- Autoinfanticide Paradox: that is, going back in time and killing oneself as a baby.
- Black hole Information Paradox: It results from the combination of quantum mechanics and general relativity. It suggests that physical information could "disappear" in a black hole. It is a contentious subject since it violates a commonly assumed tenet of science -that information cannot be destroyed.
- EPR paradox: It is a thought experiment. In quantum mechanics which challenges
long-held ideas about the relation between the observed values of physical
quantities and the values that can be accounted for by a physical theory.
The EPR experiment yields a dichotomy. Either
1. The result of a measurement performed on one part A of a quantum system
has a non-local effect on the physical reality of another distant part B,
in the sense that quantum mechanics can predict outcomes of some measurements
carried out at B; or...
2. Quantum mechanics is incomplete in the sense that some element of physical
reality corresponding to B cannot be accounted for by quantum mechanics
(that is, some extra variable is needed to account for it.)
- Grandfather paradox: It is a paradox of time travel: suppose a man travelled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox. The grandfather paradox has been used to argue that backwards time travel must be impossible. However, a number of possible ways of avoiding the paradox have been proposed, such as the idea that the timeline is fixed and unchangeable, or the idea that the time traveller will end up in a parallel timeline, while the timeline in which the traveller was born continues to exist.
- Hubble law: It is a statement in physical cosmology which states that the redshift in light coming from distant galaxies is proportional to their distance. It is considered the first observational basis for the expanding space paradigm and today serves as one of the most often cited pieces of evidence in support of the Big Bang.
- Kepler's Laws of Planetary Motion are three mathematical laws that describe
the motion of planets in the Solar System. Kepler used the observations
of the Danish astronomer Tycho Brahe. Kepler's laws challenged Aristotelean
and Ptolemaic astronomy and physics. His assertion that the Earth moved,
his use of ellipses rather than epicycles, and his proof that the planets'
speeds varied, changed astronomy and physics. Nevertheless, the physical
explanation of the planets' behaviour came almost a century later, when
Isaac Newton deduced Kepler's laws from Newton's own laws of motion and
his law of universal gravitation.
Kepler's three laws are:
1. The orbit of every planet is an ellipse with the sun at one of the foci.
Thus, Kepler rejected the ancient Aristotelean, Ptolemaic, and Copernican
belief in circular motion.
2. A line joining a planet and the sun sweeps out equal areas during equal
intervals of time as the planet travels along its orbit. This means that
the planet travels faster while close to the sun and slows down when it
is farther away. This contradicts the Aristotelean theory that planets have
uniform velocity.
3. The squares of the orbital periods of planets are directly proportional
to the cube of the semi-major axis (the "half-length" of the ellipse)
of their orbits meaning that larger orbits have longer periods and that
the speed of a planet in a larger orbit is lower than in a smaller orbit.
- Law of conservation of energy: It states that the total amount of energy in any isolated system remains constant but cannot be recreated, although it may change forms, e.g. friction turns kinetic energy into thermal energy. In thermodynamics, the first law of thermodynamics is a statement of the conservation of energy for thermodynamic systems. This states that energy can not be created or destroyed, it can only be changed from one form to another.
- Law of conservation of mass/matter (or the Lomonosov-Lavoisier law): It states that the mass of a closed system will remain constant, regardless of the processes acting inside the system. An equivalent statement is that matter cannot be created/destroyed, although it may be rearranged. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. The law of mass/matter conservation may be considered as an approximate physical law that holds only in the classical sense before the advent of special relativity and quantum mechanics.
- Olbers' paradox: It is the argument that the darkness of the night sky conflicts with the supposition of an infinite and eternal static universe. It is one of the pieces of evidence for a non-static Universe such as the current Big Bang model. This "paradox" is sometimes also known as the "dark night sky paradox".
- Schrödinger equation: It is an equation that describes how the quantum state of a physical system varies. Schrödinger's equation is primarily applied to microscopic systems, such as electrons and atoms, but is sometimes applied to macroscopic systems (such as the whole universe). The Schrödinger equation is commonly written as an operator equation describing how the state vector evolves over time. By specifying the total energy (Hamiltonian) of the quantum system, Schrödinger's equation can be solved, the solutions being quantum states.
- Stefan-Boltzmann law, also known as Stefan's law, states that the total
energy radiated per unit surface area of a black body in unit time is directly
proportional to the fourth power of the black body's thermodynamic temperature
T (absolute temperature):
A more general case is of a grey body, the one that doesn't absorb or emit
the full amount of radioactive flux. Instead, it radiates a portion of it,
characterized by its emissivity, ?:
The constant of proportionality ?, called the Stefan-Boltzmann constant
or Stefan's constant, is non-fundamental in the sense that it derives from
other known constants of nature. The value of the constant is
where k is the Boltzmann constant, h is Planck's constant, and c is the
speed of light in a vacuum. The law was deduced by Jožef Stefan (1835-1893)
in 1879 on the basis of experimental measurements made by John Tyndall and
was derived from theoretical considerations, using thermodynamics, by Ludwig
Boltzmann (1844-1906) in 1884.
- Twin paradox: It refers to a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he or she has aged less than an identical twin who stayed on Earth. This result appears puzzling, since the situation seems symmetrical, as the latter twin can be considered to have done the travelling with respect to the former. In fact, there is no contradiction and the apparent paradox is explained within the framework of relativity theory, and has been verified experimentally using precise measurements of clocks flown in airplanes.
- Wave function or wavefunction: It is a mathematical tool used in quantum
mechanics to describe any physical system. It is a function from a space
that maps the possible states of the system into the complex numbers. The
laws of quantum mechanics (i.e. the Schrödinger equation) describe
how the wave function evolves over time. The values of the wave function
are probability amplitudes the squares of the absolute values of which,
give the probability distribution that the system will be in any of the
possible states.