The Physical Sciences
Chapter 5
Analysis and Physics
1
You have doubtless often been asked of what good is mathematics and whether these delicate constructions entirely1 mind-made are not artificial and born of our caprice.
Among those who put this question I should make a distinction; practical people ask of us only the means of money-making. These merit no reply; rather would it be proper to ask of them what is the good of accumulating so much wealth and whether, to get time to acquire it, we are to neglect art and science, which alone give us souls capable of enjoying it, ‘and for life’s sake to sacrifice all reasons for living.’
Besides, a science made solely2 in view of applications is impossible; truths are fecund3 only if bound together. If we devote ourselves solely to those truths whence we expect an immediate4 result, the intermediary links are wanting and there will no longer be a chain.
The men most disdainful of theory get from it, without suspecting it, their daily bread; deprived of this food, progress would quickly cease, and we should soon congeal5 into the immobility of old China.
But enough of uncompromising practicians! Besides these, there are those who are only interested in nature and who ask us if we can enable them to know it better.
To answer these, we have only to show them the two monuments already rough-hewn, Celestial6 Mechanics and Mathematical Physics.
They would doubtless concede that these structures are well worth the trouble they have cost us. But this is not enough. Mathematics has a triple aim. It must furnish an instrument for the study of nature. But that is not all: it has a philosophic7 aim and, I dare maintain, an esthetic8 aim. It must aid the philosopher to fathom9 the notions of number, of space, of time. And above all, its adepts10 find therein delights analogous11 to those given by painting and music. They admire the delicate harmony of numbers and forms; they marvel12 when a new discovery opens to them an unexpected perspective; and has not the joy they thus feel the esthetic character, even though the senses take no part therein? Only a privileged few are called to enjoy it fully13, it is true, but is not this the case for all the noblest arts?
This is why I do not hesitate to say that mathematics deserves to be cultivated for its own sake, and the theories inapplicable to physics as well as the others. Even if the physical aim and the esthetic aim were not united, we ought not to sacrifice either.
But more: these two aims are inseparable and the best means of attaining14 one is to aim at the other, or at least never to lose sight of it. This is what I am about to try to demonstrate in setting forth15 the nature of the relations between the pure science and its applications.
The mathematician16 should not be for the physicist17 a mere18 purveyor19 of formulas; there should be between them a more intimate collaboration20. Mathematical physics and pure analysis are not merely adjacent powers, maintaining good neighborly relations; they mutually interpenetrate and their spirit is the same. This will be better understood when I have shown what physics gets from mathematics and what mathematics, in return, borrows from physics.
2
The physicist can not ask of the analyst22 to reveal to him a new truth; the latter could at most only aid him to foresee it. It is a long time since one still dreamt of forestalling23 experiment, or of constructing the entire world on certain premature24 hypotheses. Since all those constructions in which one yet took a na?ve delight it is an age, to-day only their ruins remain.
All laws are therefore deduced from experiment; but to enunciate25 them, a special language is needful; ordinary language is too poor, it is besides too vague, to express relations so delicate, so rich, and so precise.
This therefore is one reason why the physicist can not do without mathematics; it furnishes him the only language he can speak. And a well-made language is no indifferent thing; not to go beyond physics, the unknown man who invented the word heat devoted26 many generations to error. Heat has been treated as a substance, simply because it was designated by a substantive27, and it has been thought indestructible.
On the other hand, he who invented the word electricity had the unmerited good fortune to implicitly28 endow physics with a new law, that of the conservation of electricity, which, by a pure chance, has been found exact, at least until now.
Well, to continue the simile29, the writers who embellish30 a language, who treat it as an object of art, make of it at the same time a more supple31 instrument, more apt for rendering32 shades of thought.
We understand, then, how the analyst, who pursues a purely33 esthetic aim, helps create, just by that, a language more fit to satisfy the physicist.
But this is not all: law springs from experiment, but not immediately. Experiment is individual, the law deduced from it is general; experiment is only approximate, the law is precise, or at least pretends to be. Experiment is made under conditions always complex, the enunciation34 of the law eliminates these complications. This is what is called ‘correcting the systematic35 errors.’
In a word, to get the law from experiment, it is necessary to generalize; this is a necessity imposed upon the most circumspect36 observer. But how generalize? Every particular truth may evidently be extended in an infinity37 of ways. Among these thousand routes opening before us, it is necessary to make a choice, at least provisional; in this choice, what shall guide us?
It can only be analogy. But how vague is this word! Primitive38 man knew only crude analogies, those which strike the senses, those of colors or of sounds. He never would have dreamt of likening light to radiant heat.
What has taught us to know the true, profound analogies, those the eyes do not see but reason divines?
It is the mathematical spirit, which disdains39 matter to cling only to pure form. This it is which has taught us to give the same name to things differing only in material, to call by the same name, for instance, the multiplication40 of quaternions and that of whole numbers.
If quaternions, of which I have just spoken, had not been so promptly41 utilized42 by the English physicists43, many persons would doubtless see in them only a useless fancy, and yet, in teaching us to liken what appearances separate, they would have already rendered us more apt to penetrate21 the secrets of nature.
Such are the services the physicist should expect of analysis; but for this science to be able to render them, it must be cultivated in the broadest fashion without immediate expectation of utility — the mathematician must have worked as artist.
What we ask of him is to help us to see, to discern our way in the labyrinth44 which opens before us. Now, he sees best who stands highest. Examples abound45, and I limit myself to the most striking.
The first will show us how to change the language suffices to reveal generalizations46 not before suspected.
When Newton’s law has been substituted for Kepler’s we still know only elliptic motion. Now, in so far as concerns this motion, the two laws differ only in form; we pass from one to the other by a simple differentiation48. And yet from Newton’s law may be deduced by an immediate generalization47 all the effects of perturbations and the whole of celestial mechanics. If, on the other hand, Kepler’s enunciation had been retained, no one would ever have regarded the orbits of the perturbed49 planets, those complicated curves of which no one has ever written the equation, as the natural generalizations of the ellipse. The progress of observations would only have served to create belief in chaos50.
The second example is equally deserving of consideration.
When Maxwell began his work, the laws of electro-dynamics admitted up to his time accounted for all the known facts. It was not a new experiment which came to invalidate them. But in looking at them under a new bias51, Maxwell saw that the equations became more symmetrical when a term was added, and besides, this term was too small to produce effects appreciable52 with the old methods.
You know that Maxwell’s a priori views awaited for twenty years an experimental confirmation53; or, if you prefer, Maxwell was twenty years ahead of experiment. How was this triumph obtained?
It was because Maxwell was profoundly steeped in the sense of mathematical symmetry; would he have been so, if others before him had not studied this symmetry for its own beauty?
It was because Maxwell was accustomed to ‘think in vectors,’ and yet it was through the theory of imaginaries (neomonics) that vectors were introduced into analysis. And those who invented imaginaries hardly suspected the advantage which would be obtained from them for the study of the real world, of this the name given them is proof sufficient.
In a word, Maxwell was perhaps not an able analyst, but this ability would have been for him only a useless and bothersome baggage. On the other hand, he had in the highest degree the intimate sense of mathematical analogies. Therefore it is that he made good mathematical physics.
Maxwell’s example teaches us still another thing.
How should the equations of mathematical physics be treated? Should we simply deduce all the consequences and regard them as intangible realities? Far from it; what they should teach us above all is what can and what should be changed. It is thus that we get from them something useful.
The third example goes to show us how we may perceive mathematical analogies between phenomena54 which have physically55 no relation either apparent or real, so that the laws of one of these phenomena aid us to divine those of the other.
The very same equation, that of Laplace, is met in the theory of Newtonian attraction, in that of the motion of liquids, in that of the electric potential, in that of magnetism56, in that of the propagation of heat and in still many others. What is the result? These theories seem images copied one from the other; they are mutually illuminating57, borrowing their language from each other; ask electricians if they do not felicitate themselves on having invented the phrase flow of force, suggested by hydrodynamics and the theory of heat.
Thus mathematical analogies not only may make us foresee physical analogies, but besides do not cease to be useful when these latter fail.
To sum up, the aim of mathematical physics is not only to facilitate for the physicist the numerical calculation of certain constants or the integration58 of certain differential equations. It is besides, it is above all, to reveal to him the hidden harmony of things in making him see them in a new way.
Of all the parts of analysis, the most elevated, the purest, so to speak, will be the most fruitful in the hands of those who know how to use them.
3
Let us now see what analysis owes to physics.
It would be necessary to have completely forgotten the history of science not to remember that the desire to understand nature has had on the development of mathematics the most constant and happiest influence.
In the first place the physicist sets us problems whose solution he expects of us. But in proposing them to us, he has largely paid us in advance for the service we shall render him, if we solve them.
If I may be allowed to continue my comparison with the fine arts, the pure mathematician who should forget the existence of the exterior59 world would be like a painter who knew how to harmoniously60 combine colors and forms, but who lacked models. His creative power would soon be exhausted61.
The combinations which numbers and symbols may form are an infinite multitude. In this multitude how shall we choose those which are worthy62 to fix our attention? Shall we let ourselves be guided solely by our caprice? This caprice, which itself would besides soon tire, would doubtless carry us very far apart and we should quickly cease to understand each other.
But this is only the smaller side of the question. Physics will doubtless prevent our straying, but it will also preserve us from a danger much more formidable; it will prevent our ceaselessly going around in the same circle.
History proves that physics has not only forced us to choose among problems which came in a crowd; it has imposed upon us such as we should without it never have dreamed of. However varied63 may be the imagination of man, nature is still a thousand times richer. To follow her we must take ways we have neglected, and these paths lead us often to summits whence we discover new countries. What could be more useful!
It is with mathematical symbols as with physical realities; it is in comparing the different aspects of things that we are able to comprehend their inner harmony, which alone is beautiful and consequently worthy of our efforts.
The first example I shall cite is so old we are tempted64 to forget it; it is nevertheless the most important of all.
The sole natural object of mathematical thought is the whole number. It is the external world which has imposed the continuum upon us, which we doubtless have invented, but which it has forced us to invent. Without it there would be no infinitesimal analysis; all mathematical science would reduce itself to arithmetic or to the theory of substitutions.
On the contrary, we have devoted to the study of the continuum almost all our time and all our strength. Who will regret it; who will think that this time and this strength have been wasted? Analysis unfolds before us infinite perspectives that arithmetic never suspects; it shows us at a glance a majestic65 assemblage whose array is simple and symmetric; on the contrary, in the theory of numbers, where reigns66 the unforeseen, the view is, so to speak, arrested at every step.
Doubtless it will be said that outside of the whole number there is no rigor67, and consequently no mathematical truth; that the whole number hides everywhere, and that we must strive to render transparent68 the screens which cloak it, even if to do so we must resign ourselves to interminable repetitions. Let us not be such purists and let us be grateful to the continuum, which, if all springs from the whole number, was alone capable of making so much proceed therefrom.
Need I also recall that M. Hermite obtained a surprising advantage from the introduction of continuous variables into the theory of numbers? Thus the whole number’s own domain69 is itself invaded, and this invasion has established order where disorder70 reigned71.
See what we owe to the continuum and consequently to physical nature.
Fourier’s series is a precious instrument of which analysis makes continual use, it is by this means that it has been able to represent discontinuous functions; Fourier invented it to solve a problem of physics relative to the propagation of heat. If this problem had not come up naturally, we should never have dared to give discontinuity its rights; we should still long have regarded continuous functions as the only true functions.
The notion of function has been thereby72 considerably73 extended and has received from some logician-analysts74 an unforeseen development. These analysts have thus adventured into regions where reigns the purest abstraction and have gone as far away as possible from the real world. Yet it is a problem of physics which has furnished them the occasion.
After Fourier’s series, other analogous series have entered the domain of analysis; they have entered by the same door; they have been imagined in view of applications.
The theory of partial differential equations of the second order has an analogous history. It has been developed chiefly by and for physics. But it may take many forms, because such an equation does not suffice to determine the unknown function, it is necessary to adjoin to it complementary conditions which are called conditions at the limits; whence many different problems.
If the analysts had abandoned themselves to their natural tendencies, they would never have known but one, that which Madame Kovalevski has treated in her celebrated75 memoir76. But there are a multitude of others which they would have ignored. Each of the theories of physics, that of electricity, that of heat, presents us these equations under a new aspect. It may, therefore, be said that without these theories we should not know partial differential equations.
It is needless to multiply examples. I have given enough to be able to conclude: when physicists ask of us the solution of a problem, it is not a duty-service they impose upon us, it is on the contrary we who owe them thanks.
4
But this is not all; physics not only gives us the occasion to solve problems; it aids us to find the means thereto, and that in two ways. It makes us foresee the solution; it suggests arguments to us.
I have spoken above of Laplace’s equation which is met in a multitude of diverse physical theories. It is found again in geometry, in the theory of conformal representation and in pure analysis, in that of imaginaries.
In this way, in the study of functions of complex variables, the analyst, alongside of the geometric image, which is his usual instrument, finds many physical images which he may make use of with the same success. Thanks to these images, he can see at a glance what pure deduction77 would show him only successively. He masses thus the separate elements of the solution, and by a sort of intuition divines before being able to demonstrate.
To divine before demonstrating! Need I recall that thus have been made all the important discoveries? How many are the truths that physical analogies permit us to present and that we are not in condition to establish by rigorous reasoning!
For example, mathematical physics introduces a great number of developments in series. No one doubts that these developments converge78; but the mathematical certitude is lacking. These are so many conquests assured for the investigators79 who shall come after us.
On the other hand, physics furnishes us not alone solutions; it furnishes us besides, in a certain measure, arguments. It will suffice to recall how Felix Klein, in a question relative to Riemann surfaces, has had recourse to the properties of electric currents.
It is true, the arguments of this species are not rigorous, in the sense the analyst attaches to this word. And here a question arises: How can a demonstration80 not sufficiently81 rigorous for the analyst suffice for the physicist? It seems there can not be two rigors82, that rigor is or is not, and that, where it is not there can not be deduction.
This apparent paradox83 will be better understood by recalling under what conditions number is applied84 to natural phenomena. Whence come in general the difficulties encountered in seeking rigor? We strike them almost always in seeking to establish that some quantity tends to some limit, or that some function is continuous, or that it has a derivative85.
Now the numbers the physicist measures by experiment are never known except approximately; and besides, any function always differs as little as you choose from a discontinuous function, and at the same time it differs as little as you choose from a continuous function. The physicist may, therefore, at will suppose that the function studied is continuous, or that it is discontinuous; that it has or has not a derivative; and may do so without fear of ever being contradicted, either by present experience or by any future experiment. We see that with such liberty he makes sport of difficulties which stop the analyst. He may always reason as if all the functions which occur in his calculations were entire polynomials.
Thus the sketch86 which suffices for physics is not the deduction which analysis requires. It does not follow thence that one can not aid in finding the other. So many physical sketches87 have already been transformed into rigorous demonstrations88 that to-day this transformation89 is easy. There would be plenty of examples did I not fear in citing them to tire the reader.
I hope I have said enough to show that pure analysis and mathematical physics may serve one another without making any sacrifice one to the other, and that each of these two sciences should rejoice in all which elevates its associate.
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entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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solely
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| adv.仅仅，唯一地 | |
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fecund
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| adj.多产的，丰饶的，肥沃的 | |
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immediate
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| adj.立即的；直接的，最接近的；紧靠的 | |
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congeal
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| v.凝结，凝固 | |
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celestial
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| adj.天体的；天上的 | |
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philosophic
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| adj.哲学的，贤明的 | |
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esthetic
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| adj.美学的，审美的；悦目的，雅致的 | |
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fathom
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| v.领悟，彻底了解 | |
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adepts
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| n.专家，能手( adept的名词复数 ) | |
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analogous
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| adj.相似的；类似的 | |
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marvel
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| vi.(at)惊叹vt.感到惊异；n.令人惊异的事 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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attaining
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| （通常经过努力）实现( attain的现在分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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forth
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| adv.向前；向外，往外 | |
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mathematician
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| n.数学家 | |
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physicist
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| n.物理学家，研究物理学的人 | |
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mere
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| adj.纯粹的；仅仅，只不过 | |
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purveyor
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| n.承办商，伙食承办商 | |
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collaboration
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| n.合作，协作；勾结 | |
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penetrate
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| v.透(渗)入；刺入，刺穿；洞察，了解 | |
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analyst
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| n.分析家，化验员；心理分析学家 | |
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forestalling
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| v.先发制人，预先阻止( forestall的现在分词 ) | |
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premature
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| adj.比预期时间早的；不成熟的，仓促的 | |
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enunciate
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| v.发音；（清楚地）表达 | |
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devoted
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| adj.忠诚的，忠实的，热心的，献身于...的 | |
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substantive
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| adj.表示实在的；本质的、实质性的；独立的；n.实词，实名词；独立存在的实体 | |
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implicitly
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| adv. 含蓄地, 暗中地, 毫不保留地 | |
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simile
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| n.直喻，明喻 | |
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embellish
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| v.装饰，布置；给…添加细节，润饰 | |
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supple
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| adj.柔软的，易弯的，逢迎的，顺从的，灵活的；vt.使柔软，使柔顺，使顺从；vi.变柔软，变柔顺 | |
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rendering
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| n.表现，描写 | |
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purely
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| adv.纯粹地，完全地 | |
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enunciation
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| n.清晰的发音；表明，宣言；口齿 | |
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systematic
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| adj.有系统的，有计划的，有方法的 | |
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circumspect
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| adj.慎重的，谨慎的 | |
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infinity
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| n.无限，无穷，大量 | |
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primitive
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| adj.原始的；简单的；n.原(始)人，原始事物 | |
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disdains
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| 鄙视，轻蔑( disdain的名词复数 ) | |
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multiplication
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| n.增加，增多，倍增；增殖，繁殖；乘法 | |
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promptly
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| adv.及时地，敏捷地 | |
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utilized
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| v.利用，使用( utilize的过去式和过去分词 ) | |
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physicists
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| 物理学家( physicist的名词复数 ) | |
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labyrinth
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| n.迷宫；难解的事物；迷路 | |
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abound
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| vi.大量存在；(in，with)充满，富于 | |
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generalizations
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| 一般化( generalization的名词复数 )； 普通化； 归纳； 概论 | |
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generalization
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| n.普遍性，一般性，概括 | |
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differentiation
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| n.区别，区分 | |
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perturbed
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| adj.烦燥不安的v.使（某人）烦恼，不安( perturb的过去式和过去分词 ) | |
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chaos
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| n.混乱，无秩序 | |
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bias
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| n.偏见，偏心，偏袒；vt.使有偏见 | |
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appreciable
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| adj.明显的，可见的，可估量的，可觉察的 | |
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confirmation
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| n.证实，确认，批准 | |
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phenomena
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| n.现象 | |
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physically
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| adj.物质上，体格上，身体上，按自然规律 | |
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magnetism
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| n.磁性，吸引力，磁学 | |
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illuminating
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| a.富于启发性的，有助阐明的 | |
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integration
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| n.一体化，联合，结合 | |
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exterior
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| adj.外部的，外在的；表面的 | |
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harmoniously
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| 和谐地，调和地 | |
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exhausted
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| adj.极其疲惫的，精疲力尽的 | |
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worthy
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| adj.(of)值得的，配得上的；有价值的 | |
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varied
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| adj.多样的，多变化的 | |
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tempted
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| v.怂恿（某人）干不正当的事；冒…的险（tempt的过去分词） | |
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majestic
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| adj.雄伟的，壮丽的，庄严的，威严的，崇高的 | |
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reigns
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| n.君主的统治( reign的名词复数 )；君主统治时期；任期；当政期 | |
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rigor
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| n.严酷，严格，严厉 | |
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transparent
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| adj.明显的，无疑的；透明的 | |
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domain
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| n.(活动等)领域，范围；领地，势力范围 | |
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disorder
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| n.紊乱，混乱；骚动，骚乱；疾病，失调 | |
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reigned
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| vi.当政，统治（reign的过去式形式） | |
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thereby
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| adv.因此，从而 | |
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considerably
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| adv.极大地；相当大地；在很大程度上 | |
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analysts
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| 分析家，化验员( analyst的名词复数 ) | |
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celebrated
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| adj.有名的，声誉卓著的 | |
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memoir
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| n.[pl.]回忆录，自传；记事录 | |
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deduction
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| n.减除，扣除，减除额；推论，推理，演绎 | |
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converge
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| vi.会合；聚集，集中；(思想、观点等)趋近 | |
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investigators
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| n.调查者，审查者( investigator的名词复数 ) | |
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demonstration
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| n.表明，示范，论证，示威 | |
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sufficiently
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| adv.足够地，充分地 | |
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rigors
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| 严格( rigor的名词复数 )； 严酷； 严密； （由惊吓或中毒等导致的身体）僵直 | |
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paradox
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| n.似乎矛盾却正确的说法；自相矛盾的人(物) | |
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applied
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| adj.应用的；v.应用，适用 | |
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derivative
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| n.派(衍)生物；adj.非独创性的，模仿他人的 | |
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sketch
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| n.草图；梗概；素描；v.素描；概述 | |
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sketches
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| n.草图( sketch的名词复数 )；素描；速写；梗概 | |
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demonstrations
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| 证明( demonstration的名词复数 )； 表明； 表达； 游行示威 | |
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transformation
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| n.变化；改造；转变 | |
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