Principle Definition and 1000 Threads

A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed. The principles of such a system are understood by its users as the essential characteristics of the system, or reflecting system's designed purpose, and the effective operation or use of which would be impossible if any one of the principles was to be ignored. A system may be explicitly based on and implemented from a document of principles as was done in IBM's 360/370 Principles of Operation.
Examples of principles are, entropy in a number of fields, least action in physics, those in descriptive comprehensive and fundamental law: doctrines or assumptions forming normative rules of conduct, separation of church and state in statecraft, the central dogma of molecular biology, fairness in ethics, etc.
In common English, it is a substantive and collective term referring to rule governance, the absence of which, being "unprincipled", is considered a character defect. It may also be used to declare that a reality has diverged from some ideal or norm as when something is said to be true only "in principle" but not in fact.

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  1. D

    Deriving geodesic equation using variational principle

    I am trying to derive the geodesic equation using variational principle. My Lagrangian is $$ L = \sqrt{g_{jk}(x(t)) \frac{dx^j}{dt} \frac{dx^k}{dt}}$$ Using the Euler-Lagrange equation, I have got this. $$ \frac{d^2 x^u}{dt^2} + \Gamma^u_{mk} \frac{dx^m}{dt} \frac{dx^k}{dt} =...
  2. K

    Pascal's Principle: Valid w/o Gravity?

    Is Pascal's Principle valid with no gravity?
  3. T

    Question about Huygen's principle

    Huygens principle states that every point on a wave is a source of a secondary wavelet, so when you shine a light at a single slit, you get fringe patters. How come when you shine a normal light at a wall (without any slit), you don't get fringe patters? Surely, if every point on a wave is a...
  4. Stephanus

    Determinism and Uncertainity Principle

    Dear PF Forum, Determinism. 1. Is there determinism? 2. Because we can't measure the electron position, it doesn't mean there's no determinism? Heisenberg uncertainity principle states, per wiki: But because we can't measure it, it doesn't mean that the universe itself is indeterministic...
  5. Strilanc

    Criticism of blog post on quantum pigeonhole principle

    I wrote a blog post explaining the quantum pigeonhole paper (but critical of their interpretation of that result). As part of trying to improve at writing, I'm requesting criticism. What was confusing, what was good, what was rushed, what was wrong, etc. Also discussion about the quantum...
  6. D

    Non-Euclidean geometry and the equivalence principle

    As I understand it, a Cartesian coordinate map (a coordinate map for which the line element takes the simple form ##ds^{2}=(dx^{1})^{2}+ (dx^{2})^{2}+\cdots +(dx^{n})^{2}##, and for which the coordinate basis ##\lbrace\frac{\partial}{\partial x^{\mu}}\rbrace## is orthonormal) can only be...
  7. F

    Use the uncertainty principle for momentum vs. position to e

    Homework Statement Please see the attachment for a better picture The energy of an electron in a hydrogen atom is: E = p^2/2m - αe2/r; where p is the momentum, r the orbital radius, me the electron mass, e the electron charge, and α the Coulomb constant. Use the uncertainty principle for...
  8. F

    How does the Pauli Exclusion principle explain ionization E?

    How does the Pauli-exclusion principle explain ionization energy trends? Is it just that as you move down the periodic table, the electrons experience repulsion between each other, and thus the atoms get bigger?
  9. C

    What if the Principle of Least Action were different?

    What would the world be like if the Principle of Least Action were different? Let's say that it minimized a different quantity than KE - PE. EF Taylor et al argue that if the quantity minimized were KE + PE, physical systems would accelerate apart from one another. Here's their short articlet...
  10. S

    Principle of least action in field theory

    In page 15, Peskin and Schroeder states that The principle of least action states that when a system evolves from one given configuration to another between times ##t_1## and ##t_2##, it does so along the path in configuration space for which ##S## is an extremum. What is the definition of...
  11. N

    Archimedes Principle: Calculating Relative Density for a Block of Oak and Lead

    . Homework Statement A block oak weighs 90 N in air. A piece of lead weighs 130 N in water. The two weigh together 100N in water. Calculate the relative density of the wood.Homework Equations F=mxg F(archimedes)= density x Volume x g The Attempt at a Solution F (g)oak = 90 N F= mxg m= 90N/9.81...
  12. C

    Archimedes principle and passing gas

    I used a tub for my hemorrhoid. I learned the archimedes principle which the buoyant force is equal to the mass of water displaced. The tub was filled with water 4/5. I accidently farted in a tub and suddenly the water flew into the hole which is located on the top of tub.does the fart gas...
  13. G

    Huygens principle in odd/even dimensional flat space

    A well known math theorem says that - if the spatial dimension is odd - D'Alembert equation gives rise to a solution containing a term which is completely supported on the light cone. A mathematical wrap up could be the following: "in dimension 3 (and in fact, for all odd dimensions), the...
  14. D

    Archimedes principle boat displacement

    Homework Statement 35m^3 of a boat's volume is submerged under water. What does the boat weigh? The boat is 14 metres long. Homework Equations pVg=mg The Attempt at a Solution pVg=mg pV=m m=pV=1000*35 This is what my teacher said, but I don't understand it. The submerged volume of the boat...
  15. V

    Breakdown of Correspondence Principle: Null Dust Case

    In both quantum and general relativity theories we are used to provide results in the "limited" conditions to demonstrate a correspondence between new and old formalism. For instance deflection of light of a star due to Sun in GR is double the amount given in classical theory. Yet I have...
  16. Negatratoron

    Pairs of observables in uncertainty principle

    Observables are paired up in the uncertainty principle such that we can't measure both to a high degree of accuracy. Specifically, ## \sigma_x \sigma_y>\frac{\hbar}{2} ## where ## \sigma_x ## and ## \sigma_y ## are the standard deviations of our measurements. I've got two lines of questions...
  17. P

    Fermat's principle can be derived from Maxwell equations?

    As we know, the Fermat's principle states: Light takes the path of least time. I wonder whether Fermat's principle can be derived from Maxwell equations. If it can, then Fermat's principle is included in Maxwell equations, or Fermat's principle is not an independent postulate.
  18. J

    Principle of relativity and invariance of c

    First postulate (principle of relativity) and 2. Second postulate (invariance of c), affect not we continue investigating new things?
  19. bcrowell

    A Andrei Lebed, equivalence principle

    Andrei G. Lebed, "Breakdown of the equivalence between gravitational mass and energy for a composite quantum body," http://arxiv.org/abs/1404.4044 [corrected a mistake in the following paragraph] He seems to argue that hydrogen atoms moving from one region of space to another, with a different...
  20. entropy1

    Perceived contradiction in non-locality principle

    I have had some criticism on a post of mine in another topic. Since I don't want to pollute that thread with my own discussion, and since I am a layman and am really curious about the answer, I'll pose my question here. Consider two polarisation-entangled photons A and B measured by Alice and...
  21. preitiey

    Uncertainty principle at the macroscopic level

    what will be the scenario if Heisenberg's uncertainty principle is applied at macroscopic level?
  22. D

    Principle behind electrostatic shielding?

    if we have a solid conducting sphere with charges around it, then the elctric field inside the sphere is zero otherwise the electrons of the sphere would not be in equilibrium as there would be a net force acting on it. however if its a hollow sphere then why does the electric field inside the...
  23. Anton Alice

    Fermat's principle seems indefinite

    Hello forum, please take a look at the following picture: It's a salt solution, with increasing refractive index, as you go down the solution. How can I explain this with Fermat's principle? Let's set the starting point A to the point, where the laser beam penetrates the left wall of the...
  24. J

    Correspondence principle and its use

    Can you give me some examples, link of correspondence principle use in life. All I have found out are theoretical examples such as Bohr's model... If I understand right, correspondence principle says that simple equations used for classical examples with a few data and quantum equations used...
  25. S

    Einstein equivalence principle

    I can not understand what exactly this principle says.while reading a book about it ,it was written that einstein abolished gravity by saying the lift is free falling can some explain clearly what does that mean?
  26. A

    Violation of Pauli's Exclusion Principle possible?

    I was reading some simplistic explanations of Pauli's Exclusion Principle (PEP) to explain a group of non-science people, and I came across this: For Fermions, even as pressure builds, no two can be located in the same energy state. This causes them to "stack up" in effect. Only under great...
  27. nazmus sakib

    Uncertainty principle (stationary state)

    I have to derive I did all the way but stuck with the ">=" to "=" sign. what is the logic behind it ? is it safe to write "approximate" while taking the square-root on both side ? or the energy term "V" has gone during the calculation so it has only momentum "p" ?
  28. R

    Some questions on the Cosmological Principle

    Firstly, I am just a very interested layman so please forgive my ignorance and non mathematical approach. As I understand it, the cosmological principle states that on larger scales the universe is homogeneous and isotropic. So could someone help me to understand the following: 1) Does this...
  29. W

    Uncertainty Principle And Collapse Wavefunction

    Upon a measurement of the position, the wavefunction collapses to a spike centered at x0 https://farside.ph.utexas.edu/teaching/315/Waveshtml/img3240.png I encounter similar spike pictures numerous times, but there is an uncertainty in position , it can't be a spike right. First thing I see...
  30. A

    Why Don't Spherical Waves Interfere: Exploring Huygens Principle

    Hi according to Huygens principle every point on the wave front acts as a spherical source. so if a we emit monochromatic light on a screen without passing light from grating, we should see interference pattern but we don't. why don't these spherical waves interfere with each other? is this...
  31. Multiple_Authors

    Does the Heisenberg Uncertainty Principle Imply Energy Nonconservation?

    Multiple_Authors submitted a new PF Insights post Does the Heisenberg Uncertainty Principle Imply Energy Nonconservation? Continue reading the Original PF Insights Post.
  32. H

    Is the equivalence principle related to the principle of relativity?

    Here is what I understand of the equivalence principle. However, given that the concept is still a bit cloudy to me, please tell me if I am wrong: The equivalence principle is a phenomena where all inertial objects experience the same laws of physics. In other words, if you were going at...
  33. V

    Solving Force Exerted by Liquid Using Archimedes Principle

    Homework Statement Homework EquationsThe Attempt at a Solution Applying Archimedes priniciple the force exerted by liquid on the upper hemisphere would be ## F = \frac{2}{3}\pi R^3 \rho g## . Now I am not sure , what does it mean by force due to gauge pressure ? Any help is appreciated ...
  34. S

    Width of principle maxima in n slit diffraction

    I was working on derivation to find the witdh of principle maxima in n slit diffraction Angular width=2Δθm=2λ/(Ndcosθm) Where d:distance between slits θm:angle at which mth order principle maxima is present. N:no. Of slits #in the diagram given below why the minima just adjacent to mth...
  35. T

    Bernoulli principle and fluid particle

    Hello, Bernoulli principle describes the flow of a fluid for steady, incompressible flow along a streamline. But it is said for a particle of a fluid along a streamline. My question is a particle of fluid refers to a molecule or a group of molecules? Thank you
  36. DeldotB

    Principle Ideals of a Polynomial Quotient Ring

    Homework Statement Let A be the algebra \mathbb{Z}_5[x]/I where I is the principle ideal generated by x^2+4 and \mathbb{Z}_5[x] is the ring of polynomials modulo 5. Find all the ideals of A Let G be the group of invertible elements in A. Find the subgroups of the prime decomposition.Homework...
  37. E

    I What is the Principle of Equivalence and how was it determined?

    Equivalence principle says that gravitational forces are equivalent physically to inertial forces. Can someone explain what is meant by that and how was it concluded?
  38. W

    Geometric optics and Fermat's principle

    Homework Statement A ray travels as shown in the image attached below. In this case, Fermat's principle may be written as ##A =\frac{n(1+ay)}{\sqrt{1+(y')^2}}## Where y' is dy/dx, n is the index of refraction and A is a real constant. The trajectory of a ray of light is given by ##y =...
  39. A

    Heisenberg's Uncertainty Principle in quantum computing

    My question is how does the uncertainty principle relate to quantum computers? Does it hinder the theoretical production of a quantum computer?
  40. Urs Schreiber

    Insights Higher Prequantum Geometry II: the Principle of Extremal Action - Comments

    Urs Schreiber submitted a new PF Insights post Higher Prequantum Geometry II: The Principle of Extremal Action - Comonadically Continue reading the Original PF Insights Post.
  41. ohwilleke

    LQG has not been shown to violate the holographic principle

    Backreaction has the story: http://backreaction.blogspot.com/2015/09/no-loop-quantum-gravity-has-not-been.html debunking this article: Violation of the Holographic Principle in the Loop Quantum Gravity Ozan Sargın, Mir Faizal http://arxiv.org/abs/1509.00843 and also showing the authors behaving...
  42. C

    A Question on Goldstein and D'Alembert's Principle

    Hey all, I am reading Goldstein and I am at a point where I can't follow along. He has started with D'Alembert's Principle and he is showing that Lagrange's equation can be derived from it. He states the chain rule for partial differentiation: \frac{d\textbf{r}_i}{dt}=\sum_k \frac{\partial...
  43. H

    Uncertainty principle and double slit

    sir, what if we use Heisenberg's uncertainty principle to explain double slit experiment? i was reading about uncertainty principle for last few days n i was reading about double slit experiment and i realize that we can explain why we get electrons everywhere on the screen (getting...
  44. GiantSheeps

    Experimental Error in Archimedes Principle Lab

    Homework Statement I need to find three potential reasons for my .5% error in a lab where I used the Archimedes Principle to measure the density of Iron. We only used a graduated cylinder full of water and an iron mass. I measured the water level from the meniscus. We used two different...
  45. amjad-sh

    Principle of least action ...some confusion

    Principle of least action states that the particle choose the path which optimizes its action.Where the action is defined by S=t1∫t2(L)dt and L is the Lagrangian of the system.This leads to δS=0 and it is a condition to optimize S. I will summarize what confuses me about this principle in a...
  46. Samuel Williams

    Inclusion-Exclusion principle problem

    Use inclusion-exclusion to find the number of ways to arrange the six numbers 1, 2, 3, 4, 5, 6 such that either 1 is immediately followed by 2, or 3 is immediately followed by 4, or 5 is immediately followed by 6. I believe that this can be solved using unions. By setting the sets to be the...
  47. 24forChromium

    What is the principle of algebra called?

    I don't even know what is the general name for the thing whose specific name I am trying to find out, but in an algebra question, say 3x = 6y, there is the "assumption" that by dividing both side by 3, we will get x = 2y and this equation is as correct as the original one. What is thing that...
  48. isnainidiah

    Fermat's Principle to explain lenses and mirrors

    How do we use Fermat's principle of least time to explain the required shapes of lenses and mirrors?
  49. B

    Pascal's Principle Homework: Pressure Variation w/ Height & Diameter

    Homework Statement A small tube is connected to the top of a larger one and the whole thing is filled with water. The small tube has height a and the larger tube has height b. What happens to the pressure at the bottom of the larger tube as (1) a is varied, and (2) a is held constant but the...
  50. yazanhomsi

    Who First Conceived the Idea of the Cosmological Principle?

    Good day all, A question that I haven't really found the answer for yet: "Whom did first come up with the idea of the cosmological principle?" I almost looked everywhere for the answer but still cannot find it. Was it Friedmann or Lemaitre? Or was it some other great physicist? But there...
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