Three Roads To Quantum Gravity Quotes

“So, in the end, the most improbable and hence the most puzzling aspect of space is its very existence. The simple fact that we live in an apparently smooth and regular three dimensional world represents one of the greatest challenges to the developing quantum theory of gravity. If you look around at the world seekimg mystery, you may reflect that one of the biggest mysteries is that we live in a world in which it is possible to look around, and see as far as we like. The great triumph of the quantum theory of gravity may be that it will explain to us why this is so.”
― Lee Smolin, Three Roads To Quantum Gravity

“if physics is much simpler to describe under the assumption that space is discrete, rather than continuous, is not this fact itself a strong argument for space being discrete? If so, then might space look, on some very small scale, something like Wilson's lattice.”
― Lee Smolin, Three Roads To Quantum Gravity

“The search for the meaning of temperature and entropy of matter led to the discovery of atoms. The search for the meaning of the temperature and entropy of radiation led to the discovery of quanta. In just the same way, the search for the meaning of the temperature and entropy of a black hole is now leading to the discovery of the atomic structure of space and time.”
― Lee Smolin, Three Roads To Quantum Gravity

“Whatever is happening on very small scales near the horizon of the black hole will be enlarged by the effect whereby the wavelengths of light are stretched as the light climbs up to us. This means that jf we can observe light coming from very close to the horizon of a black hole, we may be able to see the quantum structure of space itself.”
― Lee Smolin, Three Roads To Quantum Gravity

“The problem is that while quantum theory changed radically the assumptions about the relationship between the observer and the observed, it accepted without alteration Newton’s old answer to the question of what space and time are. Just the opposite happened with Einstein’s general relativity theory, in which the concept of space and time was radically changed, while Newton’s view of the relationship between observer and observed was retained. Each theory seems to be at least partly true, yet each retains assumptions from the old physics that the other contradicts.”
― Lee Smolin, Three Roads To Quantum Gravity

“In the end, what is most satisfying about the picture of space given by loop quantum gravity is that it is completely relational. The spin networks do not live in space; their structure generates space. And they are nothing but a structure of relations, governed by how the edges are tied together at the nodes. Also coded in are rules about how the edges may knot and link with one another. It is also very satisfying that there is a complete correspondence between the classical and quantum pictures of geometry. In classical geometry the volumes of regions and the areas of the surfaces depend on the values of gravitational fields. They are coded in certain complicated collections of mathematical functions, known collectively as the metric tensor. On the other hand, in the quantum picture the geometry is coded in the choice of a spin network. These spin networks correspond to the classical description in that, given any classical geometry, one can find a spin network which describes, to some level of approximation, the same geometry (Figure 27).”
― Lee Smolin, Three Roads To Quantum Gravity

“The loop approach to quantum gravity is now a thriving field of research. Many of the older ideas, such as supergravity and the study of quantum black holes, have been incorporated into it. Connections have been discovered to other approaches to quantum gravity, such as Alain Connes's non-commutative approach to geometry, Roger Penrose's twistor theory and string theory.”
― Lee Smolin, Three Roads To Quantum Gravity

“Louis and I worked on two projects. In the first we tried to formulate a gravitational theory based on the dynamics of interacting loops of quantized electric flux. We failed to formulate a string theory, and as a result we published none of this work, but it was to have very important consequences. In the second project we showed that a theory in which spacetime was discrete on small scales could solve many of the problems of quantum gravity. We did this by studying the implications of the hypothesis that the structure of spacetime was like a fractal at Planck scales. This overcame many of the difficulties of quantum gravity, by eliminating the infinities and making the theory finite. We realized during that work that one way of making such a fractal spacetime is to build it up from a network of interacting loops. Both collaborations with Louis Crane persuaded me that we should try to construct a theory of spacetime based on relationships among an evolving network of loops. The problem was, how should we go about this?”
― Lee Smolin, Three Roads To Quantum Gravity

“One can take the colour-electric field as the fundamental entity, and then try to understand the picture of a string stretched between the quarks as a consequence of space having properties that make it something like an electric version of a superconductor. This is the route taken by those physicists who work on QCD. For them, the key problem is to understand why empty space has properties that make it behave in certain circumstances like a superconductor. This is not as crazy as it sounds. We understand that in quantum theory space must be seen to be full of oscillating random fields, as discussed in Chapter 6. So we may imagine that these vaccum fluctuations sometimes behave like the atoms in a metal in a way that leads to large-scale effects like superconsuctivity.”
― Lee Smolin, Three Roads To Quantum Gravity

“Many of the important principles in twentieth century physics are expressed as limitations on what we can know. Einstein's principle of relativity (which was an extension of a principle of Galileo's) says that we cannot do any experiment that would distinguish being at rest from moving at a constant velocity. Heisenberg's uncertainty principle tells us that we cannot know both the position and momentum of a particle to arbitrary accuracy. This new limitation tells us there is an absolute bound to the information available to us about what is contained on the other side of a horizon. It is known as Bekenstein's bound, as it was discussed in papers Jacob Bekenstein wrote in the 1970s shortly after he discovered the entropy of black holes.”
― Lee Smolin, Three Roads To Quantum Gravity

“In practice, the greatest amount of information that may be stored behind a horizon is huge - 10^66 bits of information per square centimetre.”
― Lee Smolin, Three Roads To Quantum Gravity

“Both loop quantum gravity and string theory assert that there is an atomic structure to space. In the next two chapters we shall see that loop quantum gravity in fact gives a rather detailed picture of that atomic structure. The picture of the atomic structure one gets from string theory is presently incomplete but, as we shall see in Chapter 11, it is still impossible in string theory to avoid the conclusion that there must be an atomic structure to space and time. In Chapter 13 we shall discover that both pictures of the atomic structure of space can be used to explain the entropy and temperature of black holes.”
― Lee Smolin, Three Roads To Quantum Gravity

“Similarly, a law of physics that allows information to be converted into geometry, and vice versa, but gives no account of why, should not survive for long. There must be something deeper and simpler at the root of the equivalence. This raises two profound questions: Is there an atomic structure to the geometry of space and time, so that entropy of the black hole could be understood in exactly the same way that the entropy of matter is understood: as a measure of information about the motion of atoms? When we understand the atomic structure of geometry will it be obvious why the area of a horizon is proportional to the amount of information it hides? These questions have motivated a great deal of research since the mid-1970s. In the next few chapters I shall explain why there is a growing consensus among physicists that the answer to both questions must ne 'yes'.”
― Lee Smolin, Three Roads To Quantum Gravity

“To understand what we mean when we say that space is discrete, we must put our minds completely into the relational way of thinking, and really try to see and feel the world around us as nothing but a network of evolving relationships. These relationships are not among things situated in space - they are among the events that make up the history of the world. The relationships define the space, not the other way around.”
― Lee Smolin, Three Roads To Quantum Gravity

“So we may now pose a question which could be answered only if we have a quantum theory of gravity. What is the nature of the information trapped in a quantum black hole? Keep this in mind as we go ahead and explore the different approaches to quantum gravity, for a good test of a theory of quantum gravity is how well it is able to answer this question.”
― Lee Smolin, Three Roads To Quantum Gravity

“For simple black holes, which do not rotate and have no electric charge, the values of the temperature and entropy can be expressed very simply. The area of the horizon of a simple black hole is proportional to the square of its mass, in Planck units. The entropy S is proportional to this quantity. In terms of Planck units, we have the simple formula S = .25 A / h G. Where A is the area of the horizon, and G is the gravitational constant.”
― Lee Smolin, Three Roads To Quantum Gravity

“What those two young physicists did remains the most important step yet made in the search for quantum gravity. They gave us two general and simple laws, which were the first physical predictions to come from the study of quantum gravity. They are: Unruh's law. Accelerating observers see themselves as embedded in a gas of hot photons at a temperature proportional to their acceleration. Bekenstein's law With every horizon that forms a boundary separating an observer from a region which is hidden from them, there is associated an entropy which measures the amount of information which is hidden behind it. This entropy is always proportional to the area of the horizon.”
― Lee Smolin, Three Roads To Quantum Gravity

“The entropy of the hot radiation she observes as a result of her acceleration turns out to be exactly proportional to the area of her horizon! This relationship between the area of a horizon and entropy was discovered by a Ph.D. student named Jacob Bekenstein, who was working at Princeton at about the time that Bill Unruh made his great discovery. Both were students of John Wheeler who a few years before had given the black hole its name. Bekenstein and Unruh were in a long line of remarkable students Wheeler trained, which included Richard Feynman.”
― Lee Smolin, Three Roads To Quantum Gravity

“The whole history of the world is then nothing but the story of huge numbers of these processes, whose relationships are continually evolving. We cannot understand the world we see around us as something static. We must see it as something created, , and under continual recreation, by an enormous number of processes acting together. The world we see around us is the collective result of all those processes. I hope this doesn't seem too mystical. If I have written this book well then, by the end of it, you may see that the analogy between the history of the universe and the flow of information in a computer is the most rational, scientific analogy I could make. What is mystical is the picture of the world as existing in an eternal three-dimensional space, extending in all directions as far as the mind can imagine. The idea of space going on and on for ever has nothing to do with what we see. When we look out, we are looking back in time through the history of the universe, and after not too long we come to the big bang. Before that there may be nothing to see- or, at the very least, if there is something it will most likely look nothing like a world suspended in a static three-dimensional space. When we imagine we are seeing into an infinite three-dimensional space, we are falling for a fallacy in which we substitute what we actually see for an intellectual construct. This is not only a mystical vision, it is wrong.”
― Lee Smolin, Three Roads To Quantum Gravity

“We are very used to imagining that we see a three-dimensional world when we look around ourselves. But is this really true? If we keep in mind that what we see is the result of photons impinging on our eyes, it is possible to imagine our view of the world in quite a different way. Look around and imagine that you see each object as a consequence of photons having just travelled from it to you. Each object you see is the result of a process by which information travelled to you in the shape of a collection of photons. The farther away the object is, the longer it took the photons to travel to you. So when you look around you do not see space-instead, you are looking back through the history of the universe. What you are seeing is a slice through the history of the world. Everything you see is a bit of information brought to you by a process which is a small part of that history.”
― Lee Smolin, Three Roads To Quantum Gravity

“One readon why it has taken so long to construct a quantum theory of gravity is that all previous quantum theories were background dependent. It proved rather challenging to construct a background independent quantum theory, in which the mathematical structure of the quantum theory made no mention of points, except when identified through networks of relationships. The problem of how to construct a quantum theoretic description of a world in which space and time are nothing but networks of relationships was solved over the last 15 years of the twentieth century. The theory that resulted is loop quantum gravity, which is one of our three roads.”
― Lee Smolin, Three Roads To Quantum Gravity

“The geometry of a universe is very like the grammatical structure of a sentence. Just as a sentence has no structure and no existence apart from the relationships between the words, space has no existence apart from the relationships that hold between the things in the universe. If you change a sentence by taking some words out, or changing their order, its grammatical structure changes. Similarly, the geometry of space changes when the things in the universe change their relationships to one another.”
― Lee Smolin, Three Roads To Quantum Gravity

“There is no meaning to space that is independent of the relationships among real things of the world. [...] Space is nothing apart from the things that exist. [...] If we take out all the words we are not left with an empty sentence, we are left with nothing.”
― Lee Smolin, Three Roads To Quantum Gravity

“The geometry of a universe is very like the grammatical structure of a sentence. Just as a sentence has no structure and no existence apart from the relationships between the words, space has no existence apart from the relationships that hold between the things in the universe. If you change a sentence by taking some words out, or changing their order, its grammatical structure changes. Similarly, the geometry of space changes when the things in the universe change their relationships to one another.”
― Lee Smolin, Three Roads To Quantum Gravity

“This tells us that M theory, if it exists, cannot describe a world in which space os continuous and one can pack an infinite amount of information into any volume, no matter how small. This suggests that whatever it is, M theory will not be some direct extension of string theory, as it will have to be formulated in a different conceptual language. The present formulation of string theory is likely, then, to be a transitional stage in which elements of a new physics are mixed up with the old Newtonian framework, according to which space and time are continuous, infinitely divisible and absolute. The problem that remains is to separate out the old from new and find a coherent way to formulate a theory using only those principles that are supported by the experimental physics of ghe twentieth and twenty-first.centuries.”
― Lee Smolin, Three Roads To Quantum Gravity

“If a surface can be seen as a kind of channel through which information flows from one region of space to another, then the area of the surface is a measure of its capacity to transmit information. This is very suggestive.”
― Lee Smolin, Three Roads To Quantum Gravity

“The fact that the amount of missing information depends on the area of the boundary of the trapped region is a very important clue.”
― Lee Smolin, Three Roads To Quantum Gravity

“Expressed in Planck units, the temperature T of a black hole is inversely proportional to its mass, m. This is a third law, Hawking's law: T = k/m. The constant k is very small in normal units. As a result, astrophysical black holes have temperatures of a very small fractionnof a degree.”
― Lee Smolin, Three Roads To Quantum Gravity

“So, Einstein's theory of gravity is a theory of causal structure. It tells us that the essence of spacetime is causal structure and that the motion of matter is a consequence of alterations in the network of causal relations. What is left out from the notion of causal structure is any measure of quantity or scale.”
― Lee Smolin, Three Roads To Quantum Gravity

“Time is described only in terms of change in the network of relationships that describes space. This means that it is absurd in general relativity to speak of a universe in which nothing happens. Time is nothing but a measure of change-it has no other meaning. Neither space nor time has any existence outside the system of evolving relationships that comprises the universe. Physicists refer to this feature of general relativity as background independence. By this we mean that there is no fixed background, or stage, that remains fixed for all time. In contrast, a theory such as Newtonian mechanics or electromagmetism is background dependent because it assumes that there exists a fixed, unchanging background that provides the ultimate answer to all questions about where and when.”
― Lee Smolin, Three Roads To Quantum Gravity