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TOPIC: Arrow of Time


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Time. There is nothing with which we are so familiar, and yet when you try to pin it down you find only a relentless torrent of questions. Why does time appear to flow? What makes it different from space? What exactly is it? It's enough to make your neurons misfire, then sizzle and smoke.
You are not alone. Physicists have long struggled to understand what time really is. In fact, they are not even sure it exists at all. In their quest for deeper theories of the universe, some researchers increasingly suspect that time is not a fundamental feature of nature, but rather an artefact of our perception. One group has recently found a way to do quantum physics without invoking time, which could help pave a path to a time-free "theory of everything". If correct, the approach suggests that time really is an illusion, and that we may need to rethink how the universe at large works.

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In a classical view of the world, space and time are smooth. The minimum scales at which, according to quantum mechanics, the smoothness breaks down the Planck length and time can be derived from other quantities, but they have not been tested experimentally, nor would they be, given their impossibly small size.

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A new theory suggests that the essential fuzziness of time may be the limiting factor for a German gravitational-wave detector.
Poets have long believed the passage of time to be unavoidable, inexorable and generally melancholic. Quantum mechanics says it is fuzzy, ticking along at minimum intervals within which the notion of time is meaningless. And Craig Hogan claims he can 'see' it in the thus far unexplained noise of a gravitational-wave detector.

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Title: The arrow of time and the Weyl group: all supergravity billiards are integrable
Authors: Pietro Fré, Alexander S. Sorin

In this paper we show that all supergravity billiards corresponding to sigma-models on any U/H non compact-symmetric space and obtained by compactifying supergravity to D=3 are fully integrable. The key point in establishing the integration algorithm is provided by an upper triangular embedding of the solvable Lie algebra associated with U/H into SL(N,R) which always exists. In this context we establish a remarkable relation between the arrow of time and the properties of the Weyl group. The asymptotic states of the developing Universe are in one-to-one correspondence with the elements of the Weyl group which is a property of the Tits Satake universality classes and not of their single representatives. Furthermore the Weyl group admits a natural ordering in terms of L(T), the number of reflections with respect to the simple roots and the direction of time flows is always towards increasing L(T), which plays the unexpected role of an entropy.

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Caltech physicist Sean M. Carroll has been wrestling with the mystery of time. Most physical laws work equally well going backward or forward, yet time flows only in one direction. Writing in this months Scientific American, Carroll suggests that entropy, the tendency of physical systems to become more disordered over time, plays a crucial role. Carroll sat down recently at Caltech to explain his theory.

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It is the invisible presence that governs your world. Trailing you like an unshakeable shadow, it ticks and tocks incessantly - you can sense it in your heartbeat, in the rising and setting of the sun, and in your daily rush to make meetings, trains and deadlines. It brings order to our lives through the categories of past, present and future.
Time. There is nothing with which we are so familiar, and yet when you try to pin it down you find only a relentless torrent of questions. Why does time appear to flow? What makes it different from space? What exactly is it? It's enough to make your neurons misfire, then sizzle and smoke.
You are not alone. Physicists have long struggled to understand what time really is. In fact, they are not even sure it exists at all. In their quest for deeper theories of the universe, some researchers increasingly suspect that time is not a fundamental feature of nature, but rather an artefact of our perception. One group has recently found a way to do quantum physics without invoking time, which could help pave a path to a time-free "theory of everything". If correct, the approach suggests that time really is an illusion, and that we may need to rethink how the universe at large works.

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For a decade, scientists have puzzled over a surprising phenomenon: Supernovae stars viewed at extreme distances seem to be moving away from us faster than those nearby.
Most researchers have assumed that the stars have somehow accelerated or that, more precisely, the rate of the expansion of the post-Big Bang universe itself has accelerated over time.


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It can drag or it can race, but what if time stopped altogether? It now seems that time could disappear from our universe - and we may already have found evidence of its forthcoming demise.
When astronomers observed a decade ago that supernovae are apparently spreading apart faster as the universe ages, they assumed that something must be causing the expansion of the universe to speed up. But so far, nobody has been able to explain where the "dark energy" causing this acceleration comes from.
Now José Senovilla at the University of the Basque Country in Bilbao, Spain, and his colleagues have a radical answer - we are fooled into thinking that the expansion of the universe is accelerating, because time itself is slowing down.

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Scientists have come up with the radical suggestion that the universe's end may come not with a bang but a standstill - that time could be literally running out and could, one day, stop altogether.
The idea that time itself could cease to be in billions of years - and everything will grind to a halt - has been set out by Professor José Senovilla, Marc Mars and Raül Vera of the University of the Basque Country, Bilbao, and Univerisity of Salamanca, Spain.

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Title: The Scientist as philosopher
Author: Friedel Weinert

time9
time13
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-- Edited by Blobrana at 15:53, 2007-11-22

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