Hi Ed,
in the same line of thinking one might say that photons do not exist at
all. They are virtual and a means to express the electromagnetic
interaction between charged particles in terms of quantum mechanics.
With this view photons don't interact - or is there an experiment that
shows the interaction of photons in the absence of electromagnetic
interaction of matter?
;-)
Cheers,
Tim
On 08/20/2015 03:36 PM, Edwin Pozharski wrote:
> 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]>
> 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
>>
>>
>>
>
--
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Paul Scherrer Institut
Dr. Tim Gruene
OFLC/107
CH-5232 Villigen PSI
phone: +41 (0)56 310 5297
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