Readers ask about the moon’s slow escape from Earth and more

Feel the burn

The discovery of a protein receptor in nerve cells that detects heat and capsaicin — the compound that makes chili peppers spicy — was one of the 2021 Nobel Prize winners in physiology or medicine, Tina Hesman Saey and Freda Kreier reported in “This year’s Nobel Prizes awarded” (SN: 11/6/21, p. 14).

Reader Joe Gadway wondered how people can tell the difference between high temperatures and spiciness if the same protein detects both.

“A lot of people ask me this question,” says Nobel laureate David Julius, the molecular physiologist at the University of California, San Francisco who discovered the receptor, called TRPV1. “Sometimes it is difficult to discriminate between [whether] your mouth is on fire because of capsaicin or because something’s hot-hot.”

Other receptors in nerve cells that can sense thermal heat but not capsaicin may help distinguish spiciness from temperature, Julius says. But these receptors’ contributions to heat sensation haven’t been worked out as thoroughly as TRPV1’s role has.

Thermal heat suppresses cold-sensing nerve fibers whereas capsaicin does not, he says. Quieting signals from the cold-sensing nerves may be another clue to help the brain figure out the source of the heat.

Speed could play a role in telling the two sensations apart as well, Julius says. Nerve cells register thermal signals faster than they do chemical signals. While a “hot” signal goes away once you stop touching a hot object, the burn of chili peppers endures. So those temporal clues may also help people tell thermal heat from chemical heat.

Great escape

Moon rock samples retrieved by China’s Chang’e-5 mission reveal that lava oozed on the lunar surface just 2 billion years ago, Freda Kreier reported in “Moon rocks record young lava flows” (SN: 11/6/21, p. 6).

Kreier reported that the moon was a lot closer to Earth 2 billion years ago and has been slowly inching away from the planet. Reader Rick Mott wanted to know how the moon manages to move away from Earth.

“The moon and Earth are caught in a constant gravitational tug-of-war,” Kreier says. Earth’s gravity keeps the moon orbiting Earth while the moon’s gravity tugs on the planet. The force of this tugging allows the moon to steal some rotational energy from Earth, making the natural satellite’s orbit bigger. “This is pushing the moon away at a rate of about 3.78 centimeters per year, roughly the same rate our fingernails grow,” she says.

This tug-of-war is also slowing down Earth’s spin, gradually making our days longer, Kreier says. In 100 years, a day on Earth will be about two milliseconds longer than it is today.


In the November 6 issue’s Feedback, a reader asked how scientists contained fusion reactions in a recent experiment (SN: 9/11/21, p. 11). Our response noted that fusion reactions don’t need containment because they fizzle out on their own. This also means that, in the future, generating energy using fusion would have no risk of a runaway reaction, which could cause an explosion. While fission reactions don’t stop on their own as fusion reactions do, we neglected to make clear that modern fission reactors are designed to prevent runaway reactions. Meltdowns in fission reactors are still possible if heat can’t be removed from the reactor.


A serious look at psychedelics” (SN: 12/4/21, p. 20) incorrectly identified the drug that Aldous Huxley popularized in his book The Doors of Perception. Rather than LSD, the drug was mescaline, a psychedelic compound made by certain cacti. And the book was published in 1954, not 1953.