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R. N. Mohapatra
5.2 Black Holes and Gravity
According to Newton’s theory, gravity does not have any effect on light.
Einstein’s theory introduced the new idea that gravity bends space-time, and
all paths, including that of light, follow the curved space (called geodesics).
This means that in a space near a massive object, the path of light gets bent
(or equivalently, light gets affected by gravity). This follows from the special
theory of relativity by Einstein, which says that mass and energy are equivalent
and since a light beam has energy, it also gets affected by gravity, just like an
object with mass. Extending this idea, one can imagine that if the mass of
an object is really large, light can get bent back and will not leave the object.
Thus the object will look black since no light is coming from it. Those massive
objects from which light ray cannot escape are called black holes. The existence
of such objects within Einstein’s theory of general relativity was first noted by
Karl Schwarzchild in 1916, and much later, the name black hole was coined
by John Wheeler of Princeton. As we discuss later, the end stage of life of
certain massive stars is a black hole. Steven Hawking and Jacob Bekenstein
pointed out that black holes also emit radiation, like a black body, known
as Hawking–Bekenstein radiation. As the black holes radiate, they lose their
mass. Hawking–Bekenstein radiation from black holes, however, has not been
established experimentally.
After a black hole has formed, it can continue to grow by absorbing mass
from its neighborhood, like surrounding stars. Thus supermassive black holes
of millions of solar masses may form. When a supermassive black hole gobbles
up massive stars, it emits a lot of radiation including neutrinos. There is a
general consensus that supermassive black holes exist in the centers of most
galaxies. An image of the first such supermassive black hole was presented by
the Event Horizon Telescope in April, 2019. The black hole, called M87, is
at a distance of 16 megaparsecs from us and has a mass of six billion solar
masses. This is an exciting finding. The fact that things that were only in our
imaginations for so long have been proven to exist is truly gratifying and is a
testament to the power of physics. Gravitational waves from as many as ten
binary black hole systems have been detected by LIGO as of February 2019.
R. N. Mohapatra
5.2 Black Holes and Gravity
According to Newton’s theory, gravity does not have any effect on light.
Einstein’s theory introduced the new idea that gravity bends space-time, and
all paths, including that of light, follow the curved space (called geodesics).
This means that in a space near a massive object, the path of light gets bent
(or equivalently, light gets affected by gravity). This follows from the special
theory of relativity by Einstein, which says that mass and energy are equivalent
and since a light beam has energy, it also gets affected by gravity, just like an
object with mass. Extending this idea, one can imagine that if the mass of
an object is really large, light can get bent back and will not leave the object.
Thus the object will look black since no light is coming from it. Those massive
objects from which light ray cannot escape are called black holes. The existence
of such objects within Einstein’s theory of general relativity was first noted by
Karl Schwarzchild in 1916, and much later, the name black hole was coined
by John Wheeler of Princeton. As we discuss later, the end stage of life of
certain massive stars is a black hole. Steven Hawking and Jacob Bekenstein
pointed out that black holes also emit radiation, like a black body, known
as Hawking–Bekenstein radiation. As the black holes radiate, they lose their
mass. Hawking–Bekenstein radiation from black holes, however, has not been
established experimentally.
After a black hole has formed, it can continue to grow by absorbing mass
from its neighborhood, like surrounding stars. Thus supermassive black holes
of millions of solar masses may form. When a supermassive black hole gobbles
up massive stars, it emits a lot of radiation including neutrinos. There is a
general consensus that supermassive black holes exist in the centers of most
galaxies. An image of the first such supermassive black hole was presented by
the Event Horizon Telescope in April, 2019. The black hole, called M87, is
at a distance of 16 megaparsecs from us and has a mass of six billion solar
masses. This is an exciting finding. The fact that things that were only in our
imaginations for so long have been proven to exist is truly gratifying and is a
testament to the power of physics. Gravitational waves from as many as ten
binary black hole systems have been detected by LIGO as of February 2019.