Event horizon (noun, “Ee-VENT hur-AYE-zon”)

An event horizon is a boundary surrounding a black hole. Beyond this boundary, nothing — not matter, energy or any kind of information — can escape.

A black hole is an ultradense object with extremely strong gravity, which draws in matter and energy around it. The speed that something needs to escape an object’s gravity is called the escape velocity. (Something has to blast off Earth at about 11 kilometers per second or 7 miles per second, for instance, to escape into space.) But at a certain point around a black hole, matter or energy would have to travel faster than the speed of light to escape. And this is impossible. That point of no return is the event horizon.

An event horizon would never be directly visible to us. That’s because light would have to leave it in order to reach our eyes or be captured by a telescope. But the shadowy outline of this boundary has been captured in images.

In April 2019, the Event Horizon Telescope released the first image of a black hole. It revealed the black hole at the center of the Messier 87 galaxy. This galaxy is about 55 million light-years from Earth. In 2022, scientists shared the first image of the black hole at the center of our own galaxy.

An image of a black hole looks like a black sphere encircled by a ring of light. The ring of light is the bright disk of gas orbiting the black hole. The black, pupil-like center shows the region where light is trapped and cannot escape. This region is called the “black hole shadow.” The event horizon lies within that shadow. Light bends around the black hole in such a way that the dark region appears at least twice the size of the actual event horizon.

In a sentence

In 2019, the first image of a black hole revealed a shadowy region, within which lies the mysterious event horizon.

Check out the full list of Scientists Say.

Fluoride (noun, “FLOOR-ide”)

Fluoride is an electrically charged form of the element fluorine.

An atom that carries such an overall charge is called an ion. Fluoride is a negative ion with a -1 charge. That’s because each fluoride ion contains nine (positively charged) protons and 10 (negatively charged) electrons.  

Fluoride strengthens the enamel that makes up the outer layer of our teeth. That makes it an important component of toothpastes and other dental products. Many places also add fluoride to their drinking water. They don’t add much. The recommended level is just three drops of fluoride per 200 liters of water. But even that amount can reduce dental cavities by about 25 percent. That’s according to the U.S. Centers for Disease Control and Prevention (CDC).

In 1945, Grand Rapids, Mich., became the first city to add fluoride to its water. By 2015, about three-quarters of Americans had access to tap water that strengthens teeth. The CDC lists fluoride-treated water as one of the 10 great public health achievements of the 20th century.

Fluoride strengthens enamel thanks to its excellent ability to snag positively charged particles. Compared to other atoms or ions, a fluoride particle is tiny. That means all its negative charge is packed into a small area. In other words, fluoride comes with high charge density. And that allows it to form strong bonds with other particles.

Fluoride binds and holds onto positively charged particles, such as calcium ions. In our mouths, this process forms calcium-based minerals that reinforce enamel. That provides a barrier against acids that would dissolve our teeth.

In a sentence

When fluoride-amended water treatments drop, dental problems rise.

Check out the full list of Scientists Say.