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PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 736 July 6, 2005 by Phillip F. Schewe, Ben Stein
WHY IS THE SKY BLUE, AND NOT VIOLET? The hues that we see in the sky are not only determined by the laws of physics, but are also colored by the human visual system, shows a new paper in the American Journal of Physics. On a clear day when the sun is well above the horizon, the analysis demonstrates, we perceive the complex spectrum of colors in the sky as a mixture of white light and pure blue. When sunlight enters the earth's atmosphere, it scatters (ricochets) mainly from oxygen and nitrogen molecules that make up most of our air. What scatters the most is the light with the shortest wavelengths, towards the blue end of the spectrum, so more of that light will reach our eyes than other colors. But according to the 19th-century physics equations introduced by Lord Rayleigh, as well as actual measurements, our eyes get hit with peakamounts of energy in violet as well as blue. So what is happening?
Combining physics with quantitative data on the responsiveness of the human
visual system, Glenn Smith of Georgia Tech (glenn.smith@ece.gatech.edu) points
to the way in which our eye's three different types of cones detect color.
As Smith shows, the sky's complex multichromatic rainbow of colors tickles
our eye's cones in the same way as does a specific mixture of pure blue and
white light. This is similar to how the human visual system will perceive
the right mixture of pure red and pure green as being equivalent to pure
yellow. The cones that allow us to see color cannot identify the actual
wavelengths that hit them, but if they are stimulated by the right combination
of wavelengths, then it will appear the same to our eyes as a single pure
color, or a mixture of a pure color and white light. (Smith, American Journal
of Physics,July 2005) PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 728 April 20, 2005 by Phillip F. Schewe, Ben Stein
AN OCEAN OF QUARKS. Nuclear physicists have now demonstrated that the material essence of the universe at a time mere microseconds after the big bang consists of a ubiquitous quark-gluon liquid. This huge insight comes from an experiment carried out over the past five years at the Relativistic Heavy Ion Collider (RHIC), the giant crusher of nuclei located at Brookhaven National Lab, wherescientists have created a toy version of the cosmos amid high-energy collisions. RHIC is of course not a telescope pointed at the sky but an underground accelerator on Long Island; it is, nevertheless,in effect, a precision cosmology instrument for viewing a very early portion of the universe, a wild era long before the time of the first atoms (which formed about 400,000 years after the big bang),before the first compound nuclei such as helium (about a minute after the big bang), before even the time when protons are thought to have formed into stable entities (ten microseconds). In our later, cooler epoch quarks conventionally occur in groups of two or three. These groupings, called mesons and baryons,respectively, are held together by particles called gluons---which act as agents for the strong nuclear force. Baryons (such as protons and neutrons), collectively called hadrons, are the normal building blocks of any nucleus. Could hadrons be melted or smashed into their component quarks through violent means? Could a nucleus be made to rupture and spill its innards into a common swarm of unconfined quarks and gluons? This is what RHIC set out to show. Let's look at what happened. In the RHIC accelerator itself two beams of gold ions, atoms stripped of all their electrons, are clashed at several interaction zones around the ring-shaped facility. Every nucleus is a bundle of 197 protons and neutrons,each of which shoots along with an energy of up to 100 GeV. Therefore, when the two gold projectiles meet in a head-on "central collision" event, the total collision energy is 40 TeV (40 trillion electron volts). Of this, typically 25 TeV serves as a stock ofsurplus energy---call it a fireball---out of which new particles can be created. Indeed in many gold-gold smashups as many as 10,000 new particles are born of that fireball. Hubble-quality pictures of this blast of particles (http://www.bnl.gov/RHIC/full_en_images.htm), shows the aftermath of the fireball, but not the fireball itself. The outward streaming particles provide all the forensic evidence for determining the properties of the fireball. To harvest this debris, the RHIC detectors must be agile and very fast. The recreation of the frenzied quark era is ephemeral, lasting only a few times 10^-24 seconds. The size of the fireball is about 5 femtometers, its density about 100 times that of an ordinary nucleus, and its temperature about 2 trillion degrees Kelvin or (in energy units) 175 MeV. RHIC was built to create that fireball. But was it the much-anticipated quark-gluon plasma? The data unexpectedly showed that the fireball looked nothing like a gas. For one thing, potent jets of mesons and protons expected to be squirting out of the fireball, were being suppressed. Now, for the first time since starting nuclear collisions at RHIC in the year 2000 and with plenty of data in hand, all four detector groups operating at the lab have converged on a consensus opinion. They believe that the fireball is a liquid of strongly interacting quarks and gluons rather than a gas of weakly interacting quarks and gluons. The RHIC findings were reported at this week's April meeting of the American Physical Society (APS) in Tampa, Florida in a talk delivered by Gary Westfall (Michigan State) and at a press conference attended by several RHIC scientists. Brookhaven physicist Samuel Aronson said that having established the quark-gluon-liquid nature of the pre-protonic universe, RHIC expected to plumb the liquid's properties, such as its heat capacity and its reaction to shock waves. The liquid is dense but seems to flow with very little viscosity. It flows so freely that it approximates an ideal, or perfect, fluid, the kind governed by the standard laws of hydrodynamics. At least in its flow properties the quark liquid is therefore a classical liquid and should not be confused with a superfluid, whose flow properties (including zero viscosity) are dictated by quantum mechanics. One of the reasons for RHIC's previous hesitancy in delivering a definitive pronouncement was concern over the issue of whether the observed nuclear liquid was composed of truly deconfined quarks and gluons or of quarks confined within hadrons, or maybe even a mixture of quarks and hadrons. According to William Zajc (Columbia Univ.and spokesperson for the PHENIX detector group at RHIC), the patterns of particles flying out of the fireball, including preliminary data on heavier, charm-quark-containing particles such as D mesons, support the quark liquid picture. To summarize, the main stories here are (1) that based on theevidence of the RHIC data, the universe in the microsecond era would seem to consist of a novel liquid of quarks and gluons; (2) that RHIC has reproduced small fragments of this early phase of the universe for detailed study; and (3) that these results are vouched for by all four RHIC groups. If there had been delays in making an announcement of the results or if the exact nomenclature for the novel nuclear matter had been left unsettled, the RHIC physicists at the press conference seemed more interested in pursuing their new kind of experimental science---a sort of fluid-dynamical cosmology. (All four groups are also concurrently publishing "white paper" summaries of their work in the journal Nuclear Physics A. Preprints are available as follows: BRAHMS,http://arxiv.org/abs/nucl-ex/0410020 ; PHENIX, http://arxiv.org/abs/nucl-ex/0410003 ; PHOBOS, http://arxiv.org/abs/nucl-ex/0410022 ; and STAR, http://arxiv.org/abs/nucl-ex/0501009)
CIRCUIT ELEMENTS FOR OPTICAL FREQUENCIES. Electric circuit elements, among them resistors, capacitors, and inductors, come in a variety of sizes to deal with a variety of applications at a range of frequencies. The familiar electrical grid, for example, operates at a frequency of 60 Hz. A circuit designed to process radio signals operates at the 100-megahertz range. A typical frequency domain for computers is 1 GHz. Higher still, microwave applications often operate at the 10-GHz (10^10 Hz) level. Nader Engheta (engheta@ee.upenn.edu, 215-898-9777) and his Penn group would like to extend the circuit concepts up to optical frequencies, around 10^15 Hz. To do this, instead of just shrinking the classic circuit elements to fraction of the typical wavelength of the optical signal being processed (around 500 nm), the Penn proposal is to make nano-inductors, nano-capacitors and nano-resistors out of sub-wavelength nano-particles, fashioned from appropriate materials on a substrate with lithographic techniques. Possible applications would include direct processing of optical signals with nano-antennas, nano-circuit-filters, nano-waveguides, nano-resonators, and even nano-scaled negative-index optical structures. (Engheta et al., Physical Review Letters, upcoming article; http://www.ee.upenn.edu/~engheta/)
STRENGTHENING QUANTUM CRYPTOGRAPHY BY PUTTING ON BLINDERS. AKorea-UK team (contact Myungshik Kim, Queen's University, Belfast,m.s.kim@qub.ac.uk , or Chilmin Kim, Paichai University) has introduced a method for preventing several clever attacks against quantum cryptography, a form of message transmission that uses the laws of quantum physics to make sure an eavesdropper does not covertly intercept the transmission. Making the message sender and receiver a little blind to each other's actions, the researchers have shown, can bolster their success against potential eavesdroppers. In quantum cryptography, a sender (denoted as Alice) transmits a message to a receiver (called Bob) in the form of single photons each representing the 0s and 1s of binary code. If an eavesdropper (appropriately named Eve) attempts to intercept the message, she will unavoidably disturb the photon through the Heisenberg uncertainty principle, which says that even the gentlest observation of the photon will perturb the particle. This will be instantly detectable by Alice and Bob, who can stop the message and start again. Quantum cryptography is already being used in the real world and is even available commercially as a way for companies to transmit sensitive financial data. But in its real-world implementation, a weak pulse of light (rather than a perfect stream of single photons) is sent down a transmission line that is "lossy,"or absorbs photons. So feasible attacks on quantum cryptography include the pulse-splitting attack (in which Eve splits a transmitted pulse into two pulses and examines one of them for information), the pulse-cloning attack (in which a transmitted pulse is copied to relatively high accuracy and then inspected for its information), and the "man-in-middle" or impersonation attack, in which Eve could impersonate Alice or Bob by intercepting the transmission and acting as sender or receiver. A new paper proposes a solution to these three attacks by proposing a technique called "blind polarization." In this technique, Alice and Bob verify their identities to each other in a rather paradoxical way, by performing some actions that is their own private information. Yet these actions make the message completely indecipherable to a third party. Alice creates a pair of pulses, but with random polarizations (polarization indicates the direction or angle in which each pulse's electric field points relative to some reference, such as a horizontal line) Alice sends the pulses to Bob, who does not know the polarizations. Nonetheless, without measuring the polarization values, Bob is able to rotate the polarization of one pulse by one amount and the other pulse by another amount, but he doesn't tell Alice which pulses got which treatment. Alice receives the pulses, and then encodes them with a message (representing the binary value 0 or 1, which could stand for "no" or "yes), then blocks one of the pulses, without telling Bob which one was blocked. Bob then reverses the various polarizations by a certain amount to get the desired message. The various polarization adjustments are designed in such a way that either pulse Alice sends will yield the desired information. According to researcher Myungshik Kim, Alice has her own private information on which pulse is blocked, while Bob has his own private information on which pulse he rotated by a given amount. Once Alice begins the transmission, there is no way for Eve to have this private information which makes their protocol effective against the man-in-middle and other attacks. (Kye et al., Physical Review Letters, upcoming article). This paper is the latest in a wave that plugs up potential vulnerabilities in quantum cryptography (for an example of using"quantum decoys" to thwart attacks, see Lo et al, Physical ReviewLetters, 17 June 2005)
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics
News Number 744 September 6, 2005 by Phillip F. Schewe, Ben Stein HOW WELL NANOTUBES CONDUCT ELECTRICITY depends a lot on their environment. Hongjie Dai and his colleagues at Stanford have made the first electrical measurements of currents flowing under high voltage (high bias) through single-walled carbon nanotubes suspended like miniature power lines. They discovered that in suspended form a micron-scale-long nanotube could carry about 5 micro-amps of current, whereas lying in the plane of a substrate the same tube can carry about 25 micro-amps. The reason for the better in-the-plane performance is that the substrate helps to dampen "optical phonons," high-energy vibrations of the nanotube atomic lattice. Dai (650-723-4518, hdai1@stanford.edu) believes that with careful engineering of the interface between a nanotube and a substrate, maximum currents could be raised to higher levels than previously possible, which might make carbon nanotubes useful for applications in high-power transistors and even nanoscale transmission lines. To make the kind of transmission lines you see in the countryside out of nanotubes, you'd have to develop a process for producing km-length carbon tubes, which is not feasible for the foreseeable future. (Pop et al., Physical Review Letters, upcoming article) |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 765 February14, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
ATTACK OF THE TELECLONES: Should quantum cryptographers begin to worry? In contrast with everyday matter, quantum systems such as photons cannot be copied, at least not perfectly, according to the "no-cloning theorem." Nonetheless, imperfect cloning is permitted,so long as Heisenberg's Uncertainty Principle remains inviolate.
According to Heisenberg, measuring the position of a particle disturbs it, and limits the accuracy to which its complementary property (momentum) can be determined, making it impossible to reliably replicate the particle's complete set of properties.
Now, quantum cloning has been combined with quantum teleportation in the first full experimental demonstration of "telecloning" by scientists at the University of Tokyo, the Japan Science and Technology Agency, and the University of York (contact Sam Braunstein, schmuel@cs.york.ac.uk and Akira Furusawa,akiraf@ap.t.u-tokyo.ac.jp). In ideal teleportation, the original is destroyed and its exact properties are transmitted to a second,remote particle (Heisenberg does not apply because no definitive measurements are made on the original particle). In telecloning,the original is destroyed, and its properties are sent to not one but two remote particles, with the original's properties reconstructed to a maximum accuracy (fidelity) of less than 100%.
(Heisenberg limits the ability to make clones as otherwise researchers could keep making copies of the original particle and learn everything about its state.)
In their experiment, the researchers didn't just teleclone a single particle, but rather an entire beam of laser light. They transmitted the beam's electric field, specifically its amplitude and phase (but not its polarization) to two nearly identical beams at a remote location with 58% accuracy or fidelity (out of a theoretical limit of 66%). This remarkable feature of telecloning stems from the very magic of quantum mechanics: quantum entanglement. Telecloning stands apart from local cloning and from teleportation in requiring "multipartite" entanglement, a form of entanglement in which stricter correlations are required between the quantum particles or systems, in this case three beams of light. (An example of a multipartite entanglement is the GHZ state between three particles that was featured in Update 414.)
In addition to representing a new quantum-information tool,telecloning may have an exotic application: tapping quantumcryptographic channels. Quantum cryptographic protocols are so secure that they may discover tapping. Nonetheless, with telecloning, the identity and location of the eavesdropper could be guaranteed uncompromised. (Koike et al., Physical Review Letters, 17February 2006; for an earlier partial demonstration of telecloning,between an original photon and one clone at a remote location and another clone local to it, see Zhao et al., Phys Rev Lett, 13 July2005)
STOCK MARKET CRITICALITY. In the months before and after a major stock market crash, price fluctuations follow patterns similar to those seen in natural phenomena such as heartbeats and earthquakes,physicists find in a study to appear in Physical Review Letters. A University of Tokyo team studied the Standard & Poor's S&P 500 index, focusing on small deviations from long-term index trends.
Such up-and-down blips in stock prices are usually "Gaussian," or "normally" random, at least when measured over sufficiently long time scales---for example, for more than one day. That means that fluctuations are likely to be small, while larger fluctuations are less likely, their probabilities following a bell curve. But when the team looked at 2-month periods surrounding major crashes such as the Black Monday event of October 19, 1987, they saw a different story: Fluctuations of all magnitudes were equally probable. As a consequence, the graph of index fluctuations looked statistically similar if plotted over different time scales, anywhere between time scales of 4 minutes and two weeks. Such behavior is called critical in analogy with a ferromagnetic metal at the "critical temperature," when regions form where the metal's atoms arrange their spins in the same direction, and these regions look similar at different levels of magnification. This self-similarity is also seen in the time intervals between heartbeats, or between earthquakes.
Mathematically, however, the stock market case differs in that the probabilities do not change with the size of the event, while in other cases of non-critical self-similarity, the probabilities usually follow a so-called power law. It is unclear what individual trading decisions lead to criticality in the stock market, co-author Zbigniew Struzik (zbigniew.struzik@p.u-tokyo.ac.jp) says, although he and the team at the University of Tokyo are working on finding explanations. Also unclear is whether the findings could one day lead to an early-warning system to predict crashes, and if such a system would precipitate a crash -- or create one artificially -- by inducing panic. "It could compensate for or neutralize the crashes,or make them worse," Struzik says. (Kiyono et al., Physical Review Letters, 17 February)
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