Computer models of merging neutron stars predicts how to tell when this happens
FRANKFURT. According to modern particle physics, matter produced when neutron stars merge is so dense that it could exist in a state of dissolved elementary particles. This state of matter, called quark-gluon plasma, might produce a specific signature in gravitational waves. Physicists at Goethe University Frankfurt and the Frankfurt Institute for Advanced Studies have now calculated this process using supercomputers. (Physical Review Letters, DOI 10.1103/PhysRevLett.124.171103)
Neutron stars are among the densest objects in the universe. If our Sun, with its radius of 700,000 kilometres were a neutron star, its mass would be condensed into an almost perfect sphere with a radius of around 12 kilometres. When two neutron stars collide and merge into a hyper-massive neutron star, the matter in the core of the new object becomes incredibly hot and dense. According to physical calculations, these conditions could result in hadrons such as neutrons and protons, which are the particles normally found in our daily experience, dissolving into their components of quarks and gluons and thus producing a quark-gluon plasma.
In 2017 it was discovered for the first time that merging neutron stars send out a gravitational wave signal that can be detected on Earth. The signal not only provides information on the nature of gravity, but also on the behaviour of matter under extreme conditions. When these gravitational waves were first discovered in 2017, however, they were not recorded beyond the merging point.
This is where the work of the Frankfurt physicists begins. They simulated merging neutron stars and the product of the merger to explore the conditions under which a transition from hadrons to a quark-gluon plasma would take place and how this would affect the corresponding gravitational wave. The result: in a specific, late phase of the life of the merged object a phase transition to the quark-gluon plasma took place and left a clear and characteristic signature on the gravitational-wave signal.
Professor Luciano Rezzolla from Goethe University is convinced: “Compared to previous simulations, we have discovered a new signature in the gravitational waves that is significantly clearer to detect. If this signature occurs in the gravitational waves that we will receive from future neutron-star mergers, we would have a clear evidence for the creation of quark-gluon plasma in the present universe."
Publication: Post-merger gravitational wave signatures of phase transitions in binary mergers. Lukas R. Weih, Matthias Hanauske, Luciano Rezzolla, Physical Review Letters Physical Review Letters DOI 10.1103/PhysRevLett.124.171103 https://link.aps.org/doi/10.1103/PhysRevLett.124.171103
Visualisation of merging neutron stars: https://www.youtube.com/watch?v=rj-r-YA9d6E&t=1s
This simulation shows the density of the
ordinary matter (mostly neutrons) in red-yellow. Shortly after the two stars
merge the extremely dense centre turns green, depicting the formation of the
Pictures may be downloaded here: http://www.uni-frankfurt.de/87973606
Caption Montage: Montage of the computer simulation of two merging neutron stars that blends over with an image from heavy-ion collisions to highlight the connection of astrophysics with nuclear physics. Credit: Lukas R. Weih & Luciano Rezzolla (Goethe University Frankfurt) (right half of the image from cms.cern)
Caption Simulation: Shortly after two neutron stars merge a quark gluon plasma forms in the centre of the new object. Red yellow: ordinary matter, mostly neutrons. Credit: Lukas R. Weih & Luciano Rezzolla (Goethe University Frankfurt)
Further information: Goethe University Frankfurt, Prof. Dr. Luciano Rezzolla, Chair of Theoretical Astrophysics, Institute for Theoretical Physics, +49-69-79847871/47879, email@example.com, https://astro.uni-frankfurt.de/rezzolla/
Psychologists at Goethe University Frankfurt research the short-term memory of visual impressions
FRANKFURT. When we look at the same object in quick succession, our second glance always reflects a slightly falsified image of the object. Guided by various object characteristics such as motion direction, colour and spatial position, our short-term memory makes systematic mistakes. Apparently, these mistakes help us to stabilise the continually changing impressions of our environment. This has been discovered by scientists at the Institute of Medical Psychology at Goethe University. (Nature Communications, DOI 10.1038/s41467-020-15874-w)
This is, however, not at all true. Our short-term memory deceives us. When looking to the left the second time, our eyes see something completely different: the bicycle and the car do not have the same colour anymore because they are just now passing through the shadow of a tree, they are no longer in the same location, and the car is perhaps moving more slowly. The fact that we nonetheless immediately recognise the bicycle and the car is due to the fact that the memory of the first leftward look biases the second one.
Scientists at Goethe University, led by psychologist Christoph Bledowski and doctoral student Cora Fischer reconstructed the traffic situation – very abstractly – in the laboratory: student participants were told to remember the motion direction of green or red dots moving across a monitor. During each trial, the test person saw two moving dot fields in short succession and had to subsequently report the motion direction of one of these dot fields. In additional tests, both dot fields were shown simultaneously next to each other. The test persons all completed numerous successive trials.
The Frankfurt scientists were very interested in the mistakes made by the test persons and how these mistakes were systematically connected in successive trials. If for example the observed dots moved in the direction of 10 degrees and in the following trial in the direction of 20 degrees, most people reported 16 to 18 degrees for the second trial. However, if 0 degrees were correct for the following trial, they reported 2 to 4 degrees for the second trial. The direction of the previous trial therefore distorted the perception of the following one – “not very much, but systematically," says Christoph Bledowski. He and his team extended previous studies by investigating the influence of contextual information of the dot fields like colour, spatial position (right or left) and sequence (shown first or second). “In this way we more closely approximate real situations, in which we acquire different types of visual information from objects," Bledowski explains. This contextual information, especially space and sequence, contribute significantly to the distortion of successive perception in short-term memory. First author Cora Fischer says: “The contextual information helps us to differentiate among different objects and consequently to integrate information of the same object through time."
What does this mean for our traffic situation? “Initially, it doesn't sound good if our short-term memory reflects something different from what we physically see," says Bledowski. “But if our short-term memory were unable to do this, we would see a completely new traffic situation when we looked to the left a second time. That would be quite confusing, because a different car and a different cyclist would have suddenly appeared out of nowhere. The slight 'blurring' of our perception by memory ultimately leads us to perceive our environment, whose appearance is constantly changing due to motion and light changes, as stable. In this process, the current perception of the car, for example, is only affected by the previous perception of the car, but not by the perception of the cyclist."
Publication: Context information supports serial dependence of multiple visual objects across memory episodes. Cora Fischer, Stefan Czoschke, Benjamin Peters, Benjamin Rahm, Jochen Kaiser, Christoph Bledowski. Nat. Commun. 11, 1932 (2020). https://doi.org/10.1038/s41467-020-15874-w
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