Hawking radiation,Black-Hole Evaporation and black hole explosions! Stephen Hawking theorized that a black hole may not be so black. He showed that when you apply the laws of quantum mechanics to classical physics, you find that they shine. They give off photons. How is this possible given that nothing escapes from black holes? In classical physics, the mass of a black hole cannot decrease, it can either stay the same or increase. To Stephen Hawking and others, this idea looked similar to the 2nd law of thermodynamics which is, “In any natural process, the entropy of a closed system always increases or remains constant, it never decreases.” Similar to the 2nd law, there are also ways to state the other 3 laws of thermodynamics such that they are true for black holes as well. In 1972, physicist Jocob Bekenstein, proposed that the surface area of a black hole is its entropy. The larger a black hole is, the more stuff it has consumed. Since everything it consumes has information, the greater the entropy of the system. Why? Because the more disordered a system is, the more information is required to describe it. The analogy with thermodynamics suggest that black holes are a thermal body. In thermodynamics, there is something called a black body, which doesn’t transmit or reflect any radiation, it only absorbs it. A black hole, too, is something doesn’t transmit or reflect any radiation. If a black hole can be thought of as a black body, then it must have a temperature, because all black bodies have a temperature, which means it must shine. When Hawking saw these ideas, he found the idea of shining black holes to be preposterous. He set out to prove why they would NOT shine. But when he applied the laws of quantum mechanics to general relativity, he realized the opposite. In 1974, he published a paper outlining a mechanism for this shine. All of space is teaming with virtual particles that come in and out of existence all the time. This is based on the Heisenberg uncertainty principle. One version of this can be written as the following: Delta-E*Delta-t ~= h/4*pi. This equation says is that the uncertainty in energy and uncertainty in time are inversely proportional to each other. But it also means that you can get particles with an energy, if it occurs for a very short period of time, or particles can exist that violate this uncertainty principle. Particle/antiparticle pairs borrow temporary energy from the present, and give it right back in the future by annihilating themselves. This occurs over a shorter time than can be measured. Can we measure this? It manifests as a force in something called the Casimir effect, in which the quantum foam outside a set of two plates is greater than the pressure inside the plates, and this creates a force pushing the plates together. As particle-antiparticle pairs get created near the event horizon, one particle can get sucked into the black hole and another is released before the two can annihilate each other. This is what we perceive as Hawking radiation. From our perspective outside the black hole, the particle we got is positive, but this means that the black hole got negative energy. In other words it lost energy. This is the same thing as the black hole losing mass. #hawkingradiation #stephenhawking The biggest problem with this explanation is that the radiation from black holes is not in all wavelengths, as would be expected with this mechanism. The radiation actually has a wavelength equal to the size of the black hole. So smaller black holes emit shorter wavelengths, or more energy, than larger black holes. The better explanation is that waves coming in from infinity and being disrupted because of the black hole event horizon, as it is forming. Certain vibrations of waves are deflected by the gravitational field of the black hole as it forms in the past. The waves entering the event horizon are disrupted in a way that the wave on the other side, carried away energies corresponding to the size of the black hole. This corresponds to an energy spectrum analogous to a black body at a certain temperature. So this is why Black holes radiate. Can we measure it? Probably not, but one prediction is made. As the black hole evaporates over time, the temperature rises, and the evaporation rate goes to infinity. This would be seen as a burst of high energy photons or gamma rays. So do we detect Gamma Ray bursts? We do - about one gamma ray bursts per day. But the pattern of gamma rays do not fit with what we would expect to see in a black hole explosion. What we see are bursts with variations in brightness, from bright to dim to bright again. The black hole evaporation should look like a steady increase in luminosity until a final explosion. But despite the lack of evidence, Hawking Radiation perfectly fits within the laws of quantum mechanics, and few physicist dispute its existence.