## Saturday, April 12, 2014

### Black Holes and Gravitational Waves (chaps 7-10)

The next installment of The Perfect Theory: A Century of Geniuses and the Battle over General Relativity. The book just came out this year but, amazingly, chapter 10 is already outdated!

Here are my earlier posts:
Chapter 1: Einstein in 1907
Chapter 2: The General Theory of Relativity Born
Chapters 3-6 Expanding Universes, Collapsing Stars, Cuckoo Einstein, and Steady States

Chapter 7: Wheelerisms
The namesake of this chapter is John Wheeler, who was known for turns of phrase like, "mass without mass" and "charge without charge." He's the one who popularized the term "black hole." He came up with the notion of a "wormhole" that bypasses space and time.

One of his main contributions to relativity was the way in which he helped rejuvenated interest in it. In the 1950s, physics was far more interested in quantum matters than general relativity. You could experiment with the quantum. General relativity was more a matter of distant space. Wheeler's support helped get some conferences on relativity going.

So there was the Institute of Field Physics, funded by a couple rich guys who were interested in gravity. Wheeler supported them and their appointment of Bryce DeWitt and his wife as the first employees. They set up meetings on gravitation in Chapel Hill, North Carolina, in the late 50s.

When Richard Feynman (quantum man extraordinaire and former student of Wheeler) arrived for the first conference in Chapel Hill not knowing directions, he helped the taxi driver figure out where it was by suggesting there would have been other attendees in the back of the taxi saying "gee mu nu, gee mu nu" ($G_{\mu\nu}$). In the words of Ferreira (author of the book), "The driver knew where to go" (109).

Another meeting of this sort came out of oil money and the University of Texas at Austin. The result was the Texas Symposium on Relativistic Astrophysics, first held in 1963 in Dallas, just after Kennedy was shot. One of the things discussed at this symposium were "quasi-stellar radio sources" that were being detected by people like Maarten Schmidt. After the conference, they would be called quasars. They were super-massive objects that emitted lots of energy.

Chapter 8: Singularities
The 60s were the "Golden Age of General Relativity," according to Kip Thorne, one of Wheeler's students. Roger Penrose was a player in the decade. He showed that the collapse of stars after they burned out always ended in singularities or black holes, as they would come to be called.

This was also the decade where the background radiation of the universe was discovered. It showed that the steady state theory was false. The universe had a beginning. Stephen Hawking emerged at this time, showing that the universe would not only end with singularities but had also begun with one.

This was also the decade where pulsars were discovered, "pulsating radio stars." These are neutron stars, stars made up almost completely of neutrons. I'm getting a better picture now of what some of the earlier chapters were talking about. There are white dwarfs that Eddington knew of. These are smaller suns that burn out but they are not massive enough to become singularities from which light cannot escape.

There are black holes. These are the super-massive stars that, when they burn out, collapse into a relativistic nightmare from which nothing can ever escape. From our perspective, they become frozen in time. Neutron stars, of which pulsars are an example, are somewhere in between in mass, more massive than white dwarfs but not so massive as a black hole.

Chapter 9: Unification Woes
Relativity and quantum mechanics have always been difficult to fit together. Einstein couldn't do it. Paul Dirac couldn't do it, although he did it with the electron. The Dirac equation had been a landmark in the history of physics. It had predicted the existence of antiparticles, for example.

This chapter takes a bit of a detour into quantum physics, since it is on the continued attempts to fit quantum physics with relativity and gravitation in particular. The 50s and 60s saw the putting together of quantum electrodynamics (QED) and what is now called the "standard model" of physics.

Dirac, like Einstein, never accepted some aspects of quantum physics. He accepted more than Einstein. For example, he showed that the approaches of Heisenberg and Schroedinger really said the same thing in two different ways. In his later career, like Einstein, Dirac became somewhat of a recluse, a celebrated landmark who refused to stay with where the program had gone. In the summer of 1983, while at Boy's State, I touched his office door at Florida State, the year before he died. He had withdrawn from the mainstream and had become a shadowy figure from the past.

In particular, QED leads to a lot of infinities that are ignored. So even though the equations point to an infinite mass for an electron, QED "normalizes" the mass of an electron by substituting the actual measured value for the infinity. Frankly, this bothers me too and surely speaks to some inadequacy in the theory, says the guy who doesn't have a degree in physics.

Hawking did some work to show some additional places where quantum physics and general relativity might come together. Hawking showed that black holes slowly evaporate, "black hole radiation."

Chapter 10: Seeing Gravity
It is amazing to me that this book is already needing to be updated. I bought this book on March 1, 2014 at the IU Memorial Union Bookstore in Bloomington. It just came out in early February of this year. By March 17, chapter 10 was out of date. That was the date that it was announced that gravitational waves had been observed.

Einstein predicted the existence of gravitational waves, ripples in spacetime, as early as 1916. Eddington rejected the idea, and Einstein himself backed off on the idea in 1936. But Hermann Bondi made a compelling case for them at the watershed 1957 meeting at Chapel Hill. Feynman agreed.

A guy named Weber was also there and would spend the rest of his life trying to prove it experimentally. Unfortunately, he saw them everywhere. Eventually he was marginalized by the scientific community and died a bitter man in 2000. He used very imprecise measuring tools, compared with the laser interferometry that is currently used.

The idea that large objects might give off "gravitational radiation," however, was supported indirectly in 1978. Taylor and Hulse used the very equations Einstein created and whose results he then later rejected to examine two neutron stars orbiting each other. The chapter ends with LIGO in North America trying to find gravitational waves using laser interferometry. Unfortunately for them, they do not seem to be the ones that discovered them.

A final feature of interest in this chapter is the rise of "numerical relativity." For decades, attempts to solve Einstein's field equations in relation to colliding black holes, using computers, would break down the computers. The computer power just wasn't powerful enough yet. Frans Pretorius cracked that one in 2005. He solved Einstein's equations for two colliding black holes on a computer without the process shutting down--90 years after Einstein set them out.

As a side note, the drive to do numerical relativity and the need for more computing power apparently played a role in the implementation of the internet, so that multiple computers across distances could collaborate together. That was in the mid-80s when Larry Smarr was convincing the US government to fund a network of supercomputing centers.

It's quite clear that some of these discoveries would not have happened without a willingness on the part of the government to fund scientific research without immediate results. Such funding seems essential to the long term ability of the US to stay ahead of the curve. The problem is that it will not always pay off and, even when it does pay off, it can be a long time later. We just have to have the foresight to commit to scientific research without immediate results.

#### 1 comment:

Martin LaBar said...

Yes, we should fund promising basic research. That doesn't mean that we shouldn't be careful in how the money is used, but we shouldn't expect some research to be immediately of use, or, perhaps, ever of use. (Whatever "use" means.)