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Simple, like a neutron star
by Staff Writers
Rome, Italy (SPX) Mar 27, 2014

Like being inside a star
Rome, Italy (SPX) Mar 27, 2014 - Some experiments are really difficult to perform in practice. To gain a detailed understanding of the behaviour of molecular hydrogen (H2), for example, we would have to produce such high pressures as those occurring within the core of gaseous planets like Jupiter and Saturn or inside stars.

If such conditions cannot be created, an alternative method is to simulate them on the computer, but the model has to be accurate. A group of research scientists from the International School for Advanced Studies (SISSA) in Trieste used a simulation model that is far more accurate than previously used, and carried out an experiment to test a hypothesis about the behaviour of hydrogen that is splitting the scientific community.

"We developed this simulation method here at SISSA over the past ten years", explains Sandro Sorella, a SISSA professor and co-author of the paper. "It's a highly accurate technique based on the quantum Monte Carlo method - a family of algorithms but usually limited to a small number of particles - that we have developed in order to consider now a large number of atoms, and obtain an almost realistic situation. A great advantage".

"We used the simulation to verify the Wigner and Huntington prediction", adds Guglielmo Mazzola, from SISSA and first author of the paper.

In 1935 Eugene Wigner and Hillard Bell Huntington conjectured that at very high pressures, when hydrogen makes the transition from the "molecular" phase to the "atomic" phase (when the atoms are so close to each other that the molecular structures can no longer be distinguished), hydrogen acquires metallic properties.

"In recent years, attempts to verify this hypothesis both theoretically and experimentally have yielded conflicting results with regard to the pressure required to achieve 'metallization'", comments Mazzola.

"Our simulation, in the liquid phase, showed that we might indeed be very far from being able to observe this transition experimentally. According to our findings, metallization can only take place at pressures approaching 500 gigapascal. This is an enormous value, which only occurs in the innermost layers of gaseous planets and cannot be achieved with currently available experimental equipment".

"A detailed understanding of the phase diagram of hydrogen", concludes Sorella, "is not only important for studies in the field of astrophysics, but also for learning how this element behaves and, for example, under what conditions it becomes a superconductor".

The research was conducted in collaboration with the advanced research institute AICS-Riken in Tokyo, which provided the computational resources of one of the most powerful supercomputers in the world, the K-computer.

In how many ways can one describe an object? Take an apple: by just looking at it we can easily estimate its weight, shape and colour but we are unable to describe it at any other level, for example, to evaluate the chemical composition of its flesh.

Something similar also applies to astronomical objects: until today one of the challenges facing scientists was to describe neutron stars at the nuclear physics level. The matter these stars are made up of is in fact extremely complex, and several complicated equations of state have been proposed.

However, to date there is no agreement as to which is the correct (or the best) one. A theoretical study conducted by SISSA (the International School for Advanced Studies of Trieste), in collaboration with Athens University, has demonstrated that neutron stars can also be described in relatively simple terms, by observing the structure of the space-time surrounding them.

"Neutron stars are complex objects owing to the matter that composes them. We can picture them as enormous atomic nuclei with a radius of about ten kilometres", explains Georgios Pappas, first author of the study carried out at SISSA. "A neutron star is what remains of the collapse of a massive star: the matter inside it is extremely dense and mostly consisting of neutrons".

"The nuclear physics required to understand the nature of the matter contained in these astronomical objects generally makes their description very complicated and difficult to formulate," continues Pappas.

"What we have demonstrated, by using numerical methods, is that there are properties that can provide a description of some aspects of neutron stars and the surrounding space-time in a simple manner, similar to the description used for black holes".

Black holes are truly unique objects: they have lost all matter and are only made up of space and time. Just like neutron stars they are the result of the collapse of a bigger star (in this case much bigger than the stars giving rise to neutron stars) and in the implosion all the matter has been swept away.

"They are considered to be the most perfect objects in the Universe and the expression 'hairless' that was coined by John Archibald Wheeler to indicate their simplicity has become famous. According to our calculations even neutron stars can be depicted in a very similar manner".

Scientists use "multipole moments" as parameters to describe objects. The moments required to describe a black hole are two, mass and angular momentum (the speed at which it rotates around its axis). For neutron stars three moments are needed: mass, angular momentum and quadrupole moment, that is, a coefficient that describes the deformation of the object produced by its rotation.

"Our calculations revealed two unexpected findings. First, we discovered that these three parameters are sufficient since higher levels moments are not independent and can be derived from the first three", explains Pappas. "The second surprising finding is that the description based on these parameters is independent of the equation of equation of state, or rather: we don't even need to know which is the equation of state".

In practice, we can have a description of a neutron star that is independent of the matter that forms it. "This has major implications", concludes Pappas. "In fact, by using the data collected with astrophysical observations - for example, the radiation emitted by a neutron star, or information about objects gravitating around the star or other information - we can reconstruct the features of a neutron star".


Related Links
International School of Advanced Studies (SISSA)
Stellar Chemistry, The Universe And All Within It

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