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Listen: https://soundcloud.com/astrophiz/a184-hyerin-cho-black-holes?
Meet Hyerin Cho, an inspiring young astrophysicist from Korea who is doing her PhD at the Harvard Center for Astrophysics in Cambridge, Massachusetts.
Full Transcript:
< Intro music: Gravitational Wave audio of black holes colliding, then ‘Radio Waves’>
Brendan: Welcome to the Astrophiz podcasts!
My name is Brendan O’Brien and today is Friday the 1st of December.
We’d like to acknowledge Australia’s first astronomers, the Aboriginal and Torres Strait Islander people, the traditional owners and custodians of the land we are on.
This episode is produced on Yorta Yorta and Wurundjeri country.
And we’re asking you to influence your local politicians with the message that we really need to change our energy policies and move to renewable energy sources to mitigate the effects of climate change.
And each month we love bringing you two fabulous episodes;
On the first of each month, our friend, molecular pharmacologist, toxicologist and amateur astronomer, Doctor Ian ‘Astroblog’ Musgrave brings you his monthly SkyGuide with all the essential observational highlights for telescopers, astrophotographers and naked eye observers.
Each month, Ian also includes “Ian’s Tangent” where he takes us on a short journey of astronomical wonder.
In the middle of each month. We bring you an exclusive and in depth interview with a noted astrophysicist, astronomer, particle physicist, radio telescope engineer, data scientist or space scientist.
Now because Ian has included his December SkyGuide in his November Skyguide, which we published two weeks ago, we’re giving our listeners a special treat by introducing you to Hyerin Cho.
So right now, we’re zooming over 16 time zones to Harvard on the east coast of the USA to speak with a very amazing and very generous astrophysicist whose latest research is revealing previously unknown secrets of black holes that live at the center of every galaxy. She is so wonderful!
Meet Hyerin Cho (pronounced H’erin)
Brendan: Hello, Hyerin!
Hyerin: Hello, Brendan!
Today, listeners, you’re invited to a very special conversation and today we’re lucky enough to introduce you to Hyerin Cho from Korea who is doing her PhD at the Harvard Center for Astrophysics in Cambridge, Massachusetts.
Hyerin has been using her formidable computational skills to simulate still-mysterious black holes using supercomputers, as well as revealing the mysteries and enigmatic fast radio bursts AKA FRBs.
She produces simulations of plasma accreting onto black holes and is on an already successful quest to discover what really makes these wonderful phenomena tick …
… and congratulations on your latest awards and your PhD scholarship … and thanks for having a chat with us today Hyerin.
Hyerin: Thank you so much for having me.
Brendan: It’s a pleasure. And before we talk about your current black hole projects and your earlier work on FRBs and your international internships, can you tell us where you grew up please Hyerin, and could you tell us how you first became interested in science and space?
Hyerin: Yeah. Sure. I grew up in Korea. It’s a very populated country. So it was very hard for me to watch stars. I think I would assume in Australia it’s very easy to look at the stars because you have very little light pollution. But in Korea that was not the case.
So I first became really interested in science and space. When I first went to an observatory in Korea, it was my first time actually like staying overnight at the observatory when I was around 12. And I didn’t know there were so many stars in the universe. Like that was my first time seeing that many stars in the night sky.
So I got really interested and coincidentally that was a time when the school, like the … uh … middle school, was teaching me about astrophysics and how stars are made.
So I think it’s all started from there. So, yeah, here I am studying the most extreme versions of the stars and black holes. And yeah, that’s how I got interested.
Brendan: Fantastic. Would you like to tell us a little about those school days and your earliest ambitions and how did those early ambitions change and evolve over time?
Hyerin: Yeah. Sure. So as I noted before, it all started with my fascination about stars, they were beautiful and the physics behind the formation of the stars was very fascinating to me. So I knew I wanted to pursue a career in astrophysics in the future even when I was very young.
Brendan: Wow!
Hyerin: However, I went to a university where we had no astrophysics professors or no astrophysics research going on. So I was doing … uh … physics during my undergraduate bachelor’s degree. So I wasn’t sure because I hadn’t tried astrophysics research at all.
I wasn’t really sure if I really liked astrophysics or not. So I just decided to do a bunch of international internships in order to work with astrophysicists all around the world.
So I think by doing those internships, I was able to figure out which part of astrophysics I really like. And it turned out to be I was very interested in the most energetic phenomena in the universe.
So yes, I think slowly over time, I grew interested in black holes or fast radio bursts and some events that are the most energetic parts or events in the universe I got interested in
Brendan: Fantastic! … and it looks like you’ve really steered your career path. And after your successful school career, you were awarded your first Bachelor’s Science degree with honors in physics and maths.
So that was at GIST, the Guangzhou Institute of Science and Technology at Guangzhou City in South Korea where you won more than a few awards and scholarships, including the outstanding thesis award, the career Talent award, and a fully funded study abroad scholarship which sent you around the world.
Then you did some amazing research in the US at Caltech and then you flew down to the southern hemisphere to visit us here down in Perth in Western Australia to Curtin University. Then over to OzGrav at the Swinburne University of Technology in Melbourne Australia to work with FRB data from the ASKAP array in the vast deserts over in western Australia.
Now we have a great Australian bias here at Astrophiz … understandably … so before we talk about your PhD work in progress at the CfA, could you tell us about your Australian journey, your adventures ‘down under’ and who you worked with at Curtin University in Western Australia and also over at OzGrav in Melbourne … and for our early career researchers who are listening, could you also first tell us, how did you line up those fabulous placements?
Hyerin: Yeah. So the start of all this … start of my Astrophysics research was the exchange student to Caltech. And that’s when I first met astrophysicists in my whole life. Yeah. And there was one professor called Sterl Phinney who was very happy to take me as a summer intern the next year to conduct research on black holes.
So actually my first research was on black holes. And while I was at Caltech, as you may know, Caltech is an Institute that has a big very active Astrophysics research.
So many astrophysicists actually visit Caltech to give talks and that’s where I met the Australian team working with FRBs. They visited Caltech to give a talk on FRBs.
And I was totally fascinated by the talk. It really fascinated me because it’s a new event that has been recently discovered, but no one really knew the origin back then.
Brendan: Yep.
Hyerin: So it really fascinated me. So I told my professor at Caltech that I’m very interested and I would like to do some internships there … so Professor Sterl immediately emailed two teams in Australia.
And I guess those emails contained … like very good words about me. So the two teams immediately agreed to have me as an intern the next summer.
So that was one in Curtin University. I worked with Jean Pierre MacQuart and Clancy James. And in Melbourne, I worked with Professor Matthew Bailes, Ryan Shannon and Adam Deller. So amazing! It was an amazing team and that’s how I could get all these internships … Um …
When I first went to Australia, I just remember that I could not understand anything that the Australians were saying because I … I was taught English with an American accent.
So I just could not understand any Australian! So I was very frustrated with myself at first … after a two weeks or so, then I could start understanding what … what people were saying.
Brendan: Heh heh!
Hyerin: I was very blessed to have wonderful people because they had a ton of patience for me even though I asked them to repeat their words and they were very, very, very OK with this.
So, yes, that’s how I got those internships in the US and also the two internships in Australia.
Brendan: Fantastic! Thank you. Yes, and we were all very shocked and saddened when JP suddenly passed away and the ripples went right through the Astro community here and internationally, and is still felt today … and we did episode 35 with him … I think back in 2017. And it’s a great reminder to us of what a generous and brilliant researcher was Dr JP MacQuart.
Now … look …we must return to your astro journey… Uh … You’re three quarters of the way through your doctorate at the Harvard and Smithsonian Center for Astrophysics at Harvard University.
And what big questions are you asking?
And what problems are you working on Hyerin?
Hyerin: Yeah. So as I said, I … I was very interested in black holes. So I actually returned to studying black holes during my PhD.
The question that now I’m trying to answer on black holes is actually the … uh … the most massive black hole in our galaxy or in any Galaxies.
So to set the picture, there are many Galaxies in our universe and for each galaxy, there is a central very massive black hole in the center of each galaxy. And the mass of that massive black hole can range all the way from million times our sun to billion times our sun.
So they are very, very massive. What I’m interested in is even though they’re very massive, they occupy a very small portion in our galaxy.
However, for some reason from our observations, we see that the black hole that’s very, very small compared to the size of the galaxy seems to be impacting the galaxy as a whole.
So what I’m trying to answer is how can that small but very massive black hole impact the galaxy as a whole?
So yeah, this is the problem that I’m trying to answer. And I think we’re getting closer.
Brendan: Fantastic. OK, great. Look, just to follow up on that, we interviewed your colleague Daniel Palumbo at the Event Horizon Telescope … uh … a month or two ago. And he later told me about the fabulous simulations that you were doing on how plasma accretes onto black holes.
Now without going into the actual code, can you tell us how you create those simulations? And what are your simulations telling you about these supermassive black holes?
Hyerin: Yeah, sure. So I simulate, as you said, plasma accreting onto the black holes.
But this is a very hard problem because we know that near a very massive black hole, the classical mechanics invented by Newton does not apply.
So we need to use a more difficult or more complicated theory, which is General Relativity by Einstein. So we need to use the simulation that can actually solve for equations in gravitation of general relativity.
So this is the type of simulations that I am using. And with this simulation that can simulate general relativistic effects, it can naturally simulate the very, very relativistic, very fast outflow from the black hole.
This is all from a pure outcome of the general relativistic effect and this cannot be simulated with just classical mechanics.
So this simulation that I’m running right now is telling me that with the interplay with general relativity and the plasma, it can explain how much outflow is occurring from the small and massive black holes.
Brendan: Oh Fantastic! OK. Look … um … this might be an ignorant question here. So do you get to measure the accuracy of your simulations by comparing the simulation you create to physical radio or optical data that’s obtained from selected targets?
Hyerin: Yes … we are … I think that’s our … one of the big goals that we have by running these types of simulations. We do have a lot of predictions. We have predictions of what we are going to observe when we observe supermassive black holes in the centers of the Galaxies.
Even though I am not doing actual observations, I use the … the other observations that have been made by other researchers.
So for example, you might already know the Event Horizon Telescope that really zoomed in right next to the supermassive black hole of our own galaxy and the neighbouring galaxy … from that observation … that was a radio observation … we can get how much mass is being accreted onto the black hole and so on.
So actually, we compare that rate of accreting mass onto the black hole from observations to our simulations.
And so far I’m finding consistent results with my simulation and the observations.
This is one way to compare … with observations. And in the future, I’m hoping to do more comparison to observations.
Brendan: Beautiful! OK. OK. Now let’s do a quick peek at your pipeline codes that you used back with your FRB research.
Why is Python such a popular coding language for astrophysicists like yourself? And what is a pipeline?
Hyerin: Yes, the pipeline is when you get all this data stream from the telescope, we need to run it through some kind of software to transform into data that we can understand … into scientific language.
So we need to convert all the data, which is basically just a data point with numbers into some quantities that scientists can understand.
Yes, that’s the pipeline and I have developed a pipeline to do this process, but I focused on retrieving the highest time-resolution of the data. So that was a pipeline that I have worked on as an undergraduate in Australia.
Brendan: Yep.
Hyerin: … and for your first part of the question, why is Python such a popular coding language? Yes, I’ve also used Python to make my pipeline. Python is actually really popular among astrophysicists mostly because of the simplicity. So it is very easy to use as a person who didn’t have a strong computational background.
Brendan: Yep.
Hyerin: … more accessible to students. So I think that’s why … um …students first learn Python instead of … uh … learning other more complicated computational languages such as C or C++ … if you want to do really, really heavy calculations, or if you want to really run fast calculations, you might want to use C++ instead.
But otherwise for just plotting your figures or doing the simple calculation, Python is the fastest and easiest way to do. Also a lot of astrophysicists are using Python. Most of the packages or modules such as like most of the software packages are developed in Python. So it’s usually easier to use the already developed package.
It’s sort of like … um … I’m not sure if you use this word or phrase in English, but it’s sort of like being on top of the giant’s shoulder … is that, is this the phrase in English?
Brendan: Oh Yes, Standing on the shoulders of giants …
Hyerin: OK. Yeah. So building up on what other people have developed already. So this is another reason why Python is very popular.
Brendan: Yeah. Fantastic. And I’ve … I’ve seen there’s a lot of nice packages up on GitHub that are being shared now.
OK. Look, let’s hear a bit about your work on your dissertation for your PhD.
You gave us a big picture earlier. But could you share with us some of the details of a particular part of your work that you’re working on right now, that’s driving you crazy or is beautifully exciting or perhaps it’s even both things what’s going on here.
Hyerin: OK. Yeah. So I was recently very excited about being able to simulate the galaxy and the black hole at the same time. Nice, as I told you before …
Observationally, and naturally in nature, we know that the black hole does impact the whole galaxy and the galaxy impacts the black hole in return.
But we were never able to achieve this study. But theoretically because the range of scales ranging from the very small black holes to all the way to the large galactic scales, the scale separation is very large.
So it was never … we were never able to simulate the whole galaxy with the black hole scales.
But we recently came up with a novel technique that can bridge this gap.
So we call it bridging gaps between this black hole and the galaxy.
And I’ve actually uploaded my paper last night.
So we were for the first time able to resolve the galaxy and the black hole and study how they interact with each other.
So this is a very exciting thing because observation was way ahead of us, and theory couldn’t catch up.
But we now have a theoretical tool that can explain the observations. This is really exciting these days.
Brendan: That’s sensational! Fantastic! Theory has caught up with practice and … and often it’s the other way around. OK … Uh … look … What about the nature of your work at the CfA?
I know that you produce your dissertation there and for you to do that, it means a lot of coding, a lot of spending a lot of time alone in front of your computer screen and the keyboard. Are you also working on some team research projects there?
Hyerin: OK. Yeah. Yes. So it would be great if I can show you my office! I basically just have one computer in front of me, and a lot of screens in front of me.
So yes, this involves a lot of coding in my private office where I just share with one other grad student. So yeah, you’re correct. I spend most of my time alone coding and simulating and looking at my simulation data and so on.
But I do also take time at least once a week to meet with my advisor, my professor that I work with to discuss what I’m finding and discuss what exciting science that we found.
And on top of that, I also have a team that I’m leading.
I’m the only graduate student and we have two professors and four postdocs working with me. So I have a team that I’m leading to do this research and I’m meeting with them once in every week to report the progress and discuss all the science.
So this is great. I actually … before uh working on simulations, I worked on analytical stuff, meaning that I solve equations with pen and paper mostly.
So that was very lonely job without having a large team. But now that I’m working on a different topic and simulations, I have a big team that I can always talk about my results with.
Brendan: Fantastic. Thank you Hyerin … OK. Yeah. So I’ve had a look at some of your published papers and I noticed that you wrote some of them when the COVID pandemic was at its peak in 2021.
Now … how did COVID affect you and your family?
And what was the impact on your Astrophysics research?
Hyerin: OK. Yeah. it was a terrible time. Fortunately, our family wasn’t affected and I also wasn’t affected health wise, but it did impact a lot in my Astrophysics research.
COVID happened when I was in my first year of my PhD program. So actually even before my first year of PhD program, so most of the grad schools that I got into that I got accepted to, I couldn’t really travel to the US to look at the campus and see what it’s like … at all of the universities.
So it was very hard for me, first of all to make the decision which graduate school that I want to go to. Everything had to be done by Zoom. I had to do the campus tour by Zoom and talk to people by on Zoom. So it was a very difficult time also. Yeah. On top of that, my whole first year, I just couldn’t travel to the United States. So I’ve had to take all the classes and do all my research back in Korea, which is basically on the other side of the world from the US.
So the time to schedule a meeting with the people here was very difficult … taking classes and 8 a.m. in the morning and then 10 pm at night, it was very difficult too.
So I think that in my first year I didn’t have a lot of progress in my research. So that was a very difficult time.
But yeah, I think when I came to the US in my second year, everything changed so much. Like my productivity went up and going to classes and keeping a regular schedule, everything worked out really well for me.
So I think it was just that one year that affected me a lot.
Brendan: Yeah, difficult times. And hopefully we all learn a lot from it. Now you told us about your work, writing and sharing code. But I did see on your website at HyerinCho-DOT-com that you’re a hip-hop instructor. Would you like to tell us about your love of dance and what does dance mean for you?
Hyerin: OK. Yeah. Yeah, I … I realized that I put up that I instructed a hip-hop class on my website. I started dancing when I was in my undergraduate college just for fun. I started with hip-hop dance. So ever since I’ve been dancing in a team, so dance is a very huge part of me because whenever I am burnt out from work or taking classes or if anything goes wrong with my research, I get very, very stressed but actually having another hobby that sort of prevents me from thinking about work and thinking about academic work and thinking about classes.
Yeah. It makes me really move away from all that thinking. So, it’s a very good hobby for me to have to relieve stress. Yeah. I’ve also taught a hip-hop class to the things I choreographed.
I’m … I’m actually still continuing right up until now in graduate school.
I realized I definitely need it because one year of not dancing was horrible.
So I decided to join a hip-hop club in Harvard as well.
So I’m still dancing. I’m still choreographing. It means a lot to me.
Brendan: Yeah, that’s fantastic. Thank you. Now, you’ve done some teaching and mentoring of students. You’re leading a research team, you’re giving public talks. You’re leading study groups … Is outreach an important part of being an astrophysicist Hyerin?
Hyerin: Oh, definitely. I think it’s a very important part. Um Even for our program like astronomy … the Harvard Astronomy program … the requirement to graduate is actually having the outreach experience.
You need to at least do one public outreach. Um … I feel like people in astrophysics really think this outreach is an important part because … including you Brendan … many people in the world are very interested in astrophysics.
It’s sort of like our natural human curiosity … like what’s out there in the universe?
What’s the most energetic thing in the universe?
I think this is a question that everyone at least had at least once in their lives.
So it is a really big … it … it is a really important thing for us … an important job for an astrophysicist to reach out to the public, to assuage their curiosities and answer those questions … the big questions that people might have.
So, yes, we think this is very, very important.
Brendan: Yeah, sharing curiosity is a great gift. Thank you very much Hyerin.
OK. Yeah. OK. Well, finally, the mic is all yours and you’ve got the opportunity now to give us your favourite rant or rave about one of the challenges that we face in science, in equity, in representations of diversity or in science denial, which is one of my favourites … or science career paths … or your own passion for research or as you mentioned, uh our huge human quest for new knowledge, the microphone is all yours.
Hyerin: Yeah. Thank you. I think I would like to maybe share my passion towards black holes. And I really, really love studying the black holes even considering that this is actually the most energetic, the most extreme conditions in the universe. I hope to sort of spread my enthusiasm towards black holes to the listeners because it’s sort of like how Newtonian physics come to an end and how gravitational … and general relativity …. sort of arose as a new theory … Um … We only can learn new things … we can only come up with a new theory … when we look at the most extreme conditions.
So I hope listeners are also very excited about the research happening near the black holes and about the black holes because that’s actually really challenging our knowledge right now.
And we are always continuously questioning ourselves … is general relativity even enough? Do we need a new theory?
And all of those questions are related to what’s happening near the black holes or even maybe inside the black hole.
So I hope I have made all our listeners excited about black holes after this podcast.
Brendan: Fantastic! Well, I certainly am! So … super massive questions, super massive black holes and hopefully we’ll also see some super massive answers.
Thank you very much. Now … thank you …
Is there anything else that we should watch out for in the near future?
We are definitely going to keep our eye on super massive black holes.
But what else are you keeping your eye on?
Hyerin: Yes … I would say I think you actually covered this in your previous podcast, but definitely the JWST, the James Webb telescope.
It is bringing very exciting new results to our Astrophysics community. So this is definitely something that you want to keep your eye on.
We’re basically … by having a really, really good telescope such as JWST … we are able to look further and further into the past of our universe and therefore, it’s actually revealing what the ancient universe looked like and how these supermassive black holes that I’ve been talking about throughout this podcast actually how they started … how they even gather that mass in the time available …
We’re beginning to see that process by looking deeper and deeper into the universe. So I would say yes, definitely … JWST is something to watch out for!
Brendan: Fantastic! Watch this space!
Well, thank you so much PhD candidate Hyerin Cho! … On behalf of all of our listeners and especially from me, I’ve learned so much today.
It’s been really fabulous to be speaking with you and I’ll be watching out for your dissertation when it’s published and your papers with great interest.
I just had a look on the ArXiv server and I saw you’ve had your latest paper published three days ago.
So we’ll recommend that our listeners go and find that and … um … I even found that you write so clearly I can even understand some of the abstract!
Thank you! … especially for your time and you’ve got a gruelling schedule there and the pressures to finalize your dissertation and good luck with your next adventures and your future travels.
Hopefully you’ll come back to Australia one day !
And meanwhile listeners can tune in to Hyerin’s research papers and follow her personal and science journey at HyerinCho-DOT-com.
That’s h …y … e …r … i … n … c … h … o …DOT com.
May your career continue to be supermassive and stellar!
Thank you so much Hyerin!
Hyerin: Thank you so much, Brendan!
Brendan: This is wonderful. Thank you. Bye now.
Hyerin: Bye.
Brendan: And remember Astrophiz is free and unsponsored.
But we always recommend that you check out Dr Ian Musgrave’s Astroblogger website to find out what’s up in the night sky.
OK … a couple of notes about our episodes over the upcoming holiday season.
Now, because Ian has already included his December Sky guide in his November Sky guide, we will be taking our usual summer holidays and taking a few weeks astro-break from Astrophiz.
But rest assured Ian will be back in February to give us our February Sky Guide.
And we’ve got some amazing interviews lined up for you for 2024.
I’m looking forward to starting 2024 by attending the TDU Conference. The Transients Down Under Conference is in Melbourne and I’m interviewing some researchers from all over the world who are attending.
Transients are phenomena that rapidly change their brightness. And researchers strive to understand the mechanisms that cause Supernovas, FRBs, fast radio bursts, GRBs, gamma ray bursts, AGNs, active galactic nuclei, TDEs, tidal disruptive events, Kilonovas … when black holes or neutron stars collide.
There’s so many transients … and yes, I’ll be ready to drink some acronym soup at TDU.
But the great thing about Astrophiz interviews is that we always get our interview guests to break down their research into something that even I can understand.
I can’t wait, keep looking up!”
< Outro music: Radio waves >
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