Sunday, March 27, 2016

The Roach Motel for everything

kw: book reviews, nonfiction, physics, black holes, history of science

In 1905 Albert Einstein published four small monographs in scientific journals. Their subjects were

  • The Photoelectric Effect, in which the color of the light, and thus its frequency, were directly related to the voltage of electrons emitted from a sensitive surface, and depending on the "activation potential" of various surfaces, there was a threshold below which no emission could occur. This proved that light is quantized as particles now called Photons.
  • Brownian Motion, in which tiny, lightweight items such as pollen grains, suspended in water and viewed through a microscope, are seen to jiggle continuously. He showed that this is a statistical effect of jostling by molecules of water, the first empirical evidence for the existence of atoms and molecules.
  • Special Relativity, in which he established that the speed of light and all effects of the interaction of light with spacetime are the same as measured in any non-accelerating reference frame, regardless of that frame's velocity with respect to any other. This implies that everything except light is variable when measured between reference frames moving at differing velocities, particularly mass, length, and the passage of time.
  • Mass-Energy Equivalence, expressed in the formula E=Mc², in which he showed that Maxwell's laws imply that as energy is added to a system its mass increases. The parameter c is a large number, 300,000 km/s, and its square is thus so huge that simply heating a kg of iron, for example, between 0°C and the melting point of the iron, will only increase its mass by something like a few billionths of a billionth of a gram. But the equation as stated provides a hint, later well defined by the scientists of the Manhattan Project, that nuclear reactions which confer a reduction of mass yield enormous energy release.

Guess which of these discoveries led to Einstein receiving the Nobel Prize in 1922? It is the Photoelectric Effect, which laid a foundation for Quantum Physics, by effectively discovering the Photon, the first quantum particle to be so defined.

Ten years later, Einstein published articles on his General Theory of Relativity, usually just called General Relativity, which extended Special Relativity to accelerating motion and, in particular, to motions in a gravitational field. The set of equations at the core of the theory show that gravity is a consequence of curvature in spacetime caused by mass. As John Wheeler states it, "Mass tells Spacetime how to curve, and Curved Spacetime tells Mass how to move."

It wasn't long before scientists, striving to find exact solutions to the theory's equations, determined that the end point of gravitational collapse was a singularity. This had been hinted at by scientists as far back as 1783, when John Mitchell first calculated the mass needed for a Sol-sized star to prevent the escape of light, and thus be rendered invisible to a (safely) distant observer.

The events along the way between 1783 and 1915, and those since, form the structure of Black Hole: How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved by Marcia Bartusiak. The author's aim is not to discuss the physics of black holes to any great extent—though a certain amount is necessary—but to trace the history of the idea, from an idea that made classical physicists queasy to the modern understanding of the way black holes have shaped the universe. That "queasiness" led to general relativity being neglected for most of fifty years. Finally, improvements in astronomical instruments and methods forced recognition that such "supercollapsed" objects as black holes might indeed be real.

The first astronomical object to be generally accepted as a black hole is Cyg X-1, in the constellation Cygnus (the Swan), which weighs about 15 times as much as the Sun. It and a blue supergiant star orbit one another closely, with a period of 5.6 days. The supergiant sheds mass in irregular fashion, and some of the gas is drawn into an accretion disk circling the black hole, where frictional and compressional heating raise temperatures to millions of degrees and cause flashing and flickering at primarily X-ray wavelengths. Such a black hole is called a "stellar black hole" because it formed from the collapse of a single star.

A much larger event or series of events must underlie the formation of the very large black holes at the centers of galaxies. It is very likely that a "supermassive black hole" is at the core of every galaxy. The stars in our own galaxy, the Milky Way, orbit a black hole with a mass of about 4 million Suns, or about 270,000 times the mass of Cyg X-1. Larger galaxies, or galaxies with larger central bulges, and large elliptical galaxies which are all central bulge, have central black holes up to several billion Suns in mass.

Do these galactic black holes also shine or flash like Cyg X-1? Whenever they have a source of infalling matter, they do. Quasars are extremely bright and very tiny (compared to a galaxy) objects that are seen in light, radio, and X-rays caused by large amounts of matter in their accretion disks. "Active galactic nuclei" are dimmer than quasars, from our perspective, but are probably quasars when seen from a special perspective, because the emissions of black holes are directional.

Black holes all spin, and they all have magnetic fields. This is because they were formed from spinning matter (and accretion adds to their spin), and all plasmas in space are magnetic; the magnetic field is retained after collapse within the event horizon. Accreted matter, heated to a plasma, is spun by the spinning magnetic field and is compressed into a pair of jets which emit primarily along the spin axis of the black hole. A quasar is what we see when we are looking "down the barrel" of one of these jets. An active galactic nucleus is seen when we are off to the side. Cyg X-1 is apparently also pointed right at us.

When we read of a quasar that has an apparent brightness of trillions of stars, we must remember that the "trillions" figure is calculated by assuming equal brightness in all directions. The actual beam is a degree or two across, and a sphere has an angular area of more than 41,000 square degrees. So the "trillions" become "billions" or "hundreds of millions", nearly all concentrated into those two beams and so amplified from our viewpoint. That is still really, really bright!

The first quasar had a spectrum too weird to fathom, at first, until it was finally realized that the spectral lines were those of hydrogen, shifted far toward the red end, implying a cosmological distance of about 2 billion light-years. The distance to a quasar is actually rather tricky to calculate, because of three effects (this is just me now; the second and third factors are not mentioned in the book):

  1. The cosmological red shift caused by the expansion of space.
  2. The gravitational red shift caused by the tremendous gravitational potential close to the event horizon (where the gravitational red shift would be infinite!). This increases the total red shift and by itself will cause us to over-estimate the distance to the quasar.
  3. A blue shift caused by the relativistic velocity of the superheated matter beam, which might be as much as 0.2c to 0.5c pointed toward us, and perhaps even more. This counteracts some of the red shift from expansion and gravity.

Of these three factors, it is generally considered that the first is the greatest, but I have not read a definitive analysis of the second and third factors for any particular quasar…and I've been looking.

The book is fascinating and enjoyable. A timeline in an appendix helps tie events together, and traces the contributions of many scientists to the understanding of general relativity and gravitational collapse and its implications. A well-researched and well-written book, it rounds out the scientific story into a fascinating human story.

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