10 Answers to Questions You May Have About Black Holes

It may not seem like it, but black holes are simple: objects that have been so squeezed by gravity that they occupy an infinitesimal volume, but without losing their mass. However, it is difficult to understand what happens within their event horizons—the point of no return, from which even light cannot escape—for the simple reason that they are invisible. Therefore, there are many questions on the subject, but scientists have already found some answers.

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  • Albert Einstein was not the first scholar to propose the existence of black holes, but it was the Theory of General Relativity that made the topic so popular. More precisely, the solutions to his equations proposed by physicists like Karl Schwarzschild, praised by Einstein for the simple way to solve problems. Since then, science fiction speculates wonders and nightmares that could be witnessed by anyone approaching a black hole.

    Today, science knows much more about these objects than a few decades ago. We even already have a real photo of a black hole, an unprecedented feat that guaranteed the responsible team a millionaire prize. If you are curious about these mysterious titans, check out the answers to 10 questions you could ask about black holes.

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    1. If black holes are invisible, how do you find out about them?
    The hole black CID-844 has grown a lot faster than its host galaxy. It lies at the center of the galaxy and is nearly 7 billion times the mass of our Sun, one of the most massive black holes ever discovered. The mass of the galaxy, however, is considered normal (Image: Reproduction/M. Helfenbein/Yale University/OPAC)

    It is true that no light, of any kind, can escape the event horizon of a black hole, which makes them invisible. But if it is active, that is, it is feeding on any kind of matter, a lot of observable things happen around them.

    When a gas is sucked by the black hole, for example, the matter is stretched and starts to spin around it before falling into the event horizon. You can simulate this by letting a good amount of water run down a tank drain so that a eddy forms above where the water flows. If you dribble some paint, you will see a colored wire spiraling towards the center before going down the drain.

    In the case of a black hole, this process accelerates matter to high speeds and heats it to millions of degrees, resulting in something called an accretion disk. This causes a glow in X-rays and radio to be emitted, and it is this glow that scientists commonly observe by astronomical instruments. In addition to the accretion disk, active black holes can have jets emitted at almost the speed of light.

    The immense gravity of black holes also distorts space itself, so you can see the influence of an invisible gravitational pull on stars and other objects—that’s how astronomers discovered there is a supermassive black hole in the center of the Milky Way, for example. Finally, scientists are able to “hear” the echo of a collision between two black holes, events that form a new hole and produce gravitational waves.

    two. Is it possible for a black hole to “swallow” an entire galaxy?

    (Image: Reproduction/Salvatore Orlando/Sketchfab)

    There is no way for a black hole to devour an entire galaxy, because the gravitational range of supermassive black holes in the middle of galaxies is not large enough to reach all objects in the galaxy. In fact, it can’t even swallow the stars closest to the galactic center.

    Our Solar System, for example, is about 26 thousand light years away from Sagittarius A*, the supermassive black hole at the center of the Milky Way. This distance is more than safe. For comparison, there are some stars that apparently orbit safely around Sagittarius A*, one of them being S2, which is about 25 thousand light years away from us.

    To better understand how the gravitational pull of a black hole works, think about those that form from collapsing stars: their gravity field remains the same as the star had before the collapse. If the Sun, for example, could become a black hole (which it won’t, because it doesn’t have enough mass to do so), all the orbits of planets, comets and asteroids in the Solar System would remain the same. Almost nothing would be devoured.

    3. What would happen if you fell into a black hole?
    (Image: Reproduction/NASA/CXC/M. Weiss)

    There is some recent disagreement on this subject, but what is generally accepted about the interior of black holes comes from the Theory of General Relativity. If we observe a black hole from afar, we can only see regions outside the event horizon, but if someone fell there, they would experience another “reality”. On the event horizon, your perception of space and time would change completely.

    This is because black holes distort space and time due to their high density. This causes something known as gravitational time dilation. If you observe from a distance an object that falls into a black hole, you will see this process at a reduced speed. The object will also appear to shrink as it approaches the event horizon, taking a seemingly infinite amount of time to reach it.

    Another curious effect is that the light reflected by the object will appear redder and darker, an effect known as gravitational redshift. If the object is a giant clock, the viewer will see its hands spinning much more slowly than his own clock. The falling object will shrink, become redder and darker, and then disappear so it can no longer be seen.

    If you are indestructible and dive into a black hole out of pure curiosity, you won’t notice any of the above. Your watch will tell time normally and you will cross the event horizon after a time nothing like infinity. However, in classical general relativity, you could not know the location of the event horizon by diving into it, due to Einstein’s equivalence principle.

    Lastly, the immense density of the black hole would compress its body horizontally and stretch it vertically, like noodles, due to the effects of tidal forces: the part of its body closest to the black hole will be pulled much harder than the farthest part. Scientists call this phenomenon “spaghification”.

    4. Have black holes influenced our planet?
    A Our galaxy’s central region, the Milky Way, with a collection of objects, including the Sagittarius Asupermassive black hole, clouds of gas at temperatures of millions of degrees, neutron stars and white dwarfs tearing material from companion stars (Image: X -Ray: NASA/CXC/UMass/D. Wang et al.; Radio: SARAO/MeerKAT)

    When a massive star explodes, it distributes elements necessary for life, such as carbon, nitrogen and oxygen, through space. Mergers between two neutron stars, two black holes or a neutron star and a black hole also scatter elements like these, which may one day become part of new planets.

    The shock waves from stellar explosions can also trigger the formation of new stars and new stellar systems. So, in a sense, we owe our existence on Earth to explosions and collision events that formed black holes a long time ago.

    On a larger scale, most galaxies appear to have supermassive black holes in their centers. The connection between the formation of these supermassive black holes and the formation of galaxies is not yet known, but it is possible that black holes played an important role in the formation of galaxies, including our own Milky Way.

    5. What is the most distant black hole ever detected?

    The most distant black hole ever discovered is in a galaxy about 00,1 billion light years from Earth. The age of the universe is currently estimated at 10, 8 billion light years, so this black hole has only existed for about 690 million years after the Big Bang.

    It was possible to detect it because this supermassive black hole is what astronomers call a “quasar”. That is, large amounts of gas are being poured into the black hole so quickly that the energy output is a thousand times greater than that of the galaxy itself, and this produces an extreme glow, allowing astronomers to detect it.

    6. Can collisions between galaxies create dangerous black holes?
    An artistic concept showing the most distant supermassive black hole ever discovered. It is part of a quasar that has emerged 37 millions of years after the Big Bang (Image: Robin Dienel/Carnegie Institution for Science)

    As we have seen before, the size of the region over which a specific black hole has significant gravitational influence is quite limited compared to the size of a galaxy. This applies even to supermassive black holes at the center of galaxies. These black holes have probably already “swallowed” most or all of the stars that were close enough to be captured by the titans’ gravity, so they won’t be able to grow much further than they already did.

    There is a way to create new black holes that are even bigger than supermassive ones, however: galaxy collisions will cause the black holes at their centers to grow. But collisions won’t happen indefinitely because the universe is large and expanding, causing galaxies to move farther and farther apart. Therefore, it is unlikely that any kind of runaway effect from black holes will occur, although it is not impossible.

    Furthermore, even if some galaxies merge, as will likely happen with Milky Way and Andromeda, their central black holes will not collide with each other. What can happen is that they, in the chaotic interactions between objects during galactic fusion, find new food, perhaps even new stars to swallow. But the chances of two black holes meeting are slim.

    7. Can black holes get smaller?

    The graph on the left is a model of an astrophysical black hole and how the Howking Radiation particles would behave; on the right is a diagram of an experiment involving a black hole analog done in the laboratory to prove Hawking’s theory (Image: Reproduction/Nature/Nova)

    Stephen Hawking proposed that, however, r black holes grow larger by devouring material, they also shrink slowly because they’re losing small amounts of energy called “Hawking radiation.” This radiation occurs because space is not really a vacuum, it is not exactly empty. In fact, it’s a sea of ​​particles that constantly appear and disappear.

    In a revolutionary study, the physicist showed that if a pair of these virtual particles were created arbitrarily (following the Uncertainty Principle of Heisenberg) near a black hole, there is a chance that one of them will be pulled into it before they both cancel each other out. In that case, the other particle will escape into space. The energy to compensate for this comes from the black hole itself, so it slowly loses energy and mass through this process.

    Eventually, in theory, black holes should evaporate through the Hawking radiation . But it would take much longer than the entire age of the universe for this to happen to most black holes we know. Black holes, even ones that are sometimes the mass of the Sun, will remain for a long, long time. So we can still learn more and more about them.

    8. Can black holes be portals to other dimensions?

    Concept of a wormhole (Image: Reproduction/ESO/L. Sidewalk)

    We would all like the answer to this question to be “yes”, simply because it is so much fun to imagine that there are dimensional portals all over the universe. But this is probably not the case. There are several scientists who publish studies on the possibilities of having a wormhole inside a black hole, for example, but there are good reasons not to get too carried away.

    So why the theorists publish studies claiming it is possible? And how do they reach these conclusions? It’s that in General Relativity, spacetime can be distorted and compressed by any matter that has mass — and this distortion is the phenomenon we call gravity. If we can compress spacetime enough, it would be possible to reach a greater distance, traveling less.

    This makes sense when we think of the analogy of the folded sheet of paper (which works like the image above), but researchers rely mainly on mathematical calculations of Einstein’s equations. Depending on how these problems are solved, a wormhole can form, creating a “shortcut” in spacetime.

    This means that if you fell into a wormhole, maybe would reach a place to millions of light years away in just a few minutes, or hours. If you traveled that distance at the speed of light without the help of a wormhole, it would take 00 millions of years to reach the same destination.

    Unfortunately, in practice, things are quite different. One of the problems with wormholes is that they would require a lot of energy to remain stable for a long time, and even if this difficulty were overcome, there would be other problems for traveling in wormholes. Even so, studies sometimes still appear suggesting that these “gates” may exist inside black holes or vice versa.

    9. How big can black holes be?

    (Image: Reproduction/ESO)

    The size of a black hole depends on something called the Schwarzschild radius, which is basically a ratio between the body’s mass and the minimum size that body can be. There is a complicated formula for calculating the Schwarzschild radius of anything, as long as you know the mass: the Schwarzschild radius of the Sun, for example, is about 3 km, and the Earth’s is about 9 mm.

    This means that the Sun can be squeezed to a radius of only 3 km (ie 6 km in diameter) without the risk of becoming a black hole, but what happens if the mass is compressed into an even smaller space? Well, the gravity of this object would be so great that to escape it would have to exceed the speed of light.

    Currently, astronomers don’t know exactly how big a black hole can be, because for General Relativity, black holes of any size and mass can exist. But there is certainly a limit to everything in the universe, even these cosmic titans. Furthermore, there is a lower limit: there is no known mechanism capable of forming black holes with less than ten solar masses, approximately.

    Finally, if we are talking about the lower limit for black holes , we can take into account that a star needs to have about ten solar masses for it to collapse. The Schwarzschild radius for an object with ten solar masses is 37.54 km — the smallest black holes must have a radius smaller than this value.

    . And the uniqueness? (Image: Reproduction/Andrew Hamilton/Jila/University Of Colorado)

    When scientists do studies on black holes, they can choose to use a theoretical model (like the Schwarzschild black hole, or some other) or what is known as an astrophysical black hole, which it takes into account only what can be certain about these objects. The singularity is not one of those things, so it is sometimes left out of the calculations.

    Gravitational singularity is the point where all the mass has flattened out to form the black hole. This point is so small that it is less than Planck’s length (13-10 times the radius of the proton), with a density that tends to infinity. The problem with this is that physics cannot deal with infinities, nor with things smaller than Plank’s length. Therefore, the existence or not of the singularity is something currently discussed.

    *This article was originally published in 29/00/1024, being updated and republished in 17/09/360616

    Source: With information from NASA

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