Changing and unchanging cycles
On New Year’s Eve, as we hum “Auld Lang Syne” and drink a cup of kindness, we celebrate the end of one year and the beginning of another. It might feel like a big deal to us here on earth, but from an astronomical perspective, it’s just a day like any other — as one planet circles one star, the sun, out of the galaxy’s billions. But since we arbitrarily impose a beginning and an end on our equitable distribution of days, let’s reflect on some of the cycles that dominate our existence: the big ones over which we have no control, the smaller ones we might hope to influence, and those about which we don’t yet know. This way, we can make some resolutions that will outlast the echoes of our song.
Patterns and repetition are comforting because they help us remember and understand events. Phenomena that occur on a regular basis generally have an underlying reason. The cosmos provides us with numerous examples. Isaac Newton deduced the law of gravity that allows us to predict elliptical orbits around a central object's gravitational pull. This is how the Milky Way attracts the sun, which takes roughly 240 million years to orbit the galaxy's center. And as we all know, the earth, along with all the other planets in our solar system, orbits the sun. The earth's cycle takes a year — hence the holiday and festivities before us.
The planets in the solar system are joined by objects with lesser celebrity status, such as asteroids and comets. Dwarf planets orbit too. "Dwarf planet'' is a new category that includes the demoted Pluto, but also other objects of similar size that have been discovered since the 1990s — objects that are big enough to take notice of (and are spherical) but not quite big enough to orbit alone. Like John Mayer, who formerly headlined on his own but now tours with the Grateful Dead, Pluto is no longer a solo act.
The earth's rotation on its axis, creating the 24-hour cycle we know so well, is why Annie who gets her gun (back when that could be considered amusing) has the sun in the morning and the moon at night. It's also why orphan Annie knows the sun will come out tomorrow. (The moon rotates too — but at a rate commensurate with that of its orbit. This results in a side of the moon we never see, sometimes colloquially, and inaccurately, called "the dark side of the moon." Being invisible to us does not make it dark — the sun still shines on this hidden hemisphere.)
Seasons result from the tilt of the earth's axis of rotation, which makes regions subject to differing amounts of the sun's radiation as the earth orbits around it. Long-term climate trends depend on other cyclical patterns too, such as the various Milankovitch cycles of thousands of years, named for the Serbian astronomer-mathematician Milutin Milankovitch, who realized that the periodic changes in the earth's ellipticity — how far from circular the orbit is — as well as changes in the earth's tilt, can result in climate changes that lead, about every 100,000 years, to ice ages and to shorter periodic weather patterns, too. However, these many-thousand-year cycles don't explain what is happening on time scales determined by human activity in the last few hundred years.
Within the earth, too, patterns repeat themselves. Over a few hundred million years of geological activity, the plates of the earth are regenerated, and, over a shorter time scale, the ocean floor is reconfigured too. Movements of the continental plates are one of the drivers of another important cycle, the carbon cycle, so essential to life on our planet. The cycle of carbon in the atmosphere — critical to our climate, and to life itself — results in part from carbon that is released into the atmosphere through volcanoes and hotspots and is absorbed back into the earth.
There is weak evidence that large-impact craters on earth — those that arose from a kilometer-wide asteroid or comet hitting the earth — also happen periodically. Research by my collaborators and me suggests that it could be the gravitational pull of dark matter in the midplane of the Milky Way that dislodges weakly bound comets at the edge of the solar system so that they are more likely to hit the earth in a 32-million-year cycle. Measurements being done currently will show whether such a dark matter disc exists.
Though we are safe from any such hypothesized cataclysmic impact for another 30 million years, there are other cycles, ones over which we can exercise some control, that present much more immediate threats. We are repeatedly alerted to melting glaciers and increasingly dangerous storms. News about the Middle East changes in detail, but the general patterns remain the same. Gun shootings in America happen so regularly that the cycles of outrage and pushback are barely noticeable.
Yet despite the recurring carousel of time, we enter each new year with optimism. It will be easier to maintain that belief in a better future if we think a little harder about how and when we can eject ourselves from undesirable patterns. The Paris climate agreement and its ancillary benefits can help us begin to make real progress in slowing down changes to the earth's climate, a cycle we have only a limited time to affect. Maybe we can curb, or at least slow down, other seeming inevitabilities, too. In the universe, cycles won't last forever as the solar system and the galaxy undergo changes. If we can keep in mind that humanity has the potential to change the cycles that we impose on ourselves, then maybe this new year we'll break that most reliable cycle of all — making resolutions we don't have the fortitude to keep.
Lisa Randall is professor of physics at Harvard and author of "Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.''