Guest post by Leonie Mueck, Associate Editor, Nature
In last week’s post you heard about beautiful experiments with light featured in our poll from the turn of the century. This week, we will talk about the time until the 1950s. And, while so many turning points in politics and history fall into that period, advances in optics and photonics were a bit betwixt and between. Scientists were modernizing their methods and instruments but still didn’t have modern-day tools like the laser, which in the 1960s would completely transform light-related research.
They did have highly advanced telescopes. When Edwin Hubble arrived at Mount Wilson Observatory in California it was 1919. By a lucky coincidence something else arrived there at around the same time: the Hooker Telescope, which allowed Hubble to perform detailed investigations on spiral nebulae. Thanks to his measurements, we now know that those nebulae are in fact distant galaxies. As jaw dropping as this finding was to his contemporaries, Hubble went on to show something even more ground-breaking. Looking at the Doppler shift of as many galaxies as possible, he found that the shift was proportional to the galaxies’ distance in 1929. The only plausible explanation for this phenomenon was that we live in an expanding Universe!
Hubble never got the Nobel Prize as his achievements were not considered to be “physics” enough back then. The scientist behind our next experiment, Dennis Gabor, did not have this problem. In 1971, he received the most prestigious award in science for the invention of holography. The sound of “holography” probably makes you think of optical holograms as seen in science-fiction films. But when Gabor filed the corresponding patent in 1947, he didn’t have the image of Princess Leia appearing in front of Luke Skywalker in mind. He rather presented his invention as a new principle for electron microscopes, which would allow recording the entire information about an electromagnetic field, namely amplitude and phase, and not just the intensity. Optical holograms would only become possible with the invention of the laser.
Likewise, the last experiment in this blog post was an inch ahead of its time. In 1956, Robert Hanbury Brown and Richard Q. Twiss published a paper entitled “A test of a new type of stellar interferometer” in Nature. The scientists had observed that the signals of two photomultipliers placed 6 meters apart were correlated and exploited this effect to determine the size of the star Sirius. If you think of light as a classical wave this observed correlation, or “photon bunching”, is simply an interference effect. But soon fellow scientists started to have doubts. The problem was that the classical picture wasn’t really applicable in this situation, with optical wavelengths and just a few photons. In 1963, later Nobel Prize winner Roy Jay Glauber was able to give the answer in a quantum mechanical explanation and his seminal paper is widely considered as marking the birth of quantum optics.
Experiments covered this week:
1929 Hubble’s discovery of an expanding universe from Doppler shift
1948 Gabor’s invention of holograms
1956 Hanbury Brown and Twiss photon correlation measurement