The conventional wisdom among people who know a little bit of quantum mechanics is that quantum mechanics is weird.
The conventional wisdom is wrong. Quantum mechanics is not weird. Interpretations of quantum mechanics are weird.
My thinking on this has changed over the years. In high school I read everything I could about the “weirdness” of our universe: Schrödinger’s cat, wave-particle duality, the collapse of the wave function, many-worlds theory, the Heisenberg uncertainty principle.
Then a strange thing happened: I went to college. I studied physics. And guess what? None of that stuff gets more than the briefest mention in the physics classroom. Why?
Because those things are beside the point. Quantum mechanics works. How you interpret quantum mechanics is your problem.
There’s a dichotomy here which is the source of most people’s confusion. Theories are different from interpretations of theories. A theory is a mathematical model that allows us to make predictions. An interpretation is a philosophical construct that allows us to sleep at night; it is a squishy heuristic that helps us unimaginative humans make sense of the math before us. Theories get things done. Interpretations never helped anybody, not really.
Let’s say that in an abandoned shack you discovered a notebook with the word “PHYSICS” written by hand over and over, thousands of times, apparently filling every page. You haven’t looked at the last few pages, but your theory is that these pages will also have the word “PHYSICS” written out. Each time you turn a page, your theory is validated: “PHYSICS” is there, as predicted.
Next to this notebook is another that looks just like it. You open the first page, and are not surprised to see “PHYSICS PHYSICS PHYSICS” again. What’s going on? Did some crazy person live in this shack? Such speculation doesn’t really matter, since you can still hypothesize that “PHYSICS” fills this notebook as well. In fact, you have a stronger theory: every notebook in this shack is filled with “PHYSICS”.
You perform an experiment: you turn the page. “PHYSICS PHYSICS PHYSICS”. The experiment supports your theory. You find more notebooks; same results. Every notebook in the shack is filled, apparently, with “PHYSICS”. But guess what? There are dozens of possible interpretations. And in the absence of further data, you can never know which one is “correct”.
Maybe the shack was once inhabited by a crazy person, who wrote “PHYSICS” precisely 250,001 times in a futile attempt at summoning Cthulhu from his ancient slumber.
Maybe a student misspelled “physics” on a test, and her cruel teacher punished her in the most depraved way possible.
Maybe Matt Damon filled the notebooks, in a method-acting attempt to get into the mindset of an OCD scientist.
Which of these interpretations is the “truth”? Without further data you cannot really say. Arguing about which is right and which is wrong is futile at best, and annoying at worst.
Of course, new data may turn up. We might find out that the notebooks are 75 years old, ruling out our Matt Damon interpretation. That interpretation is no longer a valid interpretation of the data.
Which brings me to my next point: there is no official arbiter of what constitutes a theory versus what constitutes an interpretation. Different philosophers and scientists have used the words differently at different times. All you can hope for is that a particular author is consistent in his/her use of the terms. I personally use the word “interpretation” to describe competing theories that cannot currently be differentiated by any known scientific experiment. If two different interpretations make different, testable predictions, then they are promoted to being totally different theories. (Caveat: others use the words slightly differently. Deal with it.)
So what does this have to do with quantum mechanics?
Quantum mechanics is an entirely mathematical theory. Its postulates are logical, concise, and powerful. We can use quantum mechanics to invent cell phones, computers, lasers, and iPods. Quantum mechanics doesn’t care if you “understand what it really means”, or not. It is arguably the most successful and powerful theory to come out of the 20th century.
Now, the mathematics of quantum mechanics are abstract and hard to visualize. Nevertheless, people insist on trying to visualize anyway. And the result is all kinds of weirdness: Schrödinger’s cat, wave-particle duality, the collapse of the wave function, many-worlds theory, the Heisenberg uncertainty principle. These ideas are all mental hoops that people have jumped through to explain some unambiguous, concrete, abstract linear algebra. The math is just math, and it works; what it means is anyone’s guess.
There’s no crying in baseball, and there’s no philosophy in quantum mechanics.
Don’t like the many-worlds interpretation? Fine. Be a Copenhagenist. Don’t like pilot waves? Great. Stick to your pet idea about superluminal communication. Just remember that all of these competing interpretations make the exact same predictions, so for all practical purposes they are the same. Some people go so far as to say, just shut up and calculate. [Note added 3-19-14: there are problems with pilot wave theories that in my view rule them out as being a valid interpretations of quantum mechanics. But there are hoops people can jump through to try and “force” pilot wave theories to be consistent with, say, Bell’s theorem. My broader point is that there are multiple interpretations of QM and that all have followers to this day, but that none of the interpretations really have any distinct implications for our lives.]
I don’t usually go that far. I actually think that the many-worlds interpretation is a testable theory, not an interpretation (hence the name of this blog). I think many-worlds is falsifiable. (If we ever observe a wave function collapsing, then many-worlds will have to be discarded.) But I don’t think that will happen: many-worlds is too elegant, and too powerful, to not be true.
But we’ll see.
If you think it’s absurd that a cat can be alive and dead at the same time…if you think that it’s crazy to hypothesize other universes…if you think that God does not play dice with the universe…don’t blame quantum mechanics. Blame the philosophers who try to interpret it.
Quantum mechanics works. Otherwise, you’d be reading this on an actual piece of paper.