Editing Physical constant (section)

== Tests on time-independence ==
{{main article|Time-variation of fundamental constants}}
By definition, fundamental physical constants are subject to [[measurement]], so that their being constant (independent on both the time and position of the performance of the measurement) is necessarily an experimental result and subject to verification.

[[Paul Dirac]] in 1937 speculated that physical constants such as the [[gravitational constant]] or the [[fine-structure constant]] might be subject to change over time in proportion of the [[age of the universe]]. Experiments can in principle only put an upper bound on the relative change per year. For the fine-structure constant, this upper bound is comparatively low, at roughly 10<sup>−17</sup> per year (as of 2008).<ref>
{{cite journal |author=Rosenband |first=T. |display-authors=etal |year=2008 |title=Frequency Ratio of Al<sup>+</sup> and Hg<sup>+</sup> Single-Ion Optical Clocks; Metrology at the 17th Decimal Place |url=https://zenodo.org/record/1230892 |journal=[[Science (journal)|Science]] |volume=319 |issue=5871 |pages=1808–12 |bibcode=2008Sci...319.1808R |doi=10.1126/science.1154622 |pmid=18323415 |s2cid=206511320 |doi-access=free}}</ref>

The gravitational constant is much more difficult to measure with precision, and conflicting measurements in the 2000s have inspired the controversial suggestions of a periodic variation of its value in a 2015 paper.<ref name="anderson2015">{{citation |author1=Anderson |first=J. D. |title=Measurements of Newton's gravitational constant and the length of day |date=April 2015 |journal=EPL |volume=110 |issue=1 |pages=10002 |arxiv=1504.06604 |bibcode=2015EL....11010002A |doi=10.1209/0295-5075/110/10002 |s2cid=119293843 |author2=Schubert |first2=G. |author3=Trimble |first3=V. |author4=Feldman |first4=M. R.}}</ref> 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<sup>−10</sup> per year for the gravitational constant over the last nine billion years.<ref>{{citation |author1=Mould |first=J. |title=Constraining a Possible Variation of G with Type Ia Supernovae |date=2014-04-10 |journal=Publications of the Astronomical Society of Australia |volume=31 |pages=e015 |arxiv=1402.1534 |bibcode=2014PASA...31...15M |doi=10.1017/pasa.2014.9 |s2cid=119292899 |author2=Uddin |first2=S. A.}}.</ref>

Similarly, an upper bound of the change in the [[proton-to-electron mass ratio]] has been placed at 10<sup>−7</sup> over a period of 7 billion years (or 10<sup>−16</sup> per year) in a 2012 study based on the observation of [[methanol]] in a distant galaxy.<ref name="Science-20121213">{{cite journal |last1=Bagdonaite |first1=Julija |last2=Jansen |first2=Paul |last3=Henkel |first3=Christian |last4=Bethlem |first4=Hendrick L. |last5=Menten |first5=Karl M. |last6=Ubachs |first6=Wim |title=A Stringent Limit on a Drifting Proton-to-Electron Mass Ratio from Alcohol in the Early Universe |date=December 13, 2012 |journal=[[Science (journal)|Science]] |doi=10.1126/science.1224898 |bibcode = 2013Sci...339...46B |volume=339 |issue=6115 |pages=46–48 |pmid=23239626|hdl=1871/39591 |s2cid=716087 |url=https://research.vu.nl/ws/files/668474/Science-2013-Bagdonaite-46-8.pdf }}</ref><ref name="Space-20121213">{{cite web |last=Moskowitz |first=Clara |title=Phew! Universe's Constant Has Stayed Constant |url=http://www.space.com/18894-galaxy-alcohol-fundamental-constant.html |date=December 13, 2012 |publisher=[[Space.com]] |access-date=December 14, 2012 |url-status=live |archive-url=https://web.archive.org/web/20121214081926/http://www.space.com/18894-galaxy-alcohol-fundamental-constant.html |archive-date=December 14, 2012 }}</ref>

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 units is arbitrary, making the question of whether a constant is undergoing change an artefact of the choice (and definition) of the units.<ref name="hep-th1412.2040">{{cite journal|first=Michael |last=Duff |title=How fundamental are fundamental constants?|arxiv=1412.2040|doi=10.1080/00107514.2014.980093|author-link=Michael Duff (physicist)|url=https://www.tandfonline.com/doi/abs/10.1080/00107514.2014.980093|journal=Contemporary Physics|volume=56|issue=1|pages=35–47|year=2015|bibcode=2015ConPh..56...35D|hdl=10044/1/68485 |s2cid=118347723 }}</ref><ref>{{cite arXiv |eprint=hep-th/0208093 |first1=Michael J. |last1=Duff |title=Comment on time-variation of fundamental constants |date=13 August 2002}}</ref><ref>{{cite journal |last1=Duff |first1=M. J. |last2=Okun |first2=L. B. |last3=Veneziano |first3=G. |title=Trialogue on the number of fundamental constants |journal=Journal of High Energy Physics |date=2002 |volume=2002 |issue= 3|pages=023 |arxiv=physics/0110060 |bibcode=2002JHEP...03..023D |doi=10.1088/1126-6708/2002/03/023|s2cid=15806354 }}</ref>

For example, in [[SI units]], the speed of light was given a defined value in 1983. Thus, it was meaningful to experimentally measure the speed of light in SI units prior to 1983, but it is not so now. Similarly, with effect from May 2019, the Planck constant has a defined value, such that all [[SI base units]] are now defined in terms of fundamental physical constants. 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.

Tests on the immutability of physical constants look at ''dimensionless'' quantities, i.e. ratios between quantities of like dimensions, in order to escape this problem. Changes in physical constants are not meaningful if they result in an ''observationally indistinguishable'' universe. For example, a [[variable speed of light|"change" in the speed of light]] ''c'' would be meaningless if accompanied by a corresponding change in the elementary charge ''e'' so that the expression {{math|''e''<sup>2</sup>/(4π''ε''<sub>0</sub>''ħc'')}} (the fine-structure constant) remained unchanged.<ref>{{citation |last=Barrow |first=John D. |title=The Constants of Nature; From Alpha to Omega – The Numbers that Encode the Deepest Secrets of the Universe |year=2002 |url=https://archive.org/details/constantsofnatur0000barr |publisher=Pantheon Books |isbn=978-0-375-42221-8 |author-link=John D. Barrow |url-access=registration}}.</ref>