TAC_black-hole-orbits

Ajouté : 2026-05-19

[00:00:00][Music] so thanks for coming everyone to this presentation and to the teen astronomy cafe i hope everyone will have a good time so my name is pierre christian and i study black holes i'm an astronomer and what we are going to learn about today is what exactly are black holes and what do we do like how how do we use black holes to study our theory of gravity okay so the first thing i want to talk about is myself okay so uh a little bit about myself so i did my undergraduate degree my i went to college at berkeley and then in for graduate school for my doctorate i went to harvard and then after that i spent a couple years at the university of arizona working with a team of scientists there and i just moved recently to connecticut so but before i was in the united states uh going to college i was actually in indonesia that's where i grew up that's where i was born and it's a country in the equator in the pacific ocean and it's very you know i live in a big city in which the sky looks like this okay so i've actually didn't see stars until i was quite you know i was quite old like this is all i see and then you might be you know you might be it's it might like you know you might be questioning like uh so why are you an astronomer how did you become an astronomer if you never even see stars when you were growing up so i i chose to become an astronomer an astrophysicist so an astrophysicist is just someone who applies physics to astronomy uh and i did that because i realized that the most interesting things are in space you know there are a lot of cool things on earth but you know space is much bigger there are way more interesting things in space so here are some examples of my favorite things uh there are exoplanets in space exoplanets are uh planets in outer space and some of them could be habitable so they are like alien worlds outside of the soil system so that's pretty cool uh there's also galaxies i really like galaxy so what a galaxy is it's a conglomeration of many many many stars i could be billions of stars and they look really you know they look really nice in space and they are actually really interesting objects so that's also like another thing that's really interesting uh that's in space that you can study if you work in you know if you end up you know like studying astronomy or physics in college but what really caught my attention are these things called black holes so black holes are this really weird exotic object and they are at the same time really simple and we know everything about them and so complex that we don't know anything about them so these these are the the objects that i spend my time studying and black holes are also like you know like uh very often misrepresented and so like in this cafe i would like to tell you what black holes actually are and how they actually work okay and one caveat this is not a picture of a black hole this is just an artist condition okay just some person went and draw this uh but we do actually have a picture of a black hole that's in space like we took a photograph of it and i'm going to show you to you in a little bit okay so uh what are black holes what exactly are black holes so first off in order to understand black holes we need to understand uh what is gravity because black holes are gravitational objects so there is this theory of gravity that's uh discovered by this guy this dude albert einstein pretty smart guy uh and that theory of gravity is what's called general relativity okay don't worry about why it's called general relativity it's just like you know it's just a name that he came up with one day it's just a theory of gravity okay a theory of and you might you probably have learned before about gravity in your classes uh you probably learn what's what's called newtonian gravity which is like a different theory of gravity an older one the stereo gravity this general relativity is kind of the most uh the most accurate theory of gravity that we know until now so until now we haven't been able to supplant einstein's theory one thing well basically the only thing you need to know about general relativity to understand black holes uh in this stuff is that even light you know light beams even light is affected by gravity so not just a ball or not just you know like not just like human beings or animals uh even light is affected by gravity even light is attracted you know if it falls down to the earth after some time uh so here's the here's the question uh if i throw a ball from the surface of planet right it will fall back down right if i throw a ball i just throw a ball i'm just standing on the surface of this planet i fall i throw a ball up and it will keep going up until a certain point and then it will stop and then it will fall down fall back down right so here's a question if light is affected by gravity can there be objects where light also falls back down due to gravity you know like is there an object where light can behave much like this ball that i throw if i just stand on on the surface of this object and i should light up and it will just like go back down like that like is there such an object so by the way just a little caveat like i've been saying that light is affected by gravity like this ball uh there is some certain qualifications that you need to make to make this through so uh but you don't need to really worry about that just think of it that's like you know you have a ball but this ball you know your light is like a like a baseball and you know like you can fall it goes up and then it can come down now can there be objects where light also falls back down due to gravity well uh certainly we can think of such an object right we can like imagine we can imagine such an object and this object that we imagine is what people call black holes so black holes is a region of such strong gravity that even light cannot escape so if you imagine light light actually moves pretty fast right because if i just have a laser pointer and i turn it on if i turn on my laser pointer you know the the the point will just appear like almost instantaneously at the other side of the wall so that just says that light just moves really really fast so if it moves really fast the the object that can trap it must have really strong gravity and one thing we learned about the gravity is is that the denser something is in other words it's you know like how big the planet is like if you have a planet if you can squish it in it will become more dense if you have something that is dense you get something that is stronger in gravity so here i have a question for you if i have uh the earth like this right on the left hand side of the earth and on the right hand side i have the same earth but i squeeze it in i just squeeze it in so it's the same mass but it's it's confined in smaller volume it is denser which ones do you think have higher gravity so it is true that far away you know like they have the same mass so they have the same gravity but actually if you're standing on the earth you're standing on a surface the one on the right it's actually uh actually has higher gravity because they are denser so gravity doesn't just depend on how massive something is actually but like also like how dense they are if you have an object that is really really dense and you just like stand on on the surface of it like if you stand on on this earth on the left hand side and then you just keep standing on the surface and then i squeeze this earth to something smaller actually you'll feel stronger gravity on the surface of the earth and so the right hand side is actually denser the denser something is the higher the gravity in general you know so squeezing the earth causes it to have a strong gravity because you have the same mass but you can find it a smaller volume which means the density goes up you're completely right if you are standing far far away it doesn't matter what the density is what really matters is the mass but if because you are standing on the surface of the earth uh the density actually matters if you go smaller and smaller it becomes more dense gravity goes up so uh light must be really really dense then sorry like this object that can trap light must be really really dense then because uh light moves really fast you know so the only way you can have something that's that moves really fast and still uh trap it it that needs to have a really high gravity and so it needs to be really dense so in fact we often say that there is an object that is a point of infinite density okay so infinity is a super high number imagine the highest number you can think of it's higher than that okay now a singularity is just a point of infinite density where density i put in quotation marks because again there's some caveats not exactly the density that you like to think of but the idea is the same so imagine this point of like a super super high density it's called a singularity it's just like a tiny point it turns out that a singularity is covered by this wall this membrane called the event horizon the event horizon is kind of this boundary this spherical boundary that surrounds a singularity and it's a semi-permeable membrane so what that means is something can go in but it cannot get out so once you get inside the even horizon you're trapped you're in trap inside the event horizon and this is what we call a black hole a black hole is just a singularity this point of infinite density that is surrounded by this membrane surrounded by this wall this one-way wall where you can get in but you cannot get out and once you get into the event horizon we call that area here the area here inside the black hole interior okay once you're inside the black hole in the rear you cannot get out okay so why is it why is it why do we have this uh semi-permeable membrane like why do we have this wall really and i'm going to give you an explanation of how black holes are formed once you have a singularity like once you have singularity you'll go you're going to have this event horizon and to understand this we need to think about uh a simpler situation so imagine uh just standing outside you know on a parking lot you know just yes you're just standing like at the parking lot in a flat plane in a flat like plane like a flat parking lot and then take out your laser pointers and then you shoot light to the left and to the right of you okay so this is just me standing on a parking lot and i shoot light to the left and to the right of me and light of course as i mentioned before light moves at some speed it is not infinitely fast it moves at some finite speed and what that means is just that it takes like time to move some distance so like light is just moving away from me so on this on this plot here on this graph the x-axis is position it's where i am in this parking lot and the y-axis is time so what that means is that you know let's wait until so that's today right let's wait until tomorrow tomorrow i don't have anything to do with my life you know so i just stand around at this parking lot until tomorrow and i'll just so because i'm standing at the same position you see i'm not moving in terms of position but the light beam moves the light beam that i shot yesterday will have would have moved here today okay it's a really really big parking lot and then if i keep waiting i will still be standing still but the light beam will be moving away and away away from me and this uh kind of like you can draw lines on this like beam so this is like a triangle right but actually it's a cone this cone so we call this cone uh my causal cone and i'll tell you why we called him that first understand that light moves very fast and in fact it moves so fast that there is nothing that can travel faster than light light is the fastest thing you can move at so because i can only move within this cone okay i can only move within within these lines because in if for me to move outside of this code i need to outrace the light beam i need to move faster than the light beam okay so because i cannot run faster than light there is no way for me from here to move outside of the cone i have to be you know like i have to like like the fastest i can move is like you know i can move here maybe but i cannot move faster than that indeed everything that i can affect once in where from here is within these cones so this cone is called the causal cone because this cone defines my future this is my causal future okay so also this is just a fancy word that just means like it's something related to time okay so causal future is just my future so really i am like everything outside of this cone like might as well not exist for me you know uh in physics we call this area here and the elsewhere it's where dreams come from uh for me i really like you know i'm really trapped in the in this within these lines because i cannot run faster than life so here's a quiz here's a quiz for you can i move here uh here i can move here right you say i can move here now here uh i can also move here because it's still within this line now here's the question here's another question can i move here that's correct you cannot move here you cannot move here because in order to move here i need to outrace this light beam i need to run faster than light and as we know we cannot move faster than light nothing can move faster than light so i cannot go here good now so i'm already trapped inside this light cone what we call this cone this causal future this cone here i'm already trapped within this light cone there is no black hole yet it's not a black hole that causes me to be trapped it's just the mere fact that i cannot run faster than light okay now here's the same picture near a black hole now the x-axis here is the distance away from the singularity here i put the singularity here this point of infinite density and this axis again is time now uh this singularity doesn't move it's just like staying there so it just like goes up like this with time far away from the singularity here is with my cone this cone that defines my future this light cone away from the singularity really really far away it looks exactly the same as if i was in the parking lot why because we know gravity is a force that goes weaker and weaker with distance so really really far away from the singularity i shouldn't be able to uh feel the gravity of the singularity and thus i should my my my light cone should look exactly the same as if there is no black hole as if there is no singularity it should look exactly the same uh but in general relativity we know that light is affected by gravity so imagine this cone and there's some gravitational attraction that's attracting it from that side i have this cone and there's this gravitational attraction extracting into that side so here's a question to you what would you guess if i move closer and closer you know closer and closer to the singularity like what would what would you guess could happen you know just take a guess the way to answer this question in the theory of general relativity now we are using general relativity the way to answer this question in general relativity is just to think of this light is effective affected of gravity and this cone is composed of light so this cone is affected by gravity so now you imagine if you have a cone you know a cone and then there's a gravitational attraction from that side what's going to happen the cone is going to start falling it's going to start falling and when it falls it's going to tip it's going to tip towards the singularity it's going to tip tip tip tip keep on tipping until at certain point it becomes a right angle it becomes perpendicular uh or it becomes like you know i completely straight like this and here's the thing here's the thing remember i can only move within these lines right remember before i can only move within these lines i cannot get out of this line so if i'm here yeah i can move here i can move here you know i can move wherever here i want but i cannot move here okay i can move here here here i cannot move here what does that mean remember where the x-axis is the x-axis is distance away from the singularity once i'm here i cannot move away from the singularity because to move away from the singularity means i have to move here outside of this light or outside of this this line here i am trapped within this line and then i'm just stuck here you know i'm stuck here and that is the event horizon that is this wall that is this one way wall once you got there you cannot move away from it it's like the best prison you can have okay you put something in here and then they're just stuck they cannot move out of it and all of that is because you cannot move faster than light and thus you cannot move away from so you cannot move outside of this cone okay now look if i keep look what what happened here before over here on my parking lot i say that this cone defines my future right future goes here up here my cone that defines my future tips like this so what's going on here how is it that my future seems to move to the you know to the side instead of like up and down well in fact in a technical sense inside an event horizon inside the black or of a black hole uh for this kind of black hole time and space switches places okay so in fact the singularity is actually not a position it's not a location in space but it's actually a moment in time the singularity is a moment in time so people often ask me if you're inside the black hole can you like you know use a rocket or something to dodge the singularity and the answer is no because you cannot dodge a moment in time you cannot dodge mondays no matter how much you try the singularity is much like a monday it's a moment in time you cannot like move away to dodge your singularity so you know once you're inside an event horizon once you're inside a black hole you're just doomed you're gonna hit the singularity if the elsewhere is where dreams come from the singularity is where dreams go to die okay and uh this is not exactly true for all black holes but for like this type of really simple black holes that's the case and i want to tell you tell you something else about uh about about gravity in general remember this is me back on my parking lot right i'm at the parking lot again and i'm not moving because i'm not moving at the parking lot right because and because this axis is just time i just move up like this straight in between these two lines right i'm just moving straight in between these two lines turns out this is always true even close to singularity if i don't move i'm going to move straight in between of these two lines so here i am straight in between these two lines wait a minute you say you said i am not moving but me not moving causes me to actually move into the black hole into the singularity this is what we call gravitational attraction this is actually what gravitational attraction actually is you might have learned in high school about gravitational being a force and there's this like formula that you can plug in that's all a lie this is what gravitational attraction actually is okay uh okay so now remember that this distance away from the singularity is actually uh points you know like there's this wall at every angle at every angle around the singularity this point of infinite density so this wall this even horizon actually is a spherical wall it's a wall that is a sphere that just covers the entire you know like all angles around the singularity okay and once you get in you cannot get out and because once you get in you cannot get out often astronomers you know we are astronomers often we just like don't care about what's inside the black hole anymore so you know throw it there and like forget about it so we just like often draw a black hole like this we just block it out everything inside even horizon is just like completely blotted out okay so that's all the black hole is now you ask me a question now you can ask me a question well pierre before you say that we took a photograph of a black hole how do we take photographs of a black hole if light cannot escape a black hole right they should be completely dark how can we see them well it turns out that while a black hole itself is invisible the region around it is actually bright gas particles like these gas particles there's a bunch of gas in space speeds up as they get sucked and sucked and sucked into the black hole okay and as these gas particles bump into each other they causes friction and friction causes the gas particles to be hot so if you you wrap your hands together like this friction is going to cause your hands to be quite hot the same happens with this gas particles because they bum this friction heats the material and it heats them so much they become bright and hot and we call this this material plasma okay and this plasma forms a disk around the black hole this bright disk it's like a dinner plate that's really bright and this this is what we call the accretion disk so it's what what the telescope actually sees when we take a picture quote-unquote picture of a black hole it's actually not the black hole itself we see this bright dinner plate this bright this and then we see a hole in the middle and that's like how we take a picture of a black hole now the last thing i want to talk about is how do we use black holes to understand gravity well another way to say this is let's check whether einstein is high you know whether the theory that he's discovered was actually correct and the way we do it is a follow einstein using einstein's theory of general relativity again general relativity is just this theory of gravity we can model black holes by that i mean einstein came up with what he thinks of black hole looks like einstein just came out here's what a black hole looks like now the question is how do we know if this black hole that einstein just dream up is actually the black holes that are in space okay in other words can we divide some sort of astrophysical or astronomical observation to test which of these are true whether you know einstein was right that's the black hole looks like that or it's like something is completely like you know like completely different than what einstein thinks okay and the way we test it is the following we have we use this this idea of gravitational lensing so as we said before light is affected by gravity right so if you throw a baseball you know and close to a massive object like a black like a black hole it's going to curve because it's attracted by gravity the same turns out to be true with light if you throw light near the massive object like a black hole is also going to bend like so so gravitational lensing is like a prediction of general relativity that is quiet robust and indeed we have we have actually seen this we have actually seen gravitational lensing before so here's a picture here a couple pictures of light being bend not by a black hole but by a massive object in in the middle of these pictures okay so this is like like being bent and this here is like the same picture but it's more fun you know it looks like a smiley face okay and uh how is this relevant for black holes well uh black holes as i said have this like bright accretion this around it and this brighter creature in this because it's bright it produces light and the black hole lenses its own accretion desk okay so the light from the from its own like a christian disc because the presence of the black hole here is going to like bend okay and then we can detect you know like we can detect how it looks like on earth so here we have my favorite planet earth it's where i keep all my stuff and here on earth we have a camera like a camera and you can just take a picture of what this light looks like and it turns out you know unsurprisingly i suppose that light that is you know if einstein was right that the black hole looks like this like the einstein what einstein thing a black hole looks like it's going to give a certain picture if i have a black hole that looks like oh some garber or you know some whatever that looks completely different to what einstein thinks uh the picture is going to look different right it's going to produce different looking images and here's what einstein's prediction is what the picture looks like einstein says black hole looks like a circle it looks circular you know this is what the what a black hole looks like it's just a circle so the question is when we take a picture of a black hole would the black hole look like a circle or would it look like this this is the experiment the test that you want to make and indeed we actually did take a picture of a black hole this is the picture this is a picture that's taken by a group a lot of the group uh was well is in the university of arizona and i'm part of that group we just went outside one and took a picture of the black hole okay and this is a really really really like uh uh this is like really really like difficult to do you know like this is not not easy like a lot of people spend their entire scientific really trying to do this but here we found this nice picture we took a nice picture of a black hole a natural black hole in space and all we need to do is like is this circle is ask the question is this a circle is this circular and we just you know you know you just draw a little like circle here is it a circle and you know to me it looks quite circular so indeed yes so far einstein is vindicated at least in this aspect uh the black holes in space do resemble the black holes that he think uh that he he thought of that he came up with his his black his model so in this way we can use black holes to test our theory of gravity so okay that's all i have thank you so much [Music] you