Poincaré on the constancy of light

[Histspec]

It's notable that Poincaré many years before Einstein had very interesting ideas on the constancy of light. For example: In his paper http://en.wikisource.org/wiki/The_Measure_of_Time" Poincaré wrote in 1898:

When an astronomer tells me that some stellar phenomenon, which his telescope reveals to him at this moment, happened nevertheless fifty years ago, I seek his meaning, and to that end I shall ask him first how he knows it, that is, how he has measured the velocity of light...He has begun by supposing that light has a constant velocity, and in particular that its velocity is the same in all directions. That is a postulate without which no measurement of this velocity could be attempted. This postulate could never be verified directly by experiment; it might be contradicted by it if the results of different measurements were not concordant. We should think ourselves fortunate that this contradiction has not happened and that the slight discordances which may happen can be readily explained...The postulate, at all events, resembling the principle of sufficient reason, has been accepted by everybody; what I wish to emphasize is that it furnishes us with a new rule for the investigation of simultaneity, entirely different from that which we have enunciated above...We have not a direct intuition of simultaneity, nor of the equality of two durations. If we think we have this intuition, this [36] is an illusion. We replace it by the aid of certain rules which we apply almost always without taking count of them. But what is the nature of these rules? No general rule, no rigorous rule; a multitude of little rules applicable to each particular case. These rules are not imposed upon us and we might amuse ourselves in inventing others; but they could not be cast aside without greatly complicating the enunciation of the laws of physics, mechanics and astronomy. We therefore choose these rules, not because they are true, but be cause they are the most convenient, and we may recapitulate them as follows : "The simultaneity of two events, or the order of their succession, the equality of two durations, are to be so defined that the enunciation of the natural laws may be as simple as possible. In other words, all these rules, all these definitions are only the fruit of an unconscious opportunism."

Abraham Pais (in Subtle is the Lord) said that "These lines read like the general program for what would be given concrete shape seven years later." In 1900, Poincare wrote in http://www.physicsinsights.org/poincare-1900.pdf" :

Let us suppose that there are some observers placed at various points, and they synchronize their clocks using light signals. They attempt to adjust the measured transmission time of the signals, but they are not aware of their common motion, and consequently believe that the signals travel equally fast in both directions. They perform observations of crossing signals, one traveling from A to B, followed by another traveling from B to A. The local time t is the time indicated by the clocks which are so adjusted. If [c] is the speed of light, and v is the speed of the Earth which we suppose is parallel to the x axis, and in the positive direction, then we have: t'=t-vx/c².

And in 1904 in his paper http://en.wikisource.org/wiki/The_Principles_of_Mathematical_Physics" :

The most ingenious idea has been that of local time. Imagine two observers who wish to adjust their watches by optical signals; they exchange signals, but as they know that the transmission of light is not instantaneous, they take care to cross them. When the station B perceives the signal from the station A, its clock should not mark the same hour as that of the station A at the moment of sending the signal, but this hour augmented by a constant representing the duration of the transmission. Suppose, for example, that the station A sends its signal when its clock marks the hour 0, and that the station B perceives it when its clock marks the hour t. The clocks are adjusted if the slowness equal to t represents the duration of the transmission, and to verify it the station B sends in its turn a signal when its clock marks 0; then the station A should perceive it when its clock marks t. The time-pieces are then adjusted. And in fact, they mark the same hour at the same physical instant, but on one condition, namely, that the two stations are fixed. In the contrary case the duration of the transmission will not be the same in the two senses, since the station A, for example, moves forward to meet the optical perturbation emanating from B, while the station B flies away before the perturbation emanating from A. The watches adjusted in that manner do not mark, therefore, the true time; they mark what one may call the local time, so that one of them goes slow on the other. It matters little, since we have no means of perceiving it. All the phenomena which happen [612] at A, for example, will be late, but all will be equally so, and the observer who ascertains them will not perceive it, since his watch is slow; so, as the principle of relativity would have it, he will have no means of knowing whether he is at rest or in absolute motion...From all these results, if they are confirmed, would arise an entirely new mechanics, which would be, above all, characterized by this fact, that no velocity could surpass that of light, any more than any temperature could fall below the zero absolute, because bodies would oppose an increasing inertia to the causes, which would tend to accelerate their motion; and this inertia would become infinite when one approached the velocity of light. Nor for an observer carried along himself in a translation he did not suspect could any apparent velocity surpass that of light; there would then be a contradiction, if we recall that this observer would not use the same clocks as a fixed observer, but, indeed, clocks marking "local time”.

It would be very interesting to know, whether Einstein read some of Poincaré's papers.

Regards,

 

[phyti]

An interesting paper, and another example of the inquisitive mind. There is always more than one person contemplating some part of the world around us, and the one who gets published first gets the notoriety.

 

[RandallB]

I think the point here is that in 1900 with the progress Planck, Poincare, Lorentz, and others were making it was inevitable that special relativity would have been found.

And no sour grapes from any of these three, as all acknowledged at the time it was Einstein that first pulled the key pieces together to solve the puzzle.

 

[Naty1]

In an interesting passage Peter Bergmann in THE RIDDLE OF GRAVITATION, 1992, says

In 1905 Einstein combined Lorentz's and Poincare's ideas into a new approach to the issues of frames of reference and so was able to explain why no experiment had uncovered the absolute motion of the Earth without contradiciting Maxwell's theory of electricity and magnetism.

Einstein recognized from the frame sensitivity of the gravitational field it would be more complex than the electromagnetic and guessed the gravitational field is a tensor field..

Einstein found he could construct several different theories that appeard to conform equally well to his requirements...these theories led to some effects that differed from theory to theory but those were so minute

they could not be detected experimentally at the time...

Some other point of view was needed to lead without ambiguity to a relativist theory of gravitation. Evenetually Einstein discovered this new point of view in the principle of equivalence

betweeen gravity and acceleration.