Press releases – April 2020

 

Apr 30 2020
13:58

Computer models of merging neutron stars predicts how to tell when this happens

Gravitational waves could prove the existence of the quark-gluon plasma

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

Video: 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 quark-gluon plasma.

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, rezzolla@itp.uni-frankfurt.dehttps://astro.uni-frankfurt.de/rezzolla/

 

Apr 28 2020
12:59

Psychologists at Goethe University Frankfurt research the short-term memory of visual impressions 

How mistakes help us recognise things 

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)

 We learned it as children: to cross the street in exemplary fashion, we must first look to the left, then to the right, and finally once more to the left. If we see a car and a cyclist approaching when we first look to the left, this information is stored in our short-term memory. During the second glance to the left, our short-term memory reports: bicycle and car were there before, they are the same ones, they are still far enough away. We cross the street safely.

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

Further information:
Goethe University Frankfurt
Dr Christoph Bledowski
Institute for Medical Psychology
Tel.: +49 69-6301-4533
bledowski@em.uni-frankfurt.de
http://imp-frankfurt.de/bledowski.html#welcome

 

Apr 14 2020
19:34

​Frankfurt researchers solve puzzle of Compton scattering – new approach for testing theories in quantum mechanics

Particle billiards with three players

FRANKFURT. Light can be used to knock electrons out of atoms, with light particles and electrons bouncing off each other like two billiard balls – Compton scattering. Why electrons can even be ejected from an atom when the light does not actually have enough energy to do so has now been discovered by a team of physicists headed by researchers from Goethe University Frankfurt. (Nature Physics, DOI 10.1038/s41567-020-0880-2)

When the American physicist Arthur Compton discovered that light waves behave like particles in 1922, and could knock electrons out of atoms during an impact experiment, it was a milestone for quantum mechanics. Five years later, Compton received the Nobel Prize for this discovery. Compton used very shortwave light with high energy for his experiment, which enabled him to neglect the binding energy of the electron to the atomic nucleus. Compton simply assumed for his calculations that the electron rested freely in space.

During the following 90 years up to the present, numerous experiments and calculations have been carried out with regard to Compton scattering that continually revealed asymmetries and posed riddles. For example, it was observed that in certain experiments energy seemed to be lost when the motion energy of the electrons and light particles (photons) after the collision were compared with the energy of the photons before the collision. Since energy cannot simply disappear, it was assumed that in these cases, contrary to Compton's simplified assumption, the influence of the nucleus on the photon-electron collision could not be neglected.

For the first time in an impact experiment with photons, a team of physicists led by Professor Reinhard Dörner and doctoral candidate Max Kircher at Goethe University Frankfurt have now simultaneously observed the ejected electrons and the motion of the nucleus. To do so, they irradiated helium atoms with X-rays from the X-ray source PETRA III at the Hamburg accelerator facility DESY. They detected the ejected electrons and the charged rest of the atom (ions) in a COLTRIMS reaction microscope, an apparatus that Dörner helped develop and which is able to make ultrafast reactive processes in atoms and molecules visible.

The results were surprising. First, the scientists observed that the energy of the scattering photons was of course conserved and was partially transferred to a motion of the nucleus (more precisely: the ion). Moreover, they also observed that an electron is sometimes knocked out of the nucleus when the energy of the colliding photon is actually too low to overcome the binding energy of the electron to the nucleus. Overall, the electron was only ejected in the direction one would expect in a billiard impact experiment in two thirds of the cases. In all other instances, the electron is seemingly reflected by the nucleus and sometimes even ejected in the opposite direction.

Reinhard Dörner: “This allowed us to show that the entire system of photon, ejected electron and ion oscillate according to quantum mechanical laws. Our experiments therefore provide a new approach for experimental testing of quantum mechanical theories of Compton scattering, which plays an important role, particularly in astrophysics and X-ray physics."


Publication: Kinematically complete experimental study of Compton scattering at helium atoms near the ionization threshold. Max Kircher (Goethe University Frankfurt, Germany (GU)), Florian Trinter (Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin), Sven Grundmann (GU), Isabel Vela-Perez (GU), Simon Brennecke (Leibniz Universität Hannover, Germany), Nicolas Eicke (Leibniz Universität Hannover, Germany), Jonas Rist (GU), Sebastian Eckart (GU), Salim Houamer (University Sétif-1, Algeria), Ochbadrakh Chuluunbaatar (Joint Institute for Nuclear Research, Dubna, Russia (JINR); National University of Mongolia, Ulan-Bator), Yuri V. Popov (Lomonosov Moscow State University, Russia; JINR), Igor P. Volobuev (Lomonosov Moscow State University, Russia), Kai Bagschik (DESY) M. Novella Piancastelli (Sorbonne Universités, Paris, France; Uppsala University, Sweden) Manfred Lein (Leibniz Universität Hannover, Germany), Till Jahnke (GU), Markus S. Schöer (GU), Reinhard Dörner (GU)
Nature Physics, DOI 10.1038/s41567-020-0880-2; https://www.nature.com/articles/s41567-020-0880-2

Pictures may be downloaded here: http://www.uni-frankfurt.de/87402622

Caption Graphics: Artist view of the process and cross section for Compton scattering (front) and the COLTRIMS reaction microscope which enabled the experiment (back). Photons (wiggly line) hit an electron in the atom in the centre of the COLTRIMS reaction microscope knocking out an electron (red ball) and leaving an ion (blue ball) behind. Both particles are guided by electric and magnetic fields toward detectors (red and blue discs.) Copyright: Goethe University Frankfurt, Germany

Caption Photo: Selfie of Max Kircher in front of the COLTRIMS reaction microscope.

Further information:
Professor Reinhard Dörner
Institute for Atomic Physics
Goethe University Frankfurt
Max-von-Laue-Strasse 1
60438 Frankfurt
Telephone +49 69 798 47003
doerner@atom.uni-frankfurt.de
http://www.atom.uni-frankfurt.de

 

Apr 3 2020
09:58

​Joint study by scientists from Frankfurt, Berkeley and Berlin on the socio-economic consequences of social distancing

How to manage the costs of the corona crisis?

FRANKFURT. The governmental decision on when to loosen social distancing measures should not only depend on the latest number of cases. A joint study in theoretical physics, economics and medicine at Goethe University, the University of California Berkeley, and the Vivantes Hospital in Berlin shows that other criteria urgently need to be taken into account.

Governments should pay attention to the overall situation and not just to the new case numbers reported daily. If measures are loosened too early, the epidemic would have a stronger impact and the overall costs would rise substantially according to the authors of the new study, which is being published online in advance due to its explosive nature. According to the study, the optimum would be to maintain strict social distancing at least until the case numbers have sunk sufficiently in proportion to testing capacities to allow a comprehensive tracking of individual cases.

For the further management of the current COVID-19 pandemic, scientifically based estimates of the subsequent costs for different management strategies are crucial. A combination of numerical simulations and economic cost calculations are necessary, as Professor Claudius Gros and Professor Rose Valenti from the Institute for Theoretical Physics at Goethe University have elaborated in a study together with Dr Daniel Gros (Visiting Professor UC Berkeley/Director CEPS Brussels) and Kilian Valenti (Vivantes Hospital Berlin). The researchers discovered that a policy that reacts to the continually growing number of cases leads to higher overall costs than a policy that is oriented on the total number of past cases and which also takes other factors into account.

The COVID-19 epidemic is having an effect of previously unknown proportions on society and economics, which have been largely “shut down": schools are closed, only businesses in certain branches may remain open, and people are supposed to stay home as far as possible. Epidemiological models have to take the feedback of this social distancing and other reactions onto the dynamics of the spread of the virus. In the paper, which was presented in advance on 2nd April, the authors introduce a new epidemiological model which expands the typical SIR (susceptible, infected, recovered) model by another feedback parameter. The new model makes it possible to study two different strategies, depending on whether political actors direct their attention at the daily number of cases (“short-sighted") or if decisions are made based on the overall development of the epidemic (“history-aware"). The authors demonstrate that only the second strategy has the potential to extensively contain the epidemic. The current lockdown measures should not simply be loosened due to falling case numbers unless it is possible to replace them with alternate measures that have comparable containment potential.

“Herd immunity" is a term widely used in the public, signifying the point at which the number of new infections ceases to rise. According to many estimates, this point would be reached for COVID-19 when 66 percent of the population has been infected. It is frequently assumed that the epidemic will be essentially overcome at this point. The authors of this study point out, however, that even though herd immunity means the number of daily new infections will sink, the total number of cases will continue to grow, and another 28 percent of the population will be infected. Only six percent would be spared an infection.

The total economic costs have four components: lost working hours, medical costs, “value of life" (the expected remainder of life that no longer takes place is included here as a loss), and social distancing costs (i.e., the economic losses due to limited economic activity). If the epidemic were allowed to freely run its course, it would result in total costs of approximately 1.1 trillion euros – not to mention the ethical issues – which corresponds to 30 percent of the German gross domestic product (GDP). If the economic value of life is not included, the costs of an unchecked epidemic would still amount to 14 percent of the GDP, or about 480 million euros. Strict measures reduce this value by half. However, the social costs of social distancing measures have to be accepted.

If only real costs are considered and the economic value of human life is excluded, middle strategies fare the worst according to this publication. When it comes to a global pandemic, the middle course is therefore not golden. Based on their calculations, the authors therefore argue that the strict measures should be kept in place until the number of new infections has gone down sufficiently to make a testing of the complete environment possible. This, however, would require significant increases in testing capacities.

Publication: Claudius Gros, Roser Valenti, Kilian Valenti, Daniel Gros, Strategies for controlling the medical and socio-economic costs of the Corona pandemic (2020); Link to early publication: https://arxiv.org/abs/2004.00493

A graphic can be downloaded here: www.uni-frankfurt.de/87170074

About the graphic: The x-axis shows the total number of cases, the y-axis the new cases per day. The progression is shown.

Further information: (on modelling/theory) Prof. Dr. Claudius Gros, Institute for Theoretical Physics, Riedberg Riedberg, Email gros07@itp.uni-frankfurt.de; Prof. Dr. Roser Valenti, ebd., valenti@itp.uni-frankfurt.de; (on socio-economic and political aspects) Dr. Daniel Gros, Center for European Politics Studies (CEPS), Brussels, Belgium, Email daniel@ceps.eu.