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3841 - COSMIC
RAYS - where do they come from? -
Cosmic rays produce extensive particle showers that send a cascade of
electrons, photons, and muons to Earth's surface. They were miss named in the beginning. They are not rays but atomic particles
traveling at near light speed.
-------------- 3841 - COSMIC RAYS - where do they come from?
- After the discovery of radiation by French
physicist Henri Becquerel in 1896, scientists believed atmospheric ionization
(where an electron is stripped from an air molecule) occurred only from
radioactive elements found in ground rocks or from radioactive gases.
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- Austrian physicist Victor Hess found an
additional source in 1912, when he strapped three electrometers into a balloon
and measured atmospheric radiation at an altitude of about 15,000 feet during a
total solar eclipse.
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- Physicists initially believed cosmic rays
were gamma rays, high-energy radiation produced by radioactive decay. During
the 1930s, however, experiments revealed that cosmic rays are mostly charged
particles.
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- In 1937, French physicist Pierre Auger
(1899–1993) found that extensive particle showers occur when cosmic rays
collide with particles high in the atmosphere, producing a cascade of
electrons, positrons, photons, muons , particles similar to electrons but 200
times as massive, and other particles that reach Earth’s surface.
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- Auger found an ionization rate about four
times greater than at ground level. Hess could explain the variant observations
only if a powerful source of radiation were penetrating the atmosphere from
above. Much later, in 1936, Hess received the Nobel Prize in physics for the
discovery of what we now call cosmic rays.
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- The Earth is being constantly bombarded
from space by cosmic rays of an unknown origin. Mysterious cosmic rays traveling at speeds
approaching that 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. The atmospheric crashes rain down
gigantic showers of secondary particles to the surface of our planet.
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- But despite being discovered more than a
century ago, physicists still don’t know where cosmic rays come from. 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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- One thing we certainly do know about
cosmic rays is that they are comprised of extremely energetic charged
particles , like protons, alpha particles, and atomic nuclei like helium and
iron, with 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). For reference, 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.
-
-Meanwhile, an
alpha particle emitted during the decay of Uranium-238 possess 4.27*10^6 eV of
energy. Compare that to a cosmic ray
proton, has an energy of some 10^20 eV.
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- Imagine a proton that is accelerated so
that it has an energy of 100 Joule. 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.
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- We know that the processes that accelerate
cosmic rays to such astounding energies must result from powerful and violent
events, which considerably narrows down possible sources.
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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 shock, a sudden change in the properties of the
medium. In the case of the universe, the
changed properties are velocity and density, and even magnetic fields. Luckily
for the cosmic rays, 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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- What could produce such a shock front? One
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.
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- The phenomena of a travelling shock front
can also be found in active galaxies, where huge plasma jet exist. A better
understanding cosmic rays, as well as their origins, is expected to open an
important window into tremendously powerful and cosmic events, such as
supernovae and collisions between black holes and neutron stars.
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- At a nosebleed-inducing altitude of 3.3
miles, ionization rates of the air were three times that measured at sea level.
Hess 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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- However, the upper atmosphere isn’t the
most convenient laboratory to investigate the high-energy particle collisions
they produce. 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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- The discovery of gravitational
waves are ripples in space-time predicted by Einstein’s theory of general
relativity. They have made a new form of
astronomy possible, allowing us to investigate events and objects that we could
never hope to observe in the electromagnetic spectrum alone.
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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.
January 24, 2022 COSMIC RAYS
- where do they come from?
3841
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--------------------- --- Wednesday, January 25, 2023 ---------------------------
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