anti quantum mechanics
this is taken from a longer blog post filled with equations i can’t quite follow. but the points he makes in the discussion after are full of insights, and not only into quantum mechanics, but the minds trying to encompass it, or really, trying to dismiss and conquer it. thanks lubos motl
It means that if you want to preserve the idea of some “realism”, you must abandon the assumption of locality (which won’t be enough, as argued below: I have to say it in advance). The individual spins are actually not encoded in some “local variables”. There has to be a communication which guarantees that the value of an individual spin σ1,2,3x,y that you measure depends on what other experimenters in the Solar System just decided to measure and what outcomes they received.
If this were true, you would of course have to give up relativity because there would now be fully physical superluminal signals instantly propagating through the Solar System. I designed the position of the three laboratories so that they were spacelike separated: not even light was fast enough to exchange the information.
But of course, the people who are eager to deny quantum mechanics, whatever the costs are, are also eager to deny Einstein’s relativity when it’s needed. And indeed, it’s needed. Fine. So they may believe that quantum mechanics is fundamentally wrong – it just appears to be correct – and in the same way, relativity has to be an illusion, too. At the fundamental level, objects do communicate by signals that are not just “slightly faster than light” but they are arbitrarily fast. All the major sharp qualitative insights that have been found in physics during the 20th century may be just illusions – and there is some mundane classical “mechanism” that is faking quantum mechanics and that is faking relativity.
What would it really mean if the measurement of a single electron’s spin depended on everything in the Solar System that has occurred so far, including measurements done on another planet a picosecond ago? If you really believe this thing – and it is on par to the belief that Young Earth creationism is fundamentally correct and the fossils are just a conspiracy to make the world look different than what it actually is – then you inevitably have to assume that there is a preferred reference frame in which the “real material physics” is actually taking place.
This attitude of yours, namely attempts to find arbitrarily contrived “mechanisms” whose only goal is to deny something we almost directly observe – could be used in any other context in science and outside science, too. You may deny that people have visited the Moon, that the species have evolved, that the Earth has been here for billions of years, that the events in the Middle Ages actually took place. With enough dishonesty, you may deny anything.
You may hope that the answer is Yes but from the beginning, it’s clear that you need to make billions of particular choices that don’t really exist in Nature and that have no observable consequences. And for you to isolate the “faked quantum mechanics” inside a more general class of similar theories, most of which don’t fake any quantum mechanical theory, you will need to adjust an infinite number of parameters. You will need an infinite amount of fine-tuning. Just to be sure, the amount of dishonesty you need to claim that such a proposed theory is a “real alternative” to proper quantum mechanics is much larger than the amount of dishonesty that a Young Earth creationist needs to dismiss the importance of fossils. The creationists only needs to “overlook” or “reinterpret” a finite number of fossils. You need to claim that infinitely many quantities predicted to be almost certainly nonzero by your non-quantum, non-relativistic theory (all the observable consequences of preferred inertial systems and material collapses) just happen to be accidentally zero. This denial of everything that physics knows about the natural phenomena is needed for you to protect your pet medieval belief in realism.
Obviously, they need to say that there is a real “wave function” or “density matrix” that should be treated in the same wave as a classical electromagnetic wave. Then there are some GRW-like collapses that make the world encoded in the “wave function as hidden variables” look like the real one. The wave functions must sometimes “literally collapse” and they suddenly shrink “in a particular reference frame”.
You need to make the collapse physical and because the wave function quickly “shrinks” in this mode of reasoning, you need to specify a particular reference frame in which it “shrinks”. Aside from choosing a particular reference frame, you will also have to choose a particular border above which things “really behave classically”. I say it’s artificial as well because according to quantum mechanics, the same quantum mechanical rules fundamentally apply to all systems, and not just the small ones.
The Bohmian theories say that the wave function “objectively exists” but it is just a “pilot wave” that drives another “objectively real” degree of freedom, the particle’s actual coordinate and velocity. However, in the case of the spin, the GHZM experiment unambiguously shows that the “actual value of the spins” can’t exist in the classical sense, so the particle-like portion of the Bohmian degrees of freedom or “beables” have to be thrown away, anyway. If you want to describe spins of particles, you may only use the “wave function” part of the Bohmian “beables” and this wave function spreads just like any other one.
Quantum mechanics has no problem with the spreading of this wave function because the wave function only encodes a half-baked complexified probability distribution which should be interpreted as the state of someone’s knowledge, not as objective reality. If you want to say that the wave function describes some features of the objective reality, you have to offer some actual material mechanisms that “keep it from spreading”. In the articles about the GRW collapse theories, including the recent one about Steven Weinberg, I’ve explained that any mechanism of “collapses” that you add into your theory to produce big enough effects to make the “objective wave function” more classical will inevitably introduce new perturbations that are safely excluded experimentally. So you will fail, anyway.
At the end, I think that even with the unlimited but finite amount of dishonesty, you simply can’t construct a “realist” model that will fake quantum mechanics. The ultimate problem is that observers are quantum mechanical systems as well – and they may interfere with themselves.
the “moment” when the collapse occurs is completely subjective.
This statement has many aspects. The simplest one to understand was mentioned at the beginning: if there were a “real collapse”, it would mean that the wave function of a particle that was just absorbed must “instantly disappear” from the rest of the space. However, the term “instantly” requires you to specify a particular reference frame because special relativity guarantees that “now” has a different meaning for observers in different states of motion.
When you look how Nature actually works, i.e. when you study these issues quantum mechanically, the reference frame in which the wave function collapses “now” is completely unphysical because the portions of the wave functions that “disappear” can’t influence anything that latter happens, not even in principle! They just describe some a priori possible outcomes that didn’t materialize; they only exist in the head of a physicist who calculates the predicted probabilities. When a different outcome becomes a fact, we have to switch to conditional probabilities so the outcomes violating the corresponding conditions just don’t affect any future predictions or outcomes. By the very definition of conditional probabilities, the influence of the later-disappeared portions of the wave function on any observable phenomena is exactly zero, something that wouldn’t be possible in any natural “realist” theory.
A few paragraphs ago, I also mentioned another aspect of this “subjectivity” of the collapse: less accurate observers may want to “perceive an outcome of a measurement” and they may do so prematurely. More accurate observers are calculating things more accurately so they still describe the rest of the world, including the sloppy/smaller observers from the previous sentence, by superpositions, so that they don’t forget about their potential to interfere. In quantum mechanics, this aspect of the subjectivity of the collapse is unsurprising and free of problems, too. The “collapse” just means a subjective process by which “a physicist is taking recently observed facts into account” (he modifies his state of knowledge about the world) and it doesn’t matter when he does so. Different observers predicting the same measurable final outcomes may do so at different stages and they will still agree about the predictions of events that they may consider as shared facts. These collapses don’t have to have objectively unique properties, moments, and shapes – and indeed, they demonstrably don’t: they’re a part of a subjective cannon allowing physicists to make valid probabilistic predictions.
Any “realist” description of the collapse must inevitably choose a point beyond which the collapse is “strictly real” and the potential for interference is lost after that point. But in the world around us, as described by quantum mechanics, this never happens. In principle, it is always possible to think about a more accurate observer who calculates the motion of all particles – including us – using the exact wave function with arbitrary superpositions etc. Any “realist” picture of the “wave function collapse” must introduce a new “bureaucratic intervention” after which the world is “obliged to behave classically”. For the GRW collapse proponents, this intervention is the “flash” that shrinks a wave function. For the many-worlds advocates, it’s the moment when you “objectively split the world” into many.
But according to quantum mechanics, such events when the properties of the physical systems become “exactly classical” never occur. As we study larger objects, the classical approximation becomes more tolerable, but it is never fundamentally right. So the collapses and the events in which the many worlds split never occur. And they don’t need to occur because the amplitudes only have a probabilistic interpretation and probabilistic distributions are allowed to spread without the underlying truth’s becoming fuzzy.
What I finally want to emphasize is that all this redundant and “objectively real but totally unobservable” superstructure – from many worlds to extra invisible Bohmian positions of particles (which can’t help in the case of spin or particle production, anyway) or other hidden variables to GRW collapses prescribed from above – is only being invented because certain people behave as bigots who are unable to admit that the physics research in the 20th century has irreversibly falsified all intrinsically classical models of the reality. All the new “fanciful stuff” with tons of choices and processes (superluminal communication, preferred frames, collapses, the length scale to which the GRW collapses shrink the wave function, the frequency of such flashes etc.) that can never be observed and with the infinite amount of fine-tuning and obfuscation that is needed for it to fake the real, relativistic quantum world (to guarantee that none of the new predictions is really observed) is only being proposed because some people’s bigotry has no limits. Their dogmas about “realism” are more important for them than any amount of empirical evidence, more important for them than everything that science has actually found.
…forces us to understand science as a gadget to organize our observations rather than to promote the idea about an “independent real world”