Black holes are a mysterious and fascinating cosmic phenomenon that challenge all laws of physics. From the ability to completely swallow a star to twisting the fabric of space-time and generating immense gravitational forces, black holes seem to keep us questioning the unnatural aspects associated with them.
In a presentation at the World Science Festival, Prian Greene, a publishing partner of Britannica, revealed that the story of black holes originates from the German meteorologist, mathematician, and astronomer Karl Schwarzschild.
During World War I, he was sent to the Eastern Front to assist in calculating bombing trajectories. “And somehow, amidst the battlefield, he came across materials related to Einstein’s theory of relativity and performed some calculations based on it,” Greene said. Through those calculations, Schwarzschild realized that when massive spheres collapse and are crushed into very small fragments, the gravitational force would become so strong that nothing within the influence of a black hole could escape its pull. Just as Einstein’s theory predicted: black holes twist, bend, and distort space-time.
Black holes twist, bend, and distort space-time.
To understand why time slows down as an object approaches a black hole, you need to grasp what time dilation is. Einstein, obsessed with time and space, was the first to realize that time is relative. After more than a decade of research, Einstein published his theory of relativity in 1915, shocking followers of Newtonian physics and sparking a scientific revolution.
Einstein argued that while the laws of physics are invariant in the universe, the rate or motion of time and space is not constant but relative. But relative to what? The answer lies in the perspective of the observer.
Einstein often spoke about trains and the differences in the perception of time and speed between those on and off a moving train. For instance, he pointed out that an accelerating train moves much faster relative to a person standing on the platform than to someone chasing it on another train running parallel to it. This led to a series of experiments conducted with clocks and atomic clocks, and the conclusions drawn proved that Einstein was correct: time is not invariant, and it can dilate.
However, to be scientifically accurate, time does not change based on the position of an observer; it changes due to variations in gravitational force. Scientists have demonstrated these time changes by measuring atomic clocks placed at the top of skyscrapers and on the ground, or on satellites in orbit and on Earth. So, if gravitational forces can alter time, what happens to time in the presence of the immense gravitational forces of a black hole?
Only an extremely massive object, like a black hole, can create a significant difference in the flow of time.
When it comes to the relationship between time and black holes, we have the clearest example of time dilation, as black holes are the densest objects in the universe and possess the greatest gravitational pull. NASA’s Jet Propulsion Laboratory (JPL) has studied black holes to understand the interaction between space and time. The team explains that as an object approaches a black hole, time will slow down from the perspective of an observer who is experiencing the flow of time unaffected by the black hole.
For instance, if a star approaches a black hole and we can see that star from Earth through a telescope, we would see it slow down until it effectively freezes in time, and we would never witness it falling into the black hole! However, from the perspective of that star, time continues to flow normally.
Einstein’s theory of relativity states that time will slow down near any large object due to its gravitational force. This phenomenon is known as gravitational time dilation, which increases exponentially near black holes. Black holes were once dying stars, sometimes with masses hundreds or thousands of times that of our Sun, but collapsed and compressed into a very small size. “Only an extremely massive object, like a black hole, can create a significant difference in the flow of time. There is still much to learn about what happens to time and space inside a black hole,” said Ota Lutz from NASA JPL.
