Exotic Possibilities
Objects that physicists are just beginning to imagine
Scientists seek to extend our understanding of the laws of physics by speculating about what new phenomena might be found out in the Universe, and then looking to see if they do in fact exist. Gravitational-wave astronomy is one way we might be able to find these new objects, by find. Among the most interesting possibilities are superstrings and exotic stars.
Superstrings are the fundamental objects of string (String: The fundamental object in String Theory, which replaces the notion of a particle in standard Quantum Mechanics. Rather than being a simple point-like object, fundamental particles become tiny strings or loops. The vibrations of these strings result in various properties like spin (Spin: An intrinsic property of particles. (That is, a property which does not change. Mass and electric charge are examples of intrinsic properties.) Spin is related to the usual notion of spin, though it is a little more difficult to understand. Spin comes in units of 1/2, so that a particle may have a spin of 0, 1/2, 1, 3/2, and so on. A particle's spin determines whether it is a Fermion or a Boson.).) theory (String Theory: A theory of physics taking the String as its fundamental object. This theory attempts to solve problems in standard Quantum Mechanics and Quantum Field Theory. It actually predicts the existence of gravity.), one possible successor of quantum mechanics (Quantum Mechanics: A modern physical theory which is vital to describing extremely small objects, like electrons around an atom. Though this theory also applies to larger objects, its effects become very similar to those of Newton's theory — which is typically much easier to use and understand. One of the most important ideas in Quantum Mechanics is the Uncertainty Principle (Uncertainty Principle: The principle of Quantum Mechanics — as well as Quantum Field Theory and String Theory — which says that an observer can never know both the position and velocity of a particle with perfect precision. Specifically, the more certain an observer is of the position, the less certain that observer must be of the velocity, and vice-versa.).) and general relativity (General Theory of Relativity: Einstein's version of the laws of physics, when there is gravity. Building on the Special Theory of Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity (Special Theory of Relativity: Einstein's version of the laws of physics, when there is no gravity. The two fundamental concepts in the foundation of this theory are equality of observers, and the constancy of the speed of light. The first of these means that the laws of physics must be the same, no matter how quickly an observer is moving. The second means that everyone measures the exact same speed of light. This theory is useful whenever the effects of gravity can be ignored, but objects are moving at nearly the speed of light. It has been successfully tested many times in particle accelerators, and orbiting spacecraft. For objects moving much more slowly than light, Special Relativity becomes very nearly the same as Newton's theory, which is much easier to use. ) becomes very nearly the same as Newton's theory, which is much easier to use. ), this theory generalizes Einstein's work so that the laws of physics must be the same for all observers (Observer: A person or piece of equipment that measures something in physics. Frequently, we speak of an observer measuring time or a distance in a particular place. ), even in gravity. Einstein showed that gravity is best understood as a warping of the geometry of spacetime (Spacetime: A concept in physics which merges our usual notion of space with our usual notion of time.), rather than as a pulling of objects on each other. The crucial idea is that objects move along geodesics — which are determined by the warping of spacetime — while spacetime is warped by massive objects according to the formula \(G = 8 π T\). ). If they exist, they might act much like the cosmic strings we encountered on the previous page. By listening for the distinctive whip-like crack of superstrings, we might actually be able to observe effects of string theory — a feat that is currently impossible.
Though superstrings could be hard to observe even if they do exist, another class of hypothetical objects that could be easy to observe are exotic stars: quark (Quark: An Elementary Particle which makes up the Neutron and the Proton, as well as many more exotic particles. These particles are Fermions, and have charges of either 2/3 as much as an electron's charge, or -1/3. They come in "flavors" of up, down, strange, charmed, bottom, and top, as well is in "colors" of red green and blue. The "flavor" and "color" are just fanciful names given to describe intrinsic properties of these particles — similar to charge, mass, or spin.) stars, boson (Boson: A type of particle with "integral angular momentum" — a spin of 0, 1, 2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions (Fermion: A type of particle with "odd half-integral angular momentum" — a spin of 1/2, 3/2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are electrons, neutrons, and protons. The other type of particle is the boson. ) are photons (Photon: An Elementary Particle which carries the energy of light. The photon is a Boson, and has no mass. It always moves at the Speed (Speed: For a wave, the speed of a particular point (such as its crest).) of Light (Speed of Light: A constant of Nature. This speed is precisely 299,792,458 meters per second, or roughly 670,616,629 miles per hour. One of the most unusual discoveries of science has been the fact that all Observers measure light as moving at exactly this speed, even if those observers are moving relative to each other. This fact is one of the basic ingredients in Einstein's Special Theory of Relativity.). ) (which are the particles which give us light) and gravitons (which give us gravity). The other type of particle is the fermion (Fermion: A type of particle with "odd half-integral angular momentum" — a spin of 1/2, 3/2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are electrons, neutrons, and protons. The other type of particle is the boson. ). )}) stars, fermion stars, and other possibilities. For example, a quark star might be something like a neutron (Neutron: One of the particles in an atomic nucleus. These particles have no electric charge, but they hold together the protons (positive particles in a nucleus), and account for roughly half of the particles in the nucleus. Neutrons are fermions, and are believed to form the majority of the matter in a neutron star.) star (Neutron Star: A type of star which is very old, having cooled off and stopped nuclear fusion reactions. When gravity pulls the star down on itself, the electrons and protons are squeezed together, leaving just neutrons. The star is then supported against gravity by "neutron degeneracy pressure" (no two neutrons can be in the same place at the same time). These are produced when a star is too heavy to be a white dwarf (White Dwarf: A type of star which is very old, having cooled off and stopped nuclear fusion reactions. A white dwarf is supported by "electron degeneracy pressure" (no two electrons can be in the same place at the same time). These are produced when a star is not heavy enough to turn into a Neutron Star or a Black Hole. )})|white dwarves}), but not heavy enough to turn into a Black Hole. )}). But if our understanding of physics in the extreme environment of a neutron star isn't quite right, it is possible that all the neutrons might basically merge into one giant soup of quarks, which are usually bound together inside the neutron in sets of three. A quark star would be much denser than a neutron star, and would thus have very different properties. The difference between the gravitational waves (Gravitational Wave: A gravitational disturbance that travels through space like a wave. This type of wave is analogous to an Electromagnetic Wave. Gravitational waves are given off by most movements of anything with mass. Usually, however, they are quite difficult to detect. Physicists are currently working hard to directly detect gravitational waves. Experiments like LIGO and LISA are designed for this purpose. ) given off by a quark star and those given off by a neutron star would be very easy to see in a gravitational-wave detector. These objects would be difficult or impossible to measure in more traditional ways. While they might not even exist, the possibility that they might be discovered by gravitational-wave detectors is motivation to build these exquisite machines.
The cracking whip of a cosmic string (Cosmic String: A long, heavy object from Quantum Field Theory or String Theory, which is very thin. They may have been created in the early life of the Universe, and would now stretch across the entire Universe. No cosmic string has ever been observed. They may or may not exist.)