| Topic: |
Science > Philosophy |
| User: |
"Sir Frederick" |
| Date: |
15 Nov 2004 04:08:59 PM |
| Object: |
On Timescales |
Theoretical Physics: On Timescales
ScienceWeek http://scienceweek.com
Our Universe, according to the "Big Bang" theory, is
approximately 14 billion years old. The ultimate timescale
("Planck time") of quantum cosmology 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.
THEORETICAL PHYSICS: ON TIMESCALES
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
--------------------------------
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
ScienceWeek http://scienceweek.com
--
Best,
Frederick Martin McNeill
Poway, California, United States of America
mmcneill@fuzzysys.com
http://www.fuzzysys.com
http://members.cox.net/fmmcneill/
*************************
Phrase of the week :
For the real amazement, if you wish to be amazed, is this
process: You start out as a single cell derived from the coupling
of a sperm and an egg; this divides in two, then four, then
eight, and so on, and at a certain stage there emerges a single
cell which has as all its progeny the human brain. The mere
existence of such a cell should be one of the great astonishments
of the Earth. People ought to be walking around all day, all
through their waking hours calling to each other in endless
wonderment, talking of nothing except that cell. -- Lewis Thomas (1913-1993)
:-))))Snort!)
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