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What is this mind of ours: what are these atoms with consciousness? Last week’s potatoes!
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Richard Feynman
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the things that never vary no matter what.
“Einstein’s deep view was that space and time are basically built up by relationships between things happening,” said the physicist Robbert Dijkgraaf, director of the Institute for Advanced Study in Princeton, New Jersey, where Einstein spent his final decades.
The relationship that eventually mattered most to Einstein’s legacy was symmetry. Scientists often describe symmetries as changes that don’t really change anything, differences that don’t make a difference, variations that leave deep relationships invariant. Examples are easy to find in everyday life. You can rotate a snowflake by 60 degrees and it will look the same. You can switch places on a teeter-totter and not upset the balance. More complicated symmetries have led physicists to the discovery of everything from neutrinos to quarks — they even led to Einstein’s own discovery that gravitation is the curvature of space-time, which, we now know, can curl in on itself, pinching off into black holes.
Over the past several decades, some physicists have begun to question whether focusing on symmetry is still as productive as it used to be. New particles predicted by theories based on symmetries haven’t appeared in experiments as hoped, and the Higgs boson that was detected was far too light to fit into any known symmetrical scheme. Symmetry hasn’t yet helped to explain why gravity is so weak, why the vacuum energy is so small, or why dark matter remains transparent.
“There has been, in particle physics, this prejudice that symmetry is at the root of our description of nature,” said the physicist Justin Khoury of the University of Pennsylvania. “That idea has been extremely powerful. But who knows? Maybe we
really have to give up on these beautiful and cherished principles that have worked so well. So it’s a very interesting time right now.”
Light
Einstein wasn’t thinking about invariance or symmetry when he wrote his first relativity papers in 1905, but historians speculate that his isolation from the physics community during his employment in the Swiss patent office might have helped him see past the unnecessary trappings people took for granted.
Like other physicists of his time, Einstein was pondering several seemingly unrelated puzzles. James Clerk Maxwell’s equations revealing the intimate connection between electric and magnetic fields looked very different in different frames of reference — whether an observer is moving or at rest. Moreover, the speed at which electromagnetic fields propagated through space almost precisely matched the speed of light repeatedly measured by experiments — a speed that didn’t change no matter what. An observer could be running toward the light or rushing away from it, and the speed didn’t vary.
Einstein connected the dots: The speed of light was a measurable manifestation of the symmetrical relationship between electric and magnetic fields — a more fundamental concept than space itself. Light didn’t need anything to travel through because it was itself electromagnetic fields in motion. The concept of “at rest” — the static “empty space” invented by Isaac Newton — was unnecessary and nonsensical. There was no universal “here” or “now”: Events could appear simultaneous to one observer but not another, and both perspectives would be correct.