Physical constants
- Acceleration of gravity, symbolized as g, has an average value of 9.8
metres (32.2 feet) per second per second near the surface of the Earth.
- Age of the universe: 1 or 2*10E+10 years
- Galaxies: there are10 E+11 can be seen; each of them may contain more
than 10 E+11 stars.
- Metre (definition): it is the distance travelled by light in 0.000000003335640952
second.
- Milky Way galaxy: it is about 10 E+5 light-years across and is slowly
rotating. Its stars orbit around the centre about once every several 10
E+8 years.
- Moon, has a radius of 1,738 kilometres.
- Proxima Centauri, star nearest to earth: four light-years away, 23 E+12
miles.
- Size of the observable universe: 10 to the 24th power miles.
- Solar system: Mercury (innermost planet of the solar system, average distance
from the Sun is about 58 10E+6 km, except for Pluto, it is the smallest
of the nine major planets, having a diameter of about 4,870 km), Venus (Venus
comes closer to the Earth than any other planet, approaching to within about
42 10E+6 km, diameter is about 12,103 km, its mass is approximately 81%
of the mass of the earth), Earth (only planetary body in the solar system
that has conditions suitable for life, a nearly spherical body with an equatorial
radius of slightly more than 6,378 kilometres , one satellite called moon),
Mars (mean distance from the Sun is 228 million km, Mars is a small planet,
having a mean diameter of 6,790 km ), Jupiter (equatorial radius of 71,492
km 318 times as massive as the Earth, its volume exceeds that of the earth
by more than 1,500 times, it has at least 16 satellites,), Saturn (equatorial
radius of 60,268 km, about 95 times as massive as the Earth and has more
than 700 times the volume, it has at least 19 icy satellites), Uranus (equatorial
diameter of 52,000km, 15 times as massive as the Earth, and its volume exceeds
that of the latter by more than 50 times., Neptune (4,504 10E+9 km from
the Sun, equatorial diameter 49,528 km, 17 times as massive as the Earth,
and its volume exceeds that of the latter by 44 times), Pluto (5.9 10E+9
km fro Sun, diameter, 2,300km, one satellite Charon)
- Speed of light in vacuum: 299,792,458 metres per second or 186,282 miles
per second.
- Sun: eight light-minutes away. It is an ordinary yellow star of average
size. It is the dominant body of the system, constituting more than 99 percent
of its entire mass. The Sun is a source of an enormous amount of energy,
a portion of which provides the Earth with the light and heat necessary
to support life. The Sun is a sphere of luminous gas 1,392,000 km in diameter.
Its mass is 1.99 10E+33 grams, or about 330,000 times the mass of the Earth.
The Sun generates energy by nuclear fusion reactions in its core at a rate
of 3.86 10E+33 ergs per second. Although its core temperature is close to
15,000,000 K, the temperature of the surface of the Sun is only about 5,800
K.
- Waves of wavelength:
of a metre or more, radio waves
of a few centimetre, microwaves (?)
of more than a ten thousandth of a centimetre, infra-red
of forty to eighty millionths of a centimetre, visible light
shorter wavelengths known as ultra-violet, X rays and gamma rays
NB: Until 2006 the popular view was that there were nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. However many scientists believe that Pluto has not the proper characteristics to be described as a planet. The International Astronomical Union decided in August 2006 to remove Pluto's status as a planet. From now on, according to this decision, a planet is a body that orbits the sun, is so large that its own gravity makes it more or less spherical and, most important, it dominates its region of the solar system. This is not the case of Pluto. From now on there are only eight planets instead of nine as has been the case for 76 years when Pluto was discovered. Together with three other bodies orbiting the sun (Charon -Pluto's moon-, Ceres and another body named 2003UB313 or Xena) will now be classified as " dwarf planets. The International Astronomical Union is monitoring twelve other possible bodies that could be classified as "dwarf planets" later on. Among these candidates to be upgraded as planets are Varuna, Quaor and Sedna.
Definitions
- Anthropic principle of which there are two versions
. The weak "Anthropic Principle" says that in a large or infinite
in space/time universe the conditions necessary for the development of intelligent
beings is limited to specific regions limited in space and time.
. The strong version of the "Anthropic Principle" states that
there are either many different universes or many different regions in a
single universe, each with its own configuration and, perhaps, its own physical
laws. Only in a few of these many universes or regions are right for the
development of life and intelligent beings.
- Black holes: they are cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole can be formed by the death of a massive star. When such a star has exhausted its internal thermonuclear fuels at the end of its life, it becomes unstable and gravitationally collapses inward upon itself. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity. The singularity constitutes the centre of a black hole and is hidden by the object's "surface," the event horizon. Inside the event horizon the escape velocity exceeds the speed of light, so that not even rays of light can escape into space. Only the most massive stars -those of more than three solar masses- become black holes at the end of their lives. Stars with a smaller amount of mass evolve into less compressed bodies, either white dwarfs or neutron stars. Black holes are difficult to observe on account of both their small size and the fact that they emit no light. They can be "observed," however, by the effects of their enormous gravitational fields on nearby matter.
- Chandrasekhar limit: Chandrasekhar calculated that a cold star of more than 150% of the mass of the sun would not be able to avoid total collapse under its own gravity.
- Confinement principle: the strong nuclear force always binds particles in combinations that have no colour.
- Cosmological constant: a modification introduced by Einstein in his general theory of relativity to make the universe appearing to be static and not expanding. It had the effect of an anti-gravity force without any source but build in the core of space-time.
- Deuterium: heavy hydrogen atom composed of one proton and one neutron.
- Electron volt, the energy acquired by an electron in an electron field of one volt.
- Escape velocity: let us consider a rocket shot vertically from the earth surface. Usually it will reach a certain altitude then falls back on the earth. However if the rocket speed is high enough it will escape in space. The minimum speed for this to happen is called "the escape velocity".
- Euclidean geometry is a mathematical system attributed to the Greek Mathematician Euclid of Alexandria. The method consists of assuming a small set of intuitively appealing axioms and then proving many other propositions (theorems) from those axioms. Although many of Euclid's results had been stated by earlier Greek mathematicians, Euclid was the first to show how these propositions could be fitted together into a comprehensive deductive system.
- Exclusion principle, discovered in 1925 by the Austrian physicist Wolfgang Pauli, states that two similar particles cannot exist in the same state, they cannot have the same position and velocity, within the limits of the uncertainty principle.
- Fundamental postulate of special relativity: it says that the scientific laws should be the same for all freely moving observers and this independently of the speed they are moving at.
- Galilean system of coordinates. In the special theory of relativity certain co-ordinate systems are given preference for the description of the four-dimensional space-time continuum. They are called "Galilean co-ordinate systems." In these systems four coordinates x, y, z and t determine an event, in other words a point, of the four-dimensional continuum. For the transition from one Galilean system to another that is moving uniformly by reference to the first, the equations of the Lorentz transformation are used.
- GeV or gigaelectronvolt means 10E+9 electronvolt (eV).
- Grand unification energy is the energy at which the strong nuclear forces (that decrease at increasing energies) and the electromagnetic and weak nuclear forces (that become stronger when their energies is increased) have the same strength and could then be thought to be three different aspects of the same force. At that energy the matter particles with ½ spin -quarks and electrons- would become the same.
- Parallax: in astronomy, the difference in direction of a celestial object as seen by an observer from two widely separated points. The measurement of parallax is used to find the distance of the body from the Earth. The two positions of the observer and the position of the object form a triangle; if the base line between the two observing points is known and the direction of the object as seen from each has been measured, the apex angle (the parallax) and the distance of the object from the observer can be found simply.
- Planck constant. In 1900, Max Planck was working on the problem of how the radiation an object emits is related to its temperature. He assumed that the energy of a vibrating molecule was quantised -that is, it could only take on certain values. The energy would have to be proportional to the frequency of vibration, and it comes in little "chunks" of the frequency multiplied by a certain constant h, known as Planck's constant and it has the value 6.626*10E-34 Joule/second or 4.135*10E-15 electronvolt/second. This constant does not depend on how the measurements are made, or on the particle type.
- Primordial black holes: the existence of another kind of non-stellar black hole has been proposed by the British astrophysicist Stephen Hawking. According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equal to that of an asteroid or less, might have been created during the big bang. These so-called mini black holes, unlike the more massive variety, lose mass over time and disappear. Subatomic particles such as protons and their antiparticles may be created very near a mini black hole. If a proton and an antiproton escape its gravitational attraction, they annihilate each other and in so doing generate energy -energy drained from the black hole. If this process is repeated again and again, the black hole evaporates, having lost all of its energy and thereby its mass, since these are equivalent.
- Principle of equivalence: this is the extension, in General relativity, of the fundamental postulate of Special Relativity according to which the scientific laws should be the same for all freely moving observers, and this independently of the speed they are moving at. The principle of equivalence goes one step further and applies to observers who are not freely moving but are under the influence of a gravitational field. In summary, it means that in small regions of space, it is impossible to tell if you are at rest in a gravitational field or accelerating uniformly in space.
- Principle of relativity in the restricted sense: if, relative to K and K' we have a non-rotating uniformly moving co-ordinate system, then the same general laws apply to both K and K' in exactly the same way.
- Pulsar, a rotating neutron star emitting pulses of radio waves due to the interaction between their magnetic field and the matter around them.
- Quantum, Quanta, in physics meaning a discrete natural unit, or packet, of energy, charge, angular momentum, or other physical property. Light, for example, appearing in some respects as a continuous electromagnetic wave, is emitted and absorbed in discrete amounts, or quanta; and for light of a given wavelength, the magnitude of all the quanta emitted or absorbed is the same in both energy and momentum. These particle-like packets of light are called photons, a term also applicable to quanta of other forms of electromagnetic energy such as X rays and gamma rays. Sub-microscopic mechanical vibrations in the layers of atoms comprising crystals also give up or take on energy and momentum in quanta called phonons. All phenomena in sub-microscopic systems (the realm of quantum mechanics) exhibit quantisation: observable quantities are restricted to a natural set of discrete values. When the values are multiples of a constant least amount, that amount is referred to as a quantum of the observable. Thus Planck's constant h is the quantum of action, and (i.e., h/2p) is the quantum of angular momentum, or spin.
- Quasars, cosmic objects of high luminosity as well as strong radio emission observed at extremely great distances. Quasars are no more than a light-year or two in size, but they are up to 1,000 times more luminous than giant galaxies that have a diameter of about 10E+5 light-years. The tremendous brilliance of quasars allows them to be observed at distances of more than 10E+10 light-years. This enormous amount of radiation is released from a small area at the centre of a quasar. These emission lines are always shifted toward the red, corresponding to large Doppler velocities of recession.
- Scientific theory: A theory is a model of an event or a field of interest
(for example the universe, or part of it) and rules that relates parameters
in the model to observations or experiences. A theory is acceptable if the
model, on the base of its limited parameters:
. Describes with accuracy the observations already available.
. Predicts with accuracy future observations.
If one observation disagrees with the predictions of the model, the theory
becomes invalid and must be discarded or modified. Most physical theories
are provisional, there are hypothesis that cannot be proven. More often
that not they are also extensions of previous theories. It has been proven
impossible, until now, to find a theory that describes accurately the whole
of the universe events. Scientists had to limit themselves to produce partial
theories and these have been very successful in their limited field of application.
For example:
. Aristotle theory that everything is made of four elements (earth, air,
fire and water) does not allow any prediction and for this reason is useless.
. On the other hand, Newton's theory of gravitation allowed scientists to
predict the sun, moon and planets' trajectories with a reasonable accuracy.
This was an acceptable theory at its time.
. Einstein's theory of relativity is more accurate that Newton's theory
to calculate the motion of celestial bodies. It is therefore a better theory.
In this case, however, the differences between both predictions are very
small and for many applications Newton's theory, that is much simpler, is
still acceptable and used.
. Exponents: 10 at the 9th power will be written 10 E+9; 10 at the nth power,
10 E+n; 10 at the -nth power (or 1/10 at the nth power), 10 E-n.
- Uncertainty principle, proposed in 1926 by the German physicist Werner Heinsenberg. It says that the more accurately one measures the position of a particle, the less accurate is the measurement of its velocity, and vice versa. Moreover the uncertainty in the position of the particles multiplied by the uncertainty of its velocity multiplied by the mass of the particle can never be smaller than a certain value that is known as Planck's constant.
Most new theories are extension or modifications of older ones.