Hello, everyone. Welcome to the Mindscape Podcast. I'm your host, Sean Carroll.

Quantum Gravity is a topic that we return to again and again,

in part because I think it's really interesting. It's part of what I do and my research career,

but also because it's a great example of science inaction,

or at least theoretical physics inaction. Theoretical physics might not be representative as a science,

but it's an example of a science.

And we know that there's quantum mechanics. Those are the fundamental ways that the world works.

We know there's gravity. It exists. So somehow they need to be reconciled and we're not sure how.

If you've read the big picture you've heard me talk about the laws of physics underlying everyday life,

you know that we have enough idea of how quantum gravity works to explain simple conditions,

like the solar system, why apples fall from trees, but when things get extreme in black holes or the big bang,

we don't have the full theory, so we don't know exactly what to say.

We do have a set of rules for taking a classical theory, like Einstein's general relativity,

and quantizing it, but those rules don't work for gravity, or at least not in any ordinary straightforward way.

So by following progress in quantum gravity, you can kind of see how science works when we don't know the answer

and also for that matter when there's not a lot of detailed experimental evidence.

There is experimental evidence, namely all of the experiments that say the gravity is real

and all the experiments that say the quantum mechanics is how the world works.

But that's not a lot of guidance when it comes to reconciling them.

And of course we know that there's different strategies for doing this.

Loop quantum gravity is something that is still popular.

The very second episode we ever did of Minescape was Carlo Orvelli, who talked a little bit about that.

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