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The Straight Dope


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How can a person "see stars"? When you exert yourself physically and then stop, you have these hundreds of little BBs zooming in space in front of you. I have actually believed I could touch them. Try staring at the ground about ten feet in front of you and follow one of the lights out of the corner of your eye until it blinks out. It's kind of a kick. Explain how this physical change of sight and mind can occur as easily as doing a cartwheel. --Terry, Waunakee, Wisconsin

Not being one for cartwheels, Terry, Cecil has been trying to see what kind of business he could drum up doing somersaults. This has moved him to thank God he doesn't have any kids around to see what kind of weirdo their father is, particularly after that session last month with the Hula Hoop. I did, however, get a passel of zooming BBs on one occasion, along with a monster headache. The little spots of light, which are to be distinguished from the opaque spots or threads we've discussed in the past, persist for perhaps five or ten seconds and appear to swim around. But I only got them after a particularly crazed gymnastic exhibition. What do you do, conclude your cartwheels by slamming into a wall?

BBs, "stars," and other "nonphotic" visual stimuli (i.e., those not actually produced by light) are called photopsia or phosphenes. They're believed to be caused by mechanical stimulation of the nerves of the eye. Another example is the sensation of light produced by pressure on the eye, a phenomenon once described by no less an authority than Aristotle.

Young people see stars every once in a while as a result of a blow to the head or some sudden exertion. But the little BBs don't really become common until retirement age. What happens is that the eyeball fluid, which is contained in a sort of sac, starts to pull away from the back of the eye. This is called "posterior vitreous detachment," and it usually occurs suddenly, often following a jolt to the head. You see stars and spots and your vision is blurred and distorted. The stars may persist for weeks or even years as fibers from your eyeball sac continue to tug on your retina.

Grim though it sounds, vitreous detachment is normal, occurring in maybe half the population. Apart from stars and spots, your vision usually winds up about the same. But stars (and spots) can also herald a detached retina, which is bad news indeed. I made a New Year's resolution, however, that I was not going to sow panic amongst the populace, a resolution that has already been sorely tested by that bit about penile fractures. You want to pursue this, go ask somebody else.

How do they know with any degree of certainty that no two snowflakes are alike? When I took statistics I was taught that to draw a valid conclusion one had to take a representative sample of the entire population. But considering the impossibly large number of flakes in a single snowfall, let alone that have ever fallen, how could snowologists have possibly taken a sample large enough to conclude that no two are alike? --Leslie B. Turner, San Pedro, California

They didn't, of course. Chances are, in fact, that there are lots of duplicates. What the snowologists really mean is that your chance of finding duplicates is virtually zero. It's been calculated that in a volume of snow two feet square by ten inches deep there are roughly one million flakes. Multiply that by the millions of square miles that are covered by snow each year (nearly one fourth of the earth's land surface), and then multiply that by the billions of winters that have occurred since the dawn of time, and it's obvious we're talking unimaginable googols of flakes. Some of these are surely repeats. On the other hand, a single snow crystal contains perhaps 100 million molecules, which can be arranged in a gigajillion different ways. By contrast, the number of flakes that have ever been photographed in the history of snow research amounts to a few paltry tens of thousands. So it seems pretty safe to say nobody's ever going to get documentary evidence of duplication. Still, it could happen, and what's more, Leslie, it could happen to you. The way I figure, anybody who could dream up a question like this has got to have a lot of time on his hands. Get out and get looking.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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