Tuesday, February 11, 2014

Quantum Mechanics for skeptics, redux

When I was an undergrad, I gave an informal talk on quantum mechanics for my friends in the skeptical student group.  Unfortunately, the website hosting the slides went defunct, so it's no longer available.  I offered to give a similar talk to the atheist student group at my current university.  So I'm redesigning it as a chalk talk.  It will probably be a bit more serious this time, since I'm not an undergrad, but it's still very informal.

To organize the talk, it helps me to write a blog post along the same lines.  So that's what you're getting.  Apologies if it's a bit rough.


Quantum Mechanics for Skeptics

I.  Introduction

(These quotes will be handed out on cards)
I think I can safely say that no one understands quantum mechanics.
-Richard Feynman

The physical world, including our bodies, is a response of the observer. We create our bodies as we create the experience of our world.
-Deepak Chopra

The physical process of making a measurement has a very profound effect. 
-David Albert

We're all connected by an energy field.  We swim in a sea of light, basically, which is the zero point field.
-Lynn Mc Taggart

Light and matter are both single entities, and the apparent duality arises in the limitations of our language.
-Werner Heisenberg

I wake up in the morning and I consciously create my day the way I want it to happen... and out of nowhere little things happen that are so unexplainable, I know that they are the process or the result of my creation.
-Joe Dispenza

There's all sorts of universes sitting on top of each other, and they're splitting apart and differentiating as time moves on.
-Sean Carroll

A shift in quantum state brings a parallel lifetime. The relationship to you and your environment is lifted... You are now in a parallel existence.
-Ramtha, channeled by J.Z. Knight
Some of these quotes are from physicists, and some are nonsense. I do not intend for them to be difficult to distinguish.  Most of the nonsense comes from people interviewed in the documentary What the Bleep Do We Know?

Richard Feynman of course was a famous physicist.  But despite what he said, it is clear that some people understand quantum mechanics better than others.  Now, most of you don't study physics (are there any physics majors in the audience?), so you probably don't understand quantum mechanics.  The question is, how can you tell the nonsense from the science?  Can you do it without deferring to an expert?


II. A quick overview of quantum mechanics

A. The context of quantum mechanics in physics

First I need to give some context.  I am not a quantum physicist.  The fact is that quantum physics is very well established, and isn't a topic of cutting edge research.  Almost every physicist uses quantum physics as the framework to study something else.  I'm a condensed matter physicist; I apply quantum theory to extremely large numbers of atoms.  The fundamental rules of the game are well understood, it's scaling it up that's hard.

But yes, there are some things about quantum theory that are not well understood.

But quantum gravity isn't really relevant to this talk.  Everything here is well understood.  In fact I'll stick mostly to quantum mechanics.

B. The wavefunction and measurement

Quantum mechanics describes matter as made of things that are like particles and also like waves.  Take for instance the electrons in atoms.  The electrons can be in many possible states which we describe with a "wavefunction".  This is usually represented with pictures of orbitals around the nucleus of the atom.  But it's actually just some mathematical function, which I'll plot as a function of position.

The first consequence is that the possible energies of an electron are discrete.  The energy of the electron is roughly related to the number of times this wave wiggles up and down.  It needs to wiggle up and down an integer number of times, so there are discrete energy levels.  In particular, there's a lowest energy level, which is a good thing.  Otherwise the electrons would collapse into lower and lower energies, causing all atoms to implode.

There's the question of what this wavefunction actually represents.  Well say that you had an ultra-precise way of measuring the position of the electron.  The probability of finding the electron in any position is equal to the square of the wavefunction.  So even if you prepare lots of electrons in the same way, you can never predict exactly where they are.

And here's where it gets weirder.  Say that you make two measurements, one right after the other.  The second measurement will agree with the first.  So even though the position was uncertain to begin with, by measuring it you make its position certain.  One way to describe this is by saying that the wavefunction has changed after measuring it.  This is referred to as wavefunction collapse.

C. Quantum uncertainty vs classical uncertainty

The picture I've just drawn sounds a little bit like we just don't know where the electron is.  After we measure it, and then we know where it is.  I call this "classical uncertainty".  But the uncertainty in quantum mechanics is different.

For instance, let's say that we don't know whether an electron is in a 1s state or a 2p state.  But it's not just that we don't know in the classical sense, let's say we don't know in the quantum sense.  In quantum mechanics, you represent this by adding the wavefunctions together.  Now we can take two kinds of measurements of this system.  If you try to measure the energy, sometimes you'll get the 1s energy and sometimes you'll get the 2p energy.  Then the electron will collapse into the 1s or 2p state according to what you measured.

But suppose that we instead measure the position of the electron.  We would mostly see the electron on the left side here and not on the right side.  Now if the electron were really in the 1s state, we'd see it on the right and left sides equally.  And if it were in the 2p state, we'd see it in the right and left sides equally.  But it's not merely that we don't know whether it's in 1s or 2p, it's that in some sense it's in both states.

This, by the way, is entirely a thought experiment.  Practically speaking, we wouldn't be able to control whether the electron was in a 1s + 2p state or a 1s - 2p state.  If it's 1s + 2p, the electron would appear on the left, and if it's 1s - 2p, it would appear on the right.  Since we don't know which one it is, we're back to the situation of classical uncertainty rather than quantum uncertainty.  But there are other experiments that really do demonstrate that quantum uncertainty is special.

D. Entanglement

One of the strange consequences of quantum mechanics is that you can have correlations between particles, even if those particles are far away from each other.  For instance, there's a way to prepare two photons such that they have the same polarization, even though we don't know the polarization of either individual photon.  Again, it's not that we are ignorant of the polarization, it's that it's actually in a superposition of vertical/vertical and horizontal/horizontal

Sometimes people use entanglement to argue that if we think positive, positive things will come to us by the law of entanglement.   But generally, if far-apart particles are correlated at all, there's no particular reason they would be correlated vs anticorrelated.  If the particles are interacting with a random environment, they would switch between correlation and anticorrelation such that, effectively, there's no correlation at all.


III. How to recognize quantum nonsense

A. Vocabulary

Quantum nonsense uses lots of complicated terminology in order to confuse people.  People also feel afraid to challenge it because maybe they just don't understand.  The problem is that real science also uses lots of terminology, and if you're not an expert in the field, you may not be able to tell the difference.

It's difficult to make a rule of thumb to tell the difference, but here's what I propose:  Look at who the intended audience is.  If scientists are talking to other scientists, they need terminology in order to communicate precisely.  If a scientist is speaking to the public, they may use terminology because they're not really sure how to say it in plain language.  But plain language would be ideal.

In contrast, pseudoscientists are almost always talking to the public, and use scientific terminology intentionally.  It's not that they don't know clearer ways of speaking, they actually want you not to understand.

B. Ignoring scale

In What the Bleep do We Know? there's a clip where they show a basketball bouncing in many places on a court.  Then the basketball player looks at it, and it's only in one place.  This is an okay illustration of quantum mechanics, but they neglected to explain how this only occurs on very small scales.

The appropriate scale is the atomic scale.  When you have electrons in an atom, you don't know where the electron is, but there's an extremely high probability that it's not very far from the nucleus.  The uncertainty of the basketball is on the same scale (smaller, really, since it's a heavier object).

In fact the picture is very much complicated by a system which is made by more than a few particles. As I said earlier, in my research I apply quantum physics to very large numbers of particles, such as what you would find in a grain of dust.  Quantum physics has a big impact (for one thing, the atoms aren't imploding), but large objects do not behave like small ones.  Unless the system is really cold (ie at the very limits of our cooling technology), there's too much randomness.  This randomness turns quantum uncertainty into classical uncertainty.

C. Observers

My favorite part of What the Bleep was the following argument.  In order to cause wavefunction collapse we need conscious observers.  Human cells can cause wavefunction.  Therefore, human cells are conscious beings.  What follows is a computer-animated segment with anthropomorphic human cells.  And when you think negative thoughts, the human cells have decadent parties and destroy your health.  Long story short, you should throw out your medication and just think positive.

But seriously, there's nothing in quantum mechanics that requires conscious observers.  Really what you need is some large complicated system, such as a grain of dust which introduces randomness.  This makes a quantum system behave classically, and that's what wavefunction collapse is, more or less.  Quantum mechanics doesn't say you're special (although you're free to think you're special anyway).

D. Quantum Interpretations

Now there are a few different interpretations of quantum mechanics, as to what it really all means, on the bottom of it.  The most popular interpretations are the Many Worlds Interpretation and the Copenhagen interpretation.

The Copenhagen interpretation is more or less what I've already described.  There's a quantum system which follows certain rules.  And you can measure or observe the system, which causes the system to change.  In the Many Worlds interpretation, there's nothing fundamentally different about the observation process.  The system just interacts with a measurement device, and becomes a superposition of two states.  These two states don't really interact and for all intents and purposes are independently evolving worlds.

These two interpretations are equivalent to each other, at least experimentally.  There is no experiment that can be performed to distinguish between these two.  So if someone says something that makes sense in one interpretation, but totally contradicts the experimental predictions of the other interpretation, then it's probably nonsense.

For example, when someone says that quantum mechanics requires conscious observers, you know that's wrong because there are no observers whatsoever in the Many Worlds Interpretation.  When someone says that you interact with the parallel worlds, you know that's nonsense because in the Copenhagen interpretation there are no parallel worlds to interact with.


IV. Conclusion

Quantum Mechanics is a little strange.  Quantum uncertainty is fundamentally different from what we usually think of as uncertainty.  We can have correlations between far away particles.  But it does not make conscious observers special, and nobody "chooses" reality.  I hope this helps you to distinguish quantum science and quantum nonsense.  But if not, you can ask an expert.  I can take questions now.