Origins of twentieth century Physics
The origins of modern Western science can be traced from the ancient Babylonians, through Copernicus, Galileo and Newton, and are discussed in more detail here.
Isaac Newton (1642 – 1727) gave us the relationship between force and acceleration, the inverse square law of gravity and the idea that gravity is an attractive force communicated by a still unknown mechanism.
By 1850, physics was certain that light was a wave, thanks to the observations of Young and others, and the analysis in particular of Huygens and Fresnel. This is detailed here.
The principles of theoretical science, inherited by Maxwell, Darwin, Mendel, Wegener, Mendeléev and others are detailed here.
The above conclusions, and the theoretical methodology that created and sustained them came into question in the second half of the nineteenth century.
Between 1861 and the publication of his definitive Treatise on Electricity and Magnetism in 1873, James Clerk Maxwell developed his analysis of light and electromagnetism, based on the expectation that a ‘luminiferous medium’ existed, and was fluidic in nature. This was possibly the last attempt, up to the present day, to create a mundane mechanical model as a basis for light and electromagnetism. Maxwell’s was a remarkable success in terms of the mathematical model he created, but as explanatory science in the manner of Darwin, it failed. Because of the significance of that failure and important features of that analysis that have subsequently been overlooked, Maxwell’s efforts are detailed here.
Michelson and the luminiferous ether
Albert Abraham Michelsoni in the 1880s invented an instrument known as an interferometer, and used it to search for evidence that the Earth is travelling through Fresnel and Maxwell’s ‘luminiferous ether’. As a result of this, he became the first American citizen to earn a Nobel Prize.
The interferometer splits light into two rays that travel back and forth on two paths at right angles, and then recombines them to create an interference pattern. It doesn’t measure the speed of light as such, but it should be very sensitive to any differences in light speed along the four orthogonal directions that light travels within the instrument, differences that should be apparent with changes in the Earth’s direction of motion. These differences were not found.
This observed constancy of the speed of light was a shock to physicists. If a car is travelling at speed, then we can drive alongside it and make it appear stationary. In an adjacent lane, a car appears to be travelling more slowly than it does to an observer on a bridge. We can do this with material objects and we can do it with sound, so why not with light, which until then appeared to be a similar phenomenon?
At the very least, this failure made the relativity theory of Poincaré, Lorentz and Einstein seem much more theoretically attractive.
The Michelson-Morley experiment is commonly cited as being the definitive evidence that ‘everything is relative’ but this is not quite correct. Hendrik Antoon Lorentz (1853 – 1928) and the Irish physicist George Francis Fitzgerald (1851 – 1901) proposed an alternative explanation, the Lorentz-Fitzgerald contraction. If our measuring equipment contracts along the direction of motion, then we will be unable to find any real change in light speed.
While this had the advantage of being a physical explanation, it was ad hoc, meaning invented for a particular theoretical purpose, had no certain basis in electromagnetic theory or observation, and appeared to be impossible to check. Relativity provided a simpler explanation, predictions that could be verified, new and exciting science and a philosophical basis for scientific failure. Gradually, however, the Lorentz-Fitzgerald contraction came to be viewed as an optional adjunct to special relativity, and hence forgotten as an alternative to it.
In my view, the theoretical failure was the more crucial. If Maxwell had produced a coherent physical model for both electromagnetism and light, one that was compatible with the contraction hypothesis, this would have been a powerful alternative theory, and one that was deterministic in nature.
The stubborn ether
Michelson’s failure to find the expected ether and Maxwell’s failure to create a credible physical description of its properties meant that the logical way forward was to look more favourably on the alternative, the theory of special relativity. This accepted that there was no background, no ether.
Despite this, the idea simply will not lie down. Dirac required a ‘background’ full of electrons and positrons popping in and out of existence. Phrases such as ‘noisy vacuum’, ‘zero-point energy’ and ‘quantum foam’ describe this phenomenon, central to quantum electrodynamics. There are arguments about how much energy it contains, with the epic tome Gravitationii suggesting that:
‘No point is more central than this, that empty space is not empty. It is the seat of the most violent physics. The electromagnetic field fluctuates. Virtual pairs of positive and negative electrons, in effect, are continually being created and annihilated, and likewise … pairs of other particles.’
It suggests that the background energy is ten-to-the-power-of-eighty times larger than the energy in matter, and concludes that ‘elementary particles do not form a really basic starting point to the description of nature. Instead they represent a first order correction to vacuum physics.’
Dirac’s view was that ‘with the new theory of electrodynamics we are rather forced to have an aether.’iii
The Higgs particle, if it really exists, continues this tradition. It is not a particle like any other, but a static background that influences the motion of some other particles. The search at CERN is not for a free-moving particle but for this background, the ‘Higgs field’. It is called a particle only because those who seek it are particle physicists, who have concluded that everything is a particle, including light and gravity and electromagnetic interaction, and therefore can picture the Higgs field in no other way.
No ether, no background, no wave
Nevertheless, the principle of relativity, that no observer holds a privileged position over any other, required this conclusion, that there is no ether. Any background, be it ether or a more recent incarnation, would provide a unique frame of reference, a ‘preferred frame’, as it has been called. The acceptance of no background led to special relativity.
General relativity (considered here) is based on a different idea stemming from special relativity, and does not take a clear position on whether light is a wave or a particle.
The absence of a background requires a relativistic approach, and has another consequence. This is that light cannot be a wave, as a wave requires a background to wave in. This was the crucial impetus to concluding that light is composed of particles, called photons, and the application of this idea within quantum theory.
If light, which has all the characteristics of a wave, is really a particle, then probably everything is. This leads to the discipline of particle physics and the standard model of physics. This has been useful in investigating and categorising particles, but is not examined in detail on this site.
The failure of the ether is absolutely central to what we are taught today. If we find insoluble problems in modern theory, then we will be required to reconsider that conclusion.
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i. Albert A. Michelson, Edward W. Morley, On the Relative Motion of the Earth and the Luminiferous Ether, American Journal of Science 34 (1887) page 333
ii. Charles W Misner, Kip S Thorne, John Archibald Wheeler, ‘Gravitation’ (WH Freeman, NY, 1973). Both quotes from page 1202
iii. Paul Dirac, letter to Nature 168 (1951) page 906