How can gravity turn into light
The cosmos trembles
How researchers successfully search for gravitational waves
September 14, 2015 will go down in history. On that day, researchers received gravitational waves for the first time - 100 years after Albert Einstein presented his general theory of relativity, which predicts such distortions in space-time. The sensation was achieved with the Advanced LIGO system. Their sensitivity to the delicate tremors from space is largely based on techniques and methods that scientists at the Max Planck Institute for Gravitational Physics in Hanover and Golm have worked out.
Text: Helmut Hornung
With the discovery on September 14th - it was 11:51 a.m. CEST when the waves rushed through two detectors at the Livingston and Hanford observatories in the USA - the research history of gravity reached its preliminary climax. General relativity has now passed its final test with flying colors. In addition, there is finally the opportunity to examine cosmic mass monsters in detail: Because models say that the observed gravitational waves come from two merging black holes with a mass of 29 and 36 solar masses about 1.3 billion light years away.
But what about the waves from space? Let us go to the roots of modern gravitational research - to Switzerland in 1907. There, at the Bern patent office, a “second class expert” is thinking intensively about gravity. Two years earlier he worked for the magazine Annals of Physics five works submitted, one of them entitled “On the Electrodynamics of Moving Bodies”. In the essay, the recreational researcher shakes the foundations of physics as well as in the three-page addendum "Does the inertia of a body depend on its energy content?"
The author is called Albert Einstein. The two works are later called Special Theory of Relativity. In it, Einstein breaks with Isaac Newton's dogma of absolute time and refutes the claim that speeds always add up directly. In addition, according to Newton's theory, the change in the gravitational effect of a body should be immediately noticeable in the entire universe. That means: gravity acts immediately everywhere. This is incompatible with Einstein's statement that there is a natural speed limit for the propagation of forces of any kind - the speed of light (c = 300,000 km / s).
So Einstein sets out to put the laws of gravity on a new basis. He later remembers: “It was in 1907 when the happiest thought of my life occurred to me (...) The gravitational field only has a relative existence, because for an observer who falls freely from the roof of a house - at least in his surroundings - none Gravitational field exists. In fact, all objects dropped by this observer remain in a state of rest or uniform motion, regardless of their chemical or physical nature. "
Einstein's trick can be reduced to a simple denominator: He simulates gravity with acceleration! Because acceleration also generates forces such as those that occur in a fast-moving elevator. If its cabin were soundproof and lightproof, people could believe that the earth's attraction had suddenly increased.
The realization that gravity is at least partially a question of the frame of reference led Albert Einstein to revolutionary ideas, which he presented in his general theory of relativity in autumn 1915 after eight years of work. This results in tiny deviations from Newton's model for the planetary movements. They occur most clearly in the case of Mercury, which is close to the Sun and orbits rapidly. This is how its so-called perihelion rotation can be precisely explained and calculated: "For a few days I was beside myself with excitement," writes Einstein after he had solved this riddle.
The general theory of relativity is ultimately a field theory. In it, the accelerated movement of masses in the gravitational field leads to disturbances that move through space at the speed of light. These disturbances are called gravitational waves. They are by no means hypothetical. Anyone jumping up and down on the trampoline, for example, loses energy (not just in the form of calories) and creates such waves in space-time. Now a person has a small mass and hops comparatively slowly. Therefore, the gravitational waves it emits are immeasurably small.
In space, on the other hand, there are large masses - and even a trampoline: space-time. Everything is in motion in it, because not a single heavenly body remains in one place at rest. The earth bulges out into space as it orbits the sun and emits gravitational waves with a power of 200 watts. But even these gravitational waves are still too weak to detect them with a detector.
Fortunately, there are much more violent vibrations of space-time in the universe: When two neutron stars or black holes run around each other extremely quickly or even collide with each other. Or when a massive star explodes as a supernova. Such cosmic events generate gravitational waves with an energy of around 1045 Watt - energy that is lost to moving bodies.
In fact, the two American astronomers Russell Hulse and Joseph Taylor have shown that the orbital time of the two neutron stars PSR 1913 + 16 decreases because the double system circling each other loses energy - and sends it out as gravitational waves. For this, the researchers received the 1993 Nobel Prize in Physics. But how can these waves be measured in space-time?
How do you make yourself noticeable? To do this, we mentally cut a rubber blanket that two experimenters - let's call them Albert and Isaac - hold onto the two opposite corners. Now Albert and Isaac pull on the cloth at the same time by taking two or three steps back. As they move away from each other, keep your arms close to your body. The blanket becomes longer and at the same time narrower.
Next, Albert and Isaac walk towards each other again, stretching their arms away from their bodies: the rubber blanket becomes shorter and wider at the same time. Finally, the two experimenters return to the starting position. During the experiment, a portrait of Albert Einstein painted on the rubber blanket would stretch and compress in exactly the same way as if a gravitational wave traveling from bottom to top through the plane of the rubber blanket had distorted the space.
In a second attempt, we paint two circles as far apart as possible on the rubber blanket. We call the one Start finish, the other Turning point. Then we organize a group of well-trained ants. We put everyone in the circle Start finish and leave one after the other at regular intervals to the Turning point and run back again. Because the ants are traveling at a constant speed, they all reach the target circle at the same rate and distance as they left it at the start.
Now Albert and Isaac stretch the blanket twice. As a result, the marching formation of the ant troop is pulled apart, the distances between the ants grow: the ants arrive back at their destination twice as long. This time delay occurs only temporarily, because it only affects those ants that are currently on the route. If the cloth remains taut by a factor of two, the starting ants also return in time. The (simulated) gravitational wave causes the ants to follow one another sometimes a little faster, sometimes a little slower than expected.
As described above, a gravitational wave changes the distance between the objects contained in space perpendicular to the direction of propagation. It is extremely difficult to measure that. Let's imagine a worst-case scenario in our galaxy: the explosion of a massive star. The gravitational waves emitted by this collapse - if they reach the solar system after a few thousand years of operation - would cover the distance between the sun and earth (1.5 x 1011 Meter) only by the diameter of a hydrogen atom (10-10 Meter).
Albert Einstein therefore considered the detection of gravitational waves to be impossible. And yet scientists have devised instruments that have succeeded in doing this. The first generation devices in the 1960s consisted of aluminum cylinders weighing tons and equipped with sensitive sensors. Gravitational wave pulses would have to make them vibrate like a clapper a church bell. But in spite of sophisticated amplifiers, such resonance detectors did not produce any results.
Therefore, the researchers constructed even more sensitive receivers. Their principle is based on the thought experiment with the rubber blanket. To do this, we replace the circle Start finish by a laser den Turning point through a mirror and think of the ants as the wave crests of a light signal. In order to demonstrate the tiny delays in the arrival time, a second beam path must be created perpendicular to the first so that the light waves from these two arms overlap. The measurement signal finally falls on a photodiode and can now be evaluated (Fig. 1).
Such a Michelson interferometer has in principle been around since 1882; it was originally built to test the constancy of the speed of light. Equipped with the latest technology, it is now ideally suited for the detection of gravitational waves. The Advanced LIGO system (Fig. 2), with which they have now been discovered, also works on the principle of the Michelson interferometer.
In the 1970s, Max Planck researchers began to further develop the technology of the interferometer and to adapt it to the requirements of research. The decades of work resulted in the construction of GEO600. This detector in a field in Ruthe near Hanover is one of several earthly listening posts that listen to the concert of the stars. The core of this is several diode lasers, which are similar to those of a CD player. A small crystal transforms the light into an infrared laser beam, the power of which, after high-precision preparation and filtering, is only ten watts - much more than a laser pointer, but also far too weak for meaningful measurements.
This is why detectors such as GEO600 or Advanced LIGO in Livingston (US state Louisiana) and Hanford (Washington) work with “power recycling”: a mirror sends part of the light back towards the laser, which then sends it back into the interferometer. This cycle repeats itself several times and increases the circumferential light output considerably.
The LIGO laser developed in Hanover, for example, originally achieved a basic output of 200 watts, which, thanks to the recycling trick in the interferometer, “feels” like almost one megawatt (106 watts). This increases the sensitivity of the detector considerably. The lasers are also extremely stable: for months and years they produce light of the same amplitude and frequency.
The two arms of Advanced LIGO each form four kilometers long tubes, with GEO600 they are 600 meters long. The laser beams must run between the mirrors undisturbed by external influences. It is important to eliminate vibrations caused by traffic or natural seismics. Seismometers therefore measure the vibrations, which are then compensated by piezo actuators.
In addition to this active system, all optical parts are equipped with a passive system: the mirrors, for example, are suspended as multi-level pendulums, and also have rubber and stainless steel dampers and leaf springs. In order to keep the thermal fluctuations of the air density within the system as small as possible, the interferometers are placed in evacuated stainless steel tubes. With GEO600, for example, turbomolecular pumps create an ultra-high vacuum better than 10-11 bar.
In order to discover the tiny gravitational wave signals in the mass of data, scientists need to know what to look for in the first place. That is why a department at the Max Planck Institute for Gravitational Physics is calculating what the signals look like for all possible sources of gravitational waves - merging black holes or neutron stars. The researchers then use these templates to analyze the measurement data from the detectors on high-performance large computers.
Once the signals have been found, further questions arise: Where exactly is the source located? What's behind it? Black holes or neutron stars? What is their mass? Now is the time for those experts who calculate theoretical models and compare them with the observed data. Finally, through the close interplay of experiments, simulations, analytical calculations and data analysis, the scientists bring light into the dark universe.
While Advanced LIGO and GEO600 are already working, the VIRGO detector with three-kilometer measuring arms is currently being expanded near the Italian city of Pisa, and Japanese scientists are constructing the KAGRA underground detector of the same size. An observatory is also planned in India. And in the year 2034 the interferometer eLISA will listen from space for low-frequency gravitational waves from the entire visible universe and thus complement the earth-based detectors (Fig. 3). The hunt for gravitational waves is in full swing all over the world.
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