Les trous noirs — Science étonnante #19
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Les trous noirs — Science étonnante #19

August 15, 2019


Hello everyone, today I would like to talk about those strange and mysterious that we call black holes So first I would like to explain : What is it ? Where do we find them ? And then, what is the right attitude to have if we ever meet one ? The black holes may be very complex objects whose structure has not yet been elucidated, today I would like to explain to you that we can understand their existence thanks to a simple little calculation that only uses high school physics. So imagine that you are on the ground and throwing a tennis ball in the air. Well the ball will go up to a certain height and then it will go down. If you throw the ball harder, it will go higher, but presumably, it will eventually come down again. The question we can ask is how fast should we launch the
tennis ball so that it never falls back that is, it escapes
the earthly attraction. Well like in the video I had
done on the pole vault, one can get to answer that question
by doing a simple calculation of energy. When you throw a ball at a certain
speed, we give it an energy which is called kinetic energy and which is even greater than the
speed is high. This kinetic energy she’s going to be
spent to defeat the strength of gravity that holds the ball on the ground and to which we can also associate
an energy: the gravitational energy. The condition for the ball not to
never falls back and pulls off the terrestrial attraction it’s just that the kinetic energy is superior to gravitational energy. So obviously there are equations
to express all this: the kinetic energy is half a “m” “v” square where “v” is the speed of the ball and “m” its mass and the gravitational energy is small “m” times big “M” “G” on “R” where big “m” is the mass of the Earth, “R” is its radius and “G” is the universal gravitational constant. So if we take this equation and
that we fiddle with it a little, we can come out that the minimum speed that the
ball must own to snap out of
the terrestrial attraction it’s root of 2 “G” “M” on “R”. If you throw the ball under this
speed she will eventually fall back, if you launch it above this
speed she will go into space and not will never fall again. This minimum speed to possess for
tear off the earthly attraction, it’s called the release speed and you see that it depends on the
mass of the planet and its radius so we can have fun trying to
calculate its value for a few well known stars. For example for the earth this speed
of liberation, it is worth 11 km per second. It’s really a lot eh, you see that before
to throw a ball at that speed, Well, you’ll have to get up early. For the moon for example the speed of
release is weaker, it’s only 2.4 km per second. Well it’s not very surprising since we know that
gravitational attraction to the moon’s surface, it is weaker
than on the surface of the earth. If now we have fun doing the math
for the sun, we find a value much higher, huh, the speed of
release it is 617 km per second, it’s been over two million
km / h. Now imagine a planet that
would have the mass of the sun but which would be only 1 km radius. Well, why not after all, huh? If we have fun calculating the speed of
release to the surface of this planet there are 515,000
kilometers per second. You see what’s wrong? Yeah, it’s more than the speed of light. and we know since Einstein that nothing
can go faster than light. It means that if you are at the
surface of this hypothetical planet, you will never be able to escape. and what we have is one of the possible definitions of a black hole:
a black hole is a star whose release speed is bigger than the speed of the
light, so that nothing can escape. Fortunately for us
this situation is still very uncommon, eh, you see that the speed of release,
it is all the more important as the mass is important and that the radius
is small, so it corresponds to bodies that are extremely dense,
much denser than planets or normal stars. But we think that
this situation can still happen produce for some stars at the
end of their life. You must know that when a star has finished
to exhaust all the fuel nuclear power that allows him to shine, she begins to collapse on
herself and so she becomes more in more dense, and under the effect of the attraction
gravitational, it becomes denser, it gets denser and she can finish
dense enough to form a black hole. This is called stellar black holes. So you have to know that it’s not the only one
possible scenario for the end of life stars but we think it’s this
that happens for stars whose mass exceeds several times the mass of the sun and besides if the destiny of the stars
you are interested I recommend you to go watch the videos of the channel
“The sense of wonder” who already talked about death in particular
sun and supernovas. There is a point that is really very
important to understand for good perceive the evil side of the holes
black, is that there is no need to be at
the surface of the star to become trapped. It should be known
that concept of speed of liberation, it does not apply only
when you are on the surface but it applies also near the star. We can resume our calculation just now, with a planet that has a radius “R” and a mass “M”. Imagine that instead of being at the
surface, you are at a distance “D” from the center of the planet. Well, we can redo exactly the
same calculation as just now except that this time in the energy
gravitational that will have to overcome “R” will be replaced by “D” and you see that the speed of release
will become root of 2 “GM” on “D”. Earlier we saw that speed
of liberation on the surface of the earth it was 11 km per second. Well, if I’m 25.000 km
orbit above the surface it will only be about
5 km per second. We can see that the speed of
release when one is at a certain distance from the star, it will always be
lower than it is when we are on the surface. But we can imagine that if a star is dense enough and we are not very far even if we are not on the surface, the
release speed can be more great than the speed of light and so we could still
find trapped. So we can write a condition for
to know if we are going to be trapped or not. If we have a star of mass “M”, the speed of release she will be
greater than the speed of light as soon as you are at a distance
smaller than a critical value that is worth 2 “G” “M” on “c”². Basically, if you have a mass star
“M” and that you are at a distance less than this critical distance
from him, and you’re cooked. And in fact that’s true even if the
radius of the star is much smaller than how far you are
find. So you see that what counts in
this case is not so much the physical radius of the star, it is rather
this critical distance below which one is sure to be trapped. This critical distance is called the
Schwarzschild radius of the star. It’s a tribute to the mathematician who
the first solved the equations of general relativity in the case
of a spherical body. As soon as a star has a physical radius that
is smaller than the Schwarzschild radius, you have a black hole, and so if you
are at a distance less than this radius of Schwarzschild, even if you are not
on the star and ben you are trapped. This radius of Schwarzschild, it delimits
so a surface Called the horizon of the black hole. It’s an immaterial border that
mark the point of no return. So I insist that this
border it is immaterial because only at the horizon of the hole
black, there is nothing special. It’s just the limit of the zone in
below which you are cooked. So all that is fine but it’s still theory, especially since if nothing escapes from a black hole, how do we observe them? So stellar black holes,
those that result from the collapse of a star, it has never been observed with absolute certainty. We have a number of very good candidates who we do not see very well what it could be other than black holes, but there still remains a little doubt. On the other hand, there is another category of black holes, which I have not talked about yet, and for which we are much more confident. These are called galactic black holes. Galactic black holes are giant black holes which is thought to be in the center from just about every galaxy, and especially ours: the milky way. So if you want to contemplate
a black hole or more or less look in his direction you can the summer evenings turn you south and watch the trace of the milky way near the Sagittarius constellation. So it’s a constellation that is well known amateur astronomers because its main stars draw a kind of stylized teapot And slightly above the beak of the teapot is the direction of center of the Milky Way. And it’s there we find a gigantic black hole called Sagittarius A * (A-star) and which is believed to weigh about 4 million times the mass of the sun. So here you go tell me:
how are we sure there is a black hole at this place because we can not see it directly? So it’s true that we can not see directly the black hole itself, but there are 2 things that we can observe. The first is the matter that is around the black hole and that is falling towards the black hole without still have passed the point of no return. This material is very strongly accelerated and that makes him emit electromagnetic radiation we capture from this direction of the sky. The other thing you can see is the trajectory of stars that are very close to
the black hole and which turn around.
Recently there is astronomers who have analyzed
the trajectory of these stars and that allowed them to calculate
the mass of the black hole and estimate its size. Now that we have seen what a black hole is
and we know that they exist one wonders what is going on if we ever meet one. Well if you cross a black hole and you start falling to him, it’s not going to be very, very hectic in fact. You will fall in free fall but since there is no air, to make friction it’s going to be very, very different from a free fall when we fall from an airplane for example. In fact you will fall and have the feeling of being in total weightlessness. It will be rather nice …
There is just a problem in this story is that if you fall for example the feet in front, your feet are going to be slightly closer to the black hole than your head, and so they will undergo a gravitational pull who will be a little more important that suffered by your head And the problem is that by dint of falling, this little bit, it will become bigger and bigger and he will start to stretch you. So, this phenomenon of stretching is
what is called the tidal force So, it’s not at all specific to black holes and it is precisely he
who explains the phenomenon of tides on earth. When the moon is on one side of the planet it goes attract a little more water that is close to her that water that is far from it, and so she is going deform the surface of the oceans, and that’s fine cause high tides in some places and low tides in other places. And you when are you going to fall towards the black hole, well these tidal forces they go start to stretch you stretch you stretch until you spread. This is sometimes called spaghettification. Contrary to popular belief the fact of to be spaghetti has nothing to do with the fact to cross the horizon of the black hole. Spaghettification can take place well before or after the horizon. Besides, when you go through
the horizon of the black hole, nothing special will happen, you will not even realize it. On the other hand, if you have left a friend in orbit a little further, he’s going to see some funny stuff. When you fall to the black hole,
your friend he can see you fall
because there are rays bright that you emit
and that reach him. And the closer you get to the black hole more the light rays that you emit will take a long time to reach him. And so he will see you fall more and more slowly. And in fact from his point of view to him
your fall is going so much more and more to slow down that he will watch you fall until the end of time. In fact he will see you closer and closer to the horizon but he will never be able to see you cross it. That was for your friend who stayed away from the black hole and who is watching you. On the other hand, once you have crossed on the horizon it will start
to become a lot less cool. Well already you will fall faster and faster, the spaghettification forces
will be more and more important and presumably you will be torn apart. So let’s imagine that you manage to survive, you will discover
that the structure of space-time has completely changed. So it’s something
that’s pretty hard to describe without really look at the equations,
but there is still a way to understand it. You know for us on earth
we say that time goes by, it means that time passes and we can do absolutely nothing, we can not decide to stay at one time fixed time. On the other hand for the space
it is much simpler: if I want to stay
in a fixed position of space, nobody stops me. And well on the other side
of the black hole, it’s impossible to stay at one position fixed space. Space flows, a bit of the same way time passes for us It means that whatever you do, you inevitably fall towards
the center of the black hole. If you fell into the black hole
by being equipped of a jet pack or while aboard a rocket with a motor, you may be tempted to use it to try to slow down your fall a bit. But in fact it’s a bad idea, because we can show
that if you ever try to struggle like this, the time that separates you
from the center of the black hole will shorten. If you want the time
of your fall to the center the black hole takes
as much time as possible, in fact the best thing to do
is to do nothing. You have to drop in free fall. Once in the center of the black hole, we must admit that
we do not know very well what is going on. Then Einstein’s theory of general relativity tells us that at the center of
the black hole there is a point where the density and the curvature of space-time
become infinite, what is called a singularity. Except in practice, one suspects that in the vicinity of this singularity the theory of general relativity alone,
it no longer works. We must take into account
the effects of quantum mechanics, and to do that properly,
we need to have of a unifying theory quantum mechanics and general relativity. Today we have no theory like that but, I have already told you,
there are several attempts. There is one that is
the string theory on which I made a video and there is another one
that is loop quantum gravity which I will talk about soon. But it must be admitted
that at present none of these 2 theories does give an answer really very very satisfying to know what’s going on in the neighbourhood of the singularity, and at the moment the black holes have not yet delivered all their mysteries. Well, that’s all for today ! So as usual I wrote a little note
that goes with this video and who specifies a thing or 2. In particular, you must know
that the argument that I have given to justify the existence of black holes, the one based on the speed of release, in fact it is not correct. This is an argument called “with the hands”. But to really show that black holes exist, we have to do the equations of general relativity, we can not escape otherwise. If you want to understand
where is the flaw in this argument and understand a little bit
about why general relativity explains the existence of black holes, you can go read this post. Another post that I wrote
some time ago and which may interest you wonders if we can create black micro-holes in the LHC, you know, CERN’s particle accelerator. So a priori there is little chance but there is still some version of the theory ropes that predicts that perhaps at the LHC we could create black micro-holes. Thanks for watching this video! As usual if you liked it
do not hesitate to share it to help me make the channel known, you can subscribe,
you can find me on social networks, Facebook, Twitter. Those who wish can support me on Tipeee. Thank you very much to all the tipeeers who support me, and you can also find me
on my amazing Science blog. Thank you and see you soon !

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  1. J’adore tes vidéos, tu donnes les arguments scientifiques mais d’une manière très très simple, chapeau!

  2. Très bonne vidéo. L'utilisation de la vitesse de libération de la lumière est en effet très utilisée pour décrire en première approche un trou noir en utilisant la physique classique.
    Mais, cette propriété classique ne peut être utilisée pour les trous noirs. D'ailleurs, vous l'indiquez rapidement à la fin.
    Par exemple, je peux très bien quitter la surface de la Terre temporairement en sautant sans pour autant atteindre la vitesse de libération de la Terre de 11.2 km par seconde. Autre exemple, les fusées qui envoient des satellites dans le système solaire arrive à le faire sans pour autant attendre la vitesse de libération (une accélération régulière est nécessaire néanmoins);

    Hors, pour un trou noir, la lumière ne peut quitter l'horizon des événements (même temporairement) au niveau même de l'horizon des événements et non pas à une distance finie ou infinie du trou noir (vitesse de libération de physique classique = c).

    La loi de Newton ne permet pas de décrire un horizon des événements. Il faudrait pour cela que la force de gravité soit infinie au niveau de l'horizon du trou noir pour que rien ne puisse sortir de l'horizon (hors ce n'est pas le cas). En utilisant Newton, le seul moyen serait de ce situer sur la singularité au centre du concept trou noir et non pas au niveau de son horizon.
    Les trous noirs sont des objets uniquement décrits par la relativité générale (actuellement). C'est dommage que ce point ne soit pas souvent mentionné avec un peu de détail car il me semble accessible.
    Merci pour toutes ces vidéos de qualité.

  3. Si on serait capable d'observer un trou noir avec une excellente résolution est-ce qu'on pourrait voir tout ce qu'il y a absorbé durant toute son existence ? 9:53

  4. Même pas 30 sec après l’explication de la force de libération via la terre est expliquée …… x1000/sec sois décevant/sec j’aime cette chaume aussi c’est flou

  5. Jolie vidéo sympa. Imaginer le matériau dont est fait le cœur d'un trou noir ce doit être d'une solidité à tout épreuve, dommage que l'ont puissent y accéder. La matière la plus dense de l'univers inconnu sur terre.

  6. Puisque mes connaissances limitées ne me permet pas de comprendre un truc qui m'agace … de ce que j'ai compris . La lumieres c'est des photons , les photons n'ont pas de masse ce qui permet leur vitesse .. Si les photons n'ont pas de masse , comment la gravité les affectes ?? .. comment le trou noirs les capture ??

  7. La singularité de l' avant "MUR de PLANCK" produit une vitesse bien supérieure à la vitesse de la lumière

  8. Bonjour je n'ai pas compris pourquoi il y a une zone critique. Sur terre pour lancer ariane 5 on la met pas a 11km/s faut juste que son accélération depasse l'accélération terrestre. Alors pourquoi on ne pourrait pas faire pareil avec les trou noirs ?

  9. Alors que la plupart des primates sextaxient devant " d'hypothétiques " trous noirs…la plupart des races et espèces animales et végétales ont disparues sur terre…avec en guise des espèces hybrides destructrices de notre équilibre…la régression de l'humanité s'amplifie inéluctablement !

  10. rien ne prouve que la vitesse de la lumière ne peut être dépassé puisque rien et personne n'est capable le prouver. Einstein s'étant trompé avec sa théorie lié à la gravité alors qu'il existe qu'autres forces qu'il a ignoré. Comme le disait Etienne Klein, tous les physiciens échaffaudent des théories qui, comme son nom l'indique peuvent être faussent. Mais ils en sont tellement persuadés qu'ils affirment ce qu'ils ont compris sans être sûr d'avoir tous les paramètres en mains. Ceci dit, la vulgarisation scientifique est quand même intéressante pour comprendre globalement les choses dont on a jamais eu l'idée de l'existance.

  11. Ma question : si je franchis l'horizon des évènements du trou noir cela veut dire que la lumière que mon corps émet ne peut aller seulement que dans une seule direction : vers la singularité. Donc comment un observateur extérieur peut voir mon corps jusqu'a la fin des temps car si pour moi le temps s'arrête ou presque ce n'est pas le cas de l'observateur extérieur…?

  12. 11:50, Alors pourquoi est-ce qu'au lieu de "voir" des trou noirs.. On voit pas pleins de petit points qui sont (de notre point de vue) en train de chuter.. puis s'immobiliser?
    >
    (donc un trou noir… rempli (wtf)?)

  13. Cette vitesse de libération ne concerne apparemment que les vols balistiques. Il me semble qu'un vol propulsé pourrait quitter n'importe quel corps céleste à n'importe quelle vitesse, non ?

  14. Les étoiles, une fois tout le carburant épuisé, s’effondrent et deviennent de plus en plus denses. Pourquoi les planètes n'en font elles pas autant ?

  15. Suis vraiment obstiné ! Je veux comprendre. Energie gravitationelle, orbite, vitesse de liberation , OK Mais le trou noir.
    Non, ce n est pas un "aspirateur."…
    Elle est ou cette masse ? Dans la singularité. ? Jusque la OK. Mais apres ….
    Ai regardé les trous de ver,…. Je veux comprendre. Lavoisier s'est trompé ?

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