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发表于 2005-11-19 23:33:09
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翻译页面地址：http://boinc.equn.com/einstein/ask/archive/lamonte-hawaii.html
Q:问：
I had always thought that there is no up or down in the outer spaces of the Universe. This now seems not to be correct if matter causes curvature and other objects fall towards the curves. What am I forgetting here?
Submitted by Lamonte from Waikiki, Hawaii
我总是认为在外太空之中是没有上下之分的，这好像不对，因为物质好像能导致时空的弯曲而其它物体会向弯曲方向运动。那我到底忘了什么导致我的错误呢？
A:答：
Matter does indeed cause curvature, and this curvature is present even out in space. But that's not the same as the notion of up or down.
物质当然会导致时空的弯曲，甚至在外太空这依然成立。但是这与上下无关。
Sitting at home, you have a clear idea of which way is down. Hold an apple at arm's length and let go. Down is whichever direction it moves. That also happens to be in the direction of the Earth.
当你坐在家里的时候，你当然对“下”有一个清晰的概念。拿一个苹果再放开它，“下”就是那苹果移动的方向，那就是地球所在的方向。
Riding the space shuttle in low Earth orbit (with the windows shuttered), you don't have a clear idea of down. If you let an apple go, it stays in mid-cabin where you left it. The curvature is less than at the surface of the Earth, but only by a few percent so that doesn't explain it. The key is that the apple isn't moving relative to you, which is where the everyday idea of down comes from.
在一艘低地轨道上运行的带窗户的航天飞机上，你不会对“下”有一个清晰的概念。如果你放开一个苹果，它会停留在你放开它的哪个位置上。在航天飞船上的时空弯曲比在地面上要小，但是只是几个百分点的差异，不足以解释这个现象。关键是苹果和你之间没有相对运动，你日常的“下”的概念就是从这里来的。
Now about those curves: Objects try to follow special curves called geodesics that are as straight as possible given that spacetime itself is curved. In the space shuttle cabin, you are moving on a geodesic, and when you let the apple go it sits there relative to you since it's moving on a neighboring geodesic that's very similar to yours. At home, your geodesic would lead to the center of the Earth, but you are constantly pushed off your geodesic by several thousand miles of rock getting in the way. This gives you a sensation of weight. When you let the apple go it moves (briefly) on a geodesic, which diverges from your curve, and you see it go flying away from your hand until it too encounters the Earth. So you have a well-defined notion of down, which turns out to mean "towards the geodesic I would follow if there weren't all this matter in the way."
现在我们再来谈论那些时空的弯曲。物体在运动时会试图沿着被称为测地线的曲线运动，这些曲线就是在卷曲的时空里的尽可能直的线。在航天飞船舱里，你沿着测地线运动，当你放开苹果时，苹果原地不动是因为苹果在你附近的测地线上运动，而它跟你在的那一条非常接近。在家里，你的测地线会到达地心，但是你被厚达几千公里的岩石推离了测地线。这就给了你一个可以感受到的重量。当你放开苹果时，苹果大致沿测地线运动，从你的测地线岔开出去，你就会看见它飞离你的手直到接触地面。所以你会有关于“下”的清晰概念，那就是“当我不受阻碍时的测地线方向”。
That's one of the counter-intuitive conclusions of Einstein's theory: that the idea of down and the sensation of weight are not actually due to gravity, but rather to all the other forces that are responsible for matter. In space there is still some curvature due to distant matter, but with no matter piled up to push you off your geodesic, you don't feel any "down."
这就是爱因斯坦理论的一个反直觉的推论。“下”的概念和重量的感觉并非源于重力，而是源于作用与物体的其他所有作用力。在外太空中，由于物质的存在，仍然会有弯曲的时空。但是如果没有物体把你推离测地线，你不会有任何“下”的感觉。 |
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