Black holes always seem to be in the news – especially when scientists reveal the very first photograph of one or when an Israeli researcher has created an artificial black hole (a kind of hole) in his lab.
Black holes are probably the craziest – and certainly the most enigmatic – objects in the universe. And yet black holes are known in an unusual way and play a prominent role in pop culture (both Matthew McConaughey and Homer Simpson had dangerous encounters with them). But what exactly is the essence of this bizarre phenomenon? We know the following – and we do not know it.
What is a black hole?
A black hole is a region of space in which gravity exerts such a great pull that nothing, not even light, escapes. That's the simple definition of a black hole. However, when you talk to a physicist, he will also describe a black hole as a region with very strongly curved space-time ̵
This idea of curved space-time goes back to Einstein. Just over 100 years ago, Einstein introduced his theory of gravity, known as general relativity. According to theory, matter bends or distorts the fabric of space. A small object like Earth causes only a slight distortion. a star like our sun causes more dislocations. And what about a very heavy, dense object? According to Einstein's theory, if you push enough mass into a small enough space, it will collapse and form a black hole. The degree of rejection becomes infinite.
The black hole boundary is called the "event horizon" – the point where there is no return. Matter that transcends the event horizon can never return to the outside. In that sense, the interior of a black hole is not even part of our universe: whatever happens there, we can never know because no signal can get inside out. According to the general theory of relativity, the center of a black hole contains a "singularity" – a point of infinite density and infinite curved space-time.
How does a black hole come about?
Black holes are available in different sizes. When a sufficiently massive star depletes its nuclear fuel supply, that is, when it can no longer generate energy in its nucleus by fusion reaction, it explodes (this is called a supernova), where the star releases material from its outer layers); the remaining core then contracts due to gravity. If the star is more than 20 times the size of the Sun, nothing can stop that contraction, and the star collapses until it is smaller than its own event horizon and becomes a black hole. We speak of black holes with stellar masses, since their masses are equal to the masses of the stars. But there are also huge black holes whose masses correspond to those of millions of stars. It is believed that these "supermassive" black holes are located in the centers of most galaxies, including our own Milky Way. Theorists believe that they have evolved along with the galaxies that house them. It is also speculated that microscopic or "original" black holes may have been created at the time of the Big Bang.
Can you see black holes?
Because black holes do not emit light, there is no way to see them directly. Astronomers, however, were able to deduce their existence from observations of ordinary stars orbiting a black hole as part of a binary star system. Sometimes the black hole "swallows" material from the companion star. As this material swirls around the black hole, it heats up due to friction; As a result, it emits X-rays that can be detected by the earth. (The x-rays are sent out before the material crosses the black hole event horizon.) So the first black hole to be recognized, known as Cygnus X-1, was found.
Can a black hole kill you?  Since black holes extend both time and space, an astronaut unlucky enough to fall into the hole sees something quite different than what an observer would observe from a safe distance. From the point of view of the unfortunate astronaut, things are not going well. In the case of a black hole with stellar masses, she feels something called tidal forces – the unequal pull on her feet compared to her head (assuming she enters the hole first). The astronaut would be stretched out like spaghetti, as Stephen Hawking put it vividly. In the case of a supermassive black hole, the tidal forces at the event horizon are lower. The astronaut may not feel that something unusual happens when she crosses it. Nevertheless, she is doomed to failure. When approaching the singular, the tidal forces will inevitably tear them apart before they fall into oblivion.
However, the view from the outside is very different. Because of the time dilation – physicists call it "time dilation" – sees an observer who is far away from the event horizon, the astronauts never meet their fate. Instead, we see that it is getting closer and closer to the event horizon, but never exceeds it. If we could see her clock, we would see it ticking slower and slower. She would end up "frozen" at the edge of the black hole. There is no right or wrong answer to the question "How is the astronaut?". It really depends on your frame of reference.
Can you escape a black hole?
The short answer probably is not. However, physicists have speculated about the existence of "wormholes" – a kind of space-time tunnel that connects one black hole to another. When Carl Sagan worked on his novel Contact he asked the physicist Kip Thorne to propose a method by which the heroine of the story could travel quickly from Earth to the star Vega (about 26 light-years away). Thorne considered the matter and finally suggested that a wormhole could do the trick. That was good enough for Sagan's book (later on a movie with Jodie Foster) – but as Thorne would later admit, wormholes are a highly speculative idea and he doubts that wormholes are actually going to be in our universe. (Thorne would provide his expertise to filmmakers for the year 2014 Interstellar in which black holes play a central role.)
When do black holes die?
Before the Work of Stephen Hawking As far as we know, black holes were left forever in the 70s. However, Hawking, along with physicist Jacob Beckenstein, showed that black holes actually radiate some sort of radiation (now known as Hawking radiation). This radiation carries energy away, which means black holes should simply evaporate to nothing over very long time scales. (Theorists who have compressed the numbers believe that this process should take billions of billions of years – the era of "black hole evaporation" is in the distant future, compared to the present age of our universe – about 14 billion years – just a blip.)
The announcement, Jeff Steinhauer, physicist at the Technion-Israel Institute of Technology in Haifa, Israel, has created an artificial black hole analogue directly on the subject of black hole evaporation. Steinhauer's experiment did not use gravity. Instead, he used a tube filled with ultracold atoms, which in a special condition were called Bose-Einstein condensate. Then it accelerated the atoms so that they moved faster than the sound (but still pretty slow, as the sound in such a condensate can only move slowly), creating an "audible" event horizon, as the researchers describe it. Think of it as a sip, not a light, like a black hole. The experiment produced more than just an event horizon – it was equivalent to Hawking radiation, says Steinhauer.
If the experiment withstood close scrutiny, this could be seen as an argument for the evaporation of black holes. The physics community reacted cautiously. Silke Weinfurter from the University of Nottingham in the UK said Nature : "This experiment … is really amazing, [but] it does not prove that Hawking radiation exists around astrophysical black holes."
Do it No matter if black holes evaporate? If you are a physicist, this is the case. The problem has to do with "information". According to quantum mechanics, information – the numbers that describe how massive a particle is, how fast it rotates, etc. – can neither be created nor destroyed. However, when something falls into a black hole, the information in it seems to disappear. Worse, when the black hole evaporates, the radiated Hawking radiation is confused. the original information seems to be lost forever. Although a number of possible solutions have been proposed, this information loss paradox remains one of the most pressing problems in theoretical physics.
How are black holes examined?
In 2016, scientists announced the discovery of gravitational waves through a pair of black holes (and a few months later a second pair of colliding black holes was announced). Gravitational waves are waves in space-time; Although predicted by General Theory of Relativity, they did not escape for a century and were successfully detected only with the completion of the LIGO detectors (Laser Interferometer Gravitational Wave Observatory). As with the earlier observations, the proof is indirect – we do not see the black holes – but the strength and profile of these gravitational waves fit perfectly with Einstein's theory and the well-known physics of black holes.
What & # 39; s next (event) horizon?
On April 10, 2019, thanks to the Event Horizon Telescope, we got an insight into the horizon of a black hole. With the concentrated power of the entire world of radio telescopes, astronomers obtained a detailed picture of the radiation emitted by gas and dust just before they crossed the event horizon of a black hole in the Messier 87 galaxy, some 55 million light-years from Earth.
The next main objective of the Event Horizon Telescope will be the supermassive black hole at the center of our galaxy – an object known as Sagittarius A *. Since it is so far from the earth (about 25,000 light-years), it appears as a mere pinprick in the sky. No single telescope has the resolution to show in detail what happens.