Albert Einstein, In a Newly Discovered Letter, Bridges the Fields of Science and Medicine, Discussing with a Doctor the Potential Use of Relativity in the Care of Patients

Relativity is a special case requiring bigger, cosmic applications, he explains: “The speed of light plays a special role in the whole theory [of relativity].”

 

Acquired from the heirs of the recipient, this remarkable letter does not appear in any of the known correspondence of Einstein

 

This letter incorporates special relativity, the Lorentz Factor, and the Equivalence Principle

 

It is fascinating to see doctors around the country looking to relativity to better understand medical ailments and the medical profession

In 1905, while a young patent clerk and physicist in Bern, Switzerland, Albert Einstein obtained his doctorate and published a paper that explained his newly developed Special Theory of Relativity. This unlocked many mysteries of the universe, and introduced the world to “E=mc2,” equating mass and the speed of light with energy. It established that time and space are not fixed, and in fact change to maintain a constant speed of light regardless of the relative motions of sources and observers. It showed space as a four dimensional universe, with time added as the fourth dimension. Just 10 years later, in 1915, Einstein published his General Theory of Relativity, in which gravitational effects are explained by the warping of space-time. In this theory, Einstein incorporated gravity as a geometric property of space-time. This expanded upon Special Relativity to describe the overall structure of the universe and the impulses of a gravitational universe. The two theories built on each other and were intimately related.

Before Einstein, there existed the Equivalence Theory, which basically stated that the force felt by gravity and the force exerted by acceleration were interchangeable, and that someone, for instance, inside an elevator or box could not know whether he or she was on Earth in a stationery elevator, subject to gravity, or in space, inside that same box, being subject to acceleration at the same force as gravity. The forces were identical in number and felt the same.

Much of Einstein’s work with Special Relativity relates to his work on the speed of light as a constant the relativistic nature of space around it. One element of this is the principle that the closer an object gets to the speed of light the shorter it appears to an outside observer. This shortening effect is also know as length contraction or the Lorentz contraction, referring to the apparent length contraction of an object in Einstein’s special theory of relativity. Moreover, Einstein contributed further to the concept of Equivalence by adding a fixed speed of light as an additional requirement to the universe, leading to predictions that light reacts to the effects of gravity in this theory, because the Earth’s size and its gravity bend space-time itself.

Dr. Heinrich Kobrak was a doctor specializing in the functioning and understanding of the mechanisms of sound and the ear. He wrote books on the subject and spent time researching treatment of hearing dysfunction. In 1941, he wrote his brother, Erwin Kobrak, also a doctor, evidently (since the original of this letter does not survive) wondering whether perhaps the effect of the Equivalence Principle, much of which could be found in Newtonian Physics, would affect ear function. And he also wondered whether Einstein’s theory of special relativity, specifically the shortening effect, might apply to sound waves and how humans perceive them. In other words: could relativity help heal someone with a hearing problem and could sound waves perhaps be subject to the shortening effect of this Special Relativity. He was in effect asking how the typical length contraction you expect for relativistic speeds affects sound detection by the ear since the frequency might be expected to shift if space-time gets shorter.

It is remarkable to think that doctors were looking to Einstein and Relativity for solutions to medical problems. The basic question being: can you use relativity to understand sound and the functioning of the ear and treat this medical condition?

Erwin forwarded Heinrich’s letter to Einstein.

Einstein responded, detailing how the Equivalence principle can play a role in how you would model the ear, reference the classical mechanics that governs such understanding, and also note that the speed of light plays a very special role in his theory of relatively and is not needed to understand sound and its effect on the ear. In essence, the effects predicted by the Special Theory of Relativity only occur for objects traveling near the speed of light, and would appear [insert space] infinitesimally small in everyday life.

Typed letter signed, Princeton, April 3, 1941, to Erwin Kobrak, in German. “Dear Dr. Kobrak: I would like to comment on your brother’s letter as follows:

“1 . It is true that the equivalence principle of the theory of gravitation can be applied to the theory of the ear function, since gravitational field and translational acceleration are generally equivalent . This is of course also contained in the old formulation of mechanics.

“2 . The application of the contraction formula to the sound propagation is unimportant . Those relations apply exclusively to light , in that the speed of light plays a special role in the whole theory.

“With highest esteem. A. Einstein”

This letter was acquired from the descendants of the recipient and has never been offered for sale. Moreover, it is unpublished and no record of its having been sent exists in the Papers of Albert Einstein. Its content was unknown until now. We thank the Papers Project of Albert Einstein out of Caltech for their help in researching this piece.