Once people thought that the Universe was eternal and unchanging. It came as a shock to see stars move against the night sky and sometimes explode. If they had observed for longer they would have observed the whole life cycles of stars and how they all seemed to be moving away from some central point.
In the same way, we have only been measuring fundamental physical constants for a few hundred years, compared to around fifteen billion years for the age of the Universe. Many physical constants have no inherent rationale for their values according to our theories – they are measured, not deduced. Is there anything in our physical theories to say that physical constants must remain constant?
The model for this discussion could be Hubble's constant. We know it's value is changing. As the Universe expands, the mutual gravitational attraction of all the matter and energy in it slows down the rate of expansion gradually. At the moment Hubble's constant has a value of about 75km/s per Megaparsec, wherever in the Universe you measure it, and since the rate of expansion of the Universe is slowing down, Hubble's constant is decreasing. Moreover, at any point in time it takes the same value everywhere in the Universe.
Scientific experiments have not yet pinpointed any definite evidence that physical constants are changing, although they have placed upper bounds on the maximum possible relative change per year at very small amounts (between roughly 1 part in
and 1 part in
).
It is disputed whether any changes in dimensional physical constants such as
or
are operationally meaningful;however, a sufficient change in a dimensionless constant such as
the fine structure constant is generally agreed to be something that would definitely be noticed. If a measurement indicated that a dimensional physical constant had changed, this would be the result or interpretation of a more fundamental dimensionless constant changing, which is the salient metric. From John D. Barrow 2002:
An important lesson we learn from the way that pure numbers likedefine the world is what it really means for worlds to be different. The pure number we call the fine structure constant and denote byis a combination of the electron charge,
the speed of light,
and Planck's constant
At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If
and
were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value ofremained the same, this new world would be observationally indistinguishable from our world. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value you cannot tell because all the purenumbers defined by the ratios of any pair of masses are unchanged. P >