Author: dkagan

RIP Stephen Hawking 1942 – 2018

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I found out just before going to bed in the early morning today that Stephen Hawking, one of the great theoretical physicists of the 20th century, passed away today at the age of 76. Hawking had been diagnosed with ALS (amyotrophic lateral sclerosis, also called Lou Gehrig’s disease) when he was 21 and was told that he would probably have only two more years to live. During the 55 years between his date of diagnosis and his actual death today, he (together with others) proved the inevitability of singularities in classical general relativity, united quantum theory and GR to predict the existence of radiation from black holes, helping to establish the theory of black hole thermodynamics. He developed path integral methods for dealing with quantum gravity, proposed fascinating and fruitful ideas in the area of cosmology. In short, his was a remarkably productive life, purely from the perspective of theoretical physics.

Of course, Hawking also became the most iconic physicist of the late 20th century. Bound to a wheelchair, and forced to speak first through an interpreter, then through a synthesizer, he became a pop-cultural phenomenon. His book (and Errol Morris’ documentary of the same name), A Brief History of Time was an inspiration to me when I read it at age 13, and the subsequent eight re-readings over the next several years. I was able to measure my own growth as a physicist through the additional fractions of his book that I was finally able to understand.

Hawking’s writings and life story sparked my dream of becoming a theoretical physicist, and my goal of studying at Cambridge. Amazingly, both of these things somehow happened. While there, I had the great good fortune of meeting Hawking one-on-one for an hour. We exchanged a precious few ideas, and I gained a memory I will treasure for a lifetime.

Stephen Hawking never directly mentored me, but he nevertheless was as significant a role model and inspiration as anybody in my life. Thank you Stephen. I will miss you.

Roger Penrose, one of Hawking’s close collaborators and intellectual sparring partners has written an obituary here.

John Preskill, a theoretical physicist at Caltech, and winner of bets against Stephen Hawking has written a lovely poem to honor him.

John Baez, a mathematical physicist, has some reflections on Hawking here.

Nathan Myhrvold, a former postdoc of Hawking’s (he left physics to pursue other interests), describes working with him here.

 

Reading Assignment 1: (Ch. 0)

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The assignment was to read chapter 0 of our textbook (QFT for the Gifted Amateur) and then to hand in a note with the top two topics that you would like to review from that chapter.

The winning topics were Lorentz/Poincaré symmetry (lumping together related things like 4-vectors, tensors, etc…) and Fourier transformations. These are great choices!

There were also some questions inspired by the subsection “What is a field?” I think it’d be great to talk about these questions, but I’m going to save that for another time, or perhaps for a write-up on the course blog.

In our review class today (1/26/18), I’ll do some Fourier transform review, with an eye toward applying it to QFT in particular. I’ll be fairly light on the underlying intuition, but you can find a wonderful discussion that should really help here:

https://betterexplained.com/articles/an-interactive-guide-to-the-fourier-transform/

By the way, I can’t recommend betterexplained.com highly enough! The people putting that site together have done a marvelous job.

I will do some review of special relativity, probably during next week’s review class (Fri, Feb 2). That said, one question/topic request had to do with choices of metric signature. The two competing choices are the East Coast convention \((-,+,+,+)\) and the West Coast convention \((+,-,-,-)\). People get pretty passionate about which one is the “correct” choice. My sympathies lie with the East Coast convention (I’m a New York City boy, after all!), mainly because I think it’s simpler to deal with one minus sign than three (it also extends more easily to higher spatial dimensions since no new signs are introduced).

Despite my personal preferences, its common for particle physics and QFT books to use the West Coast convention. I think that if there is a reason other than just blind tradition, then it is likely due to the fact that the 4-momentum squared is proportional to rest-mass squared in this convention rather than negative rest-mass squared. Folks probably find that negative sign annoying, and if you’re doing lots of calculations involving four-momenta, it may be more practical to go this route.

For an amusingly high-minded argument for the superiority of the East Coast convention, take a look at:

http://www.math.columbia.edu/~woit/wordpress/?p=7773

Most folks agree that these things are called conventions since there isn’t a truly correct choice between them. But Woit makes a good case for going with the East Coast convention that goes beyond simple convenience.