Thursday, April 17, 2014

Dark Matter, String Theory, and the End of Spacetime (chaps 11-14)

I suppose we all wish we could live several lives. In one of them, I would teach physics.

I finished the last few chapters of The Perfect Theory: A Century of Geniuses and the Battle over General Relativity and enjoyed it immensely. There's rarely a book that holds my attention, but this one pulled me on like a novel. I envy the brilliance of the characters in this story and wish I could even catch a glimpse of their thoughts.

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
Chapters 7-10: Black Holes and Gravitational Waves

Chapter 11: The Dark Universe
This chapter is dominated by the rise of the notion that the universe is full of dark matter, as well as the rise of the idea of dark energy. The key figure of the chapter is Jim Peebles of Princeton, who is still living at almost 80. He retired in 2000.

Peebles devoted most of his career to trying to figure out how the galaxies of the universe hang together. What role do galaxies play in Einstein's relativity? Basically, he explored the middle part, from the Big Bang to what they look like now. He was scooped trying to find background radiation from the Big Bang but he was a key predictor of it.

Throughout his career, he followed the dictum of his mentor, Bob Dicke--good observations trump mediocre theories. This would lead him and others to conclude that something they eventually called dark matter existed. Space should curve quickly but it is relatively smooth. Why does the universe lean Euclidean rather than non-Euclidean? The "cold dark matter" model or CDM suggested that 96% of the universe's stuff isn't seen because it doesn't interact with light.

Hard not to think of the ether that Einstein disproved... or did he?

At the same time, dark matter doesn't account for why the universe doesn't expand even more quickly. Since Einstein abandoned his constant (lambda), the idea that there might be a number in his equation to keep the universe from expanding had been considered an embarrassment. Einstein invented the possibility of a constant in his equations to keep the universe from expanding and he did it because he didn't like the idea of an expanding universe. It was a classic example of fiddling with the data because you didn't like the conclusion.

But by 1996, at a meeting meant to celebrate Peebles 60th birthday by a series of debates, the constant was brought up again as a possible explanation for why the universe wasn't flying apart. Michael Turner from the University of Chicago argued for it. Reluctantly, many cosmologists have finally accepted it.

Or have they? We are witnessing the rise of speculation about another dark entity--dark energy.

Chapter 12: The End of Spacetime
I've never had the slightest interest in string theory. It's a bizarre thing but the very idea that fundamental reality is a bunch of vibrating strings is really annoying to me. Quite irrational, I know. I side with Leslie Winkle in Big Bang Theory, an episode mentioned in this chapter. :-) As Einstein once said of pure math, I find string theory "superfluous erudition" that is pointless because it's untestable, narcissistic, and self-feeding.

I have a similar reaction to Stephen Hawking too. I admire his genius, of course. From what I can tell, he seems a lot smarter than Einstein ever was. His main contribution seems to be his finding that black holes radiate and ultimately evaporate, that they satisfy the second law of thermodynamics and have a temperature.

He's made and lost other bets. Most recently, he bet against the Higgs boson, which seems to have been discovered. Hawking would rather see confirmation of M-theory with its strings and membranes, multiverses and things popping in and out of nothing. Another book on my shelf is Lee Smolin's The Trouble with Physics, which seems to be part of a wave of "fed-up-ness" with string theory, a theory that has produced nothing of any tangible benefit for physics. I'm thinking it's a colossal waste of two decades.

Smolin is one of the founders of Leslie Winkle's "loop quantum gravity." The notion here is in part that space itself is quantized, that if you get down far enough, space doesn't exist on the quantum level. Now there's a more attractive theory.

Bryce DeWitt summarized the two main approaches to gravity in 1967. The covariant approach is that of the string theories. Gravity is just another force carried by a particle, dubbed the graviton. I'm not sure why I find this approach so annoying, maybe because it was a departure from classical general relativity. The canonical approach sees gravity as a function of the geometry of spacetime. It just seems so much more profound than some dinky particle or vibrating strings.

Chapter 13: A Spectacular Extrapolation
In this chapter Ferreira looks at the minority report, various alternative theories of quantum gravity that simply have not won the day or have been overlooked. The title of the chapter comes from Peebles' sense that while Einstein's general relativity has done well to describe the motion of planets within our solar system, it is quite spectacular to suggest it applies on the level of the universe.

Just as Newtonian physics broke down when things approached the speed of light or the subatomic level, so relativity breaks down on the quantum level and in the farthest reaches of space. Are we really to think that 96% of the universe is "dark matter" or is this as silly a suggestion as the ether was at the end of the 1800s? In Ferreira's words, general relativity is due for a fresh look (211). What about Dirac's work near the end of his life? Was Sakharov right to suggest that gravity emerges from the quantum nature of matter? (214) What of the discarded proposals of Milgrom and Bekenstein?

I wish I were smart enough to grasp all the nuances. I have a strong hunch is that we are in such deep waters here that a lot of the theorists don't even understand each other's proposals. Who will be the new Einstein and Hawking, the person who looks at these questions in completely fresh and different way, one that goes against all our sensibilities?

Chapter 14: Something is Going to Happen
So Ferreira hopes. Meanwhile, the US is falling behind because we aren't willing to spend the kind of money it takes to do this sort of research. We can't even get to the space station now without the help of the Russians--who by the way aren't helping us get there right now. Congress has cut funding to LISA (for measuring gravity waves). But at least the EU is still funding this sort of research.

Meanwhile, we're afraid CERN is going to cause a black hole to swallow up the earth because we saw Dan Brown's Angels and Demons.

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