Wednesday, May 2, 2012

From Quasars to Black Holes

Last week Scott Tremaine, a professor at the Institute for Advanced Study in Princeton, visited the University of Zurich & ETH. He is the only person I know who has an asteroid (3806 Tremaine; no worries it's quite small) named after him. He also wrote a standard textbook called "Galactic Dynamics" that is still used by many universities around the world.  I went to two of the three seminars he presented. This post is inspired by his talk on Black Holes in Nearby Galaxies (the slides are available on his website). 

Quasar core. Credit: Hubble Telescope
 What are quasars?
The word Quasar comes from quasi-stellar radio source. Quasars are among the most luminous and most energetic objects in the universe. They are about 10 000 000 000 000 times more luminous than the sun or 100 to 1000 times more luminous than a galaxy. This huge luminosity is produced by gas that heats up due to friction and glows brightly as it gets closer, and closer to the really dense object at the quasar core.

Why Quasars require black holes?
These are the top three reasons:
A. The radio jets that come from quasars are absolutely straight over timescales of 100 000 years. The fact that they can maintain their orientation over such a long timescale suggests that the source could be a spinning black hole - which acts like a gyroscope.  Needless to say, I was glad to hear that the mechanism that helps my toy helicopter run also governs spinning black holes.

B. The velocity of the radio jets is relativistic, i.e., close to the speed of light. These jets actually appear to be superluminal, but this is just an optical illusion due to the jet making a very narrow angle with the line of sight of the observer. This is consistent with the theory that quasars contain black holes because black holes are tiny and can eject mass at relativistic velocities.

C. This enormous amount of light comes from a tiny area in the sky of less than a few mili-parsecs. A mili-parsec is about 200 times the distance between our Earth and the Sun.
On the right we can see what is call "Einstein's cross". Four images of the same quasar appear around a foreground galaxy due to strong lensing. Gravitational lensing occurs because heavy objects bend the space-time, and as a result light rays coming from a background source are bent. When the lensing mass in complex such a galaxy, the observer can see multiple images of the same source. The fact that we see these four images puts a limit on the size of emitting source, which is the quasar - it has to be less than something called the Einstein Radius (Wambsgans 2006).

Of course, they could also be other kinds of compact objects like boson stars that do not contain a singularity, but, so far, no particles that could condense to make up these stars have been detected.

Where are these Quasars today?
The number of observed quasars per volume (or more precisely their co-moving number density) peaks at redshift of 2. The universe was a little more than 3 billion years old at the time. By now 13 billion years have passed since the Big Bang and we don't observe any quasars, but we observe massive black holes in the center of many galaxies and we believe they are 'dead' quasars.

Why are massive black holes at the center of galaxies? and how about multiple black holes?
 Dynamical Friction causes the orbit of massive bodies to spiral towards the center of the galaxy. In fact, more than one black hole could spiral towards the center of galaxy, which causes scientists to think that binary black holes and hopefully black hole mergers are common at galactic centers. I say hopefully because it can take a very long time (sometimes more than the age of the universe; this is called the final parsec problem) for these black holes to get closer and closer together due to gravitational wave emission and eventually merge.  The rate of black hole merges is still unknown. A gravitational wave observatory in space such as LISA would be the only way to get some statistics on these binaries.

How many black holes at galactic centers have been observed so far?
About 40-50 with masses ranging from millions to billions of solar masses.

Which galaxies have black holes?
Roughly speaking, most large galaxies have black holes at their center. More technically, almost all galaxies that have a "hot" component, which is the bulge of elliptical and some types of spiral galaxies, also harbor black holes at their centers. Their masses are correlated with properties of the host galaxy - in particular with velocity dispersion.

Why are black holes at the centers of galaxies important?
They play an important role in galaxy formation.

What do we know about the black hole at the center of the Milky Way, our own galaxy?
The Milky Way's black Hole: Sagittarius A*; Image credit: Chandra





The mass of 'our' black hole is about 4 million solar masses in an area of 0.5 mili-parsecs, which is about 3 x the distance to Neptune or about 100 times the average distance between the Earth and the Sun. The radius of a black hole of this mass would be more than one thousand times smaller. The presence of the black hole is suggested by the fact that the orbits of the stars around the center of the galaxy are closed ellipses (Ghez et al. 2008, Gillessen et al. 2009).  The smallest orbital period is only 16 years.

Since there is still so much to learn about quasars, I end with the poem "Quasar" by George Gamow. I sometimes recite it to my nephew, David and to my son, Edward instead of the standard "Twinkle, Twinkle Little Star" song and even they get confused:


"Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns
Twinkle, twinkle, quasi-star
How I wonder what you are."
George Gamow, "Quasar" 1964.
 

Saturday, March 24, 2012

Letters to past (and future) selves

 This painting is inspired by my mom
Today I read a cool letter that my former grad school colleague at Cornell, Ann Martin, wrote to her 14 year old self. Other letters to past selves are available on that site, too. All were interesting, and they inspired me to write this post where I answer questions from my former self. I will perhaps redo this in a few years to see how my opinions change as I grow older. 

Most of the letters I read sounded like everyone knew roughly what they wanted to do and did it in the end. My aspirations changed as I grew. I first wanted to become a lumberjack, later a forest engineer, and eventually professor AND a doctor. I have decided I liked physics in college.

The main message passed by all the letters is, however, very timely.   It is an encouragement that in the end everything will be well. Teenagers are, in general, more insecure than adults. Yet many of us still need to hear that message today. So, in some sense, these are timeless letters to our current and future selves as well as to today's children who relate to the insecurity.

Me at 16, my great aunt - Tusa Tavi -, and Ciupi.
When I was in middle school many of my friends (only the girls) had notebooks in which they would ask their classmates questions - starting from "What is your favorite color?" to "What do you think you'll become when you grow up?" to "How many children do you want to have?" and "At what age do you want to get married?".  While owning such a notebook and gathering memories was considered a girly pursuit, all children would write in the notebooks. This included the boys, the girls and me.  Even though I thought they were a good idea, I did not have such a notebook myself because I was shy. However, before writing down my aspirations, I would read what the other children wrote. I remember how enthusiastic they were in their writings. All the girls wanted to have careers and families. They wanted to marry by 21 or 22 years of age, and yet become doctors, engineers, and teachers. Somehow children have an innate knowledge on how to live life more fully than adults. This knowledge gets lost in time due to repeated discouragements from the world around us. Time passes so quickly that it does not make sense to over-analyze every aspect of life so much. It would instead be better to have courage to take decisions, and live more of it. 

The pictures in this post are from when I was in 10th grade - about 16.  The dresses I was wearing had been worn by my mother on her wedding day  (although in some of the pictures with Ciupi I wear a long skirt that is tied up be dress-like). I was excited that they had just started to fit me. My grandmother and Tusa Tavi (my great aunt) were taking pictures of me "all dressed up" and, of course, Ciupi, who was a very important member of the family at the time, had to join. The picture below also included Motzi - my favorite hen - but I cut her out of the picture because I wanted to look serious and all grown-up. Because I know she was there, I can still see a bit of her feathers in the lower right corner.

Below I answer some sample questions from my former self.

"If you could meet your past self, what advice would you give her?"
I would tell her to hug grandma and my great aunt, Tusa Tavi, again, and again for me and spend more time with the people around her because many of these people will soon be gone. The time spent with them will be cherished later much more than other activities thought to be so important at the moment. I would tell her that it's good to have fun, to be outside, to spend time with family and play with pets. There is no need to feel guilty about this.

Also, do not feel guilty when you have  courage. This courage is part of what will make you successful in the future.  In the incident this year where you turned around at the beauty contest and grabbed the microphone from the presenter to say you wanted to be both a doctor and a professor you were brave. It is not something to be ashamed of - while you were not demure and lady-like, your statement has likely been what people remembered at the end of the day. You are a doctor now (although not a medical doctor), and you might still become a professor.  It is important to have big dreams even if they often seem as far as the Moon, and to be brave enough to fight for those dreams.

"Did you succeed in becoming both a mathematician and a medical doctor?" No, few people (if any) would be able to do both in the same time and I remember people telling me this even then. However, I now think there was some deeper thought in this - I liked science, but also wanted to do something useful that helps people. I am still working at finding a way to merge such ambitions. I am not a mathematician, but my work is theoretical and abstract. However, it is by far not as abstract as mathematics. I now find aspects of science that are somewhat practical more interesting than abstract thoughts.

"In addition to a successful career, are you married to someone you love? do you have five children?"
No, No and No or at least not yet.  I have a career and I still love children and my family very much, though. I have one child whom I love dearly, and I plan to have more if possible. I also spend significant time with David, who is Mihai's son.

"Do you own a horse or at least lots of pets?" No, and No. I still love animals, but horses are big and difficult to care for just like mom says. Animals, in general, are a big responsibility without grandma (or potentially someone else) to care for them. It's fun for a few days, but afterwards it's work and the proper people and "infrastructure" have to be in place so that they are cared for property. I know this sounds boring and unreasonable to you, but it is true and pets do not fit well with someone who is most of their time at work.
With my best friends from highschool

"Do you understand everything or close to everything at least in some area of science?" No, many times when I read an article in astrophysics I feel I do not understand much. I have to read it many times and also look at related ideas/papers/books to truly understand it. Talking to people who have more experience and a deeper understanding of science can also make a tremendous difference in improving my understanding. However, I often do not have the time or the patience to read so much in random directions anymore. I read when I find articles and books that I think are important to understand or directly relevant to projects I work on. The reading I do in random directions is when I read to my children, which often happens to be my most interesting and memorable reading of the day even when it is in German.

"Did you continue to feel like the weird one among your classmates and friends/colleagues?" 
Yes, I did and I still do. Only now I understand that each person is special and feels their individuality, which is the "I am weird" feeling. However, scientists are sometimes weirder than other people. It is a problem that is richly described in the "The Big Bang Theory" show - abet in an extreme fashion. The main difference between me now and me as a teenager/you is that I can understand better that all people are human (including me) and make mistakes. I accept that occasional imperfect behavior as a natural part of life and of social interactions, and I am getting closer to my goal of being pleasant most of the times if not all the time. I know now that the latter is impossible. I also know now that a one-time mistake does not and should not brand people for life. I understand how lucky I am in so many ways.  I have an amazing family that has always supported me 100% in everything I did or tried to do. Professionally, I have had advisors I respected and loved as an undergraduate, as a graduate student and afterwards as a postdoc - and I felt this way about them at all times - even when I disagreed and disobeyed just like I feel about my parents.

"Are you more responsible than me?"
Perhaps a little. I understand the things I can and cannot do better, but I still try to push my boundaries beyond what seems possible. Sometimes that works and sometimes it does not. However, I try to be responsible with things I define as important and I think I always did - even as a teenager. So, you should give yourself some credit for that, too.

"Who are the most important people in your life now?"
While I have met quite a few amazing people who have influenced my life and my career, the most important people in my life are still my family. I did have a period when I thought friends were close to everything, but I think I am passed that. When I had my son, I realized that (1) the only person I can count 100% on is my mother, (2) I am very lucky to have her, and (3) her love for me deserves credit even though she will be there for me as long as she lives independent of such credit or of what I do or do not do.  Of course, other people have very special places in my life/heart just like in yours. However, you should not lose sleep over their dependability because some selfishness is natural for most of us and it does not make people 'bad'. It's just something that we all deal with. Also, people will come in and go out of your life. This is normal and it's part of growing up (and of moving a lot & no stable job/home/etc).

"Are you much smarter than me?"
While you still have a lot to learn and that will be true for as long as you live, you are already very smart. So, continue to make the best of it. Your interests and your list of priorities will change.  You will learn a lot in the next 15 years and perhaps even more later on. You will invariably need to learn how little you know of the vast mass of knowledge that's out there. However, remember that Mihai once told us that there are things that make you smart and things that make you appear smart.  In the end it all amounts to having the courage to do many things, guessing what needs to be done, and doing the work as well as possible and sometimes better than it initially seemed possible. This courage is something you already have as a built-in feature. Some of the shine and enthusiasm does go away with age, and so it's important to enjoy it while it lasts and to live in your present. You will succeed in many ways, but just like up to now you will also fail sometimes, and you will learn to make the most of both successes and failures without getting to disappointed. Things will always work out and often for the best.

"Do you finally know what you favorite color is?"
No. I prefer bright, happy colors. I like contrast. However, I still do not have a favorite color. I now think all colors can be beautiful in the right light/tone just like people.

Disclaimer 1. I did not live on a farm. Although I think I would have liked farm-life. Most children would love it as long as the animals are treated well. My grandparents lived in a Victorian house with high ceilings and huge doors in Lugoj.  They did have a small yard and a small garden. We had a few pets including chickens, some pigeons, Ciupi the lamb, a cat, and a dog. In Timisoara, we only had a cat.

Disclaimer 2. I did not look like a nerd - at least not in pictures. I have never considered myself a nerd. I was not into computers games, science fiction books or into breaking & fixing things. I liked being outside, and I loved my family, most animals, swimming, and, of course, reading all sorts of books, and doing math. I did believe being smart AND beautiful was a valid option, and I have worked hard all my life to become smarter.  Beauty was something that happened to be there. I wanted people to like me, but I never worried too much about my manners or my looks. I did not typically wear make-up or stuff like that, but somehow I did want to be a model as well as a doctor and mathematician. Of course, I did not end up joining any of those professions.

Wednesday, February 1, 2012

New Blog

 I have started a new blog today about my brilliant science reading. So far I just posted titles and authors of papers I find interesting on the arXiv.  In the future, I will read some of them and explain what I understand from them.  Eventually, I might organize them by topic and not by date and maybe expand to books I read, and then I'll have to switch to a wiki. For now, the point is to get in the habit to read more and understand what I read better (and sometimes why I read on a given topic).  Once this is accomplished, the means of communicating is relevant only if enough other people read the information. At this point, I am not convinced that my readings and thoughts will be interesting enough for the real world and so the documentation is mostly for myself.

The link is below:
 Ruxandra's Science Reading

Why a new blog? Because I did not want to clutter this one with 20 to 30 papers a week. I want to keep this blog for more important posts or at least important for me.

The Sultanate of Oman?

Mihai went for an interview in Dublin for a position at a military college in the Sultanate of Oman. This is a country I did not hear of before he received this job interview. Its neighbors are Saudi Arabia, the United Arab Emirates and Yemen. My brother is the ideal person to hang out with if you want to have an exciting life!

We do not know any of the details yet, but it appears that he will get the job - or at least this is what he understood from the interview. He would be department head in physics at an university in Muscat, the capital of Oman.  Once/IF he receives an offer, we will try to understand the contract and the region better. I am curious and hopeful that such a position will come with the freedom to build a good physics department, but Mihai seemed very worried and afraid to go to a 'military institution', which would involve teaching the mechanics of missiles and other related stuff.

Update: I still want to visit Oman or Dubai at some point, but Mihai never got the job offer. Maybe we'll visit next winter.