Wikipedia:Reference desk/Archives/Science/2019 May 22

= May 22 =

Photons

 * < thinnest and best aimed beam that <
 * < doesn't violate laws of physics < <
 * < a bullseye
 * < doesn't violate laws of physics < <
 * < a bullseye
 * < a bullseye
 * < a bullseye
 * < a bullseye
 * < a bullseye

Where will half the photons fall? A wavelength wide circle? (open edit window for unscrambled ASCII art) (asked by 107.242.117.33 14:25, 22 May 2019‎)
 * "ASCII art" is not enough (at least for me) to understand what you mean
 * but anyway looks to me you'll find to be happy in Diffraction-limited system
 * Gem fr (talk) 14:51, 22 May 2019 (UTC)


 * Photon waves are used for equations. Light doesn't bounce up and down in a wave as it flies through the air. 12.207.168.3 (talk) 16:01, 22 May 2019 (UTC)


 * If the OP is interested in modern theory and practice to describe where we will detect photons, An Introduction to Modern Optics is a good introductory book. After reading through a book like that, let us know if you have more specific questions.  Nimur (talk) 17:10, 22 May 2019 (UTC)


 * Basically I want to know how wide the photon equivalent of an electron cloud is. The part of the cloud that has the photon half of the time that is. Maybe the cloud won't look like a spherical galaxy since photons can't go faster than c so I made the photons fall on a 2D target. No matter how wide and perfect a laser and lens are you can't focus a laser spot sharper than this photon equivalent of an electron cloud right? 107.242.117.17 (talk) 20:01, 22 May 2019 (UTC)
 * You're looking for a specific technical parameter - something like a collision cross-section - to describe the interaction between a photon and some other object; but that's not a common way to model a photon. Most physicists don't talk about the photon's position in the same way that they talk about the electron cloud.  And what's worse - you've somehow concluded that this number, if you could deduce it, would tell you something about "beam width."  With all due respect, that's just wrong physics!  So ... if you ask a wrong-physics question, there's no way to get a right-answer.  In fact, this "characteristic" length scale of a photon - if we could even concoct it - would not be the thing that limits how well-focused a beam of light is, nor how wide the beam would appear if you looked at it or photographed it or otherwise sampled it.
 * The position of an individual photon is not well-defined until after it interacts. Instead, we use a probability density function to estimate where the photon may be, and we use experimental measurements of a large population of photons to describe where most of them will be, most of the time.  This is the whole premise of quantum-mechanically correct physics.  There is a small but finite probability that the photon is anywhere.  We can write a wave-function to describe how improbable it would be for us to find the photon at any specific coordinate, and we can even provide characteristic scale-lengths for the wave function.
 * It would be wise to do some homework: study the common models that physicists use to study the behavior of light, so that you can make sure that you're asking a well-formed question.
 * If you're deep in it, here's Solutions of the Maxwell equations and photon wave functions, (2010), which was also published in the peer-reviewed journal Annals of Physics, authored by Peter Mohr, a professional physicist employed at NIST. This is 75 pages of very math-heavy, advanced quantum-mechanically-correct physics that answers your question.  If you are not already deep in to the physics, I recommend that you start with a much simpler book, like the introductory text I linked earlier.  A more simple geometric model would be the Airy disk, or the beam profile or geometry for a beam of collimated light; but these models are applicable to ensembles and not to individual photons.
 * Part of me wants to remind you to go back and review every permuted variations of the slit experiment, ... but part of me wonders if you've ever seen those before; if this picture doesn't jog your memory - or if you're seeing it for the first time, if it doesn't enlighten you a little bit - then you really really need to go back to studying the basic physics material.
 * Nimur (talk) 20:19, 22 May 2019 (UTC)
 * I think the OP is looking for an area containing 50% probability, if such a thing exists.--Wikimedes (talk) 22:12, 22 May 2019 (UTC)
 * The diffraction limit is kind of toast -- see superlens. I won't claim to know exactly when and how well you can beat the limit with what technologies.  I suppose the fundamental Heisenberg uncertainty principle governs how well you can say where a photon will strike, relative to the uncertainty in its momentum ... however, if you are willing to tolerate substantial uncertainty in the left-right direction only (you don't know when the photon will hit and you don't know what wavelength it is) I don't see a way to use that to constrain the accuracy of the targeting. Wnt (talk) 22:26, 22 May 2019 (UTC)
 * If you want to build machines that defy physical limitations and push the boundaries of optical performance, it's a good idea to understand all aspects of the physical limitations, in theory and in practice ... this is why cameras and optical equipment companies employ cross-functional teams comprised of physicists, electronic engineers, materials and mechanical engineers, and other experts... shameless plug: we're hiring on multiple continents!  And per my usual advice - it'd be a real competitive advantage if the skilled optical physics candidate is additionally a software expert, because the truth of the matter is, most of the time, software plays a huge role in controlling and interpreting the real-world limits imposed by the laws of physics.
 * Anyway,... I have to return to my day-job, laboriously counting incident photons ... if only I could train some kind of machine to do this excruciating work for me... I'd have more time to pontificate about physics on the internet!  But, I've got bills to pay...  Good luck with the suggested-readings!
 * Nimur (talk) 16:54, 23 May 2019 (UTC)
 * Or I could just look up some news stories about how good they've gotten. this proposal wants 30 nm resolution from 193 nm light... in 2007.  But it's for the military, which may not have published.  This one from 2012 got 45 nm out of 365 nm light, and says that 193 nm was down to 22 nm resolution for lithography. [This one from 2014] predicts 10 nm resolution.  I don't doubt the skill of the practitioners, but I do doubt the diffraction limit is going to stop them soon. Wnt (talk) 03:13, 24 May 2019 (UTC)

Peach's substance
What substance in peach causes slightly burning or acrid sensation when eaten (especially on lips, similar to citruses)? 212.180.235.46 (talk) 19:33, 22 May 2019 (UTC)


 * Via PubMed search, I found Response to major peach and peanut allergens in a population of children allergic to peach (2015). "We study the sensitization to relevant allergens, such as peach and peanut LTP, Prup3 and Arah9, and seed storage protein, Arah2."
 * Pru p 3 seems to be the biological chemical name for the most common allergen in peach. This is a lipid transfer protein.
 * In at least one case, Anaphylaxis induced by nectarine, a peach-allergy patient reacted to nectarine, suffering a severe reaction including flush, wheals, and anaphylactic shock.
 * It is important to emphasize that there's a huge variety among the population. Any individual might be sensitive to other compounds, or there could be another cause for sensitivity that is totally unrelated to exposure to the peach fruit.
 * Nimur (talk) 19:47, 22 May 2019 (UTC)


 * I would be very hesitant to diagnose an allergy. Naturally peaches often have an acidic "tang" that simply results from low pH.  The comparison to citruses invites this interpretation.  I looked up the peach trichomes thinking there might be some silica in them (I think, could be wrong, that kiwi fruit can annoy people's lips and tongue by this means) but this paper, which goes into absolutely pornographic detail on peach skin, doesn't mention any silica I could see. Wnt (talk) 22:46, 22 May 2019 (UTC)


 * If you're concerned, see your doctor. ←Baseball Bugs What's up, Doc? carrots→ 23:40, 22 May 2019 (UTC)
 * I mean that tangy taste mentioned by Wnt. I'm healthy, but noticed it may slightly burn the lips. Probably that tangy substance also causes that subtle burning sensation. 212.180.235.46 (talk) 16:51, 23 May 2019 (UTC)
 * I've never experience that. So if you're concerned, see your doctor. ←Baseball Bugs What's up, Doc? carrots→ 17:32, 23 May 2019 (UTC)
 * Agreed with Bugs. It's probably nothing, but that tingling sensation is potentially a sign of a food allergy. I certainly get it from time to time when I eat something I shouldn't. No harm in getting it checked out. Matt Deres (talk) 19:00, 23 May 2019 (UTC)


 * Most likely citric acid, which causes the low pH (acidity), AKA "tartness". If you happen to have small cracks on your lips, which is common in the case of chapped lips, or small tears from accidentally biting your lip or from sharp food, like pretzels with salt crystals on the outside, that can sting. Of course, as the name implies, citric acid is common to all citrus fruit, with lemons and limes being perhaps the tartest. SinisterLefty (talk) 21:27, 27 May 2019 (UTC)