Theory Definition and 1000 Threads

Prof Wetterich has proposed that atoms are shrinking rather than the Universe is expanding. Here is a 2013 Nature News article describing his theory: https://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379 Here is his 2013 paper "A Universe without expansion"...

Hi, I needed some help with robotics but, strangely, found only a small number of threads about robotics topics. I don't know the reason for this but I think it'd nice if PhysicsForums had a separate sub-forum for robotics. Anyway, which sub-forum should be used to post questions about robotics...

Hello, The sigma $(\sigma)$ molecular orbitals are symmetrical around the bond-axis while pi $(\pi)$ molecular orbitals are not symmetrical. For example, the linear combination of 1s orbitals centered on two nuclei produces two molecular orbitals which are symmetrical around the bond-axis. Such...

Usually, I saw that string theory (perturbative, or matrix models) are made in a fixed background. Even if you consider that the metric is quantized and etc. there is an apparent physically motivated need for making a sum over topologies (manifolds, conifolds, orbifolds, and etc), for example...

I was reading about the renormalization of ##\phi^4## theory and it was mentioned that in order to renormalize the 2-point function ##\Gamma^{(2)}(p)## we add the counterterm : \delta \mathcal{L}_1 = -\dfrac{gm^2}{32\pi \epsilon^2}\phi^2 to the Lagrangian, which should give rise to a...

Hi, I've a doubt about the applicability of the substitution theorem in circuit theory. Consider the following picture (sorry for the Italian inside it :frown: ) As far I can understand the substitution theorem can be applied to a given one-port element attached to a port (a port consists of...

At least according to Tim Anderson Ph.D who wrote the paper in Physics Review. https://news.knowledia.com/US/en/articles/a-5th-dimension-may-explain-quantum-theory-the-infinite-universe-medium-6f1d6fd371e068a07f357b9babe9ab2eec06d034 What do you make of this? "The paper simply presents...

I usually don't read papers on philosophy of quantum field theory, but this one is really good: http://philsci-archive.pitt.edu/8890/ In particular, the prelude which I quote here is a true gem: "Once upon a time there was a community of physicists. This community be- lieved, and had good...

In Dynamical Systems Theory, a point in phase space is interpreted as the state of some system and the system does not exist in two states simultaneously. Can some phase spaces be given an additional interpretation as describing a field of values at different locations that exist...

I read this paper where if you take the alcubierre metric calaculations while including torsion in GR you get positive energy spin requirements instead of exotic matter. Here is the link: https://arxiv.org/abs/1807.09745 Could it be because a spinning quantum vacuum will be less stiff like a...

Hello! Let's say we have 2 states of fixed parity ##| + \rangle## and ##| - \rangle## with energies ##E_+## and ##E_-## and we have a P-odd perturbing hamiltonian (on top of the original hamiltonian, ##H_0## whose eigenfunctions are the 2 above), ##V_P##. According to 1st order perturbation...

https://arxiv.org/abs/0805.3819 Approximating the Standard Model and gravity with dust on \mathbb{R}^{0|18} Robert N. C. Pfeifer [Submitted on 25 May 2008 (v1), last revised 11 Jul 2020 (this version, v14)] This article describes a single species of non-interacting massless dust on...

The mathematical structure called a "field" is an algebraic structure that doesn't capture the properties of what physicists called a "field". What's the name for a mathematical structure that does represent the general concept of physicist's field? Is a mathematical version of a physicist's...

Hi I am aware that QM isn't deterministic. Should a theory of everything be deterministic to be a theory of everything, and if yes, then how can it be when QM as a part of it is not deterministic? Thanks in advance Seyed

Let ##\phi_{+}=\frac{d^3k}{2\omega_k (2\pi)^3}\int(\hat{a}^{\dagger}(\overrightarrow{k})e^{ikx})## and ##\phi_{-}=\frac{d^3k}{2\omega_k (2\pi)^3}\int(\hat{a}(\overrightarrow{k})e^{-ikx})##. Then ##\phi^4=\phi_{1}\phi_{2}\phi_{3}\phi_{4}=(\phi_{1+}+\phi_{1-})(\phi_{2+}+\phi_{2-}...

Dear Ladys and Gentlemans, I want to measure the sheet resistance R_square on a printed product. My measuring equipment consists a SMU to supply and measure and a four point Probe by Jandel. As output size I get the electrical resistance R (basically the measured Voltage U divided by the supply...

Quite a long time ago, I read about length of strings in string theory. And, if I remember correctly, expected length was in light years. I looked at Wikipedia today, and I see currently expected length of string is very small, around Plank size. Is it changed over time, or the expected length...

All physical laws have to be Lorentz invariant according to a lecture I just watched. Why is general covariance (which is more general than Lorentz invariance) not a requirement for all laws of physics? Are there any quantum gravity theories that take the approach of adding general covariance to...

These days I've taken an interest in lithium ion batteries and the like, but I don't really know where to go to start learning about them. I saw the general engineering sources and the materials/chemical engineering sources threads, but there was nothing specifically about batteries- Does anyone...

In the context of group theory, there's a theorem that states that for a given positive integer \(n\) there exist finitely different types of groups of order \(n\). Notice that the theorem doesn´t say anything of how many groups there are, only states that such groups exist. In the proof of this...

Let $p$ be an odd prime number such that $p\equiv 2\pmod{3}.$ Define a permutation $\pi$ of the residue classes modulo $p$ by $\pi(x)\equiv x^3\pmod{p}.$ Show that $\pi$ is an even permutation if and only if $p\equiv 3\pmod{4}.$

I have started reading about Claudia de Rham's theory of massive gravity (https://www.imperial.ac.uk/people/c.de-rham) and how it can explain the mechanism of the expansion of the universe, among other thing. Lots of infos available on the above link. Has anyone found flaws in the theory or...

Hi, I have been studying Quantum Field Theory this semester! It seems that Shwartz and Peskin are the most popular choices when it comes to studying QFT. But apparently my professor have another "old" preference. He strongly suggested that we learn QFT from Zuber's book. I have looked at the...

All books in analytical mechanics explain the case of a particle moving on a given static surface. But what happen if, for example, the surface is having some deformation?. I imagine that the principle of virtual work, and hence, D'Alembert are no longer valid since the normal force by the...

Hello! I recently had a discussion with a person who's well-read on quantum physics and I was suprised by his claim that "light is in no sense regarded as a wave" in quantum mechanics. His support for this claim was that there are no wave crest or wave trough, there is nothing moving. What...

What is known about the eigenstates of the ##\phi^4## theory in QFT? Is there an informal understanding of how these states are organized in the nonperturbative regime? For example, are there known to be any bound states in any dimensions? How does the energy of a multiparticle eigenstate (if...

This is QFT for the gifted amateur, chapter 19, which is generating the various Feynman diagrams and rules. Some calculations are given but I encounter various problems when trying to work them all out. The starting point is that we want to calculate: $$\langle q| \hat S | p \rangle = (2\pi)^3...

I couldn't do it

This is about possible problems with the equations of EM theory of Faraday and Maxwell. EM theory states that when an electron moves a magnetic field is generated. Does this field move with electron in the direction of the electron (if the electron is in motion)? Or, does it stay still in the...

Hello, I request inputs about selecting a journal to submit an article in High Energy Physics. This is in formal theory as opposed to Particle Phenomenology. The article is more than ten pages so PRL is not an option. How would you choose between JHEP and Physical Review D? Which is rated higher...

As you all know: Planck evidence for a closed Universe and a possible crisis for cosmology https://www.nature.com/articles/s41550-019-0906-9 If confirmed, it would bring back the Big Bounce as a possible hypothesis for the evolution of the universe. This has long-term consequences being that...

Summary:: What are the prerequisites to study quantum field theory? What are the prerequisites to study quantum field theory?

Hello All, I am in need of an optics expert on imaging, and in particular a fresnel lens is being used as for an imaging device. From my understanding a condenser lens can be considered equivalent to a plano-convex lens. Setup: A condenser fresnel lens is used, for which both the fresnel and...

So this is for my car. If I have a bolt that I don't know what the ideal torque is supposed to be because the manufacturer of the car doesn't publish a torque, it's possible to calculate it? This can be done just give the specification of the bolt? Regardless of what the bolt is fastening? I'm...

I am reading Group Theory in a Nutshell for Physicists by A. Zee. I have big problems when learning chapter IV.1 Tensors and Representations of the Rotation Groups SO(N). It reads I can understand why ##D\left ( R \right )## is a representation of SO(3), but I hardly can see why the tensor T...

I have searched in vain for a detailed calculation of the frequency 9192631770 Hz from basic physical constants using Quantum Mechanics. Can anyone help with this please

Obviously the parenthetical part of the definition of ##F## means ##B\subset C## but we are not allowed to use ##\subset##. I do not know how to express implication with only union, intersection, and set minus without the side relation ##B\cap C = B\Leftrightarrow B\subset C##. This is using the...

Here is the Code File in an txt. I can on request provied the whole Program, which includes the PSE, AtomFunctions and many useful but not all implemented Funtions to solve the Many Worlds Problem in C#. Please feel free to ask questions via here or email [e-mail address deleted by Mentors] I...

If string theory is correct does it mean that elementary particles like photons, electrons, and quarks don't really exist or does it mean they do but are made of cosmic strings and so therefore are not elementary?

I derive the quadratic form of Dirac equation as follows $$\lbrace[i\not \partial-e\not A]^2-m^2\rbrace\psi=\lbrace\left( i\partial-e A\right)^2 + \frac{1}{2i} \sigma^{\mu\nu}F_{\mu \nu}-m^2\rbrace\psi=0$$ And I need to find the form of the spin dependent term to get the final expression $$g...

In Griffith's Introduction to Elementary Particles, he provides a very cursory introduction to group theory at the start of chapter four, which discusses symmetries. He introduces SO(n) as "the group of real, orthogonal, n x n matrices of determinant 1 is SO(n); SO(n) may be thought of as the...

Hello! Where can I find the source for this statement (i.e. a citation for it, ideally the original one): "any smooth k-sphere embedded in ##R^n## with 2n − 3k − 3 > 0 is unknotted". Thank you!

Well there was today some lecture on String theory which I got late since I was reading something else in Condensed matter physics and then I looked at a book that I have of stochastic Integration with jumps and by associativity of ideas I searched google for Stochastic String Theory and got to...

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I understand how do 3 no. equation come from 1 & 2 no. equation. But I am struggling to understand how do 4 no. equation come from 3 no. equation. Will anyone do the steps between 3 no. equation and 4 no. equation, please ?

How did you find PF?: Searching a way to share own thoughts I was wondering if we could gather all the laws of the universe and try to link anything to everything. I'm thinking of the possibility of finding links between quantum physics and the theory of evolution. It reminds me of the...

Suppose we have a hamiltonian $$H_0$$ and we know the eigenvectors/values: $$H_0 |E_i \rangle = E_i|E_i \rangle $$ We then add to it another perturbing Hamiltonian: $$H’$$ which commutes with $$H_0.$$ According to nondegenerate first order perturbation theory: $$\langle H \rangle \approx...

Summary:: I say the answer is A because these are reactive components that take and give back energy from the circuit so no voltage drop across the 2- L & C. Please let me know yours thoughts- thanks Hello I am a newby to electronics taking a class. Please review my thinking on this problem. I...

Hi, smart people This quarantine has got me thinking: Can we get people in the air? I have 3 questions that may or may not help answer this question. But I do appreciate any insight. 1. How would one accelerate (the average human) 100 feet in the air and sustain their position? Under normal...