- 3539 - COSMIC RAYS - mysteries for a century? Mysterious cosmic rays traveling at speeds approaching the speed of light constantly pelt Earth’s upper atmosphere from the depths of space, creating high-energy collisions that dwarf those produced in even the most powerful particle colliders here on Earth.
--------------------- 3539 - COSMIC RAYS - mysteries for a century?
- Cosmic rays constantly bombard our upper atmosphere, and they might help astronomers understand the universe’s most powerful events. Cosmic explosions like the one that created the Crab Nebula could be a potential source of cosmic rays, highly energetic charged particles that pummel the Earth’s upper atmosphere.
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- These atmospheric crashes rain down gigantic showers of secondary particles to the surface of our planet. Despite being discovered more than a century ago, physicists still don’t know where cosmic rays come from.
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- Charged cosmic-ray particles are redirected by the magnetic fields they pass through on their long journey through space. As magnetic fields in space have local, small, randomly oriented structures, a prediction of the exact path of a cosmic-ray particle is impossible.
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- Cosmic rays are comprised of extremely energetic charged particles, protons, alpha particles, and atomic nuclei like helium and iron, and miniscule proportions of antiparticles.
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- The energies of these particles were monumental in comparison to those of every other particle. The average energy of a solar photon is approximately 1.4 electron volts (eV). A flying mosquito has an energy of about 1 trillion eV, or 10^12 eV, but a mosquito is also much, much larger than a single particle.
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- An “alpha particle” emitted during the decay of Uranium-238 possess 4.27*10^6 eV of energy.
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- Compare that to a cosmic ray proton, which has an energy of some 1*10^2 eV.
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- Imagine a proton that is accelerated so that it has an energy of 100 Joule. This energy corresponds to that of a tennis ball smashed with a velocity of around 124 miles per hour. Only, the tennis ball is 10^29 times heavier than the proton.
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- That means a proton can only reach that extreme, macroscopic energy by traveling at almost the speed of light. The universe must be able to accelerate particles to these energies, but we still do not know how it does it. The processes that accelerate cosmic rays to such astounding energies must result from powerful and violent events.
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- One of the best ways of accelerating particles is a shock front that occurs when a medium with a large velocity runs into a slower one, producing a shockwave, a sudden change in the properties of the medium.
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- In the case of the universe, the changed properties are velocity and density, and even magnetic fields. The field becomes highly turbulent in that process. And the combination of a shock front with turbulence is a great particle accelerator.
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- To produce such a shock front a likely suspect is supernovae. As a shell of shocked material blasted away from an exploding star, it hits the cool interstellar medium that lies between stars, almost like a cosmic tsunami. The phenomena of a traveling shock front that is one of the main reasons why supernova remnants and active galaxies are the most promising candidates for cosmic-ray acceleration.
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- It was August 1912 when Austrian-American physicist Victor Hess began a series of flights to the upper atmosphere in a hydrogen-filled balloon equipped with an electroscope. His aim was to take measurements of ionizing radiation.
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- At the time, it was widely believed that radiation from the Earth itself was responsible for this phenomenon of knocking electrons off atoms, that is ionization. Should this be the case, however ionization should be strongest near the planet’s surface.
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- That’s not what Hess found. Hess discovered something startling. At a nosebleed-inducing altitude of 3.3 miles, ionization rates of the air were three times that measured at sea level. He concluded that the source of this ionization was not coming from below our feet, but instead from above. Further measurements made during a solar eclipse also showed the Sun wasn’t the source of this ionization radiation.
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- During the course of seven balloon trips, Hess discovered cosmic rays confirmed and named by Robert Millikan in 1925 were coming from beyond our solar system. But, while the detection of cosmic rays has been associated with balloon flights ever since, the upper atmosphere isn’t the most convenient laboratory to investigate the high-energy particle collisions they produce.
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- To study the collisions caused by cosmic rays, particle physicists retreated below ground, employing increasingly monstrous particle accelerators to slam together particles in an attempt to replicate the collisions that cosmic rays spark in the upper atmosphere.
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- This quest has culminated with CERN’s “Large Hadron Collider’ (LHC) with a 16-mile circumference deep beneath the French/Swiss border. Yet, despite its impressive size, power and utility, the LHC still can’t reach the energies produced by cosmic ray collisions.
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- At roughly the same time a thrilling balloon ride changed our perspective of the universe forever, a physicist named Einstein was working on a wild theory that would radically change our understanding of the fabric of space-time. And this theory, many decades later, could provide the next step to decoding cosmic rays.
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- The discovery of “gravitational waves” which are ripples in space-time predicted by Einstein’s theory of general relativity.
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- This combination of electromagnetic or “traditional” astronomy and gravitational-wave detections (along with detecting neutrinos, which are ghost-like particles with virtually no mass or electric charge) is known as “multi-messenger astronomy“. And it has a significant role to play in future investigations of cosmic rays, and, by extension, the high-energy universe.
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- The gravitational-wave signal “GW170817” came from the merger of two neutron stars and was observed in 2017. It was significant for both multi-messenger astronomy and identifying potential sources of cosmic rays. Not only did this violent merger become the first such event to be detected in both gravitational waves and electromagnetic radiation, but it also confirmed that the merger of compact stellar remnants can accelerate particles to great speeds, creating cosmic rays.
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- There is no other way than multi-messenger astronomy to understand the origin and impact of cosmic rays. Cosmic rays alone cannot give an answer, neither can gamma-rays or neutrinos for themselves.
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- All three messengers have unique properties and show different parts of a big puzzle. Only by putting together all the pieces, can we see the full picture.
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April 11, 2022 COSMIC RAYS - mysteries for a century? 3539
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