The Last Word On Nothing

A neighborhood kid, maybe 10 years old, doesn’t have the usual relationship with gravity.  I know it’s her even when I can’t see her clearly by the way she moves through space: even when she’s not running, just walking, she looks like she might re-connect with the earth but also she might not.  She reminds me of the young dog who blew past me on my walk yesterday, ears back, head down, feet folded under its body and more off the ground than on.  She reminds me of the ballet dancer who was asked how he managed such high, long jumps and who answered, “I just go up in the air and stay there a while.”

I was interviewing an astrophysicist about the haloes of gas that surround galaxies, about the arguments over whether the gas was drifting down into the galaxies or just hanging up in space and staying there a while.  She said, “Matt thinks the halo clouds are sub-virial.”  Sub-virial.  I hadn’t heard that before, I could guess what it meant, and I’ve been looking for that word for my whole life.

Sub-virial is another of the metaphors with which science graces our lives.  The virial theorem is some math and science that’s probably not complicated but I look at the numbers and words, and giant fog bank moves over my brain and I have to remind myself to breathe.  I’m pretty sure it’s a way of describing some system (like solar system or a galaxy or a cluster of galaxies) that’s full of moving parts (like planets or stars or galaxies).  If the system is massive enough and the parts are moving slowly enough and and packed together in a small enough area – called the system’s virial radius — the system can’t fly apart and the parts can’t leave it; the system is gravitationally bound.   That makes sense.  I think it’s a refined way of describing gravity:  if I want to get away from the sun, I’d have to be an extremely fast planet, achieve escape velocity, cross the solar system’s virial radius, and get the hell out of Dodge.

The astrophysicist was worrying about whether the gas in the halos around galaxies would stay up there or would fall back in.  Her colleague Matt thought the gas was inside the galaxies’ virial radii, it would fall in, it was sub-virial.

I’m not sure why that idea is so delightful.  I certainly don’t want to go super-virial (I made that up) and leave the earth: it’s dark out there, I don’t want to be shooting around in space all by myself.  Being bound is nice.  The earth is bound to the sun and the stars to their galaxies and the galaxies to their clusters.  In fact, that quality of being bound gives the universe its orderliness.

But the ballet dancer, the dog, and the neighborhood kid — something about the ability to look like gravity might be ignorable makes me want to jump around. I did that once when I got good news, I stood in my little dark office and jumped all round it. Maybe almost-not-bound is best, still grounded but hair stuck straight out about to lift off — like the little neighborhood girl, just barely sub-virial.

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Photo by Elo Vazquez via Flickr