The cosmic landscape: chapter 1
This is my interpretation of chapter 1 of Leonard Susskind's book The Cosmic Landscape: String Theory and the Illusion of Intelligent Design.
He introduces evolution as the origin of modern cosmological ideas, which prepares the intellectual ground for the arguments that I suspect he will use later on in the book. He then goes on to say that cosmology must use universal rules that have nothing to do with our existence, citing by analogy Richard Dawkins' "blind watchmaker".
Up until now the science of cosmology has assumed that the same laws of nature hold everywhere in the universe, but the new cosmology has the universe embedded in a landscape of alternative universes (which Susskind calls a megaverse), where the "constants" of nature depend on which universe you inhabit in the megaverse.
He then notes the "coincidence" that the laws of nature are consistent with our existence, which is called the anthropic principle. At this point he does not mention the important distinction between the weak and strong versions of the anthropic principle.
I found this rather disconcerting, because it could mislead some readers into thinking that he is about to embark on an "intelligent design" argument for our existence, which would be the strong anthropic principle in which the laws of nature are selected a priori to be hospitable for our existence. Fortunately, he actually uses the weak anthropic principle, in which the laws of nature are selected a posteriori conditioned on our existence, as is detailed below.
He points out that the theory of inflation implies a megaverse, and that there is experimental evidence to support the theory of inflation. He also points out that string theory implies a landscape of alternative universes; this is where the "frontier" of string theory is currently to be found. He notes that the extremely small observed value of the cosmological constant is not predicted by theory, but this value could be "explained away" by the anthropic principle, where we inhabit a part of the megaverse in which the cosmological constant happens to have the observed value, whose smallness is a prerequisite for our existence.
He then gives a fairly detailed and conventional summary of particle physics:
- Clockwork Newtonian physics is replaced by intrinsically uncertain quantum physics, giving the uncertainty principle, where measuring something with greater precision causes the sytem to be disturbed more.
- The smaller the wavelength of a photon the larger its energy, so big accelerators are needed to probe small objects. Use this to discover that protons and neutrons are made of quarks.
- Quantum mechanics implies zero point energy, where confining particles causes them to have "quantum jitters". Conjugate quantities which are subject to the uncertainty principle are position and velocity, energy and time, and electric and magnetic fields; this last example is what gives rise to the vacuum energy all around us.
- QM implies discreteness, where energy comes in packets.
- QM implies interference, as famously exemplified by the two slit experiment, where very low light intensity still gives rise to interference effects, even though there is only one photon at a time passing through the apparatus. Therefore photons must be both particles and waves at the same time. There is no point trying to understand this using everyday intuition; it is a property of the universe that lies outside our everyday common sense.
- Nature seems to be organised hierarchically, in which you break large objects down into smaller pieces, and so on. This is called "reductionism". Thus far in experiments we have followed down the hierarchy as far as elementary particles, which are described by quantum field theory, and intuitively visualised using Feynman diagrams to describe interactions between the particles.
- He then summarises quantum electrodynamics (QED), the nucleus, Feynman diagrams (propagators, vertices, coupling constants), antimatter, fine structure constant, quantum chromodynamics (QCD), weak interactions, and that all of physics is derived from simple underlying particle creation and annihilation events.
- The use of reductionism in physics has truly been a success story. Note that neither I nor Susskind assert that the reductionist approach should a priori be assumed to work in all cases.
He then goes on to make observations and ask questions that prepare the ground for later on:
- He notes that there are 3 generations of elementary particles, and that the extra generations of particles appear to be unnecessary. Thus he says that elementary particle physics is not simple and elegant, despite what particle physicists would try to tell you; it is messy and it is more like zoology or botany. To be fair, particle physics is in much better shape now than it ever was before, but Susskind makes a valid point about the "spin" that particle physicists put on the elegance of their subject.
- He points out that the "standard model" describes properties with incredible precision, but it needs around 30 experimentally measured constants of nature to achieve these results; this is too many constants for a fundamental theory. We have no theory that says why the standard model is right and not some other model.
- Are there deeper laws that govern the standard model?
That final question is the focus of Susskind's book.