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 Question Two slit experiment ( Physics Help and Math Help - Physics Forums sci.physics.research )Updated: 2008-09-17 03:15:49 (57)
 Two slit experiment I've been looking for an answer to two questions regarding the 2-slit experiment (showing both wave and particle properties of light). I hope someone here might know how to respond. Quick review of what I believe I know: Shooting a single photon (or electron) at a wall that contains two slits will result in an interference pattern on the wall (detector plate) beyond the holes. Blocking one hole results in an accumulation pattern. When both holes are open, detecting which of the two holes the electron passes through results in an accumulation pattern (uncertainty is violated). (1) Does obtaining an interference pattern depend at all on the timing of the electron gun shooting each new electron? Or would the interference pattern still obtain even if you shot one electron a day for many days (and were able to record the impacts of electrons on the detector plate)? (2) If, in the case where one determines through which slit each electron passes, one were to replace the detector plate with ANOTHER wall containing two slits, would uncertainty be restored? That is, knowing through which slit electrons first pass should produce an accumulation pattern, but if that pattern were displayed on a wall containing two slits (in this case the experimenter does NOT determine through which slit the electron passes) would an interference pattern occur? So, in this double-two-slit experiment, determining the first "choice" made by the pasing electron, but not the second... would there be an accumulation pattern followed by an interference pattern? Thanks!
 Answers: Two slit experiment ( Physics Help and Math Help - Physics Forums sci.physics.research )
 Two slit experiment drspeg wrote: > I've been looking for an answer to two questions regarding the 2-slit > experiment (showing both wave and particle properties of light). I hope > someone here might know how to respond. [[...]] > (1) Does obtaining an interference pattern depend at all on the timing of > the electron gun shooting each new electron? Or would the interference > pattern still obtain even if you shot one electron a day for many days (and > were able to record the impacts of electrons on the detector plate)? Yes, you would still get an interference pattern. That is, the (same) interference pattern is still there even at *arbitrarily low* intensities (&& correspondingly long exposure times). I posted a bunch of references on this to this newsgroup on 29.June.2001: http://www.lns.cornell.edu/spr/2001-06/msg0033834.html ciao, -- -- "Jonathan Thornburg -- remove -animal to reply" Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut), Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam
 Two slit experiment drspeg wrote: > > I've been looking for an answer to two questions regarding the 2-slit > experiment (showing both wave and particle properties of light). I hope > someone here might know how to respond. > > Quick review of what I believe I know: Shooting a single photon (or > electron) at a wall that contains two slits will result in an interference > pattern on the wall (detector plate) beyond the holes. Blocking one hole > results in an accumulation pattern. When both holes are open, detecting > which of the two holes the electron passes through results in an > accumulation pattern (uncertainty is violated). A double slit pattern is not the sum of two single slit patterns. For one thing, the double slit maximum is directly opposite the center barrier. You can look it up in a physics text book, http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html http://www.motionmountain.net/ > (1) Does obtaining an interference pattern depend at all on the timing of > the electron gun shooting each new electron? Or would the interference > pattern still obtain even if you shot one electron a day for many days (and > were able to record the impacts of electrons on the detector plate)? Timing is irrelevant. Each individual moiety's wavefunction passes through both slits - photon, electron, or huge giant really big spatially extended rigidly multiply-connected massive lump, http://www.quantum.univie.ac.at/rese...c60/index.html C60 diffraction > (2) If, in the case where one determines through which slit each electron > passes, one were to replace the detector plate with ANOTHER wall containing > two slits, would uncertainty be restored? That is, knowing through which > slit electrons first pass should produce an accumulation pattern, but if > that pattern were displayed on a wall containing two slits (in this case the > experimenter does NOT determine through which slit the electron passes) > would an interference pattern occur? Google "quantum eraser" 15,400 hits But why stop there? http://en.wikipedia.org/wiki/Delayed...quantum_eraser > So, in this double-two-slit experiment, determining the first "choice" made > by the pasing electron, but not the second... would there be an accumulation > pattern followed by an interference pattern? Here's a nice Gedankenexperiment. We take an organic molecule that degenerately rearranges at a tremendous rate, like semibullvalene (Ea=5.5 kcal/mol), http://faculty.juniata.edu/reingold/rsch.html http://en.wikipedia.org/wiki/Semibullvalene and we do the C60 experiment with it. However... when we thin-film fabricate the diffraction grating we apply an alternating Peltier heater so alternate slits are cold and hot. When a semibullvalene wavefunction passes through the slits it has different rearrangement rates at the different temperatures. Find a set of conditions that dephases the wavefunction by exactly 180 degrees slit vs. slit. When the two halves recombine on the other side... destructive interference! Where is the molecule? -- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz3.pdf
 Two slit experiment Jonathan Thornburg -- remove -animal to reply wrote: > drspeg wrote: > > I've been looking for an answer to two questions regarding the 2-slit > > experiment (showing both wave and particle properties of light). I hope > > someone here might know how to respond. > [[...]] > > (1) Does obtaining an interference pattern depend at all on the timing of > > the electron gun shooting each new electron? Or would the interference > > pattern still obtain even if you shot one electron a day for many days (and > > were able to record the impacts of electrons on the detector plate)? > > Yes, you would still get an interference pattern. That is, the > (same) interference pattern is still there even at *arbitrarily low* > intensities (&& correspondingly long exposure times). I posted a > bunch of references on this to this newsgroup on 29.June.2001: > http://www.lns.cornell.edu/spr/2001-06/msg0033834.html Here are a couple of other references: http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm http://www.hqrd.hitachi.co.jp/em/movie.cfm, number (2) The first link describes a double slit experiment using a very low emission electron gun. The second one shows a movie of how the interference pattern is actually accumulated. The interference pattern appears even when the electron gun essentially fires a single eletron at a time. Igor
 Two slit experiment * Igor Khavkine: > Jonathan Thornburg -- remove -animal to reply wrote: >> drspeg wrote: >>> (1) Does obtaining an interference pattern depend at all on the timing of >>> the electron gun shooting each new electron? Or would the interference >>> pattern still obtain even if you shot one electron a day for many days (and >>> were able to record the impacts of electrons on the detector plate)? >> Yes, you would still get an interference pattern. That is, the >> (same) interference pattern is still there even at *arbitrarily low* >> intensities (&& correspondingly long exposure times). I posted a >> bunch of references on this to this newsgroup on 29.June.2001: >> http://www.lns.cornell.edu/spr/2001-06/msg0033834.html > > Here are a couple of other references: > > http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm > http://www.hqrd.hitachi.co.jp/em/movie.cfm, number (2) > > The first link describes a double slit experiment using a very low > emission electron gun. But are there experiments of good standing that directly rule out any memory of past events in the detector, e.g. with hours between single events, thermal randomization between events, or the like? I think that that, not single particle at a time, was essentially what the OP (drspeg ) asked. The reference list directly provided by Jonathan didn't seem to provide any such reference, and in fact, of the four URLs listed in that article, the three last URLs didn't work (probably moved). And although there were many follow-ups to the posting that Jonathans's was a response to, all of these claiming an abundance of experimental evidence, not one provided a reference to such an experiment. The closest was perhaps . But seemingly also that that was just another statistically-one-at-a-time experiments. Thus, my curiosity is pickled[1]! ;-) [1] -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail?
 Two slit experiment Igor Khavkine writes >The first link describes a double slit experiment using a very low >emission electron gun. The second one shows a movie of how the >interference pattern is actually accumulated. The interference pattern >appears even when the electron gun essentially fires a single eletron >at a time. These are all very important experiments. 1) The self-interference of both photons and electrons (and much larger things like fullerenes). These have all been demonstrated. 2) There is also the interference of two sources each going through one slit **when intensity is so low that the probability of finding two 'photons' in the apparatus at the same time is very low. Now this has been demonstrated for photons. However the laser light used for each beam (separate lasers) had extremely long coherence lengths (times). To do this with electrons might be problematic (ie "challenging") but would be interesting. Anybody any ideas how it could be done? -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment "drspeg" wrote in news:R62dnftk_8SSZQrZnZ2dnUVZ_qadnZ2d@libcom.com: > I've been looking for an answer to two questions regarding the 2-slit > experiment (showing both wave and particle properties of light). I hope > someone here might know how to respond. > > Quick review of what I believe I know: Shooting a single photon (or > electron) at a wall that contains two slits will result in an > interference pattern on the wall (detector plate) beyond the holes. Wrong. A single photon will produce a single point response somewhere on the detector plate. The probability of hitting any one point can be determined by accumulating data from a series of single photons(or electrons). A series of single photons will build up a pattern similar to that seen when a continuous stream of photons is used. > Blocking one hole results in an accumulation pattern. When one hole is blocked, and a series of single photons is used, the accumulation pattern will look similar to that seen when a continuous stream of photons is used with a single slit. > When both holes > are open, detecting which of the two holes the electron passes through > results in an accumulation pattern (uncertainty is violated). > > (1) Does obtaining an interference pattern depend at all on the timing > of the electron gun shooting each new electron? Or would the > interference pattern still obtain even if you shot one electron a day > for many days (and were able to record the impacts of electrons on the > detector plate)? Appears to be rate independent. Photon(Electron) multiplyer arrays can be used to capture and record the location of the impacts over a long period of time. > > (2) If, in the case where one determines through which slit each > electron passes, one were to replace the detector plate with ANOTHER > wall containing two slits, would uncertainty be restored? That is, > knowing through which slit electrons first pass should produce an > accumulation pattern, but if that pattern were displayed on a wall > containing two slits (in this case the experimenter does NOT determine > through which slit the electron passes) would an interference pattern > occur? Perform the experiment and see. > > So, in this double-two-slit experiment, determining the first "choice" > made by the pasing electron, but not the second... would there be an > accumulation pattern followed by an interference pattern? > > Thanks! > -- bz please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+spr@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
 Two slit experiment Alf P. Steinbach wrote: > [...] > But are there experiments of good standing that directly rule out any > memory of past events in the detector, e.g. with hours between single > events, thermal randomization between events, or the like? I remember having read about a double-slit experiment where the whole setup was disassembled and then (after some days) reassembled between the single events. It was even set up at different places. After a year (or so) they could clearly see the interference pattern. I cannot think of any memory effect in such a setup. Unfortunately, I don't know the reference. Could have been some Japanese people. Has anybody a clue of where to find this reference? Andreas.
 Two slit experiment Andreas Most > I remember having read about a double-slit experiment > where the whole setup was disassembled and then > (after some days) reassembled between the single > events. This one? http://leifi.physik.uni-muenchen.de/...r/taylor_e.htm In 1909 Geoffrey Ingram Taylor conducted an experiment in which he showed that even the feeblest light source could lead to interference fringes. "The longest experiment took 3 months, corresponding to the intensity of a candle more than a mile away" It seems that this experiment led to Dirac's famous koan "each photon then interferes only with itself".
 Two slit experiment scerir writes >It seems that this experiment led to Dirac's famous koan >"each photon then interferes only with itself". Except we know this is not correct. Experiments have been done where two separate lasers fired through two separate slits produces an interference pattern. Furthermore this happened even if "the probability of two photons being in the apparatus at one time approached zero". In any case anyone who has listened to MW radio knows that separate radio transmitters on the same frequency can readily cause interference patterns. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment bz wrote: > Wrong. A single photon will produce a single point response somewhere > on the detector plate. The probability of hitting any one point can be > determined by accumulating data from a series of single photons(or > electrons). Nop, that is a misconception or mischaracterization of the empirical facts in photon experiments. The facts are that the response of an array of photodetectors is statistically indistingushable from droplets statistics of an array of dripping faucets. Specifically, the best (the most narrow distribution of triggers) that you will ever get here is a Poissonian distribution (as long as QED vacuum is there and its vacuum fluctuations). There isn't anything 'single-photon-like' about it that can distinguish it from a thresholded EM field measurements (with photo-electron current measurements results reduced via AD conversion by the 'pulse analayzer & discriminator' electronics to a single bit precision, 0/1 values, based on the detector thresholds) of a classical field. Nothing distinctly non-classical or non-local (such as the conjectured instant collapse of a remote EM field) occurs with photo-detections in double slit/beam splitter experiments. (Otherwise, you wouldn't need Bell inequality violations tests as the exclusive QM prediction capable of making such distinction). When a detector placed on photon path A triggers, the detector at a remote path B will trigger or not trigger entirely independently (and solely based on its local field intensities) of what happened with the detector in path A. In other words, the trigger statistics on A and B detectors is exactly as if the two wave packet fragments travelled each on its own to its detector, then triggered it or not based solely on the total incident EM energy on a photo-cathode within the sampling window (the 'incident field' consists of the 'signal' EM field superposed to the ZPF or vacuum fluctuations at the cathode). For example if you take the array of pair results (A,B) which are (0,0), (0,1), (1,0) and (1,1) and split it into two sub-arrays based on results on A, hence as (1,x) and (0,y) sub-arrays, you won't find statistically smaller proportion of 1's among x values measured on B (the so-called collapse on B when A triggers) than among the y values measured on B. Of course, as with dripping faucets, you can make the detectors virtually never produce double trigger in a given sequence of sampling time windows, but you can do exactly the same with the faucets in the exactlly same way -- either shrink the sampling windows enough or reduce the input intensity/water pressure enough. In both cases if you reduce double triggers to single triggers ratio by a factor f, you will reduce the single trigger to no-trigger ratios by at least the same factor f, i.e. a setup that virtually never has a double trigger among triggers, virtually never has a single trigger among non-triggers, either. It is a trivially unsurprising phenomenon of Poissonian distribution in the limit p -> 0. There were a various experments (since 1950s) trying to establish "single photon" phenomenon i.e. the trigger statistics which would be distinguishable from that of the array of dripping faucets. It just doesn't happen. You can read on the very latest one such experiment claiming demonstration of "single photons" and its detalied analysis (showing exctly how it cheats) here: PhysicsForum thread: Photon "Wave Collapse" Experiment... http://www.physicsforums.com/showthread.php?t=71297 The experiment paper: 1. J.J. Thorn, M.S. Neel, V.W. Donato, G.S. Bergreen, R.E. Davies, M. Beck "Observing the quantum behavior of light in an undergraduate laboratory" Am. J. Phys., Vol. 72, No. 9, 1210-1219 (2004). http://marcus.whitman.edu/~beckmk/QM.../Thorn_ajp.pdf The experiment Home Page http://marcus.whitman.edu/~beckmk/QM/ > A series of single photons will build up a pattern similar > to that seen when a continuous stream of photons is used. Again, read or see what actually happens. Nothing statistically distingushes the photo-detector triggers from drips of an array of faucets (e.g. you can have interference of the water waves above the faucets and its effects on the drop counts below). That conjectured phenomenon (demonstration of a "single photon" via "sub-Poissonian distribution) has been a holy grail of the marble-photon branch of Quantum Optics since 1950s when Hanbury Brown and Twiss experiment showed that it's not how the EM field behaves, be it in experiments or theoretically (clasically or in QED). Such demonstration, experimental or theoretical (within QED formalism), has yet to be done. You can see for yourself how miserably the 2004 experiment, the pinnacle of nearly five decades of pursuit, performed and marvel at the ingenuity, the means and the lengths at which the authors went to create the appearance that it finally worked. If you do know of some experiments or a QED derivation (but not some handwaved elementary/popular QM stories) that can distinguish double slit/beam splitter photo-detection trigger statistics from the trivial classical phenomena such as dripping faucets in the Poissonian limit p->0, you are welcome to cite it.
 Two slit experiment On Thu, 29 Jun 2006, Oz wrote: > scerir writes > > >It seems that this experiment led to Dirac's famous koan > >"each photon then interferes only with itself". > > Except we know this is not correct. Experiments have been done where two > separate lasers fired through two separate slits produces an > interference pattern. That just shows that each photon comes from both sources. A photon is the quantum of excitation of the EM field, the EM field is the sum of the fields individually produced by the sources, so each photon comes from both sources. It is an excellent demonstration that photons aren't like classical billiard balls. -- Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/ E-prints: http://eprint.uq.edu.au/view/person/...,_Timo_A..html Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
 Two slit experiment Oz wrote: > scerir writes > >> It seems that this experiment led to Dirac's famous koan >> "each photon then interferes only with itself". > > Except we know this is not correct. Experiments have been done where > two separate lasers fired through two separate slits produces an > interference pattern. Hmm. Interesting. Do you have a reference to a paper or webpage? > Furthermore this happened even if "the probability of two photons > being in the apparatus at one time approached zero". To mee, this indicates that somehow the photon must interfere with itself. > In any case anyone who has listened to MW radio knows that separate > radio transmitters on the same frequency can readily cause > interference patterns. I don't think that this is the same kind of interference. If you add up two amplitude modulated waves of the same frequency, then obviously the result will be garbled, but this has a different explanation than the double slit experiment has.
 Two slit experiment > >It seems that this [Taylor's] experiment led > >to Dirac's famous koan "Each photon then > >interferes only with itself." Oz writes: > Except we know this is not correct. > Experiments have been done where two > separate lasers fired through > two separate slits produces an > interference pattern. Yes, but this does not mean that Dirac's koan is wrong. Even a photon emitted by two sources (in such a way that you cannot say which of the two sources emitted the photon) interferes only with itself. When the different possible photon paths, from sources to detector, are indistinguishable, then we have to add the corresponding amplitudes before squaring to obtain the probability. The second part of the koan (see below) seems obscure (or it needs a reformulation), since we have two-photon interference, that is to say the interference of photons emerging from _independent_ sources. s. "Each photon then interferes only with itself. Interference between two different photons can never occur." - P.A.M. Dirac, Principles of Quantum Mechanics, Clarendon, Oxford, 1930, p.15.
 Two slit experiment Thus spake Oz >Igor Khavkine writes >>The first link describes a double slit experiment using a very low >>emission electron gun. The second one shows a movie of how the >>interference pattern is actually accumulated. The interference pattern >>appears even when the electron gun essentially fires a single eletron >>at a time. > >These are all very important experiments. > >1) The self-interference of both photons and electrons (and much larger >things like fullerenes). These have all been demonstrated. > >2) There is also the interference of two sources each going through one >slit **when intensity is so low that the probability of finding two >'photons' in the apparatus at the same time is very low. > >Now this has been demonstrated for photons. However the laser light used >for each beam (separate lasers) had extremely long coherence lengths >(times). To do this with electrons might be problematic (ie >"challenging") but would be interesting. Anybody any ideas how it could >be done? > It can't be done. You probably don't remember or didn't follow it, but you induced me to calculate interference effects between wave functions for different photons using qed. The same argument did not apply to electrons, which are fermions and which are conserved in interaction. Regards -- Charles Francis substitute charles for NotI to email
 Two slit experiment Oz wrote: > Blackbird writes [...] >> I don't think that this is the same kind of interference. If you >> add up two amplitude modulated waves of the same frequency, then >> obviously the result will be garbled, but this has a different >> explanation than the double slit experiment has. > > Why? Radio waves are perfectly good EM waves just like light but with > different frequency. Sure, so I'll try to explain my point here a little better. Say we have a vertical receiving antenna, and two MW transmitters that transmit waves of the same frequency, but perfectly out of phase (relatively shifted by 1/2 the wavelength) at the location of the antenna. The waves transfer energy inducing electrons in the antenna to accelerate. According to the theory, this energy transfer (and thus the acceleration) is quantized, hence "photons". Now, any random electron will from time to time absorb a photon that either accelerates it in the "up" direction, or in the "down" direction, and since we have two sources with cancelling phases, for any finite (and sufficiently large) interval of time, the electron will absorb approximately as many "up" as "down" photons. The electron will thus exhibit a random walk, and no signal will be detected. This, however, does not mean that the *photons* interfere with eachother. Interference, as in the double slit experiment, would mean that photons from the two different sources cancelled each other (or more precisely, they would be more likely to show up at another location), thus there would be no energy absorbtion by the electron whatsoever.
 Two slit experiment "Timo A. Nieminen" wrote in message news:Pine.WNT.4.64.0607010837130.1212@serene.st... > On Fri, 30 Jun 2006, Oz wrote: > > Clearly (to me at any rate) the 'size' of a massive particle is > > determined (like the photon) by its environment (typically > > quantised). So an electron, say, can have a different physical size > > when undisturbed in an orbital This can in some circumstances be > > very large indeed when orbitals become macroscopic, for example in > > conductors. > > Don't confuse localisation with size. If a photon is "large", it > should be able to interact with and be detected by two spatially > separated detectors at the same time. I don't think we actually know the answer to that "question" since "large" photons might be radio wave photons and so far individual detection of such photons is not possible. Do you know of any experimental limits that we might have for this? IOW, what is the lowest frequency at which individual detection is experimentally possible with current technology? FrediFizzx Quantum Vacuum Charge papers; http://www.vacuum-physics.com/QVC/qu...uum_charge.pdf or postscript http://www.vacuum-physics.com/QVC/qu...cuum_charge.ps http://www.arxiv.org/abs/physics/0601110 http://www.vacuum-physics.com
 Two slit experiment Oz writes > >Personally I vote for experimental results. My question is whether >anyone has any idea how it could be done? Personally I would bet you ?50 >it will show the same result as the photon experiment. Bet rescinded.... It would need to be necessary to have two moving electron CLOUDS (that is covering a large area) that emulate a laser beam. I'm fairly sure that an electron beam does NOT have this characteristic. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Oh No writes >Thus spake Oz >> >>You have been known to make errors, later corrected (like all of us). > >I've given a demonstration again, in response to scerir. It is not >difficult, and nicely illustrates that observables, such as interference >effects, rely on the properties of operators, not just states, and thus >what is wrong with the view of superposition given in Dirac's koan. Yes. Nicely done, I even mostly understood it . >>Personally I vote for experimental results. My question is whether >>anyone has any idea how it could be done? Personally I would bet you ?50 >>it will show the same result as the photon experiment. > >I'll take you up on that. Will you accept this as an electronic >handshake? Earlier today I rescinded same.... >>The initial aim would be to produce extremely monochromatic beams of >>electrons. > >Ahem, you can't have an electron equivalent of a laser. They are >fermions and cannot all be in the same state. Absolutely true, hence (luckily for me) no such experiment can be made. >> Given that we are looking for exceedingly low beam intensity >>I suspect that the problems associated with mutual repulsion of >>electrons within a beam will be negligible (or am I being >>oversimplistic?). > >You are forgetting the exclusion principle, which is rather more >encompassing than charge. Absolutely. Perhaps a massive boson beam would be better. I guess that probably means they would be entangled so as to get the long thin coherent beam. Hmm.... >>One is almost tempted to start with very low energy electrons, perhaps >>separating them by time-of-flight (via a chopper) > >No you wouldn't be allowed to do that either. Remember, when they do >this for photons the stipulation that only one photon comes through per >amount of time is statistical. In one of these low energy laser thingies >any photon can come through at any time, but it is only fed so much >energy so there is only one per amount of time. This was to ensure a very precise *energy*, but I wasn't thinking very carefully and imagining a neutron spallation source sort of thing which is inappropriate. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment > >When the different possible photon paths, > >from sources to detector, are indistinguishable, > >then we have to add the corresponding amplitudes > >before squaring to obtain the probability. Oz writes: > Hmmm.... how very convenient.... Try 'Quantum effects in one-photon and two-photon interference'by L. Mandel (Rev.Mod.Phys.,vol.71,n.2, page S274). It is online (use Scholar Google). > > The second part of the koan (see below) > >seems obscure (or it needs a reformulation), > >since we have two-photon interference, that > >is to say the interference of photons > >emerging from _independent_ sources. > > Which is precisely my point. > The below can be reformulated (I am absolutely confident mathematicians > can do this), basically by saying "take two independent sources, and > call them one source", which gives the right answer at some > philosophical cost. No, the actual meaning of interference changes here. See Mandel's paper or, i.e., http://www.arxiv.org/abs/quant-ph/0603048 Regards, s.
 Two slit experiment On Fri, 30 Jun 2006, Oz wrote: > Timo Nieminen writes >> On Thu, 29 Jun 2006, Oz wrote: >> >>> scerir writes >>> >>>> It seems that this experiment led to Dirac's famous koan >>>> "each photon then interferes only with itself". >>> >>> Except we know this is not correct. Experiments have been done where two >>> separate lasers fired through two separate slits produces an >>> interference pattern. >> >> That just shows that each photon comes from both sources. > > Hmmm.... > > As an explanation I think Mr Occham would find this improbable. While William of Oakham/Occam would likely have found it improbable, due to not knowing about either electromagnetic fields or photons, it is not incompatible with his razor. As I wrote: >> A photon is the >> quantum of excitation of the EM field, the EM field is the sum of the >> fields individually produced by the sources, so each photon comes from >> both sources. > > A contortion IMHO. > A much more plausible explanation is that photons are completely > wavelike. The EM field is completely wavelike. Much, even most, stuff about photons comes from purely classical theory. However, by definition photons are _not_ completely wavelike, being the quanta of excitation/de-excitation of the field. Your "much more plausible" completely ignores all of the observational, experimental, and theoretical evidence for the existence of photons. But yes, the original two-slit experiment did held overthrow the old corpuscular theories of light and replace them with a wave theory of light. But note well that the old corpuscules of light (proto-photons?) were thought of as classical particles, and the modern photon is not a classical particle. > One then looks to apparent quantum behaviour in the emitters > and/or detectors where of course one finds them. One looks _at_ quantum behaviour in the emitters/detectors. Where else will you find excitation/de-excitation of the EM field? An October issue of Optics and Photonics News (iirc, in 2003) had a nice special section on "What is a photon?". Read it. Also Lamb, "Anti-photon", Applied Physics B from 1995. > NB I must logically also conclude that massive particles are also waves > and once again the quantised behaviour is due to something else. Very de Broglie. It's been done. The "something else" is "that's the way that nature works". If photons are waves, and massive particles are waves, then why is the exchange of energy between matter and EM fields quantised? Observably, it is. "Why" might be a deeper question than we can answer (yet). Asking why hbar is non-zero and has the value we measure is like asking why is c non-infinite with the value we measure(d). > Clearly > (to me at any rate) the 'size' of a massive particle is determined (like > the photon) by its environment (typically quantised). So an electron, > say, can have a different physical size when undisturbed in an orbital > This can in some circumstances be very large indeed when orbitals become > macroscopic, for example in conductors. Don't confuse localisation with size. If a photon is "large", it should be able to interact with and be detected by two spatially separated detectors at the same time. -- Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/ E-prints: http://eprint.uq.edu.au/view/person/...,_Timo_A..html Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
 Two slit experiment Blackbird writes >Oz wrote: >> scerir writes >> >>> It seems that this experiment led to Dirac's famous koan >>> "each photon then interferes only with itself". >> >> Except we know this is not correct. Experiments have been done where >> two separate lasers fired through two separate slits produces an >> interference pattern. > >Hmm. Interesting. Do you have a reference to a paper or webpage? It was posted here, probably on one of the "length of a photon" threads. More likely one of the longer-standing resident experts will know anyway. >> Furthermore this happened even if "the probability of two photons >> being in the apparatus at one time approached zero". > >To mee, this indicates that somehow the photon must interfere with itself. Maybe, however bear in mind that each laser points at only ONE slit. The light beams (one from each laser-slit) is only combined in the apparatus. single SLIT | --------------------------------------------------------> In any case anyone who has listened to MW radio knows that separate >> radio transmitters on the same frequency can readily cause >> interference patterns. > >I don't think that this is the same kind of interference. If you add up two >amplitude modulated waves of the same frequency, then obviously the result >will be garbled, but this has a different explanation than the double slit >experiment has. Why? Radio waves are perfectly good EM waves just like light but with different frequency. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment bz writes >What is usually heard there is the 'beat frequency' [hetrodyne] as signals >of two slighly different frequencies (or phases) are 'combined' in the >receiver's detector. Indeed. >A signal arriving over multipaths can interfer with itself as the phase of >the multipath signals varies due to changes in path length as ionospheric >conditions vary. No, I am NOT talking about that. I am talking about two SEPARATE transmitters, which still interfere. >Of course a HUGE number of photon is involved as MW radio frequency photons >each carry very little energy. So what? Its still photons from different sources interfering. Or would you claim that the interference will reduce as amplitudes reduce? Surely you aren't suggesting that? -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Oh No writes >Thus spake Oz >>2) There is also the interference of two sources each going through one >>slit **when intensity is so low that the probability of finding two >>'photons' in the apparatus at the same time is very low. >> >>Now this has been demonstrated for photons. However the laser light used >>for each beam (separate lasers) had extremely long coherence lengths >>(times). To do this with electrons might be problematic (ie >>"challenging") but would be interesting. Anybody any ideas how it could >>be done? >> >It can't be done. You probably don't remember or didn't follow it, but >you induced me to calculate interference effects between wave functions >for different photons using qed. The same argument did not apply to >electrons, which are fermions and which are conserved in interaction. You have been known to make errors, later corrected (like all of us). Personally I vote for experimental results. My question is whether anyone has any idea how it could be done? Personally I would bet you ?50 it will show the same result as the photon experiment. The initial aim would be to produce extremely monochromatic beams of electrons. Given that we are looking for exceedingly low beam intensity I suspect that the problems associated with mutual repulsion of electrons within a beam will be negligible (or am I being oversimplistic?). One is almost tempted to start with very low energy electrons, perhaps separating them by time-of-flight (via a chopper) or even ballistically as the take a parabolic path in a gravitational field (ok, VERY low energy). From there one could pass them through two identical (but separate) accelerators to the slit and target. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Thus spake Oz >Oh No writes >>Thus spake Oz > >>>Now this has been demonstrated for photons. However the laser light used >>>for each beam (separate lasers) had extremely long coherence lengths >>>(times). To do this with electrons might be problematic (ie >>>"challenging") but would be interesting. Anybody any ideas how it could >>>be done? >>> >>It can't be done. You probably don't remember or didn't follow it, but >>you induced me to calculate interference effects between wave functions >>for different photons using qed. The same argument did not apply to >>electrons, which are fermions and which are conserved in interaction. > >You have been known to make errors, later corrected (like all of us). I've given a demonstration again, in response to scerir. It is not difficult, and nicely illustrates that observables, such as interference effects, rely on the properties of operators, not just states, and thus what is wrong with the view of superposition given in Dirac's koan. You appreciate that there is a paradox here which needed resolution. What Dirac says is trivially true of superposition of quantum states. Hence it also would be true if we strictly observed superposition of the wave function. But we do not. We observe the behaviour of the position observable, and that is subtly different. In the instance of the electron, it boils down to the same formula. In the instance of the photon, it does not. > >Personally I vote for experimental results. My question is whether >anyone has any idea how it could be done? Personally I would bet you ?50 >it will show the same result as the photon experiment. I'll take you up on that. Will you accept this as an electronic handshake? >The initial aim would be to produce extremely monochromatic beams of >electrons. Ahem, you can't have an electron equivalent of a laser. They are fermions and cannot all be in the same state. > Given that we are looking for exceedingly low beam intensity >I suspect that the problems associated with mutual repulsion of >electrons within a beam will be negligible (or am I being >oversimplistic?). You are forgetting the exclusion principle, which is rather more encompassing than charge. >One is almost tempted to start with very low energy electrons, perhaps >separating them by time-of-flight (via a chopper) No you wouldn't be allowed to do that either. Remember, when they do this for photons the stipulation that only one photon comes through per amount of time is statistical. In one of these low energy laser thingies any photon can come through at any time, but it is only fed so much energy so there is only one per amount of time. Regards -- Charles Francis substitute charles for NotI to email
 Two slit experiment Timo Nieminen writes >On Thu, 29 Jun 2006, Oz wrote: > >> scerir writes >> >> >It seems that this experiment led to Dirac's famous koan >> >"each photon then interferes only with itself". >> >> Except we know this is not correct. Experiments have been done where two >> separate lasers fired through two separate slits produces an >> interference pattern. > >That just shows that each photon comes from both sources. Hmmm.... As an explanation I think Mr Occham would find this improbable. >A photon is the >quantum of excitation of the EM field, the EM field is the sum of the >fields individually produced by the sources, so each photon comes from >both sources. A contortion IMHO. A much more plausible explanation is that photons are completely wavelike. One then looks to apparent quantum behaviour in the emitters and/or detectors where of course one finds them. All detectors I know about rely on an irreversible energy step which is by nature quantum. >It is an excellent demonstration that photons aren't like classical >billiard balls. I would agree, they are waves. NB I must logically also conclude that massive particles are also waves and once again the quantised behaviour is due to something else. Clearly (to me at any rate) the 'size' of a massive particle is determined (like the photon) by its environment (typically quantised). So an electron, say, can have a different physical size when undisturbed in an orbital This can in some circumstances be very large indeed when orbitals become macroscopic, for example in conductors. The only real problem I have with this is why masses (which one could say is the wavelength in the time direction) for free elementary particles is most certainly quantised. If the particles were in a box of fixed length in the time direction then of course there would be no problem at all. Ideally some interaction between mass and box-size should result in only a few integral solutions that express the particles that we see. Clearly this is grossly oversimplified because really we should exchange the word 'energy' for 'mass' in all the above and there are at least two broad energy systems, that is electromagnetic and colour, and we don't seem to have very good theoretical constructs unifying them. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Oz wrote in news:T1db1MScyioEFwLk@farmeroz.port995.com: > scerir writes > >>It seems that this experiment led to Dirac's famous koan >>"each photon then interferes only with itself". > > Except we know this is not correct. Experiments have been done where two > separate lasers fired through two separate slits produces an > interference pattern. > > Furthermore this happened even if "the probability of two photons being > in the apparatus at one time approached zero". > > In any case anyone who has listened to MW radio knows that separate > radio transmitters on the same frequency can readily cause interference > patterns. > What is usually heard there is the 'beat frequency' [hetrodyne] as signals of two slighly different frequencies (or phases) are 'combined' in the receiver's detector. A signal arriving over multipaths can interfer with itself as the phase of the multipath signals varies due to changes in path length as ionospheric conditions vary. Of course a HUGE number of photon is involved as MW radio frequency photons each carry very little energy.
 Two slit experiment nightlight wrote in news:1151472082.880906.31520@p79g2000cwp.googlegro ups.com: > If you do know of some experiments or a QED derivation (but > not some handwaved elementary/popular QM stories) that can > distinguish double slit/beam splitter photo-detection trigger > statistics from the trivial classical phenomena such as dripping > faucets in the Poissonian limit p->0, you are welcome to cite it. I don't know of any that have definitely done so, but I believe the 'single photon' lasers that are designed to only produce a single photon at a time, have the potential of being the basis of such an experiment. Some are based on the idea that a single excited molecule can only produce a single 'lase' photon per incoming excitation photon. The 'buzz' in secure communications is the use of an 'untappable' stream of single photons, where the loss of even a single photon, due to attempts at tapping into the circuit, would render the stream of encrypted data as clearly compromised. Of course, simply attenuating a stream of photons from a candle or laser until the frequency of photons detected is so low as to 'assure' single photon events, really gives no such assurance. So the availability of inexpensive sources of true 'single photon' streams will undoubtably lead to some interesting observations on dual slit experments. As for producing streams of guaranteed 'single electron' or 'single He molecules', that may or may not be easier than guaranted single photon streams. I expect that given a stream of true single photons, each photon will only excite a single grain on a photographic film(or a single element on a ccd). The PATTERN of excited grains, over time, dependent on the slit arrangement.
 Two slit experiment Thus spake scerir >> >It seems that this [Taylor's] experiment led >> >to Dirac's famous koan "Each photon then >> >interferes only with itself." > >Oz writes: >> Except we know this is not correct. >> Experiments have been done where two >> separate lasers fired through >> two separate slits produces an >> interference pattern. > > Yes, but this does not mean that Dirac's >koan is wrong. > Even a photon emitted by two sources (in such >a way that you cannot say which of the two sources >emitted the photon) interferes only with itself. >When the different possible photon paths, >from sources to detector, are indistinguishable, >then we have to add the corresponding amplitudes >before squaring to obtain the probability. > The second part of the koan (see below) >seems obscure (or it needs a reformulation), >since we have two-photon interference, that >is to say the interference of photons >emerging from _independent_ sources. > >s. > >"Each photon then interferes only with itself. >Interference between two different photons >can never occur." >- P.A.M. Dirac, Principles of Quantum Mechanics, >Clarendon, Oxford, 1930, p.15. > Dirac's koan is wrong. He wrote this when quantum mechanics was in its infancy and quantum electrodynamics had hardly even been born. He was thinking, quite naturally, of the structure of one particle Hilbert space in which the quantum superposition, |f>+|g>, appears in the addition of states of one particle. It makes no sense to add the state of one particle to the state of another, because they are strictly described by states in different Hilbert spaces. However in quantum electrodynamics we have to think of the wave function not in terms of the probability for where a photon is, but rather the probability for where a photon is annihilated (detected) and we get a different result. As a concession to ASCII I shall ignore spin and simplify the formula as much as possible, and just show the bones of the argument. The photon field operator is, A(x) = |x> + creates the state |x>, and is <|A(x)|f> = so that ||^2 is the probability for detection of a photon at x. Now consider a two photon state |f;g> = |f>|g> + |f>|g>. For simplicity I am assuming no entanglement, and that the photons are distinguishable at some time in the past (they come from different sources) so that =0. When one of the photons is detected the other remains as it was, but we don't know which one is detected and which one remains. Then the final state is either |f> or |g>, i.e. it is |f> + |g>, so the probability for detection at x is given by ( = ( + ) = (|g> + |f>) where I have lost the inner product between states of different numbers of particles (it is zero). Using =0 and ==1 this comes down to + Which shows the superposition between states of different photons. +++++ This argument does not work for electrons. I shall ignore spin. For electrons the field operator is Psi(x) = |x> + creates an electron and + + is replaced by |x> = = (|f> - |g>) Again the =0 and = =1 so this reduces to = ||^2 + ||^2 So in this case we do not have interference, but simply the sum of two amplitudes. Regards -- Charles Francis substitute charles for NotI to email
 Two slit experiment scerir writes > Yes, but this does not mean that Dirac's >koan is wrong. hmmm... > Even a photon emitted by two sources (in such >a way that you cannot say which of the two sources >emitted the photon) interferes only with itself. Hmmm.... given the experiment. >When the different possible photon paths, >from sources to detector, are indistinguishable, >then we have to add the corresponding amplitudes >before squaring to obtain the probability. Hmmm.... how very convenient.... > The second part of the koan (see below) >seems obscure (or it needs a reformulation), >since we have two-photon interference, that >is to say the interference of photons >emerging from _independent_ sources. Which is precisely my point. The below can be reformulated (I am absolutely confident mathematicians can do this), basically by saying "take two independent sources, and call them one source", which gives the right answer at some philosophical cost. "Each photon then interferes only with itself. Interference between two different photons can never occur." - P.A.M. Dirac, Principles of Quantum Mechanics, Clarendon, Oxford, 1930, p.15. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Timo A. Nieminen wrote: > The EM field is completely wavelike. Much, even most, stuff about photons > comes from purely classical theory. However, by definition photons are > _not_ completely wavelike, being the quanta of excitation/de-excitation of > the field. I take it then the very simplest model of a field excited/de-excited by "photons" would be the QHO. Or possibly this is the complete model, in general only more complexified by additional modes, and additional complication added by the fact that the electromagnetic field has degrees of freedom not completely analogous with a simple mass on a spring? Hit, or a miss? I also seem to recall hearing it said that the description of the excitation of the field in terms of photons had an arbitrary element, depending how we selected the modes. Any truth in this? If the photons are "quanta of excitation/de-excitation of the field", maybe it doesn't even make sense to ask whether one photon only interferes "with itself" as opposed to "with other photons". Surely it is the total excitation of the field which is interfering with itself, which is to say "arriving at a value at each event which can be decomposed into the result of various independent processes, whose resultants add", IOW "behaves linearly", or yet IOW "behaves like a vector". If we are constrained to excite/de-excite a QHO by "quanta", is it not still the single value of psi, the wave function, which is doing the interfering -- i.e., evolving linearly via the hamiltonian -- and not the excitations/de-excitations? [I seem to be taking sides with Oz, though I didn't really start out to argue one side or the other. Such is the attractive pull of rhetoric, even when one set out merely to grope.]
 Two slit experiment Ralph Hartley wrote: > Blackbird wrote: >> In the radio wave case, with two different transmitters, you should >> be able to pick up one of the signals with a directional antenna. > > A directional antenna cannot have zero size. In order to resolve the > two transmitters, the antenna would need to be larger than the nodes > in the interference pattern. > > Ralph Hartley Yeas, you are right on that point. After my post, I did a back-of-the-envelope calculation on the interference pattern from two geostationary staellites with 3 degrees separation transmitting 2,5 cm waves. It seems that the distance between destructive regions in the pattern is approximately 0,5 m, and that is about the diameter you need for a satellite dish to give good separation between two satellites that close.
 Two slit experiment Blackbird writes >Oz wrote: >> Blackbird writes >>> Sure, so I'll try to explain my point here a little better. Say we >>> have a vertical receiving antenna, and two MW transmitters that >>> transmit waves of the same frequency, but perfectly out of phase >>> (relatively shifted by 1/2 the wavelength) at the location of the >>> antenna. The waves transfer energy inducing electrons in the antenna >>> to accelerate. According to the theory, this energy transfer (and >>> thus the acceleration) is quantized, hence "photons". Now, any >>> random electron will from time to time absorb a photon that either >>> accelerates it in the "up" direction, or in the "down" direction, >>> and since we have two sources with cancelling phases, for any finite >>> (and sufficiently large) interval of time, the electron will absorb >>> approximately as many "up" as "down" photons. >> >> Pah! That would mean you got increased random noise, which you do NOT >> get. The environment would be 'hotter'. > >Yes, and this is exactly what I believe will happen. The energy absorbtion >should lead to a small rise in the temperature of the antenna, even though >no information will be detected. No, because the field strength is demonstrably zero. It can be measured. >>> The electron will thus >>> exhibit a random walk, and no signal will be detected. This, >>> however, does not mean that the *photons* interfere with eachother. >> >> Its by far the simplest explanation. The em field is zero and no >> photons are detected. >> >>> Interference, as in >>> the double slit experiment, would mean that photons from the two >>> different sources cancelled each other (or more precisely, they >>> would be more likely to show up at another location), thus there >>> would be no energy absorbtion by the electron whatsoever. >> >> Which is precisely the effect of radio waves. >> Think about it. > >In the radio wave case, with two different transmitters, you should be able >to pick up one of the signals with a directional antenna. This is >impossible in the double slit experiment (with equipment analogous to a >directional antenna, say a focusing lens), since seen from the points of >destructive interference, both slits look dark. There is simply no energy >reaching these points. That would be an interesting example to do. However you will then see a signal simply because there is NOT a path from one slit to the detector which cannot see the other slit. This would be equally true whether you used two transmitters (lasers, stable, very) or one transmitter and reflected the radio wave (or light) to the same location as the second slit. You certainly cannot differentiate between self-photon interference and separate-photon interference by this method. In essence you CAN tell which slit the light came from because your directional antenna detects the direction (momentum). -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Oz wrote: > Blackbird writes >> Sure, so I'll try to explain my point here a little better. Say we >> have a vertical receiving antenna, and two MW transmitters that >> transmit waves of the same frequency, but perfectly out of phase >> (relatively shifted by 1/2 the wavelength) at the location of the >> antenna. The waves transfer energy inducing electrons in the antenna >> to accelerate. According to the theory, this energy transfer (and >> thus the acceleration) is quantized, hence "photons". Now, any >> random electron will from time to time absorb a photon that either >> accelerates it in the "up" direction, or in the "down" direction, >> and since we have two sources with cancelling phases, for any finite >> (and sufficiently large) interval of time, the electron will absorb >> approximately as many "up" as "down" photons. > > Pah! That would mean you got increased random noise, which you do NOT > get. The environment would be 'hotter'. Yes, and this is exactly what I believe will happen. The energy absorbtion should lead to a small rise in the temperature of the antenna, even though no information will be detected. > >> The electron will thus >> exhibit a random walk, and no signal will be detected. This, >> however, does not mean that the *photons* interfere with eachother. > > Its by far the simplest explanation. The em field is zero and no > photons are detected. > >> Interference, as in >> the double slit experiment, would mean that photons from the two >> different sources cancelled each other (or more precisely, they >> would be more likely to show up at another location), thus there >> would be no energy absorbtion by the electron whatsoever. > > Which is precisely the effect of radio waves. > Think about it. In the radio wave case, with two different transmitters, you should be able to pick up one of the signals with a directional antenna. This is impossible in the double slit experiment (with equipment analogous to a directional antenna, say a focusing lens), since seen from the points of destructive interference, both slits look dark. There is simply no energy reaching these points.
 Two slit experiment Timo A. Nieminen writes >Don't confuse localisation with size. If a photon is "large", it should be able >to interact with and be detected by two spatially separated detectors at the >same time. In a sense it can, and in a sense it cannot. Given exited enough detectors of course one can. Exited enough detectors are by definition noisy and nobody can tell if two transitions are due to the EM wave tripping two or whether one (or both) are noise. We can only tell that on average the detector is best modelled by it making quantum transitions. This is unsurprising given that detectors are quantised. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Paul Danaher writes >Your "large" photon brings me back to my puzzle about when >absorption occurs - is it on the arrival of the leading edge of the wave >packet/probability density function, or at a point at which this reaches >some threshold value? When it is detected, that is, when an effectively irreversible transition has occurred. I don;t see how this can be denied. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Timo A. Nieminen writes >The claim regarding electrons having different sizes depending on whether "they >are undisturbed in an orbital" reads to me as an identification of size with >spread of wavefunction. Its a very excellent description of size. We are talking size in meters here, not energy. >This being independent of wavelength (but see below), >low frequency isn't necessary. In fact, it's best to use the highest frequencies >available, so that the energy required for detection is only a small fraction of >the total energy. Er, yes, but such an object has a very short wavelength so is inherently more localised (or 'smaller'). >Put a gamma source in the middle of some, preferably many, >detectors. Each gamma photon can go in any direction, the radiation field of >each emission is spherically symmetric (well, perhaps a dipole field, but >spherically symmetric averaged over many). I'm not actually completely in agreement with this statement. On average this is so, and on average (that is, over many counts) you get the right answer. But the uncertainty of momentum may not be spherical because you could measure the recoil of the source (if the gamma is energetic enough), which would destroy the spherical symmetry. >I guess that you're wondering about size of photons as it might depend on >wavelength. The above means that it's hard to answer experimentally. I think >that illuminating a group of atoms, all within a wavelength, and only one of >them absorbs and re-emits, is conclusive - the "size" of a photon is no larger >than an atom. Compton can be interpreted as saying that the size of a photon is >the size of an electron, ie zero AFAWCT. Absolutely not, as diffraction will confirm. Remember that an aerial does NOT have to be a wavelength long to receive. In fact its normal for low frequencies to put appropriate L & C on a short aerial so as to tune it to the required low frequency/long wavelength. Similarly you can see an absorbing (or emitting) atom as tuned to its transition frequency, with the masses of the electron/nucleus and associated electromagnetic field interaction tuning it to the very low (in wavelength terms) transition frequency. Just as a properly tuned small aerial can transmit a very long wavelength signal, so a very small atom is perfectly capable of being tuned to a very long wavelength em wave. Clearly most transitions take very many cycles of emission before the transition is completed. >But do consider the above gamma experiment. Data is good for you. All >theoreticians should be forced into labs at some point! -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment Blackbird writes >Sure, so I'll try to explain my point here a little better. Say we have a >vertical receiving antenna, and two MW transmitters that transmit waves of >the same frequency, but perfectly out of phase (relatively shifted by 1/2 >the wavelength) at the location of the antenna. The waves transfer energy >inducing electrons in the antenna to accelerate. According to the theory, >this energy transfer (and thus the acceleration) is quantized, hence >"photons". Now, any random electron will from time to time absorb a photon >that either accelerates it in the "up" direction, or in the "down" >direction, and since we have two sources with cancelling phases, for any >finite (and sufficiently large) interval of time, the electron will absorb >approximately as many "up" as "down" photons. Pah! That would mean you got increased random noise, which you do NOT get. The environment would be 'hotter'. >The electron will thus >exhibit a random walk, and no signal will be detected. This, however, does >not mean that the *photons* interfere with eachother. Its by far the simplest explanation. The em field is zero and no photons are detected. >Interference, as in >the double slit experiment, would mean that photons from the two different >sources cancelled each other (or more precisely, they would be more likely >to show up at another location), thus there would be no energy absorbtion by >the electron whatsoever. Which is precisely the effect of radio waves. Think about it. -- Oz This post is worth absolutely nothing and is probably fallacious.
 Two slit experiment On Sat, 1 Jul 2006, FrediFizzx wrote: > "Timo A. Nieminen" wrote: >> On Fri, 30 Jun 2006, Oz wrote: > >>> Clearly (to me at any rate) the 'size' of a massive particle is >>> determined (like the photon) by its environment (typically >>> quantised). So an electron, say, can have a different physical size >>> when undisturbed in an orbital This can in some circumstances be >>> very large indeed when orbitals become macroscopic, for example in >>> conductors. >> >> Don't confuse localisation with size. If a photon is "large", it >> should be able to interact with and be detected by two spatially >> separated detectors at the same time. > > I don't think we actually know the answer to that "question" since > "large" photons might be radio wave photons and so far individual > detection of such photons is not possible. Do you know of any > experimental limits that we might have for this? IOW, what is the > lowest frequency at which individual detection is experimentally > possible with current technology? The claim regarding electrons having different sizes depending on whether "they are undisturbed in an orbital" reads to me as an identification of size with spread of wavefunction. This being independent of wavelength (but see below), low frequency isn't necessary. In fact, it's best to use the highest frequencies available, so that the energy required for detection is only a small fraction of the total energy. Put a gamma source in the middle of some, preferably many, detectors. Each gamma photon can go in any direction, the radiation field of each emission is spherically symmetric (well, perhaps a dipole field, but spherically symmetric averaged over many). Count, and look for coincidences. Has this been done? I don't know offhand. You should look; if it hasn't been done, it could be a cheap and useful entry into experimental physics for you. But yes, there is a problem with trying to make sub-wavelength individual photon detectors. Except for single atoms/molecules, but then you still need an individual photon detector to detect the re-emission. IME, this is in the near IR. I guess that you're wondering about size of photons as it might depend on wavelength. The above means that it's hard to answer experimentally. I think that illuminating a group of atoms, all within a wavelength, and only one of them absorbs and re-emits, is conclusive - the "size" of a photon is no larger than an atom. Compton can be interpreted as saying that the size of a photon is the size of an electron, ie zero AFAWCT. But do consider the above gamma experiment. Data is good for you. All theoreticians should be forced into labs at some point! -- Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/ E-prints: http://eprint.uq.edu.au/view/person/...,_Timo_A..html Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
 Two slit experiment Thus spake Oz >Absolutely. Perhaps a massive boson beam would be better. I guess that >probably means they would be entangled so as to get the long thin >coherent beam. Hmm.... This gets pretty confusing. Lasers are weird things, and I don't profess to fully understand them, but I think they are pretty dependent on the properties of photons as fundamental particles. IIRC It has been remarked here, by those more qualified than I, that the properties of a bose gas consisting of fundamental bosons are not the same as one consisting of bosons which are themselves compound particles consisting of fermions, the reason being that if compound particles were in exactly the same state it would imply that the fermions of which they consist would be in the same state, yielding a contradiction. Looking through the derivation I gave it seems to me that the critical distinction was that photons are annihilated when they are detected, so that the amplitude for detection at x is <|A(x)|f>, a pure number. We can add numbers, so we can get interference effects between different photons. Fullerenes, for example are not annihilated, and the position operator is |x>, a state. We still cannot superpose states of different particles, so that means we can expect interference effects between different photons, but not interference effects between different fullerenes. Regards -- Charles Francis substitute charles for NotI to email
 Two slit experiment FrediFizzx wrote: > "Timo A. Nieminen" wrote in message > news:Pine.WNT.4.64.0607010837130.1212@serene.st... >> On Fri, 30 Jun 2006, Oz wrote: > >>> Clearly (to me at any rate) the 'size' of a massive particle is >>> determined (like the photon) by its environment (typically >>> quantised). So an electron, say, can have a different physical size >>> when undisturbed in an orbital This can in some circumstances be >>> very large indeed when orbitals become macroscopic, for example in >>> conductors. >> >> Don't confuse localisation with size. If a photon is "large", it >> should be able to interact with and be detected by two spatially >> separated detectors at the same time. > > I don't think we actually know the answer to that "question" since > "large" photons might be radio wave photons and so far individual > detection of such photons is not possible. Do you know of any > experimental limits that we might have for this? IOW, what is the > lowest frequency at which individual detection is experimentally > possible with current technology? > > FrediFizzx This seems to bring up again the problem of the dual answers to my question about how long it takes to absorb a photon. One answer was that the wave function collapses instantaneously, another that it depends on the frequency of the photon. Your "large" photon brings me back to my puzzle about when absorption occurs - is it on the arrival of the leading edge of the wave packet/probability density function, or at a point at which this reaches some threshold value? (Thank you for the further puzzles in the Diether paper you cite, with his comment that the photon energy density volume for a radio wave could be as big as a house or bigger.)