Gravity is geometry

 

With his theories of relativity Einstein laid the foundation for much of modern physics. His special theory of relativity is based on the idea that the speed of light is constant regardless of the motion of the observer. It tells us that measurements involving space and time lead to surprising results if one travels near the speed of light. Time appears to slow down and objects seem to contract. This shows that space and time are not the rigid concepts that we are used to in everyday life.  In the general theory of relativity, which reconciles the principles of relativity with gravitation, space and time become even more flexible – changing as the world changes around them.

 

Black holes

 

In Einstein’s theory gravity is no longer simply a force that pulls falling apples to the ground. Instead, gravity is geometry. The presence of matter alters the geometry of space and time, and the geometry in turn determines how matter moves. One of the most fascinating predictions of general relativity is the existence of black holes. Black holes are made from matter, but they are not matter. They are formed when massive stars run out of nuclear fuel and collapse under their own weight. The collapse leads to a region of extreme space-time curvature. Objects can fall into this region – the black hole – but nothing can escape. Not even light.

 

 

Powerful radio-jets emerge from the cores of many active galaxies. In this case, the jet provides evidence for a gigantic black hole at the centre of M87 [copyright STScI].

 

Massive black holes grow when galaxies collide and their central black hole merge. Such collisions generate strong gravitational-wave signals  [copyright STScI]

 

 

 

 

We have strong astronomical evidence that black holes are common in the Universe. Their presence may be central to the formation of galaxies.  In fact, we believe that virtually all galaxies harbour a gigantic black hole in their centres. Yet we do not know how these black holes work. Apart from having relatively good estimates of their masses and some evidence that they may rotate very fast, we know very little. Black holes are dynamical objects that interact with the environment. They may form binary systems where two black holes orbit each other. According to general relativity, such systems will evolve towards a final black-hole collision – an event involving extreme space-time deformations.

 

 

Waves of gravity

 

Einstein's geometric theory of gravity predicts that changes in gravity propagate through the Universe in the form of waves. These gravitational waves, often thought of as “ripples” in space and time, are created whenever masses accelerate. They have not yet been detected directly, but we have strong indirect evidence that they exist. The best evidence comes from a double neutron star system called PSR1913+16. As the stars orbit each other the system radiates gravitational waves and loses energy. Observations, now spanning more than three decades, show that the orbit of the double neutron star system shrinks at exactly the rate predicted by general relativity.

 

The GEO600 detector, located in a field outside Hannover in Germany, is part of an international network of large laser interferometers listening to space for gravitational waves with frequencies between 10 and 10,000Hz. [copyright; GEO600]

 

Gravitational waves convey less a picture than a sound. Just as sound waves contain information about the musical instrument that created them, the gravitational waves carry an imprint of the event in which they were generated.

 

 

 

 

The strongest gravitational-wave signals come from the most violent events in the Universe, involving the acceleration of large masses in small regions of space.  Because of their small size to mass ratio, black holes are particularly promising sources.  If we could detect these signals, we would be able to find black holes and study them in detail. However, even though the events that generate the waves may be extremely powerful, the waves wane with distance. Since most cosmological events occur far from the Earth the gravitational waves that bathe our planet are very weak.

 

In order to catch these waves we must develop very sensitive detectors. One must be able to detect changes of about a thousandth of the diameter of the proton in a kilometer-sized detector. This is like comparing the width of a human hair to the distance to the nearest star! Cutting edge laser interferometers has been developed to detect these tiny stretches in space-time. An international network of extremely sensitive detectors is now tracking the changes in the space-time geometry, listening for tiny changes in gravity.

 

 

       

 

 

 

 

 

In GEO600 laser beams are used to measure minute changes of length in 600m long vacuum tubes. The precision of the experiment makes high demands on the quality of the measuring system. This image shows silica suspension wires being welded to special synthesized fused quartz mirrors. [copyright: GEO600]

 

 

 

 

The results of a computer simulation of the gravitational waves produced when two black holes merge to form a larger black hole. The gravitational-wave intensity is represented by different colours. [copyright: AEI Golm]

 

Sophisticated tools are needed to dig the very weak signals out of the detector noise. This poses further challenges. The development of such analysis tools requires a good theoretical understanding of gravitational-wave sources. Unfortunately, even though the mathematical equations of general relativity, encoding the interaction between matter and space-time geometry, are easy to write down they are notoriously difficult to solve. In most situations one has to resort to simplifications or costly supercomputer simulations.

 

After several decades of effort, there has recently been great progress on simulating the inspiral and final merger of two black holes. The results of these simulations provide insights into the detailed interaction of two black holes. They also provide researchers analysing the detector output with reliable signal templates.

 

 

 

 

 

 

Gravitational-wave astronomy

 

We study gravity, but gravity is also the messenger.  By detecting gravitational waves from black holes we hope to measure black-hole parameters, provide accurate maps of the shape of space-time, and test Einstein’s theory with high precision.

 

At the end of the day, the weak interaction between gravitational waves and matter may be a blessing. The gravitational waves that reach our detectors are virtually unaltered since their generation. This means that we will be able to study regions of space that cannot be investigated with the traditional tools of astronomy. Gravitational-wave astronomy will open a new window through which we will probe that dark side of our fascinating Universe.

 

     

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