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The Big Picture: A 4D Slinky

Light and gravity behave like a simple harmonic oscillator, or slinky, in four dimensions. A thought experiment: Imagine a cup that could hold neutrinos still. Turn that cup over. The neutrinos would wobble through the Earth as a SHO, cycling to the other side of the Earth and back every 88 minutes. Because the acceleration is in the direction of the velocity, this is a longitudinal wave.