Each day when you wake up, the world is, for the most part, unchanged from the day before.
The sun rises again in the east. Your underwear falls if you drop it. The water in the sink spirals down the drain like always. Just as important, your mattress won't turn into a sports car and you can't jump into the air and fly like Superman.
Reality, in other words, seems pretty stubborn, pretty fixed — and pretty much independent of whatever is going on in your head.
But is it? Is it really all those things?
Now, you might think these are questions for philosophers pondering the imponderable alone in their ivory towers. Certainly, these can't be questions for physicists who are supposed to describe what's happening right in front of us? Remarkably, it turns out these kinds of questions are exactly what physicists have to confront when they seek to understand the best of their own handiwork — the domain of quantum mechanics.
Quantum mechanics (or "quantum physics") is the body of knowledge related to the nanoworld of molecules, atoms and the component parts. It's the most powerful and accurate theory human beings have ever, ever, ever developed. The computer you're reading these words on now wouldn't be possible without quantum physics. But beneath all that power is a remarkable paradox that should never be forgotten: No one knows what quantum mechanics is talking about.
Now, let me be clear what I mean by this statement. Theories in physics are expressed in the language of mathematics. Before quantum mechanics, the mathematics for a given theory (like Newton's mechanics or electromagnetism) might have been hard, but it was still readily interpreted. That meant you could still create a "picture" in your head about what the math was describing: billiard balls colliding; planets moving in orbit; waves propagating through space.
But when physicists began probing the realm of the atom, the behavior they found was very weird and very different from what's observed for macro-scale objects like billiard balls and planets. In response to this dilemma, the founders of quantum created a new kind of mathematical physics that could describe what was seen in experiments. More importantly, this mathematics predicted the outcome of experiments with astonishing accuracy — basically the equivalent to firing a rifle bullet from New York City at a target in Los Angeles and nailing the bull's eye to within the width of a dime. It was that good.
The only problem was that no one knew how to interpret the mathematics.
This meant there was no simple way for understanding what the mathematics was describing. It couldn't tell us what, for example, an electron was — in-and-of-itself. And if we couldn't picture the stuff making up reality (like electrons) then we must still be in the dark about reality itself.
One can, of course, ignore all the metaphysical questions and simply "shut up and calculate." That approach works fine for creating computers and other powerful gadgets from quantum physics. But, for a lot of us, ignoring reality was not why we got into this business.
So throughout the past 100 years, physicists have proposed a lot of different ways to interpret their mathematics and, in the process, explain what quantum theory tells us about the fundamental nature of "The Real." These interpretations tend to fall into one of two camps.
For the first camp, the mathematics directly describes a reality that is independent and objective. In this view, quantum mechanics is an ontological theory (ontology is the branch of philosophy dealing with what truly exists). For the second camp, however, the mathematics of quantum mechanics describes only our knowledge of the world. For these folks, quantum physics is an epistemological theory (epistemology is the branch of philosophy dealing with what human beings know and how they know it).
Ontology vs. Epistemology: the world in-and-of-itself vs. just our knowledge about the world. The split between these camps can get pretty contentious. That's because most physicists start off as ontologists. When we're young we get excited about our equations. We think they are so powerful they seem to be like "thoughts in the mind of God," pointing to a truth that lies beyond the daily concerns and limitations of human life.
But the encounter with quantum weirdness can shake that vision for some physicists. Many of the founders of quantum theory were convinced that their new theory was telling them that ontology was no longer possible for physics. For them, physics was the act of learning about our interactions with the world, not the world in-and-of-itself.
The problem with all these interpretations is that, in general, there remains no way to distinguish between them experimentally. All anyone can do is argue philosophical positions based on, well, philosophy.
In a few cases, however, so-called "no-go" theorems have been proven — turning out to be enormously powerful. A no-go theorem tells scientists when certain kinds of physical situations are fundamentally impossible to achieve given the laws we understand. For example, in 1964 John Bell derived a set of relations (the Bell Inequalities) that could distinguish between true quantum weirdness and the possibility of a more classical "normal" reality hidden beneath what was seen in experiments. Experiments using Bell's no-go theorem eventually showed that quantum weirdness ruled.
More recently there has been so-called PBR theorem (which has nothing to do with hipster beer but, instead, was named after its creators Matthew Pusey, Jonathan Barrett and Terry Rudolph). PBR is also a no-go theorem that appeared to eliminate an entire class of epistemological interpretations for quantum physics. It was a very big deal — and its meaning is still being debated. But the PBR theorem didn't eliminate the most epistemological of epistemological interpretations. This is the so-called Copenhagen view that claims there is no way to talk about the world having any properties in-and-of-itself. In the Copenhagen interpretation, electrons don't have intrinsic properties like position or spin. It's only the act of measurement that makes the electrons take on specific values of these properties.
So which is it? Does the world have an intrinsic ontology? Is there something out there independent of us that has specific properties in-and-of-it? Or is it all a mush of potential and possibility about which only our knowledge takes on a stable form?
The fundamental question remains. How real is reality?
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4