In February 2016 the world was electrified with the information crucial for the development of physics – an international team of scientists officially confirmed the existence of gravitational waves predicted by Albert Einstein. We ask dr Arkadiusz Błaut from the Institute of Theoretical Physics of the University of Wrocław, one of the members of the team, what this breakthrough means for science.
What does this feel like – to take part in the project which confirms the principles of Albert Einstein’s 100-year-old general theory of relativity?
Arkadiusz Błaut: There is satisfaction but proportional – it’s an international project with over 1300 participants. I was aware that scientists were trying to detect gravitational waves, I’ve been working on this myself for a few years, but nonetheless I was surprised when we succeeded. The moment when the news started circulating in the media was very pleasant.
Let’s explain to our readers the structure of the international project which led to the confirmation of the existence of gravitational waves.
The beginnings date back to the late 1980s. It was the idea of several American scientists, a project thought to be implemented in parts – and so it was. First high-sensitivity laser detectors were constructed around 2000 in the United States. The further, improved versions appeared around 2006. According to plan the advanced LIGO model began operation in 2015.
Simultaneously, from the beginning of the 2000s, the Virgo detector, located in Italy, was being constructed, along with the GEO detectors – in Germany – and TAMA – in Japan. Independent teams merged two main projects into one between 2006 and 2007, ever since known as LIGO-Virgo, bringing together over a thousand scientists. One of its parts is a team of Polish physicists POLGRAW, constisting of engineers, astronomers, mathematicians – fifteen people in total.
When did you join the team?
Not that long ago, in 2013. I’d got interested in gravitational waves before, in 2008. The implementation of projects on terrestial detectors had been at full steam at the time. My tasks were and still are related to the next stages of the project, that is building detectors working in Space – LISA, and later eLISA. Currently, since NASA has withdrawn its co-operation, the project belongs to the European agency ESA. Its aim is to transfer, improve, and modify technical advances developed in terrestial detectors in a way that would allow to send these devices into space. The project is expected to be completed in 2034. I deal with the theoretical work related to the capabilities of a detector operating in space.
What are the tasks of the other members of the Polish team?
First of all Poles take part in data analysis, but for example Mgr inż. Adam Kutynia from Wrocław, graduate of the Wrocław University of Science and Technology, is an engineer working with the Virgo detector. He’s involved in modifying and improving the optical elements of the detector, among others, mirrors reflecting the laser beam.
There is a lot of data coming from the detector. Wave detection has this specificity that a potential signal, very often weak, which the detector has the chance to register, is deeply hidden in noise. On the basis of the sequence of numbers provided by the devices one has to develop methods of extracting the signal, allowing to specify the probability of its existence. One also has to estimate the parameters of the source, that is the object which produced the waves.
It is worth noting here that the methods of detection and statistical analysis were developed at the beginning of the 1990s by Prof. Andrzej Królak from Warsaw PAS, who is also the chief of our team. Another person who plays a crucial role is Prof. Piotr Jaranowski from Białystok. He also dealt with data analysis, but is even more known for developing the methods for approximate solutions of the Einstein’s theory equations applicable to real systems ring in space, such as, for example, binary systems of black holes emitting gravitational waves. Just writing the equations needed for this took many people, including Einstein, several decades. So on one hand we have Prof. Królak and the statistical analysis of data, and on the other Prof. Jaranowski and the recognition of wave shape itself – their work has direct application to the analysis of the data coming from working detectors.
Astrophysics is a relatively new branch of science. Was its development possible thanks to the dynamic advancement of technology, especially in recent decades?
The general theory of relativity dates back to 1915. Four years later the first effects confirming the theory, such as the bending of light beams, were detected. Partial verification came in the 1950s and 1960s thanks to, among others, recently developed lasers and increasingly precise instruments. However, those were still experiments in weak gravitational fields. In the 1970s, once satellites were sent to space, we could finally examine signals betwen the satellite and the Earth, check if anything happened with the signal, whether the Sun really modified its path or delayed it, whether time and space got warped. So on one hand some effects were predicted very early, but on the other they got confirmed after fifty years. This demonstrates that the theory itself in the moment of its creation based on some general considerations concerning how the physics of gravitation should work. In this sense it’s the triumph of thought preceding technical capabilities and experiments – not the other way around.
One of the pioneers in wave detection is Prof. Kip Thorne, famous physicist and media figure (recently he has been a consultant on the set of Interstellar, a Hollywood film by Christopher Nolan – editorial note). Together with other prominent physicists, Wheeler and Misner, he wrote a very well known Gravitation, a textbook for students and academics. It contains a section on gravitational waves and an exercise with the resulting conclusion: to calculate the effect occurring after the passing of the wave which probably can’t ever be detected (laughs). After fifteen years – in this period laser interferometry was developed – the same man saw the light at the end of the tunnel. Together with his collaborators he took the risk, dedicated several decades of his life, invested his own and other’s time, and their project turned out a success. It is a proof that it’s worth it to take risks, try to anticipate the advancements in technology, which in thirty years can perhaps allow us to achieve what we plan today. Thorne and his colleagues were successful. We owe this to their knowledge and courage. The team consisting at the end of the 1990s of several dozen people is now the international collaboration with 1300 members.
We can’t avoid the question about gravitational waves. How can we explain this phenomenon – and its impact – in layman’s terms?
It is usually said that gravitational waves are wrinkles in the fabric of space-time, moving at the speed of light.
That’s true – it’s the first sentence of Wikipedia definition.
Because it’s a very good illustration that captures our imagination. However, we should begin with what the general theory of relativity says. Before Einstein created it, gravitation had been known only in Newton’s terms – objects interact with each other via forces, but time and space is a „place” where certain dynamics of these objects take place. The general theory of relativity changes that picture. Gravitation stops being a force in space, but becomes a property of space itself. Let’s imagine a stage on which a play is being set. If, according to Newtonian mechanics, this stage is space and time, then since the creation of Einstein’s theory the stage itself and its boards can start moving and taking part in the play (laughs).
Space-time warp can be explained in terms of a rubber surface which can be bent, stretched, thickened etc. Space-time also bends. Time can pass faster or slower. The general theory of relativity presents it in equations. It explains that the source of warps is mass and energy. The same equations tell us how matter warps space-time, and how the same matter moves in what it has warped. For example, the orbital motion of the Earth around the Sun is described by the theory of relativity precisely as free movement – but in space-time warped by the Sun.
The general theory of relativity in some sense reproduces Newton’s theory in the case of small masses. In some other area, however, it’s revolutionary: it diametrically changes our vision of time and space, if only we move away from small masses and speeds. Black holes and the expanding Universe, which is after all dynamic, come up. It is exactly in the regime of the objects of large mass and large speeds where something which we call a gravitational wave appears.
What are the sources of those waves? Let’s imagine that instead of the Sun and the Earth we are dealing with larger objects located relatively close to each other, set in orbital motion, moving with speed so great that one rotation takes a single second. These are the regimes where gravitational waves can be observed. The Earth system orbiting around the Sun also sends out gravitational waves, but they are so weak that there is no chance to detect them.
In Newtonian mechanics two objects oribiting around each other would continue the motion endlessly. In Einstein’s theory the orbiting objects constantly generate space warps changing in time – we can picture them as the circles appearing on the surface of water when we throw a stone in it. Warped space moves in waves, gets further from the source, the specific system, and the gravitational waves created in this way carry energy.
Let’s note that in the beginning we weren’t so sure if the effect was real – it could have been an illusion caused by poorly chosen coordinates. In the 1930s Einstein himself doubted the existence of the waves, but he managed to correct his error before he died.
If a gravitational wave carries energy away from the system, this system loses energy. If it loses energy, the objects move closer and closer to each other – this scenario differs from Newton’s theory. It’s not elliptical but spiral movement.
How a gravitational wave detector works?
A moving gravitational wave causes alternate stretching and shrinking of space in a plane perpendicular to the direction of wave’s propagation. All you have to do is place two objects – ideally in Space – at some distance from each other, and wait until this distance starts to alternately increase and decrease. The idea is very simple. The only problem is with how big those oscillations are. The length of the LIGO detector’s arm is 4 kilometres. In this case the magnitude of oscillation for an average wave from a typical source, e.g. two black holes, is 4 x 10 -18 m – very small.
How can errors be avoided with such small values?
4 x 10-18 is less than one thousandth of proton radius and this is why, among others, confirming the existence of gravitational waves once seemed impossible. Instead, despite technological challenges, we managed to register the wave with the two American LIGO detectors, about 10 light-miliseconds apart from each other. In the next few months the Virgo detector will begin operation. When the ground-breaking observation was happening the Italian detector was being upgraded. The only two operated in the United States. Increasing the number of detectors improves our chances to detect signals and makes it easier to verify them.
Is this why the world learned about the breakthrough from September of last year in February? Because verification was difficult?
Not entirely. All expected that the detected signal will be hidden deeply in noise. But the one registered on 14th September was noticed in real time. Those who observed it were doubtful at first.
They thought it was a mistake?
Or some unannounced tests, conducted in order to check the readiness of the staff operating the devices. The effect was so strong and had the shape similar to the one predicted by the theory – let’s not forget that such signal had not been previously registered – that it was virtually visible to the naked eye even before detailed data analysis started.
Did you know about the finding already in September?
No, I learned about it later. It’s worth noting that all project participants had to keep it secret. The candidate for the gravitational wave was just too good to be true. The signal was too strong, resembled the model too much – all the details were in place.
If everything looks fine, it sometimes raises suspicion.
That’s why the results were so meticulously examinated, source parameters were estimated on the basis of the shape of the registered wave. This allowed to specify what objects and how far and fast moved.
What is the meaning of this finding? What is the aim of the research on gravitational waves?
The objective is to confirm the general theory of relativity. It’s been known for a hundred years, but it fundamentally changes the vision and lies at the basis of many notions in physics. During the last century Einstein’s theory has repeatedly been confirmed, but it’s relativistic regime – of largest masses and speeds – hasn’t, not directly.
Some signals indicating the existence of gravitational waves were noticed in the past. Pulsars system and binary system in which one of the elements was additionally sending off electromagnetic signals were observed. On the basis of these signals it was recognised that it is a binary system of stars moving on a spiral orbit – so getting rid of its energy somewhere. During twenty years of observations it was determined that this system was losing energy in a way keeping in line with the general theory of relativity and predicted level of gravitational waves emission. This finding was awarded the Nobel Prize.
A direct confirmation is wave detection. First, the significance of this finding lies in the fact that it is yet another confirmation of the general theory of relativity, which in turn is one of the pillars of physics, along two others: the standard particle model and quantum mechanics. Second, we’ll be detecting more and more waves. In not too distant future there will be detectors in Japan and India, which will increase the number of observed signals. A next generation detector will be built deep under the ground in order to eliminate seismic noise. The next step is coming into Space.
Terrestial detectors have limited capability. Their maximum sensitivity is around 100 Hz. When we manage to lower the frequency of detector’s operation to 1Hz or even some miliherz values, the chances to detect binary systems will significantly increase.
My simulations – I work on white dwarf systems in our galaxy – show that with a new instrument tested in theory it will be possible to detect around six thousand so-called close binary systems in our galaxy. And it’s still only a part of the whole. There are also, for example, supermassive black holes which mass equals ten million masses of the Sun, and they’re very distant in cosmological distances. They will expand the research capabilities of cosmology. We can also examine Einstein’s theory itself and compare it to some alternative ones.
Very powerful tool for science.
Indeed. The observation that a window to the Universe is being opened is not a metaphor nor exaggeration, it’s a fact. We’ve gained a new sense allowing us to explore the Universe. Of course the most interesting will be the things we don’t expect to see. Perhaps we’ll suddenly report a wave shaped in such a way that explaining it using the general theory of relativity will be impossible. And here’s the real significance of the finding: the opportunity to study objects which cannot be detected in any other way, objects at cosmological distances, and – perhaps – previously unknown.
Our world is expanding again.
Yes. It will be like Magellan’s journey. Just as he did, we know and at the same don’t know where we’re headed.
Will the confirmation of the existence of gravitational waves make it easier for us to travel in Space?
At this point there is no connection between the two.
So will it be possible to create more detailed maps of the Universe?
If it comes to maps, yes.
So we’re truly back to the era of explorers.
We can certainly say so.
Thanks to gravitational waves we’re looking way forward, but the finding may also tell us more about our world’s past.
Exactly! One of the sources which next generations of instruments will be able to register is the so-called stochastic gravitational background radiation. Relic, electromagnetic radiation is often mentioned when showing the oldest picture of the Universe – CMB map in which lighter and darker areas illustrate the temperature of the Universe when it was 300 000-years-old. We cannot look further into the past, because before that photons had been trapped in the thick matter of the early Universe. Taking an earlier picture of the Universe is therefore impossible – with electromagnetics. But it may happen using gravitational waves.
At this point we can of course study the scenario of the Big Bang at its early stages in a theoretical way, but here we start speculating a bit; we go beyond the limits of prevailing theories, because the ones developed up to this point can no longer explain some concepts. Currently we suffer from an overabundance of models rather than a lack of them (laughs). The most popular notion, the theoretical model of what happened, it precisely the Big Bang model, and earlier – the inflationary model. If in that era – and before – gravitational waves were produced, if we’ll be able to register them, it means we can make a photo of the really early stages of the Universe’s existence.
How early? Can we predit it?
If we consider the inflationary model as correct, we can even study gravitational waves from the transition period between the inflationary phase and the one we call the Big Bang. Moreover – we can examine different scenarios. One of them postulates the existence of a great Universe which collapsed and then exploded creating a new one – ours. Traces of this event can be hidden in the spectrum of the so-called stochastic background of gravitational waves. We aren’t able to do this today, and even in ten or twenty years, but in thirty – perhaps. The level which must be reached to test the inflationary scenarios using gravitational waves is far ahead of us – at this point it’s a million times too little if we consider instrument sensitivity. Launching a detector into Space in two decades will decrease the difference by three orders. And there are, of course, projects even more ambitious…
Interview by Michał Raińczuk
Arkadiusz Błaut (born on 7th March, 1968 in Wrocław). A member of the academic staff of the Institute of Theoretical Physics of the University of Wrocław. Between 1988 and 1993 he sudied physics at the University of Wrocław, where in 1993 he was granted M.S. degree, and in 1998 received doctoral degree. He specialises in physics of gravitational interactions; the scope of his research includes, among others, cosmology and gravitational wave detection. Since 2013 he has been a member of the POLGRAW group, a part of the LIGO-Virgo project.
Photo by Dominika Hull
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