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Hi Tim, Colin, Bernhard, Ethan, This thread reminded me of the case of 'reflection from a hard boundary', ie which involves a 180 degree phase change. (various instructive simulations available to view via google). Is that what explains the 180 degrees phase change in X-ray reflection/re-scattering from an electron? In reply to an earlier thread, referred to in this thread re QM, a useful source For one's imagination I find is George Gamow's "Mr Tomkins" book eg 'what if Planck's constant had a value of 1?' ...'it would make the game of snooker.....'. Greetings, John John R Helliwell On 6 Nov 2015, at 07:59, Tim Gruene <[log in to unmask]> wrote: > Hi Colin, Hi Ethan, > > I thought that the 180 degree shift is explained with the negative charge of > the electron, i.e. I expect the wave scattered by the nucleus to be in phase > (one would need really high resolution data to measure this, though). > > The phase flip that Ethan describes should only happen at high resolution, > where f' becomes greater than f0. At 2theta = 0, usually |f0|>|f'|, so that > there is no phase shift due to the negative f', right? > > The differential equation for the damped oscillator results in the same > equation we use to describe anomalous scattering. Maybe that's why someone has > thought of explaining one with the other. But as with all models there are > limits, and when comparing phenomena at nuclear level with a mechanistic > model, that limit is reached rather quickly. > > Best, > Tim > > On Thursday, November 05, 2015 11:32:09 PM Colin Nave wrote: >> For scattering from a single electron isn't there a 180 degree phase change >> between the incident and scattered wave? The spring demo shows this when >> the driver frequency is higher than the resonant frequency. I think the >> strong resonance in the spring demo corresponds to a strong white line. For >> the real component of the dispersive correction there will then be a change >> of sign across the white line as in the demo. >> >> The damped driven oscillator is a common method used to describe x-ray >> scattering around resonance. However, I doubt whether the demo corresponds >> in detail to this. I am also unsure about the degree of damping in the demo >> (need another knob on the control box for this) but assume that the >> driving force, amplitude of oscillation and damping all balance out to give >> a steady state. As you imply, more frequency points would be useful. >> >> Colin >> >> >> From: Ethan Merritt [mailto:[log in to unmask]] >> Sent: 05 November 2015 17:33 >> To: Nave, Colin (DLSLtd,RAL,LSCI) >> Cc: ccp4bb >> Subject: Re: [ccp4bb] AW: [ccp4bb] AW: [ccp4bb] Diffraction as a >> Single-Photon Process; was RE: [ccp4bb] Twinning Question >> On Thursday, 05 November 2015 11:51:49 AM > [log in to unmask]<mailto:[log in to unmask]> wrote: >>> Ethan >>> >>> My understanding is that one would have to have separate springs for each >>> electron in the atom. Only some would be at resonance for a particular >>> driving frequency. One would apply some sum for the total scattering of >>> the atom. >> Sure. The problem is that the phase in the physical demonstration does not >> match up >> >> with the phase seen for anomalous scattering even when considered one >> electron at a time. >> >> >> >> In anomalous scattering the f' term is maximum negative exactly at the >> resonance point. >> >> So far as I can see, that strong negative component "flips the phase" so >> that the >> >> 180° phase shift is seen at (or very near) to the resonance frequency. As >> the >> >> frequency increases from there, the f' term returns to near 0 and the f" >> term >> >> reaches its maximum. This corresponds more or less to a 90° phase shift as >> the >> >> imaginary component dominates over the real component. At even higher >> frequency >> >> the f" term also decays toward zero and the phase gradually returns to the >> original >> >> value. Thus the sequence of phase values is just plain different in the >> anomalous >> >> scattering case and the motor-driven-spring oscillator case. >> >> >> >> So as the frequency increases, the physical demo highlights an induced phase >> >> below -> edge -> above -> high >> >> 0 -> 90 -> 180 >> >> Whereas anomalous scattering shows >> >> 0 -> 180 -> 90 -> 0 >> >> >> >> If I were to show this video while teaching, and a student asked me to >> explain >> >> in more detail how it relates to anomalous scattering, I'd be flummoxed. >> >> >> >> I am inclined to think the narration in the video is simply wrong, or at >> least >> >> misleads the viewer to an incorrect conclusion. The caption on YouTube >> doesn't help. >> >> It's not that the phase is locked at 0 below and 180 above the resonance >> point; >> >> it's just that far from resonance point the input and output phases are >> decoupled. >> >> The camera happened to catch times at which the driver and the suspended >> object >> >> were "in phase" or "out of phase" and the narrator pointed that out, but >> neither >> >> state is a general phenomenon. I think. But maybe I'm confused. >> >> >> >> Ethan >> >>> Of course this is trying to give some physical description for the >>> electromagnetic field when I was complaining about a similar thing for >>> quantum mechanics. A nice article by Freeman Dyson illustrates the >>> difficulty of doing this for both approaches. >>> >>> http://www.damtp.cam.ac.uk/user/tong/em/dyson.pdf >>> >>> >>> >>> Colin >>> >>> -----Original Message----- >>> >>> From: Ethan A Merritt [mailto:[log in to unmask]] >>> >>> Sent: 04 November 2015 21:59 >>> >>> To: Nave, Colin (DLSLtd,RAL,LSCI) >>> >>> Cc: ccp4bb >>> >>> Subject: Re: [ccp4bb] AW: [ccp4bb] AW: [ccp4bb] Diffraction as a >>> Single-Photon Process; was RE: [ccp4bb] Twinning Question> >>> On Wednesday, 04 November, 2015 09:48:13 Colin Nave wrote: >>>> Domenico >>>> >>>> Thanks for the kind words! >>>> >>>> >>>> >>>> I still don't like descriptions such as " Therefore, the anomalous >>>> scattered photon will still be able to resonate with another anomalous >>>> scatterer within the crystal" This is an attempt to describe what >>>> happens to a photon before it has been observed and is therefore an >>>> attempt to interpret Quantum Mechanics. As Feynman said about his >>>> formulation - it is ""merely a mathematical description, not an attempt >>>> to describe a real process that we can measure. Niels Bohr "brainwashed >>>> an entire generation of physicist into believing that the whole job was >>>> done 50 years ago" as Murray Gell-Mann said. This might be a bit unfair >>>> but most physicists accepted the Copenhagen interpretation and >>>> concentrated on carrying out the necessary calculations from which we >>>> have all benefitted. Quantum Mechanics works but treat the physical >>>> descriptions of the processes with scepticism. >>>> >>>> >>>> >>>> Regarding anomalous scattering I like the classical analogy in terms >>>> >>>> of a damped driven oscillator. There is a good video of this sort of >>>> thing at https://www.youtube.com/watch?v=aZNnwQ8HJHU , for a non damped >>>> case showing the phase changes near resonance.> >>> I like the video, but it leaves me scratching my head a bit. >>> >>> One comes away from it expecting that there will be a 180° change in the >>> phase of every Bragg reflection just from choosing a "long" or "short" >>> wavelength x-ray source. [Or to be more precise a 180° change in the >>> contribution of the anomalous scattering atoms to every Bragg >>> reflection]. >>> >>> >>> >>> I realize that if the phase were to flip for all atoms then by Babinet's >>> principle the same underlying structure should be recoverable with either >>> choice of phases, but does this really happen? Anyhow, that would not >>> apply when only a subset of atoms in the structure have an absorption >>> edge that is spanned to the two wavelengths in question. So does this >>> demo really match up to what happens in an X-ray experiment? >>> >>> >>> >>> Ethan >>> >>>> A bit of damping is probably apparent but if anyone knows of a better >>>> example for a damped oscillator I would be interested. >>>> >>>> >>>> >>>> Colin >>>> >>>> >>>> >>>> >>>> >>>> -----Original Message----- >>>> >>>> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of >>>> >>>> Dom Bellini >>>> >>>> Sent: 03 November 2015 18:05 >>>> >>>> To: ccp4bb >>>> >>>> Subject: Re: [ccp4bb] AW: [ccp4bb] AW: [ccp4bb] Diffraction as a >>>> >>>> Single-Photon Process; was RE: [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> Dear All, >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> Sorry for bringing back this old topic but I think I might have an >>>> explanation to satisfy the original query, which I believe was not >>>> conclusively put to rest in the end. >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> I think the problem that me and the original poster, Jacob, were having >>>> was that we were confusing energy with amplitude (at least I did). >>>> I.e., anomalous scattering affects/reduces the amplitude of the atomic >>>> form factor (or structure factor in case of a crystal), but not the >>>> energy (or wavelength) of the scattered photon, which is the same as >>>> that of the incident photon. Therefore, the anomalous scattered photon >>>> will still be able to resonate with another anomalous scatterer within >>>> the crystal, without breaking any conservation of energy theory. Since >>>> anomalous scattering is an elastic effect, if one accepts the >>>> explanation model of "photon interfering with itself" and "mini-waves" >>>> in the case without resonators, then this model could be equally valid >>>> even in the presence of more than one anomalous scatterer. >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> I would like to thank Colin Nave to make me realize that I was mixing up >>>> anomalous scattering with inelastic scattering. I am pretty sure I had >>>> it clear while doing my PhD many moons ago. >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> I hope I understood correctly the original question and that this >>>> explanation to the query might make some sense to other people as well, >>>> rather than just me :-). >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> Best, >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> and sorry again for bringing this back, >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> D >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> ________________________________ >>>> >>>> From: CCP4 bulletin board [[log in to unmask]] on behalf of >>>> >>>> [log in to unmask]<mailto:[log in to unmask]> >>>> [[log in to unmask]] >>>> >>>> Sent: 31 August 2015 14:12 >>>> >>>> To: ccp4bb >>>> >>>> Subject: [ccp4bb] AW: [ccp4bb] AW: [ccp4bb] Diffraction as a >>>> >>>> Single-Photon Process; was RE: [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> Dear Jacob, >>>> >>>> >>>> >>>> You are not the only one who does not believe in quantum mechanics. >>>> Albert Einstein was probably the most famous non-believer. >>>> >>>> >>>> >>>> I agree with you that since we observe interference and diffraction >>>> patterns, there must occur interference somewhere. Although Niels Bohr >>>> claimed that you cannot say anything about a quantum system between two >>>> measurements, my strong feeling is that we see interference between the >>>> different superimposed quantum states. This is for me the truly spooky >>>> part of quantum mechanics: instead of a single foton, as long as we do >>>> not measure, there can be hundreds of fotons haunting our crystal. >>>> However, the moment we switch on the light, we find only one. The >>>> position of this foton will have been influenced by all other spooky >>>> fotons. >>>> >>>> >>>> >>>> I do not see how quantum mechanics would not conserve energy, but would >>>> be interested to learn. >>>> >>>> >>>> >>>> HS >>>> >>>> >>>> >>>> >>>> >>>> Von: Keller, Jacob [mailto:[log in to unmask]] >>>> >>>> Gesendet: Montag, 31. August 2015 13:06 >>>> >>>> An: Schreuder, Herman R&D/DE; >>>> [log in to unmask]<mailto:[log in to unmask]> >>>> >>>> Betreff: RE: [ccp4bb] AW: [ccp4bb] Diffraction as a Single-Photon >>>> >>>> Process; was RE: [ccp4bb] Twinning Question >>>> >>>>> This means that when a foton at the same moment interacts with 100 >>>>> scatterers (or resonators), there are 100 or more different states and >>>>> in each state the foton interacts with a different scatterer. In each >>>>> state, one foton interacts with only one scatterer. The moment the >>>>> measurement is made, we find only one discrete foton, corresponding to >>>>> one of these states. The distribution of the states, and therefore the >>>>> possible outcomes, depend on the presence of all scatters/resonators >>>>> within coherent range.> > >>>> Then I don't see how interference or diffraction patterns can occur >>>> without resorting to what others have said, which I don't understand >>>> really: that interference is not really happening at all, but something >>>> else with spooky probability distributions which don't need to >>>> subscribe to conservation of energy. >>>> >>>> >>>> >>>> JPK >>>> >>>> >>>> >>>> >>>> >>>> Von: CCP4 bulletin board [mailto:[log in to unmask]] Im Auftrag von >>>> >>>> Keller, Jacob >>>> >>>> Gesendet: Donnerstag, 20. August 2015 20:42 >>>> >>>> An: >>>> [log in to unmask]<mailto:[log in to unmask]<mailto:CCP4BB@JISCMA >>>> IL.AC.UK%3cmailto:[log in to unmask]>> >>>> >>>> Betreff: Re: [ccp4bb] Diffraction as a Single-Photon Process; was RE: >>>> >>>> [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> What I don't understand is how a single photon, which I thought by >>>> definition was an indivisible quantum of energy, can be split up >>>> arbitrarily amongst a number of scatterers into these "mini-waves." >>>> Doesn't that self-contradict QM's concept of quanta? >>>> >>>> >>>> >>>> One might say that somehow there are two energy-related characteristics >>>> to the photon: >>>> >>>> >>>> >>>> 1. the actual amount of total energy in the photon, and then >>>> >>>> >>>> >>>> 2. the "type" or "color" or "frequency" of the photon's energy. >>>> >>>> >>>> >>>> If you will allow me this dichotomy, then I can understand how it can be >>>> distributed to different atoms-small portions of energy of the same >>>> "color" are distributed to all of the resonators. One would also have >>>> to presuppose another thing, that the resonators themselves are able to >>>> accept packets of energy of size 1/n, as long as it's of a certain >>>> color. The problem is, however, that allowing photons and resonators to >>>> do these things violates what I thought was the central tenet of QM, >>>> that there are indivisibles known as quanta. >>>> >>>> >>>> >>>> Maybe, then, one can just drop the bit about there being quanta, or at >>>> least put a star by it? >>>> >>>> >>>> >>>> JPK >>>> >>>> >>>> >>>> >>>> >>>> From: >>>> [log in to unmask]<mailto:[log in to unmask]<mailto:hofkris >>>> [log in to unmask]:[log in to unmask]>> >>>> >>>> [mailto:[log in to unmask]] >>>> >>>> Sent: Thursday, August 20, 2015 2:03 PM >>>> >>>> To: Keller, Jacob; >>>> [log in to unmask]<mailto:[log in to unmask]<mailto:CCP4BB@JISCMA >>>> IL.AC.UK%3cmailto:[log in to unmask]>> >>>> >>>> Subject: RE: [ccp4bb] Diffraction as a Single-Photon Process; was RE: >>>> >>>> [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> Valid questions. >>>> >>>> >>>> >>>> The phenomenon of resonance needs some explanation here, in terms we can >>>> imagine: >>>> >>>> >>>> >>>> Take first the normal case: let all the n resonating electrons gain 1/n >>>> in energy from the disappearing photon. These n resonating electrons >>>> emit partial waves or whatever you want to call them totaling n*1/n in >>>> energy, which recombines into the new photon. But what happens when the >>>> phases lead to extinction? Where does the energy go? Well, it just does >>>> not happen, it won't get scattered in THAT direction. So in the >>>> probabilistic picture again, IF a photon does gets elastically >>>> scattered, then it WILL appear again. WHERE it might appear, is given >>>> by its probability distribution, aka the Fs. No contradiction here, >>>> although I fully admit that the mini-wave picture results from the need >>>> to explain, in experience-accessible terms, a non-experienceable >>>> process. That is the QM conundrum, but not a contradiction. >>>> >>>> >>>> >>>> Now the anomalous (n.b.: not inelastic!) case: In this case the net >>>> effect is a change in the fs and thus Fs, and again all it does is >>>> change the probability distribution accordingly and above picture >>>> holds. But wait - where does the X-ray fluorescence come from, and if >>>> the photon uses all its energy to kick a photoelectron out, how can it >>>> reappear? It does not. The unlucky photon that generated the >>>> photoelectron is DEAD, otherwise we violate energy conservation. That >>>> photoelectron then causes either fluorescence via outer to core >>>> transitions or can be directly measured in case it manages to escape, >>>> or make Auger electrons, whatever satisfies energy conservation. The >>>> lucky photons, passing close to absorption energy, experience only the >>>> change in scattering factor. If you look at the theoretical QM >>>> calculations for absorption spectra (Cromer Lieberman etc.), you see >>>> that the dispersion curves actually show a singularity at precisely the >>>> orbital excitation energy. That absorption curve is again simply a >>>> probability function for photon death at a given energy. In solids, >>>> this curve can be more complicated and have more detail, but still the >>>> same. >>>> >>>> >>>> >>>> So, you cannot simultaneously measure diffraction and fluorescence of >>>> one and the same photon. The fluorescence scan does not come from the >>>> anomalously but elastically scattered photons. It comes from the >>>> absorbed dead ones. There is no difference between the normal and >>>> anomalous 'miniwave' picture other than a change in fs and Fs. >>>> >>>> >>>> >>>> Radiation damage, btw, is just a process cascade caused by that photon >>>> death. >>>> >>>> >>>> >>>> I abstain from digressing into inelastic/incoherent processes. >>>> >>>> >>>> >>>> Best, BR >>>> >>>> >>>> >>>> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of >>>> >>>> Keller, Jacob >>>> >>>> Sent: Thursday, August 20, 2015 9:48 AM >>>> >>>> To: >>>> [log in to unmask]<mailto:[log in to unmask]<mailto:CCP4BB@JISCMA >>>> IL.AC.UK%3cmailto:[log in to unmask]>> >>>> >>>> Subject: [ccp4bb] Diffraction as a Single-Photon Process; was RE: >>>> >>>> [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> Do you have any explanation of how a single photon, which contains x >>>> amount of energy, can cause multiple electrons (at least 1000's!) in >>>> anomalously-scattering atoms to resonate at that energy? >>>> >>>> >>>> >>>> We don't find that the presence of different numbers of resonant >>>> scatterers requires x-rays of different energy; so why, if the energy >>>> is being divided into different numbers of resonators, does the same >>>> energy of x-rays work? >>>> >>>> >>>> >>>> I believe that BR's book says that the photon disappears or annihilates >>>> briefly, then re-emerges. This must be true, then, across thousands of >>>> electrons at once, both normal and anomalous? >>>> >>>> >>>> >>>> Jacob >>>> >>>> >>>> >>>> >>>> >>>> From: Edwin Pozharski [mailto:[log in to unmask]] >>>> >>>> Sent: Thursday, August 20, 2015 9:36 AM >>>> >>>> To: Keller, Jacob >>>> >>>> Subject: Re: [ccp4bb] Twinning Question >>>> >>>> >>>> >>>> typo indeed. The point, of course, stands - with older sources there are >>>> *no* photons inside the crystal for over 99% of the time. (Notice that >>>> diffraction pattern is still present, Bragg's law satisfied, etc)) >>>> X-ray diffraction is, for all intents and purposes, a single photon >>>> experiment. Even with the brightest and most coherent sources, when you >>>> could have multiple photons within a large crystal, these are still >>>> separated in space by a distance that is at least 100x the coherence >>>> length. Thus, X-ray photons do not interact with each other (and even >>>> if they would, it's still does not make them a wave, just good ole >>>> photons that due to their high spatial density would have detectable >>>> probability to engage in multi-photon events).> > >>>> On Wed, Aug 19, 2015 at 5:13 PM, Keller, Jacob > <[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]:[log in to unmask]>>> > wrote: >>>>> Also, if your X-ray source is not exactly the brightest synchrotron, >>>>> you are probably looking at ~10^9 photons/sec at best (I am estimating >>>>> here that it would take at least 15-20 minutes of data collection >>>>> using early 2000s "RAxisIV" in-house system to get diffraction image >>>>> of intensity similar to 0.5s exposure at 12-2). That is one photon >>>>> every nanosecond. Let's continue to ignore the fact that most photons >>>>> just fly through. A photon zips through a 1mm crystal in about 3fs. >>>>> Think about this - at a moderate intensity home source (and I can go >>>>> to sealed tubes), the process of crystal illumination by X-rays is >>>>> more like single photons flying through with about 300x long pauses >>>>> between events. To scale this, imagine that a single photon spends a >>>>> whole second inside a crystal, probing it's electron density. You >>>>> would then have to wait five minutes for the next photon to arrive.> > >>>> I was re-thinking through this, and I think one of these numbers is >>>> wrong, viz, "A photon zips through a 1mm crystal in about 3fs." The >>>> speed of light is 3x10^8 m/s, so this leads to ~3.3 ps for a 1 mm path, >>>> and not 3 fs, a difference of ~10^6. Maybe it was just a typo? Anyway, >>>> it may not make a huge difference, since this would still make for an >>>> average of ~1 photon in the crystal at a time, assuming a high flux of >>>> 10^12 photons per second. But of course there would be some Poisson >>>> statistics involved, and there would be several photons a significant >>>> part of the time. >>>> >>>> Also, I wonder about relativistic effects: in the famous train-in-tunnel >>>> thought experiment, a large train can fit in a short tunnel if it's >>>> going close to the speed of light. Is this applicable here, such that >>>> many photons are in some sense in the crystal at once? Or maybe this is >>>> a red herring. >>>> >>>> But, to change topics a bit: part of the reason I am wondering about >>>> this is anomalous scattering. Since the resonance energy of an atom is >>>> a fixed amount, how can one photon provide that energy simultaneously >>>> to the requisite number (at least thousands, I would think) of resonant >>>> scatterers? Something's very funny here. >>>> >>>> Or, come to think of it, perhaps resonant scattering is no worse than >>>> normal scattering: if the energy is divided up between the all the >>>> normally-scattering electrons, you even have a problem with the >>>> one-photon picture, since the emerging radiation is still of the same >>>> energy. You want to have everything being scattered with a certain >>>> energy, but you also want all the scatterers to scatter. The concept of >>>> "energy" seems to get strange. Does one then need two terms, in which >>>> "energy" is just a characteristic of radiation, like a color, and then >>>> there is some other attribute like "probabilistic intensity," which >>>> describes how much "photon" is there? >>>> >>>> It is striking to me how much depth these everyday occurrences really >>>> have when one starts wondering about them. >>>> >>>> Jacob >>> >>> -- >>> >>> Ethan A Merritt >>> >>> Biomolecular Structure Center, K-428 Health Sciences Bldg >>> >>> MS 357742, University of Washington, Seattle 98195-7742 >> >> -- >> >> mail: Biomolecular Structure Center, K-428 Health Sciences Bldg >> >> MS 357742, University of Washington, Seattle 98195-7742 >> >> -- >> This e-mail and any attachments may contain confidential, copyright and or >> privileged material, and are for the use of the intended addressee only. 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