What is Theory: Definition and 1000 Discussions

A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may either be scientific or other than scientific (or scientific to less extent). Depending on the context, the results might, for example, include generalized explanations of how nature works. The word has its roots in ancient Greek, but in modern use it has taken on several related meanings.
In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that scientific tests should be able to provide empirical support for it, or empirical contradiction ("falsify") of it. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge, in contrast to more common uses of the word "theory" that imply that something is unproven or speculative (which in formal terms is better characterized by the word hypothesis). Scientific theories are distinguished from hypotheses, which are individual empirically testable conjectures, and from scientific laws, which are descriptive accounts of the way nature behaves under certain conditions.
Theories guide the enterprise of finding facts rather than of reaching goals, and are neutral concerning alternatives among values. A theory can be a body of knowledge, which may or may not be associated with particular explanatory models. To theorize is to develop this body of knowledge.The word theory or "in theory" is sometimes used erroneously by people to explain something which they individually did not experience or test before. In those instances, semantically, it is being substituted for another concept, a hypothesis. Instead of using the word "hypothetically", it is replaced by a phrase: "in theory". In some instances the theory's credibility could be contested by calling it "just a theory" (implying that the idea has not even been tested). Hence, that word "theory" is very often contrasted to "practice" (from Greek praxis, πρᾶξις) a Greek term for doing, which is opposed to theory. A "classical example" of the distinction between "theoretical" and "practical" uses the discipline of medicine: medical theory involves trying to understand the causes and nature of health and sickness, while the practical side of medicine is trying to make people healthy. These two things are related but can be independent, because it is possible to research health and sickness without curing specific patients, and it is possible to cure a patient without knowing how the cure worked.

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  1. T

    Boussinesq theory (load per unit area)

    in question 10.19 , i use equation 10.26 to do the question . I have all the values of all variables , except q ( load per unit area) ... Homework EquationsThe Attempt at a Solution How to get q ? It's not stated in the textbook . Is there something to do with the homework =4m ?? I assume q =...
  2. S

    A Gravitational anomalies in gauge theory

    Dear All Can anyone explain for me what is meant by gravitational anomalies in gauge theory? What is the difference between it and gauge anomalies? Thank you
  3. Spinnor

    B Branes, gluons, time reversal, and string theory.

    So a gluon in string theory may be an open string whose ends live on one or two branes? These strings have an orientation along the string? Does the orientation change direction if we reverse time, some where I thought I read that one can think of a flow along the string so if you reverse...
  4. F

    A Please verify integral and approximation, boundary theory

    I used Newtons method and taylor approximations to solve this equation $$f'''+\frac{m+1}{2}ff''+m(1-f^{'2})=0$$ It solves for velocity of air over a flat plate. The velocity is a constant ##u_e## everywhere except in a boundary layer over the plate, where the velocity is a function of distance...
  5. Sophrosyne

    Scientific law vs. theory, Newton vs. Einstein

    My son is taking a chemistry class in high school, and he was telling me this morning that their teacher had taught them there was some kind of fundamental distinction between scientific laws and theories. He said the teacher had told them that laws are just fundamental regularities observed in...
  6. T

    Griffith Intro to QM, 1st order perturb theory in time-dep

    Homework Statement ca(0)=1, cb(0)=0 zeroth: ca(t)=1, cb(t)=0 1st: ca(t)=1, cb(t)=i/hbar*integral(H'(t) exp(iwt)) dt ca^2+cb^2=1 to 1st order of H'.: What does it mean? it is evidently not 1 at all. Homework EquationsThe Attempt at a Solution
  7. T

    A Quantum Gravity: Renormalization vs. Effective Field Theory

    In quantum gravity, I get 'mixed signals' as regards renormalizability. My state of confusion is being caused, I suspect, by an incomplete understanding of what is covered under t'Hooft's 1972 proof that non-Abelian gauge theories are renormalizable. ( = Nobel Prize 1999). Specifically, some...
  8. T

    Learning number theory for cryptography

    Hi everyone, I am trying to learn the underlying number theory concepts behind cryptography, and I was wondering if anyone knows of good resources for learning about number theory as applied to cryptography. I was hoping to practice writing proofs as well. Thanks!
  9. T

    I Does Feynman's Work on Gravitons Clarify Quantum Gravity?

    Back in the 1960s, Richard Feynman worked on quantum gravity for a few years, and most of his notes are collected in the 'Feynman Lectures on Gravitation'. His approach was that of a particle physicist applying the principles of QED to GR, notably the concept of gravitons mediating the force of...
  10. B

    Studying Is Studying String Theory Worthwhile for a Future in Theoretical Physics?

    Hello, I am studing theoretical physics and i would like to focus my work in string theory, but my fellows say that is a waste of time. ¿Do you think that is worthwhile to spend my time and effort un string theory? Greetings
  11. J

    A Defining the membership relation in set theory?

    My main question is regarding whether the membership relation is taken as an undefined concept (as is usually hinted in set theory books) or if the membership relation can be defined within the language of first order predicate theory. Let me describe a method to define the membership relation...
  12. davidge

    I (Apparent?) paradoxes - Classical/Quantum theory predictions

    I want to discuss about two apparent contradictions that are on my mind for awhile. 1st: General Relativity predicts no radiation out of black holes. Quantum Mechanics does. If we perform an experiment and see that say, black holes radiates outwards, does that mean General Relativity is wrong...
  13. Spinnor

    I Electric and Color charge in String Theory

    I get a gist of how electric charge comes about in string theory here, https://physics.stackexchange.com/questions/5665/electric-charge-in-string-theory How much needs to be amended to the above answer to understand how color charge comes about in string theory? I assume there are interesting...
  14. Z

    A question about the Dynamo Theory

    I'm studying Dynamo Theory, and have just a few questions. Today's theory says that currents in the molten outer core of the Earth give rise to magnetic fields. Now it would seem that the theory does not mean "molten iron currents", since together that is an electrically neutral substance, but...
  15. G

    I Generalization of measure theory to uncountable unions

    Hi. Is there some kind of measure theory generalized to uncountable unions? Of course one needs to take care how to make sense of sums over an uncountable index set. I was thinking about following formulation of the additivity property of the "measure": $$\mu\left(\bigcup_{i\in\ I}...
  16. S

    Iron Man - Palladium Reactor Theory

    I'm curious about something. And most of you are probably going to click away as soon as I say, or type this, I guess. But for those of who haven't, thanks. In the Iron Man comics, movies and tv shows the arc reactor keeps Tony Stark from lethal heart damage and keeps the Iron Man Armour...
  17. K

    I Force on an Object according to the General theory of relativity

    What is the force acting on an object according to general theory of relativity? If there is a such a force, can we predict the motion of an object in general relativity just using the modified Newtons laws of mechanics i.e using relativistic mass of an object instead of rest mass ?
  18. A

    High Energy High Energy Physics and Group Theory Book Recommendations

    Hey there! I just want to ask if there are any books you would like to recommend that helps in studying high energy physics and HEP data analysis? Also can you recommend a good book for group theory and symmetry? I would be glad if you have links to free downloadable books. Thanks in advance!
  19. parshyaa

    I What if the mystery of unified theory is solved?

    What if the mystery of unified theory is solved (i.e we found a TOE)? What would be the next mystery in physics(will their be any or will the context of physics would be completed(i don't think so,what do you think?).
  20. Urs Schreiber

    Insights Introduction to Perturbative Quantum Field Theory - Comments

    Urs Schreiber submitted a new PF Insights post Introduction to Perturbative Quantum Field Theory Continue reading the Original PF Insights Post.
  21. Mr Davis 97

    Simple set theory question

    Homework Statement Prove that if ##A \subseteq B##, then ##\bigcup A \subseteq \bigcup B##. Homework EquationsThe Attempt at a Solution This is a simple problem, but I just want to make sure I am writing out the proof correctly: Suppose that ##A \subseteq B##. We want to show that ##\bigcup A...
  22. T

    I Inflation theory and General Relativity

    Does General Relativity predicts that in the early universe vacuum energy was converted into matter? How does it relates to the Inflation Theory by Allan Guth? I'm asking this because I remember reading in a book on GR that there are ways of calculating the total amount of energy in the...
  23. A

    B Einstein's theory of relativity and time travel

    Hey so from my understanding of Einsteins theory of relativity the closer you travel to the speed of light the slower time is for you, is that right? if it is right does that give to reason that there could be a critical speed past the speed of light for which time for the object traveling at...
  24. Mr Davis 97

    Show Set Theory Subset Relationship: x, y $\in$ B

    Homework Statement Assume that ##x## and ##y## are members of a set ##B##. Show that ##\{ \{x\}, \{x,y\} \} \in \mathcal{P} \mathcal{P} B## Homework EquationsThe Attempt at a Solution I know that ##\{ \{x\}, \{x,y\} \} \in \mathcal{P} \mathcal{P} B## iff ##\{ \{x\}, \{x,y\} \} \subseteq...
  25. T

    I Why is Quantum Gravity theory still not "finished"?

    Physicists had success in developing a relativistic quantum mechanics, also Quantum Field theory. I wonder why it's not the same with a quantum gravity theory. I suppose this is mainly because we do not have good emphirical information from a place where Einstein's theory fails: black holes...
  26. S

    B Exploring the Theory of Space: Understanding the Nature and Changes in Space

    Is there any theory law explaining the nature of space and describing what is space and what happens to space when some event take place in space.
  27. ISamson

    B Unify General Relativity & Quantum Theory | Stephen Hawking

    Hello, I have read somewhere that the next major step in physics will be unifying general relativity with the quantum theory. Does anyone know about such? I wanted to know a bit more. I have read about such in the book by Stephen Hawking "A Brief History of Time". Thanks, I. P.S. My apologies...
  28. Mr Davis 97

    Is the Number of Elements in a Set Equal to Its Power Set?

    Homework Statement Show that ##\bigcup \{\mathcal{P} X : X \in A \} \subseteq \mathcal{P} \bigcup A## Homework EquationsThe Attempt at a Solution Suppose that ##c \in \bigcup \{\mathcal{P} X : X \in A \}##. Then by definition this means that ##\exists a \in A## such that ##c \in \mathcal{P}...
  29. tom.stoer

    A String Theory and Einstein-Cartan Gravity

    I know two essential points where General Relativity plays a central role in String Theory: i) definition of the theory using a target spacetime with some Riemannian background metric in the Polyakov action and ii) recovery of the Einstein field equations as conditions regarding conformal...
  30. RJLiberator

    Bohr-Sommerfeld model question "Old" Quantum Theory

    Homework Statement In analogy to the Bohr theory of the hydrogen atom, develop a quantum theory of Earth satellites, obtaining expressions for the orbit radius, r_n, and the energy, E_n in terms of the quantum number n and other relevant parameters. Use the "Old" Quantum Theory. A satellite of...
  31. Mr Davis 97

    I Addition of exponents proof in group theory

    Just out of curiosity, what would a proof of ##a^m a^n = a^{m+n}## amount to? Of course obviously if you have n of one thing and m of another you get m+n, but I am wondering if this is rigorous enough, or if you need induction.
  32. M

    Homework Help Regarding Newton's Theory

    Hello all; this my first time ever taking physics, and I am in dire need of some help with homework. I've had an attempt at part A (please take a look and provide feedback), but have no clue where to even begin on part B (how would I go about calculating questions 5, 6, and 7, to be precise?)...
  33. T

    I Relativistic Quantum Mechanics vs Quantum Field Theory

    What's the difference between relativistic quantum mechanics and quantum field theory? In principle, my guess is that to do the former, one needs to express the Hamiltonian in a relativistic, Lorentz invariant, form, because it seems to be the only frame-related term in the wave equation. (Is...
  34. A

    I The box notation and Lagrangians in field theory

    I have some questions about scalar field Lagrangians, using the box notation defined as \Box \equiv \frac{\partial^2}{\partial t^2} - \nabla^2 . It's a basic, perhaps silly issue, but somehow I've managed to sweep it under the rug for a long time. So, usually, the Lagrangian of a free scalar...
  35. D

    B Does this theorem “theoretically” falsify macrorealism?

    Hello! Recently I found this article: http://quantum-journal.org/papers/q-2017-07-14-13/pdf/ Being familiar with some basis of quantum formalism, I, nevertheless, experienced several difficulties with understanding of the theorem described in this paper. I would really appreciate if someone...
  36. Lunct

    B Is Our Observable Universe the Actual Size of Our Universe?

    In the multiverse theory, I keep on hearing that our observable universe is one of many universes. Is that saying that our universe ends at the point we can no longer see it - at the end of the observable universe. That would mean that the Earth is the literal centre of our universe. Am I...
  37. learn.steadfast

    I Band theory and qualitative character of bands

    I'm interested in the the band theory of solids for semiconductor modeling. I haven't solved the Schrodinger equation for multiple atoms, but I'd like to know some details that experienced physicists might already know. Several texts show how the bands develop from molecular coupling of...
  38. V

    Classical electromagnetic theory

    According to classical electromagnetic theory,for acceleration a particle must not only change its direction but it should also increase its speed. But Maxwell said that a particle accelerates then it radiates energy.(this is with reference to Rutherford model). But actually the particle is not...
  39. J

    Applied Zee and Georgi Group Theory books

    Hello. I will be attending a course on Group theory and the book that the professor suggests is Georgi's Lie Algebras in Particle Physics. As I liked Zee's book on General Relativity, I thought that it would be a blast to also use his Group theory textbook for the course. Problem is that I don't...
  40. A

    B Is Time Travel Possible? Exploring the Speed of Light and the Nature of Time

    I just wanted to share this idea with other people who may be interested: I watched a PBS Space Time episode about the speed of light more accurately described as the speed of causality. And I Submit! That "time" is simply a interaction between atoms. And though an interaction can be...
  41. S

    I Normal modes using representation theory

    Hello! I am reading some representation theory (the book is Lie Algebra in Particle Physics, by Georgi, part 1.17) and the author solves a problem of 3 bodies connected by springs forming a triangle, aiming to find the normal modes. He builds a 6 dimensional vector formed of the 3 particles and...
  42. L

    Two computational theory language questions - multi choice

    Homework Statement ##1)## Which one of the following is an example of a set ##S## such that the language ##S^*## has the same number of 8-letter words as 4-letter words? 1) ##S = \{aaaa \quad bbbb\}## 2) ##S = \{bbbb \quad bbbbbb\}## 3) ##S = \{aa \quad bb\}## 4) ##S = \{a \quad bbbb\}##...
  43. parshyaa

    I Is It Valid to Cancel Sets in Set Theory?

    We can prove that When A and B are two sets(A≠B) (A-B) = (A∩B') = (A-(A∩B)) {We can also confirm them using venn diagram} From first and third relation A-B = A - (A∩B) By cancelling A from both side I get B = (A∩B) Which is only possible when A and B are same set. What is wrong in my proof , is...
  44. K

    I Is there an alternative theory to dark matter?

    My physics teacher, who dislikes the idea of dark matter, told me that a physicist created an alternative explanation to the phenomena caused by dark matter. Is there something I missed on the news? What is the alternative theory to dark matter and how does it explain 'things' ?
  45. S

    A QCD as a classical field theory?

    Hi everyone, I have a question that, when came to me, sounded a bit silly to me as well, but then I realized, I myself maybe don't understand the logic behind this 100%, so why not discussing with you about it. So my question is the following. Usually we are used to do quantum field theory...
  46. H

    I Can Emission Theory Produce Doppler-Shift Formula?

    Emission theories propose that the velocity of light depends on the velocity of the light source. But the ordinary Doppler effect assumes the velocity of light remains as ##c## with respect to the ether medium, even when the light source is moving at speed ##v## with respect to the ether. They...
  47. mpolo

    B Einstein's Unified Field Theory - Details

    I saw a documentary the other day where Michio Kaku said something that really peaked my interest. He said that Einstein in his last days was working on something having to do with how small geometries of some sort being the cause of gravity. Does anyone know the details of Einstein's Unified...
  48. Aslet

    I How Does the Inclusion-Exclusion Principle Relate to Probability Theory?

    Hello everyone! I'm studying the physics of complex systems and I'm approaching probability theory. I understand that we need a ## \sigma-algebra ## and the Kolmogorov axioms in order to define a probability space but then I bumped into the following relation: $$ p(A_1 \cup A_2 ) = p( A_1 ) + p(...
  49. DaveC426913

    B Knot theory: closed loops in n dimensions

    I'm not a math machine, but I dabble in dimensional stuff. I think this falls under knot theory. I have built several prototypes of a tesseract. Each of them sits in a little case in my office. One of them is made from truncated cubes, held together with elastic cord: In theory, the...
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