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 reversiblethe 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.

CP violation does imply that time symmetry must be violated to preserve CPT symmetry. But CP violation occurs only in a very small number of specific interactions. Some physicists have explored the possibility that this quirk of the subatomic world might be linked to the macroscopic arrow of time, but such efforts did not succeed in finding a convincing explanation. —Tom Siegfried

In the essay “Law and disorder,” time is not defined at the outset, or anywhere else in the article. I remember the definition “time is the separation between two events.” Is there another that applies to most of the discussion in this article?

Another comment about this article, as well as many others I have read in Science News: Many people think that math models are reality. The “Law and disorder” essay reads: “Wolfram cited computer simulations that he claimed were evidence that the second law is simply wrong.”

Math models or simulations don’t “prove” a thing. I have put together math models, but I never assumed that they were reality. They were my best guess at how things might behave.
Ron Erickson, Missoula, Mont.

Time for a new law?
I’m sure that Tom Siegfried is more than familiar with the Law of Conservation of Energy. From his editorial (“For source of time’s arrow, consult a good dictionary,” SN: 6/19/10, p. 2), it would appear that he is less familiar with the Law of Conservation of Italics.
Harry Frank, Ann Arbor, Mich.

The problem of time’s arrow and our equations probably lies in our equations, not in the dictionary. Just as the laws of relativity enlarged our perspective while preserving the everyday of classical physics, so, I would wager, new and better equations will preserve the dictionary even as they enlarge our perspective on the world. Perhaps I quibble.
Charles R. Smith, Fort Collins, Colo.

Real hype over artificial life
The hype about the Venter Institute’s “synthetic,” “man-made,” “made-from-scratch” genome (“Genome from a bottle turns one bacterium into another,” SN: 6/19/10, p. 5) is overblown. The only thing the Venter scientists made from scratch was a DNA molecule. The actual genome — the gene sequence encoded by that molecule — was a copy-and-paste job (with minor modifications) from an existing, naturally occurring organism.

To be sure, the synthesis of that DNA molecule and its insertion into a functioning cell is an amazing technical feat, comparable perhaps to the Gutenberg Bible. But Gutenberg did not write that Bible; he merely printed it out, as these scientists have done with their synthetic DNA. A truly synthetic genome would imply the ability to design organisms from first principles, which science is still a long, long way from achieving.

Gregory Kusnick , Seattle, Wash.

From the Nature Index

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