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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.
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That was 30 years ago.
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I'm not afraid of black holes anymore, but I am afraid that they have been misunderstood.
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So here are 10 things that you should know about black holes.
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First things first, what is a black hole?
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A black hole contains a region from which nothing can ever escape,
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because to escape you would have to move faster than the speed of light, which you can't.
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The boundary of the region from which you cannot escape is called the horizon.
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In the simplest case, the horizon has the form of a sphere.
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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.
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How large are black holes?
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The diameter of a black hole is directly proportional to the mass of the black hole.
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So the more mass falls into the black hole, the larger the black hole becomes.
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Compared to other stellar objects though, black holes are tiny because enormous gravitational pressure has compressed their mass into a very small volume.
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For example, the radius of a black hole with the mass of planet
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Earth is only a few millimeters.
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What happens at the horizon?
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A black hole horizon does not have substance.
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Therefore, someone crossing the black hole horizon does not notice anything weird going on in their immediate surroundings.
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This follows from Einstein's equivalence principle, which implies that in your immediate surrounding you cannot tell the difference between acceleration in flat space
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and curved space that gives rise to gravity.
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However, an observer far away from a black hole who watches somebody fall in would notice
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that the infalling person seems to move slower and slower the closer they get to the horizon.
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It appears this way because time close by the black hole horizon runs much slower than far away from the horizon.
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That's one of these odd consequences of the relativity of time that Einstein discovered.
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So, if you fall into a black hole, it only takes a finite amount of time to cross the horizon,
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but from the outside it looks like it takes forever.
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What you would experience at the horizon depends on the tidal force of the gravitational field.
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The tidal force is, loosely speaking, the change of the gravitational force.
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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.
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The tidal force at the horizon is inversely proportional to the square of the mass of the black hole.
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This means the larger and more mass of the black hole, the smaller the tidal force at the horizon.
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Yes, you heard that right.
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The larger the black hole, the smaller the tidal force at the horizon.
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Therefore, if the black hole is only massive enough, you can cross the horizon without noticing what just happened.
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And once you have crossed the horizon, there is no turning back.
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The stretching from the tidal force will become increasingly unpleasant as you approach the center of the black hole.
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Eventually it will rip everything apart.
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In the early days of general relativity, many physicists believed that there is a singularity at the horizon,
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but this turned out to be a mathematical mistake.
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What is inside a black hole?
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Nobody really knows.
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General relativity predicts that inside the black hole is a singularity, that's a place where the tidal forces become infinitely large.
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But we know that general relativity does not work nearby the singularity, because there the quantum fluctuations of space and time become very large.
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To be able to tell what is inside a black hole, we would need a theory of quantum gravity.
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And we don't have one.
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Most physicists believe that such a theory, if we had it, would replace the singularity with something else.
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How do black holes form?
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We presently know of four different ways that black holes may form.
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The best understood one is stellar collapse.
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A sufficiently large star will form a black hole after its nuclear fusion runs dry,
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which happens when the star has fused everything that could be fused.
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Now, when the pressure generated by the fusion stops, the matter starts falling towards its own gravitational center,
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and thereby it becomes increasingly dense.
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Eventually the matter is so dense that nothing can overcome the gravitational pull on the surface of the star.
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That's when a black hole has been created.
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These black holes are called solar mass black holes, and they are the most common ones.
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The next common type of black holes are the supermassive black holes that can be found in the centers of many galaxies.
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Supermassive black holes have masses about a billion times that of solar mass black holes, and sometimes even more.
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Exactly how they form is still not entirely clear.
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Many astrophysicists think that supermassive black holes start out as solar mass black holes, and, because they sit in densely populated galactic centers,
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they swallow a lot of other stars and grow.
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However, it seems that the black holes grow faster than this simple idea suggests
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and exactly how they manage to do that is not well understood.
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A more controversial idea are primordial black holes.
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These are black holes that might have formed in the early universe by large density fluctuations in the plasma.
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So, they would have been there all along.
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Primordial black holes can in principle have any mass.
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While this is possible, it is difficult to find a model that produces primordial black holes without producing too many of them,
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which is in conflict with observation.
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Finally, there is the very speculative idea that tiny black holes could form in particle colliders.
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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.
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How do we know black holes exist?
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We have a lot of observational evidence that speaks for very compact objects with large masses that do not emit light.
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These objects reveal themselves by their gravitational pull.
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They do this, for example, by influencing the motion of other stars or gas clouds around them, which we have observed.
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We furthermore know that these objects do not have a surface.
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We know this
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because matter falling onto an object with a surface would cause
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more emission of particles than matter falling through a horizon and then just vanishing.
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And since most recently, we have the observation from the Event Horizon Telescope, world, which is an image of the black hole shadow.
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This is basically an extreme gravitational lensing event.
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All these observations are compatible with the explanation that they are caused by black holes, and no similarly good alternative explanation exists.
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Why did Hawking once say that black holes don't exist?
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Hawking was using a very strict mathematical definition of black holes, and one that is rather uncommon among physicists.
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If the inside of the black hole horizon remains disconnected forever, we speak of an event horizon.
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If the inside is only disconnected temporarily, we speak of an apparent horizon.
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But since an apparent horizon could be present for a very long time, like billions of billions of years,
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the two types of horizons cannot be told apart by observation.
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Therefore, physicists normally refer to both cases as black holes.
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The more mathematically inclined people, however, count only the first case with an eternal event horizon as black hole.
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What Hawking meant is that black holes may not have an eternal event horizon, but only a temporary, apparent horizon.
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This is not a controversial position to hold, and one that is shared by many people in the field, including me.
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For all practical purposes though, the distinction that Hawking drew is irrelevant.
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How can black holes emit radiation?
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Black holes can emit radiation
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because the dynamical space -time of the collapsing black hole changes the notion of what a particle is.
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This is another example of the relativity in Einstein's theory.
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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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Từ vựng và ghi chú luyện nói cho bài học này
Bài luyện nói này dựa trên video “10 things you should know about black holes”. Những từ được nhắc lại nhiều nhất trong bài: hole, horizon, radiation, mass, gravitational. Video này có 31 câu và 1878 từ để luyện shadowing. Phần lời nói dài 12:13. Người nói nói với tốc độ tự nhiên, khoảng 154 từ mỗi phút, gần với hội thoại hằng ngày. Chỉ 80% số từ nằm trong 3.000 từ tiếng Anh thông dụng nhất, nên từ vựng khá khó. 11 trong 31 câu là câu hỏi, nên bài này hợp để luyện ngữ điệu câu hỏi và cách trả lời ngắn.
Từ vựng quan trọng trong video
15 từ đáng học trong video, kèm phiên âm và nghĩa:
| Từ | Phiên âm | Nghĩa |
|---|---|---|
| horizon danh từ | /həˈɹaɪ.zən/ | chân trời |
| radiation danh từ | /ˌɹeɪ.diˈeɪ.ʃən/ | phóng xạ, bức xạ |
| particle danh từ | /ˈpɑɹtək(ə)l/ | hạt |
| galaxy danh từ | /ˈɡæl.ək.si/ | thiên hà |
| depend động từ | /dɪˈpɛnd/ | phụ thuộc |
| observation danh từ | /ˌɑbzɚˈveɪʃn̩/ | sự quan sát |
| quantum danh từ | /ˈkwɑntəm/ | số lượng, lượng |
| radius danh từ | /ˈɹeɪ.di.əs/ | xương quay |
| principle danh từ | /ˈpɹɪn.sɪ.pəl/ | nguyên lí, nguyên tắc |
| collapse động từ | /kəˈlæps/ | đổ, sập |
| notion danh từ | /ˈnoʊʃən/ | khái niệm |
| curve danh từ | /kɚv/ | khúc quanh |
| vacuum danh từ | /ˈvæ.kjuːm/ | chân không |
| observe động từ | /əbˈzɜːv/ | quan sát |
| orbit danh từ | /ˈɔɹ.bɪt/ | quỹ đạo |
Ngữ pháp trong video
Những cấu trúc người nói dùng nhiều nhất, kèm đúng cụm từ trong video:
| Cấu trúc | Trong video |
|---|---|
| Câu bị động be + quá khứ phân từ — nhấn vào việc xảy ra, không phải người làm | have been misunderstood · is called · is known |
| Thì hiện tại hoàn thành have/has + quá khứ phân từ — việc đã xảy ra nhưng còn liên quan đến hiện tại | have been misunderstood · has compressed · have crossed |
| Mệnh đề quan hệ who / which + mệnh đề — thêm thông tin về người hoặc vật | world, which is · temperature which is · thermal, which means |
Phát âm cần chú ý
Người nói dùng 6 dạng rút gọn, ví dụ don't, can't, I'm. Hãy nói theo dạng ngắn đúng như bạn nghe.
- Âm “th”: mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, furthermore /ˈfɜː(ɹ).ðə(ɹ)ˌmɔː(ɹ)/, thermal /ˈθɜːməl/, thereby /ðɛɹˈbaɪ/
- Âm “sh” và “zh”: radiation /ˌɹeɪ.diˈeɪ.ʃən/, observation /ˌɑbzɚˈveɪʃn̩/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, notion /ˈnoʊʃən/
- Từ dài — đặt trọng âm cho đúng: radiation /ˌɹeɪ.diˈeɪ.ʃən/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, temporary /ˈtɛmp(əˌɹ)ɛɹi/
Những âm người Việt thường gặp khó:
- Cụm phụ âm cuối — đọc đủ từng âm, đừng nuốt âm cuối: depend /dɪˈpɛnd/, apparent /əˈpæɹənt/, collapse /kəˈlæps/, dense /ˈdɛns/, paradox /ˈpæ.ɹəˌdɒks/
- Âm /tʃ/, /dʒ/ và /ʒ/ — tiếng Việt không có, đừng đọc thành “ch” hay “gi”: fusion /ˈfjuː.ʒən/, occasion /əˈkeɪ̯ʒən/, stretch /stɹɛt͡ʃ/, generate /ˈd͡ʒɛn.ə.ɹeɪt/, fluctuation /flʌkt͡ʃuːˈeɪʃən/
- Âm /æ/ — mở miệng rộng hơn âm “e”: galaxy /ˈɡæl.ək.si/, apparent /əˈpæɹənt/, gravity /ˈɡɹævɪti/, collapse /kəˈlæps/, vacuum /ˈvæ.kjuːm/
Cách luyện với video này
- Nghe hết video một lần, chưa cần nói, và ghi lại những từ bạn chưa biết.
- Bắt đầu ở tốc độ 0,75×, nói đuổi từng câu, rồi quay lại tốc độ bình thường khi đã quen.
- Ghi âm giọng mình rồi so với bản gốc, chú ý các từ như horizon, radiation, particle.
Phương Pháp Shadowing Là Gì?
Shadowing là kỹ thuật học ngôn ngữ có cơ sở khoa học, ban đầu được phát triển cho chương trình đào tạo phiên dịch viên chuyên nghiệp và được phổ biến rộng rãi bởi nhà đa ngôn ngữ học Dr. Alexander Arguelles. Nguyên lý cốt lõi đơn giản nhưng cực kỳ hiệu quả: bạn nghe tiếng Anh của người bản xứ và lặp lại to ngay lập tức — như một "cái bóng" (shadow) đuổi theo người nói với độ trễ chỉ 1–2 giây. Khác với luyện ngữ pháp hay học từ vựng bị động, Shadowing buộc não bộ và cơ miệng phải đồng thời xử lý và tái tạo ngôn ngữ thực tế. Các nghiên cứu khoa học xác nhận phương pháp này cải thiện đáng kể phát âm, ngữ điệu, nhịp điệu, nối âm, kỹ năng nghe và độ lưu loát khi nói — đặc biệt hiệu quả cho người luyện IELTS Speaking và muốn giao tiếp tiếng Anh tự nhiên như người bản ngữ.