chapter 7

 The Apparent Incompatibility¹ of the Law of Propagation of Light with the Principle of Relativity

 

There is hardly a simpler law in physics than that according to which light is propagated in empty space. Every child at school knows, or believes he knows, that this propagation takes place in straight lines with a velocity² c equals 300 comma 000 km./sec. At all events we know with great exactness that this velocity is the same for all colours, because if this were not the case, the minimum of emission³ would not be observed simultaneously⁴ for different colours during the eclipse of a fixed⁵ star by its dark neighbour. By means of similar considerations based on observations of double stars, the Dutch astronomer⁶ De Sitter was also able to show that the velocity of propagation of light cannot depend on the velocity of motion of the body emitting the light. The assumption that this velocity of propagation is dependent on the direction “in space” is in itself improbable.

 

In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) is justifiably⁷ believed by the child at school. Who would imagine that this simple law has plunged⁸ the conscientiously⁹ thoughtful physicist¹⁰ into the greatest intellectual difficulties? Let us consider how these difficulties arise.

 

Of course we must refer the process of the propagation of light (and indeed every other process) to a rigid¹¹ reference-body (co-ordinate system). As such a system let us again choose our embankment. We shall imagine the air above it to have been removed. If a ray of light be sent along the embankment, we see from the above that the tip of the ray will be transmitted with the velocity c relative to the embankment. Now let us suppose that our railway carriage is again travelling along the railway lines with the velocity v, and that its direction is the same as that of the ray of light, but its velocity of course much less. Let us inquire about the velocity of propagation of the ray of light relative to the carriage. It is obvious that we can here apply the consideration of the previous section, since the ray of light plays the part of the man walking along relatively¹² to the carriage. The velocity W of the man relative to the embankment is here replaced by the velocity of light relative to the embankment. w is the required velocity of light with respect to the carriage, and we have

w equals c minus v period

The velocity of propagation of a ray of light relative to the carriage thus comes cut smaller than c.

 

But this result comes into conflict with the principle of relativity set forth¹³ in Section V. For, like every other general law of nature, the law of the transmission of light in vacuo [in vacuum] must, according to the principle of relativity, be the same for the railway carriage as reference-body as when the rails are the body of reference. But, from our above consideration, this would appear to be impossible. If every ray of light is propagated relative to the embankment with the velocity c, then for this reason it would appear that another law of propagation of light must necessarily hold with respect to the carriage—a result contradictory¹⁴ to the principle of relativity.

 

In view of this dilemma¹⁵ there appears to be nothing else for it than to abandon either the principle of relativity or the simple law of the propagation of light in vacuo. Those of you who have carefully followed the preceding discussion are almost sure to expect that we should retain the principle of relativity, which appeals so convincingly to the intellect because it is so natural and simple. The law of the propagation of light in vacuo would then have to be replaced by a more complicated law conformable to the principle of relativity. The development of theoretical physics shows, however, that we cannot pursue this course. The epoch-making theoretical investigations¹⁶ of H. A. Lorentz on the electrodynamical and optical phenomena¹⁷ connected with moving bodies show that experience in this domain¹⁸ leads conclusively¹⁹ to a theory of electromagnetic phenomena, of which the law of the constancy of the velocity of light in vacuo is a necessary consequence. Prominent theoretical physicists²⁰ were therefore more inclined to reject the principle of relativity, in spite of the fact that no empirical data had been found which were contradictory to this principle.

 

At this juncture²¹ the theory of relativity entered the arena²². As a result of an analysis of the physical conceptions of time and space, it became evident that in reality there is not the least incompatibility between the principle of relativity and the law of propagation of light, and that by systematically²³ holding fast to both these laws a logically rigid theory could be arrived at. This theory has been called the special theory of relativity to distinguish it from the extended theory, with which we shall deal later. In the following pages we shall present the fundamental ideas of the special theory of relativity.

 

1 incompatibility   

n.不兼容

参考例句：

• One cause may be an Rh incompatibility causing kernicterus in the newborn. 一个原因可能是Rh因子不相配引起新生儿的脑核性黄疸。
• Sexual incompatibility is wide-spread in the apple. 性的不亲合性在苹果中很普遍。

2 velocity   

n.速度，速率

参考例句：

• Einstein's theory links energy with mass and velocity of light.爱因斯坦的理论把能量同质量和光速联系起来。
• The velocity of light is about 300000 kilometres per second.光速约为每秒300000公里。

3 emission   

n.发出物，散发物；发出，散发

参考例句：

• Rigorous measures will be taken to reduce the total pollutant emission.采取严格有力措施，降低污染物排放总量。
• Finally,the way to effectively control particulate emission is pointed out.最后，指出有效降低颗粒排放的方向。

4 simultaneously   

adv.同时发生地，同时进行地

参考例句：

• The radar beam can track a number of targets almost simultaneously.雷达波几乎可以同时追着多个目标。
• The Windows allow a computer user to execute multiple programs simultaneously.Windows允许计算机用户同时运行多个程序。

5 fixed   

adj.固定的，不变的，准备好的；(计算机)固定的

参考例句：

• Have you two fixed on a date for the wedding yet?你们俩选定婚期了吗？
• Once the aim is fixed,we should not change it arbitrarily.目标一旦确定，我们就不应该随意改变。

6 astronomer   

n.天文学家

参考例句：

• A new star attracted the notice of the astronomer.新发现的一颗星引起了那位天文学家的注意。
• He is reputed to have been a good astronomer.他以一个优秀的天文学者闻名于世。

7 justifiably   

adv.无可非议地

参考例句：

• There General Walters would come aboard to greet me, justifiably beaming with pride at his arrangement. 在那儿沃尔特斯将军会登上飞机来接我，理所当然为他们的安排感到洋洋得意。 来自辞典例句
• The Chinese seemed justifiably proud of their economic achievements. 中国人似乎为他们的经济成就感到自豪，这是无可非议的。 来自互联网

8 plunged   

v.颠簸( plunge的过去式和过去分词 )；暴跌；骤降；突降

参考例句：

• The train derailed and plunged into the river. 火车脱轨栽进了河里。
• She lost her balance and plunged 100 feet to her death. 她没有站稳，从100英尺的高处跌下摔死了。

9 conscientiously   

adv.凭良心地；认真地，负责尽职地；老老实实

参考例句：

• He kept silent,eating just as conscientiously but as though everything tasted alike. 他一声不吭，闷头吃着，仿佛桌上的饭菜都一个味儿。 来自《简明英汉词典》
• She discharged all the responsibilities of a minister conscientiously. 她自觉地履行部长的一切职责。 来自《简明英汉词典》

10 physicist   

n.物理学家，研究物理学的人

参考例句：

• He is a physicist of the first rank.他是一流的物理学家。
• The successful physicist never puts on airs.这位卓有成就的物理学家从不摆架子。

11 rigid   

adj.严格的，死板的；刚硬的，僵硬的

参考例句：

• She became as rigid as adamant.她变得如顽石般的固执。
• The examination was so rigid that nearly all aspirants were ruled out.考试很严，几乎所有的考生都被淘汰了。

12 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

13 forth   

adv.向前；向外，往外

参考例句：

• The wind moved the trees gently back and forth.风吹得树轻轻地来回摇晃。
• He gave forth a series of works in rapid succession.他很快连续发表了一系列的作品。

14 contradictory   

adj.反驳的，反对的，抗辩的；n.正反对，矛盾对立

参考例句：

• The argument is internally contradictory.论据本身自相矛盾。
• What he said was self-contradictory.他讲话前后不符。

15 dilemma   

n.困境，进退两难的局面

参考例句：

• I am on the horns of a dilemma about the matter.这件事使我进退两难。
• He was thrown into a dilemma.他陷入困境。

16 investigations   

（正式的）调查( investigation的名词复数 )； 侦查； 科学研究； 学术研究

参考例句：

• His investigations were intensive and thorough but revealed nothing. 他进行了深入彻底的调查，但没有发现什么。
• He often sent them out to make investigations. 他常常派他们出去作调查。

17 phenomena   

n.现象

参考例句：

• Ade couldn't relate the phenomena with any theory he knew.艾德无法用他所知道的任何理论来解释这种现象。
• The object of these experiments was to find the connection,if any,between the two phenomena.这些实验的目的就是探索这两种现象之间的联系，如果存在着任何联系的话。

18 domain   

n.(活动等)领域，范围；领地，势力范围

参考例句：

• This information should be in the public domain.这一消息应该为公众所知。
• This question comes into the domain of philosophy.这一问题属于哲学范畴。

19 conclusively   

adv.令人信服地，确凿地

参考例句：

• All this proves conclusively that she couldn't have known the truth. 这一切无可置疑地证明她不可能知道真相。 来自《简明英汉词典》
• From the facts,he was able to determine conclusively that the death was not a suicide. 根据这些事实他断定这起死亡事件并非自杀。 来自《简明英汉词典》

20 physicists   

物理学家( physicist的名词复数 )

参考例句：

• For many particle physicists, however, it was a year of frustration. 对于许多粒子物理学家来说，这是受挫折的一年。 来自英汉非文学 - 科技
• Physicists seek rules or patterns to provide a framework. 物理学家寻求用法则或图式来构成一个框架。

21 juncture   

n.时刻，关键时刻，紧要关头

参考例句：

• The project is situated at the juncture of the new and old urban districts.该项目位于新老城区交界处。
• It is very difficult at this juncture to predict the company's future.此时很难预料公司的前景。

22 arena   

n.竞技场，运动场所；竞争场所，舞台

参考例句：

• She entered the political arena at the age of 25. 她25岁进入政界。
• He had not an adequate arena for the exercise of his talents.他没有充分发挥其才能的场所。

23 systematically   

adv.有系统地

参考例句：

• This government has systematically run down public services since it took office.这一屆政府自上台以来系统地削减了公共服务。
• The rainforest is being systematically destroyed.雨林正被系统地毀灭。