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Black Hole Survival Kit

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

A black hole is an object whose gravitational pull is so strong that nothing can escape it. Black holes are in the news a lot these days. And they are definitely at the forefront of science research (see near the bottom of this page for a calendar of recent discoveries).


In this post I have tried to assemble a survival kit that summarizes how black holes were thought of, and then eventually found; where you might encounter them; and how to avoid getting sucked into one if you come near it.

Who thought of them?


People like John Mitchell and Pierre-Simon Laplace speculated about them before Einstein put things on a quantitative basis with his equations of the General Theory of Relativity in 1915. Though not immediately evident, over time, it was found that Einstein's equations predicted black holes. In his theory (combined with the laws of electromagnetism) black holes were nominally simple objects, since they displayed only three physical characteristics: mass, electric charge and angular momentum. Physicists often paraphrase this statement by saying that 'a black hole has no hair' - meaning it has no other attributes than the three mentioned above. There is, in fact, a 'no-hair' theorem.


Karl Schwarzchild found the black hole solution with only mass (no charge or angular momentum) about a year after Einstein's equations came out. It took 50 more years before Roy Kerr found the rotating solution with no electrical charge. Then, in 1965, Ezra Neumann found the solution that was rotating as well as charged.


How are they formed?


Black holes are formed when stars collapse on themselves, drawn inwards by the gravitational pull of their own constituents. As for our sun, in some stars, the generation of heat due to nuclear fusion balances gravity. Once the nuclear fuel runs out, Fermi pressure due to electron degeneracy (the fact that the Pauli exclusion principle does not allow electrons to pile up on top of each other) can balance gravity for small stars (this will happen to the sun in about 6 billion years). This results in a white dwarf. Fermi pressure due to neutrons can also do the trick, as in a neutron star.


But Chandrasekhar showed that a white dwarf with more than about one and a half solar mass would eventually collapse into a black hole. And Tolman, Oppenheimer and Volkoff showed the corresponding limit for neutron stars is about two solar masses. Conclusion: based on their known masses, one out of every thousand stars in the universe will become a black hole.


When were they first observed?


Cygnus X-1, about seven thousand light years away from us, but still in our galaxy the Milky Way, was the first one to be observed in 1971. Its identity as a black hole was in doubt for some time, leading to a friendly bet between Stephen Hawking and Kip Thorne. Hawking, who had wagered that it was not actually a black hole, eventually lost.


Where else can you expect to run into them?


There's one at the center of our galaxy! It was recently in the news for being 'photographed'. Millions more - varying in size from small to supermassive - are suspected to exist in our galaxy alone. A list of about a hundred black holes identified thus far is available and should definitely be consulted before you start hitchhiking in the universe.


How do you survive a black hole encounter?


The best way to survive a black hole is simply to avoid it. If this is not possible, a good way is to orbit it - that will keep you from being sucked in and dismembered by the tidal forces of gravity. But you need to keep the radius of your orbit large so the radiation emitted by other material being eaten up by the black hole (its accretion disk) does not kill you. If you are interested in this option, further details are provided here.


References


Popular - Black Hole Blues by J. Levin.

Textbook - Gravity by J. B. Hartle.

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