Classical physics was invented by Galileo, Kepler, and Newton to deal with everyday macroscopic objects that you can see with the naked eye or with the assistance of telescopes. Classical physics—equipped with calculus and its associated equations—can describe the precise location, speed, direction, and trajectory of any visible object, from airplanes and cannonballs to stars and planets.
If you were to take a snapshot of the solar system at this moment in time, you could measure, using the equations of classical physics, the position and velocity of each planet and could predict the precise location of any one planet at any future time (within a small margin of error up to a limited but significant amount of time). The mechanics of the equations are complex, but the problems are fully soluble.
As Lee Smolin explains in his new book, Einstein’s Unfinished Revolution, this turns out to not be the case at the smallest of scales. When you start asking what matter is made of—atoms, protons, electrons, photons, quarks, etc.—a new type of physics is required, quantum physics. Quantum mechanics was invented in the early twentieth century to explain quantum physics, and seeks to describe how quantum particles behave and interact with each other.
If you think of the atom as a miniature solar system (with the nucleus as the sun and the electrons as planets), you would expect quantum mechanics to be able to chart the trajectory of an electron in the same way classical physics charts the trajectory of a planet. But this is not what you find, because electrons (along with atoms and other subatomic particles) do not behave like planets (or any other macroscopic object). Experiments have confirmed that particles exhibit the following odd behaviors and properties:
- Superposition – quantum particles exhibit weird dualities: electrons, for example, can be in two different places and embody two different properties simultaneously. Electrons are both particles and waves. And if you measure an electron’s exact position, you can’t precisely measure its speed and direction. Conversely, if you measure its speed and direction, you can’t precisely measure its location (this is the uncertainty principle). Electrons pop into and out of existence and change their properties at random. Two electrons may start out in identical states and end up in different positions. That means that you only ever have half of the information you need to make predictions that are anything more than probabilistic. This is contrary to classical physics, and is analogous to being able to only calculate the position or velocity of a vehicle but not both. Imagine driving a car and having no method available to know where it will end up based on its speed and direction. (Here’s a great video that further explains superposition.)
- Entanglement – classical physics relies on the concept of locality. This means that objects cannot influence other objects unless they directly impact them or transmit a force over distances. Distant objects can communicate with each other, but not faster than the speed of light. Enter quantum mechanics, where two particles can influence each other instantaneously at large distances faster than the speed of light. This is the principle of nonlocality. It would be like the earth suddenly reversing its rotation and having an instantaneous impact on the rotation of Mars.
- The Measurement Problem – In classical physics, we can measure the speed and position of objects without altering their course. Each object has a definite position and velocity. Not so in quantum mechanics. Particles only have a definite position after we measure them, and exist in a superposition of possibilities prior to measurement. Further, each measurement impacts the quantum state we’re trying to measure. It would be like the act of measuring the orbit of Mars actually changing Mars’s orbit.
So what are we supposed to make of this? We know that even though quantum mechanics has weird properties and paradoxes and can only speak in probabilities, we also know that it makes extremely accurate predictions and is responsible for the development of many technologies.
It would seem that we have two choices. One is to accept the findings of quantum mechanics as they are, admitting that the underlying reality at the quantum level is unknowable. Quantum mechanics is a shorthand, a useful way to make predictions but that is incapable of describing a reality that either does not exist independent of our minds or that is beyond our comprehension.
Another approach is simply to say that if quantum mechanics disagrees with our deepest intuitions, perhaps quantum mechanics is wrong—or at least incomplete.
The first approach was adopted by the likes of Niels Bohr and Werner Heisenberg, who, along with others, developed the “Copenhagen interpretation” of quantum mechanics, which essentially states that quantum mechanics is complete and that we have to deal with whatever it tells us, even if we don’t like it. This is the anti-realist position. Here’s how Smolin describes the philosophy (in which he does not agree, as we’ll see in a minute):
“Bohr called the new philosophy complementarity. Here is how he talked about it: Neither particles nor waves are attributes of nature. They are no more than ideas in our minds, which we impose on the natural world. They are useful as intuitive pictures that we construct from observing large-scale objects such as marbles and water waves. Electrons are neither. Electrons are microscopic entities that we cannot observe directly, and so we have no intuition about them. To study electrons we must construct big experimental devices to interact with them. What we observe is never the electron itself; it is only the responses of our big experimental devices to the tiny, invisible electrons.”
This interpretation dominated physics for the entirety of the twentieth century, despite the existence of other plausible theories that are starting to be taken more seriously today.
The alternative view, espoused by Einstein, Schrodinger, Smolin, and others, simply states that quantum mechanics is incomplete. In this view, there IS an underlying reality that makes sense, we’ve just yet to find it. This is the realist position, which states that there is a reality that exists independent of the mind, that we can understand it, and that it is the job of science to create intelligible explanations.
Most of Smolin’s book seeks to convince the reader of the realist position and that it is a tragedy that the anti-realist position (the Copenhagen interpretation) was adopted dogmatically for an entire century. Smolin’s main argument is that we have to move beyond quantum mechanics to develop a realist model that is experimentally accurate and coherent. To this end, promising lines of realist research include pilot-wave theory (which proposes the existence of both particles and waves, with the waves guiding the location of the particles) and various collapse models. None have yet proven entirely satisfactory.
So which view is correct? The realist or anti-realist (Copenhagen) interpretation?
While the quantum puzzle remains unsolved—and may remain unsolved indefinitely—I find myself sympathetic to Smolin’s views. To be a realist only requires that you think 1) there is a universe that exists independent of the mind, and 2) that we can understand it in coherent terms.
Regarding the first premise, it’s rather absurd to think that the universe only exists when we contemplate it. As evolved primates, human beings are relative newcomers; the universe existed for billions of years without us and there’s no reason to suppose that it will cease to exist when we’re gone. Our evolution implies an environment available for us to evolve within and adapt to, hence an independent reality. (Of course, perhaps we’ve adapted only to navigate the macroscopic world and therefore have no capacity to comprehend the world at its smallest scales. If that’s the case, then the anti-realist position is, in fact, true, not in the sense that there is no independent reality, but in the sense that we can’t comprehend it, like a person born blind not being able to comprehend sight. We cannot discard this as a possibility, but let’s move on.)
Second, science has made tremendous progress under the realist philosophy. If you chart the progress of science, if something didn’t make sense, we continued to investigate it until it did. If a theory was incoherent, that meant it was wrong.
Think about it this way. You can construct a theory of the universe that has the earth at the center and still make relatively accurate predictions about things like the motions of planets and the predictions of eclipses. But, despite these otherwise accurate predictions, the theory is still wrong; our earth-centered solar system doesn’t describe reality independent of the model.
It seems, to me, that quantum mechanics is like the earth-centered model of the solar system. We can make calculations and predictions with it, but this doesn’t, in itself, mean that the theory accurately describes reality.
(A side note: for all of those who like to invoke quantum mechanics in support of whatever bizarre supernatural theory they’re peddling, they are doubly confused. First, quantum properties do not apply to macroscopic or emergent properties, so even if quantum mechanics is correct, the effects don’t scale up to macroscopic phenomena or emergent properties like consciousness. Second, quantum mechanics could be wrong itself, even in its descriptions of the quantum world. So, remember this: anytime a non-physicist brings up quantum mechanics during a non-related topic, your bullshit meter should be going haywire.)
Still, the anti-realist position could be right, and if it is, then we will never have a coherent theory, and we will be searching forever in vain. The anti-realist position declares the quantum state unknowable, so if that’s true, we will never understand quantum reality, but we will also never get to the point where we know that we can’t understand quantum reality with certainty. This discouraging possibility cannot be ruled out.
But there’s some reason for hope. We’ve encountered difficult problems before, problems that defied explanation. And out of those problems came theories like calculus and relativity, theories that both make accurate predictions and are coherent. In that respect, along with Smolin and in the true spirit of science, I’m holding out hope that we will one day coherently solve the quantum puzzle.
Seven Brief Lessons on Physics by Carlo Rovelli
Reality Is Not What It Seems: The Journey to Quantum Gravity by Carlo Rovelli
The Order of Time by Carlo Rovelli
How to Teach Quantum Physics to Your Dog by Chad Orzel
Philosophy Of Physics by Lawrence Sklar