Speed of light

• What Is the Speed of an Electromagnetic Wave in a Medium?
  In a simple medium, an electromagnetic wave's phase speed is 1/√(εμ), or approximately c/n; dispersion and absorption require extra care.
• Is Einstein’s Second Postulate an Axiom?
  Einstein’s second postulate is an empirical physical principle in the 1905 formulation of special relativity, not a mathematical axiom proven from nothing.
• Would a Car’s Headlights Become Gamma Rays Near Light Speed?
  A headlight remains visible in the car’s rest frame, but a forward observer sees relativistic blueshift; calculate the speed needed for gamma rays.
• 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.
• Why Does Every Moving Observer Measure the Same Speed of Light?
  A moving observer still measures light in vacuum at c because Lorentz transformations change space and time together, preserving light’s speed.
• Does Matter Become Energy at the Speed of Light?
  Matter does not turn into energy at light speed: massive objects cannot reach that speed, and their rest mass remains part of their total energy.
• Do Echolocating Bats Experience the Terrell Effect?
  Bats do not experience the relativistic Terrell effect, though sound-travel delays and Doppler shifts can influence their echo perception.
• Do the Laws of Physics Apply to Non-Observers?
  Relativity’s laws do not require a conscious observer: they apply to physical events, while reference frames describe how those events are measured.
• Could the Speed of Light Vary While Time Is Absolute?
  Special relativity makes vacuum light speed invariant and time frame-dependent. An absolute-time model is possible.
• At What Speed Is the Universe Expanding?
  The universe has no single expansion speed: its current rate is about 70 km/s per megaparsec, so more distant galaxies recede faster.