Your research group recently achieved outstanding results. Can you explain to us how the research on gravitational waves has been conducted and why the results are so important?
The attempt to detect gravitational waves is a scientific enterprise which has been conducted over many years. The beginning of the scientific debate on the experimental activity targeted at gravitational waves dates back to the beginning of the 1960s. Before then a lively debate took place in theoretical physics: scientists wondered whether the solution to Einstein’s equations, which substantially led to the prediction of the existence of gravitational waves, lay in a series of choices in developing the mathematical tools employed to solve such equations. The concern was whether the expected phenomenon would disappear given other methodological choices. The theoretical debate matured at the end of the 1950s thanks to the work of Herman Bondi and Felix Arnold Pirani who designed to show that the gravitation signal transferred energy and hence that it was a genuine signal, which could be somehow detected.
The pioneer of this research was Joseph Weber, who had already made a major contribution to the development of masers, the precursors of lasers. By the end of the 1960s, Weber presented a series of results, which suggested that he had detected gravitational signals using “Weber bars”, that is, two large cylinders, which were supposed to vibrate very weakly if set in motion by gravitational waves. In the paper “Evidence of Discovery of Gravitation Radiation”, Weber claimed that he had reached his goal. Subsequently, a series of parallel experiments were started by other scientists, both in the US and in Europe. Although they did not detect anything, they disproved previous alleged results and provided a tremendous stimulus for further research partly guided by the idea that, had the cylinders been cooled down, their sensitivity was likely to increase significantly. A further research phase started, which pushed detector technology further, largely due to international collaborations. The gravitational signal was supposed to be associated with the acceleration of great masses and hence, coming from very far away, would hit the whole earth as an extremely weak signal, which could be confused with noise. Nowadays, the simultaneous detection of the signal by different experimental devices in different points on Earth helps us to isolate it as well as to identify the position in the sky where the signal comes from. The crucial role of international cooperation has been pretty clear for a long time now. Scientists are well aware that research must be carried on by coordinated research groups worldwide.
The second generation detectors were ten times as sensitive as Weber bars but still failed to detect anything. Some announcements were made that had then to be retracted. Scientists learned to be very cautious – especially to prevent errors, which, though characteristic of any scientific enterprise, tend to be misperceived as failures by the general public and by funding agencies, which is ultimately damaging to the project itself. Errors are part and parcel of the history of research on gravitational waves, as of any scientific enterprise in which an original theoretical hypothesis is to be tested by empirical data. Scientists typically face both theoretical and experimental errors – especially when, at the frontiers of research activity, detection technology is pushed to its limit. Caution is thus to be recommended. Often, the first discovery receives much emphasis, even though the second test may actually be more important as it confirms the results and makes them more robust. Any discourse on what the scientific method is like, any discourse on how science must proceed must stress such features. Instead, they are often neglected or wrongly interpreted in the context of scientific dissemination for the general public, in scientific journalism and in the widespread search for easy scoops.
When gravitational waves were detected, our research team – involving the LIGO and the VIRGO research communities – announced the discovery only after all the steps designed to control the soundness of the results had been taken. Gravitational waves were observed on 14th September 2015 but, due to the extremely careful control procedure, the announcement was made only on 11th February 2016. The analysis of further data, concerning an event detected in December 2015, provided further confirmation and indicated that the first had not been a fake event. We are now confident that further events of the same kind will be observed in the future.
The discovery was presented as a “direct observation” of gravitational waves. Can you comment on this?
The whole research group, from Europe to the United States, discussed at length whether the word “direct” (which, however, does not appear in the title of the main paper announcing the discovery) should be used. We have ripples in the fabric of spacetime. To detect them, we need some “translation”, and that’s what an interferometer does. Observation is always conditional on a detection system. By “direct” we mean that we designed and built that instrument whose aim was the detection of this specific category of signal. It was a direct observation in the sense that we had a specific goal and we obtained the information we were looking for. We observed the fluorescence of black holes.
The use of the term “observation” was also the subject of vivid discussion within the research group. The alternative was “detection”. Finally, “observation” won since it indicates that we had found a way to understand what goes on in the distant universe. We were inferring the dynamics of black holes’ fluorescence from the data, observing the distant sky with a sort of brand new Galileo’s telescope. We were pointing our telescope very far away, discovering a new way to transmit information, and, in the end, suggesting a new way to address reality. Reality is always the ultimate target for a scientist, who is well aware that it can surpass our detection capacities. From the standpoint of an experimental physicist, any attempt to neglect reality is tantamount to an attempt to kill physics itself. Our daily work is an attempt to control what we think the universe is like with what the universe out there actually is like. This is ultimately, I believe, what an experimental physicist does. When I obtain a piece of information from a detection tool, this very fact tells me there is something out there, which sent the signal.
The discovery has been claimed to start a “new era” for astrophysics. In which sense is it so?
We observed some phenomena that could not be observed with the traditional detection instruments. Our knowledge of the universe is grounded in our capacity to detect electromagnetic waves, which span from radio waves to gamma waves, including the light we see. All the scientific instruments that have been developed in time – from Galileo’s telescope, to radio telescopes, gamma and X-ray satellites – allow us to obtain information through electromagnetic waves. Instead, we observed a signal that is due to a process involving black holes, i.e. objects, which do not emit any light. Such compact objects bend space, and, given their velocity, such bending in turn propagates in the universe and reaches us. We detected this bending, and the information hidden there – information concerning how the process generating it occurred, how two black holes progressively closed and then blended at an amazing velocity. We were able to extract all this extraordinary information, which was previously inaccessible. This substantially means that we now have a sort of new Galileo’s telescope, working on the basis of totally different principles.
What will this discovery lead to?
For certain our level of knowledge of the universe, of what occurs around us, will increase. We can now say that information can be transmitted through a “new channel”, even in the absence of light. This was one of the controversial issues in the debate back in the 1950s, regarding the possibility, or impossibility, of extracting information from this kind of process. Now not only do we know that gravitational waves transmitting energy in principle exist, but we also know that we can actually extract it by a direct observation of the signal itself. It is in this sense that we have been claiming that we now “see the invisible” by virtue of this new bridge to previously inaccessible information.
Which do you reckon have been the most critical and the most significant steps in the history of this research, both from a theoretical and from a technical point of view?
To consider the roots of research on gravitational waves we need to go back in time. In 1916-1918, Einstein had claimed that gravitational waves were so weak that they could not be detected. This claim must be understood in the light of the technological constraints of that time. In the 1930s, Einstein wrote a paper, where he claimed that gravitational waves were a signal that did not transfer energy, and that they were thus not detectable in principle. He submitted the paper to Physical Review, which had it refereed by an anonymous referee who claimed that Einstein was wrong. Einstein was very annoyed, and published the paper elsewhere. Later on, he realized that the referee was actually right. As this shows, research on gravitational waves underwent a long series of different phases, with many ups and downs and with many outstanding scholars believing that gravitational waves could not be observed. Weber and colleagues aimed to detect gravitational waves resulting from the explosion of a supernova, and that was the goal for a long time. A decisive step forward came with the idea that more robust emissions could be detected by pointing to binary systems which coalesce. This idea shifted the focus of the investigation. It was successful also because it was accompanied by significant technological progress: in order to hunt for this kind of phenomenon the sort of instrument we nowadays use, i.e. the interferometer, proves much better. Interferometers can detect a wide range of signals, covering a much larger frequency band than previous instruments, and they are much more adaptable and sensitive (especially at very low frequencies). The theoretical shift was thus associated with the development of a totally new detection technique, which has proved absolutely crucial to the success. Without the interferometers and the range of signal they can “catch”, we would not have detected a coalescence phenomenon.
What about “cleaning” the signal from noise and amalgamating evidence from different experimental devices? What problems arise?
In these kinds of experiments noise is our “great enemy”. In this context, we must distinguish stationary from non-stationary noise, that is, between noise whose statistical features remain constant in time and noise whose statistical features change. The signal is isolated through various techniques, mostly borrowed from elsewhere – especially from research in physics, mathematics and engineering on telecommunications and radar systems. The use of different detectors counts as a highly positive fact, since it is the easiest way to get rid of non-stationary noise (e.g. a train or an aeroplane passing by a given device can be easily identified as noise). Using two detectors therefore helps. Three or more would be even better. When combining data from different detectors, we must pay attention to the fact that they can differ in sensitivity, but nowadays we have standard techniques to adjust that. What is actually very challenging is the functioning of the experimental devices themselves: we work at the very limit of technological possibilities, pushing technological devices to their limits, and trading-off between sensitivity and robustness. In the last stage of our research, we collected data for 4 months, but the useful days in terms of data were just 48. This is because we must multiply the efficiency of one detector by the efficiency of the other; were we to add a third detector, the efficiency would drop even lower. Overall though, in terms of trade-off, the more detectors there are the better.
The results of the research on gravitational waves have also resonated among ordinary people. Why do you think this was the case?
Our research has had a major impact, actually much greater than we expected, and we have been invited to give a number of public lectures and conferences in schools. The whole research group has worked hard on dissemination (VIRGO is a collaboration project involving France, Italy, The Netherlands, Poland and Hungary; LIGO involves Germany, Spain, UK, USA). I take the success among the lay public as a sign of hope with respect to the relationship between science and society. In the end, people want to understand what scientists are working on, and overall they devote more attention to scientific enterprises than they used to. The impact of our research has to be understood in the context of a general increase of public interest in science. The announcement of the discovery of gravitational waves, in turn, served to further enhance such interest. The discovery itself concerns issues which can be disseminated in rather simple, non-technical terms and can thus be made accessible. Even if perhaps vaguely, people understood what the discovery was about. The notions of wave and spacetime attract attention because we all have some intuitions in these respects. Furthermore, black holes are highly suggestive: talking of this sort of “Gargantua” strikes a chord in the imagination. On top of that, scientists are increasing their efforts to deliver the results of their experiments in accessible terms, also outside academia, to make people understand what contents and which concepts are at play. In short, I believe the desire to understand rationally what is going on around us, what reality is like, is increasing overall. In this respect, we still need to learn thoroughly the importance of communicating scientific results to the public at large, of outreach activity as an essential part of any scientific project.
What theoretical concepts are worth exploring further in this context, and to what extent can an interaction with philosophical work help here?
During public lectures, the audience often raised questions which were only apparently naive, but which actually hid quite deep reflections. For instance, people asked: In the end, what is gravity? How can there be a spacetime wave? Can we conclude that there is a sort of “aether”, in which space-time propagates? I believe numerous concepts underlying the research on gravitational waves need to be investigated further. For instance, with regard to gravity, my colleagues and I tend to choose a shortcut, replying: “as experimental physicists, we deal with the question how it works, not with the question what it is”. The same holds for spacetime: we just know what it looks like. Questioning what gravity is, is questioning what spacetime is. The universe is expanding. What this means is that space and time are being generated. What is generating them? Is it mass-energy? Causation is involved as well, insofar as it can be related to the propagation of spacetime in new terms, not only to the propagation of electromagnetic waves. Furthermore, the description of spacetime is related to the observer. How general can it be? This is all very challenging and our standard categories need some re-thinking. A deeper reflection is needed not only for lay people, but for physicists too. Probably the research on gravitational waves has also gained much success because it touches upon categories of space and time, and it tries to raise awareness on the following crucial issue: our standard way of conceiving space and time must be relentlessly, inevitably and radically modified.