Physics Discussion Thread

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Amber G.
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Re: Physics Discussion Thread

Postby Amber G. » 19 Feb 2020 22:38

Meanwhile: Good Morning!
>> Signal in LIGO/Virgo data. Most likely GravitationalWaves from a binary black hole source. Observed a few hours ago!
Image
More at link: https://gracedb.ligo.org/superevents/S200219ac/

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Re: Physics Discussion Thread

Postby Amber G. » 28 Feb 2020 23:19

BREAKING: Famed Physicist Freeman Dyson has died at the age of 96 his daughter Mia has confirmed.
RIP.
(Very sad. He was one of my guru).

****
There are many videos of Dyson but the web of stories collection runs to almost 9 hours and the interviews are conducted by Sam Schweber, one of the most distinguished historians of science. This is the most in depth portrait of Dyson, in his own words...
https://www.youtube.com/playlist?list=PLVV0r6CmEsFzDA6mtmKQEgWfcIu49J4nN

***
Prof Dyson (along with Prof Wigner) was also responsible for Inviting Dr. Madan Lal Mehta ( Who was then just relatively unknown grad student, ) to Princeton.

Here he talks about this in the above link.


And
Last edited by Amber G. on 29 Feb 2020 01:29, edited 1 time in total.

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Re: Physics Discussion Thread

Postby ranjan.rao » 29 Feb 2020 01:01

very sad to hear...he was the one who worked with Sir Feynmann on Quantum electrodynamics and the term Dyson sphere for an advanced civilization that can build a sphere around its star to tap the energy of parent star...
wow thats great to hear...

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Re: Physics Discussion Thread

Postby Vayutuvan » 01 Mar 2020 04:12

Very sad news. I heard it yesterday on NPR.

I think he should be added to the list of the greats who were overlooked by the Nobel Committee (others being Gandhi, John von Neumann, Dantzig, SN Bose, JC Bose, MK Narayan, Fred Hoyle).

Dyson's denial of AGW might have played a role. In case of Fed Hoyle, his personality. Same with JvN as well.

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Re: Physics Discussion Thread

Postby Mort Walker » 02 Mar 2020 07:48

Dyson passed away on Friday 28 FEB 2020. He put the mathematical theoretical work for the development of the Feynman diagrams. It is too bad we've lost Dyson and I can only imagine if Feynman lived another 20 years, what he and Dyson would have uncovered.

Amber G,
My utmost parnam if you were one of Dyson's students.

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Re: Physics Discussion Thread

Postby SriKumar » 02 Mar 2020 08:09

The position of Venus* in the sky seems a bit unusual last few days/weeks. It hangs around and is visible (near the moon) for much longer in the night sky than what I've seen in the past- for hours. It also seems a bit bigger than what I can recall. Is it closer to earth the last few days?

(* the bright un-twinkling object next to the moon is what I am calling 'VEnus').

Found it:
https://www.forbes.com/sites/jamiecarte ... 45e3f47134

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Re: Physics Discussion Thread

Postby ranjan.rao » 03 Mar 2020 00:48

https://www.livemint.com/science/news/p ... 59022.html

The explosion was so large it carved out a crater in the hot gas that could hold 15 Milky Ways, said lead author Simona Giacintucci of the Naval Research Laboratory in Washington The explosion occurred in the Ophiuchus galaxy cluster, about 390 million light-years from Earth

Using four telescopes including the Giant Metrewave Radio Telescope (GMRT) in Pune, astronomers have discovered the biggest explosion seen in the universe since the Big Bang.

The blast, which released five times more energy than the previous record holder, came from a supermassive black hole at the centre of a galaxy hundreds of millions of light-years away.

The other telescopes used for the discovery included NASA's Chandra X-ray Observatory, European Space Agency's (ESA) XMM-Newton and the Murchison Widefield Array (MWA) in Western Australia.

"We've seen outbursts in the centres of galaxies before but this one is really, really massive," said Professor Melanie Johnston-Hollitt from the Curtin University node of the International Centre for Radio Astronomy Research in Australia.

"And we don't know why it's so big. But it happened very slowly - like an explosion in slow motion that took place over hundreds of millions of years," she added.

The explosion occurred in the Ophiuchus galaxy cluster, about 390 million light-years from Earth. It was so powerful it punched a cavity in the cluster plasma -- the super-hot gas surrounding the black hole.

The blast was similar to the 1980 eruption of Mount St. Helens, which ripped the top off the mountain, said lead author of the study Simona Giacintucci from the Naval Research Laboratory in the US.

"The difference is that you could fit 15 Milky Way galaxies in a row into the crater this eruption punched into the cluster's hot gas," she said.

The cavity in the cluster plasma had been seen previously with X-ray telescopes, Johnston-Hollitt said.

But scientists initially dismissed the idea that it could have been caused by an energetic outburst, because it would have been too big.

"People were sceptical because the size of outburst," she said. "But it really is that. The Universe is a weird place."

The researchers only realised what they had discovered when they looked at the Ophiuchus galaxy cluster with radio telescopes.

"The radio data fit inside the X-rays like a hand in a glove," said co-author Maxim Markevitch, from NASA's Goddard Space Flight Centre.

"This is the clincher that tells us an eruption of unprecedented size occurred here," Markevitch said.

Professor Johnston-Hollitt likened the finding to discovering the first dinosaur bones.

"It's a bit like archaeology," she said.

"We've been given the tools to dig deeper with low frequency radio telescopes so we should be able to find more outbursts like this now," Johnston-Hollitt said.

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Re: Physics Discussion Thread

Postby Amber G. » 15 Mar 2020 03:18

Mort Walker wrote:Dyson passed away on Friday 28 FEB 2020. He put the mathematical theoretical work for the development of the Feynman diagrams. It is too bad we've lost Dyson and I can only imagine if Feynman lived another 20 years, what he and Dyson would have uncovered.

Amber G,
My utmost parnam if you were one of Dyson's students.

Thanks. Strange as it may seem, I am still trying to digest this. It seems quite a few people from that era died in a short time.
Dyson was not my thesis adviser, but I have worked with him and learned a lot.. He was a remarkable fellow - some of his closest friends were Indian Grad students/postdoc etc. He was often present in many student's seminar - often nodding off :) but his knowledge in so many different fields was amazing and he was very easy to talk to - always very helpful to give insightful suggestions.

His Cambridge adviser was Hardy, he was one of the leading expert on Ramanujan's work. Not only math, but Chemistry, Medical Science, molecular biology .. virtually every field his knowledge was superb.

Interestingly with COVID-19 in the news now, he was the one who did leading work about genome sequencing etc.. (for DNA structures etc.. Along with Watson and Crick main work was done by people like Feynman and Dyson.)

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Re: Physics Discussion Thread

Postby Amber G. » 15 Mar 2020 03:27

Vayutuvan wrote:Very sad news. I heard it yesterday on NPR.

I think he should be added to the list of the greats who were overlooked by the Nobel Committee .

He once said something to the effect: ("For Nobel one has to focus, stay in one field for about 10 years.. he was not cut out for that, he enjoyed jumping from one field to other..)

By the way he never finished his PhD from Cornell.
Story is (heard from his office-mate in Cornell) - After his course work , he will buy a newspaper, cover his head with it and go to sleep. Everyday. After a few months of spending all time in sleeping he woke up and wrote the famous QED paper.

(His habit of sleeping - nodding off in lectures etc became a legend)

He did not take qualifying exams - requirement for PhD. Cornell later wanted to offer him a PhD but he did not want it.

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Re: Physics Discussion Thread

Postby Vayutuvan » 15 Mar 2020 07:47

PHD is just a degree what did it mean during the time of gauss? even at the time of Ramanujan with hardy, hardy didn't have PhD himself.

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Re: Physics Discussion Thread

Postby Amber G. » 28 Apr 2020 03:54

Here is a chance for you to try out for the USA Physics Olympiad!
(This year USA is not fielding a team for the international physics olympiad. But the final-round qualifying exam will be open to the public)
The exam will be posted TOMORROW (04/28) at the link below at 9:00am EST.
<link>

For serious students - Check out some good schools who may have a webminar to go over and discuss the solutions later. and these webminars (which some of use are doing) are now open to all.

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Re: Physics Discussion Thread

Postby Amber G. » 16 May 2020 06:38

This is an excellent article by Frank Wilczek (Nobel Prize in Physics, MIT prof), worth reading to appreciate why many of us physicists miss Freeman Dyson.

An excellent Retrospective on Freeman Dyson in
ScienceMagazine

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Re: Physics Discussion Thread

Postby SriKumar » 16 May 2020 16:32

Behind a paywall.

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Re: Physics Discussion Thread

Postby Amber G. » 16 May 2020 22:15

A sad day for India and Physics:

A generous, kind hearted scholar with total dedication to building of science and scientific Institutions in India with an extraordinary impact spanning many decades-- that's how an always smiling face of Dr. Shri Krishna Joshi will remain forever in our memories.

May the dreams of his generation for a modern, empowered nation which is yet firmly rooted in her wisdom are fulfilled.

किमत्र परिशिष्यते । एतद्वै तत् ॥

"What is left here? Truly, this is that"
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Re: Physics Discussion Thread

Postby Amber G. » 17 May 2020 04:16

^^^ Happier times ... Dr. S. K. Joshi Sir at Devasthal telescope of ARIES/DST .... He was the Chairman of the Governing Council when this largest optical telescope of Asia.

Devastha, for those who have seen the place - name is quite appropriate ..Truly abode of gods!
(At this interesting atavistic space where five mountain tribes gather once a year to celebrate a festival).
(Photo Credit: DST/GoI)
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Re: Physics Discussion Thread

Postby Prem » 22 May 2020 03:26

https://www.news.com.au/technology/scie ... 45c1aabcbc


A cosmic ray detection experiment has found particles that just could be from outside our universe.The Antarctic Impulsive Transient Antenna (ANITA) researchers used a giant balloon to get electronic antennas high into the cold, dry air above Antarctica to identify cosmic-ray showers.They discovered particles may be moving backwards in time, indicating there is a parallel universe, where the rules of the standard model of physics do not apply.“What we saw is something that looked just like a cosmic ray, as seen in the reflection of the ice sheet, but it wasn't reflected,” Peter Gorham, a physics professor at the University of Hawaii, told Times Now.“It was as if the cosmic ray had come out of the ice itself. A very strange thing.He said the discoveries suggested signals came from upward-moving particles that tunnelled through the earth before erupting from the ice.The simplest explanation for the phenomenon is that at the moment of the Big Bang 13.8 billion years ago, two universes were formed.


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Re: Physics Discussion Thread

Postby Amber G. » 22 May 2020 06:04

^^^
Prem wrote:https://www.news.com.au/technology/science/space/nasa-reveals-existence-of-parallel-universe-where-time-runs-backwards/news-story/4ba8516b17d87bc5abd24d45c1aabcbc


A cosmic ray detection experiment has found particles that just could be from outside our universe...<snip>


This is one of the worst kind of idiotic news by tabloids quoting each other.

(Seen it all over the internet abounds stories from tabloids like the New York Post, Express and the Daily Star—mostly quoting each other—some even claiming thatNASA scientists have evidence that could prove the existence of parallel universes and what not)

To point it politely - It’s all way overblown and misrepresents what the research in question is about. Scientists actually found evidence of fundamental particles that defy our current understanding of physics. It might even be an issue with how particles interact with ice... BUT..there is ZERO evidence of *anything* even remotely suggesting time running backwards or parallel universe...

No wonder, I saw a funny tweet from the lead author of the original paper regarding ANITA . "
NASA has discovered that y'all should not be getting your news from the new york post


(There is a really interesting science story here, but it's not the one being peddled. The ANITA experiment is mind-boggling in its own right. It looks for "ghostly" particles that pass through most matter. It has detected something unusual and unexpected. There are plenty of competing theories that aren't explored in the quick news hits -- something like the idea the Antarctic ice may itself be giving rise to these anomalous events)
... Meanwhile in our own universe - go out after sunset and see Venus and Mercury -- pairing up quite closely today.

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Re: Physics Discussion Thread

Postby Amber G. » 22 May 2020 06:45

^^^ Okay - some more background about the above bizarre way the "time going backward" theory came... The science article is behind a paywall so other rags , I think, just copied the first story ...
BTW the event they are talking about happened in 2016 and the way Physics works, it takes lot of time to analyze the data. They were still trying to figure it out all the data...

May be in 2020, after Covid-19, some body joked, won't it be nice to go back to 2019.. :) :) ...
Anyway there are much simpler possible theories which could fit the data.. one for example here (something I was talking about - someone tweeted me the article) :
https://www.cambridge.org/core/journals/annals-of-glaciology/article/reflections-on-the-anomalous-anita-events-the-antarctic-subsurface-as-a-possible-explanation/33C7346D70528285E88B5B909DF87ADB

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Re: Physics Discussion Thread

Postby sudarshan » 24 May 2020 07:17

From "Policy Changes in India" thread:

sudarshan wrote:Jalandhar to Dhauladhar - ~200 km. North Bihar to Himalayas - ~200 km.

Take the average scattering coefficient for dry air at various visible wavelengths - violet (0.4 mu-m), blue (0.45 mu-m), green (0.5 mu-m), yellow (0.55 mu-m), red (0.7 mu-m) and calculate how much of the incident energy makes it to a distance of 200 km. Rayleigh scattering (which is the relevant phenomenon here) coefficient decreases with the fourth power of wavelength, so there's a very strong (inverse) dependence on wavelength. This is why sunsets and sunrises are red - the blues and greens get totally scattered away over the long path through the atmosphere.

For 200 km distance, assuming dry air, about 37% of red light, 9% of yellow light, 2.7% of green light, 0.4% of blue light, and 0.012% of violet light can be expected to make it through - the rest is simply scattered away. Even a little bit of water will greatly reduce these numbers. This is why one sees hazy objects through fog. Particulates and pollutants will make that a lot worse - which is why one can't see even a few hundred meters through Delhi smog.

The blues and greens from those Himalayan peaks, making it to the eyes of viewers 200 km away, indicates that the air is practically clean of particulates (also quite dry). In the right circumstances, visible distance is a great semi-quantitative estimator of pollution levels.


Amber G. wrote:Speaking of light scattering.. a question, I often ask to physics students - why far away mountains look blue while sun around sunset is red.
(Blue light has smaller wavelength than red).


This is a topic I've looked up before. The various explanations that people give are often conflicting, and the explanations also stop without explaining all aspects of the phenomenon (i.e., partial explanations, ignoring certain questions which immediately pop up, which makes the explanation seem more like opinion than fact).

First the facts, which I'm mostly sure about:

With the sky at sunset, what we're doing is looking directly at the sunset. So the blues and greens get scattered out of our view, and only the oranges and reds reach us, since the path through the atmosphere is much greater when the sun is close to the horizon.

When we look at the daytime sky, we are not directly looking at the light source (the sun), instead, we are looking at the blues and greens which have been scattered away from somebody else's direct view, into our eyes (by the gazillions of little mirrors - N2 and O2 molecules in the air). So we don't see reds and oranges in the daytime sky, those go to the eyes of whoever is actually in the direct path of those beams (those guys will be looking directly or close to directly at the sun).

Now with the mountains, the best explanation I could come across for why they appear blue from a distance (which would also make what I said in my quoted part above partially wrong):

Viewing from a distance, the mountain is actually a dark object. The blues we see are what are indirectly scattered into our eyes from the intervening air molecules, against that dark background.

What I don't get about this explanation - why should the mountain be a dark object? If it reflects reds at the viewer, and if (according to my above calculation, which I'm pretty confident about) 37% of that reflected red should reach a viewer 200 km away (assuming clean dry air), then the viewer should see that red also. One explanation - the mountain isn't reflecting red, but absorbing it. But this doesn't seem to be true, because when we go close, we do see reds and yellows from the soil on that same mountain.

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Re: Physics Discussion Thread

Postby Mort Walker » 24 May 2020 10:41

sudarshan wrote:What I don't get about this explanation - why should the mountain be a dark object? If it reflects reds at the viewer, and if (according to my above calculation, which I'm pretty confident about) 37% of that reflected red should reach a viewer 200 km away (assuming clean dry air), then the viewer should see that red also. One explanation - the mountain isn't reflecting red, but absorbing it. But this doesn't seem to be true, because when we go close, we do see reds and yellows from the soil on that same mountain.


Is it because of the color sensitivity of the human eye around 550 nm? Towards the green to yellow color.

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Re: Physics Discussion Thread

Postby SriKumar » 24 May 2020 22:02

I think part of the answer depends on the 'usual color' of mountains. Many of them are covered with trees (green) and many of them are rocky which are mostly darkcolored (like granite). I have to belive that some of the scattered blue light in the sky relfects off the mountains (and everything else, really) so it tends to give everything a blue tinge, including seas/lakes/oceans.

The sun is a light source that is sitting outside the atmosphere and in that sense is not seeing any blue 'reflected' on it from the atmosphere. And lastly I dont know how much this is afactor but sunlight (during sunset) travels through a lot more atmosphere (And dust) than most mountains we can see on a routine basis. So the red wavelength which can 'travel the farthest' (cant explain this fully) gets through to our eyes more so relative to other wavelenghths. Mountains are not a light sournce and 'work' only from Light impinging on them, which likely will not have the full spectrum of colors to begin with (as the sun does).

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Re: Physics Discussion Thread

Postby Vayutuvan » 26 Jun 2020 02:32

https://www.techbriefs.com/component/content/article/tb/supplements/md/stories/insider/37166?utm_source=TB_Motion_News
Atomic scale structure of the single 4-atom acetylene-rotor molecule (grey-white spheres) on the chiral (i.e. having handedness) PdGa surface (blue spheres are Palladium and red spheres are Gallium).
The smallest motor in the world — consisting of just 16 atoms – measures less than one nanometer or about 100,000 times smaller than the diameter of a human hair. The rotor rotates on the surface of the stator and can take up six different positions.

The special feature of the motor is that it moves exactly at the boundary between classical motion and quantum tunneling.


More here with a animation and STM video

https://www.empa.ch/web/s604/molecular-motor

This is extraordinary.

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Re: Physics Discussion Thread

Postby Amber G. » 28 Jun 2020 04:54

^^^ Thanks for posting this ..
An anecdote: In Sixties (IIRC - but story is well known so exact date etc could be filled) Feynman talked about these small machines.. actually much bigger than above but still much smaller - nano-size - than anything existed then). He offered a prize of $1000 to the first guy who could make an electric motor of few hundred nanometer scale. This was in a lecture.

After the talk, Don Glaser (Nobel prize - bubble chamber) told Feynman: "You should have made it smaller." as "you are going to loose the bet"..Feynman did lose the bet as one of his own students were able to produce it in a few years (before his dead-line expired). Feynman lost $1000 but he was still happy - he was given that prototype and at that time that was displayed at Caltech,

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Re: Physics Discussion Thread

Postby Amber G. » 28 Jun 2020 05:03

Meanwhile there are reports from Nordic and places (Netherlands who analyzed the data is also reporting this) that there is a increase level of radioactivity - some where in Western Russia there is a damaged fuel element at a nuclear power plant.

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Re: Physics Discussion Thread

Postby Mort Walker » 28 Jun 2020 22:04

Amber G. wrote:Meanwhile there are reports from Nordic and places (Netherlands who analyzed the data is also reporting this) that there is a increase level of radioactivity - some where in Western Russia there is a damaged fuel element at a nuclear power plant.


I read some of those reports, but how does nuclear fuel get outside the reactor chamber without it not being shut down immediately? Would you not notice a loss of pressure and temperature long before any leak in the reactor? Any released of radioactive gasses would also create all sorts of alarms.

Some tabloids suggested it was a nuclear test:
https://www.dailystar.co.uk/news/world- ... s-22261778

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Re: Physics Discussion Thread

Postby Amber G. » 05 Jul 2020 23:29

This is a *major* item in USA with contribution from Indian Scientists


The PIP-II project at Fermilab includes construction of a 215-meter-long particle accelerator that will accelerate particles to 84% of the speed of light. The new particle accelerator will enable Fermilab to generate an unprecedented stream of neutrinos.

This is a major project by USA with international cooperation from India and also UK, Poland, France and Italy.


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Re: Physics Discussion Thread

Postby SriKumar » 06 Jul 2020 01:00

Amber G....Any responses to 'why moutains look blue' question posed above?

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Re: Physics Discussion Thread

Postby Amber G. » 06 Jul 2020 02:55

SriKumar wrote:Amber G....Any responses to 'why moutains look blue' question posed above?

Hi, there is already some discussion and explanation(s). Sorry if I am repeating some points or putting well known facts. Will just emphasize a few points in details, to clarify. Hope this helps. (Sorry for a long post :) )

There are two main points here, first physics of light and second - physics of eye vision,

First physics of light:

 Higher energy photon (higher frequency waves – or shortwave lengths) scatters more than lower energy photon, or longer wavelengths. Check out Rayleigh Scattering in Wiki, Very roughly speaking this scattering is proportion to 4th power of frequency. Roughly Blue/violet end of the spectrum (400 nm) scatters about 9.4 times more than red part (700 nm) of the spectrum. IOW all other things being equal, two laser rights of the same intensity far away , blue light will have about 10x less photons.
 This is a quite rough analysis, other scattering like Mie or Raman does play quite a bit part too. But this is good enough for basic calculations.
 At sunrise or sunset, when sun’s light passes through longer layer of atmosphere, most of the blue light is scattered away and you see a red (or redder) sun.
 Meanwhile the “scattered photons” make the sky looks blue – these scattered photons are coming from all directions (not from the original source).
 For Mountains - closer ones, naturally you can see more details, including colors – so one can see green trees, colored flowers etc..
 Far away mountains (or anything else for that matter – even nothing) has blue tinge due to those scattered blue light (coming from sun and other sources). Basically one is *not* seeing any color details from the mountain ( even redder color present on the mountain is not that bright), everything you see is blue haze due to those scattered photons which makes the sky blue.
 (If the far away mountain had a light house with a bright red LED light one can see it from far away – much farther than one can see a blue light but if there are not bright objects – which produces photons you see very little details)
 (Roughly speaking, it’s not the mountains which turned into blue color – the blue haze is all over and one see dim mountain in that haze).

Physics of the Eye vision:

This aspect we do not teach much in ordinary physics class.
Basically with music our ear can detect full range of frequencies and we “hear” music very similar to what a wave analyzer will measure.

For light/color perception: we really do not see “frequency” of “color” of light as physicist define.

One suspects this, for example, orange color is “between” red and yellow (frequency wise) and when one mixes yellow and red one get orange. Not the same for purple color which eye sees as “mixture of red and blue”.

Basically, as Frank Wilczek once explained we are “tone-deaf”… our eye can just detect a very narrow band in three different frequencies and the whole color sense is quite complicated. (Even a cheap spectrometer is much better in detecting actual “color” (or frequency) of light than our naked eye). What color we see is, sometimes, quite different than one would “guess” from the frequency spectrum.

But that’s a different story. BTW, our own CV Raman did a lot of work in later part of his life, to help our understanding of physiology of eye and color vision.
(Long ago I enjoyed reading "Longing for the Harmonies" by Frank Wilczek who in my opinion is a very good explainer - Highly recommend the book as, IIRC, it nicely cover the chapter on physics of eye vision ).

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Re: Physics Discussion Thread

Postby SriKumar » 06 Jul 2020 03:05

THanks. Appreciate your explanations. It appears that 'sudarshan' provided the most complete response. A couple of my points were on the right track but without the fundamentals on scattering.
Feel free to post more such questions....

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Re: Physics Discussion Thread

Postby Amber G. » 06 Jul 2020 03:20

FWIW, Other parts are correct and explained pretty well ... let me add a few comments ..
sudarshan wrote:
What I don't get about this explanation - why should the mountain be a dark object? If it reflects reds at the viewer, and if (according to my above calculation, which I'm pretty confident about) 37% of that reflected red should reach a viewer 200 km away (assuming clean dry air), then the viewer should see that red also. One explanation - the mountain isn't reflecting red, but absorbing it. But this doesn't seem to be true, because when we go close, we do see reds and yellows from the soil on that same mountain.


Red LED light on top of mountain may produce enough photons which you will be able to see from 200 Km away.
Other red objects - reflecting or scattering light may produce *very* few photons, which you can see if you are close (inverse square law - closer you are more photons your eye will capture) but not far away.

The main point is, as you said, red (or other color) is not "transferring" into blue .. it is just that further away one gets, dimmer it gets - dim enough that one can't see the color difference and the whole thing is washed out in the blue scattered light.

----
Not important in this discussion but calculating scattering etc may be more complicated than simple formulas in actual practice. Rayleigh scattering depends on frequency a lot, some (Like Mie) not that much.. and of course the amount of dust particle, humidity etc makes the whole thing pretty complex. But all this is *very* important as our sats in the sky have to understand all this while photos etc..:)

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Re: Physics Discussion Thread

Postby Amber G. » 06 Jul 2020 03:54

^^^ Hi Sudharshanji - Let me add - may be of interest in your type of calculations or at least some basic physics of light scattering.
(Of course, for details just look it up in reputable sources)

- There are two main/important type of scattering:
Rayleigh - It is frequency dependent - highly favor blue light which it scatters away. But it scatters the light in, roughly speaking, all directions.

MIe - It does not depend (again roughly speaking) on frequency, and most of the scattered part is in forward direction. (Mainly when there are large particles in the air).

So if you look away from the light source, like overhead around sunrise -- you see blue sky. (Mie scattering can be ignored as very few photons scattered that way are coming back in your direction).

When you look toward the light source, like towards sun at sunrise (or toward a "bright" mountain) - the Mie scattering is important. This is why you see a white glare around the sun in afternoon sun (when there is little Rayleigh scattering) .. or you will see a "halo" or glare around a bright point source on top of the mountain. ... Basically difference between red/blue light scattering is not as much as you will get using Rayleigh formula alone -- IOW the difference between red house and a green tree on a reflected light on the mountain will be quite different than you may get by using your Rayleigh scattering formula alone, (I have not calculated how much this difference will typically be in the case you were considering but it is not going to be as dramatic as red sunset).
Last edited by Amber G. on 06 Jul 2020 04:05, edited 3 times in total.

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Re: Physics Discussion Thread

Postby Amber G. » 06 Jul 2020 03:56

SriKumar wrote:THanks. Appreciate your explanations. It appears that 'sudarshan' provided the most complete response. A couple of my points were on the right track but without the fundamentals on scattering.
Feel free to post more such questions....

Thanks.

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Re: Physics Discussion Thread

Postby Amber G. » 06 Jul 2020 11:15

Also in recent news.. fairly exciting (but now routine) happenings in LIGO world/discoveries ..There was an announcement of GW190814 - the merger of a BlackHole and a mystery Compact Object...
Some details here about this https://www.ligo-india.in/outreach/detections/gw190814/
(Some details about contribution from LIGO - India too in above)

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Re: Physics Discussion Thread

Postby Amber G. » 07 Jul 2020 02:14

Let me add a few more comments .. (Please read my earlier comments)
sudarshan wrote:
What I don't get about this explanation - why should the mountain be a dark object? If it reflects reds at the viewer, and if (according to my above calculation, which I'm pretty confident about) 37% of that reflected red should reach a viewer 200 km away (assuming clean dry air), then the viewer should see that red also. One explanation - the mountain isn't reflecting red, but absorbing it. But this doesn't seem to be true, because when we go close, we do see reds and yellows from the soil on that same mountain.

In addition to what I said before, a few specific points -- key part here "clean dry air"

- Due to lock down, the critical difference is *not* due to clean dry air part (one still has 200 Km of air) - The *big* difference is (lack of) particulates, (pollen / dust/ water droplets/pollution etc

- The basic difference here is instead of N2, O2 molecules (the one we normally associate with Rayleigh scattering) the particles are much larger (about the wavelength of light)

Rayleigh scattering (on clear dry air) produces blue sky -- red sun at sunrise etc.
That part still makes the blue color - in the direction of the mountains (scattered light from sun etc) - you still see it,

The Mie scattering (the dust/smog/pollution etc) used to make the mountains completely invisible. Because light coming from the mountains was completely scattered away. Now there is virtually no particulates - we start seeing the mountains.. less scattering.

This particular part does not (roughly speaking) depend on "color" (or wavelength of light). it affects whole spectrum almost the same way. Whole thing becomes hazy..

(More like what you see through thick fog (water droplets are *much* bigger than N2/O2 molecules).

----
BTW - these kind of measurements (against known frequency / distance / brightness of the source) is one of the methods scientists use to measure air pollution (kind / size etc). So this kind of scattering has practical applications ... for example I can estimate the level of pollution by the stories of mountains being visible etc... specially if I can have access to actual photographs and some other details - may be someday an exam or homework problem based on this theme :) )

Added later: There are probably quite a few articles/tex_book chapters, here is one - this of similar one - may be of interest, if one is interested in plugging in the numbers and explore this more:
https://www.gfdl.noaa.gov/wp-content/uploads/files/user_files/pag/lecture2008/lecture3.pdf

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Re: Physics Discussion Thread

Postby sudarshan » 07 Jul 2020 08:12

^ Thanks for the explanation Amber ji. My mistake was in assuming that just because red frequency light could travel 200 km with minimal attenuation, it would be visible at that distance. Like you pointed out, collimated red light would be visible (laser) but otherwise the light would be subject to the inverse-square law.

Rayleigh scattering is in the limit when the wavelength of the light is much longer than the size of the particles, which is true with N2 or O2 molecules. Mie scattering is the opposite range, like soot particles, water droplets, etc. You're right that just considering the Rayleigh scattering phenomenon isn't sufficient unless the air is clean and dry.

As an interesting aside - a professor (of German origin) told our class group that he went all his life believing Mie was Chinese, but only recently (at that time - I think he was in his fifties) learned that Mie was a fellow German.

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Re: Physics Discussion Thread

Postby Amber G. » 07 Jul 2020 10:55

^^Thanks.
As an interesting aside - a professor (of German origin) told our class group that he went all his life believing Mie was Chinese, but only recently (at that time - I think he was in his fifties) learned that Mie was a fellow German.

Interesting ..:) Wonder if he knew the first name "Gustav" and also it's correct pronunciation. It is pronounced as "Mee" (as in German language Mie ought to be pronounced). (Even google home was pronouncing it incorrectly when I just checked :) ).
Another similar famous physicist/mathematician is Lie. We all study Lie algebra or Lie groups. The name is pronounced as "Lee". Many mispronounce his name and think he is not European ( Lie is actually Norwegian).

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Re: Physics Discussion Thread

Postby sudarshan » 08 Jul 2020 09:27

Auf englisch konnte es wohl "Goostaff" geschrieben werden, und ich meine, dass ihm war das bekannt (weil er gut deutsch sprach). Nur das Name "Gustav Mie" wusste er nicht, erst "Mie."

Sie sprechen deutsch, dann?

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Re: Physics Discussion Thread

Postby Amber G. » 08 Jul 2020 20:40

^^^:) No, not as fluent as my daughter-in-law (who is German-American). :)

Meanwhile, I liked this article describing some work in Indian Institute of Astrophysics.Many here will enjoy this.

Scientific works such as this, published recently in the journal Nature Astronomy, is a compelling example of the profound discovery science being done in the India and encourage our younger scientists.

Sun like stars in their later life hold key for Li increase in Universe

In case anybody is wondering Here "Li" is pronounced as Lithium" :)

(Another paper in Nature Astronomy showed the presence of Carbon essential for Life comes as the byproduct formed after the death of a Star. We are from White Dwarf - formed at the end of a massive Star's death. The study published in Nature Astronomy Published (02 March 2020) points to a basic and imperative conclusion, that carbon has been trapped in the universe for more than 4.6 billion years, when our Solar System was formed. And without Carbon there is no Life. So lithium just could be precursor to Carbon in the White Dwarf)

As Carl Sagan once famously said - "We are all made out of star-stuff".

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Re: Physics Discussion Thread

Postby SriKumar » 09 Jul 2020 04:23

^^^ Indeed, it blew my mind when I read/was told that steel atoms are produced from super novae explosions, or that gold was produced in a ever more intense process. It is quite something to know that the metal you hold in your was wrung through such terrible processes (of extreme temperatures and pressures) and traveled vast distances to get where it got. I had a new-found respect for the lowly, rusted iron nail.

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Re: Physics Discussion Thread

Postby Amber G. » 09 Jul 2020 05:05

^^^ Yes, all the gold came from Kilonova - merger of two neutron stars. Here is an earlier post from 2017
Amber G. wrote:To best of our understanding -
Yes heavier elements come from star core's but only up till Iron.
- Elements H, and He were there / got created around big bang...
- Elements from Li to Fe (Iron) are formed inside stars (Carl Sagan's words "We are star stuff")

Elements heavier than elements came from Nova/super Nova/ or Kilonova -- neutron stars collide. {new discovery}

- Theories / observed spectrums of Super Nova explosions accounted for nearly half of heavier metals nicely but not everything fit
-till the recent observation (observed from spectrum) sheds new light. About half of the heavier elements (like Gold, Platinum, Uranium) are from Neutron start collision...
And this discovery is quite important.. very big deal.



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