Starship Asterisk*

by **Chris Peterson** » Thu Jun 18, 2015 2:12 pm

Atomictom64 wrote:I recognised the Moon as a Southern Hemisphere image. And, Bingo! The city of La Plata is within one minute of latitude of my city of Adelaide, South Australia. I love the southern moon with its "rabbit" image - I think that's an African legend. Viva el sur :)

You can't tell what hemisphere an image of the Moon is made from if you don't have a horizon for reference. And even so, you may not be able to tell, since the orientation of the Moon changes over the night and it depends on knowing which horizon you are using for reference. You see the same rabbit, in the same orientation, from the northern hemisphere, just not at the same time.

by **Atomictom64** » Thu Jun 18, 2015 12:03 pm

I recognised the Moon as a Southern Hemisphere image. And, Bingo! The city of La Plata is within one minute of latitude of my city of Adelaide, South Australia. I love the southern moon with its "rabbit" image - I think that's an African legend. Viva el sur

by **JohnD** » Wed Jun 17, 2015 8:49 pm

And now (tarantara!), an Earthbound Corona.

See the Earth Science Picture of the Day for today 17th June 2015 at http://epod.usra.edu/
Same effect, allegedly, except with pollen grains!

John

by **DavidLeodis** » Tue Jun 16, 2015 9:47 pm

In the information brought up through the 'Strawberry Moon' link it mentions that Saturn and Antares were close to the Full Moon on June 2 2015, so I wonder if the obvious bright spot below the Moon is one of those (possibly Saturn) or just an image artefact.

by **Chris Peterson** » Tue Jun 16, 2015 3:25 pm

Markus Schwarz wrote:
Chris Peterson wrote:
Markus Schwarz wrote:However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.
No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.
Chris, now you confused me. To clarify, I talked about classical particles (a tennis ball, for example). The de Broglie wavelength of the tennis ball is much smaller than the ball itself. This means that the wave nature of the tennis ball is practically unobservable. We agree on that? Where do you observe diffraction of classical particles?

As I noted elsewhere, a QM analysis generally fails for a large collection of particles. So we are in agreement that the wave nature of a tennis ball is effectively unobservable. But diffraction is observed for single photons and single electrons using carefully designed experiments. In this realm, the analysis is based on QM, on the description of a particle by its wavefunction, not by its classical parameters.

On the other hand, an elementary particle, e.g. an electron, can display wave-like behaviour, like diffraction. In that case, the diffraction is described by QM, of course. This is probably best discussed in the framework of quantum field theory, which "unifies" particle and wave treatment.

I don't think we're in any disagreement, we're just using slightly different terminology. I just wanted to clarify a possible misconception that calling something a wave phenomenon might be understood to mean that it demanded a classical treatment (not your misunderstanding, but a possible misreading in the context of the preceding discussion).

by **Markus Schwarz** » Tue Jun 16, 2015 3:18 pm

Chris Peterson wrote:
Markus Schwarz wrote:However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.
No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.

Chris, now you confused me. To clarify, I talked about classical particles (a tennis ball, for example). The de Broglie wavelength of the tennis ball is much smaller than the ball itself. This means that the wave nature of the tennis ball is practically unobservable. We agree on that? Where do you observe diffraction of classical particles?

On the other hand, an elementary particle, e.g. an electron, can display wave-like behaviour, like diffraction. In that case, the diffraction is described by QM, of course. This is probably best discussed in the framework of quantum field theory, which "unifies" particle and wave treatment.

But the transition from the quantum to the classical domain is not sharp and depends on the sensitivity of the detector and the isolation of the setup. Then it is possible to observe diffraction of even complex molecules.

by **Chris Peterson** » Tue Jun 16, 2015 2:46 pm

JohnD wrote:So is this more like the diffraction seen from oil on water? That's another place where successive colour 'rainbows' are seen.

The colors we see on an oil film are caused by interference between reflections off the top and bottom surfaces of the film. Diffraction is not involved.

And refraction is not a quantum effect, while diffraction is?

Either can be analyzed by classical or quantum methods. Classical analysis tends to fall apart at small scales or for a small number of particles, quantum analysis tends to fall apart at large scales or a large number of particles. The phenomena are what they are, neither classical nor quantum. These concepts refer not to the phenomena themselves, but to how we can best model them.

by **Chris Peterson** » Tue Jun 16, 2015 2:30 pm

Markus Schwarz wrote:However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.

by **Chris Peterson** » Tue Jun 16, 2015 2:29 pm

Markus Schwarz wrote:I haven’t found a reference to Mie theory yet, but from the above article, Mie theory is a solution of the classical Maxwell equations and does not involve QM in any way.

Depends what you mean by "Mie theory". This is true if you're discussing the theory developed by Mie, but Mie scattering is a physical phenomenon that can be treated by either classical or QM analyses.

by **Markus Schwarz** » Tue Jun 16, 2015 12:33 pm

JohnD wrote:And refraction is not a quantum effect, while diffraction is?

No. Whether a phenomenon needs to be described by classical physics or QM depends on the scales of the problem. One such quantity is the de Broglie wavelength of the system. If the spatial extend of the system is comparable to it's de Broglie wavelength, QM must be used.

Both refraction and diffraction can occur in classical as well as quantum mechanics. A classical example is the diffraction of water waves, while a rainbow is a classical example for refraction of light rays. The double-slit experiment with electrons is an example of a quantum mechanical diffraction, which is caused by the wave-like nature of the electron.

However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

by **JohnD** » Tue Jun 16, 2015 11:11 am

Chris Peterson wrote:
JohnD wrote:Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.
This is not a rainbow. It isn't caused by refraction, dispersion, and internal reflection, but by diffraction, a completely different process. So there's no reason to think the structure should be similar.

(If I have time later, and nobody jumps in first, I'll try to devise a simple explanation for how diffraction describes what we're seeing. It's not complicated, but it's not easy to put into words, either.)

Thank you, Chris! And subsequent answerers!
So is this more like the diffraction seen from oil on water? That's another place where successive colour 'rainbows' are seen.

http://hyperphysics.phy-astr.gsu.edu/hb ... lfilm.html

And refraction is not a quantum effect, while diffraction is?

John

by **Markus Schwarz** » Tue Jun 16, 2015 10:37 am

I did some searching and found this helpful article about lunar coronae by Les Cowley et al.. Here I give a brief summary:

A lunar corona results from diffraction of moon light off water droplets. It is different from a rainbow in that the latter results from reflection, refraction, and dispersion in water.

Whether diffraction or refraction dominates depends on the ratio between the wavelength in question and the size of the droplet. Visible light has a wavelengths of about 500 nm. Raindrops have sizes of about 2mm, much larger than the wavelength of visible light, and reflection and refraction dominate (resulting in rainbows). The droplets causing the luna corona have a size of about 10micro meters, wich is only about a factor of 10 larger than the wavelength. Hence, diffraction plays a more dominant role.

There are different levels of rigour when it comes to diffraction calculations. As a first step (also mentioned by Neufer) on can use Frauenhofer far-field diffraction. In this limit, the diffraction pattern from a single droplet is the the same as that of a disk of the same size (Babinet’s theorem). The pattern is then given by the Airy disk.

When all drops have the same size, the resulting corona looks like today’s APOD. When the drops size has a narrow distribution the corona can be noncircular. An extreme case is iridescence.

A more general approach, that is needed when the drop size is “small”, is Mie scattering theory. Computer codes can be used to solve the equations and yield the correct corona properties. The article gives several examples.

(end of summary)

I haven’t found a reference to Mie theory yet, but from the above article, Mie theory is a solution of the classical Maxwell equations and does not involve QM in any way. It is a general Ansatz for (classical) scattering theory, but requires computers to solve effectively. If it successfully describes lunar coronae I don't understand why one would want to say that they are a QM phenomenon. But this does not mean that all scattering problems can be described by classical physics. As a rough estimate, I would say that classical scattering theory breaks down once the wavelength of light becomes comparable to the size of molecules and atoms.

by **Chris Peterson** » Tue Jun 16, 2015 1:04 am

Boomer12k wrote:I would call this something else...

I'd say treat "lunar corona" as a compound word, not the word "corona" modified by "lunar". I think the existing name is here to say, mildly confusing or not.

by **Boomer12k** » Tue Jun 16, 2015 12:55 am

I would call this something else....because The Sun's Corona...is Different....not the Diffraction through thin clouds... This is a Lunar AURA....or Solar Aura....as apposed to the extension of the Sun's surrounding plasma....which is the Sun's "atmosphere"...as seen in Solar Eclipses...

This is an Earth Atmospheric Effect...and we need a different term... But Rainbow...does not quite work as it is not really from a RAIN...but is from the moisture of the clouds, (and Moisturebow, is too weird)... so it could fit..."Lunar Rainbow". BUT AS CHRIS POINTED OUT...NOT THE SAME PROCESS....THUS... We could call it a Lunar SPECTRUM....generally if it were out further, we would call it a HALO... a Lunar Halo...but in close we call it something else....sooooooo...any suggestions?

Personally "Lunar Aura" should be fine, as it is basically used in the description...and even the definition of "Corona"...

:---[===] *

by **neufer** » Tue Jun 16, 2015 12:28 am

https://en.wikipedia.org/wiki/Airy_disk wrote:
[b][size=150]Airy pattern for circular obstruction[/size][/b]
<<In optics, the Airy pattern is the diffraction pattern resulting from a uniformly-illuminated circular [obstruction].

The angle at which the first [Airy pattern] minimum occurs, measured from the direction of incoming light, is given by the approximate formula:

where θ is in radians, λ is the wavelength of the light
and d is the diameter of the [water droplets].>>

The APOD is essentially a superposition of different colored Airy patterns with the central peak 'Airy disk' (along with the direct moon image) optically (or digitally?) reduced in intensity.

With the first [green Airy pattern] minimum at ~ 2.2 lunar diameters
(i.e., the first [green Airy pattern] maximum at ~ 3 lunar diameters)
the water droplets are ~32 microns in diameter using the

formula.

by **MarkBour** » Tue Jun 16, 2015 12:11 am

I believe I can make out 3 sequences from bluish to reddish-orange as you work out from the center of the image. I wonder if with color enhancement of the image one can see any more. Also, incidentally, is that small white dot in the image (at an orientation of about 260 degrees using standard polar coordinates with the Moon at the origin) a star ? And I too, am wondering about the blur at the edge of the Moon in the image.

Wonderful and instructive picture, today! I'm looking forward to Chris' further description.

by **Chris Peterson** » Mon Jun 15, 2015 5:52 pm

JohnD wrote:Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.

This is not a rainbow. It isn't caused by refraction, dispersion, and internal reflection, but by diffraction, a completely different process. So there's no reason to think the structure should be similar.

(If I have time later, and nobody jumps in first, I'll try to devise a simple explanation for how diffraction describes what we're seeing. It's not complicated, but it's not easy to put into words, either.)

by **JohnD** » Mon Jun 15, 2015 5:47 pm

Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.
http://www.atoptics.co.uk/rainbows/sec.htm
Red should face red, if the inner ring is part of some secondary effect, not purple face orange.

Can anyone explain this, or is it an artifact?
John

by **julianm3** » Mon Jun 15, 2015 5:16 pm

Nice shot Sergio, thanks

by **Chris Peterson** » Mon Jun 15, 2015 4:41 pm

Markus Schwarz wrote:Do you know a reference that discusses the lunar corona with QM?

No. I'll mention, however, that in a previous life I designed nephelometers used for performing immunological assays- watching the rate characteristics of scattered light in solution as antibodies and antigens reacted. Some of these instruments monitored the scatter at two different wavelengths. I worked out the Mie scattering involved from QM first principles. Classical theory was inconsistent with observation except when a narrow wavelength source was used (and the instrument used a white light source).

by **RJN** » Mon Jun 15, 2015 4:20 pm

Chris Peterson wrote: I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

OK. Thanks Chris. Yes. I have now removed "purely" from the APOD text.

- RJN

by **Markus Schwarz** » Mon Jun 15, 2015 4:11 pm

Chris Peterson wrote: [...] Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. [...]
I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Yes, the interaction with light and matter is, when you get down to it, of quantum mechanical nature. But I don't understand why you need to employ QM to describe diffraction. You need QM if you want to calculate material properties that are important to diffraction, such as the polarisability of the atoms. Once you take these properties as given, you can employ classical scattering theory.

Also, classical Rayleigh scattering applies when the scatterer is much smaller than the wavelength in question. This is why it successfully describes the blue sky, etc, where the wavelength of visible light is a few hundert nanometers compared to the size of the air molecules (~0.1 nm). Only when the wavelength becomes comparable to the atom radius (e.g. x-rays) do you need QM.

The works by Rayleigh were published in the 1870, well before QM, and those of Mie around 1910 (most likely still classical, but I didn't look up the references).

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I looked up Mie scattering on Wikipedia and in Hecht. In Mie scattering, the cross section is independent of the wavelength, while today's APOD shows an effect which is wavelength dependent. My guess is that it is related to Rayleigh scattering (which, of course, is a limiting case of Mie scattering). Do you know a reference that discusses the lunar corona with QM?

by **merryjman** » Mon Jun 15, 2015 4:07 pm

Chris Peterson wrote:
MattF wrote:
Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.
I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.
It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Okay, let me try again (and thanks to everyone, especially Chris, who's replied so far):

What is observable in the image that can't be explained using classical wave ideas? That's what I really meant to ask but I guess it wasn't very clearly stated. The person who wrote the caption seemed to think it an important point to make. Chris, you mentioned color effects - which ones in particular? Thank you

by **Craine** » Mon Jun 15, 2015 3:10 pm

Chris Peterson wrote:
MattF wrote:
Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.
I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.
It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Maybe the issue is one of level of understanding.
In the sense that the effect can be somewhat described using Wave Mechanics, but only fully understood using Quantum Mechanics.
And since many of us....well, I...can't even pretend to understand Quantum Mechanics I tend to stick with Wave mechanics. It gets me by.

by **Chris Peterson** » Mon Jun 15, 2015 2:45 pm

MattF wrote:
Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.
I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.

It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Starship Asterisk*

APOD: A Colorful Lunar Corona (2015 Jun 15)

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