[3] In a more philosophical context, the conclusion that these quantities are constant raises the question of why they have the specific value they do in what appears to be a "fine-tuned universe", while their being variable would mean that their known values are merely an accident of the current time at which we happen to measure them.
[4] It is problematic to discuss the proposed rate of change (or lack thereof) of a single dimensional physical constant in isolation.
The reason for this is that the choice of a system of units may arbitrarily select any physical constant as its basis, making the question of which constant is undergoing change an artefact of the choice of units.
Thus, it was meaningful to experimentally measure the speed of light in SI units prior to 1983, but it is not so now.
Changes in physical constants are not meaningful if they result in an observationally indistinguishable universe.
[10][11] An upper bound of 10−17 per year for the time variation, based on laboratory measurements, was published in 2008.
[12] Observations of a quasar of the universe at only 0.8 billion years old with AI analysis method employed on the Very Large Telescope (VLT) found a spatial variation preferred over a no-variation model at the
Accepted classical theories of physics, and in particular general relativity, predict a constant speed of light in any local frame of reference and in some situations these predict apparent variations of the speed of light depending on frame of reference, but this article does not refer to this as a variable speed of light.
Various alternative theories of gravitation and cosmology, many of them non-mainstream, incorporate variations in the local speed of light.
Attempts to incorporate a variable speed of light into physics were made by Robert Dicke in 1957, and by several researchers starting from the late 1980s.
[14] 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
[18] Possible variations of the cosmological constant over time or space are not amenable to observation, but it has been noted that, in Planck units, its measured value is suggestively close to the reciprocal of the age of the universe squared, Λ ≈ T−2.