Comptes Rendus de l'Académie des Sciences - Series IV - Physics-Astrophysics最新文献

The space project LISA is approved by ESA as a cornerstone mission in the field of ‘fundamental physics’, sharing its goal and principle of operation with the ground-based interferometers currently under construction: the detection and measurement of gravitational waves by laser interferometry. Ground and space detection differ in their frequency ranges, and thus the detectable sources. At low frequencies, ground-based detection is limited by seismic noise, and yet more fundamentally by ‘gravity gradient noise’, thus covering the range from a few Hz to a few kHz. On five sites worldwide, detectors of armlengths from 0.3 to 4 km are being built, two of them in Europe (GEO and VIRGO). They will progressively be put in operation between 2001 and 2003. Future improved versions are being planned, with data not until 2008, i.e. near the launch of the space project LISA. It is only in space that detection of signals below, say, 1 Hz is possible, opening a wide window to a different class of interesting sources of gravitational waves. The project LISA consists of three spacecraft in heliocentric orbits, forming a triangle of 5 million km sides.

The Equivalence Principle (EP) is not one of the ‘universal’ principles of physics (like the action principle). It is a heuristic hypothesis which was introduced by Einstein in 1907, and used by him to construct his theory of general relativity. In modern language, the (Einsteinian) EP consists in assuming that the only long-range field with gravitational-strength couplings to matter is a massless spin-2 field. Modern unification theories, and notably string theory, suggest the existence of new fields (in particular, scalar fields: ‘dilaton’ and ‘moduli’) with gravitational-strength couplings. In most cases the couplings of these new fields ‘violate’ the EP. If the field is long-ranged, these EP violations lead to many observable consequences (variation of ‘constants’, non-universality of free fall, relative drift of atomic clocks, etc.). The best experimental probe of a possible violation of the EP is to compare the free-fall acceleration of different materials.

The test of the equivalence principle can be performed in space with orders of magnitude better resolution than in the laboratory, because of the outstanding steady and soft environment of the in-orbit experiment. The expected new experimental results will contribute to the unification of the four interactions, demonstrate the existence of extra scalar interaction or participate in the research for a quantum gravity theory. The MICROSCOPE space mission is being developed within the framework of the Cnes scientific program with the objective of testing the universality of free fall with a 10⁻¹⁵ accuracy. The concept and the design of the experiment are discussed and the major performance drivers of the room temperature instrument are pointed out. The launch of the drag-free satellite is scheduled for late 2004. By its specific technology demonstration, the mission will open the way to even more accurate acceleration measurements for other space missions in fundamental physics.

We study experimentally the shape of dry patches inside a film flowing along an inclined plane at relatively high value of the contact angle θ. Their radius of curvature R near apex, is given by R/l_c∼l_cV_c/(Γ sinα)−F(α,θ), where Γ is the flow rate per unit length, α the plate slope, and F(α,θ) is a correction that increases with θ and decreases with α (l_c and V_c are the capillary length and the capillary velocity). A simple model allows us to recover this correction and also the existence of a critical flow rate above which dry patches disappear.

We discuss the possible existence of new long-range forces mediated by spin-1 or spin-0 particles. By adding their effects to those of gravity, they could lead to apparent violations of the equivalence principle. While the vector part in the couplings of a new spin-1 U boson involves, in general, a combination of the B and L currents, there may also be, in addition, an axial part as well. If the new force has a finite range λ, its intensity is proportional to 1/(λ²F²), F being the extra U(1) symmetry-breaking scale.

Quite surprisingly, particle physics experiments can provide constraints on such a new force, even if it is extremely weak, the corresponding gauge coupling being extremely small (⪡10⁻¹⁹!). An ‘equivalence theorem’ shows that a very light spin-1 U boson does not in general decouple even when its gauge coupling vanishes, but behaves as a quasi-massless spin-0 particle, having pseudoscalar couplings proportional to 1/F. Similarly, in supersymmetric theories, a very light spin-$\text{3}\text{2}$ gravitino might be detectable as a quasi-massless spin-$\text{1}\text{2}$ goldstino, despite the extreme smallness of Newton's gravitational constant G_N, provided that the supersymmetry-breaking scale is not too large.

Searches for such U bosons in ψ and ϒ decays restrict F to be larger than the electroweak scale (the U actually becoming, as an axion, quasi ‘invisible’ in particle physics for sufficiently large F). This provides strong constraints on the corresponding new force and its associated EP violations. We also discuss briefly new spin-dependent forces.

The knowledge of the gravity field of the Earth and of an associated reference surface of altitudes (the geoid) is necessary for geodesy, for improving theories of the physics of the planet interior and for modeling the ocean circulation in absolute. This knowledge comes from several observing techniques but, although it benefited from the artificial satellite approach, it remains incomplete and erroneous in places. Within a reasonable future, a substantial improvement can only come from new space techniques. Thanks to the intense lobbying by the concerned geoscientists, the coming decade will see the advent of three techniques already proposed in the seventies and to be implemented by different space agencies; these are the CHAMP, GRACE and GOCE missions.

We present a concise summary of some of the fundamentals and of our work in the field of elastography over the past 12 years. This summary is not exhaustive, since several recent reviews of this area are available. We begin by presenting some relevant background material from the field of biomechanics, which forms the foundation of this work. We then proceed to discuss the basic principles and limitations that are involved in the production of strain images (elastograms) of biological tissues. Early results as well as current results from biological tissues in vitro and in vivo are shown. We conclude with some thoughts regarding the potential of elastography for medical diagnosis.

The stability of a thermocapillary flow in an extended cylindrical geometry is analyzed. This flow occurs in a thin liquid layer with a disk shape when a radial temperature gradient is applied along the horizontal free surface. Besides the aspect ratio, a second parameter related to the local curvature is introduced to describe completely the geometrical effects. We recover classical hydrothermal waves as predicted by Smith and Davis, but the properties of these waves are shown to evolve with the curvature parameter, thus leading to a nonuniform pattern over the cell. Moreover, it is shown that the problem is not invariant with respect to the exchange of the hot and cold sides.

Based on photon migration the new goal of diffuse optical imaging is to reveal optical contrasts in the depth of biological tissues. We discuss first the origin of contrast mechanism (absorption, fluorescence and scattering) used on diffuse optical imaging and spectroscopy. Then, various experimental approaches are described based on CW, pulsed and modulated light excitation and detection. Theoretical models which provide solutions for direct and inverse problems are presented using random walk theory. Finally two studies on breast imaging and on the use of fluorescence exogeneous markers are discussed in detail.