Coincidences confound us. If lightning strikes the church and stops its clock just as old Aunt Betty dies, most of us would link the two events. Folks with a New Age mindset even claim that everything is intimately linked with everything else. Yet this morning if, just as you brushed your teeth, a gust blew down a tree in Siberia, would you really believe those events were connected? Â
Coincidences are catnip to scientists, who see them as signals for an investigation. Take the famous star cluster, the Seven Sisters, also known as the Pleiades. Newborn stars are always created in dusty, blue-colored hydrogen clouds. Sure enough, the young Pleiades stars float in a gorgeous blue nebula that certainly seems the cosmic embryonic fluid in which they were born. Years of studies finally revealed that the lovely wispy gas cloud moves in a different speed and direction. They’re not related! The Pleiades are merely passing through in a one-time event. Seems a crazy, unlikely coincidence, but there they sit and that’s how it is.Â
Consider that our sky has just two disks, Sun and Moon, that both appear the same size even though they’re actually wildly different, with one 400 times larger than the other. This enables those amazing total solar eclipses, nature’s finest spectacle. And we have a north star that just happens to be the brightest and most precisely aligned since before the last ice age.
More common than such true coincidences are the celestial match-ups that come with rational explanations. People are often amazed that the moon rotates in the same period in which it revolves around Earth, making the same hemisphere forever point our way. But tidal stresses always slow a satellite’s rotation until it’s locked in place. Every moon of every planet has this same synchronous spin. There’s similar gravitational physics behind Neptune’s orbital period being 165 Earth-years while Pluto’s year is 248 Earth years. Unless you’re a math savant you probably didn’t realize that this means Neptune circles the Sun exactly three times for every two orbits of Pluto.
What about the fact that the strong nuclear force, the gravitational constant and 40 other universal physical parameters, which could have been set to any value at the Big Bang, are each within 1% of what’s needed for the formation of chemical elements, nuclear fusion in stars and the existence of life?Â
Too impossible to be a coincidence, that’s for sure, but what’s the explanation? We have two alternatives. One is the Anthropic Principle: Only in our kind of universe would stars, planets and sentient beings be able to form. So if things weren’t this way, we wouldn’t be around to wonder about it, but since we do wonder because here we are, then the universe had to be this way. Yogi Berra would be proud of this reasoning, but it has many adherents. The other choice is biocentrism – that life and conscious awareness are fundamental aspects of the cosmos, whose parameters are thus automatically subservient to it.
It’s sometimes hard to separate coincidence from correspondence. Take the eleven-year sunspot cycle. Powerful solar storms must do something to us. But only a couple dozen sun-cycles have been observed since we became aware of them, against which we can hunt for thousands of potential rhythms and match-ups from political events to climate changes to earthquakes. Which are related and which are merely the mischief wrought by the law of averages?Â
Checking back we find that the solar cycle swings in harmony with the fashionable length of women’s skirts, the rabbit population of Australia, the party that controls Congress, the position of the Gulf Stream, and the altitude of the top of our atmosphere. Only those last two are probably related to the Sun, but you see the problem. Ultimately, the tools of statistics and probability let researchers make the call, and it’s often non-intuitive.Â
 When 25 people occupy the same room, the odds are better-than-even that two will share the same birthday. I used to demonstrate this in my classroom by asking each student to announce their birthday in turn. When the match-up arrived, it always drew howls of amazement, since it would seem that with 366 possible dates, a group of 25 would be too small to make a shared birthday likely. Try it sometime, at your next social gathering. With 25 guests, a sharing is likely; with 30 or more it’s almost a certainty.
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