News & Analysis Physics World  January 2018
(Karan Jani / Georgia Tech)

Multimessenger work bags award

The Physics World 2017 Breakthrough of the Year goes to an international team that ushered in a new era of multimessenger astronomy, as Hamish Johnston reports

The Physics World 2017 Breakthrough of the Year has been awarded to the international team of astronomers and astrophysicists that made the first ever multimessenger observation involving gravitational waves. On 17 August 2017 the LIGO–Virgo gravitational-wave detectors in the US and Italy, plus the Fermi Gamma-ray Space Telescope and the INTEGRAL gamma-ray space telescope detected nearly simultaneous signals. They came from the merger of two neutron stars – an event now called GW170817 (Phys. Rev. Lett. 119 161101).

This was the first time that LIGO–Virgo scientists had seen a neutron-star merger, but five hours later they had already worked out the location of the source in the sky (see November 2017). Over the next hours and days, more than 70 telescopes were pointed at GW170817 and a wealth of observations were made in the gamma-ray, X-ray, visible, infrared and radio portions of the electromagnetic spectrum (ApJL 848 L12). Astrophysicists also searched for neutrinos, but none were seen.

These co-ordinated observations have already provided a vast amount of information about what happens when neutron stars collide in what is called a “kilonova”. The observations have yielded important clues about how heavy elements, such as gold, are produced in the universe. The ability to measure both gravitational waves and visible light from neutron-star mergers has also given a new and independent way of measuring the expansion rate of the universe. In addition, the observation settles a long-standing debate about the origin of short, high-energy gamma-ray bursts.

The observation of GW170817 is a shining example of how our knowledge of the universe can move forwards when people from all over the world join together with a common scientific cause

While some awards, notably the Nobel prizes, are given to individuals and not groups, Physics World recognizes that science is a collaborative effort. Furthermore, the multimessenger observation of GW170817 epitomizes the collaborative nature of science and is a shining example of how our knowledge of the universe can move forwards when people from all over the world join together with a common scientific cause.

  • On a recent trip to LIGO Livingston, we spoke to LIGO scientist Amber Stuver.

Amber Stuver introduces LIGO

Highly commended research

The top 10 breakthroughs were chosen by Physics World editors from a shortlist based on the fundamental importance of the research; representing a significant advance in knowledge; having a strong connection between theory and experiment; and being of general interest to all physicists. Physics World also picked nine other pieces of research that were highly commended, which follow in no particular order.

In quantum physics, Hatim Salih of the University of Bristol, UK, and colleagues and Jian-Wei Pan of the University of Science and Technology of China and colleagues carried out theoretical and experimental work into the realization of transmitting information using quantum physics without exchanging any particles. Salih and colleagues first proposed a new quantum-communication scheme that does not require the transmission of any physical particles four years ago (Phys. Rev. Lett. 110 170502). While some physicists were sceptical, a team, led by Pan, created such a system in the lab and used it to transfer a simple image while sending (almost) no photons in the process. Dubbed “counterfactual imaging”, the technique could prove handy in imaging delicate pieces of ancient art that cannot be exposed to direct light (Proc. Natl Acad. Sci. 114 4920).

Another breakthrough in quantum physics came from Sascha Agne and Thomas Jennewein of the University of Waterloo, Canada, and colleagues, and Stefanie Barz, Steve Kolthammer and Ian Walmsley of the University of Oxford, UK, and colleagues for independently measuring quantum interference involving three photons. Seeing the effect is difficult because it requires the ability to deliver three indistinguishable photons to the same place at the same time and also to ensure that single-photon and two-photon interference effects are eliminated from the measurements. Three-photon interference could also be used in quantum cryptography and quantum simulators (Phys. Rev. Lett. 118 153603 and Phys. Rev. Lett. 118 153602).

Meanwhile, Boubacar Kanté and colleagues at the University of California, San Diego, US, created the first “topological laser”. The device involves light snaking around a cavity of any shape without scattering – much like the motion of electrons on the surface of a topological insulator (Science 358 636). The laser works at telecom wavelengths and could lead to better photonic circuits or even protect quantum information from scattering.

In applied physics, Ronggui Yang and Xiaobo Yin of the University of Colorado, Boulder, US, and colleagues created a new metamaterial film that provides cooling without the need for a power source (Science 355 1062). Made out of glass microspheres, polymer and silver, the material uses passive radiative cooling to dissipate heat from the object that it covers. It emits the energy as infrared radiation, so it can travel through the atmosphere and ultimately into space. The material also reflects sunlight, which means that it works both day and night. But perhaps most importantly, it can be produced cheaply at an industrial scale.

Work on cosmic rays resulted in two entries in the top 10. The Pierre Auger Observatory collaboration showed that ultrahigh-energy cosmic rays come from outside the Milky Way. For decades, astrophysicists have believed that the sources of cosmic rays with energies greater than about 1018 eV could be worked out from the arrival directions of these particles. This is unlike lower-energy cosmic rays, which appear to come from all directions after being deflected by the Milky Way’s magnetic fields. Now, Pierre Auger’s 1600 Cherenkov particle detectors in Argentina have revealed that the arrival rate of ultrahigh-energy cosmic rays is greater in one half of the sky (Science 357 1266). Moreover, the excess lies away from the centre of the Milky Way – suggesting that the cosmic rays have extragalactic origins.

Meanwhile, the ScanPyramids collaboration used cosmic-ray muons to find evidence for a hitherto unknown large void in Khufu’s Pyramid at Giza, Egypt. By placing different types of muon detectors in and around the pyramid, the team measured how showers of muons were attenuated as they passed through the huge structure. Computer algorithms analysed the data and revealed an unexpected and very large void deep within the pyramid (Nature 10.1038/nature24647).

In imaging, Francisco Balzarotti, Yvan Eilers, Klaus Gwosch, Stefan Hell and colleagues at the Max Planck Institute for Biophysical Chemistry, Uppsala University, Sweden, and the University of Buenos Aires, Argentina, developed a new type of super-resolution microscope that can track biological molecules in living cells in real time (Science 355 606). The new technique is called maximally informative luminescence excitation probing (MINFLUX) and it combines the merits of two Nobel-prize-winning techniques – one of which was developed by Hell. MINFLUX attains nanometre-scale resolution more quickly and with fewer emitted photons than previously possible.

In atomic physics, Christopher Monroe at the University of Maryland, US, and colleagues and Mikhail Lukin of Harvard University, US, and colleagues are credited for their independent creation of “time crystals”. Like conventional crystals, which spontaneously break translational symmetry, time crystals spontaneously break discrete time symmetry. Time crystals were first predicted five years ago and now two spin-based systems with properties resembling time crystals have been created. Lukin used spins in diamond defects (Nature 543 221) while Monroe’s spins were trapped ions (Nature 543 217).

Finally, Teruaki Enoto of Kyoto University, Japan, and colleagues provided the first detailed, convincing evidence that lightning strikes can lead to the synthesis of radioactive isotopes in the atmosphere (Nature 551 481). Physicists already knew that lightning strikes can produce gamma rays and neutrons, and had suspected that interactions between this radiation and nitrogen nuclei in the air could create radioactive nuclei. Enoto and colleagues confirmed this by measuring a gamma-ray signal indicative of nuclear decay that peaked about 1 minute after a lightning strike. This, they say, is evidence for the production of radioactive nuclei such as nitrogen-13.

  • For more on the winning work, see “A new cosmic messenger” by Imre Bartos