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I am just finishing up Adam Becker’s book What Is Real? about the relationship between quantum physics and the real world it is supposed to represent. Becker tells a good story, especially as an introduction to the world of quantum physics, the players over the years, and the intellectual principles involved. His basic premise is that, while the equations that physicists use to predict the outcome of their experiments—and so test the value of those equations as representations of the underlying world of the very small—have consistently proven their worth, the physicists themselves remain in doubt as to whether the world that they are describing actually exists.
Without going into the entire book chapter by chapter, the issue seems to be one of describing a world so small that we cannot detect it without changing it. Atoms and their component protons, neutrons, and electrons—plus all the other subatomic particles in the Standard Model—are not fixed in space like pins on a board. As with everything else, they move, as do galaxies, stars, and planets. However, instead of occupying observable orbits and tracks across the night sky, atoms mostly vibrate with the energy of what’s called “Brownian motion,” and electrons buzz frantically and randomly around their nuclei like flies in a cathedral.
We can detect the larger celestial bodies—and even masses as small as freight trains and automobiles—with visible light without the danger moving or deflecting them much. Bounce a few hundred thousand photons off a teacup, and you will not move it one millimeter. But the subatomic particles are so small that the wavelength of light we can see is so long that it misses the particle entirely, passing over and under it with no impact. Imagine that the wavelength is a long piece of rope that two girls are spinning in a game of Double Dutch. If a human-sized person enters the game and performs unskillfully, the rope has every chance of hitting—that is detecting—his or her body. But if a flea jumps through the game area, the chances of that long, curved rope ever touching its body become vanishingly small.
To detect subatomic particles, physicists must use other particles, as if in a game of subatomic billiards, or photons with much shorter wavelengths and thus having much higher energies. A high-energy photon impacting a moving electron or proton will change its direction of motion. So the issue in quantum physics is that when you locate the particle you are observing here, it’s now no longer there but going somewhere else. In quantum physics terms, no particle has an exact position until it’s observed, and then it has some other position or direction of movement in response to the observation. Mathematically, the particle’s supposed position can only be defined by probability—actually, a continuous wave function that defines various probable positions—and this wave “collapses” into a single definite position at the place and time of your observation.
Well and good. This is what we can know—all that we can know for sure—in the world of the very small.
The first issue that Becker’s book takes up is that most of the original proponents of quantum physics, including Niels Bohr and Werner Heisenberg, adopted this lack certain knowledge to an extreme. Called the “Copenhagen interpretation,” after Bohr’s institute in Denmark, their view insists that the entire point of quantum physics is the manipulation of the results of observation. The measurements themselves, and the mathematics that makes predictions about future measurements, are the only things that have meaning in the real world. The measurements are not proof that subatomic particles even exist, and the mathematics are not proof that the particles are doing what we think they’re doing. To me, this is like calculating the odds on seeing a particular hand come up in a poker game, or counting the run of cards in a blackjack game, and then insisting that the cards, the games, and the players themselves don’t necessarily exist. It’s just that the math always works.
Other physicists—including Albert Einstein—have been challenging this interpretation for years. Mostly, they pose thought experiments and new mathematical formulas to prove them. But the Copenhagen interpretation persists among quantum physicists.
A second issue in the quantum world is the nature of “entanglement.” Here two particles—two atoms, two electrons, two photons, or two other bits of matter that is sometimes energy, or matter that oscillates with wave-like energy, or waves that at the instant of detection appear as singular objects—become joined so that what one of them does, the other will do. This joining and the parallel actions persist through random occurrences—such as passing through a polarized screen—and are communicated instantly across distances that would violate the limit of light-speed travel for any object or piece of information. Here is the sort of “spooky action at a distance” that Einstein derided as a violation of general relativity.
A third issue in quantum physics is the nature of Schrödinger’s cat. To illustrate the limitations of measurement, Erwin Schrödinger proposed the thought experiment of putting a cat in a sealed box with an apparatus that releases a poison when triggered by the decay of an atomic isotope. Since the atomic decay is unpredictable, the cat in the box might be alive or already dead. It was Schrödinger’s point that until an observer opens the box, the cat exists in two “superposed” states—both alive and dead at the same time, expressed by a wave function of probability—and that the wave function does not collapse and reveal the cat’s final nature until the box is opened. As a thought experiment, this is a metaphor for measurement and observation. But some physicists insist that the superposition is real. The actual cat is physically both alive and dead until discovered.
This superposition has led some physicists to describe a splitting of the universe at the point of the box’s opening: one universe proceeds with a physicist holding a live cat; the other with a physicist mourning a dead cat. This is the “many worlds” interpretation. Both universes are equally valid, and both continue forward in time until the next quantum change that forces each universe to split again in some other way.1
Now, I freely confess that I do not have the mathematical skills to understand the equations of quantum physics. And mercifully, Adam Becker’s book does not focus on or discuss the math in detail, just the thought experiments and their supposed meaning. I also confess that I do not understand what condition enables two particles or two waves to become “entangled,” or how they interact at a distance in this state, or what might be required to untangle them. Becker does not explain any of this, either. Further, I confess that I can sometimes be simpleminded, rather literal and obvious about what I see, hear, and know, and oblivious to distinctions and nuances that other people perceive easily.
But, that said, it would seem to me that what we have here is a misinterpretation of a metaphor. The limitations of observation and measurement, as expressed in colliding particles and probabilistically dead cats, are simply reminders that we do not have direct perception of the quantum world in the same way that we can see, hear, touch, and taste, if necessary, a steam locomotive or a billiard ball. That’s a good thing to keep in mind: we don’t have all knowledge about all things. However, to insist that this metaphorical reminder means that quantum physicists are simply doing math, and that their calculations—no matter how enticingly predictive—have no meaning in the real world, that quantum physics is just a mind game … that’s taking things too literally.
I have criticized the use of mathematics to prove the improbable before.2 And I insist again that, if all you’ve got is a series of equations to prove your point, you may just be playing mind games with yourself and your fellow physicists. But the reverse is also true: the real world must exist at the quantum level. If the math works out, if the vision behind it holds together, then it must be describing something that has actual substance and energy. The details may not be exactly as we understand them. The description may be missing some elements, forces, or bits of math that we haven’t worked out yet. But the world must exist in the smallness of subatomic particles as much as it does in the vastness of stars and galaxies.
The math doesn’t exist in a quiet vacuum. The cards, the game, and the players must also exist to give the calculations meaning.
1. I have cheerfully used the many-worlds interpretation in my novel The Children of Possibility, about time travelers from the far future, and in its prequel The House at the Crossroads. But I know I’m having fun and don’t take this stuff too seriously. So much fun, in fact, that I’m now working on the sequel that picks up where Children left off.
2. See Fun with Numbers (I) and (II) from September 19 and 26, 2010.