NIST ion clock sets new record for most accurate clock

Original Article ↗ Hacker News Discussion ↗

Gravitational time dilation and sensing

  • Commenters note the new clock can resolve height differences of a few centimeters via GR time dilation (≈gh/c²), vastly better than cesium’s ~1 mile scale.
  • People speculate about detecting nearby human-scale masses or submarines via their gravitational effect on clock rate.
  • Linked work on gravity-based submarine detection concludes required sensitivity (~1e-13) makes it impractical; neutral buoyancy cancels first-order anomalies, leaving only weak higher-order “dipole” effects.

Gravity, potential, and relativity subtleties

  • Debate over whether time dilation is tied to gravitational acceleration or potential.
  • Example: clocks at Earth’s core vs surface — no net force at the center, but deeper in the potential well, so time runs slower.
  • Explanations use GR language: free-fall along geodesics vs accelerated observers held at the surface; “no net force” ≠ “no potential difference.”
  • Gravitational redshift is cited as direct evidence that deeper potentials run slower.

How quickly can you see a difference?

  • Disagreement over claims that you could detect a centimeter height change “instantly.”
  • Clarifications: the physical effect is immediate, but measurement requires integration time because of noise and finite SNR (Allan variance).
  • Key points:
    • You don’t wait for a whole extra “tick”; you measure frequency/phase differences of continuous waves.
    • Optical clocks operate at ~10¹⁴–10¹⁵ Hz; a 1e-18 fractional shift is mHz-scale and in principle resolvable in ~1s, but practical stability demands hours–days of averaging.
    • Reference to prior NIST experiment measuring 33 cm height difference over ~140,000 s.

Building and operating optical clocks

  • For a well-equipped lab, the main barriers are expensive lasers and frequency combs plus high expertise, not just raw materials.
  • Frequency combs remain “call for pricing” lab gear; most are customized, slowing commoditization, though integrated/on‑chip combs are emerging.
  • To validate performance, you typically need multiple clocks (ideally using different physical implementations) or access to a better reference.

Time standards, “most accurate,” and what accuracy means

  • The SI second is still defined via cesium; optical clocks are candidates for a future redefinition using a faster, more stable transition (e.g., Al⁺).
  • Several comments distinguish:
    • Accuracy: closeness to the defined standard (or to a modeled ideal transition once adopted as the standard).
    • Precision/stability: how consistently a clock reproduces its own frequency (how slowly two nominally identical clocks drift relative to each other).
  • Clarifications on “measuring the accuracy of the most accurate clock”:
    • You treat a well-understood atomic transition as the invariant reference and quantify environmental/noise shifts.
    • Comparing multiple identical clocks reveals noise via relative drift (random walk).
    • Comparing different species (e.g., Al⁺ vs others) can probe possible changes in “fundamental constants.”

Relativity, absolute time, and “clock vs clock signal”

  • Discussion notes there is no absolute cosmic time reference in modern physics; time-translation symmetry implies only differences are observable.
  • Global time scales (like TAI) are human-defined constructs built from ensembles of national lab clocks.
  • Optical ion and lattice clocks don’t themselves output a continuous GHz “clock signal”; a laser locked to the atomic transition, plus a frequency comb, provides a usable, divided-down electronic signal.
  • Analogy with computers: long-term stability from external reference (atomic transition), short-term from a cavity-stabilized laser.

Potential applications and limits

  • Speculation about:
    • “Einsteinian altimeters” that use local time rate as a height sensor; local density variations (geology) would perturb readings but could also enable precision gravity mapping.
    • Time-based gravitational wave detectors (“TIGO”), though commenters note you’d need clocks separated by at least a wavelength and waves would have to be very low-frequency.
    • Using time-dilation mapping like radar to sense large moving masses; judged “plausible only” with more orders of magnitude and miniaturization.
  • For GPS and navigation:
    • Better satellite clocks reduce one error term, but dominant GPS errors are ionosphere/troposphere propagation and satellite ephemeris.
    • Centimeter-level GNSS already exists using augmentation (base stations/subscriptions), but robust local sensing (optical, lidar, buried guides, etc.) is still needed for lane keeping.

NIST services, infrastructure, and policy

  • Side thread on NIST’s authenticated NTP: keys require postal mail or fax and replies are postal-only, which is awkward for non‑US users.
  • NTS (NTP over TLS) is mentioned, but current NIST/FIPS rules around AES-SIV make that unlikely for now.
  • Broader concern that while US labs lead in clock science, China is ahead in diversified, resilient timing distribution (BeiDou, eLoran, fiber). GNSS jamming/spoofing incidents highlight single-point-of-failure risks.

Funding and institutional context

  • Several comments praise US publicly funded basic science for enabling advances like this and express worry about proposed budget cuts and lab closures (including NOAA/NIST facilities).
  • Some tension over what counts as “real” vs “pretend” science; others argue that political interference in topic selection undermines the high-ROI “fund broadly, let experts decide” model.