Health & Science

Time travel: Not in this universe?

Sorry, science-fiction fans: Time travel is impossible. That is the sobering conclusion, anyway, of a Hong Kong–based team of physicists. They found that the maximum speed of a single photon, the basic unit of light, “obeys the traffic law of the universe,” Agence France-Presse reports. The photon cannot go faster than the speed of light—186,282 miles per second­—and thus provides no way around the law of physics that an effect can’t come before its cause. Previously, scientists had noticed that in certain mediums, light appeared to exceed its set limit. But when the Hong Kong researchers shot photons through atoms chilled to one ten-thousandth of a degree above absolute zero, they could make more-precise measurements of the waves they generate in the material ahead of them. It turns out even those “optical precursors” can’t break the speed-of-light barrier. Lead researcher Du Shengwang says that means the “information carried by a single photon” can only exist in the present. But the finding doesn’t refute other prospects for time travel, such as “wormholes,” which could serve, in theory, as cosmic short-cuts between distant times and places.

The price of a big brain

Subscribe to The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

Sign up

As humans get older, our brains shrink by about 15 percent, making us vulnerable to memory loss, dementia, and depression. But the brains of our closest relatives, chimpanzees, stay the same size throughout life, a new study has found. And while 50 percent of Americans over 85 suffer from Alzheimer’s, elderly chimps appear to have no similar problems. “The million dollar question” is why, study author Chet Sherwood, an evolutionary neuro­anatomist at George Washington University, tells Science. Brain atrophy may be the cost humans pay for living longer and having more gray matter than other species do. Human brains are three times as big as chimps’ and require much more energy. And unlike chimps, humans far outlive their ability to give birth. Sherwood speculates that our longevity may have evolved in part to allow humans to help raise their children’s children, since big-brained human offspring mature very slowly. But he says a long life also stresses our neurons, which “have the odds really stacked against them after long years of high energy consumption.”

Prosperous and unhappy

Residents of rich countries are more likely to be depressed than those who live in poorer nations. That’s the startling conclusion World Health Organization researchers reached when they interviewed nearly 90,000 people in 18 countries about their mental health. France and the U.S. were the most depressed; 21 percent of French people and more than 19 percent of Americans have suffered from the disorder. By contrast, many low- to middle-income countries had strikingly low depression rates. Only 8 percent of Mexicans and 6.5 percent of Chinese people say they’ve ever been depressed. Just because wealthy nations “have a high income doesn’t mean there isn’t a lot of stress in the environment,” lead researcher Evelyn Bromet tells Health​.com. And the study shows that depression “is strongly linked to social conditions.” Cultural differences may make certain nationalities less likely to admit to depression—for instance, though less than 7 percent of Japanese people say they’ve been depressed, the country has a higher suicide rate than the U.S. But other statistics held true worldwide: Women are twice as likely as men to be depressed, and the most common cause of the disorder is the loss of a partner due to death or divorce.

A breakthrough on infertility

A mutant gene may help explain why as many as 15 percent of couples experience infertility, The New York Times reports. One in five men has two defective copies of the gene called DEFB126, which can have “a very dramatic impact” on the ability to conceive, says Gary Cherr, a toxicology professor at the University of California, Davis. Cherr and his colleagues studied 500 young Chinese couples and found that men with the mutation were 30 percent less likely to impregnate their partners over a two-year period. The defect prevents sperm from receiving a protein coating that helps them penetrate cervical mucus to reach a woman’s eggs. The defective sperm, which look fine and otherwise behave normally, may be a reason why current tests can’t explain some 70 percent of male infertility cases. Scientists say a simple screening for the defect would allow many couples to skip more expensive testing, and that many men with DEFB126 mutations can conceive eventually. Study co-author Theodore Tollner thinks the fact that so many men carry marred copies of the gene means the mutation probably confers a reproductive benefit. “But we’re not sure what that advantage might be yet,” he says.