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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.
Chasing after a light beam produced another curious effect, the subject of Einstein’s second relativity paper, “Does the Inertia of a Body Depend Upon Its Energy Content?” The answer was yes. The faster you chase, the harder it is to go faster. Resistance to change becomes infinite at the speed of light. Since that resistance is inertia, and inertia is a measure of mass, the energy of motion is transformed into mass. “There is no essential distinction between mass and energy,” Einstein wrote.
It took several years for Einstein to accept that space and time are inextricably interwoven threads of a single space-time fabric, impossible to disentangle. “He still wasn’t thinking in a fully unified space-time sort of way,” said David Kaiser, a physicist and historian of science at the Massachusetts Institute of Technology.
Unified space-time is a difficult concept to wrap our minds around. But it begins to make sense if we think about the true meaning of “speed.” The speed of light, like any speed, is a relationship — distance traveled over time. But the speed of light is special because it can’t change; your laser beam won’t advance any faster just because it is shot from a speeding satellite. Measurements of distance and time must therefore change instead, depending on one’s state of motion, leading to effects known as “space contraction” and “time dilation.” The invariant is this: No matter how fast two people are traveling with respect to each other, they always measure the same “space-time interval.” Sitting at your desk, you hurtle through time, hardly at all through space. A cosmic ray flies over vast distances at nearly the speed of light but traverses almost no time, remaining ever young. The relationships are invariant no matter how you switch things around.
The law of nature obeys a symmetry, but the solution you’re interested in does not.
- Mark
Trodden
https://www.quantamagazine.org/einstein-symmetry-and-the-future-of-physics-20190626/