Monday, August 07, 2017

Rovelli 5: Fundamental Constants of the Universe

This is my fifth post on Carlo Rovelli's new book, Reality Is Not What It SeemsThe first three posts were:
1. Chapters 8-11 are fairly short. Some of it covers basic material from cosmology but there are some helpful synthetic thoughts too.

Chapter 8 fills in some gaps with regard to the Big Bang. He gives high praise to Georges Lemaître, a Belgian priest, for supporting the idea of a "primordial atom" even though Einstein strongly disagreed. Einstein's equations seemed to suggest that the universe was expanding, but he didn't want to believe it. In fact he added a "cosmological constant" to his equations to fix it (Λ). Lemaître turned out to be right about expansion.

Then Einstein lamented adding the cosmological constant and wanted to remove it. Again, Lemaître suggested he should leave it. Lemaître proved to be right again over Einstein. So in both cases Rovelli writes, "It doesn't fall to everyone to disprove Einstein" (204).

Then Lemaître stopped the Pope, apparently, from making the Big Bang official church belief. Again Rovelli says, "It is not given to everyone to disprove the pope" (205). This falls under the principle of not inserting God too dogmatically into your scientific theories, because theories change.

2. Rovelli favors something he calls "the Big Bounce." The idea here is that "our universe could be the collapse of a previous contracting universe passing across a quantum phase, where space and time are dissolved into probabilities" (208). This seems to me to be a form of the oscillating Big Bang theory.

I thought that the current sense of things was that there was not enough matter in the universe to pull everything back together and thus that the universe was headed for a "Big Rip" at the end of things. This is also different from the multiverse idea that other books I've read have suggested, namely, that our universe is just one of an infinite number of universe bubbles.

Meanwhile, Rovelli nicely, I think, counters Lee Smolin's sense that the universe is all there is by definition. Rovelli, much more soundly says, "The word 'universe' has assumed another meaning in cosmology: it refers to the spacetime continuum that we see directly around us, filled with galaxies and history of which we observe. There is no reason to be certain that, in this sense, this universe is the only one in existence" (208).

Take that Smolin, who says in the first chapter of Quantum Gravity, "By definition the universe is all there is" (17). Let's just say Rovelli is a much better philosopher, although I don't always agree with him.

The dissolution of spacetime into a cloud of possibilities when you have that much mass at a quantum size is an intriguing idea.

3. Chapter 9 asks if we have any experimental evidence for loop quantum gravity. A number of times he pushes back both against those who say you cannot talk about anything that you cannot now experimentally show and those who wildly speculate detached from current trajectories. To me this positioning makes perfect sense.

On the one hand, a theory should proceed to experimentation. "A theory lacking empirical confirmation is a theory that has not yet passed its exams" (212). On the other hand, he disagrees with wild hypotheses. "Many theoretical physicists are today looking for new theories by picking arbitrary hypotheses... I don't think that this way of doing science has ever produced good results" (215-16).

Rather, all the experimental evidence has been confirming the three cornerstones of modern physics: general relativity, quantum mechanics, and the Standard Model within quantum mechanics. The new findings have brought a complete absence of surprise. Hawking was disappointed.

The three big findings of this decade are 1) the confirmation of the Higgs boson, 2) the cosmic measurements of the Planck satellite, and 3) the detection of gravitational waves.

Also, the fact that CERN has not discovered supersymmetry is a blow to string theorists, which is why Sheldon on Big Bang Theory went looking for something else to study. :-)

4. Mapping of the cosmos has given us a sense of the lay of the background radiation left not too long after the so called Big Bang. The idea here is that it took some time for the universe to cool down enough for the light (photons) of creation to be released.

The remnants of this release are called "cosmic background radiation" (CBR), alleged to have happened some 380,000 years after the Big Bang.

Apparently, if LQG is correct (loop quantum gravity), then there should also be a gravitational background radiation. An experiment called LISA involving three satellites around the sun, might be able to test for these.

5. Chapter 10 looks at quantum black holes. There are black holes at the centers of most galaxies and, in at least one theory, they may account for what it otherwise called dark matter.

The horizon of a black hole is the point where you might stay out. Past that, nothing can get out. Time stops at the horizon. Stephen Hawking's claim to fame was his discovery that black holes slowly evaporate. Eugenio Bianchi showed that loop quantum gravity can also demonstrate Hawking's formula for the heat of a black hole.

What LQG would show is that, since spacetime is not infinitely divisible--since it never can reach a singularity--at some point a black hole should explode in a miniature version of the Big Bang. From our perspective outside a black hole, this would take billions years, even if it is only moments inside the black hole. Since the universe is allegedly 14 billion years old, we might find some of these. Rovelli suggests that some "fast radio bursts" detected by radio telescopes could be such.

6. Chapter 11 is called the end of infinity. The common sense of the suggestion here is so obvious I've thought of it for some time now and I'm not even a scientist. Why didn't Dirac and Feynman? Quantum mechanics and relativity are plagued with infinities. Feynman the pragmatist simply substituted the experimental values for certain infinities to get his equations to work.

But LQG, because it sees space as quantized, eliminates the infinities. This seems so obvious to me that it is surprising it is not a fundamental working assumption of modern physics.

"Putting a limit to infinity is a recurrent theme in modern physics. Special relativity may be summarized as the discovery that there exists a maximum velocity for all physical systems. Quantum mechanics can be summarized as the discovery that there exists a maximum of information for each physical system. The minimum length is the Planck length LP, the maximum velocity is the speed of light c, and the total information is determined by the Planck constant h" (232).

Now we are getting somewhere. This is what I've been thinking and looking for someone to put succinctly like this. "The existence of these minimum and maximum values for length, velocity, and action fixes a natural system of units. Instead of measuring speed in kilometers per hour... we can measure it in fractions of the speed of light.. In the same way, we can posit L= 1 by definition and measure length in multiples of Planck's length. And we can posit h = 1 and measure actions in multiples of Planck's constant. In this way, we have a natural system of fundamental unities from which the others follow" (233).

One more seems to complete Rovelli's set, namely, the cosmological constant (Λ) used in relativity. I have a book called Just Six Numbers that is also on my reading list. I'm hoping it will help me understand the importance of the ratio between the cosmological constant and the Planck length.

My next post should finish the book.

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