I'm Steve Strogatz, and this is The Joy of Why.

A podcast from Quantum Magazine that takes you into some of the biggest unanswered questions in math and science today.

In this episode, we're going to be talking about what it means to be alive.

What is life? Can you define it?

Scientists don't actually agree on a definition.

It sounds weird, right?

I mean, most of us would say with some confidence that a bird is alive and a chair is not.

But going deeper, scientists ask questions like this.

To be considered alive, does something have to be able

to reproduce? Does it have to be a product of evolution through natural selection? Does it

need to have a metabolism and be able to process energy? Any definition along these lines is

riddled with exceptions. For instance, is a virus alive?

Well, viruses do evolve, but they don't replicate on their own. They use the host's cellular

machinery to make more copies of themselves. The question of what life is also matters, because

if we're going to be looking for life on other planets, don't we need to at least have some idea of what life is also matters, because if we're going to be looking for life

on other planets, don't we need to at least have some idea of what we're looking for?

Later on in this episode, we'll hear from Sharif Manzi, Professor of Chemistry and the Department

of Chemistry at the University of Alberta. But first, joining me now is Robert Hazen. He's a mineralogist, astrobiologist,

and senior staff scientist at the Carnegie Institution's Earth and Planet's Laboratory.

Bob, thanks so much for joining us today. Oh, it's a pleasure. Thanks so much, Steve.

Great. Well, let's jump right into this. Why is it so hard for scientists to agree on something that commonsensically, most people would say they already understand. Like, we know that a plant is alive and a rock is not. Why is it so hard to come to some agreement about the definition of life? Yeah, that seems strange, doesn't it? Because we all know things that are alive,

and we all know things that aren't alive. And yet it's that gray area in between. So when we

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