쉐도잉 연습: 10 things you should know about black holes - 영상으로 영어 말하기 배우기

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When I first learned about black holes, I was scared that one would fly through our solar system and eat us all up.

That was 30 years ago.

I'm not afraid of black holes anymore, but I am afraid that they have been misunderstood.

So here are 10 things that you should know about black holes.

First things first, what is a black hole?

A black hole contains a region from which nothing can ever escape,

because to escape you would have to move faster than the speed of light, which you can't.

The boundary of the region from which you cannot escape is called the horizon.

In the simplest case, the horizon has the form of a sphere.

Its radius is known as the Schwarzschild radius, named after Karl Schwarzschild, who first derived black holes as a solution to Einstein's general relativity.

How large are black holes?

The diameter of a black hole is directly proportional to the mass of the black hole.

So the more mass falls into the black hole, the larger the black hole becomes.

Compared to other stellar objects though, black holes are tiny because enormous gravitational pressure has compressed their mass into a very small volume.

For example, the radius of a black hole with the mass of planet

Earth is only a few millimeters.

What happens at the horizon?

A black hole horizon does not have substance.

Therefore, someone crossing the black hole horizon does not notice anything weird going on in their immediate surroundings.

This follows from Einstein's equivalence principle, which implies that in your immediate surrounding you cannot tell the difference between acceleration in flat space

and curved space that gives rise to gravity.

However, an observer far away from a black hole who watches somebody fall in would notice

that the infalling person seems to move slower and slower the closer they get to the horizon.

It appears this way because time close by the black hole horizon runs much slower than far away from the horizon.

That's one of these odd consequences of the relativity of time that Einstein discovered.

So, if you fall into a black hole, it only takes a finite amount of time to cross the horizon,

but from the outside it looks like it takes forever.

What you would experience at the horizon depends on the tidal force of the gravitational field.

The tidal force is, loosely speaking, the change of the gravitational force.

It's not the gravitational force itself, it's the difference between the gravitational forces at two nearby places, say at your head and at your feet.

The tidal force at the horizon is inversely proportional to the square of the mass of the black hole.

This means the larger and more mass of the black hole, the smaller the tidal force at the horizon.

Yes, you heard that right.

The larger the black hole, the smaller the tidal force at the horizon.

Therefore, if the black hole is only massive enough, you can cross the horizon without noticing what just happened.

And once you have crossed the horizon, there is no turning back.

The stretching from the tidal force will become increasingly unpleasant as you approach the center of the black hole.

Eventually it will rip everything apart.

In the early days of general relativity, many physicists believed that there is a singularity at the horizon,

but this turned out to be a mathematical mistake.

What is inside a black hole?

Nobody really knows.

General relativity predicts that inside the black hole is a singularity, that's a place where the tidal forces become infinitely large.

But we know that general relativity does not work nearby the singularity, because there the quantum fluctuations of space and time become very large.

To be able to tell what is inside a black hole, we would need a theory of quantum gravity.

And we don't have one.

Most physicists believe that such a theory, if we had it, would replace the singularity with something else.

How do black holes form?

We presently know of four different ways that black holes may form.

The best understood one is stellar collapse.

A sufficiently large star will form a black hole after its nuclear fusion runs dry,

which happens when the star has fused everything that could be fused.

Now, when the pressure generated by the fusion stops, the matter starts falling towards its own gravitational center,

and thereby it becomes increasingly dense.

Eventually the matter is so dense that nothing can overcome the gravitational pull on the surface of the star.

That's when a black hole has been created.

These black holes are called solar mass black holes, and they are the most common ones.

The next common type of black holes are the supermassive black holes that can be found in the centers of many galaxies.

Supermassive black holes have masses about a billion times that of solar mass black holes, and sometimes even more.

Exactly how they form is still not entirely clear.

Many astrophysicists think that supermassive black holes start out as solar mass black holes, and, because they sit in densely populated galactic centers,

they swallow a lot of other stars and grow.

However, it seems that the black holes grow faster than this simple idea suggests

and exactly how they manage to do that is not well understood.

A more controversial idea are primordial black holes.

These are black holes that might have formed in the early universe by large density fluctuations in the plasma.

So, they would have been there all along.

Primordial black holes can in principle have any mass.

While this is possible, it is difficult to find a model that produces primordial black holes without producing too many of them,

which is in conflict with observation.

Finally, there is the very speculative idea that tiny black holes could form in particle colliders.

This can only happen if our universe has additional dimensions of space, insofar there has not been any observational evidence that this might be the case.

How do we know black holes exist?

We have a lot of observational evidence that speaks for very compact objects with large masses that do not emit light.

These objects reveal themselves by their gravitational pull.

They do this, for example, by influencing the motion of other stars or gas clouds around them, which we have observed.

We furthermore know that these objects do not have a surface.

We know this

because matter falling onto an object with a surface would cause

more emission of particles than matter falling through a horizon and then just vanishing.

And since most recently, we have the observation from the Event Horizon Telescope, world, which is an image of the black hole shadow.

This is basically an extreme gravitational lensing event.

All these observations are compatible with the explanation that they are caused by black holes, and no similarly good alternative explanation exists.

Why did Hawking once say that black holes don't exist?

Hawking was using a very strict mathematical definition of black holes, and one that is rather uncommon among physicists.

If the inside of the black hole horizon remains disconnected forever, we speak of an event horizon.

If the inside is only disconnected temporarily, we speak of an apparent horizon.

But since an apparent horizon could be present for a very long time, like billions of billions of years,

the two types of horizons cannot be told apart by observation.

Therefore, physicists normally refer to both cases as black holes.

The more mathematically inclined people, however, count only the first case with an eternal event horizon as black hole.

What Hawking meant is that black holes may not have an eternal event horizon, but only a temporary, apparent horizon.

This is not a controversial position to hold, and one that is shared by many people in the field, including me.

For all practical purposes though, the distinction that Hawking drew is irrelevant.

How can black holes emit radiation?

Black holes can emit radiation

because the dynamical space -time of the collapsing black hole changes the notion of what a particle is.

This is another example of the relativity in Einstein's theory.

Just like time passes differently for different observers depending on where they are and how they move.

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The notion of particles, too, depends on the observer and on where they are and how they move.

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Because of this, an observer who falls into a black hole thinks that he is falling in vacuum.

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But an observer far away from the black hole thinks that it is not vacuum but full of particles.

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And where do the particles come from?

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They come from the black hole.

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This radiation that black holes emit is called Hawking radiation, because Hawking was the first to derive that this should happen.

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The radiation has a temperature which is inversely proportional to the black hole's mass.

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So, the smaller the black hole, the hotter it is.

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For the stellar and supermassive black holes that we know of,

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the temperature is well below the temperature of the cosmic microwave background and cannot be observed.

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What is the information loss paradox?

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The information loss paradox is caused by the emission of Hawking radiation.

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This happens because Hawking radiation is purely thermal, which means it is random, except for having a specific temperature.

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In particular, the radiation does not contain any information about what formed the black hole.

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But while the black hole emits radiation, it loses mass and shrinks.

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So, eventually the black hole will be entirely gone,

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and all you are left with is this random radiation which depends only on the initial mass of the black hole.

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It does not at all depend on the details of the matter that formed the black hole or whatever fell in later.

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Therefore, if one only knows the final state of the evaporation, one cannot tell what formed the black hole.

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Such a process is called irreversible, and the trouble is that there are no such processes in quantum theory.

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Black hole evaporation radiation therefore is inconsistent with quantum theory as we know it, and something has to give.

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Somehow this inconsistency has to be removed.

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Most physicists believe that the solution is that the Hawking radiation somehow must contain information after all.

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So will a black hole come and eat us all up?

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It's not impossible, but very unlikely.

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Most Most stellar objects in galaxies orbit around the galactic center because of the way that galaxies form.

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It happens on occasion that two solar systems collide and a star

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or planet or black hole is kicked onto a strange orbit,

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leaves one solar system, and travels around until it gets caught up in the gravitational field of some other system.

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But the stellar objects in galaxies are generally far apart from each other,

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and we sit in an outer arm of a spiral galaxy where there isn't all that much going on.

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So, it's exceedingly improbable that a black hole would come by on just exactly the right path to cause us trouble.

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We would also know of this long in advance, because we would see the gravitational pull of the black hole acting on the outer planets.

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If you have any questions about black holes that this video did not answer, please leave me a comment.

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이 레슨의 어휘와 말하기 포인트

이 말하기 레슨은 영상 “10 things you should know about black holes”을(를) 바탕으로 합니다. 가장 자주 반복되는 단어는 다음과 같습니다: hole, horizon, radiation, mass, gravitational. 이 영상에는 섀도잉할 문장 31개와 단어 1878개가 있습니다. 말하는 구간의 길이는 12:13입니다. 화자는 분당 약 154단어로, 일상 대화에 가까운 자연스러운 속도로 말합니다. 영어에서 가장 많이 쓰이는 3,000단어에 속하는 단어가 80%뿐이라 어휘가 어려운 편입니다. 전체 31문장 중 11문장이 질문이어서 의문문 억양과 짧은 대답을 연습하기에 좋습니다.

이 영상의 핵심 어휘

영상에 나오는 익혀 둘 만한 단어 15개를 발음, 뜻과 함께 정리했습니다.

단어	발음	뜻
horizon 명사	/həˈɹaɪ.zən/	지평선, 수평선
radiation 명사	/ˌɹeɪ.diˈeɪ.ʃən/	방사, 폭사
tidal 형용사	/ˈtaɪ.d(ə)l/	간만
particle 명사	/ˈpɑɹtək(ə)l/	입자, 립자
galaxy 명사	/ˈɡæl.ək.si/	은하, 은하계
depend 동사	/dɪˈpɛnd/	의존하다, ...에 달려 있다
observation 명사	/ˌɑbzɚˈveɪʃn̩/	관찰
radius 명사	/ˈɹeɪ.di.əs/	노뼈, 요골
proportional 형용사		비례의, 비율의
explanation 명사	/ˌɛkspləˈneɪʃən/	설명
principle 명사	/ˈpɹɪn.sɪ.pəl/	원리, 원칙
controversial 형용사	/ˌkɑn.tɹəˈvɝ.ʃəl/	논쟁의 여지가있는
notion 명사	/ˈnoʊʃən/	개념
curve 명사	/kɚv/	곡선
eternal 형용사	/ɪˈtɝ.nl̩/	영원하다

이 영상의 문법

화자가 가장 많이 쓰는 문형을 영상 속 실제 표현과 함께 정리했습니다.

문형	영상 속 표현
수동태 be + 과거분사 — 누가 하는지보다 무슨 일이 일어나는지에 초점	have been misunderstood · is called · is known
현재완료 have/has + 과거분사 — 과거의 일이 지금도 관련이 있을 때	have been misunderstood · has compressed · have crossed
관계절 who / which + 절 — 사람이나 사물에 대한 추가 정보	world, which is · temperature which is · thermal, which means

주의할 발음

화자는 don't, can't, I'm 같은 축약형과 약화된 형태를 6번 사용합니다. 들리는 대로 짧게 발음하세요.

“th” 소리: mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, furthermore /ˈfɜː(ɹ).ðə(ɹ)ˌmɔː(ɹ)/, thermal /ˈθɜːməl/, thereby /ðɛɹˈbaɪ/
“sh”와 “zh” 소리: radiation /ˌɹeɪ.diˈeɪ.ʃən/, observation /ˌɑbzɚˈveɪʃn̩/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, notion /ˈnoʊʃən/
긴 단어 — 강세 위치에 주의: radiation /ˌɹeɪ.diˈeɪ.ʃən/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, temporary /ˈtɛmp(əˌɹ)ɛɹi/

한국어 화자가 어려워하는 소리:

/f/ — ㅍ(/p/)으로 바꾸지 말고 윗니를 아랫입술에 대기: fusion /ˈfjuː.ʒən/, fuse /fjuːz/, refer /ɹɪˈfɜː/, differently /ˈdɪf.ə.ɹənt.li/, furthermore /ˈfɜː(ɹ).ðə(ɹ)ˌmɔː(ɹ)/
/v/ — ㅂ(/b/)과 구별하기: observer /əbˈzɝvɚ/, observation /ˌɑbzɚˈveɪʃn̩/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, gravity /ˈɡɹævɪti/, curve /kɚv/
/z/ — ㅈ이 아니라 성대를 울리는 /s/: horizon /həˈɹaɪ.zən/, observer /əbˈzɝvɚ/, observation /ˌɑbzɚˈveɪʃn̩/, observe /əbˈzɜːv/, fuse /fjuːz/

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먼저 말하지 않고 영상을 끝까지 듣고 모르는 단어를 적어 둡니다.
0.75배속으로 한 문장씩 섀도잉을 시작하고, 익숙해지면 보통 속도로 돌아갑니다.
자신의 목소리를 녹음해 원본과 비교하고, horizon, radiation, tidal 같은 단어에 특히 주의합니다.

쉐도잉이란? 영어 실력을 빠르게 키우는 과학적 방법

쉐도잉(Shadowing)은 원래 전문 통역사 훈련을 위해 개발된 언어 학습 기법으로, 다언어 학자인 Dr. Alexander Arguelles에 의해 대중화된 방법입니다. 핵심 원리는 간단하지만 매우 강력합니다: 원어민의 영어를 들으면서 1~2초의 짧은 지연으로 즉시 소리 내어 따라 말하는 것——마치 '그림자(shadow)'처럼 화자를 따라가는 것입니다. 문법 공부나 수동적인 청취와 달리, 쉐도잉은 뇌와 입 근육이 동시에 실시간으로 영어를 처리하고 재현하도록 훈련합니다. 연구에 따르면 이 방법은 발음 정확도, 억양, 리듬, 연음, 청취력, 말하기 유창성을 크게 향상시킵니다. IELTS 스피킹 준비와 자연스러운 영어 소통을 원하는 분들에게 특히 효과적입니다.

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