Radiate accelerating particles from Why
Every second or so, a subatomic particle penetrates the outermost layers of our atmosphere with the force of a forcefully thrown stone. According to this, there must be forces somewhere in space which, for example, give protons (the simply positively charged hydrogen nuclei) a hundred million times as much energy as the largest accelerators for exploring the microcosm. But where? And how?
Astrophysicists have been dealing with these questions since the phenomenon was discovered in 1912: When the Austrian Victor Franz Hess (1883 to 1964) allowed himself to be carried by a balloon up to a height of five kilometers with an ionization chamber and the device reacted increasingly, he concluded that it was necessary probably "take refuge in a new hypothesis" - "either invoke the existence of unknown matter at great heights or assume a penetrating radiation of extraterrestrial origin"; for this he received the Nobel Prize in 1936. Independently of Hess, Werner Kolhörster, who worked in Berlin, made corresponding observations in 1913 and demonstrated the corpuscular character of the high-energy medium. Nevertheless, the term cosmic radiation has been retained. It is now known that most of the particles are protons.
In the interstellar medium there are atomic nuclei of all elements of the periodic table, which move under the influence of electromagnetic fields. Without the shielding effect of the atmosphere, they and especially the ultra-fast particles would be a serious risk to life on earth; But even in this way, mountain dwellers or air travelers are already exposed to the consequences of bombarding our air envelope - a measurably increased dose of radiation.
It is particularly noteworthy that no natural upper limit has yet been discovered for the energy spectrum of cosmic rays (also known as cosmic rays). Most known sources of electrically charged particles in space - for example the sun with its solar wind - simply do not accelerate particles beyond a certain speed (Spektrum der Wissenschaft, June 1995, p. 50). On the other hand, cosmic rays occur, albeit with decreasing frequency, at the highest energies that can be measured. The data collected so far ends up at 300 billion times the energy of the proton rest mass by chance, because there is currently no detector large enough to detect the presumably very small number of particles falling even faster.
Nevertheless, signs of extremely high-energy cosmic particles have been found at intervals of several years: When they enter the earth's atmosphere, they collide with the atomic nuclei of the air molecules and generate countless - more easily detectable - secondary particles and electromagnetic radiation, including visible light (Fig. 1). For example, a specialized observatory in the desert of the US state of Utah registered such a shower on October 15, 1991, which must have come from a particle with an energy of 50 joules (3 x 1020 electron volts or eV for short). The cosmic particle flux generally decreases with increasing energy, but to a lesser extent when approaching 1016 eV; from this it can be concluded that the fastest particles have a different origin than the slower ones (Fig. 2).
In 1960, Bernard Peters of the Tata Institute in Bombay (India) speculated that lower-energy cosmic rays were mainly generated in the Milky Way system, while the higher-energy rays came from more distant sources. Because a proton of more than 1019 eV would hardly be deflected by the magnetic fields normally prevailing in a galaxy and would therefore have to move almost in a straight line; if such particles came from our galaxy, different numbers would race earthward from different directions, because the Milky Way is by no means arranged symmetrically around us. In fact, the distribution of their impacts - like that of the less energetic particles, which are therefore widely scattered by galactic magnetic fields - is practically isotropic.
Such uncertain conclusions show how little is really known about the origin of this radiation. The astrophysicists only have more or less plausible models. One reason for this may be the almost unimaginable difference between the conditions on earth and those in cosmic zones, where matter is accelerated so vehemently.
The interstellar space contains only about one atom per cubic centimeter - much less than the best possible technically producible vacuum. In addition, it is filled with extensive electromagnetic fields that interact closely with charged particles; their number is much smaller than that of the neutral atoms. Nevertheless, things are by no means quiet in these apparently empty spaces: In a medium of such low density, the electromagnetic forces work unchecked over immensely greater distances and periods of time than in the solid matter, liquids and gases of our environment. That is why galactic space is filled with an energetic and turbulent mixture of partially ionized atoms and molecules.
Because of the astronomical distances, this violent activity is difficult to prove even in the years that a person is able to devote to its observation. On the other hand, even moderate forces can have an impressive effect because of these enormous distances: A particle whizzes through an earthly accelerator in a few millionths of a second, but in its cosmic counterpart it could be accelerated for years or even millennia. Meanwhile, the time scales at the extreme speeds of high-energy particles are distorted due to relativistic effects; if we could observe such a particle for 10,000 years, only a single second would pass in its own frame of reference.
It has long been assumed that the majority of cosmic rays - namely those below 1016 eV - are caused by supernovae. Because the energy required for the observed amount of cosmic radiation particles in our galaxy is only insignificantly lower than the average kinetic energy which the roughly three supernova explosions per century supply to the galactic medium. There is hardly any other such powerful source in the Milky Way (Spektrum der Wissenschaft, October 1989, p. 86).
If a massive star collapses because the nuclear fuel inside it is running low and the balance between the internal pressure generated by nuclear fusions and its own gravitational attraction suddenly collapses, its outer regions can be thrown away at speeds of up to 10,000 kilometers per second and more. A similar amount of energy is released when a white dwarf is torn apart in a thermonuclear detonation. In both types of supernova, the matter ejected at supersonic speeds spreads out into the surrounding medium with the formation of a violent shock wave. Presumably this collision front accelerates individual atomic nuclei so much that they drift away as cosmic radiation particles. Since they carry charges, they describe complicated orbits on their way through the interstellar magnetic fields; therefore, from the directions of incidence observed on earth, it is no longer possible to deduce the original place of origin (Spektrum der Wissenschaft, June 1991, p. 102).
The synchrotron radiation emanating from some supernova remnants provides stronger evidence for the effect of supernovae as cosmic particle accelerators. It is characteristic of the electrons that are forced into a circular path by the strong magnetic fields of an earthly accelerator (a synchrotron) and thereby emit energy - mostly in the X-ray range; their appearance in some supernova remnants indicates particularly high energies. (In earthly devices, this radiation limits the particle energy that can be achieved, because the emission increases with increasing speed, until it finally equals the energy supplied to the particle.)
The Japanese X-ray satellite "Asca" recently took pictures of the shell of the supernova that appeared in 1006. In contrast to the radiation from the interior of the star remnants, the X-ray radiation of the shell shows typical characteristics of synchrotron radiation. From this it can be concluded that electrons are accelerated up to 1014 eV there.
Furthermore, the EGRET detector on board the "Compton" space observatory was used to investigate gamma ray point sources which apparently originate from supernova remnants (Spektrum der Wissenschaft, February 1994, page 64). The observed intensities and spectra (up to one billion eV) suggest the decay of neutral pions as the cause. These elementary particles could be formed when the cosmic rays from the shell of the exploding star collide with surrounding interstellar gas. Interestingly, however, the Whipple Observatory on Mount Hopkins in Arizona has looked in vain for gamma radiation of much higher energy, which would have to emanate from the same remains if particles were to be accelerated to 1014 eV or more there.
The connection between supernovae and high-energy cosmic rays can also be investigated using their particle mix. Because the orbital radius of a charged particle in a magnetic field is proportional to its momentum per unit charge, heavier atomic nuclei have higher energy with the same orbital radius. Every process that restricts the radius of the orbit (e.g. an acceleration area of limited extent) enriches such nuclei in excess.
Ultimately, however, we not only want to gain more and more vague and indirect knowledge: We want to assign characteristic element distributions to special supernova types in the particle radiation emanating from them. For example, the detonation of a white dwarf accelerates all atomic nuclei that occur in the local interstellar medium; on the other hand, the explosion of a massive star primarily accelerates the surrounding stellar wind, the composition of which is typical of that of the outer layers of this star in earlier stages of development. In some cases the wind can contain an increased proportion of helium, carbon or even heavier atomic nuclei.
Such profiles are almost completely lost, however, when the high-energy cosmic radiation collides with atoms and molecules in the earth's atmosphere and creates showers of secondary particles. The nuclear composition can therefore only be reliably determined if the measurements are carried out above the denser air layers. However, in order to detect 100 cosmic particles with energies of around 1014 eV, a ten square meter detector would have to orbit the earth for three years; the measurements that are currently achievable, however, correspond more to one square meter of detector area and three days of observation.
One tries to get around this problem with ingenious experiments. For example, the US aerospace authority NASA is able to use high-altitude balloons to bring large payloads (around three tons) into the upper atmosphere for many days. That only costs a tiny fraction of the corresponding detectors on board satellites.
The most successful missions of this type have taken place in Antarctica, where the winds of the upper atmosphere blow pretty much in circles. A payload launched from the American research station at McMurdo Sound wanders around the South Pole with an almost constant radius and finally returns to the vicinity of the starting point. Some balloons circled the continent for ten days (Fig. 4).
One of us (Swordy) is now working with Dietrich Müller and Peter Meyer from the University of Chicago (Illinois) on a ten-square-meter detector that can measure heavy cosmic particles up to 1015 eV during such a flight. In addition, one wants to extend the measurement time to around 100 days in the future by using larger orbital radii.
Ground measurements on air showers
The investigation of cosmic rays of even higher energy - generated by previously unknown sources - requires huge detector systems on the ground. The problem of the low number of highest-energy cosmic particles is avoided by monitoring a wide effective area for months or years.
Of course, one must first identify the type of cosmic projectile from so-called air showers - cascades of collision fragments, including electrons, muons, pions, neutrons and protons, many of which in turn collide with atomic nuclei in the air and, depending on the energy, electromagnetic radiation ranging from ultraviolet to gamma -Release area of the spectrum. Such indirect methods cannot provide the atomic number of each individual primary atomic nucleus, but only statistical information about its approximate composition.
The bundle of many millions of secondary particles released by a single cosmic particle hits the ground over thousands of square meters. It would be very time-consuming to observe such large areas without gaps; therefore, one usually seeks to capture segments of the air showers with a few hundred detectors at separate locations on a random basis.
The most modern of these devices are able to extract extremely complex information from every shower. The CASA-MIA-DICE experiment in Utah, in which two of us (Cronin and Swordy) are involved, measures the distribution of electrons and muons on the ground. It also registers the Cherenkov radiation generated at different heights by the shower particles (the visible effect of an optical shock wave caused by charged particles when their speed exceeds that of light in the medium in question). Based on this data, we can reconstruct the shape of the shower more reliably and thus better estimate the type and energy of the cosmic particle that triggered it.
The third of us (Gaisser) works with a device that measures the cascade showers that hit the South Pole area. This experiment is being carried out in conjunction with AMANDA (the Antartic muon and neutrino detector array), which detects high-energy muons from the same showers by recording the Cherenkov radiation generated by them deep in the inland ice. The main goal of AMANDA, however, is the detection of neutrinos from cosmic accelerators, uncharged elementary particles that hardly interact with ordinary matter, but occasionally generate showers that are directed upwards after passing through the earth.
In addition, increasingly detailed computer models for the formation of air showers are being developed. These simulations help to better understand the possibilities and limits of soil measurements. And finally, one seeks to extend the direct detection of cosmic rays to higher energies so that detectors on the ground and on balloons can detect the same types of particles and the soil data can be calibrated more precisely.
Cosmic particles with energies above 1020 eV only hit the atmosphere around once per square kilometer and year. That is why you need an air shower detector of truly gigantic proportions for their research. In addition to the event observed in Utah in 1991, such extremely high-energy particles have also been detected by other groups in the USA as well as in Akeno (Japan), Haverah Park (Great Britain) and Jakutsk (Russia).
Everyone who deals with it, of course, gets into a dilemma. On the one hand, these fastest fragments of matter probably originate from areas of space outside our Milky Way system, since no known acceleration mechanism could generate them and since they come from all directions. (The galactic magnetic field would be too weak to bend their orbits.) On the other hand, their sources cannot be more than 30 million light years away (diameter of the galaxy: almost 100,000 light years), because otherwise the particles would have to interact with the microwave background - the residual radiation of the Big Bang - to be slowed down again. Under the relativistic conditions that apply to these exotic cosmic particles, the collision with a single background radiation quantum already deprives them of a large part of their energy.
If their sources were evenly distributed in space, the interaction with the microwave background would have to cause a steep drop in the number of particles at energies above 5 x 1019 eV; But that is not the case. So far, not enough events have been observed beyond this hypothetical threshold to be able to say with certainty what is going on there, but the meager data alone give rise to interesting speculations. Because the weak intergalactic magnetic fields hardly deflect these particles, one would obtain clear indications of their places of origin by measuring the direction of movement of a sufficient number of particles.
Three recent hypotheses about the nature of the sources illustrate the variety of possible explanations: accretion disks around black holes in galaxy centers, gamma-ray bursts and topological defects in the space-time structure of the universe.
It is believed that black holes with a billion or more solar masses, which suck up matter in the center of active galaxies, are the cause of so-called relativistic jets - currents of matter that shoot far into intergalactic space at almost the speed of light; they were detected with radio telescopes. As Peter L. Biermann from the Max Planck Institute for Radio Astronomy in Bonn assumes, the particularly radiation-active hot spots in it are actually shock waves that accelerate subatomic particles to extremely high energies. There are some indications that the directions of incidence of the highest energy cosmic particles correspond more or less to the distribution of the radio galaxies in the sky.
The second hypothesis is based on the assumption that gamma-ray bursts are produced by relativistic explosions as a result of the union of neutron stars. Mario Vietri from the Astronomical Observatory in Rome and Eli Waxman from the University of Princeton (New Jersey) have independently pointed out that the energy released in such cataclysms corresponds somewhat to that required for the observed flow of highest-energy cosmic rays. According to this, the extremely fast shock waves generated by these explosions could act as cosmic accelerators.
Particularly interesting - third hypothesis - is the speculation, according to which the ultra-fast particles arise from the decay of magnetic monopoles, strings, interfaces (domain walls) or other topological defects. These hypothetical objects are supposed to be remnants of a more symmetrical initial phase of the universe, in which gravitation, electromagnetism and weak and strong nuclear forces were united as one elementary force; in it, as it were, infinitesimal remnants of the universe would be preserved in the original state.
If such relic niches collapse in space-time and the symmetry of the forces in them is lost, according to current theoretical understanding, the energy stored in them is released in the form of extremely massive particles that instantly decay and form particle jets; their energies should be up to a hundred thousand times greater than those of the highest energy cosmic rays known to us. With it we would ultimately only observe the comparatively meager end products of primeval cosmological particle cascades.
In order to find out more about the actual sources, one now wants to analyze so many powerful air showers that the primary particles can be precisely assigned to extra-galactic objects. The AGASA system in Japan currently has an effective area of 200 square kilometers, and the new high-resolution fly's eye detector (Fly's Eye HiRes experiment) in Utah will even effectively cover around 1000 square kilometers (Fig. 3); Similar to the compound eye of an insect, it consists of a multitude of directional light receivers that monitor the sky for weak luminous phenomena in the event of particle collisions. Nevertheless, both device ensembles are only able to register a few ultra-high energy events per year.
More recently, Cronin and Alan A. Watson from the University of Leeds (Great Britain) got involved on an even larger project, named after the French physicist Pierre Victor Auger (1899-1993), who was the first to correlate particle showers based on the low-energy electrons eventually generated has investigated. The plan provides for detectors to be arranged on areas of 9,000 square kilometers in order to be able to measure hundreds of high-energy events per year. A detector field should consist of many stations that form a network with a mesh size of 1.5 kilometers; a single event would activate dozens of stations.
A preparatory meeting of experts in 1995 showed that such a system would be quite easy to build with the most modern, but already marketable technology - including solar cells, cell phones and receivers for the global positioning system. With an effective size of around 3000 square kilometers (the Saarland comprises 2570) it would cost around 75 million marks. To cover the entire sky, of course, two such detectors would have to be installed, one in the northern and one in the southern hemisphere.
While the researchers are busy building and operating such gigantic detector networks, one fundamental question remains unanswered: Is nature capable of generating even higher energy particle beams - or are we already starting to investigate the particles that hurry through the universe the fastest?
- Introduction to Ultrahigh Energy Cosmic Ray Physics. By Pierre Sokolsky. Addison-Wesley, 1988.
- Cosmic Rays and Particle Physics. By Thomas K. Gaisser. Cambridge University Press, 1990.
- High Energy Astrophysics. Volume 1, second edition. By Malcolm S. Longair. Cambridge University Press, 1992.
- Cosmic Ray Observations below 1014 eV. By Simon Swordy in: Proceedings of the XXIII International Cosmic Ray Conference. Edited by D.A. Leahy, R.B. Hicks and D. Venkatesan. World Scientific, 1994.
From: Spektrum der Wissenschaft 3/1997, page 44
© Spektrum der Wissenschaft Verlagsgesellschaft mbH
This article is contained in Spectrum of Science 3/1997
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