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  3. From the beginning, the idea of a finite universe ran into its own obstacle, the apparent need for an edge, a problem that

From the beginning, the idea of a finite universe ran into its own obstacle, the apparent need for an edge, a problem that

admin2009-05-19  51
问题          From the beginning, the idea of a finite universe ran into its own obstacle,
     the apparent need for an edge, a problem that has only recently been grappled
     with. Aristotle’s argument, that the universe is finite, and that a boundary was
     necessary to fix an absolute reference frame, held only until scientists wondered
(5)   what happened at the far side of the edge. In other words, why do we not
     redefine the "universe" to include that other side?
         Riemann ingeniously replied by proposing the hypersphere, the three-
     dimensional surface of a four-dimensional ball. Previously it was supposed that
     the ultimate physical reality must be a Euclidean space of some dimension, and
(10)  thus if space were a hypersphere, it would need to sit in a four-dimensional
     Euclidean space that allows us to view it from the outside. But according to
     Riemann, it would be perfectly acceptable for the universe to be a hypersphere
     and not embedded in any higher-dimensional space; nature need not therefore
     cling to the ancient notion. According to Einstein’s powerful but limited theory
(15)  of relativity, space is a dynamic medium that can curve in one of three ways,
     depending on the distribution of matter and energy within it, but because we are
     embedded in space, we cannot see the flexure directly but rather perceive it as
     gravitational attraction and geometric distortion of images. Thus, to determine
     which of the three geometries our universe has, astronomers are forced to
(20)  measure the density of matter and energy in the cosmos, whose amounts appear
     at present to be insufficient to force space to arch back on itself in "spherical"
     geometry. Space may also have the familiar Euclidean geometry, like that of a
     plane, or a "hyperbolic" geometry, like that of a saddle. Furthermore, the
     universe could be spherical, yet so large that the observable part seems
(25)  Euclidean, just as a small patch of the earth’s surface looks flat.
         We must recall that relativity is a purely local theory: it predicts the
     curvature of each small volume of space--its geometry--based on the matter
     and energy it contains, and the three plausible cosmic geometries are consistent
     with many different topologies: relativity would describe both a torus and a
(30)  plane with the same equations, even though the torus is finite and the plane is
     infinite. Determining the topology therefore requires some physical
     understanding beyond relativity, in order to answer the question, for instance,
     of whether the universe is, like a plane, "simply connected", meaning there is
     only one direct path for light to travel from a source to an observer. A simply
(35)  connected Euclidean or hyperbolic universe would indeed be infinite--and seems
     self-evident to the layman--but unfortunately the universe might instead be
     "multiply-connected", like a torus, in which case there are many different such
     paths. An observer could see multiple images of each galaxy and easily interpret
     them as distinct galaxies in an endless space, much as a visitor to a mirrored
(40)  room has the illusion of seeing a huge crowd, and for this reason physicists have
     yet to conclusively determine the shape of the universe.
选项 A、searching for an accurate method of determining whether the universe is finite or infinite
B、discussing problems and possibilities involved in providing a definite picture of the shape of the universe
C、declaring opposition to the notion that spherical geometry is a possible model for the shape of the universe
D、criticizing discredited theories about the possible topologies of the universe
E、refuting the idea that there is no way to tell whether the universe is finite and if so what shape it has
答案B
解析
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