The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [185]
I don’t know if this is how things will turn out. No one does. But it’s only through fearless engagement that we can learn our own limits. It’s only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality.
*Note, as in Chapter 7, that an airtight observational refutation of inflation would require the theory’s commitment to a procedure for comparing infinite classes of universes—something it has not yet achieved. However, most practitioners would agree that if, say, the microwave background data had looked different from Figure 3.4, their confidence in inflation would have plummeted, even though, according to the theory, there’s a bubble universe in the Inflationary Multiverse in which those data would hold.
Notes
Chapter 1: The Bounds of Reality
1. The possibility that our universe is a slab floating in a higher dimensional realm goes back to a paper by two renowned Russian physicists—“Do We Live Inside a Domain Wall?,” V. A. Rubakov and M. E. Shaposhnikov, Physics Letters B 125 (May 26, 1983): 136—and does not involve string theory. The version I’ll focus on in Chapter 5 emerges from advances in string theory in the mid-1990s.
Chapter 2: Endless Doppelgängers
1. The quote comes from the March 1933 issue of The Literary Digest. It is worth noting that the precision of this quote has recently been questioned by the Danish historian of science Helge Kragh (see his Cosmology and Controversy, Princeton: Princeton University Press, 1999), who suggests it may be a reinterpretation of a Newsweek report from earlier that year in which Einstein was referring to the origin of cosmic rays. What is certain, however, is that by this year Einstein had given up his belief that the universe was static and accepted the dynamic cosmology that emerged from his original equations of general relativity.
2. This law tells us the force of gravitational attraction, F, between two objects, given the masses, m1 and m2, of each, and the distance, r, between them. Mathematically, the law reads: F = Gm1m2/r2, where G stands for Newton’s constant—an experimentally measured number that specifies the intrinsic strength of the gravitational force.
3. For the mathematically inclined reader, Einstein’s equations are Ruv– ½ guvR = 8πGTuv where guv is the metric on spacetime, Ruv is the Ricci curvature tensor, R is the scalar curvature, G is Newton’s constant, and Tuv is the energy-momentum tensor.
4. In the decades since this famous confirmation of general relativity, questions have been raised regarding the reliability of the results. For distant starlight grazing the sun to be visible, the observations had to be carried out during a solar eclipse; unfortunately, bad weather made it a challenge to take clear photographs of the solar eclipse of 1919. The question is whether Eddington and his collaborators might have been biased by foreknowledge of the result they were seeking, and so when they culled photographs deemed unreliable because of weather interference, they eliminated a disproportionate number containing data that appeared not to fit Einstein’s theory. A recent and thorough study by Daniel Kennefick (see www.arxiv.org, paper arXiv:0709.0685, which, among other considerations, takes account of a modern reevaluation of the photograph plates taken in 1919) convincingly argues that the 1919 confirmation of general relativity is, indeed,