Posted on 07/21/2003 12:28:54 PM PDT by Monty22
I don't think this is quite correct. I pretty sure you cannot get an interference pattern with only one photon. I have always had the impression this experiment was a bit misleading. There is an interference pattern, which is the distribution of photons. If there is only one slit, the photons are distributed somewhat evenly. If two, they are distruted in what appears to be an interference pattern.
As for a detector being on only one slit, it's obvious, if a photon is detected at the slit, it doesn't go through the slit (or at least past the detector). What does it mean to "detect" a photon. They don't make a breeze.
Why do you suppose there is nothing in the literature about putting a detector on both slits?
Hank
Any interaction that transfers information about the state causes the wavefunction to collapse. For some measurements, a single atom of impurity in a crystal is sufficient. There is no requirement that a quantum mechanical "observer" be conscious.
Of course you can't get the entire pattern, but the probability of a single photon arriving at a certain position on a screen, after going through an interferometer, is the same as the pattern one gets with a million photons. If there is a dark fringe somewhere in the million photon case, and you arrange the photon arrival rate such that it's just one at a time through the interferometer, then you will not get any single photons at the dark place one at a time, either. It suggests that photons interfere with themselves. Feynman has made the apparently outrageous claim that photons only interfere with themselves, and never with other ones.
As for a detector being on only one slit, it's obvious, if a photon is detected at the slit, it doesn't go through the slit (or at least past the detector). What does it mean to "detect" a photon. They don't make a breeze.
There are nonlinear optical effects in which a photon can be detected without being absorbed: passage of a photon alters the refractive index of a material, that can be detected with other photons that you don't mind absorbing. (This is called "quantum non-demolition.") But when you non-destructively detect a photon in this way, you alter its phase, just enough to destroy the interference fringes that would have otherwise happened.
Why do you suppose there is nothing in the literature about putting a detector on both slits?
But there is - you can put a non-demolition detector on each slit. If they are arranged to have enough sensitivity to see every photon that passes through, the fringes on the screen completely vanish. If they miss some photons, then you get weak fringes with depth corresponding to how many you didn't detect.
If a tree fell in the woods, and nobody heard it would it still make a sound?
Is light a wave or a particle within a black hole?
... when you non-destructively detect a photon in this way, you alter its phase, ...
Of course, this is really my point. If you "alter" something, it is not the same. A two slit experiment that is not the same cannot be expected to produce the same results.
If they miss some photons, then you get weak fringes with depth corresponding to how many you didn't detect.
But ... if you didn't detect them, how do you know you missed them? (I know, I suppose they show up at the target but weren't counted.)
Just out of curiosity. Does the intereference pattern change depending on the wave length of the photons? If it is a true interference pattern I assume it does.
Thanks! Hank
In college optics the use of a laser made the experiments a lot easier to do. It may work to shine one of those laser pointer gadgets through a narrow slit in an index card and observe the diffraction pattern on an index card beyond that. Try about 2 feet from the laser to the index card, and another 2 feet to the target card.
Then repeat with two slits about an eigth of an inch apart.
That is true, and fundamental to quantum mechanics: an observation necessarily interferes with an experiment, in every case. (And, furthermore, you can't beat the uncertainty principle.)
But ... if you didn't detect them, how do you know you missed them? (I know, I suppose they show up at the target but weren't counted.)
Precisely. You know you missed them because they showed up at the target, but you didn't record which slit they went through. Lo and behold, those photons give interference fringes, but the ones you did notice going through which slit, don't.
Just out of curiosity. Does the intereference pattern change depending on the wave length of the photons? If it is a true interference pattern I assume it does.
Yes it does. The shorter the wavelength, the smaller the fringes. Eventually the wavelength gets so small that the fringes become impossible to see - in the realm of gamma rays and cosmic rays. (Such as far smaller than the diameter of a proton - how are you going to see that?!) This is the case with most kinds of matter waves: BBs or cars are so massive, that their wavelengths are so small, that the fringes are impossibly fine and they wash out and the result looks just like the classical, fringe-free case. This is part of the Correspondence Principle: All quantum phenomena reduce to their classical approximations in the limit of large quantum number. On the flip side, radio wave photons have such a large wavelength, and small energy, that it's hard to see the particle-like properties of them at all, you almost always see the fringes and interference and little else. (They are, for example, to weak to eject electrons in the photoelectric effect.)
Part of this, I think, is a mistake in science, a kind of reification from the principles of elctromagnetic and subatomic "world" to the actual material world of experience. The part I mean is the, "BBs or cars are so massive, that their wavelengths are so small ...."
It is the world of BBs and cars that science studies and discovers the principles that become, eventually, quantum mechanics and relativity. To attribute to the material existence, as it is perceived, the attributes of the material world as it is understood scientifically, invalidates that which is the source of the data science is based on. If cars and BBs aren't what they appear to be, then neither are the meter readings and computer print-outs which are considered "proof" of the scientific principles.
The real world is the one we actually experience. The world of the scientist is only an explanation of how that real world works. The scientists world is not "more real" than the one we perceive. It is real enough, but its reality depends on the one we perceive, not vice-versa. Take away the world of cars and BBs and what science studies is nothing.
(The wave/particle duality, for example, is an explanation for the seeming contradictory behavior of light, observed not at the level this phenomena actually occurs at, but in the world we actuall see. No "light waves" are actually observed, only phenomena which a wave interpretation fits. No light "photons" are actually obeserved, only phenomena photons are appropriate ways of picturing.
The math involved in these interpretations is correct, and the "pictures" (photon particles versus electromagnetic waves) are good ones, though inventions. Nevertheless, they seem to get scientists and a lot of philosophers into trouble when they make the mistake of taking those pictures and attributing them to the material world actually observed. This is especially true of some absurd interpretations of quatum mechanics, and misapplications of the uncertaintly principle and non-locality. [It is the same mistake Pythagoras made about numbers and the reason incommesurables almost destroyed him.]
By the way, I appreciate your thoughtful and helpful responses. My concern is that some ways of interpreting science today are actually undercutting the validity of science, and that some of that is the fault of scientists themselves.
I am very interested in your comments.
Hank
That's an entirely reasonable connection. Everyday objects are made of atoms too - and maybe the laws that govern things you can see continue to apply at small scales (or, very high speeds). Newton wondered whether the force that makes an apple fall is the same thing that keeps the moon in orbit. But, then again, the days of "everyday experience" are long gone. The microscope is many hundreds of years old, the telescope likewise. The spectroscope as well. You can already get a glimpse of the atomic theory of matter from just a microscope, seeing the Brownian movements of dust particles in air or water: If matter were continuous, then there wouldn't be the statistical fluctuations that make particles jiggle. And you can see these jiggles pretty easily with a microscope. Similarly, astronomers, doing nothing other than writing down the positions of planets and stars from time to time, and compared those positions to calculations. They noticed that the orbit of Mercury was precessing, and couldn't explain away the motion like they could for Saturn, by postulating an as-yet unseen planet.
It is the world of BBs and cars that science studies and discovers the principles that become, eventually, quantum mechanics and relativity. To attribute to the material existence, as it is perceived, the attributes of the material world as it is understood scientifically, invalidates that which is the source of the data science is based on.
I disagree. Science is about coming up with hypotheses, and seeing which ones are able to withstand tests. Although classical physics is wrong, it applies to the world of our direct experience astonishing well, and in fact is correct in the limit of low velocity and large quantum number. (This the the Correspondence Principle: all QM and relativistic theories MUST asymptotically approach the classical results in these limits - because the classical theory is right in this domain.)
If cars and BBs aren't what they appear to be, then neither are the meter readings and computer print-outs which are considered "proof" of the scientific principles.
Well, they are and they aren't. Matter waves can be observed in physical objects. There exist submicron diameter polystyrene spheres - little balls of plastic - and I believe scattering experiments have been done with them that show diffraction, meaning that they exhibit wavelike properties. But you can see these balls under an optical micrscope (barely) and under an electron microscope easily. They are man-made objects, far larger than atoms. They differ from "real" balls only by being a lot smaller. And they diffract.
The real world is the one we actually experience.
But, we can experience microscopes and diffraction gratings, so we can see Brownian motion or spectral lines. We can see flames change color when you put different salts in it. We can see a superconductor float on a magnet, or watch liquid helium suddenly fall through a solid ceramic plate. True, most people haven't seen all of these things (but, I have) but in principle, all of these things are available as direct experiences to any person's senses. And some of these things I describe, nearly all people have seen. Scientists propose a model that explains these things - though the model is complex and takes some years in school to even begin to comprehend.
The world of the scientist is only an explanation of how that real world works. The scientists world is not "more real" than the one we perceive. It is real enough, but its reality depends on the one we perceive, not vice-versa. Take away the world of cars and BBs and what science studies is nothing.
I disagree. Even regular people can slide a strong magnet down an aluminum or copper plate, and see that it slides slower than the friction can possibly account for. Magnets and metal plates are things in the real world, that regular people have access to. I have an explanation for what they are seeing - eddy current damping from Lenz's law - what is their explanation? Just because they didn't take Electricity and Magnetism doesn't mean the effect I describe isn't part of a potential everyday experience, because most people have magnets (on the fridge) and flat pieces of copper or aluminum (serving trays, pots and pans, cookie sheets). It helps that the magnets are very strong, but even these are sold, they are not rare like liquid helium is.
No "light waves" are actually observed, only phenomena which a wave interpretation fits. No light "photons" are actually obeserved, only phenomena photons are appropriate ways of picturing.
That is true and well stated.
Nevertheless, they seem to get scientists and a lot of philosophers into trouble when they make the mistake of taking those pictures and attributing them to the material world actually observed. This is especially true of some absurd interpretations of quatum mechanics, and misapplications of the uncertaintly principle and non-locality. [It is the same mistake Pythagoras made about numbers and the reason incommesurables almost destroyed him.]
I disagree here too, at least in part. It is not a mistake to correct for relativity or QM for regular objects, though the correction is impossibly small. It's a waste of time, perhaps, but not a mistake. That said, some of the ideas about "quantum consciousness" and other such mumbo-jumbo is absurd, and I will agree the people talking about it probably don't know what they are talking about. At least they aren't able to come up with falsifiable hypothesis, which would bring them into the realm of science.
My concern is that some ways of interpreting science today are actually undercutting the validity of science, and that some of that is the fault of scientists themselves.
The "validity of science" is pretty clear to those who understand what science is and what it tries to accomplish. It's true, things appearing to be science might actually be political agitprop (Re: Arthur Kellerman's legendarily fraudulent gun grabbing studies, "A gun in the home is 43 times more likely to kill friend or family than an unknown intruder ...") but when a scientist tells you that a particle of mass M traveling at velocity V has a de Broglie wavelength of h / (M V), it may be difficult to demonstrate that he is wrong, but there is a lot of evidence that he is right.
BBs or cars. I totally agree, everything is made of atoms (or parts of atoms, to include plasmas), and to the extent the wave/particle properties pertain to subatomic particles, of course they pertain to those things comprised of them.
Science is about coming up with hypotheses, and seeing which ones are able to withstand tests. It's a little more than that, like discovery and identification, but I would accept that.
Although classical physics is wrong... If you had said, "in terms of what we know today, classical physics is inaccurate," I might have agreed. Classical physics was not wrong, and it's discoveries are still valid. Classical physics did not address everything (it couldn't), but what it addressed, it got right. In fact, if it were wrong, all of modern physics, which is built on it, is wrong too.
What was wrong with classical physics were those who thought classical physics had all the answers. In that vain, modern physics is just as wrong.
(This the Correspondence Principle: all QM and relativistic theories MUST asymptotically approach the classical results in these limits - because the classical theory is right in this domain.)
Yes, more eloquently put.
But, we can experience microscopes and diffraction gratings, so we can see Brownian motion or spectral lines. We can see flames change color when you put different salts in it. We can see a superconductor float on a magnet, or watch liquid helium suddenly fall through a solid ceramic plate.
Yes, exactly
Scientists propose a model that explains these things
That's right. (But they are only models. The models are never more real then the things they explain.)
You quoted me, No "light waves" are actually observed, only phenomena which a wave interpretation fits. No light "photons" are actually obeserved, only phenomena photons are appropriate ways of picturing.
Then said: That is true and well stated.
I guess I should have stopped there, because that is my whole point. The "models" and explanations of science, I believe are correct, when they have been tested (especially by falsifiability), and no evidence contradicts the conclusions. My point is, while the theories are correct, especially in their mathematical basis, the "pictures" of "models" are often mistaken for actualities in themselves, as though light really was little hard corpuscles and transverse waves at the same time, and not what it actually is, a phenomena that can be pictured that way, which is convenient for attaching the numbers to.
The "validity of science" is pretty clear to those who understand what science is and what it tries to accomplish. (I agree) It's true, things appearing to be science might actually be political agitprop (Re: Arthur Kellerman's legendarily fraudulent gun grabbing studies, "A gun in the home is 43 times more likely to kill friend or family than an unknown intruder ...") (True, but I was not talking about junk science.) ...but when a scientist tells you that a particle of mass M traveling at velocity V has a de Broglie wavelength of h / (M V), it may be difficult to demonstrate that he is wrong, but there is a lot of evidence that he is right (of course I agree, but if that same scientist tells me my cat is not dead until someone has looked, I know it is time for him to retire.
Hank
I don't think so. Classical physics has stated matter can't be created or destroyed ... but that ignores pair production and radioactive decay - up to and including atomic bombs. Classical physics says you can rotate an object at any angular velocity, such that angular momentum can assume any value. Not true: angular momentum is quantized. Classical physics has no mechanism to alter the passage of time at high speed, or to alter mass at high speed. But time dilation and relativistic mass increase are both well-known - and time dilation can be observed on satellites, especially GPS satellites whose job is to keep very good time. These are the sorts of things that I mean when I say "classical physics is wrong." It is an approximation to the expermentally observable truth, it is not exact or "correct." Admittedly it is a very, very good approximation applicable to a wide variety of phenomena, but sometimes it is wrong and sometimes greiviously so. Negative thermal expansions are out of the question, classically. Another serious problem was the orbit of Mercury. Atoms and spectra were a third. Classical physics made predictions, and they were simply wrong.
In fact, if it were wrong, all of modern physics, which is built on it, is wrong too.
Not true. The fact that previous versions of theories are wrong doens't make the final modifactions wrong. The Law of Cosines isn't made wrong simply because the Pythagorean theorem, which it is a "correction to," doesn't apply to triangles that aren't right triangles. Relativity cleans up the high-speed and high-gravity cases of classical physics, and QM addresses the small-particle, low quantum number cases. Even so, the present theories are also incomplete (or, wrong) because a quantum theory of gravity hasn't yet been formulated that correctly incorporates known facts, AFAIK. Furthermore, gravitons have never been observed even in the way in which photons have been.
What was wrong with classical physics were those who thought classical physics had all the answers.
This isn't a problem with classical physics, it's a problem with people with false hopes.
In that vain, modern physics is just as wrong.
That may well be true.
My point is, while the theories are correct, especially in their mathematical basis, the "pictures" of "models" are often mistaken for actualities in themselves, as though light really was little hard corpuscles and transverse waves at the same time, and not what it actually is, a phenomena that can be pictured that way, which is convenient for attaching the numbers to.
QM has a convenient and irritating answer to this: it states that you can not talk about things that you can't measure. We can't talk about what light "is" - we can only talk about, if you put a screen with holes here And here, and put a detector there, you will see this pattern if you move the detector side to side - unless you try to see which hole the photon went through, in which case you will see this other pattern instead. We still don't know what light is - say, wave or particle or what, but we can state how it behaves under any situation.
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