Physical constants

• Is the Planck Mass About the Mass of One Eyebrow Hair?
  The Planck mass is about 22 micrograms—comparable to a small hair, but the match depends strongly on a hair’s length and thickness.
• Is the Bohr Radius Deprecated?
  The Bohr radius is not deprecated: it remains a standard physical constant and atomic unit, though the Bohr model is only an approximation.
• Is the Fine-Structure Constant Really Constant?
  The fine-structure constant has a fixed low-energy value, but quantum electrodynamics predicts that its effective value increases with energy.
• Has the Speed of Light Changed Over Time?
  The vacuum speed of light is exactly 299,792,458 m/s by SI definition; whether it varied cosmologically is a separate, dimensionless, unconfirmed question.
• What Is the Gravitational Constant in Higher Dimensions?
  In D spacetime dimensions, Newton’s constant has dimension-dependent units; compactifying extra dimensions relates it to the observed four-dimensional constant.
• Is Gravity 9.8 m/s² at Every Height?
  Gravity is about 9.8 m/s² near Earth's surface, but decreases with altitude according to the inverse-square law; learn when the approximation works.
• Can Planck’s Constant Be Derived From Maxwell’s Equations?
  Maxwell’s equations predict classical electromagnetic waves, not Planck’s constant; quantum theory supplies the action scale needed to describe photon energies.
• What Is Planck’s Constant, Simply Explained?
  Planck’s constant sets the scale of quantum effects and relates a photon’s energy to its frequency through the equation E = hf.
• Can the Accuracy of Planck’s Constant Be Increased?
  Planck’s constant has an exact numerical value in the SI since 2019; experiments can improve its practical realization, not its defined value.