In May 2024, astronomers observed a “waterfall” region of space where light and matter plunge irrevocably into the abyss of a black hole. General relativity theory states that when particles are close enough to a black hole, they cannot follow circular orbits and plunge toward the black hole at nearly the speed of light.
“Einstein’s theory predicted that this final plunge would exist, but this is the first time we have demonstrated it happening,” said Oxford University’s Andrew Mummery, who led the study. “Think of it like a river turning into a waterfall – hitherto, we have been looking at the river. This is our first sight of the waterfall.”
What is a Black Hole?
A black hole is a region in space where matter is concentrated so densely that its gravitational gradient prevents anything that falls into it from escaping – including light, which is why it appears black.
A black hole forms from a collapsing star of sufficient mass. The star’s mass (and therefore gravity) remains the same but compressed to only 1/200,000,000th of its original size; the gravitational gradient is magnified enormously. The gravitational gradient near a black hole can be millions of times stronger than we experience on Earth. It can rip planets and stars apart that venture too close. These extreme gravity conditions make black holes ideal for studying their effects on matter, light, time and space.
How a Black Hole Forms
A star produces energy by nuclear fusion. The energy released in this process creates outward pressure that keeps the star from collapsing under its own gravity. When the star exhausts its nuclear fuel and can no longer generate outward pressure, the core collapses, compressing subatomic particles together and converting gravitational potential energy into kinetic energy and heat. A shock wave can propel the star’s outer material into space. In some cases this results in a supernova so powerful it outshines entire galaxies for a brief period. The energy of a supernova is a nonillion (1,000,000,000,000,000,000,000,000,000,000) times more powerful than an atomic bomb. In total collapse, the core shrinks to a singularity—a point of infinite density—surrounded by a ”waterfall” region from which nothing can escape being drawn into the black hole.
The “Waterfall” Region
The “waterfall” region visually represents the event horizon, where space flows like a river, as the illustration above depicts. At this boundary around a black hole, matter is inexorably drawn toward and into the black hole. The flow of space speeds up, approaching the speed of light at the event horizon. All paths lead inward, and escape is impossible.
The gravitational field near the event horizon causes time to slow down dramatically relative to an observer farther away. An external observer would see an object falling toward the black hole appear to be moving ever more slowly toward and frozen in time at the waterfall region. Light waves are stretched longer toward the red end of the spectrum as they escape the gravitational field. The redshift becomes so extreme that the light’s wavelength approaches infinity and becomes undetectable to an outside observer, causing the object to fade from view.
The difference in the gravitational pull between points closer and farther from the black hole creates tidal forces so great that any object that enters the black hole will be “spaghettified” as it is stretched and compressed.
The black hole’s gravitational field also bends the path of light from behind the black hole, a phenomenon known as gravitational lensing. This effect can create multiple images of background objects and result in spectacular visual effects, such as the Einstein ring, where light from a background star or galaxy forms a ring around the black hole.
The Singularity
The singularity is a point where spacetime curvature and gravity become infinite. Eintein’s general relativity theory predicts that all matter falling into a black hole is crushed into this infinitely dense point. But neither general relativity nor quantum mechanics can describe what happens as the gravitational forces become infinitely strong. The event horizon hides the singularity from view, so its true nature and what happens there remain unknown. One theoretical possibility is that the singularity results in a Big Bang, creating another universe. Another possibility is that the singularity is never reached since it would take infinite time to arrive there due to gravitational time dilation. Yet another is that as-yet-unknown quantum effects prevent a singularity from forming. Perhaps a theory of quantum gravity theory that incorporates both quantum and relativistic effects will provide an answer in the future.
How Do We Know Black Holes Exist?
This will be the next blog topic, including a brief history of our knowledge about black holes.
More About Albert Einstein
Albert Einstein’s creative mind produced the theory of general relativity over a century ago. This theory describes virtually everything modern science knows about the universe. To learn more about Albert Einstein and how he transformed our understanding of physical reality, read the prize-winning photobiography EINSTEIN: The Man and His Mind.
