From Eternity to Here: The
Quest for the Ultimate Theory of Time
全名肖恩•迈克尔•凯若尔（Sean Michael Carroll），生于1966年10月5日，是一位宇宙学家和物理学教授，研究领域包括暗能量、广义相对论、量子力学、引力作用、统计力学和宇宙学。他的兴趣宽广，涉及哲学、复杂性理论和信息论。他是美国加利福尼亚理工学院物理系的一位研究教授，是物理学博克“宇宙方差”的供稿者，在科学期刊杂志（如《自然》、《纽约时代》、《天空和望远镜》、《新科学家》）上发表过文章。
凯若尔是一位活跃的科学传播者，自2004年以来就经常在博克中发表文章。他出现在美国历史频道的《宇宙》、科学频道的《与摩甘•弗瑞曼一起穿过虫洞》和喜剧中心的《科尔伯特报告》等节目上。他已为教育公司发行了一套讲义，是关于暗物质和暗能量的；另一套讲义是关于时间的本性。他曾作为影片（如《雷神和创神：遗产》（Thor and TRON: Legacy））以及电视秀（如《波纹和骨架》（Fringe and Bones））的科学顾问。
TheArrow of Time
By N. BARTLETTon 24 Feb. 2012
This is a book that explores the nature oftime and in so doing takes the reader in some unexpected directions. It is alsoquite a hard book.
Although Sean Carroll throws in quips andtouches of levity here and there, he is no Bill Bryson writing `A Short Historyof Almost Everything'. `From Eternity to Here' is a book you have to work at -but I found the effort worthwhile.
The author links the idea of time closelyto that of entropy, the quality of orderliness in matter and in particular theSecond Law of Thermodynamics: "The entropy of a closed system neverdecreases". Crude examples to illustrate the basic concept are theomelette (high entropy) that cannot be put back in the egg (low entropy), orthe cream and coffee (low entropy) which when stirred together (high entropy)cannot be separated.
Extending this concept to the universe, itis believed to have started as a tiny something (low entropy) and has evolvedto today's observable universe of stars and galaxies (higher entropy). Thisone-way process is seen as a analogue to time: it goes one way. We know aboutthe past but we do not know about the future: it goes one way.
However this description of entropy I foundcounter-intuitive. Surely in the past the universe was high entropy with aprimeval soup of basic particles and energy? While today is it not low entropywith stars, solar systems, galaxies and a sense of colossal order? It was notuntil page 166 that this paradox was explained.
"The culprit in this case is gravity.We're going to have a lot to say about how gravity wreaks havoc with oureveryday notions of entropy..."
Aha! Now we begin to make progress. DrCaroll's constant refrain is the question as to why the universe is relativelylow entropy. He thinks a `natural' state would be space with an even spread ofparticles and energy with high entropy. Instead we have an organised universeas we have seen which is low entropy.
Dr Carroll takes the reader down some verystrange meanderings to seek answers. To be fair he is attempting to explaincomplex issues by resorting to simple analogies and examples. Yet I found manyof them so trite that I would have accepted the original complex argument onits face value without the having the sweat of relating analogy to argument.
Having given the Second Law a good run out,Carroll then turns to quantum field theory, quantum mechanics and much later onto quantum gravity. In the meantime we meet Stephen Hawking, the world'sgreatest expert on black holes. His discovery that black holes emit radiationtransformed the understanding of how the universe could evolve. As black holes,of which there are believed to millions, including very large examples in thecentre of each galaxy, suck in matter they become larger and suck in more. Onetheory is that eventually (trillions of years) all matter will have beenabsorbed into black holes. But as black holes are emitting radiation, thenafter even greater lengths of time, they will steadily dissipate into energy.
The end of the universe will simply be avast energy field empty of all matter.
But there could be an epilogue. Because ofhow energy and matter behaves at the quantum level, random fluctuation mightcreate every million or ten million years a node or bubble of false vacuum. Itcould split off from the main field and then:
"Now we have a baby universe...all setto undergo inflation and expand to a huge size. If the properties are justright...the energy will eventually be converted into ordinary matter andradiation, and we'll have a universe that evolves according to theinflation-plus-Big Bang story".
So finally we get to the concept of themultiverse, something so beyond possible demonstration or discovery that itwill forever be a mystery. Yet it would resolve the issue of a low entropyorigin - it was spawned from a high entropy field. And that's how time started.
Read the book and be absorbed.
Thebest current commentator on cosmology
By futz on 27 Dec. 2012
This is a fine treatment of a current topicin cosmology, by a premiere theoretician in the field - a point lost on atleast one reviewer - he is not a Deepak Chopra, pulling arrows out of his rear.But my main reason for commenting is to point folks to an amazing work ofCarroll's. If you want an excellent tour of cosmology, get a hold of hisTeaching Company lecture series Dark Matter, Dark Energy - you will not regretit.
Thephysical reality of time
By Rama Rao on 28 Jan. 2010
The behavior of matter (or energy) in spaceand time is described by the laws of physics, but the puzzling thing aboutphysical reality is that space and time behave differently. Space is the samein all directions and it never changes, but time has preferred direction; pastto future and the cause-effect relationship runs parallel to this. There is nosuch thing as special place (space) in the universe but there is a specialtime. This is a mystery because physical laws governing the fundamentalparticles are mostly time-symmetric (it can function thermodynamically oranti-thermodynamically), but the time-asymmetry observed in many macromolecularprocesses is thermodynamic and it has an arrow of time. Examples include, aglass bottle breaking into pieces or hot water becoming cold are attributed tothe second law of thermodynamics which seem to set this arrow of time. Thus thephysical reality is not only governed by laws of quantum physics andrelativity, but also by the second law of thermodynamics which requires thatthe entropy (a measure of disorderliness) of a closed system, such as thisuniverse, increase with time. This implies that the past has more order thanfuture, hence the state of orderliness was probably the highest (or the entropywas the lowest) at the origin of the universe (big bang). The problem ofjustifying this arrow is not so much showing that the entropy of isolatedsystems increased, but explaining why there was low entropy in the past. Whileinflationary theory proposed by Alan Guth explains many key features of theearly universe but it doesn't explain low entropy.
In this book, the author looks for clues inseveral areas such as, properties of black hole; information-loss paradox andHawking radiation, string theory, inflationary epoch, multiverse cosmology andbaby universes. He argues that a classical de Sitter background (motherspace-time where vacuum energy is positive) does not fluctuate, but the spacewould be expanding and quantum fields will be fluctuating in a classicalfashion. But if quantum gravity is taken into consideration then de Sitterspace is itself susceptible to quantum fluctuations and this result in not onlystretching and bending of spacetime as required by general relativity but alsothey could splice into multiple pieces. These pieces first appear as bubbles ofspacetime, and then they grow and splice off to form baby universes. The babyuniverse created in a background de Sitter space is inclined both towards itspast and to the future, but each baby universe starts in a dense low entropystate and exhibits a local arrow of time as it expands and cools. The babyuniverses born in the past have an arrow of time pointing in the oppositedirection to those in the future, but for each universe, the time is directedtowards increasing entropy and the multiverse manifests overall time symmetry.The author's hypothesis sharply contrasts the idea that big bang represents theboundary to space and time, and it dispels the notion that space and time werecreated at this time. He distances himself from other physicists like LarrySchulman who suggests that the universe switched to a highly ordered state atabout 380,000 years when the universe became