Albert Michelson was the first American to win a Nobel Prize in science. He was awarded the Nobel Prize in physics in 1907 for the invention of his eponymous interferometer and for its development as a precision tool for metrology. On board ship traveling to Sweden from London to receive his medal, he was insulted by the British author Rudyard Kipling (that year’s Nobel Laureate in literature) who quipped that America was filled with ignorant masses who wouldn’t amount to anything.
Notwithstanding Kipling’s prediction, across the following century, Americans were awarded 96 Nobel prizes in physics. The next closest nationalities were Germany with 28, the United Kingdom with 25 and France with 18. These are ratios of 3:1, 4:1 and 5:1. Why was the United States so dominant, and why was Rudyard Kipling so wrong?
At the same time that American scientists were garnering the lion’s share of Nobel prizes in physics in the 20th century, the American real (inflation-adjusted) gross-domestic-product (GDP) grew from 60 billion dollars to 20 trillion dollars, making up about a third of the world-wide GDP, even though it has only about 5% of the world population. So once again, why was the United States so dominant across the last century? What factors contributed to this success?
The answers are complicated, with many contributing factors and lots of shades of gray. But two factors stand out that grew hand-in-hand over the century; these are:
1) The striking rise of American elite universities, and
2) The significant gain in the US brain trust through immigration
Albert Michelson is a case in point.
The Firestorms of Albert Michelson
Albert Abraham Michelson was, to some, an undesirable immigrant, born poor in Poland to a Jewish family who made the arduous journey through the Panama Canal in the second wave of 49ers swarming over the California gold country. Michelson grew up in the Wild West, first in the rough town of Murphy’s Camp in California, in foothills of the Sierras. After his father’s supply store went up in flames, they moved to Virginia City, Nevada. His younger brother Charlie lived by the gun (after Michelson had left home), providing meat and protection for supply trains during the Apache wars in the Southwest. This was America in the raw.
Yet Michelson was a prodigy. He outgrew the meager educational possibilities in the mining towns, so his family scraped together enough money to send him to a school in San Francisco, where he excelled. Later, in Virginia City, an academic competition was held for a special appointment to the Naval Academy in Annapolis, and Michelson tied for first place, but the appointment went to the other student who was the son of a Civil War Vet.
With the support of the local Jewish community, Michelson took a train to Washington DC (traveling on the newly-completed Transcontinental Railway, passing over the spot where a golden spike had been driven one month prior into a railroad tie made of Californian laurel) to make his case directly. He met with President Grant at the White House, but all the slots at Annapolis had been filled. Undaunted, Michelson camped out for three days in the waiting room of the office of an Annapolis Admiral, who finally relented and allowed Michelson to take the entrance exam. Still, there was no place for him at the Academy.
Discouraged, Michelson bought a ticket and boarded the train for home. One can only imagine his shock when he heard his name called out by a someone walking down the car aisle. It was a courier from the White House. Michelson met again with Grant, who made an extraordinary extra appointment for Michelson at Annapolis; the Admiral had made his case for him. With no time to return home, he was on board ship for his first training cruise within a week, returning a month later to start classes.
Fig. 1 Albert Abraham Michelson
Years later, as Michelson prepared, with Edmund Morley, to perform the most sensitive test ever made of the motion of the Earth, using his recently-invented “Michelson Interferometer”, the building with his lab went up in flames, just like his father’s goods store had done years before. This was a trying time for Michelson. His first marriage was on the rocks, and he had just recovered from having a nervous breakdown (his wife at one point tried to have him committed to an insane asylum from where patients rarely ever returned). Yet with Morley’s help, they completed the measurement.
To Michelson’s dismay, the exquisite experiment with the finest sensitivity—that should have detected a large deviation of the fringes depending on the orientation of the interferometer relative to the motion of the Earth through space—gave a null result. They published their findings, anyway, as one more puzzle in the question of the speed of light, little knowing how profound this “Michelson-Morley” experiment would be in the history of modern physics and the subsequent development of the relativity theory of Albert Einstein (another immigrant).
Putting the disappointing null result behind him, Michelson next turned his ultra-sensitive interferometer to the problem of replacing the platinum meter-bar standard in Paris with a new standard that was much more fundamental—wavelengths of light. This work, unlike his null result, led to practical success for which he was awarded the Nobel Prize in 1907 (not for his null result with Morley).
Michelson’s Nobel Prize in physics in 1907 did not immediately open the floodgates. Sixteen years passed before the next Nobel in physics went to an American (Robert Millikan). But after 1936 (as many exiles from fascism in Europe immigrated to the US) Americans were regularly among the prize winners.
List of American Nobel Prizes in Physics
* (I) designates an immigrant.
1907 Albert Michelson (I) Optical precision instruments and metrology
1923 Robert Millikan Elementary charge and photoelectric effect
1927 Arthur Compton The Compton effect
1936 Carl David Anderson Discovery of the positron
1937 Clinton Davisson Diffraction of electrons by crystals
1939 Ernest Lawrence Invention of the cyclotron
1943 Otto Stern (I) Magnetic moment of the proton
1944 Isidor Isaac Rabi (I) Magnetic properties of atomic nuclei
1946 Percy Bridgman High pressure physics
1952 E. M. Purcell Nuclear magnetic precision measurements
1952 Felix Bloch (I) Nuclear magnetic precision measurements
1955 Willis Lamb Fine structure of the hydrogen spectrum
1955 Polykarp Kusch (I) Magnetic moment of the electron
1956 William Shockley (I) Discovery of the transistor effect
1956 John Bardeen Discovery of the transistor effect
1956 Walter H. Brattain (I) Discovery of the transistor effect
1957 Chen Ning Yang (I) Parity laws of elementary particles
1957 Tsung-Dao Lee (I) Parity laws of elementary particles
1959 Owen Chamberlain Discovery of the antiproton
1959 Emilio Segrè (I) Discovery of the antiproton
1960 Donald Glaser Invention of the bubble chamber
1961 Robert Hofstadter The structure of nucleons
1963 Maria Goeppert-Mayer (I) Nuclear shell structure
1963 Eugene Wigner (I) Fundamental symmetry principles
1964 Charles Townes Quantum electronics
1965 Richard Feynman Quantum electrodynamics
1965 Julian Schwinger Quantum electrodynamics
1967 Hans Bethe (I) Theory of nuclear reactions
1968 Luis Alvarez Hydrogen bubble chamber
1969 Murray Gell-Mann Classification of elementary particles and interactions
1972 John Bardeen Theory of superconductivity
1972 Leon N. Cooper Theory of superconductivity
1972 Robert Schrieffer Theory of superconductivity
1973 Ivar Giaever (I) Tunneling phenomena
1975 Ben Roy Mottelson The structure of the atomic nucleus
1975 James Rainwater The structure of the atomic nucleus
1976 Burton Richter Discovery of a heavy elementary particle
1976 Samuel C. C. Ting Discovery of a heavy elementary particle
1977 Philip Anderson Magnetic and disordered systems
1977 John van Vleck Magnetic and disordered systems
1978 Robert Wilson Discovery of cosmic microwave background radiation
1978 Arno Penzias (I) Discovery of cosmic microwave background radiation
1979 Steven Weinberg Unified weak and electromagnetic interaction
1979 Sheldon Glashow Unified weak and electromagnetic interaction
1980 James Cronin Symmetry principles in the decay of neutral K-mesons
1980 Val Fitch Symmetry principles in the decay of neutral K-mesons
1981 Nicolaas Bloembergen (I) Nonlinear Optics
1981 Arthur Schawlow Development of laser spectroscopy
1982 Kenneth Wilson Theory for critical phenomena and phase transitions
1983 William Fowler Formation of the chemical elements in the universe
1983 Subrahmanyan Chandrasekhar (I) The evolution of the stars
1988 Leon Lederman Discovery of the muon neutrino
1988 Melvin Schwartz Discovery of the muon neutrino
1988 Jack Steinberger (I) Discovery of the muon neutrino
1989 Hans Dehmelt (I) Ion trap
1989 Norman Ramsey Atomic clocks
1990 Jerome Friedman Deep inelastic scattering of electrons on nucleons
1990 Henry Kendall Deep inelastic scattering of electrons on nucleons
1993 Russell Hulse Discovery of a new type of pulsar
1993 Joseph Taylor Jr. Discovery of a new type of pulsar
1994 Clifford Shull Neutron diffraction
1995 Martin Perl Discovery of the tau lepton
1995 Frederick Reines Detection of the neutrino
1996 David Lee Discovery of superfluidity in helium-3
1996 Douglas Osheroff Discovery of superfluidity in helium-3
1996 Robert Richardson Discovery of superfluidity in helium-3
1997 Steven Chu Laser atom traps
1997 William Phillips Laser atom traps
1998 Horst Störmer (I) Fractionally charged quantum Hall effect
1998 Robert Laughlin Fractionally charged quantum Hall effect
1998 Daniel Tsui (I) Fractionally charged quantum Hall effect
2000 Jack Kilby Integrated circuit
2001 Eric Cornell Bose-Einstein condensation
2001 Carl Wieman Bose-Einstein condensation
2002 Raymond Davis Jr. Cosmic neutrinos
2002 Riccardo Giacconi (I) Cosmic X-ray sources
2003 Anthony Leggett (I) The theory of superconductors and superfluids
2003 Alexei Abrikosov (I) The theory of superconductors and superfluids
2004 David Gross Asymptotic freedom in the strong interaction
2004 H. David Politzer Asymptotic freedom in the strong interaction
2004 Frank Wilczek Asymptotic freedom in the strong interaction
2005 John Hall Quantum theory of optical coherence
2005 Roy Glauber Quantum theory of optical coherence
2006 John Mather Anisotropy of the cosmic background radiation
2006 George Smoot Anisotropy of the cosmic background radiation
2008 Yoichiro Nambu (I) Spontaneous broken symmetry in subatomic physics
2009 Willard Boyle (I) CCD sensor
2009 George Smith CCD sensor
2009 Charles Kao (I) Fiber optics
2011 Saul Perlmutter Accelerating expansion of the Universe
2011 Brian Schmidt Accelerating expansion of the Universe
2011 Adam Riess Accelerating expansion of the Universe
2012 David Wineland Atom Optics
2014 Shuji Nakamura (I) Blue light-emitting diodes
2016 F. Duncan Haldane (I) Topological phase transitions
2016 John Kosterlitz (I) Topological phase transitions
2017 Rainer Weiss (I) LIGO detector and gravitational waves
2017 Kip Thorne LIGO detector and gravitational waves
2017 Barry Barish LIGO detector and gravitational waves
(Note: This list does not include Enrico Fermi, who was awarded the Nobel Prize while in Italy. After traveling to Stockholm to receive the award, he did not return to Italy, but went to the US to protect his Jewish wife from the new race laws enacted by the nationalist government of Italy. There are many additional Nobel prize winners not on this list (like Albert Einstein) who received the Nobel Prize while in their own country but who then came to the US to teach at US institutions.)
Immigration and Elite Universities
A look at the data behind the previous list tells a striking story: 1) Nearly all of the American Nobel Prizes in physics were awarded for work performed at elite American universities; 2) Roughly a third of the prizes went to immigrants. And for those prize winners who were not immigrants themselves, many were taught by, or studied under, immigrant professors at those elite universities.
Elite universities are not just the source of Nobel Prizes, but are engines of the economy. The Tech Sector may contribute only 10% of the US GDP, but 85% of our GDP is attributed to “innovation”, much of coming out of our universities. Our “inventive” economy is driving the American standard of living and keeps us competitive in the worldwide market.
Today, elite universities, as well as immigration, are under attack by forces who want to make America great again. Legislatures in some states have passed laws restricting how those universities hire and teach, and more states are following suite. Some new state laws restrict where Chinese-born professors, who are teaching and conducting research at American universities, can or cannot buy houses. And some members of Congress recently ambushed the leaders of a few of our most elite universities (who failed spectacularly to use common sense), using the excuse of a non-academic issue to turn universities into a metaphor for the supposed evils of elitism.
But the forces seeking to make America great again may be undermining the very thing that made America great in the first place.
They want to cook the goose, but they are overlooking the golden eggs.
It may be hard to get excited about nothing … unless nothing is the whole ball game.
The only way we can really know what is, is by knowing what isn’t. Nothing is the backdrop against which we measure something. Experimentalists spend almost as much time doing control experiments, where nothing happens (or nothing is supposed to happen) as they spend measuring a phenomenon itself, the something.
Even the universe, full of so much something, came out of nothing during the Big Bang. And today the energy density of nothing, so-called Dark Energy, is blowing our universe apart, propelling it ever faster to a bitter cold end.
So here is a brief history of nothing, tracing how we have understood what it is, where it came from, and where is it today.
With sturdy shoulders, space stands opposing all its weight to nothingness. Where space is, there is being.
Friedrich Nietzsche
40,000 BCE – Cosmic Origins
This is a human history, about how we homo sapiens try to understand the natural world around us, so the first step on a history of nothing is the Big Bang of human consciousness that occurred sometime between 100,000 – 40,000 years ago. Some sort of collective phase transition happened in our thought process when we seem to have become aware of our own existence within the natural world. This time frame coincides with the beginning of representational art and ritual burial. This is also likely the time when human language skills reached their modern form, and when logical arguments–stories–first were told to explain our existence and origins.
Roughly two origin stories emerged from this time. One of these assumes that what is has always been, either continuously or cyclically. Buddhism and Hinduism are part of this tradition as are many of the origin philosophies of Indigenous North Americans. Another assumes that there was a beginning when everything came out of nothing. Abrahamic faiths (Let there be light!) subscribe to this creatio ex nihilo. What came before creation? Nothing!
500 BCE – Leucippus and Democritus Atomism
The Greek philosopher Leucippus and his student Democritus, living around 500 BCE, were the first to lay out the atomic theory in which the elements of substance were indivisible atoms of matter, and between the atoms of matter was void. The different materials around us were created by the different ways that these atoms collide and cluster together. Plato later adhered to this theory, developing ideas along these lines in his Timeaus.
300 BCE – Aristotle Vacuum
Aristotle is famous for arguing, in his Physics Book IV, Section 8, that nature abhors a vacuum (horror vacui) because any void would be immediately filled by the imposing matter surrounding it. He also argued more philosophically that nothing, by definition, cannot exist.
1644 – Rene Descartes Vortex Theory
Fast forward a millennia and a half, and theories of existence were finally achieving a level of sophistication that can be called “scientific”. Rene Descartes followed Aristotle’s views of the vacuum, but he extended it to the vacuum of space, filling it with an incompressible fluid in his Principles of Philosophy (1644). Just like water, laminar motion can only occur by shear, leading to vortices. Descartes was a better philosopher than mathematician, so it took Christian Huygens to apply mathematics to vortex motion to “explain” the gravitational effects of the solar system.
Otto von Guericke is one of those hidden gems of the history of science, a person who almost no-one remembers today, but who was far in advance of his own day. He was a powerful politician, holding the position of Burgomeister of the city of Magdeburg for more than 30 years, helping to rebuild it after it was sacked during the Thirty Years War. He was also a diplomat, playing a key role in the reorientation of power within the Holy Roman Empire. How he had free time is anyone’s guess, but he used it to pursue scientific interests that spanned from electrostatics to his invention of the vacuum pump.
With a succession of vacuum pumps, each better than the last, von Geuricke was like a kid in a toy factory, pumping the air out of anything he could find. In the process, he showed that a vacuum would extinguish a flame and could raise water in a tube.
His most famous demonstration was, of course, the Magdeburg sphere demonstration. In 1657 he fabricated two 20-inch hemispheres that he attached together with a vacuum seal and used his vacuum pump to evacuate the air from inside. He then attached chains from the hemispheres to a team of eight horses on each side, for a total of 16 horses, who were unable to separate the spheres. This dramatically demonstrated that air exerts a force on surfaces, and that Aristotle and Descartes were wrong—nature did allow a vacuum!
1667 – Isaac Newton Action at a Distance
When it came to the vacuum, Newton was agnostic. His universal theory of gravitation posited action at a distance, but the intervening medium played no direct role.
Nothing comes from nothing, Nothing ever could.
Rogers and Hammerstein, The Sound of Music
This would seem to say that Newton had nothing to say about the vacuum, but his other major work, his Optiks, established particles as the elements of light rays. Such light particles travelled easily through vacuum, so the particle theory of light came down on the empty side of space.
Statue of Isaac Newton by Sir Eduardo Paolozzi based on a painting by William Blake. Image Credit
1821 – Augustin Fresnel Luminiferous Aether
Today, we tend to think of Thomas Young as the chief proponent for the wave nature of light, going against the towering reputation of his own countryman Newton, and his courage and insights are admirable. But it was Augustin Fresnel who put mathematics to the theory. It was also Fresnel, working with his friend Francois Arago, who established that light waves are purely transverse.
For these contributions, Fresnel stands as one of the greatest physicists of the 1800’s. But his transverse light waves gave birth to one of the greatest red herrings of that century—the luminiferous aether. The argument went something like this, “if light is waves, then just as sound is oscillations of air, light must be oscillations of some medium that supports it – the luminiferous aether.” Arago searched for effects of this aether in his astronomical observations, but he didn’t see it, and Fresnel developed a theory of “partial aether drag” to account for Arago’s null measurement. Hippolyte Fizeau later confirmed the Fresnel “drag coefficient” in his famous measurement of the speed of light in moving water. (For the full story of Arago, Fresnel and Fizeau, see Chapter 2 of “Interference”. [1])
But the transverse character of light also required that this unknown medium must have some stiffness to it, like solids that support transverse elastic waves. This launched almost a century of alternative ideas of the aether that drew in such stellar actors as George Green, George Stokes and Augustin Cauchy with theories spanning from complete aether drag to zero aether drag with Fresnel’s partial aether drag somewhere in the middle.
1849 – Michael Faraday Field Theory
Micheal Faraday was one of the most intuitive physicists of the 1800’s. He worked by feel and mental images rather than by equations and proofs. He took nothing for granted, able to see what his experiments were telling him instead of looking only for what he expected.
This talent allowed him to see lines of force when he mapped out the magnetic field around a current-carrying wire. Physicists before him, including Ampere who developed a mathematical theory for the magnetic effects of a wire, thought only in terms of Newton’s action at a distance. All forces were central forces that acted in straight lines. Faraday’s experiments told him something different. The magnetic lines of force were circular, not straight. And they filled space. This realization led him to formulate his theory for the magnetic field.
Others at the time rejected this view, until William Thomson (the future Lord Kelvin) wrote a letter to Faraday in 1845 telling him that he had developed a mathematical theory for the field. He suggested that Faraday look for effects of fields on light, which Faraday found just one month later when he observed the rotation of the polarization of light when it propagated in a high-index material subject to a high magnetic field. This effect is now called Faraday Rotation and was one of the first experimental verifications of the direct effects of fields.
Nothing is more real than nothing.
Samuel Beckett
In 1949, Faraday stated his theory of fields in their strongest form, suggesting that fields in empty space were the repository of magnetic phenomena rather than magnets themselves [2]. He also proposed a theory of light in which the electric and magnetic fields induced each other in repeated succession without the need for a luminiferous aether.
1861 – James Clerk Maxwell Equations of Electromagnetism
James Clerk Maxwell pulled the various electric and magnetic phenomena together into a single grand theory, although the four succinct “Maxwell Equations” was condensed by Oliver Heaviside from Maxwell’s original 15 equations (written using Hamilton’s awkward quaternions) down to the 4 vector equations that we know and love today.
One of the most significant and most surprising thing to come out of Maxwell’s equations was the speed of electromagnetic waves that matched closely with the known speed of light, providing near certain proof that light was electromagnetic waves.
However, the propagation of electromagnetic waves in Maxwell’s theory did not rule out the existence of a supporting medium—the luminiferous aether. It was still not clear that fields could exist in a pure vacuum but might still be like the stress fields in solids.
Late in his life, just before he died, Maxwell pointed out that no measurement of relative speed through the aether performed on a moving Earth could see deviations that were linear in the speed of the Earth but instead would be second order. He considered that such second-order effects would be far to small ever to detect, but Albert Michelson had different ideas.
1887 – Albert Michelson Null Experiment
Albert Michelson was convinced of the existence of the luminiferous aether, and he was equally convinced that he could detect it. In 1880, working in the basement of the Potsdam Observatory outside Berlin, he operated his first interferometer in a search for evidence of the motion of the Earth through the aether. He had built the interferometer, what has come to be called a Michelson Interferometer, months earlier in the laboratory of Hermann von Helmholtz in the center of Berlin, but the footfalls of the horse carriages outside the building disturbed the measurements too much—Postdam was quieter.
But he could find no difference in his interference fringes as he oriented the arms of his interferometer parallel and orthogonal to the Earth’s motion. A simple calculation told him that his interferometer design should have been able to detect it—just barely—so the null experiment was a puzzle.
Seven years later, again in a basement (this time in a student dormitory at Western Reserve College in Cleveland, Ohio), Michelson repeated the experiment with an interferometer that was ten times more sensitive. He did this in collaboration with Edward Morley. But again, the results were null. There was no difference in the interference fringes regardless of which way he oriented his interferometer. Motion through the aether was undetectable.
(Michelson has a fascinating backstory, complete with firestorms (literally) and the Wild West and a moment when he was almost committed to an insane asylum against his will by a vengeful wife. To read all about this, see Chapter 4: After the Gold Rush in my recent book Interference (Oxford, 2023)).
The Michelson Morley experiment did not create the crisis in physics that it is sometimes credited with. They published their results, and the physics world took it in stride. Voigt and Fitzgerald and Lorentz and Poincaré toyed with various ideas to explain it away, but there had already been so many different models, from complete drag to no drag, that a few more theories just added to the bunch.
But they all had their heads in a haze. It took an unknown patent clerk in Switzerland to blow away the wisps and bring the problem into the crystal clear.
1905 – Albert Einstein Relativity
So much has been written about Albert Einstein’s “miracle year” of 1905 that it has lapsed into a form of physics mythology. Looking back, it seems like his own personal Big Bang, springing forth out of the vacuum. He published 5 papers that year, each one launching a new approach to physics on a bewildering breadth of problems from statistical mechanics to quantum physics, from electromagnetism to light … and of course, Special Relativity [3].
Whereas the others, Voigt and Fitzgerald and Lorentz and Poincaré, were trying to reconcile measurements of the speed of light in relative motion, Einstein just replaced all that musing with a simple postulate, his second postulate of relativity theory:
2. Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence …
Albert Einstein, Annalen der Physik, 1905
And the rest was just simple algebra—in complete agreement with Michelson’s null experiment, and with Fizeau’s measurement of the so-called Fresnel drag coefficient, while also leading to the famous E = mc2 and beyond.
There is no aether. Electromagnetic waves are self-supporting in vacuum—changing electric fields induce changing magnetic fields that induce, in turn, changing electric fields—and so it goes.
The vacuum is vacuum—nothing! Except that it isn’t. It is still full of things.
1931 – P. A. M Dirac Antimatter
The Dirac equation is the famous end-product of P. A. M. Dirac’s search for a relativistic form of the Schrödinger equation. It replaces the asymmetric use in Schrödinger’s form of a second spatial derivative and a first time derivative with Dirac’s form using only first derivatives that are compatible with relativistic transformations [4].
One of the immediate consequences of this equation is a solution that has negative energy. At first puzzling and hard to interpret [5], Dirac eventually hit on the amazing proposal that these negative energy states are real particles paired with ordinary particles. For instance, the negative energy state associated with the electron was an anti-electron, a particle with the same mass as the electron, but with positive charge. Furthermore, because the anti-electron has negative energy and the electron has positive energy, these two particles can annihilate and convert their mass energy into the energy of gamma rays. This audacious proposal was confirmed by the American physicist Carl Anderson who discovered the positron in 1932.
The existence of particles and anti-particles, combined with Heisenberg’s uncertainty principle, suggests that vacuum fluctuations can spontaneously produce electron-positron pairs that would then annihilate within a time related to the mass energy
Although this is an exceedingly short time (about 10-21 seconds), it means that the vacuum is not empty, but contains a frothing sea of particle-antiparticle pairs popping into and out of existence.
1938 – M. C. Escher Negative Space
Scientists are not the only ones who think about empty space. Artists, too, are deeply committed to a visual understanding of our world around us, and the uses of negative space in art dates back virtually to the first cave paintings. However, artists and art historians only talked explicitly in such terms since the 1930’s and 1940’s [6]. One of the best early examples of the interplay between positive and negative space was a print made by M. C. Escher in 1938 titled “Day and Night”.
1946 – Edward Purcell Modified Spontaneous Emission
In 1916 Einstein laid out the laws of photon emission and absorption using very simple arguments (his modus operandi) based on the principles of detailed balance. He discovered that light can be emitted either spontaneously or through stimulated emission (the basis of the laser) [7]. Once the nature of vacuum fluctuations was realized through the work of Dirac, spontaneous emission was understood more deeply as a form of stimulated emission caused by vacuum fluctuations. In the absence of vacuum fluctuations, spontaneous emission would be inhibited. Conversely, if vacuum fluctuations are enhanced, then spontaneous emission would be enhanced.
This effect was observed by Edward Purcell in 1946 through the observation of emission times of an atom in a RF cavity [8]. When the atomic transition was resonant with the cavity, spontaneous emission times were much faster. The Purcell enhancement factor is
where Q is the “Q” of the cavity, and V is the cavity volume. The physical basis of this effect is the modification of vacuum fluctuations by the cavity modes caused by interference effects. When cavity modes have constructive interference, then vacuum fluctuations are larger, and spontaneous emission is stimulated more quickly.
1948 – Hendrik Casimir Vacuum Force
Interference effects in a cavity affect the total energy of the system by excluding some modes which become inaccessible to vacuum fluctuations. This lowers the internal energy internal to a cavity relative to free space outside the cavity, resulting in a net “pressure” acting on the cavity. If two parallel plates are placed in close proximity, this would cause a force of attraction between them. The effect was predicted in 1948 by Hendrik Casimir [9], but it was not verified experimentally until 1997 by S. Lamoreaux at Yale University [10].
Two plates brought very close feel a pressure exerted by the higher vacuum energy density external to the cavity.
1949 – Shinichiro Tomonaga, Richard Feynman and Julian Schwinger QED
The physics of the vacuum in the years up to 1948 had been a hodge-podge of ad hoc theories that captured the qualitative aspects, and even some of the quantitative aspects of vacuum fluctuations, but a consistent theory was lacking until the work of Tomonaga in Japan, Feynman at Cornell and Schwinger at Harvard. Feynman and Schwinger both published their theory of quantum electrodynamics (QED) in 1949. They were actually scooped by Tomonaga, who had developed his theory earlier during WWII, but physics research in Japan had been cut off from the outside world. It was when Oppenheimer received a letter from Tomonaga in 1949 that the West became aware of his work. All three received the Nobel Prize for their work on QED in 1965. Precision tests of QED now make it one of the most accurately confirmed theories in physics.
Richard Feynman’s first “Feynman diagram”.
1964 – Peter Higgs and The Higgs
The Higgs particle, known as “The Higgs”, was the brain-child of Peter Higgs, Francois Englert and Gerald Guralnik in 1964. Higgs’ name became associated with the theory because of a response letter he wrote to an objection made about the theory. The Higg’s mechanism is spontaneous symmetry breaking in which a high-symmetry potential can lower its energy by distorting the field, arriving at a new minimum in the potential. This mechanism can allow the bosons that carry force to acquire mass (something the earlier Yang-Mills theory could not do).
Spontaneous symmetry breaking is a ubiquitous phenomenon in physics. It occurs in the solid state when crystals can lower their total energy by slightly distorting from a high symmetry to a low symmetry. It occurs in superconductors in the formation of Cooper pairs that carry supercurrents. And here it occurs in the Higgs field as the mechanism to imbues particles with mass .
Conceptual graph of a potential surface where the high symmetry potential is higher than when space is distorted to lower symmetry. Image Credit
The theory was mostly ignored for its first decade, but later became the core of theories of electroweak unification. The Large Hadron Collider (LHC) at Geneva was built to detect the boson, announced in 2012. Peter Higgs and Francois Englert were awarded the Nobel Prize in Physics in 2013, just one year after the discovery.
The Higgs field permeates all space, and distortions in this field around idealized massless point particles are observed as mass. In this way empty space becomes anything but.
1981 – Alan Guth Inflationary Big Bang
Problems arose in observational cosmology in the 1970’s when it was understood that parts of the observable universe that should have been causally disconnected were in thermal equilibrium. This could only be possible if the universe were much smaller near the very beginning. In January of 1981, Alan Guth, then at Cornell University, realized that a rapid expansion from an initial quantum fluctuation could be achieved if an initial “false vacuum” existed in a positive energy density state (negative vacuum pressure). Such a false vacuum could relax to the ordinary vacuum, causing a period of very rapid growth that Guth called “inflation”. Equilibrium would have been achieved prior to inflation, solving the observational problem.Therefore, the inflationary model posits a multiplicities of different types of “vacuum”, and once again, simple vacuum is not so simple.
Energy density as a function of a scalar variable. Quantum fluctuations create a “false vacuum” that can relax to “normal vacuum: by expanding rapidly. Image Credit
1998 – Saul Pearlmutter Dark Energy
Einstein didn’t make many mistakes, but in the early days of General Relativity he constructed a theoretical model of a “static” universe. A central parameter in Einstein’s model was something called the Cosmological Constant. By tuning it to balance gravitational collapse, he tuned the universe into a static Ithough unstable) state. But when Edwin Hubble showed that the universe was expanding, Einstein was proven incorrect. His Cosmological Constant was set to zero and was considered to be a rare blunder.
Fast forward to 1999, and the Supernova Cosmology Project, directed by Saul Pearlmutter, discovered that the expansion of the universe was accelerating. The simplest explanation was that Einstein had been right all along, or at least partially right, in that there was a non-zero Cosmological Constant. Not only is the universe not static, but it is literally blowing up. The physical origin of the Cosmological Constant is believed to be a form of energy density associated with the space of the universe. This “extra” energy density has been called “Dark Energy”, filling empty space.
The bottom line is that nothing, i.e., the vacuum, is far from nothing. It is filled with a froth of particles, and energy, and fields, and potentials, and broken symmetries, and negative pressures, and who knows what else as modern physics has been much ado about this so-called nothing, almost more than it has been about everything else.
[2] L. Peirce Williams in “Faraday, Michael.” Complete Dictionary of Scientific Biography, vol. 4, Charles Scribner’s Sons, 2008, pp. 527-540.
[3] A. Einstein, “On the electrodynamics of moving bodies,” Annalen Der Physik 17, 891-921 (1905).
[4] Dirac, P. A. M. (1928). “The Quantum Theory of the Electron”. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 117 (778): 610–624.
[5] Dirac, P. A. M. (1930). “A Theory of Electrons and Protons”. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 126 (801): 360–365.
[6] Nikolai M Kasak, Physical Art: Action of positive and negative space, (Rome, 1947/48) [2d part rev. in 1955 and 1956].
[7] A. Einstein, “Strahlungs-Emission un -Absorption nach der Quantentheorie,” Verh. Deutsch. Phys. Ges. 18, 318 (1916).
[8] Purcell, E. M. (1946-06-01). “Proceedings of the American Physical Society: Spontaneous Emission Probabilities at Ratio Frequencies”. Physical Review. American Physical Society (APS). 69 (11–12): 681.
[9] Casimir, H. B. G. (1948). “On the attraction between two perfectly conducting plates”. Proc. Kon. Ned. Akad. Wet. 51: 793.
[10] Lamoreaux, S. K. (1997). “Demonstration of the Casimir Force in the 0.6 to 6 μm Range”. Physical Review Letters. 78 (1): 5–8.
Light is one of the most powerful manifestations of the forces of physics because it tells us about our reality. The interference of light, in particular, has led to the detection of exoplanets orbiting distant stars, discovery of the first gravitational waves, capture of images of black holes and much more. The stories behind the history of light and interference go to the heart of how scientists do what they do and what they often have to overcome to do it. These time-lines are organized along the chapter titles of the book Interference. They follow the path of theories of light from the first wave-particle debate, through the personal firestorms of Albert Michelson, to the discoveries of the present day in quantum information sciences.
Thomas Young was the ultimate dabbler, his interests and explorations ranged far and wide, from ancient egyptology to naval engineering, from physiology of perception to the physics of sound and light. Yet unlike most dabblers who accomplish little, he made original and seminal contributions to all these fields. Some have called him the “Last Man Who Knew Everything“.
Thomas Young. The Law of Interference.
Topics: The Law of Interference. The Rosetta Stone. Benjamin Thompson, Count Rumford. Royal Society. Christiaan Huygens. Pendulum Clocks. Icelandic Spar. Huygens’ Principle. Stellar Aberration. Speed of Light. Double-slit Experiment.
1629 – Huygens born (1629 – 1695)
1642 – Galileo dies, Newton born (1642 – 1727)
1655 – Huygens ring of Saturn
1657 – Huygens patents the pendulum clock
1666 – Newton prismatic colors
1666 – Huygens moves to Paris
1669 – Bartholin double refraction in Icelandic spar
1670 – Bartholinus polarization of light by crystals
1671 – Expedition to Hven by Picard and Rømer
1673 – James Gregory bird-feather diffraction grating
1801 – Young Theory of Light and Colours, three color mechanism (Bakerian Lecture), Young considers interference to cause the colored films, first estimates of the wavelengths of different colors
1802 – Young begins series of lecturs at the Royal Institution (Jan. 1802 – July 1803)
1802 – Young names the principle (Law) of interference
Augustin Fresnel was an intuitive genius whose talents were almost squandered on his job building roads and bridges in the backwaters of France until he was discovered and rescued by Francois Arago.
Topics: Particles versus Waves. Malus and Polarization. Agustin Fresnel. Francois Arago. Diffraction. Daniel Bernoulli. The Principle of Superposition. Joseph Fourier. Transverse Light Waves.
1665 – Grimaldi diffraction bands outside shadow
1673 – James Gregory bird-feather diffraction grating
There is no question that Francois Arago was a swashbuckler. His life’s story reads like an adventure novel as he went from being marooned in hostile lands early in his career to becoming prime minister of France after the 1848 revolutions swept across Europe.
Topics: The Birth of Interferometry. Snell’s Law. Fresnel and Arago. The First Interferometer. Fizeau and Foucault. The Speed of Light. Ether Drag. Jamin Interferometer.
No name is more closely connected to interferometry than that of Albert Michelson. He succeeded, sometimes at great personal cost, in launching interferometric metrology as one of the most important tools used by scientists today.
Albert A. Michelson, 1907 Nobel Prize. Image Credit.
Topics: The Trials of Albert Michelson. Hermann von Helmholtz. Michelson and Morley. Fabry and Perot.
1810 – Arago search for ether drag
1813 – Fraunhofer dark lines in Sun spectrum
1813 – Faraday begins at Royal Institution
1820 – Oersted discovers electromagnetism
1821 – Faraday electromagnetic phenomena
1827 – Green mathematical analysis of electricity and magnetism
1830 – Cauchy ether as elastic solid
1831 – Faraday electromagnetic induction
1831 – Cauchy ether drag
1831 – Maxwell born
1831 – Faraday electromagnetic induction
1836 – Cauchy’s second theory of the ether
1838 – Green theory of the ether
1839 – Hamilton group velocity
1839 – MacCullagh properties of rotational ether
1839 – Cauchy ether with negative compressibility
1841 – Maxwell entered Edinburgh Academy (age 10) met P. G. Tait
1842 – Doppler effect
1845 – Faraday effect (magneto-optic rotation)
1846 – Stokes’ viscoelastic theory of the ether
1847 – Maxwell entered Edinburgh University
1850 – Maxwell at Cambridge, studied under Hopkins, also knew Stokes and Whewell
1852 – Michelson born Strelno, Prussia
1854 – Maxwell wins the Smith’s Prize (Stokes’ theorem was one of the problems)
1855 – Michelson’s immigrate to San Francisco through Panama Canal
Learning from his attempts to measure the speed of light through the ether, Michelson realized that the partial coherence of light from astronomical sources could be used to measure their sizes. His first measurements using the Michelson Stellar Interferometer launched a major subfield of astronomy that is one of the most active today.
R Hanbury Brown
Topics: Measuring the Stars. Astrometry. Moons of Jupiter. Schwarzschild. Betelgeuse. Michelson Stellar Interferometer. Banbury Brown Twiss. Sirius. Adaptive Optics.
1838 – Bessel stellar parallax measurement with Fraunhofer telescope
1868 – Fizeau proposes stellar interferometry
1873 – Stephan implements Fizeau’s stellar interferometer on Sirius, sees fringes
1880 – Michelson Idea for second-order measurement of relative motion against ether
1880 – 1882 Michelson Studies in Europe (Helmholtz in Berlin, Quincke in Heidelberg, Cornu, Mascart and Lippman in Paris)
1881 – Michelson Measurement at Potsdam with funds from Alexander Graham Bell
1881 – Michelson Resigned from active duty in the Navy
1883 – Michelson Joined Case School of Applied Science
1889 – Michelson moved to Clark University at Worcester
Stellar interferometry is opening new vistas of astronomy, exploring the wildest occupants of our universe, from colliding black holes half-way across the universe (LIGO) to images of neighboring black holes (EHT) to exoplanets near Earth that may harbor life.
Image of the supermassive black hole in M87 from Event Horizon Telescope.
Topics: Gravitational Waves, Black Holes and the Search for Exoplanets. Nulling Interferometer. Event Horizon Telescope. M87 Black Hole. Long Baseline Interferometry. LIGO.
1947 – Virgo A radio source identified as M87
1953 – Horace W. Babcock proposes adaptive optics (AO)
From the astronomically large dimensions of outer space to the microscopically small dimensions of inner space, optical interference pushes the resolution limits of imaging.
Topics: Diffraction and Interference. Joseph Fraunhofer. Diffraction Gratings. Henry Rowland. Carl Zeiss. Ernst Abbe. Phase-contrast Microscopy. Super-resolution Micrscopes. Structured Illumination.
The coherence of laser light is like a brilliant jewel that sparkles in the darkness, illuminating life, probing science and projecting holograms in virtual worlds.
What is the image of one photon interfering? Better yet, what is the image of two photons interfering? The answer to this crucial question laid the foundation for quantum communication.
Topics: The Beginnings of Quantum Communication. EPR paradox. Entanglement. David Bohm. John Bell. The Bell Inequalities. Leonard Mandel. Single-photon Interferometry. HOM Interferometer. Two-photon Fringes. Quantum cryptography. Quantum Teleportation.
1900 – Planck (1901). “Law of energy distribution in normal spectra.” [1]
There is almost no technical advantage better than having exponential resources at hand. The exponential resources of quantum interference provide that advantage to quantum computing which is poised to usher in a new era of quantum information science and technology.
David Deutsch.
Topics: Interferometric Computing. David Deutsch. Quantum Algorithm. Peter Shor. Prime Factorization. Quantum Logic Gates. Linear Optical Quantum Computing. Boson Sampling. Quantum Computational Advantage.
1980 – Paul Benioff describes possibility of quantum computer
[10] B. R. Mollow, R. J. Glauber: Phys. Rev. 160, 1097 (1967); 162, 1256 (1967)
[11] J. F. Clauser, M. A. Horne, A. Shimony, and R. A. Holt, ” Proposed experiment to test local hidden-variable theories,” Physical Review Letters, vol. 23, no. 15, pp. 880-&, (1969)
[15] R. Ghosh and L. Mandel, “Observation of nonclassical effects in the interference of 2 photons,” Physical Review Letters, vol. 59, no. 17, pp. 1903-1905, Oct (1987)
[16] C. K. Hong, Z. Y. Ou, and L. Mandel, “Measurement of subpicosecond time intervals between 2 photons by interference,” Physical Review Letters, vol. 59, no. 18, pp. 2044-2046, Nov (1987)
[18] D. Deutsch, “QUANTUM-THEORY, THE CHURCH-TURING PRINCIPLE AND THE UNIVERSAL QUANTUM COMPUTER,” Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences, vol. 400, no. 1818, pp. 97-117, (1985)
[19] P. W. Shor, “ALGORITHMS FOR QUANTUM COMPUTATION – DISCRETE LOGARITHMS AND FACTORING,” in 35th Annual Symposium on Foundations of Computer Science, Proceedings, S. Goldwasser Ed., (Annual Symposium on Foundations of Computer Science, 1994, pp. 124-134.
[20] F. Arute et al., “Quantum supremacy using a programmable superconducting processor,” Nature, vol. 574, no. 7779, pp. 505-+, Oct 24 (2019)
[21] H.-S. Zhong et al., “Quantum computational advantage using photons,” Science, vol. 370, no. 6523, p. 1460, (2020)
Further Reading: The History of Light and Interference (2023)
This history of interferometry has many surprising back stories surrounding the scientists who discovered and explored one of the most important aspects of the physics of light—interference. From Thomas Young who first proposed the law of interference, and Augustin Fresnel and Francois Arago who explored its properties, to Albert Michelson, who went almost mad grappling with literal firestorms surrounding his work, these scientists overcame personal and professional obstacles on their quest to uncover light’s secrets. The book’s stories, told around the topic of optics, tells us something more general about human endeavor as scientists pursue science.
Interference: The History of Optical Interferometry and the Scientists who Tamed Light, was published Ag. 6 and is available at Oxford University Press and Amazon. Here is a brief preview of the frist several chapters:
Chapter 1. Thomas Young Polymath: The Law of Interference
Thomas Young was the ultimate dabbler, his interests and explorations ranged far and wide, from ancient egyptology to naval engineering, from physiology of perception to the physics of sound and light. Yet unlike most dabblers who accomplish little, he made original and seminal contributions to all these fields. Some have called him the “Last Man Who Knew Everything”.
Thomas Young. The Law of Interference.
The chapter, Thomas Young Polymath: The Law of Interference, begins with the story of the invasion of Egypt in 1798 by Napoleon Bonaparte as the unlikely link among a set of epic discoveries that launched the modern science of light. The story of interferometry passes from the Egyptian campaign and the discovery of the Rosetta Stone to Thomas Young. Young was a polymath, known for his facility with languages that helped him decipher Egyptian hieroglyphics aided by the Rosetta Stone. He was also a city doctor who advised the admiralty on the construction of ships, and he became England’s premier physicist at the beginning of the nineteenth century, building on the wave theory of Huygens, as he challenged Newton’s particles of light. But his theory of the wave nature of light was controversial, attracting sharp criticism that would pass on the task of refuting Newton to a new generation of French optical physicists.
Chapter 2. The Fresnel Connection: Particles versus Waves
Augustin Fresnel was an intuitive genius whose talents were almost squandered on his job building roads and bridges in the backwaters of France until he was discovered and rescued by Francois Arago.
The Fresnel Connection: Particles versus Waves describes the campaign of Arago and Fresnel to prove the wave nature of light based on Fresnel’s theory of interfering waves in diffraction. Although the discovery of the polarization of light by Etienne Malus posed a stark challenge to the undulationists, the application of wave interference, with the superposition principle of Daniel Bernoulli, provided the theoretical framework for the ultimate success of the wave theory. The final proof came through the dramatic demonstration of the Spot of Arago.
Chapter 3. At Light Speed: The Birth of Interferometry
There is no question that Francois Arago was a swashbuckler. His life’s story reads like an adventure novel as he went from being marooned in hostile lands early in his career to becoming prime minister of France after the 1848 revolutions swept across Europe.
At Light Speed: The Birth of Interferometry tells how Arago attempted to use Snell’s Law to measure the effect of the Earth’s motion through space but found no effect, in contradiction to predictions using Newton’s particle theory of light. Direct measurements of the speed of light were made by Hippolyte Fizeau and Leon Foucault who originally began as collaborators but had an epic falling-out that turned into an intense competition. Fizeau won priority for the first measurement, but Foucault surpassed him by using the Arago interferometer to measure the speed of light in air and water with increasing accuracy. Jules Jamin later invented one of the first interferometric instruments for use as a refractometer.
Chapter 4. After the Gold Rush: The Trials of Albert Michelson
No name is more closely connected to interferometry than that of Albert Michelson. He succeeded, sometimes at great personal cost, in launching interferometric metrology as one of the most important tools used by scientists today.
Albert A. Michelson, 1907 Nobel Prize. Image Credit.
After the Gold Rush: The Trials of Albert Michelson tells the story of Michelson’s youth growing up in the gold fields of California before he was granted an extraordinary appointment to Annapolis by President Grant. Michelson invented his interferometer while visiting Hermann von Helmholtz in Berlin, Germany, as he sought to detect the motion of the Earth through the luminiferous ether, but no motion was detected. After returning to the States and a faculty position at Case University, he met Edward Morley, and the two continued the search for the Earth’s motion, concluding definitively its absence. The Michelson interferometer launched a menagerie of interferometers (including the Fabry-Perot interferometer) that ushered in the golden age of interferometry.
Chapter 5. Stellar Interference: Measuring the Stars
Learning from his attempts to measure the speed of light through the ether, Michelson realized that the partial coherence of light from astronomical sources could be used to measure their sizes. His first measurements using the Michelson Stellar Interferometer launched a major subfield of astronomy that is one of the most active today.
R Hanbury Brown
Stellar Interference: Measuring the Stars brings the story of interferometry to the stars as Michelson proposed stellar interferometry, first demonstrated on the Galilean moons of Jupiter, followed by an application developed by Karl Schwarzschild for binary stars, and completed by Michelson with observations encouraged by George Hale on the star Betelgeuse. However, the Michelson stellar interferometry had stability limitations that were overcome by Hanbury Brown and Richard Twiss who developed intensity interferometry based on the effect of photon bunching. The ultimate resolution of telescopes was achieved after the development of adaptive optics that used interferometry to compensate for atmospheric turbulence.
And More
The last 5 chapters bring the story from Michelson’s first stellar interferometer into the present as interferometry is used today to search for exoplanets, to image distant black holes half-way across the universe and to detect gravitational waves using the most sensitive scientific measurement apparatus ever devised.
Chapter 6. Across the Universe: Exoplanets, Black Holes and Gravitational Waves
Moving beyond the measurement of star sizes, interferometry lies at the heart of some of the most dramatic recent advances in astronomy, including the detection of gravitational waves by LIGO, the imaging of distant black holes and the detection of nearby exoplanets that may one day be visited by unmanned probes sent from Earth.
Chapter 7. Two Faces of Microscopy: Diffraction and Interference
The complement of the telescope is the microscope. Interference microscopy allows invisible things to become visible and for fundamental limits on image resolution to be blown past with super-resolution at the nanoscale, revealing the intricate workings of biological systems with unprecedented detail.
Chapter 8. Holographic Dreams of Princess Leia: Crossing Beams
Holography is the direct legacy of Young’s double slit experiment, as coherent sources of light interfere to record, and then reconstruct, the direct scattered fields from illuminated objects. Holographic display technology promises to revolutionize virtual reality.
Chapter 9. Photon Interference: The Foundations of Quantum Communication and Computing
Quantum information science, at the forefront of physics and technology today, owes much of its power to the principle of interference among single photons.
Chapter 10. The Quantum Advantage: Interferometric Computing
Photonic quantum systems have the potential to usher in a new information age using interference in photonic integrated circuits.
A popular account of the trials and toils of the scientists and engineers who tamed light and used it to probe the universe.
When Galileo trained his crude telescope on the planet Jupiter, hanging above the horizon in 1610, and observed moons orbiting a planet other than Earth, it created a quake whose waves have rippled down through the centuries to today. Never had such hard evidence been found that supported the Copernican idea of non-Earth-centric orbits, freeing astronomy and cosmology from a thousand years of error that shaded how people thought.
The Earth, after all, was not the center of the Universe.
Galileo’s moons: the Galilean Moons—Io, Europa, Ganymede, and Callisto—have drawn our eyes skyward now for over 400 years. They have been the crucible for numerous scientific discoveries, serving as a test bed for new ideas and new techniques, from the problem of longitude to the speed of light, from the birth of astronomical interferometry to the beginnings of exobiology. Here is a short history of Galileo’s Moons in the history of physics.
Galileo (1610): Celestial Orbits
In late 1609, Galileo (1564 – 1642) received an unwelcome guest to his home in Padua—his mother. She was not happy with his mistress, and she was not happy with his chosen profession, but she was happy to tell him so. By the time she left in early January 1610, he was yearning for something to take his mind off his aggravations, and he happened to point his new 20x telescope in the direction of the planet Jupiter hanging above the horizon [1]. Jupiter appeared as a bright circular spot, but nearby were three little stars all in line with the planet. The alignment caught his attention, and when he looked again the next night, the position of the stars had shifted. On successive nights he saw them shift again, sometimes disappearing into Jupiter’s bright disk. Several days later he realized that there was a fourth little star that was also behaving the same way. At first confused, he had a flash of insight—the little stars were orbiting the planet. He quickly understood that just as the Moon orbited the Earth, these new “Medicean Planets” were orbiting Jupiter. In March 1610, Galileo published his findings in Siderius Nuncius (The Starry Messenger).
Page from Galileo’s Starry Messenger showing the positions of the moon of Jupiter
It is rare in the history of science for there not to be a dispute over priority of discovery. Therefore, by an odd chance of fate, on the same nights that Galileo was observing the moons of Jupiter with his telescope from Padua, the German astronomer Simon Marius (1573 – 1625) also was observing them through a telescope of his own from Bavaria. It took Marius four years to publish his observations, long after Galileo’s Siderius had become a “best seller”, but Marius took the opportunity to claim priority. When Galileo first learned of this, he called Marius “a poisonous reptile” and “an enemy of all mankind.” But harsh words don’t settle disputes, and the conflicting claims of both astronomers stood until the early 1900’s when a scientific enquiry looked at the hard evidence. By that same odd chance of fate that had compelled both men to look in the same direction around the same time, the first notes by Marius in his notebooks were dated to a single day after the first notes by Galileo! Galileo’s priority survived, but Marius may have had the last laugh. The eternal names of the “Galilean” moons—Io, Europe, Ganymede and Callisto—were given to them by Marius.
Picard and Cassini (1671): Longitude
The 1600’s were the Age of Commerce for the European nations who relied almost exclusively on ships and navigation. While latitude (North-South) was easily determined by measuring the highest angle of the sun above the southern horizon, longitude (East-West) relied on clocks which were notoriously inaccurate, especially at sea.
The Problem of Determining Longitude at Sea is the subject of Dava Sobel’s thrilling book Longitude (Walker, 1995) [2] where she reintroduced the world to what was once the greatest scientific problem of the day. Because almost all commerce was by ships, the determination of longitude at sea was sometimes the difference between arriving safely in port with a cargo or being shipwrecked. Galileo knew this, and later in his life he made a proposal to the King of Spain to fund a scheme to use the timings of the eclipses of his moons around Jupiter to serve as a “celestial clock” for ships at sea. Galileo’s grant proposal went unfunded, but the possibility of using the timings of Jupiter’s moons for geodesy remained an open possibility, one which the King of France took advantage of fifty years later.
In 1671 the newly founded Academie des Sciences in Paris funded an expedition to the site of Tycho Brahe’s Uranibourg Observatory in Hven, Denmark, to measure the time of the eclipses of the Galilean moons observed there to be compared the time of the eclipses observed in Paris by Giovanni Cassini (1625 – 1712). When the leader of the expedition, Jean Picard (1620 – 1682), arrived in Denmark, he engaged the services of a local astronomer, Ole Rømer (1644 – 1710) to help with the observations of over 100 eclipses of the Galilean moon Io by the planet Jupiter. After the expedition returned to France, Cassini and Rømer calculated the time differences between the observations in Paris and Hven and concluded that Galileo had been correct. Unfortunately, observing eclipses of the tiny moon from the deck of a ship turned out not to be practical, so this was not the long-sought solution to the problem of longitude, but it contributed to the early science of astrometry (the metrical cousin of astronomy). It also had an unexpected side effect that forever changed the science of light.
Ole Rømer (1676): The Speed of Light
Although the differences calculated by Cassini and Rømer between the times of the eclipses of the moon Io between Paris and Hven were small, on top of these differences was superposed a surprisingly large effect that was shared by both observations. This was a systematic shift in the time of eclipse that grew to a maximum value of 22 minutes half a year after the closest approach of the Earth to Jupiter and then decreased back to the original time after a full year had passed and the Earth and Jupiter were again at their closest approach. At first Cassini thought the effect might be caused by a finite speed to light, but he backed away from this conclusion because Galileo had shown that the speed of light was unmeasurably fast, and Cassini did not want to gainsay the old master.
Ole Rømer
Rømer, on the other hand, was less in awe of Galileo’s shadow, and he persisted in his calculations and concluded that the 22 minute shift was caused by the longer distance light had to travel when the Earth was farthest away from Jupiter relative to when it was closest. He presented his results before the Academie in December 1676 where he announced that the speed of light, though very large, was in fact finite. Unfortnately, Rømer did not have the dimensions of the solar system at his disposal to calculate an actual value for the speed of light, but the Dutch mathematician Huygens did.
When Huygens read the proceedings of the Academie in which Rømer had presented his findings, he took what he knew of the radius of Earth’s orbit and the distance to Jupiter and made the first calculation of the speed of light. He found a value of 220,000 km/second (kilometers did not exist yet, but this is the equivalent of what he calculated). This value is 26 percent smaller than the true value, but it was the first time a number was given to the finite speed of light—based fundamentally on the Galilean moons. For a popular account of the story of Picard and Rømer and Huygens and the speed of light, see Ref. [3].
Michelson (1891): Astronomical Interferometry
Albert Michelson (1852 – 1931) was the first American to win the Nobel Prize in Physics. He received the award in 1907 for his work to replace the standard meter, based on a bar of metal housed in Paris, with the much more fundamental wavelength of red light emitted by Cadmium atoms. His work in Paris came on the heels of a new and surprising demonstration of the use of interferometry to measure the size of astronomical objects.
Albert Michelson
The wavelength of light (a millionth of a meter) seems ill-matched to measuring the size of astronomical objects (thousands of meters) that are so far from Earth (billions of meters). But this is where optical interferometry becomes so important. Michelson realized that light from a distant object, like a Galilean moon of Jupiter, would retain some partial coherence that could be measured using optical interferometry. Furthermore, by measuring how the interference depended on the separation of slits placed on the front of a telescope, it would be possible to determine the size of the astronomical object.
From left to right: Walter Adams, Albert Michelson, Walther Mayer, Albert Einstein, Max Ferrand, and Robert Milliken. Photo taken at Caltech.
In 1891, Michelson traveled to California where the Lick Observatory was poised high above the fog and dust of agricultural San Jose (a hundred years before San Jose became the capitol of high-tech Silicon Valley). Working with the observatory staff, he was able to make several key observations of the Galilean moons of Jupiter. These were just close enough that their sizes could be estimated (just barely) from conventional telescopes. Michelson found from his calculations of the interference effects that the sizes of the moons matched the conventional sizes to within reasonable error. This was the first demonstration of astronomical interferometry which has burgeoned into a huge sub-discipline of astronomy today—based originally on the Galilean moons [4].
Pioneer (1973 – 1974): The First Tour
Pioneer 10 was launched on March 3, 1972 and made its closest approach to Jupiter on Dec. 3, 1973. Pioneer 11 was launched on April 5, 1973 and made its closest approach to Jupiter on Dec. 3, 1974 and later was the first spacecraft to fly by Saturn. The Pioneer spacecrafts were the first to leave the solar system (there have now been 5 that have left, or will leave, the solar system). The cameras on the Pioneers were single-pixel instruments that made line-scans as the spacecraft rotated. The point light detector was a Bendix Channeltron photomultiplier detector, which was a vacuum tube device (yes vacuum tube!) operating at a single-photon detection efficiency of around 10%. At the time of the system design, this was a state-of-the-art photon detector. The line scanning was sufficient to produce dramatic photographs (after extensive processing) of the giant planets. The much smaller moons were seen with low resolution, but were still the first close-ups ever to be made of Galileo’s moons.
Voyager (1979): The Grand Tour
Voyager 1 was launched on Sept. 5, 1977 and Voyager 2 was launched on August 20, 1977. Although Voyager 1 was launched second, it was the first to reach Jupiter with closest approach on March 5, 1979. Voyager 2 made its closest approach to Jupiter on July 9, 1979.
In the Fall of 1979, I had the good fortune to be an undergraduate at Cornell University when Carl Sagan gave an evening public lecture on the Voyager fly-bys, revealing for the first time the amazing photographs of not only Jupiter but of the Galilean Moons. Sitting in the audience listening to Sagan, a grand master of scientific story telling, made you feel like you were a part of history. I have never been so convinced of the beauty and power of science and technology as I was sitting in the audience that evening.
The camera technology on the Voyagers was a giant leap forward compared to the Pioneer spacecraft. The Voyagers used cathode ray vidicon cameras, like those used in television cameras of the day, with high-resolution imaging capabilities. The images were spectacular, displaying alien worlds in high-def for the first time in human history: volcanos and lava flows on the moon of Io; planet-long cracks in the ice-covered surface of Europa; Callisto’s pock-marked surface; Ganymede’s eerie colors.
The Voyager’s discoveries concerning the Galilean Moons were literally out of this world. Io was discovered to be a molten planet, its interior liquified by tidal-force heating from its nearness to Jupiter, spewing out sulfur lava onto a yellowed terrain pockmarked by hundreds of volcanoes, sporting mountains higher than Mt. Everest. Europa, by contrast, was discovered to have a vast flat surface of frozen ice, containing no craters nor mountains, yet fractured by planet-scale ruptures stained tan (for unknown reasons) against the white ice. Ganymede, the largest moon in the solar system, is a small planet, larger than Mercury. The Voyagers revealed that it had a blotchy surface with dark cratered patches interspersed with light smoother patches. Callisto, again by contrast, was found to be the most heavily cratered moon in the solar system, with its surface pocked by countless craters.
Galileo (1995): First in Orbit
The first mission to orbit Jupiter was the Galileo spacecraft that was launched, not from the Earth, but from Earth orbit after being delivered there by the Space Shuttle Atlantis on Oct. 18, 1989. Galileo arrived at Jupiter on Dec. 7, 1995 and was inserted into a highly elliptical orbit that became successively less eccentric on each pass. It orbited Jupiter for 8 years before it was purposely crashed into the planet (to prevent it from accidentally contaminating Europa that may support some form of life).
Galileo made many close passes to the Galilean Moons, providing exquisite images of the moon surfaces while its other instruments made scientific measurements of mass and composition. This was the first true extended study of Galileo’s Moons, establishing the likely internal structures, including the liquid water ocean lying below the frozen surface of Europa. As the largest body of liquid water outside the Earth, it has been suggested that some form of life could have evolved there (or possibly been seeded by meteor ejecta from Earth).
Juno (2016): Still Flying
The Juno spacecraft was launched from Cape Canaveral on Aug. 5, 2011 and entered a Jupiter polar orbit on July 5, 2016. The mission has been producing high-resolution studies of the planet. The mission was extended in 2021 to last to 2025 to include several close fly-bys of the Galilean Moons, especially Europa, which will be the object of several upcoming missions because of the possibility for the planet to support evolved life. These future missions include NASA’s Europa Clipper Mission, the ESA’s Jupiter Icy Moons Explorer, and the Io Volcano Observer.
Epilog (2060): Colonization of Callisto
In 2003, NASA identified the moon Callisto as the proposed site of a manned base for the exploration of the outer solar system. It would be the next most distant human base to be established after Mars, with a possible start date by the mid-point of this century. Callisto was chosen because it is has a low radiation level (being the farthest from Jupiter of the large moons) and is geologically stable. It also has a composition that could be mined to manufacture rocket fuel. The base would be a short-term way-station (crews would stay for no longer than a month) for refueling before launching and using a gravity assist from Jupiter to sling-shot spaceships to the outer planets.
By David D. Nolte, May 29, 2023
[1] See Chapter 2, A New Scientist: Introducing Galileo, in David D. Nolte, Galileo Unbound (Oxford University Press, 2018).
[2] Dava Sobel, Longitude: The True Story of a Lone Genius who Solved the Greatest Scientific Problem of his Time (Walker, 1995)
[3] See Chap. 1, Thomas Young Polymath: The Law of Interference, in David D. Nolte, Interference: The History of Optical Interferometry and the Scientists who Tamed Light (Oxford University Press, 2023)
[4] See Chapter 5, Stellar Interference: Measuring the Stars, in David D. Nolte, Interference: The History of Optical Interferometry and the Scientists who Tamed Light (Oxford University Press, 2023).
Galileo Unbound 
