Sean Carroll

Marie Roscu says in episode 268 with Matt Strassler, you were talking about terms in physics that are not accurate, like we say particle, but we mean wave, etc., as we just saw.

And Matt said at one point that he found the word matter interesting.

being problematic and unclear.

And then later I heard Jacob Brandes, another Mindscape guest, in one of his talks saying pretty much the same, and he put it like this.

Anything that is not the gravitational field, that can source gravitational fields or respond to gravitational fields, we can refer to as matter.

Where do you stand on that matter?

Yeah, you know, this is something where I'm pretty forgiving, actually, like in this sort of use of nomenclature.

And maybe I shouldn't be.

I don't necessarily defend my easygoing nature on this particular issue.

But the point is, it's absolutely true.

that physicists use the word matter and mean different things.

Physicists use almost every word in multiple meanings and expect that as long as they're talking to other people who know what they're doing, they will understand the context that you're talking about.

You know, astronomers refer to any atom bigger than helium as a metal.

So oxygen is a metal and so forth, right?

But they know that, you know, they talk about the metallicity of a stellar atmosphere or something like that because they assume that you know what they mean.

Likewise, if a cosmologist talks about matter in the context of the accelerating or the expanding universe, they're distinguishing matter from radiation.

They mean any kind of radiation.

source of energy density with the property that its energy density decreases as the scale factor to the minus 3.

That is to say, the energy density goes down exactly as the volume goes up, because that's what slow-moving particles do, like

dark matter particles or stars or galaxies, okay, or black holes for that matter.