What Is a Black Hole? Birth, Event Horizon, Singularity, Observational History, and Evaporation

According to the video, black holes are thought to be born when a massive star reaches the end of its life and collapses under its own gravity. For most of its life, a star keeps its shape because the outward pressure from nuclear fusion balances the inward pull of gravity. But this equilibrium does not last forever.

In stars with roughly eight times the mass of the Sun or more, this delicate balance breaks down catastrophically once the nuclear fuel runs out. The iron core at the center is crushed by gravity, and immediately afterward a shock wave blows away the outer layers. This is a supernova explosion, which the video describes as one of the most energetic explosions in the universe. If the core left behind after the explosion cannot withstand gravity and collapses without limit, it can become a black hole.

However, if conditions such as the star's mass are not right, what remains is a neutron star made up almost entirely of neutrons. Black holes can also be born when these neutron stars collide with each other.

As an origin other than stellar collapse, the video also introduces the hypothesis that within one second of the Big Bang, regions of extremely high matter density formed and directly became black holes. These are called primordial black holes, but none has been discovered so far.

The video states that a black hole is actually an object whose mass has been squeezed into an extremely small region, and is far from what the word "hole" suggests.

The key concept brought in to explain its nature is escape velocity. Every object in the universe has a minimum speed needed to break free of its gravity. For Earth it is about 11 km per second; for the Sun, about 600 km per second. For a black hole, it exceeds the speed of light, about 300,000 km per second. Since nothing in the universe can exceed the speed of light, anything that gets too close to a black hole can never return. Light and other forms of electromagnetic radiation are no exception, which is why, the video explains, observing black holes directly is extremely difficult.

The black holes discovered so far are divided into stellar black holes, with masses from a few to around 100 times that of the Sun, and supermassive black holes, with masses of several hundred thousand times the Sun's or more. Intermediate-mass black holes, with masses in between, have not been definitively discovered yet and remain one of astronomy's mysteries.

The event horizon is the boundary beyond which light and matter can never escape. Anything that falls inside it, even light, never comes back out. The video explains that, according to general relativity, the interior of the event horizon can never be observed, for all eternity.

For a non-rotating black hole, the size of the event horizon can be calculated exactly. Its radius is the Schwarzschild radius, which is proportional to the object's mass. For the Sun, it would be about 3 km. The video puts this in terms of Tokyo: if there were a black hole with the same mass as the Sun, the Sun's entire mass of about 2×10^27 tons would be concentrated within an area centered on Tokyo Station and extending roughly to Tokyo Tower, Tokyo Dome, and the Ryogoku Kokugikan.

On the other hand, the event horizon is not the "surface" of an object. It does not represent the black hole itself, and there is thought to be no wall there. The video says that even if a person were to cross the event horizon, they would most likely feel nothing in particular.

Black holes tend to be imagined as swallowing everything, but according to the video, as long as you don't get too close, they can be treated like any other celestial object. As an example, if the Sun were replaced by a black hole of exactly the same mass, the orbits of Earth and the other planets would not be affected at all.

So what happens if you get too close? The experience differs greatly depending on the black hole's mass. With a relatively low-mass stellar black hole, you would meet a gruesome end long before reaching the event horizon. If you approached feet first, the difference in gravity acting on your feet and your head would grow larger and larger until it eventually became strong enough to tear your body apart.

According to the figures given in the video, for a typical black hole of five solar masses, at a distance of 20 km from the center, about 5 km from the event horizon, the difference in gravity across just 1 mm reaches 17,000 times Earth's gravity. Your body would be stretched out more finely than words can describe and disappear into the black hole in a single straight line. This phenomenon is called "spaghettification," and it is experienced by any object that gets too close to a black hole.

With a supermassive black hole, by contrast, the difference in gravity due to differences in distance is very slight even at the event horizon. In this case, you would pass through the event horizon without incident and enter the black hole's interior. From then on, however, you could never escape, and it would be theoretically impossible even to send information to the outside. All that remains is to move toward the singularity thought to lie at the center, the video explains.

The theory of gravity widely accepted in physics today is Einstein's general relativity. What does this theory predict about the interior of the event horizon, which can never be observed?

According to the video, inside the event horizon, the nature of spacetime as described by general relativity is thought to differ greatly from the outside. This is often expressed as "the roles of time and space are swapped," but the video explains that, more precisely, the direction toward the center of the black hole becomes as unavoidable as the forward march of time is outside. Just as we cannot stop or resist the flow of time, inside the event horizon you cannot avoid heading toward the center. General relativity predicts that no matter how you move, you will ultimately reach the singularity.

So what lies at the center? The video states frankly that it is no exaggeration to say current physics has no idea at all. According to general relativity, there is a singularity there, whose density is calculated to be infinite. But many scientists believe this simply means the region near a black hole's center lies beyond the domain where general relativity applies, and that a theory of gravity to replace general relativity is needed.

Predicting what the singularity is like with neither observations nor theory is extremely difficult, but the video introduces several new theories that have been actively studied in recent years.

One of them is loop quantum gravity. According to this theory, time and space have smallest units that cannot be divided any further. This makes it possible to avoid the calculated result that the singularity's density becomes infinite. Furthermore, some physicists, building on this theory, believe that spacetime continues beyond the singularity and connects to a white hole. In that case, matter that falls into a black hole would be crushed finely and then spat out somewhere else in the universe, or perhaps into an entirely different universe. While noting that no such observational evidence has been found, the video says loop quantum gravity is seen as a promising theory of gravity to replace general relativity.

There are also several hypotheses holding that general relativity is inadequate not only for the singularity but even for the event horizon. One of these is the fuzzball hypothesis in string theory. String theory holds that everything in the universe arises from the vibrations of extremely tiny strings. If a black hole is regarded as a tangled collection of strings, the problem of the singularity can be resolved. Even so, since observation is nearly impossible, the video expects that it will be a long time yet before humanity unravels the mystery of the singularity.

For a long time, black holes were merely theoretical entities. Soon after general relativity was published, it was known that objects from which not even light could escape could exist on paper, but whether they actually existed remained completely unknown for a long time.

The turning point the video cites is the 1964 observation of an object emitting intense X-rays near the constellation Cygnus. Then in 1971, it was found that a nearby star appeared to be wobbling. In other words, that star was orbiting "something" that could not be observed at all through light or any other electromagnetic radiation. The mass of the invisible object, calculated from the star's mass and orbital period, exceeded the theoretical limit for a neutron star. An object with dozens of times the mass of the Sun, unobservable, and not a neutron star: today it is considered virtually certain that this is a black hole.

Then in 2019, humanity succeeded in imaging a black hole, something whose form was supposedly impossible to see. The image was captured by the Event Horizon Telescope, which achieves extremely high resolution by treating multiple radio telescopes around the Earth as a single telescope.

The first black hole imaged was the supermassive black hole at the center of the galaxy M87, about 54 million light-years away. Its mass is about 6.5 billion times that of the Sun, and its Schwarzschild radius reaches 18 billion km. The image shows superheated gas orbiting around a pitch-black central region, which the video describes as "everything exactly as theory predicted." Although their very existence was once doubted, the video says that today hardly any astronomers doubt that black holes exist.

From among the black holes that have been observed, the video introduces several distinctive examples.

Gaia BH1 is known as the closest confirmed black hole to Earth. It is about 1,560 light-years away and has 9 to 10 times the mass of the Sun. It forms a binary with a star of almost the same mass as the Sun, and the two quietly orbit each other at a separation roughly equal to the distance between the Sun and Earth.

What has caught astronomers' attention is how extraordinarily quiet this black hole is. Because it is not actively swallowing surrounding matter and has not formed an accretion disk, it emits almost no strong electromagnetic radiation such as X-rays. The video quotes the researchers who discovered it as explaining that if you put this black hole where the Sun is and put the Sun where Earth is, you would have this binary system.

Ordinarily, it is thought that such a neat, quiet binary would be unlikely to survive the supernova explosion that precedes a black hole's formation. Yet it actually exists. From this, the video says, there may still be much that humanity does not understand about how black holes are born and evolve.

Another example is the distant, ancient galaxies captured by the James Webb Space Telescope (JWST), which began observations in 2022. Because the speed of light is finite, the farther away a galaxy we observe, the further into the past we are seeing it. Some of the galaxies JWST has observed date from only about 700 million years after the birth of the universe. And even at that point, there were already supermassive black holes at their centers that were disproportionately large.

According to the video, before these results came to light, it was thought that supermassive black holes and galaxies grew together in the early universe. But JWST's observations are forcing major revisions to this hypothesis. Did supermassive black holes grow remarkably fast in the early universe, or were they born with large masses from the start rather than growing from stellar black holes? The video poses these two possibilities as open questions and says that black holes play a major role in understanding how the universe took on its present form.

General relativity predicts that anything swallowed by a black hole can never get out again. But according to the video, black holes themselves are not eternal either; they are thought to eventually evaporate. Under the theory proposed by Stephen Hawking, a black hole gradually emits electromagnetic radiation, loses energy, and ultimately disappears.

The process, however, is extremely slow. According to the predictions shown in the video, a stellar black hole would take 10^67 years (roughly 1,000 fukashigi years) to evaporate, and a supermassive black hole 2×10^100 years. Compared with the current age of the universe, about 10^10 years, black holes will vanish from the universe over a mind-bogglingly long span of time.

In the mainstream cosmology of today, stars are expected to disappear from the universe long before black holes evaporate. This is because gas is consumed by star formation and heated by events such as supernova explosions, making it harder for new stars to be born. The video depicts the universe at that time as a far darker, lonelier, and overwhelmingly vast place than it is now.

In a universe without stars, the video says, the last hope for life may be black holes. Life needs energy, and it is known to be theoretically possible to extract energy from a black hole by throwing an object close to it and letting part of it break off and escape.

The video imagines that our distant descendants might be living quietly around a black hole as the last life in the universe. And it closes with the outlook that in the far future, when even that black hole has evaporated, there will be practically nothing left in the universe.