Decades And Discoveries: Defining The Eras Of Physics History


As 2019 winds down, we’re seeing a lot of stories offering wrap-ups of “the 2010’s,” with best-of lists of albums, movies, tv shows, and pretty much any other pop-culture phenomenon that exists in more than one example. (Strictly speaking, the new decade doesn’t start until 2021, as the year-numbering system we use doesn’t have a Year 0, but as a practical matter, that cause has been lost for, well, decades.) There will, of course, be stories written about the top stories of the decade in physics and other sciences, but it feels a little tacky to do that in November.

However, it’s not unreasonable to set up a decadal wrap-up in November, by looking back at more distant decades, to provide a basis for comparison. Or maybe I should say “decades” in scare quotes, because when we engage in this sort of exercise in pop culture, we’re really talking about notional decades, not strictly calendrical ones. That is, when we talk about a “decade” in pop-cultural terms, we don’t usually mean a strictly bounded set of exactly ten years whose first three digits are the same, but a period that’s both fuzzier and more restricted.

When people talk about the notional 1960’s, for example, they don’t really mean the years 1960-1969 inclusive. As a pop-culture phenomenon, “The 60’s” don’t really start until after The Beatles hit it big; most of the really iconically 60’s stuff is post-Revolver, so starts in 1966 or so. Similarly, “The 50’s” as we think of them don’t really start until 1955-ish, when Elvis gets rolling, and “The 80’s” in the popular imagination is really the stretch between the launch of MTV in late 1981 and 1986 or so.

This leaves some orphan periods that fall in a calendrical decade, but don’t really belong to the notional decade. The last few years of the calendrical 1980’s, for example, where pop-culture was dominated by hair metal and the emergence of rap, don’t really feel of a piece with the notional 80’s, which are more about synth pop. And despite taking place in 1962, most people wouldn’t call Animal House a movie set in “the 60’s,” because it’s pre-hippie. Notional decades can also spill across calendrical boundaries— you can argue, for example, that the notional 60’s don’t really end until Nixon’s re-election in 1972, or that the aforementioned Animal House really belongs to the notional 50’s.

All of that is a long way of saying that what I’m going to do in this post is to set out a bunch of notional decades in the history of 20th century physics. These are blocks of time analogous to the notional decades of pop culture— they may not fall within strict calendrical boundaries, or even be a full ten years in length, but these are decade-ish blocks of time defined, in the minds of physicists looking backwards, as being all about a particular idea or set of ideas. I’ll try to give approximate start and end points to the notional decades of physics, and talk a little about the most consequential discoveries to emerge from each.

The 1900’s: This notional decade starts with Einstein’s “miracle year” of 1905, which saw him produce four incredibly influential papers on Brownian motion, the photoelectric effect, and what we now call Special Relativity. The period after this is defined by furious activity around these ideas of Einstein’s: using his results to confirm the reality of atoms, introducing the idea of spacetime as a unified thing and beginning to consider it geometrically, starting to think seriously about quantum light etc. This arguably extends until almost 1915, when Einstein completed General Relativity, which means it overlaps slightly with:

The 1910’s: These are the years of the Old Quantum Theory, which get started with the Bohr model of the atom in 1913, and kicked into high gear with Millikan’s experiments confirming Einstein’s model of the photoelectric effect, forcing people to take photons seriously. Einstein does some really important work here on the statistical properties of light, and generally speaking there’s a lot of work on understanding the basic internal structure of atoms. This sets the stage for:

The 1920’s: The notional 1920’s in physics are really the period from about 1924-1930, from de Broglie introducing the idea of electrons as waves and Pauli the exclusion principle (all that stuff to be thankful for), to Dirac producing his eponymous equation merging quantum ideas with Special Relativity. The really critical stuff here is the development of matrix mechanics and then the Schrödinger equation of wave mechanics, both of which were in 1925. The latter half of the decade is dominated by physicists applying these new tools to begin making really good quantitative predictions for the first time, and having epic arguments about the philosophical foundations of quantum physics.

The 1930’s: A lot of what happened in the early part of the calendar decade sort of belongs with the 20’s, carrying over arguments and discoveries associated with the emergence of quantum mechanics. Antimatter was discovered in 1932, but that’s usually noted as confirming a prediction by Dirac. Both the Einstein-Podolsky-Rosen paper and Schrödinger’s infamous cat are in 1935, but those are usually thought of as the culmination of arguments from the Solvay Conference in 1927.

To the extent that physicists talk about the 1930’s as an era, it’s remembered for the birth of nuclear physics. Chadwick’s discovery of the neutron in 1932 (like Anderson’s discovery of the positron, something that had first been hinted at by work done in the Joliot-Curie lab), Lawrence’s invention of the cyclotron, the Joliot-Curies discovering “artificial radioactivity,” and Cockcroft and Walton splitting atoms with protons from a linear accelerator are all forerunners of major developments to come. And of course there’s the late-in-the-decade demonstration of uranium fission by Hahn and Strassman (with theory from Meitner and Frisch), which leads into:

The 1940’s: As in pop culture, the 1940’s are the World War II years for physics, dominated by two enormously important projects: radar and the atomic bomb. These both had incredibly far-reaching consequences: nuclear weapons obviously defined the Cold War, and the technology developed for radar systems during the war proved essential for unlocking a lot of discoveries in the years ahead. Beyond that, the giant wartime efforts showed the power of coordinated large-scale efforts in physics, signalling the arrival of the era of Big Science.

In many ways, the most consequential discovery of the calendrical 1940’s wasn’t any of these, though, but the invention of the transistor in 1947 at Bell Labs. Like many technological developments, though, that didn’t fully pay off for many years.

The 1950’s: This is probably the longest notional decade of the lot, starting a bit early with the Shelter Island conference in 1947, where the Lamb shift and anomalous magnetic moment of the electron first started to be addressed head-on, and extending into the early 1960’s, when a bunch of different people working independently came up with what’s now called the Higgs boson (because Peter Higgs was the only one of the several people who had the basic idea to explicitly mention a particle associated with it). This is the era of quantum field theory, and incorporates the development of QED by Feynman, Schwinger, Tomonaga, and Dyson, the rise of fundamental symmetries as the defining principles of physics, the unification of electromagnetism and the weak interaction, and the beginnings of quark theory.

The 1960’s: Since the notional 50’s run long, the notional 60’s end up a bit short. Probably the most consequential development of the calendrical 60’s was the invention of the laser in 1960, but again, technology developments often take a while to play out. In terms of cultural impact within physics, I think this notional decade needs to be associate with the Space Race. The actual physics involved isn’t especially groundbreaking, but as with the Manhattan Project a couple decades earlier, the rise of NASA is important for further cementing Big Science as a path to success. And, of course, space-based observatories of various sorts later became central to all sorts of discoveries in physics and astrophysics.

The 1970’s: This decade (extending slightly forward into the early 1980’s) is arguably the Golden Age of accelerator physics, with a bunch of new particle discoveries (the tau lepton, the J/psi, the W and Z bosons) generating an enormous amount of excitement.

Of course, these years are also notable for what they didn’t detect, namely any of the particles predicted by Grand Unified Theories and other promising ideas for bringing all the known forces together into a single physical framework. This is the first real manifestation of a problem that continues to bedevil particle physics to this day.

The 1980’s: While there was also a lot of particle-physics activity in this decade, I think the most consequential discovery from the 80’s is high-temperature superconductivity, first seen in 1986 in cuprate ceramics. This was an absolute bombshell, and to this day there isn’t a fully satisfactory theoretical explanation of why these materials behave as they do. It’s an enormously significant question both on the basic physics side (whatever’s going on is interestingly weird and quantum), and in terms of potential technology. If a theory of high-temperature superconductivity can be worked out, that might lead to the development of materials that become superconducting at temperatures high enough to be useful in commerically practical devices, which would be absolutely revolutionary in terms of the ways we consume energy. That’s why there’s so much effort going on both theoretically and in experiments on unusual materials, because if this problem can be cracked, it’s an enormously big deal.

The 1990’s: I’m opening myself up to accusations of bias here, but I think there’s a solid argument that the era-defining discovery of this notional decade is Bose-Einstein Condensation (BEC) in 1995. Though, really, the story starts a bit earlier, with the development of laser cooling tools in the 1980’s.

The techniques of laser cooling, and especially BEC once it was achieved, have absolutely transformed atomic, molecular, and optical (AMO) physics from an area seen as kind of boring and backward-looking (endless measurements of spectra) to a vital and active part of some of the most exciting emerging areas in physics. AMO systems provide astonishingly clean realizations of systems needed to study the fundamentals of quantum optics, test ideas in quantum information, and simulate condensed matter systems. It’s an incredibly exciting field, and we’re still finding new ways BEC and tools developed in pursuit of BEC to open up new areas of inquiry.

So, that’s my rough attempt to divide physics into “decades,” the way we do for pop culture. This is, it should go without saying but doesn’t, just my opinion of what’s most consequential from each of these periods, and what a physicist with a bit of interest in history would immediately think of if you said “1950’s physics.” If you want to take issue with any of these, well, feel free to yell at me on social media…

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