EPR, Aspect, Bell, and Understanding Quantum Weirdness

[Schneibster]

As long as you're at it, I would appreciate your thoughts (and anyone else's who actually understands what's goin' on) as to what you think of the "Afshar" experiment, especially with regard to his position that he has falsified both Copenhagen and many-worlds by demonstrating that both wave function and particle states exist simultaneously and can be observed existing in this universe.

I'll start far afield. This should be fun and informative.

First of all, let's understand that quantum mechanics makes some really strange claims about reality at its most basic level, but it also claims that reality changes when we deal with enough quanta that we have a statistical universe into the classical physics that we observe in everyday life. Basically, individual quanta behave in ways that are impossible for everyday objects, but it is necessary that quanta do so in order that everyday objects do what they do.

Physicists have been struggling with these concepts since the quantum theory was first proposed. It's worth taking a moment to understand what the quantum theory is. The quantum theory is a theory that energy doesn't just flow out continuously from objects that emit it, or into objects that absorb it; instead, it comes in little "packets," of a determined size, one at a time. Max Planck came up with this idea to explain the blackbody radiation, as follows: the blackbody radiation is emitted at all frequencies. Now, if all frequencies are emitted with equal probability, this would obviously result in hot objects immediately radiating all their energy in gamma rays, with almost nothing lower down, because so much energy would be so easily radiated. Lord Kelvin referred to this as the "ultraviolet catastrophe," because nothing of the kind is observed. It is therefore obvious that there must be a higher probability for lower frequencies to be emitted by a hot body, and a lower probability for higher ones. Max Planck theorized that hot atoms emitted quanta of radiation, and that the energy to build one had to be collected up before it could be emitted; obviously, the quantum would be more likely to be emitted when the amount of energy was still small, rather than waiting until there is enough to make a big one. This accounted for the observed radiation peak of blackbody radiation with temperature quite nicely, but many physicists ignored it because it didn't do much else.

All that changed when Einstein came along. Einstein realized while he was considering something called the photoelectric effect that this too could be explained by quanta. What happens is that metals exposed to light form an electric charge. Now, the amount of charge varies with the amount of light; but the voltage, which is the push behind the charge, does not. Why is this? Einstein theorized that electrons were being knocked out of the atoms and this formed the charge. But he went further. He showed that the fact that below a certain frequency of light, there was no effect, but as the frequency increased, at first there was a charge that varied with the amount of light, but whose voltage was very low; as the frequency increased, the voltage increased along with it, irrespective of the amount of charge built up. Einstein thought about this and realized that below a certain frequency, the quanta didn't have enough kick to knock the electrons free, but once they did, everything above that in terms of frequency would increase the velocity of the electrons, but not their quantity; only the light intensity would increase the quantity, and only the frequency would effect their voltage. Suddenly, instead of explaining only one phase of energy transmission, quantum theory explained both emission and absorption, and from there on, it was an accepted theory. Einstein won his Nobel Prize for this, not for Relativity.

Now, with all of this going on, Schroedinger had come up with wave functions, and Max Born had proposed that they represented the probability of the quantum being at a particular location or having a particular other parameter at a particular value. Soon, Louis de Broglie (pronounced "de-broy-ah" with the "ah" rather strangled and almost silent) proposed that not only energy had waves, but matter did too. Now, quantum theory was the theory of all matter and all energy, and the fun really started.

So the quantum theory is the theory that all matter and all energy are made up of indivisible elementary particles called "quanta," and that all of physics can be explained in terms of interactions of these quanta. So far it has been incredibly successful; only gravity remains outside its grasp. A "quantum mechanics" is a description of the interactions of various quanta with one another. Generally, the quantum mechanical theories are divided into one for each force; the mechanics describes the interaction of the quanta of that force with matter and with other forces' quanta. There is also a field theory for each force, with electromagnetism and gravity the best-understood ones; finally, for electromagnetism, there is also a quantum-field theory, the most advanced type of theory which can theoretically completely describe the interaction of the quanta of that force with all other quanta, and their operation as a field.

The whole of this edifice is generally described as "quantum mechanics" to non-physicists.

OK, so Heisenberg proposed that there were certain parameters of quanta that could not be simultaneously measured. At first, he was a proponent of the idea that this was because the measurement of one quantity would disturb the measurement of another, but soon the math told him and others that in fact, those other values simply didn't exist. It had to be that way.

Einstein hated this. He believed that this showed that quantum mechanics could not be a complete and consistent description of reality, rather an ironic position since he was one of the inventors of the theory and had won a Nobel Prize for it. He famously asked if the Moon existed when no one was looking at it, and went about trying to prove that Heisenberg and the others were wrong, and that these parameters had values even though we could not measure them. He came up with a series of gedankenexperimenten (thought or mind experiments) devised to show inconsistencies in quantum mechanics, or at least incompletenesses; Niels Bohr rather famously (at least in physics circles) managed to answer every one. The last and most effective of these, however, although Bohr had an answer, turned out to be deeper than anyone had expected. This one Einstein didn't invent on his own, but in collaboration with a pair of students named Podolsky and Rosen. The gedankenexperiment is therefore called the "EPR" experiment.

Here is a generalized and simplified explanation. Einstein, Podolsky, and Rosen proposed that one consider a pair of photons emitted simultaneously from an atom under very carefully controlled conditions. Because of these conditions, they could show that the spins of the two photons had to be opposite from one another, due to conservation of angular momentum; only in this way could they add up to zero and account for the lack of change in the momentum of the atom, a necessary condition.

One would expect that upon measuring the spin on one photon, one would always measure the opposite spin in the same axis on the other. This is easily accounted for by conservation of momentum. However, what EPR proposed involved far more than this.

Spin on different axes, as I have mentioned elsewhere, is complementary under uncertainty. What EPR proposed was that we measure the spin on different axes in the two photons. And what they asserted was that because the relation between the photons guaranteed opposite spins on the same axis, this was tantamount to measuring the spin of each photon on two axes, thus violating uncertainty. Furthermore, they asserted that any other position proposed to violate conservation of angular momentum, which would threaten the foundations of all of physics, not just quantum mechanics. They hoped by this to show that despite the fact that we could not measure them, conjugate parameters had real values.

Unbeknownst to either EPR or for that matter anyone else, they had devised the first hidden variables interpretation of quantum mechanics. This turned out to be very important later on.

Bohr answered that in fact, the parameters did not have any value, and asserted that their complementarity ensured it, and that since they could not be measured, there was no paradox. EPR responded that in the absence of any value until a measurement was carried out, there was no explanation other than what Einstein denigratingly called "spooky action at a distance" for the correlation between the spins when measurements were made in only one axis; for these parameters to have no value until they were measured, the second photon's spin had to "spring into existence" instantaneously at the moment the first was measured, in violation of locality and relativistic causality. Bohr basically ignored this.

Now, this led to a long debate over the meaning of wave function collapse, and the reality of the wave function, and the importance or lack thereof of measurement. All of this was lumped together in the minds of many physicists as the "measurement problem," and it was very much in vogue among the QM community to assert that all of this fussing over interpretations was meaningless and that the only proper description of reality lay in the equations. This debate continued until John Bell came on the scene in the 1960s.

Bell considered the situation in EPR carefully and realized that everyone had missed something important. What he showed was that although the spin on a second axis was not completely dependent upon the spin on the first, the probability of a certain spin was different if the other spin had some known value than if it did not have any value. In other words, Bell figured out how to tell whether the second spin had any value, or whether the distribution of spins on the first axis was totally random. This opened a whole new door for quantum mechanics, and has resulted in a complete questioning of the nature of reality as a result. This indirect measurement was and is not dependent upon uncertainty; it is, in fact, a direct measurement of the existence or nonexistence of a value, not a determination of that value in violation of the uncertainty principle.

This opened a door; physicists could not yet step through it, because the equipment wasn't sufficiently precise, but the door was open. And some of them knew it. A theoretical physicist named John Clauser began thinking about it, and pretty soon he was working with a friend named Michael Horne; before they were done, they had added Abner Shimony and Richard Holt, and they were doing EPR experiments in real labs instead of in their heads. These early attempts had holes in the data that were collected, and many physicists, both theorists and experimenters, dismissed their results. But they did accomplish an important task: they showed that there was a precise way to interpret Bell's Theorem that allowed the use of inequalities to measure the existence or non-existence of conjugate parameters under uncertainty. Bell had shown the way, and had said that he knew how to do this, but had not shown the way before he passed away (it was an untimely death). They had also constrained the situation fairly closely; there was only a small chance left that EPR were right, and that these parameters had real values. Their tests and inequalities are often called "CHSH" referring to their initials.

In 1982, the time was finally ripe. Alain Aspect had an idea for a test of the CHSH inequality derived from Bell's Theorem. He enacted it, and proved beyond reasonable doubt that in fact the distribution of values on the measured axis was inconsistent with the existence of a real value consistent with the value measured on the other photon for that axis. The value did not exist, in other words; and "spooky action at a distance" was in fact reality.

So something about EPR's assumptions had to be wrong. Of course, none of the three ever imagined that anyone would find a way to constrain the unmeasurable; Bell did that. And they never even dreamed that their experiment would become possible, and would actually measure the existence or non-existence of definite values where those values could not be measured. But their assumptions were these:
1. Locality, or relativistic causality; that is, that local causes have local effects, that no cause can have an effect outside its light cone.
2. Local realism; that is, that all parameters have actual values whether they can be measured or not; that reality has independent existence apart from whether it is (or can be) observed. This is very closely related to counter-factual definiteness.
3. Completeness; that is, that quantum mechanics is a complete description of quantum reality.
EPR, of course, hoped to prove that the last was inconsistent with the facts; however, Bell opened the way to differentiate between the first and second assumptions, and the third, and test them. And what Aspect showed, using CHSH's inequalities and Bell's test, was that either the first or second had to be wrong. Either influences could reach out beyond the light cone, or unmeasurable complementary values of measured parameters were not merely unmeasurable but non-existent.

One of the differences between the interpretations is how they deal with this situation. Consistent histories merely asserts that these values do not exist, just as strict Copenhagen and decoherent Copenhagen do; Many Worlds asserts that the parameters have values but only in other worlds where their complements were not measured; Bohm asserts that an actual physical but undetectable influence, the pilot wave, connects these events at superluminal speed, violating locality but invisibly so as not to violate causality; TI asserts that unmeasurable causality violations occur backwards in time. All of these violate either causality or local reality. It is a conjecture of mine that any interpretation consistent with measured reality will violate one of these two in some way.

I'll address the DCQE next, and finally move on to Afshar's experiment.

 

[Schneibster]

John Wheeler had an interesting idea; he proposed to show that quantum mechanics was nonlocal. He went about this using a gedankenexperiment he called the "delayed choice" experiment. But before I can tell you how it works, I first have to explain the dual slit experiments; this is a general class of experiments in which two or more slits are used to show interference in quantum phenomena. They are of varying weirdness, with Afshar's being one of the most interesting and strangest conducted to date. Richard Feynman said of the dual slit experiment done with electrons that one could observe the most important facts about quantum mechanics from it.

The story begins with Sir Isaac Newton. Despite his fame for his Laws of Motion and of Universal Gravitation, Sir Isaac had other interests in physics; among them was light and optics. He practically single-handedly invented spectroscopy, one of the most important components of astronomy today, the primary tool of astrophysicists. Newton conceived of light as particles, "corpuscles" he called them, and due to his fame, so it remained until the beginning of the nineteenth century.

A man named Thomas Young performed an experiment that would change our perception of light forever. It was the first dual slit experiment, and with it, Young showed that Newton could not possibly be right. What he showed was that if light was directed through two slits, it showed interference.

Now, Huygens had studied waves extensively. He was an opponent of Newton's "corpuscles," and believed (with what later turned out to be excellent reasons) that light was not particles, but waves. He had done lots of study, and had come up with some very interesting and advanced ideas about how waves worked. One of these ideas was interference, and he had done some very convincing experiments on it, as well as some very convincing math. And he had done a great deal of math that showed how interference worked. So when Young saw the fateful interference pattern from his experiment, he knew precisely what it meant; and he wasted no time telling everyone about it. Light was waves, not "corpuscles," and this was the proof of it.

This state of affairs lasted until Planck came up with the quantum theory. Of course, everyone knew that quanta couldn't be particles the way we already conceived of them; Young's experiment proved that. So it was clear even from the outset that quanta had to have some very weird characteristics. But it would take another dual slit experiment to show just how weird.

The quantum theory intervened, and Schroedinger, Heisenberg, and Born did their stuff; and then came de Broglie. De Broglie hypothesized that matter could have wave-like characteristics, that it in fact was made of quanta, just like energy. Einstein's SRT had already shown that there was a deep connection between matter and energy, through [latex]E=mc^2[/latex]. De Broglie proposed that the same constant that determined the size of energy quanta, Planck's constant, should work to figure out the wavelength of matter quanta.

In 1927 at Bell Labs, Clinton Davisson and Lester Germer devised an experiment to look for interference in electrons, to prove or disprove de Broglie's hypothesis. But that was not all they found; for this interference was extremely odd. They fired electrons from a Compton accelerator at crystalline nickel. How, you ask, is that associated with a dual slit experiment? It turns out that a grating, which is very fine lines ruled very close together on a glass plate cause interference. The reasons are complicated, but it works out to the same sort of situation as Young's experiment. This interference only happens for waves, of course, so anything that shows interference patterns when shone on a grating must be wave-like. Crystals consist of atoms lined up in precise order; and in nickel, the order creates a grating of the appropriate size to cause electron beams to show interference. Davisson and Germer demonstrated this interference, and thereby proved de Broglie's hypothesis. Quantum theory took over the world.

But there was more.

An important question comes up about this experiment. What if we shoot single electrons into the nickel crystal? Will they still show interference? Quantum physicists by and large believed that they would show interference, but it is hardly clear how, at least in our understanding of everyday particles and waves. But more importantly, they believed (and it was proven) that if we did the experiment in such a way that we could tell which path the electrons took, one slit or the other, they would stop behaving like waves and stop interfering.

It was already astonishing enough that matter should show waves; and amazing that this should happen when single electrons passed through the apparatus. But this was absolutely astonishing. By simply looking to see which hole the electrons passed through, and thereby making them behave like particles instead of waves, the interference disappeared! Talk about the observer influencing the experiment proliferated, and the EPR experiment added fuel to the fire, stoking the measurement controversy (from what had been the measurement problem).

You can now see why Feynman said what he said. It's all there; matter waves, single particles interfering with themselves, and the measurement problem. Quantum mechanics in a bottle.

In 1978, John Wheeler, already famous for amusing conundrums, proposed the following gedankenexperiment: do the dual slit experiment on photons that have passed a massive object out in space, light years away. If the source of the photons is directly behind the massive object, then they will pass to one side or the other. Now, you can either focus a single telescope so that photons from both sides produce an interference pattern, or focus two telescopes so that you can see which side of the object they went by and see the particle distribution. But here's the weirdness: how did the photons know when they passed the massive object, years ago, whether you would look at these photons with one telescope, and those with two, so they could decide whether to interfere or not???!!? What if you switch telescopes right in the middle? But the results are precisely what QM predicts: look with one telescope, you get interference. Look with two, you get none. How the hell do they know??!!?

Now you understand how weird quanta can be. This situation simply cannot arise in classical physics, and we have a great deal of trouble even explaining it in quantum mechanical terms that make any sense at all. You can see how weird interpretations are de rigeur in a situation where such weirdness can exist.

Next, the mindf&%k to end them all: the DCQE. And finally, Afshar's experiment; still controversial. Ain't quantum mechanics wild?

 

[Schneibster]

So, now on to real weirdness.

Quantum erasure. How does this work?

First a little technology. I talked a while back about crystals and their ability to sort photons by their spins, creating polarized light. But it turns out that you can do much more interesting things than this. If you use a special crystal called a beta-barium borate crystal, it can do something really interesting: it splits the photon into two photons, each with half the frequency (and, of course, twice the wavelength) of the incoming photon. But these photons are coupled in a special way: their polarizations, that is their spins, are opposite to one another. So if you know the spin in any plane of one one of the photons, you can figure the spin of the other. The process is called "Spontaneous parametric down conversion," or SPDC. The crystal is abbreviated BBO.

So let's take one of the photons and run it through a dual slit experiment, and the other and send it to a detector. Let's suppose that this detector can determine the photon's polarization as well, which of course determines the polarization of the photon that went into the dual slit experiment. Now, under normal circumstances, that won't tell us anything. So we see interference at the dual slit experiment, just as we'd expect. But now let's make it a bit more interesting.

Let's suppose that we put a quarter-wave plate in front of the slits, oriented one way for one slit, and the other way for the other. This will cause the photon to have different polarization for one slit than the other, which gives us "which-path" information. However, note that in order to determine this which-path information, we need the other photon's polarization; the effect of the quarter-wave plate is dependent upon that. The interference now disappears, just as QM leads us to expect. As soon as we can determine the slit the second photon went through, the photons behave like particles, and the interference disappears.

Now let's erase the which-path information. I have already noted on another thread that the polarization of a photon is reoriented when it passes through a polarizer at 45 degrees to the original plane of polarization; we'll therefore place such a polarizer in the path of the photons going to the first detector. Now we no longer know the polarization of the first photon, and therefore even though the polarization of the second photon can be measured, its original value cannot be determined, and as a result we don't know any more which slit it went through.

The interference reappears.

Now, this is weird, because we didn't change ANYTHING in the second photon's path, yet it shows or doesn't show interference depending on what we can find out about the first photon's polarization. We insert the polarizer in the first photon's path, or remove it, and the second photon shows or does not show interference; but again, we haven't changed anything we're doing to the second photon.

We've "erased" the which-path information; that's the only change. That's why this is called the "quantum eraser."

Think about this again. We take a photon, generate two photons that have complementary characteristics from it, so that by measuring one, we measure the other. We send the one to be measured, and the other through a dual slit experiment. Then we put a device in front of the dual slit experiment that allows us to correlate the polarization of the first photon with the slit the second photon chose. Simply by measuring the first photon, which stopped being causally connected with the second when they left the BBO crystal, we cause the interference of the second to disappear, long after their interaction in the BBO crystal. Weirder yet, we can erase the information by re-polarizing the first photon, again well after both have left the BBO crystal, and we see the interference again.

But that's not the end. The end is the delayed choice quantum eraser.

Take the two photons from the BBO crystal. Send one immediately to a dual slit detector. Send the other into a labyrinth. In this labyrinth, send the photons to a splitter; a device that sends half of the one way, and half the other. One way, you detect which-path information, using the sort of quarter-wave plate arrangement you did for the quantum eraser; the other way, you run them through a polarizer and erase the which-path information from them.

Now, you're not even detecting the which-path information until after the interference already happened- or didn't.

HOW DO THEY KNOW??!!?

 

[Jeremy]

*Brain asplodes*

 

[wipeout]

I can add to that. Need to read it all first, though. The Afshar thing I remember looking at, figuring out what was wrong and then forgetting what I figured out.

 

[SusanB-M1]

*Brain asplodes*

Thank you - that really made me laugh! I have just listened (i.e. set the screen reader to read continuously) to #1-3 with much interest. I do not attempt to achieve any understanding, but just pick up, I hope, the vaguest of gists. For years I have listened to Science programmes on radio and occasionally on TV and have read Bill Bryson's, 'Short History..' and 'The Bluffer's Guide to the Quantum Universe'; also a book about Einstein, Bohr et al. I think it is all absolutely fascinating, but as an ancient person enjoy just being a bystander. I do admire tremendously those who understand even a part of it.

 

[Apathia]

Thanks a bunch, Schneibster!

I recently read this:
http://sciencenow.sciencemag.org/cgi/content/full/2007/216/4?etoc&eaf

The article contains this unfortunate sentence that hints why many Skeptics would rather come at all this from a 19th Century perspective.

Rather, the experimenter decides only later, when he decides whether to put in the second beam splitter. In a sense, at that moment, he chooses his reality.

Just what the Bleep people ran to the bank with.

In reality, the quanta know nothing and no choces are made.
Reality is a seemless dynamic of which we are totally integral.
There is still a good deal of athropomorphizing we need to get out of our system. But then can we ever actually understand outside our fishtank of human sensory experience?

 

[Pragmatist]

I'll start far afield. This should be fun and informative.

Well, it would be if it was at all accurate...

First of all, let's understand that quantum mechanics makes some really strange claims about reality at its most basic level, but it also claims that reality changes when we deal with enough quanta that we have a statistical universe into the classical physics that we observe in everyday life. Basically, individual quanta behave in ways that are impossible for everyday objects, but it is necessary that quanta do so in order that everyday objects do what they do.

Physicists have been struggling with these concepts since the quantum theory was first proposed. It's worth taking a moment to understand what the quantum theory is. The quantum theory is a theory that energy doesn't just flow out continuously from objects that emit it, or into objects that absorb it; instead, it comes in little "packets," of a determined size, one at a time. Max Planck came up with this idea to explain the blackbody radiation, as follows: the blackbody radiation is emitted at all frequencies. Now, if all frequencies are emitted with equal probability, this would obviously result in hot objects immediately radiating all their energy in gamma rays, with almost nothing lower down, because so much energy would be so easily radiated. Lord Kelvin referred to this as the "ultraviolet catastrophe," because nothing of the kind is observed. It is therefore obvious that there must be a higher probability for lower frequencies to be emitted by a hot body, and a lower probability for higher ones. Max Planck theorized that hot atoms emitted quanta of radiation, and that the energy to build one had to be collected up before it could be emitted; obviously, the quantum would be more likely to be emitted when the amount of energy was still small, rather than waiting until there is enough to make a big one. This accounted for the observed radiation peak of blackbody radiation with temperature quite nicely, but many physicists ignored it because it didn't do much else.

Wrong. The real story is here: http://physicsweb.org/articles/world/13/12/8

All that changed when Einstein came along. Einstein realized while he was considering something called the photoelectric effect that this too could be explained by quanta. What happens is that metals exposed to light form an electric charge. Now, the amount of charge varies with the amount of light; but the voltage, which is the push behind the charge, does not. Why is this? Einstein theorized that electrons were being knocked out of the atoms and this formed the charge. But he went further. He showed that the fact that below a certain frequency of light, there was no effect, but as the frequency increased, at first there was a charge that varied with the amount of light, but whose voltage was very low; as the frequency increased, the voltage increased along with it, irrespective of the amount of charge built up. Einstein thought about this and realized that below a certain frequency, the quanta didn't have enough kick to knock the electrons free, but once they did, everything above that in terms of frequency would increase the velocity of the electrons, but not their quantity; only the light intensity would increase the quantity, and only the frequency would effect their voltage. Suddenly, instead of explaining only one phase of energy transmission, quantum theory explained both emission and absorption, and from there on, it was an accepted theory. Einstein won his Nobel Prize for this, not for Relativity.

Wrong. Einstein was the first to propose quanta and particularly with regard to the photoelectric effect.

Now, with all of this going on, Schroedinger had come up with wave functions, and Max Born had proposed that they represented the probability of the quantum being at a particular location or having a particular other parameter at a particular value. Soon, Louis de Broglie (pronounced "de-broy-ah" with the "ah" rather strangled and almost silent) proposed that not only energy had waves, but matter did too. Now, quantum theory was the theory of all matter and all energy, and the fun really started.

Wrong. Schrodinger's work came 20 years later. And Schrodinger's work was based on De Broglie's.

So the quantum theory is the theory that all matter and all energy are made up of indivisible elementary particles called "quanta," and that all of physics can be explained in terms of interactions of these quanta.

Wrong. There are no elementary particles called "quanta" - quantization is a process in which we find that dynamic systems change according to discrete states rather than continuously.

So far it has been incredibly successful; only gravity remains outside its grasp. A "quantum mechanics" is a description of the interactions of various quanta with one another. Generally, the quantum mechanical theories are divided into one for each force; the mechanics describes the interaction of the quanta of that force with matter and with other forces' quanta. There is also a field theory for each force, with electromagnetism and gravity the best-understood ones; finally, for electromagnetism, there is also a quantum-field theory, the most advanced type of theory which can theoretically completely describe the interaction of the quanta of that force with all other quanta, and their operation as a field.

The whole of this edifice is generally described as "quantum mechanics" to non-physicists.

This is nonsense. A quantum mechanics is a system of mechanics in which certain "properties" of the system appear to be quantized into discrete states. There are no magical entities known as "quanta".

OK, so Heisenberg proposed that there were certain parameters of quanta that could not be simultaneously measured. At first, he was a proponent of the idea that this was because the measurement of one quantity would disturb the measurement of another, but soon the math told him and others that in fact, those other values simply didn't exist. It had to be that way.

Wrong. Heisenberg proposed that certain qualities or properties of matter could not be simultaneously measured with absolute precision, there was an inherent uncertainty in all measurements which was related to the quantum of action. He was also always a proponent of the idea that measurement of one quantity would disturb the measurement of another, but his argument also went beyond that. The math never "told him and others that in fact, those other values simply didn't exist", that it is simply one interpretation that might explain what is actually observed.

Einstein hated this. He believed that this showed that quantum mechanics could not be a complete and consistent description of reality, rather an ironic position since he was one of the inventors of the theory and had won a Nobel Prize for it. He famously asked if the Moon existed when no one was looking at it, and went about trying to prove that Heisenberg and the others were wrong, and that these parameters had values even though we could not measure them. He came up with a series of gedankenexperimenten (thought or mind experiments) devised to show inconsistencies in quantum mechanics, or at least incompletenesses; Niels Bohr rather famously (at least in physics circles) managed to answer every one. The last and most effective of these, however, although Bohr had an answer, turned out to be deeper than anyone had expected. This one Einstein didn't invent on his own, but in collaboration with a pair of students named Podolsky and Rosen. The gedankenexperiment is therefore called the "EPR" experiment.

Here is a generalized and simplified explanation. Einstein, Podolsky, and Rosen proposed that one consider a pair of photons emitted simultaneously from an atom under very carefully controlled conditions. Because of these conditions, they could show that the spins of the two photons had to be opposite from one another, due to conservation of angular momentum; only in this way could they add up to zero and account for the lack of change in the momentum of the atom, a necessary condition.

One would expect that upon measuring the spin on one photon, one would always measure the opposite spin in the same axis on the other. This is easily accounted for by conservation of momentum. However, what EPR proposed involved far more than this.

Spin on different axes, as I have mentioned elsewhere, is complementary under uncertainty. What EPR proposed was that we measure the spin on different axes in the two photons. And what they asserted was that because the relation between the photons guaranteed opposite spins on the same axis, this was tantamount to measuring the spin of each photon on two axes, thus violating uncertainty. Furthermore, they asserted that any other position proposed to violate conservation of angular momentum, which would threaten the foundations of all of physics, not just quantum mechanics. They hoped by this to show that despite the fact that we could not measure them, conjugate parameters had real values.

Unbeknownst to either EPR or for that matter anyone else, they had devised the first hidden variables interpretation of quantum mechanics. This turned out to be very important later on.

Wrong. Einstein, Podolsky and Rosen set out to show that the wavefunction could not describe physical reality, they proposed an argument based on measurements of the position and momentum of a pair of unspecified particles. They did not mention photons or spin.

Bohr answered that in fact, the parameters did not have any value, and asserted that their complementarity ensured it, and that since they could not be measured, there was no paradox. EPR responded that in the absence of any value until a measurement was carried out, there was no explanation other than what Einstein denigratingly called "spooky action at a distance" for the correlation between the spins when measurements were made in only one axis; for these parameters to have no value until they were measured, the second photon's spin had to "spring into existence" instantaneously at the moment the first was measured, in violation of locality and relativistic causality. Bohr basically ignored this.

Wrong. Bohr's argument is complex and is based on unavoidable disturbances between the systems under measurement. Some parts of the later, refined argument do imply the non-simultaneous existence of the quantities but it was not explicit in the original argument. Bohr also did not ignore the idea of non-locality, he was explicitly opposed to it.

Now, this led to a long debate over the meaning of wave function collapse, and the reality of the wave function, and the importance or lack thereof of measurement. All of this was lumped together in the minds of many physicists as the "measurement problem," and it was very much in vogue among the QM community to assert that all of this fussing over interpretations was meaningless and that the only proper description of reality lay in the equations. This debate continued until John Bell came on the scene in the 1960s.

Wrong. The debate began before EPR, EPR did not lead to the debate. And this is not the "measurement problem" - which is concerned with the resolution of superposition of states as well as the effect of disturbances on the system.

Bell considered the situation in EPR carefully and realized that everyone had missed something important. What he showed was that although the spin on a second axis was not completely dependent upon the spin on the first, the probability of a certain spin was different if the other spin had some known value than if it did not have any value. In other words, Bell figured out how to tell whether the second spin had any value, or whether the distribution of spins on the first axis was totally random. This opened a whole new door for quantum mechanics, and has resulted in a complete questioning of the nature of reality as a result. This indirect measurement was and is not dependent upon uncertainty; it is, in fact, a direct measurement of the existence or nonexistence of a value, not a determination of that value in violation of the uncertainty principle.

This is nonsense. Bell's argument had nothing to do with mutual dependence of spin states on different axes.

This opened a door; physicists could not yet step through it, because the equipment wasn't sufficiently precise, but the door was open. And some of them knew it. A theoretical physicist named John Clauser began thinking about it, and pretty soon he was working with a friend named Michael Horne; before they were done, they had added Abner Shimony and Richard Holt, and they were doing EPR experiments in real labs instead of in their heads. These early attempts had holes in the data that were collected, and many physicists, both theorists and experimenters, dismissed their results. But they did accomplish an important task: they showed that there was a precise way to interpret Bell's Theorem that allowed the use of inequalities to measure the existence or non-existence of conjugate parameters under uncertainty. Bell had shown the way, and had said that he knew how to do this, but had not shown the way before he passed away (it was an untimely death). They had also constrained the situation fairly closely; there was only a small chance left that EPR were right, and that these parameters had real values. Their tests and inequalities are often called "CHSH" referring to their initials.

Wrong. Bell set out the inequalities in his original paper and argued them through to a complete conclusion. Bell presented his paper as a complete proof in itself against EPR.

In 1982, the time was finally ripe. Alain Aspect had an idea for a test of the CHSH inequality derived from Bell's Theorem. He enacted it, and proved beyond reasonable doubt that in fact the distribution of values on the measured axis was inconsistent with the existence of a real value consistent with the value measured on the other photon for that axis. The value did not exist, in other words; and "spooky action at a distance" was in fact reality.

Wrong. Aspect proved nothing beyond reasonable doubt. The experiment has been contested many times and repeated many times with increasing accuracy, although it's probable the results are valid, there are still outstanding and unanswered objections to it. And the argument about the reality of the values is a different one to the non-locality argument.

So something about EPR's assumptions had to be wrong. Of course, none of the three ever imagined that anyone would find a way to constrain the unmeasurable; Bell did that. And they never even dreamed that their experiment would become possible, and would actually measure the existence or non-existence of definite values where those values could not be measured. But their assumptions were these:
1. Locality, or relativistic causality; that is, that local causes have local effects, that no cause can have an effect outside its light cone.
2. Local realism; that is, that all parameters have actual values whether they can be measured or not; that reality has independent existence apart from whether it is (or can be) observed. This is very closely related to counter-factual definiteness.
3. Completeness; that is, that quantum mechanics is a complete description of quantum reality.
EPR, of course, hoped to prove that the last was inconsistent with the facts; however, Bell opened the way to differentiate between the first and second assumptions, and the third, and test them. And what Aspect showed, using CHSH's inequalities and Bell's test, was that either the first or second had to be wrong. Either influences could reach out beyond the light cone, or unmeasurable complementary values of measured parameters were not merely unmeasurable but non-existent.

One of the differences between the interpretations is how they deal with this situation. Consistent histories merely asserts that these values do not exist, just as strict Copenhagen and decoherent Copenhagen do; Many Worlds asserts that the parameters have values but only in other worlds where their complements were not measured; Bohm asserts that an actual physical but undetectable influence, the pilot wave, connects these events at superluminal speed, violating locality but invisibly so as not to violate causality; TI asserts that unmeasurable causality violations occur backwards in time. All of these violate either causality or local reality. It is a conjecture of mine that any interpretation consistent with measured reality will violate one of these two in some way.

I'll address the DCQE next, and finally move on to Afshar's experiment.

In the immortal words of Pauli - so bad it's, "Not even wrong".

 

[Ziggurat]

In the immortal words of Pauli - so bad it's, "Not even wrong".

I can see Schneibster pissed you off in that other quantum mechanics thread. It's funny, he's quick to hurl out the insults if you disagree with him, but he's also remarkably thin skinned. I think he put me on ignore because I pointed out he was wrong in a relativity thread, even though I hadn't insulted him but he had insulted me. My guess is you'll probably get similar treatment soon - not that I consider it punishment or anything.

 

[The Atheist]

In the immortal words of Pauli - so bad it's, "Not even wrong".

God, I wish you'd been able to say that at the start. I read all that! Now, I have to go read your physics web pages to get back on track.

Hope SusanB-M1 reads your reply...

 

[Pragmatist]

I can see Schneibster pissed you off in that other quantum mechanics thread. It's funny, he's quick to hurl out the insults if you disagree with him, but he's also remarkably thin skinned. I think he put me on ignore because I pointed out he was wrong in a relativity thread, even though I hadn't insulted him but he had insulted me. My guess is you'll probably get similar treatment soon - not that I consider it punishment or anything.

You're right - he did!

And I agree with you absolutely. I've seen a few recent examples of his behaviour in various threads and I'm getting fed up with it. It's one thing when someone is arrogant because they're always right, it's another when all they do is spew BS, mislead laypeople and then throw a childish tantrum when others actually ask to see some evidence for their claims.

And this is just the tip of the iceberg, I'm sure you saw this in the Politics section since you posted in the relevant thread:

By the way, you DO realize how stupid you look making snide comments about someone who posts page-long essays on physics that you're obviously incapable of understanding, don't you?

The behaviour in that thread is so far beyond the pale, I think someone needs an urgent reality check!

 

[Pragmatist]

God, I wish you'd been able to say that at the start. I read all that! Now, I have to go read your physics web pages to get back on track.

Hope SusanB-M1 reads your reply...

The problem is that a lot of his explanations are riddled with errors, but it takes ages to go through them all point by point. I'm not even going to try for most of them, I simply urge everyone to check things out themselves and not to believe what they're being fed by an arrogant poseur.

 

[Ziggurat]

And this is just the tip of the iceberg, I'm sure you saw this in the Politics section since you posted in the relevant thread:

Actually I missed it because I stopped paying attention to that thread. But that's just hillarious!

 

[The Atheist]

The problem is that a lot of his explanations are riddled with errors, but it takes ages to go through them all point by point. I'm not even going to try for most of them, I simply urge everyone to check things out themselves and not to believe what they're being fed by an arrogant poseur.

Thanks for doing what you did so far! The trouble with this type of subject, as Susan stated, is that most of us on the forum only have a vague idea about quantum physics, or any type of high-level science really and it's easy for soemone else to appear very knowledgeable and be completely wrong. I wouldn't have known where to start looking, but I certainly don't accept much in life at face value. I'm certainly not taking yours at face value, either, although the response is silly enough to suggest your reply is a bit close to the marrow.

I just can't wait until Articulett gets to read all this - I'd PM her, but she has me on ignore.

 

[kjkent1]

Well, this is just great. Everyone's managed to offend each other, but no one's bothered to discuss my question, so I'll offer it again -- only with a bit more explanation. Recently, this paper (http://arxiv.org/abs/quant-ph/0702188) was published, in which the authors suggest that they can obtain the which-way information of a quantum particle while maintaining its associated wave function, by using a "non-perturbative" (non-thermodynamic) method.

The experiment appears to be successful, although one can certainly argue that without a direct thermodynamic interaction between the subject particle and the observer, that the experiment is not the same as the classic double-slit experiment, which attempts to actively measure the subject particle.

Anyone have thoughts on the above? Does the new experiment falsify the Copenhagen and many-worlds interpretation of quantum mechanics, or is this experiment perhaps showing something entirely new different, i.e., a direct observation of undisturbed wave-particle duality?

Or, do I just have no idea of what I'm talking about (a genuine possibility)?

 

[Schneibster]

I'm workin on it, kjkent1- you gotta understand what we already know in order to understand what Afshar's experiment shows.

 

[wipeout]

Well, this is just great. Everyone's managed to offend each other, but no one's bothered to discuss my question, so I'll offer it again -- only with a bit more explanation. Recently, this paper (http://arxiv.org/abs/quant-ph/0702188) was published, in which the authors suggest that they can obtain the which-way information of a quantum particle while maintaining its associated wave function, by using a "non-perturbative" (non-thermodynamic) method.

The experiment appears to be successful, although one can certainly argue that without a direct thermodynamic interaction between the subject particle and the observer, that the experiment is not the same as the classic double-slit experiment, which attempts to actively measure the subject particle.

Anyone have thoughts on the above? Does the new experiment falsify the Copenhagen and many-worlds interpretation of quantum mechanics, or is this experiment perhaps showing something entirely new different, i.e., a direct observation of undisturbed wave-particle duality?

Or, do I just have no idea of what I'm talking about (a genuine possibility)?

The answer is ultimately quite straightforward.

Once the particle's two possible histories -- that it went through one hole or the other -- interfere with each other, neither single history can EVER be said to show which slit or hole the particle went through.

At Wikipedia, you can see in figures 1 and 3 that this happens as the two possible histories start to overlap and before the particles reach the lens or even the wires:

http://en.wikipedia.org/wiki/Afshar_experiment

Just because you can later assign that it went through the top or bottom hole now means nothing, quantum mechanically. There is no "which way" information. The overlap kills that. The interference means the particle came through both holes, wherever you later find it.

Quantum mechanics survives unscathed, as it always seems to.

 

[kjkent1]

I would like to share your confidence in the survival of quantum mechanics, unscathed. However, the article explains that the photons are sufficiently spaced apart such that there is very little probability of interference at the crossover points.

If this is true, then it seems that while the Afshar experiment may not completely falsify quantum uncertainty, it does seem to demonstrate that a qualifying measurement for the purposes of invoking quantum uncertainty, must be thermodynamically active, rather than merely obstructive/passive.

If true, then I wonder if there is some sort of phase cancellation which occurs during an active observation, and which causes the wave function to vanish.

I also wonder whether the experiment would be different if it were conducted with particles other than photons.

 

[wipeout]

If there wasn't self-interference for a particle on its own at the crossover points in Afshar's experiment, the particle wouldn't make it through the wires in figure 3 at Wikipedia.

The flaw in the experiment is believing a particle can be said to have gone through the upper hole if found in the lower detector or through the lower hole if found at the upper detector once interference is involved. And the positioning of the wires shows only particles that self-interfere will make it through to the lens and beyond so interference definitely is involved.

Like I say, this interference means the particle came through both holes, wherever we later find it.

 

[Cuddles]

The trouble comes from viewing particles as particles and waves as waves. The fact is that particles can behave like waves and waves can behave like particles, so it makes little sense to view them as seperate entities. Generally, people have always treated them as seperate, so even when looking at experiments like the double-slit they are viewed as either particles or as waves. Either they interfere or they have a definite position. There is always the assumption that while waves and particles can act interchangeably, at any given point we can only see them acting as either a wave or a particle. However, there is nothing inherent in quantum mechanics that says this, it is simply an assumption that feels nice and has generally been observed to be the case. This experiment, if true, challenges this assumption, but does very little else. We may have to rethink our ideas on when is something a wave or a particle, but the fundamentals of quantum physics remain unchanged.