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In one of Einstein's<br>
famous thought experiments

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he realized that<br>
gravity and acceleration 

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appear to be<br>
the same phenomenon.

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Think about<br>
what would happen 

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if an elevator suddenly<br>
went into a free-fall.

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The person inside would<br>
effectively weigh nothing.

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Next, imagine that<br>
same person

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in a motionless rocketship,<br>
so far from earth

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that the force of gravity<br>
is practically zero.

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Just like in the<br>
free-falling elevator

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he would also weigh nothing.

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Now, put the rocket in motion.

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As the rocket speeds up

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the passenger's<br>
weight increases.

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To him, it feels<br>
just as if gravity 

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had planted his feet<br>
firmly on the floor.

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Einstein realized<br>
that the force of gravity 

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is actually the<br>
acceleration you feel

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as you move<br>
through space-time.

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