In physics, the accuracy of measured quantities depends on various factors, including the specific measurement techniques, experimental setup, and the inherent uncertainties associated with the physical quantities being measured. However, there are certain fundamental constants and quantities that have been measured with extremely high precision. Some examples include:
Speed of Light (c): The speed of light in vacuum is a well-known constant with a defined value of approximately 299,792,458 meters per second. It has been measured with high precision using a variety of techniques, including interferometry and cavity resonators.
Planck's Constant (h): Planck's constant is a fundamental constant of nature that relates the energy of a photon to its frequency. It has been measured with high accuracy using methods such as the quantum Hall effect and atom interferometry.
Electron Charge (e): The elementary charge of an electron is a fundamental constant that characterizes the charge of a single electron. It has been measured with remarkable precision using techniques such as the Millikan oil drop experiment and electron beam measurements.
Fine-Structure Constant (α): The fine-structure constant is a dimensionless quantity that characterizes the strength of electromagnetic interactions. It has been measured with high precision through various experiments, including spectroscopy and quantum electrodynamics calculations.
Electron Magnetic Moment (g-factor): The electron's magnetic moment is a measure of its response to an external magnetic field. It has been measured with exceptional accuracy using techniques like electron magnetic resonance and Penning trap experiments.
It's important to note that the accuracy of measurements can improve over time as technology and experimental techniques advance. Therefore, the most accurately measured quantities in physics may change as new breakthroughs occur and more precise measurements are made.