19th-century constraints on properties of the aether

[bcrowell]

Did physicists ever try to put empirical constraints on the properties of the aether? I recall reading a popularization by Isaac Asimov in which he remarked that the aether would have had to have a very low density, and then to explain the high speed of light it would have had to have a stiffness many orders of magnitude higher than that of steel. I wonder whether this is really the way 19th-century physicists would have approached the issue, or whether Asimov was just making up an imaginary analysis and retrospectively putting it into their minds.

If the aether isn't entrained, then there is no obvious way to measure its inertia. But since experiments required partial entrainment, it seems like there should have been observable forces that were proportional to the density of the aether. E.g., if I accelerated a macroscopic object, it would have had to partially entrain some of the aether, and the aether's inertia should have added onto the physically observed inertia. But if the amount of aether entrained is proportional to the mass of the object (not its volume) then this is just a renormalization of the mass of all objects, and therefore not observable.

From the WP article "Luminiferous aether," it sounds like this type of model also would have problems accounting for the wavelength-dependence of the index of refraction.

Supposing that one can put an upper bound on the density of the aether, do we then get a lower bound on its shear modulus or something? Did anyone actually do this?

I also seem to recall descriptions of failed attempts to make mechanical models of electromagnetic waves. Again, this was probably in some popularization. At the time I got the impression that these were actual mechanical models containing springs and gears, but I suppose that in reality they would have been models on paper -- if they existed at all ...? Actually it's not obvious to me that, even given modern knowledge, you can't make an actual mechanical model of a light wave, reproducing the transverse polarization and the superposition of waves. Is there some kind of no-go theorem that proves that this is impossible? It seems unlikely that such a model could correctly reproduce all the predictions of Maxwell's equations, since Maxwell's equations implicitly have SR built in, including things like relativistic addition of velocities.

 

[Histspec]

Some "properties" were given by Maxwell in 1878 in his Encyclopædia Britannica article on the aether. See sections "Elasticity, tenacity, and density of the aether" and "The aether distinct from gross matter".
http://en.wikisource.org/wiki/Encyclopædia_Britannica_Ninth_Edition/Ether

For much more details see also Oliver Lodge, "The ether of space", 1908. Sections V-IX. There he elaborates on energy, density, viscosity etc.
http://archive.org/details/etherofspace00lodguoft

 

[bcrowell]

Thanks, Histspec! Wow, amazing that an entire erudite physics book can be written on a nonexistent thing...

 

[bobc2]

Interesting ideas, bcrowell. A couple of times I've started to do a finite element model of ether that would allow me to examine results using various combinations of mass density, Young's modulus and shear modulus. I could then enforce various dynamic stimuli. I might try it using properties suggested by Hitspec.

 

[sardar]

Therefore, it seems like any mechanical model would have to be based on an alternative theory that reproduces the same results as Maxwell's equations, but does so without SR.

I can say that physicists in the 19th century did indeed try to put empirical constraints on the properties of the aether. However, it is important to note that the concept of the aether was largely a theoretical construct and there was no direct evidence for its existence. Therefore, any constraints placed on its properties were based on theoretical considerations rather than experimental data.

One approach to constraining the properties of the aether was through the analysis of the speed of light. Asimov's popularization does accurately reflect the thinking of some 19th-century physicists, who believed that the aether would have to have a very low density and a high stiffness in order to explain the observed speed of light. However, this was just one of many theories and there was no consensus among scientists on the exact properties of the aether.

Another approach was through the study of the index of refraction. As you mentioned, the aether model would have had difficulties explaining the wavelength-dependence of the index of refraction. This was a major problem for the aether theory and many physicists attempted to find alternative explanations.

Regarding your question about putting an upper bound on the density of the aether and its shear modulus, I am not aware of any specific attempts to do so in the 19th century. However, as I mentioned earlier, these constraints were largely based on theoretical considerations rather than empirical data.

As for mechanical models of electromagnetic waves, there were indeed attempts to create such models in the 19th century. However, these were mostly thought experiments and not actual physical models. It is also worth noting that any mechanical model would have to be based on a different theory than Maxwell's equations, as you mentioned. While it may be possible to create a mechanical model that reproduces some aspects of electromagnetic waves, it would not be able to fully explain all the phenomena observed and predicted by Maxwell's equations.

In conclusion, 19th-century physicists did try to put constraints on the properties of the aether, but these were largely based on theoretical considerations rather than empirical data. The concept of the aether was eventually disproven by the famous Michelson-Morley experiment in 1887, leading to the development of the theory of relativity.