“Seeing in the Dark,” by Joshua Frieman, discusses the effort to confirm whether the accelerated expansion of the universe is because of dark energy. Is it possible to test the hypothesis that our known universe is part of a much larger, more massive universe that might account for the accelerated expansion?

Jack W. Hakala
via e-mail

Frieman points out that a difficulty with one of the four candidates for dark energy, the quantum-mechanical contribution to the vacuum energy, is that its quantity is not yet determined, because calculations give about 10120 times more than what currently appears to exist. This embarrassing miscalculation does not, however, negate the principle that its contribution is likely to be nonnegligible.

David Reid
Haifa, Israel

FRIEMAN REPLIES: In response to Hakala: The notion of a multiverse continues to fascinate theorists. One idea for explaining the smallness of the cosmological constant (why it is not 120 orders of magnitude larger) posits that in different regions of the multiverse, the cosmological constant takes on different values. In most of those regions, it is much too large to enable the conditions for life to form; only in regions where it is small, as we observe it to be, can a 14-billion-year-old universe filled with galaxies and life form. Whether we can test this anthropic selection principle is still open to question.

Reid raises a good point. Whatever dark energy turns out to be, it's likely that we will still be left trying to understand the smallness of the vacuum energy. Whether measurements of the properties of dark energy from the Dark Energy Survey or other projects will help illuminate this problem is as yet unknown.


As a one-time contributor to Scientific American and long-time subscriber, I am surprised to see “Disease Detector,” by Shana O. Kelley, presented as a scientific article when it actually is a cleverly disguised advertisement for a commercial enterprise. As stated in the article, despite being a Distinguished Professor of chemistry at the University of Toronto, Kelley also happens to be “director of, and holds equity, in Xagenic, a company that commercializes technology described in this article.” This is an “infomercial.” I expect more from the most prestigious popular scientific magazine in the U.S.

Editor, Nutrition and Cancer

THE EDITORS REPLY: Scientific American invites leading investigators to write about cutting-edge research. Once in a while, an author is also involved with companies that hope to commercialize the work. In such cases, we disclose relevant financial ties, as we did in Kelley's biography. Her article is based on peer-reviewed scientific literature, cited in her story, and also presents competing technology.


“Baby Talk,” by Patricia K. Kuhl, discusses a period of about six months to seven years in which children are able to quickly learn “a language or two.” In my experience, babies and young children are, in fact, capable of acquiring several languages simultaneously, if they are provided with the right family and social environment.

My wife and I were fortunate to provide such an environment for our four daughters, who acquired German, Arabic, French and English during their early years. My German wife and I, a native Arabic speaker, have communicated in English since we met. We brought up our daughters in France, where they went to a primary school that was taught in French and English and were looked after, for a few hours a week, by a French speaker. We then moved to Spain when they were three to eight years old. They acquired Spanish within three months of starting school.

For this proficiency to be achieved, it is vital that each individual communicate with the children in their own native language. And children should be discouraged from mixing languages when they speak to a person. Young children will associate each person with his or her first language.

Azzam Qasrawi
Ronda, Spain


I'm curious about George Musser's reference to gravity traveling at the speed of light in his article “Where Is Here?” Has this been confirmed?

Watching a gorgeous sunset some years ago, someone piped up, “Of course, you know, this really happened eight minutes ago.” While I was showering the next morning, it dawned on me that if gravity traveled at the speed of light, Earth's orbit would be “off” (geometrically with the sun) by eight minutes, multiplied by our orbital velocity.

Is it not reasonable to assume that gravity could be an instantaneous manifestation, sort of like nonlocality?

Syd France
Kirkland, Wash.

In discussing nonlocality, Musser talks about the reshaping of spacetime in terms of the movement of tectonic plates. I think a more apt and universal reference for the point he is trying to make is that Earth is always both spinning on its axis and revolving around the sun, and the solar system itself is revolving around the center of our galaxy, and so on. Therefore, any attempt to define a location on Earth as being “here” in spacetime is an inherently nonsensical concept.

Dennis Coleman
Allendale, N.J.

MUSSER REPLIES: In answer to France's question: Isaac Newton did think that gravity propagated instantaneously, but in our modern understanding, thanks to Albert Einstein, gravity, per se, does not travel: it is a consequence of the curvature of spacetime at the location of an object. Changes in curvature (namely, gravitational waves) do travel, at light speed, as has been confirmed indirectly by observations of the orbits of binary pulsars and, more controversially, by the gravitational lensing caused by Jupiter.

Coleman is right that the universe appears to have no preferred frame of reference, so it's meaningless to talk of “the” position or velocity of, say, a baseball. It is moving at 80 miles an hour relative to the field but far faster relative to the sun or the center of the galaxy. That idea goes back at least as far as Galileo, and Einstein formalized it in special relativity. But the tectonics metaphor was intended to get across a different idea: that even if you decide on a frame of reference, position has no fixed meaning. That is the innovation of general relativity.


I appreciate hearing about any medical research concerning food allergies, such as in “Overreaction,” by Ellen Ruppel Shell [The Science of Health]. My son, who is 21, has several well-documented IgE-mediated food allergies.

In discussing a study involving peanut allergies, Shell casually refers to “proteins found in nuts.” Peanuts are legumes, not tree nuts.

Mary Ellen Clark
via e-mail