Everybody remembers the picture of the hungry black hole swallowing all matter in its vicinity. Not even light has a chance to escape. However, closer inspection reveals that feeding a black hole is far from trivial. Typically, the matter around black holes has organized itself into so-called accretion disks.
And just as the Earth is not falling into the sun, the black hole cannot just scarf up the matter encircling it. Before being swallowed by the black hole, the gas in the disk has to be slowed down in order to weaken the centrifugal forces, which keep the gas rotating. But how does one put the brakes on matter in an accretion disk? Since this problem not only applies to black holes, but also to normal stars, it is of fundamental importance for the formation of cosmic structures.
Balbus and Hawley proposed the solution to this problem in 1991. They showed mathematically that stable rotating flows can be destabilized by external magnetic fields. This effect, now known as magnetorotational instability (MRI), enables sufficient angular momentum transport in accretion disks, which is essential for the mass concentration in stars and black holes.
Laboratory simulation of star formation
For approximately five years, teams throughout the world have tried to create this instability in a laboratory. Two recent papers in the journal “NATURE” (November 16, 2006) underlined the urgent need for an experimental demonstration of MRI by showing evidence that hydrodynamics alone is not capable of producing turbulence in accretion disks. At the Forschungszentrum Dresden-Rossendorf the experiment PROMISE (Potsdam Rossendorf Magnetic InStability Experiment) was set-up and carried out as a joint project between scientists from Dresden and from the Astrophysikalisches Institut Potsdam (AIP). The experimental set-up contains unusual details, such as the use of a simple wastewater tube carrying the coil that produces the vertical magnetic field. Within this tube, there are two co-axial copper cylinders with a two to one ratio in radius. As long as the rotation rate of the outer cylinder is larger than a quarter of the rate of the inner cylinder, the liquid metal flow between them is stable. The flow field is measured with ultrasonic velocity sensors. The interesting point is that this initially stable flow is destabilized by an externally applied spiral magnetic field. For the first time, this experiment allowed the observation of a magnetorotational instability in a laboratory.
The figure shows the measured axial flow as a function of the vertical position and time for three different currents in the coils. In each case, the azimuthal magnetic field is produced by an axial current of 6000 Ampères. In good agreement with numerical simulations, an upward traveling wave is observed only for certain levels of current in the coils. Moreover, the measured frequency of the traveling wave agrees well with the numerical prediction.
The results were published recently in “Physical Review Letters” and “Astrophysical Journal Letters”.
Meteoritic stardust unlocks timing of supernova dust formation
19.01.2018 | Carnegie Institution for Science
Artificial agent designs quantum experiments
19.01.2018 | Universität Innsbruck
On the way to an intelligent laboratory, physicists from Innsbruck and Vienna present an artificial agent that autonomously designs quantum experiments. In initial experiments, the system has independently (re)discovered experimental techniques that are nowadays standard in modern quantum optical laboratories. This shows how machines could play a more creative role in research in the future.
We carry smartphones in our pockets, the streets are dotted with semi-autonomous cars, but in the research laboratory experiments are still being designed by...
What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how...
For the first time, scientists have precisely measured the effective electrical charge of a single molecule in solution. This fundamental insight of an SNSF Professor could also pave the way for future medical diagnostics.
Electrical charge is one of the key properties that allows molecules to interact. Life itself depends on this phenomenon: many biological processes involve...
At the JEC World Composite Show in Paris in March 2018, the Fraunhofer Institute for Laser Technology ILT will be focusing on the latest trends and innovations in laser machining of composites. Among other things, researchers at the booth shared with the Aachen Center for Integrative Lightweight Production (AZL) will demonstrate how lasers can be used for joining, structuring, cutting and drilling composite materials.
No other industry has attracted as much public attention to composite materials as the automotive industry, which along with the aerospace industry is a driver...
Scientists at Tokyo Institute of Technology (Tokyo Tech) and Tohoku University have developed high-quality GFO epitaxial films and systematically investigated their ferroelectric and ferromagnetic properties. They also demonstrated the room-temperature magnetocapacitance effects of these GFO thin films.
Multiferroic materials show magnetically driven ferroelectricity. They are attracting increasing attention because of their fascinating properties such as...
08.01.2018 | Event News
11.12.2017 | Event News
08.12.2017 | Event News
19.01.2018 | Materials Sciences
19.01.2018 | Health and Medicine
19.01.2018 | Physics and Astronomy