The Lorentz Ether Theory (LET) is the commonly used name for the original interpretation of the theory which was originally named Lorentz-Einstein theory and is known today as Special Relativity (SR). The spacetime interpretation of relativity, which is now the standard interpretation, was proposed by Minkowski only later.

In the Lorentz ether, there exists an absolute space as well as absolute time - these basic concepts of classical physics have not been changed. The space is filled with an ether, which is homogeneous. static, and incompressible. As usual in ether theories, the speed of light is simply the speed of sound waves of the ether.

The clocks as well as the rulers are distorted if they move relative to the ether. A clock which moves relative to the ether is time-dilated, goes slower by a factor \(\gamma = \sqrt{1-\frac{v^2}{c^2}}\). A ruler shrinks by the same factor in the direction of movement. As a consequence of these distortions of our measurement instruments, it becomes impossible to measure the own velocity relative to the ether. This is the consequence of Lorentz symmetry. If an observer moves with constant velocity, he has no possibility to detect that he moves. He can, erroneously, assume that he is in rest, and that the distorted time measured with his clock is true time. This error is combined with another error, namely an error identifying those events which happen at the same time. If one is at rest, it makes sense to synchronize time with far away events using Einstein synchronization: One sends a light signal to a mirror and back and measures the time. Then the moment when the light ray arrives at the mirror and starts to go back will be after half of the time the light ray needs for the whole way. If the observer moves, this idea fails, thus, one obtains a wrong synchronization if one uses this Einstein synchronization. But above errors compensate each other nicely, so that one cannot even identify by some measurements with rulers and clocks that one has made an error assuming that one is at rest. This is the equivalence principle.

The mathematical formulas for all observable effects are those of Special Relativity (SR). Once one cannot find out by measurement if one is at rest or not, and, as a consequence, it is impossible to measure true time and undistorted distances, true time becomes, together with the correct identification of contemporaneity and true distances in space, a hidden variable. Thus, the Lorentz ether is a Hidden Variable Theory.

Let's note here the major objections against the Lorentz ether:

In the Lorentz ether, there exists an absolute space as well as absolute time - these basic concepts of classical physics have not been changed. The space is filled with an ether, which is homogeneous. static, and incompressible. As usual in ether theories, the speed of light is simply the speed of sound waves of the ether.

The clocks as well as the rulers are distorted if they move relative to the ether. A clock which moves relative to the ether is time-dilated, goes slower by a factor \(\gamma = \sqrt{1-\frac{v^2}{c^2}}\). A ruler shrinks by the same factor in the direction of movement. As a consequence of these distortions of our measurement instruments, it becomes impossible to measure the own velocity relative to the ether. This is the consequence of Lorentz symmetry. If an observer moves with constant velocity, he has no possibility to detect that he moves. He can, erroneously, assume that he is in rest, and that the distorted time measured with his clock is true time. This error is combined with another error, namely an error identifying those events which happen at the same time. If one is at rest, it makes sense to synchronize time with far away events using Einstein synchronization: One sends a light signal to a mirror and back and measures the time. Then the moment when the light ray arrives at the mirror and starts to go back will be after half of the time the light ray needs for the whole way. If the observer moves, this idea fails, thus, one obtains a wrong synchronization if one uses this Einstein synchronization. But above errors compensate each other nicely, so that one cannot even identify by some measurements with rulers and clocks that one has made an error assuming that one is at rest. This is the equivalence principle.

The mathematical formulas for all observable effects are those of Special Relativity (SR). Once one cannot find out by measurement if one is at rest or not, and, as a consequence, it is impossible to measure true time and undistorted distances, true time becomes, together with the correct identification of contemporaneity and true distances in space, a hidden variable. Thus, the Lorentz ether is a Hidden Variable Theory.

Let's note here the major objections against the Lorentz ether:

- The positivistic objection: What really exists, should be observable. Once we cannot distingush, by observation, if we are at absolute rest, there is no such thing as absolute rest. Once we cannot meaure, via Einstein synchronization, the contemporaneity of events, no contemporaneity exists in reality. All what exists is the spacetime as a whole.

- The conspiracy: To assume that clocks and rulers are somehow distorted would be unproblematic. But that these distortions conspire in such a way that we cannot even identify if we are at rest or not seems unnatural.

- Occam's razor: The Lorentz ether has to postulate the existence of an additional element of reality, namely the preferred subdivision of spacetime into absolute space and absolute time.

- The irrelevance of interpretations: Once different interpretations of SR make the same predictions about all observables, the two interpretations are equivalent empirically, as physical theories. Therefore interpretations are irrelevant. So, it makes no sense to discuss them, let's follow the one everybody knows, which is the spacetime interpretation.

- No extension to gravity: SR is no longer relevant, it has been replaced by GR. For GR, we have only the spacetime interpretation.

- Positivism is wrong, and has to be replaced by the Popperian method. It does not contain a requirement that all objects of theories are observable.

- Against the conspiracy, already Lorentz has found an argument. If the force which holds matter together is the electromagnetic force, or some other force which follows a similar equation, then the equations which describe matter, that means, also our rulers and clocks, have the same symmetry as the Maxwell equations of electromagnetism. But these equations are Lorentz-symmetric. Therefore, the equations describing clocks and rulers also would have to be Lorentz-symmetric. This is already sufficient to prove time dilation and length contraction.

- Occam's razor could be, as well, applied against the spacetime. In the Lorentz ether, only space exists, filled with the ether, and what is in space changes in time. This is sufficient. There does not have to exists some four-dimensional spacetime. So it is the Minkowski spacetime which introduces entities - a whole additional dimension - without necessity.

- It appears that the two interpretations are not completely equivalent. In the spacetime interpretation, one can prove Einstein causality, and, as a consequence, Bell's inequality for space-like separated events. The Lorentz ether allows hidden causal influences faster than light, but from past into future in true, absolute time. This makes it impossible to proof Bell's inequality. Given that Bell's inequality is violated, this becomes a strong argument in favor of the Lorentz ether.

- An extension of the Lorentz ether interpretation to gravity exists. It can be considered and discussed here too. One can reasonably assume that the fact that this ether interpretation for the GR equations was unknown was a decisive argument against the ether. But this decisive argument appears invalid.