Chemistry of Light with Einstein, Will Read This Book with a Shock of Surprise

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may put a dent in inflation’s armour. The tion of mass, and the observed temperature inflationary models favoured by theorists are fluctuation level — roughly one part in a lauded for the simplicity of their predictions, hundred thousand — reflects a similar level but if these new results withstand further of density fluctuations. The distribution scrutiny then inflation either cannot be so appears at first to be roughly consistent with simple, or must be discarded. a Gaussian. But there are only a small num- 100 YEARS AGO Among inflation’s predictions is that ber of independent 7ǂ7-degree regions on A complimentary dinner was given to the distribution of density perturbations in the sky, so there is an error in each point in Prof. Virchow at the Hôtel Métropole on the early Universe should be Gaussian (or a histogram constructed from the data,8 Wednesday in last week. ... Lord Lister, ‘normal’): a histogram of the density at dif- simply from sample variance. Detector noise in proposing the toast of the evening, ferent points in the Universe should produce provides another source of uncertainty. dwelt upon the versatility of the genius of the familiar bell-shaped curve. Gaussianity There must, therefore, be some deviations the distinguished guest, his eminence as also implies specific forms for the correlation from Gaussianity. Are they statistically a pathologist being equalled by his between densities at several different significant, or just sampling and noise reputation as an anthropologist and points. The new work provides preliminary fluctuations? antiquarian. He referred particularly to evidence that contradicts these predictions. Several searches have yielded null results. Virchow’s “Cellularpathologie,” which The Differential Microwave Radiometer But there are an infinitude of flavours of work, he remarked, “swept away the false experiment on the Cosmic Background non-Gaussianity: a distribution might be and barren theory of a structureless Explorer (COBE) satellite has mapped the wider or narrower, have longer tails than a blastema, and established the true and temperature of the CMB with an angular res- Gaussian, be skewed and so on. Still more fertile doctrine that every morbid olution of seven degrees5 (Fig. 1). The pho- flavours are allowed when correlations structure consists of cells which have tons come to us from roughly 15 billion light between regions are considered — and any been derived from pre-existing cells as a years away; they were produced about deviation in the histogram from a perfect progeny. Cellular pathology is now 300,000 years after the Big Bang, when the bell curve poses problems for inflation. universally recognised as a truth. Even Universe became transparent and the pho- Ferreira et al.4 have developed a tech- those morbid structures which deviate tons last scattered from electrons in the pri- nique to test for a specific form of non- most from the normal structure are mordial cosmic plasma. Photons from over- Gaussianity. They search for this particular known to be derived as a progeny from dense regions of the Universe lost more ener- correlation between three points on the sky. normal tissue — from normal cells, driven gy as they climbed out of deeper gravitation- When applied to the data from the Differen- to abnormal development by injurious al potential wells and are thus redder than tial Microwave Radiometer, the search turns agencies.” photons reaching us from underdense up a positive result at the 98% confidence From Nature 13 October 1898. regions. level. As it probes a different form of non- The distribution of temperature fluctua- Gaussianity, the result is perfectly consistent 50 YEARS AGO tions thus reflects the primordial distribu- with prior null results. “Einstein: His Life and Times.” By Philipp Photonics Frank. — Those who, like the present reviewer, are personally unacquainted Chemistry of light with Einstein, will read this book with a shock of surprise. While Dr. Frank’s This picture is of a ‘photonic molecule’, the sympathies are all with Einstein, the latest development in our increasingly portrait presented to us is not altogether sophisticated control of photons. Bayer a pleasant one. We see a man and colleagues (Phys. Rev. Lett. 81, developing early into the traditional type 2582–2585; 1998) have built a range of of nineteenth-century ‘professor’. He these structures to test how pure modes of regards himself as free to develop any trapped light behave in them. eccentricity of behaviour, whether those Until a few years ago, our manipulation about him like it or not, and to talk shop of light was crude: creating photons of semiconductor and the surrounding air in season or out of season, a visible light by heating matter until its allows photons to leak only slowly out of characteristic illustrated by the electrons vibrate violently enough to the sides; this confinement means that only description of a courtesy visit to a non- radiate; filtering them; bouncing them off resonant photon modes are excited. This mathematical colleague in Berlin, in mirrors. The light is generally incoherent, technology may be used to make efficient which, after subjecting his hosts to a and has a wide range of colours. Lasers are semiconductor lasers with a single excited forty-minutes discourse on relativity, a much more refined tool, producing a mode. Einstein left abruptly. This lack of narrow beam of nearly monochromatic, To take the idea a stage further, Bayer et appreciation of the fact that ideas and coherent (in-phase) light. al. linked pairs of gallium-arsenide interests which did not happen to interest Even cleverer are photonic-bandgap photonic atoms with narrow bridges. As him might still be as valuable as those materials, where scattering off periodic the bridge length was reduced, new that did may well explain Einstein’s arrays of a dielectric prevents certain photonic states emerged that are similar to difficulties in the Berlin Academy, or his wavelengths of light from propagating. the bonding and antibonding orbitals of failings as a teacher. Always, apparently, Such materials can be used to bend light the hydrogen molecule. ready to lecture on his researches of the around corners, and trap it in cavities. The aim now is to build more moment or to deliver popular discourses, Light can also been trapped in ‘photonic complicated photonic molecules to study Einstein was not prepared to teach his atoms’. A semiconductor layer is this photon–matter interaction, and students systematically what they had sandwiched between two mirrors, and an perhaps to learn something, by need to learn. — G. C. McVittie island is etched out of the overall structure. analogy, about electronic states in real From Nature 16 October 1948. The refractive-index barrier between the molecules. Stephen Battersby