In theory, we should be surrounded by beautiful symmetry. Gravity and electromagnetism, the forces that most visibly shape the universe, operate the same way in every direction. Yet there are many puzzling inequalities.
Take Earth’s orbit. You might assume that the number of days from one equinox to the other—spring to autumn, or autumn to spring—would be identical. It’s not even close: The autumn-to-spring half is a week shorter than the other stretch. This imbalance exists because Earth’s orbit is not a circle but rather an ellipse that carries us closest to the sun on January 3 or 4. As a result, we speed through space during the Northern Hemisphere’s cold months and loaf along during the warm ones. Some calendar hanky-panky conveniently disguises the imbalance. The extra summertime week is hidden because brief February lies within the short segment of the year and because the fall equinox occurs later in September (the 23rd) than the spring equinox does in March (the 20th).
Now consider Earth’s grossly mismatched hemispheres. Many children notice that they can hold up a globe so that almost nothing but water shows, and then turn it around to see mostly land. This inequality results from irregularities in the way heat flows out from Earth’s interior. Currents of hot rock churning inside our planet have tended to pull all the continents together and then cast them apart again over hundreds of millions of years.
Mismatched halves show up throughout the solar system. Astronomers were stunned by the first images of the moon’s farside, captured by the Soviet spacecraft Luna 3 in 1959. The two hemispheres seemed like different worlds. The face we see has fewer large craters and far greater areas of smooth, dark, frozen lava. Nobody really knows why. The latest theory is that a giant, lopsided plume of hot rock tore through the lunar interior some 3.5 billion years ago, smothering the Earth-facing side with enormous volcanic flows that erupted from the interior.
Inequalities pop up in the far reaches of space as well. Gravity tends to pull stars and planets into tidy spheres, but these shapes can be altered by random motions and interactions. Take Achernar, a bright star visible just above the southern horizon this month from Hawaii and southern parts of Texas and Florida. Astronomers recently discovered that it is the flattest star known, measuring more than 50 percent wider across the equator than from pole to pole; rapid rotation is responsible. Spiral galaxies, too, often deviate from symmetry. A new Hubble image shows that the spiral known as ESO 510-13 is beautifully warped, probably by an encounter with another galaxy.
The most puzzling imbalance is the composition of the universe as a whole, which in theory should contain equal amounts of matter and antimatter. What happened to the “anti” half? A tantalizing clue came in 1998, when researchers at CERN in Geneva and at Fermilab outside of Chicago confirmed the prediction that certain subatomic particles change into their antimatter analogues at a lower rate than conversions in the opposite direction. Physicists had theorized that the transformations were asymmetrical. This experiment revealed, for the first time, this subtle natural bias against antimatter. A lovely showpiece of symmetry, Saturn, is on prime display this month. When it reaches opposition on New Year’s Eve, its rings will be maximally tilted toward Earth. Those who bought telescopes for summer’s Martian visit can now enjoy a far more spectacular sight, high up at midnight. Aided by proximity and that wide-open view of the rings, Saturn offers the biggest and brightest appearance of its entire 29 1/2-year orbit. This spectacle is due to the ringed world’s 27-degree tilt—one irregularity in our favor.