Causality is not a part of the actual world

This is an update of a point that I have written about in two essays,

• Could have, would have, should have (2020)
• Reality and reality boxes (2023)

but in the form of a presentation for the Fermi Society of Philosophy, and focusing on just the point about causality: that it's a feature of possible worlds, not the actual world.

Here is a video of this talk, as it was presented on February 15, 2024.

To view the slideshow

Click on this link:

and once JupyterLite has loaded, click on the small, orange jupyterlab-deck icon on the top toolbar:

Select cells and control-enter (Windows, Linux) or command-enter (Mac) to run them in the slideshow. Use the arrows on the bottom-right to navigate between slides.

You will probably also want to increase your browser window's zoom to 150‒200%, using control-+ (Windows, Linux) or command-+ (Mac).

Abstract

"Causality" and "to cause" are common words in physics, particularly in relativity and quantum mechanics. This would make it seem like causality is a quantity in physics, akin to energy or gravitational strength, but it is not. For "A causes B" to mean "B would happen if A happens," a causality relation has to range over sets of possible worlds, not just the actual world in which physical quantities reside.

In this talk, I'll argue in favor of causality as a property of a set of possible worlds, both at the qualitative level that it is usually presented in modal logic, but also with computer simulations that serve as a mock-up of a set of physical worlds. In particular, I'll show a case of two physically identical worlds that are products of different physical laws with a different causal structure, to underscore the point that causality requires more than the actual world to be sensible.