Gravitation: The dominant force in our universe
Gravitation is the dominant force in our universe – as Joan Centrella explains to Annalie in this interview. Fascination with gravity has driven scientific research since its earliest days. Two of the giants of modern scientific research, Sir Isaac Newton and Albert Einstein, made their most important contributions to physics by explaining gravity and how it acts on objects. Newton’s law for the force of gravity made sense of the motions of planets for the first time and served astronomers well for more than two centuries.
Einstein completely rewrote the book on gravity with his theory of General Relativity, certainly one of the most important scientific achievements of the last two hundred years. It describes how matter warps space and time and creates gravitational influences on a cosmic scale. But for Einstein there is no force of gravity pushing bodies around. Instead, a moving body simply follows the straightest possible path through the gravitationally warped space-time. The result is not only Newton’s familiar orbital motions of the planets, but much much more.
Black holes and gravitational waves
Einstein found that his theory of warped space-time predicted, among other things, the existence of black holes and gravitational waves. Black holes are the most extreme kind of warping, one so strong that it traps matter inside it. Gravitational waves are ripples in the warping that travel through space at the speed of light. Just as water waves ripple outward whenever the surface of a pond is disturbed, so also gravitational waves ripple outward whenever the warping of spacetime is changed.
As Annalie has heard from many of the scientists who study black holes, one of the most important goals of research today is to prove Einstein´s predictions by making the first direct measurement of gravitational waves, and black holes are a very strong source of them.
The waves from black holes don’t come out from inside the holes, because nothing can do that; however, if two black holes orbit one another and finally merge together, the distortions that they make in the surrounding space-time are extreme, and they send out strong ripples in all directions. Remarkably, as Annalie learned from her interviews with Günther Hasinger and Stefanie Komossa, astronomers expect such merger events to happen often enough to be detectable in the very near future.
Computer simulations of black hole mergers
To understand just what General Relativity predicts will happen when two black holes merge is a big challenge. Einstein’s equations are just too complicated to be solved with pen and paper for such a situation, so for many years physicists have been developing sophisticated computer programs to simulate the merger. As Joan Centrella explains, since we can’t experiment with black holes ourselves, our computers become the black hole laboratory. By changing the starting conditions for a merger (the masses of the holes, their spins, their initial orbits around one another), physicists can use computers to find out what happens: what the emitted gravitational waves look like in detail, and what the final merged object is.
For many decades, as Ed Seidel explained to Annalie in one of his interviews, computers were too slow and small to provide accurate answers, and scientists’ programs also could not follow the merger process for long without developing large errors. But computer hardware and software have finally improved to the point where reliable answers, checked and cross-checked by many research groups, are now coming out. Joan Centrella’s group at NASA, along with the group led by Manuela Campanelli at the Rochester Institute of Technology (and formerly at the University of Texas at Brownsville), put the last piece of the software puzzle together and finally succeeded in performing long-lasting simulations (see here and here) that ran fast enough that research groups could study many simulations over the course of several months.
What have scientists learned from the simulations?
The most important result from these simulations is simply that mergers of black holes produce bigger black holes. This has long been expected: as Annalie learned from Cliff Will, the British physicist Roger Penrose formalized this expectation with his Cosmic Censorship conjecture, that there are no “naked singularities”. The computer simulations fully support this, although by themselves they can’t prove it will happen in all circumstances.
The next most important result of the simulations is that, in general, the final black hole can get a very big “kick”; in other words, it shoots off at high speed from the place where it was formed. This speed is essentially a recoil due to the emission of gravitational waves during the last part of the merger process. These waves carry energy and momentum, and they do not radiate out uniformly in all directions. There is stronger radiation in some directions than in others, and the result is that the final black hole moves off in the opposite direction to the one where the radiation was strong.
The speeds are huge: more than 1% of the speed of light if the starting conditions are right. Importantly, the “right” conditions depend on the initial spins and orbits but not on the overall masses of the holes: supermassive black holes can be accelerated to such speeds just as easily as “ordinary” stellar-mass black holes.
Speeding black holes
No other objects in the universe travel at such immense speeds, apart from the tiny elementary particles that astronomers call cosmic rays and neutrinos. To get an object with the mass of a star, let alone millions or billions of stars, up to these speeds requires extraordinary accelerations, which no force of Nature other than black-hole gravity seems able to produce.
It could be science fiction: imagine a black hole hurtling through space at thousands of kilometers per second, crossing a distance comparable to the size of the Earth in just a few seconds. But this is science fact, a prediction of careful computer simulations. And as Annalie learned in her interview with Stefanie Komossa, astronomers may already have observed just such a supermassive black hole in the process of leaving a galaxy. There could be thousands of black holes, each with a mass billions of times the mass of our Sun, streaking through the vast empty spaces between galaxies all over the universe.
A black hole that moves at a substantial fraction of the speed of light has a huge energy and momentum, and that means that the gravitational waves that were emitted by the merger event and which accelerated the black hole must also have been immensely strong. This explains why gravitational wave astronomers are so interested in black holes: they are probably the strongest sources of gravitational waves anywhere.
Using simulations to detect gravitational waves from black holes
To improve their chances of detecting these waves, gravitational wave astronomers need the third important result of the simulations that the numerical groups are performing: detailed predictions of the exact wave-forms (shapes) of the waves as they arrive at the detectors. With this knowledge the scientists are able to recognize the waves even when they are obscured by detector noise. Annalie learned more about this in her interview with Badri Krishnan, who converted the predicted wave-forms into sounds and played them for her to listen to. The first detections by ground-based detectors will be made easier and will happen earlier because of the computer simulations of the mergers.
Confronting General Relativity with gravitational wave observations by LISA
But for Joan Centrella and her colleagues at NASA’s Goddard Space Flight Center, the most interesting observations of all will be performed by the space-based gravitational wave detector LISA, which is a joint project between NASA and the European Space Agency (ESA). LISA, currently planned for launch around 2020, will be sensitive to gravitational waves with the frequencies that come from the mergers of supermassive black holes in the centers of galaxies. Most important, LISA will have super-high sensitivity: the wave-forms will stand out clearly, and the importance of the simulations will be turned around: instead of helping make the detections, the simulations will be compared with the observations in order to test General Relativity itself. If the observations do not correspond to the detailed predictions of the numerical simulations, then scientists will have to go back to the theoretical drawing board and start re-thinking gravitation theory. For the third time!
Watch this numerical simulation of two black holes that shows how energy is radiated in the from of gravitational waves.
The most spectacular project in gravitational wave detection
is LISA: a gravitational wave detector in space consisting of three satellites in solar orbit connected by laser arms 5 million km long. LISA will be the largest-ever man-made construction and will survey the universe to pick up signals from the first massive black holes. It will register every merger of two supermassive black holes that occurs anywhere in the universe during its mission. From these and other measurements, LISA will test General Relativity with unprecedented accuracy, help astronomers understand how galaxies and their massive central black holes evolved, study with high precision the mysterious “dark energy” that pervades the universe, and potentially reveal whether the “dark side” of our universe contains other kinds of dark but massive objects, like cosmic strings, boson stars, or things scientists have not yet even imagined!