Samir’s Selection 11/02/2015 (p.m.)

  • tags: thoughtexperiment Einstein relativity physics gravity acceleration light

    • THIS month marks the 100th anniversary of the General Theory of Relativity, the most beautiful theory in the history of science, and in its honor we should take a moment to celebrate the visualized “thought experiments” that were the navigation lights guiding Albert Einstein to his brilliant creation. Einstein relished what he called Gedankenexperimente, ideas that he twirled around in his head rather than in a lab. That’s what teachers call daydreaming, but if you’re Einstein you get to call them Gedankenexperimente.
    • As these thought experiments remind us, creativity is based on imagination. If we hope to inspire kids to love science, we need to do more than drill them in math and memorized formulas. We should stimulate their minds’ eyes as well. Even let them daydream.
    • “Events that are simultaneous with reference to the embankment are not simultaneous with respect to the train,” wrote Einstein. Here’s the fun part: There is no reason to decree that the man is right and the woman wrong, because there’s no reason to assume that the embankment is “at rest” and the train “in motion.” The man, woman, train, Earth, solar system, galaxy, etc., are all in motion relative to one another, and none of them can claim the privileged status of being at absolute rest. So there is no “real” or “right” answer. What is “simultaneous” is relative, depending on your state of motion.
    • That means time is relative. If you travel near the speed of light, time slows down.
    • This relativity of space and time became known as the Special Theory, because it applied only to a special case: an observer moving at a constant velocity. It’s harder to make the more general case that the same principles apply to a person who is accelerating or turning or rotating. It would take Einstein a decade more to come up with a General Theory that applied to all forms of motion.
    • Once again, his path was lit by a thought experiment. “I was sitting in a chair in the patent office at Bern when all of a sudden a thought occurred to me,” he recalled. “If a person falls freely, he will not feel his own weight.” He later called it “the happiest thought in my life.”
    • The effects produced by gravity and the effects produced by acceleration are equivalent, Einstein postulated. Thus they must have the same cause. “The effects we ascribe to gravity and the effects we ascribe to acceleration are both produced by one and the same structure,” he declared.

      In his Special Theory, Einstein had shown that space and time were not independent, but instead formed a fabric of “space-time.” Now, with his general version of the theory, which became known as the General Theory of Relativity, this fabric of space-time became not merely a container for objects. Instead, it had its own two-way dynamics: moving objects would curve the fabric, and the curves of the fabric would influence how objects moved.

    • This can be visualized through yet another thought experiment: Imagine rolling a bowling ball onto a trampoline. It will curve the fabric. Then put some billiard balls onto it. They will gradually roll toward the bowling ball — not because the bowling ball exerts some mysterious attraction at a distance, but instead because it has curved the fabric of the trampoline.
    • As the physicist John Wheeler put it: “Matter tells space-time how to curve, and curved space tells matter how to move.”
    • One consequence of the equivalence between gravity and acceleration is that gravity should bend a light beam. Einstein showed this through yet another thought experiment. Imagine a chamber that is being accelerated upward. A laser beam comes in through a pinhole on one wall. When it hits the opposite wall, it will be at a spot closer to the floor, because the chamber has moved upward. If you could track it, the trajectory would seem curved, because the upward motion is accelerating. According to the equivalence principle, the effect of gravity is the same as that of acceleration, so light should curve as it goes through a gravitational field.

      It was almost four years before scientists were able to conduct a convincing test of the theory.

    • “When a blind beetle crawls over the surface of a curved branch, it doesn’t notice that the track it has covered is indeed curved,” he said. “I was lucky enough to notice what the beetle didn’t notice.”
    • In fact, Einstein did more than just notice what the blind beetle couldn’t see. He was able to imagine it by conjuring up thought experiments. That ability to visualize the unseen has always been the key to creative genius. As Einstein later put it, “Imagination is more important than knowledge.”

Posted from Diigo. The rest of my favorite links are here.

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