New York City
THE EDITORS REPLY: Numerous readers reacted to Graffin’s assertion by sending us great examples of science songs. In addition to Monty Python’s immortal song, others include “Mammals” by the band They Might Be Giants, John Boswell’s album “Science Is the Poetry of Reality” and many songs by Tom Lehrer. Presumably none of the songs satisfies Graffin’s taste to qualify as “good.”
DARK AND STILL
In “Dark Worlds,” Jonathan Feng and Mark Trodden explain that the dark matter candidates called super-WIMPs interact only through gravity. That means they cannot undergo the type of collisions that dissipate energy (or hardly any collisions at all) the way ordinary particles do, primarily turning kinetic energy into electromagnetic energy, in the form of photons.
“When created, super-WIMPs would have been moving at a significant fraction of the speed of light,” the authors write, adding that “they would have taken time to come to rest.” In purely gravitational interactions, energy is nearly conserved. The only possible mechanism for individual super-WIMPs to lose kinetic energy is to convert a tiny bit of it into gravitational radiation. If super-WIMPs essentially cannot interact, how can they come to rest?
La Crescenta, Calif.
FENG AND TRODDEN REPLY: If the universe were not expanding, super-WIMPs would indeed have no way of slowing down. In an expanding universe, however, all matter comes to rest eventually, meaning that its motion ultimately is owed entirely to the expansion of the universe (technically, this means that it comes to rest in co-moving coordinates, which expand with the expansion of the universe). Thus, this is the sense in which super-WIMPs slow down. Incidentally, the same principle applies to the slowdown of WIMPs. The weak interactions that WIMPs possess (and super-WIMPs lack) do not have any appreciable impact on how long they take to come to rest or how well they seed galaxy formation.
HUMANS AND PARASITES
I read with interest in Mary Carmichael’s “Halting the World’s Most Lethal Parasite” the idea of vaccinating mosquitoes by using a human carrier to pass the vaccine on to the mosquito. Couldn’t you use other mammals such as livestock instead, thereby eliminating the ethical dilemma of vaccinating people who will not directly benefit?
CARMICHAEL REPLIES: Vaccinating animals is an intriguing idea and one that is clearly more applicable for vector-borne diseases with nonhuman reservoirs (for example, vaccinating dogs to control both canine and human visceral leishmaniasis transmission). Still, the two major human malaria parasite species, Plasmodium falciparum and P. vivax, are restricted in their “choice” of vertebrate host. Also, to reiterate one of the points made in the article, a vaccination campaign using only a malaria-transmission blocking vaccine (TBV) would indeed confer direct benefit to the immunized individual. The benefit is not immediate but simply delayed. No one is envisioning using TBV alone, however. It would most likely be used in combination with antimalarials and other vaccines.