Aberration, Continued
In a short paper it is impossible to enumerate those fruitless efforts of three centuries, all trying to establish incontrovertibly the veracity of Galileo’s legendary “Eppur Si muove!”. Those interested in particulars will find them sprinkled throughout the extensive literature dealing with the issues involved.(15) For the purpose at hand we may restrict ourselves – as a cursory view of history clearly intimates – to a crucial experiment at the crossroads of classical and relativistic science. To wit, as already mentioned, the test performed in 1871 by Airy, a test more than a century earlier suggested by a forgotten genius, Ruggiero Guiseppe Boscovich (1711 -1787).
Since the readers for whom this essay is intended range, professionally grouped, from interested laymen to doctors in astrophysics, I am compelled to be popular without sacrificing correctness and to elaborate where for those “in the know” a single sentence would suffice. Only one mental favour I must ask all of them to grant me. It is that for the sake of argument they suspend or forget for a few minutes a fact they already “know” or are convinced of, i.e. that the Earth is no more than an aimlessly through space wandering relatively miniscule satellite of a typical minor star. Those reluctant – very unscientifically, I assert! – to do this I remind of the fact that, so to say, Einstein’s general Theory of Relativity has democratized the starry world by conferring equal status to all celestial contenders for the place of kingpin in the heavenly merry-go-round. If however they already balk at the idea that, after all, the Earth could be a strong candidate for this unique position, I regretfully advise them to refrain from even a mere perusing of the argumentation developed in the following analysis of Air/s failure to verify Bradley’s account of aberration, in which I point out the purblindness of the scientists’ refusal to see the plain truth.
Grasping that nettle I follow, for a short explanation of what said aberration is all about, the Dutch physicist and Nobel prize winner J.D. van der Waals (1837 – 1923), who during his lifetime witnessed the demise of classical science and the emergence of relativity physics, 16) (see figure 1).
Imagine somewhere on Earth a closed box ABCD with a pinhole P in the top through which a light ray, from a source S stationed in a tower, touches the bottom DC in S. Now suppose that we set our box in motion towards the right. Then the light in a straight line moving ray SS still needs a fraction of time after passing through the pinhole to reach the bottom DC. But during this split second the box has moved to position A1, B1, C1, D1, and “inside” the box S will hence have veered to S1 at the left of S. Further: it is not difficult to see that, when we fasten our frame of reference on the box, the path of the light ray will show a slant.
Next we now fill the box with water and repeat our Gedankenexperiment. With light source and box both at rest, relative to us and the Earth, nothing alters, but as soon as we again set the box in motion we observe a change. In water the speed of light is about three fourths of its speed in air. Consequently the “wavicles” emanated by S need more time to traverse the box. As seen by an observer situated at the bottom of that box their trajectory is, it follows, more slanted than it was on our first trial run, and they will reach the bottom at S2.
So far, so good. However, now the action shifts in space and time to a duo of astronomers who became convinced that they had found a phenomenon capable of removing the last lingering doubt whether Copernicus had indeed the right sow by the ear. In December A.D. 1725 we see James Bradley and Samuel Molyneux manipulating a telescope fixed to a chimney stack and directed at the star Gamma Draconis, almost vertically overhead. Neglecting for brevity’s sake the finer points of the affair: prolonged observation showed the two stargazers that Gamma Draconis, relative to the eartbound chimney of Molineux’s house, in the course of a year described a small circle. By the light of the foregoing their conclusion is easy to grasp and crystal clear: the Earth is moving, and in fact revolving relative to Gamma Draconis and hence relative to all fixed stars, the Sun included. More: taking into account the speed of light and the observed angle of aberration, simple trigonometry shows our orbiting home to have exactly the velocity that Bradley already “knew” it had of more than one hundred thousand km/hr. The slightest skepticism remaining about the truth of Copernican astronomical gospel could therefore be laid to rest.
Well, not totally! Logically considered, this conclusion uses that invalid theoretical syllogism, the modus ponendo ponens. If situation P is the case, we agree, then we shall observe the phenomenon Q. Now indeed we observe Q. Does it therefore follow that P is the factual state of affairs? By no means necessarily, for Q may be caused by a variety of other circumstances. As one of my textbooks of logic remarks: “We shall have frequent occasions to call the reader’s attention to this fallacy. It is sometimes committed by eminent men of science, who fail to distinguish between necessary and probable inferences, or who disregard the distinction between demonstrating a proposition and verifying it”.(17)
“Aberration”, to quote van der Waals, “may equally well be squared with the supposition that the stars indeed describe circlets. And though we find the latter explanation improbable and prefer the first, the question may arise: is it in no way possible by means of observations to decide which of the two suppositions is the right one?”(18)
Boscovich, sensibly and objectively not inclined to put all his theoretical eggs in Bradley’s logically bottomless basket, saw a chance to do just that. And many an astronomically non-conversant reader, having followed the discourse thus far, may already have realized that chance also. Fill a telescope with water and measure the aberration angle for any fixed star. If the angle in this manner obtained is larger than the one measured by Bradley, the Earth indeed orbits, relative to firmament and Sun. If no different value is registered, then the starry sphere swings, with the Sun on which it appears to be centered, around that beautifully blue-and-white marbled “planet” Gea.