The measurement problem, Copenhagen, pilot-wave, and many worlds

With quantum physics, we have a situation where a quantum object, such as a photon, electron, atom or similar scale entity, acts like a wave, spreading out in a superposition, until we look at it (by measuring it in some manner), then it behaves like a particle.  This is known as the measurement problem.

Now, some people try to get epistemic about this. Maybe the wave isn’t real but just epistemic probabilities. The issue, shown in the double-slit experiment, is that the wave interferes with itself, something those who want to relegate the wave to completely non-real status have to contend with.

An important point is that if the wave is very spread out, say light years, and any part of it is measured, the whole thing collapses to a particle, apparently faster than light.  This appears to violate relativity (and hence causality), which was Albert Einstein’s chief beef with quantum physics, and the impetus behind the concept of entanglement explored in the EPR paradox.

Now, we have an equation, the Schrodinger equation, that models the evolution of the wave.  Its accuracy has been established in innumerable experiments.  But when we actually look at the wave, that is, attempt to take a measurement, we find a particle, that subsequently behaves like a particle.  The math appears to stop working, except as a probabilistic prediction of where we’ll find the particle.  This is often called the wave function collapse.

The Copenhagen interpretation handles this by saying that quantum physics only applies to small isolated systems.  As soon as something macroscopic is involved, such as a measuring device, the rules change.  Kept to a minimal instrumental version, I think this interpretation is underrated.  Bare bones Copenhagen doesn’t attempt to explain reality, only describe our interactions with it.  It could be seen as an admission that the metaphors of our normal scale existence are simply inadequate for the quantum realm.

Of course, people can’t resist going further.  Copenhagen is actually more a family of interpretations, some of which involve speculation about consciousness causing the collapse.  Reality doesn’t congeal until we actually look at it.  I think the challenges of quantum computing rule this out, where engineers have to go to extreme efforts to preserve the wave to get the benefits of that type of computation.  They’d probably be very happy if all they had to do was prevent any conscious mind from knowing the state of the system.  But it’s an idea many people delight in, so it persists.

The pilot-wave interpretation, often referred to as De Broglie-Bohm theory, posits that there is both a particle and a wave the entire time.  The wave guides the particle.  When we look / measure, the wave becomes entangled with the environment, it loses its coherence, and so the particle is now free to behave like a particle.  This idea actually predates Copenhagen, although it wasn’t refined until the 1950s.

Pilot-wave initially looks promising.  We preserve determinism.  But we don’t preserve locality.  Looking at the wave, anywhere in its extent, still causes the whole thing to decohere and free up the particle, even if the particle is light years away.  So, Einstein wasn’t happy with this solution, since relativity appears to still be threatened.

Hugh Everett III looked at the above situation and asked, what if the math doesn’t in fact stop working when we look?  Our observations seem to indicate that it does.  But that’s failing to account for the fact that macroscopic systems, including us, are collections of quantum objects.

As it turns out, the Schrodinger equation does predict what will happen.  The wave will become entangled in the waves of the quantum objects comprising the measuring device.  It will become entangled with the environment, just as pilot-wave predicted, but unlike pilot-wave, Everett dispenses with the particle.

Crucially, rather than collapsing, the superposition of the wave will spread, just as it seems to do before we look.  Why does it appear to collapse?  Because it has spread to us.  We have gone into superposition.  Every branch of that superposition will now continue to spread out into the universe.  But the branches are all decohered from each other, each no longer able to interfere with the other.  They are essentially causally isolated.

So each of those branches could be romantically described as being in its own separate “world”, resulting in many worlds, the many worlds interpretation.

The appearance of the collapse, under the many worlds interpretation, is because we are now on one branch of the wave function, observing the small fragment of the original wave that became entangled with this branch of the environment.  Under this interpretation, there is a different version of us in each other branch seeing differing parts of the wave, which we now refer to as a “particle”.

Which of these interpretations is true?  Copenhagen, pilot-wave, many worlds, or some other interpretation?  They all make the same observable predictions.  (The ones that don’t were discarded long ago.)  It’s the predictions they make beyond our ability to observe that distinguish them from each other.

We could ask which has the fewest number of assumptions.  Most people (often grudgingly) will admit that many worlds has the most elegant math.  (Evoking comparisons with Copernicus’ heliocentric model in relation to Ptolemy’s ancient geocentric one.)  And it does preserve realism, locality and determinism, just not one unique reality.  Whether that mounts to fewer assumptions than the others is a matter of intense debate.

Each interpretation has a cost, often downplayed by the proponents of that interpretation, but they’re always there.  Quantum physics forces us to give up something: realism, locality, determinism, one unique reality, or some other cherished notion.  As things stand right now, you can choose the interpretation that least threatens your intuitions, but you can’t pretend there isn’t a cost.

Unless of course I’m missing something.

An excellent explanation of quantum decoherence, and how it might lead to many worlds

Matt O’Dowd is a first class science communicator.  In this latest video, he does an excellent job explaining decoherence, and why the MWI (many worlds interpretation) ends up being so tempting when you see it through.

Of course, this doesn’t mean MWI is the right interpretation, but it does demonstrate why many find it tempting.  (At least once they get over the visceral reaction we all seem to initially have for it.)

But it’s worth noting that decoherence actually is compatible with pilot-wave and many other interpretations, and reportedly even some versions of Copenhagen.  So don’t let MWI cause you to dismiss it!

Is reality an illusion? If so, does it matter?

Hoffman_1KDonald D. Hoffman, a psychologist at the University of California, Irving, has been getting a lot of attention recently for his views, that evolutionary evidence indicates that reality is an illusion, that the only thing that exists are conscious minds.

This is a modern version of an ancient concept, called idealism.  The earliest writings about it come from the ancient Greeks, although they were almost certainly influenced by the Indian ascetics and sages.

Hoffman’s version is built using ideas of modern science.  He starts with the observation that we aren’t evolved to perceive reality as it is, but in a way that is adaptive, that ensures our survival.  He then brings in quantum physics, noting the often held understanding that consciousness is what causes quantum decoherence (aka wave function collapses).  From this, he deduces that the external world is an illusion, that conscious minds are all that exist.  (This is admittedly a very quick and dirty summation.  Read the articles for a more thorough, and perhaps fairer summary, or watch one of his talks or debates.)

Hoffman has a very valid point about our minds not being evolved to accurately comprehend reality.  This is something that has been understood for a long time.  The idea that we can’t naively trust our perceptions is ancient, and has been well known throughout the history of modern science.  (For example, see Francis Bacon’s Idols of the Mind.)  Reality is not as we perceive it, although it’s a large jump from that common understanding to concluding that reality overall is an illusion.

Image credit: Dhatfield via Wikipedia
Image credit: Dhatfield via Wikipedia

To bridge that gap, Hoffman brings in quantum physics.  To be clear, quantum physics is bizarre.  The more people seem to understand it, the more bizarre it seems to them.  However, Hoffman’s understanding of consciousness causing quantum decoherence might be a bit dated.

Quantum decoherence is now commonly described by physicists to happen from any interaction with the environment.  Only isolated quantum objects, such as individual subatomic particles before they interact with anything (such as hitting the screen in the double-slit experiment), or molecules kept in very isolated laboratory conditions, can remain in a superposition wave state.  In other words, Schrodinger’s Cat either lived or died before the box was opened, since a cat is definitely a noisy system in terms of quantum interference.

This makes sense when you consider why scientists and engineers are struggling to build quantum computers.  Keeping qubits, quantum computing’s version of binary bits, in superposition, preventing them from decohering, is a major challenge.  It’s also why most proto-quantum processors have to operate at near absolute zero temperature, to avoid disturbing the superposition state of the processor’s internals.  If preventing conscious interaction were all that were necessary to prevent decoherence, it seems like these designs could be much simpler.

There’s also the broader difficulty that using sensory experience to conclude that all sensory experience is an illusion, that is, using data from that illusion to conclude it is an illusion, seems problematic.  Of course, someone could say they find logical contradictions or absurdity in that data, hence the illusion conclusion.  But then I have to ask, contradicting what?  Or absurd by what standard?

But, just for the sake of argument, let’s bracket those difficulties, and assume Hoffman is right.  Idealism is true.  One of the other issues I’ve always seen with idealism is that it exempts other minds from this logic.  If everything else is an illusion, what prevents any other minds I perceive to be out there from also being an illusion?  Maybe I’m the only mind that exists.  Or maybe you are and I’m the illusion.  Of course, this is solipsism.  But I’ve  never been able to see what stops idealism from being a logical slippery slope into solipsism.

Furthermore, if we’re going to engage in this kind of skepticism, we can’t even be sure about our own memories.  Maybe everything outside of your mind is an illusion.  Maybe you came into existence 30 seconds ago together with your existing memories.  Perhaps you are in fact a Boltzmann brain, a conscious entity that emerged from random fluctuations a few seconds ago, and will disappear back into those fluctuations in a few more seconds.

All of this is aside from the issue that if reality is an illusion, that illusion appears to exact unpleasant, often painful consequences for not taking it seriously, including consequences for aspects of that illusion we don’t know about, or forget about, or any of the other problems caused by what we normally think of as objective, mind independent reality.

Essentially this makes the illusion our reality, our universe.  And that all the people who insist that there is a reality outside of the universe are right.  We can only hope, if anyone is controlling this illusion, that they’re kindly disposed toward us, or at least not hostile.  The border between this line of reasoning and theology seems like a blurry one.

All of which is to say, if reality is an illusion, then we have little choice but to play the game.  It makes sense to play that game as well as we can, which means understanding it in the most reliable way we can, which brings us back to the methods of science and philosophy for understanding what we commonly call objective reality.

Unless, of course, I have logical holes in my reasoning.

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.