A Pause for Reflection
Let us put what you have just learned in perspective. If all we are interested in is the structure of the world around us; we can get along nicely with the electron, the neutrino, the neutron, and the”rotten. As someone has said, we can operate “Spaceship Earth” quite well with just these particles. We can see a few of the more exotic particles by loo ing for them in the cosmic rays; however, to see most of them, we must build massive accelerators and look for them at great effort and expense. The reason we must go to such effort is that-measured in energy terms live in a world of very low temperatures. Even at the center of the Sun, the value of kT is only about I keV. To produce the exotic particles, we must be .able to accelerate protons or electrons to energies in the GeV an~ TeV range and hi.ghe~. nce upon “..time the temperature everywhere was high enough to provide Just such energies’and far greater. That time of extremely high temperatures occurred in the big bang beginning of the universe, when the universe (and both space and time) first existed. Thus, one reason why scientists study particles at high energies is to understand what the universe was like just after it began. As we shall discuss shortly, all of space within the universe was initially tinyin extent, and the temperature of the particles within that space was incredibly high. With time, however, the universe expanded and cooled to lower temperatures, eventually to the size and temperature we see today.Actually, the phrase “we see today” is complicated: When we look out into space, we are actually looking back in time, because the light from the stars and galaxies has taken a long time to reach us. The most distant objects that we can detect are quasar (quasi-stellar objects), which are the extremely bright cores of galaxies that are as much as 14 X 109ly from us. Each such core contains a gigantic black hole; as material (gas and even stars) is pulled into one of those black holes, the material heats up and radiates a tremendous amount of light, enough for us to detect in spite of the bug distance. We “see” a quasar as it once was, when that light began its journey to us billions of years ago.