Timekeeping as a practice has evolved through centuries from observing natural forces like the Sun, Moon, water etc to progressing to measure atoms and their nuclei in the present time. The advent of advanced  technology entails the development of next generation devices ie. Nuclear Clocks.

• Three Major Developments have been reported in building functional Nuclear Clocks: 
• • A laser to excite thorium-229 nuclei to a specific higher energy state
  • A way to link a thorium-229 nuclear clock with an optical clock
  • A precise estimate of the excitation energy. The nucleus’s de-excitation emission has a frequency of 2,020 terahertz, alluding to an ultra-high precision.

About Clocks

• Clocks are devices that measure the passage of time and display it. 
• Parts: Modern versions of Clocks contain a power source, resonator, and counter.
• A clock measures the amount of time that has passed by tracking something that happens in repeating fashion, at a fixed frequency.
  • Example: 
    • The sundials of ancient times tell time by casting shadows of changing lengths against sunlight.
    • Water Clock: The water would slowly fill a vessel, with its levels at different times indicating how much time had passed. 
      • The water clocks were fitted with additional water tanks, gear wheels, pulleys, and even attached musical instruments to the point where they were practically developing rudimentary analog computers.
    • Modern Clocks: They use quartz crystal. 

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Modern Clocks: Evolution of Mechanical Clocks

• Invention of the Verge Escapement Mechanism: It was the first major revolution in timekeeping  invented in the 13th century, paving the way for modern clocks opening the door for the development of mechanical clocks. 
• Process: The fundamental element here was a gear that, through a combination of mechanical arrangements, could only move in fixed intervals. 
  • Escape Wheel: The first gear was called an escape wheel if it was circular.
  • Balance Wheel: A second gear, called the balance wheel, was enmeshed with the first such that when the escape wheel moved forward one gear tooth at a time, the balance wheel would oscillate back and forth.
  • This oscillation would drive the ‘hands’ of a clock on a clock face as long as some force was applied on the balance wheel to keep it moving.
• Invention of Spring-Driven Clocks:
  • Using Coiled Spring: Clockmakers developed and improved on the previous mechanical devices between the 15th and 18th centuries by replacing the suspended weight that applied the force on the balance wheel with a coiled spring. 
    • The balance spring would return the balance wheel to its neutral position after every ‘tick’ motion before the ‘tock’ motion towards the other side. As a result, the clocks lost a few minutes a day versus a few hours a day before.
  • New Mechanisms like the Fusee were developed to ensure the spring always delivered a uniform force and does not become inaccurate as the spring unwound. 
    • The idea to couple a balance spring with the balance wheel also led to the advent of pocket watches
• Pendulum Clock: The Clock was invented in the mid-17th century, by the Dutch inventor Christiaan Huygens.
  • The clock also used the escapement mechanism, Huygens made an important contribution by working out a formula to convert the pendulum’s swings to the amount of time passed.
• The Marine Chronometer: It was developed in the 18th century in 1761 AD by a  carpenter named John Harrison and delivered to the British government for its longitude prize.
  • Need: For measuring time on a ship. A ship needs to accurately know its latitude, longitude, and altitude for it to locate itself on the face of the earth.
  • Measuring the Longitude required  an accurate clock onboard each vessel. The Marine Chronometer featured mechanisms to ensure the clock’s operation wasn’t affected by the ship’s rocking, the force of gravity, and some temperature changes.
• Electric Clocks:  They were developed in the 19th century whose energy source was a battery or an electric motor rather than suspended weights or springs, although the former and latter were attached to improve the effciency of existing designs.

The 20th Century Clocks

It heralded the development of two important types of clocks ie. the Quartz clock and the Atomic clock with a similar fundamental setup ie. consisting of a power source, a resonator, and a counter.

• Quartz Clocks: Quartz clocks are inexpensive to make and easy to operate, and their invention led to watches and wall-clocks becoming very common from the mid-20th century.
  • Quartz Crystal: The resonator here is a quartz crystal. The power source sends electrical signals to a quartz crystal, whose crystal structure oscillates due to the piezoelectric effect. 
  • The signal’s energy can be tuned to make the crystal oscillate at its resonant frequency, making it the resonator. The counter counts the number of periodic oscillations and converts them into seconds (depending on the crystal’s period). 
  • A digital display shows the counter’s results.
• Atomic Clocks: The resonator here  is a group of atoms of the same isotope and the power source is a laser. 
• The laser imparts a specific volume of energy for the atom to jump from its low energy state to a specific higher energy state. The atom releases radiation with a well-established frequency when it jumps back down.
  • Example:  The Caesium atomic clock uses caesium-133 atoms as the resonator. These atoms release radiation of frequency 9,192,631,770 Hz when they excite and  de-excite.
    • The counter records one second has passed when it detects 9,192,631,770 full waves of the radiation.
  • Time Standard: Atomic clocks are distinguished by their resonator and each such clock is called a time standard. 
    • Example:  India’s time standard is a caesium atomic clock at the National Physical Laboratory, New Delhi, which maintains the Indian Standard Time.
  • Range: The Frequency  emitted in a caesium atomic clock is in the microwave range (gigahertz), and the resulting clock loses or gains a second only once in 20 million years or so.

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Future Clocks

• Next -Generation Optical Clocks: The radiation in the next-generation clocks is in the optical range (hundreds of terahertz).
  • These devices use strontium or ytterbium atoms as resonators and don’t miss a second in more than 10 billion years.
• Nuclear Clocks: Their resonators are the nuclei of specific atoms rather than the whole atom. Atomic clocks need to make sure the resonator atoms aren’t affected by energy from other sources, like a stray electromagnetic field. 
  • An atom’s nucleus, however, is located well within each atom, surrounded by electrons, and thus could be a more stable resonator.

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