A physicist who bets that gravity can’t be quantized

The Everett Interpretation, aka Many Worlds, holds that both observer and observed are quantum mechanical systems. From this it follows that the act of observation does not produce a "collapse", but it does separate the observer into multiple observers that can't interact with each other again.

So Schrödinger's cat is in a superposition of alive and dead before the box is opened, and after it is opened the observer is in a superposition of one who saw the cat alive, and the other who saw the cat dead. It feels bizarre, but there are no contradictions. And it is what quantum mechanics predicts.

For those who believe in this interpretation, there is a simple test of quantum gravity. Set up 2 Cavendish experiments to measure gravity. Based on whether there is a click in a Geiger counter, choose which one to put a cannon ball next to. Measure both.

This has been done, and we only see the gravity from the cannon ball that we placed, and not the alternate location it might have been placed at.

But believers in other interpretations of quantum mechanics will disagree that this experiment tests anything at all.

From what this article says, his assumption is that gravity is classical, but "fuzzy" or probabilistic: you can't precisely measure the gravitational field of a sufficiently small object.

In the last years we've seen progressively bigger objects being put in quantum superposition. The theory from this article is incompatible with this process continuing indefinitely. When and if we create a sufficiently big object that we are able to entangle it with something else via gravitational interaction, this would immediately disprove this theory.

So the good news is that this theory is clearly falsifiable, possibly even without creating a particle accelerator the size of the solar system.