A Brief History Of Time

Galileo is considered the first modern scientist. He believed people could learn how the world works by observing it carefully. His support for the Copernican theory that the Earth revolves around the sun was deemed heretical by the Catholic Church, and the pope forbade Galileo from espousing it. Galileo finally convinced a later pope to let him write an even-handed book about the Copernican and Earth-centered theories; this book, Dialogue Concerning the Two Chief World Systems, was published in 1632 to great acclaim and helped convince the world that the sun, and not the Earth, was at the center of the solar system (195).

Galileo was placed under house arrest for being too convincing, and he was forced publicly to recant his beliefs. Privately, he smuggled his next book, Two New Sciences, to Holland for publication. It made an even bigger impression on European readers and helped launch modern physics.

The single most important physics book, Principia Mathematica, was written by Englishman Isaac Newton, who also became president of the scientific Royal Society and was the first scientist to be knighted.

Newton also was famous for his quarrels. He feuded with Astronomer Royal John Flamsteed, who provided important data for the Principia but later refused to cooperate with Newton; Newton seized the information he wanted but was restrained by the courts from publishing it.

Newton invented calculus years before the German philosopher Gottfried Leibniz also did so. Leibniz published first, causing a big feud.

The Appendix is divided into six sections, each titled with a term that is listed below in bold.

Dark Energy and the Accelerating Expansion of the Universe. In 1998, astronomers found that the universe is expanding faster and faster. This rules out the possibility of the universe contracting back into a “big crunch.” Einstein believed there should be a “cosmological constant” that pushes back against gravity. He withdrew the idea when better ones came along, but now the universe’s accelerating expansion calls for just such a constant. Scientists don’t know why the acceleration is happening; they use the placeholder term “dark energy.”

Hawking’s no-boundary theory posits that infinite universes exist, and, by the anthropic principle, humanity exists in one with an anti-gravity “dark energy” value that’s tolerable and doesn’t make the universe fly apart.

Microwave Background Radiation and the No Boundary Proposal. Microwaves have become a major test of cosmological theories. The microwave background noise discovered in 1965 showed that the early universe was very hot, and that inflation and expansion have smoothed out most, but not all, of the irregularities, as predicted by the uncertainty principle of quantum mechanics and by Hawking’s no-boundary proposal.

Hawking’s idea is that, going back in time, the universe resolves to a point, much as the lines of longitude on the Earth resolve to a point called the North Pole; asking what happened before the Big Bang is “like asking what lies north of the North Pole” (202). Space and time are meaningless before the Big Bang.

Still unconfirmed are the gravitational waves predicted during the inflationary period, a fraction of a second after the Big Bang. These waves would polarize the microwave background radiation; studies are ongoing.

Meanwhile, the microwave data is so good that it’s beginning to teach cosmologists more about the conditions of the very early universe. Among other things, the evidence suggests that the energies at which all particles behave the same are a trillion times higher than those that the CERN Large Hadron Collider can study.

Eternal Inflation and the Multiverse. A multiverse might erupt from a single big bang. Slight variations in the expansion process, as predicted by the quantum uncertainty principle, would cause some areas to slow down while others continue to expand, resulting in “many different universes […] possibly with different local laws of physics and chemistry” (204). Some physicists who are skeptical of a multiverse have proposed that the inflationary period was a low-energy affair that wouldn’t create lots of universes. Recent satellite observations of the microwave background seem to support a high-energy inflation, meaning the multiverse is still possible.

Gravitational Waves. Gravitational waves were first proposed in Einstein’s General Theory of Relativity in 1916. Since then, even Einstein doubted their existence, but the theory predicts that gravity waves remove energy from orbiting systems, and, indeed, such objects tend to move inward toward each other. In 2016, the LIGO Collaboration detected gravity waves for the first time from the collision of two black holes. This new technique will give scientists a look at black holes that they can’t get from telescopes and other detectors. The future of black hole astronomy is very promising: A vast catalog of gravity-wave information from these most extreme regions of the universe will help cosmologists refine their theories of how the cosmos started and how it runs.

The Information Paradox. All major scientific theories of reality, from Newton to quantum mechanics, expect that the information contained in the universe must always be preserved. Black holes absorb particles and their information, but it’s not clear that the information can be retrieved, even if the holes evaporate. Hawking at first assumed that information absorbed into a black hole would be lost. Black holes retain some information at their event horizons, but any such knowledge is available in a distorted format. It’s like a burned-up book that can be retrieved, but only as smoke and ashes, which are hard to interpret.

Outlook. Much progress has been made since the second edition of Brief History, some of it completely unanticipated, like dark energy. The idea of a multiverse seems hard to accept, but humanity has already learned to live with the idea that they exist on a tiny planet in one of many galaxies.