astro
07-04-2005, 12:48 PM
The Dimensions Are Indeed Moving!!!
LEE SMOLIN says so!!!
LEE SMOLIN a theoretical physicist, is concerned with quantum gravity, "the name we give to the theory that unifies all the physics now under construction." More specifically, he is a co-inventor of an approach called loop quantum gravity. In 2001, he became a founding member and research physicist of the Perimeter Institute for Theoretical Physics, in Waterloo, Ontario. Smolin is the author of The Life of The Cosmos and Three Roads to Quantum Gravity.
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
Another reason that string theory cannot be the final word is that in string theory one studies strings moving in a fixed classical spacetime. Thus, string theory is what we call a background-dependent approach. It means that one defines the strings as moving in a fixed space and time. This may be a useful approximation, but it cannot be the fundamental theory. One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the unprecedented accuracy of eleven decimal places—tells us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up. The fundamental theory must unify quantum theory with a completely dynamical description of space and time. It must be what we call a background-independent theory. Loop quantum gravity is such a one; string theory is not.
The debate between proponents of background-dependent and background independent theories is in fact just the modern version of an ancient debate. Since the Greeks, the argument has raged between those who believed that space and time have an eternally fixed, absolute character and those who thought space and time are no more than relations between events that themselves evolve in time. Plato, Aristotle, and Newton were absolutists. Heraclites, Democritus, Leibniz, Mach, and Einstein were relationalists. When we demand that the quantum theory of gravity be background-independent, we are saying we believe that the triumph that general relativity represented for the relational point of view is final and will not be reversed.
Much of the argument between string and loop theorists is a continuation of this debate. Most string theorists were trained as elementary-particle physicists and worked their whole lives in a single fixed spacetime. Many of them have never even heard of the relational/absolute debate, which is the basic historical and philosophical context for Einstein's work. Most people who work in loop quantum gravity do so because at some point in their education they understood the relational, dynamical character of spacetime as described in general relativity, and they believe in it. They don't work on string theory because they cannot take seriously any candidate for a quantum theory of gravity that is background-dependent and hence loses (or at best hides) the relational, dynamical character of space and time.
Similarly, at first string theorists were resistant to the idea that the fundamental theory must be background-independent. However, I think that by now almost all string theorists have come around. They did so because there are reasons internal to string theory to believe that the fundamental theory must be background-independent. This is because string theory turned out to be non-unique. While the original hope, back in the 1980s, was that mathematical consistency would suffice to determine the unified theory, it turns out that in fact there are a huge number of equally consistent string theories. Each is as consistent as any other and each depends on a different choice of fixed background. Further, in spite of the huge numbers of string theories we know about, none of them agree with observations on the three points I mentioned above.
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
LEE SMOLIN says so!!!
LEE SMOLIN a theoretical physicist, is concerned with quantum gravity, "the name we give to the theory that unifies all the physics now under construction." More specifically, he is a co-inventor of an approach called loop quantum gravity. In 2001, he became a founding member and research physicist of the Perimeter Institute for Theoretical Physics, in Waterloo, Ontario. Smolin is the author of The Life of The Cosmos and Three Roads to Quantum Gravity.
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
Another reason that string theory cannot be the final word is that in string theory one studies strings moving in a fixed classical spacetime. Thus, string theory is what we call a background-dependent approach. It means that one defines the strings as moving in a fixed space and time. This may be a useful approximation, but it cannot be the fundamental theory. One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the unprecedented accuracy of eleven decimal places—tells us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up. The fundamental theory must unify quantum theory with a completely dynamical description of space and time. It must be what we call a background-independent theory. Loop quantum gravity is such a one; string theory is not.
The debate between proponents of background-dependent and background independent theories is in fact just the modern version of an ancient debate. Since the Greeks, the argument has raged between those who believed that space and time have an eternally fixed, absolute character and those who thought space and time are no more than relations between events that themselves evolve in time. Plato, Aristotle, and Newton were absolutists. Heraclites, Democritus, Leibniz, Mach, and Einstein were relationalists. When we demand that the quantum theory of gravity be background-independent, we are saying we believe that the triumph that general relativity represented for the relational point of view is final and will not be reversed.
Much of the argument between string and loop theorists is a continuation of this debate. Most string theorists were trained as elementary-particle physicists and worked their whole lives in a single fixed spacetime. Many of them have never even heard of the relational/absolute debate, which is the basic historical and philosophical context for Einstein's work. Most people who work in loop quantum gravity do so because at some point in their education they understood the relational, dynamical character of spacetime as described in general relativity, and they believe in it. They don't work on string theory because they cannot take seriously any candidate for a quantum theory of gravity that is background-dependent and hence loses (or at best hides) the relational, dynamical character of space and time.
Similarly, at first string theorists were resistant to the idea that the fundamental theory must be background-independent. However, I think that by now almost all string theorists have come around. They did so because there are reasons internal to string theory to believe that the fundamental theory must be background-independent. This is because string theory turned out to be non-unique. While the original hope, back in the 1980s, was that mathematical consistency would suffice to determine the unified theory, it turns out that in fact there are a huge number of equally consistent string theories. Each is as consistent as any other and each depends on a different choice of fixed background. Further, in spite of the huge numbers of string theories we know about, none of them agree with observations on the three points I mentioned above.
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html