This is a book on ‘Time’ written by physicist-author Sean Carroll who works at Caltech University, USA. Time is an extremely useful idea intricately woven into our lives; however, its true definition and implications on a larger scale eludes most of us. Even physicists find trouble to come to grips with time and its nuances. What is time and what exactly is meant by the flow of time? Sean Carroll in this wonderful book seeks to answer that with some individual insights.
Newtonian physics sees time as a stretch from an infinite past to an infinite future where events unfold. Einstein replaced it with the relativistic notion of time where there are no absolutes. Each observer in the universe has his own idea of time flow depending on his place and his speed relative to some other object. A person or a clock traveling exceedingly fast will have slowing of their times-the time dilatation effect as noted by an observer standing still and having a clock of his own. Speeds near that of light distorts time in very curious ways.
Yet, most of us have an intuitive idea of a general arrow of time from the past to the future. How does this arrow of time come about? We do not remember the future, and neither can a scrambled egg unscramble to a complete egg. We do not grow younger every day leading to birth one day. Those things happen only in the ambit of science fiction. There seems to be something true about an arrow of time despite all laws in physics being time reversible, implying that they work in either direction of time. Time is irrelevant in describing the laws of nature whether at a microscopic or a macroscopic level. But, there is one law which works only one way from the past to the future and is the essence or definition of time arrow- the second law of thermodynamics.
This is the most inviolable law of nature which states that the entropy of a closed system increases or stays constant with time. Entropy of the universe always increases from low entropy to high entropy giving time a direction. Always. Entropy is a measure of disorder of things and a low entropy is a condition of high order. High entropy is a state of low order or rather disorder. The time arrow is thus simply from order to disorder.
It is a law yet to be broken which says that entropy of a closed system, the disorder of constituent atoms, will always increase. Complexity like stars, planets, galaxies, life represent local areas of low entropy and seem to break the law; but these local areas of complexity or lower entropy is always accompanied by increase in the total entropy of the system and an ever-increasing fragility. Complexity is always fragile and in danger of going extinct. Nature always is an attempt to make complex things simple. A huge message for humans who represent maximum complexity today.
Anyway, the second law allows us to have a time arrow-from low entropy to high entropy; from a period of high order to a period of low order. The universe thus has a very well-defined arrow of time. The problem which now confounds the cosmologists is that of extremely low entropy of the Big bang moment. The entropy today is higher than yesterday and why is that so? The answer is because it was even lower the day before. Not a very scientific answer one might say, but that is indeed the best possible explanation today. The universe was created in the Big bang about 14 billion years ago. The bang was an epochal event, and together with another theory called ‘inflation’ as proposed by Alan Guth, a complete explanation of the universe today. But in long regress of lower and lower entropy backward in time, we reach the primary event and must wonder why the Big bang started with lowest entropy.
Entropy is defined by the number of ways its constituent atoms can be placed- the microstates- without changing the macrostate. This is a particularly important insight by one of the key scientists in the field of thermodynamics-Boltzmann. For example, let us see an ice cube in water. The number of ways the atoms in the ice-cube can be combined without a change of the overall state of an ice cube is a certain number. Now, the ice-cube melts in water. The number of microstates of the atoms in the water body which can possibly exist without changing the overall macrostate of a water body is considerably more than the previous number for the ice cube. The ice-cube has a limited number of arrangements of its atoms or molecules in which it can exist as compared to the final dissolved state of the ice-cube. Entropy increases always in the system and hence an ice cube dissolves in water giving a direction in time. A movie in reverse showing an ice cube forming in water would be immediately detected by a discerning viewer.
Hence, an increase in entropy, a measure of the possible microstates without changing the macrostate, is the most constant measure of the arrow of time. In quantitative terms, the measure of entropy at the Big bang moment was 1088 and the entropy of the universe now is 10 101 (or ten googols). The maximum entropy of the observable universe when it is stretched into vast emptiness is predicted to be 10 120 . These numbers may not appear significantly large, but that is the power of exponential expression.
Dark energy as a player in cosmology was discovered in the 1990s and is now having huge implications in the understanding of universe. The universe was initially thought of as being static forever. Then Big bang physics showed that the universe, with its hundred billion galaxies and a hundred billion stars in each galaxy, is expanding from a single point in space and time. The idea of the universe changed from being suspended in eternity to a definite origin in time. Space also began at this point. A question of what was before the bang is like asking what is north of North-pole. It does not make sense, so shut up.
However, scientists thought that the final fate of the universe would be a collapse. The universe would expand, fighting against gravity all the time, till it reaches a decisive point. At this place, the universe would turn inward and then start collapsing to the Big Crunch moment. However, dark energy made a huge paradigm change in understanding the final fate of the universe. Dark energy is an anti-gravitational force which seems to be expanding the universe at an accelerating rate! It is a runaway universe. The fate of the universe is now predicted to be vast emptiness as matter will slowly merge into black holes and black holes in turn would evaporate. However, there is no need to panic. The period for this event is a google number of years- 10100 that is! We are only 1010 years from the start of the universe, at the very early phase of the history of the universe and that is surprising to the author.
The author then makes his boldest prediction based on the present understanding of the laws of thermodynamics. Why was the entropy so low at the Big bang moment and why do we find ourselves so close to the origin point-1010 years- rather than anywhere else in the possible 10100 of the overall history? The author says that the explanation may be that universes are forming all the time- the multiverse model. Baby universes are forming all the time.
The author speculates/ predicts that there is a uniform state of complete emptiness or increasing entropy in the background multiverse space. Quantum mechanics state that empty space is not completely empty. It is bubbling with activity because of quantum fluctuations. One of the quantum fluctuations can give rise to a Big bang type low entropy moment and this expands as an enclosed spacetime bubble. A baby universe is born. This low entropy beginning of the baby universe from a larger multiverse becomes a Big bang moment of that universe. Later, gravitational energy, inflation and other mechanisms take over to form the planets, galaxies, and sometimes life. In this scenario, there is nothing special about our universe. We are one of the many infinites. String theory, the newest and the most controversial child of physics predicts 10500 of universes like ours embedded in this so-called multiverse.
The amazing thing about gravity is that it is negative energy. The total energy content of the entire universe, if the negative gravitational energy is considered too, is precisely zero! From nothing we come forth and into nothing we go. The universe is the biggest illusion perhaps. The universe has been defined as the ultimate free lunch by some physicists. The baby universe would expand to the point of maximum emptiness and entropy, and this would form other baby universes (the grandchildren universes). In this scenario of constant forming of universes, there is no surprise in the early low entropy state of the Big bang. It is happening all the time.
In some universes, there may be no life possible; in some like ours, organisms may evolve to ask the question of why all this. It is a fantastic proposition and the author is hopeful that the next few decades may see some indirect experimental proof of this scenario. Multiverse theory is not ‘falsifiable’, and that is a key definition of improper science according to Karl Popper, one of the doyens in the field of philosophy of science. Our energies presently are no way capable of testing the multiverse theories and the problem child called string theory. The author is however optimistic. In this scenario, the entropy of the total multiverse would be either remain constant or increase as per the second law. The creation of a baby universe is not a fiery moment for the observer in the background universe. The baby universe silently pinches itself off and grows into an expanding bubble completely disconnected from its parent universe.
The concept of using thermodynamics as a definition of time was interesting. The book should be read very carefully; and it is easy to lose the way in the voluminous tome. The only criticism is that the author should have simplified even more. At places, quite a few in fact, the author in his excitement seems to be talking to other physicists and even himself forgetting the lay reader! Such moments are frightening because the reader might simply drop the book. However, the general idea and theme of the book remains intact. The reader is advised to hence go slow and not get deterred by the occasional steep climbs into complex technical language. Michio Kaku, Brian Greene, Carl Sagan, Hawking and Brian Cox are the prototypes of skilled physics writing; the author should perhaps take a few tips from their style of writing only to make things a little breezier for the lay reader. The stuff is there, the fun is there, the thrill of learning something new is there, but the language at some places is a little tough. However, I do wish the author keeps the books rolling in the future for the readers. The book is worth a buy.
RANDOM MUSINGS 