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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

• "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,

• "Relative to what, Captain?"

• 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.

• 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.

• 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

• 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, &quot;Do you know how fast you were going?&quot;

• 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,

• &quot;But some things are absolute!&quot;

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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