A new way to turn light into matter

May 21, 2014

Updated: May 21, 2014 22:47 IST

Shubashree Desikan

The title begs the question – did older ways to turn light into matter exist? Well, that is exactly what photo-electric effect does! So what’s new here is that we are talking about using direct, pairwise, photon-photon interactions to produce electron-positron pairs, which has not been possible to achieve, experimentally. Writing in Nature Photonics, O.J. Pike et al of Imperial College, London and Max-Planck Institut, Heidelberg, have come up with a suggestion for an experiment in which this can be achieved. They further reiterate their idea by running a Monte Carlo simulation which demonstrates that the dominant channel, with proper design of the experimental apparatus, is the two-photon (Breit-Wheeler) process of pair production.

Quantum electrodynamics does allow for the interconversion of photons and matter, namely, electron-positron pairs can annihilate to give photons; pairs of photons can interact to give electron-positron pairs and even vacuum, or empty space, can undergo polarisation to give out electron-positron pairs.

There are various energy constraints that make some of these processes difficult to achieve experimentally, especially that of two photons colliding to give out a pair.

Pike et al have proposed an experimental scheme to build a table-top photon-photon collider which will produce electron-positron pairs. They suggest shooting ultra-relativistic electrons into a gold target where, due to retardation, the electrons emit high-energy photons. This beam is fired into a vacuum holhlraum (a hohlraum is a cavity whose walls are in equilibrium with the radiation inside. So that the radiant energy that comes out of a small hole in it is like a blackbody radiation.) Inside the vacuum holhlraum, the light beam interacts and produces the electron-positron pairs, which, in turn, can be picked up using a magnetic field outside the cavity.

The authors suggest that the dominant mechanism in the pair production will be the two-photon collision process, and this is an experimental feat that has not been achieved so far. Further they perform Monte Carlo simulations and solve the cross-section equations to corroborate this.

Though this simulation does not bring in anything new theoretically, it will be quite a feat to have a table-top photon-photon collider as they appear to suggest.

However, since they are suggesting that the energies of the pairs produced are in the range of 100 MeV to a GeV, it is debatable how useful this would be in high-energy physics experiments. It may be useful to probe quark resonances, but then, these have already been probed in other particle accelerators.

Keywords: Photon-photon interactions, electron-positron pairs, High energy physics experiments, quantum electrodynamics, high energy photons

May 26, 2014

The science of science writing

readerseditor@thehindu.co.in

Last week there was an exchange that brought out the clear distinction between writing about social sciences and science. On May 22, 2014, Shubashree Desikan wrote an interesting piece titled ‘A new way to turn light into matter’ in the weekly Science and Technology section. Her article was about a scientific paper published in a reputed journal, Nature Photonics , in which scientists from the Imperial College, London and Max-Planck Institut, Heidelberg, had argued for building a tabletop photon-photon collider that will produce electron-positron pairs.

One of our readers from Chennai, Mr. Raghavan, was quick to point out an erroneous comparison. The first paragraph of the article read: “The title begs the question — did older ways to turn light into matter exist? Well, that is exactly what photo-electric effect does!” Mr. Raghavan wrote to say that photo-electric effect is not turning light into matter, but is generation of electricity due to the influence of light and has nothing to do with the pair production and annihilation of matter explained in the article. Shubashree was quick to regret the confusion her piece created between photo-electric effect and pair production. She said: “Photo-electric effect does not create electron-positron pairs and this was not meant to be implied. In the sense that photo-electric effect and pair production, as we observe these, are only allowed by a quantum theory, they are equally contrary to a classical perception and that was the intention of mentioning the photo-electric effect as an example. It was not to say that photo-electric effect involved electron-positron pair production.”

The inherent strength of The Hindu is its commitment to cover a range of topics and develop in-house expertise in handling various domains of knowledge. From diplomacy and strategic affairs to banking, from sports to macroeconomics, from law and constitutional affairs to agriculture, from rural affairs to development models, from politics of governance to the politics of freedom of expression, the newspaper has invested in talents who can handle these important issues with expertise and élan.

Challenging field

The science and technology section is an important one in the newspaper. Apart from having a specialised weekly page on the topic, the newspaper covers many critical developments in the fields of science and technology. The Council for the Advancement of Science Writing rightly states that of all the journalism specialties, science writing may well be the most challenging and rewarding. Science journalists cover an impressive array of fields where advances are rapid, their impact on the society wide-reaching and the boundaries are constantly being redefined. This field is challenging because no writer can be qualified in any university to cover the range that includes genetics and astrophysics, atomic science and pharmacology, marine biology and organic chemistry, toxicity and environmental sciences, and human body and beyond. It is rewarding because science journalism helps us understand the new possibilities that these sciences have opened up for us, and also address a range of ethical issues. It takes enormous skill to present arcane and complex news of scientific discoveries and explorations in a language that is accessible to the general reader. The distinctiveness of science journalism lies in producing a prose that is simple but not simplistic, that respects the intelligence of a reader without being condescending. A science journalist is a truly multi-tasking person as the subjects he/she covers are very wide and deep both in their scope and content.

Role of science journalist

At the World Congress of Science Journalists held in Helsinki last year, the leading theme was critical thinking in the public sphere. The conference spoke about the most burning ethical issues and the values ingrained in the everyday work of a science journalist. From basic human rights and codes of conduct to matters of accessibility of scientific knowledge and political debates, it asked the participants: what is the significance of science journalism and science communication in the realisation of shared scientific advancement and culture in different parts of the world? It also looked at the question of vulnerability in the face of natural disasters, the world economic crisis, cultural changes and questions of health and well-being and the role of a science journalist in providing credible scientific information that can lead to informed choices.

The fundamental difference between writing about social sciences and science and technology is in the use of analogous examples to explain and elucidate a point or a view. While employing an analogous example enriches our understanding in social sciences, it may backfire as in the case of Subashree’s report because of extreme specificity of each subject. An analogy in a science story forces the writer to add annotations and afterwards to explain the intended meaning. Being a science journalist is demanding. It is not easy to explain Albert Einstein’s work in the language of Ernest Hemingway.

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