Sink holes and dust jets on comet 67P

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by Paul Weissman from Nature 523, 42–43 (02 July 2015) doi:10.1038/523042a


Analyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation.
When do 18 holes not make for a pleasant afternoon playing golf? When the 18 holes are located on the surface of a comet speeding through the Solar System. Vincent et al.(1) describe the holes, also called pits, that comprise one of the many discoveries of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P). The Rosetta spacecraft went into orbit around 67P in August 2014, and the surprises have been coming fast since then. Vincent et al. propose a mechanism for the formation of the pits and identify them as one of the sources of active dust jets.
Comets are the most primitive bodies in the Solar System; they are the remnants of its formation process. Comets therefore retain a physical and chemical record of the conditions and materials in the solar nebula — the gas and dust cloud out of which the Sun and planets formed 4.56 billion years ago. Conveniently, comets have spent most of that time in two very cold storage locations: the Kuiper belt beyond the orbit of Neptune and the spherical Oort cloud outside the planetary region, stretching halfway to the nearest stars. The distant Oort cloud is the source of the long-period comets that have orbital periods ranging up to millions of years. The Kuiper belt is the source of the Jupiter-family comets, such as 67P, which typically have periods of less than 20 years and orbital dynamics that are strongly affected by Jupiter.
As a comet approaches the Sun and warms up, the central solid part, known as the cometary nucleus (comprised of volatile ices and primitive meteoritic material), begins to sublimate and becomes enveloped by a freely outflowing atmosphere called the coma. One of the first surprises for Rosetta, the first ever comet-rendezvous mission, was the odd shape of the target comet’s nucleus (Fig. 1a)(2). Although some nuclei comprised of two large pieces and looking like a bowling pin had been observed before by fly-by missions to other comets, the two lobes of 67P sit on top of each other, with a narrow ‘neck’ in between. There is intense speculation as to how this odd configuration may have formed. Did two cometary nuclei gently collide randomly in the solar nebula, or is the nucleus a single piece that has been oddly sculpted by sublimation processes? Although the former is the more likely scenario, some scientists on the mission suspect the latter.

Vincent et al.(1) analysed images of comet 67P taken by the Optical, Spectroscopic and Infrared Remote Imaging System cameras on the Rosetta spacecraft. a, The complex nucleus topography includes large, flat-floored basins (indicated by white arrows). A large, circular pit is visible just above the centre of the image (red arrow). b, A string of pits dot the surface of the cometary nucleus. In active pits such as these, bright jets of dust are seen being emitted from the sunlit walls. The contrast of this image has been enhanced to highlight the interiors of the pits and the jets. As a result, the cometary surface looks very bright, but in reality it reflects only about 6% of the incoming sunlight — roughly the same as the black toner particles in a laser printer cartridge. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Vincent et al.(1) analysed images of comet 67P taken by the Optical, Spectroscopic and Infrared Remote Imaging System cameras on the Rosetta spacecraft. a, The complex nucleus topography includes large, flat-floored basins (indicated by white arrows). A large, circular pit is visible just above the centre of the image (red arrow). b, A string of pits dot the surface of the cometary nucleus. In active pits such as these, bright jets of dust are seen being emitted from the sunlit walls. The contrast of this image has been enhanced to highlight the interiors of the pits and the jets. As a result, the cometary surface looks very bright, but in reality it reflects only about 6% of the incoming sunlight — roughly the same as the black toner particles in a laser printer cartridge.
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta’s camera system, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), is comprised of narrow-angle and wide-angle digital cameras. As the OSIRIS team of scientists2 began to map the surface of the nucleus using the cameras, they discovered 18 pits on the surface, which Vincent et al. now describe more thoroughly. The cometary nucleus has a diameter of approximately 4 kilometres. The pits are typically about 200 metres in diameter and about 180 metres deep. Pit-like features have been observed on other cometary nuclei, but the morphology of the pits on 67P has not been seen before. They typically have cylindrical shapes with circular openings and near-vertical walls (although at least one pit seems to be lying at a steep angle). And some of the pits are clearly active: images of pits that are illuminated by sunlight show dust jets emanating from their walls and/or floors (Fig. 1b).
How did the pits form? Vincent et al. suggest that they are ‘sink holes’, which formed when material near the surface of the nucleus collapsed into the low-density interior. Rosetta’s Radio Science Investigation team has found(2) that the nucleus has an average bulk density of only 470 ± 45 kilograms per cubic metre, about half the density of solid water ice. But the Grain Impact Analyser and Dust Accumulator instrument has measured(3) a dust-to-ice mass ratio of 4 ± 2, suggesting that silicates and organics, rather than ices, make up about 80% of the mass of the nucleus. This in turn implies that 75–85% of the nucleus interior is empty space, a parameter known as porosity. A high porosity is predicted by the leading scenarios for the internal structure of cometary nuclei, which suggest that they are aggregates(4) of smaller, icy bodies that gently came together in the solar nebula. These aggregates are also referred to as rubble piles(5). This concept has provided insights into the behaviour of comets, such as random and other splitting events.
The morphology of 67P’s surface is dominated in some areas by large, flat-floored basins, similar to features seen on the nucleus of comet(6) Wild 2. It has been suggested that these are sublimation basins that slowly widen as the walls sublimate, leaving large, non-volatile particles that cover the basin floor. The basins cannot be impact craters because they have the wrong size distribution (there are too many large ones), and because not many impact craters are expected on a small cometary nucleus such as 67P.
Could the pits described by Vincent et al. be the precursors of the basins, slowly widening as their walls sublimate? Many of the pits found by OSIRIS are located in the same region on the nucleus where many of the large sublimation basins are found. Both comet 67P and comet Wild 2 are relatively young — that is, they have only recently (within the past 60 years) been perturbed by the gravitational field of Jupiter to perihelion distances (the point in their orbit closest to the Sun) at which it is warm enough for water ice in the nucleus to sublimate, and at which the activity that manifests itself as the bright cometary coma and tails begins. If this is so, why are sublimation basins not observed on other, perhaps older, Jupiter-family comets such as Tempel 1 and Hartley 2? Older nuclei may have accumulated thicker layers of non-volatile materials that have buried the sublimation basins and substantially lowered the activity levels of those comets.
Rosetta has already indicated that it has more surprises for us. On 13 June 2015, the orbiter began receiving signals from the Philae lander, which is on the surface of the comet nucleus and was last heard from in November 2014. With its batteries recharging, Philae probably has much more information to transmit about its final landing location. Also, the activity of the nucleus is expected to reach a maximum soon after the comet passes through perihelion at 1.25 astronomical units from the Sun (a point about 25% farther from the Sun than Earth’s orbit) in mid-August 2015. Rosetta will then follow 67P away from the Sun as cometary activity begins to wane. What changes will we see on the nucleus surface? And how will this alien golf course look from Rosetta’s vantage point then?


(1) Vincent, J.-B. et al. Nature 523, 63–66 (2015).
(2) Sierks, H. et al. Science 347, aaa1044 (2015).
(3) Rotundi, A. et al. Science 347, aaa3905 (2015).
(4) Donn, B. & Hughes, D. in 20th ESLAB Symp. Exploration of Halley’s Comet (eds Battrick, B. et al.) 523–524 (ESA, 1986).
(5) Weissman, P. R. Nature 320, 242–244 (1986).
(6) Kirk, R. et al. in 46th Lunar and Planetary Science Conf. Abstr. 2244 (2015).

Bounty of dark galaxies found

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from Nature 523, 9 (02 July 2015) doi:10.1038/523009b

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Astronomers have discovered more than 850 faint galaxies in a galaxy cluster that could be made mostly of dark matter.
Using archived images from the Subaru Telescope in Hawaii, a team led by Jin Koda at Stony Brook University in New York searched for observations of the Coma galaxy cluster, which is roughly 101 million parsecs (330 million light years) away. The team found 854 ultra-diffuse galaxies, a class of faint galaxy that can be as large as the Milky Way, but which has only 0.1% the number of stars. For these galaxies to remain gravitationally bound together, the researchers show that more than 99% of their mass must be dark matter.
This suggests that the crowded environment sucks gas away from these galaxies, leaving them largely unable to form stars.


Paper: Approximately a thousand ultra-diffuse galaxies in the coma cluster by Jin Koda, Masafumi Yagi, Hitomi Yamanoi, and Yutaka Komiyama

From generation to generation

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by Robert Kowalewski from Nature Physics 11, 705–706 (2015) doi:10.1038/nphys3464


A new measurement from the LHCb experiment at CERN’s Large Hadron Collider impinges on a puzzle that has been troubling physicists for decades namely the breaking of the symmetry between matter and antimatter.

Experimental constraints on the unitarity triangle. Each band shows the allowed region (at 95% confidence level, CL) based on specific measured quantities. The quantities η and ρ are functions of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, which allow the triangle to have a base of unit length oriented along the ρ axis. The angles α, β and γ correspond to the blue and tan bands, and are measured from matter-antimatter-violating asymmetries in B meson decay. The circular arcs centred on (10) show the constraints from the mass differences, Δmd and Δms, measured in studies of B-B oscillations. Measurements of matter-antimatter violation in the kaon system determine εK, which is a measure of the admixture of the CP-even eigenstate in the long-lived neutral kaon, and result in the green band. The dark green semi-circle centred on (0,0) shows the constraint from the measurement of the ratio IVubl/IVcbl, where Vub describes the transition of a b quark to a u quark. Image courtesy of the CKMfitter group.

Experimental constraints on the unitarity triangle. Each band shows the allowed region (at 95% confidence level, CL) based on specific measured quantities. The quantities η and ρ are functions of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, which allow the triangle to have a base of unit length oriented along the ρ axis. The angles α, β and γ correspond to the blue and tan bands, and are measured from matter-antimatter-violating asymmetries in B meson decay. The circular arcs centred on (10) show the constraints from the mass differences, Δmd and Δms, measured in studies of BB oscillations. Measurements of matter-antimatter violation in the kaon system determine εK, which is a measure of the admixture of the CP-even eigenstate in the long-lived neutral kaon, and result in the green band. The dark green semi-circle centred on (0,0) shows the constraint from the measurement of the ratio |Vub|/|Vcb|, where Vub describes the transition of a b quark to a u quark. Image courtesy of the CKMfitter group.

We learn early that the matter in and around us is made up of three particles: electrons, and the up and down quarks found in nuclei. Add in the electron neutrino and we also account for nuclear fission and fusion and the stellar furnace that fuels life on Earth. But nature is not that simple. It replicates this four-particle structure in ‘generations’ of heavier, but otherwise similar, particles. The first evidence for this was the discovery of the muon in 1936. Other second-generation particles were subsequently discovered, as was another unexpected phenomenon: the violation of matter-antimatter (CP) symmetry in neutral kaons(1). Now, writing in Nature Physics, the LHCb collaboration(2) provides fresh evidence to fuel the ongoing discussion surrounding CP violation.
In 1973, Makoto Kobayashi and Toshihide Maskawa proposed a mechanism whereby mixing between the mass and weak eigenstates of quarks would, if there were three generations, result in an irreducible complex phase that could be responsible for CP violation(3).
The discovery of the first third-generation particle, the tau lepton(4), came a year later, followed in 1977 by the discovery of the third-generation ‘b’ quark(5). With the advent of high-intensity electron-positron colliders at the start of the twenty-first century, studies of CP violation in the decays of B mesons (which contain a b quark) at the BaBar and Belle experiments validated Kobayashi and Maskawas proposal, for which they shared in the 2008 Nobel Prize in Physics.
The CKM matrix – introduced by Kobayashi and Maskawa, following the formative work of Nicola Cabibbo – describes the mixing of quark mass and weak eigenstates in the standard model of particle physics. It is unitary and can be fully specified with four parameters: three real angles and one imaginary phase. This unitarity condition is the basis for a set of testable constraints in the form of products of complex numbers that sum to zero – for example, V*ud Vub + V*cd Vcb + V*td Vtb = 0 where Vub describes the transition of a b quark to a u quark. The triangle in Fig. 1 provides a convenient graphical representation of this equation. The unitarity condition connects a large set of measurable quantities in the standard model, including CP-violating asymmetries, which depend on the imaginary phase, and mixing strengths, which are magnitudes such as |Vub| and |Vcb|. In the standard model, all the bands corresponding to the different measurements in Fig. 1 should overlap at a unique point, which they do at the current level of precision. The presence of new particles or interactions would contribute to these measurable quantities in different ways, resulting in bands that fail to converge at a point. The ratio of matrix elements |Vub|/|Vcb| corresponds to the length of the side of the ‘unitarity triangle’ opposite the angle labelled β, which is well determined from measured CP-violating asymmetries. The precise determination of this ratio is a crucial ingredient in providing sensitivity to new particles and interactions.
Experiments at electron-positron colliders have measured |Vub| and |Vcb| for many years using two complementary methods based on the decays of a B meson to an electron or muon, its associated neutrino and one or more strongly interacting particles. The first method measures exclusive final states whose decay rates are proportional to |Vqb|2 (where q = u, c), and uses lattice quantum chromodynamic (QCD) calculations of form factors to determine |Vqb|. The second inclusive method requires only the presence of an electron or muon and sums over many exclusive final states. These summed rates are also proportional to |Vqb|2, the determination of |Vqb| in this case relies on perturbative QCD calculations and auxiliary measurements. Although these two methods have improved significantly in precision over the years, the values determined for both |Vub| and |Vcb| from the inclusive method persistently exceed those from the exclusive method by two to three standard deviations. This has prompted speculation that the familiar left-handed charged weak interaction has a right-handed counterpart that contributes
to this difference.
With this backdrop, the new measurement of the ratio |Vub|/|Vcb| from the LHCb experiment at CERN’s Large Hadron Collider (LHC) is a welcome addition to the literature(2). It is based on a different exclusive decay mode than can
be measured at the electron-positron collider experiments, namely that of a baryon containing b, u and d quarks (a heavier version of the neutruon) that decays into a proton, a muon and a neutrino. Particle physicists have been surprised that these decays, where the missing neutrino prevents reliance on kinematic constraints, can be distinguished from the huge background inherent in proton-proton collisions at the LHC This new result, which makes use of very precise spatial measurements of the decay vertices of short-lived particles and uses innovative analysis techniques, is a noteworthy achievement.
What have we learned? The new experimental information, instead of resolving the inclusive-exclusive puzzle, deepens it. The measurement and corresponding lattice QCD calculation lead to a value for |Vub|/|Vcb| that is lower than both the pre-existing exclusive and inclusive determinations. The consistency of the three determinations with a single value is only 1.8%, indicating that particle physicists have more work to do in this area. On a more positive note, the LHCb measurement, when combined with previous measurements, strongly disfavours the hypothesis of a right-handed weak interaction.


(1) Christenson, J. H., Cronin, J. W. Fitch, V L. & Turlay, R. Phys. Rev. Lett. 13, 138-140 (1964).
(2) The LHCb collaboration Nature Phys. 11,743-747 (2015).
(3) Kobayashi, M. & Maskawa, T. Prog. Theor Pinys. 49, 652-657 (1973).
(4) Perl, M. L. et al Phys. Rev. Lett 35,1489-1492 (1975).
(5) Herb, S. W. et al Phys. Rev Lett. 39, 252-255 (1977).

Dust-poor galaxies at early times

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by Veronique Buat from Nature 522, 422–423 (25 June 2015) doi:10.1038/522422a


Observations of galaxies that formed early in the Universe’s history reveal much lower dust levels than are found in sources from a slightly later era. It seems that galaxies underwent rapid change during a relatively short period.


The study of the most distant galaxies, observed as they were about 1 billion years after the Big Bang, is crucial for our understanding of the star-forming activity and the physical processes at work in these young systems. Capak et al.(1) present a study of nine such galaxies using a linked-up telescope array. They find that the dust and gas properties in these systems hint at an interstellar medium (ISM) that is much less evolved than in galaxies about 2 billion years older. This suggests that there was a rapid change in the overall properties of galaxies during the early life of the Universe.
The expansion of the Universe shifts the ultraviolet (UV) light emitted by newly formed stars in remote systems to longer (visible and near-infrared) wavelengths that, unlike UV light, can be observed by ground-based telescopes. The most distant objects known today are detected as a result of a break in the continuum of their redshifted spectra at wavelengths of around 0.1 micrometres; this is due to the absorption of UV photons by neutral hydrogen in the intergalactic medium. The absorption occurs for photons with energies corresponding to wavelengths shorter than the Lyman-α line of hydrogen (1,216 nm), and galaxies whose distances have been estimated by this method are known as Lyman break galaxies (LBGs). The most comprehensive surveys undertaken so far have led to detections of very young LBGs that formed approximately 0.5 billion years after the Big Bang(2).
The presence of interstellar dust complicates the study of galaxies, and affects measurements of fundamental, observationally derived properties such as the star-formation rate. This is because dust is efficient at absorbing the energetic UV photons (a proxy for the star-formation rate) that are emitted by young stars and at re-emitting their energy in the infrared domain, at wavelengths longer than 5 μm. This is a complex process that depends not only on the amount of dust present, but also on its distribution relative to the stars and on its composition(3). Overall, dust substantially reduces the intensity of stellar light reaching the telescopes(4).
A straightforward method to account for the UV light produced in galaxies involves observing the radiant energy that is absorbed by dust, is re-emitted and is then redshifted in the far-infrared and submillimetre domains. However, the low sensitivity of detectors, combined with the poor spatial resolution achieved by single-dish telescopes, make surveys of high-redshift galaxies at these wavelengths less efficient than those at optical or near-infrared wavelengths. Even the Herschel Space Observatory, which detected(5) the infrared emission from dust in galaxies at redshifts of up to 2–3 (corresponding to a time roughly 2 billion to 3 billion years after the Big Bang), was able to detect only hyper-luminous sources at much larger distances(6).
Given that directly measuring the long-wavelength emission from dust is so challenging, astronomers resort to empirical relations to derive dust’s infrared luminosity. One such relation links this luminosity to the stellar UV luminosity(7) for a representative sample of nearby, actively star-forming galaxies, for which both luminosities have been accurately measured. Unfortunately, this recipe is not universally applicable because it depends on the properties of the ISM (such as the composition of dust and its distribution relative to the stars), as well as on the stellar populations in the galaxies(8). Despite these caveats, however, it is extensively used to estimate the level of obscuration of the stellar UV light by ISM dust for galaxies across a wide redshift range. It will therefore be important to check the validity of this relationship — especially for young, high-redshift systems. A critical evaluation could also yield clues to the properties of the ISM at those early times.
Capak et al. used the Atacama Large Millimetre Array (ALMA), which was designed to overcome both the resolution and sensitivity problems (Fig. 1). Being an interferometer (a series of telescopes linked up to combine astronomical observations), ALMA has a small field of view that is suitable for observing well-centred sources, and it can detect the weak submillimetre emission originating from dust in ordinary galaxies at high redshifts. The authors used 20 of ALMA’s antennas in unison to observe the dust and gas emissions of 9 typical LBGs located at redshifts 5–6; these correspond to a time when the Universe was about 1 billion years old.

Capak et al.(1) used 20 of ALMA's antennas to study the interstellar medium (ISM) of 9 galaxies that were present when the Universe was only about 1 billion years old. The authors found that their sources contain a smaller amount of dust than expected. Some of the galaxies in the sample may have an ISM similar to that of the Small Magellanic Cloud (a satellite galaxy of the Milky Way), which is visible here (right of centre) as the smaller of the two Magellanic Clouds above the antennas.

Capak et al.(1) used 20 of ALMA’s antennas to study the interstellar medium (ISM) of 9 galaxies that were present when the Universe was only about 1 billion years old. The authors found that their sources contain a smaller amount of dust than expected. Some of the galaxies in the sample may have an ISM similar to that of the Small Magellanic Cloud (a satellite galaxy of the Milky Way), which is visible here (right of centre) as the smaller of the two Magellanic Clouds above the antennas.

Capak and colleagues selected their sample from the Cosmic Evolution Survey field, a two-square-degree area that has been extensively observed by most of the major telescopes, from the ground and from space. ALMA detected the thermal dust emission in four galaxies, and an ISM spectral line emitted from gaseous carbon at a wavelength of 158 μm in all nine of them; the carbon feature is the dominant ISM emission line of galaxies in the far-infrared domain. Such a high detection rate is outstanding, because previous attempts failed to simultaneously detect the carbon feature and thermal dust emission(9).
The authors’ study argues for a very low dust content and stellar-light obscuration in these systems. The four galaxies whose thermal dust emission was detected may harbour an ISM similar to that of the Small Magellanic Cloud (a satellite galaxy of the Milky Way), which is characterized by a low abundance of elements heavier than helium. The upper limits put on the dust emission of the remaining five sources call for an even more extreme situation with a much lower infrared emission. That seems to be at odds with the observed UV luminosity of these systems. The enhanced carbon emission-line intensities also suggest low dust levels relative to the gas present in these early galaxies, although other explanations cannot be excluded.
An immediate consequence of these findings is that the classical calculations used to derive obscuration due to dust from the observed UV continuum luminosity are unlikely to be valid for LBGs in the early Universe. As a result, the star-formation rate considered to be appropriate for these galaxy types is likely to have been overestimated by factors of between two and four in previous studies. Last but not least, this pioneering work paves the way for future observational campaigns. Although observing the low levels of dust emission from large samples of high-redshift galaxies may prove challenging even for ALMA, Capak and co-workers’ finding of enhanced carbon emission lines should become a useful tool in the study of star-forming galaxies at those early epochs.


(1) Capak, P. L. et al. Nature 522, 455–458 (2015).
(2) Bouwens, R. J. et al. Astrophys. J. 803, 34 (2015).
(3) Witt, A. N. & Gordon, K. D. Astrophys. J. 528, 799 (2000).
(4) Burgarella, D. et al. Astron. Astrophys. 554, A70 (2013).
(5) Gruppioni, C. et al. Mon. Not. R. Astron. Soc. 436, 2875–2876 (2013).
(6) Riechers, D. A. et al. Nature 496, 329–333 (2013).
(7) Meurer, G. R., Heckman, T. M. & Calzetti, D. Astrophys. J. 521, 64 (1999).
(8) Boquien, M. et al. Astron. Astrophys. 539, A145 (2012).
(9) Maiolino, R. et al. Mon. Not. R. Astron. Soc. (in the press); Preprint at arXiv

Evaporation drives engine

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from Nature 522, 259 (18 June 2015) doi:10.1038/522259b


An engine fuelled only by water evaporation can power a miniature car and lights.

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Ozgur Sahin at Columbia University in New York and his colleagues applied bacterial spores to thin plastic strips. The spores absorb and release water with changes in relative humidity, so the strips curl and straighten. The team stacked the strips and formed them into a water-containing engine so that the strips were exposed to recurring periods of high and low humidity, acting like oscillators to power the engine. When attached to a generator, the engine powered light-emitting diodes. A rotary version attached to two pairs of wheels (pictured, left) pushed a 100-gram car forwards (pictured, right).
The engine could be used in devices in areas that have scarce electricity, the authors say.


P.S.: interessting: the news is close, the paper is open!

Two-atom bunching

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by Lindsay J. LeBlanc from Nature 520, 36–37 (02 April 2015) doi:10.1038/520036a


The Hong–Ou–Mandel effect, whereby two identical quantum particles launched into the two input ports of a ‘beam-splitter’ always bunch together in the same output port, has now been demonstrated for helium-4 atoms.
All particles, including photons, electrons and atoms, are described by a characteristic list of ‘quantum numbers’. For a pair of particles whose lists match, there is no way of telling them apart — they are perfectly indistinguishable. One of the more intriguing consequences of quantum mechanics arises from this indistinguishability, and was exemplified(1) in an experiment by Hong, Ou and Mandel (HOM) in the 1980s. The researchers showed that, although a single photon approaching an intersection along one of two input paths exits in one of two output paths with equal probability, identical pairs brought to the intersection simultaneously from different paths always exit together. Lopes et al.(2) now demonstrate this manifestation of two-particle quantum interference for two identically prepared — and thus indistinguishable — helium-4 atoms. The result provides an opportunity to extend advances made in quantum optics to the realm of atomic systems, especially for applications in quantum information.
As a graduate student faced with finding a wedding present for my labmate, I decided that the HOM experiment was a fitting analogy to marriage: from two separate paths, this couple’s lives were intersecting and would continue along a single path together. Along with the formalism describing the effect tucked into the card, I gave them a glass ‘beam-splitter’ to represent a key ingredient in the optical demonstration of the effect: this glass cube could act as the intersection, at which half the light incident on any of the four polished faces is transmitted, with the remaining half being reflected; for single particles, the probabilities for transmission and reflection are both 50%. All HOM experiments require a ’50:50 beam-splitting’ mechanism that sends quantum particles incident along one of two input paths to one of two output paths with a 50% probability (Fig. 1a).

Each beam-splitter (blue) is represented as two input paths (left) and two output paths (right); here we consider '50:50' beam-splitters, for which the probability of each output is of equal magnitude. A particle is represented by a red circle, and its wavefunction's phase by the position of the black dot on the grey circle. Individual phases cannot be measured directly. a, Possible outcomes for a single particle entering either of the input paths; the probabilities for particle transmission and reflection are both 50%. In the case of reflection, the phase changes by 90°. b, For incoming particles at both inputs, there are four possible outcomes. However, the overall probability of the outcomes is determined by adding the individual probabilities using rules of quantum mechanics. For bosonic particles such as photons and helium-4 atoms, the subject of Lopes and colleagues' study(2), the first two outcomes (transmit/transmit and reflect/reflect) cancel. The only outcomes remaining are the third and the fourth.

Each beam-splitter (blue) is represented as two input paths (left) and two output paths (right); here we consider ’50:50′ beam-splitters, for which the probability of each output is of equal magnitude. A particle is represented by a red circle, and its wavefunction’s phase by the position of the black dot on the grey circle. Individual phases cannot be measured directly. a, Possible outcomes for a single particle entering either of the input paths; the probabilities for particle transmission and reflection are both 50%. In the case of reflection, the phase changes by 90°. b, For incoming particles at both inputs, there are four possible outcomes. However, the overall probability of the outcomes is determined by adding the individual probabilities using rules of quantum mechanics. For bosonic particles such as photons and helium-4 atoms, the subject of Lopes and colleagues’ study(2), the first two outcomes (transmit/transmit and reflect/reflect) cancel. The only outcomes remaining are the third and the fourth.

Careful analysis shows that there must be a well-defined relationship between the beam-splitter’s inputs and outputs that is demanded by energy conservation in the classical picture of the beam-splitter3, or by a property known as unitarity in the quantum view4: for classical waves, this relationship fixes the relative positions of the output waves’ peaks and valleys with respect to those of the input waves, whereas for quantum particles this relationship manifests as a relative ‘phase’ between the particles’ input and output wavefunctions. Although the probability of finding a particle in a particular output path depends only on the amplitude of its wavefunction, the phase is important when determining the output wavefunction, and corresponding output probability, for two or more particles.
If two particles enter such a 50:50 beam-splitter, naively one would expect one of four possible outcomes: two in which the particles exit along a path together, and two in which they exit along different paths (Fig. 1b). In these cases, the single-particle output-wavefunction phases accumulate in an overall output phase. The HOM result is a consequence of the particles’ indistinguishability, which means that there is no measurable difference between the two outcomes in which the particles exit along different paths. The overall output phases of these indistinguishable outcomes are opposite to each other, and when added together using quantum rules for bosons (particles with integer spin, a quantum property common to both photons and helium-4 atoms), these two possible outcomes interfere and cancel. The only outcomes remaining are those with two particles in a single output. As a result, simultaneous single-particle detections (‘coincidence counts’) at both outputs are forbidden.
Lopes et al. demonstrate two-particle quantum interference with helium-4 atoms. In their experiments, the atoms’ paths are related to their speeds, which are manipulated by selectively transferring momentum to and from light in absorption and emission processes(5, 6). First, the researchers prepared a ‘twin pair’ by removing from an atom reservoir indistinguishable atoms with different speeds. Second, they used light pulses to modify the atoms’ momenta and cause the pair to meet; the atom in the first path travels with velocity v1 and the atom in the second path with v2. A beam-splitting mechanism implemented reflection and transmission by changing the atoms’ speeds with 50% probability from v1 to v2 and vice versa.
The atoms continued to travel until they hit a time-resolved, multipixel atom-counting detector, at which an atom with v1 would arrive at a different time from one with v2. Lopes and colleagues prepared many twin pairs in a short interval and recorded the precise location and timing of the atoms’ arrivals at the detector: a coincident count would be the measurement at a particular location of a particle at time t1 followed by a measurement at t2. Although the researchers found that the arrivals from the many pairs were distributed in two time windows (corresponding to the two output paths), they found a striking lack of instances among these random outcomes when the time difference was exactly t2t1, indicating that the atoms from a twin pair must be exiting the beam-splitter with the same velocity. This ‘anticorrelation’ is the signature of a HOM experiment.
As in quantum-optics demonstrations of the HOM effect, the present result demonstrates that pairs of identical, ‘quantum-entangled’ particles have been produced. The unique capabilities of this apparatus, including the combination of condensed metastable helium-4 atoms and the atom-counting detector, offer a spatial and temporal resolution unavailable to others. Protocols for transmitting and processing quantum information, analogous to those used in optical systems, can now be implemented with new capabilities in atomic systems: atoms, unlike photons, may interact with one another, and because they have mass, their mechanical properties, such as momentum, can be varied and used as experimental parameters.
Furthermore, because atoms can also be fermions (particles with half-integer spin, such as electrons), they could exhibit a quantum-interference effect that is the fermionic equivalent of the HOM effect(4). Evidence for this mechanism has already been seen in electronic systems(7). The bosonic HOM effect demonstrated here, and its fermionic counterpart, may offer new possibilities for implementing quantum-information protocols and for exploring the foundations of quantum physics.


(1) Hong, C. K., Ou, Z. Y. & Mandel, L. Phys. Rev. Lett. 59, 2044–2046 (1987).
(2) Lopes, R. et al. Nature 520, 66–68 (2015).
(3) Ou, Z. Y. & Mandel, L. Am. J. Phys. 57, 66 (1989).
(4) Loudon, R. Phys. Rev. A 58, 4904–4909 (1998).
(5) Campbell, G. K. et al. Phys. Rev. Lett. 96, 020406 (2006).
(6) Bonneau, M. et al. Phys. Rev. A 87, 061603 (2013).
(7) Neder, I. et al. Nature 448, 333–337 (2007).

‘Tatooines’ may be common

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from Nature 523, 9 (02 July 2015) doi:10.1038/523009d

Title: STAR WARS ¥ Pers: HAMILL, MARK ¥ Year: 1977 ¥ Dir: LUCAS, GEORGE ¥ Ref: STA039PE ¥ Credit: [ LUCASFILM/20TH CENTURY FOX / THE KOBAL COLLECTION ]

Title: STAR WARS ¥ Pers: HAMILL, MARK ¥ Year: 1977 ¥ Dir: LUCAS, GEORGE ¥ Ref: STA039PE ¥ Credit: [ LUCASFILM/20TH CENTURY FOX / THE KOBAL COLLECTION ]

Planets orbiting a binary star system — like Tatooine, the fictional home planet of Luke Skywalker in Star Wars — could form with surprising ease.
Most known circumbinary planets orbit close to their stars, where the competing gravitational forces from the two stars make the orbits of nearby objects unstable or intersect. This prevents debris from clumping together to form planets. But Benjamin Bromley of the University of Utah in Salt Lake City and Scott Kenyon of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, show with simulations that a zone exists near the host stars where the orbits of debris wobble, but do not cross, allowing for planet formation.
This suggests that Earth-sized ‘Tatooines’ could be common and that more are likely to be discovered soon.

Paper: Planet formation around binary stars: Tatooine made easy by Benjamin C. Bromley and Scott J. Kenyon

Space-rock alert

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by Alexandra Witze from Nature 522, 418 (25 June 2015) doi:10.1038/522418a


Step aside, Armageddon and Deep Impact: two films are in competition to be this generation’s seminal asteroid movie. Both offer crucial information about the asteroid threat to new audiences.
Disaster Playground and 51° North are released widely on 30 June, the inaugural celebration of Asteroid Day — the anniversary of the 1908 meteorite explosion over Tunguska, Siberia. The event is meant to spread awareness of the threat of space rocks, repackaging the alarms that a small group of skywatching scientists have been issuing for decades. The novelty comes in how these films do it.

In Disaster Playground, firefighters train for an asteroid impact.

In Disaster Playground, firefighters train for an asteroid impact.

Both explore the ramifications of what might happen if an asteroid were discovered on a collision course with Earth. Both feature a narrator who interacts with real-life asteroid experts in a variety of offbeat settings. Both are a far cry from conventional documentaries, being more mash-ups of scientific information with fictionalized narratives.
Disaster Playground, directed by Nelly Ben Hayoun, is a visually arresting roller-coaster ride through California and the American Southwest, featuring interviews with high-impact scientists from NASA, the Sandia National Laboratories and elsewhere. It opens in Texas, with a cowboy and his horse startled by a huge blast. Ambulances and firefighters race through the debris left by the impact. Within minutes, meteor scientist Peter Jenniskens of NASA’s Ames Research Center in Moffett Field, California, is presiding at an official-looking podium, sombrely reading a statement about the disaster.
In real life, Hayoun has the impressive title “designer of experiences at the SETI Institute”, the organization in Mountain View, California, that explores life’s place in the Universe. The film is certainly a designed experience. Hayoun ferries a bright-green toy dinosaur and a huge red telephone from interview to interview to represent the threat of mass extinction and the alerts needed to save the world from apocalypse. She uses offbeat camera angles, a pulsing soundtrack from rave band The Prodigy and title screens with giant capital letters to create a frenetic pace.
Hayoun coaxes scientists into the closest thing they may ever achieve to performance art. Ames astronomer David Morrison recreates his influential 1993 speech to Congress on the asteroid threat; his boss, Pete Worden, breaks out a Viking helmet and shield as a backdrop for an otherwise staid office interview. Hayoun’s own appearances bind it all together as she hurries from shoot to shoot, pausing only to buy a pair of cowboy boots on her way to explore Disaster City, an emergency-response training centre in Texas, where search and rescue teams work their way through intentionally collapsed buildings. There is no time to wait when disaster strikes.
Grigorij Richters51° North is similarly kinetic but much more self-absorbed. It centres on a fictional YouTube star called Damon, who becomes obsessed with the asteroid threat. His reality-show audience is gradually alienated as he turns from taping his girlfriend and his dogs to making show after show about space-rock disasters. That all changes when — well, no spoilers. But given the title and the fact that it is about asteroids, you can guess what happens to London.
Richters has created a true millennial approach to the asteroid threat. Damon is constantly on camera: taping, uploading, tweeting, instagramming, sharing every moment. Brian May, Queen guitarist and astrophysicist, provides the soundtrack.
There is some science. Damon interacts with many of Britain’s leading asteroid experts, and even tours the Spaceguard Centre in Knighton, the United Kingdom’s leading asteroid-information centre. He works in smooth explanations of the various strategies for diverting an asteroid on a collision course with Earth. Footage from the many Russian car cameras that recorded the Chelyabinsk meteorite strike in February 2013 fits the video-focused approach. Unlike in Disaster Playground, there is no wider discussion of what the end of the world means for anyone other than the self-obsessed Damon.
Today, the cutting edge of asteroid science lies in pushing the limits of detection, searching for smaller and smaller risky space rocks. Scientists have pretty much nailed the chances of spotting any world-ending asteroid, a rock big enough to wipe out an entire region. What is left are the ones that can slip through the current net of telescopes and hit with little to no warning.
As New Year celebrations welcomed in 2014, an asteroid a few metres wide entered the atmosphere and disintegrated over the Atlantic Ocean — all while many of the astronomers who would normally track it were on a holiday break. We do live in a disaster playground.