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What Is Spacetime, Really?

blog.stephenwolfram.com

111–120 of 122 posts

Re: What Is Spacetime, Really?

#111
post #37

Earlier quoted context omitted.

It seems to me that the fundamental thing that Wolfram is saying is that reality is non-local. Locality itself is an emergent phenomenon. This is his clever way of getting around Bell's Theorem. His networks don't require space to exist, they are space. When we think of spacetime, we think of a manifold, and he's saying that what looks like a manifold at a macro level is really just a discrete network at a micro leve…

But a theory is supposed to model something that actually exists, yes? So for instance when philosophers made the conjecture for the existence of the atom, their theory was either correct or not, on the basis of whether the atom actually existed in nature. So if Wolfram is correct, then there exists at the lowest level of nature a network, upon which everything else follows. Wolfram is trying to describe reality, som…

This is a fundamental philosophical question. The same question can be asked about more traditional physical models, e.g. what is Einstein's spacetime composed of, and how did his equations of general relativity come to be? Are they real, or are they just a model that we use to describe reality?

Over time, physicists have come to take models very seriously (thanks in part to Einstein's 1905 papers). That is, when our model says that electrons act like particles and waves, we assume that electrons really are these odd things that are somewhere in between a particle and a wave.

So, if it turns out that Wolfram's ideas have merit, and there really is an underlying spacetime network which can be used to model and derive all of the known equations of physics (and likely some unknown equations), then at that point we would have to take very seriously the existence of such a network. We would be forced to think of a seemingly continuous spacetime as a special facet or feature of this underlying network, in the same way that we now think of space as a special feature of the more general spacetime.

Re: What Is Spacetime, Really?

#112
post #13
post #7

Earlier quoted context omitted.

I am not a physicist, but Wolfram covers this. "Because there’s a theorem (Bell’s Theorem) that says that unless there’s instantaneous non-local propagation of information, no such “hidden variables” model can reproduce the quantum mechanical results that are observed experimentally." Wolfram's continues by saying he is allowing instantaneous non-local propogation because network nodes have no coordinates; coordinate…

Scott Aaronson argues that, based on what we know about quantum mechanics, Wolfram's "long-range thread" cannot reproduce special relativity and Bell inequality violations: www.scottaaronson.com/papers/nks.ps

It seems to me that Aaronson's argument can be easily bypassed by adding non-local hidden variables in the form of a loosely connected network that essentially injects pseudo-randomness into the approximately flat Minkowski spacetime network. Note that Bell's inequality does not rule out non-local hidden variables as a viable explanation of quantum mechanics.

Re: What Is Spacetime, Really?

#113

Earlier quoted context omitted.

You can impose a one-dimensional ordering on a finite state of any size. Drawing a parabola, for instance, yields a 2-D figure. But the process that draws it (on a computer) is 1-D: Start at the origin Move right X Move up X*X Make a dot If X = Screen.Width Stop X=X+1 Repeat There you go: a 2-D thing from a discrete sequence of 1-D steps. I hope you can see that it generalizes to N-D things.

I think the question here, which has been raised in other parts of the comments here, is what the complexity of the operation to serialize/deserialize is and how one might think about translating between computational complexity and dimensionality.

My contention is that all things are understood one step at a time, and I believe this is influencing theoretical physics in a way that the theoreticians themselves are not fully aware of. Especially Wolfram's theory.

Re: What Is Spacetime, Really?

#114
Here's my attempt at a summary. Wolfram speculates that the universe is a network of nodes and connections. This network changes over time by simple substitution rules. Specific patterns in the network give rise to effects that on the larger scale that we experience as the physical world. He has shown that if you model the universe this way, you can neatly derive special relativity and general relativity. He is also doing a brute-force search through networks to look for one that exhibits properties like our universe.

I'm curious if I got it wrong.

Re: What Is Spacetime, Really?

#115
post #85
post #63

Earlier quoted context omitted.

1: they aren't. They don't need to be, and they cannot be. Simulate the first step of the first machine, then the first step of the second machine, then the second step of the first machine, then the first step of the third machine, then the second of the second, then the third of the first, then the first of the fourth, and so on. Every step of every program/machine is eventually reached. For the same reason that th…

Chaotic functions of continuous real variables are not computable. Computable physics assumes a discrete lattice, which the universe is not proven to be.

Can't find any proof of that claim. So far it seems more likely that all physical systems are computable. Although you may need infinite time to achieve perfect accuracy when computing them in a simulation, but matematically that's still computable.

Computability makes no assumption on the geometry, only on the rules the system follows.

Re: What Is Spacetime, Really?

#116
post #94

Earlier quoted context omitted.

No, I don't think that's it. He is saying that physical reality is literally a network of interconnected nodes at its fundamental level. I think you're still assuming that these graphs have to be embedded in some space to be real. If we get fundamental enough, we can go below geometry, into algebra. For a less abstract example, what's the radius of an electron in modern physics?

These graphs have to be embedded into a computation device to be real and changing in time. To build such a computation device, maybe you need time and space. It could even be that our Universe is of the minimum complexity required to build a computation device capable of computing the graphs that describe our Universe.

> These graphs have to be embedded into a computation device to be real and changing in time.

No, no, no! These graphs are meant to be more fundamental than spacetime. They can change in the sense that there are algebraic rules for stepping them from one state to another, but this step isn't in our time. Our space and time should be generated by operating on these graphs (let's say "stepping" them, as long as we remember that it's not a time-step in our spacetime, but just an algebraic operation).

EDIT> I should say that this whole idea is a model. The question of whether a model is real or not independent of the system being modeled is a philosophical one that we can't really do justice to in an HN comment thread.

Re: What Is Spacetime, Really?

#117
post #7

Maybe a physicist could help me out: the idea here of treating space as a simple discrete network, and particles just being emergent behavior sounds very interesting. But doesn't this violate Bell's Theorem, that no local hidden variables could completely explain quantum mechanics?

I am not a physicist, but Wolfram covers this. "Because there’s a theorem (Bell’s Theorem) that says that unless there’s instantaneous non-local propagation of information, no such “hidden variables” model can reproduce the quantum mechanical results that are observed experimentally." Wolfram's continues by saying he is allowing instantaneous non-local propogation because network nodes have no coordinates; coordinate…

Bell's theorem assumes that spacetime is locally Euclidean so it is not relevant when there are networks and rules that allow for causal connections that appear to be an observer at a distance.

Re: What Is Spacetime, Really?

#118

Maybe a physicist could help me out: the idea here of treating space as a simple discrete network, and particles just being emergent behavior sounds very interesting. But doesn't this violate Bell's Theorem, that no local hidden variables could completely explain quantum mechanics?

Loop Quantum Gravity (LQG) and Causal Dynamical Triangulations (CDT) are two theories of quantum gravity that describe spacetime as arising from a network of sorts. For LQG, spacetime emerges from a spin foam quite elegantly. And for CDT things are also quite elegant. I'm not aware of either violating Bell's Theorem, but quantum gravity isn't my specialty. Also, physicists don't generally take Wolfram's claims about…

I read another Wolfram blog post where he answered those criticisms. http://blog.stephenwolfram.com/2012/05/living-a-paradigm-shi...

I don't think Bell's theorem raises a problem in either LQG and CDT because they have the traditional quantum theory built into them. They are not deterministic theories like Wolfram's. Bell's theorem discounts the possibility of quantum (as it is observed) if there were no long range connections defying locality.

Re: What Is Spacetime, Really?

#119
post #22

Things like this always leave me feeling stupid. How is one supposed to wrap their mind around the idea of space emerging from an underlying "network"? What exactly is being networked; what is the "stuff" of the network? He describes the network in terms of connections, which in my mind requires a topology, or space within which to exist; yet space does not apparently exist at this level. If there is no space, then w…

Wolfram is proposing a radically different way of interpreting the universe. His main argument is that if his cellular automatons can achieve emergent behaviour similar to real physics, then it is evidence for it being the underlying mechanism in the universe. For a (really long) discussion on how this simulation parallels physics, check his book, A New Kind of Science, out. Then maybe you would want to read Aaronson…

More precisely, simple rules can have emergence. Wolfram's most common example is simple cellular automata. Cellular automata do not need to be simple. Other rules like Turing machines can have emergence.
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