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  • How fast are you moving right now?

  • That seems like an easy question.

  • The first tempting answer is,

  • "I'm not moving."

  • Upon further reflection,

  • you realize that maybe the Earth's motion counts.

  • So, a second tempting answer is,

  • "19 miles/second around the Sun."

  • But then you recall learning that the Sun

  • moves around the center of the Milky Way galaxy,

  • and the Milky Way moves within the Local Group of galaxies,

  • and the Local Group moves within the Virgo Cluster,

  • and the Virgo Cluster moves within...

  • "How fast are you moving?"

  • is not an easy question.

  • When Mission Control tells astronauts

  • how fast they're going,

  • there's always an assumed standard of rest.

  • At the start of the voyage,

  • speeds are given relative to the launchpad.

  • But later, when the launchpad is

  • just one more arbritrary place

  • down there on Earth's spinning surface,

  • speeds are given relative to

  • the idealized, non-spinning pinpoint center of Earth.

  • On their way to the Moon,

  • Apollo astronauts had a hard time

  • answering the question,

  • "How fast are you moving?"

  • Speed away from Earth was one thing,

  • and speed toward the Moon was quite another.

  • That's because the Earth and the Moon

  • move relative to one another.

  • Ah, of course!

  • Speed is a relative quantity.

  • When Captain Kirk ask Lieutenant Sulu

  • if the Starship Enterprise has reached a speed of warp 7,

  • Sulu should reply,

  • "Relative to what, Captain?"

  • Such a sassy reply

  • may get subordinate Starfleet officers in trouble,

  • but it is the only good answer

  • to the question, "How fast are you moving?"

  • This is basic relatively talking.

  • Not fancy Einsteinian relativity,

  • but good old fashioned (and still correct)

  • Galilean relativity.

  • Galileo seems to have been the first person

  • to realize that there is no such thing

  • as an absolute speed.

  • Speeds are relative.

  • This means that speeds only have meaning

  • when they are referred to a reference frame.

  • Presumably that reference frame is itself at rest.

  • But then we have to ask again,

  • "At rest relative to what?"

  • Because even the concept of rest

  • has lost any hint of absolute meaning.

  • Speed is relative, and rest is relative.

  • Earth's speed is 19 miles/second relative to the Sun.

  • The Enterprise's speed is warp 7

  • relative to the center of the Milky Way galaxy.

  • Your speed is zero relative to your easy chair.

  • But depending on where you sit,

  • it is hundreds of miles/hour relative to Earth's center.

  • When we furrow a brow and ask,

  • "But how fast is Earth really moving?"

  • we imagine Spaceship Earth

  • plowing through the ocean of space

  • as it orbits the Sun.

  • But space is not an ocean.

  • It has no substance as water does.

  • Space is not a thing;

  • space is nothing.

  • Space is no thing.

  • You can move between two points in space,

  • say between Earth and Mars,

  • but you can't move through space.

  • There's nothing to move through.

  • It's like trying to say how much a hole weighs.

  • A hole weighs exactly nothing

  • because a hole is nothing.

  • It's a void, and so is space.

  • To move relative to nothing is meaningless.

  • The concepts of speed and of rest

  • have only relative meaning.

  • They are absolutely meaningless.

  • They mean something

  • only with respect to arbitrarily chosen,

  • artificial frames of reference.

  • If, someday, you are buckled into your spaceship,

  • and you see from the side window, say,

  • a space station whizz by at constant speed,

  • there is no way to know which of you is really moving.

  • Neither of you is really moving

  • because there is no deep reality about constant speed.

  • Constant speed in a straight line

  • has only relative meaning,

  • a kind of relative reality.

  • Does this mean that all motion is relative?

  • No! Some motions have only relative meaning,

  • but some motions have absolute meaning,

  • are absolutely real.

  • For example, constant speed is relative,

  • but change in speed is absolute.

  • Calling something absolute in science

  • means that arbitrary standards are not used

  • in its measurement.

  • It is unambiguously measurable.

  • When your spaceship fires its engines,

  • your change in speed is beyond doubt.

  • You feel it in your stomach,

  • and your ship's sensors can measure it.

  • Outside your window,

  • the passing space station

  • may seem to be changing speed,

  • but the beings inside the station will not feel it.

  • And no sensors can measure it.

  • You are really changing speed,

  • and they are really are not.

  • There's something absolutely real

  • about changes in speed.

  • The same goes for rotation.

  • If your spaceship is spinning,

  • you can feel it,

  • and your ship's sensors can measure it.

  • The space station outside

  • may seem to be going around you,

  • but it is you who feels queasy,

  • not the folks in the space station.

  • You are really spinning,

  • and they really are not.

  • There's something absolutely real about rotation.

  • So, some motions are relative, and some are not.

  • There is no deep reality about constant speed,

  • but changes in speed are deeply real,

  • and so are rotations.

  • We have to be thoughtful

  • in our analysis of everyday experience

  • in order to identify what is deeply real.

  • Since we can be fooled by perceptions

  • as basic as speed,

  • maybe every perception deserves careful scrutiny.

  • This is what inspired Einstein

  • to his incredible insights

  • about the speed of light and forward time travel.

  • Knowing how to identify

  • what is deeply real

  • is tough and important work.

  • If a police officer ever pulls you over for speeding

  • and asks, "Do you know how fast you were going?"

  • an insightful, though perhaps unwise, reply

  • would be, "Relative to what?"

  • And then, as you sit in the backseat of the police car

  • and feel it accelerate toward jail,

  • you can add,

  • "But some things are absolute!"

How fast are you moving right now?

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B1 TED-Ed relative speed space earth quot

【TED-Ed】How fast are you moving right now? - Tucker Hiatt

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    阿多賓 posted on 2014/03/06
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