HST and 'dark matter' |
HST and 'dark matter' |
Guest_PhilCo126_* |
May 11 2007, 05:13 PM
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Guests |
ASA Updates Plans for Hubble 'Ring Of Dark Matter' Briefing
GREENBELT, Md. - NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal. |
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Mar 11 2010, 05:37 PM
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Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
From Inertia Theory -- Paul Davies:
QUOTE Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from.
Newton believed that inertia is an innate property of matter manifesting itself whenever matter accelerates (this includes rotation) relative to absolute space. You could think of the space in the vicinity of the accelerating body as somehow reacting to its motion to produce inertial forces. Newton never explained how, but took it to be a law of nature. Newton's arch rival, Gottfried Leibniz, rejected this. Space, being empty, provides no reference against which a body can be said to accelerate. How can something move with respect to nothingness? We can judge motion, claimed Leibniz, only relative to other material bodies. Take the Earth's rotation. We observe the daily progression of the sun and stars across the sky. Our ancestors believed it was the heavens that turned, not the Earth. But suppose there were no sun and stars? Suppose Earth were alone in infinite space? Would it then make any sense to say it was rotating? The debate raged on. Newton's hypothesis of absolute space predominated, but champions of the alternative "relative" view fought back, first in the guise of the Irish philosopher Bishop George Berkeley, then Austrian physicist-philosopher Ernst Mach - he of the Mach numbers. Mach, whose ideas greatly influenced Einstein when formulating his theory of relativity at the start of the century, insisted that acceleration can be defined only relative to the distant stars, a statement that came to be dignified with the name of "Mach's principle". Mach's principle faced a thorny problem. Acceleration produces inertial forces. How can the distant stars be responsible for those? Could it really be that the child riding the roundabout is being tugged at by far-flung galaxies? Einstein and others sought a mechanism to explain how a rotating body might experience a centrifugal force as a result of some sort of interaction with all the distant matter in the universe. A clue came from the theory of gravitation: after all, centrifugal force is sometimes even called artificial gravity. Viewed from the roundabout, it is the rest of the universe that is rotating. We know that when electric charges circulate around a loop the resulting electric current produces a magnetic field. Could it be that the apparent rotation of the universe produces a gravitational version of a magnetic force that plucks at the clinging child? To test the idea, Einstein considered a small body at rest inside a rotating shell of material in otherwise empty space. Using his theory of relativity, he calculated what would happen. It turns out that the body should indeed feel a tiny gravito-magnetic force. Further evidence in favour of Mach's principle comes from cosmology. If rotational motion is purely relative, then it is clearly nonsensical to talk about the rotation of the universe as a whole, for with respect to what would it rotate? In Newton's theory, it is entirely possible for the entire cosmos to spin about some axis. Given that almost all astronomical systems are observed to rotate to some extent, we might expect, if Newton is right, to observe a universal rotation too. Astronomers find no evidence for a systematic rotation of the universe. Their observations imply that the universe cannot have turned by even one degree since the big bang. If rotation is absolute, the absence of a universal rotation seems to be a very special and contrived state of affairs, but if as Mach claimed it is relative, then the observations are explained. |
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