Justifiable Herbicide?
In “No-Till: The Quiet Revolution,” David R. Huggins and John P. Reganold argue for no-till farming as a more sustainable alternative to plow-based agriculture and describe how herbicide use has enabled growers to effectively practice no-till on a commercial scale. I cannot believe that anyone other than the herbicide manufacturers is seriously proposing that flooding the earth with lethal chemicals is any solution to the problems of agriculture. Their effect on humans and other animals is known, and their effect on soil and groundwater is potentially disastrous (even if the Environmental Protection Agency gives out assurances about the latter). Producing enough food to feed the world’s population without harming the earth is a hard question; this is certainly not the answer.

Louise Tremblay Cole
Galt, Calif.

THE AUTHORS REPLY: As we have stated, reliance on herbicides is a weakness of no-till as it is currently practiced. We contend, however, that no-till is a positive step in the evolution toward more sustainable farming. In addition, we support and are actively engaged in research efforts that integrate no-till into strategies that limit or even eliminate synthetic pesticide use, as in organic production.

Tillage exposes fertile topsoil to the ravages of water and wind, resulting in soil erosion rates that are greater than soil renewal rates in many parts of the world. Sadly, this practice imperils worldwide food security and environmental quality. The fall of past civilizations has been “written on the land,” as soil erosion claimed their capacity to produce food. No-till agriculture preserves precious topsoil, bringing erosion rates into line with those of soil formation. But no-till currently comes with the trade-off of using herbicides.

We share Cole’s skepticism about herbicides—not all is known about their impacts—and support science-based efforts to assess the short- and long-term effects of such chemicals as well as other agricultural tools and practices within the guiding precepts of sustainable agriculture. We also share Cole’s concern of feeding the world’s population while protecting the environment. We believe we will not only need appropriate emerging technologies and traditional conservation farming practices but also good government policies, smart business models and social ingenuity to accomplish this.

Universal Units?
In “The Self-Organizing Quantum Universe,” Jan Ambjørn, Jerzy Jurkiewicz and Renate Loll describe how, in looking to reconcile quantum theory with Einstein’s general theory of relativity, they developed a new approach to quantum gravity called causal dynamical triangulations. In this approach, on the smallest scales spacetime has only two dimensions (approximated as a series of triangles), but on larger scales it smoothly transforms to three, then four dimensions (approximated as the triangles constructing curved shapes).

Could this fact mean that quantum mechanics would apply only to particles that experience less than four dimensions and that relativity would apply only to the four-dimensional universe?
If so, there would seem to be no point in looking for a mathematical framework that can join these two pillars of physics.

Howard Wolowitz
via e-mail

Ambjørn, Jurkiewicz and Loll state that “space keeps its overall form as time advances; it cannot break up into disconnected pieces.” How, then, is the expansion of the universe explained by the geometric spacetime structures they describe? Perhaps new pieces keep getting created in between and push away the others?

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Fuat Bahadir
Omaha, Neb

THE AUTHORS REPLY: As to the first question, all known elementary particles—those that are in an energy range that allows us to observe them directly in particle accelerators—behave according to the rules of both quantum mechanics and four-dimensional space, so there is no contradiction here. The phenomenon of the change in the spectral dimension observed in the theory of causal dynamical triangulations happens on much shorter scales and crucially needs the input of both general rela­tivity and quantum theory in a unified way. This unusual behavior will affect particles (matter) as well as the dynamical behavior of spacetime itself at these scales.

Regarding the second question, when we wrote about the “overall form” of space, we were referring to the way it “hangs together” as a whole (what a mathematician would call its topology). This approach still leaves the freedom for space to grow or shrink, to bend, deform or develop bumps in places, and so on, which indeed will depend on how the microscopic pieces fit together, appear and disappear. The important point is that three-dimensional space cannot break up into several pieces or develop additional handles to change its overall connectedness.

Beat and the Beast
I disagree with Steven Brown and Lawrence M. Parsons’s assertion in “The Neuroscience of Dance” that humans are the only species that can dance or display rhythm. Other species are quite capable of displaying rhythm. Horses performing dressage, for instance, are accompanied by music. And parrots and other birds move to music. (A simple Google search for “animal dancing” returns many videos of animals exhibiting rhythmic movement as well.) Although one can argue that these animals are displaying learned behaviors, they must have a predisposition for rhythmic movement.

Dennis Carrasquillo
via e-mai

THE AUTHORS REPLY: Our point on unique human rhythm capacity was not about synchronization per se but about the ability to entrain to an isochronous (steady) beat, as is typical in group dance and music. We are not aware of any evidence that nonhuman animals naturally show coordinated group movements to an isochronous beat. Since completing our article, however, two teams of colleagues (led by Aniruddh D. Patel of the Neurosciences Institute in San Diego and by Marc D. Hauser of Harvard University) have reported studies showing evidence that parrots, cockatoos and macaws can move in synchrony to a limited range of musical beats.

Thus, the ability of an individual to synchronize to an isochronous beat may not be limited to humans. Such a capacity in birds is latent, though, because in the wild they do not generate isochronous sounds and hence have no opportunity to entrain to beats. Research in this area is developing rapidly, so we will all need to stay tuned.

Note: This article was originally printed with the title, "Letters".