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

Registration to the Gravity! on-line course opens again

The registrations to the course Gravity!  have just reopened on the Futurelearn platform . 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, black holes. And no doubt that gravitational waves will be centre stage on this course!

Classes start on Monday 9 May for six weeks.  You may register here on  the FutureLearn platform. The course is free and registration is open to everyone.

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”.


LISAPathfinder arrives at its destination

After a six-week journey, LISA Pathfinder arrived today at its destination, the Lagrange point L1, a point on the virtual line joining the Earth to the Sun, some 1.5 million km from Earth, where the gravitational effects are balanced by the centrifugal force.

LISA Pathfinder’s arrival came after a final thruster burn using the spacecraft’s propulsion module on 20 January. The small, 64-second firing was designed to slightly change its speed and just barely tip the craft onto its new orbit about L1. “We had planned two burns to get us into final orbit at L1, but only one was needed,” says Ian Harrison, Spacecraft Operations Manager at ESA’s ESOC operation centre in Darmstadt, Germany, where the mission control and science teams are located.

LISAPathfinder journey from the Earth to L1

LISAPathfinder journey from the Earth to L1

Since launch, the propulsion module raised the orbit around Earth six times, the last of which kicked it towards L1.


The propulsion module separated from the science section at 11:30 GMT (12:30 CET) after the combination was set spinning for stability.

“Heat and vibration from the regular, hot thrusters on the propulsion module would cause too much disturbance during the spacecraft’s delicate technology demonstration mission,” notes Ian. “Primary propulsion during the rest of the mission will be provided by microthrusters to keep us at L1.”

These small thrusters were used in the hours after separation to kill the spin and stabilise the spacecraft.

Next week, LISA Pathfinder’s trajectory will be fine-tuned with a series of three micro-thruster bursts, taking it onto its final orbit, a 500 000 km × 800 000 km orbit around L1.

The next delicate step will be the final release of the test masses on 15 and 16 of February.


Pierre Binétruy and the Gravity! team meet you in the Paris Science Museum

Pierre Binétruy and members of the Gravity! team are in residence at the Palais de la Découverte, the Paris science museum located in the Grand Palais at the bottom of the Champs-Elysées.

This is part of the celebration of the 100th anniversary of General Relativity.

Meet us in person on some of the afternoons of January till March 2016, and perform with us some simple experiments revealing the true nature of gravity. We are located in the entrance rotunda: un chercheur, une manip (“one researcher, one experiment”).



Time 2.30pm-5pm on the following dates:


Wed 6         Antoine Petiteau

Sun 10        Pierre Binétruy

Wed 13       Antoine Petiteau

Sun 17        Pierre Binétruy

Sat 23         Hubert Halloin

Sun 24        Henri Inchauspe

Wed 27        Philippe Bacon

Sat 30         Philippe Bacon


Wed 3          Hubert Halloin

Sat 6           Pierre Binétruy

Sun 7          Pierre Binétruy

Sat 13         Pierre Binétruy

Sun 14        Henri Inchauspe

Wed 17        Pierre Binétruy

Tue 23         Hubert Halloin

Fri 26           Hubert Halloin

Sat 27          Pierre Binétruy

Sun 28         Pierre Binétruy


Sat 5             Henri Inchauspe

Sun 6             Hubert Halloin




Brzmienia/Sonoridades: Gorka Alda celebrates Chillida in Wroclaw

The composer of the music and sounds of Gravity!, Gorka Alda, has conceived two sound installations for the exhibition “Brzmienia/Sonoridades” (Sonorities) dedicated to the sculptures of Eduardo Chillida in Wroclaw, European Capital of Culture 2016.



Gravitación, Eduardo Chillida

The theme of gravitation is recurrent in the work of Eduardo Chillida (1924-2002). Surprisingly, he is using paper for the sculptures on this theme, and the vacuum between the different paper sheets plays a significant role as well.

The exhibition opens this Friday 15  at Awangarda Gallery, in Wroclaw, and continues until March 13 (curators: Inés R. Artola and Ignacio Chillida).

A new particle hint at CERN? Could it tell us something about gravity?

Last December 15, the two experiments which have discovered the Higgs particle at CERN, ATLAS and CMS, have presented the first results of the LHC collider at the highest 13 TeV collision energy and they both announced an excess of 2-photon events at 750 GeV.


This might be the sign of the existence of a new particle with a mass energy of 750 GeV, decaying into two photons like the Higgs. But the evidence is still very preliminary: one only sees small bumps when one plots the number of events versus the energy.


In technical terms, one talks of standard deviations: ATLAS has seen a 3.6 standard deviation, and CMS a 2.6 standard deviation, when the scientific community agrees to talk of a discovery only for standard deviations larger than 5. The excitement comes from the fact that both experiments see an excess at the same mass. But one will probably have to wait another year to accumulate more data and see whether the excess builds up… or disappears if it was just an unhappy coincidence.


But theoretical papers have rushed to provide interpretations of these events. Nature has recently counted almost one hundred papers appearing on the web arXive where the scientific community uploads their papers (108 to this date, see here). Why such an interest? And has it anything to do with gravity?


Well, the Higgs particle discovered in 2012 was the last building block of the Standard Model, which realizes the unification between the electromagnetic and the weak nuclear force. A new particle of mass energy 750 GeV (decaying into two photons) is not accommodated by the Standard Model; it would be a clear sign that this Standard Model has to be revised, or rather enlarged: one would have to go beyond the Standard Model, as we physicists say.


For many of us in the scientific community, this is expected because the Standard Model answers very fundamental questions but leaves many other open. For example, it does not provide a candidate for a dark matter particle. Could the new hypothetical particle be this dark matter particle? Most probably not, but many theoretical papers stress that this first signal of new physics would lead to the discovery of further particles. There is the hope to discover among them this dark matter particle, and thus to solve a puzzle which has been with us since the 1930s (remember that the only signs of dark matter have been so far gravitational, which led some to propose modifications of general relativity to account for the observations).


Another motivation is of a more theoretical nature. The Standard Model only realizes the unification of two fundamental forces (electromagnetic and weak nuclear forces). What about the two others (strong nuclear force and gravitation)? Well, a larger unification requires new dynamics and thus new particles. But the Higgs particle, which provides the masses of all other particles, is very special: its own mass is destabilized by the quantum fluctuations associated with the new particles. In other words, if there is indeed a new particle of mass energy 750 GeV, this would tend to increase the Higgs mass to the same value. But the Higgs mass is measured to be 125 GeV, that is six times smaller. It will then be fascinating to see what protects the Higgs mass from such fluctuations. A new symmetry, for example? In any case, this will give first hand information on the dynamics that may eventually lead to a unification of the microscopic forces with gravity, at a much higher energy. The long sought marriage of quantum physics with general relativity.


2016 thus appears full of promises of major discoveries in fundamental physics. Will it fulfil its promises?


Best wishes to all for this new and exciting year.


Pierre Binétruy

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