Pulsars
Pulsars are very small, very massive—and thus very dense—stars that rotate at very high speeds. Unlike most stars, which emit light all around them, pulsars are thought to emit light in small beams as they rotate, creating a sort of lighthouse effect (see Figure 5.8). They are then detected by searching for periodic pulses of light in the sky. Physically, they represent the very dense cores of massive stars that collapsed in on themselves due to gravity, also known as neutron stars. They are thought to be about as massive as our Sun, but only about 20 km wide. As they collapse, the stars reach some of the highest densities known (second only to black holes); they rotate incredibly fast and generate some of the largest magnetic fields known in the Universe. Because of all these properties, neutron stars are considered to be excellent “laboratories” that can help us learn about the laws of physics and how matter behaves under such extreme conditions.
Figure 5.8[1]
Quantum electrodynamics
The exciting theories of special relativity and quantum mechanics were discovered in the first few decades of the 19th century. These theories taught us that objects that are either moving extremely fast or very small can work very differently from objects in the slow (that is, compared to the speed of light) and “macroscopic” or human-scale world we are used to. Quantum mechanics governs things like electrons, but because these have very small masses (again, compared to you or another human or even a proton), they can move at relativistic speeds close to the speed of light, the fastest possible speed for any particle. This means that to describe them we have to combine quantum mechanics and special relativity. This is only possible in a multi-particle theory called quantum field theory, where particles are represented by fields (sets of numbers) that take some value at every point in space-time.
Quantum electrodynamics (QED) is a theory that describes electromagnetism using quantum field theory. This means it describes the possible states and interactions of particles like electrons, which have electric charge. In nature, we observe that charged particles interact with each other. In QED, this interaction is described by the exchange of photons, or little packets of light. Two electrons that carry the same charge will repel each other if they are brought together, as you already know. In QED, this process can also be thought of as one electron “throwing” a photon at another, causing them to move apart, like when billiard balls collide. QED is a very successful theory, called “the jewel of physics” because of its ability to make extremely accurate predictions. Figure 5.9 shows two electrons being repelled, or interacting, via an exchanged photon. (The progression of time is represented from left to right).
Figure 5.9[2]
Radioactivity
Heavier elements are inherently unstable and can change into other elements: this is called transmutation. You might start out with an atom of uranium, which will change into thorium, then radium, radon, polonium and so on. This process, called radioactive decay, can be very fast for some elements and much slower in others. It is the basis for certain dating methods in geology and other historical disciplines. The product nuclei are called daughter nuclei. An example of a radioactive “family tree” is shown below:
Radium226(Ra226) ® Radon222(Rn222) ® Polonium218(Po218) ® Lead214(Pb214)
The numbers refer to atomic weights: the total number of nucleons (protons plus neutrons) present in a nucleus. Isotopes of an element have the same number of protons (i.e., the same atomic number), but different numbers of neutrons and therefore different atomic weights. The isotope of lead in the chain above is not stable but will decay further. What is the trend in atomic weights through this chain? Do the numbers increase or decrease?
If an element turns into another element with a lower atomic weight, you might think that it must be losing something. Indeed it is: this is the radiation that first led people to discover radioactivity and the reason radioactive elements can be dangerous, both to people (see the “Radioactivity and health” section) and as starting points for atomic weapons.
The radiation that is given off by a radioactive substance was found early on to be of three types, which can be separated by a magnetic field (see Figure 5.10). They were named alphabetically: alpha rays, beta rays and gamma rays. It is now known that alpha rays are actually helium nuclei, and beta rays are electrons. Gamma rays are another kind of electromagnetic radiation.
The energy of a wave of electromagnetic radiation is given by E = hc/L, where L is the wavelength of the wave, and h and c are constants (c is the speed of light, around 300 million metres per second). Given this, are gamma rays of higher or lower energy than visible rays? What about radio waves and microwaves? Which do you think are worse for you?
Figure 5.10[3]
Radioactivity and health
Radioactive elements occur naturally: they are present in the food we eat, the air we breathe and in the earth. This means they are in building materials, like the walls of your bedroom, and in you. In addition, cosmic rays are constantly passing through us. These are energetic particles that bombard the earth from space. People have lived with these sources of radiation forever, and though they may sometimes be harmful, there is no way to avoid them. How harmful radioactivity can be was only realized in the 19th century when people began to study concentrated amounts of highly radioactive materials. Of course, these are much more dangerous and therefore helped us to understand how radioactivity works and how best to handle it. In fact it can be used for good (cancer treatment and nuclear power) as well as bad (poisoning and atomic bombs), just like a knife: it depends on the intentions of the user. The important thing is to understand how it works, so that we can use it responsibly.
Early on, the harmful effects of radiation were not fully realized. Pierre Curie experimented with the burns radioactive elements left on his skin by deliberately exposing his arm to radioactive barium for hours to see what would happen. The skin was burnt and then formed a wound that took 52 days to heal, but left a grey patch of dead tissue. Although he did not know it, this occurred because when radiation enters the body, it can harm healthy cells. As radiation passes through the body, large amounts of energy (like a kick or punch) are transferred to molecules in the region, disrupting their structure. The cells are injured or die. Pierre Curie’s hands became so damaged by exposure to radiation that for a time he struggled to dress himself, and in the years before his death, he suffered from debilitating bone pain. It may have been such pain or weakness that prevented him from getting up after he was knocked down by a horse on a busy street in Paris in 1906. Instead, he was run over by the 30-foot wagon the horse was pulling and died instantly. His wife Marie Curie suffered from weakness, dizziness, anemia and cataracts, all of which were probably caused by her long exposure to radioactivity.
It was known during Pierre Curie’s lifetime that radium had proved effective in the treatment of cancers—because it kills cells, which should have been a clue. However, it was hard to track its effects as a poison because it could take a long time before any negative effects were seen, and it did not affect everyone in the same way. Before the mechanism was understood, many products containing radium were marketed as good for your health including toothpaste, bath salts and suppositories! Eventually, careful precautions were put in place to protect people working with radioactivity.
Symmetries in physics
Symmetries play a key role in modern physics. A conservation law corresponds to every type of symmetry. For instance, since we believe that the laws of physics are the same at any point in space, we deduce that momentum is conserved. As the laws of physics are the same in all directions, we deduce that angular momentum is conserved. Since the laws of physics are the same at all moments in time, energy is conserved. These are continuous symmetries. There are also, however, discrete symmetries. The parity (P) operation is the one that replaces a system of fundamental particles with its mirror image. If the parity operation were a true symmetry of nature, then a spin particle decay would produce the same particle rates in one direction as in the opposite direction. All directions are equivalent, right? Not quite so. Chien-Shiung Wu (Madame Wu) showed in 1956 that the decay of cobalt-60 violates this symmetry. Parity was the first symmetry found not to be conserved. Later, the charge parity operation, the one that exchanges particles and antiparticles, was also discovered to be asymmetric.
Biographical information
Jocelyn Bell Burnell (1943–)
Jocelyn Bell Burnell (born Susan Jocelyn Bell) was born in Belfast, Northern Ireland, on July 15, 1943. Her father was an architect for the Armagh Observatory, which was close to their home. She grew up with a love for astronomy. Although she failed the entrance tests to study at British schools, she was sent to a boarding school in England to continue her studies. She noted that her science teachers were very important to her decision to have a career in science. One particular teacher convinced her to study physics by telling her: “You don’t have to learn lots and lots […] of facts. You just learn a few key things, and […] then you can apply and build and develop from those.”
While she was doing her PhD at the University of Cambridge, she helped build a radio telescope and was involved in the discovery of pulsars. During her career she has contributed to the study of many astrophysical objects at all wavelengths of the electromagnetic spectrum and has been awarded many prestigious prizes (though not the Nobel Prize, to the disappointment of many scientists). She has also received numerous honorary degrees. She was Professor of Physics at the Open University for 10 years and then a visiting professor at Princeton University. Recently she was Dean of Science at the University of Bath between 2001 and 2004, and was President of the Royal Astronomical Society between 2002 and 2004. As of 2007, she is Visiting Professor of Astrophysics at Oxford University. She was created a Dame of the Order of the British Empire in June 2007.
Jocelyn Bell Burnell[4]
Subramanyan Chandrasekhar (1910-1995)
Subramanyan Chandrasekhar was born on October 19, 1910, in Lahore, India (now in Pakistan). His family later moved to Madras (now known as Chennai) where he studied at Presidency College and demonstrated an avid interest in astrophysics. In 1930, he received his Bachelor of Science degree in Physics, with honours. Because of his academic achievements, Chandrasekhar was awarded a scholarship by the government of India to attend graduate school at the University of Cambridge, in England. It was on his way to England that he started his work on the life cycle of stars. He obtained his PhD in 1933 and worked in England until 1937 when he moved to the University of Chicago, where he worked for the rest of his life. He became the Distinguished Service Professor of Theoretical Astrophysics in 1947 and attained emeritus status in 1985.
During his long and successful physics career, Chandrasekhar worked on challenging astrophysical problems including the life cycle of stars, how light is created and transported from the inside to the outside of stars, and the role of magnetic fields in galaxies. He was awarded the Gold Medal of the Royal Astronomical Society in 1953 and the Royal Medal of the Royal Society in 1962. In 1983, he was awarded the Nobel Prize for his groundbreaking work on the structure and evolution of stars. He died in 1995. NASA named the third of its four Great Observatories after Chandrasekhar. The Chandra X-ray Observatory was launched and deployed by Space Shuttle Columbia on July 23, 1999. The telescope is designed to study the extremely dense neutron stars and black holes that Chandrasekhar worked on throughout his life.
Subramanyan Chandrasekhar[5]
Marie Curie (1867-1934)
Marie Curie (born Maria Sklodowska) was born in 1867 into an intellectual but impoverished family in Warsaw, then under Russian rule. She grew up in a house where learning was play and which, at one point, even served as a boarding school. She was an exceptional student, but women were not allowed to study at the University of Warsaw at the time. Still, both Marie and her older sister Bronia felt they were entitled to the higher education their brother Jozef was getting. The only way to get it was to go abroad, to France. This would be expensive, so the sisters decided to study in relay: first Marie would work while Bronia studied medicine in Paris, and then once Bronia was set up, she would support Marie in her studies. Marie worked for six years as a governess, far from home. She persevered with the tiring and demanding job, made more difficult by a love affair with the oldest son of the family she worked for, which was frowned upon by his parents. In the evenings, she studied alone so as to be better prepared for university once she got there; she also started and ran a peasant school to teach poorer children how to read and write in Polish, their native tongue—a potentially dangerous political activity at the time. She read and worked through texts in all subjects but eventually realized science was the discipline she most loved.
At the age of 24, Marie finally made her way to Paris to begin her studies at the Sorbonne. As one of a handful of women, mostly foreign, she ranked first in her science degree and second in her mathematics degree. She had planned to return to Poland to teach, but an opportunity arose to work for the Society for the Encouragement of National Industry in Paris on the magnetic properties of certain hardened steels. She accepted the offer, and while trying to find lab space was introduced to Pierre Curie, also a deeply committed physicist whose first love was work. When she married him in 1895, it was in a dress she had chosen to serve as a lab uniform. When he became interested in her doctoral research, an idyllic period of collaboration followed. Curie later wrote that she would never be able to express the joy she felt at the time, with days devoted to gruelling hard work and nights enchanted by the tubes glowing like fairy lights in the makeshift lab. For this work, the Curies, together with Henri Becquerel, were jointly awarded the Nobel Prize in Physics in 1903.