Physics Discussion Thread

[Amber G.]

Meanwhile: Good Morning!
>> Signal in LIGO/Virgo data. Most likely GravitationalWaves from a binary black hole source. Observed a few hours ago!

More at link: https://gracedb.ligo.org/superevents/S200219ac/

[Amber G.]

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

[ranjan.rao]

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...

[Vayutuvan]

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.

[Mort Walker]

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.

[SriKumar]

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

[ranjan.rao]

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.

[Amber G.]

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.)

[Amber G.]

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.

[Vayutuvan]

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.

[Amber G.]

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.

[Amber G.]

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

[SriKumar]

Behind a paywall.

[Amber G.]

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"

[Amber G.]

^^^ 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)

[Prem]

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.

[Amber G.]

^^^

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.

[Amber G.]

^^^ 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

[sudarshan]

From "Policy Changes in India" thread:

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.

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.

[Mort Walker]

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.

[SriKumar]

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).