Archive for International System of Units

Time-Varying Constants

Posted in The Universe and Stuff with tags , , , on April 1, 2019 by telescoper

Two serious questions crossed my mind during Friday’s very interesting talk about the redefinition of SI units. One is that the changeover to the new system takes place on 20th May, which is right in the middle of the examination period at Maynooth University. We will therefore have to supply two differents set of physical constants, one to go with examination papers taken before 20th May and the other for those taken afterwards. This will even affect those of us who like to use natural units in which, for example, Planck’s Constant is set equal to unity: after the redefinition of Planck’s constant on 20th May 2019, we will have to set its value in natural units to be equal to 0.99999999987.

The other question is that the new system of SI units presupposes that the constants of physics are actually constant and can therefore provide a stable framework. Many theories have been suggested in which the fundamental constants change with time. In the present context I feel obliged also to point out as an example the classic paper of Scherrer (2009) (PDF available here) the abstract of which reads:

We examine the time variation of a previously-uninvestigated fundamental dimensionless constant. Constraints are placed on this time variation using historical measurements. A model is presented for the time variation, and it is shown to lead to an accelerated expansion for the universe. Directions for future research are discussed.

This casts grave doubt on the motivation for the new system of SI units, at least until 12 noon.

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A Change of Units

Posted in The Universe and Stuff with tags , , , , on March 29, 2019 by telescoper

To round off a very strange week I’ve just been to an interesting talk by Dr Bajram Zeqiri of the National Physical Laboratory in Teddington (UK) about imminent changes to the International System of Units (usually known as SI units). In a nutshell, what is to happen is that the current seven base units are to be redefined in terms of fundamental constants. In effect this means that the these constants will fix the standard units rather than the other way round. For more details, see here. The change is due to come into effect on 20th May 2019.

Our speaker Dr Zeqiri is nearing the end of a short tour of Ireland speaking about these changes. Before giving the third talk on this subject talk today, 29th March 2019, thought to be the date on which the United Kingdom would leave the European Union, he wondered whether he might be able to claim political asylum in Ireland. Fortunately, today is not Brexit Day and following today’s events in Westminster it is by no means certain when that might be or indeed whether Brexit will even happen at all…