Friday, December 7, 2012

ACMEScience News Video Interview

Mihai and I talked to Samuel Hansen of ACMEScience News Now about the applicability of atomic clocks to every day life. This was our first video interview over skype. The lighting in our house was not great, neither was our attire and I mumbled a little, but the video does get the main ideas across if you are interested in atomic clocks and I hope we'll do better next time.

Samuel is a great host! You should support his series of interviews because they give a glimpse of some of the best science available to the general public!

The movie can be found on

and on youtube (you will also find it if you just search for Mihai Bondarescu or Ruxandra Bondarescu on youtube):

The animation in the video was created by Thomas Glauninger and Daniela Isch of the University of Zurich IT team.

I wanted to post the video below as well, but Sam's description does not show up and I am not sure if the pageviews are counted on youtube if I post it here.

Tuesday, September 11, 2012

Mapping the substructure of the Earth with atomic clocks?

My first geophysics paper (published in Geophysical Journal International; arXiv and widely mentioned in the news) discusses how one could use the tick rate of atomic clocks to find density anomalies within the Earth (e.g., water, oil, mineral deposits). This method provides the most direct, local mapping of the geoid and could be combined with existent geophysical surveying to improve our understanding of the internal structure of the Earth. This work was described in the press in English (MIT's Technology Review, New Scientist,, University of Zurich press release), French (Futura Sciences), German (Welt der Physik), Swiss German (Sonntag Zeitung and the German version of the UZH press release) and Russian (Science Compulenta). Once the university press release appeared, the paper was picked up again by various sites: spacedaily, sciencedaily,, sciencenewsonline,,, redorbit,,, rdmagazine, IEEE Inside Technology Spectrum, etc - eventually, I lost count. However, my favourite is the EEE Spectrum article (Note: Gold would be very difficult to find using this method because it's mixed up with sand and other materials; so while it sounds cool, it is not a good example).

So, what is this paper about? We point out that super-accurate time measurements could help us gain better understanding of the interior of the Earth on the local scale by measuring minute changes in the tick rate of clocks due to their proximity to heavy objects. As optical atomic clock technology improves, networks of atomic clocks and/or portable clocks could monitor the underground. Learning more about the interior of the Earth would lead to better mapping of mineral deposits, monitoring of movements of tectonic plates (perhaps help with earthquake prediction?), mapping of magma dynamics under volcanoes, and much more.

Gravity and Time
The slow down of time near massive objects is a direct prediction of General Relativity.  Time at the bottom of a cup of milk is slower than time at the top (if you waited for 15 million years, the clock at the bottom of the cup will be about 10 seconds late compared to the clock at the top.) This minute time difference is very important in our every day life. If it did not exist there would be no gravity and the milk would not stay in the cup. Current atomic clocks are accurate enough to measure such tiny time differences: already in 2010 in "Optical Clocks and Relativity" (published in Science) Chou et al. were able to measure the difference in the gravitational potential for two clocks - one placed 33 centimeters above the other.

Clocks and the Geoid
An observer at the mean sea level observes constant tick rate of clocks (after corrections for tidal effects, etc). The surface of constant tick rate is called the geoid, i.e., the equipotential surface that corresponds to the mean sea level. Clocks above the mean sea level tick faster, while clocks below this sea level tick slower. The geoid is extended from oceans to land, but there it is more difficult to determine.  Clocks measure local geopotential differences and are thus the most direct possible way to determine the geoid. One could place a clock at sea level and another one somewhere on the continent, and use the change in tick rate to measure the geopotential difference and deduce the geoid.

Clocks and the interior structure of the Earth
Clocks are not only affected by their distance from the center of the Earth, but also by underground structure. A clock located over a heavy rock is slower than one placed over an empty cave. We want to understand the internal structure of the Earth to find minerals, gas, oil and water. We would like to predict when and understand why earthquakes and volcanoes happen, and also how we as humans affect the mean sea level, which accounts for the effects of all subsurface density variations. It is possible to build a network of optical-atomic clocks. Fiber optics provide extremely accurate transmissions of better than one part in 1019. The technology of atomic clocks is on the doorstep of reaching an accuracy level in clock rate (frequency ratio inaccuracy of 10−18) equivalent to 1 cm in determining equipotential surface (including geoid) height.  Our calculations suggest that such clocks are sensitive to geoid perturbations caused by spherical structures of a radius of about 1 km if the structure is close to the surface and has a 20% density contrast with the upper crust. Bigger structures can be resolved to higher depths of tens of kilometers. Disclaimer: These are very simple theoretical estimates that suggest that further study is needed.

Joint Gravity and Optical Clock surveying?
 Atomic clock geopotential surveys could be used together with relative gravity data to benefit from their different depth sensitivities. We learn in highschool (and the in the first physics course in college) that the potential of a point mas is proportional with 1/R  and the gravitational acceleration g is proportional to 1/R2, where R is the distance to the point mass.  A sphere generates the same change in the gravitational potential and in the acceleration as a point mass as long as the observer sits above the center of the sphere. In this simple example, once can divide the potential by g to find the depth to the center of the object. Finding the distance to all underground structure would amazing! However, we do not know the shape of density anomalies and so, in general, determining the depth to an underground structure of unknown shape & composition is much more challenging. Therefore clock measurements are just another piece of the puzzle in solving the inverse problem that determines the shape of underground structure.

Clarification for article about this work:  The advantage of atomic clocks is that they provide the most direct possible geoid mapping at the location of the clock.  Atomic clocks measure potential differences and not derivatives of the potential. The disadvantage of satellite geoid maps is that they provide low spatial resolution limited by the distance between the satellite and Earth (i.e., a few hundred kilometers). A direct measurement of the geoid from the ground bypasses that problem.  However, it is wrong to say that the accuracy of atomic clock measurements would be higher than that of gravimeters or than that of satellites. Gravimeters have very high accuracy, but they measure the derivative of the potential in one direction (that pointing down).  The direction of g cannot be measured accurately, and thus the change in geopotential is poorly defined. It is also important to keep in mind that local measurements are unlikely to replace satellite measurements.

Atomic clocks can calibrate satellite measurements and add detail to satellite maps, but not  replace them because it is physically difficult to deploy instruments and move them around the Earth. Clocks can be placed at locations with high seismic or volcanic activity for better monitoring. Portable clocks may be transported in trucks in regions of interest. Atomic clock measurements should be used in conjunction with gravity and gradiometric measurements. It is difficult to understand and ultimately map underground structure with data from instruments at the surface of the Earth. In order to improve our understanding of the interior of our planet we need to combine instruments to obtain as much information as we can get.

Have I ever seen an atomic clock?
An atomic clock
Yes, see picture. I visited Dr. Steve Lecomte's laborator in  Neuchâtel. They miniaturize clock parts for space missions and other applications.  His group developed a prototype for the Hydrogen Maser that will be placed on the  the International Space Station in 2017 as part of ESA's Atomic Clock in Space (ACES) mission.  I learned that building instruments for space is much more difficult than for Earth-based experiments because all components have to space qualified, which means they can never risk by using the latest technology.

The most accurate atomic clocks to date, which have a frequency inaccuracy close to 10−18, are, at this point, still laboratory size devices, which typically fit in a room with a big table. CSEM also builds very tiny atomic clocks of the size of a regular watch, but these have much lower accuracy (~ 10−11). So far they have not been able to build clocks that are both tiny AND highly accurate AND reliable. There are ultraprecise (~ 10−17  over integration periods of a few hours) atomic clocks that are small enough to fit in a truck, have been tested for portability, and can perform measurements of geophysical interest. However, they are still very expensive and not reliable enough.

How would the portable clock be synchronized to a reference clock?
One could connect atomic clocks via fiber optics wire, which have reached accuracies beyond those of the clocks, and use them to map the geoid of the Earth. They could also use a telecommunication satellite. The challenge here is the transmission through the atmosphere, which can be turbulent and introduces errors. In some areas, a dense network of clocks can use wireless communication, which has been tested over 2 km in free space at 10−18.

Of course, the GPS has been used in various areas of science including geophysics (e.g., to study how much the ground moves on top of volcanoes).  The accuracy of the GPS clocks is about 4 orders of magnitude below that of the best optical atomic clocks. However, they are used to measure distances. They shine light receivers on Earth to measure the position of a given object relative to the satellites. Here we talk about clocks measuring the slow down of time due to presence of mass, which is a different effect.

How did Mihai and I ended up working on this topic? What is the story of this paper? (Warning: stories about articles are typically boring. Proceed at your own risk.)
 Highly precise clocks with a frequency inaccuracy of 10−18 have been proposed for the space mission STE-QUEST, which could have been one of the five medium size missions to fly in 2022+. Prof Phillipe Jetzer, who is my postdoc advisor, was appointed on the science team of this mission. So, he asked us to read the STE-QUEST science document and think of potential applications for the technology developed for this mission, and for potential science that could be tested by STE-QUEST. The atomic clock work falls in the former category. My brother, Mihai, who is a highly creative person with a Caltech PhD, immediately thought that we could find oil with portable atomic clocks of this accuracy, and potentially bound the dark matter halo of the Earth because the satellite is supposed to have an elliptic orbit (The direct bound on the dark matter halo around the Earth is weak, but that is another story). If a clock can be placed on a space mission, then it has to be portable.

After Mihai and  I convinced Phillipe that this idea was relevant through a back of the envelope calculation, the three of us then talked to Lapo Boschi, a geophysicist at ETH-Zurich. Lapo mentioned a seminar by another colleague, Gyorgy Hetenyi, who is involved in gravity measurements. After more discussion with Lapo and Gyorgy, we concluded that the paper was worth writing with just a simple example vs. a complicated simulation. So, I ended up writing this paper in collaboration with them and with Prof. Jayashree Balakrishna, a long-term collaborator of mine from Harris Stowe State University, who patiently read through geophysics books and articles with me.

The old telescope
View from Neuchâtel observatory

Sunday, August 26, 2012

Edward turns two!

With Grandma
My son turned two in August 2012! Overall, I am proud of my little guy and thankful for having him. Both Edward and David (my nephew) are two healthy little boys who like to play outside. They run, and jump, and yell, and spit sometimes, too - they like to spit upwards and then run and laugh.

The following Edward-facts are true at this point:   He loves his cousin David and sometimes calls David's name even in his sleep. His worst nightmare seems to be dreaming that David does not share a toy with him.  David is only 5 himself and is really good at sharing, but I try to not have unreasonable expectations.

Edward speaks fluently in Romanian, and also speaks some English. He responds in English if he is spoken to in English. He says most words correctly, but every once in a while refers to "biloza" instead of "buldozer", "papac" instead of "capac", "pingea", etc. He still refers to himself as "tu/you".
He has been diaper-free since April 2012, and we have had very few accidents since then.

 We live close to the lake in Fallanden and so Edward loves playing in water, and trying to swim.  He still needs to be pushed along to advance in water, but I am glad he accepts the water and that my mother and I can swim around him vs. sit on the shore.
In the lake
On the bike
The boys.

Edward is learning to roller-blade and rides a bike without pedals.  Another thing he loves is having us draw animals, insects, and planes and cars for him. His favorite animal is the cat - both drawn and in real life. He is starting to be able to draw himself also and his drawings do make sense - although at the end he adds a number of random lines to them.

The birthday cake
When it rains and the snails come out, Edward asks me to pick every one of them to throw them in some green area so that they do not get stepped on. He likes counting them, too:  "3, 4, 2". In the evening or when the weather is not good enough for swimming, we go hiking.  Edward can walk to the top of the hiking trail behind our house and back without asking to be picked up. Of course, "can" is the key word here, which means he does not always want to walk on his own. Nevertheless, he is doing well for a little fellow of about two years of age.

Tuesday, July 24, 2012

Detecting Gravitational Waves in Space?

The LISA Space-Craft. Credit: AEI/Milde Marketing/Exozet.
 Gravitational waves are extremely weak vibrations of the space-time. The fabric of space-time is very stiff. By the time they are close to Earth, the gravitational wave strain for some of the strongest sources is less than 10-20.   These waves travel through the universe at the speed of light and pass unaffected by obstacles. Some are believed to carry information from the beginning of the universe. This post is inspired by the LISA Symposium I attended in May 2012 in Paris. I have waited longer than I should have before writing. So, I cannot give too much detail. I will summarize what I remember from that symposium almost three months after the meeting. The talks are available online if you are looking for more details or for other aspects of the gravitational wave science that were presented and are not mentioned here.

What I learned in Paris
My brother, Mihai, and I
The US side: Since LISA was not selected to fly either in the US or in Europe, there was a call to re-examine the LISA concept from NASA with the aim to develop a concept for a cheaper mission. A series of potential space based gravitational wave detectors emerged. However, most were LISA-like, and all cost estimates were around one billion dollars. They showed it is so far not possible to make a cheaper gravitational wave mission in space.  It did emerge that perhaps there could be multiple LISA-type missions in the future launched by different countries.

The European side: 
Attendants of the Paris meeting. Proof that we were there.

 Even though eLISA/NGO was not selected as the next large European mission, its science was selected unanimously by ESA reviewers as the best among the three proposed large missions. The technology preparation for this mission is going forward with the LISA Pathfinder, which is half a billion Euros enterprise with a planned launch of 2014/2015. So, LISA is not 'dead'. It is going forward with the LISA Pathfinder.  If the Pathfinder is successful, it seems unavoidable that a LISA mission will fly afterwards. The waiting and the preparation process is extremely difficult for any mission. The first LISA-concept was proposed in the 1980s. The Pathfinder will be launched in 2015+ and the mission itself maybe in 2026+. I have only been to one LISA Symposium so far. The enthusiasm built is beautiful to watch, but I also felt some of the pain of the many people who have studied this mission for most of their lives and may not even live long enough to see it fly.

Most space missions take very long to plan and to be launched. This is natural. However, to me as a human it is very difficult to imagine the 2030s. How much of the technology developed now will still be useful then? In 2032 I will be 60.  I've turned 30 in September 2012. I cannot imagine myself being that old yet...perhaps I'll look like my mother, but how will the world around me look? will we have quantum computers by then? will that change our way of life entirely?

Other countries
China and India seemed interested to launch the satellite for free, i.e., in exchange for technology on building the launch vehicle. At this point it is unclear if a future LISA will fly from Russia, the US, China, or India. China has some LISA-type research going on. They plan a pathfinder that will fly in a low orbit to measure the geoid of the Earth. The geoid is called the true figure of the Earth - it the equipotential surface at sea level.  A LISA-type mission will not be in a low orbit and so their naming scheme is confusing. However, the European pathfinder contains an accelerometer that is several orders of magnitude more precise than the 'state of art' accelerometers used in geophysics and, if flown in a low orbit, could potentially measure the geoid. A more accurate geoid measurement will help us map Earth's sub-structure, i.e., find water, oil reservoirs, predict  Earthquakes, etc, and so flying such a satellite would be of high impact.

What about Earth-based detectors? Pulsar Timing?
Scientists have built several gravitational wave detectors on Earth: two LIGO detectors in the US and VIRGO in Italy. They were operational until last year, and they are currently being upgraded. We had an updates from the LIGO and VIRGO teams, and it seemed that the design sensitivity is expected to be reached later ~ 2018 and that they expect a few sources per year. Other detectors are being planned in India, Japan and Australia, but there was not much discussion about that.

Pulsars are also natural gravitational wave detectors. If a gravitational wave passes between a pulsar and the Earth, it will affect the timing between pulses. However, it is important to remember that Ground based, space based and pulsar timing gravitational wave detectors are all sensitive to different parts of the frequency spectrum, which makes these three different techniques complementary and not in competition with each other.

Why build a detector in space?
When a gravitational wave passes by a detector, it compresses and expands the space around the detector by very tiny amounts. On Earth, the compression and expansion measurements are contaminated by deep noise due to the activities going on around the detector, e.g., people cutting trees nearby, traffic, Earthquakes. This noise is very difficult to remove from the data, but scientists have made significant progress in that direction in previous LIGO and VIRGO runs.  However, no gravitational wave have been found to date. The data from the Earth-based detectors is not public in order to avoid false claims of detection and loss of credibility.

On the other hand, space is a quiet environment. However, building a detector in space is much more difficult than building one on Earth. It has to work perfectly from the beginning.  Once it has flown it is very expensive to impossible to fly up there to fix it or make adjustments. So, scientists chose to build the Earth-based detectors first. The hope in the gravitational wave community is that the space based gravitational wave detectors will be built and will fly in the next three decades. We expect to detect many gravitational waves with both Earth-based and space based detectors that will open new windows to the universe, and we hope to hear some of the many untold parts of its history.

Tuesday, July 10, 2012

Visiting Stockholm in the Kingdom of Sweden

The Golden Room in The City Hall
In the first week of July,  I went to the 13th Marcel Grossman meeting in Stockholm. I presented a short seminar on the "Physics of the Far Future" and I will have to submit a conference proceeding about it by February. Initially, I wanted to go alone, but in the end I decided to take the children and my mother. Of course, Mihai and Andy came to attend the meeting. So, everyone was there and we had a reasonably good time. Traveling is a nice way to learn new things, but somehow I always feel relieved and happy to be back home and for the next two years home is in Switzerland.

First impression of Stockholm 
The Blue Room
Stockholm in July is warm and very beautiful. The Kingdom of Sweden is a rocky country with many lakes, and trees, and lovely parks. The average height in Sweden in 1.82 meters for men. I am 1.78 meters and female and Sweden is the only country I have visited where I did not feel too tall. We visited the Skansen museum, the Stockholm's City Hall, the university where the meeting was, and some parks around the hotel. Skansen is a famous Swedish open air museum with a nice zoo and many traditional old houses, which give a glimpse of Sweden before electricity and industrialization.

The City Hall
The Nobel Prize ceremony is held in the Blue Room of the City Hall in December every year. This is where the reception for our conference was held as well. The room is not blue. The architect changed his mind when he saw the beautiful red brick walls of the Blue Hall and decided against painting them blue. After the reception, we visited the Golden Hall (see picture), which is another room in this beautiful building. The golden part of the name comes from the 18 million pieces of gold mosaic and glass covering its walls.  The mosaic figures on the walls looked like the painted walls in cathedrals to me. However, they do not have much to do with religion. They represent important events in Sweden and in the world.

Familiar with dynamite?
On our way to the meeting we were told that we have two minutes to pass on fairly long stretch of road before 'the blast'. We hurried and made it in the two minutes, and than we heard THE BOOM. There was a siren that was announcing the blast as well. It turns out that dynamite is used frequently in Sweden due to the rocky nature of the soil, and we just happened to be along when they had to blow up something. This placidity toward dynamite shocked me a little, which shows I am not of Swedish ancestry. It shocked Andy, too, and he is a quarter Swedish.
An old bike at the Skansen Museum
The Reindeer

The Big Cats and their litter
The snake - this fake animal is what the kids loved most

Sleeping upright?
I learned that beds were really short in the 18th century because people would sleep sitting up tied with some kind of leather belts; they did have some pillows to lean on.  Retrospectively, while it still is surprising, it makes some sense. The reason for this apparent self-imposed torture is that they were afraid of dying in their sleep. It is strange that I have never seen this type of sleeping in a movie yet; in fact I have never heard of humans sleeping in upright or sitting positions all the time and out of choice before this visit to the Skansen museum. It seems that due to the various diseases for which there was little medicine, it was easier to breathe sitting or standing than lying down, which is true if one's lungs are filled with fluid. My 0th order guess is that people must have felt more comfortable this way when they were sick, and that they were sick for so long that they became used to this sleeping position.

Conclusion: in the light of the habits of our ancestors, planes, buses and trains are a natural places to sleep. Oh... and one could say the same about lecture halls and meeting rooms, which explains a lot.

The Discovery of the Higgs announced on the 4th of July
Peter Higgs was invited to CERN after the discovery of the Higgs boson was officially announced. The Higgs boson gives mass to all other particles when it interacts with them, and is a crucial pillar of the Standard Model, which describes electromagnetic, weak and strong nuclear interactions among subatomic particles. Note that even though most people think only of Peter Higgs when they mention the Higgs boson and the Higgs mechanism, the seminal papers of 1964 that described how particles could acquire mass were written by six people: Robert Brout and François Englert, Peter Higgs, and Gerald Guralnik, C. Richard Hagen, and Tom Kibble; Higgs was the only one who was the sole author on his paper and had the name that became associated with this new particle. These six scientists received the Sakurai Prize in 2010, and some of them will receive the Nobel prize in the next few years once it is confirmed that this particle is truly the Higgs boson. 

Some of the first seminars about the Higgs boson were at the Marcel Grossman meeting.  The mass of this new particle is about 125-127 GeV (133 times the mass of the proton). The two experiments at CERN: ATLAS and CMS reported slightly different masses with CMS announcing a mass of 125.3 ± 0.6 GeV and ATLAS a mass of ~ 126.5 GeV both with a 5 sigma significance. The corresponding percentage for 5 sigma is 99.9994, which means that pure statistical fluctuations will give a result in the 5-sigma range 0.0006 percent of the time. So, scientists are sure they have found a new particle, but there are still 10+ years of work ahead of the Large Hadron Colider at CERN. They will continue to study the Higgs and its channels of interactions over the next few years to understand which version of the Standard Model is true. A Higgs found with this mass does not rule out supersymmetry. The hope is that the Higgs is not part of a "vanilla Standard Model" and that exciting new physics is very near.

Monday, July 2, 2012

Two Weeks in Romania

This was my first visit to Romania in four years. It was fun to be back! I saw some of the places I had loved, spent some time with my father and spent more time with Edward and David. However, four years is a long time to be away ... too long.

Old Diary
I found my dairy from when I was 12 in my grandparents' house. It only has a few pages. I must have not liked writing too much at the time, but it is interesting to read now.  It has many "exact" facts. For some events I wrote the exact hour and minute, but I did not write the day, month or year. I also liked drama and so I wrote about a big storm and the 1989 revolution and not much about my every day life and thoughts, which I would have found interesting now. The facts trend can still be seen in my writing today. It's difficult to write about real people and feelings without offending and I still avoid writing that is too personal.

Excerpt from my 1994 diary
"It was 6:05 in the evening when Mihai reached the train station in Timisoara. Mother was waiting for him. In five more minutes a terrible storm started. It was suddenly pitch black. The wind started blowing so hard that the big trees on the streets were falling to the ground. Any birds caught in flight were thrown down and hit the asphalt. The rain was very heavy. Water reached 1.9 meters on some roads in less than an hour. Under the bridge on which the railway passes, the cars were completely under water. The submerged cars created an alley on which people could step.

Mother and Mihai reached home safely. The next morning mother had to go to the hospital for work. Mother is a doctor and so it is very important that she reaches the hospital. She had to swim part of the way or to jump from tree to tree just like Tarzan. Up to now, it was reported that three people were found drowned and 26 were injured and taken to the County Hospital. During the storm the town leaders left for safer places, but have returned since and the government is evaluating the loses.

I have concluded that it is most important to be healthy, and I am very thankful that mother and Mihai made it out of this storm safely. Grandfather passed away this year. His death was the saddest thing that has happened to me so far.  He must have asked God to let him watch over mother and Mihai during the storm, and continues to watch over all of us. However, I also think that the watching over family on Earth cannot be all he does and that he must very busy up there with things that we cannot understand."
Ruxandra (at 11-12 years of age)

Recovered "Work of Art" (nowadays everything can be called art)
The 'work of art' on the left is from that period. It is proof that I was terrible at drawing and I still am.  It is meant to show a young princess with a crown among flowers, green grass and trees. The blue balloon-like things must have been UFOs. My brother was fascinated by UFOs, extraterrestrial beings and the paranormal in that period, and would read tons of books in these areas. Such books seemed boring, arid and technical to me. The facts/stories presented, which were too poorly understood for the claims made, never seemed worth the effort of reading the books. Retrospectively, these topics are a natural inclination for a boy entering his teenage years with a vivid imagination and an interest in science. Mihai had read all of Jules Verne's books that were available to us (over a dozen) long before he entered high school. However, he was not interested in the more modern science fiction books like most of my current colleagues because they did not have much connection with reality. Instead he transitioned from Jules Verne to reading about UFOs and  paranormal in potentially real lives, and later on to real science.
How did it feel to be back?
In many ways it feels as if I had never left. When I visited before, I was in graduate school and my grandmother and great aunt had recently died. The house felt empty and full of shadows of a past with more people and more laughter in it.  In addition to feeling sad, I wanted badly to go back to a time when my life had less work and 'busyness' in it and was fuller of people I love and miss dearly, but I knew I could not go back and felt I could not go forward in the less busy direction - at least not yet.  To continue the busyness while at home, I helped former classmates and children of family friends with college and graduate school applications for US schools. However, there was still a strong feeling that while I was very busy, my life was very empty.

(Note that I am a physicist and I do know it is impossible to go back in time. Going back in time implies traveling faster than light and that violates the founding principle of Einstein's Theory of Relativity.)

Now, with the children here, it's all joyful again and it feels so right to be back. The children are happy to have a private back yard and other children of similar age with whom they can run around the block.  There are a bunch of stray cats whom we have been feeding over the past few weeks, and we have a tenant who has a very friendly dog and several friendly neighbors who love children and know how to behave around them. Edward saw chickens, rabbits, a goat, and a horse in addition to dogs and cats.  He enjoyed feeding our cats and playing with the various toys bought for David and his brother Danny, which are new to him and kept him occupied in the evenings or when it was too warm to be outside. I still spent some time helping a neighbor with her highschool graduation exam in Chemistry. She passed the exam, and helping her prepare for it took way less than helping someone prepare 20+ college applications.

My father
My father looks the same as I remember him. He is just a bit older with whiter hair and a whiter beard. He is less interested in life than he used to be, which could be a sign that life is slowly slipping away from him, but he still comes across as a very stubborn person with a strong personality.  He spends too much time watching TV. However, he is taking care of the garden, of Puppy, a former stray dog that my sister-in law brought home when David was younger, and he also partially manages a house we are renting. He does not seem to see or feel an imminent end to his life and I hope this means he still has at least a few more years to live. One thing that surprised me when I returned to Romania this time is how much Mihai behaves like my father did when I was little - in some of the pleasant and of the less pleasant ways. It's sometimes feels so much like going back in time that it is scary. I guess genetics is very important after all, and it is difficult to impossible to stray too much from one's roots. Somehow with all this travel and education obtained all over the world we move forward while being still tied to the past and to our family through parts of ourselves more than through other people and material things.

More of the past two weeks
Edward went swimming for the first time! He also got his first fancy haircut. It was really more of an adjustment; he was not too patient, but our hairdresser was able to cut the uneven parts off his hair; David, my mom and I also have new hairdos. I have short hair now, which fits the really hot weather.  Oh...and  I was told I still look 16 by our next door neighbor. It felt good to hear I don't look as if I am 100 - I am turning 30 in September. The compliment may be related to the fact that I have been wearing some my clothes from before I left home. Some still fit, but not all.

My mother, Tusa Tavi and me in 1983
On Sunday we visited the house in Lugoj where my grandparents had lived. It is a lovely Victorian home with high ceilings. The yard is overgrown with weeds at the moment. It used to be full of beautiful flowers that seemed to only get bigger and lovelier as the years passed (see picture below; my mom was about 6 years older than I am now).

The children had fun sitting on the porch, which is still cool even in really hot summer days, and Mihai changed some broken shingles in the attic.  The roof is leaking in one of the rooms. I hope we will be able to fix it before the fall comes. The house has been empty since my grandmother and great aunt passed away, and I still have a feeling of time stopped in place every time I go there. It is still full of old things that my grandparents had or that Mihai and I used as children.

How long have I been away and where is "home" now?
At the end of July I am entering the 12th year since I left Romania to go to college in the US. In all this time 'Planet Earth' has been my true home and the target of my various jobs/job applications. I live in Switzerland now, but I still feel home in Romania in so many ways that are difficult to explain and I am considering returning to Romania in the future.

My family lived in Timisoara from when I was 6 until I left home at 18. While growing up, my brother and I had spent all vacations with my grandparents in Lugoj, a small, quiet city about 60 kilometers from Timisoara.

Why the four year gap between my visits to Romania?
 Where was I in the past four years?  I was a postdoctoral scholar in the US at Pennsylvania State University from 2008 until October 2011 when I moved to Zurich. The Penn State astrophysics and physics programs and the scientists there are pretty amazing. I loved Penn State just as I had loved Cornell as a graduate student, and the University of Illinois in Urbana-Champaign as an undergraduate. I highly recommend all three schools.  In addition to the brilliant local faculty, we had many external speakers whom we were encouraged to meet and significant freedom to travel and work on projects of our choosing, which is partly why the topics of my research are so varied. The other reason for my all-over-the-place-research is my personality. I find it difficult to work on one project at a time.

After I moved to Zurich, it took some time to find a house, to get accommodated, etc and the 2011 year passed very quickly.

Why so difficult to travel from the US to Romania?
As a student I did not have problems, but the work visas are so much more painful to obtain and extend. On an H-1 visa somehow the number I entered the country on was used to pay me and exiting on an existent visa meant invalidating any future-already approved visas.   Oh...and I had to change visas every year and re-apply 6 months before changing it. My last H-1 visa required significant work on the part of my postdoc advisor and the secretaries to prove to US immigration that I was not qualified and could receive the minimum payment allowed by this visa. Nobody can do much to improve the situation at this point because immigration is so unpopular in the declining US economy.

My parents have visitor visas for the US. My father did come to visit me once in the fall of 2010 and my mom came twice over this four year period. However, my father has a heart condition and such long trips are very tiring for him. So, the distance itself is a problem and I am glad to be on the same continent now.

In Switzerland, there are work permits just like in the US, which need to be renewed every year for the first two years, and then I perhaps can receive a longer permit.  However, these forms have no connection to how many times I exit the country. So, I can go to professional meetings outside Switzerland and visit my father as well; and, of course, I can also travel for fun.

How was the trip from Zurich, Switzerland to Timisoara, Romania and back?
Long and tiring. There is no direct flight from Zurich to Timisoara. On the way there: Edward (1 year and 10 months at this point) and I took a train to Milano Centrale (about 5 hours total) through Bern. Then we took a bus from Milano Centrale to Bergamo. Afterwards I took a cab to the nearest hotel, and another cab to the airport the next morning, and then a flight to Timisoara at 7 a.m.  The way back was all in one day leaving Timisoara at 6:00 A.M., reaching Bergamo at 7 A.M., and then taking a bus and several trains and other buses. In general, there is a direct train from Milano to Zurich, but there was a rockfall and land slide on the way and a bus goes around that. Slow, regional trains have to be boarded afterwards. So, after leaving our house in Timisoara at 3:30 AM (we did not sleep at all that night), we reached Zurich at 4 PM. Going through all this with two small children was not fun.

We went to Romania again at the middle of August and left in September. My mother and I drove this time.  It took two days each way, and it was an equally long and tiring trip. The first day the children were OK, but the second day seemed just too much for them. Perhaps I will try taking the train next time.

Friday, June 8, 2012

The Transit of Venus

Credit: NASA Solar Dynamics Observatory (SDO)
The transit of Venus was over on June 6 and I did not have time to finish this post while it was happening. However, this transit is rare enough that it's worth mentioning even belatedly. Indeed, another transit of Venus will not occur this century.

A few basic facts
The transit of Venus occurs when Venus passes between the Sun and the Earth and becomes visible against the solar disk. The planet  can be seen as a small black disk that moves across the sun obscuring small portions of the solar disk for a duration of a few hours.  In 2012, the transit lasted six hours and 40 minutes and was observed over two days: June 5 and June 6. The radius of Venus is a little over 6000 km. Venus is a rocky planet of about the same size as the Earth, and significantly larger than the Moon. However, Venus appears to obscure a much smaller portion of the Sun than the Moon from Earth because it's further from us. For this reason, the transit also is slower than a solar eclipse.

A bit of History
The last transit of Venus occurred 8 years ago in June 2004. These transits are predictable. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 year. The eight year gap is because the length of eight Earth years is almost the same as 13 years on Venus, so every eight years the planets are in roughly the same relative positions. The next transit of Venus will occur in December 2117, and the previous one was in 1882. The transits are so rare because of the inclination of the orbit of Venus relative to the Earth. The first transit was observed in 1639 and was used by Jeremiah Horrocks to correct Kepler's prediction for the orbit of Venus. Many people observed the transit of Venus in 2012 the same way Jeremiah did it so many years ago by focusing the image of the Sun through a simple telescope onto a piece of paper.

 How can scientists learn from this event today?
Scientists will use this event to test methods used to characterize extrasolar planets from transits events against what we already know about Venus, and to learn more about Venus. We can see how accurate is the prediction for the diameter of Venus relative to the diameter of the Sun from the dip in the sun's brightness. This event is also expected to improve our understanding of the atmosphere and climate of Venus, and to provide a testbed for methods of finding the atmospheric composition of extrasolar planets from transit events.

Did I see the transit of Venus in Zurich?
Not really. It was cloudy and too early in the morning. I have to confess I am not much of a practical astronomer/observer. I am more interested in the science that we can learn from events by analyzing data from instruments much more powerful than my eyes rather than in observing events myself. I do love to look at the beautiful images and videos that NASA provided for this transit.

For those who remember the transit of Venus in 2004, there was not so much talk about it then even though the transit looked similar by eye/small telescope/fancy glasses. What makes this event more dramatic for me is that we have the technology to learn a lot from it now, while we did not eight years ago. Of course, it's also the last time we'll see Venus transiting in front of the Sun in our lifetimes, and I am glad 'the world' is making the most of it.

More beautiful images and videos
If you search for the transit of Venus on youtube you will find several videos from NASA including one where they photograph it from the International Space Station.

There are also images and videos that can be downloaded directly from NASA website:

Venus and dogs
This is a topic that everyone and their dog* is writing about (quote from David Tsang, postdoctoral scholar at Caltech)**.

* some of my astrophysicist friends at Caltech have dogs
**Dave's comment was related to my suggestion of writing a paper on the two solar mass neutron star that was observed in 2010. Of course, neutron stars are not directly related to Venus, but Dave's expression was something that popped into my head while writing this post. Why? Because it's applicable to this post as well. There is a lot of information on the transit of Venus already on the web (and in books), but I wrote about it because it made me understand it better.

Thursday, June 7, 2012

A Week in Paris

Chasing the birds in Paris

In May I went to Paris to attend the LISA Symposium (read the gravitational waves in space post if you are interested in the science; however, the trip to Paris deserves a post of its own.) The conference was held at the Bibliotheque Francois Mitterand, the National bibliotheque in Paris. It is a beautiful building that was named after France's longest serving president. It hosts the globes of Luis XIV, the longest reigning king in European history.  He reigned for 72 years and 110 days and became king before he was five years old. It is difficult to imagine trusting my five year old nephew, David, to be king and to lead a country. Of course, king Luis XIV had plenty of help in the beginning - with his mother and some cardinal being the actual rulers. However, he was considered of age at fifteen, went through the ritual of coronation, and eventually ended up being the Sun King, one of the most successful kings of France. So, it maybe that in the current epoch we ask too little of our children.

The globes of King Louis XIV are huge and show surprising detail. The non-explored coasts are blurry, while the explored coasts are drawn in solid line. They show a good knowledge of Africa, America and Asia, and even some knowledge of New Zealand. France had many colonies at the time and so geography was important.

My mom, David and Edward came to the bibliotheque when the meeting ended. It has very many wooden steps, and then a big platform on which the children fed and chased birds. They also saw the globes and some exhibitions from various donors. The latter included papers that looked like what I draw when I am bored with the difference being that those people have money and so the things they write or draw are valuable. Still, I am not sure I would donate my drawings to an exhibition even if I had tremendous amounts money (Why? This is a sample of my in-existent talent from grade school. It speaks for itself ... and note that my ability to draw has not improved since.)

We were not allowed to take pictures of the exhibits. Part of the reasoning behind this is that the value of old objects is maintained if they are kept in their original condition. The light from the numerous cameras can affect the paintings in similar ways in which the sun affects our skin;  it can change the colour of the image.

A small island on lac Daumesnil
The meeting was fairly long - from morning until 6:30 in the evening and by that time everything was closed. However, we skipped a day when there were mostly experimental talks and when to the park at the Lac Daumesnil where we rented a classic wooden boat with oars. The children loved the boat ride. We saw small turtles and a beautiful cave in the middle of the lake.  The lake also had what seemed like patches of oil in the water, which made us wonder what had been washed in the lake.

On Thursday, we went to a restaurant that Philippe selected whose former guests included Picasso and Francois Mitterand, who had ordered a curry. Mihai and I shared a fish dish, which was delicious. In addition to Philippe (my current postdoc advisor) and our research group in Zurich, we were also joined by a graduate student from Netherlands. She was originally from Nepal. We learned a bit about the tallest mountains in the world:  8 out of 10 of  the tallest mountains are in Nepal, which also has extremely high real estate prices (of the order a thousand of dollars per square meter) even though the population there is still fairly poor. There was no communism there and so land has always been thought as valuable. On the way back we saw the Eiffel tower from the metro, which goes overground in some areas in Paris.

First Impressions of Paris
Collective nursing...

Mihai at the oars
 Paris is a big city that feels very much alive with traffic and people that appear busy and are moving fast. For example, Parisians like to walk on the stair-like elevators in the metro/train stations.  In the US they always stand on those and I think in Switzerland people also seem relaxed. Time and customer service did not seem to matter so much at our hotel/apartment building, which was nice otherwise. They did not include the code to open the front door or to access our room, and forgot to mention that nobody would be there on Sunday. We arrived on Sunday, and  had to wait for several hours. Eventually, we received the code and were able to enter. The next day we received 2 Euros for our trouble. In the US, they would have apologized and given us at least a free night. However, I was pleasantly surprised that the conference was flawlessly organized, and the hotel was walking distance to the bibliotheque. It was also well catered and the food was particularly good.

The smoke behind the hotel
As expected, Paris is dirtier than Zurich.  I like Zurich much better for raising children because Zurich because it is a safe and relatively clean city. The walls, the corners and doors on most streets in Paris smell badly like pee. The non-touristic areas are full of poor people. Some are homeless. They live in tents or on the vents where the warm air exits the metro station, and seem to move surprisingly little. I guess they are afraid of losing their spots. Near various cement mixing places on the river Seine there were large groups of people looking for work in construction. We have also seen the police stopping (foreign) cars in search for drugs.

Next to a statue with soldiers and slaves/prisoners of war
In spite of all this dear reader, if you love art, you should visit Paris. The museums, the parks with plenty of beautiful statues and small lakes with boats to rent, and the National Biblioteque are all superb. Paris really is a center for art. Weather-wise, I think Paris is close enough to London to get its share of rain. It did rain the first day we were there, and we did not have appropriate attire for that. Luckily, the rest of the time was sunny just like it was predicted on the weather channel.

Walking along the Seine
Mihai was singing these two variants of the song below as we were walking through Paris. I am not exactly sure where he got it from - and I am also not sure I have my tenses right in French, but these verses are the closest that I can get to an appropriate ending for this post today.

"Le Parisienne, Le Parisienne, ...
   ils vivre sur la Seine."


"Le Parisienne, Le Parisienne,  
ils vivent sous le ciel près de la Seine"

Here it's important to point out that both Mihai and I are tone deaf. But even though the French are socialists - they still have streets named after Lenin, but not one with Stalin's name - they believe in the freedom of expression in all forms of art.

Tuesday, May 29, 2012

Like a Diamond in the Sky?

Spitzer measurements suggest the atmosphere of planet WASP 12-b has carbon monoxide, excess methane, and not much water vapor. Image credit: NASA/JPL-Caltech/CFHT/MIT/Princeton/UCF

I have learned that there could be some astronomical truth in the picture conjured when singing the refrain of "Twinkle, Twinkle Little Star". David and Edward love this song and so do many other generations of children.  I love it, too and I am glad to have an excuse to include it in a post in some form.

"Twinkle, twinkle, little star,
     How I wonder what you are.
     Up above the world so high,
     Like a diamond in the sky."
              (Ann and Jane Taylor, 1806)

  Do stars have a diamond-like composition?... well,  not as far as I know, but some planets do. At the end of April I went to a seminar on the chemical characterization of Extraplanetary Atmospheres by Nikku Madhusudhan, a prize postdoctoral fellow at Yale University. I found out that the atmosphere and the core of some planets have more Carbon than Oxygen. In fact, this is not so new anymore, but it is the first time I have heard of it.

It is important to remember that planets do not have any light of their own, but they do masquerade as stars because they reflect the light of the sun. However, in general, they do not appear to twinkle in the sky.  The stars twinkle because we see them through many layers of thick, turbulent air known as the Earth's atmosphere. As the light from the star travels through the atmosphere the light is bent or refracted. This bending of light many times and in random directions causes the twinkling effect. Planets do not appear to twinkle unless the air is extremely turbulent because they are much closer to us. Stars do not twinkle when viewed from the Moon, for example, because the Moon does not have an atmosphere or from the International Space Station.
Hubble Image of a young sun-like star surrounded by a disk
  Let's get back to what Nikku said. He pointed out that models for planet formation and planetary atmospheres assume solar abundance ratio. Our star, the sun, has Carbon to Oxygen ratio of 0.5, i.e., twice as much oxygen as Carbon.  If we assume this ratio, water should be the most abundant element in planetary atmospheres. Nikku and his collaborators found several planets with atmospheres that are Carbon rich vs. Oxygen rich. So, this assumption needs to be relaxed.

How do scientists measure the composition of atmospheres of extrasolar planets? Telescopes use spectroscopic measurements of light reflected by the planetary atmosphere, i.e., they measure the intensity of the light as a function of wavelength, to find the chemical composition. One opportunity to make this type of measurements is when we observe the planet transiting across its parent star. The planet blocks some of the star's light and produces a noticeable dip in the measured light output. The advantage of this method is that scientists can probe the atmospheres of planets discovered in this way, and obtain information on their chemical composition, in addition to that of the star. They can also look at the depth of the transit and estimate the ratio of the radius of the planet with respect to that of the star. The disadvantage of this method is that there are relatively few sources because we can only detect planetary systems that are close to edge-on with respect to the observer.

 Another way to study the atmospheres of planets is to directly image them. However, planets are extremely faint sources relative to their parent stars. So, telescopes need to block the light from the star while leaving the planet detectable in order to detect the planets this way.  So, while the number of planets detected this way is growing, this method remains very challenging. 

What about planets in our solar system?
The Galileo satellite was destroyed during a controlled impact with Jupiter in 2003. Its findings are consistent with a Carbon to Oxygen ratio greater than or equal to 1. So, we could have diamond planets/moons in our own solar system. However, it is believed that the satellite landed in a particular dry spot on Jupiter.  There are several missions that will tell us more about Jupiter and its moons in the near future. The Juno mission, which was launched last year, will test this hypothesis in a few years. It takes 5 years to get to Jupiter in its current configuration. There is another mission that will be launched to study Jupiter's moons by the Europeans by 2022.  It's called Jupiter Icy Moon Explorer (JUICE).  However, it is important to remember that Jupiter is very different from extrasolar planets because it is cold. Jupiter is cooler than the water condensation curve, which means water would be condensed, i.e., buried deep in the atmosphere. Most of the discovered extrasolar planets are Hot Jupiters, Hot Neptunes and SuperEarths.

The first diamond planet and possible consequences
The first giant planet believed to have its atmosphere and core dominated by Carbon, WASP-12b, was discovered by astronomers in 2008. Nikku and his collaborators showed that this planet has low levels of water and high levels of methane. The carbon to oxygen ratio (C/O) was bigger than or equal to one by three standard deviations from the mean. If a planet with this composition is found orbiting a sun-like star with a C/O = 0.5, then this means that there are planetesimals with both this composition and a C/O grater than or equal to 1 composition.  In this case, the protoplanetary can not have the same composition everywhere - it must fluctuate as a function of distance. An alternative model proposes that the water condensed early on in the protoplanetary disk, while the C/O in the gas is higher (Oberg et al. 2011). Giant planets form by accreting this gas and hence would have a composition that is Carbon rich.

Can rocky planets have diamond cores?
Some authors (Rogers and Seager 2010, Fressin et al. 2012) show that rocky planets like our Earth could have Carboids instead of Si at their center. However,  SuperEarths are smaller and their atmospheres are harder to observe. Their chemical composition needs to be studied in order to understand which planets can support life and which can not. So, it's a question that hopefully will be answered sometime in the future.

Overall, the main message of the talk was that it is not OK to assume that all planets and stars have the same composition as our sun. This ratio varies and the variation can have impact on our predictions for habitability and on our understanding of planet formation.

Saturday, May 12, 2012

Afraid of Supermoon?

Birds against Supermoon. Credit: Taken by Flickr photographer Don Kittle
I am sorry to report that we did not see "The Supermoon" in Zurich this year. There were two reasons for this. The first was that David decided he was afraid of the Supermoon and the second was that we could not find it. We tried to go out at night to see it and climbed up the mountain in the back of our (rented) house to find the Moon. On the way there David refused to go any further and started crying. I failed to convince my 5 year old nephew that there was absolutely no reason to be afraid of the Moon - now that it's a little bigger. In the end he became so upset I had to give up.

It later occurred to me that the tides are stronger when Supermoon happens and so his fear might not have been unreasonable if we had lived on a house close to the ocean or been on a boat vs. in Switzerland. Of course, his fear is not unreasonable anyway because he is a five year old child and it's OK to be afraid of a bigger Moon if you are five. It just surprised me that I could not convince him to not be afraid.

When does "Supermoon" happen? and how big is it?
Source:; Sun: bright yellow, Moon: purple, Earth: Blue
The Moon is Earth's satellite and has an elliptical orbit around the Earth. The apogee is the point where the moon is the furthest from Earth and the perigee is the point when it's closest to Earth. Supermoon happens when the Earth, the perigee-Moon, and the Sun are all in line. In other words, the Supermoon is a full moon that is also closest to Earth (a full Moon happens when the moon is on the opposite side of Earth from the Sun).

Supermoon happens once a year. In general, the full moon and the perigree moon do not coincide exactly. So, the size of Supermoon does vary. However, Supermoon is not huge. NASA says the 2012 Supermoon is only about 14% larger than a typical full moon at its furthest point and about 30% brighter than the other full Moons of 2012,  which makes it seem bigger.

Does David have a point? Should we be afraid of Supermoon?
 The combined effects of the Sun and the Moon will be the strongest of the year when Supermoon happens. The tides are stronger than at any other time of the year. Supermoon did ground a few ships last year. However, no correlations have been found between Supermoon and Earthquakes or Tsunami or any other natural disaster. So, while we should be cautious if we are on a ship or on the beach and it's Supermoon night, Supermoon does not seem to be something we should fear. David did relax and forgot about his fear when we were back inside. He and Edward spent the rest of the evening jumping around the house and screaming "jumping monkey" until they were tired enough to go to sleep.

Friday, May 11, 2012

Behind The Light

This is not a post about death and life after death. It's about a different light - the bending of light caused gravitational lensing that increases the brightness of the source. This post is inspired by several talks on the subject at the ITP. In particular, Dr Sebastiano Calchi Novati (from Salerno University; a former postdoc) had "behind the light" as part of his title of his talk on microlensing and galactic astrophysics.

 What is Gravitational Lensing?
Gravitational lensing occurs when an object comes in-between the source of light and the observer and bends the light from the source affecting the amount of light that the observer sees. Sometimes multiple images of the source appear. The light rays that we would not have seen otherwise are bent from their path towards (or away from) the observer causing the far away source to change its brightness. There are several types of lensing: weak lensing, strong lensing and microlensing.

Strong Lensing

Strong lensing: five images instead of one
The most extreme bending of light occurs when the lens is massive - galaxy or cluster of galaxies - and relatively close to the source of light.  If the source is a quasar, multiple point images of the quasar will appear due to its small radius.  When the background source is another galaxy, the observer can see giant arcs or rings. Strong lensing produces some of the most beautiful images in astronomy.

Weak Lensing

Light from far-away galaxies can be bent by closer galaxies or galaxy clusters. The sources will be stretched and magnified, but the distortions are only a few percent. Very many sources have to be analyzed by scientists to look for coherent distortions. These distortions give us information about the mass distribution of the lenses. This way clusters nearby have their mass distribution measured, i.e.,  are "weighted" against far way galaxies. One of the most striking of weak lensing examples is the bullet cluster, which shows a subcluster passing through the cluster. The weak lensing contours map where most of the mass is, which is the dark matter. The Bullet cluster observations show that dark matter, stars, and gas behave differently during the collision. The gas interacts electromagnetically and slows down the most. The stars collide and slow down, and the dark matter clumps just go through each other since dark matter is collisionless. This is the first direct evidence for dark matter that it makes it more likely that dark matter is formed from particles vs just a modification of gravity.

 Light from stars in distant galaxies is bent by closer planets or other compact objects causing the distant stars to become brighter for a period of time, i.e., the planet acts as a magnifying glass for the distant star. This time period can be weeks, days or even hours. The duration of the event depends on the mass of the lens, the distance to the lens and its velocity. It is also important to point out that these are "one-time" events that do not repeat. Microlensing first emerged as a technique to find compact objects made from dark matter. However, no evidence for such dark stars exists to date.

Microlensing later morphed into a successful planet-finding method. It can find Earth-mass planets that are relatively far from their star. The distance dependence is stronger than the dependence on the mass of the lens. In fact, gravitational microlensing was the first method to find Earth-mass planets orbiting main sequence stars.  The first extragalactic exoplanet in the Andromeda Galaxy A was also found through microlensing.

Does the title make sense?
We can use the light from distant objects to learn more about closer object though the lensing effect. So, the "behind" in the title here has an allegoric meaning, i.e., we learn about what is behind the change in brightness. However, do not be confused, the lens (closer star/planet/etc) is between Earth and the further away (and older) source, and the duration of the event depends on how long the lens spends across our line of sight.