Category Archives: Physics

Maintaining standards: The triple-point cell

Our society depends on standards in countless, mostly invisible ways.   If we can’t agree on how to measure weight, length, temperature, or time we can kiss manufacturing and our manufactured world with its specified, engineered, and interchangeable parts goodbye.   Actually creating and maintaining these standards is hard and the methods change over the years.  The meter was once defined in relation to earthly distances until the realization that the earth changes over time.  Now it is defined in relation to the speed of light which we are pretty sure does not.  The second used to be defined in terms of the day, now it is based on a fundamental property of the cesium atom.  The kilogram has just been redefined in terms of Planck’s constant which then turns into a combination of the second and the meter.

Making any of these measurements is difficult and requires a lot of fancy equipment, often involving lasers, vacuum chambers, electromagnets, and/or racks of electronics.  Here’s how NIST’s new F2 atomic clock works schematically and here’s a package from its inventor on the details.   As the F2 becomes a practical albeit sophisticated standard, even fancier methods are under development for the future.

The Kelvin, fundamental unit of temperature, is a nice exception to this complexity.  It is defined in relation to the triple point of water; that temperature at which the liquid, solid, and vapor phases of isotopically controlled, gas and contaminant-free water are in equilibrium.  Measure this and the Kelvin is 1/273.16 of that.   The aptly named triple point cell requires appropriate water, a skilled glassblower, and some patience.   Thermometers can be calibrated against this standard within and across laboratories.

The Fluke Corporation, despite its name, has long been a respected supplier of a wide variety of test and measurement equipment and they sell such a triple point cell.  In the right hands, it can allow the temperature of 0.01C (the Centigrade and Kelvin are equivalent) to be measured with an uncertainty better than ± 0.0001 °C.  Here’s Fluke’s Matt Newman showing how it is done and not a laser to be seen.

Addendum 20 February 2019: The Kelvin has also been redefined as of November 2018.  It is now tied to Boltzmann’s constant, k.  NIST says that not much will change for the moment since the triple point cell is a known, reliable tool.

 

Guiding Waves to Guiding Light: The vacuum tubes among us

Before the laser came the maser and before that the radar that let civilization live long enough to create the other two.  We think that vacuum tubes have been completely overcome by their solid-state, fully integrated and integratable semiconductor rivals but they soldier on in niches where very high powers have to be sent out of antennas either to other antennas or to scatter back from targets.  Here’s a superb old video explaining the ‘klystron‘, a name fragrant with the aroma of old school pulp science fiction.   They’re still in use as are Traveling Wave Tube Amplifiers (TWTAs, pronounced ‘tweetas’) along with a few other devices that are coming up on nearly ninety years of life.

The Bell System is long gone but its manufacturing arm, the Western Electric Corporation, still has a website and offers products under its old banner.  Its ‘Historic Technical Library’ section is a goldmine of references.  Under ‘Western Electric Technology’ we can learn how to use our Picturephones and read the classic 1965 monograph, Principles of Electron Tubes.  The latter delves rigorously into the business of taking small radio signals and amplifying them to for communications, science, or surveillance.   Both klystrons and TWTAs get detailed treatment.  Fittingly, the final chapter is on gas lasers featuring the ever popular helium-neon variety  with only a brief mention of the carbon dioxide laser invented about the time the book would have gone to press.

It is easy to forget how the development of lasers and nonlinear optical devices came as logical outgrowths of the earlier work at much longer wavelengths – storing power in one medium and exchanging it to another all by playing games with resonances and the speed of light.   The Handbook allows the reader to rediscover these links, often for the first time.  It is not also surprising that places strong in the one such as Stanford; home of the brothers Varian, Edward Ginzton, and William Hansen of klystron fame, became so strong in the other with Schawlow, Hänsch, Siegman, Byer, and Harris. Of course, Bell Labs also falls into that category but it hardly bears repeating since it was so strong in so many areas.

Youtube Channel: 2020tesla

17 May 2021: Several Western Electric links updated

What’s up quark?: Eugene Khutoryansky’s physics videos

As time marches relentlessly on, it slowly erodes what you think you know – especially those things you never knew in the first place.  Despite all of its cruft, marketeering, and self-promotion there’s still a small corner of the internet that hews to its educational roots.  The structure of subatomic particles like protons and neutrons is not conceptually easy, the mathematics reserved for a few.  Eugene Khutoryansky’s colorful and surreal videos do a great service in making abstract concepts concrete.  The underlying classical music soundtrack is in subtle contrast to the extremely non-classical  physics.

Youtube Channel: Physics Videos by Eugene Khutoryansky

Not just hot air: The GE MS9001E gas turbine

Without much further comment, here’s a very deep look into an industrial gas turbine engine. The CAD/CAM work is terrific and one wonders at the design and manufacturing effort put into just this one product.

[Edited 3 September 2018: Original video was taken down by the Youtube poster. Replaced with another link]
[Edited 6 January 2019: No longer available on the backup site, either.  Takedowns suspected]
[Edited 13 March 2019: Aaand it is back.  For now]

Youtube Channel: Ahmed Gaber

 

Eric Betzig goes deep, again: 3D movies of cellular activity

Eric Betzig‘s lab at the Janelia Research Campus has just released a jaw-dropping high-definition 3D movie of cellular machinery in motion.  Words are not sufficient to describe the beauty of the data and the impact of the method which will soon be made available to researchers interested in using or developing it.

I met the man a few times during my postdoctoral life at Bell Laboratories where he was a research scientist.  An acknowledged star in a building full of brilliant people, his Near-Field Scanning Optical Microscope was considered Nobel worthy.  The Labs went down the tubes a few years later when the MBA visigoths took over.  Betzig left, reinvented himself a couple of times, and came back with even more pathbreaking ideas in microscopy that overcame what he felt were insurmountable limitations of his first breakthrough.  He went to Stockholm in 2014 for the newer inventions and the doors they opened.  The Prize has not slowed him down.

The Janelia public release has details and links to several videos, including the one below.

The technical paper appears in the latest issue of Science Magazine.

Observing the cell in its native state: Imaging subcellular dynamics in multicellular organisms
T. Liu et.al.
Science 360, eaaq1392 (2018). DOI: 10.1126/science.aaq1392
The Abstract is also available through PubMed

Youtube Channel: The Howard Hughes Medical Institute

Calling the shot: Brian Metzger on kilonovae

Soon after LIGO‘s first detection of a black hole-black hole merger, the astronomical community was hinting about a potentially more scientifically  exciting event within the interferometer’s grasp: The merging of two neutron stars. When two dark objects coalesce, the product is unsurprisingly dark. Colliding neutron stars on the other hand might emit light of some kind and the collision product need not necessarily be a black hole. More intriguingly, so-called kilonovae resulting from neutron star collisions have been proposed as the actual origin in our universe of many elements heavier than iron, challenging the conventional wisdom of these coming from supernovae.

Here’s a prescient talk by Prof. Brian Metzger of Columbia University and coiner of the term ‘kilonova’ on the consequences of neutron star binary mergers. He discusses their signatures in the gravitational wave record and across the electromagnetic spectrum to their ultimate role in nuclear synthesis. Given at Harvard on 16 March 2017, it is quite accessible for a technical colloquium presentation. A mere five months later on 17 August 2017, LIGO and its European counterpart VIRGO indeed detected the merger of two neutron stars and set of a flurry of observational activity across the globe and in space which confirmed at least qualitatively the predictions by Metzger and his group.

The details are still confusing.  For example, we can assume that it takes a long time for two neutron stars to form, presumably from the death as a supernova of each of a large, but not too large, binary pair.  These violent events will eject a lot of material into the interstellar medium.  The neutron stars then spiral slowly and combine, releasing a lot of neutrons to stick to light elements, transmuting them up the periodic table through the r-process.  But, where do these light elements come from if the ejecta from each of the progenitor stars has had a very long time to spread? (*)

Harvard’s Edo Berger has a concise summary of the multimessenger gold rush incited by the event in a special issue of Astrophysical Journal Letters.  Many of the papers are free to download.  As an aside, I was acquainted with Edo when he was an undergraduate physics student at UCLA while I was a researcher in the same department.  I had no idea then he’d become one of the Dukes of Earl of  high energy astrophysics.

(*) Addendum 20 April 2019: After a year of futility in not finding an answer to this question, I emailed Prof. Metzger and asked.  In a prompt and gracious reply he said that the ejecta from the merging neutron stars create the seed nuclei required for the r-process.  There are sufficient protons (10-30%) in the ejecta to form nuclei of mass number ~100 within milliseconds.  These then absorb further neutrons within the constraints of beta decay to create very heavy elements within a few seconds.  So,  it seems that neutron stars aren’t neutrons all the way down!

30 May 2020: New video source; prior channel was deleted.

Youtube Channel: CfA Colloquium

 

Youtube Channel: Kowch737

https://www.youtube.com/watch?v=_R0mCWaSmao

 

The Deep Space Network – Helping Voyager phone home

The Voyager Spacecraft – Image courtesy NASA and JPL

It is now official, humankind has put its toe into interstellar space as Voyager 1 crossed into the heliopause nearly a year ago. This milestone has been hotly debated in the scientific literature as heavyweight teams presented evidence pro and con. It was only in the last few weeks, however, all parties have agreed on what the data from the probe mean. And they agree that it is now past the interface between the solar wind and the great beyond.
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