0:07 [Music] welcome back to science clique today what would we see if we fell into a black hole a black hole is a spherical region of the universe a sort of space-time bubble which has an intense gravity this bubble attracts objects as if it were pulling the fabric of space not unlike a star or a planet the surface of the bubble is called the horizon of the black hole it divides the universe into two regions the exterior and the interior below the horizon the fabric of the universe is pulled so quickly that nothing not even light can escape
0:55 today we know that our universe contains many black holes they are very distant and represent no danger to us nonetheless we can wander out of curiosity what we might see if we were to fall into one of them answering this will allow us to improve our understanding of the extreme phenomena of our universe and to rectify a few common misconceptions on the subject our galaxy the milky way hosts in its core a supermassive black hole called sagittarius a star using a spaceship we approach it at a reasonable distance and we jump into the void of space here begins our fall
1:40 it will last about one hour and will be fatal to us the spaceship remains stationed orbiting the black hole and witnessing our fall [Music] before we even start falling we run into a problem the black hole is supermassive its gravity equates to a mass millions of times greater than the sun with such an attraction it captures stars and surrounding mata forming a thick disk of plasma that orbits around it [Music] this disc spins incredibly fast almost at light speed and loses energy by friction causing turbulences that heat
2:26 it up it thus produces intense radiations light ultraviolets x-rays and gamma rays which blind us instantly and risk burning us alive [Music] the plasma itself which can reach several billion degrees might also well burn us alive in what follows we will therefore assume that we are equipped with an extremely resistant spacesuit which allows us to withstand the heat and filter out any harmful radiation [Music] however despite the spacesuit the disc still seems extremely bright we can't distinguish anything we will therefore imagine that our helmet has a visor which helps reduce the brightness of the disc so that we can observe it in detail as well as see
3:13 the stars in the background [Music] at first we notice that the disc is blue within the whole electromagnetic spectrum produced our visor filters everything but visible light the light our eyes can see in this spectrum the disc emits more blue because this colour is more energetic it is the same effect that gives very hot stars a bluish tint strangely one side of the disk looks brighter than the other to understand the matter orbits very quickly such that on one side it approaches us at high speed while on the other side it moves away on one side the light waves get stacked
4:01 together we receive them with greater frequency while on the other side the light gets stretched we receive it with less energy this is called the doppler effect it's the same phenomenon that occurs with sound waves when a car crosses the street looking at the black hole we see that the back of the disc appears warped as if it were folded around the horizon forming a ring of light this is due to the strong gravity of the black hole which deflects light rays acting like a gravitational lens looking in one direction we observe objects that are actually located elsewhere [Music]
4:49 finally the stars in the background seem ever so slightly blue we do not feel it but the black hole distorts time such that our clock turns slightly slower than that of distant stars compared to our clock the light from the stars seems accelerated to the extent that we see it bluer than its original colour [Music] surprisingly during the first 10 minutes of the fall the black hole appears to be moving away we are moving closer and closer to it but its image seems to shrink more and more
5:38 imagine throwing a bull through a car the ball is thrown at 90 degrees and crosses the car side to side the passenger sees the ball travel across the car at 90 degrees but imagine now that the car is moving this time the passenger sees the ball coming from the front with a narrow angle because the car is moving objects reaching it seem to come from a different direction more from the front [Music] in our case the situation is analogous only with light rays instead of a ball we receive the light of the disc from a certain direction but because we are
6:24 moving towards the black hole the rays seem to reach us from a different direction squashed towards the front this is called aberration [Music] after these first ten minutes we are now falling very quickly at four percent the speed of light and from this point onwards the black hole starts to grow in our field of view [Music] as minutes go by we gradually descend towards the black hole our speed increases and we receive the light in front of us with an increasingly high intensity while the light behind us seems to get darker [Music] this again is the doppler effect at play
7:11 when falling towards the black hole we catch up with the light in front of us while the light behind us has more difficulty reaching us [Music] if we turned around we would notice that the clock inside the spaceship seems to be running slower than before the effect is subtle but still noticeable this is also due to the doppler effect if we imagine that the clock emits a signal each second these signals take longer and longer to reach us and we thus observe the ticking in slow motion [Music] minutes pass and we gradually fly over the disk of
7:56 matter suddenly after 57 minutes of falling the disc seems to fade out we have just reached the point beyond which mata can no longer orbit the black hole steadily the gravity is so intense that at this distance the plasma begins to spiral very quickly into the black hole things will now get much faster [Music] just two minutes later we are twice as close to the horizon from now on any light coming from outside is destined to fall into the black hole this is called the photon sphere
8:44 at this precise distance it's even possible for light to orbit the black hole [Music] as we approach more and more the passengers who remained in the spaceship see our image fade and gradually slow down until freezing on the horizon the light that we emit takes more and more time to escape the black hole it is received increasingly slowly and gradually fades away the passengers will never see us cross the horizon but for us just 24 seconds after crossing the photon sphere it is time to reach the horizon after this moment it will be impossible
9:31 for us to go back but what do we see when crossing the horizon surprisingly enough nothing special it would in fact be very difficult to determine when exactly we crossed the horizon [Music] we still receive the light of distant stars which falls with us [Music] we can still see the spaceship in orbit in which the clock even if it seems slowed down is still ticking and we can still perfectly see our body and in particular our feet because even if all light is carried inwards it can still move upwards relative to us and thus reach our eyes
10:19 this is actually a fundamental principle on a global scale space time is curved by the black hole and generates this horizon inside which objects are disconnected from the outside but at the local scale space time is almost flat and the curvature is not noticeable which means that everything behaves normally [Music] furthermore as opposed to what one might think the image of the black hole does not appear to encompass us at all even if we are now inside the black hole its image only takes up 15 percent of our field of view the rest is still filled with stars this is again due to the phenomenon of
11:05 aberration now that we move extremely fast the light that arrives from the sides and even from behind seems to come from the front our field of view is contracted ahead of us and widened behind us in particular the spaceship behind us seems to grow larger even though we move away from it we have crossed the horizon and nothing special has happened however from now on if we try to send a signal to the outside it will never reach its destination below the horizon everything is destined to fall downwards with no hope of ever coming out
11:50 but for the moment everything is fine it is after another 30 seconds just before reaching the center of the black hole that in the blink of an eye we feel an incredibly powerful force that stretches us until dislocating our body this is called spaghettification our head being further from the center than our feet the difference in pull is such that our body is very quickly torn apart [Music] in our very last moments we have the impression of falling onto a flat surface as if we were reaching the surface of an entirely black planet while a circle of light intensifies all around us a tenth of a second later the shredded
12:37 remains of our body reached the center of the black hole our journey ends here for the moment modern physics cannot describe what happens this close to the center of a black hole our current theories describe singularities points where the curvature of space-time becomes infinite but scientists suspect that these singularities do not really exist we would have to unify gravity with quantum physics if we wish to understand these extreme regions of the cosmos for now some hypotheses even question the fact that one could survive crossing the horizon moreover in reality black holes can have an angular momentum which generates
13:22 strange effects with multiple horizons and mathematical instabilities whose consequences we do not yet understand that said our current theory still give us a good idea of what we would see in particular we can correct a few widespread misconceptions for example the disc around the black hole is not orange but blue if some images show it as orange this is because they use false colours which are only intended to represent luminosity but not the real colour in visible light another idea that is widespread is that we would observe the whole history of the universe unfold before our eyes
14:07 while our field of view would get engulfed by the black hole this is also wrong as it forgets to take into account the doppler effect and the aberration of light there are however many mysteries left to solve and we will have to wait until our theories are perfected to understand what truly happens at the heart of a real black hole [Music] you
