[Paleopsych] SW: On Timescales

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Theoretical Physics: On Timescales
http://scienceweek.com/2004/sb041119-5.htm

    The following points are made by Alexander E. Kaplan (Nature 2004
    431:633):
    1) Our Universe, according to the "Big Bang" theory, is approximately
    14 billion years, or 5 x 10^(17) seconds (s) old. The ultimate
    timescale ("Planck time") of quantum cosmology --approximately
    10^(-43) s, the Big Bang's birth-flash -- is an elementary grain or
    pixel of time, within which our normal physics of four-dimensional
    space and time breaks down into a much greater number of dimensions,
    as hypothesized by the "superstring" theory.
    2) In logarithmic terms, we, with a lifetime of approximately 70 years
    (roughly 2 x 10^(9) s), exist on a scale that has more in common with
    the age of the Universe than with Planck time. We have learned how to
    keep track of time -- we could even regard ourselves as "Homo
    temporal" -- but how much of it is controlled and used by us? Although
    the "long" end of this scale is still only of academic interest, the
    "short" end is becoming a hot and bustling frontier of science and
    technology. The most familiar examples would be communication and
    computers. In the quest of higher computer performance, one of the
    major parameters is the clock frequency or, inversely, the clock
    cycle. An old 1989 UNIX computer had a clock frequency of
    approximately 17 MHz; today's off-the-shelf computers have a clock
    cycle of almost 3 GHz, or 0.3 nanoseconds.
    3) Lasers have been moving even faster into shorter time domains. Soon
    after the invention of laser in 1959, the length (duration) of a pulse
    of light passed the nanosecond (ns, 10^(-9) s) and then picosecond
    (ps, 10-12 s) thresholds, and the race was on to get to even shorter
    pulses. The sub-picosecond and femtosecond (fs, 10^(-15) s) domain
    became a field of rich research, with topics such as the registration
    of super-fast processes, time-resolved spectroscopy, characterization
    of semiconductors with sub-ps relaxation times, and the control of
    chemical reactions and fs time-resolution by powerful laser pulses.
    This domain also hosts the so-called Terahertz technology, which uses
    these pulses as, for example, a diagnostic tool to "see-through"
    opaque materials and structures.(1-5)
    References:
    1. Paul, P.M. et al. Science 292, 1689 (2001)
    2. Hentschel, M. et al. Nature 414, 509 (2001)
    3. Zewial A. Nature 412, 279 (2001)
    4. Kaplan, A.E. & Shkolnikov, P.L. Phys. Rev. Lett. 88, 74801 (2002)
    5. Greene, B. The Elegant Universe, (Random House, New York, 2003)
    Science http://www.sciencemag.org
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    Related Material:
    ON THE NEUROPSYCHOLOGY OF TIME
    The following points are made by P.A. Lewis and V. Walsh (Current
    Biology 2002 12:R9):
    1) Immanuel Kant (1724-1804) attempted to explain the special status
    of time and space in perception by arguing that our understanding of
    the Universe is limited by the way our brains process information.
    Specifically, Kant noted that we perceive all events as occurring in
    time and space, but it is not clear whether these dimensions exist in
    reality or are byproducts of our mental organization. For the
    neuroscientist, the question is slightly different: Allowing that our
    perceptions are mental constructs and therefore often differ from, or
    ignore, physical reality (e.g., illusions), the question becomes, How
    do brain structures and processes shape these perceptions?
    2) Within most sensory modalities, there is a clear starting point,
    because the dimensions being examined -- size, color, pitch, pressure,
    etc. -- can be measured using known receptor systems. For time,
    however, it is less clear how to approach the issue, since we do not
    appear to have a set of peripheral time sensors or a primary time area
    in the brain. So how do we come to be aware of time, and what
    mechanisms do we use to measure it?
    3) Psychologists and physiologists have been investigating time
    measurement since the early 17th century, and approaches they have
    used fall into two main categories: a) examination of the
    psychophysical properties of temporal estimation data, and b)
    investigations aiming to isolate the necessary brain regions using
    focal lesions or, more recently, neuroimaging. An important
    fundamental concept that has emerged from this work is that of
    multiple neural clocks. Measurement of intervals with different
    durations, or for different behavioral purposes, appears to draw upon
    quite discrete and different mechanisms in many cases.
    Current Biology http://www.current-biology.com
    --------------------------------
    Related Material:
    HISTORY OF PHYSICS: ON THE MEASUREMENT OF TIME
    Notes by ScienceWeek:
    Although a verbal definition of time (other than a purely operational
    definition) presents philosophical difficulties, from the standpoint
    of physics, time is the most accurately measured physical quantity. In
    general, there are two independent and fundamental time scales: a) the
    dynamical time scale, which is based on the regularities of the
    motions of the celestial bodies fixed in their orbits by gravitation;
    b) the atomic time scale, which is based on the characteristic
    frequency of electromagnetic radiation emitted or absorbed in quantum
    transitions between internal energy states of atoms or molecules.
    The first known device for indicating the time of day was the
    "gnomon", which appeared in approximately 3500 BC. This instrument
    consisted of a vertical stick or pillar, the length of the shadow cast
    by the stick or pillar providing an indication of the time of day. By
    the 8th century BC, more precise devices were in use. The earliest
    known sundial still preserved is an Egyptian shadow clock dating at
    least from the 8th century BC, and which consists of a straight base
    with a raised crosspiece at one end. On the base is inscribed a scale
    of 6 time divisions. The base is placed in an east-west direction with
    the crosspiece at the east end in the morning and at the west end in
    the afternoon. The shadow of the crosspiece on the base indicates the
    time.
    The Babylonian hemispherical sundial (hemicycle), apparently invented
    by the astronomer Barosus in approximately 300 BC, consisted of a
    cubical block into which was cut a hemispherical opening. To the
    opening was fixed a pointer whose end lay at the center of the
    hemispherical space. The path traveled by the shadow of the pointer
    was approximately a circular arc whose length and position varied
    according to the seasons. An appropriate number of arcs were inscribed
    on the internal surface of the hemisphere, each arc divided into 12
    subdivisions. Each day, reckoned from sunrise to sunset, had 12 equal
    intervals or "hours". Since the length of the day varied according to
    the season, these hours were known as "temporary hours".
    The Greeks developed and constructed sundials of considerable
    complexity in the 3rd and 2nd centuries BC, including instruments with
    either vertical, horizontal, or inclined dials, indicating time in
    temporary hours. The Romans also used sundials with temporary hours,
    and some of these Roman sundials were portable. The Arabs increased
    the variety of sundial designs, and at the beginning of the 13th
    century AD the Arabs wrote on the construction of sundials with
    cylindrical, conical, and other surfaces.
    In general, a "clock" is a device that performs regular movements in
    equal intervals of time, the device linked to a counting mechanism
    that records the number of movements. The first public clock that
    struck the hours was made and erected in Milan (IT) in 1335. The
    oldest surviving clock is that at Salisbury Cathedral, which dates
    from 1386. In approximately 1500, small portable clocks driven by a
    spring appeared, the dials with an hour hand only. The pendulum was
    applied as a time controller in clocks beginning in 1656, although
    Galileo had already suggested this in 1582.
    The familiar subdivision of the day into 24 hours, the hour into 60
    minutes, and the minute into 60 seconds is of ancient origin, but
    these subdivisions came into general use in approximately 1600 AD.
    When the increasing accuracy of clocks led to the adoption of the
    "mean solar day", which contained 86,400 seconds, the "mean solar
    second" became the basic unit of time.
    The adoption of the International System (SI) second, defined on the
    basis of atomic phenomena, as the fundamental time unit, occurred
    provisionally in 1964 and finally in 1967. A second is now defined as
    9,192,631,770 cycles of radiation associated with the transition
    between the two hyperfine levels of the ground state of the cesium-133
    atom. The number of cycles of radiation was chosen to make the length
    of the defined second correspond as closely as possible to that of the
    previous standard, the astronomically determined second of "Ephemeris
    Time" (defined as 1/(86,400) of the mean solar day).
    The following points are made by J.C.Bergquist et al (Physics Today
    March 2001):
    1) The authors point out that although a unit of time can be
    constructed from other physical constants, time is usually viewed as
    an arbitrary parameter to describe dynamics. The frequency of any
    periodic event, such as the mechanical oscillation of a pendulum, or
    the quantum oscillation of an atomic dipole, can be adopted to define
    the unit of time, the second.
    2) For centuries, the mean solar day served as the unit of time, but
    Earth's period of rotation is irregular and slowly increasing. In
    1956, the International Astronomical Union and the International
    Committee on Weights and Measures recommended adopting Ephemeris Time,
    based on Earth's orbital motion around the Sun, as a more accurate and
    stable basis for the definition of time. This recommendation was
    formally ratified in 1960 by the General Conference on Weights and
    Measures.
    3) Until the definition of the second in terms of atomic time in 1967,
    most work in standards laboratories was devoted to developing
    secondary standards, such as lumped-element circuits and quartz
    crystals, whose resonant frequencies could be calibrated relative to
    Ephemeris Time. But frequencies derived from resonant transitions in
    atoms or molecules offer important advantages over macroscopic
    oscillators. Any unperturbed atomic transition is identical from atom
    to atom, so two clocks based on such a transition should generate the
    same time. Also, unlike macroscopic devices, atoms do not wear out,
    and as far we know they do not change their properties over time.
    4) The basic idea of most atomic clocks is straightforward: a) First,
    identify a transition between two non-degenerate energy states of an
    atom. b) Then, create an ensemble of these atoms (e.g., in an atomic
    beam or storage device). c) Next, illuminate the atom with radiation
    from a tunable source that operates near the transition frequency. d)
    Sense and control the frequency where the atoms absorb maximally. e)
    When maximal absorption is achieved, count the cycles of the
    oscillator: a certain number of elapsed cycles generates a standard
    interval of time. But although the general idea of an atomic clock is
    straightforward, in practice there are a number of experimental
    difficulties that limit accuracy. The latest atomic clocks use a
    single ion to measure time with an anticipated precision of one part
    in 10^(18).
    Physics Today http://www.physicstoday.org