Dynamic theory of gravity
Nikola Tesla's dynamic theory of gravity is reported to be Tesla's attempt to formulate a theory relating gravity and electromagnetism, i.e. a unified field theory. No mathematical details of the theory are available, Tesla never published them, and there is no evidence that Tesla ever worked them out besides his own testimony. Because of this, and because of the achievements of Albert Einstein's theory of general relativity, Tesla's theory is not used by physicists.
Tesla believed that it is absolutely impossible to convert mass into energy, even after the publication of Einstein's work and, less than a decade later, the development of the atomic bombs.
During a 1891 lecture on the structure of the aether and electromagnetism, Tesla stated that there was "an infinitesimal world, with molecules and their atoms spinning and moving in orbits, in much the same manner as celestial bodies carrying with them static charges ...", and, there were independent carriers transmitting "... tensions or electrostatic strains ...". This predated the research of Rutherford, Bohr, and Einstein into the nature of fundamental particles. 
The bulk of Tesla's research into the aether and electromagnetism was conducted between 1892 and 1894, when he was conducting experiments with high frequency and high potential electromagnetism and patenting devices for their utilization. It was completed, according to Tesla, by the end of the 1930s. Tesla claimed that the theory provided an alternative to Einstein's general relativity, explaining gravity as a mix of transverse and longitudinal electromagnetic waves.
On his 81st birthday (July 10, 1937), Tesla released a prepared statement asserting that he had "worked out a dynamic theory of gravity" that he soon hoped to give to the world. Few details were revealed by Tesla about his theory in the announcement, and he died without publishing any further details. Portions of the 1937 statement include:
- [The Dynamic theory of gravity] ... explains the causes and motions of heavenly bodies under its influence so satisfactory that it will put to an end idle speculation and false conception, as that of curved space ... '
- Only the existence of a field of force can account for the motions of the bodies as observed, and its assumption dispenses with space curvature. All literature on this subject is futile and destined to oblivion. So are all attempts to explain the workings of the universe without recognizing the existence of the ether and the indispensable function it plays in the phenomena ...
- There is no energy in matter other than that received from the environment... It applies rigorously to molecules and atoms as well as the largest heavenly bodies, and to all matter in the universe in any phase of its existence from it: very formation to its ultimate disintegration.
- Before the electron theory was advanced, I had established that radioactive rays consisted of particles of primary matter not further decomposable...
- ... The kinetic and potential energy of a body is the result of motion and determined by the product of its mass and the square of velocity. Let the mass be reduced, the energy is diminished in the same proportion. If it be reduced to zero the energy is likewise zero for any finite velocity. In other words, it is absolutely impossible to convert mass into energy. It would be different if there were forces in nature capable of imparting to a mass infinite velocity. Then the product of zero mass with the square of infinite velocity would represent infinite energy. But we know that there are no such forces and the idea that mass is convertible into energy is rank nonsense.
The statement criticized Albert Einstein's theory of relativity, which had by that time been accepted by the physics community. Until shortly before his death, Tesla refused to accept the notion that energy could be obtained from matter (by atomic fission).
Relationship to Relativity
- ... Supposing that the bodies act upon the surrounding space causing curving of the same, it appears to my simple mind that the curved spaces must react on the bodies, and producing the opposite effects, straightening out the curves. Since action and reaction are coexistent, it follows that the supposed curvature of space is entirely impossible - But even if it existed it would not explain the motions of the bodies, as observed. 
His basic arguments against space being curved by gravitational effects seems to indicate that he failed to understand Einstein's theories. Tesla believed that many scientists substitute mathematics for experiments, and that such "mathematician scientists" would eventually build a structure which had no relation to reality.
The principles of relativity disputed by Tesla have since been confirmed by a wide variety of experiments. For instance, the relativistic relation between mass and energy has been confirmed by the energy demands of modern particle accelerators, and more directly with the observation of particle-antiparticle creation and annihilation (see, for example, Electron-positron annihilation). Effects arising from Einstein's general theory of relativity have also been observed; for instance, general relativistic corrections are required to synchronize the clocks on the GPS satellites with earth-bound ones.
-  : Tesla, Nikola, "Prepared Statement of Tesla". July 10, 1937. (Interview with press on 81st birthday observance; DOC format)
-  : Seifer, Marc J., "Wizard, the Life and Times of Nikola Tesla". ISBN 1-559723-29-7
-  : O'Neill, John H., "Prodigal Genius". ISBN 0-914732-33-1
-  : Tesla, Nikola, "Radio Power Will Revolutionize the World". Modern Mechanics and Inventions, July, 1934.
- Tesla, Nikola, "How Cosmic Forces Shape Our Destiny". New York American, February 27, 1925.