A Comet Grazing the Sun
The problem between Einstein and myself was always the same, and we were equally obstinate: he because the mathematical model coincided with such unimaginable precision with the natural events, nowhere better observable than in the celestial sphere with the planets and their satellites on the prescribed paths; I, because it appeared to me that these exact coincidences between theory and nature had been achieved at the cost of a grievous omissionof electrical charges and fields. Natural catastrophes which I discovered to have taken place were my starting point, but these catastrophes were denied, and my description of the phenomena that accompanied them evoked the accusation that I had committed an outrage against the entire house of science. Yet even independently of what I read in ancient sources, historical or legendary, the picture of the solar system in which electricity and magnetism were absent and denied a role was strange to me. Once I read that the Jewish people produced the geniuses of Marx, Freud, and Einsteinthe three men who so greatly influenced the world of todaybecause by the nineteenth century the Jewish people was mature in intellect, yet foreign to European scientific thought, and entered this domain when no longer novices in the house of learning, therefore more given to criticism, skepticism, and an original grasp of the content. If I saw things differently, it was possibly because I came in conflict with the accepted notions, being myself no longer a fledgling; I did not go through the normal process of studying geology or astronomy as a student in college, accepting everything on faith, subdued by the assertion that science in our days and since some time ago is finally on the right track, after periods of ignorance of the ancients and erroneous ideas of the pre-Newtonian days. I could not help seeing things differently.
I decided to select a case in which electromagnetic interrelations between two bodies in the solar system would be more apparent than elsewhere. Such a case would be in the passage of a comet very close to the sun, actually grazing the solar corona. In my understanding there would be a very pronounced case of electromagnetic interaction. Physical science, or, better, celestial mechanics, forbade such an interpretationand why? Because as soon as electromagnetism is given right of entry, the entire solar system with planets and satellites would be engulfed in a forbidding sweep of forces and interrelations. If a comet that goes through the corona of the sun experiences some electromagnetic effect, then what about the same comet a little distance from the sun, before it reaches perihelion, or the point of nearest approach, or after it passes it? And if there, too, there should be some electromagnetic effect, then what about still greater distances and the behavior of cometary tails in general? Cometary tails, as already mentioned on earlier pages, keep away from the sun: on approaching the sun the tail moves behind the head of the comet; at the time the comet circles the sun in perihelion, the tail sweeps the sky, almost like a stiff rod; and when the head retreats from perihelion and rushes on its orbit back into space, the tail precedes the head, again kept away from the sun. The behavior of the cometary tails is not in accord with what should have been expected on the basis of gravitational forces; the tails should be attracted to, not repelled by the sun. The problem was also in the minds of astronomers of the nineteenth century. John Herschel wrote:
But when at the beginning of the present century the Russian physicist, P. Lebedew, succeeded in demonstrating that light exerts pressure on the surface it falls upon, in agreement with the postulate of Clerk Maxwell, he wrote: this result is of importance to astrophysics as furnishing a much simpler explanation of the repulsive force of the sun than the hypothetical ones of electrical charges."2
This pressure, or repulsion, is generally much smallerin the case of the sun 20,000 times lessthan the opposite action of the gravitational attraction; but calculation shows that on particles of dust of a certain small diameter the pressure of light will exert a greater force than will gravitation, and this because gravitation acts according to the mass, and pressure according to the surface, and a small particle has more surface in relation to its mass than does a larger particle. Although celestial mechanicians never really tried to investigate the problem quantitatively, the explanation was taken over into all textbooks. A quantitative analysis would show that the force needed to drive particles away from the sun at the speed observed must be between 200 and 2,000 times more powerful than the gravitational attraction exerted by the sun, instead of being 20,000 times weaker; (both act as the inverse square of distancelight and its pressure act four times weaker on an illuminated surface when the distance from the source of the light is doubled). A comet may have a tail as long as 100 million miles and thus reach all the distance from the sun to the terrestrial orbit, or even 200 million miles and thus reach past the orbit of Mars.
Finally, the cometary tails obviously have on one hand particles larger than dust grains and on the other hand they contain gases, but it is also obvious that light cannot drive these larger particles as it drives molecules of gases, and on this alone the argument capsizesand leaves the behavior of tails unexplained.
The light of cometary tails is not just the reflected light of the sun; they glow by their own light, a fact established by spectroscopic analysis. It appeared to me that the comets are charged bodies, and possibly their tails and heads carry significantly different charges.
On the other hand the rotating sun, if it is a charged body, must create a magnetic field. Does not the corona when seen at full eclipse, or with the help of an occulation disc (coronograph), have the appearance of magnetic lines of force as they can be traced by the position of iron filings spread over a Compton paper, in the presence of a magnetic field? Then would not a comet going through the corona of the sun be subject to electromagnetic interactions? Further, is not a comet held away from the sun by its magnetic field? But if comets are subject to electromagnetic forces when close to the sun, they may be subject to the same forces when at some distance from the sun, too; and if comets respond to forces besides gravitation, are not the planets also responsive to somelarge or small or minutebut some influence emanating from the sun, besides gravitation, namely of electromagnetic nature? The consequences are innumerable: is space empty, or filled with fields and influences? This is a question not unlike the question in theology: Is there or is there not a God? But now I was like a chess player sitting opposite the world champion, I being just an amateur, a beginner, plotting my attack. I moved a pawnbut I placed it in such a position that the champion immediately grasped the implications of my strategy. Let this move stand, and one by one, the bishop, the castle, the queen, and the king himself would all be under attack. The pawn could not be left in its threatening position.
You can take a pawn from the board if you have a piece in position to do this, and if the consequences will not be harmful; Einstein made his move. It was contained in the remarks he made to a letter I wrote him on September 17, though I did not send it until eight weeks later, with Gina Plungian. I included a note for Miss Dukas. Einsteins handwritten marginal annotations on my letter are here given as footnotes.
The best I could wish was that Einstein would cede me the point; and the next best that he would answer as he did; thus he documented the position of science on the issue in 1954. Four years will pass and it will be admitted that the pressure of light cannot, by a factor of 200 to 2,000, be the cause of the repulsion of the cometary tails3;
the time will come when scientists will think it elementary that a comet crossing the solar corona could not escape electromagnetic effects; but by then it will appear self-understood that this is as it should be; and then I will need to prove that not so long ago different notions prevailed; and how much easier it will be if a man whose authority is unmatched should have written the verdict of science on the very document in which I claimed a divergent view.
Johannes Kepler, mentioned in my letter and in Einsteins notes, the discoverer of the three laws of planetary motions known by his name, was a man to whom Einstein felt a special sympathy, even affinity.
I was obstinate. I was determined to face the issue squarely on this most obvious caseof a comet going through the corona of the sun. And I had to answer the reference on Einsteins part to Keplerian laws.
Einstein also appended the following postscript to my letter: