April 27, 2007

Should be a law

Subramanyam Chandrashekhar was startled one January day in 1935. The quiet 24-year old from Madras was to speak the next day at the meeting of the Royal Astronomical Society in London. He was going to announce at the meeting a major new discovery in a field he had been studying for some years.

But on this day Chandrashekhar was astonished to find in the programme for the RAS meeting that Sir Arthur Eddington would also be speaking there, and on the same subject. Perhaps the world's premier astronomer then, Eddington was something of a mentor to Chandrashekhar at Cambridge University: the two had been meeting for months to discuss Chandrashekhar's work on stars. In fact, it was an association that might have been, well, in the stars all along: Chandrashekhar won a prize in a physics contest at Madras University that first got him interested in the subject, and that prize was Eddington's book on stellar structure.

And now Sir Eddington was going to speak too. Chandrashekhar did not know what to think. Could it be that the great astronomer was planning to upstage the young man?

Chandrashekhar's work addressed an old question: what happens to when a star has used up all its fuel and starts to die? The accepted theory said that small stars would shrink into dense little globes called white dwarfs. Larger ones? Chandrashekhar had studied their collapse and reached a profound conclusion. A star larger than about 1.4 times the Sun's size, he said, would keep shrinking beyond the white dwarf stage. It would get smaller and smaller, denser and denser, in a state he called relativistic degeneracy. Where this would end was a question Chandrashekhar left dangling tantalizingly: "A star of large mass cannot pass into the white dwarf stage, and one is left speculating on other possibilities."

Arthur Eddington now took the stage. A distraught Chandrashekhar heard him say: "...[T]he point of my paper is that there is no such thing as relativistic degeneracy." He went on to destroy Chandrashekhar's thesis, but not by arguing with his calculations or the logic. He simply could not accept what he thought was the absurd conclusion it led to: that the star shrinks till it is a only few kilometres across. Such an object could not possibly exist. Eddington finished by saying: "I think there should be a law of nature to prevent a star from behaving in this absurd way."

Returning to Cambridge that night, Chandrashekhar was afraid his career was in pieces. Years later, he remembered thinking to himself: "This is how the world ends, not with a bang but with a whimper."

Of course, his world did not end. Today we have given a name to the object Chandrashekhar's theory indicated: a black hole. That 1.4-times-solar-size figure is in every basic astronomy textbook as the Chandrashekhar Limit. He was one of the 20th Century's most celebrated astronomers; a Nobel Prize winner at the age of 71; a man of whom his close friend Martin Schwarzschild -- himself a renowned astrophysicist -- said: "There is not one field that he's worked in where we are not now daily using some of his results."

S Chandrashekhar reached great heights as a scientist. The odd thing is that he did so at least partly due to that experience in London in January 1935.

But first a little about his work on the death of stars. Throughout their lives, stars undergo a series of nuclear reactions called fusion. Fusion burns the material that makes up stars and gives off a great deal of heat. This creates a pressure that balances the natural attraction of gravity in a star. Because there is this balance, it does not collapse on itself.

Eventually, the star uses up all its fuel. There is no pressure to resist gravity, and it begins shrinking and cooling to the white dwarf stage. As it shrinks, it compresses its remaining matter. Now when any kind of matter is compressed, the empty space between its atoms is squeezed out and they are pushed closer together. In a white dwarf, the limit of this process is reached. The electrons in the atoms are so close together that, as they mutually repel each other, further compression is impossible.

However, Chandrashekhar showed there is another limit: on how much compression electron repulsion can resist. Since compression depends on the mass of the star, stars greater than a certain mass -- the Chandrashekhar limit, 1.4 times the mass of the sun -- will generate compression pressures that overcome electron repulsion. These stars will shrink to the white dwarf stage and go right on shrinking. The theory predicts that some stars will shrink so much that they form small regions of space from which nothing can escape. The gravitational pull of these bizarre objects is so high that even light is sucked in. You know the name: black holes.

Today, astronomers know of the existence of black holes. But that day in London, Sir Eddington found the very idea of them absurd.

Chandrashekhar and Eddington argued over the theory for years, though they remained good friends all along. Eventually Chandrashekhar left England for the University of Chicago and gave up the subject entirely. He turned his mind to numerous other fields: the stability of a particular kind of rotating object, the theory of relativity, why the sky is blue, and more. He did return to black holes from a new angle, and that brought him his 1983 Nobel Prize.

He always believed that it was because he was driven from his first specialty by that argument with Eddington, because he took up and enjoyed other fields of research, that he remained productive well into his 70s. Most scientists do their best and most innovative work in their 20s and 30s. After that, they rarely add significantly to science; rarely, as Chandrashekhar argued, refine their own minds.


Chandrashekhar thought it was because scientists develop an arrogance towards nature. These are often pioneers in their fields, they make far-reaching contributions. Their success makes them believe that they have a special view of science, a somehow right view of science. For example, why should Eddington have pronounced that there must be a law of nature to prevent stars from turning into black holes? How could he possibly know? How could he assume, regardless of his great work in astronomy, that he knew exactly what the laws of nature should or should not allow?

Nobody can have it all right. It's a lesson that might serve us all well. Non-scientists included.

S Chandrashekhar learned that lesson and let it fuel his own varied contributions to science. And he once wrote: "Nature has shown over and over again that the kinds of truth which underlie nature transcend the most powerful minds."

For me, that understanding is the truest measure of his greatness.


Anonymous said...

Interesting reading. Can you please supply sources for these anecdotes?

Anil P said...

That's precisely the point. Eddington applied an Astronomer's point of view to Astrophysics.

Abi said...

Chandrasekhar is known to have changed his research field every 7 or 8 years (typically, after writing a definitive account of that field). In Kameshwar Wali's book "Chandra", the Nobel laureate talks about this 'defining moment' (Eddington's hostile outburst) which forced the first change-of-fields on him. He probably saw the importance of this strategy to ensure that he never got 'arrogant with nature' in any of those fields that he worked in.

Thanks for this post, Dilip.

Just to pick nits, isn't his name spelled "Chandrasekhar"?

m. said...

point nicely made... very nice post :)

Anonymous said...

Thats an interesting anecdote.
NASA named the their XRay observatory after him too.


Saurabh said...

I had just read about Chandrasekhar's limit in school, and pretty much knew nothing else about the man's personal greatness. Thanks for sharing this.

Wavefunction said...

Nice post, but just one comment: while Chandrasekhar certainly anticipated black holes, a star with a mass slightly greater than that postulated by the Chandrasekhar limit will become a neutron star. The real limit necessary for a star to form a black hole was worked out by Robert Oppenheimer and his student Hartland Snyder in 1940.

Govar said...

Great read!

Anonymous said...

Thanks for sharing this Dilip.

Dilip D'Souza said...

Thanks for the comments, all.

browntobacco: thanks for the reminder! Abi indicates one source for some of this; I've read of the Eddington/Chandrasekhar disagreement (and how it moulded Chandrasekhar's thinking) in other places, including "The World Treasury of Physics, Astronomy and Mathematics" (Clifton Fadiman, ed).

Abi: a correct nit picked there indeed. (I must have been subconsciously thinking of one of my childhood heroes, BS Chandrashekhar). In fact, I got his first name wrong too, it's Subrahmanyam.

Ashutosh: thanks for the clarification. Care to expound on that some more? I'd love to hear it.

Sunil said...

a lot of stuff on the Chandrasekhar limit and the formation of neutron stars is actually there in Hawkings "A brief history of time", that's written in a relatively easily understandable way.

You might enjoy reading this article by CV Vishveshvara called "S. Chandrasekhar: Reminiscences and reflections". I thought it was a terrific read, and brought out a lot about Chandrasekhar.

Wavefunction said...

Hi...sorry for the late response. In the 1920s and 30s, Chandrasekhar did think about what happens to white dwarfs if they are beyond 1.4 solar masses. However, as you nicely summarised, his spat with Eddington drove him away from that kind of research. However, Fritz Zwicky in the US and Lev Landau in Russia took up the matter and predicted the existence of neutron stars. It was Oppenheimer who truly considered what would happen at a singularity, and how events would look there for an observer.
I can suggest a very good book that expounds on these matters: "Black holes and time warps: Einstein's outrageous legacy" by the famous astrophysicist Kip Thorne.