Black holes are perhaps the most mysterious objects in the universe. Unless, of course, things are hidden somewhere in the depths, the existence of which we do not know and cannot know, which is unlikely. Black holes are colossal mass and density, compressed into one point of a small radius. The physical properties of these objects are so strange that they puzzle the most sophisticated physicists and astrophysicists. Sabine Hossfender, a theoretical physicist, compiled a selection of ten facts about black holes that everyone should know.
What is a black hole?
The defining property of a black hole is its horizon. This is a border beyond which nothing, not even light, can return. If a detached area becomes detached forever, we are talking about an "event horizon." If it is only temporarily separated, we speak of the "visible horizon." But this "temporary" could also mean that the region will be separated for much longer than the present age of the universe. If the black hole's horizon is temporary but long-lived, the difference between the first and the second is blurry.
How big are black holes?
You can imagine the horizon of a black hole as a sphere, and its diameter will be directly proportional to the mass of the black hole. Therefore, the more mass falls into the black hole, the larger the black hole becomes. Compared to stellar objects, however, black holes are tiny, because mass is compressed into very small volumes under the influence of irresistible gravitational pressure. The radius of a black hole with a mass of planet Earth, for example, is only a few millimeters. This is 10,000,000,000 times less than the present radius of the Earth.
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The radius of a black hole is called the Schwarzschild radius after Karl Schwarzschild, who first deduced black holes as a solution to Einstein's general theory of relativity.
What's happening on the horizon?
When you cross the horizon, nothing special happens around you. All because of Einstein's principle of equivalence, from which it follows that you cannot find the difference between the acceleration in flat space and the gravitational field, which creates the curvature of space. However, an observer far from the black hole who is watching someone else fall into it will notice that the person will move slower and slower as they approach the horizon. As if time moves more slowly near the event horizon than far from the horizon. However, some time will pass, and the observer falling into the hole will cross the event horizon and find himself inside the Schwarzschild radius.
What you experience on the horizon depends on the tidal forces of the gravitational field. The tidal forces on the horizon are inversely proportional to the square of the black hole's mass. This means that the larger and more massive the black hole, the less force. And if only the black hole is massive enough, you can cross the horizon before you even notice that something is happening. The effect of these tidal forces will stretch you: the technical term physicists use for this is spaghettification.
In the early days of general relativity, it was believed that there was a singularity on the horizon, but this turned out to be not the case.
What's inside a black hole?
Nobody knows for sure, but definitely not the bookshelf. General relativity predicts that in a black hole there is a singularity, a place where tidal forces become infinitely large, and once you pass the event horizon, you cannot go anywhere else but into the singularity. Accordingly, it is better not to use general relativity in these places - it simply does not work. To tell what happens inside a black hole, we need a theory of quantum gravity. It is generally accepted that this theory will replace the singularity with something else.
How do black holes form?
We currently know of four different ways black holes form. The best understanding is associated with stellar collapse. A large enough star forms a black hole after its nuclear fusion stops, because everything that could already be synthesized has been synthesized. When the pressure created by fusion ceases, matter begins to fall towards its own gravitational center, becoming more and more dense. In the end, it becomes so dense that nothing can overcome the gravitational effect on the surface of the star: this is how a black hole is born. These black holes are called "solar mass black holes" and are the most common.
The next common type of black hole is the "supermassive black hole", which can be found at the centers of many galaxies and which have masses about a billion times that of solar black holes. It is not yet known for certain exactly how they are formed. It is believed that they once began as solar-mass black holes that consumed many other stars in densely populated galactic centers and grew. However, they seem to absorb matter faster than this simple idea suggests, and how exactly they do it is still a subject of research.
A more controversial idea was primordial black holes, which could have been formed by almost any mass in large density fluctuations in the early universe. While it is possible, it is difficult to find a model that produces them without over-creating them.
Finally, there is the very speculative idea that tiny black holes with masses close to that of the Higgs boson could form at the Large Hadron Collider. This only works if our universe has extra dimensions. So far, there has been no confirmation in favor of this theory.
How do we know black holes exist?
We have a lot of observational evidence for compact objects with large masses that do not emit light. These objects give themselves away by gravitational attraction, for example, due to the movement of other stars or gas clouds around them. They also create gravitational lensing. We know that these objects do not have a solid surface. This follows from observations, because matter falling on an object with a surface should cause the release of more particles than matter falling through the horizon.
Why did Hawking say last year that black holes don't exist?
He meant that black holes do not have an eternal event horizon, but only a temporary apparent horizon (see paragraph one). In a strict sense, only the event horizon is considered a black hole.
How do black holes emit radiation?
Black holes emit radiation due to quantum effects. It is important to note that these are quantum effects of matter, not quantum effects of gravity. The dynamic spacetime of a collapsing black hole changes the very definition of a particle. Like the passage of time, which is distorted next to a black hole, the concept of particles is too dependent on the observer. In particular, when an observer falling into a black hole thinks that he is falling into a vacuum, an observer far from the black hole thinks that this is not a vacuum, but a space full of particles. It is the stretching of space-time that causes this effect.
First discovered by Stephen Hawking, the radiation emitted by a black hole is called Hawking radiation. This radiation has a temperature inversely proportional to the mass of the black hole: the smaller the black hole, the higher the temperature. The stellar and supermassive black holes that we know have temperatures well below the temperature of the microwave background and therefore are not observed.
What is an information paradox?
The information loss paradox is caused by Hawking radiation. This radiation is purely thermal, that is, it has only temperature by chance and of certain properties. The radiation itself does not contain any information about how the black hole formed. But when a black hole emits radiation, it loses mass and contracts. All this is completely independent of the substance that became part of the black hole or from which it was formed. It turns out that knowing only the final state of evaporation, one cannot say from what the black hole was formed. This process is "irreversible" - and the catch is that there is no such process in quantum mechanics.
It turns out that the evaporation of a black hole is incompatible with the quantum theory as we know it, and something needs to be done about it. Eliminate the inconsistency somehow. Most physicists believe the solution is that Hawking radiation must somehow contain information.
What does Hawking propose to solve the black hole information paradox?
The idea is that black holes must have a way to store information that has not yet been accepted. Information is stored on the horizon of a black hole and can cause tiny displacements of particles in Hawking radiation. In these tiny displacements, there may be information about the trapped matter. The exact details of this process are currently unclear. Scientists are awaiting a more detailed technical paper from Stephen Hawking, Malcolm Perry and Andrew Strominger. They say it will appear at the end of September.
At the moment, we are sure that black holes exist, we know where they are, how they form and what they will eventually become. But the details of where the information goes to them still represent one of the biggest mysteries in the universe.
Ilya Khel