Black holes are 230 years old

On this day in history, 230 years ago, the world learned of the idea of a black hole.
The first known place in which a black hole is described is in a letter written by John Mitchell which was sent to Henry Canvendish in 1783. In the letter Mitchell writes:

If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity. -Mitchell

The letter was written on the 26th of May 1783. It was then read at a meeting of the Royal Society in London on the 27th of November of the same year. Finally it was published in the society’s journal, Philosophical Transactions, Royal Society, London, on the 1st of January 1784 [aside: The Royal Society, a group of natural philosophers and scientists, formed on the 28th of November 1660, making it over 350 years old!].
Mitchell reached his conclusion by using Newton’s Laws. Sir Isaac Newton had published his laws of motion and gravitation via Principia Mathematica in the year 1687, about a century before Mitchell’s letter to Cavendish. You can use Newton’s laws to ask the question: how fast do I have to be moving in order to escape the gravitational pull of Earth? The answer is found by equating your Kinetic Energy (Ek, the energy associated with your moving away from the planet) and your gravitational potential energy (Eg, the energy associated with Earth pulling on you):

(1)   \begin{equation*} Ek=Eg \end{equation*}

(2)   \begin{equation*} {1 \over 2} mv^2 = {GMm \over r} \end{equation*}

where M is the mass of the Earth, m is your mass, v is your speed, G is the universal gravitational constant, and r is the radius of the Earth. Playing with the equation a bit (and entering in the M and r of Earth), you get:

(3)   \begin{equation*} v=\sqrt{{2GM \over r}}=11.2 km/s \end{equation*}

Therefore, in order to escape Earth’s gravity, you need to have a starting speed of 11.2 km/s. This is called, happily, ‘escape velocity.’ You can use this equation on any object whose mass and radius you know (escape velocity for: the Moon=2.4 km/s, Mars=5.0 km/s, and so on).
Mitchell’s mental leap was to ask the equation: ‘what if an object’s escape velocity was the speed of light?’ What would the ratio of mass to radius of that object be? The speed of light was measured at the time to be 295 000 km/s (Bradley, 1728). So, crunching the numbers, Mitchell found that an object with the same density of the Sun, but with a radius 500x bigger, would have a surface escape velocity equal to the speed of light. Therefore, ‘all light emitted from such a body would be made to return towards it, by its own proper gravity.’
Astounding! This was the first time anyone had ever supposed there to be an object that would have a gravitational potential well deep enough to capture light. Unfortunately, Mitchell’s work was not uncovered until the 1970s. Until that time, Pierre Laplace was considered the first to propose the idea.

A slight addendum to the story: Mitchell based his proposal on the idea that light would be attracted to matter via Newton’s laws. This was accepted at the time but research into particle vs. wave theory of light in the 1800s showed that light could not act in such a way. This squashed the idea of black holes (or ‘dark stars’ as they were referred) until a gentlemen by the name of Albert Einstein developed General Relativity and showed that light follows the curved space created by a massive object. This reinstated a scientific basis for the idea of a black hole.
John Mitchell was a polymath: geologist, mathematician, physicist, astronomer, and more. He contributed deeply to many fields (including plate tectonic theory, and magnetism).

Suggested Reading:

Philosophical Transations – The original letter written to Cavendish
American Museum of Natural History – John Mitchell and Black Holes
Astronomy Society of Edinburgh – Black Holes History
The American Physical Society – John Mitchell Anticipates Black Holes

Book Review: Maus II -..and Here My Troubles Began

This is the second book by Art Spiegelman, telling the story of his father’s journey as a jew trough the second world war. I wrote about the first Maus here.

So in this one, Vladek (Art’s father), has just made it to Auschwitz. This is arguably the most well known prison camp from the holocaust. Vladek goes on to describe what it was like living here. He talks of how they treated them, how they didn’t have clothes that fit. You would have one size 13 shoe and one size 9. one spoon for food, if you lost it…tough luck.
The constant fear of being sent to the chimneys was always there. Always hiding, always in fear. There was death all around. These two books are very powerful. It’s a graphic novel, so you have pictures to go along with it.
What I found to be really well done in these books is that the story is told in pieces, and you see Art talking with his father in the mid 80s and then you’re transported to the time in Auschwitz as Vladek narrates. You constantly see the man he was, and what the experience did to him in his later life; how he dealt with it, how he’s changed.
With all the horror I read about in this book, it gave me one big smile at the end. I suggest you go looking for it!
happy reading!

Book Review: Maus

This book was both recommended and provided by my friend Vicky. I was over at her place because a mutual friend was in town, and I was staring at her case full of books. Vicky pulled Maus off the shelf and said, ‘you should read this’ in a very convincing manner. I’m not one to turn down a recommendation like this so I borrowed it and I couldn’t put it down.

The book is actually a graphic novel written and drawn by Art Spiegelman. The novel actually has a subtitle Maus: A Survivor’s Tale, I: My Father Bleeds History. The story is told through the conversations between a young man and his aging father, that are real events that happened to the author’s father.
The young man, Artie Spiegelman, was born in America, but his father, Vladek, was born and lived in Poland during the holocaust. As a jewish man, Vladek went through some of the worst the holocaust had to offer, and Artie wants him to retell it so that he can write a novel and publish it. (it looks like it worked).
I should mention that the main characters I just described are actually mice. The entire story is drawn such that all the jewish people are mice, all the Nazis are drawn as cats, and the polish people who are not jewish are pigs. This added dimension reinforces the power and control the nazis had over the jewish, and how helpless the jews were.
This installment starts in the mid 1930s when the jewish discrimination starting ramping up, and follows Vladek’s plight straight through to 1944 when he was sent to Auschwitz. This was one of the most well-known camps during the holocaust, though Vladek does not arrive here until the end.
The story really makes the holocaust real. Each story the father tells has you hanging on every word. He tells of how he was sent to work camps, and then had to pay off nazis to escape. He lived for years in bunkers built into the attics of his friends house, or into the ground under his home. He went years barely being able to find enough food for him and his wife. He tells stories of how his friends and family were all taken away by raids and sent to camps where everyone was gassed, or shot.
They’re horrific stories, but not graphic. No one in my family was in Germany or Poland during the holocaust so I have no personal viewpoint on this topic, but this novel REALLY brought the holocaust home for me. For us, the things that happened during those times to the jewish people was insane, and it’s hard to understand what it was like. But the father in this story knew nothing else, he lived every day in fear of being taken away, and it comes across in the telling. I felt connected the story in a much more personal way than when I learned about the holocaust in textbooks.
Perhaps the reason it was so impactful was that I knew it was based on real father/son conversations.
I really enjoyed it because it was a simple straightforward story of the holocaust. My friend Vicky said its great for teaching junior high/high school students with because its able to teach the facts behind the very emotional story. I would agree!
I highly recommend reading this one, there are more in the series as well. I think I might go after that one too.
happy reading!