Martin John Rees (born 1942) is a British cosmologist and astrophysicist who was Master of Trinity College, Cambridge from 2004 to 2012 and President of the Royal Society between 2005 and 2010. He has written other books such as Just Six Numbers: The Deep Forces That Shape The Universe,Our Final Hour: A Scientist's Warning,From Here to Infinity: A Vision for the Future of Science,Our Cosmic Habitat,Gravity's Fatal Attraction: Black Holes in the Universe, etc.
He wrote in the Acknowledgements section of this 1997 book, "This book presents an individual view on cosmology---how we perceive our universe, what the current debates are about, and the scope and limits of our future knowledge." In the first chapter, he adds, "Our universe, and the laws governing it, had to be... rather special to allow our emergence... The apparent fine-tuning on which our existence depends could be a coincidence. I once thought so. But that view now seems too narrow. What's conventionally called `the universe' could be just one member of an ensemble. Countless others may exist in which the laws are different. The universe in which we've emerged belongs to the unusual subset that permits complexity and consciousness to develop. Once we accept this, various apparently special features of our universe---those that some theologians once adduced as evidence for Providence or design---occasion no surprise. This line of thought---the enlarged perspective of the `multiverse'---supplies a motive for this book." (Pg. 3)
He explains, `My Cambridge colleague, Stephen Hawking, claimed in A Brief History of Time that each equation he included would have halved the book's sales. He followed that injunction, and so have I. But he (or maybe his editor) judged that each mention of God would double the sales... In that latter respect I shall not follow Stephen's lead. Scientists' incursions into theology or philosophy can be embarrassingly naïve or dogmatic. The implications of cosmology for these realms of thought may be profound, but diffidence prevents me from venturing into them." (Pg. 6)
He suggests, "If evolution on Earth could be rerun, the outcome might be quite different... But would INTELLIGENCE necessarily emerge?... Once intelligence has emerged, and developed beyond some threshold, it controls the biosphere, and natural selection no longer has free rein. Unless the dominant species wiped itself out, intelligence would never have a second independent chance to evolve. Even when simple life exists, we don't know the chances that it evolves toward intelligence, nor how long it persists even when it does emerge. Intelligent life could be `natural'; or it could have involved a chain of accidents so surprisingly rare that nothing remotely like it has happened anywhere else in our Galaxy." (Pg. 22)
He adds, "The odds may be stacked so heavily against intelligent life that none has developed at any other site in our Galaxy---conceivably there is none anywhere in the part of the universe that we can observe. Some may find it depressing to feel alone in a vast, mindless cosmos. But I would personally react in quite the opposite way. It would in some ways be disappointing if SETI searches were doomed to fail, but we could then envisage our Earth in a less humble cosmic perspective than it would merit if our universe already teemed with advanced life forms." (Pg. 25)
He states, "Our entire observable universe could be an oasis in a grand ensemble of other universes. Although we cannot observe them (and they may be forever inaccessible), other universes are a natural expectation from current cosmology. Moreover, many features of our universe that otherwise seem baffling fall into place once we recognize this. A main theme of this book will be to elaborate the concept of a `multiverse.'" (Pg. 28)
He summarizes, "The history of our universe divides into three parts: 1. The first millisecond... The relevant physics is still speculative---indeed, one motive for studying cosmology is that the early universe may offer the only real clues to the laws of nature at extreme exigencies. 2. The second stage runs from a millisecond to about 1 million years. It's an era where cautious empiricists feel more at home... There is good quantitative evidence... and the relevant physics is well tested in the lab. Part two of cosmic history, though it lies in the remote past, is the easiest to understand." (Pg. 160)
He acknowledges, "It may seem counterintuitive that an entire universe at least 10 billion light-years across... can have emerged from an infinitesimal speck. What makes this possible is that, however much inflation occurs, the total net energy is zero. It is as though the universe were making itself a gravitational pit so deep that everything in it has a negative gravitational energy exactly equal to its rest-mass energy... This realization makes it easier to swallow the concept that our entire universe emerged almost `ex nihilo.'" (Pg. 169)
He states, "Could a collapsing universe rebound phoenixlike into a new cycle? Nothing could stop the density from rising to infinity---to a `singularity.' ... Physical conditions in the `bounce' would transcend the physics we understand, so that nothing could be said about the possibility of a rebound into a new cycle---still less about what had gone before. The concept of an `arrow of time' ... breaks down under these extreme conditions." (Pg. 195)
He notes, "Nobody has proved that it is utterly impossible to create, in our present universe, the kind of warped space-time that would allow time travel... Could there be closed time-loops on some microscopic scale? Would `time machines' be so much stranger than the other strange paradoxes of the quantum world?... These issues are certainly not settled, but I would tend to believe ... that physics has a kind of internal integrity that prohibits travel into the past." (Pg. 219)
He says, "The physical laws `laid down' in the big bang seem to apply to everywhere we can now observe. But though they are unchanging (or almost so), they seem rather specially adjusted. This could be a coincidence: I used to think so. But an enlarged cosmological perspective suggests an interpretation that seems compellingly convincing. There may be other universes---uncountably many of them---of which ours is just one. In the others, the laws and constants are different. But ours if not randomly selected. It belongs to the unusual subset that allow complexity and consciousness to develop. Once we accept this, the seemingly `designed' or `fine-tuned' features of our universe need occasion no surprise." (Pg. 238)
He admits, "Scientists commonly pay obeisance to `Ockham's razor' ... which ... means `Don't multiply entities more than is absolutely necessary.' Nothing, perhaps, could seem to violate this more drastically than postulating an infinite array of universes! Nor does it, at first sight, seem properly `scientific' to invoke regions that are unobservable, and perhaps always will be. The concept of an ensemble of universes of which ours is just one member (and not necessarily a typical one) is, needless to say, not yet in sharp theoretical focus. But it helps to explain basic (and previously mysterious) features of our universe, such as why it is so big, and why it is expanding. In the broader perspective of a `multiverse,' anthropic reasoning acquires genuine explanatory force." (Pg. 247)
He speculates, "The other universes may even be completely disjoint from ours, so that they will never come within the horizon of our remotest descendants. We may be part of an infinite and eternal multiverse within which new domains `sprout' into universes whose horizons never overlap... The multiverse could encompass all possible values of fundamental constants, as well as universes that follow life cycles of very different durations... In some there could be no gravity... In others, gravity could be so strong that it crushes anything large enough to evolve into a complex organism. Some could always be so dense that everything stayed close to equilibrium, with the same temperature everywhere. Some could even have different numbers of dimensions from our own." (Pg. 247-248)
This is a well-written, thought-provoking presentation of many current cosmological ideas, that will be of great interest to anyone studying this area.
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