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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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Vocabolario e note di pronuncia per questa lezione
Questa lezione di conversazione si basa sul video “10 things you should know about black holes”. Le parole che ritornano più spesso: hole, horizon, radiation, mass, gravitational. Questo video contiene 31 frasi e 1878 parole da ripetere con lo shadowing. Il parlato dura 12:13. Chi parla ha un ritmo naturale di circa 154 parole al minuto, vicino a una conversazione quotidiana. Solo il 80% delle parole rientra nelle 3.000 più comuni dell’inglese, quindi il lessico è impegnativo. 11 delle sue 31 frasi sono domande: un buon esercizio per l’intonazione interrogativa e le risposte brevi.
Vocaboli chiave di questo video
15 parole del video che vale la pena imparare, con pronuncia e significato:
| Parola | Pronuncia | Significato |
|---|---|---|
| horizon sostantivo | /həˈɹaɪ.zən/ | orizzonte |
| radiation sostantivo | /ˌɹeɪ.diˈeɪ.ʃən/ | radiazione |
| tidal aggettivo | /ˈtaɪ.d(ə)l/ | di marea, mareale |
| particle sostantivo | /ˈpɑɹtək(ə)l/ | granello, pezzetto |
| galaxy sostantivo | /ˈɡæl.ək.si/ | galassia |
| depend verbo | /dɪˈpɛnd/ | dipendere |
| observer sostantivo | /əbˈzɝvɚ/ | osservatore |
| stellar aggettivo | /ˈstɛl.əɹ/ | stellare |
| observation sostantivo | /ˌɑbzɚˈveɪʃn̩/ | osservazione |
| apparent aggettivo | /əˈpæɹənt/ | apparente, visibile |
| radius sostantivo | /ˈɹeɪ.di.əs/ | radio |
| proportional aggettivo | proporzionale | |
| explanation sostantivo | /ˌɛkspləˈneɪʃən/ | spiegazione, esplicazione |
| increasingly avverbio | sempre più, man a mano | |
| principle sostantivo | /ˈpɹɪn.sɪ.pəl/ | principio |
I phrasal verb che sentirai
| Parola | Significato |
|---|---|
| fall through verbo | andare a monte |
| tell apart verbo | differenziare, distinguere |
La grammatica di questo video
Le strutture che chi parla usa di più, con le parole esatte del video:
| Struttura | Nel video |
|---|---|
| Forma passiva be + participio passato — conta ciò che accade, non chi lo fa | have been misunderstood · is called · is known |
| Present perfect have/has + participio passato — un’azione passata che conta ancora adesso | have been misunderstood · has compressed · have crossed |
| Frasi relative who / which + frase — un’informazione in più su una persona o una cosa | world, which is · temperature which is · thermal, which means |
Pronuncia a cui fare attenzione
Chi parla usa 6 contrazioni e forme ridotte, come don't, can't, I'm. Pronunciale nella forma breve, così come le senti.
- I suoni “th”: mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, furthermore /ˈfɜː(ɹ).ðə(ɹ)ˌmɔː(ɹ)/, thermal /ˈθɜːməl/, thereby /ðɛɹˈbaɪ/
- I suoni “sh” e “zh”: radiation /ˌɹeɪ.diˈeɪ.ʃən/, observation /ˌɑbzɚˈveɪʃn̩/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, notion /ˈnoʊʃən/
- Parole lunghe — attenzione all’accento: radiation /ˌɹeɪ.diˈeɪ.ʃən/, explanation /ˌɛkspləˈneɪʃən/, controversial /ˌkɑn.tɹəˈvɝ.ʃəl/, mathematical /ˌmæθ(.ə)ˈmæt.ɪ.kəl/, temporary /ˈtɛmp(əˌɹ)ɛɹi/
I suoni difficili per chi parla italiano:
- Consonante finale — senza aggiungere una vocale dopo: depend /dɪˈpɛnd/, apparent /əˈpæɹənt/, radius /ˈɹeɪ.di.əs/, collapse /kəˈlæps/, curve /kɚv/
- /æ/ — più aperta della “e”: galaxy /ˈɡæl.ək.si/, apparent /əˈpæɹənt/, gravity /ˈɡɹævɪti/, collapse /kəˈlæps/, vacuum /ˈvæ.kjuːm/
Come esercitarsi con questo video
- Ascolta tutto il video una volta senza parlare e annota le parole che non conosci.
- Inizia a velocità 0,75×, fai shadowing frase per frase e torna alla velocità normale quando diventa facile.
- Registrati e confronta con l’originale, facendo attenzione a parole come horizon, radiation, tidal.
Cos'è la tecnica dello Shadowing?
Shadowing è una tecnica di apprendimento delle lingue supportata da studi scientifici, originariamente sviluppata per la formazione dei traduttori professionisti e resa popolare dal poliglotta Dr. Alexander Arguelles. Il metodo è semplice ma potente: ascolti un audio in inglese di madrelingua e lo ripeti immediatamente ad alta voce — come un'ombra che segue il parlante con un ritardo di solo 1–2 secondi. A differenza dell'ascolto passivo o degli esercizi di grammatica, lo shadowing costringe il tuo cervello e i muscoli della bocca a elaborare e riprodurre simultaneamente i modelli di discorso reale. La ricerca dimostra che migliora significativamente la precisione della pronuncia, l'intonazione, il ritmo, il discorso connesso, la comprensione dell'ascolto e la fluidità del parlato — rendendolo uno dei metodi più efficaci per la preparazione alla prova di speaking dell'IELTS e per la comunicazione reale in inglese.