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首页 » 经典英文小说 » A Brief History of Time 时间简史 » CHAPTER 7 BLACK HOLES AIN’T SO BLACK
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CHAPTER 7 BLACK HOLES AIN’T SO BLACK
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Before 1970, my research on general relativity hadconcentrated mainly on the question of whether or not therehad been a big bang singularity. However, one evening inNovember that year, shortly after the birth of my daughter,Lucy, I started to think about black holes as I was getting intobed. My disability makes this rather a slow process, so I hadplenty of time. At that date there was no precise definition ofwhich points in space-time lay inside a black hole and whichlay outside. I had already discussed with Roger Penrose theidea of defining a black hole as the set of events from which itwas not possible to escape to a large distance, which is nowthe generally accepted definition. It means that the boundary ofthe black hole, the event horizon, is formed by the light raysthat just fail to escape from the black hole, hovering1 foreverjust on the edge (Fig2. 7.1). It is a bit like running away fromthe police and just managing to keep one step ahead but notbeing able to get clear away!
Suddenly I realized that the paths of these light rays couldnever approach one another. If they did they must eventuallyrun into one another. It would be like meeting someone elserunning away from the police in the opposite direction - youwould both be caught! (Or, in this case, fall into a black hole.)But if these light rays were swallowed up by the black hole,then they could not have been on the boundary of the blackhole. So the paths of light rays in the event horizon hadalways to be moving parallel to, or away from, each other.
Another way of seeing this is that the event horizon, theboundary of the black hole, is like the edge of a shadow - theshadow of impending3 doom4. If you look at the shadow cast bya source at a great distance, such as the sun, you will see thatthe rays of light in the edge are not approaching each other.
If the rays of light that form the event horizon, theboundary of the black hole, can never approach each other,the area of the event horizon might stay the same or increasewith time, but it could never decrease because that would meanthat at least some of the rays of light in the boundary wouldhave to be approaching each other. In fact, the area wouldincrease whenever matter or radiation fell into the black hole(Fig. 7.2). Or if two black holes collided and merged5 togetherto form a single black hole, the area of the event horizon ofthe final black hole would be greater than or equal to the sumof the areas of the event horizons of the original black holes(Fig. 7.3). This nondecreasing property of the event horizon’sarea placed an important restriction6 on the possible behavior ofblack holes. I was so excited with my discovery that I did notget much sleep that night. The next day I rang up RogerPenrose. He agreed with me. I think, in fact, that he had beenaware of this property of the area. However, he had beenusing a slightly different definition of a black hole. He had notrealized that the boundaries of the black hole according to thetwo definitions would be the same, and hence so would theirareas, provided the black hole had settled down to a state inwhich it was not changing with time.
The nondecreasing behavior of a black hole’s area was veryreminiscent of the behavior of a physical quantity called entropy,which measures the degree of disorder7 of a system. It is amatter of common experience that disorder will tend to increaseif things are left to themselves. (One has only to stop makingrepairs around the house to see that!) One can create orderout of disorder (for example, one can paint the house), butthat requires expenditure8 of effort or energy and so decreasesthe amount of ordered energy available.
A precise statement of this idea is known as the second lawof thermodynamics. It states that the entropy of an isolatedsystem always increases, and that when two systems are joinedtogether, the entropy of the combined system is greater thanthe sum of the entropies of the individual systems. Forexample, consider a system of gas molecules9 in a box. Themolecules can be thought of as little billiard balls continuallycolliding with each other and bouncing off the walls of the box.
The higher the temperature of the gas, the faster the moleculesmove, and so the more frequently and harder they collide withthe walls of the box and the greater the outward pressure theyexert on the walls. Suppose that initially10 the molecules are allconfined to the left-hand side of the box by a partition. If thepartition is then removed, the molecules will tend to spread outand occupy both halves of the box. At some later time theycould, by chance, all be in the right half or back in the lefthalf, but it is overwhelmingly more probable that there will beroughly equal numbers in the two halves. Such a state is lessordered, or more disordered, than the original state in which allthe molecules were in one half. One therefore says that theentropy of the gas has gone up. Similarly, suppose one startswith two boxes, one containing oxygen molecules and the othercontaining nitrogen molecules. If one joins the boxes togetherand removes the intervening wall, the oxygen and the nitrogenmolecules will start to mix. At a later time the most probablestate would be a fairly uniform mixture of oxygen and nitrogenmolecules throughout the two boxes. This state would be lessordered, and hence have more entropy, than the initial state oftwo separate boxes.
The second law of thermodynamics has a rather differentstatus than that of other laws of science, such as Newton’s lawof gravity, for example, because it does not hold always, just inthe vast majority of cases. The probability of all the gasmolecules in our first boxfound in one half of the box at a later time is many millionsof millions to one, but it can happen. However, if one has ablack hole around there seems to be a rather easier way ofviolating the second law: just throw some matter with a lot ofentropy such as a box of gas, down the black hole. The totalentropy of matter outside the black hole would go down. Onecould, of course, still say that the total entropy, including theentropy inside the black hole, has not gone down - but sincethere is no way to look inside the black hole, we cannot seehow much entropy the matter inside it has. It would be nice,then, if there was some feature of the black hole by whichobservers outside the blackhole could tell its entropy, and which would increasewhenever matter carrying entropy fell into the black hole.
Following the discovery, described above, that the area of theevent horizon increased whenever matter fell into a black hole,a research student at Princeton named Jacob Bekensteinsuggested that the area of the event horizon was a measure ofthe entropy of the black hole. As matter carrying entropy fellinto a black hole, the area of its event horizon would go up,so that the sum of the entropy of matter outside black holesand the area of the horizons would never go down.
This suggestion seemed to prevent the second law ofthermodynamics from being violated in most situations.
However, there was one fatal flaw. If a black hole has entropy,then it ought to also have a temperature. But a body with aparticular temperature must emit radiation at a certain rate. Itis a matter of common experience that if one heats up apoker in a fire it glows red hot and emits radiation, but bodiesat lower temperatures emit radiation too; one just does notnormally notice it because the amount is fairly small. Thisradiation is required in order to prevent violation11 of the secondlaw. So black holes ought to emit radiation. But by their verydefinition, black holes are objects that are not supposed to emitanything. It therefore seemed that the area of the eventhorizon of a black hole could not be regarded as its entropy.
In 1972 I wrote a paper with Brandon Carter and anAmerican colleague, Jim Bardeen, in which we pointed12 out thatalthough there were many similarities between entropy and thearea of the event horizon, there was this apparently13 fataldifficulty. I must admit that in writing this paper I wasmotivated partly by irritation14 with Bekenstein, who, I felt, hadmisused my discovery of the increase of the area of the eventhorizon. However, it turned out in the end that he wasbasically correct, though in a manner he had certainly notexpected.
In September 1973, while I was visiting Moscow, I discussedblack holes with two leading Soviet15 experts, Yakov Zeldovichand Alexander Starobinsky. They convinced me that, accordingto the quantum mechanical uncertainty16 principle, rotating blackholes should create and emit particles. I believed theirarguments on physical grounds, but I did not like themathematical way in which they calculated the emission17. Itherefore set about devising a better mathematical treatment,which I described at an informal seminar in Oxford18 at the endof November 1973. At that time I had not done the calculationsto find out how much would actually be emitted. I wasexpecting to discover just the radiation that Zeldovich andStarobinsky had predicted from rotating black holes. However,when I did the calculation, I found, to my surprise andannoyance, that even non-rotating black holes should apparentlycreate and emit particles at a steady rate. At first I thoughtthat this emission indicated that one of the approximations Ihad used was not valid19. I was afraid that if Bekenstein foundout about it, he would use it as a further argument to supporthis ideas about the entropy of black holes, which I still did notlike. However, the more I thought about it, the more it seemedthat the approximations really ought to hold. But what finallyconvinced me that the emission was real was that the spectrumof the emitted particles was exactly that which would be emittedby a hot body, and that the black hole was emitting particlesat exactly the correct rate to prevent violations20 of the secondlaw. Since then the calculations have been repeated in anumber of different forms by other people. They all confirmthat a black hole ought to emit particles and radiation as if itwere a hot body with a temperature that depends only on theblack hole’s mass: the higher the mass, the lower thetemperature.
How is it possible that a black hole appears to emit particleswhen we know that nothing can escape from within its eventhorizon? The answer, quantum theory tells us, is that theparticles do not come from within the black hole, but from the“empty” space just outside the black hole’s event horizon! Wecan understand this in the following way: what we think of as“empty” space cannot be completely empty because that wouldmean that all the fields, such as the gravitational andelectromagnetic fields, would have to be exactly zero. However,the value of a field and its rate of change with time are likethe position and velocity21 of a particle: the uncertainty principleimplies that the more accurately22 one knows one of thesequantities, the less accurately one can know the other. So inempty space the field cannot be fixed23 at exactly zero, becausethen it would have both a precise value (zero) and a preciserate of change (also zero). There must be a certain minimumamount of uncertainty, or quantum fluctuations24, in the value ofthe field. One can think of these fluctuations as pairs ofparticles of light or gravity that appear together at some time,move apart, and then come together again and annihilate25 eachother. These particles are virtual particles like the particles thatcarry the gravitational force of the sun: unlike real particles,they cannot be observed directly with a particle detector26.
However, their indirect effects, such as small changes in theenergy of electron orbits in atoms, can be measured and agreewith the theoretical predictions to a remarkable27 degree ofaccuracy. The uncertainty principle also predicts that there willbe similar virtual pairs of matter particles, such as electrons orquarks. In this case, however, one member of the pair will bea particle and the other an antiparticle (the antiparticles of lightand gravity are the same as the particles).
Because energy cannot be created out of nothing, one of thepartners in a particle/antiparticle pair will have positive energy,and the other partner negative energy. The one with negativeenergy is condemned28 to be a short-lived virtual particle becausereal particles always have positive energy in normal situations. Itmust therefore seek out its partner and annihilate with it.
However, a real particle close to a massive body has lessenergy than if it were far away, because it would take energyto lift it far away against the gravitational attraction of the body.
Normally, the energy of the particle is still positive, but thegravitational field inside a black hole is so strong that even areal particle can have negative energy there. It is thereforepossible, if a black hole is present, for the virtual particle withnegative energy to fall into the black hole and become a realparticle or antiparticle. In this case it no longer has toannihilate with its partner. Its forsaken29 partner may fall into theblack hole as well. Or, having positive energy, it might alsoescape from the vicinity of the black hole as a real particle orantiparticle (Fig. 7.4). To an observer at a distance, it willappear to have been emitted from the black hole. The smallerthe black hole, the shorter the distance the particle withnegative energy will have to go before it becomes a realparticle, and thus the greater the rate of emission, and theapparent temperature, of the black hole.
The positive energy of the outgoing radiation would bebalanced by a flow of negative energy particles into the blackhole. By Einstein’s equation E = mc2 (where E is energy, m ismass, and c is the speed of light), energy is proportional tomass. A flow of negative energy into the black hole thereforereduces its mass. As the black hole loses mass, the area of itsevent horizon gets smaller, but this decrease in the entropy ofthe black hole is more than compensated30 for by the entropy ofthe emitted radiation, so the second law is never violated.
Moreover, the lower the mass of the black hole, the higherits temperature. So as the black hole loses mass, itstemperature and rate of emission increase, so it loses massmore quickly. What happens when the mass of the black holeeventually becomes extremely small is not quite clear, but themost reasonable guess is that it would disappear completely ina tremendous final burst of emission, equivalent to the explosionof millions of H-bombs.
A black hole with a mass a few times that of the sun wouldhave a temperature of only one ten millionth of a degree aboveabsolute zero. This is much less than the temperature of themicrowave radiation that fills the universe (about 2.7? aboveabsolute zero), so such black holes would emit even less thanthey absorb. If the universe is destined31 to go on expandingforever, the temperature of the microwave radiation willeventually decrease to less than that of such a black hole,which will then begin to lose mass. But, even then, itstemperature would be so low that it would take about a millionmillion million million million million million million million millionmillion years (1 with sixty-six zeros after it) to evaporatecompletely. This is much longer than the age of the universe,which is only about ten or twenty thousand million years (1 or2 with ten zeros after it). On the other hand, as mentioned inChapter 6, there might be primordial32 black holes with a verymuch smaller mass that were made by the collapse33 ofirregularities in the very early stages of the universe. Such blackholes would have a much higher temperature and would beemitting radiation at a much greater rate. A primordial blackhole with an initial mass of a thousand million tons would havea lifetime roughly equal to the age of the universe. Primordialblack holes with initial masses less than this figure wouldalready have completely evaporated, but those with slightlygreater masses would still be emitting radiation in the form ofX rays and gamma rays. These X rays and gamma rays arelike waves of light, but with a much shorter wavelength34. Suchholes hardly deserve the epithet35 black: they really are white hotand are emitting energy at a rate of about ten thousandmegawatts.
One such black hole could run ten large power stations, ifonly we could harness its power. This would be rather difficult,however: the black hole would have the mass of a mountaincompressed into less than a million millionth of an inch, thesize of the nucleus36 of an atom! If you had one of these blackholes on the surface of the earth, there would be no way tostop it from falling through the floor to the center of the earth.
It would oscillate through the earth and back, until eventually itsettled down at the center. So the only place to put such ablack hole, in which one might use the energy that it emitted,would be in orbit around the earth - and the only way thatone could get it to orbit the earth would be to attract it thereby37 towing a large mass in front of it, rather like a carrot infront of a donkey. This does not sound like a very practicalproposition, at least not in the immediate38 future.
But even if we cannot harness the emission from theseprimordial black holes, what are our chances of observingthem? We could look for the gamma rays that the primordialblack holes emit during most of their lifetime. Although theradiation from most would be very weak because they are faraway, the total from all of them might be detectable39. We doobserve such a background of gamma rays: Fig. 7.5 showshow the observed intensity40 differs at different frequencies (thenumber of waves per second). However, this background couldhave been, and probably was, generated by processes otherthan primordial black holes. The dotted line in Fig. 7.5 showshow the intensity should vary with frequency for gamma raysgiven off by primordial black holes, if there were on average300 per cubic light-year. One can therefore say that theobservations of the gamma ray background do not provide anypositive evidence for primordial black holes, but they do tell usthat on average there cannot be more than 300 in every cubiclight-year in the universe. This limit means that primordial blackholes could make up at most one millionth of the matter in theuniverse.
With primordial black holes being so scarce, it might seemunlikely that there would be one near enough for us toobserve as an individual source of gamma rays. But sincegravity would draw primordial black holes toward any matter,they should be much more common in and around galaxies41.
So although the gamma ray background tells us that there canbe no more than 300 primordial black holes per cubiclight-year on average, it tells us nothing about how commonthey might be in our own galaxy42. If they were, say, a milliontimes more common than this, then the nearest black hole tous would probably be at a distance of about a thousand millionkilometers, or about as far away as Pluto43, the farthest knownplanet. At this distance it would still be very difficult to detectthe steady emission of a black hole, even if it was tenthousand megawatts. In order to observe a primordial blackhole one would have to detect several gamma ray quantacoming from the same direction within a reasonable space oftime, such as a week. Otherwise, they might simply be part ofthe background. But Planck’s quantum principle tells us thateach gamma ray quantum has a very high energy, becausegamma rays have a very high frequency, so it would not takemany quanta to radiate even ten thousand megawatts. And toobserve these few coming from the distance of Pluto wouldrequire a larger gamma ray detector than any that have beenconstructed so far. Moreover, the detector would have to be inspace, because gamma rays cannot penetrate44 the atmosphere.
Of course, if a black hole as close as Pluto were to reachthe end of its life and blow up, it would be easy to detect thefinal burst of emission. But if the black hole has been emittingfor the last ten or twenty thousand million years, the chance ofit reaching the end of its life within the next few years, ratherthan several million years in the past or future, is really rathersmall! So in order to have a reasonable chance of seeing anexplosion before your research grant ran out, you would haveto find a way to detect any explosions within a distance ofabout one light-year. In fact bursts of gamma rays from spacehave been detected by satellites originally constructed to lookfor violations of the Test Ban Treaty. These seem to occurabout sixteen times a month and to be roughly uniformlydistributed in direction across the sky. This indicates that theycome from outside the Solar System since otherwise we wouldexpect them to be concentrated toward the plane of the orbitsof the planets. The uniform distribution also indicates that thesources are either fairly near to us in our galaxy or rightoutside it at cosmological distances because otherwise, again,they would be concentrated toward the plane of the galaxy. Inthe latter case, the energy required to account for the burstswould be far too high to have been produced by tiny blackholes, but if the sources were close in galactic terms, it mightbe possible that they were exploding black holes. I would verymuch like this to be the case but I have to recognize thatthere are other possible explanations for the gamma ray bursts,such as colliding neutron45 stars. New observations in the nextfew years, particularly by gravitational wave detectors46 like LIGO,should enable us to discover the origin of the gamma raybursts.
Even if the search for primordial black holes proves negative,as it seems it may, it will still give us important informationabout the very early stages of the universe. If the earlyuniverse had been chaotic47 or irregular, or if the pressure ofmatter had been low, one would have expected it to producemany more primordial black holes than the limit already set byour observations of the gamma ray background. Only if theearly universe was very smooth and uniform, with a highpressure, can one explain the absence of observable numbersof primordial black holes.
The idea of radiation from black holes was the first exampleof a prediction that depended in an essential way on both thegreat theories of this century, general relativity and quantummechanics. It aroused a lot of opposition48 initially because itupset the existing viewpoint: “How can a black hole emitanything?” When I first announced the results of mycalculations at a conference at the Rutherford-AppletonLaboratory near Oxford, I was greeted with general incredulity.
At the end of my talk the chairman of the session, John G.
Taylor from Kings College, London, claimed it was all nonsense.
He even wrote a paper to that effect. However, in the endmost people, including John Taylor, have come to theconclusion that black holes must radiate like hot bodies if ourother ideas about general relativity and quantum mechanics arecorrect. Thus, even though we have not yet managed to find aprimordial black hole, there is fairly general agreement that ifwe did, it would have to be emitting a lot of gamma rays andX rays.
The existence of radiation from black holes seems to implythat gravitational collapse is not as final and irreversible as weonce thought. If an astronaut falls into a black hole, its masswill increase, but eventually the energy equivalent of that extramass will be returned to the universe in the form of radiation.
Thus, in a sense, the astronaut will be “recycled.” It would bea poor sort of immortality49, however, because any personalconcept of time for the astronaut would almost certainly cometo an end as he was torn apart inside the black hole! Eventhe types of particles that were eventually emitted by the blackhole would in general be different from those that made up theastronaut: the only feature of the astronaut that would survivewould be his mass or energy.
The approximations I used to derive50 the emission from blackholes should work well when the black hole has a mass greaterthan a fraction of a gram. However, they will break down atthe end of the black hole’s life when its mass gets very small.
The most likely outcome seems to be that the black hole willjust disappear, at least from our region of the universe, takingwith it the astronaut and any singularity there might be insideit, if indeed there is one. This was the first indication thatquantum mechanics might remove the singularities that werepredicted by general relativity. However, the methods that I andother people were using in 1974 were not able to answerquestions such as whether singularities would occur in quantumgravity. From 1975 onward51 I therefore started to develop amore powerful approach to quantum gravity based on RichardFeynrnan’s idea of a sum over histories. The answers that thisapproach suggests for the origin and fate of the universe andits contents, such as astronauts, will be de-scribed in the nexttwo chapters. We shall see that although the uncertaintyprinciple places limitations on the accuracy of all ourpredictions, it may at the same time remove the fundamentalunpredictability that occurs at a space-time singularity.

点击收听单词发音收听单词发音  

1 hovering 99fdb695db3c202536060470c79b067f     
鸟( hover的现在分词 ); 靠近(某事物); (人)徘徊; 犹豫
参考例句:
  • The helicopter was hovering about 100 metres above the pad. 直升机在离发射台一百米的上空盘旋。
  • I'm hovering between the concert and the play tonight. 我犹豫不决今晚是听音乐会还是看戏。
2 fig L74yI     
n.无花果(树)
参考例句:
  • The doctor finished the fig he had been eating and selected another.这位医生吃完了嘴里的无花果,又挑了一个。
  • You can't find a person who doesn't know fig in the United States.你找不到任何一个在美国的人不知道无花果的。
3 impending 3qHzdb     
a.imminent, about to come or happen
参考例句:
  • Against a background of impending famine, heavy fighting took place. 即将发生饥荒之时,严重的战乱爆发了。
  • The king convoke parliament to cope with the impending danger. 国王召开国会以应付迫近眉睫的危险。
4 doom gsexJ     
n.厄运,劫数;v.注定,命定
参考例句:
  • The report on our economic situation is full of doom and gloom.这份关于我们经济状况的报告充满了令人绝望和沮丧的调子。
  • The dictator met his doom after ten years of rule.独裁者统治了十年终于完蛋了。
5 merged d33b2d33223e1272c8bbe02180876e6f     
(使)混合( merge的过去式和过去分词 ); 相融; 融入; 渐渐消失在某物中
参考例句:
  • Turf wars are inevitable when two departments are merged. 两个部门合并时总免不了争争权限。
  • The small shops were merged into a large market. 那些小商店合并成为一个大商场。
6 restriction jW8x0     
n.限制,约束
参考例句:
  • The park is open to the public without restriction.这个公园对公众开放,没有任何限制。
  • The 30 mph speed restriction applies in all built-up areas.每小时限速30英里适用于所有建筑物聚集区。
7 disorder Et1x4     
n.紊乱,混乱;骚动,骚乱;疾病,失调
参考例句:
  • When returning back,he discovered the room to be in disorder.回家后,他发现屋子里乱七八糟。
  • It contained a vast number of letters in great disorder.里面七零八落地装着许多信件。
8 expenditure XPbzM     
n.(时间、劳力、金钱等)支出;使用,消耗
参考例句:
  • The entry of all expenditure is necessary.有必要把一切开支入账。
  • The monthly expenditure of our family is four hundred dollars altogether.我们一家的开销每月共计四百元。
9 molecules 187c25e49d45ad10b2f266c1fa7a8d49     
分子( molecule的名词复数 )
参考例句:
  • The structure of molecules can be seen under an electron microscope. 分子的结构可在电子显微镜下观察到。
  • Inside the reactor the large molecules are cracked into smaller molecules. 在反应堆里,大分子裂变为小分子。
10 initially 273xZ     
adv.最初,开始
参考例句:
  • The ban was initially opposed by the US.这一禁令首先遭到美国的反对。
  • Feathers initially developed from insect scales.羽毛最初由昆虫的翅瓣演化而来。
11 violation lLBzJ     
n.违反(行为),违背(行为),侵犯
参考例句:
  • He roared that was a violation of the rules.他大声说,那是违反规则的。
  • He was fined 200 dollars for violation of traffic regulation.他因违反交通规则被罚款200美元。
12 pointed Il8zB4     
adj.尖的,直截了当的
参考例句:
  • He gave me a very sharp pointed pencil.他给我一支削得非常尖的铅笔。
  • She wished to show Mrs.John Dashwood by this pointed invitation to her brother.她想通过对达茨伍德夫人提出直截了当的邀请向她的哥哥表示出来。
13 apparently tMmyQ     
adv.显然地;表面上,似乎
参考例句:
  • An apparently blind alley leads suddenly into an open space.山穷水尽,豁然开朗。
  • He was apparently much surprised at the news.他对那个消息显然感到十分惊异。
14 irritation la9zf     
n.激怒,恼怒,生气
参考例句:
  • He could not hide his irritation that he had not been invited.他无法掩饰因未被邀请而生的气恼。
  • Barbicane said nothing,but his silence covered serious irritation.巴比康什么也不说,但是他的沉默里潜伏着阴郁的怒火。
15 Soviet Sw9wR     
adj.苏联的,苏维埃的;n.苏维埃
参考例句:
  • Zhukov was a marshal of the former Soviet Union.朱可夫是前苏联的一位元帅。
  • Germany began to attack the Soviet Union in 1941.德国在1941年开始进攻苏联。
16 uncertainty NlFwK     
n.易变,靠不住,不确知,不确定的事物
参考例句:
  • Her comments will add to the uncertainty of the situation.她的批评将会使局势更加不稳定。
  • After six weeks of uncertainty,the strain was beginning to take its toll.6个星期的忐忑不安后,压力开始产生影响了。
17 emission vjnz4     
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。
18 Oxford Wmmz0a     
n.牛津(英国城市)
参考例句:
  • At present he has become a Professor of Chemistry at Oxford.他现在已是牛津大学的化学教授了。
  • This is where the road to Oxford joins the road to London.这是去牛津的路与去伦敦的路的汇合处。
19 valid eiCwm     
adj.有确实根据的;有效的;正当的,合法的
参考例句:
  • His claim to own the house is valid.他主张对此屋的所有权有效。
  • Do you have valid reasons for your absence?你的缺席有正当理由吗?
20 violations 403b65677d39097086593415b650ca21     
违反( violation的名词复数 ); 冒犯; 违反(行为、事例); 强奸
参考例句:
  • This is one of the commonest traffic violations. 这是常见的违反交通规则之例。
  • These violations of the code must cease forthwith. 这些违犯法规的行为必须立即停止。
21 velocity rLYzx     
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。
22 accurately oJHyf     
adv.准确地,精确地
参考例句:
  • It is hard to hit the ball accurately.准确地击中球很难。
  • Now scientists can forecast the weather accurately.现在科学家们能准确地预报天气。
23 fixed JsKzzj     
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。
24 fluctuations 5ffd9bfff797526ec241b97cfb872d61     
波动,涨落,起伏( fluctuation的名词复数 )
参考例句:
  • He showed the price fluctuations in a statistical table. 他用统计表显示价格的波动。
  • There were so many unpredictable fluctuations on the Stock Exchange. 股票市场瞬息万变。
25 annihilate Peryn     
v.使无效;毁灭;取消
参考例句:
  • Archer crumpled up the yellow sheet as if the gesture could annihilate the news it contained.阿切尔把这张黄纸揉皱,好象用这个动作就会抹掉里面的消息似的。
  • We should bear in mind that we have to annihilate the enemy.我们要把歼敌的重任时刻记在心上。
26 detector svnxk     
n.发觉者,探测器
参考例句:
  • The detector is housed in a streamlined cylindrical container.探测器安装在流线型圆柱形容器内。
  • Please walk through the metal detector.请走过金属检测器。
27 remarkable 8Vbx6     
adj.显著的,异常的,非凡的,值得注意的
参考例句:
  • She has made remarkable headway in her writing skills.她在写作技巧方面有了长足进步。
  • These cars are remarkable for the quietness of their engines.这些汽车因发动机没有噪音而不同凡响。
28 condemned condemned     
adj. 被责难的, 被宣告有罪的 动词condemn的过去式和过去分词
参考例句:
  • He condemned the hypocrisy of those politicians who do one thing and say another. 他谴责了那些说一套做一套的政客的虚伪。
  • The policy has been condemned as a regressive step. 这项政策被认为是一种倒退而受到谴责。
29 Forsaken Forsaken     
adj. 被遗忘的, 被抛弃的 动词forsake的过去分词
参考例句:
  • He was forsaken by his friends. 他被朋友们背弃了。
  • He has forsaken his wife and children. 他遗弃了他的妻子和孩子。
30 compensated 0b0382816fac7dbf94df37906582be8f     
补偿,报酬( compensate的过去式和过去分词 ); 给(某人)赔偿(或赔款)
参考例句:
  • The marvelous acting compensated for the play's weak script. 本剧的精彩表演弥补了剧本的不足。
  • I compensated his loss with money. 我赔偿他经济损失。
31 destined Dunznz     
adj.命中注定的;(for)以…为目的地的
参考例句:
  • It was destined that they would marry.他们结婚是缘分。
  • The shipment is destined for America.这批货物将运往美国。
32 primordial 11PzK     
adj.原始的;最初的
参考例句:
  • It is the primordial force that propels us forward.它是推动我们前进的原始动力。
  • The Neanderthal Man is one of our primordial ancestors.的尼安德特人是我们的原始祖先之一.
33 collapse aWvyE     
vi.累倒;昏倒;倒塌;塌陷
参考例句:
  • The country's economy is on the verge of collapse.国家的经济已到了崩溃的边缘。
  • The engineer made a complete diagnosis of the bridge's collapse.工程师对桥的倒塌做了一次彻底的调查分析。
34 wavelength 8gHwn     
n.波长
参考例句:
  • The authorities were unable to jam this wavelength.当局无法干扰这一波长。
  • Radio One has broadcast on this wavelength for years.广播1台已经用这个波长广播多年了。
35 epithet QZHzY     
n.(用于褒贬人物等的)表述形容词,修饰语
参考例句:
  • In "Alfred the Great","the Great"is an epithet.“阿尔弗雷德大帝”中的“大帝”是个称号。
  • It is an epithet that sums up my feelings.这是一个简洁地表达了我思想感情的形容词。
36 nucleus avSyg     
n.核,核心,原子核
参考例句:
  • These young people formed the nucleus of the club.这些年轻人成了俱乐部的核心。
  • These councils would form the nucleus of a future regime.这些委员会将成为一个未来政权的核心。
37 thereby Sokwv     
adv.因此,从而
参考例句:
  • I have never been to that city,,ereby I don't know much about it.我从未去过那座城市,因此对它不怎么熟悉。
  • He became a British citizen,thereby gaining the right to vote.他成了英国公民,因而得到了投票权。
38 immediate aapxh     
adj.立即的;直接的,最接近的;紧靠的
参考例句:
  • His immediate neighbours felt it their duty to call.他的近邻认为他们有责任去拜访。
  • We declared ourselves for the immediate convocation of the meeting.我们主张立即召开这个会议。
39 detectable tuXzmd     
adj.可发觉的;可查明的
参考例句:
  • The noise is barely detectable by the human ear.人的耳朵几乎是察觉不到这种噪音的。
  • The inflection point at this PH is barely detectable.在此PH值下,拐点不易发现。
40 intensity 45Ixd     
n.强烈,剧烈;强度;烈度
参考例句:
  • I didn't realize the intensity of people's feelings on this issue.我没有意识到这一问题能引起群情激奋。
  • The strike is growing in intensity.罢工日益加剧。
41 galaxies fa8833b92b82bcb88ee3b3d7644caf77     
星系( galaxy的名词复数 ); 银河系; 一群(杰出或著名的人物)
参考例句:
  • Quasars are the highly energetic cores of distant galaxies. 类星体是遥远星系的极为活跃的核心体。
  • We still don't know how many galaxies there are in the universe. 我们还不知道宇宙中有多少个星系。
42 galaxy OhoxB     
n.星系;银河系;一群(杰出或著名的人物)
参考例句:
  • The earth is one of the planets in the Galaxy.地球是银河系中的星球之一。
  • The company has a galaxy of talent.该公司拥有一批优秀的人才。
43 Pluto wu0yF     
n.冥王星
参考例句:
  • Pluto is the furthest planet from the sun.冥王星是离太阳最远的行星。
  • Pluto has an elliptic orbit.冥王星的轨道是椭圆形的。
44 penetrate juSyv     
v.透(渗)入;刺入,刺穿;洞察,了解
参考例句:
  • Western ideas penetrate slowly through the East.西方观念逐渐传入东方。
  • The sunshine could not penetrate where the trees were thickest.阳光不能透入树木最浓密的地方。
45 neutron neutron     
n.中子
参考例句:
  • Neutron is neutral and slightly heavier than the proton.中子是中性的,比质子略重。
  • Based on the neutron energy,the value of weighting factor was given.根据中子能量给出了相应的辐射权重因子的数值。
46 detectors bff80b364ed19e1821aa038fae38df83     
探测器( detector的名词复数 )
参考例句:
  • The report advocated that all buildings be fitted with smoke detectors. 报告主张所有的建筑物都应安装烟火探测器。
  • This is heady wine for experimenters using these neutrino detectors. 对于使用中微子探测器的实验工作者,这是令人兴奋的美酒。 来自英汉非文学 - 科技
47 chaotic rUTyD     
adj.混沌的,一片混乱的,一团糟的
参考例句:
  • Things have been getting chaotic in the office recently.最近办公室的情况越来越乱了。
  • The traffic in the city was chaotic.这城市的交通糟透了。
48 opposition eIUxU     
n.反对,敌对
参考例句:
  • The party leader is facing opposition in his own backyard.该党领袖在自己的党內遇到了反对。
  • The police tried to break down the prisoner's opposition.警察设法制住了那个囚犯的反抗。
49 immortality hkuys     
n.不死,不朽
参考例句:
  • belief in the immortality of the soul 灵魂不灭的信念
  • It was like having immortality while you were still alive. 仿佛是当你仍然活着的时候就得到了永生。
50 derive hmLzH     
v.取得;导出;引申;来自;源自;出自
参考例句:
  • We derive our sustenance from the land.我们从土地获取食物。
  • We shall derive much benefit from reading good novels.我们将从优秀小说中获得很大好处。
51 onward 2ImxI     
adj.向前的,前进的;adv.向前,前进,在先
参考例句:
  • The Yellow River surges onward like ten thousand horses galloping.黄河以万马奔腾之势滚滚向前。
  • He followed in the steps of forerunners and marched onward.他跟随着先辈的足迹前进。


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