Eversince its process of discovery in the early 1900s, Cosmic Rays have been asource of mystery and misconceptions for physicists. Even the name ‘CosmicRays’ stems from the misunderstanding of Robert Millikan in the 1920s. After aseries of ionization measurements at a range of heights, he came to themisdirected conclusion that it was caused by electromagnetic massless gammarays 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 werein fact deflected by Earth’s magnetic field, producing a higher distribution ofrays at the poles than at the equator (Compton,1934), and that 2 rows of Geiger counters separated by metal shieldingwere detecting secondary rays every time (rather than the 0.
01% chance ofdetection it would have if they were purely gamma rays) (Stanev,2004)was it realised that they were made of high energy sub-atomic particles, ratherthan photons. A lot of theories have since been postulated about the exactorigins of these cosmic rays and what can be found from them. In this essay, Iwill attempt to summarise the history of the search for the source of cosmicrays and why we should care.
Historyof the detection of cosmic raysAfterits discovery in 1896, the leading theory for the cause of ionization in theair was radioactivity from unstable elements on Earth, with some researchshowing a decrease in radiation with distance from the ground (North,2008). Using his own specially designed versionof an Electrometer (a device that relates the time taken for two gold leaves inan 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 theEiffel Tower (Gbur,2011), the largest man-made structure at the time in 1909. Ifthe ionisation was all due to the radioactivity of Radium in the ground, theintensity of its effects should halve with every 80 metres of elevation. Withthe tower standing at an impressive 300 metres, the radiation detected shouldhave been a mere fraction of what he had measured at its base, however this wasnot the case (Falkenburg,2003).Whathe discovered altered the narrative and caused scientists to look up foranswers instead of down.Thisled to Victor Hess, who is often heralded as the man who definitivelydiscovered Cosmic Rays, to conduct further readings at various larger altitudesin a hot air balloon (North,2008).
He noticed that there was “no essential change” from his readings atground level to his readings at 1100 metres and notably concluded(Dan,2012):”The results of my observation are best explained by theassumption that a radiation of very great penetrating power enters ouratmosphere from above.” Oneof his readings was even conducted during a total solar eclipse, which allowedhim to rule out the sun as the main origin of the radiation. This paired withthe fact that there wasn’t a regular variation pattern throughout the day oryear, like there is with the sun’s visible rays, triggered scientists to lookeven further out into space for answers. Hess’research earned him a share of the 1936 Nobel Prize (Stanev,2004).Primaryand Secondary Cosmic RaysWehave since learned that there are two main types of Cosmic Rays – primary andsecondary. Primary Cosmic Rays come straight from its source and someultra-high energy varieties, such as the Oh-My-God particle in 1991, can even haveenergies of up to 1020 eV. (Nerlich,2011) (Forreference the LHC is only designed to collide at about 1.
4 x 1012eV) (cds.cern.ch,2017) They predominantly consist of protons and alsocontain 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 usevery day. Some constituents of secondary cosmic rays such as muons, pions andneutrinos were previously unheard of when discovered and led a whole new worldof particle physics for scientists to investigate.
Soscientists now know the ‘air showers’ detected and theorised by Hess andscientists during his time were Secondary Rays from our own atmosphere, butwhere did the energetic Primary Rays that caused their existence come from?ExactOriginsTheexact origins of Cosmic Radiation are still not completely obvious due to thedeflection of their trajectories by magnetic fields which makes it difficult totrace them back to their source.Oneof the leading theories is that they are accelerated by supernovae shockwavesfrom the explosion of distant stars in the Milky Way. Evidence from the FermiGamma-Ray Space Telescope in 2009 suggests that charged particles get trappedbouncing in the cloud of gas and magnetic field for thousands of years,accelerating and gaining increasingly more energy with each passing through theshock wave, until they can break out into space as a cosmic ray(NASA,2013). However,ultra-high energy particles such as the Oh-My-God particle, named so because ofits astonishing energy of 99.
9999…% of the speed of light, cannot currently beexplained by this theory as, from our knowledge, supernovas cannot have themagnetic field strength or area needed to accelerate particles to such highenergies (Stanev,2004).Possiblesources 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 largeamounts of energy from their centre (possibly due to a super massive black holedrawing in and accelerating the matter there), causing a super-hot accretiondisc. AGNs that eject waves close to the speed of light perpendicular to thedisc like Radio Galaxies, Quasars and Blazars are being closely studied andcompared to known cosmic ray incidences (imagine.
gsfc.nasa.gov,2017). Although somescientists approve of this theory, others believe the sources must be fromwithin our own galaxy for some particles to have the level of energy it haswhen it reaches us (Stanev,2004).So why can’t we detect where they come from?Anexciting theory for this is that these Ultra-High Energy Cosmic Rays could besigns of a new, not yet fully understood area of Physics such as dark matter, remnantsfrom a time just after the Big Bang or even a new type of force. Just as cosmicrays paved the way for a whole new world of particle physics in the 20thcentury, some researchers believe they have the potential of leading them tounearth new fields of study today. However, although lower energy rays at LHClevel energies pass through earth about a thousand times per square kilometreper second, one high energy ray may only be spotted in one square kilometre ayear, and an ultra-high ray; once a thousand years! The Pierre AugerObservatory located in South America is made up of 16,000 small particledetectors 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 tofollow (Cham andWhiteson,2017). One innovative idea championed by the University ofCalifornia claims the key could be in our pockets already.
By developing an appto exploit the CMOS chip inside our smart phones, members of the public can bea part of the world’s most thorough detector (Palca,2015)(Whiteson,2015) There are alsoexperiments in space such as the Alpha Magnetic Spectrometer (AMS) aboard the ISSthat directly monitor primary cosmic rays for antimatter, in an attempt to linkthe radiation to dark matter and the origins of the universe (AMS02.ORG,2018)Butwhy should we care? Why dedicate so many resources to studying these invisibleparticles when its very presence was unnoticed and seemingly insignificant toour lives until about a century or so ago?Despiteits reputation for increasing the risk of cancer and complicating prolonged spacetravel (Cucinotta andDurante, 2006), Cosmic Rays have proved to be unlikely source ofknowledge when studied carefully. From being living proof of special relativitythrough its muons, to acting as galactic messengers by giving us direct information about the chemicalcomposition of far reaches of the universe (Stanev,2004), Cosmic Rays havebecome a useful tool in science.One interesting recent application has been the use of its muons as anon-invasive scanning tool, like X-rays, due to its ability to pass throughthick materials and structures. This has allowed us to find out more about theinner 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 valuabletool in archaeology too.From being dismissed as being radiation from Earth to possibly being thekey to dark matter, the study of Cosmic Rays and their origins have completed acomplete 180? since its conception. But despite an estimated 500 million or so’rays’ hitting our Earth every year (Cham andWhiteson,2017), scientists have not yet definitivelylocated their specific sources. Nevertheless, through their efforts, they havemanaged to give birth to a whole new field of physics- Particle Physics.Through analysing cloud chambers for muons, positrons, pions and neutrinos, cosmicrays have widened our understanding of the building blocks of our world fromjust protons, neutrons and electrons and have inadvertently led to many morediscoveries than Hess might have initially imagined up in his balloon.Furthermore, with experiments ranging from the AMS aboard the ISS to particledetector apps that allow us to be part of scientific history, the mystery ofCosmic Rays is still not over and with that, neither is its potential to exposemore about the universe.