge-parity, or CP, asymmetry) is too small to explain the baryon-dominated universe, folks are looking both experimentally and theoretically for other sources, often in tandem with trying to solve other mysteries (like the fact that the Standard Model doesn’t explain dark matter or gravity at all). Some classes of models do involve extra dimensions and some classes of those extra dimensional models do introduce additional sources of CP symmetry violation. But to be sure, this is to provide a way to annihilate the antimatter in the early universe so as to produce the matter-dominated situation today. It’s not a means of not annihilating it and instead tucking it away.
Oops, miscounted my zeros. Make it +0.00003. Side note: neutrinos get this “blurring of categories” as well, and the equivalent measure of asymmetry for those particles is still poorly constrained and can potentially be 1000x larger than the quarks. (Side side note: I’m sweeping a lot under the rug in how one connects these values to the actual stuff happening in early universe, where temperatures (and thus particle energies) are bonkers high. But the idea that neutrinos may be the necessary extra source of CP violation is a very popular one at the moment.)
Okay.
Yet? And sorry for my persistence.
In those models, which explain CP symmetry violation with extra dimensional models … am I correct that any portions of particles (or complete particles if any existed) not in intersecting planes (so to speak) with each other would be observable to each other only by gravity? Or is that misunderstanding on my part?
That understanding (or misunderstanding) is the crux of my imagining.
IF, and a big if it is, there are extra dimensions, then it is hard for me to think that they would empty of … stuff, and that all of that stuff would be fully aligned in the same “planes” all the time.
If such stuff not in the same planes exists and it is observable to the other only by gravity, then what would the observable universe be predicted to look like to us?
What would be different than what we currently see, if anything, and what could be predicted that could be falsified?
This depends on the specific model and what it’s trying to accomplish. Before I say more, though…
Keep in mind that if a model says that (for instance) an electron can access additional dimensions in some way, then so too can positrons, or vice versa. Even in these models, your basic matter and antimatter particles (i.e., elementary fermions like electrons and quarks) still have a twin-ness from a spacetime perspective. Symmetry violations enter the picture through other means, with extra dimensions providing a platform for building whatever complicated stuff one needs to lead to those violations.
So, back to the previous quote –
There are two diverging topics here. Topic A stems from your query about whether “any portions of particles…not in intersecting planes…would be observable to each other only by gravity?” I want to keep this separate from the original Topic B of matter/antimatter asymmetries, as they are very different things.
For Topic A, it depends. There are a lot of unsolved problems in fundamental physics, and extra dimensions have been invoked to try to solve a good number of them. The most common application does relate to gravity. In particular, it is unknown why gravity is so weak, and people have found lots of ways of hiding gravity in higher dimensions. The simplest mental picture one can get for this style of model is that all our normal particles live in our normal 4D space (3+1 dimensions), as do all of the normal gauge bosons that mediate particle interactions, but the possible graviton lives in the higher dimensional space, and its influence within the 4D space is thereby reduced, making gravity weak. It’s not entirely crazy that gravity would be special here, as we know well that gravity is deeply connected to spacetime and its geometry (n.b., general relativity, spacetime curvature, etc.), but also that trying to introduce gravity in the same way as the other forces runs into immediate trouble anyway.
So, to summarize, in the most common cases, particles live in regular 4D space and gravity lives in (4+n)D space, for some value of n. Having said that, one can also form models where other things live in the higher-dimensional space and have interactions that happen in the higher dimensional space. I can get to the experimental ramifications of this, but that will be a future post.
For Topic B, I need to introduce a couple of things.
First, interactions in the Standard Model aren’t “programmed” into the physics but instead emerge naturally from certain symmetries. You are familiar with some simple symmetries in physics, like the fact that physical laws are symmetric with respect to spatial position or orientation. That is, barring outside influence, I can reorient my enclosed experiment or move it from here to there and my measurements are unchanged.
Second, in the underlying mathematics of the Standard Model, you can introduce the normal particles like electrons/positrons or quarks/antiquarks and have them be non-interacting. That is, they can just swim around in spacetime doing nothing of interest. The particles are described in terms of quantum fields, and these fields (like fields of grass) stretch out everywhere. Quantized wiggles living in these fields are the particles we observe.
So, whence do particle interactions arise? In addition to the simple symmetries like rotational symmetry or spatial-shifting symmetry, one can require that the model respect another certain type of symmetry that has to do with how the quantum fields change under a particular mathematical operation (a “gauge” symmetry). Diving into the details would derail this discussion, but if others want more here, I can walk that topic in parallel.
For our purposes, @DSeid, what’s important is that when you ask the model to obey these gauge symmetries, it happily obliges – but with consequences. One consequence is that you get the corresponding “gauge bosons” (photon, Z, W) appearing. Another is that you get whatever complexities the emergent particle interactions require.
It is in these interaction complexities that things like CP violation show up. Not in the naked particle/antiparticle spacetime behavior but instead in the additional layer that comes about when interactions emerge.
To summarize all that: the particles themselves are boring. Their interactions are interesting. And, it’s in these interactions that matter/antimatter asymmetries arise.
Introducing additional dimensions allows more flexibility in how interactions can emerge. More complicated gauge symmetries in (4+n)D space with more complicated field structure can be considered. When these are squished down to form an effective model in 4D (e.g., to make experimental predictions), it can lead to additional sources of CP violation in the interactions.