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dark matter: In physics and cosmology, dark matter is hypothetical matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. Some scientists believe this "dark matter" to be the Aether through which all disturbances we know of as light and gravity propagate. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter and dark energy account for the vast majority of the mass in the observable universe. The observed phenomena which imply the presence of dark matter include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component."
The dark matter component has vastly more mass than the "visible" component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter makes a contribution to dark matter but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and plasma physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much as the marking of early maps with "terra incognita."
Plato's allegory of the cave: Imagine prisoners who have been chained since their childhood deep inside a cave: not only are their arms and legs immovable because of chains; their heads are chained in one direction as well so that their gaze is fixed on a wall.
Behind the prisoners is an enormous fire, and between the fire and the prisoners is a raised walkway, along which puppets of various animals, plants, and other things are moved. The puppets cast shadows on the wall, and the prisoners watch these shadows. Behind this cave there is a well-used road, and upon this road people are walking and talking and generally making noise. The prisoners, then, believe that these noises are coming directly from the shadows they are watching pass by on the cave wall.
The prisoners engage in what appears to us to be a game: naming the shapes as they come by. This, however, is the only reality that they know, even though they are seeing merely shadows of objects. They are thus conditioned to judge the quality of one another by their skill in quickly naming the shapes and dislike those who play poorly.
Suppose a prisoner's chains break, and he is able to get up and walk about (a process which takes some time, as he has never done it before). Eventually he will be compelled to explore; he walks up and out of the cave, whereby he is instantly blinded by the sun. He turns then to the shadows on the floor, in the lakes, slowly working his way out of his deluded mind, and he is eventually able to glimpse the sun. In time, he would learn to see it as the object that provides the seasons and the courses of the year, presides over all things in the visible region, and is in some way the cause of all these things that he has seen.
Once enlightened, so to speak, the freed prisoner would not want to return to the cave to free his fellow prisoners, but would be compelled to do so. Another problem lies in the other prisoners not wanting to be freed: descending back into the cave would require that the freed prisoner's eyes adjust again, and for a time, he would be one of the ones identifying shapes on the wall. His eyes would be swamped by the darkness, and would take time to become acclimated. He might stumble, Plato asserts, and the prisoners would conclude that his experience had ruined him. He would not be able to identify the shapes on the wall as well as the other prisoners, making it seem as if his being taken to the surface completely ruined his eyesight. (The Republic bk. VII, 516b-c; trans. Paul Shorey)
camera obscura (Lat. dark chamber) was an optical device used in drawing, and one of the ancestral threads leading to the invention of photography. In English, today's photographic devices are still known as "cameras".
The principle of the camera obscura can be demonstrated with a rudimentary type, just a box (which may be room-sized, or even hangar sized) with a hole in one side, (see pinhole camera for construction details). Light from only one part of a scene will pass through the hole and strike a specific part of the back wall. The projection is made on paper on which an artist can then copy the image. The advantage of this technique is that the perspective is accurate, thus greatly increasing the realism of the image (correct perspective in drawing can also be achieved by looking through a wire mesh and copying the view onto a canvas with a corresponding grid on it).
With this simple do-it-yourself apparatus, the image is always upside-down. By using mirrors, as in the 18th century overhead version (illustrated in the Discovery and Origins section below), it is also possible to project a right-side-up image. Another more portable type, is a box with an angled mirror projecting onto tracing paper placed on the glass top, the image upright as viewed from the back.
As a pinhole is made smaller, the image gets sharper, but the light-sensitivity decreases. With too small a pinhole the sharpness again becomes worse due to diffraction. Practical camerae obscurae use a lens rather than a pinhole because it allows a larger aperture, giving a usable brightness while maintaining focus.
Post je objavljen 10.09.2008. u 15:50 sati.