ARI SHAPIRO, HOST:
It's good to have people on call when you need them - a plumber, a doctor, an astrophysicist. Here at NPR we have blogger Adam Frank, and we decided to call him up on behalf of all of you, our listeners, so that you can ask your big questions and Adam will give us some understandable answers. Hi, Adam.
ADAM FRANK, BYLINE: Hey. How's it going?
SHAPIRO: Good. Thanks to everybody who tweeted their questions. You want to read the one that we've got up next?
FRANK: OK. The question this time is, if there are points in space with only three atoms per square meter, what fills in the rest?
SHAPIRO: Let me start by asking, are there points in space with only three atoms per square meter? Is that a thing?
FRANK: Oh, yeah. Actually most of space is pretty empty.
SHAPIRO: But when I think of emptiness I think of, like, air. And air is not really emptiness. What do you mean...
FRANK: Yeah, that's...
SHAPIRO: ...When you say emptiness in the context of space?
FRANK: OK. So for reference, there's about 10 to the 25 molecules in a cubic meter of air. This translates into 10 million, million, million, million bits of matter in the space about the size of a beach ball. So when you take a...
SHAPIRO: So that's a lot of stuff even if it's invisible, weightless stuff.
FRANK: Right, exactly. So when I take a deep breath (inhales), really what I'm doing is I'm filling my lungs with all those little bits of matter, all those particles of air.
SHAPIRO: And when we talk about emptiness in space, there's no air. There's maybe three atoms per square meter. And the rest...
FRANK: Yeah, well, so this is the amazing thing about space. It is remarkably empty - right? So you know, there are some places where there's just one little bit of matter in the same beach-ball-like volume. And there are some places where it's even less than that. You'd have to actually search an even larger volume of space before you found a lonely atom. So yes, space is pretty damn empty.
SHAPIRO: OK. So just to answer the listener's question, if there are points in space with only three atoms per square meter, what fills in the rest? The answer is nothing...
SHAPIRO: ...Fills in the rest.
FRANK: Right because essentially for a physicist, the absence of matter is nothing. I mean there is still space and time there, but you know, there - the absence of matter we consider to be a state of, you know, zero matter, zero energy density, is a way of putting it.
SHAPIRO: That's University of Rochester professor and NPR science blogger Adam Frank. Thank you, Adam.
FRANK: It was a pleasure.
SHAPIRO: And if you have a question about physics, astronomy or science in general, ask us, and we'll put it to our astrophysicist. The show is on Facebook and Twitter @npratc.
[POST-BROADCAST CORRECTION: The audio for this story incorrectly states that there are points in space with only three atoms per square meter. The correct distance is three atoms per cubic meter.] Transcript provided by NPR, Copyright NPR.