- Anthropic principle: It states that humans should take into account the constraints that human existence as observers imposes on the sort of universe that could be observed. In other words, the only universe we can see is one that supports life. If it were a different type of universe, we would not exist to see it.
- Computational equivalence Principle: The principle states that systems found in the natural world can perform computations up to a maximal ("universal") level of computational power. Computation is therefore simply a question of translating inputs and outputs from one system to another. Consequently, most systems are computationally equivalent. Proposed examples of such systems are the workings of the human brain and the evolution of weather systems.
- Copernican principle: It states the Earth is not in a central, specially favoured position. More recently, the principle is generalised to the relativistic concept that humans are not privileged observers of the universe.
- Cosmological Principle: It is a principle invoked in cosmology that,
when applied, severely restricts the large variety of possible cosmological
theories. It follows from the observation of the Universe on a large scale,
and states that:
. On large spatial scales, the Universe is homogeneous and isotropic.
. Or simply put, the universe is the same everywhere on a large scale.
- Equivalence principle: In relativity it refers to several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
- Heisenberg uncertainty principle: In quantum physics it is the statement
that locating a particle in a small region of space makes the momentum of
the particle uncertain; and conversely, that measuring the momentum of a
particle precisely makes the position uncertain. In quantum mechanics, the
position and momentum of particles do not have precise values, but have
a probability distribution.
A mathematical statement of the principle is that every quantum state has
the property that the root-mean-square deviation of the position from its
mean (the standard deviation of the X-distribution):
times the RMS deviation of the momentum from its mean (the standard deviation
of P):
can never be smaller than a small fixed multiple of Planck's constant:
This means that if an observer measures the position of a particle with
accuracy ?X, the state of the particle immediately after the measurement
has .
- Holographic principle: It is a speculative conjecture about quantum gravity theories claiming that all of the information contained in a volume of space can be represented by information which lives in the boundary of that region. In other words, if you have an empty sphere, all of the events within can be explained by the arrangement of information on the surface of the sphere. In a larger sense, the theory suggests that the entire universe can be seen as a two dimensional information structure "painted" on a boundary surface, and that the three dimensions we observe are illusory. The holographic principle also states that at most there is one degree of freedom (or 1 Boltzmann constant k unit of maximum entropy) for every four Planck areas in that theory, i.e. in natural units. Many physicists suggest that 11-dimensional M-theory could ultimately form a complete theory of everything.
- Galilean invariance or Galilean relativity is a principle of relativity which states that the fundamental laws of physics are the same in all inertial frames. Galileo Galilei first described this principle in 1632 in his Dialogue Concerning the Two Chief World Systems using the example of a ship travelling at constant speed, without rocking, on a smooth sea; any observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary. The fact that the earth on which we stand orbits around the sun at approximately 30 km/s offers a somewhat more dramatic example.
- Lorentz covariance Principle: It is a key property of space-time that
follows from the special theory of relativity, where it applies globally.
Local Lorentz covariance refers to Lorentz covariance applying only locally
in an infinitesimal region of space-time at every point, which follows from
general relativity. Lorentz covariance has two distinct, but closely related
meanings.
1. A physical quantity is said to be Lorentz covariant if it transforms
under a given representation of the Lorentz group.
2. An equation is said to be Lorentz covariant if it can be written in terms
of Lorentz covariant quantities. The key property of such equations is that
if they hold in one inertial frame, then they hold in any inertial frame.
This condition is a requirement according to the principle of relativity,
i.e. all non-gravitational laws must make the same predictions for identical
experiments taking place at the same space-time event in two different inertial
frames of reference.
- Mach principle: A Mach principle is any of a class of principles. The
notion is that "mass there influences inertia here". Any statement
which follows in this spirit may be classified as a "Mach principle".
Their correctness depends on the theory of gravity, though Einstein's general
relativity is the most frequently discussed theory. Here are a few examples:
Mach0: The universe, as represented by the average motion of distant galaxies
does not appear to rotate relative to local inertial frames.
Mach1: Newton's gravitational constant G is a dynamical field.
Mach2: An isolated body in otherwise empty space has no inertia.
Mach3: Local inertial frames are affected by the cosmic motion and distribution
of matter.
Mach4: The universe is spatially closed.
Mach5: The total energy, angular and linear momentum of the universe are
zero.
Mach6: Inertial mass is affected by the global distribution of matter.
Mach7: If you take away all matter, there is no more space.
Mach8: is a definite number, of order unity, where ? is the mean density
of matter in the universe, and T is the Hubble time.
Mach9: The theory contains no absolute elements.
Mach10: Overall rigid rotations and translations of a system are unobservable.
- Occam's razor or Ockham's razor: This principle states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory. This is often paraphrased as "All other things being equal, the simplest solution is the best." In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest entities.
- Pauli exclusion principle: It is a quantum mechanical principle formulated stating that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that, for two identical fermions, the total wave function is anti-symmetric. For electrons in a single atom, it states that no two electrons can have the same four quantum numbers, that is, if n, l, and ml are the same, ms must be different such that the electrons have opposite spins.
- Perfect Cosmological Principle: states that the Universe is homogenous and isotropic in space and time. In this view the universe looks the same everywhere (on the large scale) as it always has and always will. It is the principle underpinning steady-state theory and chaotic inflation theory. The Perfect Cosmological Principle is an extension of the Cosmological Principle, which accepts that the universe changes its gross feature with time, but not in space.
- Relativity Principle: It is a criterion for judging physical theories,
stating that they are inadequate if they do not prescribe the exact same
laws of physics in certain similar situations. These types of principles
have been successfully applied throughout science, implicitly in Newtonian
mechanics or explicitly in special and general relativity.