# What is General: Definition and 1000 Discussions

A general officer is an officer of high rank in the armies, and in some nations' air forces, space forces, or marines.In some usages the term "general officer" refers to a rank above colonel.The term general is used in two ways: as the generic title for all grades of general officer and as a specific rank.
It originates in the 16th century, as a shortening of captain general, which rank was taken from Middle French capitaine général.
The adjective general had been affixed to officer designations since the late medieval period to indicate relative superiority or an extended jurisdiction.
Today, the title of general is known in some countries as a four-star rank. However, different countries use different systems of stars or other insignia for senior ranks. It has a NATO rank scale code of OF-9 and is the highest rank currently in use in a number of armies, air forces, and marine organizations.

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1. ### Compton scattering in a general medium

For part one, my energy conservation equation is nhf0 + mc2 = nhf' + E my momentum conservation in x-axis is nhf0= nhf' cos(theta) + c𝒑 cos(fi) My momentum conservation in y-axis is nhf' sin(theta) = c𝒑 sin(fi) For part 2 I understand that I am supposed to get a qudratic equation in terms of...
2. ### I Finding position vector in general local basis

How do you derive the position vector in a general local basis? For example, in spherical coordinates, it's ##\vec r =r \hat {\mathbf e_r}##, not an expression that involves that involves the vectors ## {\hat {\mathbf e_{\theta}}}## and ## \hat {{\mathbf e_{\phi}}}##. But how would you show this?
3. ### Does the Universe have a finite number of fundamentals to uncover?

Fire, Gravity, Electromagnetism, Atoms, DNA, Steam power, Nuclear, Quarks. All of these things have one thing in common. They are fundamental aspects of the universe that humans have uncovered and given names. However, all of these great discoveries occured quite some time ago and as a science...
4. ### B I'm trying to find a general formula for a harmonic(ish) series

Im looking for a general formula for the partial sum of a series where the nth term is in the form of a_n = 1/(c+kn), where c and k are arbitrary constants. it "looks" like a harmonic series but not in the form I'm capable of figuring out. help.

6. ### General form of Newton II -- Not understanding this step in the derivation

For this, Does someone please know how do we derive equation 9.9 from 9.8? Do we take the limits as t approach's zero for both sides? Why not take limit as momentum goes to zero? Many thanks!
7. ### I Calculating the Hodge Star in the most general case

I have been learning a little Differential Geometry lately and recently came across the Hodge Star. So far I have been unable to find an explanation for its calculation that I can understand. I believe some ways of computing it were only valid in an orthonormal basis, but I would like to be able...
8. ### I Adler's New Book on Relativity: Misprint on Page 16?

I bought a copy of Adler's new book on relativity. Is there a misprint on page 16 regarding the Lorentz metric = diag (-1,-1,-1,-1) or am I missing something? The text itself after the equation suggests it is the same metric as on the earlier page so that index placement, two lower indices vs...
9. ### Can I find a general solution to this circuit?

TL;DR Summary: I have to find an equivalent resistance of the circuit below, dependent on the amount of ##R_3## - resistors. Here is the circuit: I think there is no general solution. When I want to calculate it, I have to do...
10. ### General relativity - Using Ricc and Weyl tensor to find the area

I have the following question to solve:Use the metric:$$ds^2 = -dt^2 +dx^2 +2a^2(t)dxdy + dy^2 +dz^2$$ Test bodies are arranged in a circle on the metric at rest at ##t=0##. The circle define as $$x^2 +y^2 \leq R^2$$ The bodies start to move on geodesic when we have $$a(0)=0$$ a. we have to...
11. ### A Special & General Relativity: CTC Metrics in Cylindrical Coordinates

I am studying metrics that exhibit CTCs. I was looking at a few different metrics... Tipler's solution Godel metric Kerr metric For starters to compare them, I am trying to convert said metrics into cylindrical coordinates. Thanks in advance for any help😃
12. ### Relativity Recommended general relativity textbook

Hi, I am looking to study general relativity at my own steam (currently finishing 1st year physics at Warwick) during the summer. What textbook(s) would you recommend? I've heard good things about A. Zee's 'Einstein Gravity in a Nutshell'- is that worth it, and would it be suitable for someone...
13. ### Deriving general specific heat capacity formula

For this, Dose anybody please know of a better way to derive the formula without having ##c = \frac{\Delta Q}{m \Delta T}## then taking the limit of both sides at ##\Delta T## approaches zero? I thought ##\Delta Q## like ##\Delta W## was not physically meaningful since by definition ##Q## is...
14. ### I Determinism & General Relativity: A Physics Primer

I know nothing about physics, to be clear. My friend was saying due to general relativity, the faster you move through space, the slower you move through time. Objects with a heavy mass (like a blackhole) can distort the fabric of space time and being near its gravitational pull means that you...
15. ### Science of Evaporation - General & Personal Questions

Wasn't sure where to categorize this thread, but thought chemistry would be the most appropriate (mods: feel free to move as you see fit). I have some questions on the science of water evaporation - first in general terms and then within the context of something that happened to me. General...
16. ### A Solving "const. K" in Gourgoulhon's SR in General Frames

In sect. 2.3.2 "Ideal Clock", p. 33, of É. Gourgoulhon's text book on "Special Relativity in General Frames" $$\tau_C [ \, \text{tick}_{j}, \text{tick}_{(j + N)} \, ] = K_C \, N.$$ (equation (2.11); notation adapted.) The only other reference to this "constant K" is on the following page...
17. ### Searching for information on the General Electric J31 engine

Hi, I am currently trying to do a project related to this engine and I am trying to find information about it. I am looking for technical papers, journals or overall description of the inner workings of the engine. I am looking forward for something more in depth than wikipedia. Also...
18. ### B Is a Spin 2 Particle the Key to Understanding Gravitation in Quantum Mechanics?

I have some questions regarding the expected exchange particles for gravitation. From my understanding the following was valid: We can linearize the equations of GTR for weak fields "Quantum mechanics" (Schrödinger, Dirac equations) are linear Those linear equations allow eigenstates and...
19. ### Relativity Lenny Susskind Has Done It Again - General Relativity

This time with General Relativity: https://www.amazon.com/dp/1541601777/?tag=pfamazon01-20 I got a copy as soon as I noticed it. And it is good - as all his books are. Notice - number one best seller. Lenny deserves a medal. There is a genuine thirst for science beyond banal...
20. ### GTRPy - A python package for General Relativity Related Calculations

Hello, everyone I am now working on this project quite a while now and I just wanted to share it with this forum, which I was a member for a long time. I am working on a python application about GR and I believe I managed to create a very user-friendly layout. It's called GTRPy, and it allows...
21. ### A Popular Pseudoscience Video on General Relativity: Analysis & Criticism

Here is the video: [link deleted by moderators] His basic idea is to take the spacetime interval and add a 5th term for the 5th dimension he is describing so it looks like: $$\Delta S^2 = c^2\Delta t^2 + c^2\Delta w^2 - \Delta x^2 - \Delta y^2 - \Delta z^2$$ where w is the difference in time...
22. ### I Modeling the Earth and Sun (2 body orbits) using general relativity?

Modeling the time evolution of the sun and earth orbiting each other using ##F = \frac{GMm}{r^2}## is straightforward. However, it appears that modeling the time evolution of the same 2 body system using general relativity seems to be a hard/intractable problem? There was in depth discussion by...
23. ### B Array Representation Of A General Tensor Question

So, I've been watching eigenchris's video series "Tensors for Beginners" on YouTube. I am currently on video 14. I, in the position of a complete beginner, am taking notes on it, and I just wanted to make sure I wasn't misinterpreting anything. At about 5:50, he states that "The array for Q is...
24. ### B Transformation Rules For A General Tensor M

So, I've been watching eigenchris's video series "Tensors for Beginners" on YouTube. I am currently on video 14. I am a complete beginner and just want some clarification on if I'm truly understanding the material. Basically, is everything below this correct? In summary of the derivation of the...
25. ### Find the general solution of the given PDE

My take; ##ξ=-4x+6y## and ##η=6x+4y## it follows that, ##52u_ξ +10u=e^{x+2y}## for the homogenous part; we shall have the general solution; $$u_h=e^{\frac{-5}{26} ξ} f{η }$$ now we note that $$e^{x+2y}=e^{\frac{8ξ+η}{26}}$$ that is from solving the simultaneous equation; ##ξ=-4x+6y##...
26. ### I Is this just a typo in Schutz' book on General Relativity?

I'm wondering is I'm missing something, or this should be " a non-zero component"?
27. ### I Metrics of GR Solutions: Comp List, Hist & Viability

Does anyone know of a comprehensive list of solutions to GR, their developmental history, and the viability for serving as a practical model for the observable universe?
28. ### B Solve General Geodesics in FLRW Metric w/ Conformal Coordinates

Once having converted the FLRW metric from comoving coordinates ##ds^2=-dt^2+a^2(t)(dr^2+r^2d\phi^2)## to "conformal" coordinates ##ds^2=a^2(n)(-dn^2+dr^2+r^2d\phi^2)##, is there a way to facilitate solving for general geodesics that would otherwise be difficult, such as cases with motion in...
29. ### A Time Travel, General Relativity & Information Paradoxes

General relativity permits some exact solutions that allow for time travel. Some of these exact solutions describe universes that contain closed timlike curves, or world lines that lead back to the same point in spacetime. I wondered if these solutions also permits Causal loops? Such as the one...
30. ### I Does Superfluid Dark Matter provide a successful explanation for MOND?

I was glancing through https://arxiv.org/abs/1605.07458 I don't fully get why scale invariance kicks in below a certain acceleration. Is this because the centripetal force becomes constant? I do see that Newton's second law with the centripetal force included is scale invariant. This symmetry...
31. ### Other General Science on modern technology and space technology.

I’m in need of recommendation of a general science book (‘general’ means just a bit of introduction and its application, not going into its detailed theoretical and technical workings) which contains the following topics (though not exhaustive) Space Technology: the basic concept of launching...
32. ### I Def Proper Time GR: Half or Integral Along Path?

In texts on General Relativity, the proper time ##d\tau^2 = -ds^2## (with an appropriate choice of metric signature) is commonly said that the time measured by a timelike observer traveling along a path is given by the integral of ##d\tau## along this path. Of course it's possible to construct a...
33. ### I General solution of 1D vs 3D wave equations

For the 1 dimensional wave equation, $$\frac{\partial^2 u}{\partial x ^2} - \frac{1}{c^2}\frac{\partial ^2 u }{\partial t^2} = 0$$ ##u## is of the form ##u(x \pm ct)## For the 3 dimensional wave equation however, $$\nabla ^2 u - \frac{1}{c^2}\frac{\partial ^2 u }{\partial t^2} = 0$$It appears...
34. ### What does G mean in general relativity?

In Newtonian mechanics, G is simply a proportionality constant or the force with which two bodies of unit mass attract each other. However, GR doesn't treat gravity as a force. So how is G defined in GR? Is it a property of spacetime or just some useless mathematical artefact? What does G...
35. ### B General Relativity & Grav. Time Dilation Qn

hello I'm korean high school student and sorry for my poor English. I saw ## t_0=t_f\sqrt{1 -\frac{ 2GM}{rc^2}} ## in wikipedia. does ## \sqrt{1 -\frac{ 2GM}{rc^2}} ## of this equation have name like lorentz factor ## \frac{1}{\sqrt{1 -\frac{v^2}{c^2}}} ##of ## t=\frac{t_0}{\sqrt{1...
36. ### I General expression of wind force on a sail....

Hello everyone, In trying to better understand how sailboats work, how they can sail upwind (not directly), how the go faster than the wind speed, I have been thinking about the magnitude of the wind force and its equation: $$F_{wind}= \frac {1}{2} \rho_{air} A v_{wind}^2$$ Instead of ##...
37. ### A Loop quantum gravity and General relativity

Hi PFs, I am reading this paper written by carlo Rovelli: https://arxiv.org/abs/1010.1939 there are many things that i fail to understand, but i would like to begin with a simple thing. Rovelli write that: It is locally Lorentz invariant at each vertex, in the sense that the vertex amplitude...
38. ### Other General Science level books for Defence technology

Additional/Optional Subject: Space and Defence technology. Began a week ago, they taught a whole week, for 2.5 hours per day on space technology: Orbiter, Rover, Ramjet, Scramjet, Air-breathing engine, Reusable Launching Vehicle and et cetra ... et cetra... Thought, I, could absorb and...
39. ### MHB Help with a question (Bernoulli General solution)

Hello guys I hope you all are doing well. :) I found below question in a book by Martin Braun "Differential Equations and Their Applications An Introduction to Applied Mathematics (Fourth Edition)" The question : The Bernoulli differential equation is (dy/dt)+a(t)y=b(t)y^n. Multiplying through...
40. ### From General Relativity to Dark Energy

If we insert the values from (2.9), (2.10), (2.11) into (2.5) & (2.6) how can we get (2.13) & (2.14) ?? I need to see the calculations step by step.
41. ### I The general structure of relativistic QFTs

In a recent thread about Bell tests etc. it has been claimed my point of view of "locality" where "non-mainstream physics". Of course, I cannot give a complete summary of the foundations in a forum posting, as demanded by @DrChinese there. The most clear treatment, particularly emphasizing the...
42. ### B General Relativity & Ether: Clarifying a Contradiction?

Hope this question can be quickly clarified: There was a statement that the General Relativity can be interpreted by speaking of an ether whose state varies from point to point. Is this correct?!
43. ### A Eigenvalue of the sum of two non-orthogonal (in general) ket-bras

We have a matrix ##M = \ket{\psi^{\perp}}\bra{\psi^{\perp}} + \ket{\varphi^{\perp}}\bra{\varphi^{\perp}}## The claim is that the eigenvalues of such a matrix are ##\lambda_{\pm}= 1\pm |\bra{\psi}\ket{\varphi}|## Can someone proof this claim? I have been told it is self-evident but I've been...
44. ### Relativity General relativity with focus on action?

I am having a class of general relativity. It seems that the professor will follow an approach which consist of achieve the action, and variate it to get the equations of motion (indeed, that's how we already got the geodesic equation, the dynamics of a particle in electromagnetism, the equation...
45. ### I Ideal Fluids & General Relativity: Why Needed?

Why in general relativity do we need the physics of perfect fluids?
46. ### I General Relativity: Is it Local?

Spacetime is a differential manifold and at each point is attached a Minkowski spacetime. There the laws of physics are the usual ones without gravity. Gravity is the curvature of spacetime. To define the concept of curvature do we need to evaluate at least one neighborhood of point P? Is...
47. ### B Robert Wald's General Relativity: Energy-Momentum Relation

Hello, this is my first thread. Robert Wald, in General Relativity, equation (4.2.8) says : E = – pa va where E is the energy of a particle, pa the energy-momentum 4-vector and va the 4-velocity of the particle. How can I see this is compatible with the common energy-momentum-relation E2 – p2 =...
48. ### I Length contraction in General Relativity

In GR, a free falling object when viewed by a distant observer appears to be length contracted and slows down as it approaches the event horizon of a black hole. The length contraction piece, however, seems counterintuitive. I would have thought that the leading edge of the object would...
49. ### A Does Spacetime Absorb Energy in General Relativity?

Some physicists prefer to explain the problem of conservation of energy in General Relativity by considering the gravitational potential energy of the universe that would cancel all the other energies and therefore the energy in the universe would be conserved this way. However, many other...
50. ### MHB Integrating Factor Method for Solving y' + y = e^{-2t}

$y'=-y+e^{(-2)t}$