Why is the universe always changing?

With all that is going on, we can count on heaven to give us a sense of stability. But maybe we are asking too much.

The Moon, which Shakespeare prophetically called “unsteady,” moves 1½ inches (3.8 centimeters) away from Earth every year. It’s not worth losing sleep over. But on Mars, billionaire astronauts may be less optimistic. The largest Martian satellite, Phobos, orbits closer to its planet’s surface than any other moon in the solar system, only 3,700 miles (6,000 kilometers) away. It’s also getting closer at about 6 feet (1.8 meters) per century. This produces increasing tidal stresses on Phobos that are creating worrisome striations. Eventually, it will fall apart so that its name, “fear” in ancient Greek, can be terrifyingly appropriate.

But much scarier is comet 109P/Swift-Tuttle. Much loved because the fragments it spews are Perseid summer meteorites, it was discovered by astronomers Lewis Swift and Horace Tuttle just three days apart in July 1862. The giant, fast-moving comet can come close to Earth’s orbit as 0.0009 astronomical unit, which makes it the most dangerous celestial object for humanity. (An astronomical unit is the average distance between the Earth and the Sun.) But calculating where it will be in the distant future is not easy. Its orbital period is 133.28 years, and if you’re a math geek you probably realize that number is suspiciously close to 11 times Jupiter’s orbital period of 11.86 Earth years. So yes, an interesting thing happened a thousand years ago: The comet got caught up in Jupiter’s repeated gravitational tugs and adopted an 11:1 resonance, circling the Sun for 11 Jovian orbits.

Will this at least give it enough stability so that we can accurately predict its future orbit? No! It’s likely to stay in the same predictable pattern for a few thousand years, sure, but after enough time, all bets are off.

At least we would like stability closer to home, as in the atoms of our bodies. But it’s a mixed bag even here. You may remember that atoms have protons and neutrons in their nuclei. Elements have varieties, called isotopes, which are characterized by their different numbers of neutrons. We breathe oxygen which mostly has eight protons and eight neutrons. But a small fraction have one or two extra neutrons. It doesn’t matter: all oxygen isotopes last forever.

Stability at last? Not so fast. “Forever” is not true for its components. A neutron is stable when it is inside an atom. But when it is free, as in the direct currents released by nuclear reactors, the average neutron decays in 14 minutes and 40 seconds. Protons fare better, with a half-life of around (some say at least) 10 trillion trillion trillion years.

What about the rest of the atoms in the universe? This is where numbers and patterns come into the picture. Of the hundreds of isotopes in the natural world, all elements with atomic numbers 1 through 82 have at least one stable isotope. And all the elements that harbor more than 82 protons do not have stable varieties.

We might assume that nature has no preference for odd or even numbers. But that’s not true. The vast majority of the most abundant elements—atomic numbers 1 through 82—are stable if they have an even number of protons and neutrons, and unstable if they have an odd number. Also, even atomic number elements are not radioactive and last forever.

Sometimes we are surprised. You can go to Amazon and buy a one pound block of pure bismuth for a few dollars. It is a beautiful item, suitable for display in your living room. What is currently in fashion is to melt it in a saucepan and let it solidify again, removing a superficial layer, which spontaneously forms surprising shapes like irregular skyscrapers. Anyway, until recently, everyone thought that bismuth was eternally stable. But measurements made in 2003 surprised French researchers, who discovered that it has a half-life of 1.9 x 1019 years. So, although Amazon doesn’t say so, half of its precious bismuth will be gone in 19 trillion years.

Ah, stability, where are you?

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