Image: Mockup Graphics

It’s an ordinary banana, which means it serves well as a graphic to help illustrate why the universe is not purely deterministic.

Many years ago, I had an argument with an old-school BBS user who claimed that life, all of it, was purely deterministic. Rewind the movie, he said, and the same things will happen again. If you shoot an arrow through the air, it doesn’t follow a random trajectory, it goes where it goes because of the laws of physics. Rewind time to when the arrow was fired and it will land in the same place every time.

When I said that on a fundamental level, when you look at the behavior of subatomic particles, they aren’t deterministic, he was like “yeah, but so what? If a single atom vibrates a little bit differently in the arrow, it’s not going to affect the arrow’s path. On a macroscopic level, quantum randomness doesn’t matter. It all cancels out.”

Enter the banana.

Every banana is naturally radioactive. Bananas contain potassium, some of which naturally occurs in the form of the radioactive isotope potassium-40.

Potassium-40 usually decays by ß- decay, releasing a high-energy electron. However, every so often, an atom of potassium-40 decays via ß+ decay, releasing a positron, an antimatter electron. This positron usually annihilates immediately with an electron, producing two high-energy gamma rays.

What does this have to do with determinism?

As near as we can tell, things like radioactive decay and decay modes are entirely stochastic. We can tell how long it takes half of a particular radioactive element to decay, but this is entirely probabilistic; we don’t know which half will decay, only that in such and such a span of time, half the atoms in the sample will decay.

This doesn’t seem to be deterministic at all. There’s no hidden variable. Rewind time and an atom that decayed last time might not decay this time.

So, back to the guy who insisted that weird quantum stochasticity can’t affect the macroscopic world:

Gamma rays are highly ionizing. If a gamma ray strikes a molecule of DNA, it can smash that molecule apart. Sometimes it can be repaired, sometimes it can’t.

It’s possible for a radioactive decay inside a cell to damage the DNA in the cell in such a way as to turn a cell cancerous. I’m sure you can see where I’m going with this.

I think that nobody will argue that “getting cancer” is a profound macroscopic event.

Cancer from radiation is well-understood. Radioactive decay is random. an atom of potassium-40 might decay in a way that causes cancer in someone who has just eaten a banana, but if you rewind the tape, if you go back in time and play it again, that atom might not decay. Or it might decay earlier or later, and the gamma rays it produces sail harmlessly past the DNA molecule without damaging it.

In other words, the universe is not deterministic on a micro or a macro level. Go back in time and replay events again, they absolutely can play out differently.

The odds with something like a banana giving you cancer are very small. But there are a lot of people, a lot of molecules of potassium-40, and a lot of time. Plus this is simply one example of how quantum-level randomness can bubble up to the macroscopic world, it’s not the only one.

Point is, if you were to stop the universe, rewind the clock, and play it forward, you’re not guaranteed to get the same results. The farther back you rewind the clock, the more likely and more numerous these differences will be.

I think that’s amazing. The unbroken thread of events that had to play out just so for all of us to be here in this moment is absolutely astonishing.