Letters
By Science News
Time’s arrow
I’ve enjoyed reading Science News since I was a kid; thanks very much for producing such a fine periodical! This is the first time I’ve felt compelled to write to you about an article you’ve published: “Law and disorder” (SN: 6/19/10, p. 26). I can’t help but feel that the time theory that Sean Carroll proposes misses the point of time. Time is the observation of forces acting on matter or energy. Take an event such as dropping a ball: Gravity acts on the ball, pulling it to the ground. If we reverse time, the ball will be repelled from the Earth. The ball can’t just do this of its own accord; some force would have to be repelling the ball from the ground. Therefore, if we reverse time, we also have to reverse gravity in order to get the required result. But, to the best of my knowledge, gravity acts only in one direction. Because gravity is a force of attraction and reversing time would make it a force of repulsion, going back in time, unlike Tom Siegfried’s article suggests, is not allowed by the laws of physics.
Joshua Feinberg, Portland, Ore.
The laws of physics are time-reversible only on the microscopic level, where molecular collision processes do appear the same in a movie running forward or backward. The reader is quite right that many macroscopic processes are not reversible—the mystery is how such irreversible phenomena can arise from the time-reversible microworld. —Tom Siegfried
Tom Siegfried’s essay and book review about time raise an issue that has troubled me for some years. I have observed that, because math is so beautifully able to describe nature, scientists in general seem to have overlooked the fact that our math and its equations are descriptive, not prescriptive. “The math allows …” is heard over and over. Simply because an equation, or even a chemical reaction, is reversible in no way means anything else is too. Nature does not obey the laws of physics. The laws only describe nature.
P.M. deLaubenfels, Corvallis, Ore.
The essay “Law and disorder” states, “Equations describing the forces that guide matter in motion work just as well going backward in time as forward.” However, it was widely believed (I don’t know if this was ever actually proven) that time (T) symmetry is violated at the subatomic level (since the symmetry of CP, the combination of charge and parity, is known to be violated, either T or CPT must also be). This is particularly interesting since a CP violation may explain why the universe contains more matter than antimatter (“Muons offer clue to why universe isn’t just space,” SN: 6/19/10, p. 8). Perhaps there is a link between these two mysteries.
Bobby Baum, Bethesda, Md.