LISA Pathfinder to successfully achieve its mission

We have exceeded not only the requirements set for LISA Pathfinder, but also the accuracy required for LISA at all frequencies: we are definitely ready to take the next step.” Karsten Danzmann*

We started our LISA Pathfinder Saga on June 18th 2015, and followed with you and the researchers involved, this exciting mission for exactly two years. The goal of LISA Pathfinder was to validate the technology that will be used for the future LISA mission. That is, having two test masses placed in each of the three spacecraft of the LISA constellation, which is designed to detect gravitational waves in space.

Nonetheless the LISA Pathfinder mission exceeded its objectives, and the LISA mission has just been selected as the third large-class mission in ESA’s Science Programme. Along with a third detection of gravitational waves emitted from the coalescence of two black holes, announced last month by the Advanced LIGO collaboration, we are definitively starting the era of Gravitational Astronomy.

The LISA Pathfinder mission performed its final tests and will receive its last command on July 18th 2017.


You can read everything about the achievements of the LISA Pathfinder mission on the ESA website:


This has been a splendid journey that we enjoyed following with you. And we can certainly expect more like this in the coming years.


*Director at the Max Planck Institute for Gravitational Physics, director of the Institute for Gravitational Physics of Leibniz Universität Hannover, Germany, Co-Principal Investigator of the LISA Technology Package, and lead proposer of LISA.

Image: LISA Pathfinder in space. Credit: ESA/C. Carreau

A new session of Gravity! starts on Monday 30th January

Classes for the third session of the on-line course Gravity! start on Monday 30th January, for six weeks. You may register here on the FutureLearn platform. The course is free and registration is open to everyone.

Since its first run, this course has attracted close to 90 000 learners, including both French and English versions. 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, dark energy, black holes, ending with gravitational waves, whose detection was announced in February 2016. 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 and the Endowment Fund Physics of the Universe from Paris Diderot University.

Two members of the Gravity! team are awarded a L’Oréal-UNESCO fellowship for Women in Science

On October 12 were announced in Paris the L’Oréal-UNESCO Fellowships for Women in Science. Each year, these awards allow talented young woman scientists to pursue promising research projects. Two members of the Gravity! team were honored at this occasion.




Valerie Domcke is a PCCP fellow at Université Paris Diderot. She received her Ph.D. in theoretical physics from the University of Hamburg, then went to Trieste for a postdoctoral position before joining the Paris Centre for Cosmological Physics (PCCP) in October 2015. She is presently working on the physics of the early Universe and on gravitational waves. She is particularly interested in the connection between particle physics and the physics of the Universe .




Eleonora Capocasa is presently doing her Ph.D. at APC laboratory in the Virgo team (as a member of the LIGO-Virgo collaboration, she signed the gravitational wave discovery paper last February). She is working on ways to increase the sensitivity of gravitational wave detectors.

Congratulations to both of them! And let us make the wish that their example will attract many more young women scientists to the field of gravitational wave astrophysics.

A proton’s life



One of our pleasures is to discover some real gems among the posts  of the Gravity! course. We already have a great collection. As a tribute to all of you who made brilliant contributions, we decided to highlight the beautiful story that Damien Pigret, a talented participant to the second session of Gravité!, proposed on the course forum. Because this is still summer,  we propose this delightful story as a series, A proton’s life, published in 4 installments every Monday. Enjoy!

Pierre Binétruy

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.

The color of stars

Most of the stars that you see in the sky are stars of our own galaxy. You can see that some are more yelow, some bluish, some reddish… But why do they have a color? Well, we first have to understand why material objects have color.

A rose is red because, if you illuminate it with some white light (which means some light with all the shades of the rainbow), it only reflects the red component.


A white rose reflects all colors, hence looks white.


A black object looks black because it absorbs all colors of light and reflects none.

If the surface of Mars looks red, it is because, illuminated by the light of the Sun, it only reflects red light.



But what makes the color of the Sun? What makes the color of stars?

The light that comes from them cannot be reflected by their surface, because there is no light source close by; this light must be emitted by themselves. Indeed, if you were sending some light beam to the surface of the Sun, it would be absorbed by this surface. In other words, the surface of the Sun -and of all stars for that matter- is a black body, in the sense that it is a perfect absorber of light. But why do we see colors?

Well, it was discovered at the end of the XIXth century that a heated black body emits electromagnetic radiation (light if you prefer), and the color of this light is characteristic of its temperature.

Somehow, you know this: take a piece of charcoal, it is black,; heat it: it will become red or even white; it thus emits a light which is characteristic of its temperature.charcoal_red




Similarly, the color of the light emitted by stars gives us precious information about the temperature of the star interior.
As for explaining this strange phenomenon called black body radiation, one had to wait till Max Planck in 1900. This was the birth of quantum mechanics…

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:

Why test the equivalence principle?

Let us first remember what is the equivalence principle.


The equivalence principle expresses a property which is at the basis of Einstein’s general relativity: the equivalence between acceleration and a gravitational field. More precisely, observations made in a system in acceleration (e.g. a rocket) are indistinguishable from those made in a gravitational field (e.g. on Earth).


This allows to understand better the notion of mass, which is actually describing two apparently independent concepts:

  • the mass of a material object characterizes how it couples to a gravitational field; for example a more massive object is submitted to a greater attraction to the Earth, a greater weight. This mass is called the gravitational mass.
  • the mass of a material object characterizes its inertia, that is its resistance to changes of motion. This mass is called the inertial mass. Since acceleration corresponds to a change of velocity, hence a change in motion, it is this mass that appears in the famous law of motion: force = mass x acceleration.

The equivalence principle tells us that gravitational mass and inertial mass are identical. This is why it is at first so difficult to disentangle the notion of weight (related with the gravitational mass) from the notion of inertia (related with the notion of inertial mass).


This principle has some important consequences. Take for example a kilogram of gold and one of platinum. They resist to changes of motion in the same way: they have the same inertial mass. Hence they have the same gravitational mass and identical motions in a gravitational field; they are attracted in the same way by the Earth. This has been checked on ground to a precision of one part in 10 000 000 000 000 (in other words 10-13).


But theorists are not fully happy with the theory of Einstein. They would like to unify general relativity with the quantum theory which describes non gravitational forces. They thus have to change, even though in a subtle way, the description of the gravitational attraction. But by doing so, they often lose the precise identification between gravitational and inertial mass.

For example, let us consider string theory where the basic objects are no longer point particles but microscopic one-dimensional objects (the strings!): our good old particles are considered as grains of energy which correspond to modes of oscillation of these fundamental strings. We are used to (violin) strings emitting (sound) waves, but remember that, in the microscopic world, waves and particles are united in a single concept (the two sides of the same coin if you prefer). This is why different types of oscillations of the fundamental microscopic strings lead to different types of particles, with different energies E, hence different masses m (E=mc2).

Now in such a theory, the gravitational force between two particles/waves is understood as an oscillation, or a series of oscillations of the underlying string. It is thus not at all obvious that the gravitational force between say two protons is identical to the gravitational force between two neutrons, or between a proton and a neutron. Thus, if two material objects have the same inertial mass but different number of protons and neutrons, they may be falling differently in the gravitational field of the Earth. They would thus have different gravitational masses. This leads to a violation of the equivalence principle.

This is exactly what the Microscope mission aims at testing, gaining two orders of magnitude over the existing experiments on Earth (10-15).

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