Event Horizon

The event horizon is the outer boundary of an interstellar object known as a black hole. A black hole is the collapsed remains of a massive dead star where gravity is so strong that not even light can escape. The event horizon marks a point of no return, past which nothing inside the black hole can ever be seen or detected from outside. Although an event horizon is spherical in shape, it is not a physical boundary; it simply marks the point at which the velocity needed to escape from the black hole’s gravity exceeds the speed of light—a speed that nothing can exceed. As would be expected, the size of a black hole’s event horizon depends on the size of the black hole itself. In relatively small black holes, anything passing the event horizon would be stretched out and ripped apart by gravity. However, in larger black holes, it would theoretically be possible to survive passing the event horizon, but such an explorer, and news of their fate, would be forever trapped inside the black hole.

Background

Stars are massive nuclear furnaces that fuse together hydrogen atoms in their core, creating the energy that powers the star. Stars are formed by the gravitational collapse of gas and dust in a massive interstellar cloud. As the gas and dust collapses at the center of the cloud, the heat and pressure eventually build up until nuclear fusion is ignited. For most of their lives, stars exist in a state of equilibrium. The nuclear fusion at their cores pushes outward, while the force of gravity tries to crush the star.

Eventually, stars begin to use up their supply of hydrogen fuel, which is turned into helium and heavier elements by the fusion process. When a star’s fuel supply is exhausted, it will meet one of three fates, depending on its initial mass. When the sun and stars of similar size use up their hydrogen fuel, gravity takes over and begins crushing the star. However, the electrons in the star’s atoms reach a point where they become so packed together that they halt the collapse. The star then becomes a white dwarf, the remains of a stellar core shrunk to the size of Earth.

Stars that are more massive than the sun die in a spectacular stellar explosion called a supernova. A supernova blasts away the outer layers of a star and crushes the star’s core beyond the point at which the packed electrons can stop its collapse. If a star is about two to three times the mass of our sun, the collapse will crush the star’s subatomic particles into neutrons, which will halt the collapse. What remains is a super-dense stellar corpse called a neutron star. Neutron stars are more massive than our sun but have been crushed to a diameter of about 10 to 12 miles (16 to 19 kilometers).

Overview

The collapse of a star more than three of four times more massive than our sun cannot be stopped by the force of packed electrons or neutrons. Nothing known in the universe can halt its collapse. The star is crushed by gravity until it reaches a point of infinite density known as a singularity. The gravity of a singularity is so strong that nothing, not even light, can escape its grasp. The singularity marks the center of what astronomers refer to as a black hole.

The event horizon marks the outer boundary of a black hole. It is the point where gravity becomes so strong that an object would need to be moving faster than light to escape its pull. However, as proven by physicist Albert Einstein, light is the ultimate speed limit in the universe; nothing can travel faster. The term event horizon was first coined in 1956 by Austrian physicist Wolfgang Rindler who described it as the horizon past which events would become unobservable.

The event horizon does not have physical properties; it is a mathematically deduced point of no return that surrounds the singularity in a spherical shape. Although no light can escape the event horizon, many black holes are surrounded by extremely hot disks of gas and dust, the matter being sucked in by the object’s gravity. In 1916, German physicist Karl Schwarzschild developed a way to determine the size of the event horizon in certain black holes based upon their mass. Known as the Schwarzschild radius, the formula measures the distance from the singularity to the outer edge of the event horizon. The larger the initial mass of the singularity, the farther away the event horizon boundary will reach.

As no information can escape from inside a black hole, scientists do not know what exists beyond the event horizon, although they have developed many theories. It is likely the laws of physics that apply elsewhere in the universe do not hold inside a black hole.

If an explorer attempted to travel past the event horizon and into a black hole, their ultimate fate would be the same, but their journey would depend on the size of the black hole. A black hole with an original stellar mass about the size of our sun would be about 2 miles (3.2 kilometers) across. Because these black holes are smaller, their gravity is more concentrated. As an explorer approached the event horizon feet first, gravity would begin affecting their legs more than their head. The gravitational force on their legs would be about one trillion times stronger than on their upper body. Gravity would begin to pull and stretch their body in a process called spaghettification. The explorer would be dead before they passed the event horizon, where what remains of their body would eventually be torn apart by the singularity’s gravity.

However, an explorer’s fate would be different if they fell into a supermassive black hole. This type of object is typically found at the heart of large galaxies and are believed to form as large black holes digest more and more matter and merge with other black holes. A supermassive black hole with about four million times the mass of our sun lies at the center of the Milky Way galaxy.

In theory, if an explorer were to venture close to such a black hole, they could pass through the event horizon away from the center of gravity and likely survive. However, what the explorer encounters inside would be lost forever with them, as they cannot send any signals to the outside universe. Eventually, they, too, would be spaghettified and torn apart.

Bibliography

Carlisle, Camille M. “What’s Inside a Black Hole?” Sky & Telescope, 28 Dec. 2016, skyandtelescope.org/astronomy-resources/whats-inside-a-black-hole/. Accessed 27 Mar. 2023.

"Event Horizon Telescope Makes Highest Resolution Black Hole Detections from Earth." Center for Astrophysics, 27 Aug. 2024, www.cfa.harvard.edu/news/event-horizon-telescope-makes-highest-resolution-black-hole-detections-earth. Accessed 13 Nov. 2024.

Fletcher, Seth. “The First Picture of the Black Hole at the Milky Way’s Heart Has Been Revealed.” Scientific American, 12 May 2022, www.scientificamerican.com/article/the-first-picture-of-the-black-hole-at-the-milky-ways-heart-has-been-revealed/. Accessed 27 Mar. 2023.

Lea, Robert. “What is a Black Hole Event Horizon (And What Happens There)?” Space.com, 3 Mar. 2023, www.space.com/black-holes-event-horizon-explained.html. Accessed 27 Mar. 2023.

Levin, Janna. Black Hole Survival Guide. Knopf, 2020.

Lutz, Ota. “Telescopes Get Extraordinary View of Milky Way's Black Hole.” Jet Propulsion Laboratory, 12 May 2022, www.jpl.nasa.gov/edu/news/2022/5/12/telescopes-get-extraordinary-view-of-milky-ways-black-hole/. Accessed 27 Mar. 2023.

Rodriguez, Leo, and Shanshan Rodriguez. “Could a Human Enter a Black Hole to Study It?” Astronomy, 4 Feb. 2021, astronomy.com/news/2021/02/could-a-human-enter-a-black-hole-to-study-it. Accessed 27 Mar. 2023.

Siegfried, Amanda. “UT Dallas Remembers Founding Faculty Member Wolfgang Rindler.” The University of Texas at Dallas, 11 Feb. 2019, news.utdallas.edu/faculty-staff/ut-dallas-remembers-founding-faculty-member-wolfgang-rindler/. Accessed 27 Mar. 2023.

“What Are Black Holes?” National Aeronautics and Space Administration, 23 Nov. 2020, www.nasa.gov/vision/universe/starsgalaxies/black‗hole‗description.html. Accessed 27 Mar. 2023.