Thursday, July 22, 2021

Measuring rods

In his popular book on relativity theory, Einstein says that distance is just what measuring rods measure. I am having a hard time making sense of this in Einstein’s operationalist setting.

Either Einstein is talking of real measuring rods or idealized ones. If real ones, then it’s false. If I move a measuring rod from one location to another, its length changes, not for relativistic reasons, but simply because the acceleration causes some shock to it, resulting in a distortion in its shape and dimensions, or because of chemical changes as the rod ages. But if he’s talking about idealized rods, then I think we cannot specify the relevant kind of idealization without making circular use of dimensions—relevantly idealized rods are ones that don’t change their dimensions in the relevant circumstances.

If one drops Einstein’s operationalism, one can make perfect sense of what he says. We can say that distance is the most natural of the quantities that are reliably and to a high degree of approximation measured by measuring rods. But this depends on a metaphysics of naturalness: it’s not a purely operational definition.

6 comments:

William said...

It is possible that Einstein was using the rigid rod definition primarily (and operationally) as a way of setting up his later claim that fully _rigid_ bodies change size (without mechanical compression) along the axis of very high velocities. Otherwise an uninformed reader might think he is merely claiming that high velocity leads to an elastic, springlike compression, which would mislead the reader.

IanS said...

This paper (Dieks: The Adolescence of Relativity...) may be of interest

https://dspace.library.uu.nl/bitstream/handle/1874/202812/dieks-minkowski_2010.pdf?sequence=1

It seems that Einstein was well aware of the issue, and that the operationalism was pedagogical.

El Filósofo said...
This comment has been removed by the author.
Alexander R Pruss said...

Ian: Thanks for the paper. That looks very helpful.

Alexander R Pruss said...

BTW, what brought on this post was reading Dewey on Einstein. Even if the operationalism isn't essential to E, it does seem essential to D.

Unknown said...

What is your alternative to operationalism?