When I took Steven Weinberg's undergrad quantum mechanics class in 1998, he said in the second or third class, "I assume you all know what an electron is, what a proton is." This chagrined me enough that I immediately spoke up and said, "I don't know what an electron is."
I'd just written a master's thesis, at what's now Texas State Univeristy-San Marcos, questioning the assumption of an electron as a "point" particle, and pointing out that the electron was/is still a mystery. Weinberg's choice of words was not ideal. If he'd said something like, "I assume you are all familiar with the properties of electrons and protons," I would not have spoken up. The endpoint of my classroom conversation with the good professor Weinberg was that I said mainly I was interested in what it means to "find" an electron.
This is one of those peculiar quantum mechanics things that are too often taken for granted: we talk, for instance, about the wavefunction giving the probability of finding an electron--the square of the wavefuntion is interpreted as a probability density--but we don't say or think much about what the physical process for "finding an electron" is. In answer to my question, Weinberg mentioned a photographic plate as one method of finding where an electron is (was), and a cloud chamber as another.
The photographic plate or just photographic film can be used as a detector in the so-called two-slit experiment, which is famous for showing the wave nature of electrons and other elementary particles. The set up is like a slide projector and a screen. Electrons go through the closely-spaced, very narrow slits (projector) and hit the faraway photographic plate (screen), leaving little dots where they hit. Like a microscopic kind of pointillism, and defying all logic, the dots form a wave interference pattern of bright and dark bands on the screen. Which slit a particular electron went through can't be determined without causing the wavelike property (interference) to disappear. So the observer--the experimenter, or photographer--can observe either property, wave or particle, but not both simultaneouusly, by her choice of experimental set-up. (More than two slits could be used, but the interference pattern is simplest with two.)
An example of the opposite or complementary case from particles behaving like waves is the photoelectric effect, where ultraviolet light hits a clean metal surface and ejects electrons from the surface. As explained by Einstein in 1905, it's necessary (and sufficient) to imagine a single-energy unit of light, now called a photon, being responsible for the ejection of a single electron. The wave description of light propagation just fails completely here, as does the particle model of an electron in explaining the interference pattern in the two-slit experiment. Which could be called the two-hole experiment if tiny round holes are used instead of slits.
So, anyway what we have in general on the sub-microscopic scale is the wave-particle duality, while on the large scale of everyday life, we have the love-hate duality in the case of the not-so-elementary psychological-physical-emotional-spiritual entity known as the person (per-SAHN). Hard to figure, also.
Today I present photos (below) not of the traces of electrons, but of an alligator gar (small) and a water moccasin (big) that I took last summer from the platform shown in the photo at left. You have to look carefully to find the snake in the mud (an S shape, bottom photo). Well, the gar too, for that matter (hint: it's horizontal, parallel to my bare foot, not much more than a shadow).
I'd just written a master's thesis, at what's now Texas State Univeristy-San Marcos, questioning the assumption of an electron as a "point" particle, and pointing out that the electron was/is still a mystery. Weinberg's choice of words was not ideal. If he'd said something like, "I assume you are all familiar with the properties of electrons and protons," I would not have spoken up. The endpoint of my classroom conversation with the good professor Weinberg was that I said mainly I was interested in what it means to "find" an electron.
This is one of those peculiar quantum mechanics things that are too often taken for granted: we talk, for instance, about the wavefunction giving the probability of finding an electron--the square of the wavefuntion is interpreted as a probability density--but we don't say or think much about what the physical process for "finding an electron" is. In answer to my question, Weinberg mentioned a photographic plate as one method of finding where an electron is (was), and a cloud chamber as another.
The photographic plate or just photographic film can be used as a detector in the so-called two-slit experiment, which is famous for showing the wave nature of electrons and other elementary particles. The set up is like a slide projector and a screen. Electrons go through the closely-spaced, very narrow slits (projector) and hit the faraway photographic plate (screen), leaving little dots where they hit. Like a microscopic kind of pointillism, and defying all logic, the dots form a wave interference pattern of bright and dark bands on the screen. Which slit a particular electron went through can't be determined without causing the wavelike property (interference) to disappear. So the observer--the experimenter, or photographer--can observe either property, wave or particle, but not both simultaneouusly, by her choice of experimental set-up. (More than two slits could be used, but the interference pattern is simplest with two.)
An example of the opposite or complementary case from particles behaving like waves is the photoelectric effect, where ultraviolet light hits a clean metal surface and ejects electrons from the surface. As explained by Einstein in 1905, it's necessary (and sufficient) to imagine a single-energy unit of light, now called a photon, being responsible for the ejection of a single electron. The wave description of light propagation just fails completely here, as does the particle model of an electron in explaining the interference pattern in the two-slit experiment. Which could be called the two-hole experiment if tiny round holes are used instead of slits.
So, anyway what we have in general on the sub-microscopic scale is the wave-particle duality, while on the large scale of everyday life, we have the love-hate duality in the case of the not-so-elementary psychological-physical-emotional-spiritual entity known as the person (per-SAHN). Hard to figure, also.
Today I present photos (below) not of the traces of electrons, but of an alligator gar (small) and a water moccasin (big) that I took last summer from the platform shown in the photo at left. You have to look carefully to find the snake in the mud (an S shape, bottom photo). Well, the gar too, for that matter (hint: it's horizontal, parallel to my bare foot, not much more than a shadow). In my much younger days, the gar and snake would have been targets for shooting with my .22 rifle. Nowadays the only living thing I shoot is an occasional minnow. Usually I shoot at a piece of wood or other floating target. I did shoot a water moccasin once a couple of years ago at the farm, where I built this platform between two large fields (very quiet out there!). It was unnecessary. I pulled the trigger "without really thinking about anything at all," like the young man who shoots the monkey at the beginning of Denis Johnson's novel Tree of Smoke.
Peace.
