When I tell people that my passion is trying to figure out what quantum mechanics means, they tend to respond with a blank stare. And if I happen to be talking to a physicist, there’s often also a hint of sadness in their eyes, as if to say, “Poor you.”
After all, quantum mechanics (or “quantum theory,” a synonym) is an obscure topic for most people. And most physicists who are aware of what I mean by the question of “what quantum mechanics means” also know that some of the smartest people in the world – including most famously, Albert Einstein and Niels Bohr – have been debating it since the theory first took shape in the mid-1920s. A common attitude is that the question is philosophical and not scientific, that there is no way to establish the right answer and that, therefore, people are destined to argue about it forever, much the way they have about the existence of God or why there is a universe at all.
But some physicists (and even philosophers) are more optimistic. The reasons why vary between people and are hard to describe without getting into a detailed description of the problem itself, some of the proposed solutions, and how those proposals connect to other topics in physics and science in general, including gravity, cosmology and even human consciousness.
Still, here in this little blog post – hopefully the first of many – I think I can give you a sense of what the issue is and why it’s important. One big reason why many people are not yet ready to give up on finding the right answer are the stakes involved, which are, I believe, very high.
To do that, I have to say a little, bit not too much, about how quantum mechanics differs from pre-quantum, or classical, physics. So here goes. Classical physics starts by describing objects in terms of properties we can see and measure. Think of Galileo letting balls of different masses roll down ramps and measuring how far they’ve traveled in a given amount of time. Other than a ball’s mass, the properties of it which mattered for Galileo’s experiment were its position in space and its speed, both of which change in time.
Galileo observed that, all other things being equal, balls of different masses traveled the same distance in the same amount of time. Isaac Newton later explained this behavior with his laws of motion and gravity, which said that i) an object’s acceleration is proportional to the force applied to it divided by its mass and that ii) the force of gravity that the earth applies to the balls is proportional to their mass. Therefore, balls rolling down identical ramps, as well as objects falling unimpeded to the ground (i.e. ignoring air resistance) all experience the same constant acceleration and travel the same distance in the same time irrespective of their mass.
That’s a classic example of classical physics at work: objects have properties such as position and speed, and the laws (or equations) of physics explain how those properties change in time in different circumstances. Physicists call the properties of an object that are subject to change (e.g. position and velocity) its “state” and use the laws of physics to explain, or predict, how an object’s state changes with time.
Now, what physicists gradually realized in the early years of the 20th century is this whole scheme of describing objects with a state and finding laws that explain how the state changes with time doesn’t work for very small objects such as atoms and the particles that they are made of – protons, neutrons and electrons. Instead, they had to come up with a different scheme, which we now call quantum mechanics.
The new, quantum scheme differs from the old, classical one in two important ways. The first is that the quantum laws of physics (again, the equations) only predict the likelihoods (or probabilities) of the outcomes of experiments (think in terms of where, say, an electron ejected from an atom ends up on a screen.) The second way that quantum physics is different from classical physics is that its equations do not describe objects such as electrons in terms of properties such as position and velocity; instead it describes them with a “quantum state” (aka the wavefunction) which is essentially a list of numbers whose meaning is not at all clear, but which can be used to compute the aforementioned probabilities of the outcomes of experiments and other observations. (I’ve packed a lot into this relatively short paragraph, so if these ideas are new to you I recommend reading it another time, or two, slowly!)
That’s really all you need to know to start appreciating the mystery of quantum theory. There’s more to it than this, but the key question is, essentially, what exactly does the quantum state mean: what is it describing and why do we need to use it to make predictions in the subatomic realm? Also: what is preventing us from describing particles such as electrons with the properties of position and speed that work perfectly well for objects that we can see, such as baseballs? Zooming out a bit, since all of the physical world is thought to be governed by quantum theory and since chemistry is built on physics and biology is built on chemistry, the meaning and scope of all the hard sciences are thrown into question by this ambiguity about what the quantum state means. What is it, ultimately, that science is describing? A freestanding physical world that is independent of our interventions? Our experiences when we interact with the world? Our beliefs about those experiences? The stark fact is that no one, at this point, knows.
Of course some people think they know, but there is no consensus amongst the experts, even after a century of theoretical and experimental progress and debate. Some believe the quantum state represents a physical entity, which implies that the baseball-like properties of position and velocity could be part of, but are not the full, story at the subatomic scale. Others believe that the quantum state describes our knowledge (and ignorance) of particle properties, rather than the properties themselves, much the way we might consider the state of a tossed coin is “50/50 heads or tails” before it lands. Still others think that quantum mechanics is only an approximation to a more accurate theory that will, once it’s found, once again describe objects in terms of good old-fashioned physical properties.
Because quantum theory underpins all the physical sciences and phenomena 1 the answer to the question about its meaning informs practically everything we know or can know about the world. One interpretation of the quantum state says the universe we know is just one in an enormous and ever-blossoming multiverse. Another says that human actions (as determined by their freely made decisions) are continually building and shaping the universe, that what we think of as physical laws aren’t actually hard-and-fast limits and that we may ultimately be able to break them by, e.g., traveling faster than the speed of light, e.g. Others say that our decisions are predetermined and that there is no free will, while still others suggest that – despite the stunningly successful progress of science over the past few centuries – we have barely scratched the surface of understanding the universe and our place in it.
So, when I say that my passion is trying to figure out what quantum mechanics means, it is also these kinds of questions that I’m trying to get some handle on. What is the fundamental nature of our universe? Is our universe just one among many? What, if anything, is special about humans and life in general? How much of science describes what’s outside of us, versus what’s happening in our minds? What is consciousness, do we have free will, what role do humans play in the fate of the universe, etc. etc. etc.?
I might be ambitious but I’m not so megalomaniacal to think that I’ll ever definitively answer, or even make meaningful progress on, such gigantic questions. But, as someone who does not believe in supernatural things, I don’t see how anyone else can hope to make real progress on them either until we have figured out what our most fundamental physical theory means.
I don’t know if I can contribute much to progress on that question either. But I’m trying, if only to experience time on one frontier of human knowledge and to not regret later that I did not give it my best shot. At the very least, I’d like to make enough progress, at least in my own mind, to have more educated and firmer opinions about the other big questions.
Since writing helps me clarify my thinking, I plan to use this blog as a kind “scratchpad” where I can scribble about what I’m learning, what my ideas are and, if others choose to chime in, what I think of their ideas as well.
1 With the exception of gravity, which could in fact be a clue to quantum theory’s meaning.
Hi. I have been a fan of your musings on the quantum world for a couple of years. I am an amateur quant (maybe with some qualifications). What fascinates me is that the views of very diverse disciplines seem to be coalescing on some answers to philosophical questions such as reality, mind, consciousness, the universe. I find the quantum physics angle especially interesting, because it is based on mathematics — the last thing one would associate with philosophical questions.