Collapse of Quantum Wavefunction Captured In Slow Motion 242
ananyo writes "It is the most fundamental, and yet also the strangest postulate of the theory of quantum mechanics: the idea that a quantum system will catastrophically collapse from a blend of several possible quantum states to just one the moment it is measured by an experimentalist. Researchers have now been able to capture that collapse through the use of weak measurements — indirect probes of quantum systems that tweak a wavefunction slightly while providing partial information about its state, avoiding a sudden collapse. Atomic and solid-state physicist Kater Murch of the University of California, Berkeley, and his colleagues performed a series of weak measurements on a superconducting circuit that was in a superposition — a combination of two quantum states. They did this by monitoring microwaves that had passed through a box containing the circuit, based on the fact that the circuit's electrical oscillations alter the state of the microwaves as they pass through the box. Over a couple of microseconds, those weak measurements captured snapshots of the state of the circuit as it gradually changed from a superposition to just one of the states within that superposition — as if charting the collapse of a quantum wavefunction in slow motion."
Information (Score:5, Interesting)
'Wavefunction collapse' is how the Copenhagen Interpretation 'explains' this phenomenon, but like many of its 'explanations' they don't provide a compelling reason for things to happen this way. Terms like 'measurement' go from a precise QM meaning (eg. matrix multiplication) to a vague, ambiguous meaning like 'a concious observer'. This leads to tenuous extrapolations and conclusions, like the distinguished position of observers, the inclusion of conciousness into the interpretation and all the quantum 'explanations' of consciousness which that has spawned.
Alternative interpretations are much less mysterious. For example, the Many Worlds Interpretation explains it via information transmission. A measument is anything which transmits information from inside the system to outside the system. When a system is measured, it doesn't 'collapse' into one state; rather, the thing which performed the measurement becomes part of the (now larger) system.
The Transactional Interpretation explains it as two-way communication between events at different times; a measurement is any event which propagates information back in time and a system is only in multiple states because the event which caused it is awaiting the information from the measurement.
Schrodinger's cat can be used to point out the difference:
The Copenhagen Interpretation says that the cat is literally both alive and dead at the same time in the box, then when a concious observer (a human) opens the box, the cat immediately becomes either alive or dead. This is very strange, for example why is a concious observer necessary?
The Many Worlds Interpretation says that the cat is literally both alive and dead at the same time in the box. Anything which interacts with it, for example photons of light, will become part of the system; ie. the light will literally be both a reflection of a living cat and a reflection of a dead cat. If those photons enter my eye then I will literally be both a human who has seen a living cat and a human who has seen a dead cat. If you talk to me, you will literally be a human who has talked to a human who has seen a living cat and a human who has talked to a human who has seen a dead cat, and so on. This propagation is exactly the flow of information between systems; there is nothing magical about humans, except that we happen to be human. A photon would get the same results as us if it repeated our experiments, with no 'concious observer' involved, except for the fact that photons don't tend to perform experiments (ie. the 'conciousness' part of Copenhagen is an anthropic bias).
Re:Information (Score:5, Interesting)
the 'conciousness' part of Copenhagen is an anthropic bias
It's worse than that. According to defenders of the Many-Worlds interpretation (of which I consider myself one), Copenhagen's collapse has several problems. Less Wrong's Eliezer Yudkowsky has written an extensive introduction to QM from the perspective of the Many-Worlds Interpretation [lesswrong.com] and as part of the series he's extensively criticized the collapse postulate, summarizing its problems [lesswrong.com] thus:
Given the above considerations, whatever the experiment detected is most certainly not collapse.
Trivial kind of local collapse (Score:5, Interesting)
This is not the non-local collapse which some QM physicists (mystical school of thought) believe in. Everything in this experiment is local, the two superposed wave components which collapse into one are fully overlapped. Hence it is no more mysterious than your radio antenna collapsing superposed waves of thousands of radio stations striking it, into one component, that of a station you tuned in.
The real controversy is about existence of non-local collapse i.e. when two components and detectors are "far apart" (at space like distance), so that detection by detector D1 (supposedly) instantly collapses the remote field component causing the remote detector D2 to fail to detect it. Most recent experiment claiming to demonstrate such phenomenon with photon on a beam splitter actually cheated (see discussion here [physicsforums.com]). In that claim they basically tweaked the timings on two coincidence circuits well out of manufacturer's specs so that they could never trigger D1 and D2 simultaneously.
Non-local collapse, which was never demonstrated empirically, does not follow from the Quantum Field Theory (discussion here [physicsforums.com]) but is merely a hypothesis in the QM "measurement theory", which is the speculative, soft and fuzzy, part of the theory that has been debated among physicists, philosophers and mystics for nearly a century without getting anywhere so far.
Re:Information (Score:4, Interesting)
Do you actually know what you're talking about, or have you just read too much science fiction? I don't remember the Copenhagen Interpretation being related to "consciousness", either in the requirement that an observer be "conscious" or in using quantum mechanics to explain consciousness. Are you sure that's not just some whacky interpretation of the Copenhagen Interpretation that people have come up with more recently, due to misunderstanding?
Well I've got a Master's degree in Physics, which doesn't make me an expert in all things Physicsy, but the Copenhagen Interpretation isn't exactly cutting edge.
The Copenhagen Interpretation itself doesn't mention conscious observers, but it makes a vague distinction between measurements (which collapse the wavefunction) and non-measurements (which don't), without actually explaining what the difference is.
For example, in the classic Young's Slits experiment, light shines on to a surface containing two slits. If we measure which slit a photon passes through, it will appear on the other side as expected. If we don't measure which slit a photon passes through, it will pass through both and cause an interference pattern. But what consitutes a measurement and, more importantly, why does it depend on something 'we' do?
According to (Heisenberg's formulation of) the theory, everything can be modelled as matrix transformations. A unitary transformation is not a measurement, so it will not collapse the wavefunction. A non-unitary transformation is a measurement and will cause collapse (specifically, the system will collapse to an eigenstate in the basis of the transformation). All well and good *in theory*, but it's difficult to apply to the real world. Is "looking at something" a unitary transformation? Is "poking with a stick" a unitary transformation? It's difficult to tell. What's more, transformations aren't just caused by actions, they depend on the configuration of the whole system. For example, we can collapse a wavefunction by measuring where a particle *isn't*!
So, in the case of Young's Slits, why is it that when *we* try to interact with a photon its wavefunction collapses (ie. a measurement is made), but it *doesn't* happen when the slits themselves interact with the photons, or electrons in the air absorb and re-emit the photon, etc.? The Copenhagen Interpretation doesn't say anything about this; it makes the distinction and leaves it at that. But if there's a distinction, what is it? What's the only thing that sets apart all of the possible measurement-making things and all of the possible non-measurement-making things? In all seriousness, the answer to that question is consciousness. This has perplexed Physicists for decades, leading to philosophies like "shut up and calculate".
However, it turns out that this is the wrong question to be asking in the first place! According to Many Worlds, measurement is all about information flow. If information is transmitted between two things, they become part of a QM system with a corresponding wavefunction; it doesn't matter whether those things are electrons, slits, screens or humans.
An analogy would be to think of the boundary of the system as being like a bubble, expanding to engulf anything which interacts information-theoretically with the contents. What Copenhagen calls 'wavefunction collapse' would be the moment when that bubble engulfs the experimenter. According to Copenhagen, when the bubble hits the experimenter it bursts, causing the entire contents of the bubble to instantly take a fixed configuration (as an aside, since QM systems can be any size, this line of thinking has also lead to the 'faster than light' nonsense, since the bubble must burst instantly everywhere as soon as anyone measures its contents). Accoring to Many Worlds, engulfing an experimenter is just like engulfing an electron, or a table, or a planet; the bubble just keeps growing as long as information keeps flowing.
*From the point of view of the observer* these inter