Resources - Glossary

Big Band Theory: It is the simplest theory that agrees with the cosmological principles and most observational facts in our universe. It states that when the age of the universe was about just 1ms, the universe was very small, with very high density and temperature (about 10, 000, 000, 000, 000 K). From that time onward, the universe expanded and its temperature decreased. The cosmic microwave background at 3K is what was left. In certain sense, the expansion is similar to the baking of bread. The size of the bread increases and the distances between the raisins also increase. If you sit on one of the raisins, you will see that all the raisins are receding from you. Besides, the farther away a raisin locates, the faster it recedes. It also explains the cosmological redshift. If the galaxy is far away from us, the light it radiates takes a longer time to reach us. During the time of traveling, the universe expands and stretches out the wavelength of the photons. The Big Bang Theory can tell us the age of the universe. We know how fast the galaxies are receding from us. So, if we run the movie of the universe backward, we can calculate how long it takes for the galaxies to get back together. From this, the age of the universe is about 15 billion years.

Black Body Radiation: A black body is an object which can absorb completely any electromagnetic radiation of any frequency falling on it. The electromagnetic radiation emitted by this object is called black body radiation. It is found that the wavelength at which the most energy is emitted is inversely proportional to the absolute temperature of that object. If the object is so hot that it can emit visible light. When the temperature increases, the wavelengths of the radiation emitted become shorter.

Black Hole: If the remnant of a supernova explosion is heavier than two or three solar masses, the remnant will collapse to a black hole. Black hole is where the gravitational force is so strong that it bends light to the greatest extent: light cannot escape. We call the center of a black hole a singularity. We do not know the physical laws that govern the singularity and have no idea what happens there.

Coefficient of friction: It is the quotient obtained by dividing the value of the force necessary to move one body over another at a constant speed by the weight of the body. For example, if a force of 10 newtons is needed to move a body of 100 newtons over a horizontal surface at a constant speed, the coefficient of friction between these two materials is 10/100, or 0.1. Different materials in contact give different values of coefficient of friction.

Falsifiability: It is an essential concept in the philosophy of science. It was first developed by Karl Popper in the 1930s. To convey the concept roughly, we can say that for an assertion to be falsifiable, it must be in principle possible to make an observation or do a physical experiment that would show the assertion to be false. For example, the assertion "All crows are black" could be falsified by observing a red crow.

Galaxy: They are the basic building blocks of our (visible) universe. We, including our Sun and the solar system are in one galaxy, the Milky Way Galaxy, which is a fairly typical spiral galaxy. Galaxies come with many sizes and shapes. They are classified into three types: the elliptical, the spiral and the "none of the above" irregular. Almost all the stars visible by naked eyes are in our galaxy, the Milky Way Galaxy. It contains about 200 billion stars. Most of them are on the disk, which we call the "galactic disk". The diameter is about 100,000 ly (light year) while the thickness is just 1,000 ly. Thus, the disk is very thin compared with the diameter.

Horsepower: unit of power, 1 horsepower = 746 watts.

Light Year: To talk about the distances between stars, we could use light year (ly), which is the distance traveled by light in one year. Note that light year is a distance scale, not a time scale. 1 ly is about 9, 460, 000, 000, 000, 000m.

Mass deficit: It is the difference in mass between the free protons and neutrons of which the nucleus is made, and the mass of nucleus itself.

Neuron (Neurone): A cell specialized to operate in the nervous system and for the transmission of electrical signals over long distances. Neurons receive input from sensory cells or other neurons and send output to muscles or other neurons. Neurons with sensory input are called sensory neurons, neurons with muscle outputs are called motor neurons, and neurons that connect only with other neurons are called interneurons.

Neutron Star: If the mass of the core after a supernova explosion is more than 1.4 but less than about 3 solar masses, the star will contract to a size even smaller than the white dwarf. The electrons are squeezed into the nuclei and are combined with the protons to form neutrons. A "neutron star" is formed. A neutron star composes mostly of neutrons, about 95-99%. Yet there is a trace amount of electrons and protons. Its typical size is about 8 to 16 km in radius, which is roughly the size of Hong Kong Island. Since the gravitational field on the surface is very strong, no hills or mountains can be formed on a neutron star.

Piezo-electric transducer: When a mechanical stress is applied to a solid dielectric, a voltage will be produced between its surfaces and it is called piezo-electric effect. On the contrary, when a voltage is applied to this material, it will be deformed mechanically.

Plasma: It is the state of matter at very high temperature. The material is fully ionized into positive ions and negative ions. A plasma conducts electricity and can be affected by magnetic fields.

Population Inversion: Terms used in the field of LASER. Take the ruby laser as an example. An atom firstly absorbs energy and transits to an excited state. The atom stays at the excited state only momentarily and then falls to an intermediate state called metastable state. It stays at the metastable state for a rather long time. Its prolonged stay at the metastable state causes the number of atoms at the state being larger than that at the ground state, and such a phenomenon is called population inversion. Population inversion is a key to producing laser, because it ensures that the number of atoms returning from the metastable state to the ground state by stimulated emission is more than that transiting from the ground state to the metastable state by spontaneous absorption, so that the number of photons in the medium will increase and hence there is a laser output.

Quanta: In quantum theory introduced by Planck at 1900, energy exists in discrete units and it is called quantum (in plural form, quanta). This discrete amount of energy is the product of the Plank constant and the frequency of radiation it represents. The idea of quanta of energy was applied to wave, so that we can investigate the particle nature of waves. For example, the quanta of electromagnetic waves are called photons.

Quasars: It is the short form of the quasi-stellar object. They are small and dim, just like a star. However, near the infrared and radio, they are quite bright. Their spectra show large amount of redshift indicating that they are very far away from us. Thus, we conclude that they radiate enormous amount of energy, even larger than those active galaxies nearby. Quasar is probably also the supermassive black hole at the center of a young galaxy.

Semiconductor: A material that has conductivity between that of a conductor and that of an insulator. Charges can flow through the semiconductor carried by the electrons and by holes. Semiconductors are used to produce such electronic devices as diodes, transistors, and computer memory devices.

Shear Wall: It is an important structure for an anti-earthquake building. It always placed besides the elevator and it is thicker than usual walls (thicker than 25cm, some thicker than 50cm). In Taiwan, since most of the buildings are made of steel-reinforced concrete, the shear walls also made of steel-reinforced concrete.
Because of its higher strength, the shear wall can bear most of the lateral force of the building structure. They can absorb most of the energy by deformation at the beginning of earthquake. So the structure of the building cannot be affected, the resistant to earthquake can be greatly increased.

Sonoluminescence: When a sound wave or ultra-sound wave passes through a liquid, the bubble inside will become luminous. Until now we do not have a perfect theory for this phenomenon.

Surface Tension: It is a property of liquid. There is a tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. The phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid. The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the center of the liquid mass by this net force. The surface then appears to act like an extremely thin membrane, and the small volume of water that makes up a drop assumes the shape of a sphere, held constant when an equilibrium between the internal pressure and that due to surface tension is reached.

Transition: In Quantum Mechanics, the electrons have different "energy levels" in each element. The electron can occupy any one of energy levels, just like you can put a book on any levels of a bookshelf. When an electron changes its energy level, it is called "transition". ("Transition" includes the actions that an electron jumps from a higher energy state to a lower energy state and vice visa.) When an electron in a higher energy level jumps to a lower energy level, energy will be emitted as photon. The energy of the emitted photon is equal to the energy difference between these two levels. While an electron in a lower energy level jumps to a higher energy level, it needs a photon of which energy must equal to the energy difference of these two levels. Each element has many energy levels. We can obtain the "emission spectrum" and "absorption spectrum" from measuring the frequencies of the radiation emitted or absorbed in transitions. As the spectra of each element are unique, we can find out the elements of an unknown object by analyzing its spectra.

Tsunami: Series of catastrophic ocean waves generated by submarine movements, which are caused by earthquakes, volcanic eruptions, or landslides beneath the ocean. In the open ocean, tsunamis may have a height of 30 meters and more. Tsunamis can cause severe destruction on coasts and islands.

White dwarf: It is a stage of a star’s life. When a solar mass star become dead, it will first expand and become a “red giant”. When there is no more nuclear burning, it shrinks. It grows fainter as well as hotter and becomes a "white dwarf". If a star is less massive than about 0.4 solar mass, it will become a white dwarf directly when it dies. A typical white dwarf is slightly smaller than the Earth, but with about the same mass as our Sun. Its density is about 300,000 times of a rock. After it has radiated away all its residue energy, it becomes a "black dwarf".