An observer outside of a black hole can only see the event horizon-the interior is hidden. A spectacular image of the black hole sitting in the galaxy M87-a huge black hole weighing almost 7 billion times as much as our sun-was recently produced by the Event Horizon Telescope collaboration !Īs a graduate student at Princeton in the 1970s, Jacob Bekenstein pondered the following puzzling question. In fact, now you can even see a black hole-or more precisely, its surrounding neighborhood. For instance, to compress the Earth to the size required to make a black hole, you would have to reduce its radius from today’s 4,000 miles to ~9 mm! Happily, Nature has provided us with real life examples of black holes, so we do not need to do the work of making one. We are talking about really dense matter. It is okay if you do not follow the details of the equation the basic point is that if you put an amount of mass M or larger in a sphere of radius smaller than R S( M), then voila-it will become a black hole. Where G is the gravitational constant and c is the speed of light. Therefore, its interior workings remain a mystery to the outside world, including us! If you want to know in more detail what a “small enough region” means, we can say A black hole is a clump of matter so dense that even light cannot escape its surface, or event horizon. Einstein’s theory of gravity says that when you squeeze it into a small enough region of space-with a size known as the Schwarzschild radius, R S( M)-the object becomes a qualitatively new kind of beast, a black hole. What is a black hole? Imagine taking a lump of matter at some fixed mass M, and crushing it. To the great surprise of theoretical physicists, it was discovered in the early 1970s that to this list of commonplace system with an entropy, you can add black holes. We know what these systems are made of, and how to count configurations of the underlying parts. The entropy of a system is defined as the logarithm of the number of possible configurations of its constituents (atoms for matter shirts and shorts for the messy room) that would look roughly the same, to a casual observer.įor conventional objects like tables, rooms, or slabs of lead, it is no surprise that one can associate an entropy to the system. In a messy bedroom, shirts and shorts can be flung about the room willy nilly. The atoms can be flying about every which way in hot matter. In contrast, matter at high temperature, or a messy room, has high entropy. In a very clean room, every piece of clothing is folded and stacked in a drawer. At low temperature, in a chunk of material, every atom sits at a place that minimizes the energy of the system. In both cases this is because the constituents of the system have no choice about where to be. Systems in very special states-almost any hunk of normal matter at very low temperatures, or a very clean bedroom, for instance-have low entropy. The entropy in a physical system is a measure of the amount of messiness or arbitrariness characterizing the system. Physicists call this the black hole entropy. In this article we discuss current thoughts about one of the mysteries of black holes-how one can explain the number of distinct (but similar) physical states they hide behind their event horizon. Black holes are both commonplace-our own Milky Way galaxy hosts a giant black hole in its center!-and mysterious. This is an object where matter has been packed so densely that even light cannot escape gravity’s pull at its surface, or event horizon. This means that in diverse physical systems, ranging from stars at the end of their lives burning nuclear fuel, to gas clouds which collapse under the weight of their own gravity, a natural endpoint can be the development of a black hole. Gravity is a universally attractive force, which tries to cause matter to clump.
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