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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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And don't forget to subscribe.

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Лексика и советы по произношению к этому уроку

Этот урок разговорной практики построен на видео «10 things you should know about black holes». Чаще всего повторяются слова: hole, horizon, radiation, mass, gravitational. В этом видео 31 предложений и 1878 слов для шедоуинга. Речь длится 12:13. Говорящий говорит в естественном темпе, около 154 слов в минуту, близком к повседневной речи. Только 80% слов входят в 3000 самых частых слов английского языка, поэтому лексика сложная. Вопросов здесь 11 из 31 предложений, так что это хорошая тренировка вопросительной интонации и коротких ответов.

Ключевая лексика этого видео

15 слов из видео, которые стоит выучить, с произношением и значением:

Слово	Произношение	Значение
horizon существительное	/həˈɹaɪ.zən/	горизо́нт, небоскло́н
radiation существительное	/ˌɹeɪ.diˈeɪ.ʃən/	излуче́ние
tidal прилагательное	/ˈtaɪ.d(ə)l/	приливный
particle существительное	/ˈpɑɹtək(ə)l/	части́ца, крупи́ца
galaxy существительное	/ˈɡæl.ək.si/	гала́ктика
depend глагол	/dɪˈpɛnd/	зави́сеть
observer существительное	/əbˈzɝvɚ/	наблюда́тель, наблюда́тельница
stellar прилагательное	/ˈstɛl.əɹ/	звёздный, звездообра́зный
observation существительное	/ˌɑbzɚˈveɪʃn̩/	наблюде́ние
quantum существительное	/ˈkwɑntəm/	коли́чество
apparent прилагательное	/əˈpæɹənt/	ви́димый
radius существительное	/ˈɹeɪ.di.əs/	лучева́я кость
proportional прилагательное		пропорциона́льный
explanation существительное	/ˌɛkspləˈneɪʃən/	объясне́ние, разъясне́ние
increasingly наречие		всё бо́льше и бо́льше, всё бо́лее и бо́лее

Фразовые глаголы, которые вы услышите

Слово	Значение
tell apart глагол	различа́ть, различи́ть

Грамматика в этом видео

Конструкции, которые говорящий использует чаще всего, с точными словами из видео:

Конструкция	В видео
Пассивный залог be + причастие прошедшего времени — важно, что происходит, а не кто это делает	have been misunderstood · is called · is known
Present Perfect have/has + причастие прошедшего времени — прошлое действие, важное сейчас	have been misunderstood · has compressed · have crossed
Придаточные определительные who / which + предложение — уточнение о человеке или предмете	world, which is · temperature which is · thermal, which means

Произношение, на которое стоит обратить внимание

Говорящий использует 6 сокращённых и редуцированных форм, например don't, can't, I'm. Произносите их коротко, так, как слышите.

Звуки «th»: mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, furthermore /ˈfɜː(ɹ).ðə(ɹ)ˌmɔː(ɹ)/, thermal /ˈθɜːməl/, thereby /ðɛɹˈbaɪ/
Звуки «sh» и «zh»: radiation /ˌɹeɪ.diˈeɪ.ʃən/, observation /ˌɑbzɚˈveɪʃn̩/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, notion /ˈnoʊʃən/
Длинные слова — следите за ударением: radiation /ˌɹeɪ.diˈeɪ.ʃən/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, temporary /ˈtɛmp(əˌɹ)ɛɹi/

Звуки, трудные для русскоязычных:

/w/ — губы округлены, это не /v/: quantum /ˈkwɑntəm/, consequence /ˈkɑn.sɪ.kwəns/, swallow /ˈswɑ.loʊ/, microwave /ˈmaɪkɹoʊˌweɪv/, equivalence /ɪˈkwɪv.ə.ləns/
Звонкие согласные в конце слова — не оглушайте их: depend /dɪˈpɛnd/, curve /kɚv/, observe /əbˈzɜːv/, fuse /fjuːz/, derive /dɪˈɹaɪv/

Как заниматься с этим видео

Прослушайте всё видео один раз молча и выпишите незнакомые слова.
Начните со скорости 0,75×, повторяйте предложение за предложением и вернитесь к обычной скорости, когда станет легко.
Запишите себя и сравните с оригиналом, обращая внимание на такие слова, как horizon, radiation, tidal.

Что такое техника Shadowing?

Shadowing — это научно обоснованная техника изучения языка, изначально разработанная для подготовки профессиональных переводчиков и популяризированная полиглотом доктором Александром Аргуэльесом. Метод прост, но эффективен: вы слушаете аудио на английском от носителей языка и немедленно повторяете вслух — как тень, следующая за говорящим с задержкой в 1–2 секунды. В отличие от пассивного прослушивания или грамматических упражнений, Shadowing заставляет мозг и мышцы рта одновременно обрабатывать и воспроизводить реальные речевые паттерны. Исследования показывают, что это значительно улучшает точность произношения, интонацию, ритм, связную речь, понимание на слух и беглость речи — что делает его одним из самых эффективных методов для подготовки к IELTS Speaking и реального общения на английском.

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