The Universe Is Not a Computer by Ken Wharton

Thanks, glad it caught your interest. I suspect we're talking about two very different things, however. Perhaps you're concerned with objects that in principle follow some well-defined dynamical equations, but those equations just happen to be too complex to calculate to the required precision. I'm concerned with global rules that apply on the level of the Lagrangian density and the action, but have no corresponding Newtonian-style equations that always can describe time-evolution from a given state.

Thanks, Michael -- that was the hope!

Hi Peter,

I'm afraid that I still can't find the commonality between our ideas, but thanks for your nice words, and best of luck in the contest.

Dear Ben,

Thank you very much for the thoughtful questions. You're asking a lot of the same questions that I am, as I pursue this research program. Here are some attempts at answers, based on my current thinking. (Numbers match to your questions.)

1) The key question is why the path integral works in the first place. If you have to sum first, and then square, then you're correct: one needs all the paths to be "real". But there's another way to "double" the paths so that you never need to square the sum (Sinha and Sorkin, 1991), in which case one can imagine that the universe is just choosing one of these possibilities (weighted appropriately), and the unchosen paths aren't "real". The problem with this, as noted in that paper, is that if one is taking the particle perspective, then some of those probability weights have to be negative, in order to get interference. BUT -- I currently think that a field-configuration-path view can solve this problem, especially if a similar "doubling" is used. (To see my arguments for fields vs. particles, you can try my previous essay in this series, "quantum theory without quantization".) This is actually my main research focus right now.

2) Feynman's approach does indeed take the Lagrangian that classically yields a real, second-order Euler-Lagrange equation, and (via the path integral) makes equivalent predictions to a complex, first-order Schrodinger equation. (Dropping one order is offset by the real->complex transition.) But I'm not interested in generating NSU-style equations... The question is how to quantize a classical Lagrangian (such as GR's) without ever making such a step in the first place. No, it's not known how to do this yet, and I don't know of anyone else who is really working on it. (I'm looking forward to reading your paper, and hope to get to it soon.)

3) The latter. I mean, look at how beautiful GR is, with a few brilliant principles guiding the whole thing. Then look at the conceptual mess of QM and QFT. Which is more likely to be on the right track, especially when it comes to spacetime? Sure, maybe GR is also incorrect at some level, but it seems to be to be closer to the ultimate truth than QM (again, especially when it comes to spacetime).

4) Well, in any unified theory along the lines that I envision, the metric would also be solved "all at once", along with any matter fields. So an external hypersurface boundary condition on a manifold would generate the structure of the spacetime within that boundary as an effect. Is this giving up traditional causality? Sure. But traditional causality is just another aspect of the NSU. (Also, see the recent piece on my work with Huw Price.)

Maybe one way to think about it is to make an analogy with a high-Q laser cavity that contains a standing-wave EM field. Flipping time and space, the analogy to the NSU view would be that the left mirror causes the field, and the nodes of the field then determine where the right mirror is allowed to be. (Sort of a left-to-right causal order.) The LSU view is that both mirrors determine the allowed modes of the field that can be inside the cavity. Extending the field to include the spacetime metric, this solution would also determine how far apart the mirrors were in the first place. The global solution might then still look like a "left-to-right causal" relationship, but it would be an illusion.

5) Yes, I like the continuum -- and here I'll again point you to my last essay, 'quantum theory without quantization'. If it's not still on the fqxi site, a copy is here. Is this anthropocentric? I don't see how, but maybe it's a blind spot of mine. (Furthermore, I don't like Planck scale arguments for discreteness, because those scales aren't Lorentz invariant. Doubly special relativity doesn't solve the problem, I don't think, without introducing bigger problems at macroscopic scales.)

As far as why we live in 4D, sure, I'd like to know that, but would be content with an ultimate theory that took that as a given. My guess is that once we found an ultimate theory, we could experiment with other dimensionalities and at least narrow down the coherent possibilities. But it seems tough to do that without a working ultimate theory.

Thanks again for some great questions!

Ken

Hi Edwin,

Thanks for the kind words! I'll be looking forward to your comments.

Ah, in that case there might be more overlap than I thought. I'll do some reading and let you know if I find any connections.

Thanks, Peter -- good luck to you as well.

Frank, I'm afraid I don't really follow much of your comment, but good luck developing your ideas. Friedemann is an adjunct faculty member in our department, but he doesn't teach here, and is mostly at SETI. He's working on some very interesting things these days...

Hi Armin,

Yes, the LSU assumes a block universe, but so does the NSU and all of modern physics, as I see it. (With the exception of the measurement problem as applied to the standard quantum state; for more on this telling mismatch you'll have to dig up my entry to the very first FQXi contest.)

Denying a block universe is basically saying that we're parameterizing physics all wrong, and we shouldn't have functions that look like the classical Electric field E(x,y,z,t), but instead things like E(x,y,z,t_clock,t_obs), where t_clock is usual time and t_obs is the time at which one is discussing E. Maybe someone will develop such a physical theory, but until they do, physics assumes a block universe.

As for your second question, it depends on what you mean by "violations in causality". See the recent piece on my work with Huw Price.

Also, I'm not trying to "rationalize" away entanglement -- I'm trying to come up with a spacetime-based LSU model that quantitatively yields all of the same correlations as the standard configuration space version. I don't have it yet, but there are no conceptual show-stoppers. As for whether I'm 'giving up too soon'... I'm actually just exploring an alternate path towards the same goal. I don't think the century-old NSU-approach to quantum phenomena would suffer if a few researchers headed off in a new, promising direction. :-)

If your entry concerns the path integral, I'll put it near the top of my to-read-list... Thanks for the pointer!

Hi Michael,

Yes, lots of points of agreement. But let's take the disagreements one point at a time.

First, you bring up the block universe. This is a red-herring that has little to do with NSU, LSU, or anything directly related. In my mind, the arguments against the block universe (arguments for presentism, or the growing block) are simply are not consistent with SR, GR, or how we parameterize physical events (see my post to Armin above). I know that lots of philosophers (and some physicists) seem to think that presentism or the growing-block is potentially viable, but the burden of proof is on them, not me. Smolin is trying to change physics to add a "now", which is fine, but physics does not currently have a "now". And most of the anti-block arguments sneak in multiple time dimensions in practically every sentence. Even the way you phrased it --" But from this it doesn't follow that the future is equally real or 'already there' ", is using the word "already" that treats time as a subject while using the word "future" that treats time as an object. Those are two different times (the time at which the future is discussed, and the time of the future in question), ergo one needs a model with two time dimensions to even have a physics-based discussion about presentism.

I'm confused about your sentence that reads: " As far as we can see, someone could (and they do) still believe in NSU/reject block universe and adopt all your other suggestions." This baffles me, and leads me to think I didn't write a very good essay, given that my primary suggestion is to reject NSU. And while I maintain that ordinary NSU physics implies a block universe, LSU *certainly* implies a block universe. What do future boundary conditions mean to someone who doesn't think the future is "real"? Who is "someone"?

For your next big point, I grant that I have no detailed model, just some of the pieces. You guys are way ahead of me on that front. (Actually, I have a beautifully simple model (hinted at the end of my essay), but I'm having a devilish time working out the math, so I don't yet know if it generates the right probabilities.) I also would like to see how my (future) model deals with all these things, but it's always nice to map out potential show-stoppers before one gets that far. In my experience, all of the proposed show-stoppers come from people implicitly assuming NSU without realizing it. Therefore I wrote this essay to help lay out the fact that NSU is a pervasive assumption, not a necessary logical position.

You write: " you still assume there would always be a 3+1 type explanation of a NM sort between the measurements, you just have to work backwards to get it.", but that's not quite right. In an LSU model, once you work backward to fill in the 4D spacetime, you can translate that 4D description to a 3+1D *description* of what is going on between measurements. But I do not think it will be "Newtonian", in that it will not always be a solution to some master differential equation , and I do not think it will be an "explanation", just a movie-like description of what actually happened. For an *explanation*, one will have to consider the whole 4D-LSU picture. Hopefully this makes it clearer how much difference there is between the NSU and the sort of LSU that I envision.

You end that part with "Also notice that the use of future boundary conditions doesn't require that QM systems have definite worldlines and the only reason to assume they must is some sort of NSU prejudice." First of all, you know I'm looking for field descriptions, not particle descriptions, so "worldlines" is a bit misleading. But I'm not sure what you're getting at, exactly... asking the universe to have some particular reality between measurements doesn't seem NSU or LSU, just *realist*. Again, that intermediate solution need not, in general, solve a differential equation.

Finally, you seem to imply that your RBW gives up "more of the NSU-type" picture than I want to myself, but I think you're conflating much of the realism of standard classical physics with the Newtonian Schema, as defined in my essay. Classical physics works on a continuum; does this make the continuum in any way part of the Newtonian Schema? No -- see the definition of NSU in section I. (After all, my main example of the NS is a computer, which are typically digital!) I grant that RBW gives up more of standard physics than I want to, but let's not throw out perfectly valid concepts unless we need to. What quantum phenomena force a spacetime-realist to give up is the NSU, not spacetime itself. Maybe spacetime will have to go as well, but I'm never going to make that leap without first undertaking a much better exploration of the spacetime-realist-LSU landscape.

Best, Ken

You certainly don't have to be friendly to GR or SR -- many quantum theorists take the same view.

For me, I guess I just have too much respect for Einstein's hard-won insights, and too little respect for our human intuitions about time.