Sunday, November 19, 2017

Snow White, Snow White five years later and the genius goat

From left to right: Snow, Lady David, Edward, and Lady Edward
This summer we spent the equivalent of $80 on ... goats. Mihai purchased them at my request from the village of Tormac. Their family had gone to work in Italy and was dismantling their farm. So, In June 2017, Snow White, Snow White Five Years later (the mother of Snow While; also known the Fairest of them all or as the Lady David), and the genius goat (Lady Edward) have joined our family.

Lady Edward and Lady David are like most mothers: friendly, highly intelligent and provide the best milk I have ever tasted. David milks them twice (and sometimes three times) a day. They are too amazing for words -- when one considers my writing ability. They are, however, described in almost every sentence Edward writes for his homework. 

My phone's collage of Edward (at a petting zoo & this summer)
Andy is not too happy with our naming scheme or with my latest acquisition. He said that he will leave me and never return if I buy a cow. This summer he spent his time helping the children milk the goats -- during and in-between gravitational wave detections -- AND claiming not to remember which one is Lady Edward and which one is Lady David (He also does not know whether he/LIGO is seeing black holes or boson stars; nobody knows that yet for sure.) His job was to hold the goats still, but before running to catch them he always asked us to tell him the colour of the goat he was after. Their colours are really very plain. One is white and the other is grey. In his school writing Edward  refers to his goat as 'Edward -- the goat' or 'Goat Edward'.  He mostly has to write in German and told me 'Frau Edward' would sound too strange for school, and I solemnly agreed.

The Genius Goat can open knots with her mouth and use a fork. She was observed to steal David's cup with left-over rice pudding, which had extra sugar and cacao in it. She put it gently on a a box, and started to take stuff out of the cup with David's fork. The door did not close well and to prevent the goats from coming inside and eating the grain (or sitting on my bed) we tied it with a knot. After I closed the gate as I was driving out with the children, we saw Lady Edward climb on her hind legs and gently open the knot with her mouth. Lady David is bigger, and very strong. She mostly pushes stuff with her head. Snow White climbs everywhere -- it's because she is the lightest. She can even climb on the car.

Edward turned seven this summer. David is ten, and little James just turned one. I stopped writing dedicated birthday posts. The children and our animals take so much care that I have not had the energy to write much before today, but tonight I could not sleep.  So, today after re-lighting the fire, I felt the need to write and not for useful, precise, science writing. I should, however, try to get some sleep to have the strength to wake up soon enough to take on the new day with school, animals and a one-year old AND people coming to install central heating (this time it's gas based). I promise to write more later ... after I do some work on boson stars and take some better pictures of David and his goat/the goats. I will first re-check the fire.

Thursday, November 9, 2017

Measuring Planetary Spin from Spacecraft Timing

Exploring spacetime with general relativity


The trajectory of NASA's Juno Mission
General relativity is slowly becoming a tool that teaches us about the objects that curve spacetime instead of a hindrance to be corrected for. Planets are heavy and bend the fabric of spacetime affecting the orbits of satellites that go around them and the paths of light sent. Knowing very precisely when the light arrived, and when it was sent is part of space-craft timing. For Juno and Cassini this timing is already sensitive to higher order general relativistic effects like frame dragging. In the future, such measurements could be used to determine the spin of planets to within a percent, which can tell us about their interior. For missions in eccentric orbits relativistic effects are kick-like and can only be observed when the satellite is within a few hours of its pericentre.  We argue that instead of performing cumulative frame dragging measurements over many orbits as was done for Earth, high eccentricity missions like Juno and Cassini need the specific time dependence of each relativistic effect to aid in recovery.

A few words about gravity...

Bent spacetime affecting the orbit of a satellite & the light it sends
General relativity is the theory of gravitation. It says we live in four dimensions -- 3 spatial and 1 temporal -- and that space and time are connected. Gravitation itself is a consequence of the curvature of the spacetime. A planet is heavy and curves the spacetime around it causing it to seem like it pulls objects towards/around it.  This is gravity.

Redshift contribution from frame dragging for Juno. Zoom in.
Looking at relativistic effects
Missions like Juno and Cassini present new possibilities for measuring relativistic effects around the giant planets in our solar system. Relativistic effects are amplified if the orbit of the satellite is eccentric because the spacecraft moves faster and the satellite passes by the planet very closely where the gravitational field is stronger.  This coupled to the larger size of the planet causes frame dragging accelerations that are a few hundred times larger than those near Earth. Since the effects are larger, they might be easier to detect than around Earth.

Most planets have higher spins than black holes. The angular momentum per unit mass for black holes is less than 1, whereas for planets it can be of order hundreds. Earth has a spin on about 800 while Saturn's is about 1000. Precise frame dragging measurements can constrain planetary spin providing an independent estimate of the internal structure of the planet. This structure is relatively uncertain for the gas giants, which are believed to have an internal core of unknown size that rotates at a different rate than the surface.

 We simulate the trajectory of a satellite in a curved spacetime and find the path of the light it sends to receiving stations on Earth. Both follow 'straight lines' in their spacetime also called geodesics. The dynamics of a satellite orbiting a planet can largely be described by Newtonian physics with general relativity providing only small contributions.  The equations of motion are expanded in velocity orders to separate Newtonian and relativistic effects.

The biggest general relativistic effect is time dilation. GPS satellites are sensitive to time dilation and correct for it -- if they would not, the GPS would be off by about 10 km every day. Moving clocks tick slower than stationary clocks. So do clocks in a gravitational field. The ground station has its own time dilation and the difference between its tick signals and those arriving from the satellite are known as the redshift. We compute this redshift for satellites around Earth (Galileo and a proposed mission in an eccentric orbit) and for eccentric orbits around Jupiter (Juno) and Saturn (Cassini). Galileo satellites and the Atomic Clock Ensemble in space -- an ensemble of two atomic clocks that will be placed on the International space station in 2018 -- provide an even better measurements of time dilation, which tests the equivalence principle.

We are interested in higher order relativistic effects like frame dragging in which a spinning mass drags the spacetime in its vicinity affecting any orbiting satellite. The orbital plane of the satellite precesses about the spin axis of the planet. Historically, this effect was first predicted by in 1918 by Einstein, Lense and Thirring. They studied Amalthea, the third moon of Jupiter, and found that it precesses by 1'53'' per century.

Existent Measurements of frame dragging 
Orbital perturbations due to frame dragging have been measured using laser ranging by LARES and LAGEOS. Gravity Probe B measured the effects of frame dragging on the orientation of onboard gyroscopes. The effect is typically averaged over multiple orbits. It is then buried in much larger non-relativistic precession making it very hard to identify the relativistic contribution. E.g., Mercury's observed precession is mostly due to Newtonian planetary perturbations with the relativistic contribution being only about 7% of the total.

Relativistic effects for the Juno orbiter
Instead of averaging we compute each higher order relativistic effect as a function of time and find that they alter the orbit in a kick-like manner at the pericentre. For Juno the kick due to frame dragging could be measured for about two hours. We argue that technology has advanced enough so that we might be able to filter out these effects if we knew their specific time dependence.



This post sketches the results from our paper published in the Frontiers Journal this year. You can find details of the work below.

Andreas Schaerer, Ruxandra Bondarescu, Prasenjit Saha, Raymond Angelil,  Ravit Helled and Philippe Jetzer, "Prospects for measuring Planetary Spin and Frame dragging in Spacecraft Timing Signals", Frontiers in Astronomy 4, 11 (2017).



Sunday, October 22, 2017

Observing gravitational waves AND light from the same source

artist conception
It's been so exciting to see, hear and read through the physics news in the past week that I have not had time to write! More gravitational waves have been detected and this time telescopes have seen light emitted from what's believed to be the closest observed merger of compact objects to date. The collision happened 130 million years ago, when dinosaurs still roamed the Earth, in a galaxy from the Hydra constellation. LIGO has seen gravitational waves from this inspiral on August 17, 2017.  The event was made public on October 16 together with a suite of technical articles.

Edward and his goats
Those of you who know us might guess Andy has been pretty stressed out this summer, but also excited and proud. I was mostly in charge of the children, house repairs and the many animals we have acquired -- taking time off now-and-then to prepare a talk and a fellowship application. Our son, Edward, had a gravitational wave appear on his birthday (the binary black hole also seen by VIRGO) and the neutron star merger 3-days later. So, a picture of Edward next to that of the gravitational waves is somewhat appropriate. His 3-goats also sneaked in the picture. The announcement happened while we were celebrating Edward Seidel's 60th birthday.

What happened on August 17, 2017? The first light from this event was picked up by Fermi's Gamma Ray Burst monitor. Independently, LIGO Hanford saw the event, LIGO Virgo saw nothing, LIGO Livingston saw a huge glitch, which meant data would have been removed around it if there had been no event. The Hanford detector triggered. When the trigger was checked, they saw that Fermi has seen gamma rays some two seconds after LIGO saw the merger of the neutron stars.  Gravitational waves travel at the speed of light towards Earth to shake LIGO's mirrors, while light moves slower through air than vacuum (and is delayed when passing near massive objects), which makes it travel a bit slower. The short time delay meant it could be from the same event, but they had to find its location to be sure.

The waveform was long with the highest signal to noise ratio observed to date. It looked like the first neutron star - neutron star collision or the first neutron star - black hole collision was observed. The strength of the signal meant the colliding stars were close to Earth. So, it was important to get the localization as accurate as possible and look for other kinds of light with all telescopes that could see it.

After running a code developed by Andrew Lundgren at the LVC meeting in 2010 to remove huge  glitches, they uncovered the signal in LiIGO Livingston. The fact that Virgo saw nothing meant the event was near one of its blind spots.  It took four more hours of work to obtain sky localization. When the location was out it was not night in Chile yet, and so scientist spent time planning observations. 

Telescopes might have ignored  the event altogether if LIGO had not seen it because the burst of radiation from the colliding neutron stars was off-axis, i.e., it was not pointed towards Earth. This is why this very close collision was not bight in Gamma rays. It may be that we have missed other sources like this before. Yet only when gravitational wave observatories run for longer we'll be able to tell.

Days later Chandra saw X-rays at this sky location, and telescopes are still observing radio waves. So, stayed tuned for more!!! It will also take time to analyze and understand the data.

The US LIGO detectors are upgrading. If they succeed in reaching design sensitivity, they could see up to an event a week in a year or so. The down side is that we'll miss detections in this period. I wish they could let one detector running in the US and LIGO Virgo, while upgrading the other.

All gravitational waves signals seen to date (LIGO collaboration)
Where was Andy when the universe burped emitting gravitational waves? We were celebrating Edward's seventh birthday belatedly. So, Andy was in Chizatau - in Transylvania. He was watching his computer screen while cooking in our lightless kitchen. This kitchen has no window, but we can leave the door open for light. If the door is open, we can hear noise from the main road (lots of trucks) and the goats. So, the lightless state is at times preferred. I took the children swimming to let him work and cook. When we returned, he was very animated. Nothing was burning in the kitchen. So, while I knew he was not allowed to tell me until the official announcement, I thought this must have been the first gravitational wave LIGO has seen while we were in Chizatau.  We've seen gravitational waves from a black hole binary on August 14, but black holes are...well... black... and this time the universe thought we needed light!  

To remember: Let your children dream! Today one can observe the universe while cooking in Transylvania, and lead/work with a multi-national collaboration after-and-before goat milking! (Many thanks to LouAnne Lundgren for pointing this out).

Note:  The LIGO collaboration has about 1000 people -- many of whom I am proud to call my friends. Work was done in collaboration with observers and with theorists (more friends and people I respect) for this event. I don't mean to take credit or discredit work done by others. This is simply meant to be a fun personal post.

Monday, July 17, 2017

I married a professor after all...


Andy & Andy-Chick
less serious
 Andy received a reader position at the Institute of Cosmology and Gravitation from the University of Portsmouth. He is joining the institute in October. Needless to say: I am very proud of him! This is the equivalent of an associate professor position in the US. It means he will build his own research group and that the position is permanent. Furthermore, if all is well, it is expected that he will be promoted to full professor in a few years. 

While we do have two children together, Andy and I are not actually married. A marriage certificate would have made our already multi-national lifestyle even harder to deal with. Certificates need to be apostilated according to the Hague convention and presented when dealing with real estate issues and with children. The apostile is only valid for a limited period of time. It is easier to simply not be married. Then one can just present their ID. However, given that we  have a family together, the relationship is somewhat equivalent and title seemed catchy.

me, Electron & Positron
As I write this I am surrounded by Electron and Positron. James also just woke up. So, I have to conclude this post to help him use the potty. He is only 8 months old and doing a reasonable job of being diaper-free.

Will Andy's position suddenly brighten my career prospects? No. I am receiving a visiting position that is unpaid and the support to apply to fellowships if/when I find the time.