Fundamentals of Time and Relativity

Light principle

  • Measured in Einstein time, the light velocity is by definition equal to the constant c in any inertial frame. This is Einstein’s light principle.
  • The light principle allows distances to be most accurately determined by time measurements.

It is in reality neither a supposition nor a hypothesis about
the physical nature of light,
but a stipulation which I can make of my own free will
in order to arrive at a definition of simultaneity.
Albert Einstein, 1916

Einstein’s definition of clock synchronization is equivalent to saying that synchronicity is achieved when clocks are set such that the velocity of light, measured with their help, is the same in the direction from A to B as in the reverse direction from B to A. In other words, Einstein assumes that the one-way speed of light is the same as the well-known round-trip speed of light, and defines time so that this assumption automatically holds true. This is Einstein’s light principle.

It is unclear whether Einstein got the idea of assuming an isotropic speed of light from the earlier writings of Poincaré, or that he arrived at it independently. To give credit to both, this assumption of isotropy is often referred to as the Einstein-Poincaré convention.

There is a close connection with the S.I. definition of the unit of length (meter). Since 1984, this unit is defined as the fraction 1/299.792.458 of the light-second (i.e., the distance traversed by a photon in vacuum in one second). This means that, without any ambiguity, the time difference in (3.1) determines the (Euclidean) distance AB ¯ = BA ¯ between the points A and B:

3.2

AB ¯ = 1 2 c( t ¯ A − t A )

with c:=299.792.458 m/s  the fixed S.I. value of the light velocity. The last equation may be interpreted as ‘time measures distance’, which has become standard practice in modern technology, like in GPS satellite navigation.

  • The constant speed of light c is a conversion factor between time and space according to the system of units employed.
  • Einstein gave the light principle the status of an independent postulate in order to transcend Maxwell’s electrodynamics. In 1908 Hermann Minkowski elevated the invariant light velocity to an intrinsic property of spacetime; see Minkowski Space.