[Music] thank you now a number of people have been credited with the quote that no doubt you have heard black holes are where God divided by zero Einstein has been credited with Hawking has been credited with I believe the correct attribution is to the comedian Steven Wright as it turns out but regardless the quip does highlight something true and important which is this within Einstein's own general theory of relativity there exists an entity whose physical properties break the very rules on which physics more broadly and general relativity more specifically are based that is when you
get to the center of a black hole to the singularity of a black hole Einstein's equations they break down and they break down in a manner in fact that is not unlike dividing by zero now because of this unsurprisingly Einstein himself he was deeply skeptical that black holes were real but by now that ship has long since sailed we have incontrovertible evidence that black holes are real yet that does not mean that we fully understand their structure or their formation and our guest for this conversation has been really thinking very deeply about black holes their
properties and their formation and has made a really bold suggestion about black holes that we will get to which might just be receiving observational support which would be a potentially wonderful step forward so I am so thrilled to introduce you to priia nadarajan who is the Joseph s and Sophia s fruton professor of astronomy and physics at Yale and studies the formation of black holes over the course of cosmic history she was named by Time Magazine as one of the hund most influential people of 20 24 so great to see you priia thanks for joining
us for this conversation and I'd like to kind of Jump Right In to this subject of black holes because you know everybody's heard of these monstrous structures out there in the cosmos and yet there are so many deep puzzles that still remain I thought just to kind of set context ultimately for the work of yours that I want to get to if we can just sort of walk through the history of thinking that led to this concept of black holes and ultimately for its observational support so I mean where would you pick up the story
with with Carl Schwarz is that sort of a natural place or is there another spot first of all thank you so much for inviting me I'm so delighted to be here absolutely and talking to all of you um I think we may want to start strangely perhaps uh with the black hole of Kolkata oh so it turns out I know there's an Indian connection right so Indians are notorious for this we always kind of find a connection back so um so it turns out it was an Infamous prison when the British East India Company was
trying to make its way and the local NOAB had actually imprisoned some officers overnight and it was a place of no return so uh it was a very apt term that then got picked up by John Wheeler much later to describe these astrophysical objects but you're right truly where the Story begins is with Einstein's um theory of general relativity and swash child finding the first kind of exact solution Einstein never imagined that there might be an exact solution so this just really corresponds in a simple way right to the shape of space around a very
compact distribution of matter and um it's a very peculiar very strong Distortion in the geometry of space and Einstein of course one of the amazing things he did with his theory was to show that the geometry the contents and the fate of the universe and of any entity in the universe were interrelated and and an exact solution would mean the shape of space around distributional matter now it's an important point you make there that Einstein himself was not the first person to solve Einstein's equations absolutely and he actually they're so messy uh that he never
really imagined there would be an exact solution yeah so he like got these approximate solutions that were pretty good got the bending of Starlight by the Sun from the approximate Solutions yeah but then Schwarz child yeah within six months right Schwarz child was actually fighting in The Trenches World War I but Einstein had given this lecture at the Prussian Academy of Sciences so I child heard about it in the trenches and he found he was a course he really good hearing out there and yeah not damaged yet right and and can you imagine like just
nerding out when you're like fighting and then coming back home and doing sort of in the evening uh maybe it was you know a way of dealing with the stress yeah but he found the solution and he wrote to Einstein and Einstein thought it was fantastic with solution but he didn't think it was real he thought this is like a nice cute mathematical solution and I think that's the way they remained for quite a while the basic idea he was assuming you had a spherical Mass yeah you crush it sufficiently small and you bring it
pretty much to a point reduce it pretty much to a point and then you would have an extreme Distortion of space time right literally like a puncture in space time in fact they have this formula if we can just bring it up as a point of reference tell us what we're seeing there so this is the formula of the a very um defining quantity for a black hole and that's called The Event Horizon so this is sort of a sacred boundary for the black hole where essentially Beyond this event horizon the change in shape the
geometry of space time is so extreme that anything that crosses this boundary never makes it back out including light not just anything material but also light so and you know so this is this an enigmatic property in many ways I have to admit right this is one of the reasons the fact the existence of an event horizon is what kind of tantalized me when I was a kid reading about this right but it also turned off Einstein in a sense because if you plug in for instance the mass of the Sun into this formula it
tells you how much you'd have to squeeze the sun three kilometers which is ludicrous sounding you plug the Earth in there and you have to squeeze it down to what a couple centimeters or something like a penny or something yeah so so how could you possibly do that and yet we were going to talk about ways that we think nature does nature does it and there maybe novel ways yeah if indeed this happens I mean just to get sort of a a picture of what the resulting region of space would look like absolutely and I
think um what is really remarkable is that from all the skepticism including from Einstein himself know kind of proposed the theory right now we're finding pretty much every galaxy in the universe Harbors one of these mysterious objects and its heart and these black holes that are seen in the centers of galaxies like the visualization you're all seeing is about often millions of times the mass of the Sun could even be billions for billions as which is sort of mindboggling scales and following again the history and root to having confidence that these objects were real before
observation that will come to really nailed the case Chandra sear was one right who played a vital role can you just describe how he Advanced our understanding here yeah so I think um Chandra shikar was the first person to kind of make the connection of a class of existing astronomical object stars that were pretty well understood and showed that any star that by birth started off life with a mass about 8 to 10 times the mass of the Sun or larger would inevitably live its life out and explode and end up as a black hole
um this was a theoretical prediction that he made um and in fact he sort of was able to work out that the Stellar corpses there were three kinds of Stellar corpses compact objects you know neutron stars white dwarfs and black holes the most extremal would be the black holes you know his it was a theoretical idea but there was a lot of opposition to that idea for quite a while so it took once again it's the peculiar properties of the black hole that cause skepticism and you know as you mentioned right away that one of
in your introduction one of the peculiar properties other than the Event Horizon is the sort of breakdown of all physical laws as we understand them once you edge closer and closer to the ultimate Singularity for which we don't quite even today have the right language to describe I think there's been a lot of progress but um so it's very enigmatic so there's a lot of opposition to that idea so we certainly won't answer the question of the singular unless you're going to break some news here today I want to stick to the real black holes
well but the real black holes have something going on down there that ultimately we have to address because once we know they're real they've got to make sense and you know a singularity is Nature's way of telling us that our mathematics is wrong right right or also so the limits of our current understanding and so again these are the kinds of things I mean even Einstein himself I think this is paper is what is it 1939 wrote papers trying to argue that sure under these simplified highly symmetric assumptions perhaps you can get this mathematical solution
but it's not a real physical thing in the real world but can you imagine Brian a scientist actually writing a paper like contradicting what his own Theory predicts I mean Einstein was really he yeah he was definitely um a liberal thinker you know um you know such a nice put he did more and he did it more than once absolutely with the expansion of the universe he was a hold out he did not like he aesthetically he was so attached to the idea of fixity he did not want uh to hear about the evidence ultimately
Hubble convinced him because there was enough observational evidence that you know Einstein could not be hold out anymore yeah and so he absolutely followed where the evidence took him but en root certainly had these interesting moments of not trusting the results from the equations that he himself had had given the world which is a absolutely interesting combination now I'd say among the next important steps in trying to understand whether black holes are real or not you know there is work in fact by op Heimer yeah who now you know I think most people were at
least somewhat aware of Oppenheimer until you know reason now everybody is which is which thanks to Christopher noan yeah which is a great thing um but I guess it was Oppenheimer and Snider perhaps is the the the key paper yeah so I think uh this was an important paper uh demonstrating numerically computationally actually following the um explosion of a star all the way really close to as far as they could at the time that Numerica allowed them and they were able to show that asically you're going to end up with an object that's going to
be a gravitationally collapsed object so and I think that it's a very interesting point and a trajectory which I think you know when we talk about new developments we'll see it's the same trajectory so you have a theoretical proposal then you actually need a numerical demonstration because you know in cosmology and astrophysics we just can't do controlled experiments with the universe itself so we simulate the universe in a box the conditions and once that's demonstrated then there's faith in going and trying to look for these objects and and I think that's sort of the same
trajectory we follow still and I think the Oppenheimer Snider paper is really important because it kind of validated looking for signatures of a black hole and then it was a dig um the discovery of Sigma X1 from the mathematical standpoint though there was still an argument that people could make that whatever you're doing it's still simplified right and maybe if the star that's collapsing has enough irregularities or asymmetry you know that that something will prevent this black hole from forming and then Roger Penrose you know comes along and of course ultimately the work earns him
a share of the Nobel Prize but this I think had a dramatic impact on people's thinking yeah absolutely and I think he was again able to make a much stronger connection with the formation of the singularity uh with the Distortion in SpaceTime and and I think it's the work of pendus and then Hawking after that that really sort of strengthened the idea and in a way normalized you know this concept of sort of extremal geometric Distortion around a compact Mass but with with with his results all of a sudden was what you know regardless of
the impurity of the system you were inevitably going to find yourself in the state of a black hole and then you already mentioned you know John Wheeler was the fellow who picked up the name yeah wherever the original origin may have came but the black hole of Kolkata do you know do you know where it was that John Wheeler where he was giving a talk when an audience member said hey it sounds like you're talking about a black hole and he yeah black hole that's a good name I thought it was the relativistic meeting Texas
meeting right I don't think it was I think it was anybody here you know you must be you know I think Mara Bak had tracked this down here it is here it is watch this right here it was there what no really you see Tom's Diner right there right that's the old Seinfeld you know corner so that's the Gard Institute for space studies above it and I'm pretty certain that was at a lecture in that building that this so that's where the term black hole comes from so it was Ed first there okay that I
now I could be wrong but I'm pretty sure that that's correct uh if I am wrong we'll cut this out of the digital version you're absolutely certain about that you know now again we're still sort of in the realm of of theory yeah and then Andrea gz and collaborators they to do exactly what you described they start to look carefully at the center of our galaxy yeah and starting to map essentially the shape of space and the impact of the shape of space on the Motions of stars right around the black hole Yeah so this
is just incredibly beautiful work I mean every time I see this movie I'm still like blown away that this is real data of real stars that have been followed for a couple of decades now and so this very you can visually demonstrates this is sort of looking like the solar system and orbits and you therefore know there is a very massive object at the focus of that they're all whipping around and basically it has to be a black hole to account for the Motions that you're seeing and then you know just to really completely nail
the case oh yeah then you have direct Imaging finally as close as up close as we will ever get to a black hole so this is squirting just you know one and a half to two times uh outside the Event Horizon where you're still able to see light that is sort of swirling around and is getting dramatically Bent by the gravity of the black hole and you're catching that so this is uh just a beautiful it's my favorite dut I'm with you on that although I'm not a great fan of donuts but uh you know
but it's a wonderful story of success absolutely coming from Pure mathematics that turns across you know a hundred years yeah of human thought ultimately resulting in a direct confirmation that's kind of the dream that each and every one of us has and speaking of that dream you're heading in the potential direction of a version of that dream and so I thought it' be good to now turn to the work that has been occupying you for quite some time which is to think about alternate ways in which black holes could form yeah right and so give
us a sense of this is super it's super exciting as you said right this Arc of proposing an idea having you know working on it for a while trying to persuade people that it might work and then have people go out look out for it and have all of that sort of close within the lifetime of a single scientific career yeah it's just stunning so I think the um idea that we had so this is work that I did with one of my post talks in 2005 2006 jpe lato and so at this time what
was happening was people finding these quazars which are basically actively feeding black holes super massive ones farther and farther back in the universe and they're super bright they're like Cosmic lighthouses and we can infer the masses of those black holes that are putting out that kind of Beacon and these were already sometimes a billion times the mass of the sun when the universe was barely a gig year two gig years old you know the entire universe today is 13.8 gig a year so just to give you a sense it was really pretty close uh so
about a decade ago so then the question was if the seed black hole is the end state of a star that blows up like Chandra told us do we have enough time in the universe to grow it from you know 10 times the mass of the Sun maybe 50 times mass of the sun to a billion times the mass of sun do we have enough time to make that happen with the physics that we currently understand about how black holes gr so it's basically eating up material in its environment and occasionally crashing into each other
maybe merging with another one yeah so um so it started to look challenging so we said oh this is interesting can we just somehow make a black hole byp passing the formation of a star and make it be really massive from the get-go like maybe can you start life can a black hole start life 10,000 100,000 times the mass of the Sun from its birth because then it alleviates the feeding timing kind of problem and so we found actually just kind of thinking um thinking about the physics you know not worrying at that time about
you know whether it's realistic whether it can happen in our universe just can we get some Physics to work and we found we could uh you needed some very special conditions um so we were able to show that in a very early Galaxy before to form any stars in this particular Galaxy there's a setting where this would work so you have a Galaxy that has formed its first stars but it has a little gassy satellite that's moving around and in this gassy satellite you don't form Stars yet instead you have a lot of gas so
that would settle in because the gas has some spin some angular momentum it would settle down into a dis and that this disc could then be made unstable by phenomena that are available some departure in symmetry from spherical symmetry which is expected and then the whole thing would go unstable kind of pulls on itself and then you know and I think the analogy that works really nice and you know up to a point is when you pull the plug out of a bathtub and you see that Vortex of material water going really fast something like
that the akin to pulling the plug happens in the universe and all the gas can get siphoned in very rapidly right into the center and this is really what we need to make a black hole in fact we have some visuals just to give a sense you know so if you begin this is the more conventional yeah so you have a star you have a massive star that starts its life out then goes Supernova and then leaves behind a black hole and then it could only grow by sucking in nearby material merging with another and
and that mass its birth mass is expected to be you know it's uncertain we don't know the masses of the first Stars yet they could have been much more massive than stars that are forming today nearby so we think maybe that could be the way to make a black hole seed that's 50 times maybe 100 times not not not a not 10,000 times yeah and then the new idea if you can take us through that so you have this gas disc that is swirling around in the satellite Galaxy and you have direct collapse and you
are starting to form so no star no star yet and just guas and you form a black hole but because of the particular configuration you kind of need a Galaxy nearby that has already formed stars to make this happen because ultimately this object is going to crash and merge into that parent Galaxy in fact I think we have the next phase of that as well so the black hole is formed and then it can do what you're saying yeah it inevitably actually crashes into the parent Galaxy yep and when it does so then it's then
it produces transiently a very special type of galaxy so this is what we actually unimaginatively at the time called an over massive black hole Galaxy and OBG where the mass of the black hole far exceeds the mass of the Stars which is the opposite of what we find in the nearby Universe for example for the Milky Way so the mass of the black hole in the Milky Way is 4 million times the mass of the Sun and the mass of the Stars around it in just the inner Region Three orders of magnitude larger so the
gravity of our galaxy in the entire scheme of things the black hole doesn't count as much except when you get right next to the CER and this would be different this would be different and so I think there's this nice visual that helps give a sense of the distinct timeline of formation so help us understand what we're seeing here so this is a sequence showing you if you formed a light seed and you form this heavy seed from direct collapse then at at an epic say between 400 to 600 million years after the big bang
well I mean the magic of this epic is James web Space Telescope can take us right back to then so the James web Space Telescope would could potentially so that's what we predicted so you're predicting that the lower sequence may have happened in the early universe and it would have Yi over what was the exact name OBG you I just my brain keeps going YN afterwards but um but um but um so never occurred to me yeah um but but you get this uh this overly large black hole Galaxy so this penultimate uh frame that
you see in the sequence right that's that's the difference if you started if this galaxy started life with a light black hole versus one that formed with direct collapse and that third that's penultimate frame the fourth frame there is an epic that is now accessible to us with James Webb so we wrote this paper with a specific predictions you know the original ideas 2005 2006 it took a long while for us to for this problem to be computationally tractable he demonstrate it can happen and that the conditions in the universe are available for this to
happen and then we predicted in 2017 what James Webb should see and and that if James web happens to look back to this epic say 400 to 600 million years after the big bang and nature does produce these objects we should see this very peculiar object and did you have like a particular spectral signature that you'd be looking for yeah absolutely so there was a clear-cut signature between one and three microns uh which is sort of in the infrared these are the this is the window wavelength window that the James web opened up to us
you new eyes into the Universe in that range so there was very clearcut signature but one of the other important things is because this object the OBG has an over mass of black hole in the center it means that the light output that's coming from there would be dominated by the mass the dying gasps of gas that are being pulled into the black hole instead of the Starlight so that's a skewed kind of budget of where the light's coming from and that has a very special signature I see and the signature is that it should
also glow in the x-rays and these objects would be quite fake this is from the accretion disc around the black hole producing I see and so it should be seen both by James web and in the X-ray and so that naturally takes us to the question of looking now that James web is up there and this is something that you have begun to do yeah so we have uh very compelling evidence so with um an x-ray astronomer collaborator and friend uh Akosh Bogdan once we knew that James web was going to be scanning a bit
of the sky you know an opportune piece of Sky because you know these objects you would like Nature's telescopes these cluster lenses that magnify the distant universe and bring them into view ideally because these objects are far away if you can get help additional help from nature that would be great and so James Webb was looking behind those regions and so we proposed to look at that some uh same region in x-rays as well so Akosh suggested you know putting in a proposal to get time on Chandra and to look because you know you the
incontrovertible evidence was going to be a set of properties including detection in the X-ray and so both of these devices would be useful to get corroborating evidence for and what is really beautiful about this work is in fact to originally calibrate that lensing and the magnifying effect Buble data was used so this was you know us using everything that's up there absolutely and so we have some of the imagery that no doubt has gotten you extremely excited so this is James web and maybe walk us through I think this is the uh not quite the
closest image that we have yeah so what you saw there was that large lens that was going to bring the background interview yes and that is the object that little spe if we zoom in I think you have a zoom we have even we can yes I know it looks really Blobby and Tiny but it's real and it's there and um and then you go back and look at the Chandra data yeah so I think we have some of that too so let's just take a look at this data if you can bring up the
Chandra and in fact we can also I think label it as well yeah and so when you study the details and then you compare it to your prediction how is that looking unbelievable I think um I remember distinctly when akos called me so because the key thing was that it had to be detected in the X-ray yeah so James had James Webb had seen that object we had the we had the sort of the spectral energy distribution of that object and the shape looked uncannily like what we had predicted and and it was at the
right kind of distance remember I kept saying that there's kind of an opportune sort of timeline where these kinds of transient objects have to exist so it was in that window um and it satisfied so there were two properties one was x-ray detection and how bright it would be in the X-ray compared to James web okay so again it's the balance of energy the balance of energy in the X-ray versus the infrared and and uh so when I saw this image you know that you could do that calcul a because it was detected to this
to this date this is the most distant black hole where we have detected x-rays as well and you know x-rays have always been the kind of you know sure fireproof of the existence of an accreting black yeah absolutely and so how has the community responded is this controversial is it broadly accepted is this done deal yeah well I think it's pretty much done deal when we published the paper last year it was in know the detection threshold we were at like 4.6 Sigma and we didn't have the entire x-ray data yet so now we have
it in hand and we've crossed five Sigma so it is real for sure um and I think the community has come around to I think the the skepticism was actually much earlier on when the as we we saw historically right a lot of the push back is for the theoretical idea but that started shifting already in Say by seven eight years ago when we made this first concrete prediction of what should be seen I think uh it would be fair to say black hole seeding became a bandwagon and now it's a very active area of
research and you know everybody wants to get on get in on trying to see okay how do you understand I know black hole seating became a thing like when we first wrote our papers and direct collapse people were kind of skeptical they said oh it's a cute piece of physics and it works but is it real and so we come back to that same Arc right right and so now that we have the evidence I think what is super exciting about this is that uh this is of course established and so what it really tells
you is that nature makes black holes in multiple different ways not just what we thought before Stars run out of nuclear fuel they collapse and so forth but you're saying there just more ways of doing that yeah can there be other ways yes absolutely so I think another way that I am super excited um and open to is the idea of primordial black holes so you could have made these very tiny black holes very very early on in the universe during the very early you know just post inflation of the universe and that they could
have survived throughout these singularities in SpaceTime and sort of sit around sit around till our universe becomes dominated by matter and then start growing and maybe they could be SE so this is a picture that I've investigated with others it's a very active kind of question but the you know but once again with this other seating there has to be a very compelling piece of evidence showing that that is the origin part of the problem is once matter or anything goes into a black hole there is no memory so you see a black hole it
contain you cannot extract any information about how it Formed therefore it is the relationship between the black hole and its environment the Galaxy that it's sitting in uh configuration its mass ratio with the Stars etc those are the signatures that can tell you more about how the blackf for yeah I mean because one of the Striking properties of black holes is that there's a way in which they are the most simple objects in the universe you label them by their Mass their charge and their angular momentum how much they're spinning absolutely and any two pretty
much modular subtleties that are still at the Forefront of research any two that agree on these properties they're effectively indistinguishable as objects in their own right and so to understand about them you you're you to know the environment and the history by which they came and hopefully that is imprinted in some wake of exactly and the evolution right so we are now we have since we have snapshots of data from the universe throughout its age we're able to track not the same object obviously but objects in different stages and so we have these theories that
now connect the evolution from the formation and of course the the issue is whether there is a slightly different growth path that you know one scenario may give you compared to the other right so it's a totally active area of research but I think you know for direct collapse I think it would be fair to say that now we have incredibly compelling evidence especially since it's now sort of five Sigma Beyond five Sigma well that is an amazing story congratulations it really is very much thank you and thank you so much for conversation thank you
everybody prajan [Applause] [Music] n [Music]
