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Ted began to position the pieces of fruit。 〃This orange is the sun。 And these are the planets; which move in circles around the sun。 So we have the solar system on this table。〃
〃Okay。〃
''85'' 〃Fine;〃 Ted said。 〃Now; the sun〃…he pointed to the orange in the center of the table…〃is very large; so it has a lot of gravity。〃
〃Right。〃
Ted gave Norman a ball bearing。 〃This is a spaceship。 I want you to send it through the solar system; so it passes very close to the sun。 Okay?〃
Norman took the ball bearing and rolled it so it passed close to the orange。 〃Okay。〃
〃You notice that your ball rolled straight across the flat table。〃
〃Right。〃
〃But in real life; what would happen to your spacecraft when it passed near the sun?〃
〃It would get sucked into the sun。〃
〃Yes。 We say it would fall into the sun。 The spacecraft would curve inward from a straight line and hit the sun。 But your spacecraft didnt。〃
〃No。
〃So we know that the flat table is wrong;〃 Ted said。 〃Real space cant be flat like the table。〃
〃It cant?〃
〃No;〃 Ted said。
He took the empty bowl and set the orange in the bottom。 〃Now roll your ball straight across past the sun。〃
Norman flicked the ball bearing into the bowl。 The ball curved; and spiraled down the inside of the bowl until it hit the orange。
〃Okay;〃 Ted said。 〃The spacecraft hit the sun; just like it would in real life。〃
〃But if I gave it enough speed;〃 Norman said; 〃itd go right past it。 Itd roll down and up the far side of the bowl and out again。〃
〃Correct;〃 Ted said。 〃Also like real life。 If the spacecraft has enough velocity; it will escape the gravitational field of the sun。〃
〃Right。〃
〃So;〃 Ted said; 〃what we are showing is that a spacecraft passing the sun in real life behaves as if it were entering a ''86'' curved region of space around the sun。 Space around the sun is curved like this bowl。〃
〃Okay 。。。〃
〃And if your ball had the right speed; it wouldnt escape from the bowl; but instead would just spiral around endlessly inside the rim of the bowl。 And thats what the planets are doing。 They are endlessly spiraling inside the bowl created by the sun。〃
He put the orange back on the table。 〃In reality; you should imagine the table is made out of rubber and the planets are all making dents in the rubber as they sit there。 Thats what space is really like。 Real space is curved…and the curvature changes with the amount of gravity。〃
〃Yes。。。〃
〃So;〃 Ted said; 〃space is curved by gravity。〃
〃Okay。〃
〃And that means that you can think of gravity a
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