General Relativity & Gravity

Earlier on, we talked about how curvature of space can arise in our models. But at that point we were just talking about “empty space”. Now we can go back and also talk about how curvature interacts with mass and energy in space.

In our earlier discussion, we talked about constructing spherical balls by starting at some point in the hypergraph, and then following all possible sequences of?rconnections. But now we can do something directly analogous in the causal graph: start at some point, and follow possible sequences of?t?connections. There’s quite a bit of?mathematical trickiness, but essentially this gets us “volumes of light cones”.

If space is effectively?d-dimensional, then to a first approximation this volume will grow like?t^(d+1). But like in the spatial case, there’s a correction term, this time proportional to the so-called Ricci tensor Rμv. (The?actual expression?is roughly?

?where the?ti?are timelike vectors, etc.)

OK, but we also know something else about what is supposed to be inside our light cones: not only are there “background connections” that maintain the structure of space, there are also “additional” causal edges that are associated with energy, momentum and mass. And in the limit of a large causal graph, we can identify the density of these with the so-called energy-momentum tensor Tμv. So in the end we have two contributions to the “volumes” of our light cones: one from “pure curvature” and one from energy-momentum.

Again, there’s?some math involved. But the main thing is to think about the limit when we’re looking at a very large causal graph. What needs to be true for us to have?d-dimensional space, as opposed to something much wilder? This puts a constraint on the growth rates of our light cone volumes, and when one works everything out, it implies that the following equation must hold:

But this is exactly?Einstein’s equation?for the curvature of space when matter with a certain energy-momentum is present. We’re glossing over?lots of details?here. But it’s still, in my view, quite spectacular: from the basic structure of our very simple models, we’re able to derive a fundamental result in physics: the equation that?for more than a hundred years?has passed every test in describing the operation of gravity.

There’s a footnote here. The equation we’ve just given is without a so-called cosmological term. And how that works is bound up with the question of what the zero of energy is, which in our model relates to what features of the evolving hypergraph just have to do with the “maintenance of space”, and what have to do with “things in space” (like matter).

In existing physics, there’s an expectation that even in the “vacuum” there’s actually a formally?infinite density of pairs?of virtual particles associated with quantum mechanics. Essentially what’s happening is that there are always pairs of particles and antiparticles being created, that annihilate quickly, but that in aggregate contribute a huge effective energy density. We’ll discuss how this relates to quantum mechanics in our models later. But for now let’s just recall that particles (like electrons) in our models basically correspond to locally stable structures in the hypergraph.

But when we think about how “space is maintained” it’s basically through all sorts of seemingly random updating events in the hypergraph. But in existing physics (or, specifically, quantum field theory) we’re basically expected to analyze everything in terms of (virtual) particles. So if we try to do that with all these random updating events, it’s not surprising that we end up saying that there are these infinite collections of things going on. (Yes, this can be made much more precise; I’m just giving an outline here.)

But as soon as we say this, there is an immediate problem: we’re saying that there’s a formally infinite—or at least huge—energy density that must exist everywhere in the universe. But if we then apply Einstein’s equation, we’ll conclude that this must produce enough curvature to basically curl the universe up into a tiny ball.

One way to get out of this is to introduce a so-called cosmological term, that’s just an extra term in the Einstein equations, and then posit that this term is sized so as to exactly cancel (yes, to perhaps one part in 1060?or more) the energy density from virtual particles. It’s certainly not a pretty solution.

But in our models, the situation is quite different. It’s not that we have virtual particles “in space”, that are having an effect on space. It’s that the same stuff that corresponds to the virtual particles is actually “making the space”, and maintaining its structure. Of course, there are lots of details about this—which no doubt depend on the particular underlying rule. But the point is that there’s?no longer a huge mysteryabout why “vacuum energy” doesn’t basically destroy our universe: in effect, it’s because it’s what’s making our universe.

Black Holes, Singularities, etc.

One of the big predictions of general relativity is the existence of?black holes. So how do things like that work in our models? Actually, it’s rather straightforward. The defining feature of a black hole is the existence of an event horizon: a boundary that light signals can’t cross, and where in effect causal connection is broken.

In our models, we can explicitly see that?happen in the causal graph. Here’s an example:

At the beginning, everything is causally connected. But at some point the causal graph splits—and there’s an event horizon. Events happening on one side can’t influence ones on the other, and so on. And that’s how a region of the universe can “causally break off” to?form something like a black hole.

But actually, in our models, the “breaking off” can be even more extreme. Not only can the causal graph split; the spatial hypergraph can actually throw off?disconnected pieces—each of which in effect forms a whole “separate universe”:

By the way, it’s interesting to look at what happens to the?foliations observers makewhen there’s an event horizon. Causal invariance says that paths in the causal graph that diverge should always eventually merge. But if the paths go into different disconnected pieces of the causal graph, that can’t ever happen. So how does an observer deal with that? Well, basically they have to “freeze time”. They have to have a foliation where successive time slices just pile up, and never enter the disconnected pieces.

It’s just like what happens in general relativity. To an observer far from the black hole, it’ll seem to take an infinite time for anything to fall into the black hole. For now, this is just a phenomenon associated with the structure of space. But later we’ll see that it’s also the direct analog of something completely different: the process of measurement in quantum mechanics.

Coming back to gravity: we can ask questions not only about event horizons, but also about actual singularities in spacetime. In our models, these are places where lots of paths in a causal graph converge to a single point. And in our models, we can immediately study questions like whether there’s?always an event horizon?associated with any singularity (the “cosmic censorship hypothesis”).

We can ask about other strange phenomena from general relativity. For example, there are?closed timelike curves, sometimes viewed as allowing time travel. In our models, closed timelike curves are inconsistent with causal invariance. But we can certainly invent rules that produce them. Here’s an example:

We start from one “initial” state in this multiway system. But as we go forward we can enter a loop where we repeatedly visit the same state. And this loop also occurs in the causal graph. We think we’re “going forward in time”. But actually we’re just in a loop, repeatedly returning to the same state. And if we tried to make a foliation where we could describe time as always advancing, we just wouldn’t be able to do it.

Cosmology

In our model, the universe can start as a tiny hypergraph—perhaps a single self-loop. But then—as the rule gets applied—it progressively expands. With some particularly simple rules, the total size of the hypergraph has to just uniformly increase; with others it can fluctuate.

But even if the size of the hypergraph is always increasing, that doesn’t mean we’d necessarily notice. It could be that essentially everything we can see just expands too—so in effect the granularity of space is just getting finer and finer. This would be an interesting resolution to the?age-old debate about whether the universe is discrete or continuous. Yes, it’s structurally discrete, but the scale of discreteness relative to our scale is always getting smaller and smaller. And if this happens fast enough, we’d never be able to “see the discreteness”—because every time we tried to measure it, the universe would effectively have subdivided before we got the result. (Somehow it’d be like the ultimate calculus?epsilon-delta proof: you challenge the universe with an epsilon, and before you can get the result, the universe has made a smaller delta.)

There are some other strange possibilities too. Like that the whole hypergraph for the universe is always expanding, but pieces are continually “breaking off”, effectively forming black holes of different sizes, and allowing the “main component” of the universe to vary in size.

But regardless of how this kind of expansion works in our universe today, it’s clear that if the universe started with a single self-loop, it had to do a lot of expanding, at least early on. And here there’s an interesting possibility that’s relevant for understanding cosmology.

Just because our current universe exhibits three-dimensional space, in our models there’s no?reason to think that the early universe necessarily also did. There are very different things that can happen in our models:

In the first example here, different parts of space effectively separate into non-communicating “black hole” tree branches. In the second example, we have something like ordinary—in this case 2-dimensional—space. But in the third example, space is in a sense very connected. If we work out the volume of a spherical ball, it won’t grow like?rd; it’ll grow exponentially with?r?(e.g. like 2r).

If we look at the causal graph, we’ll see that you can effectively “go everywhere in space”, or affect every event, very quickly. It’d be as if the speed of light is infinite. But really it’s because space is effectively infinite dimensional.

In typical cosmology, it’s been quite mysterious how different parts of the early universe managed to “communicate” with each other, for example, to smooth out perturbations. But if the universe starts effectively infinite-dimensional, and only later “relaxes” to being finite-dimensional, that’s no longer a mystery.

So, OK, what might we see in the universe today that would reflect what happened extremely early in its history? The fact that our models deterministically generate behavior that seems for all practical purposes random means that we can expect that most features of the initial conditions or very early stages of the universe?will quickly be “encrypted”, and effectively not reconstructable.

But it’s just conceivable that something like a breaking of symmetry associated with the first few hypergraphs might somehow survive. And that suggests the bizarre possibility that—just maybe—something like the angular structure of the?cosmic microwave background?or the very large-scale distribution of galaxies might reflect the discrete structure of the very early universe. Or, in other words, it’s just conceivable that what amounts to the rule for the universe is, in effect, painted across the whole sky. I think this is extremely unlikely, but it’d certainly be an amazing thing if the universe were “self-documenting” that way.

Elementary Particles—Old and New

We’ve talked several times about particles like electrons. In current physics theories, the various (truly)?elementary particles—the quarks, the leptons (electron, muon, neutrinos, etc.), the gauge bosons, the Higgs—are all assumed to intrinsically be point particles, of zero size. In our models, that’s not how it works. The particles are all effectively “l(fā)ittle lumps of space” that have various special properties.

My guess is that the precise list of what particles exist will be something that’s specific to a particular underlying rule. In cellular automata, for example, we’re?used to seeing complicated sets of possible localized structures?arise:

In our hypergraphs, the?picture will inevitably be somewhat different. The “core feature” of each particle will be some kind of locally stable structure in the hypergraph (a simple analogy might be that it’s a?lump of nonplanarity in an otherwise planar graph). But then there’ll be lots of causal edges associated with the particle, defining its particular energy and momentum.

Still, the “core feature” of the particles will presumably define things like their charge, quantum numbers, and perhaps spin—and the fact that these things are observed to occur in discrete units may reflect the fact that it’s a small piece of hypergraph that’s involved in defining them.

It’s not easy to know what the?actual scale of discreteness in space might be?in our models. But a possible (though potentially unreliable) estimate might be that the “elementary length” is around 10–93?meters. (Note that that’s very small compared to the Planck length ~10–35?meters that arises essentially from dimensional analysis.) And with this elementary length, the radius of the electron might be 10–81?meters. Tiny, but not zero. (Note that?current experiments only tell us?that the size of the electron is less than about 10–22?meters.)

One feature of our models is that there should be a “quantum of mass”—a discrete amount that all masses, for example of particles, are multiples of. With our estimate for the elementary length, this quantum of mass would be small, perhaps 10–30, or 1036?times smaller than the mass of the electron.

And this raises an intriguing possibility. Perhaps the particles—like electrons—that we currently know about are the “big ones”. (With our estimates, an electron would have 10^35?hypergraph elements in it.) And maybe there are some much smaller, and much lighter ones. At least relative to the particles we currently know, such particles would have few hypergraph elements in them—so I’m referring to them as “oligons” (after the Greek word??λιγο??for “few”).

What properties would these oligons have? They’d probably interact very very weakly with other things in the universe. Most likely lots of oligons would have been produced in the very early universe, but with their very weak interactions, they’d soon “drop out of thermal equilibrium”, and be?left in large numbers as relics—with energies that become progressively lower as the universe expands around them.

So where might oligons be now? Even though their other interactions would likely be exceptionally weak, they’d still be subject to gravity. And if their energies end up being low enough, they’d basically collect in gravity wells around the universe—which means in and around galaxies.

And that’s interesting—because right now there’s quite a mystery about the amount of mass seen in galaxies. There appears to be a lot of “dark matter” that we can’t see but that has gravitational effects. Well, maybe it’s oligons. Maybe even lots of different kinds of oligons: a whole shadow physics of much lighter particles.