Interesting People mailing list archives
Re: My Brain Hurts!
From: David Farber <dave () farber net>
Date: Thu, 26 Jun 2008 07:52:18 -0700
________________________________________ From: Steve Manning [sjmanning () fymc com] Sent: Thursday, June 26, 2008 10:31 AM To: David Farber Subject: Re: [IP] My Brain Hurts! Dear Dave. For IP if you wish. Absolutely the most appropriate "Subject" line I have seen since the advent of commercial email. To be sure, the content/topic is a sublime tour de force in the esoteric. "Four-dimensional emptiness?" That *is* better then what "this" scribe can conjure up on the best of penmanship days, even when deliberately attempting pretentious drivel. Best. Steve. David Farber wrote: ________________________________________ From: Randall Webmail [rvh40 () insightbb com<mailto:rvh40 () insightbb com>] Sent: Thursday, June 26, 2008 9:05 AM To: dewayne () warpspeed com<mailto:dewayne () warpspeed com>; David Farber; johnmacsgroup () yahoogroups com<mailto:johnmacsgroup () yahoogroups com> Subject: My Brain Hurts! http://www.sciam.com/article.cfm?id=the-self-organizing-quantum-universe&print=true Using Causality to Solve the Puzzle of Quantum Spacetime A new approach to the decades-old problem of quantum gravity goes back to basics and shows how the building blocks of space and time pull themselves together By Jerzy Jurkiewicz, Renate Loll and Jan Ambjorn Editor's Note: Click here for the web animations mentioned in the article How did space and time come about? How did they form the smooth four-dimensional emptiness that serves as a backdrop for our physical world? What do they look like at the very tiniest distances? Questions such as these lie at the outer boundary of modern science and are driving the search for a theory of quantum gravity—the long-sought unification of Einstein's general theory of relativity with quantum theory. Relativity theory describes how spacetime on large scales can take on countless different shapes, producing what we perceive as the force of gravity. In contrast, quantum theory describes the laws of physics at atomic and subatomic scales, ignoring gravitational effects altogether. A theory of quantum gravity aims to describe the nature of spacetime on the very smallest scales—the voids in between the smallest known elementary particles—by quantum laws and possibly explain it in terms of some fundamental constituents. Superstring theory is often described as the leading candidate to fill this role, but it has not yet provided an answer to any of these pressing questions. Instead, following its own inner logic, it has uncovered ever more complex layers of new, exotic ingredients and relations among them, leading to a bewildering variety of possible outcomes. Over the past few years our collaboration has developed a promising alternative to this much traveled superhighway of theoretical physics. It follows a recipe that is almost embarrassingly simple: take a few very basic ingredients, assemble them according to well-known quantum principles (nothing exotic), stir well, let settle—and you have created quantum spacetime. The process is straightforward enough to simulate on a laptop. To put it differently, if we think of empty spacetime as some immaterial substance, consisting of a very large number of minute, structureless pieces, and if we then let these microscopic building blocks interact with one another according to simple rules dictated by gravity and quantum theory, they will spontaneously arrange themselves into a whole that in many ways looks like the observed universe. It is similar to the way that molecules assemble themselves into crystalline or amorphous solids. Spacetime, then, might be more like a simple stir fry than an elaborate wedding cake. Moreover, unlike other approaches to quantum gravity our recipe is very robust. When we vary the details in our simulations, the result hardly changes. This robustness gives reason to believe we are on the right track. If the outcome were sensitive to where we put down each piece of this enormous ensemble, we could generate an enormous number of baroque shapes, each a priori equally likely to occur—so we would lose all explanatory power for why the universe turned out as it did. Similar mechanisms of self-assembly and self-organization occur across physics, biology and other fields of science. A beautiful example is the behavior of large flocks of birds, such as European starlings. Individual birds interact only with a small number of nearby birds; no leader tells them what to do. Yet the flock still forms and moves as a whole. The flock possesses collective, or emergent, properties that are not obvious in each bird's behavior. A Brief History of Quantum Gravity Past attempts to explain the quantum structure of spacetime as a process of emergence had only limited success. They were rooted in Euclidean quantum gravity, a research program initiated at the end of the 1970s and popularized by physicist Stephen Hawking's best-selling book A Brief History of Time. It is based on a fundamental principle from quantum mechanics: superposition. Any object, whether a classical or quantum one, is in a certain state—characterizing its position and velocity, say. But whereas the state of a classical object can be described by a unique set of numbers, the state of a quantum object is far richer. It is the sum, or superposition, of all possible classical states. For instance, a classical billiard ball moves along a single trajectory with a precise position and velocity at all times. That would not be a good description for how the much smaller electron moves. Its motion is described by quantum laws, which imply that it can exist simultaneously in a wide range of positions and velocities. When an electron travels from point A to point B in the absence of any external forces, it does not just take the straight line between A and B but all available routes simultaneously. This qualitative picture of all possible electron paths conspiring together translates into the precise mathematical prescription of a quantum superposition, formulated by Nobel laureate Richard Feynman, which is a weighted average of all these distinct possibilities. With this prescription, one can compute the probability of finding the electron in any particular range of positions and velocities away from the straight path that we would expect if the electrons followed the laws of classical mechanics. What makes the particles' behavior distinctly quantum mechanical are the deviations from a single sharp trajectory, called quantum fluctuations. The smaller the size of a physical system one considers, the more important the quantum fluctuations become. 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