Is the Universe Fundamentally Indeterministic?

My answer?  Yes!! 

What does that mean exactly?  Yeah, mumble mumble mumble.

Backing up,  I recently got a question from someone named Brian, who was commenting on the previous post.  I love this question - it goes straight to one of the most profound philosophical ideas that physics leads us to ponder.  The question was:


"I was just curious if you knew whether QM's being non-deterministic also rules out determinism at higher levels (like molecules, cells, etc)."

I'm not going to pretend that I can give a complete and totally satisfying answer to this question, but I will mumble at length about it!  It is going to take a couple of steps to get to the actual question.  So, here goes...me mumble my way through my own thoughts on the subject, in several steps

Step 1:  Quantum Mechanics and Determinism:

Quantum mechanics (the physics of very small systems like particles and atoms) is based on mathematical laws that are probabilistic in nature.  For example, these laws can tell you the probability that if you measure the location of an electron in an atom, you will find it at a particular place.  But, you can never predict exactly where the electron will be.  Hence, quantum mechanics is not a deterministic theory (it does not enable you to make exact predictions of the properties of particle systems).

Now, the question is:  is the probabilistic behavior fundamental, or is this just how we characterize what we observe because our theory is incomplete?  Maybe if we knew more about electrons, and we could measure extra properties of the electron or the atom, we could predict the exact location at any time.  Perhaps there are "hidden variables" describing the system of the electron in the atom, and our theory is just missing those.

This is a tempting way to rescue determinism, but it turns out to be impossible!  In 1964, a guy named Bell used a little bit of logic to come up with a straightforward way to test whether hidden variables could work.  Interestingly, you can test this experimentally without needing to specify what those hidden properties are.  Sure enough, experimental tests of particle behaviors prove that quantum mechanics isn't missing anything - the probabilistic description is complete.

As I discovered this spring (when I taught a class in Randomness), trying to explain Bell's theorem is really hard!  And, I should also mention that it's something that continues to be the subject of enormous discussion at the intersection between philosophy and physics (see, for example, the Stanford Encyclopedia of Philosophy article on Bell's Theorem).  As well as I understand it, the experimental tests of Bell's theorem lead you inescapably to pick one of the three following conclusions:
1) The universe is "fundamentally random", such that the behavior of something like an electron in an atom does not follow deterministic rules (whether or not we know those rules)
2) The universe does follow deterministic rules, but the behavior of a single particle is influenced by the location/behavior of every other particle in the universe (hence, even in principle it would be impossible to make exact predictions anyway).
3) Basic, simple logic is wrong.

If 3 were true, a whole lot of other stuff would make no sense.  But, depending on your philosophical preferences, you can pick option 1 or 2.  I'm perfectly happy with option 1, hence my resounding "yes" at the top of this post.

Step 2:  Macroscopic Determinism:

Ok, so let's just go forward with the assumption that particle behaviors can only be predicted probabilistically, and not deterministically.  Does that preclude deterministic behavior in macroscopic systems made up of many particles?  Not necessarily.

The rules of quantum mechanics, including the fuzzy, probabilistic behavior, really only operate for small systems.  Once you start to build up systems of many particles, their collective behavior becomes much more "classical" - governed by mathematics that is fully deterministic and allows exact prediction of behavior.  Probing the transition from probabilistic rules to deterministic rules is something physicists are currently doing (see some of the material in this previous post, for example).

It's not like one set of rules just turns off at a particular size scale and the other set of rules turns on.  Basically, the deterministic laws of classical physics (Newton's laws of motion, for example) can be viewed as only approximately true.  But the larger the system, the closer the approximation gets to being pretty perfect.  By the time you are operating with an object like a grain of dust, its behavior can be predicted with mathematics that is strictly deterministic, to a very very high accuracy.

So where does that leave the behavior of cells?  REALLY INTERESTING QUESTION!  I don't think anyone knows for sure, and a lot of the detailed physics of tiny biological systems is under study right now.  Cells are large enough objects that in terms of their physics, they probably could be described with deterministic mathematics.  But, at the same time, some of the processes within cells do take place at small enough scales that quantum mechanics could matter.  So, this is science to actively watch for in the coming decades - is anyone able to figure out whether the probabilistic nature of quantum mechanics measurably translates to randomness in the behavior of cells?

Step 3:  "Effective" Indeterminism:

There's one more step in how I think through all of this in my own head.  The underlying question to me (whether or not Brian had this in mind) is whether human brains are ultimately deterministic or not.  In other words, is every thought that I have a result of deterministic clockwork-like motions in my head?  Or is some of the apparent spontaneity ultimately due to the fundamental randomness of the universe (a lovely thought, in my opinion)?

This would seem to be a straightforward either-or situation:  either my brain is deterministic or it isn't.  But, I'd like to point out that even if it is deterministic, that doesn't necessarily mean we will ever actually be able to accurately predict what a system as complex as the brain will do.  This is not just a statement of the current limitations of science, but has to do with fundamental physics as well.

Even in fully deterministic systems, the interactions of many objects (particles, cells, etc.) can lead to a level of complexity such that the behavior appears to be random, and cannot ever be reduced in practice to deterministic laws.   It may be the case that to exactly predict the behavior of one neuron (even in a deterministic brain) you would need to know the positions and motions of so many particles that it would take a computer the size of the universe just to store the numbers.  If this is the case, the brain could still be "effectively" indeterministic - it could follow deterministic rules but involve such a level of complexity that no human instrument (present, or future, or even in science fiction) could ever determine what you're going to think next.

Multiple Multiverses

Two students wrote to me in the last week asking about two separate articles, both claiming "proof" of the existence of multiple universes.  Oddly enough, the two articles are talking about completely different kinds of physics!  Both articles are guilty of journalistic abuses of science, in my opinion - especially in their use of the word "proof".  Yet they both report on reputable scientific research, so there is some validity to both ideas.  Let's disentangle them a bit.


1.  Macroscopic Quantum Mechanics and Multiverses.





(image from researcher Andrew Cleland, at UCSB), taken from a Science News article)

The first of the articles emailed to me this week has the provocative title "Freaky Physics Proves Multiple Universes Exist", from Fox News.  This is a pretty exotic-sounding title.

The research reported in the Fox News article deals with quantum mechanics.  Quantum mechanics is the physics of the very small, and it includes well-known oddities like the idea that a particle can be simultaneously in many places at once.  The act of 'observing' the particle (which always involves light or other particles interacting with it) forces it to 'pick' which position it is in.  These kinds of  behaviors have been confirmed by many experiments, it just sounds especially strange and spooky when you try to describe quantum mechanics with ordinary language.

One of the long-standing explanations for weird quantum simultaneity involves a notion of many universes.  These 'universes' are differentiated by multiple possible outcomes of every physical event.  For example, if you flip a coin and it has a truly random chance of falling heads or tails, we would envision that it actually falls as heads in 'one universe' and as tails in 'another universe'.   This helps with the quantum mechanics interpretation, because when we say that a particle behaves as if it's simultaneously in many places, what we mean is that there are simultaneously many universes in which it's doing different things.

This version of 'multiple universes' is not something you would think of as separate whole physical systems 'beyond' our universe of stars and galaxies and space.  The quantum mechanical 'universes' are infinitely many simultaneous variations on everything that has ever happened.

So what's the article about?  Basically, researcher Andrew Cleland and his collaborators have managed to create a tiny object (shown in the picture above) that displays the same kind of quantum fuzziness as individual particles do.  Even though the object (like a little vibrating fin) is quite tiny, it is still HUGE compared to individual particles, so it is essentially macroscopic.  The exciting thing is seeing that it shows the same properties of individual particles:  it behaves as though it is simultaneously vibrating in different ways.  This is a macroscopic manifestation of a particle property that is well know.  It is also a step in the direction of better understanding possible technological applications of quantum mechanics.

Is this 'proof' of multiverses?  No!  The multiverse (multiple universe) idea is an interpretation of what we see in this experiment and what we have seen in many previous experiments.  It's a valid interpretation, but there are others that don't involve multiple universes that are equally valid.



2.  Dark Flow and Multiverses



(Hubble Space Telescope image of the Coma Cluster)

In my classes on cosmology, students often ask me whether there might be other universes somehow 'beyond' ours.  Perhaps, universes containing different sets of stars and galaxies and beings.  I usually answer that I don't know how to talk about 'beyond' the universe, since the universe contains all of space as we know it.  But, then I say that it's always possible.  Even so, the question of multiple universes doesn't become a significant question for science until those other universes have an observable impact on ours.

Well, that's exactly what some other researchers have recently claimed, as reported in a National Geographic article called "New Proof Unknown 'Structures' Tug at our Universe".  This article is reporting on research by Alexander Kashlinsky and collaborators, in which they analyzed images of galaxy clusters (enormous groupings of many galaxies in clouds of gas and dark matter) and also images of the Cosmic Microwave Background.  It's tricky to explain their technique in a few sentences, but the bottom line is that they believe that they have shown that a large number of galaxy clusters all seem to be moving in the same direction.  This is called the "Dark Flow", which is just a cheesy name to designate the unexplained motion.

A number of scientists (including local supersmart grad student Ryan Keisler) have criticized the data analysis techniques that were used to find evidence for "Dark Flow".  The Dark Flow research is not something straightforward to do - galaxy clusters are far too large for us to literally see them moving.  It involves subtle calculations using a combination of different types of data, and there are a lot of places where things can go wrong.  Other scientists analyzing the exact same data fail to see this effect, so it is fair to say there is serious doubt about the "Dark Flow" result!

However, if there were to be an observed "Dark Flow", it might have a pretty cool interpretation.  One way to explain bulk motion of many large objects in the universe is to postulate something along the lines of gravity from beyond our observable universe (perhaps this counts as being from another universe?).  I have no idea how this would work, but it's intriguing.  Popular news articles like the one above are going way too far calling this 'proof' of multiple universes.  However, unexplained large-scale gravitational pulls would be one potential way that other universes could affect ours, thus leading to new theories that extend beyond our own universe. 

So, I wouldn't make too much of this.  However, for all of my students who find multiple universes so fascinating, here's an example where they are being seriously discussed in science!