Learning from the time and length redefinitions, and the metre demotion

Area of Science:

• Metrology and fundamental physics.
• Atomic physics and precision measurement.
• Space-based experimentation.

Background:

• Historical redefinitions of the meter and second provide valuable insights.
• Progress in length metrology has advanced from prototype standards to light-speed definitions.
• Atomic clocks have achieved remarkable precision, exceeding 16 digits with laser cooling techniques.

Purpose of the Study:

• To discuss challenges in redefining the kilogram.
• To draw lessons from past metrology redefinitions for future standards.
• To introduce the STAR Mission for testing space-time asymmetries.

Main Methods:

• Reviewing historical advancements in length and time metrology.
• Highlighting the impact of laser cooling and optical frequency combs on precision.
• Describing the design and goals of the STAR Mission, including onboard optical clocks and length etalons.

Main Results:

• Metrology has progressed significantly, with optical clocks now exceeding 17 digits of accuracy.
• Optical combs simplify frequency comparisons, enabling higher precision.
• The STAR Mission aims to test frequency shifts at the 1E-18 level.

Conclusions:

• Past metrology redefinitions offer crucial lessons for the kilogram's future.
• Technological advancements are paving the way for highly sensitive space-time research.
• The STAR Mission is poised to push the boundaries of fundamental physics testing.