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九年级英语新目标 Unit 1 How do you study for a test? 单元测试试题
新目标英语九年级单元练习题Unit 1
How do you study for a test?班级______姓名_________一、单项选择(
)1. In most countries, people speak English as _____ second language.A. a
)2. You should read _____ to practice your pronunciation.A. loud
D. aloudly(
)3. I don't think you can learn English well _____ this way.A. in
)4. I found _____ hard to understand what he said.A. that
)5. He wants to find a cheaper restaurant _____.A. to eat
C. to eat in
D. to eat with(
)6. Though he was new, he had no trouble _____ friends with others.A. to make
)7. _____ the help of the teachers, he passed the exam.A. Under
)8. The little girl is afraid _____ dogs.A. of
)9. He was very _____ when he heard the news.A. frustrate
B. frustrating
C. frustrated
D. frustrates(
)10. If you don't know the words, you should _____ in a dictionary.A. look up them B. look up it
C. look them up D. look it up(
)11. He teaches himself English _____ watching English movies.A. for
)12. I'm afraid that I don't _____ understand what you said.A. quite
)13. Why is there _____ noise outside? Go and see what is happening.A. much too
B. too many
C. very much
D. so much(
)14. Don't laugh _____ people when they are _____ trouble.A. in
)15. She thinks that _____ grammar is boring.A. study
B. studying
C. to studying
D. is studying(
)16. I don't feel like _____ anything now.A. eat
)17. Let's go _____ this afternoon.A. fish
B. fishing
C. to fish
D. fishes(
)18. Mr. Brown came into the classroom, _____ some papers in his hand.A. carry
B. carrying
C. to carry
D. carried(
)19. The boy was made _____ the new words ten times.A. copy
B. copying
C. to copy
D. copied(
)20. You should regard problems _____ challenges.A. as
)21. " Where is your father?"
"He _____ Shanghai on business."A. has been to B. has gone to
C. has been in D. has been at(
)22. _____ David goes, he is welcome.A. Wherever
B. Whichever
C. However
D. Whatever(
)23. I'd like to go with you, _____, my hands are full.A. but
B. however
C. whenever
D. wherever(
)24. He's never been to Australia, _____?A. is he
B. isn't he
D. hasn't he(
)25. His parents are always strict _____ him _____ everything.A. in
)26. We have nothing _____.A. to be afraid B. to afraid of
C. be afraid of D. to be afraid of(
)27. "I missed the football match between China and Japan."
"Really? _____!"A. How exciting B. What a pity C. How excited D. What a good idea(
)28. When I passed the room, I found Lucy _____ an English song in it.A. is singing
C. to sing
D. singing(
)29. _____ do the students study for an English exam?A. What
)30. Have you ever practiced _____ English with foreigners?A. speaking
C. to speak
)31. " _____ do you study for a test?"
"By listening to the tape."A. When
)32. I really need some paper _____.A. to draw
B. drawing
C. to draw with D. to draw on(
)33. Do you know the boy _____ Mike Brown?A. to call
C. calling
D. called.(
)34. _____, do you know where Jack is now?A. By the way
B. In the way
C. On the way D. To the way(
)35. My brother is _____ young that he can't go to school.A. too
)36. When _____ where she was from, the woman began to cry.A. to ask
)37. I saw so many books in the shelves, and I didn't know _____.A. which to choose
B. What to chooseC. where to choose
D. how to choose(
)38. Though he failed again, he tried _____ fifth time.A. a
)39. Last night she helped her mother _____ the housework.A. did
)40. Parents shouldn't compare their own children _____ others.A. to
D. by二、完形填空A　　Life is not easy, so I'd like to say, "When anything happens，believe in yourself."　　When I was 14, I was __1__ nervous to talk to anyone. My classmates often __2__ me. I was sad but could do nothing. Later, __3__ happened. It changed my life. It was an English speech content. My mother asked me to take part in it. What a terrible idea! It meant I had to __4__ in front of all the teachers and students of school!　　"Come on, boy. Believe in yourself. You are sure to win." Then, my mother and I talked about many different topics（题目）. At last I chose the topic "Believe in yourself". I tried my best to __5__ all the speech and practiced it over 100 times. __6__ my mother's great love, I did well in the contest. I could __7__ believe my ears when the news came that I had won the first place. I heard the cheers（欢呼）from the teachers and students. Those __8__ who looked down on me, now all said "Congratulations!" to me. My mother hugged（拥抱）me and cried __9__.　　__10__ then, everything has changed for me. When I do anything, I try to tell myself to be sure and I will find myself. This is sure not only for a person but also for a country.(
D. enough(
)2. A. made faces at B. looked after C. laughed at
D. caught up with(
)3. A. something
B. nothing
C. anything
D. everything(
)4. A. write
)5. A. remember
)7. A. almost
D. hardly(
)8. A. teachers
B. classmates
)9. A. angrily
C. quietly
D. excitedly(
)10. A. Since
D. BeforeB　　Erik was a 14-year-old boy. He thought he was a young man but his parents didn't think so. They told him, "When you begin to think about
others, you'll be a young man."　　One morning, Erik's parents gave him __2__ money to buy hamburgers at a store. He saw an old man there. The man looked very sick. Erik __3__ up to him and found that the old man had lost his money and hadn't had __4__ for his breakfast. Erik took the man to the nearest restaurant. But the old man ordered only a glass of water for himself. Erik felt __5__ for him, so he asked the waiter to __6__ the man some bread and a cup of coffee. The old man was so __7__ that he ate up all the food very soon. After that, the man told Erik that he would never forget his kindness. Erik was very __8__ when he heard the old man say, "You are a very good young man."　　__9__ their surprise, the food was a present because __10__ day was the birthday of the boss.(
)1. A. help
B. helping
C. to help
D. helped(
)2. A. a few
)3. A. went
)4. A. something
B. anything
C. everything
D. nothing(
)5. A. glad
C. ashamed
D. terrible(
)6. A. take
D. bringing(
)7. A. full
D. worried(
)8. A. sorry
B. pleased
C. disappointed D. surprised(
)9. A. For
)10. A. that
D. these三、阅读理解A　　A young boy was playing with a ball in the street. He threw it too hard and it broke the window of a house and fell inside. A woman came to the window and shouted at the boy. The boy was frightened（害怕）and ran away. But he still wanted his ball back. A few minutes later he returned and knocked at the door and said. "My father's going to come and repair（修理）your window very soon."　　After a while, a man came to the door with tools（工具）in his hand so the woman let the boy take his ball away.　　When the man finished repairing the window, he said to the woman, "Please pay ten dollars."　　"But aren't you the boy's father?" the woman asked, looking surprised. "No," he answered，looking more surprised, "Aren't you his mother?"(
)1. The boy lost his ball because _____.　　A. the ball fell into the woman's garden　　B. he went through the window of the woman's house　　C. his ball fell into the woman's house through the window　　D. his father took the ball away(
)2. The boy ran away because _____.　　A. he wanted to ask his father to repair the window　　B. he didn't want to take his ball back　　C. he went to look for his father to ask for his ball　　D. he was afraid of the woman.(
)3. The boy came back again because _____.　　A. his father came with him
B. he wanted to get his ball back　　C. the woman was very friendly
D. he wasn't afraid of the woman(
)4. The man who repaired the window wasn't the boy's father，was he?　　A. Yes, he wasn't. B. No, he wasn't. C. Yes, he wasn't. D. No, he was.(
)5. Which of the following is true?　　A. The boy was the son of the woman.　　B. The man knew that the woman was not the boy's mother.　　C. The boy was honest and paid for the woman's window.　　D. The woman thought that the man was the boy's father.B　　Students in many countries are learning English. Some of these students are small children. Others are teenagers. Many are adults. Some learn at school, others by themselves. A few learn English by hearing the language over the radio.　　Why do all these people want to learn English? It is difficult to answer this question. Many boys and girls learn English at school. Because it is one of their subjects required（要求）for study. They study their own language and math and English. Some people learn it because it is useful for their work. Many people learn English for their higher studies, because at college or university some of the books are in English. Other people learn English because they want to read newspapers or magazines in English.(
)1. People learn English _____.　　A. at school
B. over the radio C. on TV
D. not all in the same way(
)2. Different kinds of people want to learn English _____.　　A. together with other subjects
B. for different reasons　　C. for their work
D. for higher studies at college(
)3. From the passage we know that _____.　　A. we can learn English easily
B. English is difficult to learn　　C. English is learned by most people in the world　　D. English is a useful language and we must work hard to learn it(
)4. Which of the following is RIGHT?　　A. We don't want to learn any foreign language.　　B. We can do well in all our work without English.　　C. English is the most important subject in schools.　　D. We should learn English because we need to face the world.(
)5. English isn't one of the subjects required for study in our school，is it?　　A. Yes, it is.
B. No, it isn't.
C. Yes, it isn't. D. No, it is.C　　Henry works in a factory. He comes from a poor family and was in school for only four years. He has to do the hard work and he is paid less. He likes to watch football very much and spends much time on it.　　One evening there was a big football match on a playground. He borrowed some money from his friends and hurried there. Here were a lot of people. And all the tickets were sold out. He was sorry for it. He saw a pole（电线杆）outside the playground and climbed up it quickly. A policeman came and said, "It's dangerous to stay on it! Come down!"　　"Wait a minute, please!" Henry said and immediately the policeman heard the cheers on the playground and asked in a hurry, "Which team has scored a goal(进球)?"　　"Ours."　　"Wonderful! You can stay there. But be careful!" the policeman said happily and left. When the match would soon be over, he came back again and asked, "Who's won?"　　"Theirs, 3-2."　　"Come down," the policeman said angrily. "Such a match isn't worth watching!"　　Henry had to come down, but soon thy heard cheers again. The policeman said in a hurry, "Climb up quickly and see who's scored a goal!"(
)1. Henry is paid less because _____.　　A. he likes to watch football match　　B. he didn't learn much when he was young　　C. he's the youngest in the factory　　D. he doesn't go to work on time(
)2. Henry borrowed some money to _____.　　A. have supper
B. buy some medicine　　C. buy a ticket for the match
D. buy over（收买）the policeman(
)3. Henry stayed on the pole because _____.　　A. he didn't buy a ticket for the match　　B. he hoped to save some money　　C. he could see clearly there　　D. he wanted to have a joke with the policeman(
)4. The policeman soon changed his mind because _____.　　A. he thought it was safe on the pole　　B. Henry had paid money for it　　C. he hurried to go into the playground　　D. he was also interested in the match(
)5. _____, so the policeman became angry.　　A. Henry didn't listen to him
B. Henry didn't give him some money　　C. Their team had lost the game
D. Henry didn't know who had wonD　　As we know, there are differences between western culture and Chinese culture. We can see differences when we pay attention to the way words are used. Let's look at the words about animals and plants. Most expressions in Chinese about the dog, for example, "a homeless dog", "a mad dog", "a running dog" and "a dog catching a mouse", have negative（消极的）meanings. But in western countries, dogs are thought to be honest and good friends of humans. In English, people use the dog to describe positive（积极的）actions. For example, "you are a lucky dog" means you are a lucky person. And "every dog has its day" means each person has good luck sometimes. To describe a person's serious illness, they say "sick as a dog". The word "dog-tired" means very tired. However, Chinese love cats very much. But in western culture, "cat" is often used to describe a woman who is cruel（冷酷的）. There are many other examples of how "cat" is used differently as well.　　The rose is regarded as a symbol（象征）of love in both China and some western countries. People think the rose stands for（代表）love, peace（和平）, courage and friendship. And the rose is the national flower of England, America and many other countries.　　The words about plants and animals are used in positive or negative ways in different cultures. We can learn about many differences in cultures by comparing how some words are used.(
)1. The word "dog" in Chinese usually _______.　　A. shows peace
B. stands for friendship C. has a negative meaning(
)2. "Every dog has its day." means "_______".　　A. Everybody in the world is lucky.　　B. Everybody has a time in life to be lucky.　　C. If one works hard, he's sure to succeed.(
)3. Western people usually use "cat" to refer to（指代）"_______".　　A. a tired person
B. an unkind woman
C. a homeless person(
)4. Which of the following is true according to the passage?　　A. Words show differences in culture.　　B. Chinese people prefer dogs to cats.　　C. Western people think cats to be good friends.(
)5. What's the best title(标题) for the passage?　　A. Negative or Positive
B. Different countries Have Different Culture　　C. Rose Means the Same in Chinese and English四、补全对话( Chen Han and Wang Wei are classmates. One afternoon, they are talking about their future in the classroom...)Chen Hua: So what are you going to be when you grow up?Wang Wei: I'm going to be an astronaut like Yang Liwei, Fei Junlong and Nie Haisheng.Chen Hua: That sounds like an exciting job. But I don't think it's easy to be an astronaut. _1_________Wang Wei: I'm going to do some exercise every day so that I can be strong enough. And I'm going to study science at college. How about you? What are you going to be?Chen Hua: _2_________Wang Wei: Why are you going to be a teacher?Chen Hua: Because I like being with children.Wang Wei: _3________Chen Hua: Yes, I have. I'd like to work here in our hometown. I want to do something to make our hometown more beautiful.Wang Wei: _4________Chen Hua: Great! Let's work hard at our lessons to make our dream come true.Wang Wei: _5________A. I hope I can do something for our hometown, too.B. Have you thought about where to work?C. I'm going to be a teacher.D. OK. Let's start right now.E. How are you going to do that?五、单词拼写1. Have you ever talked with __________（外国人）to practice your pronunciation?2. I'm afraid that I can't __________（发音）some of the words in unit one.3. They got more __________（建议）about how to learn English on the Internet.4. He started to make his own original sentences __________（使用）the grammar he learnt.5. We will go fishing __________（除非）it rains tomorrow.六、阅读填词 根据短文内容和首字母提示，把所缺单词补充完整　　Skin-diving（潜泳）is a new (1) s
today. This sport takes you (2) i
_ wonderful world. It is like a visit to the moon. When you are under water, it is easy for you to climb big rocks（岩石）, because you are no longer (3)_h
.Here, under water, everything is blue and green. During the day, there is (4)_p
_ of light. When fishes swim nearby, you can catch them (5)_w
_ your hands.When you have tanks（罐）of air on your back, you can stay in deep water for a long time. But you must take (6)_c
_ when you dive in deep water. To catch fish is one of the most interesting (7)_p
_ of this sport. Besides, there are more uses for skin-diving. You can clean ships (8)_w
_ taking them (9)_o
_ of water. You can get many things from the deep sea.Now you see that skin-diving is (10)_b
_ useful and interesting.七、用所给词语的适当形式填空1. You must pay much attention to your __________ ( pronounce ).2. He always does wrong at school. We all think he is __________ ( frustrate ).3. I think that __________ ( watch ) English movies is the best way to learn English.4. I'm sorry I can't understand __________ ( speak ) English.5. Why not __________ ( join ) an English club to practice speaking English?6. Watching movies is __________ ( frustrate ) because the people speak too quickly.7. He made a lot of __________ ( mistake ) in the math test yesterday.8. She is an outgoing girl. She has no trouble __________ ( make ) new friends here.9. They study for the test by __________ ( work ) with friends.10. I spent about two hours ___________( do ) my homework yesterday evening.11. Do you know which is the best way __________ ( learn ) English well?12. You should try your best __________ ( study ) well.13. In order to catch the early bus, he ran as __________ ( quick ) as he could.14. Do you know the __________ ( different ) between put on and wear?15. I found this dictionary is very __________ ( help ) for us students.16. Can you giver me some good __________ ( suggestion )?17. He is a man that can __________ ( easy ) gets angry.18. What should you do if you want to be a good language __________ ( learn ).19. "Help __________ ( you ) to some fruit." Mrs. Smith said to the children.20. You __________ ( real ) did __________ ( good ) in the last math test.21. The boys took some __________ ( photo ) of __________ ( potato ).22. Don't worry about your daughter. She __________ ( call ) you as soon as she __________ ( get ) to Beijing.23. We often hear her __________ ( sing ) English songs in the next room.24. The flowers need __________ ( water ) right now.25. Hurry up! There is little time __________ ( leave )八、书面表达英语是世界上最重要的语言之一，许多人都在学习英语。作为一名学生，你打算怎样学英语呢？写一篇不低于70词的短文，谈一谈你学英语的方法。参考答案：一、1-5 ACACC
6-10 BBACC
11-15 DADAB
16-20 BBBCA　　21-25 BABCD
26-30 DBDCA
31-35 CDDAD
36-40 CAADA二、A.
BCBBA三、A.
1-5 DBDDAC.
1-5 CBBAB四、ECBAD五、1. foreigners
2. pronounce
3. suggestions
5. unless六、1. sport
7. parts　　8. without
10. both七、1. pronunciation
2. frustrated
3. watching
6. frustrating　　7. mistakes
9. working
11. to learn
12. to study　　13. quickly
14. difference
15. helpful
16. suggestions
17. easily
18. learner　　19. yourselves
21. potatoes
23. sing　　24. watering
25. left八、（略）Visionlearning | Process of Science | The Nature of Scientific Knowledge
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The Culture of ScienceThe Nature of Scientific Knowledge
by Anthony Carpi, Ph.D., Anne E. Egger, Ph.D.
Did you know?
Did you know that it was not Magellan, Columbus, or even Copernicus who first proposed that the world was round? Rather, 2,000 years before these Europeans, Greek philosophers referred to the Earth as a sphere. An accumulation of evidence over the centuries confirmed that the Earth was round long before explorers sailed around the world.
This module explores the nature of scientific knowledge by asking what science is. It emphasizes the importance of a scientific way of thinking and shows how observation and testing add to the body of scientific knowledge. Focusing on astronomy and physics, the module highlights the work of scientists through history who have contributed to our understanding of the age of the universe as a means of conveying the nature of scientific knowledge.
Terms you should know
method, series of actions or steps
support for an idea, opinion, or hypothesis
Table of contents
Glossary Terms
Figure 1: Representation of Eratosthenes' studies demonstrating the curvature of Earth and the geometry used to calculate the circumference of the planet. (Click to see additional information in larger version)
It seems preposterous to us today that people once thought that the Earth was flat. Who could have possibly thought of our planet as a giant disk with the stars and heavens above, and boulders, tree roots, and other things below? But this was the
view of Earth in much of the world before the 2nd century , though the details differed from culture to culture. And it was not explorers who sailed around the world that finally laid the idea to rest, but an accumulation of
long before this.
Greek philosophers referred to a spherical Earth as early as the 6th century . They observed that the moon appeared to be a sphere and therefore inferred that Earth might also be spherical. Two hundred years later, in the 4th century BCE, the Greek philosopher
observed that the shadow of the Earth on the Moon during a
is always curved, thus providing some of the first
that Earth is spherical. In the 3rd century BCE, the mathematician Eratosthenes observed that at noon on the summer solstice in the ancient Egyptian city of Syene, the sun was directly overhead as objects did not cast a shadow. Eratosthenes was from Alexandria, Egypt, some 500 miles to the north, and he knew that a tall tower cast a shadow in that city at the same time on the summer solstice. Using these
and measurements of shadow length and distance, he inferred that the
of the Earth is curved and he calculated a remarkably accurate estimate of the circumference of the planet (Figure 1). Some years later, the Greek geographer Strabo added to this evidence when he observed that sailors saw distant objects move downward on the horizon and disappear as they sailed away from them. He proposed that this was because Earth was curved and those sailors were not simply moving further away from the objects but also curving around the planet as they sailed.
Figure 2: Earthrise taken on December 24, 1968, from the Apollo 8 mission.
image & NASA
Aristotle, Eratosthenes, and Strabo didn't call themselves scientists, yet they were using the
of science by making
and providing explanations for those observations.
Thus, we knew that Earth was a sphere long before 's men sailed all the way around it in 1522 or before Apollo 8 astronauts sent back pictures of Earth from space in 1968 (Figure 2), documenting its spherical shape. In fact, those astronauts had to be absolutely confident that the Earth was a rotating sphere, orbiting the Sun, or they would never have been able to get into orbit. It is the nature of science and scientific knowledge that gave them that confidence, and understanding the difference between scientific knowledge and other types of knowledge is critical to understanding science itself.
What is science?
Science consists of two things: a body of knowledge and the
by which that knowledge is produced. This second component of science provides us with a way of thinking and knowing about the world. Commonly, we only see the "body of knowledge" component of science. We are presented with scientific concepts in statement form – Earth is round,
are negatively charged, our genetic code is contained in our , the
is 13.7 billion years old – with little background about the process that led to that knowledge and why we can trust it. But there are a number of things that distinguish the scientific process and give us confidence in the knowledge produced through it.
So then, what is the scientific process? The scientific
is a way of building knowledge and making predictions about the world in such a way that they are testable. The question of whether Earth is flat or round could be put to the test, it could be studied through multiple lines of , and the
evaluated to determine whether it supported a round or flat planet. Different scientific disciplines typically use different
and approaches to investigate the natural world, but testing lies at the core of scientific inquiry for all scientists.
As scientists analyze and interpret their
and Interpretation module), they generate , , or
module), which help explain their results and place them in context of the larger body of scientific knowledge. These different kinds of explanations are tested by scientists through additional , , modeling, and theoretical studies. Thus, the body of scientific knowledge builds on previous ideas and is constantly growing. It is deliberately shared with colleagues through the
module), where scientists comment on each other's work, and then through publication in the scientific literature (see our
module), where it can be evaluated and integrated into the body of scientific knowledge by the larger community. And this is not the end: One of the hallmarks of scientific knowledge is that it is
to change, as new data are collected and reinterpretations of existing data are made. Major theories, which are supported by multiple lines of , are rarely completely changed, but new data and tested explanations add nuance and detail.
A scientific way of thinking is something that anyone can use, at any time, whether or not they are in the
of developing new knowledge and explanations. Thinking scientifically involves asking questions that can be answered analytically by collecting
or creating a
and then testing one's ideas. A scientific way of thinking inherently includes creativity in approaching explanations while staying within the confines of the data. Thinking scientifically does not mean rejecting your culture and background, but recognizing the role that they play in your way of thinking. While testable explanations are a critical component of thinking scientifically, there are other valid ways of thinking about the world around us that do not always yield testable explanations. These different ways of thinking are complementary – not in competition – as they address different aspects of the human experience.
It's easy to be confident in the scientific
and our knowledge when we can provide irrefutable , as we were able to do by orbiting around the Earth in a spaceship and taking pictures of an obviously round planet. But most scientific investigations do not lead to results that are so easily supported, and yet we still rely on and trust the knowledge produced through the process of science. Why do we trust it? Because it works. Science has a long history of creating knowledge that is useful and that gives us more insight into our surroundings. Take one of the statements above: The
is 13.7 billion years old. Why should we have confidence in this statement?
Comprehension Checkpoint
The scientific process is a way of building knowledge and making predictions that
a.can be tested.
b.are accepted as scientific law.
Check answer
The age of the universe
How old is the universe? How can we possibly know the age of something that was created not simply before human history, but before our planet came into being? This is a difficult question to address scientifically, so much so that through the early 20th century many scientists assumed that the
was infinite and eternal, existing for all of time.
Machines and entropy
The first indication that the
may not have existed for all of time came from an unlikely source: the study of engines. In the 1820s,
was a young officer on leave from the French military. While taking classes at various institutions in Paris, he became interested in industrial problems, and was surprised to see that no scientific studies had been undertaken on the steam engine, a relatively new invention at the time and a poorly understood one. Carnot believed that engines could be better understood – a characteristic common to scientists is that they work to better understand things – and so he studied the transfer of
in engines. He recognized that no engine could be 100% efficient because some energy is always lost from the
(Figure 3). Carnot published his ideas in a book titled Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power, which presented a mathematical description of the amount of
that could be generated by an engine, called the
Figure 3: An infrared image of a running engine showing the temperature of various parts of the engine. Higher temperatures (red and yellow portions of the image) indicate greater heat loss. The loss of heat represents a loss of efficiency in the engine, and a contribution to the increasing entropy of the universe.
image & Epogee, Ltd
Carnot's work didn't receive much attention during his lifetime, and he died of cholera in 1832, when he was only 36 years old. But others began to realize the importance of his work and built upon it. One of those scientists was , a German physicist who showed that Carnot's
was not limited to engines, but in fact applied to all
in which there was a transfer of . Clausius' application of an explanation for one phenomenon to many others is also characteristic of science, which assumes that processes are universal.
In 1850, Clausius published a paper in which he developed the second
of thermodynamics, which states that
always flows from a high energy state (for example, a
that is hot) to a low energy state (one that is cold) (). In later work, Clausius coined the term
to describe the energy lost from a system when it is transferred, and as an acknowledgement of the pioneering work of
in providing the foundation for his discoveries, Clausius used the symbol S to refer to the entropy of a system.
But how do engines and
relate to the age of the universe? In 1865, Clausius published another paper that restated the Second
of Thermodynamics as "the entropy of the
tends to a maximum." If the universe was infinite and existed for all time, the
says that all of the
within the universe would have been lost to entropy by now. In other words, the stars themselves would have burned out long ago, dissipating their
into surrounding space. The fact that there are still active stars must mean that the universe has existed for a finite amount of time, and was created at some specific point in time. Perhaps the age of that point in time could be determined?
Comprehension Checkpoint
Science assumes that
a.natural processes are universal.
b.each process is particular to the individual system being observed.
Check answer
Redshift and the Doppler effect
At about the same time, an Austrian physicist by the name of
was studying astronomy and mathematics. Doppler knew that
behaved like a wave, and so began to think about how the movement of stars might affect the light emitted from those stars. In a paper published in 1842, Doppler proposed that the observed
of a wave would depend on the relative speed of the wave's source in relation to the observer, a phenomenon he called a "frequency shift" (Doppler, 1842). He made an analogy to a ship at sail on the ocean, describing how the ship would encounter
of the water at a faster rate (and thus higher frequency) if it were sailing into the
than if it were traveling in the same direction as the waves.
You might be familiar with the
shift, which we now call the
in his honor, if you have ever listened to the sound of traffic while standing on the side of the road. The familiar high-to-low pitch change is an example of the effect – the actual frequency of the
emitted is not changing, but the speed of the passing vehicle affects how quickly those
reach you. Doppler proposed that we would see the same effect on any stars that were moving: Their color would shift towards the red end of the
if they were moving away from Earth (called a redshift) and towards the blue end of the spectrum if they were moving closer (called a blueshift) (see Figure 4). He expected to be able to see this shift in , or pairs of stars that orbit around each other. Eventually, Doppler's 1842 paper, entitled "On the coloured
of the double stars and certain other stars of the heavens," would change the very way we look at the . However, at the time, telescopes were not sensitive enough to confirm the shift he proposed.
Figure 4: A representation of how the perceived spectrum of light emitted from a galaxy is affected by its motion (Click to see additional information in larger version).
Doppler's ideas became part of the scientific literature and by that means became known to other scientists. By the early 1900s, technology finally caught up with Doppler and more powerful telescopes could be used to test his ideas. In September of 1901, an American named Vesto Slipher had just completed his undergraduate
in mechanics and astronomy at Indiana University. He got a job as a temporary assistant at the Lowell Observatory in Flagstaff, Arizona, while continuing his graduate work at Indiana. Shortly after his arrival, the observatory obtained a three-prism , and Slipher's job was to mount it to the 24-inch telescope at the observatory and learn to use it to study the rotation of the planets in the solar . After a few months of problems and trouble-shooting, Slipher was able to take
of Mars, Jupiter, and Saturn. But Slipher's personal
interests were much farther away than the planets of the solar system. Like Doppler, he was interested in studying the spectra of , and he began to do so in his spare time at the observatory.
Over the next decade, Slipher completed a Master's
and a PhD at Indiana University, while continuing his work at Lowell Observatory measuring the spectra and Doppler shift of stars. In particular, Slipher focused his attention on stars within spiral
(Figure 5), expecting to find that the shift seen in the spectra of the stars would indicate that the galaxies those stars belonged to were rotating. Indeed, he is credited with determining that galaxies rotate, and was able to determine the
at which they rotate. But in 1914, having studied 15 different nebulae, he announced a curious discovery at a meeting of the American Astronomical
in August:
In the great majority of cas the largest
are all positive...The striking preponderance of the positive sign indicates a general fleeing from us or the Milky Way.
Slipher had found that most galaxies showed a
in their , indicating that they were all moving away from us in space, or receding (). By measuring the magnitude of the redshift, he was able to determine the recessional
or the speed at which objects were "fleeing." Slipher had made an
that put a new perspective on the , and in response, he received a standing ovation for his presentation.
Figure 5: The Andromeda galaxy, one of the spiral nebulae studied by Vesto Slipher, as seen in infrared light by NASA's Wide-field Infrared Survey Explorer.
image & NASA
Slipher continued his work with
and galaxies and published another paper in 1917, having now examined 25
and seen a redshift in 21 of them. Georges Lema?tre, a Belgian physicist and astronomer, built on Slipher's work while completing his PhD at the Massachusetts Institute of Technology. He extended Slipher's measurements to the entire , and calculated mathematically that the universe must be expanding in order to explain Slipher's . He published his ideas in a 1927 paper called "A homogeneous Universe of
and growing radius accounting for the radial
of extragalactic nebulae" (), but his paper met with widespread criticism from the scientific community. The English astronomer Fred Hoyle ridiculed the work, and coined the term "Big Bang"
as a disparaging nickname for Lema?tre's idea. And none other than
criticized Lema?tre, writing to him "Your math is correct, but your physics is abominable" ().
Einstein's criticism had a personal and cultural component, two things we often overlook in terms of their influence on science. Several years earlier, Einstein had published his general
(). In formulating the theory, Einstein had encountered one significant problem: General relativity predicted that the
had to be either contracting or expanding – it did not allow for a static universe. But a contracting or expanding universe could not be eternal, while a static, non-moving universe could, and the prevailing cultural belief at the time was that the universe was eternal. Einstein was strongly influenced by his cultural surroundings. As a result, he invented a "fudge factor," which he called the cosmological , that would allow the theory of general relativity to be consistent with a static universe. But science is not a dem it is neither the most common or most popular conclusion that becomes accepted, but rather the conclusion that stands up to the test of
over time. Einstein's cosmological constant was being challenged by new evidence.
Comprehension Checkpoint
Scientists are not influenced by their personal experiences, their beliefs, or the culture of which they are a part.
Check answer
The expanding universe
In 1929, an American astronomer working at the Mt. Wilson Observatory in southern California made an important contribution to the discussion of the nature of the . Edwin Hubble had been at Mt. Wilson for 10 years, measuring the distances to galaxies, among other things. In the 1920s, he was working with Milton Humason, a high school dropout and assistant at the observatory. Hubble and Humason plotted the distances they had calculated for 46 different galaxies against Slipher's recession
and found a
relationship (see Figure 6) ().
Figure 6: The original Hubble diagram. The relative velocity of galaxies (in km/sec) is plotted against distance to that galaxy ( a parsec is 3.26 light years). The slope of the line drawn through the points gives the rate of expansion of the universe (the Hubble Constant). (Originally Figure 1, from "A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae," Proceedings of the National Academy of Sciences, Volume 15, Issue 3, 1929: p. 172. (C) Huntington Library, San Marino, CA.)
image & The Huntington Library
In other words, their graph showed that more distant galaxies were receding faster than closer ones, confirming the idea that the
was indeed expanding. This relationship, now referred to as Hubble's , allowed them to calculate the rate of expansion as a function of distance from the slope of the line in the graph. This rate term is now referred to as the Hubble . Hubble's initial
for the expansion rate was 500 km/sec/Megaparsec, or about 160 km/sec per million-light-years.
Knowing the rate at which the
is expanding, one can calculate the age of the universe by in essence "tracing back" the most distant objects in the universe to their point of origin. Using his initial
for the expansion rate and the measured distance of the galaxies, Hubble and Humason calculated the age of the universe to be approximately 2 billion years. Unfortunately, the calculation was inconsistent with lines of
from other investigations. By the time Hubble made his discovery, geologists had used radioactive dating techniques to calculate the age of Earth at about 3 billion years () – or older than the universe itself! Hubble had followed the
of science, so what was the problem?
to revision in science. It soon became clear that there was a problem in the way that Hubble had calculated his constant. In the 1940s, a German astronomer named Walter Baade took advantage of the blackouts that were ordered in response to potential attacks during World War II and used the Mt. Wilson Observatory in Arizona to look at several objects that Hubble had interpreted as single stars. With darker surrounding skies, Baade realized that these objects were, in fact,
of stars, and each was fainter, and thus more distant, than Hubble had calculated.
Baade doubled the distance to these objects, and in turn halved the
and doubled the age of the . In 1953, the American astronomer Allan Sandage, who had studied under Baade, looked in more detail at the brightness of stars and how that varied with distance. Sandage further revised the constant, and his estimate of 75 km/sec/Megaparsec is close to our modern day estimate of the Hubble constant of 72 km/sec/Megaparsec, which places the age of the universe at 12 to 14 billion years old.
The new estimates developed by Baade and Sandage did not negate what Hubble had done (it is still called the Hubble , after all), but they revised it based on new knowledge. The lasting knowledge of science is rarely the work of an individual, as building on the work of others is a critical component of the
of science. Hubble's findings would have been limited to some interesting
on the distance to various stars had it not also built on, and incorporated, the work of Slipher. Similarly, Baade and Sandage's contribution were no less significant because they "simply" refined Hubble's earlier work.
Since the 1950s, other means of calculating the age of the
have been developed. For example, there are now
for dating the age of the stars, and the oldest stars date to approximately 13.2 billion years ago (). The Wilkinson Microwave Anisotropy Probe (see the
links on this module for more detail) is collecting
(Figure 7). Using these data in conjunction with Einstein's
of general , scientists have calculated the age of the universe at 13.7 ± 0.2 billion years old (). The convergence of multiple lines of
on a single explanation is what creates the
foundation of scientific knowledge.
Figure 7: Visual representation of the cosmic microwave background radiation, and the temperature differences indicated by that radiation, as collected by the Wilkinson Microwave Anisotropy Probe.
image & NASA/WMAP Science Team
Comprehension Checkpoint
Major ideas in science are rarely the work of
a.individuals.
b.multiple researchers.
Check answer
Why should we trust science?
Why should we believe what scientists say about the age of the universe? We have no written
of its creation, and no one has been able to "step outside" of the , as astronauts did when they took pictures of Earth from space, to measure its age. Yet the nature of the scientific
allows us to accurately state the age of the observable . These predictions were developed by multiple researchers and tested through multiple
. They have been presented to the scientific community through publications and public presentations. And they have been confirmed and verified by many different studies. New studies, or new research methods, may be developed that might possibly cause us to refine our estimate of the age of the universe upward or downward. This is how the proc it is
to change as more information and new technologies become available. But it is not tenuous – our age estimate may be refined, but the idea of an expanding universe is unlikely to be overturned. As
builds to support an idea, our confidence in that idea builds.
Upon seeing Hubble's work, even
changed his opinion of a static
and called his insertion of the cosmological
the "biggest blunder" of his professional career. Hubble's discovery actually confirmed Einstein's
of general , which predicts that the universe must be expanding or contracting. Einstein refused to accept this idea because of his cultural biases. His work had not predicted a static universe, but he assumed this must be the case given what he had grown up believing. When confronted with the , he recognized that his earlier beliefs were flawed, and came to accept the findings of the science behind the idea. This is a hallmark of science: While an individual's beliefs may be biased by personal experience, the scientific enterprise works to collect data to allow for a more objective conclusion to be identified. Incorrect ideas may be upheld for some amount of time, but eventually the preponderance of
helps to lead us to correct these ideas. Once used as a term of disparagement, the "Big Bang" theory is now the leading explanation for the origin of the universe as we know it.
There are other questions we can ask about the origin of the , not all of which can be answered by science. Scientists can answer when and how the universe began but cannot calculate the reason why it began, for example. That type of question must be explored through philosophy, religion, and other ways of thinking. The questions that scientists ask must be testable. Scientists have provided answers to testable questions that have helped us calculate the age of the universe, like how distant certain stars are and how fast they are receding from us. Whether or not we can get a definitive answer, we can be confident in the
by which the explanations were developed, allowing us to rely on the knowledge that is produced through the process of science. Someday we may find
to help us understand why the universe was created, but for the time being science will limit itself to the last 13.7 or so billion years of phenomena to investigate.
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