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Relative motion9/28/2023 (Image credit: )Īnd then Einstein took it to the next level as only Einstein could: not only are the laws of motion the same in every inertial (jargon alert: "non-accelerating") reference frame, but all physics are the same. If you're a topical expert - researcher, business leader, author or innovator - and would like to contribute an op-ed piece, email us here. That's right: we've traded a universe with no speed limits but a fixed background to one with a speed limit but free of any aether. For example, there is now a universal speed limit. By disregarding the absolute reference frame (and the aether along with it) the world has some weird properties. What's the big deal? Special Relativity is the big deal. Things can only be said to be "moving" relative to another observer. And by "settle" I mean "pick Maxwell." Einstein took the old notions of Galileo's relative motion and took them to the extreme. So that's what they mean by 'relativity'Īlong came Einstein to settle the dispute. Who wins? I mean, they're all pretty smart, so it's tough. This is what's called a "problem." On one side you have the Newton worldview, saying there must be an absolute reference frame, and on the other you have Maxwell plus the M&M (wait, that may be copyrighted, let's go with M-M) worldview, saying there is no evidence of the aether, and hence no absolute reference frame. No matter what, no matter how fast or in what direction the Earth was moving, their measured speed of light remained stubbornly fixed. That's the problem: try as they might, the Michelson-Morley experiments failed to detect any changes to the speed of light. By building a sensitive apparatus and comparing the speed of light moving in different directions at different times of day, they thought they could measure the motion of the Earth relative to the background aether. And that's what a bunch of people, including Albert Michelson and Edward Morley did. If we're moving around relative to that fixed background, we can detect our motion relative to it - and hence deduce the properties of the aether - by measuring changes to the speed of light. In other words, the aether itself was the universal Newtonian background. Since the speed of light was a constant in the vacuum, and the vacuum was filled with aether (as the thinking went back then), then the aether must be in the universal reference frame. All motion in the universe is relative to that background.īack to Maxwell. It was in this universal frame that he could write down the mathematics that he needed to get the job done. This assumption of a fixed background formed one of the bases of Newton's laws of motion. Oh, and a master clock, too, to keep perfect time. That's the place where we can measure the "total, real, final, I'm serious, guys" speed. A "background" to the universe that stays fixed and unmoving for eternity, giving us all a stage to play around on. How can we tell if we're moving if … we can't tell that we're moving? A universal referenceĮnter the universal reference frame. After being restrained by the flight attendants, reflect on the fact that the ball behaved perfectly normally, despite the fact that you're hurtling through the air at several hundred miles per hour. For example, a physics experiment performed on the train (say, testing how much my head would hurt after being hit with a ball) would not tell me if I was on a train or not. This idea of relative motion means that you can't easily figure out that final speed. So what's the real speed? The absolute, final speed? Another interested observer, way out in space, would also add in the rotation and orbit of the earth. When I say something like "the ball is traveling at 50 mph towards me," I implicitly add "relative to my head." If I'm on a train, someone standing on the platform would add the speed of the train to the speed of the ball to get the "total" speed. Galileo was the first to point out the obvious: all motion is relative. There's a reason that the word aether is unfamiliar to you post-19th century readers: Without knowing it, Maxwell's insight into the nature of light would end up destroying the entire concept.īut why did people think we needed some aether-stuff for light to wave around in the first place? If there's nothing somewhere, then that somewhere is full of aether. The stuff, the goop, the nougaty goodness that permeates the entire universe. I get it now: Light is a wave of aether! Ta-da! Right. A vacuum? Where there's absolutely nothing else? Why, that's the domain of the aether.
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