Q: GAMMA RADIATION FROM SPACE HAS VERY HIGH ENERGY LEVELS. WHAT IS THE THEORETICAL LIMIT ON THE ENERGIES OF GAMMA-RAY PHOTONS? IS THERE A CURRENT TECHNOLOGY TO DETECT AND MEASURE THE HIGHEST ENERGY LEVEL?
A: There is no strict theoretical limit on the highest energy of astrophysical gamma rays, so far as I know, but there are some practical limitations. The first is the energy of the particle that produces the gamma ray. Gamma rays result from the interactions of electrons and protons that have been accelerated to almost the speed of light. Higher-energy particles produce higherenergy gamma rays, so one limitation to the maximum gamma-ray energy is the maximum energy to which particles can be accelerated. To accelerate a particle, it must be confined to the region of space where the acceleration takes place using magnetic fields — in the same way as particles are confined to the radius of the Large Hadron Collider here on Earth. The maximum particle energy therefore depends upon the magnetic field strength and the size of the acceleration region. Pulsars, for example, are relatively small but have intense magnetic fields. Galaxy clusters, on the other hand, are enormous but have weak magnetic fields. When a particle reaches a velocity high enough that it can escape the acceleration region, the acceleration stops, and the particle gains no further energy. The other limitation is the ability of the gamma ray to survive its journey to Earth. Gamma rays are destroyed
ESO/A. ROQUETTE
John Zinke Cambria, California
Gamma-ray bursts (GRBs) are the most luminous explosions in the universe. Although their exact origin remains unknown, astronomers envision that GRBs are the result of either a massive star’s life-ending supernova, or the merging of two compact objects, such as neutron stars or black holes.
when they interact with lowerenergy photons. Since the universe is full of low-energy photons from starlight and the cosmic microwave background, gamma rays are limited in the distance they can travel. This limit changes with the energy of the gamma ray — low-energy gamma rays can travel across most of the universe; higher-energy gamma rays can only survive if they are produced inside of our galaxy, or in a near neighbor. Detecting a high-energy gamma ray is relatively simple, but high-energy gamma rays are extremely rare. A typical astronomical source produces just a few gamma rays per square meter of Earth’s surface every year — so we need a very large detector. When a gamma ray strikes the top of the atmosphere, it initiates a cascade of particles, which in turn produces a flash of blue light. Gamma-ray observatories, such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), can observe over effective areas as large as a football field by detecting the products of these particle cascades at ground level. The highest-energy
gamma rays that have been detected by these observatories have energies of around 50 teraelectron volts, or 50 billion times the energy of an X-ray. Higher-energy gamma rays certainly exist in the universe, since particles with much higher energies have been observed. Large area cosmic ray observatories, like the Pierre Auger Observatory, are searching for them. Jamie Holder Associate Professor of Physics & Astronomy, University of Delaware, Newark, Delaware
Q: YOUR OCTOBER 2016 ARTICLE ON BLACK HOLES DESCRIBES A SINGULARITY WITH INFINITE DENSITY. IT HAS BEEN MY UNDERSTANDING THAT ANYTHING WITH INFINITE DENSITY ALSO HAS INFINITE MASS AND INFINITE GRAVITATIONAL ATTRACTION. WOULDN’T ANYTHING WITH INFINITE ATTRACTION COLLAPSE THE UNIVERSE? Dan Johnston, Hermitage, Pennsylvania
A: A simple description is that the infinite density of the singularity corresponds to the finite mass of a star collapsed to a point. The density is infinite
but the volume infinitesimal, so the mass is still finite. The true description is a bit more complex due to the distortion of space-time that the concentration of mass produces. However, we believe that the general theory of relativity of Einstein stops being valid before the star shrinks to a point and has to be replaced by a more general theory that takes quantum effects into account. There is no general agreement yet on what such a theory looks like. There are some preliminary indications that such theories would replace the singularity with a region of large density and large quantum fluctuations. Jorge Pullin Professor and Hearne Chair of Theoretical Physics, Louisiana State University, Baton Rouge, Louisiana
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