Yes, that's plural. Multiverses. I have spoken many times before on this podcast about "the multiverse". Indeed it is a central theme of ToKCast and a thread running through both the Fabric of Reality and The Beginning of Infinity. But here we discuss other kinds of multiverse - Max Tegmark's 4 species of multiverse. To what extent do they count as science? Are they testable? Does that matter? I found this one a lot of fun.

As an alternative to Max Tegmark's work on all this, the lesser known but perhaps more specialised Luke Barnes (@lukebarnesastro on Twitter) focusses on Fine Tuning in cosmology. His website https://letterstonature.wordpress.com/luke/ is prolific when it comes to this issue and he takes on the problem from a vast array of perspectives.

This is Sabine Hossenfelder & Luke Barnes debating "The fine tuning of the Universe: Was the cosmos made for us?" https://www.youtube.com/watch?v=5OoYzcxzvvM

And this is him "against" Sean Carroll: https://www.youtube.com/watch?v=nJEWg1ifUCg These discussions with Luke (a relative unknown) up against "celebrity" physicists can be really interesting for a couple of reasons sociologically. Luke's no-nonsense Aussie attitude against a continental European in the first instance and an American in the second instance is just worth noticing for the subtle cultural differences (very subtle perhaps!) and also because Luke, as I say, is highly specialised on this particular problem of fine tuning of the laws of physics. Sabine and Sean understand the basics of this - but it's not their day to day work. It is Luke's and so that difference is telling at times. Finally here is Luke's discussion with Robert Kuhn of "Closer to Truth" - what I said is my favourite Youtube channel (where you can also find discussions with David Deutsch, Jaron Lanier, Paul Davies and well, almost anyone who's anyone in physics/science/cosmology/philosophy: https://www.youtube.com/watch?v=xY7Ck1y1fx4

Also see his page on the Closer to Truth website which has his background and links to lots of his videos: https://www.closertotruth.com/contributor/luke-barnes/profile Luke may be an Aussie, but I don't actually know him personally - I just happen to think he is a particularly cogent voice on these issues!

Ep 136: Things that make you go mmmmm? Part 3: Multiverses

ToKCast