since its process of discovery in the early 1900s, Cosmic Rays have been a
source of mystery and misconceptions for physicists. Even the name ‘Cosmic
Rays’ stems from the misunderstanding of Robert Millikan in the 1920s. After a
series of ionization measurements at a range of heights, he came to the
misdirected conclusion that it was caused by electromagnetic massless gamma
rays that originated from the fusion of Hydrogen in outer-space (Millikan and Cameron,1928).
It was only until several years later was the fact that primary cosmic rays were
in fact deflected by Earth’s magnetic field, producing a higher distribution of
rays at the poles than at the equator (Compton,1934), and that 2 rows of Geiger counters separated by metal shielding
were detecting secondary rays every time (rather than the 0.01% chance of
detection it would have if they were purely gamma rays) (Stanev,2004)
was it realised that they were made of high energy sub-atomic particles, rather
than photons. A lot of theories have since been postulated about the exact
origins of these cosmic rays and what can be found from them. In this essay, I
will attempt to summarise the history of the search for the source of cosmic
rays and why we should care.
of the detection of cosmic rays
its discovery in 1896, the leading theory for the cause of ionization in the
air was radioactivity from unstable elements on Earth, with some research
showing a decrease in radiation with distance from the ground (North,2008). Using his own specially designed version
of an Electrometer (a device that relates the time taken for two gold leaves in
an air tight container to discharge after being repelled apart by a charged rod),
Father Theodor Wulf famously conducted experiments at the top and bottom of the
Eiffel Tower (Gbur,2011), the largest man-made structure at the time in 1909. If
the ionisation was all due to the radioactivity of Radium in the ground, the
intensity of its effects should halve with every 80 metres of elevation. With
the tower standing at an impressive 300 metres, the radiation detected should
have been a mere fraction of what he had measured at its base, however this was
not the case (Falkenburg,2003).
he discovered altered the narrative and caused scientists to look up for
answers instead of down.
led to Victor Hess, who is often heralded as the man who definitively
discovered Cosmic Rays, to conduct further readings at various larger altitudes
in a hot air balloon (North,2008). He noticed that there was “no essential change” from his readings at
ground level to his readings at 1100 metres and notably concluded
“The results of my observation are best explained by the
assumption that a radiation of very great penetrating power enters our
atmosphere from above.”
of his readings was even conducted during a total solar eclipse, which allowed
him to rule out the sun as the main origin of the radiation. This paired with
the fact that there wasn’t a regular variation pattern throughout the day or
year, like there is with the sun’s visible rays, triggered scientists to look
even further out into space for answers.
research earned him a share of the 1936 Nobel Prize (Stanev,2004).
and Secondary Cosmic Rays
have since learned that there are two main types of Cosmic Rays – primary and
secondary. Primary Cosmic Rays come straight from its source and some
ultra-high energy varieties, such as the Oh-My-God particle in 1991, can even have
energies of up to 1020 eV. (Nerlich,2011) (For
reference the LHC is only designed to collide at about 1.4 x 1012
eV) (cds.cern.ch,2017) They predominantly consist of protons and also
contain alpha particles, electrons, and photons as well as some heavier nuclei. These ‘rays’ hit particles that make up our atmosphere,
only to be scattered into millions of secondary cosmic rays that rain over us
every day. Some constituents of secondary cosmic rays such as muons, pions and
neutrinos were previously unheard of when discovered and led a whole new world
of particle physics for scientists to investigate.
scientists now know the ‘air showers’ detected and theorised by Hess and
scientists during his time were Secondary Rays from our own atmosphere, but
where did the energetic Primary Rays that caused their existence come from?
exact origins of Cosmic Radiation are still not completely obvious due to the
deflection of their trajectories by magnetic fields which makes it difficult to
trace them back to their source.
of the leading theories is that they are accelerated by supernovae shockwaves
from the explosion of distant stars in the Milky Way. Evidence from the Fermi
Gamma-Ray Space Telescope in 2009 suggests that charged particles get trapped
bouncing in the cloud of gas and magnetic field for thousands of years,
accelerating and gaining increasingly more energy with each passing through the
shock wave, until they can break out into space as a cosmic ray
ultra-high energy particles such as the Oh-My-God particle, named so because of
its astonishing energy of 99.9999…% of the speed of light, cannot currently be
explained by this theory as, from our knowledge, supernovas cannot have the
magnetic field strength or area needed to accelerate particles to such high
sources for these types of rays could be from even further than our own galaxy.
Active Galactic Nuclei are a class of galaxy that emit extraordinarily large
amounts of energy from their centre (possibly due to a super massive black hole
drawing in and accelerating the matter there), causing a super-hot accretion
disc. AGNs that eject waves close to the speed of light perpendicular to the
disc like Radio Galaxies, Quasars and Blazars are being closely studied and
compared to known cosmic ray incidences (imagine.gsfc.nasa.gov,2017). Although some
scientists approve of this theory, others believe the sources must be from
within our own galaxy for some particles to have the level of energy it has
when it reaches us (Stanev,2004).
So why can’t we detect where they come from?
exciting theory for this is that these Ultra-High Energy Cosmic Rays could be
signs of a new, not yet fully understood area of Physics such as dark matter, remnants
from a time just after the Big Bang or even a new type of force. Just as cosmic
rays paved the way for a whole new world of particle physics in the 20th
century, some researchers believe they have the potential of leading them to
unearth new fields of study today. However, although lower energy rays at LHC
level energies pass through earth about a thousand times per square kilometre
per second, one high energy ray may only be spotted in one square kilometre a
year, and an ultra-high ray; once a thousand years! The Pierre Auger
Observatory located in South America is made up of 16,000 small particle
detectors dotted over 3000 square km, and is one way of overcoming this issue,
however, reportedly costing about $100,000,000 makes it an expensive model to
follow (Cham and
Whiteson,2017). One innovative idea championed by the University of
California claims the key could be in our pockets already. By developing an app
to exploit the CMOS chip inside our smart phones, members of the public can be
a part of the world’s most thorough detector (Palca,2015)(Whiteson,2015) There are also
experiments in space such as the Alpha Magnetic Spectrometer (AMS) aboard the ISS
that directly monitor primary cosmic rays for antimatter, in an attempt to link
the radiation to dark matter and the origins of the universe (AMS02.ORG,2018)
why should we care? Why dedicate so many resources to studying these invisible
particles when its very presence was unnoticed and seemingly insignificant to
our lives until about a century or so ago?
its reputation for increasing the risk of cancer and complicating prolonged space
travel (Cucinotta and
Durante, 2006), Cosmic Rays have proved to be unlikely source of
knowledge when studied carefully. From being living proof of special relativity
through its muons, to acting as galactic messengers by giving us direct information about the chemical
composition of far reaches of the universe (Stanev,2004), Cosmic Rays have
become a useful tool in science.
One interesting recent application has been the use of its muons as a
non-invasive scanning tool, like X-rays, due to its ability to pass through
thick materials and structures. This has allowed us to find out more about the
inner workings of volcanoes and disabled nuclear power plants. Recently, this method has even revealed a new,
previously unknown void in the Great Pyramid of Giza (Guglielmi,2017) and is becoming a valuable
tool in archaeology too.
From being dismissed as being radiation from Earth to possibly being the
key to dark matter, the study of Cosmic Rays and their origins have completed a
complete 180? since its conception. But despite an estimated 500 million or so
‘rays’ hitting our Earth every year (Cham and
Whiteson,2017), scientists have not yet definitively
located their specific sources. Nevertheless, through their efforts, they have
managed to give birth to a whole new field of physics- Particle Physics.
Through analysing cloud chambers for muons, positrons, pions and neutrinos, cosmic
rays have widened our understanding of the building blocks of our world from
just protons, neutrons and electrons and have inadvertently led to many more
discoveries than Hess might have initially imagined up in his balloon.
Furthermore, with experiments ranging from the AMS aboard the ISS to particle
detector apps that allow us to be part of scientific history, the mystery of
Cosmic Rays is still not over and with that, neither is its potential to expose
more about the universe.