10.6 Black Holes in the Real Universe
153
In Sects. 11.3–11.6 we will discuss gravitational waves. The source of the first
waves detected was the merger of two black holes from close orbit; the 2016 event
was called GW150914 and involved two black holes of about 30 solar masses each
(Abbott 2016). Since then there have been many other similar gravitational wave
events detected, including the merger of two neutron stars. It is rather remarkable
that those detections involve two of the most extraordinary things predicted by general
relativity, black holes and gravitational waves. Moreover they provide the first data
we have obtained on truly strong gravitational fields and give some of the strongest
evidence for the existence of black holes.
Black holes are not limited to stellar scale objects. There is no reason why the
processes that give rise to clusters of stars and galaxies should not also produce
black holes of much larger than stellar mass. For a star cluster the presence of a
supermassive black hole (SMBH) in the center would be signaled by rapid motion
of stars near the center; such large kinetic energy implies large potential energy,
which in turn implies a small massive object at the center. Just such clusters have
been observed. In particular the center of our Milky Way galaxy contains a very
interesting black hole of about 4 million solar masses, called Sagittarius A* or Sgr
A*: it is not visible to optical telescopes due to obscuring dust and gas but is the
object of much present research activity using radio telescopes designed to measure
the motion of stars near the central black hole. The goal is to study the system as
close to the Schwarzschild radius as possible.
It is widely thought that many galaxies, perhaps most, contain a SMBH at their
centers. There is a class of galaxies with “active galactic nuclei” (AGN) which emit
intense radiation and fit this picture. It is probable that quasars, which emit enormous
amounts of electromagnetic radiation, are powered by black holes at their centers.
The mechanism is analogous to that in Fig. 10.6, but on a larger scale. A SMBH at
the center of the galaxy Messier 87 has actually been imaged using a global network
of radio telescopes, called the event horizon telescope (EHT), set up to act like an
interferometer; the image is a rather fuzzy ring as shown in Fig. 10.7. A false-color
image and more details are available on a number of websites (EHT 2019; Wiki BH).
In summary, black holes have become very well-known among physicists and
astronomers and even the educated public. They occur at scales from stellar to galactic
and are one of the prime focus areas of current research.
10.7 Hawking Radiation from a Black Hole
All of the preceding discussion of black holes was based on classical physics, and
ignored quantum effects. Such quantum effects have been and still are the focus of
much theoretical activity. We will discuss here only a simple version of the most
notable effect, the thermal radiation emitted by a black hole, called Hawking radiation. Before we begin we emphasize that quantum effects such as Hawking radiation
have not been observed despite strong efforts and remain in the realm of theoretical
speculation.
153
In Sects. 11.3–11.6 we will discuss gravitational waves. The source of the first
waves detected was the merger of two black holes from close orbit; the 2016 event
was called GW150914 and involved two black holes of about 30 solar masses each
(Abbott 2016). Since then there have been many other similar gravitational wave
events detected, including the merger of two neutron stars. It is rather remarkable
that those detections involve two of the most extraordinary things predicted by general
relativity, black holes and gravitational waves. Moreover they provide the first data
we have obtained on truly strong gravitational fields and give some of the strongest
evidence for the existence of black holes.
Black holes are not limited to stellar scale objects. There is no reason why the
processes that give rise to clusters of stars and galaxies should not also produce
black holes of much larger than stellar mass. For a star cluster the presence of a
supermassive black hole (SMBH) in the center would be signaled by rapid motion
of stars near the center; such large kinetic energy implies large potential energy,
which in turn implies a small massive object at the center. Just such clusters have
been observed. In particular the center of our Milky Way galaxy contains a very
interesting black hole of about 4 million solar masses, called Sagittarius A* or Sgr
A*: it is not visible to optical telescopes due to obscuring dust and gas but is the
object of much present research activity using radio telescopes designed to measure
the motion of stars near the central black hole. The goal is to study the system as
close to the Schwarzschild radius as possible.
It is widely thought that many galaxies, perhaps most, contain a SMBH at their
centers. There is a class of galaxies with “active galactic nuclei” (AGN) which emit
intense radiation and fit this picture. It is probable that quasars, which emit enormous
amounts of electromagnetic radiation, are powered by black holes at their centers.
The mechanism is analogous to that in Fig. 10.6, but on a larger scale. A SMBH at
the center of the galaxy Messier 87 has actually been imaged using a global network
of radio telescopes, called the event horizon telescope (EHT), set up to act like an
interferometer; the image is a rather fuzzy ring as shown in Fig. 10.7. A false-color
image and more details are available on a number of websites (EHT 2019; Wiki BH).
In summary, black holes have become very well-known among physicists and
astronomers and even the educated public. They occur at scales from stellar to galactic
and are one of the prime focus areas of current research.
10.7 Hawking Radiation from a Black Hole
All of the preceding discussion of black holes was based on classical physics, and
ignored quantum effects. Such quantum effects have been and still are the focus of
much theoretical activity. We will discuss here only a simple version of the most
notable effect, the thermal radiation emitted by a black hole, called Hawking radiation. Before we begin we emphasize that quantum effects such as Hawking radiation
have not been observed despite strong efforts and remain in the realm of theoretical
speculation.