Timekeeping: From the Sun to the Atomic Second

Transcription

1 Timekeeping: From the Sun to the Atomic Second Dr E Anne Curtis National Physical Laboratory, Teddington, UK Celebration of Science 17 October 2015

2 Our History I believe that in the National Physical Laboratory we have the first instance of the State taking part in scientific research. The object of the scheme is, I understand, to bring scientific knowledge to bear practically on our everyday industrial and commercial life; to break down the barrier between theory and practice; to effect a union between science and commerce. - HRH the Prince of Wales 1902

3 The UK s National Measurement Institute World-leading NMI 700+ staff 500+ scientists BIS s largest science and technology asset (a government laboratory) In partnership with the universities of Strathclyde and Surrey 400 laboratories, state of the art facilities

4 Metrology SI base units: electrical quantity thermodynamic temperature length time luminous intensity mass amount of substance How did we get from the sun to the atomic second? The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom

5 Overview Historical overview of timekeeping what drives our need for clocks the technology of metrology Current state-of-the art how to make an atomic clock what do we need it for today

6 Stability - averaging data can give a more precise result Systematics - understanding the offset Accuracy vs Precision Increasing precision Increasing accuracy Inaccurate Imprecise Inaccurate Precise Accurate Imprecise Accurate Precise For clocks: accuracy getting to work on time stability/precision measuring heart beat reproducibility do you trust your clock? Reproducibility - same results?

7 What is a clock? Oscillator something periodic - repeating (pendulum, electromagnetic radiation) Counter something that can measure the oscillations rf counter Reference for accuracy and reproducibility from place to place (how do you check your clock is still on time?)

8 Two questions: Local solar time Measuring Time Why do people need to know what time it is? How do these needs change over time? based on the motion of the sun across the sky Egyptian obelisks (3500 BC), portable shadow clocks (1500 BC) Systematics: changes throughout the year overcast days ~12 hours a day without a clock! Oscillator Counter Reference

9 Measuring Time Water clocks Among the first timekeepers that didn t depend on the motion of celestial bodies Simple Egyptian clocks (1500 BC), more elaborate Greek & Roman clocks (100 BC to 500 AD) Inflow and outflow types Invented to measure time at night timing of the night watch night time religious ceremonies also used to time debate in court portable version to measure pulse rate Systematics: sloping sides only give near constant drip/flow temperature and humidity affect flow rate evaporation Calibrate with sundial

10 Measuring Time Egyptian Waterclocks Innovations: markers to measure the passing of time gears to enable mechanical readout mechanism Elaborate Su Song Chinese Waterclock (1088 AD)

11 Measuring Time Pendulum Clocks Concept credited to Galileo (1582) First clock constructed by Huygens (1656) Systematics: Period of a pendulum (small oscillation amplitude) l T~2p g l = length of pendulum g = acceleration due to gravity change in temperature/humidity (length of pendulum) change in air pressure (friction on pendulum) To compensate temperature changes: mercury pendulums (1721) maintains centre of mass bimetal construction (1726) compensates length change Error to less than 10 seconds / day

12 What is the longitude problem? Latitude is measured by the sun s angle at noon Easy to measure local time by the sun, but to find your horizontal position (longitude) you need to know the time at a known reference point Measuring Time Navigation in the 17 th century Founding of the Royal Observatory by King Charles II in 1675 Purpose to solve the problem of longitude at sea Earth circumference 360 o 24 hour day 15 o = 1 hour Pendulum clocks were accurate enough on land, but problematic at sea Large variation in humidity and temperature Rocking of the ship (storms) Corrosion from the sea air

13 Longitude Act of 1714 Measuring Time 20,000 prize for measuring longitude to 0.5 (> 2 million) error of 35 miles at equator equivalent to an accuracy of 3 seconds per day But how accurate are your seconds? Prize won by John Harrison Sea-going clock H4 accurate to 0.2 seconds / day ~30 km over the 7-week trip

14 Measuring Time Greenwich Mean Time Noon is not the same as the sun being directly overhead (±15 min over year) Instead use mean annual average of the non-uniform motion of the real sun 1 second = 1 / of the mean solar day Established as the global standard in 1884 Any standard based on the rotation of the earth was going to run into problems, as measurement technology and timing needs became more sophisticated.

15 Milliseconds seconds Variation in the earth s rotation What causes changes in earth s rotation? Conservation of angular momentum weather patterns, tidal patterns changes in mass distribution of the earth (fluid core, earthquakes) moon and earth getting farther apart The difference between Earth rotational time (UT1) and the mean solar day. Variability of Earth s Rotation Length of Day seconds Calendar year Need a better reference than the earth

16 The introduction of atomic time 1955: First caesium atomic clock produced by Essen & Parry at NPL, accurate to 1 part in : International Atomic Time (TAI) began, following the development of further caesium clocks at NBS (USA) and ON (Switzerland) Reproducibility 1967: Caesium clock adopted as the basis for the international definition of time The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. 1 part in is ~1 second error in 300 years compared to 0.2 s per day, (a few parts in 10 6 ) for the Harrison clock 1972: GMT was replaced as the international time reference by Coordinated Universal Time (UTC) maintained by an ensemble of atomic clocks around the world (40 countries, ~260 clocks) Leap seconds keep UTC and earth rotational time consistent

17 Mircowave atomic standards today Caesium fountain clock Accuracy ~ (with several days averaging time) error of 1s in 3 billion years. How do atoms take us from errors of second/day to second/billion years?

18 visible/optical Energy Atoms as references for clocks Quantum Mechanics - quantized energy levels excited UV/X-ray frequency (f) Energy (E) The atomic clock counter measures radiation ground oscillator produces radiation radio/ir feedback atomic signal to oscillator precision of the atomic reference atoms are identical reproducibility atomic reference

19 What is driving modern day clock development? Navigation - GPS + satellite control - missile guidance systems - deep space missions Synchronisation - very long base-line interferometry and arrays (radio telescopes) - global economy Standards - economic and public needs

20 Global Navigation Satellite Systems Image courtesy of ESA 1. How does satellite navigation work? 2. What do clocks have to do with any of this? Proposed constellation of Galileo satellites in space

21 Time of Arrival (TOA) Ranging Foghorn example Dx Dx 3 Dx 2 speed of sound v ~ 335 m/s v= Dx Dt (meters) (seconds) Dx = v Dt

22 Time of Arrival (TOA) Ranging timing error Foghorn example e Dx Dx 2 e timing offset = 1 s speed of sound ~ 335 m/s e = v t offset e = 335 m error! Dx 3 e Timing errors reduce the accuracy of the position measurement

23 Satellite Navigation Dx = v Dt Dx Satellite Receiver speed of sound v ~ 335 m/s speed of electromagnetic radiation v ~ m/s for e ~ 0.3 m (1 ft), nanosecond timing is necessary

24 What s next? Clocks keep getting better and better - optical clocks would not lose 1 second in the lifetime of the universe Earth-based clocks start to run into problems - gravitational redshift (frequency of clock changes depending on its distance from the geoid) - atmospheric effects (communicating from satellites to earth) Solution? Put your optical clocks in space!

25 Optical master clock in space Necessary for intercomparison of ground-based optical clocks Fundamental physics Tests of general relativity, e.g. STE-QUEST mission Geoscience Direct measurement of earth s geopotential with high resolution Tracking tectonic plate movement Navigation Upgrade of GPS/Galileo to optical clocks VLBI Very Long Baseline interferometry (LISA gravity wave detection) VLA Very Large (telescope) Arrays (Radio astronomy timing) Deep space missions Communications Space-based optical clocks What has history shown us? The better the clock, the longer the list

26 Title of Presentation Name of Speaker Date The National Measurement System delivers world-class measurement science & technology through these organisations NPL Open House 17 May pm - 8pm The National Measurement System is the UK s national infrastructure of measurement Laboratories, which deliver world-class measurement science and technology through four National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL The former National Engineering Laboratory, and the National Measurement Office (NMO).

27 SI-bot. The Movie. Are you, or do you know a budding young film maker? Or perhaps you're keen to share your science know-how? NPL's SI-bot movie challenge could win you a tour of our facilities and a cash prize of 500.Register now to take part and to receive your free SI-bot! Here at the National Physical Laboratory (NPL), to celebrate what we do, we have created a range of SI-bot figures, each one representing a different unit of measurement. The second SI-bot represents all things related to time. The challenge is to make a film about time, less than one-minute long, using the second SI-bot. To receive your SI-bot, follow the link to register and let us know where you would like it sent. Two lucky winners will receive cash prizes ( 500) to further their film-making experience. In addition, the creator(s) of the best overall film will be given a personal tour of NPL with their friends or family. Details and registration link at

28 1 second 1 1 minute 60 1hour day month year years 3.15E years 3.15E years 3.15E+10 1 million years 3.15E million years 3.15E million years 3.15E+13 1 billion years 3.15E billion years 3.15E+15