Podcaster: Richard Drumm
Organization: Astrosphere new Media
Link : http://astrosphere.org
Description: What is a black hole and will one eat the Earth?
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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Welcome to the Q&A series of the 365 Days of Astronomy. If you have a question which you’d like us to answer, visit our G+ or facebook pages and fire away!
Today’s question is “What is a black hole and will one eat the Earth?”
I’ll answer this in reverse order. Will one eat the Earth? No. There, that was easy. The likelihood of a stellar mass black hole wandering randomly past Earth and drawing it in is effectively zero. The possibility of the CERN Super Collider in Switzerland making a micro black hole is conceivable, but highly speculative, so it’s not likely, and even if it did, the black hole that it created would evaporate after 10^-25 second, so it’s not going to eat Earth or even so much as an electron. So no. A black hole is not going to eat Earth.
But back to the first question, what -IS- a black hole? Time for a bit of a history lesson. The first person to have the idea for a black hole (so far as we know) was the English scientist (or natural philosopher as scientists were called back then) John Mitchell in 1783. He wrote this letter to Henry Cavendish. Both were members of the Royal Society:
If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its … inertia, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
So even though Mitchell didn’t know what the speed of light was, and though he didn’t know that it’s not possible to go faster than light, he does see light being bent back down into the fierce gravity of the hypothetical super dense sphere.
13 years later Pierre-Simon Laplace independently came up with the idea of a “Dark star” which he put in his book “Exposition du système du Monde” for its first 2 printings. It was not included in later editions because the wave nature of light seemed to preclude gravity from having any effect on it.
It took Albert Einstein’s theory of general relativity in 1915 to establish that light can, after all, be bent by gravity. A little after Einstein published his 1915 paper describing all this, Karl Schwarzschild published a solution to Einstein’s field equations which pointed to the existence of what is now called by scientists the “Schwarzschild radius” or popularly called the “event horizon” which is the point of no return where light, radio waves & even X-rays can’t escape.
Robert Oppenheimer (of atom bomb fame) came to see the event horizon as the edge of a bubble where time has stopped. As something falls into the event horizon, we see it slowing down and (by the doppler effect) turning red till time stops and the infalling matter completely fades from view, becoming invisible even in radio waves. He called them “frozen stars” because from the outside things look frozen in time, at the instant of time where matter enters the Schwarzschild radius.
You see, they weren’t called “black holes” yet, but “dark stars” or “frozen stars”. In 1958 American physicist David Finkelstein called the Schwarzschild surface an “event horizon” and called it “a perfect unidirectional membrane: causal influences can cross it in only one direction”. It took a journalist, Ann Ewing to coin the term, writing an article titled “Black Holes in Space” in January 1964.
The super dense thing in the middle of all this is called a singularity because of the mathematical term “singular”, which means that some of the math terms in Einstein’s equations have become infinite and therefore not definable. Take the calculator in your smart phone and divide any number by zero and you’ll see what I mean. I get the word “Error” on the screen when I do it.
So, you might ask, where do black holes come from?
They come originally from type 2 supernovae, where the core of a large star (20 times or more the mass or our Sun) collapses under its gravity when it can no longer can generate energy to hold itself up. Our Sun is nowhere near large enough to do this, so we’re safe.
The black hole in the center of our galaxy (called Sagittarius A* after the constellation it’s seen in) is called a supermassive black hole (SMBH for short) as it has 4.1 million times the mass of our Sun. It’s like the glue that holds our galaxy together.
So how do astronomers arrive at this 4.1 million solar mass figure?
They’ve detected stars (in infrared light, which penetrates the galaxy’s dust very well) stars in the constellation Sagittarius that are being violently flung around by something that is that massive. Using Isaac Newton’s law of universal gravitation and knowing the mass of the nearby star called S2 through examination of the spectrum of light coming from it, the mass of the invisible black hole has been calculated.
I’ve myself have heard people ask “Won’t the black hole just suck the whole galaxy down?” Once again the answer is no. Just as our Earth safely orbits the Sun and isn’t going to fall straight into it, our Sun is safely orbiting the galaxy’s black hole and isn’t going to turn from its path and head straight to it. It just isn’t going to happen.
So we’re safe. There is, though, a gas cloud with 3 times the mass of Earth that is being disrupted by it’s 2013 close pass to the SMBH. Over the next few years a significant portion of this cloud will form an accretion disk around the SMBH and it’ll be visible in X-rays. This will have no effect on earth other than the flurry in publishing of scientific papers.
Thank you for listening to this episode of 365 Days of Astronomy. You can send me your questions via twitter by tweeting @365DaysOfAstro. Thank you!
End of podcast:
365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Astrosphere New Media. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. In the new year the 365 Days of Astronomy project will be something different than before….Until then…goodbye