Readers discuss black holes’ trippy effects on time, banned swimsuits

Time to eat

Astronomers watched in real time as the supermassive black hole at the center of a galaxy went from dim and quiet to bright and actively feeding on material, Adam Mann reported in “For the first time, scientists witness a black hole turning on” (SN: 7/13/24 & 7/27/24, p. 7).

Reader Chris Sheppard wondered how a black hole can consume material, when time seems to stop at the outer edge — the event horizon.

“The seeming paradox arises because of the extreme time dilation near the event horizon,” says theoretical physicist Eduardo Martín-Martínez of the University of Waterloo in Canada. From the perspective of a distant observer, time seems to progress more slowly near the black hole’s edge. So infalling matter appears frozen in time, never crossing the event horizon, Martín-Martínez says. Light emitted by that matter becomes increasingly stretched to longer wavelengths, or redshifted, and eventually becomes invisible.

“However, from the point of view of the infalling material itself, time is experienced normally,” Martín-Martínez says. The matter crosses the event horizon after a finite amount of time and moves toward the black hole’s center, called the singularity. “If the distant observer were to approach the horizon themselves, once they are close enough, they would see matter cross the horizon at a finite time, and they themselves would cross the horizon at a finite proper time,” Martín-Martínez says.

Getting up to speed

If elite athletes are to ever reach humans’ projected maximum speed in running or swimming, they will need perfect technique, Erin Garcia de Jesús reported in “What’s the human speed limit?” (SN: 7/13/24 & 7/27/24, p. 36).

In the 2000s, a now-banned swimsuit line from Speedo ushered a wave of new records in the 50-meter freestyle, Garcia de Jesús reported. The suits compressed swimmers’ bodies and made them more buoyant.

Given that compression tends to increase density, which decreases buoyancy, reader David H. Brands asked how the suits could have had such an effect.

These were two parallel effects, not one leading to the other, says associate news editor Christopher Crockett. The compression made the swimmers’ bodies more streamlined and thus reduced drag. At the same time, the suits also trapped air around swimmers’ bodies, increasing buoyancy, Crockett says.