Last week, I was at my graduate alma mater, the University of Maryland, College Park, to help run The Schrödinger Sessions, a workshop giving science fiction writers a "crash course" on quantum physics. The workshop was held at the Joint Quantum Institute, a collaboration between UMD and the National Institute of Standards and Technology in Gaithersburg, and the 17 writers who attended got to hear talks from faculty and post-docs about the essentials of quantum physics, and tour research labs studying ultracold atoms and trapped ions.

This was a whole lot of fun, and the presentations were outstanding. It was also thoroughly draining, so after everybody headed out, I went in search of mindless entertainment: I stopped at a movie theater on my way to the airport, and watched Ant-Man. Which, I should say up front, was a very entertaining way to pass a couple of hours, mostly thanks to the brisk caper-movie plot and the charming cast.

That said, there were some problems on the science side. I have previously discussed my low opinion of the comic-book version of science, and this movie had a lot of issues in that vein. It's also not especially consistent about the way its central gadget works, despite Rhett Allain's valiant effort to retcon the notion of shrinking objects. But one particular plot point hit a little too close, after the just-completed workshop. Hank Pym (played by Michael Douglas) explains that if not properly regulated, his shrinking process might take someone using it down to "The Quantum Realm" at which point the concepts of time and space lose all meaning. It probably won't surprise anyone with the slightest familiarity with the genre to know that this particular Chekovian gun gets fired by the end of the third act, but details of how that plays out would be a spoiler (and don't really matter for my point), so I won't say more than that.

Having just spent three days explaining *real* quantum physics, though, this rankled a bit. It's a little too close to the idea of quantum physics as magic, suspending all rules. While quantum physics is amazing in many ways, it is very definitely not magic-- the real "quantum realm" is in fact quite tightly constrained by rules that we understand very well.

The primary factor contributing to the persistent pop-culture idea of quantum physics as an "anything goes" zone is that quantum physics is all about probability. When we do calculations in quantum mechanics, the end result is always a probability distribution. We can never predict the specific outcome of a *single* measurement, just the distribution of outcomes we expect when that experiment is repeated many times with identically prepared particles.

This is an idea that classically trained physicists find deeply disturbing on a philosophical level, and famously led Einstein to say a number of things that are usually paraphrased as some variant of "God does not play dice with the universe." The entire project of classical physics, dating back hundreds of years, is based on the idea that given knowledge of the current state of the universe and the mathematical rules governing its behavior, we can project forward to some point in the future and predict where everything will be and how it's moving. The notion that two identically prepared particles sent through the same experiment might produce two completely different outcomes seems to threaten the very foundation of physics.

And yet, it's clearly true, as we see in things like this double-slit experiment with electrons done by researchers at Hitachi. While each electron starts in the same source with the same conditions, they appear in a seemingly random pattern on the detector screen. But when the experiment is repeated enough times, we end up with a clear pattern of "stripes" tracing out the probability distribution we predict for electron waves passing through their apparatus. We can't predict the spot where any particular electron will end up, but we can say with certainty that given enough electrons, they will trace out this kind of distribution.

Does this mean, then, that physics is completely overthrown and anything goes? Not at all. Quantum physics isn't deterministic in the simple and elegant way that classical physicists were taught to seek (but of course, with the development of chaos theory, we know that even purely classical physics isn't as predictable as they would've liked), but we can predict the probabilities of various outcomes with exquisite precision. The most precise measurements of the properties of electrons in magnetic fields agree with the predictions of the (inherently probabilistic) theory to about 14 decimal places.

Quantum mechanics does allow the possibility of improbable events happening, as in the phenomenon of "tunneling," where a particle moving with an energy too low to pass through some barrier may nevertheless show up on the far side, as if the barrier simply weren't there. But even in quantum theory, improbable events are *unlikely to occur*. We can use quantum physics to make a scanning tunneling microscope to study matter on the atomic scale, but an electron actually tunneling through the gap between tip and sample remains exceedingly unlikely. We see a measurable signal from an STM only because we can throw astronomically huge numbers of electrons at the barrier-- billions of billions-- so that the tiny fraction of them making it through are enough to detect. The probability is something we can predict very precisely, though, and in fact, that's the key to the operation of an STM-- because we can predict exactly how the probability of tunneling varies as we change the distance between tip and sample, we can use the measured current to determine the distance, and measure changes as small as the height of a single atom.

Somewhat ironically, this randomness also turns out to be essential for preserving another limit that Einstein cherished: that no information can travel faster than the speed of light. The phenomenon that today we call "entanglement," first clearly identified in the famous "EPR" paper by Einstein and his younger colleagues Boris Podolsky and Nathan Rosen, allows the result of measurements on two entangled particles at widely separated locations to be correlated with each other in ways that cannot be matched by a classical theory, or explained by information passing between the particles at light speed or below.

At first glance, this might seem a way to send superluminal messages-- Alice, at Point A measures the state of her particle, changing the state of Bob's particle at Point B, which Bob can then measure to receive whatever message Alice wants to send. Quantum randomness saves the day, though-- because it's impossible to predict the outcome of a *single* measurement, Alice and Bob each end up with a random string of ones and zeros, with no useful information content. The "spooky" correlation between their lists only becomes apparent after the fact, when they compare lists. *That* process must take place via classical means, though, at speeds slower than the speed of light.

So, the probabilistic nature of quantum physics is very real, it most emphatically is not a theory where anything goes. God may well be throwing dice to determine the fate of the universe, but the dungeon master tables in which He looks up the results are very tightly constrained by quantum rules that we understand and can calculate with exceptional precision.

There is another, more charitable way to interpret Ant-Man's "Quantum Realm" where time and space lose meaning, namely as an invocation of quantum gravity. Our best theory of gravity is Einstein's General Relativity, in which what we see as the force of gravity is, in fact, a warping of spacetime by the presence of matter. The path that a freely falling object-- a satellite in orbit, say-- follows through spacetime appears like a curve to us because spacetime itself is curved. The orbit is like a transatlantic airplane route seen on a Mercator projection map-- the "curved" path is, in fact, the shortest possible route once you account for the curvature of the Earth (or space-time, in the case of the satellite).

General relativity is a deterministic theory-- In John Wheeler's famous phrase, "Spacetime tells matter how to move; matter tells spacetime how to curve." Everything is smooth and predictable, and the predictions pass every experimental test. It is not, however, readily compatible with quantum physics, and many attempts to combine the two lead to fluctuations in spacetime itself at the smallest possible scales. If you squint a bit and tip your head sideways like a confused dog, this might look like space and time losing all meaning in the manner suggested by the film.

Again, though, the possibilities here are very tightly constrained by a wide range of observations, a point that Raman Sundrum made repeatedly on the last day of our workshop. The area where quantum physics and general relativity intersect will contain physics that we don't currently understand, but that area is very tightly constrained by observations.

Theories of quantum gravity are well outside my core expertise, though, so I'll leave that side of things to others. I think the core point of larger-scale quantum physics remains, though, namely that while quantum effects *allow* improbable events, those will remain highly improbable, and not completely destroy the meaning of spacetime any more than the occasional lottery winner destroys the meaning of money.

I'm an Associate Professor in the Department of Physics and Astronomy at Union College, and I write books about science for non-scientists. I have a BA in physics from

…I'm an Associate Professor in the Department of Physics and Astronomy at Union College, and I write books about science for non-scientists. I have a BA in physics from Williams College and a Ph.D. in Chemical Physics from the University of Maryland, College Park (studying laser cooling at the National Institute of Standards and Technology in the lab of Bill Phillips, who shared the 1997 Nobel in Physics). I was a post-doc at Yale, and have been at Union since 2001. My books _How to Teach Physics to Your Dog_ and _How to teach Relativity to Your Dog_ explain modern physics through imaginary conversations with my German Shepherd; _Eureka: Discovering Your Inner Scientist_ (Basic, 2014), explains how we use the process of science in everyday activities, and my latest, _Breakfast With Einstein: The Exotic Physics of Everyday Objects_ (BenBella 2018) explains how quantum phenomena manifest in the course of an ordinary morning. I live in Niskayuna, NY with my wife, Kate Nepveu, our two kids, and Charlie the pupper.