Sean Carroll’s Something Deeply Hidden

Cover of Something Deeply HiddenI’m just about finished reading Sean Carroll’s Something Deeply Hidden.  I was going to wait to post this until I’d completely finished, but all I’ve got left is the appendix, I perceive that I’ve gotten through the main points, and discussion on the previous post is veering in this direction.

As widely reported, Carroll is an advocate for the Everettian interpretation of quantum mechanics, generally known as the Many Worlds Interpretation (MWI).  I gave a primer on this back in December.  Nothing in Carroll’s book invalidated that description, so if you need the basics, check it out.

Carroll’s broad point is the MWI, in terms of the mathematical postulates, is the most austere interpretation.  Its central premise is that we should ask what happens if quantum systems evolve solely based on the Schrodinger equation.  Doing so leads to a deterministic theory that explain our observations and preserves realism and locality, which makes it broadly compatible with special and general relativity.

The basic idea is that the wave function never collapses, it just becomes entangled with the waves of other quantum systems.  We see this in experiments, where particles that interact don’t experience collapse, but merely become entangled.  And physicists have been able to isolate ever larger molecules and keep them in states of quantum superposition.

The Copenhagen interpretation posits that eventually interaction with macroscopic objects causes the wave function to collapse.  But what exactly is a macroscopic object?  At what point between large molecules and measuring devices do we cross the boundary that causes wave function collapse?

The Everettian view is, never.  The superposition of the initial quantum system never collapses, it continues spreading.  We perceive it to have collapsed, but that’s only because it’s spread to us, and we’ve become the version of us looking at one particular outcome, with other versions in other branches of the wave function looking at the other outcomes.

One thing I was hoping to get from Carroll was a description of how the MWI avoids the issues with Bell’s theorem.  That theorem notes that if all the states of entangled quantum particles are set when they become entangled, the statistics of various outcomes will be constrained in a way that they won’t be if the states aren’t set until they’re measured.

Numerous experiments have verified that the statistics match the values not being set until the measurement.  This is an issue for Copenhagen and other interpretations because under them, in order for the entangled relationships to hold, the measurement results have to be communicated to the other particle in some sort of faster than light communication, violating locality, Einstein’s spooky action at a distance.  (But as I noted in the previous post, not in any way that’s actually useful.)

The main thing Carroll says about this is what I’ve seen from other sources.  Bell’s theorem assumes a definite outcome to the measurement.  But since in MWI there are no definite outcomes, every outcome is realized, the theorem doesn’t apply.

(Maybe under MWI, the statistics aren’t constrained in any one branch of the wave function because the outcomes are spread over all the branches, but that’s my speculation.)

Another question I hoped to see an answer to is how much branching a quantum interaction causes and at what resolution?  For a binary result, like the direction of spin, the answer should be two.  But the location of a particle is spread out in a wave, and elementary particles are points in space.  How many branches does that wave result in?  Carroll admits that we don’t know.  We just don’t know how granular the universe is.  He notes that Everett himself considered it plausible that the number might be infinite, which I can’t see as a strength of the theory.

Another question is whether the branching happens all at once, an entire universe instantly created, or gradually from the interaction outward.  My primer post and description above inherently assumes that the answer is gradual.  But Carroll states that it can be accounted for both ways.  It’s hard for me to see how creating an entire universe billions of light years wide, all at once, preserves locality, so I’m not sure how this can be true, except perhaps in an instrumental manner.  But an appeal to instrumentalism here seems wrong since the whole point of the MWI is to find the reality behind the observations.

Carroll also briefly considers the question a lot of people wonder about, if reality is branching like this, where are all these branches located?  Carroll’s reply is that the branches aren’t “located” anywhere.  They’re all right here, just not in a way that they can interact with each other.  While I think I understand this point, it’s undoubtedly one that a lot of people struggle with, and I don’t think Carroll addresses it sufficiently.

Another question is the conservation of energy.  Where does the energy for all these different branches come from?  Carroll’s answer is that the original energy of the universe is constantly being spread out among the branches, albeit not evenly, but according the amplitude of each outcome in the wave function.

But here is where the infinite branching point above becomes more problematic.  If the energy is being diluted throughout the branches, and there are an infinite number of those branches, does that mean the energy started out as infinite?  These two answers don’t seem to fit together.

One point Carroll does make clear, our personal decisions do not cause branching.  The branching is caused by quantum events.  Of course, in some of those branches, we might make different decisions, so the effect might be that there are universes with us making varying decisions, but it isn’t guaranteed.  Indeed, I tend to think that most branches will look identical at the macroscopic level, with the differences only being at the microscopic quantum level.

Carroll also discusses what you need to do with Everettian physics if you don’t want the other branches, the many worlds.  You have to add something to the formalism to accomplish this.  He looks at the deBroglie-Bohm pilot-wave theory, which although older than the MWI, could be seen as Everettian physics plus a particle that reifies one of the branches as the real one.  He also looks at GRW, an interesting premise that maybe quantum systems simply spontaneously and randomly collapse, but since they do so very rarely, we usually only see the collapse in association with large macroscopic systems.

The main thing to understand is that each of these additions come with a cost.  With pilot-wave, it’s explicit non-locality.  With GRW, it’s a largely ad-hoc premise whose only purpose is to ban the other worlds.  He discusses other alternatives with similar issues.

Carroll doesn’t address it, but a common move in the physics community is to accept Everettian physics itself, but simply say that the other worlds aren’t there.   This is known as the unreal version of the interpretation.  My issue with this move, aside from the absence of any explanation for why only one of the branches is the real one, is that it reminds me of the Tychonic System.

In the decades between when Copernicus proposed the sun centered model of the solar system and Galileo was able to produce empirical observations that supported it, Tycho Brahe proposed a compromise model, where a lot of the planets orbited the sun, but the sun continued to orbit the Earth.  It was an attempt to get the benefits of the elegant mathematics of the Copernican model, while preserving the “philosophical benefits” of the Earth centered Ptolemaic system.  Today it’s obvious that this was a misguided attempt to save appearances, but it wasn’t obvious in the late 16th century.

Along similar lines, Chad Orzel recommends that we not consider the other worlds as real, but merely as metaphors, accounting devices.  This is also reminiscent of one of the most common moves during the 16th century, to say that Copernicus’ crazy claim that the Earth moves shouldn’t be taken literally.  It should be regarded merely as a mathematical convenience.  Max Planck made a similar move when he first introduced quanta into his calculations.   The claim is, not to worry, it’s not like this crazy thing is real; it’s just an accounting gimmick.

I don’t know whether the MWI is reality or not.  As I’ve noted many times, I think it’s a candidate for reality.  But if you find the mathematics of the Everettian view elegant, and want the benefit of that elegance, then I think you should either accept the consequences (many worlds) or find a good reason to reject those consequences.  (A theory of quantum gravity might eventually provide a reason, but that’s speculation.)

Toward the end of the book Carroll gets into stuff involving the possible emergence of spacetime from quantum mechanics, which I found difficult to follow.  He does point out it’s difficult to work with quantum mechanics in terms of cosmology without, at least implicitly, working under the MWI.

Finally, in the epilogue, he reveals the the title comes from something Einstein wrote describing his wonder as a child that something about the workings of a compass implied, “something deeply hidden.”

All in all, I found the book a good discussion on these topics.  That said, Carroll isn’t really striving for even handedness here.  He’s a partisan for a particular view, and it shows.

Einstein, Schrodinger, and the reluctance to give up hard determinism

Ethan Siegel on his Starts With a Bang blog has an interesting review of Paul Halpern’s new book on Einstein and Schrodinger, and their refusal to allow the implications of quantum physics to dissuade them from idea that the universe is strictly deterministic.  It’s an interesting post and one that I recommend reading in full.  I may well have to read Halpern’s book.

English: Hydrogen in (3,0,0)-state.
English: Hydrogen in (3,0,0)-state. (Diagram credit: Wikipedia)

The idea that the universe is fully deterministic is one that many people hold on to tightly, even though science has made that view questionable since the 1920s.  Things that happen with a particular quantum particle, such as an electron, can’t be predicted.  We can only assign probabilities to particular outcomes.  It’s only with populations of vast number of those particles that we begin to be able to make predictions.  Determinism appears to be an emergent phenomenon.

Many strict determinists find comfort in the notion that since the uncertainties average out over large enough scales, that we leave quantum uncertainty behind as we go up to the macroscopic scale.  And we do, to some extent.  It’s why we can use innumerable physical laws to make predictions.  But quantum uncertainty does intrude in the macroscopic world.  The very fact that we can do experiments that tell us about it is proof of that.  The question is to what extent it bleeds into macroscopic reality in natural processes.

Even if it only does so in one in a trillion interactions, within the uncertainty involved in any scientific measurement, in complex dynamic systems, chaos theory shows that that one in a trillion outcome can snowball in time to make those complex dynamic systems unpredictable, even in principle.  This means that complex dynamic systems such as the weather, economies, the human mind, and even sufficiently advanced computer systems, may have behavior that will never be predictable, at least not completely.

In my experience, those that do hold on to strict determinism, either don’t understand the implications of quantum mechanics (I won’t accuse them of not understanding quantum mechanics itself since even experts like Richard Feynman never claimed to have that understanding), choose to ignore those implications, or they tightly grasp on to interpretations of quantum mechanics that supposedly preserve determinism, such as the MWI (Many World Interpretation).

While I personally see the MWI as a candidate for reality, I’ve never been particularly impressed by the idea that it preserves determinism.  What does it mean to say that reality is deterministic when everything possible happens, but we still can’t predict what we’ll observe, even in principle, along our subjective timeline?   I’m not convinced that deserves the name “determinism.”  It certainly isn’t very useful for predicting future observations.

Anyway, Siegel’s post is a reminder that we’re all human and fallible, including the geniuses who, sometimes despite themselves, have broken new ground that call into question our most fundamental assumptions about reality.  And that reality itself has no obligation to conform to our most ingrained expectations.

New interpretation of quantum physics: Many Interacting Worlds

There’s a new interpretation of quantum mechanics: Scientists propose existence and interaction of parallel worlds: Many Interacting Worlds theory challenges foundations of quantum science — ScienceDaily.

This new interpretation appears to be similar to the MWI (Many Worlds Interpretation) where quantum superpositions don’t collapse, but spread, creating what amounts to new universes.  However, in this theory, the parallel worlds already exist and interact.

The team proposes that parallel universes really exist, and that they interact. That is, rather than evolving independently, nearby worlds influence one another by a subtle force of repulsion. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

…Professor Wiseman and his colleagues propose that:

  • The universe we experience is just one of a gigantic number of worlds. Some are almost identical to ours while most are very different;
  • All of these worlds are equally real, exist continuously through time, and possess precisely defined properties;
  • All quantum phenomena arise from a universal force of repulsion between ‘nearby’ (i.e. similar) worlds which tends to make them more dissimilar.

Dr Hall says the “Many-Interacting Worlds” theory may even create the extraordinary possibility of testing for the existence of other worlds.

The fact that this might be a testable theory makes it seem more scientific than the more speculative interpretations.  From the abstract, this new view would be completely deterministic, with probabilities only arising from our ignorance about which worlds we are in and interacting with.

The actual paper is here.  The abstract and “Popular Summary” are readable, but the rest quickly becomes greek for non-physicists.  Hopefully we’ll see a write up soon from Sean Carroll or one of the other blogging physicists.