Dec 4, 2015 09:11 AM
Controversial experiment sees no evidence that the universe is a hologram
http://news.sciencemag.org/physics/2015/...e-hologram
EXCERPT: It's a classic underdog story: Working in a disused tunnel with a couple of lasers and a few mirrors, a plucky band of physicists dreamed up a way to test one of the wildest ideas in theoretical physics—a notion from the nearly inscrutable realm of "string theory" that our universe may be like an enormous hologram. However, science doesn't indulge sentimental favorites. After years of probing the fabric of spacetime for a signal of the "holographic principle," researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, have come up empty, as they will report tomorrow at the lab. The null result won't surprise many people, as some of the inventors of the principle had complained that the experiment, the $2.5 million Fermilab Holometer, couldn't test it....
Physicists confirm that time moves forward even in the quantum world
http://www.sciencealert.com/physicists-c...ntum-world
EXCERPT: For the first time, an experiment has confirmed that the laws of thermodynamics hold true even at the quantum level – which means that even in the quantum world, you can’t unspill that glass of milk. The reason time runs the way it does in our everyday lives is because of the second law of thermodynamics, which states that over time all systems become more disordered, or increase in entropy. And that process is irreversible, which is why time only moves forward. But theoretical physicists had predicted that on the quantum level, the process might go both ways. That’s because when you start dealing with really, really small particles, the laws of physics – such as the Schrödinger equation – are 'time-symmetric' or reversible. "In theory, forward and backward microscopic processes are indistinguishable," writes Lisa Zyga for Phys.org. Now physicists led by the Federal University of ABC in Brazil have performed an experiment that confirms that those theories don’t match up with the reality, with thermodynamic processes remaining irreversible even in quantum systems. But they still don’t understand why that’s the case....
[Clarifying] Copenhagen interpretation of quantum mechanics
http://motls.blogspot.ca/2011/05/copenha...antum.html
EXCERPT: [...] The Copenhagen folks were very carefully applying positivism. That means that they refused to talk about properties of physical systems that can't be measured unless some observations or consistency of the predictions make it necessary to talk about them - which is an attitude that became essential with the birth of quantum mechanics. In this sense, they uniformly rejected the assumption of "realism". If the observations are described by a framework that doesn't contain any "real and objective" things or properties before the measurement but if the measurement may be predicted, then it is how the things should be.
Lots of fringe stuff, garbage, and crackpottery was later written by various people who weren't really part of the Copenhagen school of thought but who found it convenient to abuse the famous brand. That's why one can also hear that the Copenhagen school may (or even must) interpret the wave function as a real wave that collapses much like a skyscraper when it's hit by an aircraft on 9/11.
But nothing like that has ever been a part of the Copenhagen school of thought. If you open any complete enough description of the Copenhagen interpretation or if you look at Bohr's or Heisenberg's own texts, you will invariably see something like the following six principles:
(1) A system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system. (Heisenberg)
(2) The description of nature is essentially probabilistic, with the probability of an event related to the square of the amplitude of the wave function related to it. (The Born rule, after Max Born)
(3) It is not possible to know the value of all the properties of the system at the same time; those properties that are not known with precision must be described by probabilities. (Heisenberg's uncertainty principle)
(4) Matter exhibits a wave–particle duality. An experiment can show the particle-like properties of matter, or the wave-like properties; in some experiments both of these complementary viewpoints must be invoked to explain the results, according to the complementarity principle of Niels Bohr.
(5) Measuring devices are essentially classical devices, and measure only classical properties such as position and momentum.
(6) he quantum mechanical description of large systems will closely approximate the classical description. (The correspondence principle of Bohr and Heisenberg)
Note that the very first point says that the wave function is a collection of numbers describing subjective knowledge. That doesn't mean that in practice, everything will be always subjective - or whatever the spiritual people have attributed to quantum mechanics. Of course that constant interactions between parts of the world - and different people - pretty much guarantee that they have to agree about many "objective properties". But as a matter of principle, this rule is important for quantum mechanics and Werner Heisenberg has never left any doubts that this is how one had to interpret it.
Heisenberg would often describe his interpretation of the wave function using a story about a guy who fled the city and we don't know where he is but when they tell us at the airport they saw him 10 minutes ago, our wave function describing his position immediately collapses to a smaller volume, and so on. This "collapse" may occur faster than light because no real object is "collapsing": it's just a state of our knowledge in our brain.
In practice, everyone can use pretty much the same wave function. But in principle, the wave function is subjective. If the observer A looks at a quantum system S in the lab, he will use a wave function where S has a well-defined sharp spin eigenstate as soon as the spin of S is measured by A. However, B who studies the whole system A+S confined in a lab won't "make" any collapse, and he evolves both S and A into linear superpositions until B measures the system. So A and B will have different wave functions during much of the experiment. It's consistent for B to imagine that A had seen a well-defined property of S before it was measured by B - but B won't increase his knowledge in any way by this assumption, so it is useless. If he applied this "collapsed" assumption to purely coherent quantum systems, he would obtain totally wrong predictions.
So the wave function is surely subjective if one wants to obtain a universal description of the world. It's a collection of probability amplitudes that may be combined in various ways, before the squared absolute values of the combinations are interpretated as probabilities. All probabilities of physically meaningful events may be calculated in this way - as the squared absolute value of some linear combination of the probability amplitudes.
No collapse in the principles of the interpretation
A widely propagated myth is that the Copenhagen interpretation is all about the "collapse". However, if you look at the six principles above, there is not even a glimpse of a comment about a "collapse" because it's not needed. The notion of an objective collapse was introduced by John von Neumann in 1932 and he was clearly not a part of the Copenhagen school of thought anymore. Comments that Heisenberg later switched to an "objective wave function" or an "objective collapse" are untrue, and even if these legends were true, these new opinions wouldn't be a part of the Copenhagen interpretation and, more importantly, they wouldn't be valid.
Because the wave function is subjective, see rule 1, everything that happens with the wave function has to be subjective as well.
Consider a cat. You will evolve a wave function and the final state is "0.6 alive + 0.8i dead." (The fact that the actual state is not pure in any useful sense will be discussed later.) When you observe the cat, it's - unexpectedly - alive. Once you know that the cat is alive, it becomes a fact. You have to use this new knowledge in all your subsequent predictions and retrodictions if they're supposed to be any accurate. Or valid, for that matter. I think that the previous statement is totally obvious....
http://news.sciencemag.org/physics/2015/...e-hologram
EXCERPT: It's a classic underdog story: Working in a disused tunnel with a couple of lasers and a few mirrors, a plucky band of physicists dreamed up a way to test one of the wildest ideas in theoretical physics—a notion from the nearly inscrutable realm of "string theory" that our universe may be like an enormous hologram. However, science doesn't indulge sentimental favorites. After years of probing the fabric of spacetime for a signal of the "holographic principle," researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, have come up empty, as they will report tomorrow at the lab. The null result won't surprise many people, as some of the inventors of the principle had complained that the experiment, the $2.5 million Fermilab Holometer, couldn't test it....
Physicists confirm that time moves forward even in the quantum world
http://www.sciencealert.com/physicists-c...ntum-world
EXCERPT: For the first time, an experiment has confirmed that the laws of thermodynamics hold true even at the quantum level – which means that even in the quantum world, you can’t unspill that glass of milk. The reason time runs the way it does in our everyday lives is because of the second law of thermodynamics, which states that over time all systems become more disordered, or increase in entropy. And that process is irreversible, which is why time only moves forward. But theoretical physicists had predicted that on the quantum level, the process might go both ways. That’s because when you start dealing with really, really small particles, the laws of physics – such as the Schrödinger equation – are 'time-symmetric' or reversible. "In theory, forward and backward microscopic processes are indistinguishable," writes Lisa Zyga for Phys.org. Now physicists led by the Federal University of ABC in Brazil have performed an experiment that confirms that those theories don’t match up with the reality, with thermodynamic processes remaining irreversible even in quantum systems. But they still don’t understand why that’s the case....
[Clarifying] Copenhagen interpretation of quantum mechanics
http://motls.blogspot.ca/2011/05/copenha...antum.html
EXCERPT: [...] The Copenhagen folks were very carefully applying positivism. That means that they refused to talk about properties of physical systems that can't be measured unless some observations or consistency of the predictions make it necessary to talk about them - which is an attitude that became essential with the birth of quantum mechanics. In this sense, they uniformly rejected the assumption of "realism". If the observations are described by a framework that doesn't contain any "real and objective" things or properties before the measurement but if the measurement may be predicted, then it is how the things should be.
Lots of fringe stuff, garbage, and crackpottery was later written by various people who weren't really part of the Copenhagen school of thought but who found it convenient to abuse the famous brand. That's why one can also hear that the Copenhagen school may (or even must) interpret the wave function as a real wave that collapses much like a skyscraper when it's hit by an aircraft on 9/11.
But nothing like that has ever been a part of the Copenhagen school of thought. If you open any complete enough description of the Copenhagen interpretation or if you look at Bohr's or Heisenberg's own texts, you will invariably see something like the following six principles:
(1) A system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system. (Heisenberg)
(2) The description of nature is essentially probabilistic, with the probability of an event related to the square of the amplitude of the wave function related to it. (The Born rule, after Max Born)
(3) It is not possible to know the value of all the properties of the system at the same time; those properties that are not known with precision must be described by probabilities. (Heisenberg's uncertainty principle)
(4) Matter exhibits a wave–particle duality. An experiment can show the particle-like properties of matter, or the wave-like properties; in some experiments both of these complementary viewpoints must be invoked to explain the results, according to the complementarity principle of Niels Bohr.
(5) Measuring devices are essentially classical devices, and measure only classical properties such as position and momentum.
(6) he quantum mechanical description of large systems will closely approximate the classical description. (The correspondence principle of Bohr and Heisenberg)
Note that the very first point says that the wave function is a collection of numbers describing subjective knowledge. That doesn't mean that in practice, everything will be always subjective - or whatever the spiritual people have attributed to quantum mechanics. Of course that constant interactions between parts of the world - and different people - pretty much guarantee that they have to agree about many "objective properties". But as a matter of principle, this rule is important for quantum mechanics and Werner Heisenberg has never left any doubts that this is how one had to interpret it.
Heisenberg would often describe his interpretation of the wave function using a story about a guy who fled the city and we don't know where he is but when they tell us at the airport they saw him 10 minutes ago, our wave function describing his position immediately collapses to a smaller volume, and so on. This "collapse" may occur faster than light because no real object is "collapsing": it's just a state of our knowledge in our brain.
In practice, everyone can use pretty much the same wave function. But in principle, the wave function is subjective. If the observer A looks at a quantum system S in the lab, he will use a wave function where S has a well-defined sharp spin eigenstate as soon as the spin of S is measured by A. However, B who studies the whole system A+S confined in a lab won't "make" any collapse, and he evolves both S and A into linear superpositions until B measures the system. So A and B will have different wave functions during much of the experiment. It's consistent for B to imagine that A had seen a well-defined property of S before it was measured by B - but B won't increase his knowledge in any way by this assumption, so it is useless. If he applied this "collapsed" assumption to purely coherent quantum systems, he would obtain totally wrong predictions.
So the wave function is surely subjective if one wants to obtain a universal description of the world. It's a collection of probability amplitudes that may be combined in various ways, before the squared absolute values of the combinations are interpretated as probabilities. All probabilities of physically meaningful events may be calculated in this way - as the squared absolute value of some linear combination of the probability amplitudes.
No collapse in the principles of the interpretation
A widely propagated myth is that the Copenhagen interpretation is all about the "collapse". However, if you look at the six principles above, there is not even a glimpse of a comment about a "collapse" because it's not needed. The notion of an objective collapse was introduced by John von Neumann in 1932 and he was clearly not a part of the Copenhagen school of thought anymore. Comments that Heisenberg later switched to an "objective wave function" or an "objective collapse" are untrue, and even if these legends were true, these new opinions wouldn't be a part of the Copenhagen interpretation and, more importantly, they wouldn't be valid.
Because the wave function is subjective, see rule 1, everything that happens with the wave function has to be subjective as well.
Consider a cat. You will evolve a wave function and the final state is "0.6 alive + 0.8i dead." (The fact that the actual state is not pure in any useful sense will be discussed later.) When you observe the cat, it's - unexpectedly - alive. Once you know that the cat is alive, it becomes a fact. You have to use this new knowledge in all your subsequent predictions and retrodictions if they're supposed to be any accurate. Or valid, for that matter. I think that the previous statement is totally obvious....