Of all the strange objects discovered in the Universe, black holes remain among the most mysterious. These extraordinary regions of space contain so much mass concentrated into such a small area that their gravity can trap even light itself.
Although black holes were once considered purely theoretical, astronomers have now photographed them and observed collisions between them using gravitational-wave detectors.
From time dilation and spaghettification to Hawking radiation and supermassive black holes, here are 10 fascinating black hole facts that reveal why these cosmic monsters continue to captivate both scientists and the public alike.
1: How are black holes formed?
A massive star collapsing in upon itself – A black hole is formed when a large star starts running out of fuel and begins to collapse under its own gravity. Such a star may become a white dwarf or a neutron star, but if the star is sufficiently massive then it may continue shrinking eventually to the size of a tiny atom, known as a gravitational singularity. A black hole refers to the region in space in which the singularity’s gravitational force is so strong that not even light can escape its pull.
2: How small is a black hole singularity?
Black hole singularities are zero-dimensional – The singularity at the core of a black hole may shrink to a size smaller than an atom, with its gravitational force so strong that it forces the spacetime surrounding it to be bent to infinite curvature. How small is a gravitational singularity? While it is said to be a point of zero dimensions, our current physical theories suggest that the smallest known scale for space-time is a Planck length, beyond which our physical theories break down and don’t make sense. As American theoretical physicist Kip Thorne states in his description of a singularity, it is a “point where all laws of physics break down.”
3: How do black holes affect space-time?
Black holes distort space-time and slow time – The mass of a black hole is so dense and the gravity of its singularity so strong that, in accordance with Einstein’s Theory of General Relativity, it actually distorts the space-time around it and not even light can escape. The boundary beyond which light cannot escape the black hole’s gravity well is known as the event horizon, while its radius is called the Schwarzschild radius (see picture below for details). Once particles and light rays go past the event horizon their light cones “tip over” and point to the singularity, which now represents all future-directed paths with no escape possible.
Interestingly, by keeping a safe distance from a black hole‘s event horizon, a spaceship would experience a slower passage of time compared to the rest of the universe. A spacecraft could therefore travel centuries into the future relative to Earth, although just a few hours would seem to have passed for those people onboard the craft.
4: Why do objects appear to “freeze” near a black hole?
Objects appear to slow down near a black hole – To an outside observer with a telescope, an object passing the event horizon will appear to slow down then “freeze” in time without ever seeming to pass through the event horizon. This is because the light takes longer to escape the black hole’s gravitational pull and light signals won’t reach the viewer for an infinitely long time.
As time elapses, the light subsequently becomes red-shifted and dimmer as its wavelength becomes longer, eventually disappearing from the sight of the observer as it becomes infrared radiation, then radio waves.
5: Can you survive inside a black hole?
A person falling into a black hole would be spaghettified – If a person was able to survive long enough to describe falling into a black hole, he would at first experience weightlessness as he goes into free fall, but then feel intense “tidal” gravitational forces as he got closer to the center of the black hole. If his feet were closer to the center than his head, then his feet would feel a stronger pull until he eventually is stretched and then ripped apart. As he falls in he may observe distorted images as the light bends around him and he will also still be able to see beyond the black hole as light continues to reach him from the outside.
6: What is the gravitational pull of a black hole?
Gravitational pull same as other objects of the same mass – It is important to realize that a black hole’s gravitational field is the same as that of any other object in space of the same mass. In other words, it won’t “suck” objects in any more than any other normal star, with things being more likely to just fall into them if they got too close to their event horizon. If our Sun was replaced with a black hole of equal mass, for example, the Earth would continue experiencing the same gravitational force as before. Only when objects get too close to the black hole would the stronger gravitational force become apparent.
7: Is a wormhole and a black hole the same?
Wormholes appear similar to black holes – A traversable wormhole, known alternatively as a Lorentzian wormhole, Schwarzschild wormhole or Einstein-Rosen bridge, is a theoretical opening in space-time allowing a “shortcut” through intervening space to another location in the Universe. However, from the outside wormholes may exhibit many of the characteristics usually associated with a black hole and be virtually impossible to tell apart.
In recent times, theoretical physicists, such as Kip Thorne, have speculated that wormholes and quantum entanglement may in fact be the very same phenomenon. In other words, entangled particles may be connected by an Einstein-Rosen bridge. If this proves correct, it would represent a huge stride towards a Unified Field Theory and trying to unify the fields of quantum mechanics and gravity.
8: Who discovered black holes?
John Mitchell developed the theory of black holes in 1783 – John Michell (1783) and Pierre-Simon Laplace (1796) were the first people to propose the concept of “dark stars” or objects which, if compressed into a small enough radius, would have an escape velocity which exceeded even the speed of light. Later, the term “frozen star” was used to describe the last phase of a star’s gravitational collapse, when light unable to escape from its surface would make the star appear frozen in time to an observer. In the 20th century, John Wheeler eventually coined the phrase “black hole” as the object would absorb all the light that hits it while reflecting nothing back.
9: Can a black hole die?
Black holes eventually evaporate over time – Physicists now believe that black holes actually radiate small numbers of mainly photon particles and so can lose mass, shrink, and ultimately vanish over time. This unverified evaporation process is known as “Hawking Radiation”, after Professor Stephen Hawking who theorized about its existence in 1974. However, it is a staggeringly slow process and only the smallest black holes would have had time to evaporate significantly during the 14 billion years the Universe has existed.
10: What is the nearest black hole to Earth?
Closest known black hole is just 1,600 light-years away – Astronomers believe that most galaxies contain a supermassive black hole at their centre. At the heart of our own Milky Way lies Sagittarius A* (Sgr A*), a supermassive black hole located around 26,000 light-years from Earth and containing millions of times the mass of our Sun.
However, Sagittarius A* is far from being the closest black hole to our planet. That distinction belongs to Gaia BH1, a stellar-mass black hole located approximately 1,600 light-years away in the constellation of Ophiuchus. Despite being relatively nearby in astronomical terms, it poses no threat to Earth and was only discovered because of its gravitational effect on a companion star.
Bonus Fact #11: Can black holes be photographed?
Black holes have now been photographed – For decades, black holes were invisible objects that could only be detected indirectly through their effect on nearby stars and gas. That changed in 2019 when the Event Horizon Telescope collaboration released the first-ever image of a black hole located at the centre of the galaxy M87.
In 2022, astronomers also released an image of Sagittarius A*, the supermassive black hole at the centre of our own Milky Way galaxy. While the black hole itself remains invisible, the images reveal the glowing ring of superheated matter surrounding its event horizon. These remarkable observations provided one of the strongest confirmations yet that black holes truly exist.
Frequently Asked Questions About Black Holes
Can a black hole destroy Earth?
No known black hole is close enough to threaten Earth. Even if the Sun were somehow replaced by a black hole of the same mass, Earth would continue orbiting normally.
Are black holes real?
Yes. Astronomers have observed stars orbiting invisible massive objects, detected gravitational waves from black hole mergers, and captured images of material surrounding black holes.
What is the biggest black hole discovered?
Some of the largest known black holes contain tens of billions of times the mass of the Sun and are found at the centres of giant galaxies.
How many black holes are in the Milky Way?
Astronomers estimate that the Milky Way may contain millions of stellar-mass black holes, although only a small fraction have been detected so far.
