Guest post by Amos Martinez, Associate Editor, Nature Communications
We kick off this week’s experiments for our poll with the discovery of a special kind of light: cosmic microwave background. The story of this discovery is a beautiful example of the fortuity of scientific discovery.
In 1964, Arno Penzias and Robert Wilson were looking for radio emission from the Milky Way using an antenna originally built for radio-wave satellite communications. They soon noticed a noise in the microwave region evenly spread in all directions of space. After eliminating every known noise source, including a pigeon’s nest in the 6m antenna, they reached the conclusion that the noise could only be coming from outside this galaxy.
Elsewhere, cosmologists were debating whether the universe had a beginning and had been created by a Big Bang or had always existed. Advocates of the big bang theory Robert H. Dicke, Jim Peebles, and David Wilkinson had predicted that had the big bang taken place it would have generated an enormous blast of radiation that should still be detectable in the microwave region with a sensitive enough device. Sure enough, that persistent noise measured by Penzias and Wilson turned out not to be caused by pigeons but by radiation generated during the creation of the Universe. This discovery represented the first solid experimental proof of the Big Bang and, as Stephen Hawking put it, the final nail in the coffin of the steady-state theory.
Penzias and Wilson were awarded the Nobel Prize for Physics in 1978, for the discovery of the cosmic microwave background radiation.
Let’s now move on into the 70s, when lasers started to make their presence felt across all scientific disciplines. We will start with laser trapping and cooling, a technology that took its first steps in the 70s.
The possibility of exerting attractive or repulsive optical forces using focused laser beams on small objects was first demonstrated by Arthur Ashkin and colleagues at Bell labs. This work set the foundations for atom trapping and also to laser cooling where, by carefully engineering the energy and momentum exchange between the laser beam and the atoms, it was possible to reduce the random motion of atoms. This concept has led to the capability to cool atoms down to temperatures barely above absolute zero. This technique plays a key role in the development of the next generation of atomic clocks as well as the measurement of gravitational fields. Optical tweezers also have an impact in microbiology where live micro-organisms and even DNA molecules can be trapped and manipulated without damage, as well as in the assembly of nanostructured materials.
Steven Chu, Claude Cohen-Tannoudji and William D. Phillips were awarded the Nobel Prize in Physics in 1997 for the development of methods to cool and trap atoms with laser light.
From its advent, the great potential of lasers was clear, but scientists struggled to identify what exactly their role could be, hence lasers were often described as a solution looking for a problem. It is probable however, that spectroscopy was the first application that came to mind to those that envisioned and devised the first lasers and masers.
Sure enough, in 1972, only a few years after the first laser demonstration, Theodor W. Hänsch used laser spectroscopy to measure with unprecedented precision the transition frequency of the Balmer line of atomic hydrogen using a dye laser. However, it was not until the latter part of 1990s, when frequency combs from mode-locked lasers were used for frequency metrology, that truly game-changing precision became available. A frequency comb is a set of equidistant spectral lines that can be used as an optical ruler as long as the comb spacing and the carrier–envelope offset frequency are known and stabilized. Frequency combs can be used to measure frequency and time with extreme precision which has numerous technological applications, such as in gas sensing and optical clocks, but also in fundamental science: they are used for detecting possible variations in universal physical constants, which could challenge our understanding of physics and the universe.
John Hall and Theodor W. Hansch were awarded the Nobel Prize in Physics in 2005 for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique.
Experiments covered this week:
1964 Discovery of cosmic microwave background from faint radio waves
1970 Demonstration of optical tweezers and cooling
1970s Laser frequency combs for precision metrology
Next week Federico Levi looks at the photonic crystals and the bell test.