In “How to Find Another Earth” [Advances], Lee Billings reports on NASA’s Wide-Field Infrared Survey Telescope (WFIRST) and a “starshade” that the agency is considering using to help it image other Earths. Billings describes the two as tens of thousands of kilometers apart. Their desired relative positions are a matter of calculations that, by now, may be almost routine. But maintaining the precise relative positions of objects so far apart must be a daunting requirement. Has this been worked out?

Sarasota, Fla.

BILLINGS REPLIES: Indeed, formation flying would be crucial for any telescope hoping to make use of a starshade. It is already routine in space, notably in rendezvous maneuvers of spacecraft arriving at the International Space Station (ISS). Controlling the starshade and telescope to achieve formation flying is not thought to be a technological challenge—it would require that each spacecraft control its position with an accuracy of about one meter, whereas a spacecraft docking with the ISS must control its position to better than 30 centimeters. Instead the challenge is in sensing the lateral motions of the two far-separated spacecraft, which would each register as milliarcsecond shifts in position on the sky relative to the other. A telescope with a fine-guidance camera like that of the Hubble Space Telescope could detect the precise lateral position of a starshade bearing an LED bank and a laser beacon. Distances between the two spacecraft would be easily measured via radio transponders.


When doing research with human subjects, the control group is crucial. In “The Brain-Boosting Power of Video Games,” Daphne Bavelier and C. Shawn Green describe how they used participation in a “social game” to compare with their treatment: participation in a computer action game. If those are the only two choices, their results are trivial.

What about comparing participation in computer games with equal time in such activities as playing tennis, building a fort, cooking dinner or even reading a book? I don't doubt that computer games improve some cognitive skills. The problem is that they appear to be replacing every other childhood activity. And comparing the effects of participation in one kind of game with another isn't useful science, although I'm sure it makes the CEOs of the game corporations very happy.

Fallbrook, Calif.


Reading through John Pavlus's article “Bipedal Metal,” I can find no explanation of why a robot should have two legs (apart from possible application to human prostheses) like the ones he reports on. It seems that a three- or four-legged robot would be more stable and better able to negotiate rough terrain. And there is no reason why it couldn't have just one arm. Are we following the pattern of science-fiction writers who assumed an alien creature would follow the contingent human body type?

Professor emeritus of physics
Brooklyn College

PAVLUS REPLIES: According to the researchers I interviewed, a big reason to design “anthropomorphic” bipedal robots is to optimize their versatility and agility in human-built environments. These environments are already built to accommodate a two-armed, two-legged body, and so a robot with the same body could, ideally, exploit those same advantages. A three-legged, one-armed robot might have difficulty navigating built environments efficiently. Several of the teams in the 2015 DARPA Robotics Challenge did build highly capable nonbipedal robots, but the hope is that an effective bipedal robot would be able to do everything those unusually shaped robots did and more.


I was astonished and exasperated in almost equal measures by “Our Place in the Cosmos,” Noam I. Libeskind and R. Brent Tully's article on the Laniakea supercluster of galaxies, which includes the Milky Way. Astonished by the scale and beauty but frustrated because the authors discuss methods for determining the galaxies' radial peculiar velocities (those relative to their motion from cosmic expansion), but they give no hint of how they estimate the galaxies' transverse velocities (those perpendicular to the line of sight). Without knowing the latter, we have no way of appreciating how reliable the conclusions they draw are.


LIBESKIND REPLIES: There is no way to measure the transverse velocity of anything but the closest galaxies (and even then it is exceedingly difficult). That's because that velocity is simply imperceptible, given the huge distances that most galaxies are at. But we can estimate it based on our reconstruction methods. What we have to start with is a (relatively speaking) fairly small sampling of the radial peculiar velocity for some (random) set of galaxies. This set is an incomplete sampling of the galaxies in the heavens because we can't compute the peculiar velocity for all galaxies. Many galaxies don't have the required stars or other markers needed to compute distances. Based on the velocities we do have, we can then find the full 3-D velocity field, which is most consistent with the one-dimensional radial peculiar velocity field we measured. This “guess” also provides us with an estimate for the full 3-D density field, which takes care of the incompleteness of our sampling.

Although this technique sounds dodgy, it actually is not. We can test it by simulating the universe, using well-established techniques, and then “observing” our simulations in the same way we observe the universe, namely by obscuring regions and introducing measurement errors, poor sampling, and so on. We can then apply our suite of techniques and see how well we do. In the simulation, we have the benefit of being able to know the “real” velocities and compare them with the calculated ones. Turns out it works pretty well.


Ellen Ruppel Shell's article on “Gauging the Effects of Lead” [The Science of Health] brings some calm scientific perspective to the discussion on the effects of low-level lead exposure and the means by which blood-level standards are set. I lived nearly my entire childhood a few miles downwind of a lead smelter. My personal observation, though clearly not a scientific study, is that my exposed peers and I have developed normally. I am not suggesting that lead exposure shouldn't be avoided or mitigated but that there is a substantial adult population with similar childhood conditions who should be studied.

Even without precise blood-level measurements, reasonable estimates of these exposures could be made from residual soil levels, production records, exposure levels from smelters operating in developing countries and other sources. Studies checking for the possible effects in such exposed populations could provide a wealth of data.

Lakewood, Colo.