There Can Be No Final Theory of Gravity

There Can Be No Final Theory of Gravity

All we have are better and better approximations to reality

Brett: In almost all cases, you only ever have one theory on offer.

In the case of gravity, there literally is only one theory on offer at the moment. There’s general relativity. Previously we did have two theories. We had Newtonian gravity and we had general relativity—but we did a crucial experiment.

This idea of a crucial experiment is the cherry on top of science. You’ve got these two competing theories. If the experiment goes one way, one theory is ruled out but the other theory is not. In which case, you keep that theory for so long as no problems arise.

This vision of knowledge enables us to have an open-ended quest for progress.

This is completely unlike any other idea about knowledge. The overwhelming majority of physicists are still Bayesian. And the reason they’re still Bayesian is that this is typically what’s taught in universities and this is what passes for an intellectually rigorous way of understanding the world.

But all it is is what I would call species of scientism. It’s because they have a formula behind them, the Bayes’ theorem, which is a perfectly acceptable statistical formula. People use it all the time in perfectly legitimate ways. It’s just that it’s not an epistemology. It’s not a way of guaranteeing, or even being confident, that your theory is actually true.

My favorite example of this: Prior to 1919, every single experiment that was done on gravity showed that it was consistent with Newton’s theory of gravity. What do Bayesians say in that situation? They say you’re getting more and more confident in Newton’s theory.

How does that make sense? How do you square that the day before it was shown to be false was the day when you’re most confident in it?

Now, a Popperian doesn’t have this problem. A Popperian says, “At no point was Newton’s theory actually true. It contained some truth, but that truth isn’t a thing that we can measure.”

I say it contained some truth because it’s certainly got a more direct connection to reality than some other random person’s guess about the nature of gravity. Gravity does indeed approximately vary as the inverse square law, but not exactly. We needed general relativity to correct the errors in Newton’s theory of gravity.

And even though general relativity is our best guess right now, it can’t ultimately be the final theory of gravity. There can be no final theory of gravity. All we have is better and better approximations to reality.

Naval: I think the reason we fall into Bayesianism so easily is probably related to why we fall into pessimism so easily. We’re evolutionarily hardwired for Bayesianism.

Every other animal on the planet that can form good explanations is a Bayesian. They’re just looking at repeated events and saying, “The sun rose yesterday. The sun will rise tomorrow.” Or, “That thing I touched is hot. It’s probably going to be hot in the future.” That is how most of our biological systems and most of our evolutionary heritage worked.

It’s just now we have this neocortex that can form good explanations about the seen in terms of the unseen. That gives us a higher level of reasoning, but that higher level of reasoning is not instinctual to us. It requires effort. It requires deep thinking.

We default to Bayesianism because that is how a lot of the natural world around us seems to work at least at the purely biological level.