Friction and black holes
By R. David Whitby, Contributing Editor | TLT Worldwide September 2023
Black holes can be detected by looking for the high-energy radiation, created by friction, that surrounds them.
When a region in space has a density with a gravitational pull so strong that nothing, not even light, can escape from the region, it is known as a “black hole.” Three types of black holes have been observed and studied in detail in our Milky Way galaxy and in other galaxies.
Stellar-mass black holes, with masses from about three to dozens of times the mass of our Sun, are spread throughout many galaxies. Supermassive black holes, weighing hundreds of thousands to billions of solar masses, are found in the centers of most big galaxies, including ours. They are believed to form by absorbing nearby stars or interstellar gas and by merging with other black holes.
Intermediate-mass black holes, with masses ranging from about 100 to 10,000 solar masses, were suspected to exist. Although several candidates were identified with indirect evidence, the most convincing example was in 2019, when the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from a merger of two stellar-mass black holes 1.3 billion years ago. This event, called GW150914,1 resulted in a black hole weighing 142 Suns.
Although it is difficult to “see” a black hole, its mass and location can be determined by the orbits of any stars that are rotating around it. Astronomers have identified numerous stellar black hole candidates in binary systems using this method. They have established that the radio source known as Sagittarius A, at the center of the Milky Way, contains a supermassive black hole of about 4.3 million solar masses.
Albert Einstein’s theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. However, black holes were considered to be only a mathematical possibility, until theoretical astrophysicists’ studies in the 1960s showed they were a generic prediction of general relativity. Interest in gravitationally collapsed compact objects was sparked in 1967 by Jocelyn Bell Burnell’s discovery of pulsars, which are rapidly rotating neutron stars. Several researchers independently identified the first black hole, Cygnus X-1, in 1971. The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation.
When a star with more than 20 solar masses exhausts the nuclear fuel in its core and collapses under its own weight, the collapse triggers a supernova explosion that blows off the star’s outer layers. If the resulting core contains more than about three solar masses, no known force can stop its collapse to a stellar-mass black hole. Smaller collapsed cores form neutron stars.
The image of a black hole was captured for the first time in 2019, by the Event Horizon Telescope, an international collaboration that networks eight ground-based radio telescopes into a single Earth-sized dish. The image appears as a dark circle silhouetted by an orbiting disk of hot, glowing matter. The supermassive black hole is located at the heart of a galaxy called Messier 87 (M87), located about 55 million light years away, and weighs more than six billion solar masses.
Event horizons for black holes are widely misunderstood. The notion that black holes “vacuum up” material in their vicinity is erroneous, since black holes are no more capable of seeking out material to consume than any other gravitational attractor. As with any mass in the universe, matter must come within a black hole’s gravitational influence before it becomes possible for the two masses to amalgamate. The idea that matter can be observed falling into a black hole also is not true. The event horizon is generally understood to mean the place at which light cannot escape from the black hole, but this is usually not observable.
Astronomers can detect only accretion disks around black holes, where material moves with such velocities that friction creates high-energy radiation, which can be detected. The friction within clouds of gas or the enormous tidal forces within stars surrounding a black hole cause nearby matter to heat up to millions of degrees and to emit radio waves, X-rays and gamma rays. Some matter from the accretion disks can be forced out along the axis of spin of the black hole, creating visible jets when these streams interact with matter such as interstellar gas or when they happen to be aimed directly at Earth. This is why observers see an orbiting disk of extremely hot, glowing matter surrounding a black hole.
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David Whitby is chief executive of Pathmaster Marketing Ltd. In Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.