Black Holes


By Eddy Goss, Tan


Matter circulating around a black hole.
Matter circulating around a black hole.
For decades, black holes were objects of science fiction. Many scientists believed they were too bizarre to exist in the universe even though general relativity predicted that black holes could exist. Astronomers have now discovered several black holes in X-ray emitting binary star systems (pairs of stars that orbit about each other). They observed a normal star orbits a massive yet invisible companion and the theory is that this is a black hole. Black holes are the last evolutionarystage in the lifetime of gigantic stars at least 15-20 times the size of our sun. When these giant starsreach the final stages of their lives, the flame of thermonuclear fusion is extinguished and the gravitational collapse takes over. The star caves in on itself, swirling into a funnel that is infinite. Gravitational pull is so great in this downward thrust of crushed matter that nothing can get out, even light. These giant stars are literally crushed out of visible existence and are consequently known as “black holes”. These holes may continually pull mass from their neighboring stars, growing the black hole and shunting the other star, until the black hole is large and the companion star completely vanishes leaving the remaining star to orbit alone. Planets, light and other matter have to pass close to a black hole in order to be pulled in. The “event horizon” is a ring at the top of the funnel which if passed is a point of no return.

Theories

Most smaller stars end up as “white dwarfs” or “neutron stars” when their nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity. Black holes are the last evolutionary stage of enormous stars that have been at least 15-20 times the size of our sun. When a huge star comes to the end of its life it explodes in cataclysms known as “supernovae”. The remnants of the star are scattered into space but as there is no longer any opposing force on gravity and the core of the star starts to collapse within itself and the black hole shrinks to zero volume where it is infinitely dense, known as “singularity”. When black holes were first hypothesized in the late 1790’s, they were called “invisible stars”. John Michell of England and Pierre La Place of France independently theorized the existence of an “invisible star” using Newton’s Laws of gravity and motion. They calculated that an object needed to have an escape velocity greater than the speed of light to pass the “event horizon”. John Wheeler, an American theoretical physicist, later named these collapsed stars “black holes”.

Stages of a star collapsing into a black hole.
Stages of a star collapsing into a black hole.


Black holes neither emit light nor let light out once it passes the “event horizon”. The “event horizon” is a critical boundary that once passed, is a point of no return. The swirling material from the exploded stars gets drawn into the gravitational pull of this funnel as it crosses the “event horizon”. Radiation from in falling gas that doesn’t quite make it past the event horizon can be observed. Scientists can measure the heat and speed of the circulating matter, using this radiation and thus infer the presence of a black hole. The hot swirling disk-like matter near the “event horizon” is known as the accretion disk.

There is a theory that the “gravity well” of a black hole is so great that a gravitational time dilation occurs causing time to stop. At the center of a black hole, under the pull of infinite gravity, space-time has infinite curvature and matter is crushed to infinite density. It is at a “singularity” that time and space cease to exist as we know it. At the center of a black hole, under the pull of infinite gravity, space-time has infinite curvature and matter is crushed to infinite density. It is at a “singularity” that time and space cease to exist as we know it.

The size and mass of a black hole are directly related. The larger the black hole, the more space it takes up. It is thought that a black hole can eventually evaporate as it gets smaller, proportionate to the amount of radiation it admits. When astronomers observe areas where stars appear to circle space, the conclusion is that this is a black hole. There is a massive black hole at the center of the Milky Way galaxy 30,000 light years away.

Evidence


German astronomer, Karl Schwarzschild (1873-1916) was studying Einstein’s revelations on gravity when he discovered an incredible implication of general relativity. He showed that if the mass of a star is concentrated into a miniscule spherical area, when it’s mass divided by it’s radius is greater than a particular value, the result is a space-time warp with such gravitational pull that anything that enters this area, including light, cannot escape. These compressed stars were originally know as dark or frozen stars but American physicist, John Wheeler, re-named them black holes, black because they cannot emit light and holes because once entered, nothing can get out. As black holes cannot be seen with telescopes, astronomers look for clues from light-emitting stars that may be positioned around a black hole “event horizon”. As dust and gas from other stars fall towards the event horizon of a black hole they accelerate to nearly the speed of light. At such speeds friction generates a lot of heat and the dust and gas mix glows from visible light and X-rays. This is visible by telescope. There is evidence now that there is a black hole two and a half the size of our sun in the middle of the Milky Way galaxy.

Steven Hawking, cosmologist and theoretical mathematician and Professor at Cambridge University, UK, has shown that black holes can radiate energy, based on quantum mechanics and the uncertainty principle. Matter or energy can be created from empty space for brief moments, as truly empty space does not exist. Hawking realized that if a particle/anti-particle pair came close to the event horizon, one could fall into the hole before the other annihilates it which, if it escapes the gravitational pull of the black hole, appears as radiation as it escapes. This is known as Hawking Radiation. Hawking said, “Black holes are not really black after all, they glow like a hot body and the smaller they are, the more they glow”. Hawking’s theory is that the temperature of a black hole varies inversely to its mass. Steven Hawking proposed that as radiation is emitted from a black hole, the black hole loses mass. The more radiation the black hole emits, the smaller it gets.

A question often asked is if light has no mass, how can it be trapped by the gravitational pull of a black hole? Newton’s theory of gravity suggests that only objects with mass could produce a gravitational force on each other so as light has no mass, the force of gravity couldn’t affect it. Einstein then discovered that gravity is produced by a curved space-time and he theorized that the mass and radius of an object (it’s compactness) actually curves space-time. The stronger the gravitational field of an object, the more the space around the object is curved which means straight lines are no longer straight if exposed to a strong gravitational field, they are curved. As light normally travels in a straight line, once it passes through a strong gravitational field it follows a curved path and gets trapped in the black hole. Sir Arthur Eddington, in the 1920’s proved Einstein’s theory when he observed starlight curve when it traveled close to the Sun. This was the first successful prediction of Einstein’s General Theory of Relativity.

In Feb. 1997, the Hubble Space Telescope had a new instrument installed call the Space Telescope Imaging Spectrograph (STIS). It is the main black hole seeker
Hubble Space Telescope
Hubble Space Telescope
on the Hubble. The spectrograph splits any incoming light into prisms and diffraction gratings into a rainbow. The STIS measures ultraviolet, visible and near-infrared wavelengths, which allows it to capture a wide area at once. The placement and intensity of the spectrum can be analyzed to calculate the speed at which stars and gas swirl at a particular location and consequently calculate the mass of the object the stars are orbiting. If the stars are swirling quickly, a massive central object is found – a black hole. The observations made with the Hubble Space Telescope have provided the best evidence to date that black holes exist. The Advanced Camera for Surveys was installed on the Hubble in March 2002 representing the third generation of science instruments aboard the Hubble Space Telescope. This new camera has a wider field of view, sharper image quality and enhanced sensitivity, doubling the Hubble’s field of view and expanding its capabilities considerably. This upgrade makes the telescope ten times more effective and prolongs its useful life.

Conclusion


There are many theoretical concepts about black holes like “wormholes” which are “tunnels” (where two black holes connect) which would allow rapid travel through space and time to far places within the universe. This captures the public’s imagination and that of the movie industry. Einstein’s equations suggest that black holes should, in theory, lead to parallel universes. There is also the possibility of time travel into the past and the future. A New York University physicist,
Artistic impression of black holes with connecting wormhole.
Artistic impression of black holes with connecting wormhole.

Jia Liu, has suggested using a dark matter “jet engine to power spacecraft. Mathematicians at Kansas State University, Louis Crane and ShawnWestmoreland,claim that using an artificial black hole as a power source is possible. This would involve building a spaceship with a large parabolic reflector behind it and putting a small black hole in its focus. The “Hawking Radiation” given off by the black hole would push the spacecraft to near-light-speed within a few decades. At this near-light-speed velocity, time would slow down. This may make it possible to each the Andromeda galaxy 2.5 million light years away within a human lifetime. A mathematician from Oxford University, Roger Penrose has calculated that a technologically advanced civilization could be powered by extracting enough energy from a black hole, yet another illustration of the possibilities that lie ahead as science progresses. Discovery of black holes could lead us to many new dimensions.

References


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Brenneman, Laura. “Spinning Hearts of Darkness.” Sky & Telescope May 2011: 20-25. Print.
Greene, Brian. The Elegant Universe. London: Vintage, 1999. Print.
Hawking, Stephen. “Stephen Hawking’s Universe: Strange Stuff Explained.” PBS. N.p., n.d. Web. 2 May 2011. <http://www.pbs.org/‌wnet/‌hawking/‌strange/‌html/‌blackh.html>.
Steinberg, Daniel. “No Escape: The Truth about Black Holes.” Amazing Space. N.p., n.d. Web. 2 May 2011. <http://amazing-space.stsci.edu/‌resources/‌explorations/‌blackholes/‌teacher/‌sciencebackground.html>.