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-------------------- 2573 - BLACKHOLES - everything we know ?
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- Blackholes are some of the strangest phenomena in our universe. Strange things can happen when all that mass is packed within a specific radius called the “Schwarzschild Radius“. Then a so-called “event horizon” forms at the Schwarzschild radius and the star becomes a black hole.
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- The Schwarzschild radius ‘rs’ and the mass “M” of a spherical object have a very easy relation: They are proportional to each other, “rs = a x M“, with “a” being very small when we measure in standard units.
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- Let us for example consider an object with the mass of earth, then its Schwarzschild radius is given by only about 9 millimeters! No physical process is known in which all of earth can be compressed that much and it seams quite unlikely that there are many black holes with the mass of earth.
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- The situation changes when we consider bigger and bigger masses. This is because the volume within the Schwarzschild radius, i.e., the space where we can store all the mass to form a black hole, increases much faster. In fact, if we double the mass we get roughly eight times more volume to store it. The increase is with the cube of the radius
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- The formation of black holes becomes easier the heavier they are. It is known that there are mechanisms at the end of the lifetime of very massive stars that lead to the formation of so-called stellar black holes. Even heavier black holes can develop when stellar ones merge.
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- The black hole is not much different from any other stellar object of the same mass. The only big difference is that we do not see any light originating from the black hole. An interesting effect, that occurs when we come closer to it, is that time for us passes slower than for those who stay away from the black hole. Increased gravity slows down time.
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- the effect is not tractable in a direct way. Any clock that we carry with us behaves completely fine from our perspective. Only if we return back to a place far away from the black hole and compare the times passed we could see a difference. This effect appears for any massive object that we come close to, and is not only a feature of black holes.
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- Remember that the solution to Einstein’s equations, which in particular tells us how time behaves, outside the spherical object only depends on its mass! However, for any stellar object that is not a black hole we would at some point reach that object and enter it. Inside of it the solutions do depend on its specifics.
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- One can show that effects like “time dilation” cannot increase further arbitrarily. If we come closer and closer to the black hole the effect will grow without a bound until we reach the previously mentioned “event horizon“.
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- In the moment that we spend on the event horizon all time passes for anything outside the black hole. The end of all things happens in the rest of the universe, and in fact after the moment in which we enter the black hole, there is no way back.
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- This is also visible in a remarkable and strange feature of Schwarzschild’s solution. If we compare it inside and outside of the event horizon one observes that global time and the radial direction interchanged their meaning. In a world outside the black hole everything and everyone has to move forward in time. This is a fundamental feature of Einstein’s theory.
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- Inside the event horizon the radial direction takes the place of time with the drastic consequence that, no matter what, we have to move forward in this direction, where forward means toward the center of the black hole. There really is no way back! Not even the most powerful rocket that we could imagine can prevent us from finally reaching the very center of the black hole, where gravitational forces become immeasurably strong and at the latest there the quantum features of black holes and gravity itself overtakes everything.
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- We have seen before that Schwarzschild’s solution only depends on the mass of the object. But what happens with all the information about the object that collapsed into a black hole? It consisted of many different particles, it had a temperature, a matter distribution, a specific spectrum of radiation, and so on.
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- If we only believe in the classical world, then all that information is hidden behind the event horizon after the formation of the black hole. Then it is by no means tractable for anyone outside the black hole. In a classical world this is not a big problem. No one outside the black hole can get the information but causes no issues concerning consistency of the theory.
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- We know that our world is in fact not a classical one, and our knowledge about the quantum theories that describe at least ordinary matter in our universe is well established. Using this knowledge Stephen Hawking could show that quantum effects near the event horizon of a black hole lead to a constant flow of particles away from it. A black hole radiates and must loose mass over time. If we wait long enough a black hole evaporates either completely or until some tiny remnant of it is left.
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- Where is all the information after the evaporation? Now that we include some quantum theory in the description of black holes this question becomes very important. Thoughtless processing of quantum information can easily lead to inconsistencies.
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- For example, information has to be spread very fast inside the black hole. Otherwise it would be possible to copy quantum states, which is strictly forbidden in any consistent quantum theory. There is no real consensus among the physics community on how the black hole deals with quantum information. One possibility might be that it is hidden in its Hawking radiation.
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- If we wait long enough and collect a sufficient amount of it we might be able to regain all the information we want. Finding a convincing and self-consistent description of black holes in contact with the quantum world will undoubtedly be an important step toward a quantum description of gravity itself. This might be one of the next big steps in theoretical physics!
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- If we first consider an ordinary stellar black hole four times the mass of our Sun, then its Hawking radiation can be associated with a temperature only roughly a hundred millionth Kelvin above the absolute zero temperature. Temperature plays almost no role in describing the everyday physics of that black hole.
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- This is true for any stellar black hole. Next let us consider a coin of, say, five grams. Quantum effects of that coin do not play any significant role in its everyday physics. It can be described almost perfectly by classical theories. However, if we consider a black hole of the same mass, things look rather different. As a partially quantum object it radiates and evaporates within a tiny fraction of a second.
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- All its mass converts into energy, which results in an explosion three times stronger than the bomb dropped on Hiroshima. We see that for black holes quantum effects play a significant role much earlier than for matter under ordinary conditions.
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- Astrophysicists normally assume that huge systems like the universe, are indifferent to details of smaller systems contained within it The research has identified and fixed a subtle error that was made when utilizing Einstein's equations to model the expansion of the universe. It is now apparent that general relativity can observably connect collapsed stars to the behavior of the universe as a whole, over a thousand billion billion times bigger.
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- One consequence of this study is that the growth rate of the universe presents data about what happens to stars at the end of their lives. Astronomers typically believe that giant stars form black holes when they perish, but this is not the only potential outcome.
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- Dark energy was initially discovered back in 1998. Scientists determined that the expansion of the Universe is accelerating, consistent with the presence of a uniform contribution of dark energy.
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- Three years ago LIGO observed two black holes colliding by monitoring gravitational waves. Colliding double black hole systems were assumed to exist. The colliding systems in question, however, where approximately five times larger than predicted. The results of this recent study might also apply to this mystery.
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- Astronomers have recently found the biggest black hole ever measured. It is 40 billion times the sun’s mass, or roughly two-thirds the mass of all stars in the Milky Way. The gargantuan black hole lurks in a galaxy that’s supermassive itself and probably formed from the collisions of at least eight smaller galaxies.
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- When two spiral galaxies , like our Milky Way and the nearby Andromeda Galaxy , collide, they can merge and form an elliptical galaxy. In crowded environments like galaxy clusters, these elliptical galaxies can collide and merge again to form an even larger elliptical galaxy. Their central black holes combine as well and make larger black holes, which can kick huge swaths of nearby stars out to the edges of the newly formed galaxy.
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- The finding is exciting because it lends support to astronomers’ current understanding of quasars, distant galaxies with massive central black holes that emit huge amounts of light as they gobble up nearby matter in a process called accretion. Studying quasars made astronomers think that black holes 10 billion or more solar masses must exist for some of these faraway quasars to be so bright.
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- What happens when you are, about to leap into a black hole. What could possibly await should ,against all odds , you somehow survive? Where would you end up and what tantalizing tales would you be able to regale if you managed to clamber your way back?
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- Falling through an “event horizon” is literally passing beyond the veil. Once someone falls past it, nobody could ever send a message back. They would be ripped to pieces by the enormous gravity, so I doubt anyone falling through would get anywhere.
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- If that sounds like a disappointing and painful answer. Ever since Albert Einstein's general theory of relativity was considered to have predicted black holes by linking space-time with the action of gravity, it has been known that black holes result from the death of a massive star leaving behind a small, dense remnant core.
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- Assuming this core has more than roughly three-times the mass of the Sun, gravity would overwhelm to such a degree that it would fall in on itself into a single point, or singularity, understood to be the black hole's infinitely dense core.
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- The resulting uninhabitable black hole would have such a powerful gravitational pull that not even light could avoid it. Tidal forces would reduce your body into strands of atoms (or 'spaghettification', as it is also known) and the your body would eventually end up crushed at the singularity.
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- What about a wormhole? Black holes are strange regions where gravity is strong enough to bend light, warp space and distort time. Over the years scientists have looked into the possibility that black holes could be wormholes to other galaxies. They may even be a path to another universe.
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- Maybe a black hole leads to a white hole? Certainly, if black holes do lead to another part of a galaxy or another universe, there would need to be something opposite to them on the other side. Could this be a white hole. Unlike a black hole, a white hole will allow light and matter to leave, but light and matter will not be able to enter.
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- Scientists have continued to explore the potential connection between black and white holes. In 2014 some scientists claimed that there is a classic metric satisfying the Einstein equations outside a finite space-time region where matter collapses into a black hole and then emerges from a while hole. In other words, all of the material black holes have swallowed could be spewed out, and black holes may become white holes when they die.
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- Far from destroying the information that it absorbs, the collapse of a black hole would be halted. It would instead experience a quantum bounce, allowing information to escape. Should this be the case, it would shed some light on a proposal by Stephen Hawking who explored the possibility that black holes emit particles and radiation, thermal heat, as a result of quantum fluctuations.
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- Hawking said a black hole doesn't last forever. Hawking calculated that the radiation would cause a black hole to lose energy, shrink and disappear. Given his claims that the radiation emitted would be random and contain no information about what had fallen in, the black hole, upon its explosion, would erase loads of information.
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- This meant Hawking's idea was at odds with quantum theory, which says information can't be destroyed. Physics states information just becomes more difficult to find because, should it become lost, it becomes impossible to know the past or the future.
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- Hawking's idea led to the 'black hole information paradox' and it has long puzzled scientists. Do we go back to the concept of black holes emitting preserved information and throwing it back out via a white hole?
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- Appling “loop quantum gravity” to a black hole found that gravity increased towards the core but reduced and sent whatever was entering into another region of the universe. The results gave extra credence to the idea of black holes serving as a portal.
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- In this study, “singularity” does not exist, and so it doesn't form an impenetrable barrier that ends up crushing whatever it encounters. It also means that information doesn't disappear.
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- The “black hole firewall hypothesis“ calculation using quantum mechanics could feasibly turn the event horizon into a giant wall of fire and anything coming into contact would burn in an instant. In that sense, black holes lead nowhere because nothing could ever get inside.
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- This theory violates Einstein's general theory of relativity. Someone crossing the event horizon shouldn't actually feel any great hardship because an object would be in free fall and, based on the “equivalence principle“, that object would not feel the extreme effects of gravity.
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- It could follow the laws of physics present elsewhere in the universe, but even if it didn't go against Einstein's principle it would undermine quantum field theory or suggest information can be lost.
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- Hawking once more, in 2014, published a study in which he redefined the existence of an event horizon saying gravitational collapse would produce an 'apparent horizon' instead.
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- This horizon would suspend light rays trying to move away from the core of the black hole, and would persist for a "period of time." In his rethinking, apparent horizons temporarily retain matter and energy before dissolving and releasing them later down the line.
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- This explanation best fits with quantum theory, which says information can't be destroyed, and, if it was ever proven, it suggests that anything could escape from a black hole.
- Hawking went as far as saying black holes may not even exist. Instead, black holes should be redefined as meta-stable bound states of the gravitational field. There would be no singularity, and while the apparent field would move inwards due to gravity, it would never reach the center and be consolidated within a dense mass.
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- Anything which is emitted from the black hole will not be in the form of the information swallowed. It would be impossible to figure out what went in by looking at what is coming out. One thing's for sure, this particular mystery is going to swallow up many more scientific hours for a long time to come.
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- Astronomers recently found a black hole so big that theory strains to explain it. They have discovered a stellar-mass black hole that appears to be 68 times heftier than Earth's sun, nearly three times bigger than the heaviest such objects should be.
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- Calculations suggest that the Milky Way galaxy's stellar-mass black holes, which form after the violent deaths of giant stars, should top out at only 25 times the mass of the sun. Supermassive black holes that lurk at the hearts of galaxies are much bigger, of course, containing millions or billions of solar masses.
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- What's more, the huge black hole is also relatively close to Earth in cosmic terms. It sits at 13,800 light-years from our planet, a small fraction of the Milky Way's estimated diameter of 200,000 light-years.
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- Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution. We thought that very massive stars with the chemical composition typical of our galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant.
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- The researchers do acknowledge some caveats with the study. For example, the mass of the black hole depends on its calculated distance. Europe's Gaia space telescope, which precisely measures the movements of a billion stars, has suggested that the distance to this black hole might be only about 7,000 light-years, or roughly half the distance the Chinese team calculated. If that's true, the black hole would be only 10 times the mass of the sun.
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- That is about as much as we know about blackholes. Now you know as much as anybody else. Happy New year 2020.
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- January 2, 2020 2573
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--------------------- Friday, January 3, 2020 --------------------
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