"We have observed gravitational waves. We've done it!" Last February 11 in Washington, scientists from the LIGO-VIRGO network put an end to the suspense when they announced their discovery. It was a result that had taken the combined efforts of more than 1,000 scientists. Patrice Hello is a professor at Université Paris-Sud and heads the VIRGO team at LAL d'Orsay. He gave us a bit of background to this announcement.
UPSaclay : There's been a lot of talk about the discovery of gravitational waves, but this observation dates back to September 14, 2015. Why such a long wait before announcing it?
Patrice Hello : A discovery of this sort has to be verified before it's announced, and the only way to get an estimate of its reliability is to collect more data under the same experimental conditions. So we continued to collect data for a month after the event to make sure that what we had detected was not background noise. Next, and above all, we had to check the quality of the data, the absence of artifacts, etc. We checked our software, for instance, to make sure hackers hadn't played a trick on us.
To be honest, we were a bit paranoid because there have been precedents. Two years ago, some scientists announced the discovery of primordial gravitational waves without having verified all their data. That's why we took every possible precaution before making a public announcement.
UPSaclay : How was the LAL involved in the discovery?
PH : The LAL has always been part of the VIRGO collaboration; it’s one of VIRGO’s founding members. There are six associated laboratories in France, and since 2007, VIRGO has formed a network with LIGO to exploit the data. It's a truly global collaboration of more than 1,000 researchers.
What we've been working on at the LAL more specifically has been verification of analyses and data quality studies, two fields in which we specialize. And all the while respecting the embargo, i.e. preventing any external leaks. Unfortunately there were some leaks, but I can assure you that they did not come from the LAL!
UPSaclay : The fusion of two black holes must be an extremely rare phenomenon. Will it be possible to perceive such a signal again in the near future?
PH : Definitely. I can't tell you everything, because we still have a lot of data to analyze. But I am convinced that if we have seen one, we shall see others.
UPSaclay : It is often repeated that this observation proves Einstein's general theory of relativity, published in 1915. In fact, all that has been done is to confirm century-old theories. Isn't that a little disappointing?
PH : It's true, this is no surprise since general relativity has already been well tested: in the weak field regime of the solar system, but also with the observation of binary pulsars, a discovery which won Hulse and Taylor the Nobel Prize in Physics in 1993. But here we have verified it even more thoroughly and in a new regime, because we’re really looking at an extreme case of general relativity—there can be no stronger gravitational fields than those made when two black holes merge.
UPSaclay : So the theory of relativity has been confirmed in a slightly new regime?
PH : Exactly. But we know this theory is going to evolve. We know that today, good physics theories are quantum theories, but general relativity can’t be quantized. It’s a theoretical framework that works well on a large scale for astrophysics and the observation of the universe. But physicists’ goal is to move toward a theory of everything, which can only be done in the context of quantization. And for that, there’s still a long way to go.
What is especially important is that we have opened a new window onto the universe by discovering a new messenger that brings us unprecedented information. It is a new means of observation, complementary to light.
UPSaclay : In your opinion, what could be discovered in the coming months?
PH : It’s well known that every time a new type of telescope has come into service, new configurations and celestial objects have been discovered. So who knows? But still, it is the first time that black holes have been demonstrated directly. Until now, there had been only indirect traces and clues. A new astronomy is born.
UPSaclay : One last thing: it has been said that gravitational waves will allow us to go back even further in the history of the Universe?
PH : That's not strictly true. The further you observe, the earlier you observe; that is true. But we will never find traces of the Big Bang through gravitational waves. What we hope to observe one day is the residue of inflation in the cosmic microwave background. But the VIRGO and LIGO detectors may not be in the best position to detect it at this point.
Image credits: Wikimedia - Alain R, Wikimedia - The Virgo collaboration, Wikimedia - NASA, ESA, H. Teplitz and M. Rafelski