UPSC Prelims Practice Questions — Scientists report world’s first working nuclear clocks

Q1. With reference to the difference between an atomic clock and a nuclear clock, consider the following statements: Which of the statements given above is/are correct?

  1. In an atomic clock the reference frequency comes from electrons moving between energy levels, whereas in a nuclear clock it comes from a change in the energy state of the nucleus.
  2. A nuclear clock is less disturbed by stray electric and magnetic fields mainly because the atomic nucleus carries no net electric charge.
  3. A nuclear clock keeps time by counting the steady rate at which its radioactive isotope decays, while an atomic clock does not depend on decay.
  • A. 1 only
  • B. 1, 2 and 3
  • C. 2 and 3 only
  • D. 1 and 2 only

Q2. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. The first working nuclear clocks reported in 2026 are based on the isotope thorium-229.
  2. Thorium-229 is the fertile isotope that is converted into fissile uranium-233 in the third stage of India's nuclear power programme.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q3. Atomic clocks have long used electronic transitions, but clocks based on transitions inside the atomic nucleus were achieved only recently. Which one of the following best explains why most nuclei were unsuitable for such clocks?

  • A. Nuclear transitions are strongly disturbed by stray electric and magnetic fields in laboratories
  • B. Most nuclei are radioactive and decay before any transition can be measured reliably
  • C. Nuclear transition frequencies are too low to be counted by available electronic counters
  • D. The photon energies needed lie in the X-ray or gamma range, which is hard to control precisely

Q4. With reference to how the thorium-229 nuclear clock keeps time, consider the following statements: Which of the statements given above is/are correct?

  1. The cycles counted as the clock's ticks come from the laser's oscillation, while the thorium nuclei act as the reference against which the laser is checked.
  2. A fall in how often the thorium nuclei get excited indicates that the laser frequency has drifted away from the nuclear transition frequency.
  3. Because the laser is repeatedly corrected to match the nuclear transition, the clock's long-term accuracy is set by the nucleus rather than by the laser hardware.
  • A. 2 and 3 only
  • B. 1, 2 and 3
  • C. 1 and 2 only
  • D. 1 and 3 only

Q5. With reference to the role of precise clocks in satellite navigation systems, consider the following statements: Which of the statements given above are correct?

  1. Since navigation signals travel at the speed of light, a one-microsecond error in a satellite clock can shift a computed position by roughly 300 metres.
  2. Time signals from navigation satellites are used to synchronise telecom networks and power grids even where no position fix is required.
  3. A regional system such as NavIC is designed for positioning alone, so its users must take their time reference from GPS.
  4. Atomic clocks on navigation satellites need periodic correction, because even atomic standards drift slightly over time.
  • A. 2, 3 and 4 only
  • B. 1, 2, 3 and 4
  • C. 1, 2 and 4 only
  • D. 1 and 2 only

Q6. Consider the following facts: I. C-DOT and CSIR-NPL are developing a NavIC-based Primary Reference Time Clock that will give telecom and internet service providers direct traceability to IST. II. Under the Legal Metrology (Indian Standard Time) Rules, 2026, telecom providers, power grids and financial entities must synchronise their systems with IST. III. Transaction records from different networks are used as evidence in digital forensic analysis. Which of the following inferences can be drawn from the above?

  1. These measures aim to reduce India's dependence on a foreign-controlled satellite system for the time reference of its critical networks.
  2. Once the reference clock is deployed, telecom operators will no longer need any local clocks of their own in their networks.
  3. A common traceable time reference makes timestamps from different networks comparable, which strengthens the evidentiary value of digital records.
  • A. 1 and 3 only
  • B. 2 and 3 only
  • C. 1 only
  • D. 1, 2 and 3

Q7. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. When a leap second is introduced internationally, a leap second is also inserted into Indian Standard Time.
  2. Indian Standard Time is maintained as a fixed offset of five hours thirty minutes from Coordinated Universal Time.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q8. Ultra-precise clocks such as the thorium-229 nuclear clock are being used to probe fundamental physics. In this context, consider the following statements: Which of the statements given above is/are correct?

  1. A clock can detect a dark-matter candidate only if that dark matter emits light at the same frequency as the clock's transition.
  2. If a fundamental constant such as the fine-structure constant changed slightly, transition frequencies would shift, and two clocks with different sensitivities would drift apart.
  3. A clock-based dark-matter search looks for tiny, possibly periodic, variations in the ticking rate rather than for particles striking a detector.
  • A. 1, 2 and 3
  • B. 1 only
  • C. 2 and 3 only
  • D. 1 and 3 only