Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: rpfpatrick on 16/07/2008 21:35:56
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Does the double slit experiment work with all wavelengths of the spectrum (e.g. infrared, ultraviolet etc.)?
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Surely not all possible wavelengths have been tested. With an appropriately designed experiment though, it will probably work for any wavelength (in principle at least).
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How big do you think you would need the kit to be for 1500m broadcast signals to be investigated? But , if you were after the quantum aspect of the experiment, single photons with extremely small energies would be very difficult to look at.
But (why) should we expect em waves to behave very differently at extreme ends of the spectrum?
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I would expect the wavelength to not matter either, but it is a good question. One gets used to Quantum Mechanics only applying at a small scale. It is interesting to think about superposition of probability amplitudes at very long wavelengths. A Youngs double slit experiment where you can actually stand between the slits is an interesting thought. Of course you can still get classical wave interference. The issue would be, as Sophie says, the low energy of the photons. Are there any cases of microwaves behaving like a particle so that a suitable detector can be devised or a way of producing a single photon?
One way this can be done as a thought experiment is to imagine that there is a photon produced through an electron falling from one atomic shell to another but that the atom is arranged to be moving rapidly away from the apparatus so that the photon wavelength is stretched by Doppler effect. The photon could be detected by another atom on the other side of the apparatus moving towards the apparatus. Both atoms would see the photon as a particle but the apparatus would see it as a wave. Although difficult to engineer in practice, it would seem to show no limitations on the wavelength that would change the experimental outcome of there being wave-particle duality.
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"But (why) should we expect em waves to behave very differently at extreme ends of the spectrum?"
Does the spectrum have ends?
Diffracting gamma rays would be tricky, nothing is small enough to act as much of a grating.
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Thanks for all the replies. What really set me thinking was wondering whether something similar happens with things like radio waves and heat or is it only with 'visible light'? Is it just a question of altering the size of the 'slits'? [???]
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graham d
an electron falling from one atomic shell to another
It would be a good idea to dispell the idea that photons can only be produced by "electrons falling through shells". That is only true for a relatively narrow range of em frequencies (optical). It is much better to talk in terms of electronic charges changing their 'position' or energy state within an electric field. That allows for both extremes- electrons in a wire (Microvolts:RF) and electrons leaving a nucleus (Megavolts: gamma.
The first model for teaching quantum theory may have been the hydrogen atom but it shouldn't be used exclusively.
The 'slits' experiment is a very simple and reproducible example of diffraction with visible light. There are many other examples of diffraction which can be seen at many different frequencies. MF Radio reception depends upon it.
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Sophie, the purpose of using the "electron falling through shells" thought experiment was exactly to show that a wave-particle duality that people are familiar with, should be extendable to all wavelengths, which people are not familiar with. At high energies the photons tend to behave more like particles and at low energies they behave more like waves, but the duality still is present in the nature of the beast. It just happens that around the visible spectrum the duality is more evident. It isn't surprising that there is diffraction in radio waves but I can't think of any examples of a radio wave behaving like a particle though there must be some I guess. At the other end of the spectrum, there is X-ray diffraction through crystal lattices but direct evidence of wave behaviour at much higher energies (for photons) is less common.
It is interesting that there is a physical representation of an elecromagnetic wave, giving it a tangible quality. Here the difficulty is in seeing why it might behave as a quantum of energy. When seeing that a (tangible) particle (like an electron) can also behave like a wave, this is not so easy to understand because there seems no physical nature to the wave; it is just a probability amplitude. Perhaps there is a tangible pair of fields (in some space) responsible for the amplitude wave of an electron but I don't know what these would be. I would be interested to know if this concept falls out of any of the recent GUT theories.
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Sophie, the purpose of using the "electron falling through shells" thought experiment was exactly to show that a wave-particle duality that people are familiar with, should be extendable to all wavelengths, which people are not familiar with.
Fair enough, for the cogniscenti. But the fact is that the 'shorthand' for em interaction with matter involves the shell thing and it is responsible for a lot of misunderstanding - being so specific.
Of course, we all learned our first Quantum ideas in the context of the H atom. The problem is that many people never get any further.
And, of course, a photon interaction may well not involve 'just' one electron and an atom. Reflection at a metal surface can involve loads of them sloshing around; it's just system energy changes.
You make an interesting distinction between em and matter particles / waves - the more familiar form in each case being different. I guess it's a matter of familiarity.
The nature of an electron is more particle-like when it is free (it has a 'trajectory') but more wave-like when in the bound state (standing wave solutions satisfying the Schroedinger equation). The situation is almost the reverse for electromagnetic waves in that their behaviour during propagation is easier (?) to appreciate if you use a wave model but their interaction with matter needs to involve the quantum idea.
We have had some interesting threads about the nature of Photons and how they leave one system and 'find' another. My personal conclusion is that the wave nature of the photon implies a wavelength which is far too great wrt an atom or molecule to allow for a photon to be sent out 'in a particular direction'. It has to spread out (diffraction).
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The first post asked about the double split experiment - ask anyone who claims to know about quantum physics and if they say "yes" then probably they don't..
If we are here... then a wave goes through both splits at once as a probability field. We disturb its measurement by a measurement. Also as Karl Popper said, no theories can be proven, only dis-proven. Therefore our best experiments are those that disprove common belief (however hard that concept is for some of us)
In one thought experiment photons are released one by one and a diffraction pattern is seen at the opposite end - this requires a vector summation of field so the photon must be going through both holes at once, if it is a particle.
The curious and untested prediction is that we imagine a detector placed at say slit one - we reppeat the experiment given that no photons are detected. Do we see a diffraction pattern given than no photons were absorbed?
The popular belief is no - the diffraction pattern will not emerge based on our non measurement - the same as a measurement.
Still how could we test this in real like, more importantly how could we falsify it?
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If you atennuate the light through one of the slits then the interference pattern is modified; the 'nulls' just get shallower and shallower as the pattern approaches the diffraction pattern of a single slit.
Your photon detector in one slit would be catching some or all of the photons through it so a modified pattern would result.
I find it much easier to consider the photon aspect of em only when there is an interaction / detection involved. The rest of the time you can consider the wave. It is essential NOT to say "Yes, but which is it, really?"
To treat the matter as Popper would, sometimes you can find that it's clearly not a wave and, other times, it's not a particle. You could say that it is, in fact, neither!
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thank you for your reply sophie.
"If you atennuate the light through one of the slits then the interference pattern is modified"...
this is however a mistake on your part as "attenuation" assumes a wave and I was speaking of photons one by one. Did this escape you?
Maybe use your spell checker a bit more sophie also.
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And did my second paragraph escape you?
If you equate attenuation with the elimination of a proportion of the photons then you may see a connection. In any case, you do not have to rely on the two slits being exactly the same size. This will always have the effect of distorting the pattern for the reason I gave.
As for "photons one by one". You surely realise that, if you consider, therefore detect, each photon on its own, you have resolved the uncertainty associated with the two alternative paths and you will get NO interference pattern.
I think a sense checker for you is more important than a spell checker for me.
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unintelligible gibberish
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Ian, please try to be polite.
I think the problems with the two slit experiment are easier to fathom when you consider reducing the number of photons (or particles) to such a low level that there is only one photon in the system at any time. The interference pattern still emerges over a suficient period of time. If you place a detector in one of the slits, it will destroy the interference pattern. If the detector is such that only some photons are detected you could consider this as the same as a 100% detector being randomly moved in and out of position. This will clearly produce a combination of single and double slit patterns according to the statistics of the detector's ability to detect. This is another way of looking at it, but is the same as Sophie's answer.
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unintelligible gibberish
That's your problem, not mine, friend. Clearly graham.d understood it.
Perhaps you should do some more reading before you get cross when you don't understand.
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Ian, please try to be polite.
I think the problems with the two slit experiment are easier to fathom when you consider reducing the number of photons (or particles) to such a low level that there is only one photon in the system at any time. The interference pattern still emerges over a suficient period of time. If you place a detector in one of the slits, it will destroy the interference pattern. If the detector is such that only some photons are detected you could consider this as the same as a 100% detector being randomly moved in and out of position. This will clearly produce a combination of single and double slit patterns according to the statistics of the detector's ability to detect. This is another way of looking at it, but is the same as Sophie's answer.
Yes, tend to agree with you BUT, whether you have two slits or no slits, the detector becomes a major part of the experiment and is interacting with the light energy. If it is placed before the slits, it is in the diffraction pattern of the source. The probability of it recording a photon is set by this diffraction pattern. I have thought long and hard about this and, as far as I can see, the only way to view, satisfactorily, what is going on is to describe the situation between source and anything which 'gets in the way' (detector, screen with the slots in or the final projection screen) is to treat the em radiation as a wave.
Try to read the whole of the following before launching into a stinging reply!
You really have to address (or agree on ) the detailed nature of what we call a photon. The only thing about which we all agree is its Energy (hf).
Now, most people just say that a particular photon emerges from the source and makes its way in a predetermined direction until it 'hits' something. So how big would the photon have to be? Either it is a true point object - which we don't like very much- or its 'size' relates, somehow, to the wavelength of the radiation. But how long would it be? people describe photons as 'wave packets' which I understand means a burst of waves - in some kind of envelope. How far would the burst extend in the direction it is considered to be traveling? How wide would it be? Certainly much less than a wavelength because they can 'squeeze' through very tiny holes (radio waves can get through pinholes and be detected). Furthermore, for Photons to arrange themselves according to diffraction / interference patterns, they must 'know about' the whole situation.
I don't want to dictate how you should think about the 'true nature' of a photon and how it actually gets from A to B but any description which you think of brings you up against some real inconsistencies. For instance, if it has to be a wave as it moves then it would be subject to diffraction at its source. That would make it very broad indeed because it would be spreading out. Then, if it is spreading out , how can it interact with only one tiny particle when it reaches its destination (which could be a million light years away). It would have to shrink, instantly on arrival.
Although there are many instances where you can see an effect which can be explained in terms of 'little bullets' (gamma rays making a GM tube click, for instance) I think that the Photon is a bit of a snare and a delusion. You can talk in terms of Photons but, if you really want to get a feel of what is going, you need to treat the whole of the transmission process in terms of waves - which behave perfectly consistently, actually, even down to the effects of passing through transparent media. Then, when a system (atom, molecule, piece of wire etc,) detects one quantum of energy you can say that a Photon has arrived. You don't need to consider it actually traveling at all.
The options of where this Photon could be detected are determined by the 'pattern' of the waves emerging from the source or the slits. We refer to it as a probability function. The only problem with this idea is that everything is wide open until there is an interaction with something. Once this has happened, all other possibilities are immediately eliminated. Every other object in the whole of the Universe 'suddenly' gets the message that "this quantum of energy is spoken for - no one else can have it". This is entanglement and, although it is a difficult concept, it takes care of all the fudging which one gets when trying to use Photons to explain what goes on.
OK your turn now.
We really need to drop down into a deeper stratum of understanding to resolve this completely.
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You won't get a stinging reply from me unless I really get pi**ed off, which happens only rarely :-)
I agree. The particle aspect only occurs at the collapse of the wavefunction. As far as dimensions of a wave go, really this is of infinite extent. It is just that the probabilties get very small over most of space but are higher in what we would consider to be the "classical" regions. With a finely tuned laser the coherence length is very large (and the frequency band necessarily very narrow).
Richard Feynman once said "all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment".
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I can only repeat as I said earlier
"In one thought experiment photons are released one by one and a diffraction pattern is seen at the opposite end - this requires a vector summation of field so the photon must be going through both holes at once, if it is a particle.
The curious and untested prediction is that we imagine a detector placed at say slit one - we reppeat the experiment given that no photons are detected. Do we see a diffraction pattern given than no photons were absorbed?
The popular belief is no - the diffraction pattern will not emerge based on our non measurement - the same as a measurement."
The correspondents argue with themselves not me!
Anyway I know good people from the lesser forms.
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On being polite rather than just self parading - the original question asked was
"Does the double slit experiment work with all wavelengths of the spectrum (e.g. infrared, ultraviolet etc.)? "
Has this been answered with self promo hype?
I would have thought "yes" was correct. But it could have been explained as a particle question than as a probability wave interpretation. Both are simple enough.
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Ian, I can't tell whether you are being insulting or not, so I will give you the benefit of the doubt! If you place a detector capable of detecting a photon and it detects no photons, the logic of it would say that you know that photons have passed through the other slit. The mere presence of the detector has prevented the interference, even though it has not been troubled by a detction. I think the apparent magic is because we insist on trying to think this through as waves or particles but not as wave-particles. We do this because these beasts are not something that we can experience in everyday life and our brains are not accustomed by genetics or learning to cope with the concept. I don't pretend to grasp all the implications of this. I rather agree with my hero, Prof Feynman, on this. It is something which I just have a slight handle on having been convinced by the evidence and the maths.
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graham you say as I say
So why argue?
Maybe it's best not to chat
Quantum physics invites too many with armchair views - I prefer Occam's razor to questions.
Maybe I am insulting or maybe someone else should be so directed.
Is there truth? Or are there questions?
To fail to ask a question is stupid - to inflict failure on others is a crime.
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Ian Scott
In one thought experiment photons are released one by one and a diffraction pattern is seen at the opposite end - this requires a vector summation of field so the photon must be going through both holes at once, if it is a particle.
If you really think that then you haven't considered all its implications. If it "goes through both slots" then how might it be detected going through one of the slots? If it is somehow shared between the slots, how is there energy enough for it to be detected? Does the other 'half' of it go on and 'half' illuminate the screen? It's a Quantum, isnt it? How can you have fractions of it?
Think outside the box. Did you read the whole / any of my earlier , long, post?
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graham-d
It's no fun - you keep agreeing with me!
You have clearly been thinking deeply, too. . . .
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In one thought experiment photons are released one by one and a diffraction pattern is seen at the opposite end - this requires a vector summation of field so the photon must be going through both holes at once, if it is a particle.
"Introduction to the Quantum Theory - Third Edition" David Park. Paragraph 8.5, page 290:
"Can a Particle Be in Two Places at Once?"
I don't want to write down all it's written there with the equations, I will only write the first 3 lines and the last one:
"Thinking of the two-slits experiment one might perhaps say yes, for if it does not in some sense go through both slits there cannot be interference. So does it? Install two counters and find out.
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Detector 1 or 2 may register a count, but not both."
The reason (I add) is that if the particle would pass through both holes, then you can write down a resulting wavefunction which would give no interference and the pattern in the screen would be that of classical particles going through two holes.
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to all correspondents
It is simple to imagine a diffraction pattern from a wave emerging from two splits. What you fail to grasp is non measurement - as the act forces a particle behavior and the wave effect is lost.
Why is this hard for you?
Maybe quantum physics is too simple it needs child play
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this commentary remains absurd - from two silly slits in a piece of paper. The wave probability function "propagates" through both isn't that obvious? There is nothing hard to understand in this.
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this commentary remains absurd - from two silly slits in a piece of paper. The wave probability function "propagates" through both isn't that obvious? There is nothing hard to understand in this.
I don't think you actually know what you're talking about. I certainly don't. To quote Richard Feynman "If you think you understand quantum mechanics, then you don't understand quantum mechanics."
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I never can understand the obsession with the Two Slits Experiment. Diffraction happens everywhere. All the arguments rehearsed here apply in every em interaction in the Universe. People seem to want photons to be different when in a very simplified context.
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this commentary remains absurd - from two silly slits in a piece of paper. The wave probability function "propagates" through both isn't that obvious? There is nothing hard to understand in this.
You have written correctly: THE WAVE propagates through both slits, NOT THE PARTICLE:
so the photon must be going through both holes at once, if it is a particle
THE WAVE it's not THE PARTICLE. Can you understand the difference?
If you don't consider this important difference, you can forget talking of QM.
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I never can understand the obsession with the Two Slits Experiment. Diffraction happens everywhere. All the arguments rehearsed here apply in every em interaction in the Universe. People seem to want photons to be different when in a very simplified context.
I think it's a beautiful experiment because it was so useful in overturning previously held beliefs and contains an incredible amount of physics. The problem is that people assume that the 2-slit experiment is just one experiment that always shows wave/particle duality. All it really does is show that whatever forms the interference pattern behaves as a wave. This has been a huge step at different points in the history of physics, however, and is still one of the most dramatic ways to demonstrate wave behavior. (I'm biased here, since I do work in coherence theory, but its still a nice experiment!)
1) Classical light: The 2-pinhole experiment verifies that light is indeed a wave since it shows interference patterns. You can also investigate the coherence properties of light (i.e. how much randomness is involved--varying from very random/incoherent for black bodies to very ordered/coherent for lasers).
2) Relativistic quantum mechanics: You can explain what's going on by field theory/path integrals. The particle takes all possible paths from each slit, but because of the mathematics involved in QM, only some of the paths end up being useful. There's a great deal of similarity between this and the classical waves, because the mathematics of QM wavefunctions and the mathematics of classical waves end up being similar: i.e. waves can cancel each other out just like the probability wavefunctions. This does show wave/particle duality if you use relativistic electrons, which we already knew behave as particles in many applications.
But I agree with sophiecentaur. The strength of the experiment is in demonstrating that whatever you're shooting at it is a wave. Since most of us would have no doubt about the wave nature of photons (they make up light waves, after all), watching them behave as waves isn't really a surprise... For photons, what is interesting is watching contexts (the photoelectric effect) there they behave as particles rather than waves.
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The strength of the experiment is in demonstrating that whatever you're shooting at it is a wave. Since most of us would have no doubt about the wave nature of photons (they make up light waves, after all), watching them behave as waves isn't really a surprise... For photons, what is interesting is watching contexts (the photoelectric effect) there they behave as particles rather than waves.
More precisely, I would say that they behave as particles exactly when they are detected. What do you think?
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Generally, yes. Usually your field is strong enough to think of it as classical waves, and you then have so many photons hitting your detector at once that you can't pick out each individual hit and just see intensity fringes. If, however, you dial your field down so that you expect only one photon in the detector at a time, then you'll probably see individual "clicks" of detection. You can go back and forth between the models, relating intensity to photon hits, but it generally requires some sophisticated mathematics because you're dealing with quantum statistics.