Pharao (Project of atomic clock using in orbit atom cooling) will measure the time with such stability and accuracy performance that the scientists could test the general relativity theory with an unprecedented accuracy.
The time is measured thanks to a periodic phenomenon such as the oscillation of a pendulum. Since 1967, it has been beating to the rhythm of the atoms: one second was defined as the 9,192,631,770 period duration corresponding to a transition between two energy levels of a caesium 133 atom.
Atoms change their energy level when going through a microwave cavity which make them resonate by experiencing a transition called hyperfine structure transition. The quantum phenomenon which is used (absorption or emission of photons corresponding to this energy change) provides a more accurate and stable frequency than every other known phenomena and sets the tone, or rather the International Atomic Time (IAT). The best current clocks were built following this principle and operate using different types of atoms.
But it is necessary yet for the atoms not to be affected by the environment. Patience and length of time, every sources of error must be thoroughly tracked down.
The atoms had to be hosted behind a magnetic shielding with a low gas pressure up to the ultra high vacuum in order to avoid the collisions and even more difficult, atoms had be be slowed down to a minimum. For that purpose, the laser light has been used in a bid to cool the atoms down to a temperature close to absolute zero. The atoms' reaction time in the cavity could thus grow by at least two orders of magnitude.
But with such low speeds, the atoms felt under the effect of gravity.
Even so, the scientists took advantage of it by launching them upwards and letting them fall-out under their own weights. That is the principle of an "atomic fountain" which increased again the atom's reaction time thanks to the round-trip.
The first caesium atomic fountain was created in the 1990's in the Observatory of Paris. The accuracy and stability of this fountain were ten times better than all existing clocks. However, this unprecedented result marked the start of a new story.
What would you think about in order to free yourself from the gravity effects? The scientists had the idea of placing such a laser-cooled caesium atom clock in orbit around the Earth, into free fall. It was simple, but not that simple. The first step was to be able to sharply reduce the clock dimensions, make its working system sturdier and adapt it to the absence of gravity.
That is how CNES came sponsoring this space clock project in 1993. The parabolic aircraft operated by Novespace on behalf of CNES was particularly well placed to carry out the preliminary conception trials.
Instrument's technical design agreed
In 1992, in the Caravelle-0g, the atom trapping and cooling methods were already verified. Then, in 1997 in the Airbus-0g, the complete clock prototype gave the expected results and confirmed the instrument's technical design.
That is how the Pharao space clock project, proposed to CNES and ESA by the Kastler Brossel laboratory of the Ecole Normale Supérieure and the Syrte laboratory of the Observatory of Paris, was born in 1997.
Associated with a hydrogen maser clock, the Pharao clock will be on board the European Aces ensemble (Atomic Clock Ensemble in Space) on the outside of the International Space Station.
Thanks microwave and optical signal reception and emission, the time measured by ACES at an altitude of 400km will be compared to the time set by the best atomic clocks distributed over different points on the ground.
This project is dedicated to Albert Einstein, a clock and moving train aficionado (maybe satellites today?) who did change the time and space conception. In 1905, he abolished the absolute time notion with its special relativity theory and unified time and space in order to create a single four dimensional spacetime. In 1915, he identified the gravity with this spacetime's geometry in its general relativity theory.
Since then, the general relativity have been tried by several clock and mass motion trials in free fall and it did become part of the daily life because the GPS clocks and shortly Galieo's are being freed from the relativistic effects. But the general relativity theory comes up against another revolutionary theory of the twentieth century which describes the universe as a quantum one. It is also confronted with the observation of new unknown components in the Universe such as dark matter and dark energy which could come from anomalies noted in the laws of gravity at the very large galactic and cosmic scales.
That is why the scientists are looking to test again the general relativity theory with increased precision and build new representations of the world by introducing new dimensions, particles, symmetries, etc. These theories, such as the string theory, predict such infinitesimal effects (a physical constant variation for example) that they could not be measured yet.
The Pharao clock, with a 10-16 relative stability frequency over a few days, should gain on the Einstein effect (i.e. gravitational red shift of the frequency also called redshift) test by a factor 30 and on the test of a possible fine structure constant variation by a factor 50 before the end of the decade.
In addition to these fundamental physics tests, the spacetime characterization at such precise levels opens up outlooks in geodesy and navigation.
Pharao may not be the last. In particular if the STE-QUEST project, shortlisted as a medium-sized mission within the frame of the European Cosmic Vision programme, is selected.
A cold atom clock in absence of gravity, Ph. Laurent1, P. Lemonde1, E. Simon1, G. Santarelli1, A. Clairon1, N. Dimarcq2, P. Petit2, C. Audoin2, C. Salomon3, Eur. Phys. J. D 3, 1998, 201-204
1 BNM-LPTF, 61 avenue de l'Observatoire, 75014 Paris, France
2 Laboratoire de l'Horloge Atomique, Bâtiment 221, Université Paris-sud, 91405 Orsay, France
3 Laboratoire Kastler Brossel, Département de Physique de l'École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France