We imagine a large portion of empty space, so far removed from stars and other appreciable2 masses, that we have before us approximately the conditions required by the fundamental law of Galilei. It is then possible to choose a Galileian reference-body for this part of space (world), relative to which points at rest remain at rest and points in motion continue permanently3 in uniform rectilinear motion. As reference-body let us imagine a spacious4 chest resembling a room with an observer inside who is equipped with apparatus5. Gravitation naturally does not exist for this observer. He must fasten himself with strings6 to the floor, otherwise the slightest impact against the floor will cause him to rise slowly towards the ceiling of the room.
To the middle of the lid of the chest is fixed7 externally a hook with rope attached, and now a “being” (what kind of a being is immaterial to us) begins pulling at this with a constant force. The chest together with the observer then begin to move “upwards” with a uniformly accelerated motion. In course of time their velocity8 will reach unheard-of values—provided that we are viewing all this from another reference-body which is not being pulled with a rope.
But how does the man in the chest regard the Process? The acceleration9 of the chest will be transmitted to him by the reaction of the floor of the chest. He must therefore take up this pressure by means of his legs if he does not wish to be laid out full length on the floor. He is then standing10 in the chest in exactly the same way as anyone stands in a room of a home on our earth. If he releases a body which he previously11 had in his land, the acceleration of the chest will no longer be transmitted to this body, and for this reason the body will approach the floor of the chest with an accelerated relative motion. The observer will further convince himself that the acceleration of the body towards the floor of the chest is always of the same magnitude, whatever kind of body he may happen to use for the experiment.
Relying on his knowledge of the gravitational field (as it was discussed in the preceding section), the man in the chest will thus come to the conclusion that he and the chest are in a gravitational field which is constant with regard to time. Of course he will be puzzled for a moment as to why the chest does not fall in this gravitational field. just then, however, he discovers the hook in the middle of the lid of the chest and the rope which is attached to it, and he consequently comes to the conclusion that the chest is suspended at rest in the gravitational field.
Ought we to smile at the man and say that he errs12 in his conclusion? I do not believe we ought to if we wish to remain consistent; we must rather admit that his mode of grasping the situation violates neither reason nor known mechanical laws. Even though it is being accelerated with respect to the “Galileian space” first considered, we can nevertheless regard the chest as being at rest. We have thus good grounds for extending the principle of relativity to include bodies of reference which are accelerated with respect to each other, and as a result we have gained a powerful argument for a generalised postulate of relativity.
We must note carefully that the possibility of this mode of interpretation13 rests on the fundamental property of the gravitational field of giving all bodies the same acceleration, or, what comes to the same thing, on the law of the equality of inertial and gravitational mass. If this natural law did not exist, the man in the accelerated chest would not be able to interpret the behaviour of the bodies around him on the supposition of a gravitational field, and he would not be justified14 on the grounds of experience in supposing his reference-body to be “at rest.”
Suppose that the man in the chest fixes a rope to the inner side of the lid, and that he attaches a body to the free end of the rope. The result of this will be to stretch the rope so that it will hang “vertically” downwards15. If we ask for an opinion of the cause of tension in the rope, the man in the chest will say: “The suspended body experiences a downward force in the gravitational field, and this is neutralised by the tension of the rope; what determines the magnitude of the tension of the rope is the gravitational mass of the suspended body.” On the other hand, an observer who is poised16 freely in space will interpret the condition of things thus: “The rope must perforce take part in the accelerated motion of the chest, and it transmits this motion to the body attached to it. The tension of the rope is just large enough to effect the acceleration of the body. That which determines the magnitude of the tension of the rope is the inertial mass of the body.” Guided by this example, we see that our extension of the principle of relativity implies the necessity of the law of the equality of inertial and gravitational mass. Thus we have obtained a physical interpretation of this law.
From our consideration of the accelerated chest we see that a general theory of relativity must yield important results on the laws of gravitation. In point of fact, the systematic17 pursuit of the general idea of relativity has supplied the laws satisfied by the gravitational field. Before proceeding18 farther, however, I must warn the reader against a misconception suggested by these considerations. A gravitational field exists for the man in the chest, despite the fact that there was no such field for the co-ordinate system first chosen. Now we might easily suppose that the existence of a gravitational field is always only an apparent one. We might also think that, regardless of the kind of gravitational field which may be present, we could always choose another reference-body such that no gravitational field exists with reference to it. This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes.
We can now appreciate why that argument is not convincing, which we brought forward against the general principle of relativity at the end of Section XVIII. It is certainly true that the observer in the railway carriage experiences a jerk forwards as a result of the application of the brake, and that he recognises, in this the non-uniformity of motion (retardation) of the carriage. But he is compelled by nobody to refer this jerk to a “real “acceleration (retardation) of the carriage. He might also interpret his experience thus: “My body of reference (the carriage) remains19 permanently at rest. With reference to it, however, there exists (during the period of application of the brakes) a gravitational field which is directed forwards and which is variable with respect to time. Under the influence of this field, the embankment together with the earth moves non-uniformly in such a manner that their original velocity in the backwards20 direction is continuously reduced.”
点击收听单词发音
1 postulate | |
n.假定,基本条件;vt.要求,假定 | |
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2 appreciable | |
adj.明显的,可见的,可估量的,可觉察的 | |
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3 permanently | |
adv.永恒地,永久地,固定不变地 | |
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4 spacious | |
adj.广阔的,宽敞的 | |
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5 apparatus | |
n.装置,器械;器具,设备 | |
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6 strings | |
n.弦 | |
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7 fixed | |
adj.固定的,不变的,准备好的;(计算机)固定的 | |
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8 velocity | |
n.速度,速率 | |
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9 acceleration | |
n.加速,加速度 | |
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10 standing | |
n.持续,地位;adj.永久的,不动的,直立的,不流动的 | |
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11 previously | |
adv.以前,先前(地) | |
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12 errs | |
犯错误,做错事( err的第三人称单数 ) | |
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13 interpretation | |
n.解释,说明,描述;艺术处理 | |
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14 justified | |
a.正当的,有理的 | |
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15 downwards | |
adj./adv.向下的(地),下行的(地) | |
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16 poised | |
a.摆好姿势不动的 | |
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17 systematic | |
adj.有系统的,有计划的,有方法的 | |
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18 proceeding | |
n.行动,进行,(pl.)会议录,学报 | |
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19 remains | |
n.剩余物,残留物;遗体,遗迹 | |
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20 backwards | |
adv.往回地,向原处,倒,相反,前后倒置地 | |
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