Section 5–5 Units and standards of time

(Units of time / Standards of time / Atomic clock standards)

In this section, the three interesting points discussed are units of time, standards of time, and atomic clock standards.

1. Units of time:

“…Until very recently we had found nothing much better than the earth’s period, so all clocks have been related to the length of the day, and the second has been defined as 1/86,400 of an average day (Feynman et al., 1963, section 5.5 Units and standards of time).”

Dr. Sands explains that it is convenient to have a standard unit of time, such as a day or a second, and we can refer any period of time as a multiple or a fraction of this unit. However, the experimental determination of a unit of time has been central to the history of time-keeping and time measurement. Specifically, it was a challenge to fit the two units of time, namely month and day, into a calendar year. First, a year was the time for the earth to rotate around the sun that is measured between two successive vernal equinoxes. Second, a month was the time for the moon to rotate around the earth such that it passes a “fixed” star again. Third, a day was the time for the earth to rotate about its axis such that the “high noon” is observed repeatedly. Currently, a day is divided into 24 hours based on the ancient Egyptians’ observations of stars.

Historically, we used to define a second as 1⁄86,400 of a solar day or “the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.” (One day being 24 hours X 60 minutes X 60 seconds = 86,400 seconds). The subdivision of hours and minutes into 60 is based on the ancient Babylonians’ preference of using numbers to the base 60. Our current definition of a second is based on the 1967 Thirteenth General Conference on Weights and Measures held during 10-16 October 1967. The SI second of atomic time was defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

2. Standards of time:

“…For a long time, the rotational period of the earth has been taken as the basic standard of time (Feynman et al., 1963, section 5.5 Units and standards of time).”

Importantly, a human pulse is not reliable as a basic standard of time because the pulse rate of a person may vary throughout a day. Although the earth’s rotational period was used as a basic standard of time, it has been found that the rotation of the earth is not exactly periodic when it is compared with more accurate clocks. Furthermore, the rotational speed of the earth is not constant and gradually slowing because of atmospheric circulations, geophysical phenomena and tidal friction (due to the moon’s gravity). As a result, a “second” based on this astronomical standard of time is gradually longer than the “second” that is defined by an atomic standard of time. In a sense, the definition of a second is a historical and an arbitrary choice.

In 1967, Jocelyn discovered the first pulsar (also known as a neutron star) that rotates at a high speed and it emits radio pulses at relatively regular intervals. Currently, it is possible to create a pulsar clock (or an astronomical standard of time) by using a radio telescope which receives signals from designated pulsars. These radio signals appear like a lighthouse beacon due to the rotation of the pulsar. Using the radio telescope, astronomers can measure the arrival times of successive radio pulses to a precision of 100 to 500 nanoseconds. In other words, pulsars could be considered as a more precise astronomical standard of time.

3. Atomic clock standards:

“…since it has been possible to build clocks much more accurate than astronomical time, there will soon be an agreement among scientists to define the unit of time in terms of one of the atomic clock standards (Feynman et al., 1963, section 5.5 Units and standards of time).”

According to Sands, an atomic clock’s internal period is based on an atomic vibration which is very insensitive to the temperature or any other external effects. In the 1960s, these clocks could keep time to a precision of one part in 10⁹ or better. During that time, Professor Norman Ramsey at Harvard University designed and improved an atomic clock which operates based on the vibration of the hydrogen atom. He believed that this hydrogen clock could be 100 times more accurate. Thus, physicists predicted that atomic clocks could be much more accurate than astronomical time, and there would be an agreement among scientists to define the unit of time in terms of an atomic clock standard.

Modern atomic clocks are based on Caesium 133 atoms because they are relatively massive and more stable as compared to hydrogen clocks and ammonia clocks. An important principle of the caesium clock is that caesium atoms are flipped into a higher energy state by using microwave radiations. Furthermore, these atoms will reach a detector (or a wire) and result in an electric current that can help to calibrate (or tune) quartz crystal clocks. Interestingly, the definition of a second was made even more specific in 1997 with the stipulation that caesium atoms are at rest at a temperature of 0 Kelvin (or an environment whose thermodynamic temperature is 0 K). However, one may question how this temperature can be achieved exactly.

Note: There are at least 13 Nobel (physics) laureates that have contributed in the physics of time-keeping since the 1940s: Otto Stern, Isidor Rabi, Polykarp Kusch, Nikolai Basov, Aleksander Prochorov, Charles Townes, Alfred Kastler, Norman Ramsey, Hans Dehmelt, Wolgang Paul, Steven Chu, Claude Cohen-Tannoudji, and Williams Phillips (Jones, 2000). Einstein, of course, has also contributed in providing an operational concept of time.

Questions for discussion:

1. How would you define a unit of time?

2. How would you select a standard of time?

3. Why do metrologists prefer an atomic time standard instead of an astronomical time standard?

The moral of the lesson: the unit of time can be defined by using an atomic time standard or an astronomical time standard.

References:

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

2. Jones, T. (2000). Splitting the Second: The Story of Atomic Time. Bristol: Institute of Physics.