Slashdot Mirror
Researchers At Brown University Shattered a Quantum Wave Function
Jason Koebler writes: A team of physicists based at Brown University has succeeded in shattering a quantum wave function. That near-mythical representation of indeterminate reality, in which an unmeasured particle is able to occupy many states simultaneously, can be dissected into many parts. This dissection, which is described this week in the Journal of Low Temperature Physics, has the potential to turn how we view the quantum world on its head. Specifically, they found it's possible to take a wave function and isolate it into different parts. So, if our electron has some probability of being in position (x1,y1,z1) and another probability of being in position (x2,y2,z2), those two probabilities can be isolated from each other, cordoned off like quantum crime scenes.
Fearmongering much? Cosmic rays hitting the Earth's atmosphere produce up to 40 times higher-energy collisions (and on a continual basis) than any experiment that human physicists have ever done. If black holes were a significant risk, our planet would have long since been consumed.
This is incorrect. The kitty analogy doesn't really work well here because the idea of the box itself is what is in question. It would be closer to finding that what you thought was a box with one kitty was really a box with kitty pieces that can be spread out and opened and all still be "alive" (or dead).
I can explain briefly. Since I don't know your background, I'll break this up into sections. Skip what you know.
Scale
Depending on the lengths of space involved with a topic of study in Physics, one of three schools of thought will be used. At the macro scale, the lengths that we experience day to day, Newtonian mechanics are usually good enough. At very large scales appropriate for studying stars, planets, and so on, General Relativity comes into play. At very small length scales appropriate for studying atoms and their constituent particles, Quantum Mechanics is used. There are far more fields in Physics, but these three provide the broadest toolset in these terms.
Superposition
The world as we experience it has some fairly intuitive rules, like cause and effect. We call that determinism; that if we know the initial state of a system and the rules that it follows then we can predict what state it will end up in. You know what will happen if you life your mouse and let go: it will fall to the surface you lifted it from. In Quantum Mechanics, determinism does not apply.
One of the things required for determinism to work is that one set of initial conditions produces one outcome under the rules that govern how the system proceeds in time. The dropped mouse falls back to the surface beneath it. In Quantum Mechanics, there is no "one outcome", but instead there are many. Let's call these outcomes "states," because this applies to the initial conditions as well. When an observation is made, only one state is found, and the wave function describes the probabilities of finding each related state. Until the observation is made, every state exists or is happening simultaneously. We call that a "superposition" of states.
Wave Summations
One of the mathematical tricks used to solve for a wave function takes into consideration several possible waves and sums them. Now, this gets fairly complicated and it's well beyond the scope of an Internet forum post to explain it fully. Suffice to say that we call each wave summed to get the end wave function a "wave packet". That math is at work around you all the time; it's used to turn the analog radio signals used for broadcasts into square waves for digital broadcasts, for example. The researchers discussed in this article are not breaking down wave functions into wave packets, but I explain this because I want to impress upon readers that wave functions describe multiple states.
Finally, this article...
The researchers have found a way to isolate states in the superposition to observe them individually, which is interesting for many reasons. You may have heard of the double slit experiment, which is a good analogy for this. When particles are observed before passing through the slit, they appear as particles with determinate positions (wave function collapse) but when they're not observed, they appear as interference patterns between waves (superposition). Using that experiment purely as an analogy, these researchers have found a way to observe the particles that form the interference patterns so that each can be studied individually.
"An electron in liquid helium forces open a cavity referred as an electron bubble. These objects have been studied in many past experiments. It has been discovered that under certain conditions other negatively charged objects can be produced but the nature of these “exotic ions” is not understood. We have made a series of experiments to measure the mobility of these objects, and have detected at least 18 ions with different mobility. We also find strong evidence that in addition to these objects there are ions present which have a continuous distribution of mobility. We then describe experiments in which we attempt to produce exotic ions by optically exciting an electron bubble to a higher energy quantum state. To within the sensitivity of the experiment, we have not been able to detect any exotic ions produced as a result of this process. We discuss three possible explanations for the exotic ions, namely impurities, negative helium ions, and fission of the electron wave function. Each of these explanations has difficulties but as far as we can see, of the three, fission is the only plausible explanation of the results which have been obtained."
Research group website
Non-paywalled copy of paper
TLDR: This research group studies exotic electron effects in superfluid helium. They see a particular effect that is not currently explained. There are a few possible explanations, and they argue that a particular one is probably true.
Inaccurate "news" articles ensue.
(The physics is subtle enough that, despite reading the abstract and bits of the paper, I would not venture to try to summarize it. You can smell a mile away, though, that this article is poor understanding mixed with hyperbole. The specific flavor is, "Quantum Mechanics is Philosophical Magic".)
I took the question to be more a layperson's request for explanation. I don't think they meant to get that technical, though I could be wrong.
This isn't particle duplication. States are marked, and then the particle is observed in one state. But then the other states can be observed as well -- there just won't be an electron at any of those places except the one where it is observed.
You're confused because you're mixing up the states of a particle with the particle itself. If your wave function describes a distribution of positions, then each position continues to exist after the wave function is collapsed by observation.
That's a pretty good summary !
Some really good videos ...
Quantum Physics And How We Affect Reality!
https://www.youtube.com/watch?...
The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know
https://www.youtube.com/watch?...
Hint: There is no conspiracy -- just a Google Talk about entanglement and wave collapse
I'll venture a summary of their experiment and hypothesis, though I didn't read the paper itself and I won't swear that it's accurate:
When a single electron enters a container of helium superfluid it repels the surrounding atoms, creating a bubble of definite size, which proceeds to slowly sink to the detector at the bottom at a determinate rate based on it's size - the larger the bubble, the slower it sinks. Before those electron-bubbles reach the detector; however, it is apparently detecting additional, unexplained charges traveling at at least 18 discrete speeds and, more rarely, charges that seem to travel on a continuous spectrum of speeds. They believe it to be unlikely that there are a sufficient range of impurities in the fluid to explain such a large number of speeds, and hence an alternate explanation should be sought.
Their hypothesis is that these additional charges are in fact smaller bubbles formed by electron wave functions being partially reflected at the liquid's surface: on impact an electron may either enter the fluid, or bounce off. Or, thanks to quantum superposition, it may do both simultaneously with varying levels of probability. In the latter case the partial wavefunction that did penetrate the fluid surface could be expected to create smaller (faster) bubbles in a variety of sizes - some of the electron probability is not within the bubble, and so the repulsion effect is lower and the bubble correspondingly smaller and faster moving.
As I understand it the implication is that simply interacting with the helium is insufficient "measurement" to collapse the wavefunction, instead it gets to maintain a partial presence until such time as it interacts with the detector, which measures it's presence with sufficient definitiveness that the electron must then be wholly present or absent. This would be a revolutionary finding as it would be the first time that a superposition of states has been detected to measurably impact the interaction of a particle with its environment - in all previous QM experiments when a wavefunction collapsed and a single particle was detected, its position and velocity were consistent with the history of a single classical particle traveling along the path that ended in detection, and superposition could only be detected in the statistical distribution of detections, such as the interference patterns of a two-slit experiment.
If correct, this could be a major step forward in determining what exactly constitutes a "measurement" for the purposes of collapsing a quantum wavefunction, a question which has thus far gone almost completely unanswered and spans the complete range from the vague "interaction with the macroscopic world" to the quasi-mystical "observed by a conscious mind"
--- Most topics have many sides worth arguing, allow me to take one opposite you.
The particle (an electron in their experiment) still collapses to only one of those states. I have an analogy that may help.
Suppose that you have a groundhog to catch, and this groundhog has dug a network of tunnels around your property. Every night, he comes out and eats your veggies, and you know that he will come out of only one of his holes. You know as well that he tends to favor some holes more than others. So, you are advised to place a trap at the most likely hole and keep trying until you catch him.
Then, a scientist from Brown University calls you up, and says, "Wait! All of those holes are important! Place one trap at every hole." That's what you do, and instead of waiting however many nights it could take to catch the groundhog by chance, you catch him on the first night. Now you have a groundhog in one trap, and you have all the other traps marking the holes. That makes it easy to deal with the groundhog while keeping the holes marked for landscapers to come.
So, you can study the one groundhog and you can study all the holes, but the groundhog still only got caught in the one trap.
The electron is still only observed in one state because there's only one electron and the wave function still collapses to that one state upon observation. But every state it might have collapsed to is marked, and those states can be observed and studied even though they don't have electrons.