Physics Question #45

Serge Bellemare, a 52 year old male from the Internet asks on August 7, 1999,

Do all planets and stars rotate on themselves, and do we know why?

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The answer

Donald J. Barry answered on August 7, 1999

Rotation, like death and taxes, seems to be an essential part of the Universe. Since space itself is frictionless, there is, in general, no damping force to slow things down. So whatever rotary motion objects obtain or have, no matter how small, they generally keep.

The one exception, curiously enough, seems to be the Universe itself, taken as a whole. We cannot measure any net rotation to the entire show, although this does not mean that such a motion does not exist. It's just below our level of detection. Ernst Mach, the German philosopher, asked an unusual question in the last century. He said: "Pretend one is floating in space amongst the stars and galaxies, eyes closed. Are we spinning?" How would we find out? Well, if we extend an arm and feel it tugged away, as someone would on a merry-go-round, then we know we are spinning. So we reach into a pocket and pull out a few rocks. We throw them against the spin to slow ourselves down. We keep doing this until we no longer feel any force on our arms when extended. So now we are not spinning, at least according to the space around us. Let's open our eyes. Is the universe spinning with respect to us? No. So there is a deep connection with what we find to be "spin-free" for ourselves, and spin-free for the universe.

Why should this be? We don't really know. Einstein provided equations which tell us how the "spin-free" condition of reference must relate from here to there. But his equations permit the, admittedly bizarre, case where what is not spinning to us is spinning for someone else. But it seems that irregularities like this were smoothed out in the Universe at the very beginning. So the Universe as a whole probably doesn't have spin, at least in any even remotely significant way.

So what about spins of objects within the Universe? That's also a difficult question. Equations of nature do tell us that in a gravity-bound system, things get lumpier over time, and motions and spins all become exaggerated. But this requires that motions and spins be non-zero to begin with, otherwise there is nothing to amplify. So the Universe must have had primordial motion or lumpiness. But almost infinitesimal amounts of this are sufficient to explain the motions and spins in the Universe today. It's like a bowling ball poised atop the sharpest pin -- the vaguest hint of a disturbance topples it. And so the question of spin probes questions of the deepest past and earliest Universe. That makes it worthy of study.

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