Theories Definition and 466 Threads

I was wondering if anyone could suggest anything which describes Type I and Type IIB superstring theories, and D3-branes.

As the discovery matches templates based on GR, and the regime is of very strong gravitational fields and very high speeds (relativistic speeds), and there is a 90% match between model and measured data, this does rule out linear or quasi linear alternative theories of gravity?

I am not a scientist, but as a hobby I am summarizing different initial condition theories, specifically, eternal inflation, LGC, cyclic, and bounce theories. I need a completion time ATB where all theories produce an identical plasma. The plasma then enters the big bang process of expansion...

I like physics involving the cosmos. I was wondering what theories are out there that explain how the observable universe is expanding at an increased rate rather then slowing down due to gravity or other variables. I know the dark energy theory that basically states there's a energy that...

What role do theories, models, and laws play in the scientific process? Is there a clear dividing line between a theory and a law? Are models used in both theories and laws? All theories and laws? Do our best current models perfectly represent reality to the best of our knowledge, or are they a...

Urs Schreiber submitted a new PF Insights post Examples of Prequantum Field Theories IV: Wess-Zumino-Witten-type Theories Continue reading the Original PF Insights Post.

Urs Schreiber submitted a new PF Insights post Examples of Prequantum Field Theories III: Chern-Simons-type Theories Continue reading the Original PF Insights Post.

Urs Schreiber submitted a new PF Insights post Examples of Prequantum Field Theories II: Higher Gauge Fields Continue reading the Original PF Insights Post.

Urs Schreiber submitted a new PF Insights post Examples of Prequantum Field Theories I: Gauge Fields Continue reading the Original PF Insights Post.

Why in first order logic theories are not possible a demonstration with infinite steps?

It's non-trivial since if you do not have a background metric you cannot define the scale, with respect to which couplings are supposed to run. So new approaches to renormalization have appeared. Perimeter has a workshop on this and a number of Monday 28 Sep talks are on line...

Hello all, i would like to start at the beginning please, am i right in thinking that gravity is created by the Mass of an object and has nothing to do with its actual size?

I'm trying to read this recent paper (http://arxiv.org/abs/1507.07745) but I don't understand the definition of an "operation" in the paper. It is defined as a set of events on the first page, but I don’t know what an event is in this context. Looks like it’s some kind of function but I can’t...

Because I'm only in pre-calculus right now (I'm still not in ninth grade yet), I haven't been able to find a way to back up my theories on physics which are mostly based off of reason and logic (I know this is a bad way to back up my theories, I just want to know what you think of them and how I...

Two recent PF threads were interesting IMO. Why no time before the BB? and A Farewell to Falsifiability Here on PF I can read about collapse-rebound, quantum fluctuations, multiverses, and many more speculative theories about pre-BB eras or universes beyond the observable. But it is my...

hi~I am a beginner of Gauge theories,and I have some questions~ Why we need loacal phase invariance? Because i cannot understand why we assume phase depend on the position.Is there any theory leading to this asssumption?

Hello, So in the familiar case of non-relativistic particle Lagrangians/actions, we know the equations of motions are given by \frac{\partial \mathcal L}{\partial x^i} = \frac{\mathrm d }{\mathrm dt} \left( \frac{\partial \mathcal L}{\partial \dot x^i} \right) In the familiar case of...

[Moderator's Note: Changed level of thread to "Advanced" based on the topics being asked about, all are graduate level topics.] I feel that I have an inadequate understanding of many important concepts in condensed matter physics, so I want to try to learn at least the most basic parts. So what...

As I understand it, the BB says the universe came from a tiny ball of stuff. Is it correct that no one has an explanation as to where the ball of stuff came from? Is it also correct that the BB is still speculation and subject to revision, even potentially discarding altogether?

Was just thinking, is it ever possible that the same phenomenon can be fully explained by two theories that are not subset of each other? :rolleyes:

Hello, my name is Alex. I'm a high school student in Menomonie and I'm fascinated by the world of physics. I want to work toward getting a doctorates degree in physics and go on to work as a theoretical physicist. I'm taking AP Physics as a class and spend all of my free time learning and...

What does it mean below that in order to keep these theories from violating the principle of the conservation of energy, the mathematics requires that any collapse be incomplete? What is the meaning of complete collapse vs incomplete (and the "tails")...

Hi, Is it more than mere coincidence that we get the correct energy levels (ignoring relativistic, QFT and spin shifts) of the hydrogen atom using Bohr's model? The assumptions it is based on just don't agree with modern QM. Since I'm going to be a physics teacher, there's also a didactic...

I understand that the constancy of c is a postulate, back in the day guided by the Michelson experiment and Maxwell's equations. I was wondering, are there any (speculative) theories out there that have this fact as an outcome of something deeper/more basic?

If QM is a statistical model to approximate something underlying space time we don't quite understand yet, and there is a complex geometry underlying space time, is it possible to find other ways to simplify molecular optimizations and electron interactions in computational chemistry using...

Dear PF personnel, I am a college sophomore with double majors in mathematics and microbiology. I wrote this email to seek your advice on selecting a theoretical, proof-based textbook on the multivariable calculus. I will be taking a multivariable calculus on this Summer but it unfortunately...

I'm new to Astrophysics but some scientists say that all time is happening at one time. How is this possible if the universe is still expanding

"2 theories, same experimental predictions, can one fail but not the other ?" (above was intented to be the title, got truncated) Let’s say we have 2 physical theories, call them A and B, that make the same experimentally verifieable predictions. But they are based on different postulates. And...

Are all physics theories based on "the equivalence of energy E and mass m always reliant on the speed of light c? Are there any alternative theories related to the equivalence of energy and mass that are not reliant on "the speed of light"? I would appreciate your attention, ttvlr

Hi folks -- it is my understanding that only non-Abelian gauge theories can be asymptotically free. But can anyone provide me with a canonical reference showing that such symmetries can be dynamically broken and still retain that status? I gather that this is the case (from reading the...

Are many-particles systems theories like the Standard model of particle physics examples of local hidden variable theories?

What is the difference between Yang-Mills and QED theories? Yukawa and QCD? specially in terms of the lagrangians. I really want to get into this subject with a previously first sight.

I've been into physics for years now but no matter how much a study, i find that i always lack basic knowledge that others seem to have. I was wondering could someone provide me (if possible) with a list of fundamental concepts every physicist should know? i imagine the list might be to long or...

Could someone please explain to me the concept and theories of 'Quantum Physics', as I am quite perplexed at this topic, but simultaneously intrigued at the thought of it.

Sorry I am new to quantum physics. I am curious about what is the most accurate theory till now to predict quantum behavior. And I have several questions. 1. Is quantum field theory a "method" and can used universally in standard model? 2.What is the most accurate theory(or model) recently in...

Have theories been formed yet on how to stop a black hole?

im planing to educate myself . on quantum mechanics/physics . so could you please refer me 1 good fundamental QM text / reference book . and 1 book that covers all the theories on time ( space time , time line ...)

What theories attempt to explain the dark matter today? The force of gravity is the only thing that interacts with dark matter, the fundamental force "weaker" is gravity however, if she is not weak just reach dimensions that do not have access, dark matter could not be these heavenly bodies...

I'm not sure if this thread would belong in the Academic Guidance forum, so if it does, feel free to move it there. I'm looking for research topics that might involve thermo- or fluid dynamics, but are centered around grand unified theories. I'm particularly interested in computational fluid...

I myself don't really believe that the rules of physics break down inside a black hole or that time can go backwards. I was wondering if there is anything that some people do not believe or have serious doubts about.

Hello, I don't know much about GUT physics, but I've been wondering whether these models usually breaks the grand unified symmetry to the standard model all at once, or multiple times at different energies. And in the case of multiple breakings, how many Higgs-like scalars are needed...

A theory is a subset of a formal language together with a set of inference rules on that formal language in which the members of the theory need no premises to be true, right? So if I had a subset ##\mathcal{T}## of a formal language ##\mathcal{F}##, and a set of inference rules in which all...

I am aware that the most popular version of the quantum brain theory i.e. the Penrose and Hameroff version has been disproved experimentally many times. But I was wondering what the status of other such attempts such as Vitiello and Freeman's dialog model is, in the scientific community ? Is...

Dear friends, I am writting a simple essay about the mechanisms of scientific evolutions, where I would like to point out the main driving forces that induce scientific progress. I would like to give simple examples from history of science (e.g Physics, Astrophysics, Mathematics, etc.)...

I mean I don't see any theory actually derives gravitational interaction from actual fields of particles. I only see some manipulation of GR to make it look like a quantum field theory. So are these theories looked upon as one more step to actual unification or are they the FINAL unification...

I have been teaching myself QFT and General Relativity. The mathematics of those fields is daunting, and I find that what I have come across is very difficult to master. Of course it will take work, but can someone recommend a good text for self-leaning differential geometry with application...

Hi, I think I have several misconceptions about the theoretical framework of black holes, I'm just not sure where my intuition (or, lack of) goes wrong. So sorry if this sounds really stupid, any help is appreciated. The scenario that I find confusing is what Alice sees when she throws Bob into...

Hello, I am currently studying spontaneous symmetry breaking in qft. Several textbooks I've read prove Goldstone's theorem under supposing that 1) There exists a continuous global symmetry under which the Lagrangian is invariant. 2) The vacuum state is not annihilated by the conserved...

I am a programmer and physics fan. A speaker (Lawrence Krauss?) at https://origins.asu.edu/events/great-debate-parallel-realities-probing-fundamental-physics (not on youtube yet) said physics theories beyond the standard model are an under-determined problem. There are 7,000 theories that fit...

After reading Lee Smolin's Time Reborn and Rodney Brook's Fields of Color, a question occurred to me. Has anyone compiled a "reality" scorecard for the differing theories out there, both current and historical? For example, QFT proponents would consider the following things as real: fields...