This is the second of three entries on quantum mechanics. Read the first here.
Now that I’ve spent some time talking about how other people get quantum mechanics wrong, it’s about time I get my own chance to screw it up. Following the set of arbitrary rules I laid out in my last quantum mechanics entry, I’m going to start by listing some things you cannot do with quantum mechanics:
- You cannot influence the universe by thinking about something.
- You do not have a chance of spontaneously teleporting to Mars.
- Your ex does not have a chance of spontaneously teleporting to Mars.
- You cannot travel into alternate universes.
- You cannot travel faster than light.
This can all be summarized by stating:
- You cannot do anything impossible in “normal” physics using quantum physics.*
If you retain anything from reading this post, let it be this! Also, see that asterisk? I did say before that I’d alert you whenever I said something that is debatable, and that’s what I’m doing there. But fear not, I’ll explain it all in fine print at the end of my third QM entry; ignore it for now.
Anyways, given that little postulate stated above, you may ask what the difference between quantum physics and normal physics is. Well, back in the Renaissance, the prevailing notion was that the universe was like a complex mechanical computer; that given the state of the universe as it is now, the laws of physics would procedurally generate every future state without any ambiguity. Another way to describe this is that you could always predict everything that’s going to happen in the future if you knew everything about the universe right now. However, the basis for quantum mechanics is that this is false and you should feel bad for thinking that!
The basis of quantum mechanics is that the laws of physics are fundamentally incomplete. For certain situations, the laws of physics don’t say what should happen specifically, only what can and can’t happen. An intuitive example of this is standing a normal pencil on the pointy end; after letting it go, it will inevitably fall down, but there’s no obvious preference of direction in which it will fall down to. What Renaissance physicists would counter with is that knowledge of the pencil tip shape at the microscopic level and the motion of the air molecules in your room will always tell you how the pencil will fall down—and that is absolutely true, with the caveat that it is functionally impossible to obtain that information.
The real problem appears when you go down to the smallest scales of the universe, to a single isolated particle (or a few); there’s no hidden features there, no “tip shapes” or other properties to rely on when the laws of physics don’t explicitly tell you what’s going to happen. Of course, there are no subatomic pencils, but there are multiple processes in the quantum world (mostly involving radioactive decay) that preserve that feature of not having a precise or well-defined outcome.
This leads to two critical questions, the first being pragmatic and the second philosophical:
- What do these objects do when the laws of physics don’t determine their specific future?
- What do these objects become when the laws of physics don’t determine their specific future?
The answer to the first question, which may or may not be intuitive, is that they’ll do one of the things the laws of physics say they can do! Our quantum pencil will fall in one direction sometimes and another direction sometimes—and the best we can do is quantify those probabilities using experiments, develop some physical laws for those probabilities based on our data, and that’s that.
If you think about it, there isn’t really anything quantum-y about that at all. This is exactly the way we’d try to describe the physics of our normal pencil falling down; setting it up to fall down a bunch of times and recording the different outcomes to guess the probabilities of it going one way or the other. This is so ubiquitous in the world of science that it is its own long-standing field of physics: in short, bo-ring!
Where things get spooky is when discussing the answer to the second question I presented above. See, for normal objects, not measuring what state they’re in doesn’t affect anything about the probabilities in which you can find them. When the normal pencil falls down and you’re not around to check on it, you can still say it fell down to some specific position; you just don’t know which until you check. For a quantum pencil, this can’t be true!
This is due to the fact that, for objects with no specific future defined by the laws of physics, the probabilities of its possible outcomes can affect each other. This is completely insane! For a quantum pencil, the fact that it could fall to the right can affect its probability of falling in every other direction*. As a result, the chances that a quantum object will behave one way or another can be affected by when and how you interact with the object as you check its status! (The precise way in which those probabilities affect each other is so tricky that they not only needed to describe it indirectly by instead describing a related quantity, but they also had to use imaginary numbers to be able to describe that.)
For this reason, we can’t really say the quantum pencil exists in the same way a classical one does before you interact with it. In fact, it is literally impossible to comprehend it traditionally since it violates one of the three classic laws of thought! For the purposes of our brain, when the quantum pencil falls down and you’re not around to measure it, it’s in some weird glitch state until you interact with it, where you’ll find the quantum pencil fell down either to the left or right just like the normal one would.
To illustrate this difference from a philosophical point of view, let’s say that I was an amoral psychotic and decided to link the life or death of a cat to the direction in which a normal pencil is falling down. If the pencil falls to the left, the cat lives, while if the pencil falls to the right, the cat dies. I, meanwhile, am getting some coffee as this insane little experiment is going on.
At some point while the barista is preparing my ristretto, the normal pencil falls down and the cat either lives or dies, and will remain in that state as I go check on what happened with hot coffee in hand. Nothing weird here (other than the part where cats are dying).
Now we replace the normal pencil with a quantum one and observe that, because the life and death of the cat depend on the state of the pencil, the cat gets put into a glitch state too after the quantum pencil falls down! As a result, I can’t really say anything about whether or not the cat is alive or dead until I go check on it. (This is a very famous thought experiment). Also worth mentioning is that in normal physics, this linking of statistical outcomes between pencil and cat has a name, but in quantum mechanics they call it something fancier even though it’s functionally the same thing.
If we want to make things really interesting, we can extend this experiment a little bit. Let’s say that, when I look at the cat post-pencil drop, I have an emotional reaction that differs based on what state I find the cat in. If it’s alive, I’m happy, and if it’s dead, I’m sad. Now let’s get a bit meta and say that some AI that can detect emotions is experimenting on me experimenting on cats, and that it was installing some Windows update until after I see the outcome of my experiment. In that situation, from the point of view of the computer, both the cat and me are going to be in a glitch state until it checks what emotion I’m feeling; even though I am clearly seeing the cat as being either alive or dead!
This means the strange glitchiness is totally dependent on frame of reference and independent of whether or not something “conscious” is measuring things! This isn’t the first time we’ve run into this craziness, but it does have a lot of philosophical juice in QM that people love to spend hours debating on (presumably while getting high on something). As a result, and this is a thing a lot of people get wrong, human consciousness does not affect quantum mechanics.
Alright! That’s enough for one quantum entry. In the next one, I’ll discuss some more cute examples of counter-intuitive behavior in QM and keep trying to stay on my mission of making quantum boring again. Wish me luck!
Arnaldo Rodriguez-Gonzalez 