Quantum Magic

Nothing I write can be relevant to everyone who might read it, and determining an intended audience is an important step in the writing process.  While I like to think that many of these Tuesday posts are relevant to a general audience of interested and engaged people, some are pitched more directly to readers, or writers, or an even more specific niche.  In this case, I’m directing this post to science fiction writers – in particular, the ones who invoke the word ‘quantum’ as a kind of deus ex machina explanation for anything.  Yes, this is the long overdue ‘basics of quantum mechanics’ post to which I’ve alluded in dribbles and drabbles in various posts, like when we discussed quantum computing as a service, or narrative physics, and I thought a long time about how best to frame it.  Given what I suspect is the site’s readership, I settled on this direction.

Especially in movies and television that fall loosely under the science fiction moniker, ‘quantum’ has somehow become a stand-in for ‘magic.’  That quantum drive that makes faster-than-light travel as easy as saying ‘engage’?  Might as well call it a magic drive.  That quantum energy field enabling your trendy post-scarcity society?  Magic energy.  Quantum compression technology ala the Antman movies?  Magic size-morphing.  Need something to happen to make a plot work, and not sure how to make it realistic, technically?  Well, it must just be magic quantum.

Whether this is because the writers don’t understand quantum physics, or think their audience doesn’t, it bothers me (and it bothers my wife, too, because then I start trying to explain why that’s not really how quantum entanglement works in the middle of the movie).  Maybe it’s because quantum physics really does produce some counterintuitive effects and is a very challenging field of study for the uninitiated – I believe it was Niels Bohr who said that “when you think you’re starting to understand quantum physics, you’re farthest from understanding it, and when you think it makes no sense and can’t possibly be a real explanation of the way the universe works, you’re starting to get it.”  Regardless, quantum physics is not magic, and I hope that this post will help explain a little of the basic principles of quantum physics.

Necessarily, this will be a survey post.  We’ll examine core concepts that compose quantum physics, what unifies the field (not to be confused with unified field theory), and talk a little about specific, well-known examples of its predictions, like quantum entanglement and Heisenberg’s uncertainty principle, but we won’t be getting into the weeds on terms like quantum chromodynamics (real thing), virtual particles (real-ish thing), or loop quantum gravity (a mathematically well-defined background-independent quantization of general relativity).  Hm, so far, I’m not making a very good case for why quantum physics shouldn’t be used as a stand-in for magic.  Let’s start actually talking about quantum physics.

If there is a single key to demystifying quantum physics, it is acknowledging what the word ‘quantum’ actually means.  Coming from the Latin ‘quantus’, meaning ‘how much’, a ‘quantum’ is nothing more than a discrete unit.  The easiest way to think of this is in terms of numbers.  If you remember back to your math classes, you might recall that there are numbers, which includes imaginary numbers, there are real numbers, which excludes the imaginary numbers but includes any and all numbers on the number line, and there are whole number integers, which are numbers which can be expressed without the use of fractions.  Now, imagine you are counting or measuring something.  If the result can be a fraction, then that something is considered continuous.  On the other hand, if the result can only be a whole number, an integer, then you are dealing with something discrete – in other words, quantum.  Quantum physics, literally, is just the physics of discrete units.

For physical purposes, this usually means dealing with the universe on the smallest of scales, which is why quantum physics as a field is so often conflated with (and rightly correlated with) particle physics.  You’re likely already familiar with one of the ‘quanta’ that first gave rise to the study of the field that became known as quantum physics: the photon.  A photon is a discrete packet of light energy.  You can’t have a fraction of a photon.  Easy enough, right?  Photons are light particles.  Well…not quite.  This is where things start to get weird.

Einstein did not win a Nobel prize for the theories of relativity; he actually won it for his paper describing something called the photoelectric effect.  The photoelectric effect underpins the function of photovoltaic cells, some types of light bulbs, most digital imaging sensors, and far more in our modern world.  It describes how photons – those discrete packets of light energy – can incidence with the electrons in atoms and impart that energy.  It turns out that electrons can only absorb certain, prescribed amounts of energy, called energy levels.  Thus, when the photon strikes the electron, the electron will only absorb precisely that much energy, exciting it to that ‘level.’  Think of it like a staircase – the electron can by one stair one or stair two, but it can’t be between stairs.  Electrons can also emit energy as photons, dropping down to lower energy levels in the process.  When the electrons do this, the photons emitted have a specific frequency corresponding to the energy level, which manifests as the various distinctive element spectra.

That phenomenon – the photoelectric effect – is considered quantum physics.  Straightforward enough, right?  It is…except for a little thing called wave-particle duality.  Before Einstein told us about the photoelectric effect, Maxwell realized that electricity and magnetism were two sides of a coin and combined them into electromagnetism.  Electromagnetism propagates as waves, waves with which we are all familiar: light waves.  But we just established that light travels in discrete particle packets called photons.  So which is it?  Is light a particle, the photon?  Or is it a wave?  It turns out that the answer is both.  And not in the sense that sometimes it behaves as a wave, and sometimes as a particle, or that light waves are made up of lots of particles that together act like a wave on a macroscopic scale – that would be too easy.  No, light exists simultaneously as both a particle and a wave.  Each individual photon is also a continuous light wave.

Accepting this as fact is one thing – the evidence for wave-particle duality of light is enormous, rigorous, compelling, and approachable – understanding it is something else entirely.  It gets even more confusing when you realize that photons are not the only particles that behave in this fashion.  It turns out that even massive particles (meaning particles that have mass, not really big particles), like electrons, exhibit wave-particle duality.  The famous double-slit experiment demonstrates that, if no observer is present, a single electron will produce an interference pattern on a detector on the other side of the two slits, meaning that the particle was behaving like a wave.  If that’s not weird enough for you, when an observer is present, the single electron will act as a particle, producing no interference pattern on the detector.

This experiment is a two-slitted gateway into the truly baffling world of quantum physics.  It introduces us to the concept of a probability wave, which suggests that all particles exist as waves of probability suffusing the entire universe, with a spike in the probability that a particle will be found in a given spot at the point where conventional physics would suggest it should be.  Once you’ve wrapped your head around the idea of particles existing as probability waves with no definite positions, we get into the fact that somehow the act of observing causes the probability wave to collapse into a discrete particle, the mechanism for which continues to elude and frustrate physicists.

These concepts – probability waves and the effect of observation – lead to Heisenberg’s uncertainty principle, which states that you cannot know perfectly a particle’s position and velocity at any given time.  The more precisely you try to measure one property, the less insight you will have into the other.  So, you could narrow down a particle’s position to incredible precision, but then you wouldn’t have any idea of its velocity.  Conversely, you can identify its exact velocity, but then you would have not idea where it might be.

It’s important to remind yourself when you’re thinking about these concepts and trying to come to terms with these sometimes outlandish claims (and I encourage you to dig deeply into your own research on any of these topics) that physicists didn’t just start making these rules up to confuse people.  Every one of concepts and principles is backed up by rigorous data and observation, with experimental setups that are understandable, approachable, and replicable.  While you can get into parts of quantum physics where the phenomena under discussion are just derivations and predictions based on ‘what the math says,’ what we’re discussing today is all backed up by experiment, and so is most of the rest of the field.  Indeed, if the data weren’t so compelling, it would be easy to dismiss these notions.  Even with compelling data, it’s tempting to dismiss some of this as just too ridiculous to be real.  The famous Schrödinger’s cat thought experiment was intended to elucidate just how crazy the predictions could be, and Einstein equally famously decried quantum entanglement as ‘spooky action at a distance.’

Quantum entanglement claims that particles can become related – entangled – in such a way that they will affect each other instantly, no matter how separated they become.  You might have one particle spinning clockwise, and the other spinning counterclockwise.  Separate them by a billion light years, and then cause one to flip its spin, and the partner particle would also reverse its spin, at precisely the same instant.  Does that violate lightspeed?  Well, sort of, but not really, because the information about the change occurring would still have to travel at lightspeed.  This is why quantum teleportation wouldn’t actually break the lightspeed barrier, but quantum teleportation is a topic for another post, because this one is already getting too long.

Stephen Hawking invoked quantum entanglement to predict Hawking radiation.  At a black hole’s event horizon, conditions mimic those at the beginning of the universe, leading to spontaneous particle generation (we’re not getting into that now, either).  Those particles generate as entangled pairs, but they are right on the event horizon of a black hole.  Hawking predicted that, because of quantum entanglement and the conservation of momentum, when one of the entangled particles is inevitably pulled into the black hole, the other would be shot away from the black hole.  That particle that escapes is Hawking radiation.

I have to resist continuing with examples, as fun as it would be to try to explicate in an approachable way such concepts as virtual particles, free space energy, Planck scale turbulence, and quantum field theory, or maybe even get into string theory.  Instead, let’s bring this post back around to how we started it; quantum physics is not magic.  As bizarre as its claims can be, and as outlandish as its predictions seem, they are experimentally verifiable and a completely natural part of the world.  It is worth remembering, when quantum physics starts to see too weird, that all of this strangeness ultimately gives rise to the familiar world in which we live.  Sure, quantum physics says that there’s a vanishingly small chance that you’ll spontaneously wake up on Mars tomorrow, but you could live through a few hundred billion years and never see that happen.

This doesn’t mean you can’t use quantum physics in your storytelling – in fact, I encourage it.  Rather, it is a reminder that quantum phenomena are not magic, and throwing the word quantum in front of something does not make it automatically possible.  Throwing a ‘quantum’ into your descriptions is a copout.  If you want something technically correct, do the research and embrace the limitations (Sanderson’s Second Law of Magic, I believe it is (the one about limitations being more important than abilities), applies as much to real science as it does to imaginary magic).  If you don’t care, embrace the magic.

That was…a lot.  I debated a long time about how to do an introduction to quantum physics post, and I think this was a decent result.  We could have done something that went into much more technical detail, and we might in the future drill into specific quantum phenomena, but this provided a (hopefully) approachable survey to the topic.  My goal is to get you thinking in the tortured, convoluted ways required by quantum physics, and I think, if you take the time to engage with it, that this post does that.  As always, I am happy to discuss these ideas further in the comments.  And remember – you can either know where you are, or where you’re going, but not both.  Probably.