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Heisenberg: "Isn't that precisely what you have done with relativity?"

"Possibly," Einstein said, "but it is nonsense all the same."

Einstein had ruled out the ether in 1905 because all attempts to detect it had failed. Its removal had been a central feature of his special relativity.

Einstein then complained to an early biographer, Philipp Frank, that a new and disagreeable fashion had arisen: the new physicists were arguing that things were not real if they couldn't be observed. Frank reminded Einstein that that fashion "was invented by you in 1905."

"A good joke should not be repeated too often," Einstein replied.

By 1920, Einstein had brought back his new ether, and I'll return to that. It may yet open the door to a revisionist look at relativity.

SPECIAL RELATIVITY MAKES VERY Peculiar claims. You and I, next to one another, carry identical rulers and wear exactly synchronized watches. When I move, I see your ruler shrink and your watch slow down. You observe no such changes -- called time dilation and length contraction -- but you do see my ruler shrink and my watch slow down.

So, exactly the same question that Einstein asked of quantum mechanics could be asked of relativity.

Isaacson: "Some may be tempted to ask: Which observer is 'right'? Whose watch shows the 'actual' time elapsed? Which length of the rod is 'real'?" Mindful of the perplexing history here, he diplomatically finesses the question ("it is not a question of whether rods actually shrink or time really slows down...").

At the end of a new book called It's About Time, the recently retired physics professor N. David Mermin, who taught relativity at Cornell for decades, asks the same question. He asks of moving sticks and clocks that allegedly shrink and lag: "Do these things really happen, or are they just secondary manifestations... leading to disagreements about what constitutes a valid measurement?"

Mermin's answer is one that you might consider surprising in a book published exactly 100 years after Einstein's theory was invented:

There is by no means unanimity among practicing physicists on this question, and one frequently finds assertions that, for example, moving clocks appear to run slowly when measured by stationary ones, or that moving sticks appear to shrink.

Here is Arthur Eddington, the famous British astronomer who led the 1919 eclipse expedition that confirmed Einstein's prediction about the bending of starlight grazing the sun. Eddington wrote: "The shortening of the rod is true, but it is not really true. It is not a statement about reality (the absolute) but it is a true statement about appearances in our frame of reference."

Got that? It's in Eddington's The Nature of the Physical World (1928). There's an apocryphal story about Eddington. Someone told him he was one of only three people who understood the general theory. He is said to have replied: "I'm trying to think who the third one is." By the way, the bending of starlight is really real, and it confirms something important that Einstein had written in 1916. "The principle of the constancy of the velocity of light in vacuo must be modified." Yet that principle had been central to special relativity.

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