What is Quantum theory: Definition and 226 Discussions

In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and in condensed matter physics to construct models of quasiparticles.
QFT treats particles as excited states (also called quanta) of their underlying quantum fields, which are more fundamental than the particles. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding quantum fields. Each interaction can be visually represented by Feynman diagrams according to perturbation theory in quantum mechanics.

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  1. Narasoma

    I Lorentz invariance of quantum theory

    I read Lucien Hardy's paper whose tittle was "Quantum Mechanics, Local Realistic Theories, and Lorentz Invariant Relativistic Theories". There, he argued that lorentz invariant observables which involved locality assumption contradict quantum mechanics. I tried to follow his argument, but got...
  2. Run like hell

    Quantum Weinberg: The quantum theory of fields 2005 edition vs 1995

    Hi all, I'm about to buy the first volume of the series by Weinberg, but I'm a little worried about the edition, see I have a lot of requisites for a book before buying it. I've seen in the library the old hardcover edition and it looks fine for me: it's not written too small and the book even...
  3. T

    A Why General Relativity is non-renormalizable quantum theory?

    Hello I have not understand this.
  4. Urs Schreiber

    Insights Spectral Standard Model and String Compactifications - Comments

    Urs Schreiber submitted a new PF Insights post Spectral Standard Model and String Compactifications Continue reading the Original PF Insights Post.
  5. Jackie CH Liu

    A A Quantum Theory of Spacetime, the Physical Reality of Cross

    I just want to share my paper I recently wrote. I am trying to submit for publication: [Reference to unpublished paper deleted]
  6. Y

    What is Theoretical Physics and How Can It Be Discussed Without Math?

    Physics is one of my most favorite topics, particularly Theoretical Physics. I have no formal education in Physics but I read quite a bit Physics material and also some Physics books. I have B.Sc. in Materials Science and Engineering. I like to discuss advanced physics ideas logically and...
  7. scientepid

    I How can you visualize the Higgs field, or can you?

    Just when I thought I had finally wrapped my brain around relativity, Quantum theory took off. Then the Higgs Boson was discovered. How does the Higgs field under-pin relativity, namely space-time? How is the Higgs field distributed? Does it have curvature like space time, or is omnipresent...
  8. mertcan

    Quantum Quantum Theory Books for Understanding Derivations

    hi, I really want to dig valuable things out of quantum theory, also I have a big eagerness to see the derivation of formulas to understand the logic of this topic. Could you recommend me some nice books which may meet my needs I expressed at the beginning ?
  9. zdroide

    I Is there a real need for a theory of everything (ToE)?

    I been studying quantum field theory and standard model lately. I not see how a unified theory could fit between quantum field and special or general relativity. One being for big objects and the other for microscopic one. In fact, standard model not seem to be all proven. Many particles are not...
  10. B

    A The unification of quantum theory and relativity theory

    I was wondering if anyone here is familiar with Jim Beichler's Single(operational)Field Theory (SoFT)? In particular, I wanted to know if anyone could refute his work.. <crackpot link deleted> . More than anything, I would just like some opinions of it, as I don't have anyone to discuss such...
  11. E

    What are the branches of Quantum Theory?

    What are the branches that we can say are categorized under [Quantum Theory](http://en.wikipedia.org/wiki/Quantum_theory)? I am thinking they are QFT, Standard Model and Quantum Mechanics. How about atomic physics and Solid state physics? Also how about Quantum Gravity and Quantum Information...
  12. Bunduru

    Is Consciousness Linked to Quantum Superposition and Entanglement?

    Someone I know sent me something for validation. Is this train of thought headed in the right direction? Please forgive him for the informal language used; he doesn't have a physics background...Thanks! "Superposition simply implies that conscious beings always have choices, and that...
  13. Rapier

    Quantum Theory of Earth Satellite Analogy to Bohr Model

    Problem: In analogy to the Bohr Theory of the hydrogen atom, develop a quantum theory of Earth satellites, obtaining expressions for the orbit radius (r) and the energy (E) in terms of the quantum number (n) and the other relevant parameters. A satellite of mass 1000 kg is in a circular orbit of...
  14. V

    Physics Research career on particle physics and quantum theory

    I want to do research as a physicist on either particle physics or quantum theory.For that I wish to pursue higher degrees in world's leading institutions of physics.What should be my career path after i complete my graduation in Physics which i am presently persuing.
  15. E

    Is there a quantum theory of other entities besides particles and fields?

    QM or quantum mechanics is quantum theory of particles QFT or quantum field theory is quantum theory of fields (containing QED, QCD, and maybe QG) are there none beside particles or fields.. why is there no quantum theory of other stuff?
  16. J

    A particle near an event horizon

    Homework Statement A sub-atomic particle is near the event horizon of a black hole. Due to the nearby gravitational field, the Ricci Curvature Tensor is changing rapidly. The particle then performs quantum tunneling. Homework Equations Which version of spacetime does the tunneling particle...
  17. M

    Trying to understand electromagnetic waves in quantum theory

    I am an 8th grade science teacher who has regularly used the pre-Bohr planetary-like model of the atom (like most 8th grade science teachers and textbooks) to help students start to understand atomic physics. But in my free time, I have repeatedly tried, and failed, to really understand the...
  18. F

    (Algebra) Quantum Theory - Cauchy-Schwartz inequality proof

    Homework Statement Given two arbitrary vectors |\phi_{1}\rangle and |\phi_{2}\rangle belonging to the inner product space \mathcal{H}, the Cauchy-Schwartz inequality states that: |\langle\phi_{1}|\phi_{2}\rangle|^{2} \leq \langle\phi_{1}|\phi_{1}\rangle \langle\phi_{2}|\phi_{2}\rangle...
  19. A

    Readings for a general understanding of Quantum Field Theor?

    Hi all, I have an undergraduate degree in Physics, but I've since specialized in more environmental applications of Physics. I've taken a few quantum Physics courses as part of my degree, but, if I'm being honest, what I took from those courses was more of a mathematical view of the field, such...
  20. TrickyDicky

    Time dependence in quantum theory

    I'm trying to elucidate certain concepts about time dependence and perturbation theory in quantum mechanics and QFT. I get the impression that most of the important results that in principle can be considered to have a time dependence can actually be calculated in terms of time-independent...
  21. B

    How important is quantum theory to cosmology/astrophysics?

    I'm asking this to help with making subject choices. One thing I'd like carry away from this degree is an overall understanding of the origin and structure of the universe. I'm wondering how much quantum mechanics and quantum field theory should I take to gain this understanding. I have heard of...
  22. J

    Interested in learning quantum theory

    Hello I have an interest in assimilating new knowledge and would be most in debted if you could give me a possible crash course in the most used phrases, words and equations used in quantum therory and quantum mechanics so I can know what to study first in order to get a good grasp on the...
  23. E

    Topic: Modeling Time and Velocity Using Integers in Relation to the Real Numbers

    If a ball is held at rest on a slope and then released what is it's next velocity? How can it's velocity change from nothing to something ?If the change from zero is infinitesimally small would this contradict the Quantum Theory as it's change of energy would be continuous.
  24. W

    Quantum theory of radiation

    hi , i am not getting how harmonic oscillator is linked to the modes of radiation field- as explain by Dirac in his quantum theory of radiation.
  25. ShayanJ

    Exploring the Complexity of Single Photon QED: Questions and Confusions

    QM started as a non-relativistic theory for microscopic particles. Then there were the attempts to make it relativistic and the results of those attempts suggested that you can't have a relativistic system with a fixed number of particles. But things are a bit different for photons. A...
  26. P

    Intro help for quantum theory -Middle School

    Ok -- My son is in Middle School (Math Level Algebra 2) and currently is on the math team with an aptitude for science and is interested in Quantum Theory. He just picked up a copy of Illustrated Brief History of time and the Universe in a Nut Shell -- As a parent I am a little lost and I am...
  27. Demystifier

    What is the fundamental theory of quantum mechanics?

    Quantum theory is called "quantum" because it is supposed to be about quanta - discrete spectrum of certain quantities which in classical theory have a continuous spectrum. But actually, quantum theory does not need to be about quanta at all, because many quantum quantities actually have a...
  28. vead

    Quantum theory and electronic engineering

    I'm an electronics engineering student and I've only had a introductory one-semester course to quantum mechanics, so I don't really have much knowledge on quantum physics I'd like to get more information about the topic Quantum Information in electronics what is real use of quantum theory...
  29. F

    A new version of quantum theory

    Has anybody seen this theory before, If so, do you have more information on it. "An introduction to the tomographic picture of quantum mechanics" http://arxiv.org/pdf/0904.4439.pdf Thanks
  30. Y

    Bohr's Correspondence Principle and quantum theory

    Homework Statement Does Bohr's correspondence principle apply only to quantum theory? Can you give an example of the applications of this principle to relativity theory? Homework Equations The Attempt at a Solution According to my textbook, Bohr's Correspondence Principle simply...
  31. I

    Quantum theory and the implications on human life

    Hey guys, I’m interested in knowing if there are any known implications of quantum theory in the macroscopic scale, regarding randomness. Small particles behave probabilistically, but that has any consequence in the macroscopic or in the microscopic (but higher scale than those particles –...
  32. V

    Space-time continuity as an assumption in quantum theory

    There is an apparent conflict between relativity and quantum theory, in which case quantum theory must be redundant isn't it as it explicitly makes the assumption that spacetime is continuous whereas relativity in-fact derives the notion that spacetime is continuous from an experimentally...
  33. C

    How to Convert Between Joules and Kilojoules in Quantum Theory

    Homework Statement Calculate the size of the quantum involved in the excitation of a molecular vibration of period 20 fs. Express the results in joules and kilojoules per mole. Homework Equations ΔE = h/t The Attempt at a Solution I converted fs to seconds and used the above equation to...
  34. marcus

    Radical new foundations for both quantum theory and space-time

    This was just posted http://arxiv.org/abs/1308.2206 Energetic Causal Sets Marina Cortês, Lee Smolin (Submitted on 9 Aug 2013) We propose an approach to quantum theory based on the energetic causal sets, introduced in Cortês and Smolin (2013). Fundamental processes are causal sets whose...
  35. marcus

    Oeckl positive boundary form of quantum theory

    Robert Oeckl proposed this new formulation of QT in December of last year. http://arxiv.org/abs/1212.5571 I think it's important and worth learning about. It could replace Dirac form of QT for some (especially general covariant) quantizations. Historically it derives from 1980s work by Witten...
  36. O

    Sakurai : Approximations in the construction of Quantum theory

    From 'Modern Quantum Mechanics, revised edition' by J.J. Sakurai, In page 44 the translation (spacial displacement) operator \mathcal{T}(d \boldsymbol{x'}) is introduced. \mathcal{T}(d \boldsymbol{x'}) \mid \boldsymbol{x'} \rangle = \mid \boldsymbol{x'} + d \boldsymbol{x'} \rangle It is...
  37. Greg Bernhardt

    Quantum Consistent Quantum Theory by Griffiths

    Author: Robert B. Griffiths Title: Consistent Quantum Theory Amazon Link: https://www.amazon.com/dp/0521539293/?tag=pfamazon01-20
  38. micromass

    Quantum Lectures on Quantum Theory by Isham

    Author: Chris J. Isham Title: Lectures on Quantum Theory: Mathematical and Structural Foundations Amazon Link: https://www.amazon.com/dp/1860940013/?tag=pfamazon01-20 Prerequisities:
  39. Greg Bernhardt

    Quantum The Quantum Theory of Fields by Steven Weinberg

    Author: Steven Weinberg Title: The Quantum Theory of Fields Amazon Link: https://www.amazon.com/dp/052167056X/?tag=pfamazon01-20 Prerequisities: Contents:
  40. inflector

    Smolin's Edge Response - Deterministic Quantum Theory?

    Did anyone read Lee Smolin's response to the Edge.org question: "What are you worried about?" http://edge.org/response-detail/23778 Opening paragraph:
  41. A

    The expectation value in quantum theory

    Going from the abstract state vector lψ> and the mean-value of an observable x (operator) given by: <x> = <ψlxlψ> I want to show how that is done in the position basis: So I take: <x> = <ψlxlψ> And insert completeness in front of the state vector to get the expansion involving the...
  42. A

    On the quantum theory of radiation

    Hi, In section 3 of Einstein's "On the quantum theory of radiation" Einstein says equations (8) and (9) follow from (7) and Wien's displacement law. I don't see how that is. For example, if we replace (8) by \frac{A_m^n}{B_m^n} = \alpha \frac{\nu}{T} \nu^3 All conditions still seem to...
  43. A

    Can Quantum Theory Apply to Large-Scale Universe Phenomena?

    Can quantum be applied to the larger scales in the universe and what would be the affect of uncertainty and exclusion. If the answer is yes where can I get the answer to a calculation I am interested in.
  44. E

    Quantum Theory for an Armchair Physicist?

    Hi. I'm looking for a good "popular press" treatment of quantum theory for a bright and interested layman. Specifically, something that will explain to me Bell's Inequality in a manner that I can grok, with an emphasis on concepts and (if at all possible) a minimum of mathematics. Can...
  45. W

    Quantum Theory: Minimizing Energy Subject to Normalization Contraint

    Homework Statement In a previous problem, I derived that for a given wavefunction \Psi (x) in a potential, the energy of the state could be calculated as a functional of the wavefunction. I now need to minimize the energy, subject to the usual wavefunction normalization constraint, and show...
  46. nukeman

    Looking for suggestions for an experiment involving Quantum theory and Biology?

    Hey guys, (this is not homework). I am working on a "out of class" project right now, and I want to get some ideas on ways you can run an experiment in which you can apply quantum theory to biology? Thanks :)
  47. F

    Quantum theory vs String theory (Who's the best?)

    Just about few hours ago,my friend and i argued about which theory is the best theory nowadays ever existed. I claim that string theory is the only game in town as what i heard from Dr. Michio Kaku's TV shows, while my friend strongly disagree with it. He claimed that quantum theory is the best...
  48. W

    Quantum Theory: Operator Exponentiation

    Homework Statement Let \left|x\right\rangle and \left|p\right\rangle denote position and momentum eigenstates, respectively. Show that U^n\left|x\right\rangle is an eigenstate for x and compute the eigenvalue, for U = e^{ip}. Show that V^n\left|p\right\rangle is an eigenstate for p and...
  49. W

    Two Questions in Quantum Theory

    Quantum Theory: Commutators of Functions of Observables Homework Statement First is a question from Sakurai Modern Quantum Mechanics, 2nd ed., 1.29a. Show that [x_i,G(\mathbf{p})] = i\hbar\frac{\partial G}{\partial p_i} and [p_i,F(\mathbf{x})] = - i\hbar\frac{\partial...
  50. J

    Simplified spacetime and Emc2 vs Quantum Theory

    "simplified" spacetime and Emc2 vs Quantum Theory I tried to post this in the textbook or homework section but it kept saying i was not allowed? So, to attempt to keep to the rules of the site i will ask my question but also try and start a discussion to try and keep things interesting. Ive...
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