Sumariani | Tamblang | Physics

Filling a cavity with photons, and watching them leave

noreply@blogger.com (Made Sumariani) — Fri, 12 Dec 2008 15:14:00 +0700

Figure 1: By coupling a nonlinear system, such as an atom, to the electromagnetic field, it is possible to create Fock states (eigenstates of the harmonic oscillator). (Top) Brune et al. send atoms (left) into a cavity (center). The atoms are prepared with pulse P1 to be in a superposition of states |e〉 and |g〉 before they enter the cavity. The relative phase between these states, which is converted to probability amplitudes for |e〉 and |g〉 with pulse P2 when the atoms exit the cavity, depends on the number of photons in the cavity. (Bottom) In place of a cavity, Wang et al. create an electromagnetic field in a microwave resonator (blue). A superconducting qubit, acting as an artificial atom, couples to the center conductor.

The harmonic oscillator is one of the most fundamental systems in quantum mechanics. Equipped with its solution—one of the first that every physics student learns to calculate exactly—it is possible to describe realistic problems, from phonons in a crystal to the interaction of light with an atom. It is perhaps ironic, then, how challenging it is to actually prepare a pure harmonic oscillator state with a well-defined excitation number n, also known as a Fock state. Now, using different methods, two groups—Michel Brune and colleagues at the Laboratoire Kastler Brossel of the CRNS and Collège de France, both in Paris, and Haohua Wang and colleagues at the University of California in Santa Barbara (UCSB)—have created these nonclassical states of the harmonic oscillator and performed a detailed study of how they decay in time. The experiments, reported in Physical Review Letters [1, 2] demonstrate that the lifetime of a Fock state with excitation number n scales as 1/n, as predicted by theory.

Since it is of relevance to both experiments, consider one of the simplest realizations of the harmonic oscillator: the electromagnetic field. Its excited states are photons and a Fock state corresponds to the creation of n photons with the same energy, ħω. However, when using a classical source with a well-defined frequency (such as a laser) to generate an electromagnetic field, the result is a coherent state: a superposition of Fock states that is nearly indistinguishable from a classical state. The reason is that the energy spectrum of the harmonic oscillator is linear, such that the energy provided by the source will spread over a wide distribution of Fock states. Instead, to directly prepare a purely nonclassical photon state and observe quantum effects, we need to make a sufficiently strong interaction between the electromagnetic field and an additional, nonlinear, component. This is the heart of the experiments from Brune et al. and Wang et al.

In the work from the CNRS group, the nonlinear components are atoms with excited states that are not evenly spaced in energy. In particular, they use circular Rydberg atoms (atoms in highly excited states and with maximum angular momentum: l=|m|=n-1 ). These atoms have large dipole moments that couple strongly to the microwave photons that are used in the experiments. To enhance this coupling, the photons are confined to a cavity made out of two high-quality mirrors that face each other. The CNRS group has studied the interaction between light and matter in a cavity, also known as cavity quantum electrodynamics (QED) [3, 4, 5], with circular Rydberg atoms over the last 20 years. Thus, they have been able to carefully design the experiment so that the coupling strength between the circular Rydberg atoms and the photons in the cavity overwhelms all the decay rates of the combined system: the single-photon decay rate κ out of the cavity, the atomic decay rate Γ1, and the atomic dephasing rate Γϕ. It is worth noting that cavities can now be fabricated with extremely large quality factors such that the single photon lifetime 1/κ can be as large as 0.1 seconds [6]. This is long enough for photons to travel 39,000 km back and forth between the two mirrors separated by 2.7 cm!

The experiment by the CNRS group relies on the conditional preparation of Fock states. They first create a coherent state of the cavity field with a microwave source. Then, to prepare a pure but randomly chosen Fock state, they rely upon the magic of quantum measurements: they perform a measurement of photon number with result n, which projects the classical field to the quantum state |n〉. The CNRS team’s measurement is special in the sense that it involves no energy exchange. In these so-called quantum nondemolition measurements, atoms that are nonresonant with the photons in the cavity are sent one by one across the cavity. Before entering the cavity, each atom is prepared in a superposition of two of its internal states, labeled |g〉 and |e〉 for ground and excited states (see Fig.1, top ). During the time that the atom is in the cavity, this superposition acquires a relative phase proportional to the number of photons in the cavity. After leaving the cavity, a second pulse converts this phase information to probability amplitudes for |g〉 and |e〉, which are then measured by state-selective ionization of the atom. By repeating this process with sufficiently many atoms (roughly 110 in the experiment), the Fock state, which was randomly selected from the initial field distribution, is prepared with high accuracy.

After preparing the Fock state |n〉, every atom that is then sent through the cavity reveals information about the subsequent evolution of the cavity field. In this way, Brune et al. follow the time evolution of Fock states n=0 through n=7, and can track how these states decay, something known as quantum process tomography. As expected from theory, they find that the decay rate of a Fock state with n photons is nκ, which is n times faster than for a Fock state with n=1. This enhanced rate simply reflects the fact that each additional photon has its own probability to decay, speeding up the relaxation. Since in this experiment preparing a Fock state is a random process, completely characterizing the state is a costly enterprise, requiring up to a million single atom measurements.

In parallel, researchers have been developing an on-chip version of cavity QED, also known as circuit QED. In this system, the many Rydberg atoms are replaced by a single superconducting qubit and the cavity is a transmission-line resonator, essentially a one-dimensional superconducting coaxial cable (see Fig. 1, bottom). Gaps in the center conductor of the resonator play the role of the mirrors in cavity QED. Moreover, similarly to the cavity used by the CNRS group, excitations of the resonator are microwave photons. These essentially one-dimensional cavities have a small mode volume, resulting in a large electric field per photon. Superconducting qubits are electrical circuits based on Josephson junctions. With their well-defined energy levels, they behave as artificial atoms, providing the essential nonlinearity. In addition, superconducting qubits have a large effective dipole moment. As a result, this system can easily reach the strong coupling regime of cavity QED [7]. Groups at Yale [8] and Delft (in this last case using a different type of on-chip cavity) [9] first demonstrated strong coupling between a superconducting qubit and a microwave resonator in 2004. Because of the very strong coupling, it was predicted [10] and soon confirmed [11] that in circuit QED, Fock states could be resolved by measuring the qubit absorption spectrum. Earlier this year, the UCSB group showed they could prepare Fock states with up to n=6 photons [12].

In the new experiments from the UCSB group [2], the superconducting qubit, playing the role of the atom, is capacitively coupled to the center conductor of the resonator (Fig. 1, bottom). An advantage of this artificial atom is that the energy difference between its |g〉 and |e〉 states can be tuned into and out of resonance with the resonator frequency. Starting with the qubit out of resonance and in its ground state |g〉, a classical source is used to pump it to |e〉. This energy quantum is then transferred to the resonator by tuning the qubit so it is in resonance with the microwave resonator for an appropriate amount of time. By repeating this process, Fock states with n up to 15 have been created. The microwave resonator’s state can in turn be determined by tuning the qubit into resonance with the resonator and measuring the undriven Rabi oscillations of the qubit between |g〉 and |e〉. Since the frequency of these oscillations depends characteristically on the photon number in the microwave resonator, Wang et al. can extract information about the photon distribution of the Fock state. Similarly to the CNRS group, they find that the n-photon Fock state decays at the enhanced rate nκ.

The experiments now reported by Brune et al. and Wang et al. go beyond the groups’ earlier work in that both are able to create Fock states with large n and reach a level of precision with which to probe the decay of these states. By combining two prototypical systems—harmonic oscillators and two-level systems—cavity QED has established itself in the last 20 years as a unique test-bed for fundamental investigations of quantum mechanics. With the recent developments, such as cavities with high-quality factors and circuit QED, new ways to generate, control and measure non-classical states of light are now possible and more surprises are sure to be on their way.

http://physics.aps.org

A glassy counterpart to supersolids

noreply@blogger.com (Made Sumariani) — Tue, 9 Dec 2008 19:29:00 +0700

Figure 1: (a) A classical system of Brownian spheres may crystallize at low temperatures if cooled slowly. By contrast, a bi-disperse system (i.e., consisting of two different kinds of hard spheres) such as the one shown cannot crystallize and will jam into a glass at high densities and low temperatures. Similarly, a rapidly cooled system of hard spheres does not have enough “time” to crystallize and forms a glass instead. Using the mapping of [7], it is seen that the quantum analog of the classical hard-sphere Brownian system is that of spheres with sticky interactions. In the classical hard-sphere limit, only pair interactions appear in the corresponding quantum system. (b) The pair potential in the corresponding quantum system given by Biroli et al. [1] (for two values of λ, a parameter that adjusts how hard the classical potential is, where r is the interparticle distance and σ is the sphere diameter) and the corresponding phase diagrams, shown in (c). Top: The solid curve indicates the classical Brownian hard-sphere phase diagram (pressure P versus volume fraction ϕ) for uniform annealed systems wherein spheres forming a liquid at low densities pack into a face-centered-cubic (FCC) crystal structure at high densities. The dashed curve shows the phase diagram of a rapidly quenched or bi-disperse system in which crystallization is thwarted and the system becomes a glass instead [random close packing (RCP)]. Bottom: The corresponding quantum phases obtained by the map of [7]. The liquid-to-solid transition of the classical Brownian sphere system maps into a superfluid to supersolid transition. Similarly, the superfluid to superglass transition constitutes an analog of the classical liquid to glass transition. [ Panels (b) and (c) adapted from Biroli et al. [1].]

Glasses are liquids that have ceased to flow on experimentally measurable time scales. By constrast, superfluids flow without any resistance. The existence of a phase characterized by simultaneous glassiness and superfluidity may seem like a clear contradiction. However, in a paper in Physical Review B, Giulio Biroli (Institut de Physique Théorique, France), Claudio Chamon (Boston University), and Francesco Zamponi (École Normale Supérieure, France) prove that this is not so [1] and illustrate theoretically the possibility of a “superglass” phase. This phase forms an intriguing amorphous counterpart to the “supersolid” phase [2, 3] that has seen a surge of interest in recent years [4]. Within a “supersolid” phase, superfluidity can occur without disrupting crystalline order.

Interacting quantum particles can indeed form such a superglass phase at very low temperatures and high density, and the work of Biroli et al. confirms the earlier suggestion by Boninsegni, Prokof ’ev, and Svistunov [5] and an investigation by Wu and Philips [6] of such a phase. The superglass phase is characterized by an amorphous density profile, yet at the same time a finite fraction of the particles flow without any resistance as if they were superfluid. Thus the superglass constitutes a glassy counterpart to the supersolid phase.

The approach invoked by Biroli et al. to prove the existence of a superglass is particularly elegant. It relies on mapping [7] viscous classical systems, whose properties are well known, to new many-body quantum systems. In realizing the link between classical and quantum systems to gain insight into the quantum many-body phases, Biroli et al. nicely add an important new result to earlier investigations that built on such similar insights elsewhere. Chester [3] suggested the existence of a supersolid by relying on such a connection. In a similar fashion, Laughlin invoked a highly inspirational analogy [8] between variational (the so-called Jastrow type) wave functions describing fractional quantum Hall systems and a previously studied system of classical charged particles interacting via a logarithmic potential. By using the classical plasma analogy and using known results on it, Laughlin was able to make headway on the challenging many-body quantum problem and construct his highly successful wave functions.

The mapping used by Biroli et al. similarly enables exact results on the quantum problem of superglasses and a detailed correspondence of spatial and temporal correlations between the classical and quantum systems. The authors apply this mapping to a classical system well known to exhibit glassy dynamics—the Brownian hard sphere problem. The quantum counterpart of the classical hard sphere problem is a natural system containing hard sphere interactions [Fig. 1(a),1(b)]. On the classical side of the correspondence, the hard sphere system has been heavily investigated [9, 10, 11]. When the sphere packing density is slowly varied, the classical Brownian hard sphere system undergoes a transition from a liquid at low density to an ordered crystal at high density [9]. When crystallization is thwarted by a rapid increase of the packing density or by, for example, a change of the particle geometry, the system cannot order nicely into a crystal and instead jams into a dense amorphous glass [10, 11].

Biroli et al. noticed that by using the mapping between quantum systems with classical glass-forming systems such as the Brownian spheres, they can obtain nontrivial results. In its simplest form, the mapping of [7] casts the first-order differential equation in time for the dynamics of viscous classical particles as a Schrödinger equation with an effective Hamiltonian. Biroli et al. find that under this mapping, the glassy phase of the classical system translates into a quantum glass of a Bose system. Similarly, the classical solid maps onto a quantum bosonic crystal, resulting in an interesting phase diagram [Fig.1(c)]. The spatiotemporal correlations of the (bosonic) quantum counterpart may be computed by mapping to the classical system. Both the glassy and solid phases harbor a finite Bose-Einstein condensate fraction. Putting all of the pieces together, Biroli et al. provide an important proof of concept of the superglass phase in a simple and precise way. This route may be replicated for classical systems other than the Brownian hard sphere that also display solid and glass phases.

What physical systems might exhibit the new superglass phase? Recent experiments [4, 12] on solid helium-4 exhibit supersolid-type features and have led to a flurry of activity. In the simplest explanation of observations, a fraction of the medium becomes, at low temperatures, a superfluid that decouples from the measurement apparatus. However, the condensate fraction that is required for such an explanation to account for the data does not simply conform with thermodynamic measurement [12]. Rittner and Reppy [13] further found that the measured putative supersolid-type feature is acutely sensitive to the quench rate for solidifying the liquid, while Aoki, Keiderling, and Kojima observed rich hysteresis and memory effects [14]. All of these features can arise from glassy characteristics alone [15, 16]—precisely as in the superglass phase discussed by Biroli et al. It may be that a confluence of both superfluid and glassy features (and their effects on elastic properties of a medium) [17] is at work.

These effects should be observable as experimental consequences of (super-) glassy dynamics, such as disparate relaxation times that could be measured [15]. Typical glass formers indeed typically exhibit relaxations on two different time scales. Cold atom systems may provide another realization of a superglass state. Indeed, a supersolid state of cold atoms in a confining optical lattice was very recently achieved [18]. It is natural to expect a superglass analog of these cold atomic systems.

Superglasses may also have realizations in other areas such as superconductivity and I speculate on these below. For example, consider a lattice version of the continuum system investigated by Biroli et al.: a “lattice superglass.” For charged bosons (e.g., Cooper pairs) on a lattice, such a superglass would correspond to a superconductor with glassy dynamics. In a similar vein, a “lattice supersolid” of Cooper pairs would correspond to a superconductor concomitant with well-defined crystalline (i.e., charge-density wave) order. Indeed, in some heavy fermion compounds as well as in the cuprate and the newly discovered iron arsenide family of high-temperature superconductors [19] there are some indications of nonuniform mesoscale spatial electronic structures and glassy dynamics. Classical glass formers are known to exhibit “dynamical heterogeneities”—a nonuniform distribution of local velocities [20]. I also speculate that “quantum dynamical heterogeneities” may be derived by applying the mapping used by Biroli et al. to classical glass forming systems that exhibit dynamical heterogeneities..

“Spin superglasses” are another possibility. Quantum spin systems in a magnetic field [21] can exhibit a delicate interplay between the formation of singlet states and the tendency of spins to align with the field direction. These systems can be mapped onto a system of bosons with hard-core interactions—just as in the system investigated by Biroli et al. In some spin S=1/2 antiferromagnets in an external magnetic field, triplet states with spins aligned along the field direction can be regarded as hard-core bosons. In many other systems, interactions between quantum spins may also be mapped onto hard-core-type bosonic systems [22, 23]. Invoking these bosonic representations, if a solid or glassy phase appears in a classical Brownian system, then a mapping similar to that of Biroli et al. suggests supersolidity/superglassiness in the corresponding quantum spin system. Recently, there has been much work examining supersolidity in such spin systems, e.g., [23]. It is highly natural to expect new lattice spin superglass counterparts

Finally, even more intriguing superglasses might be possible. In transition-metal compounds, the fractional filling of the 3d atomic shells allows for cooperative orbital ordering [24]. Perhaps low-temperature Bose-condensed glasses of orbitals could appear, forming an orbital superglass. The orbital states may be described by pseudospins [24] that may be mapped to hard-core bosons [22]. The work of Biroli et al. allows us to investigate the possibility of an orbital superglass by knowing the dynamics of hard-core Bose model derived from a classical counterpart. In addition, the classical-to-quantum map of [7] may also suggest a new quantum critical point in related systems. The classical zero temperature jamming transition [25] of hard spheres or disks is a continuous transition with known (dynamical) critical exponents, e.g., [26, 27]. Replicating the mapping used by Biroli et al., we may derive an analog quantum system harboring a zero temperature transition with similar critical exponents. Thus the classical critical point [25, 26, 27] may rear its head anew in the form of “quantum critical jamming.” All of the systems discussed above were free of quenched disorder. Applying the same mapping to classical viscous systems with quenched disorder, we may further arrive at quenched super spin glass analogs of classical spin glass systems [28]. The tantalizing superglass phase may have numerous ramifications.

http://physics.aps.org

The many shapes of spinning drops

noreply@blogger.com (Made Sumariani) — Sun, 7 Dec 2008 10:56:00 +0700

Figure 1: (Top) The bifurcation and stability diagram for rotating liquid drops that are held together by surface tension. The plot depicts dimensionless angular velocity as a function of dimensionless angular momentum. The depicted shapes are stable along the solid lines and unstable along the dashed lines. All shapes with more than 2 lobes are unstable. (Bottom) The experimental setup used by Hill and Eaves. The water droplet is suspended by a magnetic field. The field also exerts a torque on a current that runs between two thin wires embedded in the droplet, thus setting the droplet into rotation. A camera monitors the droplet shape from below.

Liquids, such as water or oil, can form droplets that are held together by surface tension—a cohesive force that causes the surface of many liquids to behave as an elastic membrane. Because liquid droplets are fairly simple to study and control under different conditions, they have fascinated experimentalists and theoreticians—including Laplace, Gauss, Poincarè, Chandrasekhar, Bohr, and Wheeler—for more than two centuries. A droplet on a surface, for example, spreads out or balls up depending on its interaction with the surface. But what happens to the shape of droplets in motion? As a freely suspended droplet rotates at higher and higher velocity, the droplet will tend to get pulled apart and deform. Droplets distort according to a minimum energy principle, always seeking the lowest energy state for a given rotational frequency, and theorists predicted that they would form the series of equilibrium shapes shown in the top of Fig. 1 [1].

To see the full range of shapes shown in Fig. 1 requires a carefully designed experiment. First, the effects of gravity must be removed, since the theoretical predictions assume a freely suspended droplet. Second, the viscous drag on the droplet must be as small as possible. Finally, the droplets must spin at a controllable rate. Writing in Physical Review Letters, Richard Hill and Laurence Eaves of the University of Nottingham in the UK have been able to achieve this by ingeniously combining diamagnetic levitation of water droplets in air with a liquid electric motor technique to spin up the droplets [2]. They are able to observe several different families of rotating water droplets, compare with theoretical expectations, and even record the centimeter-sized droplets in several movies. Because liquid drop models are used to describe systems like atomic nuclei and black holes that cannot be controlled in the laboratory, their work opens the way to a whole new series of measurements of direct interest to several branches of physics.

The first experimental investigations of rotating fluid droplets were conducted by a blind Belgian physicist named Joseph-Antoine-Ferdinand Plateau [3] (Plateau, who also performed experiments with light, once kept his naked eyes fixed for too long on the sun, which brought on a choroid inflammation and total blindness in 1843). Plateau suspended olive oil in a mixture of water and alcohol that had the same density as the oil, thereby balancing gravity with the buoyant force and leaving the droplet effectively weightless. Starting with a nonrotating drop of spherical shape, Plateau found that as he rotated the droplet at higher and higher speeds, the droplet transformed from a spherical shape (at zero rotation), to axisymmetric shapes (at slow speeds), to ellipsoidal and 2-lobed “peanut” shapes, and finally a toroidal shape at very large rotational speeds. These are the same shapes shown in Fig. 1, which were fully understood theoretically about 100 years after Plateau’s experiments.

Unfortunately, in Plateau’s setup the surrounding fluid exerted a large viscous drag on the rotating droplets, which induced unwanted shape deformations. The droplets were also hard to control and to spin, therefore comparisons with theoretical results were mainly qualitative. More recently, precise experiments with silicone oil droplets have been performed in orbiting spacecraft [4], and it has been possible to see the 2-lobed droplet shape. Unfortunately, special care is needed to control and spin up the droplet using acoustic pressure waves, not to mention that it requires going into space!

Hill and Eaves found an innovative way to compensate for the effects of gravity and to make the droplets spin. They solved the gravity problem by using diamagnetic levitation (Fig. 1, bottom). Many substances, including water, are diamagnetic. When acted upon by external magnetic fields, the electrons in diamagnetic materials rearrange themselves creating small currents, which oppose the external field. In a magnetic field that is not constant in space, a diamagnet will always seek the lower field strength. Thus, diamagnetic substances, despite being weakly magnetic, can easily be levitated if the exterior magnetic field has high enough intensity and a well-defined gradient (in fact, even animals, such as frogs, can be levitated diamagnetically [5]).

To set the droplet into rotation, the Nottingham group designed a “liquid electric motor,” also shown in the bottom part of Fig. 1. They inserted two thin wire electrodes in the droplet, with one wire aligned with the droplet center and with the magnetic field. The other wire was set parallel to the first a distance d away. A current I was set to flow through these wires. It follows that a Lorentz force IdB acts on the droplet, causing a torque of IBd2/2 on the droplet that accelerates it to rotate around the vertical axis with a frequency that can be easily controlled by adjusting the current.

There is one final catch to the experiment: the off-axis electrode can excite small amplitude waves on the surface of the droplet, like a boat moving on a water surface. These excited waves have an important effect, in that they can help stabilize otherwise unstable configurations [6]. For instance, 3-lobed configurations cannot usually be observed since they are unstable and rapidly decay into another configuration (typically the 2-lobed shapes). However, if the small waves excited by the off-axis electrode are in resonance with this configuration, they are able to sustain 3-lobed configurations far longer than expected. As the rotation of the droplet increases, it progresses through an elliptical (not shown in Fig. 1) and then a 2-lobed shape. As the rotation increases even further, the droplet assumes a 3-lobed configuration, which was measured accurately for the first time in this experiment. It turns out that the critical rotational frequency at which 2 and 3-lobed shapes start to develop agrees very well with theoretical predictions (see Video 1 and 2). A sequence of beautiful movies of this experiment can be seen at http://netserver.aip.org/cgi-bin/epaps?ID=E-PRLTAO-101-065848).

Attempts to understand rotating fluid droplets have been more than an academic exercise. Since surface tension is a cohesive force, it can be used to model other cohesive forces, such as gravity, that hold together stars and planets, and the nuclear interactions that hold together nuclei [7, 8]. Higher dimensional objects (that is, objects which only exist in a large number of space-time dimensions) with an event horizon seem to behave in general as fluid membranes [7, 9]. Recently, the breakup of extended black-hole-like objects that are called “black strings” was explained in terms of the same classical fluid mechanics instability that is responsible for the breakup of fluid jets in a water tap, or for the formation of water droplets in rain [7, 10]. Simultaneously, it has been understood that many field theories have a hydrodynamic description in the limit of high energy density [11]. If we combine this description with the now famous gravity/quantum field duality [12], we are left with a gravity/hydrodynamic duality, where fluids are literally dual to black holes [13]. Therefore droplets, fluid cylinders, and other fluid configurations could teach us how gravity behaves, with the experimental solution of Hill and Eaves providing a simple way to investigate these exciting issues in table-top experiments.

http://physics.aps.org

High-energy physics in a new guise

noreply@blogger.com (Made Sumariani) — Fri, 5 Dec 2008 15:42:00 +0700

Figure 1: A magnetoelectric effect in a topological insulator. (Left) A quantum Hall effect occurs without strong magnetic field when an electric field applied in the plane of the interface between a topological (red region) and an ordinary (blue region) insulator (or vacuum) induces a precisely quantized current perpendicular to the field. (Right) A magnetic field applied perpendicular to the same interface introduces (n+1/2) electrons for each flux quantum of applied field. The shaded region corresponds to the charge density, ρ, of the electrons, which mainly concentrates around the boundary between the two insulators and is largest where the magnetic field is strongest.

In a prescient but until recently largely forgotten 1987 paper, Frank Wilczek [1] analyzed the effect of hypothetical elementary particles called “axions” on the laws of electricity and magnetism. Axions had been postulated in 1977 in an attempt to explain the absence of charge-parity (CP) violation in the strong interaction between quarks. Wilczek showed that the electrodynamics of axions can be described if one adds a term of the form ΔLaxion=θ(e2/2πhc)B⋅E to the ordinary Maxwell Lagrangian that governs the behavior of the electromagnetic field, where θ describes the strength of the axion field. Such a term is allowed by symmetry, but causes nontrivial modifications to Maxwell’s equations.

As far as we know today, axions do not occur in empty space, and the electrodynamics of these particles appeared to have gone down in history as an interesting curiosity, not relevant to the universe we live in. In a paper appearing in Physical Review B, however, Xiao-Liang Qi, Tayor Hughes, and Shou-Cheng Zhang of Stanford University [2] show that a term ΔLaxion, analogous to what was predicted in high-energy physics, is present in the theoretical description of a class of crystalline solids called topological insulators. The existence of topological insulators—materials characterized by a bulk energy gap and the presence of conducting surface states that are robust (or “topologically protected”) to impurities and defects—had been predicted in a series of recent theoretical works [3, 4, 5] and was confirmed experimentally this year in the semiconducting alloy Bi1-xSbx by a group at Princeton [6]. All of these developments have propelled axion electrodynamics from an idle curiosity to an experimentally observable reality. Aside from establishing the axion term, Qi et al. [2] provide a number of important insights into the physics of topological insulators and make connections with other known topological states of matter, notably the quantum Hall liquids.

Wilczek showed that the axion term has two important consequences: it modifies Gauss’ law by adding to the source term an extra charge density, so that ∇⋅E=ρ becomes ∇⋅E=ρ-(e2/2πhc)∇θ⋅B, and revises Ampère’s law by contributing an additional current density, so that ∇×B=∂tE+j becomes ∇×B=∂tE+j+(e2/2πhc)(∇θ×E+∂tθB).

The extra charge and current density only appear when the quantity θ varies in space or time. Of course, θ in crystalline solids describes something much different than the original “axions” that were hypothesized in high-energy physics. The Stanford group show that, provided the electrons’ equations of motion are time-reversal invariant, all three-dimensional insulating solids can be characterized by a quantized value of the axion field in their bulk: θ=2πn, with n integer in ordinary insulators, while θ=2π(n+1/2) in topological insulators. It follows then that θ must vary near any boundary between two insulators characterized by different bulk values of θ, and the effects of axion electrodynamics should become apparent in that region of space.

There is an important subtlety in the above classification of time-reversal invariant insulators. It turns out that while the boundary between either two ordinary or two topological insulators does not necessarily exhibit interesting behavior vis-à-vis axion electrodynamics, the boundary between a topological insulator and an ordinary insulator (or the vacuum, which presumably has θ=0) is a very special place where Maxwell laws do not hold in their conventional form.

The modifications to Maxwell’s equations give rise to some very interesting phenomena illustrated in Fig. 1. Perhaps the most remarkable of these is the finding that the boundary between a topological insulator and a normal insulator exhibits a quantum Hall effect: when an electric field E is applied in the plane of the boundary, a current flows in the direction perpendicular to E (Fig. 1, left). This is a direct consequence of the modification to Ampère’s law. This Hall current is dissipationless, a property that could be potentially useful in future electronic devices. The magnitude of the current is given by σHE with the “Hall” conductance quantized as σH=(e2/h)(n+1/2). The factor of 1/2 means that the surface exhibits a fractional quantum Hall effect. It is also notable that, unlike what is found in two-dimensional electron gasses, the Hall effect occurs here in the absence of a strong external magnetic field (although a weak magnetic field, or another time-reversal breaking perturbation is needed to determine the direction of the Hall current [1, 2]).

Similarly, according to the modified Gauss’ law, a magnetic field applied perpendicular to the plane of the surface leads to accumulation of charge (Fig. 1, right). The total accumulated charge corresponds to e(n+1/2) per flux quantum of the applied field. Thus, interestingly, one can think of a fractional charge, equal to a half-integer number of electrons, as being bound to each flux quantum. In this context, one can hypothesize that, in analogy with the fractional quantum Hall states, these flux-charge composites will exhibit fractional exchange statistics and thus be potentially useful in schemes that seek to implement fault-tolerant quantum computation [7, 8]. Finally, the surface of a topological insulator can rotate the polarization vector of reflected light (Kerr effect) and would be another experimental signature of axion electrodynamics in these materials.

What is the microscopic picture for these effects? The key ingredient is the presence of strong spin-orbit coupling, which typically occurs in crystalline solids that are made from the heavier elements, such as Pb and Bi. Under the right conditions, the spin-orbit terms can give rise to anomalous band structure that can in turn support topologically robust gapless states at the surfaces of a bulk insulator. This means that bands associated with the surface states are guaranteed to meet at a certain number of points in the surface Brillouin zone, while the bulk bands remain separated by a gap. These surface states are chiral—specifically, the electron spin and momentum are aligned—and at low energies resemble states of massless Dirac fermions, now familiar from the physics of graphene.

The precise definition of the topological insulator involves counting these surface states: an odd number corresponds to a topological insulator, whereas an even number implies ordinariness [3, 4]. That number N is also related to θ from the above discussion of axion electrodynamics. Specifically, it holds that N=θ/π. Remarkably, whether N is odd or even depends only on the bulk properties of the insulator. This situation is reminiscent of the integer quantum Hall systems where the number of chiral edge states is determined by the bulk Hall conductance. In fact, Qi et al. make this connection more transparent by deriving the topological invariant of a three-dimensional insulator from the fictitious four-dimensional quantum Hall system employing an ingenious procedure of dimensional reduction.

Having an odd number of chiral Dirac fermions at a surface is itself odd. In fact, the famous Nielsen-Ninomyia “no-go” theorem [9] states that under very general conditions such Dirac fermions must always come in pairs of opposite chirality, e.g., as they do in graphene. A topological insulator evades this theorem by spatially separating the states of opposite chirality so that they appear on its opposite surfaces. In this way the three-dimensional system as a whole satisfies the no-go theorem but its surfaces, when viewed in isolation, seemingly violate it. Qi et al. explain how the axion phenomenology discussed above arises at the microscopic level from the odd number of topologically robust chiral surface states.

The anomalous surface states that are characteristic of topological insulators have other possible applications that go beyond axion electrodynamics. An example is the proposal put forward by Fu and Kane [10] to pair the surface electrons of a topological insulator into a superconducting state by means of the proximity effect. Owing to the chiral nature of the surface electrons, this superconducting state has unusual topological excitations, with just the right properties to be potential fundamental blocks for quantum computers [7].

The unusual phenomenology of topological insulators arises from the interplay between their unique band structure and the well understood physics of spin-orbit coupling. Thus, remarkably, these systems can be thought of as weakly interacting in the sense that electron-electron interactions play no significant role. Ever since Anderson’s famous 1972 essay “More is different” [11], no condensed-matter physicist has doubted the virtually limitless potential for discovery of new phenomena in interacting quantum many-body systems. That something radically new can appear even in noninteracting systems came both as a great surprise and a promise that profound discoveries can be made where no one expects them.

http://physics.aps.org

Anti-Matter Goldmine

noreply@blogger.com (Made Sumariani) — Tue, 2 Dec 2008 16:51:00 +0700

Billions of anti-matter particles were recently let loose at Lawrence Livermore National Laboratory. Using a short-pulse laser, a team of researchers figured out how to produce anti-electrons or positrons faster and in greater density than ever before in the laboratory.

While positrons were the only form of anti-matter produced in the experiment, not all anti-matter particles are positrons. Every particle has its own corresponding, oppositely charged anti-particle (check out last month's post on anti-matter).

The researchers struck gold; literally. By shooting a laser through a gold sample the size of the head of a push pin, approximately 100 billion positron particles were generated, shooting out of the sample in a cone-shaped plasma "jet".

Accelerated and ionized or charged by the laser, electrons plough through the gold sample, hitting gold nuclei along the way. The electron-gold nuclei interactions serve as a catalyst to create positrons, kind of like how fertilizer assists in the growth of plants. The laser is able to produce large quantities of positrons by concentrating the energy given off by electrons in space and time.

This new ability to create enormous amounts of positrons in the lab is significant-it could one day lead to discoveries explaining why more matter than anti-matter survived the Big Bang at the nascent of the universe. That is, answer the question of why we are made of matter and not anti-matter!

www.physicscentral.com

Operatic Atom Bombs

noreply@blogger.com (Made Sumariani) — Thu, 27 Nov 2008 08:42:00 +0700

I'd wager the average person rarely (if ever), spends a Friday evening indulging in Die Meistersinger von Nurnberg, but operas aren't always long-winded scenes of voluptuous, ornately dressed characters bellowing incomprehensibly.

Producer John Adam's Doctor Atomic is a two-act opera about the making of the Atom Bomb, the nuclear weapon that was eventually dropped on Hiroshima and Nagasaki near the end of World War II.

The setting is the summer of 1945, in the desert of Los Alamos, New Mexico, where J. Robert Oppenheimer and a team of scientists gathered to build and test the bomb for the first time.

The opera focuses on renowned physicist Robert Oppenheimer and his scientific and moral dilemma surrounding the Los Alamos project-with lots of science thrown in. Created from various sources ( including declassified government documents), the text or libretto of the opera is littered with discussions on uranium and plutonium, the TNT equivalency of the bomb, and whether or not a test explosion might set the atmosphere on fire-an indisputably bad scenario.

In addition to being a brilliant theoretical physicist (he received his PhD at age 22), Oppenheimer loved the arts and culture; it is widely know that he decided to learn Sanskrit in order to read the Bhagavad Gita. He was appointed scientific director of the Manhattan Project after years of contributions to quantum mechanics, nuclear physics, spectroscopy, and astrophysics. He was first to publish a paper in the 1930s suggesting the existence of what now call black holes.

Atomic energy is created by the splitting (fission) or joining (fusion) of atoms- but only by using specific isotopes of uranium or plutonium can a massively destructive explosion be reached. The two atomic bombs detonated over Hiroshima and Nagasaki relied on fission.

Elements undergoing fission ( for example uranium) release neutrons. Some neutrons are scooped up by other uranium nuclei leading to more fission, while others escape the process altogether. If the expected number of neutrons which trigger new fissions is less than 1, a nuclear chain reaction may occur but the size will decrease exponentially.

If the expected number of neutrons is greater than 1, the chain reaction will increase exponentially. The term 'critical mass' describes the point at which the expected number of neutrons causing fission is 1 or more, thus becoming a self-sustaining chain reaction.

The bomb released over Hiroshima used TNT to blow subcritical masses of uranium 235 together, resulting in a 10 kiloton explosion. "Fat man", the bomb used against Nagasaki was a subcritical mass of plutonium 239 squeezed to bit by TNT and causing a 20 kiloton explosion.

If you find yourself in the mood for an operatic pondering of nuclear fusion and fission, Dr. Atomic will finish a run at the Metropolitan Opera in New York Thursday November 12, ad then travel to London at the English National Opera.

www.physicscentral.com

Mysteriously Speedy Dolphins: Gray's Paradox Solved

noreply@blogger.com (Made Sumariani) — Wed, 26 Nov 2008 14:47:00 +0700

Remember Gray's paradox? In 1936 the eponymous British zoologist James Gray couldn't reconcile his observations of dolphins swimming at speeds of over 20 miles per hour with his calculations, which demonstrated that dolphin muscles simply weren't built to produce enough acceleration to overcome drag. He ended up blaming this drag violation on dolphin's skin, postulating that it must have drag-reducing properties.

Fast forward decades later to this year's Annual Meeting of the American Physical Society (APS) Division of Fluid Dynamics in San Antonio, Texas, where professor Timothy Wei of Rensselaer School of Engineering announced that he and a team of researchers had solved Gray's paradox- and no, skin has nothing to do with the speediness of these adorable sea mammals.

Wei and his team are the first to provide solid evidence illustrating that dolphins actually do produce enough force to overcome drag. "The scientific community has known this for a while, but this is the first time anyone has been able to actually quantitatively measure the force and say, for certain, the paradox is solved, said Wei in a Rensselaer Polytechnic Institute press release.

Using combined force measurement tools developed for aerospace research with Digital Particle Image Velocimetry (a video-based measurement technique that captures 1,000 video frames per second), Wei tracked two bottlenose dolphins, Primo and Pula, as they swam through water heavily populated with tiny air bubbles.

The color-coded results showed the speed and direction of water flow around and behind the each dolphin, enabling the researchers o calculate precisely how much force was produced.

Turns out they produce way more force that Gray ever imagined- approximately 200 pounds of force is created by tail flapping ( in contrast, Olympic swimmers only generate 60-70 pounds of force at top speed). Good thing dolphins are considered harmless!

www.physicscentral.com

Snake Power

noreply@blogger.com (Made Sumariani) — Sat, 22 Nov 2008 13:44:00 +0700

Anyone who’s ever tried to defend a sand castle against the onslaught of a rising tide will have some notion of the enormous energy carried by the waves crashing onto our shores. It has been estimated that wave power could supply a quarter of UK energy demand, yet converting it into electricity is more difficult than it looks.

Current research efforts have branched out into many directions, each seeking to tame the ocean into an efficient commercially viable energy source. One such approach is Anaconda, an innovative wave power system invented by physicist Francis Farley and engineer Rod Rainey.

As its name suggests, Anaconda’s shape is reminiscent of a giant snake, consisting of a rubber tube 200 m long and 7 m across, filled with water and tethered below the ocean surface. Unlike its reptilian cousin however, its habitat is not the Amazon river but the UK coastline.

‘As a wave goes by and passes over the tube, it instigates another wave inside the tube, called a bulge wave,’ explains Professor Grant Hearn, who has been entrusted with the development of Anaconda along with fellow University of Southampton researcher John Chaplin.

The bulge wave stretches out the elastic walls of the tube and, as the rubber regains it shape, is pushed along its length, exactly like the pulses of blood travelling down your arteries. The energy from this wave could then be converted into electricity, for example by using a water turbine at the far end of the tube.

Thinking outside the box

‘You’re essentially using the natural resonance of fluid to transport energy,’ comments Hearn. In this way, Anaconda offers a startingly fresh approach. ‘All the other structures tend to use articulations of some sort, and you exploit the relative motion due to that articulation,’ he adds. The Salter duck for example, invented in the 1970s, uses the motion of ‘ducks’ bobbing up and down on the surface of the water.

Despite highly efficient conversion rates, other hurdles have kept the Salter duck out of water. Constantly buffeted by waves and immersed in salty water, wave power devices tend to suffer from short life spans. Anaconda neatly sidesteps this issue by having no articulations to wear down or parts to fail. ‘Rubber is a very resilient material, so it’s not likely to suffer fatigue in the same way that concrete or metal might, ’ says Hearn.

Soaring costs are also a major headache for engineers, but Anaconda’s simple rubber structure has the additional advantage of making it relatively cheap to produce.

In many ways Anaconda seems to outsmart rival technologies, but Hearn remains pragmatic about its merits, seeing it instead as just one of many innovations that will be necessary to build the perfect wave power system. ‘Each exploits a different technique, but eventually a mixture of these things will come together,’ he says.

Hearn and his team still have a lot of fine tuning to do to maximise Anaconda’s efficiency, but if all goes to plan the sea serpent might be rearing its head on the English or Scottish coast in five years' time. Looks like Nessie too might be facing some tough competition.

www.physics.org

The Restaurant at the End of the Universe

noreply@blogger.com (Made Sumariani) — Fri, 14 Nov 2008 15:30:00 +0700

What will be in astronauts' lunch boxes when they go to Mars? Soggy sandwiches, squishy bananas and crisps are unlikely to make the cut.

‘If we go to Mars, that’s a two and a half year mission, so the food would need a five year shelf life,’ explains Dr Michele Perchonok, a food scientist at NASA’s Johnson Space Center in Houston.

There is no refrigeration on board spaceships, so harmful bacteria will have to be zapped before take-off using high pressure processing or microwave sterilisation. ‘With these emerging preservation technologies, foods have a much higher quality once they’ve been processed. And with higher quality comes higher nutrition,’ adds Perchonok.

Saving space is another major consideration. Visitors to Mars will need to pack light and keeping the contents of the food cupboard down to a minimum is key. ‘We want to make things as compact as possible and as lightweight as possible,’ says Perchonok. An even bigger challenge however is ensuring that the food tastes good.

Staying down to Earth

Space travel may summon up thoughts of strange, futuristic food stuffs, but Perchonok’s team strive to ensure that astronauts’ meals taste like they would back on Earth. ‘As missions have got longer and longer, we’ve started developing foods that are closer to home,’ says Perchonok. So astronauts are more likely to be slurping up spaghetti than downing weird space shakes.

Serving food in zero gravity can still hold a few surprises. ‘The crew get to try the food before departing, but we do receive reports that once in orbit it doesn’t taste the same as on Earth,’ comments Perchonok. ‘One reason is that you are in weightlessness and so hot air does not rise. About 85 to 90% of what you taste is really what you smell so if you can’t smell these aromas you may not taste as much.’

Mars bars

As well as their pick of the standard menu items, homesick astronauts are allowed to bring a stash of their favourite Earth treats, as long as these can be stored and consumed safely. Unfortunately for some, this means no crisps or biscuits. ‘We can’t have any crumbs on board,’ says Perchonok. ‘Crumbs can not only go into the equipment and clog up filters but could also get into your nose or your eyes.’

Despite crisp deprivation, today’s astronauts should still consider themselves lucky compared to their predecessors, who endured sandwiches compressed into bite sized cubes and pureed dishes similar to baby food. ‘Now astronauts have around 160 to 180 unique items which they can choose from,’ says Perchonok.

In coming decades, space crew could be treated to the ultimate luxury on Mars – fresh food. Researchers believe that crops such as mizuna lettuce, wheat or soy beans could be grown on Mars. And who knows, if we do find alien life on Mars, it might even be quite tasty.

www.physics.org

Physics is quantitative

noreply@blogger.com (Made Sumariani) — Fri, 31 Oct 2008 21:41:00 +0700

Physics is more quantitative than most other sciences. That is, many of the observations experimental results in physics are numerical measurements. Most of the theories in physics use mathematics to express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed are more amenable to quantitative approaches than other areas. Physical definitions, models and theories can often be expressed using mathematical relations, as early as 1638, when Galileo published the law of falling bodies in his Two New Sciences.

A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by comparing the predictions of its theories with data procured from observations and experimentation, whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.

Relation to mathematics and the other sciences
Physics relies on mathematics to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.

Whenever analytic solutions are not feasible, numerical analysis and simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.

Beyond the known universe, the field of theoretical physics also deals with hypothetical issues, such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatter rather than matter.

In the Assayer (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences, as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (called in this case quantum chemistry), thermodynamics, and electromagnetism.

wikipedia.org

Applications and influence

noreply@blogger.com (Made Sumariani) — Tue, 28 Oct 2008 21:41:00 +0700

Applied physics is a general term for physics which is intended for a particular use. An applied physics curriculum usually contains a few classes from the applied disciplines, like chemistry, computer science, or electrical engineering. It usually differs from engineering in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of applied mathematics. Applied physicists can also be interested the use of physics for scientific research. For instance, people working on accelerator physics might seek to build better particle detectors for research in theoretical physics.

Physics is used heavily in engineering. Statics, a subfield of mechanics, is used in the building of bridges or other structures; the simple machines such as the lever and the ramp had to be discovered before they could be used; today, they can be taught to schoolchildren. The understanding and use of acoustics will result in better concert halls; similarly, the use of optics creates better optical devices. An understanding of physics makes for more realistic flight simulators, video games, and movies, as well as in forensic investigations (what do we know and when do we know it; what did the subject know and when did the subject know it).

Because of its historical relationship to the development of scientific method, physics reasoning can handle items which would ordinarily be mired in conundrums or uncertainty. For example, in the study of the origin of the Earth, one can reasonably model Earth's mass, temperature, and rate of rotation, over time. From these values, the chemical composition of Earth at differing epochs can be posited. Even if a precise linear timeline might be problematic, qualitative statements can then be made about the history of Earth, which are still founded in the laws of physics.

There are many fields of physics which have strong applied branches, as well as many related and overlapping fields from other disciplines that are closely related to applied physics.

Wikipedia.com

Molecules at the movies

noreply@blogger.com (Made Sumariani) — Sun, 19 Oct 2008 20:12:00 +0700

Atoms and molecules are not the most photogenic of subjects. Once you’ve zoomed in on their miniscule size, you still have to find a way of snapping electrons hurtling around at breakneck speeds of up to 10 million km/h.

Physicists at Imperial College London have however made hydrogen and methane molecules into unsuspecting movie stars thanks to a revolutionary new measurement technique.

Using a process known as high harmonic generation (HHG), this technique can capture ‘films’ of chemical processes at timescales that almost defy the human imagination.

The principle bears similarities to strobe photography, where rapid motion is freeze-framed by leaving the camera’s shutter open in a darkened environment and then producing a bright flash of light. The shorter this flash, the faster the motion that can be recorded.

Freezing the movement of a speeding bullet, for example, requires a flash of light no longer than 1 microsecond (see image above).

In HHG, pulses of laser are fired into a jet of gas and the high intensity wave of light sweeps an electron off each atom. But the electron quickly smashes back into the resulting ion. ‘In this collision, the electron’s kinetic energy is released in an extremely short but very bright burst of x-ray radiation,’ says Dr John Tisch, who worked with Imperial College colleagues Dr Sarah Baker and Professor Jon Marangos to develop this new technique.

Blink and you’ll miss it

This x-ray ‘flash’ can be squeezed down to a staggering 100 attoseconds long - pretty impressive when you consider that in the time it takes for you to blink, a whopping 300 million billion attoseconds have already slipped by.

By analysing certain characteristics of the x-ray beam, researchers build up a ‘movie’ of the molecule, for example as it changes its structure after being rapidly ionised. The time interval between the individual ‘frames’ in these ‘movies’ is also 100 attoseconds - resulting in an effective framing-rate of 10 million billion frames per second, compared to around 2000 frames per second for the super-slow motion used in TV sports coverage.

This new measurement system and related techniques being developed by a number of groups around the world could revolutionise our fundamental understanding across science.

‘The kind of dynamics that we are now beginning to be able to capture control the function of many molecules', comments Dr Tisch. And putting more complex molecules under the spotlight in years to come could yield ‘footage’ of processes that have never been seen before.

So it might not be long before scientists attend the world premiere screening of photosynthesis or entangled electron states. Such understanding could help to create quantum computers, faster electronics, designer materials or artificial photosynthesis.

Imperial College’s physicists may be the current world record holders for the fastest ever observation of molecular dynamics, but, as Dr Tisch puts it, ‘the relentless drive for shorter and shorter measurement techniques’ is still on. With molecules swiping all the leading roles, it’s clear that slow motion takes are truly no longer the preserve of film stars and football players.

www.physics.org

Boobs, babes and blood

noreply@blogger.com (Made Sumariani) — Wed, 15 Oct 2008 18:44:00 +0700

For the average physicist, you might expect typical study subjects to include atoms or distant galaxies. Visit the medical physics department at University College London however, and you’d be more likely to stumble upon premature babies or breast cancer patients in the lab.

Dr Adam Gibson and his team are developing a new medical imaging technique, known as optical tomography, which uses near infrared light to take a peek at what blood is getting up to inside the human body.

The instrument, called MONSTIR, shines light through the body and counts individual photons as they emerge at the other side. ‘Blood absorbs light very strongly,’ says Gibson. ‘So the amount of light that gets through is an indicator of the amount of blood.’

By using two different wavelengths of light and comparing how the two are absorbed, the colour of the blood - a telltale sign of its oxygen levels - can also be deduced. ‘Bright red, tomato ketchup blood carries lots of oxygen, and dull, brown blood has less,’ explains Gibson.

In itself, this is already an exciting development for medical imaging. Most imaging techniques take a snapshot of the body which then needs to be interpreted, but MONSTIR’s images provide not just pretty pictures but hard numbers.

A rush of blood to the head

Oxygenated blood fuels many processes in the body and following its path can deliver precious insight to doctors. The team at UCL has already used MONSTIR to explore what’s on the mind of premature babies. ‘The part of the brain that’s active uses more blood,’ explains Gibson. ‘So you can use this to map out brain activity.’

As well as investigating babies’ perception of pain and the way they learn language, the team have used the technique to detect bleeding inside the brain.

Breast cancer patients have also come under MONSTIR’s scrutiny. ‘Cancer is associated with an increase in blood volume to the tumour, and sometimes also changes in blood oxygenation,‘ says Gibson.

Whilst the relatively poor resolution of the images produced means the technique is not best suited to screening for cancer, it has advantages over alternative imaging techniques for other applications in the fight against cancer. In particular, MONSTIR could come in handy for monitoring the effectiveness of a particular drug which aims to starve off cancer by disrupting blood flow to the tumour.

‘With x-rays, you get ionising radiation, and MRI is big, expensive and clumsy, so it’s not convenient to use it over and over again, whereas we can use this as often as we want to,’ comments Gibson.

Finding parents willing to lend their newborn babies for a day can sometimes be harder than spotting far flung planets, but Gibson wouldn’t have it any other way. ‘One of the nice things about medical physics is the patient contact. It’s nice for us to be reminded of what we’re doing our research for.’

www.physics.org

Enigma of the singing dunes

noreply@blogger.com (Made Sumariani) — Sun, 5 Oct 2008 21:08:00 +0700

When the thirteenth century explorer Marco Polo encountered the weird and wonderful noises made by desert sand dunes, he attributed them to evil spirits. But 700 years later, scientists still don’t completely understand the causes of this eerie phenomenon.

Sand dunes can be heard ‘singing’ in more than 30 locations worldwide, and in each place the sounds have their own characteristic frequency, or note. In reality the sounds produced are less like singing and more like a low-frequency drone (low frequency corresponds to low notes; bass as opposed to treble). The sounds are emitted when sand cascades down the face of a dune in an avalanche, the cause of which can be the wind, people walking on the top of the dune or even sliding down it.

In 2001 a team of French physicists, including Stéphane Douady and Bruno Andreotti, went to Morocco to study the shape and motion of sand dunes. They became fascinated by the singing of the dunes and began to investigate it in addition to their other research. They found that avalanches they triggered manually produced the same sound as those that occurred naturally, which suggests that the wind doesn’t play a part. They also concluded that the sound is not produced by the dune resonating, as happens in the case of a musical instrument for example, because the frequency of the sound produced is the same for different sizes of dune. Thus the team focused their investigation on the motion of the sand grains, rather than on the properties of the entire dune.

Douady and Andreotti both came up with the idea that the sounds must be produced by sand grains becoming synchronised – moving in definite patterns as they move down the surface of the dune. Their hypotheses differed in that Douady believed the sounds, which after all are just vibrations of air molecules, were produced by air being squeezed out from between the synchronised grains. Andreotti proposed that the sound was due to the surface of the avalanche vibrating the air around it like a large hi-fi speaker. The pair began to follow very different lines of inquiry and ended up in complete disagreement. This, combined with a subsequent quarrel over how best to publish their findings, led to the two researchers falling out. So much so, in fact, that they now avoid each other, despite working in the same small field of physics. Their scientific adventures and disagreements were the subject of an award-winning article in the November 2006 edition of Physics World, the Institute of Physics members’ magazine.

There is still no consensus as to the exact explanation of the singing dunes. In fact, scientific papers published more recently suggest that the large-scale structure of the dune does play a part after all. The story of the differences between Douady and Andreotti, and the twists and turns in the wider investigation into the exact cause of the dunes’ song, highlight the fact that the progress of science can be affected by the vagaries of human nature. For example, why does a scientist choose a certain path of investigation? That’s something that your average physics textbook won’t ever answer, even if we do finally get to the bottom of why dunes sing.

www.physics.org

Latest Features

noreply@blogger.com (Made Sumariani) — Sat, 4 Oct 2008 11:11:00 +0700

Keep your ears peeled

What do bats, submarines and doctors have in common? They all rely on sound to ‘see’ better.

Most of the time, our eyes do a pretty good job of telling us about our surroundings. Seeing in the dark or in murky water is much trickier, but bats and dolphins have found a way around this problem, relying on sound waves to find their bearings and even to catch prey. The secret behind this ability lies in an everyday phenomenon - the echo.

All about echoes

You’re in a cave. Shout. A moment later you hear your voice’s ghostly twin - an echo. This happens because the sound waves produced by your vocal cords travel through the air, bounce off the cave walls and are picked up again by your ears.

Sound travels relatively slowly, about 343 metres per second in air, which is why there’s a slight delay before you hear the echo. But this delay is what allows bats to make use of echos.

Bats have vocal cords and ears that are fine-tuned for producing and hearing very high frequencies of sound, known as ultrasound. They emit high-pitched ‘chirps’, which are inaudible to humans, and then listen out for the resulting echos. Keeping track of how long it takes for each cry to be reflected back enables them to work out how far away an object is.

Other characteristics of the echo can also give bats an idea of the object’s size and the direction it’s moving in. By piecing together this information, they can build up an accurate image of their surroundings, and spot tasty insects in the dark.

Ultrasound waves enable bats to get a clear picture of their environment as they don’t spread out around obstacles as much as lower frequency sounds like the human voice. Imagine trying to find your way around a dark cave just by listening to the echos of your shouts.

Sound in medicine

It wasn’t long before scientists caught on to this nifty idea and adapted it to enable submarines to locate their targets and to give doctors a peek into their patients’ bodies.

Ultrasound imaging has been used in medicine for over 50 years. Specialised equipment directs pulses of high frequency sound waves into the body and, as these waves meet different layers of tissue, they are reflected back and analysed.

Millions of pulses and echoes are produced and received each second, providing information which is then processed to create an image on screen. Ultrasound scans are commonly used to check on unborn babies as well as muscles and internal organs including the heart, liver and kidneys, without harming the patient.

The applications of ultrasound don’t end there. Adapting the frequency and intensity of the waves makes them suitable for a multitude of uses, including cleaning teeth, locating oil reservoirs, destroying cancerous tumours and killing bacteria in water.

With all that high pitched racket going on, we should probably be grateful that we can’t hear ultrasound.

www.physics.org

Setting hearts aflutter

noreply@blogger.com (Made Sumariani) — Fri, 3 Oct 2008 11:40:00 +0700

Rappers and butterflies agree on at least one point: when you want to deliver a show-stopping performance, the more bling the better. While rappers have taken a shine to the glitz and glamour of diamonds, some butterflies, birds and beetles have evolved iridescent, luminous colours of their own.

Most of the colours we see in nature are produced by pigments, but a completely different mechanism underlies the shimmering hues of iridescent butterflies. Robert Hooke and Isaac Newton were the first to hypothesise that these creatures possessed miniscule physical structures which manipulated light. Fast forward a few hundred years and researchers are still busy unravelling the mysteries of what is known as structural colour.

Standing out from the crowd

A typical iridescent butterfly’s wings are carpeted in hundreds of thousands of tiny scales. ‘These scales are anything from 1 to 3 microns thick and laid out like roof tiles,’ explains Dr Peter Vukusic, a physicist who studies iridescence in nature at the University of Exeter. Each of these scales is in turn composed of several layers of ultra thin film.

‘When light hits the top surface of the film, some of the wave is reflected, and some is transmitted downwards,’ says Vukusic. The transmitted wave then strikes the lower surface of the film and bounces back up, rejoining the light that was reflected from the top surface (see diagram).

This means that the transmitted wave travels further than the reflected wave and, depending on the thickness of the film, the two waves can become out of phase – the peaks and troughs of the waves no longer line up exactly. Any phase difference causes interference between the two waves.

Vukusic compares the process to ocean waves coming into a harbour. Upon meeting, two coinciding crests can form a ‘super crest’, or a crest and a trough can cancel each other out. Similarly, the butterfly’s scales will amplify certain wavelengths of the reflected light, whilst dampening others, affecting the colour that we ultimately see.

More than just producing unusual shades, structural colours bring added intensity. ‘Pigmentary colour is often not very bright, as incoming light is not reflected in any particular direction’, says Vukusic. Structural colour, on the other hand, directs light, grabbing viewers’ attention with flashes of brilliant colour. Depending on the species, this can serve to attract the opposite sex, intimidate a rival, or ward off predators.

The Butterfly Effect

It may seem that science is finally catching up with nature’s nifty designs, but there is still plenty left to learn. ‘It’s an exciting field to be working in’, says Vukusic. ‘Every single day we are looking at species that have never been investigated before.’

An increased understanding of these systems has already inspired a number of applications, from antifraud devices on banknotes to shimmering eyeshadows. It can only be a matter of time before rappers catch on as well.

www.physics.org

Recreating the Big Bang

noreply@blogger.com (Made Sumariani) — Thu, 2 Oct 2008 11:45:00 +0700

Or almost. Switch on for the Large Hadron Collider at CERN is due to happen on 10 September, 2008, marking the beginning of a physics experiment which intends to recreate conditions last seen a trillionth of a second after the Big Bang.

The experiment involves smashing particles together in a 27 km ring, built 100m underground on the French-Swiss border. Racing at 99.99 per cent of the speed of light, particles will collide at the heart of massive detectors, allowing scientists a fleeting glimpse of tiny subatomic particles which may answer fundamental questions about the building blocks of nature.

As the universe was forming, hadrons bound together to make the larger particles that scientists have been studying since the Greek Democritus coined the term ‘atom’.

We have a relatively clear understanding of some kinds of hadrons like protons and neutrons but many of our universe’s tiniest constituents have yet to reveal their secrets.

One particle which will have theorists sitting on the edge of their seats is the Higgs Boson – if its existence is confirmed then physicists can truly claim to understand the origins of mass: in other words, why some particles are ‘heavier’ than others.

ATLAS is one of the four particle detectors being used to capture images of the fleeting, tiny but very high-energy collisions. ATLAS has been designed to take ‘pictures’ of 600 million proton collisions every second and will be the machine, if physicists are currently correct, that confirms the existence of the Higgs Boson.

Surprises in store

However, other less predictable phenomena are also causing pulses to race as we get closer to switch-on and the day the data from the detectors starts to pour in.

At present, we understand three dimensions: up and down, back and forth, and left and right. Looking into the smallest nooks and crannies of space could reveal extra dimensions, almost unimaginable, which would force us to completely rewrite today’s physics textbooks.

Physicists also hope that the LHC will elucidate some of the mysteries surrounding dark matter. This enigmatic substance makes up the majority of our universe’s mass yet we know very little about it. Physicists have deduced from gravitational effects on visible matter that it’s out there, but only by recreating the conditions present at its birth do we stand a chance of really knowing what it is.

The most exciting findings however are those that cannot yet even be predicted. As Professor Antonio Ereditato, Director of the Laboratory for High Energy Physics in Bern, says, ’This is like opening a window on an unknown view. You expect to see mountains but you may see a sea shore.’
As the LHC steers particles onto a collison course with the equivalent energy of a 400 tonne train travelling at 150 km per hour, the detectors will spew out huge amounts of data. It will then be up to physicists to diligently pore over these results until a mountain, a sea shore or just a very, very Big Bang comes into view.

From : www.physics.org

Large Hadron What?

noreply@blogger.com (Made Sumariani) — Wed, 1 Oct 2008 18:22:00 +0700

It's almost switch on time for the LHC, but how switched on are you? Taking our quiz is the only way to find out...

1. The LHC is housed in a circular tunnel 100 m below the Swiss-French border. Why was it built underground?

a. Because it was cheaper that way.
b. So that particles don't need to get out their passports at border checkpoints on the surface.
c. So that the physicists working on it couldn’t escape.

2. When working on the LHC, how did the physicists get to different points around the 27 km tunnel?

a. By Shetland pony.
b. By teleportation.
c. By bicycle.

3. Which of the following is an LHC particle detector?

a. DICTIONARY.
b. ENCYCLOPEDIA.
c. ATLAS.

4. What revolutionary invention came about at CERN?

a. Sliced bread.
b. The World Wide Web.
c. The toothbrush.

5. Which of these isn’t a real particle?

a. A Sobon.
b. A Sparticle.
c. A Gluon.

6. Quarks are one of the fundamental building blocks of matter, forming protons and neutrons. The different types of quarks are known as ‘flavours’ – what are the six flavours?

a. Left, right, odd, weird, weirder and downright bizarre.
b. Up, down, charm, strange, top and bottom.
c. Vanilla, strawberry, chocolate, lemon, pistachio and mint choc chip.

7. Why does the LHC have to be cooled down to an icy -271C?

a. To operate the superconducting magnets.
b. To save money on the heating bill.
c. To ensure that it’s the coolest experiment ever.

8. The Higgs boson is sometimes nicknamed the God particle because physicists believe it can explain:

a. Why toast always falls jam side down.
b. Why it always rains on Bank holidays.
c. How particles acquire mass.

9. Physicists hope the LHC will help them to find out more about dark matter. What do we know about it so far?

a. It hides under small children’s beds at night.
b. It makes up most of the mass of our universe.
c. It should always be washed at 30C on a delicate cycle.

10. The LHC is the biggest experiment in the world, but how much does it cost the UK each year?

a. The same as a pint of beer per adult.
b. The same as a bottle of vintage Dom Perignon champagne per adult.
c. The same as a glass of tap water per adult.

Correct answers:

1. a
2. c
3. c
4. b
5. a
6. b
7. a
8. c
9. b
10. a

If you scored

- 8 or over: Congratulations! You are the Higgs boson, the pride and joy of the physics community (assuming you do exist).

- Between 5 and 7: You are anti-matter. Produced routinely during LHC particle collisions, you’re pretty popular with CERN researchers, who are counting on you to teach them more about how our universe was formed.

- 4 or less: You are a neutron. Dirt common, to physicists you’re one of most humdrum particles of them all.

From : www.physics.org

Secret of spider-man suit revealed

noreply@blogger.com (Made Sumariani) — Wed, 1 Oct 2008 10:53:00 +0700

Physicists have found the formula for a "spider-man" suit, drawing inspiration from the amazing wall-clinging ability of geckos, and the ingenious properties of velcro.

In a paper for the Institute of Physics' Journal of Physics: Condensed Matter, Professor Nicola Pugno has described how a combination of adhesive forces could be used to make a suit to support the weight of a human body.

The suit would be covered with molecular-sized hooks, which would allow the wearer to cling to a surface and detach themselves easily. This would be used in conjunction with van der Waals forces, the microscopic forces that allow geckos to hang upside-down from ceilings. "There are many interesting applications for our theory, from space exploration and defence to designing gloves and shoes for window cleaners of big skyscrapers" said Pugno.

The theory is all the more significant because, as with spiders' and geckos' feet, the hooks and hairs are self-cleaning and water-resistant. This means that they will not wear or get clogged by bad weather or dirty surfaces and will be able to withstand some of the harshest habitats on earth, including the deep sea.

There are a number of problems that will need to be overcome before the spiderman suit becomes a reality. For example, because human muscles are different to a gecko's, research would have to be done to ensure that the suit can be used without causing muscle fatigue.

"However now that we are this step closer, it may not be long before we see people climbing up the Empire State Building with nothing but sticky shoes and gloves to support them" added Pugno.

From : www.physics.org

1900 to Present

noreply@blogger.com (Made Sumariani) — Wed, 1 Oct 2008 10:41:00 +0700

In 1895, Röntgen discovered X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Marie Curie, Pierre Curie, and others. This initiated the field of nuclear physics.

In 1897, Joseph J. Thomson discovered the electron, the elementary particle which carries electrical current in circuits. In 1904, he proposed the first model of the atom, known as the plum pudding model. (The existence of the atom had been proposed in 1808 by John Dalton.)

These discoveries revealed that the assumption of many physicists that atoms were the basic unit of matter was flawed, and prompted further study into the structure of atoms.

In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. Neutrons, the neutral nuclear constituents, were discovered in 1932 by Chadwick. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during World War II, as research was conducted by each side into nuclear physics, for the purpose of creating a nuclear bomb. The German effort, led by Heisenberg, did not succeed, but the Allied Manhattan Project reached its goal. In America, a team led by Fermi achieved the first man-made nuclear chain reaction in 1942, and in 1945 the world's first nuclear explosive was detonated at Trinity site, near Alamogordo, New Mexico.

In 1900, Max Planck published his explanation of blackbody radiation. This equation assumed that radiators are quantized in nature, which proved to be the opening argument in the edifice that would become quantum mechanics.

Beginning in 1900, Planck, Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Paul Dirac formulated quantum mechanics, which explained the preceding heuristic quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently probabilistic; the theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales. During the 1920s Erwin Schrödinger, Werner Heisenberg, and Max Born were able to formulate a consistent picture of the chemical behavior of matter, a complete theory of the electronic structure of the atom, as a byproduct of the quantum theory.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised in the late 1940s with work by Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga, and Freeman Dyson. They formulated the theory of quantum electrodynamics, which describes the electromagnetic interaction, and successfully explained the Lamb shift. Quantum field theory provided the framework for modern particle physics, which studies fundamental forces and elementary particles.

Chen Ning Yang and Tsung-Dao Lee, in the 1950s, discovered an unexpected asymmetry in the decay of a subatomic particle. In 1954, Yang and Robert Mills then developed a class of gauge theories which provided the framework for understanding the nuclear forces. The theory for the strong nuclear force was first proposed by Murray Gell-Mann. The electroweak force, the unification of the weak nuclear force with electromagnetism, was proposed by Sheldon Lee Glashow, Abdus Salam and Steven Weinberg and confirmed in 1964 by James Watson Cronin and Val Fitch. This led to the so-called Standard Model of particle physics in the 1970s, which successfully describes all the elementary particles observed to date.

Quantum mechanics also provided the theoretical tools for condensed matter physics, whose largest branch is solid state physics. It studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. The transistor was developed by physicists John Bardeen, Walter Houser Brattain and William Bradford Shockley in 1947 at Bell Telephone Laboratories.

The two themes of the 20th century, general relativity and quantum mechanics, appear inconsistent with each other. General relativity describes the universe on the scale of planets and solar systems while quantum mechanics operates on sub-atomic scales. This challenge is being attacked by string theory, which treats spacetime as composed, not of points, but of one-dimensional objects, strings. Strings have properties like a common string (e.g., tension and vibration). The theories yield promising, but not yet testable results. The search for experimental verification of string theory is in progress.

The United Nations declared the year 2005, the centenary of Einstein's annus mirabilis, as the World Year of Physics.

From : www.wikipedia.org

History

noreply@blogger.com (Made Sumariani) — Tue, 30 Sep 2008 13:25:00 +0700

Ancient Times
Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. Another mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Several theories were proposed, the majority of which were disproved. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. On the other hand, the commonly accepted works of Ptolemy and Aristotle are not always found to match everyday observations. There were exceptions and there are anachronisms: for example, Indian philosophers and astronomers gave many correct descriptions in atomism and astronomy, and the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.

Middle Ages
The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the Scientific Revolution of the late 17th century. The precursors to the scientific revolution can be traced back to the important developments made in India and Persia, including the elliptical model of planetary orbits based on the heliocentric solar system of gravitation developed by Indian mathematician-astronomer Aryabhata; the basic ideas of atomic theory developed by Hindu and Jaina philosophers; the theory of light being equivalent to energy particles developed by the Indian Buddhist scholars Dignāga and Dharmakirti; the optical theory of light developed by Arab scientist Alhazen; the Astrolabe invented by the Persian Mohammad al-Fazari; and the significant flaws in the Ptolemaic system pointed out by Persian scientist Nasir al-Din al-Tusi. As the influence of the Islamic Caliphate expanded to Europe, the works of Aristotle preserved by the Arabs, and the works of the Indians and Persians, became known in Europe by the 12th and 13th centuries.

The Middle Ages saw the emergence of experimental physics with the development of an early scientific method emphasizing the role of experimentation and mathematics. Ibn al-Haytham (Alhazen, 965-1039) is considered a central figure in this shift in physics from a philosophical activity to an experimental one. In his Book of Optics (1021), he developed an early scientific method in order to prove the intromission theory of vision and discredit the emission theory of vision previously supported by Euclid and Ptolemy. His most famous experiments involve his development and use of the camera obscura in order to test several hypotheses on light, such as light travelling in straight lines and whether different lights can mix in the air. This experimental tradition in optics established by Ibn al-Haytham continued among his successors in both the Islamic world, with the likes of Qutb al-Din al-Shirazi, Kamāl al-Dīn al-Fārisī and Taqi al-Din, and in Europe, with the likes of Robert Grosseteste, Roger Bacon, Witelo, John Pecham, Theodoric of Freiberg, Johannes Kepler, Willebrord Snellius, René Descartes and Christiaan Huygens.

The Scientific Revolution
The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of Nicolaus Copernicus's De Revolutionibus (most of which had been written years prior but whose publication had been delayed) was brought from Nuremberg to the astronomer who died soon after receiving the copy.

Further significant advances were made over the following century by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early 17th century, Galileo pioneered the use of experimentation to validate physical theories, which is the key idea in modern scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the Principia, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics. Classical mechanics was re-formulated and extended by Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange, Irish mathematical physicist William Rowan Hamilton, and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

After Newton defined classical mechanics, the next great field of inquiry within physics was the nature of electricity. Observations in the 17th and 18th century by scientists such as Robert Boyle, Stephen Gray, and Benjamin Franklin created a foundation for later work. These observations also established our basic understanding of electrical charge and current.

In 1821, the English physicist and chemist Michael Faraday integrated the study of magnetism with the study of electricity. This was done by demonstrating that a moving magnet induced an electric current in a conductor. Faraday also formulated a physical conception of electromagnetic fields. James Clerk Maxwell built upon this conception, in 1864, with an interlinked set of 20 equations that explained the interactions between electric and magnetic fields. These 20 equations were later reduced, using vector calculus, to a set of four equations by Oliver Heaviside.

In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe light. Confirmation of this observation was made with the 1888 discovery of radio by Heinrich Hertz and in 1895 when Wilhelm Roentgen detected X rays. The ability to describe light in electromagnetic terms helped serve as a springboard for Albert Einstein's publication of the theory of special relativity in 1905. This theory combined classical mechanics with Maxwell's equations. The theory of special relativity unifies space and time into a single entity, spacetime. Relativity prescribes a different transformation between reference frames than classical mechanics; this necessitated the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. Einstein built further on the special theory by including gravity into his calculations, and published his theory of general relativity in 1915.

One part of the theory of general relativity is Einstein's field equation. This describes how the stress-energy tensor creates curvature of spacetime and forms the basis of general relativity. Further work on Einstein's field equation produced results which predicted the Big Bang, black holes, and the expanding universe. Einstein believed in a static universe and tried (and failed) to fix his equation to allow for this. However, by 1929 Edwin Hubble's astronomical observations suggested that the universe is expanding. Thus, the universe must have been smaller and therefore hotter in the past. In 1933 Karl Jansky at Bell Labs discovered the radio emission from the Milky Way, and thereby initiated the science of radio astronomy. By the 1940s, researchers like George Gamow proposed the Big Bang theory, evidence for which was discovered in 1964; Enrico Fermi and Fred Hoyle were among the doubters in the 1940s and 1950s. Hoyle had dubbed Gamow's theory the Big Bang in order to debunk it. Today, it is one of the principal results of cosmology.

From the late 17th century onwards, thermodynamics was developed by physicist and chemist Boyle, Young, and many others. In 1733, Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. In 1798, Thompson demonstrated the conversion of mechanical work into heat, and in 1847 Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. Ludwig Boltzmann, in the 19th century, is responsible for the modern form of statistical mechanics.

From : www.wikipedia.org

Relation to Mathematics and the other Sciences

noreply@blogger.com (Made Sumariani) — Mon, 29 Sep 2008 14:01:00 +0700

Physics relies on mathematics to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.

Whenever analytic solutions are not feasible, numerical analysis and simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.

Beyond the known universe, the field of theoretical physics also deals with hypothetical issues, such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatter rather than matter.

In the Assayer (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences, as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (called in this case quantum chemistry), thermodynamics, and electromagnetism.

Philosophical Implications

Physics in many ways stemmed from ancient Greek philosophy. From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, to the Ptolemaic astronomy of a crystalline firmament upon which the stars rested, our view of the universe seemed static. By the twentieth century, this picture became less certain, and now a static universe is only one possibility in an array of possible universes.

Aristotle's early observations in natural history, and natural philosophy usually did not involve much fact checking or detailed observation, which allowed errors to come to rest in our knowledge of the world. When closer investigation overturned this picture of the world, philosophers came to study other possible forms of reasoning. The use of a priori reasoning found a natural place in scientific method as well as the use of experiments and a posteriori reasoning came to be used in Bayesian inference. By the 19th century physics was realized as a positive science and a distinct discipline separate from philosophy and the other sciences.

"Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things." —Isaac Newton Study of the philosophical issues surrounding physics, the philosophy of physics can be encapsulated as empiricism, naturalism, and for some, realism. The mathematical physicist Roger Penrose has been called a Platonist by Stephen Hawking, while Penrose continues to eschew quantum mechanics as a final theory about reality.

Ørsted (1811) noted that physicists readily make deductions about nature, based on their closer familiarity with experiments about nature, whereas the mathematicians and philosophers must make do with fewer positive statements about nature.

That said, there are certain statements such as Newton's Third Law of Motion., generalized into the Principle of Equivalence. This principle is the logical basis for general relativity, whose solutions give metrics for spacetime. The success of general relativity influenced Einstein to eschew quantum theory, to which he made seminal contributions, and to eventually believe that all physical theory ought to be independent of observation. He lost his position of leadership in physics as a result of his belief in determinism rather than chance.

From : www.wikipedia.org

Data Collection and Theory Development

noreply@blogger.com (Made Sumariani) — Sun, 28 Sep 2008 15:03:00 +0700

There are many approaches to studying physics, and many different kinds of activities in physics. There are two main types of activities in physics; the collection of data and the development of theories.

The data in some subfields of physics is amenable to experiment. For example, condensed matter physics and nuclear physics benefit from the ability to perform experiments. Sometimes experiments are done to explore nature, and in other cases experiments are performed to produce data to compare with the predictions of theories.

Some other fields in physics like astrophysics and geophysics are primarily observational sciences because most their data has to be collected passively instead of through experimentation. Nevertheless, observational programs in these fields uses many of the same tools and technology that are used in the experimental subfields of physics. The accumulated body of knowledge in some area of physics through experiment and observation is known as phenomenology.

Theoretical physics often uses quantitative approaches to develop the theories that attempt to explain the data. In this way, theoretical physics often relies heavily on tools from mathematics and computational technologies (particularly in the subfield known as computational physics). Theoretical physics often involves creating quantitative predictions of physical theories, and comparing these predictions quantitatively with data. Theoretical physics sometimes creates models of physical systems before data are available to test and validate these models.

These two main activities in physics, data collection and theory production and testing, draw on many different skills. This has lead to a lot of specialization in physics, and the introduction, development and use of tools from other fields. For example, theoretical physicists apply mathematics and numerical analysis and statistics and probability and computers and computer software in their work. Experimental physicists develop instruments and techniques for collecting data, drawing on engineering and computer technology and many other fields of technology. Often the tools from these other areas are not quite appropriate for the needs of physics, and need to be adapted or more advanced versions have to be produced.

The culture of physics research differs from the other sciences in the separation of theory from data collection through experiment and observation. Since the 20th century, most individual physicists have specialized in either theoretical physics or experimental physics. The great Italian physicist Enrico Fermi (1901—1954), who made fundamental contributions to both theory and experimentation in nuclear physics, was a notable exception. In contrast, almost all the successful theorists in biology and chemistry (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, though this is changing as of late.

Although theory and experiment are usually performed by separate groups, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.

From : www.wikipedia.org

Scope and Goals

noreply@blogger.com (Made Sumariani) — Sat, 27 Sep 2008 21:13:00 +0700

Physics is the discipline devoted to understanding nature in a very general sense: the fundamental characteristic of physics is that it aims to gain knowledge, and hopefully understanding, of the general properties of the world around us. As an example, we can consider asking the following question on the nature of the Universe itself: how many dimensions do we need? Given that we know the Universe to consist of four dimensions (three space dimensions, and one time dimension), we can also ask why the universe picked those particular numbers: why not have four space dimensions? The fact that a choice was made out of a possibility of many means that questions like these fall under the scope of physics. Other general properties of nature include the existence of mass (as in Newton's laws of motion), charge (as in Maxwell's equations), and spin (in Quantum mechanics), amongst others.

However, whilst physics studies the general properties of nature, it will often also study the properties of certain objects within nature. Thus it is also physics whose job it is to describe what happens to, for example, planets whose motion is affected by nearby stars. Generally, the study of the specific objects in nature are shared between the three sciences: biology is roughly responsible for the living organisms, chemistry for the study of the elements and molecules, and physics is given responsibility over all that remains (See the section Relation to mathematics and the other sciences for further information). The fact that physics is delegated all objects besides those covered by biology and chemistry means that it is responsible for the study of a wide range objects and phenomena, from the smallest sub-atomic particles, to the largest galaxies. Included in this are the very most basic objects from which all other things are composed of, and therefore physics is sometimes said to be the "fundamental science".

Generalities aside, physics aims to describe the various phenomena in nature in terms of simpler phenomena: that is, to find the mechanisms for why nature behaves the way it does. Thus, physics aims to both connect the things we see around us to a root cause, and then to try to connect these root causes together in the hope of finding an ultimate reason for why nature is as it is. For example, the ancient Chinese observed that certain rocks (lodestone) were attracted to one another by some invisible force. This effect was later called magnetism, and was first rigorously studied in the 17th century. A little earlier than the Chinese, the ancient Greeks knew of other objects (amber) that when rubbed with fur would cause a similar invisible attraction between the two. This was also first studied rigorously in the 17th century, and came to be called electricity. Thus, physics had come to understand two observations of nature in terms of some root cause (electricity and magnetism). However, further work in the 19th century revealed that these two forces were just two different aspects of one force - electromagnetism. This process of "unifying" forces continues today (see section Current research for more information).

From : www.wikipedia.org

Physics

noreply@blogger.com (Made Sumariani) — Fri, 26 Sep 2008 19:51:00 +0700

Physics, in everyday terms, is the science of matter and its motion. It is the science that seeks to understand very basic concepts such as force, energy, mass, and charge. More completely, it is the general analysis of nature, conducted in order to understand how the world around us behaves.

In one form or another, physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of astronomy. Over the last two millennia, physics has sometimes been synonymous with philosophy, chemistry and certain branches of mathematics and biology but it emerged as a modern science in the 16th century . Physics is now generally distinct from these other disciplines, even though its boundaries remain difficult to define rigorously.

Physics is significant and influential, in part because advances in its understanding have often translated into new technologies, but also because new ideas in physics often resonate with the other sciences, mathematics and philosophy. For example, advances in the understanding of electromagnetism led directly to the development of new products that have transformed society (including television, computers and domestic appliances); advances in thermodynamics led to the development of motorized transport; and advances in mechanics inspired the development of the calculus, quantum chemistry, and the use of instruments like the electron microscope in microbiology.

Today, physics is both a broad and deep subject that, in practical terms, can be split into several subfields. It can also be divided into two conceptually different branches: theoretical and experimental physics; the former dealing with the development of new theories, whilst the latter deals with the experimental testing of these new, or existing, theories. Despite many important discoveries during the last four centuries, many significant questions about nature still remain unanswered, and many areas of the subject are still highly active.

From : www.wikipedia.org