Thursday June 16: Gravity! hangout from Stanford on black holes and gravitational waves

Pierre Binétruy and George Smoot invite you to participate to the final hangout of the second session of the Gravity! course. This hangout will focus on black holes and gravitational waves. It will be broadcasted this Thursday June 16 at 19h00 UTC (20h00 London, 21h00 Paris, 12h00 California), live from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at SLAC, Stanford University.

KIPAC_logoThe Google Hangout will be streamed live on Google Hangouts and Youtube for approximately 60 minutes, where you can follow the questions and answers live. If you are not registered in this session of Gravity! you may ask your questions below or on Twitter using#FLGravity.

Two highlights for this hangout will be the recent publication of the first results by the LISAPathfinder mission, as well as the exciting new result of the LIGO-Virgo collaboration announced on June 15.

Our guests for this event will be:


Tom Abel is the director of the Kavli Institute for Particle Astrophysics and Cosmology, joint laboratory of the SLAC National Laboratory and Stanford University. His group explores the first billion years of cosmic history using ab initio supercomputer calculations. He has shown from first principles that the very first luminous objects are very massive stars and has developed novel numerical algorithms using adaptive-mesh-refinement simulations that capture over 14 orders of magnitude in length and time scales.  Most recently he is pioneering novel numerical algorithms to study collisionless fluids such as dark matter.



Roger Blandford, a native of England, held a faculty position at Caltech since 1976 when, in 2003, he moved to Stanford University to become the first Director of the Kavli Institute of Particle Astrophysics and Cosmology. He is a world-renrecognized expert in black hole astrophysics, cosmology, gravitational lensing, cosmic ray physics and compact stars.



michael_landryMichael Landry is Detection Lead Scientist with the LIGO Hanford Observatory in Washington state, and a physicist with the California Institute of Technology. Michael began work in the field of gravitational wave physics as a postdoc with Caltech in 2000, stationed at the LIGO Hanford Observatory, and has remained there as a scientist since that time. From 2010 to 2015, he led the installation of the Advanced LIGO detector at Hanford. This collaborative work, done by the LIGO Scientific and Virgo Collaborations totaling a thousand people, culminated in the first direct detection of gravitational waves from a binary black hole merger, announced Feb 11, 2016.



Stefano Vitale is the Principal Investigator (P.I.) of the LISAPathfinder mission. He is professor at the University of Trento in Italy and is a key figure of the gravitational wave community in Europe. He worked on the cryogenic acoustic detector AURIGA before joining the LISA mission where he is leading the Italian effort. He has developed in Trento a laboratory which contributed the inertial sensor onboard the LISAPathfinder mission.

LISAPathfinder exciting first results: green light for the LISA mission

The first results of the LISAPathfinder mission were presented this Tuesday June 7 in a press conference organized by ESA and published in Physical Review Letters. And they are even better than was anticipated.

This technological mission was launched on December 3 from Kourou (see the launch  and meet the technical team who participated in a Gravity! hangout  a few hours before launch).

This mission tests one key aspect of the future space gravitational wave observatory, known as LISA: it measures the variation, due to a passing gravitational wave, of distances between test masses  which are in free fall i.e. which follow purely gravitational trajectories. To protect the masses from any perturbation, there is a clever set up, known as “drag free”. It consists in using the satellite for protecting the test mass placed in its centre from any perturbation.

To understand how it works, imagine that a micrometeorite hits the satellite: the satellite moves sideways, the test mass is thus no longer in its centre, the satellite detects this anomaly through sensors, it ignites some micro thrusters to reposition itself around the test mass.

Of course, such a device is never perfect, and the test mass feels some tiny perturbations, but the goal of the LISAPathfinder is to show that the perturbations are small enough that they still allow to have confidence in the detection of gravitational waves.

Quantitatively, the goal is to minimize the stray forces acting on the test masses. But a force induces an acceleration and the goal of LISAPathfinder is to minimize the stray acceleration (one talks of an acceleration noise). The initial goal of the mission was to reach over periods of 1000 seconds a stray acceleration which is smaller than 10-13 times g, the local acceleration due to the Earth gravity.




How to realize this? In the future LISA mission, the test masses are placed at the centre of each satellite, 5 million kilometers apart  from one another. Laser beams connect the three satellites, thus forming a huge triangle. Relative variations of distance are measured by interferometry, just as for ground detectors, such as LIGO.



In LISAPathfinder, one arm of the future LISA mission is reduced to 38 cm in order to locate two test masses into a single spacecraft. The distance between these two masses is monitored by laser beams that form an interferometer very similar to the one on board LISA (apart from the distance covered by the beams).  You can find below a video provided by ESA that explains the main characteristics of the mission.


It is not possible to have simultaneously the two masses in free fall, because their orbits are very similar but not exactly identical. This is why one uses one of the two masses as a reference, whereas the other one is left free. It is on this second mass that one checks that it is in free fall, at least in the limits required for the acceleration noise.


“The measurements have exceeded our most optimistic expectations,” says Paul McNamara, LISA Pathfinder Project Scientist. “We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five better.”

Indeed, as shown on the following plot which appears in the published paper, the acceleration noise reached is 5 times smaller than what was required, basically it is already what is needed for the LISA mission, and even better for high frequencies.


“Not only do we see the test masses as almost motionless, but we have identified, with unprecedented precision, most of the remaining tiny forces disturbing them,” explains Stefano Vitale, the scientist in charge of the mission (Principal Investigator).

This success is obviously a green light for the gravitational wave observatory, the third large mission (L3) of the European Space Agency, known as LISA. This mission was originally identified for a launch in 2034 but this success, and the historic discovery of gravitational waves by the LIGO detector, offer strong arguments to advance significantly the schedule.


A new session of Gravity! starts this Monday 9 May

Classes for the new session of the on-line Gravity! course start this coming Monday 9 May for six weeks. You may register here on the FutureLearn platform. The course is free and registration is open to everyone.

The first session of this course had attracted 70 000 registered learners last Fall. This new session follows the same programme: it revisits the emergence of the main concepts from Galileo to Newton and Einstein before discussing some of the main aspects of gravity in the Universe -Big Bang, expansion and cosmic inflation, cosmic microwave background, dark matter and dark energy. We had to reshuffle the last two weeks of the course which deal with black holes and gravitational waves, to give the proper place to the exciting discovery of gravitational waves which was announced last February. This will certainly be the climax of this course!

Gravity! is for all those of you curious about the mysteries of the Universe and invites you to understand, without any prerequisite in physics, the foundations of Einstein’s theory that makes gravity “the engine of the Universe”.

The Gravity! course is produced by the Paris Centre for Cosmological Physics of Paris Diderot University, with, for this session, a guest participation of the Kavli Institute for Particle Astrophysics and Cosmology in Stanford.

Watch the launch of the Microscope mission live from Kourou

The launch of the Microscope mission which will test the equivalence principle to unprecedented precision was planned to take place Friday April 22 at 21h02 UTC (18h02 Kourou time, 23h02 Paris time) from the launch pad in Kourou, French Guyana, then reprogrammed on Sunday 24, same time, because of poor meteorological conditions. Due to a technical problem on the Soyuz launcher, it was postponed again to be finally launched on Monday April 25 at 21h02 UTC (23h02 Paris time). The launch has been a success and the satellite is now on its orbit at an altitude of 710 km.

The equivalence principle, which basically states that an acceleration is equivalent to a gravitational field,  is one of the foundations of Einstein’s theory. But modern theories which try to unify gravity with the other fundamental forces tend to violate this principle. See here why.

The principle of measurement on which is based the Microscope mission is explained here.

The broadcast of the launch can be seen below:

Gravitational Wave Fiesta : course material

Following our Gravitational Wave Fiesta, below are the slides shown in the various presentations and a record of the last session, the ultimate quizz proposed by Pierre Binétruy to the learners of the Gravity!

Thanks to all for these fruitful and friendly couple of days!

fiesta16 001

Pierre Binétruy (Paris Centre for Cosmological Physics/APC)

Introduction to the Fiesta: physicists dreamed about them for 100 years English/French

Introducing the Fiesta

Eric Chassande-Mottin (Laboratoire APC)

The story of GW150914 discovery (English)


Matteo Barsuglia (APC)

The LIGO and Virgo detectors (English)


Eric Plagnol (APC)

LISAPathfinder news (English)



Antoine Petiteau (APC)

How will we analyze the LISAPathfinder data? (French)


Joël Bergé (ONERA)

The Microscope mission and the equivalence principle (English)


Pierre Binétruy and all the participants

A last quiz: questions and answers during the last session of the “Fiesta”

The teaser:

The full recording:


Feb 29 at 16.30 UTC: hangout on the discovery of gravitational waves

On February 11th, 2016 was announced the discovery of gravitational waves by the LIGO and Virgo collaborations. We organize the Gravitational Wave Fiesta in Paris for this historical occasion. In order to allow everyone to participate, we hold a special hangout on February 29 at 16:30 GMT.

Ask your questions!

You will have the opportunity to ask your questions about this exceptional discovery.

This Google Hangout will be streamed live on YouTube and Google+ Hangouts for approximately 60 minutes, where you can follow the questions and answers live.

If you are not among our 80 lucky guests who will be present in Paris for the Fiesta, we encourage you to ask your questions and we will select the most popular ones to ask our guests during the Hangout.

There are three ways you can do this:

  • You will be able to send questions and comments before and during the event by submitting them in the Google Hangout Q&A chat window (if you have a Google account).
  • You can send us questions to our Twitter account @Gravity_Paris, using the hashtag #FLGravity.
  • If you are registered to the first session of the Gravity! course on Futurelearn,  you can leave your questions or comments in advance on the discussion of step 5.12 of the course .

What happens if I can’t watch the live Hangout?

Don’t worry!  A recording of the discussion will be made available after the live event finishes. You will be able to access the recorded video after the event right below.

Will loading the Hangout mean I appear on camera?

No, you will just watch the live stream like any other video (though Google users can submit comments via the interface).

Is a Google account required to view the Hangout?

No, you can watch the Hangout without logging in to Google.

LISAPathfinder test masses released : a major step on the road

The gravitational skies seem to be auspicious these days. While the LIGO collaboration was announcing the discovery of gravitational waves, the LISAPathfinder team was going through a very delicate process : the release of the two test masses which was completed successfully this morning.


Behind the polished press release of ESA, let me explain what was at stake, and why everyone in the eLISA project is relieved and very joyful today. Indeed, you can see in the tweet below the reaction of Stefano Vitale, the scientist in charge of the mission, and César García, the project manager (we had met him in Kourou last November a few hours before launch, see the video of the hangout).


LISAPathfinder is testing the basic principle behind the eLISA mission which is to be able to measure variation of distances between two test masses which are only submitted to gravitation, in other words which are freely floating in the cosmos. These test masses are small cubes of gold-platinum of 46 mmm side.


Test mass ©CGS SpA

During the experimental phase, these masses are floating in a cage, called electrode housing. Thanks to these electrodes, the satellite is constantly monitoring the position of the test mass, and operates its external microthrusters in order to change its own position and reposition itself in such a way that the test mass stays at the centre of its cage. In this way, the satellite protects the test mass from external perturbations.

But there is one difficulty: whereas the test mass is floating once on site, it has to be tightly locked during launch: otherwise, the strong vibrations would shake it within its housing, which would provoke irreparable damage.


(c) University of Trento

Electrode housing © University of Trento



And here is the tricky engineering problem, which gave nightmares to ESA (and, in an earlier stage, NASA) teams: this was known to everyone in the mission as the “infamous caging mechanism”. How do you release the test mass once you have tightly locked it? The difficulty is that, once tightly pressed, the test mass sticks to the metal finger that presses on it. But one has to release the mass very softly, because only very small forces can be acted upon.


The solution that was finally adopted relies on a two-stage process.


Throughout LISAPathfinder’s launch, , and the six-week cruise to its work site, each cube was held firmly in place by eight ‘fingers’ pressing on its corners. On 3 February, the locking fingers were retracted and a valve was opened to allow any residual gas molecules around the cubes to vent to space. Each cube remained in the centre of its housing held by a pair of rods softly pushing on two opposite sides.



The rods were finally released from one test mass yesterday and from the other this morning, leaving the cubes floating freely, with no mechanical contact with the spacecraft.

Congratulations to the project manager, César García, and all the technical teams involved in this success !

It will be another week before the cubes are left completely at the mercy of gravity, with no other forces acting on them. Before then, minute electrostatic forces are being applied to move them around and make them follow the spacecraft as its flight through space is slightly perturbed by outside forces such as pressure from sunlight.


On 23 February, the team will switch LISA Pathfinder to science mode for the first time, and the opposite will become true: the cubes will be in free fall and the spacecraft will start sensing any motion towards them owing to external forces. Microthrusters will make minuscule shifts in order to keep the craft centred on one mass.


The final word to Stefano Vitale: “Releasing LISA Pathfinder’s test masses is another step forward in gravitational wave astronomy within this memorable month: the test masses are, for the first time, suspended in orbit and subject to measurements”.


Pierre Binétruy

Gravitational wave fiesta 29Feb/01Mar

This is the first of the Gravity! workshops for the learners of Gravity!, all their friends and all those interesting in getting a better understanding of the mysteries of our Universe.

The workshop will of course focus on the topic of gravitational waves, with the historic event of their discovery.
What is a gravitational wave? How were they observed? What have we learnt about black holes from their discovery? What comes next? What is the present status of  LISAPathfinder? So many questions to cover with specialists of the field, with a programme of lab visits in small groups, a social event and a hangout live with the rest of the world.


To reserve your participation, please go to this website  (we ask for a modest participation of 10€ in order not to cover the expenses but to have a better idea of the number of participants).

The event takes place from Monday February 29 at 9.30am till Tuesday March 1 at 4pm.

Language: English and French
Venue: University Paris Diderot, Amphitheater Buffon, 15, rue Hélène Brion (13th arrondissement)

Metro and RER stop: Bibliothèque François Mitterrand

Programme: see below

affiche_GW Fiesta_V3

A questionnaire has been distributed to all participants to stimulate their curiosity. You may have access to it here.

Monday 29 February/Lundi 29 février

Amphitéâtre Buffon

9h30-11h00 : Gravitational waves and their discovery, an overview/Les ondes gravitationnelles et leur découverte, une introduction (P. Binétruy)

11h00-11h30 : Coffee break/Pause café

11h30-12h30 : What do you expect from MOOCs, a discussion led by P. Binétruy

12h30-14h00 : Buffet lunch/Déjeuner buffet

  • Amphithéâtre Buffon (en français)

14h00-14h30 L’histoire de la découverte de GW150914 (E. Chassande-Mottin)

14h45-15h15 Les détecteurs LIGO et Virgo (M. Barsuglia)

15h30-15h50 Les nouvelles de LISAPathfinder (E. Plagnol)

16h00-16h20 Comment va-t-on analyser les données de LISAPathfinder (A. Petiteau)

  • Bâtiment Condorcet, Salle Luc Valentin (4th floor, 454A) (in English)

14h00-14h30 The LIGO and Virgo detectors (M. Barsuglia)

14h45-15h15 Story of GW150914 discovery (E. Chassande-Mottin)

15h30-15h50 How to analyze the LISAPathfinder data (A. Petiteau)

16h00-16h20 LISAPathfinder news (E. Plagnol)

  • Amphitéâtre Buffon

16h30-17h30 Coffee break/Pause café

17h30-18h30 Hangout with the whole Gravity! community (in English)


Tuesday 1 March/Mardi 1er mars

  • Bâtiment Condorcet

9h00-10h30 : Group visits/Visites par groupe

10h30-11h00 : Coffee break/Pause café (4th floor/4ème étage)

11h00-12h30 : Group visits/Visites par groupe

12h30-14h00 : Free time for lunch/Temps libre pour déjeuner

  • Amphithéâtre Buffon

14h00-14h40 : The Microscope mission and the equivalence principle/La mission Microscope et le principe d’équivalence (J. Bergé)

14h40-16h00 : Discussion session, future actions, wrap up/Session de discussion, actions futures, conclusions (P. Binétruy)

The curtain rises on the gravitational Universe

After a long wait, the rumour has been confirmed: gravitational waves have been detected in the LIGO interferometric antenna. This is a major scientific event, not only because it concludes a century long chase : Einstein predicted the existence of such waves in 1916, only a few months after his seminal series of papers on General Relativity in November 1915. But this is also the opening of a new era: the direct observation of the gravitational Universe.


Indeed, all the discoveries of the XXth century have confirmed Newton’s point of view that gravitation is running the Universe in its largest spatial dimensions as well as its evolution with time. But all detection until now was based on light, or more generally electromagnetic waves. Not so surprising from human beings equipped with a sensitive light detector, the eye. The discovery of today opens up the exciting possibility of exploring the Universe, and our space-time with waves of gravity. We will thus get first-hand information on the most gravitational of all astrophysical objects, the black holes, but also on many violent phenomena in the Universe, and ultimately the most violent of all, the Big Bang.


Today is clearly only a beginning. Ground detectors are now in a position to observe many more stellar events and do outstanding science. The space detector eLISA, as well as the observation of pulsar timing, will open the window onto the gravitational Universe at large. It is a beautiful symbol for the future that, at the time the discovery is announced, the LISAPathfinder satellite is waking up at the Lagrange point L1, ready to test the technology of the future eLISA mission.


It is time to rejoice and to congratulate the scientists of the LIGO collaboration, but also the GEO600 and Virgo European teams who joined in the analyses, for their commitment over many years to this search, and for their careful handling of this outstanding discovery, despite the pressure from the scientific community and the media. And its is also the occasion to stress that this is an incredible achievement in terms of ultra-precise measurements.


We hope that all the learners of Gravity! feel gratified to have spent weeks on understanding better the gravitational Universe, and thus that they better share the excitement of this discovery.


We propose on this website a series of posts, under the heading Gravitational wave discovery, which presents more details on gravitational waves, their detection, and the present discovery. They will be complemented in the future. Do not hesitate to comment and ask questions.


We had also said that we would organize the first Gravity! Workshop at the end of this month. The dates are nox fixed, February 29 and March 1, and the location is Paris. And the title: Gravitational wave fiesta. Surprised? All you want to know about gravitational waves and their discovery. Reserved to Gravity! learners… and all their friends. So it is time to visit Paris for the week-end, and make a short excursion into the gravitational Universe. And if you really cannot come and meet us, we will have a hangout on the evening of February 29.

We had promised exciting news for the forthcoming months and years. It only begins.

Pierre Binétruy and George Smoot

1 2 3 4 5 6