Debugging the Quantum Physics Sequence

He very carefully wrote this post to avoid his real objections and instead come across as a neutral-point-of-view expert. I found this disingenuous, and I think that the context might be especially helpful to readers who haven't been around for all that long.

What do you mean by the technical challenge posed by relativity? As far as I can tell relativistic quantum mechanics is unusually satisfactory and uncontroversial.

Edit: Based on the responses, it appears I have confused Copernicus and Kepler pretty badly.

To the extent that "Copernicus is better than Ptolemy" does not pay rent in anticipated experience, I stand by my position that rationalists have no reason to assess the probability that the sentence is true.

Can you expand further? Because my impression was that "Copernicus is better than Ptolemy" pays rent pretty much immediately. Ptolemy can only keep up when there are no new observations. But the moment new observations occur, Copernicus says "Oval shaped orbits," while Ptolemy needs to explain why it never noticed the need for another epicycle.

In short, the need to avoid privileging the hypothesis is distinct from the possible desirability of simpler theories. Or as Karl Popper might say, "Falsifiability, baby."

I'm not sure that the project is worth the effort, since it seems clear to me that the whole QM sequence contradicts the central point of "Making Beliefs Pay Rent."

Even assuming all the physics in the QM sequence were perfect, I see no value comparing MWI to Copenhagen unless the difference matters somehow. That is, if the sentence "MWI is less wrong than Copenhagen" does not pay rent in anticipated experience, I'm unconvinced it is rational to have an opinion about the probability of that sentence. And if the sentence does pay rent in anticipated experience, why are we even thinking about which theory is more complex?

Until that issue can be resolved, searching for a particular scientific dispute to use as a case study is putting the cart before the horse.

Feel free to elaborate.

If I'm understanding this correctly, this implies that TI needs a "transaction eater" analogous to the "world eater" in the Collapse theory. This seems to be a liability of TI that isn't matched by any liability of MWI.

Yeah, it does require something like that. Proponents of the TI claim that ultimately every interpretation that actually accounts for the success of the Born rule will have to include some such component, and so MWI advocates are deluding themselves if they think they can avoid it.

There is a version of the TI (called the possibilist TI), where transactions aren't actually "eaten", in the sense that all possible transaction really occur in some possible world (and these possible worlds are real, they exist). But only one of these transaction occurs in the actual world. As for how to make sense of this, your guess is as good as mine.

After some previous disappointments, my probability that this paper answers "No" to the above questions is too small to try to read yet another one. The more so as the author is obviously taking as burdens things that physics clearly permits, like bifurcating minds (which can be done with uploads on computers, never mind MWI). Have you read it and can you confirm a "No" to all the antimagic questions?

What do you think about Relational QM? That's where I'd put most of the single-world, comprehensible-to-this-layman probability. It doesn't seem to require faster-than-light influence on a real particle or any obvious non-locality.

RQM as an alternative to MWI seems to just assert that if we take correlations as fundamental, we find that only one history holds together logically. I do not expect this to hold, because if it did then I'd need a reason not to expect a proof that MWI is impossible. But perhaps if I understood the topic better I would find it unfair to demand such a proof.

The framework of Wheeler-Feynman theory is just classical Maxwell electrodynamics with waves that converge on a charged particle as well as waves that spread from a charged particle. So it ought to be just as relativistic and local and deterministic as it usually is, except that now you're interested in solutions that have two oppositely directed arrows of time, rather than just one. (Remember that the equations themselves are time-symmetric, so "emission" of radiation can, in principle, run in either direction.)

In practice, they artificially hacked with the theory to remove self-interactions of particles (particle absorbing its own emissions at a later or earlier time), because that produced incalculable infinite forces; but then they were unable to account for the Lamb shift, which does come from self-interaction; and then somehow Feynman made the leap to path integrals, and in the quantum framework they could deal with the infinities of self-interaction through renormalization.

It may seem like a big leap from the classical to the quantum picture. But classical dynamics can be expressed as wave motion in configuration space via the Hamilton-Jacobi equation, and it's not a big step from the HJE to quantum mechanics. Also, doing anything practical with path integrals usually involves working with classical solutions to the equation of motion, which in the quantum theory have high amplitude, and then looking at corrections which come from neighboring histories.

It's quite conceivable that these quantum deviations from classicality may result from the interference of forward causality and retrocausality. Maybe the Wheeler-Feynman theory just needs some extra ingredient, like micro time loops from general relativity, in order to become consistent. We would be dealing with a single-world model which is locally causal but not globally causal, in the sense that the future would also be shaped by the distribution of micro time loops, and that's not determined by its current state. Our world would be one of an ensemble of self-contained, globally consistent "classical" histories, and the quantum probability calculus (including the Born rule) would just turn out to be how to do probability theory in a world where influences come from the future as well as from the past. For example, the Aharanov "two-state-vector formalism" might show up as the way to do statistical mechanics if you know yourself to be living in such an ensemble. There would be no ontological superpositions. Wavefunctions would just be "probability distributions with a future component".

The status of these speculations is remarkably similar to the status of many worlds. The construction of an exact theory along these lines, with a clear explanation of how it connects to reality, remains elusive, but you can assemble suggestive facts from the quantum formalism to make it plausible, and there is a long tradition of people trying to make it work, one way or another: Wheeler and Feynman, John Cramer, Yakir Aharonov.

Practical QM contains the dualism of wavefunctions and classical observables. Many worlds reifies just the wavefunction and tries to find the observables in it. Retrocausality just keeps the classical part and tries to explain the wavefunction as something to do with forwards and backwards causality. Bohmian mechanics keeps the wavefunction and then fleshes out the classical part in a way governed by the wavefunction. Nomological Bohmian mechanics keeps the classical part of Bohmian mechanics, and replaces the wavefunction with an additional nonlocal potential in the classical equations of motion. If you could obtain that nonlocal potential from a local retrocausal theory, you would finally have an exact, single-world, deterministic explanation of quantum mechanics.

Which ones are not actual properties of the collapse interpretation?

I don't think Eliezer has suggested they were properties of all possible non-Everett interpretations.

Right, I just wanted to underscore that no QM is required at any point (the point I mentioned before, but it never got any attention from EY).

there's no reason in the QM sequence to suspect that simulated humans would not be able to think or act rationally. The phenomenon of rationality is well-screened from the implementation layer of the universe.

My point exactly, QM (and hence the QM sequence) is not needed for rationality training.

Sim-EY would be correct about that implementation layer of his universe that screens off all the lower levels. That is, his statements about the universe are true of the simulation, which, as far as he can tell, is all there is. His words have valid referents. And indeed, MWI would be correct in that domain.

You lost me with the last statement. MWI would be correct in what domain? The simulated one? Hardly, it's all classical and digital.

The concept of a modal interpretation is even vaguer in its implications than many-worlds and retrocausal interpretations. The only unifying concept seems to be that other worlds exist in the discourse, but they are purely counterfactual and play no role in explaining anything that happens in the one actual world.

I don't agree that the concept of a modal interpretation is vague. The basic concept is that a quantum system can have a physical property with a definite value without its wavefunction necessarily having to be in an eigenstate of the corresponding operator. So the eigenstate-eigenvalue link is not bidirectional. That's basically it.

The only vagueness is that there are then multiple interpretations, each of which assigns different properties as the ones which have the definite values. So which properties does the system have then, and how can we tell?

There are various theorems ("Hardy theorems") proving that an ontological theory can't assign definite values to all observables in all quantum states, such that all QM expectations are satisfied simultaneously.

I think you mean Kochen-Specker theorem here (and similar results going back to Gleason's theorem)? The system can't have definite (non-contextual) values of all operators at once, because the operators don't commute. Particular interpretations build a maximal set of properties which the system can have at once. Hardy's theorem seems to be related to whether the properties can be Lorentz invariant or not.

As you say, Bohmian mechanics is one of the interpretations (based on assigning definite position states to all particles at all times), but perhaps is not the most plausible one, since the choice of the position operator as the "preferred" operator is "put in" by hand. Other interpretations try to allow the preferred operators to "drop out" of the wave function (via measurement events or other decoherence events) rather than being "put in" and are in that sense simpler (fewer assumptions) and more plausible.

Did you read the link? Most of the recent modal interpretations are based heavily on decoherence (between different branches of the wave function). So, no, the possible worlds don't "interfere" with each other.

There are different ways of splitting the wave function into "worlds" - this is why there are several modal interpretations, not just one of them. This ambiguity is a more powerful criticism by the way (exactly what are the worlds, and why should we believe one particular split rather than the others?)

In any case, I think you missed the point here. Whether the modal interpretations are right or not, the problem is that they are just not discussed in the sequence. Instead, the sequence contains a colossal non-sequitur from "wave function collapse is a silly theory" to "so all these many worlds really exist".

Ah, I must have seen that. Non-disjunctive then. Thanks.

Thanks - I've amended the final paragraph to change 'view' to 'outcome' throughout - hope it helps.

I have a problem with your Possibilist TI that I also had with original TI, and with almost every ontological interpretation except for Bohmian mechanics - I can't figure out what the ontology is; nor even what the mathematical object is, that represents reality in the theory.

If Einstein had had his way, reality would have been described by classical fields on a manifold. Mathematically the universe would be represented by some particular exact solution of the equations of motion. Even given that, we could still ask the ontological questions like, what is a property, what is a causal relation and why does it necessitate anything, and so on; but at least the mathematics would be clear.

Quantum mechanics also has a certain clarity, if you resolutely regard it as not ontological, but just as an algorithm for making predictions. The observables are what's real, but they are an incomplete description of reality, and wavefunctions etc are a recipe for making predictions, whose reasons for working are unknown and remain to be discovered.

A peculiar laxity regarding the notion of reality, and regarding what counts as an adequate specification of an ontological theory, entered physics when people started trying to regard quantum mechanics as a complete theory of reality, rather than an incomplete one; and many ontological interpretations have inherited some of these lax attitudes, even as they try to restore objectivity to physical ontology. At least, this is how I explain to myself the oddities that I keep encountering in the literature on quantum foundations.

I will give another example of an ontological interpretation whose mathematical basis I think is clear - and it's a "back-and-forth-in-time" theory like TI: Mark Hadley's idea that QM arises from subatomic time loops. Hadley's ontology is like Einstein's, fields on a manifold, but the difference is that the manifold is non-orientable, it's full of little time loops, and quantum mechanics is supposed to arise from the global consistency constraints imposed by the coexistence of innumerable coexisting causal loops. The idea may or may not work, but at least the mathematical starting point is clear.

One more example of non-clarity before I turn to TI: MWI. MWI says that reality consists of one big wavefunction or state vector - OK, that much is clear. The non-clarity in this case comes when you ask, what parts of the wavefunction or state vector correspond to observable reality? Are the "worlds" the components of the wavefunction, when decomposed in a special basis? Or do all possible basis decompositions produce another, equally real set of worlds? Etc., lots of questions which have been raised many times on this site.

Now to TI. Let me give an example of an ontological claim that might have been made about TI, which would have provided a clear starting point. It could have been claimed that what exists are particles and fields. The particles trace out world-lines, the fields do their thing. And then the TI claim could have been, that the fields can be decomposed, in some specific way, into a particular set of advanced waves and retarded waves, which can be arranged into the "pseudo-time sequence" making up a "transaction".

That sounds like a clear starting point to me. And then the challenge would be to go into the details - describe how the decomposition works, and explain why the quantum formalism is the appropriate and correct way to compute probabilities in this world where influences are going back and forth in time "simultaneously".

That is not what I found in John Cramer. Instead, his only visible mathematical foundation is just, the usual quantum formalism. Then he has a few specific physical setups, where he tries to communicate the gist of the TI way of thinking. Also, as I recall, there is a path integral formalism in which advanced and retarded waves appear.

At this point, as a "philosophy of QM", TI appears structurally very similar to CI. The math is still just the same quantum formalism, perhaps amended to include advanced waves in the path integral. There is no clear mathematical description of the ontological part of the theory. Instead, there is just a way of thinking and a way of talking about the traditional quantum formalism. In CI, it's Bohr going on about complementarity and the uncertainty principle, in TI, it's Cramer going on about pseudotime sequences.

I have not yet seen your book, but so far, I don't find, in Possibilist TI, an improvement on this situation. Instead, there seems to be just an extra layer to the talking, in which "possibilities" are ascribed an important role. It's a little odd that something nonexistent should matter so much for the understanding of that which exists, but I can let that go, it's not my main concern. My main concern is just - what is the mathematical object, that corresponds to reality? I've already given two examples of theories where there is no mystery at all about what that is - fields on a manifold, and fields on a nonorientable manifold. I've also given a clear example of a theory that does not attempt to be ontologically complete, namely, QM with observables regarded as real, and wavefunctions regarded as not real.

What I would like to know is just, what sort of mathematical object describes the actual part of Possibilist TI ontology? Is it a definite history of particles and fields, which then gets ontologically analyzed in a certain way (and perhaps that is where the "possibilities" come in)? If I open your arxiv paper, I see kets, propagators, quantum fields, squared amplitudes, and a whole pile of stuff which just looks like standard quantum formalism. So it looks like you have produced just another way of talking about the quantum formalism, rather than a clear ontology whose objects can be specified with mathematical exactness. Please prove me wrong, and show me the part where you just say "These are the entities that exist, and these are the states they can have." :-)

If not everyone can agree on what "obvious" means then it is wrong to use the word in an argument.

I agree wholeheartedly.

In the wonderful shiny future when the sequences are finally compiled into something.

I think we all agree that some form of Occam's razor makes sense

Yes.

and that it can become a quantifiable criterion if you pose it in terms of "how many bits does it take to specify this hypothesis as a predictive algorithm?"

That's a much stronger claim:

• It's unclear whether this is the proper way of formalizing Occam's razor.
• Even if it was, it is an uncomputable approach, and, as far as I know, there aren't even reasonably methods to approximate it in the general case.

You don't smuggle it in as an ansatz.

You apply the generalized anti-zombie principle. There was a reason he went there first.

The position of mainstream specialists is that MWI is plausible, but there is no compelling evidence or theoretical arguments to conclusively decide in favour of one particular interpretation, and they have personal preferences largely based on intuitive appeal. Others go further and claim that the whole QM interpretation issue is meaningless and scientifically improper.

Therefore, there is no consensus on the issue in the mainstream scientific community.

Yudkowsky attempted to resolve the issue once and for all, and I think it's uncontroversial to say that he objectively failed.

Also, I think the E.Y.'s epistemology is sound

Most of its elements (empiricism, bayesian inference, etc.) are sound and essentially uncontroversial. Some elements specific to his version ("informal" Solomonoff Induction and Kolmogorov complexity) are more questionable.