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For example, a "change" in the speed of light c would be meaningless if accompanied by a corresponding "change" in the elementary charge e so that the ratio e 2:c (the fine-structure constant) remained unchanged. [8] Natural units are systems of units entirely based in fundamental constants.
Any ratio between physical constants of the same dimensions results in a dimensionless physical constant, for example, the proton-to-electron mass ratio. The fine-structure constant α is the best known dimensionless fundamental physical constant. It is the value of the elementary charge squared expressed in Planck units. This value has become ...
The Gaussian gravitational constant used in space dynamics is a defined constant and the Cavendish experiment can be considered as a measurement of this constant. In Cavendish's time, physicists used the same units for mass and weight, in effect taking g as a standard acceleration.
The ratio of the velocity of the electron in the first circular orbit of the Bohr model of the atom, which is 1 / 4πε 0 e 2 / ħ , to the speed of light in vacuum, c. [17] This is Sommerfeld 's original physical interpretation.
For example, the atomic mass constant is exactly known when expressed using the dalton (its value is exactly 1 Da), but the kilogram is not exactly known when using these units, the opposite of when expressing the same quantities using the kilogram.
[5]: 127 He found the speed of light was slower in water than in air. This experiment did not determine the absolute speeds of light in water or air, only their relative speeds. The rotational speed of the mirror could not be sufficiently accurately measured to determine the absolute speeds of light in water or air.
Instead of working with Hubble's constant, a common practice is to introduce the dimensionless Hubble constant, usually denoted by h and commonly referred to as "little h", [29] then to write Hubble's constant H 0 as h × 100 km⋅s −1 ⋅Mpc −1, all the relative uncertainty of the true value of H 0 being then relegated to h. [46]
Fizeau's experiment showed a faster speed of light in water moving in the same direction and a slower speed when the water moved opposite the light. However the amount of difference in the speed of light was only a fraction of the water speed.