Lorentz and Fitzgerald thought that the contraction of measuring rods is caused by the movement of matter through the ether, ...Here is what Lorentz said in 1895:
Einstein suggested that the contraction is a question of "perspective" rather than a physical contraction in the moving system, and that it is mutual.
§ 91. As strange as this hypothesis would appear at first sight, nevertheless one must admit that it's not so far off, as soon as we assume that also the molecular forces, similarly as we now definitely can say it of the electrical and magnetic forces, are transmitted through the aether. If this is so, then the translation will change the action between two molecules or atoms most likely in a similar way, as the attraction or repulsion between charged particles. Now, since the shape and the dimensions of a fixed body are, in the last instance, determined by the intensity of the molecular effects, then also a change of the dimensions is inevitable.Even tho Popper belittles it, this is a brilliant and correct statement. People knew that matter was made of atoms, but the atoms could have been solid objects in physical contact with each other. In fact, they are held together by electromagnetic forces, just as Lorentz predicted.
Karl Popper was a very famous and well-respected philosopher, but his defense of Einstein is based on misquoting him. Einstein never said that the contraction is a question of "perspective". You can check Einstein's famous 1905 paper for yourself. You will not find the word "perspective", and you will not find any disagreement with Lorentz's views. Einstein gave dozens of interviews all his life about how he (supposedly) discovered relativity, and while he acknowledged having read that 1895 Lorentz paper, he never denied that the contraction was a physical contraction.
There are a lot of Einstein idolizers who argue at great length that Einstein's 1905 view was somehow different from Lorentz's, but those arguments are never based on what Einstein actually says. Since Einstein spent a lot of time and effort into trying to explain the merits of his relativity work, it is a strange to claim that he had some view that he never claimed himself.
Einstein was a patent examiner. They are trained to separate work from what went before, and to judge an inventor's originality by his own claims. Inventors only get credit for what they claim for themselves.
Though Einstein appears to have known Poincaré's Science and Hypothesis prior to 1905, there is no theory like Einstein's in this great book. Einstein could not have known Poincaré's article of 1905; and even in this article there is only a most inspiring programme sketched for a relativity theory -- not the theory itself.Poincare's 1902 book was written for the general public, and does not have any equations. It explains the principle of relativity, and denies the aether, but does not flesh out the theory, as Popper says. Poincare's 1905 article was really two articles -- a 5-page summary and a 50-page detailed paper. Einstein certainly could have known about Poincare's summary as it was published and delivered to Einstein's library three weeks before Einstein submitted his own 1905 relativity paper.
Popper was wrong to say that Poincare's 1905 paper was only a sketch and not a theory. Poincare had concepts like 4-dimensional spacetime geometry and covariance of Maxwell's equations, and these went way beyond what Einstein did.
Herbert Dingle was not so respected:
Ultimately Dingle re-focused his criticism to claim that special relativity was logically inconsistent: "The theory [special relativity] unavoidably requires that A works more slowly than B and B more slowly than A -- which it requires no super-intelligence to see is impossible."This is indeed a paradox, but Poincare and Minkowski proved that there is no logical inconsistency, by showing that the Lorentz transformations form a group preserving a geometric structure.
Popper has his critics also. A recent paper by Alan B. Whiting argues that in "his most famous work he displays misunderstandings of science and mathematics at a basic level." Popper understood science much better than most philosophers, but maybe that is not saying much.
Much of this is explained in How Einstein Ruined Physics.