The laws of physics have always existed

The physical understanding of natural processes is continuously developed. New and refined descriptions and hypotheses are specifically tested in experiments. But you have to be aware that experiments on Earth can only cover a tiny fraction of the possible conditions. We cannot operate with masses that are similar in size or larger than the mass of the earth, and we cannot experiment with energies that exceed the energy supplied by the sun. However, we have a trick at our disposal: with particle accelerators such as the Large Hadron Collider at Cern near Geneva or with large lasers such as the National Ignition Facility in Levermore (USA), considerable energy can be focused on very little matter. This means that at least physics can be researched with extremely high energy per particle - even if only individual elementary particles or atomic nuclei can be put into the desired high-energy state for a very short time.

Stars - outside and inside
For thousands of years, however, we have seen the sun almost every day, and of course you immediately suspect that the conditions there are very different from those on earth. In the 19th century, for example, people looked for the spectral lines of unknown elements in sunlight (see page 18). In fact, Joseph Norman Lockyer and Pierre Janssen found unknown spectral lines in the solar spectrum in 1868, thus postulating the discovery of a new element. It was called "helium", derived from the Greek name "Helios" for the sun. It was only 30 years later that it was found that helium also occurs on earth. Since it does not form compounds as a light noble gas and immediately rises into the atmosphere when it is released, it was not noticed earlier. But the sun is only a star. How about all the others? If we look into the cloudless night sky, the stars are always there - tiny, static and unreachably distant. However, this impression is misleading in several ways: Most stars are about as massive as the sun, some even one or two orders of magnitude more massive. Due to the great distance, your light actually appears as an indissoluble point. As with the sun, the spectrum of light contains detailed information about the surface and temperature of stars. The speed and rotation on the surface can be calculated from small frequency shifts in the spectral lines caused by the so-called Doppler effect and even, in favorable cases, the period of planetary orbits can be determined. The stars are therefore not seen as a collection of "white" static points of light, but as a "colored" world of moving, rotating and pulsating star surfaces with a determinable frequency of the elements as far as our telescopes can see. The stars get the energy for their luminosity from nuclear fusion. First of all, hydrogen is fused into helium. For stars with an initial mass of more than eight solar masses, it takes about 30 million years for neutron-rich iron group elements to accumulate in the center as end products of various fusion sequences. Whenever protons are converted into neutrons during nuclear fusions, neutrinos are produced. According to the Standard Model, these are massless particles that only react via the weak interaction and can therefore escape directly from the star's interior. In detectors with huge tanks one can detect individual neutrinos emitted by the sun and thereby "follow" the nuclear reactions in the sun. It was a decades-long problem that the measured number of neutrinos did not match the expected amount. It was only in 2002 that it was finally concluded that the neutrinos, contrary to expectations, do have a mass and that the standard model of particle physics must be revised. This is again an example of how the physical laws derived on earth could be fundamentally expanded and improved by applying them to astrophysical conditions.

Huge supernova explosions
If the iron group elements accumulating inside the star exceed a critical mass, the entire inner area of ​​the star collapses suddenly and within a second under the action of gravity. This star collapse is only slowed down when the approximately 1055 atomic nuclei - this corresponds roughly to the mass of a sun! - collide in the center and form a homogeneous mass of nuclear matter with a radius of only about 50 kilometers. The Pauli principle forbids the electrons to assume the same quantum mechanical states in such a small space. Therefore, they have to take on high energies and are so easily captured by protons, which then convert into neutrons. The many neutrons form the precursors of a neutron star or a "black hole". The outer layers of the stars are ejected in a huge explosion, a supernova. The connection between supernova explosions and neutron stars was postulated by the Glarus physicist Fritz Zwicky as early as 1934, just two years after James Chadwick discovered the neutron. The transfer of knowledge works in both directions: New results from experiments on Earth also open up new interpretive approaches for astronomical observations. As in the sun, every time a proton is converted into a neutron, a neutrino is emitted. When a supernova exploded in our neighboring galaxy, the Magellanic Cloud, in 1987, a few supernova neutrinos were discovered. This first evidence of cosmic neutrinos was honored with the Nobel Prize in 2002. Since these neutrinos can escape directly from the densest and hottest regions of the universe because of their weak interaction, they carry information about matter under extreme conditions with them that is not contained in the star spectra. At the University of Basel we are working intensively on the precise description of the transport of neutrinos through hot and dense nuclear matter and testing the course of neutrino-driven supernova explosions in computationally intensive computer models at the Swiss National Supercomputing Center. It is assumed that when the neutrinos leave the hot neutron star, they deposit a fraction of their energy in the surrounding layer and thus trigger the supernova. The exact mechanism of the explosion is not yet fully understood. As a further possibility, strong magnetic fields could also deflect the collapsing matter into an explosion. In both models, a violent shock wave is expected, which hurls the outer star shell into the interstellar medium. The high explosion energy enables the formation of specific elements that are heavier than the iron group elements. When new stars are later born from the ejected matter, the characteristic spectral lines of the heavy elements appear again in the spectrum of the new stars. In this way, the composition of long-past supernova ejections can be determined and compared with models. A large part of the heavy elements on earth and the sun were once created in explosions of supernovae.

Looking for gravitational waves
Another astrophysical window on the dynamics of dense matter will open when the gravitational wave detectors under construction reach their full sensitivity. The large mass, which moves in fractions of a second in a star collapse and a supernova explosion, sends out gravitational waves according to Einstein's theory of relativity, which propagate through space as variations in length and time mass. Gravitational waves have never been directly detected on earth. However, there is a system of orbiting neutron stars whose observed change in orbit can be traced back with extreme precision to the emission of gravitational waves. In addition, the existence of exotic states of dense matter can be investigated. It is an open question at what densities and temperatures protons and neutrons dissolve into their components, i.e. the quarks. Such a transition in the neutron star would affect the gravitational waves. Tobias Fischer's doctoral thesis at the University of Basel last year showed that such a phase transition to quark matter would cause a distinct characteristic increase in the emission of neutrinos, which can already be measured with the current neutrino detectors or falsified as soon as the next galactic supernova explodes . That can be tomorrow, but it can also take a few years or even decades. As these examples show, the generalization and refinement of the laws of physics in the mutual interplay between terrestrial experiments and the observation of astrophysical phenomena is constantly being developed. As soon as a phenomenon incompatible with the previous laws is discovered, the physical laws are improved so that they apply again everywhere in the known universe. It is therefore assumed that nature can be described uniformly in the entire universe. The detection of inconsistencies between the known physical laws and measurable natural phenomena is then the first step towards the next improvement of the theory. As a new tool of our epoch, the previously inaccessible distant astrophysical events can be put right in front of your nose using detailed computer models, thus gaining access to any size of the model at any time. However, the next discrepancies can only really be detected if one succeeds in looking through all the experimental and astrophysical windows that can be reached and carefully comparing the models with nature.