Criticism of the theory of relativity
Criticism of the theory of relativity of Albert Einstein was mainly expressed in the early years after its publication in the early 1900s, on scientific, pseudoscientific, philosophical, or ideological bases. Though some of these criticisms had the support of reputable scientists, Einstein's theory of relativity is now recognized as self-consistent, in accordance with many experiments, and moreover serves as the basis of many successful theories such as quantum electrodynamics.
Reasons for criticism of the theory of relativity were, for example, alternative theories, rejection of the abstract-mathematical method, misunderstandings, and alleged errors in the theory. Besides those reasons, antisemitic objections to Einstein's Jewish heritage occasionally played a role as well. Even today there are some critics of relativity (sometimes called "anti-relativists"), however, their viewpoints are not taken seriously by the scientific community.
Special relativity 
Relativity principle versus electromagnetic worldview 
Around the end of the 19th century, the view was widespread that all forces in nature are of electromagnetic origin (the "electromagnetic worldview"), especially in the works of Joseph Larmor (1897) and Wilhelm Wien (1900). This was apparently confirmed by the experiments of Walter Kaufmann (1901–1903), who measured an increase of the mass of a body with velocity which was consistent with the hypothesis that the mass was generated by its electromagnetic field. Max Abraham (1902) subsequently sketched a theoretical explanation of Kaufmann's result in which the electron was considered as rigid and spherical. However, it was found that this model was incompatible with the results of many experiments (including the Michelson-Morley experiment, the Experiments of Rayleigh and Brace, and the Trouton-Noble experiment), according to which no absolute motion of an observer with respect to the luminiferous aether ("aether drift") could be observed. Henri Poincaré (1902) called this impossibility "the principle of relativity". Therefore Hendrik Antoon Lorentz (1904) created a model (Lorentz ether theory) that was based on an immobile aether, and contrary to the theory of Abraham, it included effects like length contraction and local time both of which are part of the Lorentz transformation.[A 1][B 1]
Against Lorentz, Abraham (1904) said that (a) length contraction requires a non-electromagnetic force to ensure the electron's stability, which was unacceptable for a proponent of the electromagnetic worldview. And he doubted the possibility that (b) a model based on the relativity principle could be formulated at all. However, Poincaré (1905/6) could show that (b) is possible when a non-electric potential ("Poincaré stress") is assumed, which is subjected to the Lorentz transformation. Poincaré also assumed the non-electric nature of gravitation. Thus (a) and the electromagnetic worldview had to be given up. Eventually, Albert Einstein published in September 1905 what is now called special relativity, which was based on a radical new view of the relativity principle in connection with the constancy of the speed of light in all inertial frames of reference. In special relativity, space and time are relative and the aether doesn't exist at all. Although this theory was founded on a complete different basis, it was experimentally indistinguishable from the aether theory of Lorentz and Poincaré, since both theories employ the Lorentz transformation.[A 2][A 3][A 4][B 2][B 3][C 1]
Claimed experimental refutations 
To conclusively decide between the theories of Abraham and Lorentz, Kaufmann repeated his experiments in 1905 with improved accuracy. However, in the meantime the theoretical situation had changed. Alfred Bucherer and Paul Langevin (1904) developed another model, in which the electron is contracted in the line of motion, and dilated in the transverse direction, so that the volume remains constant. And while Kaufmann was still evaluating his experiments, Einstein published his theory of special relativity. Eventually, Kaufmann published his results in December 1905 and argued that they are in agreement with Abraham's theory and require rejection of the "basic assumption of Lorentz and Einstein" (the relativity principle). Lorentz reacted with the phrase "I am at the end of my Latin", while Einstein did not mention those experiments before 1908. Yet, others started to criticize the experiments. Max Planck (1906) alluded to inconsistencies in the theoretical interpretation of the data, and Adolf Bestelmeyer (1906) introduced new techniques, which (especially in the area of low velocities) gave different results and which cast doubts on Kaufmann's methods. Therefore Bucherer (1908) conducted new experiments and arrived at the conclusion that they confirm the mass formula of relativity and thus the "relativity principle of Lorentz and Einstein". Yet Bucherer's experiments were criticized by Bestelmeyer leading to a sharp dispute between the two experimentalists. On the other hand, additional experiments of Hupka (1910), Neumann (1914) and others seemed to confirm Bucherer's result. But the doubts lasted until 1940, when in similar experiments Abraham's theory was conclusively disproved. (It must be remarked that besides those experiments, the relativistic mass formula was confirmed already in 1917 in the course of investigations on the theory of spectra. And in modern particle accelerators, the relativistic mass formula is routinely confirmed.)[A 5][A 6][A 7][B 4][B 5][C 2]
In 1902-1906, Dayton Miller repeated the Michelson-Morley experiment together with Edward Williams Morley. They confirmed the null result of the initial experiment. However between 1921–1926, Miller conducted new experiments which apparently gave positive results. Those experiments attracted some attention in the media, yet they were refuted by the scientific community. Einstein ironically remarked "subtle is the Lord, but malicious He is not". Einstein, Max Born, and Robert S. Shankland pointed out that Miller hadn't appropriately considered the influence of temperature. And a modern analysis by Roberts shows, that Miller's experiment gives a null result, when the technical shortcomings of the apparatus and the error bars are properly considered. Additionally, Miller's result is in disagreement with all other experiments, which were conducted before and after. For example, Georg Joos (1930) used an apparatus of similar dimensions to Miller's, but he obtained null results. And in recent experiments of Michelson-Morley type where the coherence length is increased considerably by using lasers and masers the results are still negative.[A 8][B 6][C 3]
In 2011, the OPERA collaboration published results according to which the speed of neutrinos is slightly faster than the speed of light. However, potential sources of errors are investigated and independent confirmations are necessary as well. In March 2012 the ICARUS experiment was able to show these results to be anomalous. The ICARUS experiment results combined with errors found in the methodology of the OPERA experiment have cast serious doubt on the claim that neutrinos travel faster than light. For further information and sources see Faster-than-light neutrino anomaly.
Acceleration in special relativity 
It was also claimed that special relativity cannot handle acceleration, which would lead to contradictions in some situations. However, this assessment is not correct, since acceleration actually can be described in the framework of special relativity (see Hyperbolic motion, Rindler coordinates, Born coordinates). Paradoxes relying on insufficient understanding of these facts were discovered in the early years of relativity. For example, Max Born (1909) tried to combine the concept of rigid bodies with special relativity. That this model was insufficient was shown by Paul Ehrenfest (1909), who demonstrated that a rotating rigid body would, according to Born's definition, undergo a contraction of the circumference without contraction of the radius, which is impossible (Ehrenfest paradox). And Max von Laue (1911) showed that rigid bodies cannot exist in special relativity, since the propagation of signals cannot exceed the speed of light, so an accelerating and rotating body will undergo deformations.[A 9][B 7][B 8][C 4]
Paul Langevin and von Laue showed that the twin paradox can be completely resolved by special relativity. If two twins move away from each other, and one of them is accelerating and coming back to the other, then the accelerated twin is younger than the other one, since he was located in at least two inertial frames of reference, and therefore his assessment of which events are simultaneous changed during the acceleration. For the other twin nothing changes since he remained in a single frame.[A 10][B 9]
Another example is the Sagnac effect. Two signals were sent in opposite directions around a rotating platform. After their arrival a displacement of the interference fringes occurs. Sagnac himself believed that he had proved the existence of the aether. However, special relativity can easily explain this effect. When viewed from an inertial frame of reference, it is a simple consequence of the independence of the speed of light from the speed of the source, since the receiver runs away from one beam, while it approaches the other beam. And when viewed from a rotating frame, the assessment of simultaneity changes during the rotation, and consequently the speed of light is not constant in accelerated frames.[A 11][B 10]
As it was shown by Einstein, the only form of accelerated motion that cannot be described is the one due to gravitation, since special relativity is not compatible with the Equivalence principle. Einstein was also unsatisfied with the fact that inertial frames are preferred over accelerated frames. Thus over the course of several years (1908–1915), Einstein developed general relativity. This theory includes the replacement of Euclidean geometry by non-Euclidean geometry, and the resultant curvature of the path of light led Einstein (1912) to the conclusion that (like in accelerated frames) the speed of light is not constant in extended gravitational fields. Therefore, Abraham (1912) argued that Einstein had given special relativity a coup de grâce. Einstein responded that within its area of application (in areas where gravitational influences can be neglected) special relativity is still applicable with high precision, so one cannot speak of a coup de grâce at all.[A 12][B 11][B 12][B 13][C 5]
Superluminal speeds 
In special relativity, the transfer of signals at superluminal speeds is impossible, since this would violate the Poincaré-Einstein synchronization, and the causality principle. Following an old argument by Pierre-Simon Laplace, Poincaré (1904) alluded to the fact, that Newton's law of universal gravitation is founded on an infinitely great speed of gravity. So the clock-synchronization by light signals could in principle be replaced by a clock-synchronization by instantaneous gravitational signals. But in 1905, Poincaré himself solved this problem by showing, that in a relativistic theory of gravity the speed of gravity is equal to the speed of light. Although much more complicated, this is also the case in Einstein's theory of general relativity.[B 14][B 15][C 6]
Another apparent contradiction lies in the fact, that the group velocity in anomalously dispersive media is higher than the speed of light. This was investigated by Arnold Sommerfeld (1907, 1914) and Léon Brillouin (1914). They came to the conclusion that in such cases the signal velocity is not equal to the group velocity, but to the front velocity which is never faster than the speed of light. Similarly, it is also argued that the apparent superluminal effects discovered by Günter Nimtz can be explained by a thorough consideration of the velocities involved.[A 13][B 16][B 17][B 18]
Also quantum entanglement (denoted by Einstein as "spooky action at a distance"), according to which the quantum state of one entangled particle cannot be fully described without describing the other particle, does not imply superluminal transmission of information (see quantum teleportation), and it is therefore in conformity with special relativity.[B 16]
Insufficient knowledge of the basics of special relativity, especially the application of the Lorentz transformation in connection with length contraction and time dilation, led and still leads to the construction of various apparent paradoxes. Both the twin paradox and the Ehrenfest paradox and their explanation were already mentioned above. Besides the twin paradox, also the reciprocity of time dilation (i.e. every inertially moving observer considers the clock of the other one as being dilated) was heavily criticized by Herbert Dingle and others. For example, Dingle wrote a series of letter to Nature at the end of the 1950s. However, also the self-consistency of the reciprocity of time dilation was already demonstrated long before in an illustrative way by Lorentz (in his lectures from 1910, published 1931)[A 14] and many others - they alluded to the fact, that it is only necessary to carefully consider the relevant measurement rules and the relativity of simultaneity. Other known paradoxes are the Ladder paradox and Bell's spaceship paradox, which also can simply be solved by consideration of the relativity of simultaneity.[A 15][A 16][C 7]
Aether and absolute space 
Some physicists (like Oliver Lodge, Albert Abraham Michelson, Edmund Taylor Whittaker, Harry Bateman, Ebenezer Cunningham, Charles Émile Picard, Paul Painlevé, Herbert E. Ives) were uncomfortable with the rejection of the aether, and tried to interpret the Lorentz transformation by using a preferred frame of reference like in the older aether-based theories of Lorentz, Larmor, and Poincaré. This was in connection with the problem, as to how the theories of Lorentz and Einstein are to be distinguished. However, on one hand the aether plays the role of a preferred reference frame, but on the other hand it shall be unobservable due to a "conspiracy" of effects. This combination was considered very improbable, so (except the minority mentioned above) most physicists preferred Einstein's theory as a radical new view of space and time, and there was no place for the aether in the classical sense within modern physics anymore.[A 17][A 18][A 19][B 19][C 8][C 9][C 10][C 11]
Another attempt to re-establish some sort of aether was made by Einstein in the 1920s. This was concluded by him as a consequence of the fact, that the space-time continuum of special relativity cannot be influenced by the presence of matter and, as the aether or Newton's absolute space, consequently has an independent existence. Einstein argued, that this is partly also the case in general relativity as a consequence of the failure of Mach's principle, although contrary to Newton's absolute space, the "ether of general relativity" is affected by the presence of matter (see section on General covariance below). However, this terminology was not accepted by the scientific community, since no state of motion can be ascribed to this "aether". Also the attempts of Paul Dirac (1951) to re-interpret the quantum vacuum as an aether equipped with a state of motion, were not successful. And in his Nobel lecture, George F. Smoot (2006) described his own experiments on Cosmic microwave background radiation as "New Aether drift experiments". However, as pointed out by Smoot, this "Aether drift" is not in contradiction to special relativity or the Michelson-Morley experiment, since it only refers to a reference frame in which the CMBR is isotropic and in which the description of the Big Bang is most convenient.[A 20][B 20][B 21]
Alternative theories 
The theory of complete aether drag, as proposed by George Gabriel Stokes (1844), was used by some critics as Ludwig Silberstein (1920) or Philipp Lenard (1920) as a counter-model of relativity. In this theory, the aether was completely dragged within and in the vicinity of matter, and it was believed that various phenomena, such as the absence of aether drift, could be explained in an "illustrative" way by this model. However, such theories are subject to great difficulties. Especially the aberration of light contradicted the theory, and all auxiliary hypotheses, which were invented to rescue it, are self-contradictory, extremely implausible, or in contradiction to other experiments like the Michelson–Gale–Pearson experiment. In summary, a sound mathematical and physical model of complete aether drag was never invented, consequently this theory was no serious alternative to relativity.[B 22][B 23][C 12][C 13]
Another alternative was the so-called emission theory of light. As in special relativity the aether concept is discarded, yet the main difference from relativity lies in the fact that the velocity of the light source is added to that of light in accordance with the Galilean transformation. As the hypothesis of complete aether drag, it can explain the negative outcome of all aether drift experiments. Yet, there are various experiments that contradict this theory. For example, the Sagnac effect is based on the independence of light speed from the source velocity, and the image of Double stars should be scrambled according to this model - which was not observed. Also in modern experiments in particle accelerators no such velocity dependence could be observed.[A 21][B 24][B 25][C 14]
Principle of the constancy of the speed of light 
Some consider the principle of the constancy of the velocity of light insufficiently substantiated. However, as already shown by Robert Daniel Carmichael (1910) and others, the constancy of the speed of light can be interpreted as a natural consequence of two experimentally demonstrated facts:[A 22][B 26]
- The velocity of light is independent of the velocity of the source, as demonstrated by De Sitter double star experiment, Sagnac effect, and many others (see emission theory).
- The velocity of light is independent of the direction of velocity of the observer, as demonstrated by Michelson-Morley experiment, Kennedy-Thorndike experiment, and many others (see luminiferous aether).
Note, that measurements regarding the speed of light are actually measurements of the two-way speed of light, since the one-way speed of light depends on which convention is chosen to synchronize the clocks.
General relativity 
General covariance 
Einstein emphasized the importance of general covariance for the development of general relativity. This view was challenged by Erich Kretschmann (1917), who argued that every theory of space and time (even including Newtonian dynamics) can be formulated in a covariant way, if additional parameters are included - so stating general covariance as the basis of a theory would be insufficient. Although Einstein (1918) agreed with that argument, he argued that Newtonian mechanics in general covariant form would be too complicated for practical uses. Although Einstein's answer was incorrect (subsequent papers showed that such a theory would still be usable), there can be made another argument in favor of general covariance: it is a natural way to express the equivalence principle, i.e., the equivalence in the description of a free-falling observer and an observer at rest, and thus it is near at hand to use general covariance together with general relativity, not with Newtonian mechanics. Connected with this, also the question of absolute motion was dealt with. Einstein argued, that general covariance is connected with Mach's principle, which would eliminate any "absolute motion" within general relativity. Yet, as pointed out by Willem de Sitter in 1916, Mach's principle is not fulfilled in general relativity because matter-free solutions of the field equations are possible as well. This means that the "inertio-gravitational field", which describes both gravity and inertia, can also exist without matter (therefore Einstein for some time used the term "ether" for this field in some semi-popular lectures). However, as pointed out by Einstein, there is a fundamental difference: while Newton's absolute space is strictly separated from matter, the inertio-gravitational field of general relativity can also be influenced by matter.[A 23][A 24][B 27][B 28][B 29][B 30]
Bad Nauheim Debate 
In the "Bad Nauheim Debate" (1920) between Einstein and Philipp Lenard, the latter stated the following objections: He criticized the lack of "illustrativeness" of relativity, a condition that allegedly can only be met by an aether theory. Einstein responded that the content of "illustrativeness" or "common sense" has changed in time, so it cannot be used as a criterion for the validity of a theory. Lenard also argued, that Einstein reintroduced the aether in general relativity. This was refuted by Hermann Weyl - although Einstein used that expression in 1920, he simply referred to the fact that in general relativity, space possesses properties that influences matter and vice versa. However, no "substance" with a state motion (as the aether in the older sense) exists in general relativity. Lenard also argued, that general relativity admits of the existence of superluminal velocities. For example, in a reference frame in which the Earth is at rest, the distant points of the whole universe are rotating around Earth with superluminal velocities. However, as been pointed out by Weyl, it's not possible to handle a rotating extended system as a rigid body (neither in special nor in general relativity) - so the signal velocity of an object never exceeds the speed of light. Another issue (that was raised by both Lenard and Gustav Mie) concerns the existence of "fictitious" gravitational fields, which were introduced by Einstein within accelerated frames to guarantee their equivalence to frames in which gravitational fields exist. Lenard and Mie argued, that only forces can exist that are proportional to real existing masses, while the gravitational field in an accelerating frame of reference has no physical meaning, i.e. the relativity principle can only be valid for mass proportional forces. Einstein responded, that based on Mach's principle one can think of these gravitational fields as induced by the distant masses. In this respect the criticism of Lenard and Mie was partly justified - Mach's principle is not fulfilled in general relativity, as already mentioned above.[A 25][C 15]
Silberstein-Einstein controversy 
Ludwik Silberstein, who initially was a supporter of the special theory, objected at different occasions against general relativity. In 1920 he argued that the deflection of light by the sun, as observed by Arthur Eddington et al. (1919), is not necessarily a confirmation of general relativity, but may also be explained by the Stokes-Planck theory of complete aether drag. However, such models are in contradiction with the aberration of light and other experiments (see "Alternative theories"). And in 1935, Silberstein claimed to have found a contradiction in the Two-body problem in general relativity. However, also this claim was refuted by Einstein and Rosen (1935).[A 26][B 31][C 16]
Philosophical criticism 
The consequences of relativity, such as the change of ordinary concepts of space and time, as well as the introduction of non-Euclidean geometry in general relativity, were criticized by some philosophers of different philosophical schools. It was characteristic for many philosophical critics, that they had insufficient knowledge of the mathematical and formal basis of relativity, consequently the criticisms often missed the heart of the matter. For example, relativity was misinterpreted as some form of relativism. However, this is misleading as it was emphasized by Einstein or Planck. On one hand it's true that space and time became relative, and the inertial frames of reference are handled on equal footing. On the other hand the theory makes natural laws invariant - examples are the constancy of the speed of light, or the covariance of Maxwell's equations. Consequently, Felix Klein (1910) called it the "invariant theory of the Lorentz group" instead of relativity theory, and Einstein (who reportedly used expressions like "absolute theory") sympathized with this expression as well.[A 27][B 32][B 33][B 34]
Critical responses to relativity (in German speaking countries) were also expressed by proponents of Neo-Kantianism (Paul Natorp, Bruno Bauch, etc.), and Phenomenology (Oskar Becker, Moritz Geiger etc.). While some of them only rejected the philosophical consequences, others rejected also the physical consequences of the theory. Einstein was criticized for violating Immanuel Kant's categoric scheme, i.e., it was claimed that space-time curvature caused by matter and energy is impossible, since matter and energy already require the concepts of space and time. Also the three-dimensionality of space, Euclidean geometry, and the existence of absolute simultaneity was claimed to be necessary for the understanding of the world - none of them can possibly be altered by empirical findings. However, Hentschel (1990) and others criticized these arguments as "Strategies of Immunization". By moving all those concepts into a metaphysical area, any form of criticism of Kantianism would be prevented. Additionally, he argued that also Kant's philosophy is the product of his time, i.e. Kant used Newton's theories as the basis of many of his philosophical thoughts. Therefore, other Kantians like Ernst Cassirer or Hans Reichenbach (1920), tried to modify Kant's philosophy. Subsequently, Reichenbach rejected Kantianism at all and became a proponent of logical positivism.[A 28][B 35][B 36][C 17][C 18][C 19]
Based on Henri Poincaré's conventionalism, philosophers such as Pierre Duhem (1914) or Hugo Dingler (1920) argued that the classical concepts of space, time, and geometry were, and will always be, the most convenient expressions in natural science, therefore the concepts of relativity cannot be correct. This was criticized by proponents of logical positivism such as Moritz Schlick, Rudolf Carnap, or Reichenbach. They argued that Poincaré's conventionalism could be modified, as to bring it into accord with relativity. Although it's true, that the basis assumptions of Newtonian mechanics are simpler, it can only be brought into accord with modern experiments by inventing auxiliary hypotheses. On the other hand, relativity doesn't need such hypotheses, thus from a conceptual viewpoint, relativity is in fact simpler than Newtonian mechanics.[A 29][B 37][B 38][C 20]
Some proponents of Philosophy of Life, Vitalism, Critical realism (in German speaking countries) argued that there is a fundamental difference between physical, biological and psychological phenomena. For example, Henri Bergson (1921), who otherwise was a proponent of special relativity, argued that time dilation cannot be applied to biological organisms, therefore he denied the relativistic solution of the twin paradox. However, those claims were rejected by Paul Langevin, André Metz and others. Biological organisms consist of physical processes, so there is no reason to assume that they are not subject to relativistic effects like time dilation.[A 30][B 39][C 21]
Based on the philosophy of Fictionalism, the philosopher Oskar Kraus (1921) and others claimed that the foundations of relativity were only fictitious and even self-contradictory. Examples were the constancy of the speed of light, time dilation, length contraction. These effects appear to be mathematically consistent as a whole, but in reality they allegedly are not true. Yet, this view was immediately rejected. The foundations of relativity (such as the equivalence principle or the relativity principle) are not fictitious, but based on experimental results. Also, effects like constancy of the speed of light and relativity of simultaneity are not contradictory, but complementary to one another.[A 31][C 22]
In the Soviet Union (mostly in the 1920s), philosophical criticism was expressed on the basis of dialectic materialism. The theory of relativity was rejected as anti-materialistic and speculative, and a mechanistic worldview based on "common sense" was required as an alternative. Similar criticisms also occurred in the People's Republic of China during the Cultural Revolution. (On the other hand, other philosophers considered relativity as being compatible with Marxism)[A 32][A 33]
Relativity hype and popular criticism 
Although Planck already in 1909 compared the changes brought about by relativity with the Copernican Revolution, and although special relativity was accepted by most of the theoretical physicists and mathematicians by 1911, it was not before publication of the experimental results of the group around Arthur Stanley Eddington (1919), that relativity was noticed by the public. Einstein was praised in the mass media, and he was compared to Nikolaus Copernicus, Johannes Kepler and Isaac Newton. This fame led to a popular "relativity hype" ("Relativitätsrummel", as it was called by Sommerfeld, Einstein, and others), but it also caused a counter-reaction of some scientists and scientific laymen. The controversy (atypical for scientific discussions) was partly carried out in the press, and the criticism was not only directed to relativity, but personally to Einstein as well.[A 34][A 35]
Academic and non-academic criticism 
Some academic scientists, especially experimental physicists such as the Nobel laureates Philipp Lenard and Johannes Stark, as well as Ernst Gehrcke, Stjepan Mohorovičić, Rudolf Tomaschek and others criticized the increasing mathematization of modern physics, especially in the form of relativity theory and quantum mechanics. It was seen as a tendency to abstract theory building, connected with the loss of "common sense". In fact, relativity was the first theory, in which the inadequacy of the "illustrative" classical physics was clearly demonstrated. The critics ignored these developments and tried to revitalize older theories, such as aether drag models or emission theories (see "Alternative Theories"). However, those qualitative models were never sufficiently advanced to compete with the success of the precise experimental predictions and explanatory powers of the modern theories. Additionally, there was also a great rivalry between experimental and theoretical physicists, as regards the professorial activities and the occupation of chairs at German universities. The opinions clashed at the "Bad Nauheim debate" in 1920 between Einstein and Lenard, which attracted much attention in the public.[A 36][A 34][C 15][C 23][C 24]
In addition, there were many critics (with or without physical training) whose ideas were far outside the scientific mainstream. These critics were mostly people who had developed their ideas long before the publication of the theory of relativity and they tried resolve in a straightforward manner some or all of the enigmas of the world. Therefore, Wazeck (who studied some German examples) gave to these "free researchers" the name "world riddle solver" ("Welträtsellöser", such as Arvid Reuterdahl, Hermann Fricke or Johann Heinrich Ziegler). Their views had their quite different roots in monism, Lebensreform, or occultism. Their methods were characterized by the fact that they practically rejected the entire terminology and the (primarily mathematical) methods of modern science. Their works were published by private publishers, or in popular and non-specialist journals. It was significant for many "free researchers" (especially the monists) to explain all phenomena by intuitive and illustrative mechanical (or electrical) models, which also found its expression in their defense of the aether. Therefore they rejected the inscrutability of the relativity theory, which was considered a pure calculation method that cannot reveal the true reasons behind things. The "free researchers" often used Mechanical explanations of gravitation, in which gravity is caused by some sort of "aether pressure" or "mass pressure from a distance". Such models were regarded as an illustrative alternative to the abstract mathematical theories of gravitation of both Newton and Einstein. Additionally, also the enormous self-confidence of the "free researchers" is noteworthy, since they not only believed to have solved all the riddles of the world, but also had the expectation that they would rapidly convince the scientific community.[A 37][C 25][C 26][C 27]
Since Einstein rarely defended himself against these attacks, this task was undertaken by other relativity theoreticians, who (according to Hentschel) formed some sort of "defensive belt" around Einstein. Some representatives were Max von Laue, Max Born, etc. and on popular-scientific and philosophical level Hans Reichenbach, André Metz etc., who led many discussions with critics in semi-popular journals and newspapers. However, most of these discussions were failing from the start. Physicists like Gehrcke, some philosophers, and the "free researchers" were so obsessed with their own ideas and prejudices, that they were unable to grasp the basics of relativity, consequently the participants of the discussions were talking past each other. In fact, the theory which was criticized by them was not relativity at all, but rather a caricature of it. The "free researchers" were mostly ignored by the scientific community, but with time also respected physicists such as Lenard and Gehrcke found themselves in a position outside the scientific community. However, the critics didn't believe that this was due to their incorrect theories, but rather due to a conspiracy of the relativistic physicists (and in the 1920s & 1930s of the Jews as well), which allegedly tried to put down the critics, and to preserve and improve their own positions within the academic world. For example, Gehrcke (1920/24) held that the propagation of relativity is a product of some sort of mass suggestion. Therefore he instructed a Media monitoring service to collect over 5000 newspaper clippings which were related to relativity, and published his findings in a book. However, Gehrcke's claims were rejected, because the simple existence of the "relativity hype" says nothing about the validity of the theory, and thus it cannot used for or against relativity.[A 38][A 39][C 28]
Afterward, some critics tried to improve their positions by the formation of alliances. One of them was the "Academy of Nations", which was founded in 1921 in the USA by Robert T. Browne and Arvid Reuterdahl. Other members were Thomas Jefferson Jackson See and as well as Gehrcke and Mohorovičić in Germany. Whether other American critics such as Charles Lane Poor, Charles Francis Brush, Dayton Miller were also members, is unknown. However, the alliance disappeared already in the mid 1920s in Germany and 1930 in the USA.[A 40]
Chauvinism and antisemitism 
Shortly before and during World War I, there appeared some nationalistically motivated criticisms of relativity and modern physics. For example, Pierre Duhem regarded relativity as the product of the "too formal and abstract" German spirit, which was in conflict with the "common sense". Similarly, popular criticism in the Soviet Union and China, which partly was politically organized, rejected the theory not because of factual objections, but as ideologically motivated as the product of western decadence.[A 41][A 32][A 33]
So in those countries, the Germans or the Western civilization were the enemies. However, in Germany the Jewish ancestry of relativity proponents such as Einstein and Minkowski made them targets of racist critics, although many German critics did not have such motives. Paul Weyland, a known nationalistic agitator, arranged the first public meeting against relativity in Berlin in 1919. Lenard and Stark were known for their nationalistic opinions. While they avoided antisemitic claims in their first publications, it was clear for many, that antisemitism played a role. Reacting to this underlying mood, Einstein openly speculated in a newspaper article that, in addition to insufficient knowledge of theoretical physics, antisemitism was also a reason of their criticisms. Some critics, including Weyland, reacted angrily and claimed that such accusations of antisemitism were only made to force the critics into silence. However, subsequently Weyland, Lenard, Stark and others clearly showed their antisemitic prejudices by beginning to combine their criticisms with racism. For example, Theodor Fritsch emphasized the alleged negative consequences of the "Jewish spirit" within relativity, and the far right-press continued this propaganda unhindered. After the murder of Walther Rathenau (1922) and murder threats against Einstein, he left Berlin for some time. Gehrcke's book on "The mass suggestion of relativity theory" (1924) was not antisemitic itself, but it was praised by the far-right press as describing an alleged typical Jewish behavior. Philipp Lenard in 1922 spoke about the "foreign spirit" as the foundation of relativity, and afterward he joined the Nazi party in 1924; Johannes Stark did the same in 1930. Both were proponents of the so-called German Physics, which only accepted scientific knowledge based on experiments, and only if accessible to the senses. According to Lenard (1936), this is the "Aryan physics or physics by man of Nordic kind" as opposed to the alleged formal-dogmatic "Jewish physics". Additional antisemitic critics can be found in the writings of Wilhelm Müller, Bruno Thüring and others. For example, Müller claimed that relativity is a pure "Jewish affair" and it would correspond to the "Jewish essence" etc., while Thüring made comparisons between the Talmud and relativity.[A 42][A 43][A 44][A 34][A 45][A 46][B 40][C 29][C 30][C 31]
Accusations of plagiarism and priority discussions 
Some critics like Lenard, Gehrcke and Reuterdahl called Einstein a plagiarist, and they questioned his priority for inventing relativity. On one side, the purpose of those allegations was to allude to non-relativistic alternatives to modern physics, and on the other side, Einstein himself should be discredited. However, it was quickly seen that these accusations were unfounded, since the physical content and the applicability of those former theories were quite different from relativity. Some examples:[A 47][A 48][B 41][B 42][C 32][C 33]
- Johann Georg von Soldner (1801) was credited for his calculation of the deflection of light in the vicinity of celestial bodies, long before Einstein's prediction which was based on general relativity. However, Soldner's derivation has nothing to do with Einstein's, since it was fully based on Newton's theory, and only gave half of the value as predicted by general relativity.
- Paul Gerber (1898) published a formula for the perihelion advance of Mercury, which was formally identical to an approximate solution given by Einstein. However, since Einstein's formula was only an approximation, the solutions are not identical. In addition, Gerber's derivation has no connection with General relativity and was even considered as meaningless.
- Woldemar Voigt (1887) derived a transformation, which is very similar to the Lorentz transformation. As Voigt himself acknowledged, his theory was not based on electromagnetic theory, but on an elastic aether model. His transformation also violates the relativity principle.
- Friedrich Hasenöhrl (1904) applied the concept of electromagnetic mass and momentum (which were known long before) to cavity- and thermal radiation. Yet, the applicability of Einstein's Mass–energy equivalence goes much further, since it is derived from the relativity principle and applies to all forms of energy.
- Menyhért Palágyi (1901) developed a philosophical "space-time" model in which time plays the role of an imaginary fourth dimension. But Palágyi's model was only a reformulation of Newtonian physics, and had no connection to electromagnetic theory, the relativity principle, or to the constancy of the speed of light.
Some modern historians of science still consider the question, whether Einstein was possibly influenced by Poincaré, who offered some interpretations of Lorentz's electron theory that can also be found in special relativity.[A 49] Another discussion concerns a possible mutual influence between Einstein and David Hilbert as regards completing the field equations of general relativity (see Relativity priority dispute).
A collection of various criticisms can be found in the book "Hundert Autoren gegen Einstein" (A Hundred authors against Einstein), published in 1931. It contains very short texts from 28 authors, and excerpts from the publications of another 19 authors. The rest consists of a list that also includes people who only for some time were opposed to relativity. Besides philosophic objections (mostly based on Kantianism), also some alleged elementary failures of the theory were included, however, as some commented, those failures were due to the authors' misunderstanding of relativity. For example, Hans Reichenbach described the book as an "accumulation of naive errors", and as "unintentionally funny". Albert von Brunn interpreted the book as a backward step to the 16th and 17th century, and Einstein is reported to have said with irony, that one author alone would have been sufficient to refute him:
- "If I were wrong, then one would have been enough!" —Albert Einstein, commenting on the book 100 Authors Against Einstein
According to Goenner, the contributions to the book are a mixture of mathematical–physical incompetence, hubris, and the feelings of the critics of being suppressed by modern physicists. The compilation of the authors show, Goenner continues, that this was not a reaction within the physics community—only one physicist (Karl Strehl) and three mathematicians (Jean-Marie Le Roux, Emanuel Lasker and Hjalmar Mellin) were present—but an inadequate reaction of the academic educated citizenship, which didn't know what to do with relativity. As regards the average age of the authors: 57% were substantially older than Einstein, one third was around the same age, and only two persons were substantially younger.[A 50] Two authors (Reuterdahl, von Mitis) were antisemitic and four others were possibly connected to the Nazi movement. On the other hand, no antisemitic expression can be found in the book, and it also included contributions of some authors of Jewish ancestry (Salomo Friedländer, Ludwig Goldschmidt, Hans Israel, Emanuel Lasker, Oskar Kraus, Menyhért Palágyi).[A 50][A 51][C 34]
Status of criticism 
The theory of relativity is considered to be self-consistent, is verified by many experimental verifications, and serves as the basis of many successful theories like quantum electrodynamics. Therefore, fundamental criticism (like that of Herbert Dingle, Louis Essen, Petr Beckmann, Maurice Allais and Tom van Flandern) has not been taken seriously by the scientific community, and due to the lack of quality of many critical publications (found in the process of Peer review) they were rarely accepted for publication in reputable scientific journals. So, as in the 1920s, most of the critical works have been published in small publications houses, alternative journals (like "Apeiron" or "Galilean Electrodynamics"), or private websites (like Conservapedia).[A 52] Consequently, where criticism of relativity has been dealt with by the scientific community, it has mostly been in historical studies.[A 53][A 54]
However, this does not mean that there is no further development in modern physics. The enormous progress of technology leads to extremely precise ways of testing the predictions of relativity, and so far it has successfully passed all tests (such as in particle accelerators to test special relativity, and by astronomical observations to test general relativity). In addition, in the theoretical field there is continuing research intended to unite general relativity and quantum theory. The most promising models are string theory and loop quantum gravity. Some variations of those models also predict violations of Lorentz invariance on a very small scale.[B 43][B 44][B 45]
See also 
Historic analyses 
Relativity papers 
Critical works 
- The Newspaper clippings and works collected by Gehrcke and Reuterdahl form an important basis for historic research on the criticism of relativity;
- Milena Wazeck: Einstein's sceptics: Who were the relativity deniers?, New Scientist, 18 November 2010.
- CosmosMagazine: Was Einstein a fake?