Download Precise Measurement of Time

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Precise Measurement of Time
Norman F. Ramsey
Long before the earth supported
life, it was subject to marked periodicities: the change from day to night
and the change in seasons. These
periodicities had a profound effect on
the evolution of biological organisms;
most animals have a circadian
rhythm which leads them to need
sleep at approximately 24-hour interval even if they are confined to an
artificial environment with no periodic changes in lighting. The all-toofamiliar j t lag that human beings
experience when traveling long distances east or west demonstrates the
difficulty that they have re etting
their internal biological clocks.
The periodicities in man's physical environment and in his biological
nature clearly must have affected his
development of the concept of time.
That d velopment has in tum been
closely intertwined with the ability to
measure time, and in some respects
the very meaning of time is defined
by the methods used to measure it.
The fundamental features of any
clock are a mechanism to provide
regular periodicity, such as the rotation of the earth, the successive in:ersions of an hour glass, or the
motion of a pendulum; a source of
energy to maintain motion without
significantly affecting the periodicity;
and a means for counting the periods
and displaying the time. In discuss'omran &msey IS Hrggins Professor of Plrysics
Emmtus at Han.~t~rd Umvrn;rty and ttru recently
a visrtmg fellow at tire Joint iiiSirtute for
l.Aoomtory Astroplrysrcs m Boulder, Colorado. His
erptrimmtal work lras ranged from molecular
OCams to particle plrysics, and Ire lras concentmted
on prtcist measumnmt of tire tlectnc and
11ragnetrc properties of m1clcons, nuclei, atoms,
and molecules. He IS the mventor of the separated
OSCillatory field method used m atomic beam clocks
md co·mventor of the hydrogen masu. He IS
currently president of tire Umted Chapters of Phi
Beta Kappa . Address: Lyman Plrysrcs l.Aooratory,
Han.~t~rd Univrn;rty, Cambridge, MA 02138.
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American Scientist, Volume 76
ing the characteristics of clocks, it is
necessary to distinguish between
three different but related properties:
accuracy, reproducibility, and stability. Accuracy measures the degree to
which a clock agrees with the value
specified in the definition of the unit
of time. Reproducibility is a measure
of the extent to which properly
adjusted independent devices of the
same design agree. Stability is a measure of the degree to which a device
gives the ~e results in successive
intervals of time. Because the stabil-
Tradition has it that in 1583,
Galileo compared the motion of a
swinging lamp with his pulse and
discovered that the lamp's period of
oscillation was approximately independent of its amplitude (1). This
observation provided the scientific
basis for the use of pendulums in
clocks. Although Galileo recognized
the value of his discovery for keeping
time, it was 73 years before the Dutch
scientist and clockmaker Christian
Huygens devised a suitable mechanism, called an escapement, to maintain the oscillations and made the
first successful pendulum-controlled
clock. Clearly technology transfer
The unprecedented
was slow. Twenty years later, Huygens combined some ideas of Robert
accuracy and stability
Hooke with his own escapement
of atomic clocks
mechanism and produced a dock
whose periodicity was provided by
have revolutionized
an oscillating balance wheel retimekeeping and
strained by a spiral spring. This clock
was the forerunner of clocks and
exact measurement
watches with balance wheels and
in many fields
hairsprings. The pendulum clock
continued to provide the most accurate timekeeping for the next two
ity and accuracy of a clock are pri- and a half centuries. The accuracy of
marily detennined by the mechanism both pendulum and balance wheel
providing the periodicity, this feature clocks improved steadily up to the
is emphasized in discussions of the twentieth century, as compensations
were introduced to diminish the efprecise measurement of time.
Various devices and mecha- fects of changing external conditions
nisms have been used over the cen- such as temperature (2). By the midturies as sources of periodicity for dle of the twentieth century, the stameasuring time. Many prehistoric bility of the best gravity pendulum
monuments are oriented to detect clocks was approximately 2 x 10- 8 .
the summer solstice. Solar indicators
were used by early man to divide the Quartz crystal clocks
day into measurable subunits; as civilization advanced, these were super- Time and frequency are closely relatseded by water clocks, candles, and ed, since frequency is by definition
hourglasses, most of which had inac- the number of cycles of osdllation
curacies worse than 1,000 seconds a per second (hertz). Provided the
day (Fig. 1). Since there are approxi- counting of the cycles is correct, an
mately 100,000, or Hf, seconds in a accurate measurement of time proday, this corresponds to an accuracy vides an equally accurate measurement of frequency; conversely, a deof one part in 100, or 10- 2•
Figure 1. Human beings have searched over
the centuries for ever more precise ways to
measure time. The earliest mechanical
device for telling time-originated in
ancient Egypt and subsequently developed
by the Greeks and Romans-was the water
clock, or clepsydra. The relatively
sophisticated clepsydra shown here consists
of a reservoir for supplying water, a float in
a receiving vessel that rises with the water
level to indicate on a cylindrical scale the
passage of time, and a siphon, drum, and
gear wheels to empty the vessel at the end
of the day and rotate the scale 1/365 of a full
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vice oscillating at a stable frequency
can be the source of the periodicity
that is measured by a dock. The
earliest clocks, as we have seen, were
developed primarily for measuring
the passage of time. However, the
initial incentive for the development
of tuning forks, quartz crystal oscillators, and atomic clocks was the measurement of frequency .
The oscillatory motions of a vibrating olid can provide a measurement of frequency or periodicity that
can serve as the basis of a clock. It is
difficult to maintain the vibratory motion with most substances, but certain cry tals have the property of
piezoelectricity (literally, "pressure"
electricity). Piezoelectricity is polarization ( hift in the relative locations
of positive and negative charges) produced by mechanical strain in certain
cry tal ,· and it i al o the inverse
effect whereby a crystal become
strain d when electrically polarized.
The piezoelectric property permits a
cry tal to be coupled to an electronic
circuit to maintain the vibrations and
to count them . Although the phenomenon was discovered by Pierre
and Jacques Curie in 1880 and piezoelectric crystals were subsequently
used by Nicholson and Cady in an
electronic oscillator, the first clock
driven by a quartz crystal wa developed only in 1928 by Horton and
Morrison (3).
Of all the known crystals, single
crystals of quartz (silicon dioxideSi02) have been found to be the best
for measuring time and frequency
because of their piezoelectric properties, hardne s, low internal dissipation, durability, uniformity, availability in sufficiently large sizes, and
capability of being cut into wafers
whose characteristic vibration frequencies are nearly independent of
temperature. A quartz crystal is
shown in Figure 2, along with some
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of the cuts that provide suitable wafer .
The best quartz clocks are highly
stable (2 x 10- 13), but they are much
worse for reproducibility (1 x 10- 7)
and war e still for accuracy, since
new crystal usually have to be calibrated experimentally against other
frequency standards. Quartz clocks
do, however, have the advantage
over other more accurate and stable
devices that they are inexpensive and
portable, as evidenced by quartz
wristwatches. The adaptation of
quartz oscillators to small watches
required major innovations, includ-
ing the cutting of the crystal in a
tuning fork shape, as shown in Figure 3, to provide a convenient natural period of oscillation in a crystal of
small size.
Atomic clocks
Although quartz clocks achieve remarkable stability, the greatest accuracy, reproducibility, and stability
have been achieved by atomic clocks,
which now provide the basis for defining the second as the unit of time.
The measured periodic motion of
atomic clocks arises from the magnet1988 january-February
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i(" mler,lCllon of the hcavv nucleus of
the atom with it light electrons. The
nuclei of most atoms are magnetized
like compass needles. Just as a spinning top tilted at an angle precesses
about the vertical under the influence
of gravity, the nucleus precesses
about the field created by the electrons' magnetism. The axis and angular momenta of the spinning motions of the nucleus and electrons
and the resulting precessional motion are shown in Figure 4.
The precessional frequencies
and the angular momenta are be t
expressed in terms of quantum mechanics. For the atoms most often
used in atomic clocks, only two states
of relative orientation between the
angular momentum of the nucleus
and that of the electrons are permitted, one in which they are as nearly
parallel as possible and the other
in which th y are as nearly antiparallel as possible. If the difference
in energy between the two state is
W1 - W2, then the clas ical precesional frequency corresponds to the
quantum mechanical frequency
f = (WI - W 2) I h
Since this internal precessional
frequ ncy was first observed in optical spectroscopy as a very small splitting of spectral lines, it is often called
the hyperfine frequency. It is determined by atomic properties which
Figure 2. The Curie brothers discovered the
phenomenon of piezoelectricity-vibr~tions
in cryst.Us c~used by mech~nic~l strilin-in
1880, but it was not until the 1920s th~t ~
clock was invented to use the property as ~
source of periodicity. Single cry t.Us of
quartz an cut into w~fers (the sh~ded
rectMtgles shown here, for example) whose
frequency of vibr~tion is neuly independent
of temperature.
44
American Scientist, Volume 76
mounting pedestal
Figure 3. For use in wristw~tches, qu~rtz
cut in the shape of a tuning
fork with prongs ~bout 5 mm long x 0.8
mm wide and 0.03 mm thick. Gold tuning
pads on the ends of the prongs allow
trimming to precise frequencies.
oscill~tors ~re
are constant in time (except for small
variations due to fluctuating external
magnetic and electric fields), so it
provid~ an excellent basis for a highpreci ion dock. Before such a clock
can be built, however, there must be
a suitable means for observing and
counting this internal atomic periodicity.
The first method for accurately
measuring hyperfine frequencies
was provided by molecular beam
magnetic resonance (Fig. 5). It was
developed starting in 1937 by I. I.
Rabi and his associates (4, 5). The
apparatus is evacuated to a pressure
less than 10 7 torr, to avoid collisions
of the atoms with the background
gas. Cesium atoms are most commonly used. They are heated in the
oven to raise their vapor pressure to
a point where significant numbers of
them pass through the oven's slit
jaws, forming a beam. The inhomogeneous field of a magnet deflects
the beam, since the force on the
north pole of the electrons' magnetic
dipole moment is different in magnitude from that on the south pole. The
beam then passes through the main
part of the apparatus, where it experiences a weak static magnetic field
and an even weaker perpendicular
oscillatory magnetic field. Beyond
the field region the atoms are deflected again by a second inhomogeneous magnetic field. The beam
then strikes the hot-wire ionizer of
the detector, where each atom gives
up an electron and becomes a positive ion which is electrically counted.
If the frequency of the oscillating
magnetic field equals the hyperfine
frequency given in the equation
above, the orientation of the electrons' spin angular momentum is reversed by the resonant oscillatory
field; the force on the atom is in tum
reversed, and the beam follows the
indicated path with a maximum of
detected intensity. If, on the other
hand, the frequency of the oscillating
field is far from the hyperfine frequency, no resonant reorientation of
the electrons' magnetic moment will
occur, and the beam will be directed
downward by the second inhomogeneous magnet with the result that
the detected intensity will decrease.
The detected intensity can then be
used through a normal feedback circuit to adjust slightly the frequency
of a quartz crystal oscillator, which
provides the oscillating magnetic
field that induces the atomic transitions. In this fashion, the oscillator
frequency is stabilized to the internal
atomic prece ional frequency .
In Rabi's resonance method, the
oscillatory magnetic field extended
uniformly all the way through the
region with the static magnetic field
(see Fig. 5). However, in 1949 I
showed that there were many advantages to concentrating the oscillatory
magnetic field in two shorter regions
at the beginning and end of the region with the static magnetic field (6).
The resonances can then be much
narrower, and the first-order Doppler shift is eliminated. (The first-order
Doppler shift for electromagnetic
waves is analogous to the familiar
increase in pitch of the whistle on an
approaching locomotive; it corresponds to the compression of waves
as the source approaches an observer. The second-order Doppler shift is
much smaller than the ftrst, arising
from the well-known effect-explained by the special theory of relativity-that rapidly moving clocks
appear to run slowly.)
Although Rabi and his associates first discussed the possibility of
atomic clocks before 1945, many improvements in subsequent years by
Jerrold Zacharias and others (5, 7)
were required to achieve the present
accuracy of to- 13 • The atomic cesium
clock was so far superior to all previous clocks that in 1967 the international defmition of the second was
changed from one based on the motion of the earth about the sun to
9,192,631,770 periods of the cesium
atom.
A variety of atomic and molecu-
Jar resonance devices were developed in the 1950s as possible alternatives to the atomic cesium beam (5).
Among the most effective were those
that employed atoms such as rubidium stored along with a neutral buffer
gas such as helium in bottles whose
inner surface were coated with waxes similar to dotriacontane (C32 H 66).
The atoms were optically pumped
with circularly polarized light into
preferred states. Ammonia (NH 1)
masers (devices for amplifying microwave by stimulated emission of radiation) were developed by Townes
and others. None attained a great
accuracy a the atomic beam ce ium
clocks, but when less accuracy is
ufficient, an optically pumped rubidium clock is often used because of
it lower cost and lighter weight.
For many purpo s such as measurement
in radio a tronomy,
which need the highest pos ible stability over period of a few hours, the
Figure 4. The greatest accuracy, tabihty, and
reproducibility ue provided by atomic
dock , which take advantage of the periodic
magnetic interactions of nuclei and
electrons. In this schematic diagram, a
nudeu precesses about the field created by
the electrons' magnetism, with the vector I
representing the spin angular momentum of
the nudeu , J that of the electrons, and F
the resultant or vector um of the two. The
precessional motion is shown by the circle.
best existing clocks are atomic hydrogen masers (Fig. 6), which Daniel
Kleppner and I invented in 1960 (8).
An intense electrical discharge in the
ource converts commercially available molecular hydrogen (H 2) into
atomic hydrogen (H). The atoms
emerge from the source into a region
that has been evacuated to a pres ure
below 10 t. torr. A beam of diverging
atoms enters the state-selecting mag-
net, which has three north poles alternating in a circle with three south
poles. By symmetry the magnetic
field is zero on the axis of such a
magnet and increases away from the
axis. The low-energy hyperfme state
of atomic hydrogen, called the F = 0
state, has the characteristic that its
energy decreases with increasing
bottle for ten seconds and using the
same aperture for entrance and exit is
extremely low. Masers have very low
noise levels, especially when the amplifying element is an isolated single
atom.
The major disadvantage of the
hydrogen maser is that the atoms
collide with the wall at intervals,
In 1967 the international definition of the second was
changed from one based on the motion of the earth about
the sun to 9,192,631,770 periods of the cesium atom
magnetic field. Just as a ball runs
downhill to where the energy is lower, the atoms in the low-energy state
will move away from the axis to
where their energy is still lowerthat .is, the beam of these atoms will
be dispersed as in Figure 6. Conversely, most of the atoms in the
high-energy F = 1 state will be
drawn toward the axis, and thus
those atoms are focu d.
Atoms in the high-energy state
enter the srna!J aperture of the storage cell, a 15-cm diameter bulb coated with teflon on the inside. If these
atoms are exposed to microwave radiation at the hyperfine frequency,
more atoms will go from the higher
energy state to the lower, and the
released energy will make the microwave radiation stronger-that is, the
device will be an amplifier, a maser.
If the storage ce!J i placed inside a
tuned cavity, an oscillation at the
re onance frequency will be increased in magnitude until an equilibrium value is reached, at which
level the oscillation will continue indefinitely. The energy to maintain
the oscillation comes from the continuing supply of hydrogen atoms in
the high-energy state.
The atomic hydrogen maser has
unprecedentedly high stability due to
a combination of desirable features.
The atoms reside in the storage bulb
much longer than in an atomic beam
apparatus, and as a result the resonance line is much narrower. They
are relatively free and unperturbed
while radiating, unlike the atoms in
most optically pumped rubidium
ce!Js, which collide frequently with
buffer gas atoms. The first-order
Doppler shift is removed, since the
average velocity of atoms stored in a
changing slightly the hyperfine frequency and giving rise to a wall shift
of two parts in 10 11 • The wall shift
can be experimentally determined by
measurements with bulbs of different
diameters or with a deformable bulb
whose surface-to-volume ratio can be
altered. Since the shift is constant, its
exi tence mostly affects the maser's
accuracy rather than its stability.
Over periods of several hours, the
hydrogen maser's stability is better
than 1 X 10- IS.
Future improvements
It might appear that there is little
room for improvement in the measurement of time. That is not the
case. The state-selecting magnets of
atomic beam clocks can be replaced
with laser beams, and two beams of
atom can go through the transition
region in opposite directions to reduce the harmful effect of phase
shifts. New coating materials that
result in smaller and more stable wall
shifts may be developed for hydrogen masers, including superfluid liquid helium and teflon-like oils or
greases.
Atomic time standards based on
lasers and related quantum devices
are being developed at much higher
frequencies. Measurements to the
same fraction of a cycle at higher
frequencies correspond to smaller
fractions of the frequency. The firstorder Doppler shift is large, but various ingenious methods have been
devised to reduce or eliminate it.
Saturated absorption spectroscopy
and two-photon spectroscopy employ two laser beams at the same
frequency going in opposite directions so that the first-order shifts
1988 January-February
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be stored for long interval in trap
and since there are no wall effects,
trapped ions have attractive feature
as time and frequency standards.
Until recently, the ions had high velocities in the traps and consequently
large Doppler shifts. Used in combination with the laser cooling method
discu sed below, however, ion traps
are very effective.
More recently, various techniques have been invented to trap
neutral atoms with nonuniform laser
beams (10, 11). The forces arise in
some cases from the gradient of a
beam's electric field and in other cases from the momentum imparted to
an atom when it scatters a photon.
Although some of the traps are quite
shallow, the laser cooling techniques
help to overcome this disadvantage.
[For more on atom trapping and cooling, see the interview with Steven
Chu in this issue of American Scien-
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tist.]
Laser cooling is a method by
which a beam of light can be used to
damp the velocity of an atom or ion
(11, 12). The basic mechanism utilizes
a laser beam tuned lightly lower in
frequency than a strongly allowed
resonance
transition. When the vemagnet A
magnet 8
locity of the ion or atom is directed
against the laser beam, the light fredetector
quency in the ion's frame is Doppler
shifted closer to resonance, and the
light scattering takes place at a higher
rate than when the velocity is in the
oven
I
wave gu1de cav1ty
same direction as the laser beam.
Since
the photons are reemitted in
Figure 5. One me~ns for meuuring the hyperfine frequency of ~tom shown in Figure 4 is
random directions, the net effect over
the atomic be~m magnetic reson~nce ~ppuatus. To ~void l~rge numbers of collisions
a motional cycle is to damp the ion's
between ~toms ~nd the surrounding gu, pressure in the apparatus is kept below 10-7 torr. A
velocity by absorption of photon mobeam of atoms is boiled out of the oven, deflected by the inhomogeneous magnetic field of
mentum. Using pairs of counterprom~gnet A, md subjected to ~ we~k static m~gnetic field in the main put of the ~ppu~tus.
As the bum m~kes its first md second pu~ge through the w~ve guide c~vity, it is
pagating laser beams along each of
subjected to ~ we~ker perpendicular oscill~tory magnetic field; it is then deflected ~gain by
three mutually perpendicular axes,
the inhomogeneous m~gnetic field of m~gnet 8 ~nd fin~lly converted to po itive ions by ~
one can obtain a three-dimensional
hot-wire ionizer in the detector. The ions are counted electric~lly. In the diagram, m~gnets A laser-cooled region in which the atand 8 h~ve been rot~ted 90" ~bout ~ vertic~! axis to show the tips of their poles. The
oms move very sluggishly; such a
photogr~ph shows the cesium ~tomic be~m clock ~t the Nation~ Bure~u of Standuds. E<lch
region is frequently called "optical
end of the clock hu both ~n oven ~nd ~ detector, so that the ~ppuatus can be operated in
molasses." Atoms have been lasereither direction. (Photogr~ph courtesy of N~tion~ Bureau of Stmduds.)
cooled to less than 0.0003 K, and
narrow resonances have been obcancel (9-11). Concurrent with the can be trapped in a static uniform served with a single cooled and
development of stable oscillators at magnetic field in combination with a trapped ion. The laser cooling not
much higher frequencies, intense ef- suitable nonuniform electrostatic only overcomes the first-order Doppforts have also been made to multiply field (Penning trap) or in suitable ler broadening but also virtually elimand divide such frequencies so that nonuniform electric fields whose po- inates the second-order Doppler
valid comparisons can be made be- larities and gradients alternate in shift, which remains in all atomic
tween standards at all frequencies.
time at radio frequencies (Paul traps). clocks unless the atoms are sufficientAlthough ions cannot be stably Dehmelt and his associates (10) pio- ly cooled.
It is too soon to say which comtrapped in static electric fields, it has neered the use of traps for studying
been known for a long time that they atomic spectroscopy. Since ions can bination of trapping and cooling will
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be most effective, but there is a high
probability that one or more of the
new techniques we have examined
will lead to even more stable periodicities, which can in tum provide the
basis for even more stable atomic
docks.
Time and relativity
The measurement of time is so accurate today that relativistic effects on
clocks are directly observable. According to the special theory of relativity, if two observers with clocks
are moving relative to each other at a
constant relative velocity, each will
find that the other's clock is going at
a lower rate than his own clock. Thi
relati\ is tic effect has been demonstrated by a comparison of two atomIC dock · l'lCion! and after one of them
wa carried on a high-speed aircraft.
As a re ult of thi variation, the
idea of simultaneity for two events at
different locations 'Io es its meaning.
If two observer moving relative to
each other view two events at different locations, one observer may conclude that the events are simultaneous, while the other will be
convinced that they are not; or one
observer mav conclude that event A
precedes event B, while the second
observer concludes that B precedes
A. But the reordering of events can
never be so great that the cause of an
event seems to occur after the event
itself.
It might seem that the different
dock rates would lead to such horrible ambiguities that the concept of
time would be useless. That is not
the case. Each observer has his own
"proper time," which is the time as
measured by an accurate dock kept
with him at all times. Then, if one
knows the relative velocities of different observers, one can calculate their
proper times by the well-known
mathematical relations of the special
theory of relativity.
Einstein's general theory of relativity overcomes the limitation of his
special theory to constant relative velocities by including acceleration and
gravitation. One consequence of the
general theory is that the rate of a
dock depends not only on its apparent velocity but also on its gravitational potential-for example, a dock
far from earth will go at a slightly
faster rate than one dose to the earth.
microwave cav1ty
source
Figure 6. In the atomic hydrogen maser, which measures time with high stability over
periods of a few hours, hydrogen atoms pass through a low-pressure region (lo-• torr) into a
state-selecting magnet. In the magnet, atoms in the low-energy hyperfine state (F 0)
diverge from those in the high-energy hyperfine state (F 1). Most of the latter enter the
storage cell, a 15-cm diameter bulb whose inner surface is coated with teflon. Exposed to
microwave radiation, these atoms release their energy. The radiation takes on the extra
energy, continuing indefinitely at the frequency for which the energy transfer is greatest, the
hyperfine frequency. The oscillations of the microwave radiation that emerges through the
coaxial cable provide the periodicity required for a highly stable atomic clock. The
photograph shows an atomic hydrogen maser with the side and front panels removed. The
source of hydrogen atoms is at the top of the maser, and the storage cell is at the bottom.
Twenty-two masers like the one shown here are in use worldwide for radio astronomy,
tracking of space probes, and time keeping. (Photograph courtesy of Smithsonian
Astrophysical Observatory.)
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Time as date
The word time i used with two
different meanings. One is an interval of time, such as the number of
minute required to boil an egg. It is
this meaning that has been the subject of my discussion so far. Time is
also used in the sense of date, such
as 30 seconds past 2:00 P.M., EST, 12
December 1987. Clearly the two
meaning are closely related, since
the latter is the interval between
some agreed starting point and the
d signated time (13).
Even with international agreement on the definition of the second
as the unit of time and on a suitable
starting event for the measurement
of time as date, there are added
complications due to the variable rate
of the earth's rotation. If the day and
year were defined solely in terms of
atomic time, the calendar would
change it relation to the sun and
stars, to the confusion and inconvenience of astronomers and navigators and eventually even the casual
observer. As a result, by international agreement there are occasional
leap seconds to correct for the irregular motion of the earth.
The international time scale is
based on what is called Coordinated
Universal Time, or urc. The stan-
jects in our olar system, and from
the velocitie and gravitational potentials of transported clocks. Care is
required even in the definition of
synchronization. Ambiguities can be
avoided on the surface of the earth
by utilizing a "coordinate time" on
an underlying nonrotating system
and then calculating by known relativistic principles the relationship between this coordinate time and the
measured time on the surface of the
rotating earth (14).
Need for precise
measurement of time
It might seem that there should be no
need for measurements of uch high
accuracy as those already achieved,
much less those contemplated with
future improvements; but there are
many applications, some of which
push cprrent techniques to their limits.
In radio astronomy one looks
with a parabolic reflector at the radio
waves coming from a star, just as in
optical astronomy one looks with a
telescope at the star's light waves.
Unfortunately, the wavelength of the
radiation is about a miJJion times
longer than the wavelength of light,
so the resolution of the normal radio
telescope is about a million times
The unit of length has recently been defined as the
distance light travels in a specified time, and the basic
units for voltage and resistance soon will be defined in
terms of time
dards laboratories of a number of
major nations contribute measurements from their own cesium clocks
to the Bureau international de l'heure
in Paris, which averages them to
obtain the trrc. Leap seconds are
introduced with adequate warning at
the beginning or middle of the year
when required to prevent the urc
from differing from navigators' time
by more than 0.7 seconds. At present
a leap second is introduced approximately every year and a half.
Allowance must be made for different relativistic effects over the surface of the earth and in nearby space.
As we have seen, such effects arise
from the rotation and orbital motion
of the earth, from the gravitational
potential of the earth and other obAmerican Scientist, Volume 76
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worse than that of an optical telescope, depending as it does on the
ratio of wavelength to telescope aperture. However, if there are two radio
telescopes on opposite sides of the
earth looking at the same star and if
the radio waves entering each are
matched in time, it is equivalent to a
single telescope whose aperture is
the distance between the two telescopes, and the resolution of such a
combination exceeds that of even the
largest single optical telescope. For
precise matching in time, each of the
two radio telescopes needs a highly
stable clock.
Precision clocks are also needed
to measure the periods of pulsarsstars that emit their radiation in short
pul es-and the changes in their pe-
riods, which sometimes occur
smoothly but sometimes abruptly.
Of particular intere t are millisecond
pulsars, whose remarkable constancy of period rivals the stability of the
best atomic clocks. In fact, one of
these pulsars is so stable that it may
eventually be suitable as a standard
of time over long periods (15). Another millisecond pulsar is part of a
rapidly rotating binary star that is
slowly changing its period of rotation. This change can be attributed to
loss of energy by the radiation of
gravity waves-the first experimental evidence for the existence of gravity waves.
Accurate measurements of time
permit measurements of the variability of quantities that were once
thought to be constant. As we have
seen, the rotation period of the earth,
which once served as the ba is for
defining the unit of time, is now
known to vary by a few parts in a
hundred million from winter to summer and from year to year. Some of
the variation is regular and some
unpredictable. Different atomic clock
rates have been accurately compared
over long periods of time to see if
there might be changes in their relative rates which could correspond to
a change with time in the fundamental physical constants, but no such
change has yet been discovered.
Precision clocks make possible
an entirely new and more accurate
navigational system, the global positioning system, or GPS. A number of
satellites containing accurate atomic
clocks transmit signals at specific
times so that any observer receiving
and analyzing the signals from four
such satellites can determine his position to within ten yards and the
correct time within one hundredth of
a millionth of a second (10- 8 s).
Time can be measured so accurately that wherever possible other
fundamental measurements are reduced to time measurements. Thus
the unit of length has recently been
defined as the distance light travels
in a specified time, and the basic
units for voltage and resistance soon
will be defmed in terms of time.
Accurate clocks have provided
important tests of both the special
and general theories of relativity. In
one experiment, the periodic rate of a
hydrogen maser carried in a rocket
10,000 krn up changed with speed
and altitude by the amounts predict-
ed by the . pecial and general theories
(16, 17). In other experiment ob rvers have mea urcd the delay predicted by relativity for radio waves
passing near the sun. Future improvements in the stability of clocks
should make possible even more rigorous te ts of fundamental theories.
5. N . F. Ramsey. 1983.
References
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