Time Measurement

Summary

Time measurement is the science and practice of counting the repetitions of recurring phenomena and subdividing the intervals between each repetition into smaller units capable of being measured by various devices, including mechanical and atomic clocks. Time measurement is an important part of everyday life. It enables people to schedule activities, measure distances and speed, and navigate from place to place. The accurate measurement of immensely small units of time is essential to many fields of basic and applied science, including physics, computing, and medicine.

Definition and Basic Principles

The science of time measurement involves devising methods to count the number of iterations of phenomena that repeat themselves at regular intervals. For example, the rotation of theEarth on its axis, the shifts in the phases of the moon, and the changing of the seasons are all familiar units of time based on observable changes in material objects.

Time itself is a slippery concept with no simple scientific definition, though physicists consider it one of the four fundamental dimensions of the universe, along with length, width, and depth. Although scientists have not managed to satisfactorily define time, they have developed extraordinarily accurate ways of quantifying its passage so that individual events can be referred to on a consistent scale. The goal of time measurement is to supply information about the time of day, or the instant at which an event takes place; time interval, or the duration between two events; and frequency, or the rate at which a repeated event takes place.

One of the basic principles of time measurement is the notion that time is not a physical constant but rather can shrink or expand in response to other forces. According to Albert Einstein's general theory of relativity, strong gravitational fields can stretch the interval between the signals given by a clock. This phenomenon can affect both observations of the simultaneity of events and measurements of the duration of events. Clocks located in space, such as the ones on the International Space Station or on satellites orbiting the Earth, must be adjusted to correct for the effects of relativity. The special theory of relativity shows that two observers moving in relation to one another will observe duration and simultaneity differently. However, the differences between their observations are negligible, except when the speeds involved approach the speed of light.

Background and History

Attempts to mark the passage of days, months, and seasons using the movements of the sun, moon, and stars long predate attempts to measure shorter periods of time. Calendars, in other words, are older than clocks. Early clocks, such as sundials and obelisks, used the changing length of shadows over the course of the day to mark intervals that roughly corresponded to modern hours. Other primitive means of keeping track of the passage of time include water clocks, which were designed to drip at a relatively constant rate; hourglasses, which used the same principle but incorporated sand instead of water; and clocks that used the burning of candles or incense to measure time.

In the fourteenth century, mechanical (machine-powered) clocks began to appear. Some relied on a device known as an escapement, which controlled an unwinding spring that rotated a series of gears that, in turn, caused the hands of the clock to tick forward steadily. Others used a pendulum (a weight on a string), the back-and-forth motion of which served as a natural oscillator. Over the next several hundred years, inventors and engineers labored to reduce inaccuracies in timekeeping by compensating for such factors as friction, changes in temperature, and interference from other moving parts within the clocks.

The twentieth century saw two major advances in time measurement: the development of quartz clocks, which used electric circuits to generate constant electrical vibrations in quartz crystals, and the invention of atomic clocks, which take advantage of the natural resonance frequency of atoms to create an immensely stable oscillator. The atomic clock has become the standard tool for modern scientific timekeeping.

How It Works

Physical time scales span a dazzlingly wide range. The smallest scale that physicists can work with mathematically is Planck time, a single unit of which (about 10^–43 seconds) is defined as the length of time it takes for a photon traveling at the speed of light to cross a distance of one Planck length (about 10⁻³³ centimeters). In contrast, the cosmological time scale, on which events, such as the beginning of the universe and the formation of stars and planets are marked, consists of periods as vast as tens of billions of years.

Household Clocks. Most clocks, watches, and small electronic circuits used in everyday life are built on oscillators that use quartz crystals to generate a consistent pulse. Quartz crystals, whether natural or synthetic, exhibit a property known as piezoelectricity, which means that the crystals expand and contract—that is, vibrate—when they receive an electric force. The combination of a quartz crystal with a battery that applies and then reverses an electric voltage produces a regular oscillation, the frequency of which depends on the size of the crystal and the form into which it is cut. A quartz oscillator found in an ordinary household clock will probably have a quality factor (Q) of about 10⁴ to 10⁶ and be accurate to about a few seconds per month—perfectly adequate for everyday use.

Scientific Clocks. Scientific and industrial purposes demand atomic clocks with far more accuracy and stability than household clocks or watches. Atomic clocks make use of the fact that all atoms are capable of existing at a number of different discrete (noncontinuous) levels of energy. As an atom jumps back and forth between a higher and a lower energy level, it resonates at a particular frequency, and this frequency is exactly the same for every atom of a given element. For example, cesium-133 atoms (one of the two types of atoms most commonly used in atomic clocks; rubidium is the other) resonate between two particular energy levels at 9,192,631,770 cycles per second. In fact, the time it takes for this number of oscillations of a cesium-133 atom to take place serves as the definition of a second in the International System of Units (SI). This stable vibration can be thought of as paralleling the swinging back and forth of the pendulum in an old-fashioned mechanical clock.

There are many different forms of atomic clocks, but the basic mechanics involved are relatively consistent. A laser beam is shone into a cloud of atoms, tossing them high into the air. The frequency of light at which the laser shines is adjusted until the vast majority of the atoms experience a change in energy state. This process tunes the laser's frequency to match the resonance frequency of the atoms themselves, and it can then be used to mark the passage of time with great accuracy and reliability. Most atomic clocks also use a technique known as laser cooling, in which the movement of atoms is slowed by dropping the temperature within the clock to something very close to absolute zero. This lengthens the period of time during which the atoms can be properly observed.

Standards in Time Measurement. Within the field of time measurement, standards refer to devices or signals that serve as benchmarks for particular measurements, such as time intervals or frequencies. Standards allow other clocks to be precisely adjusted so that they all keep the same time and can be recalibrated according to the same measure if they should happen to gain or lose time. For example, the National Institute of Standards and Technology's (NIST) cesium fountain atomic clock NIST-F1, located in Boulder, Colorado, was introduced in 2000 as the standard atomic cesium clock on which all other clocks in the United States (US) were to be calibrated. NIST-F1 is one of hundreds of highly accurate atomic clocks around the world that together define the interval between each second in UTC, the official global time of day. NIST-F1 will gain or lose a second only once every hundred million years. In 2014, NIST introduced the NIST-F2, a more advanced cesium fountain atomic clock that operates alongside NIST-F1 and is approximately three times more accurate, gaining or losing a second once every three hundred million years or so. The NIST-F3 and NIST-F4 joined the NIST-F2, improving technological capabilities and accuracy.

Applications and Products

Navigation. The ability to measure time accurately and to precisely synchronize more than one clock is essential for many forms of navigation, including the US-based Global Positioning System (GPS, or NAVSTAR) used in many cars, boats, airplanes, missile-guidance systems, cell phones, watches, computer clocks, network clocks, and other devices. To determine a location, a GPS receiver calculates the time it takes for signals from four separate satellites to reach it, multiplies that time by the speed of a radio wave, and uses this figure to determine the exact distance between the receiver and each satellite. Then, the receiver creates four imaginary spheres, each of which has a radius the same length as the distance between the receiver and one of the satellites. The latitude, longitude, and altitude at the point where the spheres cut across each other is the latitude, longitude, and altitude of the device. This process is known as triangulation. The United States is not the only country with a global navigation satellite system; Russia, China, and the European Union have also developed similar applications.

Even the tiniest inaccuracies in the measurement of the time interval between when the signal is sent and when it is received can cause substantial errors—differences of several feet or more—in the triangulation calculation that determines the user's physical location. As a result, GPS receivers rely heavily on standard atomic frequency references. Each satellite has a small handful of local atomic clocks onboard (multiple clocks are used to provide a system of redundancies, a way to ensure reliability and accuracy). Each clock is calibrated to a master atomic clock measuring UTC and managed by the US Naval Observatory. In general, each clock on a GPS receiver, no matter where on Earth it is located, will be no more than 500 nanoseconds to 1 millisecond out of sync with UTC. Time measurement facilitates the movement of cars, boats, and airplanes on the earth and sea and in the sky and also is fundamental to interplanetary space travel. Every National Aeronautics and Space Administration (NASA) spacecraft is equipped with standards-referenced atomic clocks that enable accurate and reliable navigation.

Telecommunications. Every telecommunications system in use—including standard telephone systems, wireless communication networks, radio and television broadcasting systems, cable networks, satellite telephones, and the Internet—relies on the ability to synchronize exactly the signals sent between transmitters and receivers. For example, wireless telephones transmit information to each other in the form of small chunks of data called packets. The signal between the transmitter and receiver must be perfectly synchronized to identify where each packet begins and ends. In addition, the transmitter and receiver must process data at the same speed so as to prevent lags or data loss. Finally, for multiple telephones to send data using the same frequency channel, each telephone is assigned a particular time slot. Individual transmitters send their signals in series, never interfering with one another's data, even though they share the same medium of transmission. (Radios broadcast their signals using exactly the same system, which is known as time division multiple access, or TDMA.) Because cell phones move from location to location as their users travel, they also require a precise time-measurement system, usually pegged to the atomic clocks on GPS satellites. These systems help make minute changes to the timing of the cell phone signals to compensate for the fact that the distance between the telephone and the base station is changing.

Electronics and Computers. Every computer contains a small, built-in quartz-based oscillator that serves as an internal marker of time intervals for the machine. The central processing unit (CPU) uses this clock to determine the intervals at which its microprocessor is directed to complete instructions, as well as for purposes, such as scheduling automatic processes and time-stamping events. It is also important for computers that are sending and receiving information over a network or the Internet to be highly synchronized with each other for data to be transmitted accurately. Since the piezoelectric qualities of quartz crystals change with temperature, computer clocks tend to drift as the machinery inside them heats up with use. For this reason, most computer networks are equipped with a Network Time Protocol (NTP) server that uses an atomic frequency standard, such as the signal produced by NIST-F2, to synchronize the internal clocks of the computers to a more accurate and stable time signal.

Careers and Course Work

A student contemplating a career as an academic or industry-based researcher in time measurement or metrology (the study of measurement itself) or planning to enter a related field, such as astronomy, geophysics, or engineering, should begin by pursuing a rigorous course of study in science, technology, and mathematics, preferably leading to a Bachelor of Science degree in physics. Of prime importance within the field of physics are subjects, such as atomic structure, special and general relativity, electromagnetics, cosmology, the physics of the solar system, and quantum mechanics. In mathematics, geometry and trigonometry are especially instructive for the science of time measurement.

An interest in time measurement might lead down several career paths. For instance, one might enter the field of horology and become a designer of timekeeping apparatuses, such as clocks, watches, timers, and marine chronometers. One might become a staff researcher investigating the physics of time at a government or university laboratory or observatory, such as the National Institute of Standards and Technology, the US Naval Observatory, or the NMi Van Swinden Laboratorium in the Netherlands. Alternatively, one might pursue a job developing, maintaining, or repairing time measurement instrumentation for scientific or industrial purposes, in which case, coursework in mechanical and electrical engineering would also be required.

Social Context and Future Prospects

Scientists are working on a new generation of optical atomic clocks that use atoms, such as strontium-87 and mercury, to produce oscillations at optical frequencies about 100,000 times faster than conventional cesium clocks, which oscillate at microwave frequencies between 0.3 and 300 gigahertz. These clocks are still experimental but hold the potential to be about one hundred times more accurate than NIST-F2—in other words, only gaining or losing a second once every ten billion years—and capable of measuring time at higher resolutions (subdividing it into smaller units).

Time-measurement research has the potential to revolutionize a host of fields. Such devices could allow military navigators to bypass jammed GPS signals, enable emails to be encrypted at a far safer and more complex level than ever before, or be used in inexpensive portable medical imaging devices to scan patients' hearts, brains, and other organs at high resolution even outside hospitals. Optical and quantum clocks are at the forefront of emerging technologies in time measurement. These promise further increased accuracy and precision. Projects, including the NASA’s Deep Space Atomic Clock, explore space-based timekeeping. The study of time measurement will remain an expanding field accessible to many interdisciplinary fields of study. 

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