On Thursday, 05 November, 2015 23:32:09 colin.n...@diamond.ac.uk wrote:
> For scattering from a single electron isn't there a 180 degree phase change
> between the incident and scattered wave?
I don't see how to fit that into the context of diffraction from a crystal.
It gets right back to the original confusion that arises when mixing
descriptions of single-particle events with wave-like interactions.
Would your scattered photon count up all the electrons in the crystal
and choose a phase based on whether the count was even or odd?
> 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.
I am coming around to the view that the demo nicely demonstrates that
transfer of energy from the driver to the resonant object occurs only
as a certain frequency, but is not designed in such a way that it maps
onto a model for anomalous scattering. There is nothing in the demo
that corresponds to a scattered photon. The "phase" in narration is
the phase of the induced resonance relative to the driver. But what
we are interested in is the phase of the scattered photon relative to
that of the incident photon, something that is not represented by
any component of the demo that I can think of.
And the effect that it does show, the transfer of energy at the
resonance frequency, is most directly analogous to the generation
of X-ray fluorescence (incoherent scatter) rather than a coherent
scattering event. You could remove the drive motor from the demo
at that point and the oscillating weight would remain in an
excited state.
For the purpose of showing how excitation and scattering depends on the
input wavelength/frequency, maybe videos like these are more useful:
https://www.youtube.com/watch?v=gNooDfVH30g
https://www.youtube.com/watch?v=nCeoYN9exL0
Ethan
> 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:merr...@u.washington.edu]
> 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
> colin.n...@diamond.ac.uk<mailto:colin.n...@diamond.ac.uk> 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:merr...@u.washington.edu]
>
> > 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:CCP4BB@JISCMAIL.AC.UK] 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 [CCP4BB@JISCMAIL.AC.UK] on behalf of
>
> > > herman.schreu...@sanofi.com<mailto:herman.schreu...@sanofi.com>
> > > [herman.schreu...@sanofi.com]
>
> > > 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:kell...@janelia.hhmi.org]
>
> > > Gesendet: Montag, 31. August 2015 13:06
>
> > > An: Schreuder, Herman R&D/DE;
> > > CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK>
>
> > > 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:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von
>
> > > Keller, Jacob
>
> > > Gesendet: Donnerstag, 20. August 2015 20:42
>
> > > An:
> > > CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK%3cmailto:CCP4BB@JISCMAIL.AC.UK>>
>
> > > 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:
> > > hofkristall...@gmail.com<mailto:hofkristall...@gmail.com<mailto:hofkristall...@gmail.com%3cmailto:hofkristall...@gmail.com>>
>
> > > [mailto:hofkristall...@gmail.com]
>
> > > Sent: Thursday, August 20, 2015 2:03 PM
>
> > > To: Keller, Jacob;
> > > CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK%3cmailto:CCP4BB@JISCMAIL.AC.UK>>
>
> > > 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:CCP4BB@JISCMAIL.AC.UK] On Behalf Of
>
> > > Keller, Jacob
>
> > > Sent: Thursday, August 20, 2015 9:48 AM
>
> > > To:
> > > CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK%3cmailto:CCP4BB@JISCMAIL.AC.UK>>
>
> > > 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:pozharsk...@gmail.com]
>
> > > 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
> > > <kell...@janelia.hhmi.org<mailto:kell...@janelia.hhmi.org<mailto:kell...@janelia.hhmi.org%3cmailto:kell...@janelia.hhmi.org>>>
> > > 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
>
>
--
Ethan A Merritt
Biomolecular Structure Center, K-428 Health Sciences Bldg
MS 357742, University of Washington, Seattle 98195-7742
