AT ITS deepest level, nature is random and unpredictable. That, most physicists would say, is the unavoidable lesson of quantum theory. Try to track the location of an electron and you'll find only a probability that it is here or there. Measure the spin of an atom and all you get is a 50:50 chance that it is up or down. Watch a photon hit a glass plate and it will either pass through or be reflected, but it's impossible to know which without measuring it.
Where does this randomness come from? Before quantum theory, physicists could believe in determinism, the idea of a world unfolding with precise mathematical certainty. Since then, however, the weird probabilistic behaviour of the quantum world has rudely intruded, and the mainstream view is that this uncertainty is a fundamental feature of everything from alpha particles to Z bosons. Indeed, most quantum researchers celebrate the notion that pure chance lies at the foundations of the universe.
However, a sizeable minority of physicists have long been pushing entirely the opposite view. They remain unconvinced that quantum theory depends on pure chance, and they shun the philosophical contortions of quantum weirdness. The world is not inherently random, they say, it only appears that way. Their response has been to develop quantum models that are deterministic, and that describe a world that has "objective" properties, whether or not we measure them. The problem is that such models have had flaws that many physicists consider fatal, such as inconsistencies with established theories.
Until now, that is. A series of recent papers show that the idea of a deterministic and objective universe is alive and kicking. At the very least, the notion that quantum theory put the nail in the coffin of determinism has been wildly overstated, says physicist Sheldon Goldstein of Rutgers University in New Jersey. He and a cadre of like-minded physicists have been pursuing an alternative quantum theory known as Bohmian mechanics, in which particles follow precise trajectories or paths through space and time, and the future is perfectly predictable from the past. "It's a reformulation of quantum theory that is not at all congenial to supposedly deep quantum philosophy," says Goldstein. "It's precise and objective - and deterministic."
If these researchers can convince their peers, most of whom remain sceptical, it would be a big step towards rebuilding the universe as Einstein wanted, one in which "God does not play dice". It could also trigger a search for evidence of physics beyond quantum theory, paving the way for a better and more intuitive theory of how the universe works. Nearly a century after the discovery of quantum weirdness, it seems determinism may be back.
The debate over quantum theory and determinism started in the 1920s, when physicists Niels Bohr and Werner Heisenberg suggested that the unpredictability of quantum phenomena reflected an inherent fuzziness in nature. Einstein and others countered that the unpredictability might instead reflect nothing more than a lack of adequate knowledge. In principle you could predict the outcome of a coin flip, they argued, if you had perfect knowledge of the coin's initial state and surroundings.
At the historic 1927 Solvay meeting in Brussels, physicist Louis de Broglie tried to further this idea, showing how quantum randomness might arise in a non-mysterious way. He suggested that quantum particles show wave-like phenomena because they are accompanied by "pilot waves" that influence their motion in just the right way as to make them obey the Schrödinger wave equation, a cornerstone of quantum theory. However, most dismissed de Broglie's ideas, citing in particular shortcomings pointed out by the physicist Wolfgang Pauli.
Yet de Broglie's ideas would not go quietly. In the early 1950s, physicist David Bohm developed a more consistent version of the pilot-wave model, one based on the same equations as ordinary quantum theory but offering a different interpretation of them. Bohm found buried within those equations a close link to the mathematics of classical physics, which is based on Newton's laws of motion. Bohmian mechanics asserts that the outcome of an experiment isn't truly random, but is determined by the values of certain "hidden variables". For instance, in quantum theory two electrons may be "entangled" such that their states appear to have a kind of spooky link; measuring the spin of one determines the spin of the other, say. Bohm's theory suggests that they share a hidden variable governing spin. The theory also shows how probabilistic quantum measurements can always arise from specific particle trajectories.
Take a key puzzle in quantum theory: explaining how a beam of particles passing through two slits in a screen will create a wave-like interference pattern, even if the particles are sent one at a time. While mainstream quantum theory insists that you can't give any account of exactly how a given particle moves, Bohmian mechanics can. It suggests that a quantum wave associated with each particle goes through both slits and sets up a pattern of constructive and destructive interference - just like the bright and dark interference bands produced with light. This wave pattern then acts on the particles, driving them towards the "bright" bands of constructive interference (see Diagram).
In the Bohmian view, the statistical interference pattern arises from individual particles following distinct trajectories. This does away with any inherent quantum fuzziness, and shows that it's still possible to believe not only in determinism but also in the intuitive notion that particles really act like particles, having definite positions at all times. "The wave function choreographs the motion of the particles," says physicist Detlef Dürr of Ludwig Maximilian University in Munich, Germany. "As a result, while everything is deterministic, the universe evolves in such way that the appearance of randomness emerges, and precisely as described by the quantum formalism."
Still, the majority of physicists didn't take Bohmian mechanics very seriously, often suggesting that it was contrived and afflicted with technical problems. One of the biggest of these was that Bohm's original model cannot cope with the physics of Einstein's special relativity. It worked only for particles with low energies and speeds, where the number of particles in any process remains fixed. At higher energies, relativistic processes routinely create and destroy particles, as when an electron and positron annihilate one another, turning their energy into light. The simplest version of Bohm's theory could not handle such processes.
So Goldstein and others have tried to develop modified versions of the theory that can. Their work began in the 1980s and 90s as part of an effort to develop Bohmian models that describe not only quantum particles but quantum fields as well, which provide the basic framework of all modern physics. In these models, the universe consists both of particles following precise trajectories and of continuous fields that, like classical magnetic or electric fields, also evolve in a deterministic way. Over the past decade, Goldstein, working with Dürr and physicist Nino Zanghi of the University of Genoa in Italy, has shown that this picture gives a consistent view of relativistic particle processes, while reproducing the accurate predictions of quantum field theory (Physical Review Letters, vol 93, p 090402).
The most promising result to come out of this framework was published last year by Ward Struyve and Hans Westman, both at the Perimeter Institute in Waterloo, Ontario, Canada. They developed a Bohmian model that matches one of the most accurate theories in the history of science - quantum electrodynamics, the theory of light and its interactions with charged particles. In fact, Struyve and Westman found that a number of Bohmian models can easily account for all such phenomena, while remaining fully deterministic (Proceedings of the Royal Society A, vol 463, p 3115).
The researchers have not yet ironed out all the wrinkles. In particular, critics contend that Bohmian models still don't satisfy the fundamental principle of relativity, that all frames of reference are on an equal footing. Nevertheless the models appear to have no serious difficulty in coping with particles being created or destroyed, as many physicists had thought they would. "This is real progress that's happened over the past decade," says Tim Maudlin, a philosopher of physics at Rutgers. "The main objections to the theory have now either been addressed, turned out not to be serious or represent issues for the standard theory as much as the Bohm theory."
Goldstein and others have also solved another nagging problem for Bohmian models: elucidating how a deterministic theory can give rise to the fuzziness observed in quantum experiments in the first place. The uncertainty principle of quantum mechanics states that measuring the position of a quantum particle limits your knowledge of its momentum, and vice versa. The standard explanation is that the particle's state is undetermined until you measure it, but in Bohmian mechanics the state is always well defined. The trick, Goldstein says, is that measuring one variable stirs up uncertainty in the other due to interactions between the measuring device and the particle, in a way that matches the uncertainty principle.
Even so, most physicists are not yet ready to embrace the new models, because one crucial problem remains: Bohmian theory, critics point out, doesn't make any predictions that differ from those of ordinary quantum mechanics. "The theory is successful only because it keeps standard wave mechanics unchanged,"
...
******************** Even so, most physicists are not yet ready to embrace the new models, because one crucial problem remains: Bohmian theory, critics point out, doesn't make any predictions that differ from those of ordinary quantum mechanics. "The theory is successful only because it keeps standard wave mechanics unchanged," says Dieter Zeh of the University of Heidelberg in Germany. He adds that the rest of the theory is biased towards the ideas of classical physics and is "observationally meaningless".
That objection isn't really fair, say Bohmian supporters. After all, one might equally argue that the standard theory doesn't go beyond Bohm's theory. "If some historical circumstances had been only slightly different," says physicist Hrvoje Nikolic of the Rudjer Boskovic Institute in Zagreb, Croatia, "then it would have been very likely that Bohm's deterministic interpretation would have been proposed and accepted first, and would be dominating today."
"If historical circumstances had been slightly different, then the deterministic view of the universe would be dominating today"Philosopher of science Arthur Fine of the University of Washington in Seattle says that ideological objections, not technical ones, have been the main factor in the reluctance to accept Bohmian models. "There are some real criticisms one could raise," he says, "but these aren't the ones you find in the physics literature over the past 80 years." That's why he doubts further theoretical development of Bohmian mechanics will change physicists' minds. "Only new experimental results will do that," he says.
Such experiments might seem impossible, as Bohmian models are supposed to make all the same predictions as ordinary quantum theory. Yet some physicists, such as Antony Valentini of the Perimeter Institute, are now suggesting that the form of Bohmian models naturally points to ways in which the universe might depart from the standard predictions - with experimentally observable consequences. *************************
This is really what it comes down to. Either a model has some value in enabling us to make better predictions, or it doesn't. If the efforts to achieve an empirical distinction are fruitless, what then? Doesn't it tell us that we are creating an artificial distinction---that just like the cat, both dead and alive, the universe is both random and deterministic, and we can't open the box?
On Sun, 23 Mar 2008 09:00:45 -0700 (PDT), tg <tgdenn...@earthlink.net> wrote: >On Mar 23, 10:25 am, Sir Frederick <mmcne...@fuzzysys.com> wrote: >> http://www.newscientist.com/article/mg19726485.700 >> Quantum randomness may not be random >> 22 March 2008 >> From New Scientist Print Edition. Subscribe and get 4 free issues. >> Mark Buchanan
>******************** >Even so, most physicists are not yet ready to embrace the new models, >because one crucial problem remains: Bohmian theory, critics >point out, doesn't make any predictions that differ from those of >ordinary quantum mechanics. "The theory is successful only because >it keeps standard wave mechanics unchanged," says Dieter Zeh of the >University of Heidelberg in Germany. He adds that the rest of >the theory is biased towards the ideas of classical physics and is >"observationally meaningless".
>That objection isn't really fair, say Bohmian supporters. After all, >one might equally argue that the standard theory doesn't go >beyond Bohm's theory. "If some historical circumstances had been only >slightly different," says physicist Hrvoje Nikolic of the >Rudjer Boskovic Institute in Zagreb, Croatia, "then it would have been >very likely that Bohm's deterministic interpretation would >have been proposed and accepted first, and would be dominating >today."
>"If historical circumstances had been slightly different, then the >deterministic view of the universe would be dominating >today"Philosopher of science Arthur Fine of the University of >Washington in Seattle says that ideological objections, not technical >ones, have been the main factor in the reluctance to accept Bohmian >models. "There are some real criticisms one could raise," he >says, "but these aren't the ones you find in the physics literature >over the past 80 years." That's why he doubts further >theoretical development of Bohmian mechanics will change physicists' >minds. "Only new experimental results will do that," he says.
>Such experiments might seem impossible, as Bohmian models are supposed >to make all the same predictions as ordinary quantum theory. >Yet some physicists, such as Antony Valentini of the Perimeter >Institute, are now suggesting that the form of Bohmian models >naturally points to ways in which the universe might depart from the >standard predictions - with experimentally observable >consequences. >*************************
>This is really what it comes down to. Either a model has some value in >enabling us to make better predictions, or it doesn't. If the efforts >to achieve an empirical distinction are fruitless, what then? Doesn't >it tell us that we are creating an artificial distinction---that just >like the cat, both dead and alive, the universe is both random and >deterministic, and we can't open the box?
>-tg
With the recent Bohm posts, I thought another might be appropriate. I suspect that what is "really" going on is beyond our cognitive ability, but it must be fun trying. But then look at our theories, models and understandings 400 years ago. I have read enough science fiction to make the present situation rather passe. Immortality would interest me more.
<mmcne...@fuzzysys.com> wrote: >http://www.newscientist.com/article/mg19726485.700 >Quantum randomness may not be random >22 March 2008 >From New Scientist Print Edition. Subscribe and get 4 free issues. >Mark Buchanan
>AT ITS deepest level, nature is random and unpredictable. That, most physicists would say, is the unavoidable lesson of quantum >theory. Try to track the location of an electron and you'll find only a probability that it is here or there. Measure the spin of an >atom and all you get is a 50:50 chance that it is up or down. Watch a photon hit a glass plate and it will either pass through or be >reflected, but it's impossible to know which without measuring it.
>Where does this randomness come from? Before quantum theory, physicists could believe in determinism, the idea of a world unfolding >with precise mathematical certainty. Since then, however, the weird probabilistic behaviour of the quantum world has rudely >intruded, and the mainstream view is that this uncertainty is a fundamental feature of everything from alpha particles to Z bosons. >Indeed, most quantum researchers celebrate the notion that pure chance lies at the foundations of the universe.
>However, a sizeable minority of physicists have long been pushing entirely the opposite view. They remain unconvinced that quantum >theory depends on pure chance, and they shun the philosophical contortions of quantum weirdness. The world is not inherently random, >they say, it only appears that way. Their response has been to develop quantum models that are deterministic, and that describe a >world that has "objective" properties, whether or not we measure them. The problem is that such models have had flaws that many >physicists consider fatal, such as inconsistencies with established theories.
>Until now, that is. A series of recent papers show that the idea of a deterministic and objective universe is alive and kicking. At >the very least, the notion that quantum theory put the nail in the coffin of determinism has been wildly overstated, says physicist >Sheldon Goldstein of Rutgers University in New Jersey. He and a cadre of like-minded physicists have been pursuing an alternative >quantum theory known as Bohmian mechanics, in which particles follow precise trajectories or paths through space and time, and the >future is perfectly predictable from the past. "It's a reformulation of quantum theory that is not at all congenial to supposedly >deep quantum philosophy," says Goldstein. "It's precise and objective - and deterministic."
>If these researchers can convince their peers, most of whom remain sceptical, it would be a big step towards rebuilding the universe >as Einstein wanted, one in which "God does not play dice". It could also trigger a search for evidence of physics beyond quantum >theory, paving the way for a better and more intuitive theory of how the universe works. Nearly a century after the discovery of >quantum weirdness, it seems determinism may be back.
>The debate over quantum theory and determinism started in the 1920s, when physicists Niels Bohr and Werner Heisenberg suggested that >the unpredictability of quantum phenomena reflected an inherent fuzziness in nature. Einstein and others countered that the >unpredictability might instead reflect nothing more than a lack of adequate knowledge. In principle you could predict the outcome of >a coin flip, they argued, if you had perfect knowledge of the coin's initial state and surroundings.
>At the historic 1927 Solvay meeting in Brussels, physicist Louis de Broglie tried to further this idea, showing how quantum >randomness might arise in a non-mysterious way. He suggested that quantum particles show wave-like phenomena because they are >accompanied by "pilot waves" that influence their motion in just the right way as to make them obey the Schrödinger wave equation, a >cornerstone of quantum theory. However, most dismissed de Broglie's ideas, citing in particular shortcomings pointed out by the >physicist Wolfgang Pauli.
>Yet de Broglie's ideas would not go quietly. In the early 1950s, physicist David Bohm developed a more consistent version of the >pilot-wave model, one based on the same equations as ordinary quantum theory but offering a different interpretation of them. Bohm >found buried within those equations a close link to the mathematics of classical physics, which is based on Newton's laws of motion. >Bohmian mechanics asserts that the outcome of an experiment isn't truly random, but is determined by the values of certain "hidden >variables". For instance, in quantum theory two electrons may be "entangled" such that their states appear to have a kind of spooky >link; measuring the spin of one determines the spin of the other, say. Bohm's theory suggests that they share a hidden variable >governing spin. The theory also shows how probabilistic quantum measurements can always arise from specific particle trajectories.
>Take a key puzzle in quantum theory: explaining how a beam of particles passing through two slits in a screen will create a >wave-like interference pattern, even if the particles are sent one at a time. While mainstream quantum theory insists that you can't >give any account of exactly how a given particle moves, Bohmian mechanics can. It suggests that a quantum wave associated with each >particle goes through both slits and sets up a pattern of constructive and destructive interference - just like the bright and dark >interference bands produced with light. This wave pattern then acts on the particles, driving them towards the "bright" bands of >constructive interference (see Diagram).
>In the Bohmian view, the statistical interference pattern arises from individual particles following distinct trajectories. This >does away with any inherent quantum fuzziness, and shows that it's still possible to believe not only in determinism but also in the >intuitive notion that particles really act like particles, having definite positions at all times. "The wave function choreographs >the motion of the particles," says physicist Detlef Dürr of Ludwig Maximilian University in Munich, Germany. "As a result, while >everything is deterministic, the universe evolves in such way that the appearance of randomness emerges, and precisely as described >by the quantum formalism."
>Still, the majority of physicists didn't take Bohmian mechanics very seriously, often suggesting that it was contrived and afflicted >with technical problems. One of the biggest of these was that Bohm's original model cannot cope with the physics of Einstein's >special relativity. It worked only for particles with low energies and speeds, where the number of particles in any process remains >fixed. At higher energies, relativistic processes routinely create and destroy particles, as when an electron and positron >annihilate one another, turning their energy into light. The simplest version of Bohm's theory could not handle such processes.
>So Goldstein and others have tried to develop modified versions of the theory that can. Their work began in the 1980s and 90s as >part of an effort to develop Bohmian models that describe not only quantum particles but quantum fields as well, which provide the >basic framework of all modern physics. In these models, the universe consists both of particles following precise trajectories and >of continuous fields that, like classical magnetic or electric fields, also evolve in a deterministic way. Over the past decade, >Goldstein, working with Dürr and physicist Nino Zanghi of the University of Genoa in Italy, has shown that this picture gives a >consistent view of relativistic particle processes, while reproducing the accurate predictions of quantum field theory (Physical >Review Letters, vol 93, p 090402).
>The most promising result to come out of this framework was published last year by Ward Struyve and Hans Westman, both at the >Perimeter Institute in Waterloo, Ontario, Canada. They developed a Bohmian model that matches one of the most accurate theories in >the history of science - quantum electrodynamics, the theory of light and its interactions with charged particles. In fact, Struyve >and Westman found that a number of Bohmian models can easily account for all such phenomena, while remaining fully deterministic >(Proceedings of the Royal Society A, vol 463, p 3115).
>The researchers have not yet ironed out all the wrinkles. In particular, critics contend that Bohmian models still don't satisfy the >fundamental principle of relativity, that all frames of reference are on an equal footing. Nevertheless the models appear to have no >serious difficulty in coping with particles being created or destroyed, as many physicists had thought they would. "This is real >progress that's happened over the past decade," says Tim Maudlin, a philosopher of physics at Rutgers. "The main objections to the >theory have now either been addressed, turned out not to be serious or represent issues for the standard theory as much as the Bohm >theory."
>Goldstein and others have also solved another nagging problem for Bohmian models: elucidating how a deterministic theory can give >rise to the fuzziness observed in quantum experiments in the first place. The uncertainty principle of quantum mechanics states that >measuring the position of a quantum particle limits your knowledge of its momentum, and vice versa. The standard explanation is that >the particle's state is undetermined until you measure it, but in Bohmian mechanics the state is always well defined. The trick, >Goldstein says, is that measuring one variable stirs up uncertainty in the other due to interactions between the measuring device >and the particle, in a way that matches the uncertainty principle.
>Even so, most physicists are not yet ready to embrace the new models, because one crucial problem remains: Bohmian theory,
> On Sun, 23 Mar 2008 09:00:45 -0700 (PDT), tg <tgdenn...@earthlink.net> > wrote:
>>On Mar 23, 10:25 am, Sir Frederick <mmcne...@fuzzysys.com> wrote: >>> http://www.newscientist.com/article/mg19726485.700 >>> Quantum randomness may not be random >>> 22 March 2008 >>> From New Scientist Print Edition. Subscribe and get 4 free issues. >>> Mark Buchanan
>>******************** >>Even so, most physicists are not yet ready to embrace the new models, >>because one crucial problem remains: Bohmian theory, critics >>point out, doesn't make any predictions that differ from those of >>ordinary quantum mechanics. "The theory is successful only because >>it keeps standard wave mechanics unchanged," says Dieter Zeh of the >>University of Heidelberg in Germany. He adds that the rest of >>the theory is biased towards the ideas of classical physics and is >>"observationally meaningless".
>>That objection isn't really fair, say Bohmian supporters. After all, >>one might equally argue that the standard theory doesn't go >>beyond Bohm's theory. "If some historical circumstances had been only >>slightly different," says physicist Hrvoje Nikolic of the >>Rudjer Boskovic Institute in Zagreb, Croatia, "then it would have been >>very likely that Bohm's deterministic interpretation would >>have been proposed and accepted first, and would be dominating >>today."
>>"If historical circumstances had been slightly different, then the >>deterministic view of the universe would be dominating >>today"Philosopher of science Arthur Fine of the University of >>Washington in Seattle says that ideological objections, not technical >>ones, have been the main factor in the reluctance to accept Bohmian >>models. "There are some real criticisms one could raise," he >>says, "but these aren't the ones you find in the physics literature >>over the past 80 years." That's why he doubts further >>theoretical development of Bohmian mechanics will change physicists' >>minds. "Only new experimental results will do that," he says.
>>Such experiments might seem impossible, as Bohmian models are supposed >>to make all the same predictions as ordinary quantum theory. >>Yet some physicists, such as Antony Valentini of the Perimeter >>Institute, are now suggesting that the form of Bohmian models >>naturally points to ways in which the universe might depart from the >>standard predictions - with experimentally observable >>consequences. >>*************************
>>This is really what it comes down to. Either a model has some value in >>enabling us to make better predictions, or it doesn't. If the efforts >>to achieve an empirical distinction are fruitless, what then? Doesn't >>it tell us that we are creating an artificial distinction---that just >>like the cat, both dead and alive, the universe is both random and >>deterministic, and we can't open the box?
>>-tg
> With the recent Bohm posts, I thought another might be appropriate. > I suspect that what is "really" going on is beyond our > cognitive ability, but it must be fun trying. But then look at our > theories, models and understandings 400 years ago. > I have read enough science fiction to make the present situation > rather passe. Immortality would interest me more.
Perhaps space expands with an expanding consciousness while the universe remains the same? That would at least fit the empirical reality. I think we can rest assured that we will rationalize something.
New Scientist is in error what they present is NOT Bohm theory, what the Durr Group present {Sheldon Goldstein} is in fact Bells theory!
It is claimed that Bohm's theory would dispel the uncertainty principle and lead quantum physics into the realm of some kind of mechanistic certainty, well firstly the pilot wave theory put forward by Sheldon Goldstein et al is in fact a version preferred by the late John Bell and not the version championed by the late David Bohm {& Prof Basil Hiley}. While they start along similar lines, in that they both reject the notion that the unit observable is always the preferred determined observable, there are major differences. Bells theory is a first-order-in-time equation specifying the velocity of the particles in a system. The guidance formula is then taken as a new type of fundamental law. Newtonian accelerations, forces and potentials play no part in the description of the dynamics. Where as Bohm's theory (& de Broglie-Bohm) is a second-order theory in which the conceptual structure is predominantly that of (a constrained) classical mechanics. The basic differences from classical mechanics are expressed in terms of the restriction of the initial momentum, non- local potentials, forces and accelerations of quantum-mechanical origin. Now the later has had little press of late, yet there have been some powerfully insights from the formulism, such as the relation of the quantum potential to vacuum states, and more recently the work on Symplectic algebra and the wave equation. Thus leading to the discovery that the symplectic symmetry in Quantum theory is actually a double cover called a metaplectic group. Chief advocate of this approach to Bohm theory is the mathematician Maurice de Gosson, who's work not only embraces the uncertainty principle in relation to Bohm theory, but also shows that some kind of uncertainty principle may manifests itself on the classical level as well! Now contrary to myth this is in keeping with David Bohm's approach to uncertainty principle, for he envisioned that there maybe many levels to reality in which there could be an interplay between determinacy and indeterminacy. One thing that stands out from reading his work {1957-1993} is that David Bohm was in favour of non-mechanistic organic theory; and was strongly against any notion of mechanistic minimalism advocated by John Bell {& Sheldon Goldstein et al}.
