The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum.

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiation and gravitational waves). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.


It basically defines the speed of causality.

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement. The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the charge of the electron, which can be observed and measured with great accuracy.

So it says here that the base units are derived from the speed of light and the charge of the electron.

Now we are using SI units, and it defines the second as the base unit. Everything else is derived.

The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. Within the context of the SI, the second is the coherent base unit of time, and is used in the definitions of derived units.

So basically the SI itself has a contradiction in the definition of its base unit.


It is not clear if the universal constant like the speed of light is the base unit, or it is the second (based on the caesium atom). Which one is derived from which, and is the speed of light the universal constant or is it the second?

I might be saying, that the transition time between the hyperfine levels of the caesium atom are constrained by the speed of causality (speed of light), so this becomes circular.


  1. Is our physics and our SI based on the universal constant of speed of light or time (second), and which one is the derived unit?

The answer by lemon is correct. For a different (but equivalent) perspective: Since the speed of light is constant, you can define exactly two out of these three units: meter, second, and meter-per-second.

Historically, before relativity and the idea that there was a universal constant with units meter/second, people defined meter and second. That implied a definition for meter-per-second.

Now, for a variety of reasons and taking into account that the speed of light is constant, we define the second and the meter-per-second. That implies a definition of the meter.

As stated in the answer by lemon, the definition of the second is currently based on the transition period between specific levels of a Cs atom, and the meter-per-second is defined to be the value that gives the meter-per-second as 1/299792458 of the speed of light, which corresponds to the historically measured value to some precision. The meter is a derived unit from those two definitions, taking the value necessary to make both relations simultaneously true.


The unit of time (second) is derived from the caesium atom as you describe. And the unit of distance (metre) is derived from the speed of light and the caesium atom - it's the distance light travels in 9192631770 / 299792458 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

  • $\begingroup$ Thank you, so can we say that our physics is based on two base values, the speed of light, and the time (second)? So both of them are universal constants that are equally important (independent)? Do you think that the transition time can depend on the speed of light? $\endgroup$ – Árpád Szendrei Jul 25 at 15:17
  • $\begingroup$ You seem to be confusing fundamentals of physics with units of measurement, @ÁrpádSzendrei. The base values are for the SI units. They are defined in terms of physical quantities, but they are not "base values" for "our physics." $\endgroup$ – Brick Jul 25 at 20:00

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