Endless Light

In “All the Light There Ever Was,” Alberto Domínguez, Joel R. Primack and Trudy E. Bell discuss measurement of the extragalactic background light, which is made up of all the accumulated light from galaxies throughout the universe's history.

How can that background radiation still be sloshing around? If light travels in a straight line, except for gravitational lensing, why didn't it become lost long ago?

DAVID MARANZ
via e-mail

In answering the question “Why is the night sky dark?” the authors did not discuss a simple model explaining the effect, based on special relativity, that I was taught as a graduate student.

In that model, the intensity of light from a moving light source decreases the faster that source moves away from an observer. In the quantum-mechanical version, the light intensity is proportional to the light frequency and the density of photons, and they, too, decrease the faster the light source moves away.

PHILIPP KORNREICH
Syracuse University

THE AUTHORS REPLY: In answer to Maranz's question: Galaxies shine for billions of years, and most of the light they have emitted has never been absorbed because space is so empty. Thus, the light continues to accumulate, and we continue to receive it from galaxies surrounding us at all different distances. The more distant the galaxy, the longer the light has been traveling toward us and the earlier it was emitted.

In response to Kornreich: As we mention in our article, the wavelength of light expands proportionally to the expansion of the universe, which does indeed cause the intensity of light from distant sources to decrease. The fact that the sources are moving away from us and that their clocks are moving slower also causes the intensity to decrease. These general relativistic effects are always included when we calculate the light we receive from distant galaxies.

Animal Testing

In “Ending a Cruel Legacy” [Forum], Barbara J. King exposes a paradox that experimenters must ignore if they wish to preserve the illusion of behaving ethically when studying infant monkeys: Researchers perform tests on primate infants to understand the behavior of humans raised under stressful conditions. For the results to have any value, they must therefore recognize that human and monkey brains and personalities share similar developmental pathways. But if monkeys are similar enough to be useful experimental models, how can these trials be anything other than cruel and unethical?

BILL TARVER
Wokingham, England

Narcissism's Cause

Kiddo Knows Best,” by Andrea Alfano [Advances], reports that psychologists in the Netherlands led by Eddie Brummelman concluded that narcissism in children was associated with excessively praising parents. The article, however, did not rule out the possibility that children hardwired to be self-centered might have tended to seek praise from parents, who then responded accordingly. As a psychiatrist, I think it would be nearly impossible to determine the chronology of giving praise versus requesting it from a research study.

DOUGLAS M. BERGER
Tokyo

BRUMMELMAN REPLIES: As Berger points out, narcissistic children do crave admiration from others. But our work suggests that they don't lead their parents to adopt a grandiose view of them.

Our study consisted of four measurements, one every six months. In each, we assessed children's narcissism and how much parents saw their child as an extraordinary individual.

We consistently found that parents' overvaluation of their children predicted increased narcissism in those children six months later. Yet children's narcissistic traits didn't predict increased parental overvaluation in the same time period.

Planetary Priorities

In “Birth of a Rocket,” David H. Freedman reports that “the real justification for human spaceflight is to take steps toward expanding the human race's stomping grounds.” Does he mean for a few elite astronauts or humanity generally? The former may be achievable—but why bother? The latter seems just shy of delusional. Either way, wouldn't it be more sensible to take the up to $1 trillion that a trip to Mars might cost and use it to prevent the collapse of livable conditions on Earth?

EVAN FALES
Iowa City, Iowa

Inaccessible Information

David Pogue's “The Upgrade Game” [TechnoFiles] notes that we have little choice but to accept upgrades to software and devices because older versions will eventually no longer be “supported.”

Many corporations, scientific groups and governmental entities have lost ready access to information that was stored just a couple of decades ago, using software and hardware that are no longer supported or available. Society has a legitimate interest in making certain that upgrades to operating systems and essential programs continue to support their predecessors in case we need to retrieve such information.

JOHN F. ROSSMANN
Tustin, Calif.

Sleep Apnea

In “The Not So Silent Epidemic” [The Science of Health], David Noonan discusses remedies for sleep apnea, a disorder that causes snoring and can be life-threatening. He notes that the strap-on CPAP mask is effective, but many patients hate it.

The new Inspire Medical Systems device he reports on is exciting. But the way he dismisses “generic” oral appliances as targeting “the symptom, rather than … the underlying problem” is appalling. There are professionally made oral appliances that are a first-line approach to treating mild to moderate sleep apnea and an alternative for moderate to severe sleep apnea for patients who do not tolerate CPAP. An 88-month study, published in 2013 in Respirology, showed that they were equal to CPAP. Their lower efficacy is balanced by greater utilization.

IRA L. SHAPIRA
Chair, Alliance of TMD Organizations Diplomate, American Board of Dental Sleep Medicine

Inflammations's Value

Cells on Fire,” by Wajahat Z. Mehal, nicely elucidates how inflammation is now known to be triggered by structures within cells. But we must draw a distinction between exaggerated inflammatory response and subclinical inflammation. The latter is an absolute necessity: it allows fresh cells to replace degenerating older ones and helps to flush tissues out of accumulating debris that can potentially alter those tissues' functions.

ROMESH KHARDORI
Eastern Virginia Medical School

Erratum

Birth of a Rocket,” by David H. Freedman, incorrectly refers to a prototype of a Virgin Galactic suborbital rocket exploding last October. At that time, a nonprototype suborbital craft had crashed during a test flight but had not exploded.