Black Holes Explained: Course by Alexei V. Filippenko

This is a course about black holes, regions of space in which gravity is so strong that nothing, not even light, can escape. References to black holes are abounding in popular culture; most people have heard of them, but few truly understand what they are. Far from being the figments of the fertile imaginations of theoretical physicists and science-fiction writers, black holes almost certainly exist in the Universe; the observational evidence for their presence is now quite compelling. The 2 main varieties are stellar-mass black holes, produced by the collapse of the core of massive stars at the end of their lives, and supermassive black holes, which form in the centers of galaxies. But although black holes themselves are real, travel through them is probably impossible, despite their use as exotic passages in some fiction books and movies. Incredibly, black holes might not really be black; modern theories combining quantum physics, thermodynamics, and general relativity suggest that they can gradually evaporate by emitting particles.

We begin with a broad overview of black holes, explaining some of the main concepts and defining terms. Newtonian plausibility arguments for the existence of "dark stars" (black holes) were first made several centuries ago, but a complete understanding requires Einstein's general theory of relativity and the realization that matter warps both space and time. According to classical physics, matter that passes beyond the event horizon, or boundary, of a black hole collapses to form an infinitely dense singularity, and nothing ever emerges back into the outside world. Black holes are shown to be quite small, and there is virtually no chance that one will swallow Earth.

Next we consider stellar-mass black holes and their formation from certain types of massive stars. Although most massive stars end their lives by exploding as a result of the energy emitted by the final gravitational collapse of their core to a dense neutron star, in some cases the collapse proceeds further, resulting in the formation of an even denser black hole. Gamma-ray bursts, described in Lecture Three, are thought to be such "birth cries" of black holes, or (in some cases) the growth of existing stellar-mass black holes that devour a companion neutron star. Lecture Four discusses the methods used to search for stellar-mass black holes and measure their masses, which are typically 5 to 10 solar masses but can reach as high as at least 30 Suns.

We then turn to supermassive black holes that lurk in the central regions of galaxies, starting with the monster in our own Milky Way Galaxy. This object, weighing in at about 4 million solar masses, provides the best available evidence for the existence of black holes; stars in its vicinity move so quickly around such a small volume that explanation other than a black hole seems viable. The supermassive black holes in other galaxies can be even larger, reaching a few billion solar masses in some cases. As discussed in Lecture Six, they probably formed early in the history of the Universe and manifested themselves as luminous quasars. When 2 galaxies merge together, their central supermassive black holes can also coalesce into a single bigger black hole, releasing ripples in the fabric of space-time known as gravitational waves. Such waves, which are also emitted when stellar-mass black holes or neutron stars merge, are the subject of Lecture Seven; they allow us to "listen" to the Universe, obtaining information that would otherwise remain hidden.

In Lecture Eight we discuss the tidal stretching that occurs near a black hole, especially in the case of the stellar-mass variety. A hypothetical journey all the way to the singularity in a supermassive black hole is described; though doomed, the traveler would experience a truly wild ride, with many weird effects visible. However, if the traveler avoids the event horizon and returns home, he will have aged less than people who remained far from the black hole, effectively jumping into the future relative to them. Lecture Nine extends the discussion to rotating and charged black holes, which mathematically seem to imply the existence of wormholes—passages to other universes or shortcuts through our Universe. Unfortunately, it is probably impossible to traverse a real wormhole.

Next we consider Stephen Hawking's theory that black holes can actually lose mass, gradually evaporating due to quantum effects and ending their existence in colossal explosions. Most physicists have reasoned that black holes are incredibly simple objects, characterized only by their mass, electric charge, and spin; if so, however, black hole evaporation seems to violate a fundamental principle of quantum physics that no information is ever permanently lost from the Universe. Lecture Eleven examines this conundrum and its recent possible resolution; all of the information in a black hole actually resides in a membrane wrapped around the event horizon, analogous to a hologram, and is not destroyed by the evaporation process.

In Lecture Twelve, we discuss the possible production of miniature black holes in the Large Hadron Collider, and the physical implications if this were to be achieved. Contrary to common assertions, such minuscule black holes pose absolutely no threat to humans or to Earth. We end with a brief look at paths for the future of black hole research.

Taken From The Great Courses

Link recommended by Professor Alex Filippenko [link]