The study of Cosmic rays has attracted some of the finest minds of the 20th century. The story of their triumphs and failures is one of the most fascinating in all science. For instilling my interest I am indebited to Dr. Kyoung Hye Moon who shared her fascination and love for the science and who introduced me to the great statesman of cosmic ray science: Prof. Maurice M. Shapiro and told me of his School of Cosmic Ray Astrophysics. Here I give a flavor of the cosmic ray adventure in a form graspable by any interested reader.
Cosmic rays appeal not to the human lust for money and power but to a deeper inner sense; a
desire to go beyond ourselves and to understand what until recently we did not know existed. The
quest is simple: Where do the fast moving particles that continually bombard the earth's upper
atmosphere come from? How are some of them accelerated to energies that exceeds our most
powerful machine (the Tevatron at Fermi laboratory) by 100 million times? These are noble goals but
to them we must add the challenge of heroic experiments conducted in exotic locations that include: stratospheric balloons; at the
world's tallest mountains; at the bottom of oceans; in the frozen lands of the Antarctic; in space
craft beyond the atmosphere. One might think that all these diverse experiments have lead to a rapid understanding. Nothing could be
further from the truth. Almost all the 'theoretical certainty' from some of the most eminent scientist
of the 20th century has been proved wrong by experiment. This is no ordinary science.
One hundred years ago, in 1896, Becquerel began the nuclear age when, by serendipity, he discovered that certain minerals could blacken photographic emulsion. This momentous discovery would eventually change the world and answer such questions as why the sun, which every one 'knew' was made from coal, hadn't burned out. Following Becquerel's discovery the early years of the twentieth century saw an explosion in the study of radiation. Scientists made expeditions to glaciers and found that the radiation was less than near rocks but it didn't go away. The obvious answer was that it was in the air. Indeed this was 'proved' by scientists who took their detectors into caves and found no reduction in the radiation. 'Clearly' the radiation didn't come from the sky otherwise it would have decreased. At that time no one knew of radon or that this radioactive gas accumulates in caves in sufficient quantity to distort the scientists measurements. A young physicist (Viktor Hess) asked a question so deep that almost no one had thought to ask. How far did one have to go from the earth's surface to escape this radiation or background as we now call it? Asking such a question was a crucial step but Hess did much more -- he answered it although not in a way that anyone foresaw although there had been clues: experiments atop the Eiffel tower (330m) showed that the radiation didn't reduce with height as quickly as expected. In hindsight this was very significant. Hess wanted more altitude but back in 1912 there was no space shuttle, few airplanes and so he made a series of hot air balloon flights first to ~1 km (.625 miles) then later reaching a maximum altitude of ~5km (~3.125 miles). The result was not what he expected and is recorded at the Center for Scientific Culture in ERICE as:
"Here in the Erice maze,
Cosmic rays are all the craze,
Just because a guy named Hess,
When ballooning up found more not less."
It was an amazing discovery: The further one moved from the earth the more radiation there was. Hess didn't stop there though. He also made observations during a partial eclipse and found no change in the radiation. Hence Hess did two crucial things. First he discovered cosmic rays. Secondly he showed they did not come from the sun.
Many books simply stop at this point, but to do so is to miss the essence of science. Some scientists didn't believe Hess. They were not jealous and certainly not fools. Science is about truth, about others being able to reproduce results in different experiments. Doubts came from measurements made by the great American scientist Robert Millikan (who won the Nobel prize in 1923 for measuring the electron charge) and who greatly developed the technology of cosmic ray measurement. First in 1922 he used automatic recording devices on meteorological balloons that flew to 15 km but found much less radiation above 5 km. However, a large temperature effect diminished the conclusions. The next year in an expedition to Pike's peak (4.3 km) he found that 4.8 cm of lead screened out the increased radiation. Since he thought the lead was much less than the equivalent reduction in overlying atmosphere from climbing the mountain he concluded there was no radiation from above. The error was that lead is a much better radiation screen than air. Finally from measurements in lakes at high altitudes he found that the radiation fell as one went deeper into the lake. There was only one conclusion: the radiation was not in the air; it came from the sky.
Millikan's odyssey is the reality of scientific work. It is a quest to understand the truth of nature always knowing that it is easy to fool ones self. The final arbiter is not the acclaim of one's peers but the hard objectivity of reproducible results. Hess was right and 20 years after his flight he received the Nobel prize.
Following Hess's discovery speculation continued for many years. Eminent physicists (including Edward Teller who pioneered the hydrogen bomb) were convinced that it came from the sun. Yet experiments showed that cosmic rays didn't change during a solar eclipse nor was there a day/night variation. The heated debated between two Nobel laureates (Robert Millikan and Arthur Compton) lead the New York Times on the last day of 1932 to note:
Debate of rival theorists brings drama to session of nation's scientists. THEIR DATA AT VARIANCE. New findings of his ex-pupil lead to thrust by Millikan at 'less cautious' work.
Millikan having been convinced of a radiation from the sky was convinced that is must be gamma radiation: like a strong light rays and hence the name cosmic rays. However, other measurements showed that the radiation was influenced by the Earth's magnetic field while gamma rays
are not. The pupil won the day.
In passing it should be noted that gamma rays and particularly bursts of gamma rays are now an important area of research that is conducted by many friends and colleagues of Kyoung Hye, but they are not cosmic rays. So what are cosmic rays? Many were convinced it was a flux of negatively charged electrons
like the ones used in your computer. Yet the radiation was bent the 'wrong way' by the earth's
magnetic fields. So what is it? We now know it is predominantly the nuclei of the atoms that we and
everything we know of are made from. In space they come from every direction and even though
they are not electromagnetic rays Millikan's name was adopted and they are called cosmic rays.
Where do all these atoms come from? How are they accelerated? These questions are still at the cutting edge of cosmic ray research. From many experiments it appears (I use appears here because there are still experimental uncertainties) that the cosmic ray spectrum has two features: the knee and the ankle -- see figure. To understand what these mean we must first understand energy. Its most basic definition is the capacity to do work. The more energy one or a machine has the more one or the machine can do. How do we measure energy? In cosmic rays we use a unit that is called the electron volt (eV), but what does this mean? A simple way to understand this concept is to relate it to temperature. Sitting in a normal office at say 20 C (68F) the energy in eV is ~1/40 eV. If the temperature was to double to an unpleasant 40C (116F) the energy would be ~1/20 eV. So for our normal experience the unit is small. Even for the surface of the sun (~6000 C) this only corresponds to ~7.5 eV. Nuclear energies are measured in millions of eV whilst cosmic rays have been measured to ~ one hundred million million million eV. (yes three millions). At an energy of one thousand million million the spectrum seems to have a shape change called the knee whilst at ~ million million million it may have another shape change called the ankle. One of the most convincing models of cosmic ray origin was proposed by Prof. Shapiro. In this model he argues that the very numerous red dwarf flare stars (100 times more frequent than stars like the sun) and which produce flares 1000 times more powerful than solar flares are the source. The evidence for this is that the cosmic ray species detected have a dependence of the amount of energy needed to remove atomic electrons and the dependence is well fit by what is known of the temperatures of flare stars. Prof. Shapiro's model also requires a small (~2%) admixture of material from another class of stars called Wolf Rayet but that is getting in to too much detail. During the generation of heat and light in a star hydrogen nuclei are burned to form the heavier elements. In Prof. Shapiro's model these are flung off by flares. Occasionally the shock wave from a supernova (a gigantic explosion that occurs when a star that has burned all its fuel collapses) will pass by and accelerate the atoms as it collides with them - this is called Fermi acceleration after the inventor of the idea. It is believed that this mechanism can work to about the knee. How the particles are accelerated beyond that is an open question. The knee can be seen as the end of the Fermi process. However, and here is a big rub, why is there not a break in the spectrum? Why do the lines in the figure join smoothly? The answers will only come from more research.
On a dark cloudless night away from city lights one is often overwhelmed by the glow of the milky way. Although we can't see cosmic rays directly there energy density is comparable to the starlight. The great voids between the galaxies are however without cosmic rays. (We know this because we do not see gamma-rays that a universal distribution of cosmic rays would produce from the nearby Magelanic clouds). Most cosmic rays are created within the Milky Way and are mostly confined by the vast galactic magnetic fields.
One of Einstein's great advances in his special theory of relativity was his postulate that the velocity of light is a universal constant of ~186,000 miles/second. Even if you are in the space shuttle traveling at the escape velocity mach 25 (~ 5 miles/second) the light coming towards you and the light from the engines seen by your friends on the ground would still travel at 186,000 miles/second NOT 186,005 or 185,995 -- strange to us in our Newtonian universe! Why there should be such a natural speed limit is one of the great unanswered question of physics. Cosmic rays travel at almost the speed of light. In fact they travel so close to the velocity of light that most standard calculators do not have enough zero's to display the difference!
All aspects of the space program from the exposure of humans to radiation to the design of sensitive instruments are influenced by cosmic rays. Astronauts sometimes report flashes of light in their eyes. These come from energetic cosmic rays that pass through the aqueous humor in their eyes producing Cerenkov light. Many astronomical satellites, such as the HUBBLE space telescope, are designed so that the random flux of cosmic ray can be removed from the optical images.
The cosmic rays contain all the elements found on earth but from the ratios of radioactive elements we know they arrive at our earth having traveled for millions of years beginning their journeys deep within the galaxy. The study of cosmic rays thus tells us about their origin, about the immense interstellar medium through which they travel, and about the vast galactic magnetic fields that confine most of them to our galaxy. As the arriving cosmic rays smash in to the atmosphere they produce secondary nuclei. Some of these are radioactive such as an isotope of carbon (carbon-14) that is produced from atmospheric nitrogen. This radioactive element allows the age of any thing that has lived during the last approximately 60,000 years to be dated. There are many other areas where cosmic rays are used in other sciences.
You may be thinking this is some weasel statement but its not. We have no good ideas where Hess particles get their energy but it is enormous. How much energy do they carry? Well each cosmic ray is very very small. We don't know what Hess particles are, but for example if we consider hydrogen cosmic rays (the most common species) you would need 6 followed by 23 zeros before the decimal point to add up to a total weight of 0.04 oz (1 g). Nothing! that's what your thinking. Nothing that minute can have any significant amount of energy -- wrong. The energy these minute particles carry is phenomenal. The highest energy events detected have the same energy as a tennis ball traveling at 250 mph! The energy is over one hundred million times more than can be created in the largest experiments (the Tevatron at Fermi Laboratory) on earth. They are the highest energy particles known. In acknowledgment of the contribution to this science made by Victor Hess they are called Hess particles.
Back to the start of the Hess sectionHess particles are very rare. From what we know now there is on average one Hess particle per square kilometer (0.39 square miles) of earth's atmosphere per century!
Back to the start of the Hess sectionThe science of cosmic rays is glamorous but it is also extraordinarily demanding. It's study requires
a sharp intellect and a dedication to develop both theoretical and practical skills that cover a broad
range of sub-disciplines such as advanced mathematics to the ability to study faint microscopic trails
left by cosmic rays in photographic emulsions. Many of the people in cosmic ray
research have doctoral degrees. Yet although these are not easy things they are insufficient.
Beyond that one must have the
ability to work effectively in international collaborations and to interact productively with
colleagues. But even this is not enough. To be successful one must also have an intuition, an ability
to see order where others see dis-order, to see how something that others feel is impossible can be
done and cheaply too, and to convince funding agencies that your project must be supported. With
all these qualities one may do well but the giants of this field have yet another priceless gift: an ability
to ask the question that is so deep that others felt there was no question and then to answer it. These qualities
can not be learned -- they are god given gifts. Although we can never
know where Dr. Kyoung Hye Moon's passion for cosmic rays would have lead there is no doubt
that she had these qualities.
There are many current cosmic ray experiments. To get a flavor for them visit here.
The study of cosmic ray has attracted the finest minds of the twentieth century. The highest scientific honor (the Nobel Prize) has been given to four scientists for their research into cosmic rays. Several other scientists who made major contributions to the discipline have won Nobel prizes for other studies. By these great honors it is evident that the study of cosmic rays has dramatically influenced 20th century science and although prediction is fraught with uncertainty it is probable that cosmic ray research will similarly influence 21st century science.
It was 24 years following his pioneering flights that the contribution of Hess was recognized. After his discovery it was not clear what use cosmic rays would be. Anderson found an application when he discovered the positron. The first particle of anti-matter that the British theorist Dirac had predicted.
Back to the giants of cosmic ray physicsMuch early cosmic ray research was conducted with cloud chambers that had to be triggered to take a picture at just the right moment. Blackett realized that if he took two detectors one above and one below the chamber and then only took a picture when both detectors fired he would increase his chances of taking a good picture. This coincidence technique is vital component of almost all modern scientific instruments.
Powell with his colleagues in Bristol (UK) who included Peter Fowler (the grandson of Rutherford who discovered the nucleus) developed photographic emulsion techniques for cosmic ray studies that are still used today. He was awarded the Nobel prize for his study of sub-nuclear particles called mesons.
Back to the giants of cosmic ray physicsThe great Italian physicist won the Nobel prize for his studies of artificial radioactivity induced by neutrons. It was Fermi who demonstrated the first controlled nuclear reaction below squash courts at the University of Chicago in 1942. Fermi made numerous contributions to science and his regarded as one of the greatest scientists of the 20th century. He has been honored in many ways. For example nuclear sizes are measured in units called Fermi. One Fermi is 0.000,000,000,000,001 m! Fermi also provided a possible partial answer to one of the great questions of cosmic ray research. How are cosmic rays accelerated to high energies? Fermi suggested that the shock waves from supernova explosions could provide the necessary energy. Although this model is still not proven and can not explain the Hess particles it is the most widely believed mechanism.
Millikan skill as an experimenter was demonstrated when he measured the electrical charge of the electrone. He did this by an ingenious technique. Using a microscope he studied how very small oil drops with a few electrons attached fell under the action of gravity. He then applied a force to the drops with an electrical field and from the change in motion of the oil drops he deduced the electron charge -- a beautiful experiment. Millikan was awarded the Nobel prize in 1923. Millikan however did much pioneering work on cosmic rays following Hess's discovery and although his original idea that cosmic ray were like energetic light rays was wrong his name was adopted.
Back to the giants of cosmic ray physicsArthur Holly Compton was awarded the Nobel prize for his discovery of the Compton effect. Compton showed that x-rays (like strong light rays) behaved as particles whereas in other experiments they behaved as waves. This results provided evidence for the wave-particle duality of light. It showed that the human concepts of wave or particle can not be applied to quantum systems. Such systems can exhibit both types of behavior and can be thought of as wave and particle. The behavior one sees depends on how one looks. Compton also made a world wide survey of cosmic radiation in the 1930's to determine information about the variation with geo-magnetic lattiude. Compton shared his prize with Charles Wilson who devised the cloud chamber. This device which makes the trails of ionizing particles visible by creating conditions so that condensation occurs along the particle track. It gave experimenters their first view of nuclear interactions and was a crucial research tool for many years.
Back to the giants of cosmic ray physicsMany scientists who did not win the Nobel prize have made major contributions to cosmic ray research. Their stories can be found in several text books -- visit your library and search their data base for cosmic rays.
Back to the giants of cosmic ray physics
email: smithae@hiwaay.net