It is distinct from a mathematical constant, which has a fixed numerical value, but does not directly involve any physical measurement.
The discovery and verification of Maxwell's equations connected the same quantity with an entire system, electromagnetism.
When the theory of special relativity emerged, the quantity came to be understood as the basis of causality.
For example, the speed of light is defined as having the numerical value of 299792458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time.
While its numerical value can be defined at will by the choice of units, the speed of light itself is a single physical constant.
Depending on the choice and arrangement of constants used, the resulting natural units may be convenient to an area of study.
Currently, this is the theory of general relativity for gravitation and the Standard Model for electromagnetic, weak and strong nuclear interactions and the matter fields.
There is, however, no single "correct" way of enumerating them, as it is a matter of arbitrary choice which quantities are considered "fundamental" and which as "derived".
[7] However, while its value is not known to great precision, the possibility of observing type Ia supernovae which happened in the universe's remote past, paired with the assumption that the physics involved in these events is universal, allows for an upper bound of less than 10−10 per year for the gravitational constant over the last nine billion years.
[9][10] It is problematic to discuss the proposed rate of change (or lack thereof) of a single dimensional physical constant in isolation.
With this change, the international prototype of the kilogram is being retired as the last physical object used in the definition of any SI unit.
[16] Some physicists have explored the notion that if the dimensionless physical constants had sufficiently different values, our Universe would be so radically different that intelligent life would probably not have emerged, and that our Universe therefore seems to be fine-tuned for intelligent life.