Remember when I asked for suggested topics for an upcoming Bloggingheads discussion with David Albert about quantum mechanics? The finished dialogue is up and available here:
I would estimate that we covered about, say, three percent of the suggested topics. Sorry about that. But perhaps it’s better to speak carefully about a small number of subject than to rush through a larger number.
And I think the dialogue came out pretty well, if I do say so myself. (And if not me, who?) We started out by laying out our respective definitions of what quantum mechanics “is,” in terms that should be accessible to non-experts. (One user-friendly answer to that question is here.) Happily, that didn’t take up the whole dialogue, and we had the chance to home in on the real sticky issue in the field: what really happens when we observe something? This is known as the “measurement problem” — it is unique to quantum mechanics, and there is no consensus as to what the right answer is.
In classical mechanics, there is no problem at all; you can observe anything you like, and if you are careful you can observe to any precision you wish. But in quantum mechanics there is no option of “being careful”; a physical system can exist in a state that you can never observe it to be in. The famous example is Schrodinger’s cat, trapped in a box with some quantum-mechanical killing device. (Someone must write a thesis on the ease with which scientists turn to bloodthirsty examples to illustrate their theories.) After a certain time has passed, the cat exists in a superposition of states: half alive, half dead. It’s not that we don’t know; it is really in a superposition of both possibilities at once. But when you open the box and take a look, you never see that superposition; you see the cat alive or dead. The wave function, we say, has collapsed.
This raises all sorts of questions, the most basic of which are: “What counts as `looking’ vs. `not looking’?” and “Do we really need a separate law of physics to describe the evolution of systems that are being looked at?”
In our dialogue, David does a good job at laying out the three major schools of thought. One, following Niels Bohr, says “Yes, you really do need a new law, the wave function really does collapse.” Another, following David Bohm, says “Actually, the wave function doesn’t tell the whole story; you need extra (`hidden’) variables.” And the final one, following Hugh Everett, says “You don’t need a new law, and in fact the wave function never really collapses; it just appears that way to you.” This last one is the “Many Worlds Interpretation.”
I want to actually talk about the pros and cons of the MWI, but reality intervenes, so hopefully some time soon. Enjoy the dialogue.
@ slide2112 & mathematician
Consciousness isn’t required to collapse the wave function. Read up on the Stern-Gerlach experiments, especially the sequential versions.
http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment
The first experiment with two z-spin measurements show that the first measurement collapsed the wave function such that no spin-down electrons were detected by the second detector. An inhomogeneous magnetic field acts as the “observer” here, nothing conscious or even living required.
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“If a physical theory does not accommodate consciousness, …”
I don’t know what this means.
Eric wrote:
The ground hears it. That is, the ground vibrates from absorbing the impact. That’s all “hearing” would mean in the context of physics.
The egoism is caused by a reluctance to include speculation in a formal essay. A physicist trying to explain measurement using math would be seen by his peers as un-scientific. But trying to explain it in ordinary language can be excused as just dealing with the public. Once a physicist puts himself in this low-falutin mode of speech, he is vulnerable to all sorts of temptations in forming his thesis.
Explicit collapse theories are a long way from being provable, but they are a responsible endeavor into a type of rational thinking that is unfortunately not very popular anymore.
Just a thought, and I hope that the real theorists here will correct me if I am wrong…
The interpretation of Schroedinger’s Cat usually given in ‘pop’ physics books and programs tends to give rise to the suggestion, as stated above by slide2112, that ‘consioucsness’ becomes some quantum mechanism of determinancy. I, too, have a problem with that idea, for the same reasons he states. BUUUUTTT…
What if, instead of ‘consciousness’, one substitutes the concept of ‘consequence’; i.e. the conscious observation of the cat’s state leads to a consequence of some kind, and thus the waveform collapse only occurs (and can only occur) when the collapse in one direction or another has a consequence on some aspect of the local space.
That removes (IMHO) the troublesome idea of QM being predicated in some manner upon conscious observation, since that observation is only then, in this formulation, a specific flavor of consequence.
Does this make sense, or am I rambling here? 😀
@ Mark Harrison
The fact that straw-men don’t have consciousness, doesn’t mean it doesn’t exist.
Many worlds is just the most literal reading of quantum mechanics. If Bryce De Witt had not given the name “many worlds”, this would probably not even be a controversy. Environmental decoherence + many worlds interpretation is all you need to understand that quantum measurement is just quantum evolution.
And who, pray tell, is Neils Bohr?
I did not know there would be a series.
A fundamental(foundational point of view) question to me is whether the content of one, can be based on the philosophical point of view(Science Saturday: Time
Typo fixed.
Jason, thanks for answering adamk87’s question. There’s no big deal about “bumping into” the apparatus; we can always make the slits and the detector big and massive enough so that they are unaffected by the photons. The fundamental issue is that the behavior of the photons is different if we watch which slit they go through vs. when we don’t.
The fact that the interference pattern produced by interferometers involving mirrors is not affected by the mirrors recoiling, is actually explained by the fact that the mirrors themselves are described by quantum mechanics and have to obey the uncertainty principle.
If you want to detect the “which path information” using the recoil of the mirror, the mirror’s wavefunction in momentum space would have to be sufficiently sharp. But then the wavefunction as a function of position would be broader than the photon’s wavelength, therefore you cannot have an interference pattern.
More techically, the strength of the interference patern is proportional to the overlap of the wavefunction of the mirror (more generally, the wavefunction of the rest of the universe) when the photon recoils and when it doesn’t. The interference pattern will thus vanish when the overlap is zero, i.e. when the two wavefunctions are orthogonal. That in turn implies that there exists an observable such that the two wavefunctions are eigenfunctions with different eigenvalues. These eigenstates then correspond to the two paths the photon can take.
In this article some less trivial cases of which path information are considered. You can
produce entangled photons such that the “which path information” for one photon is present in another photon. Then you don’t get an inyterference pattern (you do get it when you measure certain correleations between the two photons). Unlike in the case of the recoiling mirror, the absense of the correlation cannot be explained classically.
I still think there is a problem with the decoherence idea in that it treats the decoherence mainly via measurement interference with the wave function – as if the wave function is the final reality. But the wave function is just a statistical measure of potential. This is how it is described mathematically but it gives no fundamental insight on the nature of quantum reality from a non-statistical viewpoint.
It has been shown that a pattern will build up on the target gradually of interference via the double slit experiments even if electrons are released one a time. This occurs as long as you don’t observe the path of the electrons. Obviously you can describe observing the path as creating a decoherence. But it tells you nothing fundamentally new.
Doesn’t it seem much simpler, and also elegant, to just assume a memory effect is occuring in the vacuum from each individual electron passing through. The wave equation sensibility is just describing the build up of interference of each new electron being effected by the path of the previous electron going back ad infinitum to the first electron. The vacuum remembers!! And the decoherence effect that stops the interference and makes each electron act like a particle and not a wave is caused by randomizing the vacuum energy by shining a light on that path. Remember, the experiment always takes place in the dark for the interference effect to occur at the target and not along the path.
Hey Jason, thanks for elaborating on the experiment for me, but there are still some things that remain unclear. I think Ill just have to get some books on it.
My question was in essence have all factors relating to the equipment used in the experiment been set up or examined such that what we observe is a reality and not just the equipment messing up the data. My guess is that when the experiment was first done the first thing that was questioned was the apparatus. I mean I don’t know much about physics or this experiment, but I was just wondering if either the ‘flash’ that is used to record the points or the disk that the electrons/photons are shot through could influence the data in such a way as to produce the results we observe.
Eric,
The main point to be made is that what is “real” is largely an open question. Now, we know from measurements confirming quantum mechanics that “something” that behaves like the Schroedinger equation in the non-relativistic case actually exists. There is no question about this, and every interpretation of quantum mechanics that is in any way serious affirms this.
The next question is, how much more do we need to describe observation? Decoherence provides the simplest answer: nothing else. All other interpretations assume the existence of other hypothetical entities in addition to the wavefunction that follows Schroedinger’s equation. The work done in decoherence theory seems to indicate that none of this is necessary, and since for the most part there are no observational effects from any of the added hypothetical entities from the other interpretations, they offer no new explanatory power and are simply culled out by Occam’s Razor.
Attempting to claim that there is as yet reason to believe any other interpretation of quantum mechanics is rather akin to taking a set of data points that lies on a line, and claiming that we need to use a quadratic form to properly describe those data points. There is no need: the line works just fine. Therefore, unless we find some definitive evidence that the MWI is wrong, as weird as it is, it’s the only reasonable default position we should have with quantum mechanics.
P.S. I’m sorry, but I don’t understand your statements about the vacuum.
Jason,
I think at this point we will have to agree to disagree as we both have our opinions. I will say this: If an interference occurs at the target in the two slit experiment even if electrons are released one at a time one can only conclude that the wave equation is being built up on a “memory” of the path of the last electron. There is no other way to put it.
The problem I think you are encountering is not in understanding what I’m saying but in accepting that my explanation may be simpler to understand than yours. You have to accept that the quantum vacuum is a real physical entity and not just a useful mathematical abstraction. It seems to be an intermediary in all particle interactions. Not everything is just math – the math in physics actually represents something physical.
Eric,
Errr, no memory of the paths of previous electrons is required. The probability distribution of where the first electron hits is completely independent of the probability distribution of where the second electron hits (assuming the first electron doesn’t change the apparatus, of course). In fact, simple memory tells you nothing whatsoever about the interference pattern. What is instead seen is that the behavior of the electron between the two slits and the screen is precisely as described by Schroedinger’s Equation. Somehow, until the electron hits the screen, it acts like a wavefunction. So why should I accept any explanation that claims that something different is happening at the screen, when one is available that assumes no such thing?
And I have no problem with the idea that the vacuum is a real physical entity. I just have a problem with the claim that it is needed to describe what’s going on here.
Eric & Jason,
A way to settle this vacuum memory claim is to set up an ensemble of thousands of double-slit experiments and only send one electron through each. This way, the memory of the vacuum will not affect any other electrons, since there is only one electron per experiment. Then, tally up the positions in each of the experiments and note the pattern.
If Eric is right, the distribution of the positions of the electrons should be uniform, instead of the standard double-slit fringes, since there will be no memory effect.
All I need are thousands of electron guns, slits, film and labs. Where’s my grant money?
#26 Mark Harrison
No, the measurement problem is there, read up on von Neumann measurement. The role of consciousness is either dismissed, or drop in favor of other interpretation. It is never shown to be wrong
Did anything further become of this? http://www.sciencedaily.com/releases/2008/08/080806140128.htm
The notion of “wavefunction collapse” is completely ridiculous, and it pains me that it is still taught to thousands of gullible youths.
I would no longer think wavefunction collapse is so ridiculous if someone could present me with a theory of when it happens. If you believe in wavefunction collapse, what are the real laws under which wavefunctions evolve (which are now nonunitary), when does the collapse happen, how is the theory covariant (or why does it appear so to such high precision in experiment), and how do you experimentally detect the lack of unitarity?
I think there’s a reason interpretational issues are usually discussed in the context of QM and not of QFT — in the latter context, it’s even harder to swallow the concept of wavefunction collapse.
There is no real collapse of the wavefunction. This is where decoherence is your friend. The whole point is that ONLY unitary evolution is there, and the “collapse” is an artifact of system-apparatus interaction.
The system is in a superposition of many eigenstates initially. The eigenbasis for doing the expansion depends on the measuring apparatus. (i.e., you can expand a generic spin state as spin up-down or spin left-right or spin back-front, but “measurement” gives rise to a specific final state based on which of these three the apparatus is measuring). A measuring apparatus is precisely such a device that has states which couple to specific eigenstates of the system and evolve together. Because of the large number of states of the apparatus, the combined evolution is such that the overlap between one such (coupled) eigenstate of the system+apparatus, with another, is almost zero. A little bit of math shows that when this happens, one effectively ends up with probabilities instead of amplitudes. This is all there is to collapse.
You still need many worlds here. Why? Because the eigenstates could have decohered away from each other because of coupling to apparatus-states, but still that does not tell us which branch we are in. This choice is where many worlds comes in. But this is no more an issue for (the mythical) “collapse” as it was for unitary evolution.
There are some interesting questions you can certainly ask here, but I don’t see why people take pride still in saying that they don’t “understand” quantum mechanics. Decoherence + many worlds has solved the interpretational confusions of measurement. Whats funny is that I have even heard arguments against the landscape of string theory using arguments against the many worlds of QM. Of course, the arguments that one often hears against the landscape are also mostly based on gut feelings – either against (pereceived) speculation, or based on the (premature claim of) lack of predictivity. So maybe it is not that bad.
I think that Albert did not give properly Bohr’s view. All you need for the wavefunction collapse (or reality forming if you prefer) is to give an exact boundary between quantum and classical so that you have a sharp separation between these worlds and a measurement apparatus is a well-defined object whose interaction with a quantum system provokes collapse of the wavefunction.
Marco
“I think that Albert did not give properly Bohr
I had always intuitively rejected the “many worlds” interpretation as inelegant and a bit too star trek for my tastes. However, some of this discussion seems to have recast it as simply “decoherence” produced by the regular unitary evolution of QM when a microscopic system gets coupled to a macroscopic measuring device. This sounds more promising when expressed in words, but where can I find an (accessible) exposition of the maths behind it?