|MLA Citation:||Bloomfield, Louis A. "Medical Imaging and Radiation" How Everything Works 19 Mar 2018. Page 2 of 2. 19 Mar 2018 <http://www.howeverythingworks.org/prints.php?topic=medical_imaging_and_radiation&page=2>.|
The list of fundamental particles—particles that are not known to be composed of other particles—is relatively short. It includes 6 types of quarks, which are given the arbitrary names "up", "down", "charm", "strange", "top", and "bottom". These quarks are never found by themselves but are instead used to build two major classes of subatomic particles: baryons (including protons and neutrons) and mesons. The list of fundamental particles also includes 6 types of leptons, which are given the names "electron", "electron neutrino", "muon", "muon neutrino", "tau", and "tau neutrino". These leptons are found by themselves and aren't used to build any other subatomic particles. These quarks and leptons are described as fermions and each has an associated antiparticle.
In addition to quarks and leptons, there are a number of fundamental particles that allow the fundamental fermions to interact with one another. These interaction particles are described as bosons and include the "photon", "W+ Boson", "W- Boson", "Z Boson", 8 different "gluons", and a particle called "Higgs" (which has not yet been observed but is thought to exist).
The list of subatomic particles that can be formed from the fundamental particles is extremely long and listing it here wouldn't be very enlightening. The only subatomic particles that are common in nature are protons, neutrons, electrons, and photons. Some of the others appear through nuclear or subnuclear processes in radioactive materials, nuclear reactors, particle accelerators, or celestial objects, but most of these exotic subatomic particles haven't been common since moments after the big bang.
X-ray crystallography is an important tool for materials science and molecular biology. Just as the colored interference patterns that appear on a soap bubble when sunlight reflects from that bubble tell you something about the structure of that soap bubble, so the X-rays that reflect from a crystal tell you something about the structure of that crystal. X-rays experience interference after they reflect from a crystal and the interference patterns can tell you where individual atoms are located within a crystal or within the molecules from which the crystal is made. Materials scientists use this information to understand the crystals they have produced while molecular biologists use it to understand the molecular structures of complicated biological molecules.
While fusion is somewhat more energy efficient than fission, that's not the whole reason why hydrogen bombs (thermonuclear bombs) are more powerful than uranium bombs (fission bombs). The main reason is that thermonuclear bombs can be much larger than fission bombs because there is no upper limit to the amount of hydrogen you can assemble in a small region of space. In contrast, if you assemble too much fissile uranium in a small region of space, a chain reaction will begin and the material will overheat and explode. At the height of the cold war, the Soviet Union built gigantic thermonuclear weapons with explosive yields as large as 100 megatons of TNT.
The basic calculation of critical mass is straightforward in principle, but it requires a thorough understanding of the nuclear fuel. Because you need to know how likely one nuclear fission is to cause a subsequent nuclear fission, you must know both the types of fragments you can expect from the first nuclear fission and the likelihood that each fragment will induce a subsequent fission in another atomic nucleus before that it escapes from the nuclear fuel. Because the range of possible fragments, their kinetic energies, and their paths through the nuclear fuel are so vast, an accurate calculation of critical mass is extremely complicated. As an indication of the difficulty, note that fission fragments may bounce off nuclei without inducing fission, so that you must consider bent paths as well as straight ones. Not surprisingly, the calculation of critical mass is too difficult to do exactly, even with the help of computers. In fact, one of the reasons that Germany didn't develop nuclear weapons during World War II was that its scientists miscalculated the critical mass of a fission bomb based on enriched uranium and thought that they would need many tons of enriched uranium rather than the true critical mass of about 52 kilograms. Certain that a critical mass of enriched uranium was unattainable, they didn't pursue the project.
But suppose that the bus were traveling at 99.999999% of the speed of light and you were to run toward its front at 0.000002% of the speed of light (about 13 mph or just under a 5 minute mile). Now what would happen?
First, the bus speed I quoted is in reference to some outside observer because the seated passengers on the bus can't determine its speed. After all, if the shades are pulled down on the bus and it's moving at a steady velocity, no one can tell that it's moving at all. So let's assume that the bus speed I gave is according to a stationary friend who is watching the bus zoom by from outside.
While you are running toward the front of the bus at 0.000002% of the speed of light, your speed is in reference to the other passengers in the bus, who see you moving forward. The big question is what does you stationary friend see? Actually, your friend sees you running toward the front of the bus, but determines that your personal speed is only barely over 99.999999%. The two speeds haven't added the way you'd expect. Even though you and the bus passengers determine that you are moving quickly toward the front of the bus, your stationary friend determines that you are moving just the tiniest bit faster than the bus. How can that be?
The answer lies in the details of special relativity, but here is a simple, albeit bizarre picture. Your stationary friend sees a deformed bus pass by. Ignoring some peculiar optical effects due to the fact that it takes time for light to travel from the bus to your friend's eyes so that your friend can see the bus, your friend sees a foreshortened bus—a bus that is smashed almost into a pancake as it travels by. While you are in that pancake, running toward the front of the bus, the front is so close to the rear that your speed within the bus is miniscule. Why the bus becomes so short is another issue of special relativity.
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