December Photos of the Month: Microwaves

December is a month when many celebrate holidays, and this year I discovered one I hadn't heard of: National Microwave Oven Day, celebrated on December 6th. While we don’t have anything in our collections pertaining to the microwave oven – not even in our wonderful collection of Physics of Technology books, which covers inventions ranging from the laser to the toaster – we do have quite a few things on microwave physics more generally. I realized I could stand to know more about microwaves, so December’s Photos of the Month are devoted to just that.

Malcolm Strandberg setting up apparatus for a 1945 article in "Life" magazine on microwaves. Catalog ID: Strandberg Malcolm F1. Credit: AIP Emilio Segrè Visual Archives

Link to the full issue. Strandberg’s article begins on page 93.

The end of World War II in August 1945 brought the end of the security curtain around wartime technologies. Malcolm “Woody” Strandberg had one of those wartime jobs, developing RADAR systems in the MIT Radiation Laboratory. So that September, his boss asked him to educate Life magazine readers on the properties of microwaves.

In the photo, Strandberg holds a neon light bulb which is lit by microwave radiation. The radiation is transmitted from the horn of a magnetron at the left of the photo, passes briefly through open air into the smaller horn on the serpentine tube, then travels through the tube before emerging to ionize the bulb in his hand. Years later, Strandberg noted that the level of microwave radiation used in this shoot would not be used today without good justification: “In those days the radiation was handled in a manner that would bring criticism in later years when microwave ovens were being introduced and everyone became sensitized to the possible harm the radiation could cause to human biology.” Strandberg’s website features many interesting photos not used in the magazine piece, with commentary.

The piece appeared in the November 19, 1945 of Life. It was not the cover story; the cover just proclaims “BIG BELTS”, with no word of the many other articles inside.

Marvin Chodorow, in his office at the Microwave Laboratory at Stanford University, sits next to a model of the first pulsed megawatt klystron ever built. The pictures in back of Dr. Chodorow are those of Sigurd (left) and Russell Varian, inventors of the klystron. Catalog ID: Chodorow Marvin F1. Credit: Photograph by Stanford University Photographic Department, courtesy AIP Emilio Segrè Visual Archives.

A klystron is one way of generating microwaves. Klystrons are a type of vacuum tube; they take lower frequency radio waves and increase their amplitude into higher electromagnetic ranges by adding an electron beam, then having the radio wave and the electrons resonate through cavities inside the tube. The output from a klystron can range from UHF (Ultra High Frequency, which we used to use for TV signals) up through microwave. Marvin Chodorow, pictured here, researched and developed applications for klystron microwave tubes.

While some microwave ovens historically used klystrons to generate microwaves, they typically use magnetrons today. These are a different type of microwave vacuum tube which use a strong magnet to control the direction of the electrons, and are more efficient.

Interior view of the antenna at Haystack Observatory. For scale, note the person standing at bottom right. Catalog ID: Massachusetts Institute of Technology F21. Credit: Massachusetts Institute of Technology (MIT) Lincoln Library, courtesy AIP Emilio Segrè Visual Archives, Physics Today Collection

The Haystack Microwave Research Facility was built by MIT in 1964, and is used mainly for astronomy. Its Haystack Radio Telescope is a large parabolic antenna (or dish) with a surface area of more than a quarter acre, enclosed in a round 46 meter metal-frame radome which protects it from wind and weather.

The Haystack Radio Telescope was famously used to take long-range interplanetary radar measurements in what was called the “fourth test” of Albert Einstein’s theory of relativity. This test examined the hypothesis that gravity has an effect on light, bending and slowing its path. The test was devised by Irwin Shapiro, and implemented in 1966-1967. It bounced radar signals off Venus and Mercury as each planet passed behind the sun, and confirmed that the sun’s gravity did very slightly slow these waves down. You can read more about the experiment at MIT’s site.

Cutaway illustration over a photo of Lincoln Laboratory's Haystack Microwave Research Facility in Tyngsbobo, Mass. Catalog ID: Massachusetts Institute of Technology H6. Credit: Massachusetts Institute of Technology, Lincoln Library, courtesy AIP Emilio Segrè Visual Archives, Physics Today Collection

George F. Smoot and the U2 receiver used to discover the dipole anisotropy of the cosmic microwave background radiation. Catalog ID: Smoot George F1. Credit: Lawrence Berkeley National Laboratory, courtesy of AIP Emilio Segrè Visual Archives

The cosmic microwave background (CMB) is a pervasive type of radiation found everywhere in the universe. This radiation was first detected in 1964 by astronomers who were looking for sources of noise. Physicists quickly realized that this radiation matched a 1948 prediction of a type of radiation which should be left over from the time of the big bang. The feature of this microwave radiation which makes it the best evidence for the big bang origin is that the radiation is anisotropic; that is, rather than being uniform (isotropic), we find different properties when we measure it in different directions. The CMB is directional; it emanates from a starting point.

In 1979, the U2 anisotropy experiment was designed to measure this anisotropy. The equipment in this photograph was mounted to a U2 reconnaissance plane and flown around. George Smoot was one of the researchers involved. A decade later, in 1989, Smoot was principal investigator on an experiment which took similar and more detailed readings using Differential Microwave Radiometers (DMR) aboard NASA’s Cosmic Background Explorer (COBE) satellite. Smoot received a 2006 Nobel Prize for his role in the project.

You can read more about the U2 experiment and the CMB at Lawrence Berkeley Laboratory’s website.

L-R: C.H. Townes, J. P. Gordon, and T. C. Wang. Wang stands next to the first ammonia-beam maser. The second ammonia-beam maser is front and center with its side removed so that its internal structure is visible. 1955. Columbia University. Catalog ID: Townes Charles C10. Credit: AIP Emilio Segrè Visual Archives, Hecht Collection

Not having a physics background, in my mind the word “maser” refers to the sci-fi weapons which Japan deploys against giant monsters in Godzilla movies. Parabolic antennae, usually mounted on army tanks, which project lovely but ineffective beams until the monster melts them or crushes them. I’m always excited to see these trundle onto the screen, so I owed it to myself to learn more about what real world masers actually are.

A maser (Microwave Amplification by Stimulated Emission of Radiation) produces coherent microwaves. The coherence means that emitted waves have the same wavelength, so that when they interact with each other they will form a single wave with greater amplitude. To achieve this effect, the maser stimulates atoms by adding photons to them; this creates a chain reaction, and each atom affected will produce radiation of the same wavelength. The more familiar laser (Light Amplification by Stimulated Emission of Radiation) is a subset of maser which uses higher frequency photons in the UV light or visible light spectrum.

The first maser was built by Charles Townes and his co-workers at MIT in 1953. The idea was independently developed by Nicolay Basov and Aleksandr Prokhorov at Lebedev Institute in Moscow. Both teams published in 1954, and the 1964 Nobel Prize in Physics was shared by Townes, Basov, and Prokhorov.

Bloembergen 3-level solid-state ruby maser at the Agassiz Station of the Harvard College Observatory. Catalog ID: Harvard College Observatory F3. Credit: Harvard College Observatory, courtesy of AIP Emilio Segrè Visual Archives

The first masers stimulated atoms of ammonia gas, but this was not found to be suitable for practical use. Nicolaas Bloembergen of Harvard created the first “solid state” maser, using ruby crystal instead of a gas. One of these is in use as part of a 60-foot radio telescope at Harvard College Observatory’s Agassiz Station, pictured here. The maser increased the sensitivity of the telescope ten fold. The maser-powered telescope has enabled Harvard astronomers to detect stars and exoplanets, and to study the hydrogen content of remote galaxies.  

References 

M.W.P. Strandberg. (2011 April 30). A Photographic Essay on the Properties of Microwave Radiation. http://www.hr1973.org/strand/mwpstr/www/foto/node1.html

Haystack marks physics milestone. (2005 July 14). MIT News. https://news.mit.edu/2005/einstein

U2 Anisotropy Experiment. Lawrence Berkeley Laboratory, Smoot Group Astrophysics & Cosmology. https://aether.lbl.gov/www/projects/u2/

The Cosmic Microwave Background Radiation. Lawrence Berkeley Laboratory, Smoot Group Astrophysics & Cosmology. https://aether.lbl.gov/cmb.html

Gerald R. Davidson. (1961 October 18). “Professor Receives Award For Invention of ‘Maser’". Harvard Crimson. https://www.thecrimson.com/article/1961/10/18/professor-receives-award-for-invention-of/

What is a MASER? Stanford University, Gravity Probe B: Testing Einstein's Universe. https://einstein.stanford.edu/content/faqs/maser.html