Posted on 10/12/2005 3:10:28 AM PDT by PatrickHenry
A hundred years ago, we took the first steps in recognising, at the level of elementary physical events, the dual character of nature that had been postulated in natural philosophy. Albert Einstein was the first who saw Max Planck’s quantum hypothesis leading to this dual character. Einstein suggested the photon have an electromagnetic wave character, although photons had previously been considered as particles. That was the quintessence of his work on the photoelectric effect. Later in 1926, it was deBroglie that recognised that all the building blocks of nature known to us as particles - electrons, protons, etc. - behave like waves under certain conditions.
In its totality, therefore, nature is dual. None of its components can only be considered as a particle or as a wave. To understand this fact, Niels Bohr introduced in 1923 the Complementarity Principle: simply put, every component in nature has a particle, as well as a wavelike character, and it depends only on the observer which character he sees at any given time. In other words, the experiment determines which characteristic one is measuring - particle or wave.
His whole life long, Einstein suspected [PH: shouldn't that be "doubted"?] that natural characteristics actually depend on the observer. He believed that there must be a reality independent of the observer. Indeed, quantum physics has simply come to accept as a given over the years that there does not seem to be an independent reality. Physics has ceased questioning this, because experiments have confirmed it repeatedly and with a growing accuracy.
The best example is Young’s double-slit experiment. Coherent light is passed through a barrier with two slits. On an observation screen behind it, there is a pattern made of light and dark stripes. The experiment can be carried out not only with light, but also particles - for example, electrons. If single electrons are sent, one after the other, through the open Young double slit, then a stripe-shaped interference pattern appears on the photo plate behind it. The pattern contains no information about the route that the electron took. But if one of the two slits is closed, an image appears of the other open slit from which one can directly read the path of the electron. What this experiment does not produce, however, is a stripe pattern and situation report. For that, a molecular double slit experiment is required that is based not upon position-momentum uncertainty, but on reflective symmetry.
The double-slit was voted the most beautiful experiment of all time in a 2002 poll by Physics World, published by England’s Institute of Physics. Although each electron seems to go alone through one of the two slits, at the end a wavelike interference pattern is created, as if the electron split while it went through the slit, but then was subsequently re-unified. But if one of the slits is closed, or an observer sees which slit the electron went through, then it behaves like a perfectly normal particle. That particle is only at one position at one time, but not at the same time. So, depending on how the experiment is carried out, the electron is either at position A, position B, or at both at the same time.
But Bohr’s Complementarity Principle, which explains this ambiguity, requires that one can only observe one of the two electron manifestations at any given time - either as a wave or a particle, but not both simultaneously. This remains a certainty in every experiment, despite all the ambiguity in quantum physics. Either a system is in a state of "both/and" like a wave, or "either/or" like a particle, relating to its localisation. This is, in principle, a consequence of Heisenberg’s uncertainty principle, which says that given a complementary pair of measurements - for example, position and momentum - only one can be determined exactly at the same time. Information about the other measurement is lost, proportionally.
Recently there has been a set of experiments suggesting that these various manifestations of material can be "carried over into" each other - in other words, they can switch from one form to the other and, under certain conditions, back again. This set of experiments is called quantum markers and quantum erasers. Researchers have shown in the last few years that for atoms and photons - and now, electrons - "both/and" and "either/or" exist side-by-side. In other words, there is a grey zone of complementarity. There are therefore experimentally demonstrable conditions in which the material appears to be both a wave and a particle.
These situations can be described with a duality relation. It can be seen as an extended Complementarity Principle for quantum physics; it can also be labelled a co-existence principle. It says that manifestations of material which would normally be mutually exclusive - e.g., local and not local, coherent and not coherent - are indeed measurable and make themselves evident, in a particular "transition area". One can speak of partial localisation and partial coherence, or partial visibility and partial differentiability. These are measurements that are connected to each other via the duality relation.
In this transition area the Complementarity Principle, and the complementary dualism of nature, can be extended to be a co-existence principle, a parallel dualism. Nature has thus an ambivalent character previously unassumed. Atomic interferometry provides us with examples of this ambivalence. It was first found in 1997 in atoms, which are made from an assembly of particles.
In a recent issue of Nature Max Planck researchers in Berlin, together with researchers from the California Institute of Technology in Pasadena, California, report about a molecular double-slit experiment with electrons - not assemblies of particles, like atoms. Molecules with identical, and thus reflectively symmetrical, atoms, behave like a microscopically small double-slit built by nature. Nitrogen is one such molecule. In it, each electron - also the highly localised inner electrons - stays simultaneously in both atoms. If we ionise such a molecule with a weak x-ray, we end up with a coherent - that is, wavelike - strongly coupled electron emission from both atomic sides. This is just like a double slit experiment with single electrons.
For the first time, the researchers were able to show the coherent character of electron emissions from such a molecule, in this analogue to the double slit experiment. They used a weak x-ray to destabilise the innermost, and thus most strongly localised, electrons of nitrogen from the molecule, and then followed their movement in the molecular frame of reference using ion coincidence measurements. In addition, the researchers succeeded in proving something long doubted: that a disruption of the reflective symmetry of this molecule leads to a partial loss of coherence through the introduction of two different heavy isotopes, in this case N14 and N15. The electrons begin to localise partially on one of the two, now distinguishable, atoms. This is equivalent to partially marking one of the two slits in Young’s double slit experiment. This is partial "which way" information, because the marking gives information about which path the electron took.
The experiments were carried out by members of the working group "atomic physics" of the FHI at the synchrotron radiation laboratories BESSY in Berlin and HASYLAB at DESY in Hamburg. The measurements took place using a multi-detector array for combined electron and ion proof behind what are called undulator beam pipes, which deliver weak x-rays with a high intensity and spectral resolution.
[Lead paragraph, which seems like an abstract, so to make the article read smoother, I moved it to the end.]
In something akin to a double-slit experiment, scientists at the Fritz Haber Institute of the Max Planck Society, in co-operation with researchers from the California Institute of Technology in Pasadena, California, have shown for the first time that electrons have characteristics of both waves and particles at the same time and in virtually the push of a button can be switched back and forth between these states. The researchers provided evidence that disrupting the reflective symmetry of these molecules by introducing two different heavy isotopes, in this case N14 and N15, leads to a partial loss of coherence. The electrons partially begin to localise on one of the two, now distinguishable, atoms. The results could have implications for the building and control of "artificial molecules", which are made of semiconductor quantum dots, and are a possible component of quantum computers. (Nature/i>, September 29, 2005).
Oh, there's plenty of philosophical red meat in quantum physics; it's just that most people get the philosophy wrong. Reality is weird, nonlocal, superposed, indeterminate, random, and even retroactive, but in the final analysis, everyone's measurements of this reality will agree.
Actually, there is only one electron field. Individual electrons are merely excited states of it.
Better watch out. Your grandchildren might decided to wipe you out in your childhood as part of a lab in time travel. It could happen to any of us at any --
I don't like the phrasing much. These are two different experiments with differing setups. There's no a priori reason to expect the same result.
Yep. Electrons are electrons. We assign the concept "wave" or "particle" as is convenient.
It's also a big leap to realize that wave functions are additive, not probabilities. QM probability lies not on a Boolean algebra but on a lattice. Also there are no marginals.
This statement bothered me too and was totally contradicted by the conclusion the article was trying to prove:
. . . have shown for the first time that electrons have characteristics of both waves and particles at the same time . . .
The state isn't determined by the observer, observing doesn't change anything. Both states exist at the same time and it has proved, up until this point, impossible to observe that fact.
This experiment, if it bears out, proves that there is an independent, objective reality and elementary particles exist in both states at the same time.
Only thing left to do is coin a new term to define this state, like "time-space-continuum" did for whatever this nuthouse is we exist in.
But what if the interference pattern appears when you use one electron or photon at a time?
which is how the experiment is done.
"Wave-Particle Duality" is more a description of measurements than of objects. (At least that's what I think today.)
Whatever it is, it makes a lot of experience based intuitive thinking obsolete.
When they talk about the electron double-slit experiment, do they mean GP Thomson and Davisson's experiments? One of my favorite ironies is that nearly a half century after JJ Thomson proved both the electron was a particle, his own son showed that it was also a wave.
I'm not sure. Maybe someone who knows will chime in.
Brings back memories... The lattice of wave functions would be Lebesque integrals. How about fuzzy set theory for marginals?
Bookmark for later.
Check out this quantum computer using quantum bits. They were talking about teleporting.
That is one way electrons could be in two places at the same time...
The reason they tend to think that is due to the Heisenberg uncertainty principle.
Anything you do to measure the system will interact with it. When you get down to atomic scales, your measurement will perturb the system from its original state and information about the system "as it was" gets lost.
Just remember. Schroedinger saved a fortune on kitty litter.
Cheers!
Strange stuff indeed. I've been in grad school studying the stuff for a few years now - I've gotten better at doing the math behind it, but the more I learn about quantum physics the more I realize that my knowledge of it is insignificant in the grand scheme of things, and I can't say my philosophical understanding of it is any better than it was before I starting studying it. Even the simplest results in QM are very difficult to understand thoroughly.
I do wonder what the crazies at PETA would think of Schroedinger and his dead cat if he had done his thought experiment in this day and age.
That would an atomic particle of a different color. I did not pick up that it was a single electron. Thanks for pointing that out.
I would still need to know how many electrons were measured at the target site. If a single electron is always received yet an interference pattern exists then it is out of my league. However, two or more electrons hitting the target when only one was sent can be explained depending upon the voltage potentials at the emitter, mask with the slit and the target.
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