What is Principle: Definition and 1000 Discussions

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. geshel

    I Holographic Principle and/or implications of the entropy bound

    Hi all, First post here. I'm a casual physics enthusiast, but I've been reading and thinking a lot about this topic lately. The thing I'm most interested in is the fact that black hole formation involves the simultaneous limits of two things: time dilation and the information bound. I find it...
  2. C

    I Intuitive approach to Huygens' principle

    Hi All, Huygens principle has been extended with two independent efforts in order to reform its original feature that gives rise to a back propagating wave. 1) Fresnel proposed the obliquity factor ##(1/2)(1 + \cos\theta)##. 2) Miller proposed two kinds of emissions (dephased). D. Miller...
  3. Like Tony Stark

    Applying D'Alembert's principle to a bead on an elliptical hoop

    Hi I've written D'Alembert's principle as you can see in the attached files. I computed the virtual work done by the weight and the elastic force (since the work done by the normal force is zero) and then I used the fundamental hypothesis, which states that the constraint forces can be written...
  4. E

    A Gravitational Potential Energy & the Equivalence Principle

    First, in section 20.4, after listing all the things gravitational potential energy does not do, they say the equivalence principle forbids it being localized. I thought I understood the equivalence principle, but maybe I don’t. Any comments explaining that would be appreciated. Second, they...
  5. E

    Buoyant Force and Archimedes' Principle

    Hello there, I have a quite different approach on answering the part c and d of this problem. Can you guys help me to see if my method is acceptable or logical? (I got the same answers as the back of the textbook, however my methods differ from the solutions manual). Here is my answer/attempt at...
  6. Y

    Asymmetric bending principle of second moment of A derivation

  7. J

    B Special Relativity & Einstein's Equivalence Principle

    I was recommended a paper: <https://digital.library.adelaide.edu.au/dspace/bitstream/2440/101285/3/hdl_101285.pdf>. And in the opening sentence read: "The Einstein equivalence principle (EEP) is at the heart of special relativity." To me this didn't make sense. Firstly because Einstein...
  8. P

    Principle of superposition for charges/induced charges

    The key observation to solve the above problem is that the charge Q can be dragged out into a flat capacitor plate parallel to the 2 existing plates. Apparently, while the charge distribution on the 2 existing plates changes, the total charge induced on each plate remains the same, due to the...
  9. I

    Chemistry Water and Le Chatelier's Principle

    I think that adding water here shifts the reaction to the left in favour of reactants (since the amount of product is increased). This will also decrease the concentration of [Co(H2O)6]2+, Cl-, and CoCl42− too (dilution), but I'm not sure how much that would affect the direction that the...
  10. cianfa72

    I Principle of relativity: active vs passive point of view

    Hi, starting from this thread I'm a bit confused about the content of the principle of relativity from a mathematical point of view. Basically the "Galilean principle of Relativity" puts requirements on the transformation laws between Inertial Frame of Reference (IFR); thus they have to...
  11. yucheng

    Volume of 4-ball by Cavalieri's principle?

    I tried integrating the 4-volume of a 4-hemisphere, that is, $$\int^{R}_{0} \frac{4}{3} \pi r^3 dw$$ (along w-axis), since ##r## is proportional to ##w##, where ##r=\frac{w}{R} R##, ##r=w##, thus the integral becomes $$\int^{R}_{0} \frac{4}{3} \pi w^3 dw = \frac{\pi}{3} R^4$$ The volume of a 4-D...
  12. K

    Understanding Principle Stress Planes: Exploring Shear Forces and Rotation

    All the materials that I have been reading, it says that when the element is rotated to a θ, this will be called the principle stress plane with no shear force acting on it. but visually when you rotate the element, there will be a shear force acting parallel to the plane. I am very confused...
  13. Pouramat

    Variation principle -- looking for resources to read and understand

    Summary:: Can anyone introduce an informative resource with solved examples for learning variation principle? For example I cannot do the variation for the electromagnetic lagrangian when ##A_\mu J^\mu## added to the free lagrangian and also some other terms which are possible: $$ L =...
  14. C

    Electromagnetism and the Pauli Exclusion Principle

    Electromagnetism in the atoms is why we can't pass through a bank vault. But supposed electromagnetism were canceled for an object, what would happen to the residual or remaining Pauli Exclusion principle? Would it still cause resistance to passing through the vault? On a second scenerio, what...
  15. K

    I Quantities in the Heisenberg Uncertainly Principle

    Hello, I am a Brazilian Physics student and would like to ask a question. Why are not all physical quantities related to each other by the degree of precision in the Heisenberg Uncertainty Principle? For example, why is it possible to determine the energy and position of a particle without its...
  16. F

    Does twisted light violate Fermat's principle?

    The twisted light propagate in curved line, then does twisted light violate Ferma's priciple?And then it violate refraction and refractive laws?
  17. B

    B The relativistic uncertainty principle

    Could this statement be the first step towards quantum gravity? Or is it trivial or not true at all?
  18. H

    A Holographic principle (I'm looking for a paper that I read)

    I read a paper a long time ago. And i forgot who wrote it. The author said that information was not encoded on space time but on the boundaries of regions of space time. there was 3 parts on this boundaries. a space like part (for the initial conditions) . another one for the "out" state...
  19. T

    General Query on Archimedes' Principle

    I had some questions about Archimedes' Principle. This image shows an object being gradually submerged into fluid: As far as I'm aware, when the object is floating, weight of fluid displaced by object = upthrust acting upwards on the object. When the object is fully submerged, volume of the...
  20. A

    Principle of Nuclear Fission Direct Energy Conversion

    Good day everyone, I've recently been researching direct energy conversion schemes for nuclear fission, and I have a question on the basic physics behind the device (specifically on how it converts kinetic energy to electricity). In essence, the "basic" scheme for fission DEC devices is that...
  21. R

    Deriving the Law of Reflection from Huygens Principle

    Attached is section 33.7 from my book, which introduces Huygen's principle in order to derive the law of reflection. I am more used to the ray model rather than the wave model, so I'm constantly going to try to relate everything back to rays. Making this connection also helps with completeness...
  22. iVenky

    B How do you explain a plane wavefront based on huygens' principle?

    Let's assume a plane wave going in the x-direction. Going by Huygens' principle, each point on the wavefront should act like a source. If that's the case, wouldn't plane wavefront become spherical like shown below? I am so confused
  23. Axel Togawa

    I Double slit experiment and the Uncertainty Principle

    With the double slit, experiment we show the double nature of light and matter as wave and particle. In particular, the so called "which way" thought experiment illustrate the complementary principle. In my book, this experiment is analyzed putting a series of particles in front of one of the...
  24. ohwilleke

    I Evidence of Strong Equivalence Principle Violations?

    This paper appears to be a major break though in observational evidence of a strong equivalence principle violation, something predicted in MOND and contrary to general relativity. The analysis is model dependent, but it seems to rule out the most plausible conventional GR based alternatives...
  25. DuckAmuck

    I Is the Heisenberg-Robertson Uncertainty Relation Always Consistent?

    The general uncertainty principle is derived to be: \sigma_A^2 \sigma_B^2 \geq \left(\frac{1}{2} \langle \{A,B\} \rangle -\langle A \rangle \langle B \rangle \right)^2 + \left(\frac{1}{2i} \langle [A,B] \rangle \right)^2 Then it is often "simplified" to be: \sigma_A^2 \sigma_B^2 \geq...
  26. R

    B Understanding Strong Equivalence Principle Errors & Misunderstandings

    Please help me understand all the errors and misunderstandings, contained in the following paragraphs: 1. When a gravitational wave passes through earth, it 'squashes' the form of earth, hence gravity geometry is changed, hence gravity at any given point on Earth change at that moment, be it a...
  27. patric44

    Proof of the generalized Uncertainty Principle?

    hi guys i am trying to follow a proof of the generalized uncertainty principle and i am stuck at the last step : i am not sure why he put these relations in (4.20) : $$(\Delta\;C)^{2} = \bra{\psi}A^{2}\ket{\psi}$$ $$(\Delta\;D)^{2} = \bra{\psi}B^{2}\ket{\psi}$$ i tried to prove these using the...
  28. patric44

    Is this a valid derivation of the Uncertainty Principle?

    Homework Statement:: i saw this simple derivation of the uncertainty principle in my college introductory quantum book Relevant Equations:: Δp.Δx = h hi guys i saw this derivation of the uncertainty principle in my college quantum book , but the derivation seems very simple and sloppy , i...
  29. H

    A Spin-orbital combination and the exclusion principle

    If we combine the two spin-orbital states with ##l=1, s=1/2##, we obtain the combined states with ##J=3/2## and ## J=1/2##. Also, we know that the exclusion principle forbids the two electrons with identical spin-orbital states ##|l,s,m_l,m_s>##. If we combine the two combined states with...
  30. person123

    Bernoulli Principle For Pitot Tubes

    Here is the setup: Apply Bernoulli Principle to the top (free surface) of the two pitot tubes (1 for static and 2 for dynamic with the points colored in red): $$\frac{p_1}{\rho_w g}=h+\frac{p_2}{\rho_w g}$$ The difference in air pressure would give the following:$$p_1=p_2+h\rho_{air} g$$...
  31. U

    I Equivalence principle and the Uniqueness theorem

    We work with Maxwell's equations in the frequency domain. Let's consider a bounded open domain ## V ## with boundary ## \partial V ##. 1. The equivalence theorem tells me that if the field sources in ## V ## are assigned and if the fields in the points of ## \partial V ## are assigned, then I...
  32. wrobel

    A On a formal viewpoint on the D’Alembert–Lagrange Principle

    I would like to discuss the following article: https://web.ma.utexas.edu/mp_arc/c/20/20-87.pdf This article is pure educational and it does not contain any research level results. My aim was to build the theory of the D’Alembert–Lagrange Principle in mathematically closed and modern form...
  33. C

    I Equivalence Principle & Geometry: Ben Crowell's General Relativity

    In the first sentence of Chapter 2 in Ben Crowell's "General Relativity" he states: "The geometrical treatment of space, time, and gravity only requires as its basis the equivalence of inertial and gravitational mass". This is stated as if it's an obvious fact, but I don't understand why. Why...
  34. P

    I Possible paradox in the uncertainty principle?

    In the Heisenberg's uncertainty principle ## \triangle x \triangle p \geqslant \frac{\hbar}{2} ## what happens when the uncertainty in position becomes very small is that the uncertainty in momentum becomes very large. But what happens when the spread of the uncertainty in momentum becomes...
  35. RUTA

    Insights A Principle Explanation of the “Mysteries” of Modern Physics

    Continue reading...
  36. iVenky

    I Maxwell's Demon and the Uncertainty Principle

    Maxwell's demon measures the position and velocity of the particle. How can it do that when it violates the uncertainty principle? Does that mean uncertainty principle is unavoidable otherwise we will violate II law of thermodynamics as in the case of Maxwell's demon?
  37. A

    A Why do we extremize the Lagrangian in the Hamilton principle instead of energy?

    I know that by extremizing lagrangian we get equations of motions. But what if we extremize the energy? I am just little bit of confused, any help is appreciated.
  38. Haorong Wu

    I Variational operator in the least action principle

    Hello. Since I learned the least action principle several years ago, I cannot figure out the difference between the variational operator ##\delta## in ##\delta S=0## and the differential operator ##d## in, say ##dS##. Everytime I encountered the variational operator, I just treated it as a...
  39. BruceAW

    I Apparent Violations of Principle of Relativity?

    I have for a long time been pondering the concept of 'Absolute velocity'. Or, 'Proper motion'. The velocity of an object, with respect to the center of mass of the universe, and the cosmic microwave background radiation. Ways, it seems to make more sense, than merely relative velocities, with no...
  40. H

    B Uncertainty Principle versus spin alignment

    Hello, So I know that the magnetic moments of atoms are dependent on the spin and orbital angular momenta of its electrons. Both of these quantities are limited by the uncertainty principle so that neither of their direction and magnitude can be known simultaneously with arbitrary precision...
  41. M

    How the action-reaction principle works in the case of a screw fastener and a nut?

    A screw fastener advances into a nut. Assuming there are no frictional forces (or negligible): a) Is the action force tangential to the male helical thread (screw) and the reaction tangential (in opposite direction to action) to the female helical thread (nut) b) or does the action-reaction...
  42. jaumzaum

    B Exploring Huygens' Principle and Diffraction Intensity

    Hi. I was studying Huygens Principle and I learned that diffraction usually occurs more when the slit is comparable in size with the wavelength, and this image does not leave my head: We can see that the "middle" part of the wavefront keeps traveling in the same direction, but the wavefront in...
  43. J

    I Equivalence principle: an electric charge and a coil

    The near-range magnetic field ##\vec{B}## of a point charge ##q## at distance ##\vec{r}##, moving at a non-relativistic velocity ##\vec{v}##, is given by $$\vec{B}=\frac{q}{4\pi\epsilon_0c^2}\frac{\vec{v}\times\hat{r}}{r^2}.$$ Faraday's law of induction for the induced EMF ##V_c## in a coil...
  44. wrobel

    A The Lagrange–d'Alembert principle for rigid bodies

    This post is also an invitation to compose problems of the presented below type. The Lagrange–d'Alembert principle by itself is used very seldom. It is usually used to derive the Lagrange equations and that is all. But actually it is a powerful tool in nonholonomic mechanics. As an example...
  45. C

    MHB What is the Pigeonhole Principle Problem in the African Rally?

    The following problem and solution (Problem 7 - African Rally) is presented in "the Art of Mathematics" by Bela Bollobas: Its solution is I am fine until I get to the sentence that starts ... and then I'm lost in trying to understand the use of the i and j indices. Any help...
  46. udtsith12

    Mass loss in Bernoulli's Principle?

    Hi, I could really use help. I am trying to understand what would happen to the velocity of a freestream air if it loses mass while traveling down a tube. For example, suppose that you have 1 cubic meter of air traveling at 10 m/s down a duct with a 1m^2 cross section. And then suppose that you...
  47. C

    A The Cosmological Principle means infinite Observable Universes

    I am posting to ask for any comments about a couple things. I will post a link to a thread on another forum about this. and would love to hear any and all thoughts about it. the thread begins asking about 2 things. the first one is about how fast we are moving. I,402,00 mph relative to the CMB...
  48. hyksos

    A Derive the Principle of Least Action from the Path Integral?

    Several weeks ago I had considered the question as to how one can start from the Schroedinger Equation, and after several transformations, derive F=ma as a limiting case. At some point in my investigations of this derivation, it occurred to me that this is simply too much work. While in...
  49. tanaygupta2000

    Help With Heisenberg's Energy-Time Uncertainty Principle

    So according to Heisenberg's energy-time uncertainty principle, the product of accuracies in energy and time is equal to ћ/2. In this problem, I know I have to calculate ΔE. But when I'm using Δt = 1.4e10 yrs. = 4.41e17 s, I am getting ΔE = 0.743e-33 eV, which is certainly incorrect! Where am I...
  50. A

    A Spin dynamics and the Pauli exclusion principle

    For high temperature superconductivity, people usually say two quasifree electrons are pairing, one is spin up and the other one is spin down. So, if that is the case, each two electrons will have zero spin angular momentum. Since the superconductivity is the magnetic properties and spin is the...
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