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October 13, 2005

Science 2005: Re-thinking Einstein’s thought experiment

Think about the properties of light and the absolute character of time, urged a Pitt professor at last week’s on-campus science forum.

Now rummage around in your basic physics background, which likely exposed you to the seminal theories of Albert Einstein (1879-1955), who revolutionized the field 100 years ago when he proposed his general theory of relativity, said John D. Norton, professor in history and philosophy of science who holds an adjunct appointment in philosophy.

In his “Autobiographical Notes,” Einstein wrote that “the germ of the special relativity theory” occurred to him at the precocious age of 16, some 10 years earlier than his legendary paper in 1905, said Norton, who led a presentation titled, “Chasing the Light: Einstein’s Most Famous Thought Experiment,” as part of Science 2005, Pitt’s science and technology celebration.

“The problem is how the thought experiment, famous as it is, delivers its results, because it fails to generate serious problems for an ether-based electrodynamics,” which was the prevalent scientific theory at the time, said Norton.

(In textbook physics, the ether is an all-pervading, infinitely elastic, mass-less medium, that was postulated as the medium of propagation of electromagnetic waves.)

Norton, who has published extensively detailing the steps of Einstein’s discovery of general relativity, said, “I would like to propose a new way to read the thought experiment that fits in with the stages of Einstein’s ultimate discovery.”

In the thought experiment, Einstein imagined himself pursuing a beam of light (a waveform) at velocity C (the velocity of light in a vacuum). According to Norton, Einstein reasoned that such a chase would allow him to catch up to the waveform and be moving with it, like a surfer riding a wave, and thus he would observe a frozen lightwave. Einstein wrote: “I should observe such a beam of light as an electromagnetic field at rest, though spatially oscillating. There seems to be no such thing, however.”

Norton said, “The trouble is that it is quite unclear just how this thought creates difficulties for ether theories of electromagnetism,” because in his notes Einstein offers three objections that all are answered easily by ether theory.

Instead, Norton opined, the key is not to relate the thought experiment to ether theory. “Rather, we know that Einstein devoted some effort, during the years leading up to his discovery of 1905, to so-called ‘emission’ theories of light and electromagnetism. Einstein eventually found such theories untenable,” Norton pointed out.

“I propose that Einstein’s thought experiment provided an especially cogent way of formulating those objections and thereby supported his final decision: to give up on emission theory in favor of retaining the celebrated Maxwell-Lorentz theory, but with a radically altered theory of space and time.”

In the then-standard electrodynamics of Maxwell and Lorentz, electromagnetic action always propagated at C with respect to the ether, Norton said. “The simplest example was the propagation of a lightwave. But it held equally for the action of one charge upon another,” he said. “It was this fact that made it impossible to conform the principle of relativity to electromagnetism. The ether supplied a preferred state of rest essential to the theory, but incompatible with the idea that all inertial states of motion are equivalent.”

So Einstein modified electromagnetic theory to say that electromagnetic effects always are propagated at C with respect to the source of the effect. Such a theory would no longer require an ether state of rest, because the motion of the source is added to the propagating effect and the principle of relativity is no longer threatened, Norton maintained.

“We immediately see that the three objections Einstein records present serious obstacles to an emission theory,” he said.

“Einstein’s first objection was that we don’t actually experience frozen light, while under the emission theory, a light source moving away from us at C will leave a frozen light wave behind in space. But we experience no such thing,” Norton maintained.

Einstein’s second objection was that frozen light was incompatible with Maxwell’s equations. “An emission theory allows the existence of frozen light waves,” Norton said. “But the emission theory must agree closely with the treatment of static fields in Maxwell’s theory, and Maxwell’s theory does not admit the static fields that correspond to frozen light waves.”

In his third objection, Einstein said of the observer who chases the light beam in the thought experiment, “How should the observer know or be able to determine that he is in a state of fast uniform motion?” Norton said.

“Of course, in the context of an emission theory, the ‘state of fast uniform motion’ must be read as ‘fast uniform motion with respect to the source of the light,’ he said.

To answer that question, Norton quoted Einstein’s conclusion: “All attempts to clarify this paradox satisfactorily were condemned to failure as long as the axiom of the absolute character of time, or of simultaneity, was rooted unrecognized in the unconscious. To recognize clearly this axiom and its arbitrary character already implies the essentials of the solution of the problem.”

“When Einstein abandoned an emission theory of light, he had also to abandon the hope that electrodynamics could be made to conform to the principle of relativity by any normal modifications,” Norton said. “He was willing to seek realization of his goal in a re-examination of our basic notions of space and time.”

—Peter Hart

Filed under: Feature,Volume 38 Issue 4

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