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. D

    The action principle in quantum theory

    Hello! I wonder is there exist a way to deduct the conserved current equation, generator of the symmetry transformation and so on from a total quantum L with the help of action principle? Many books i have seen deduct the physical quantity from the classical L or...
  2. PhanthomJay

    Planck's Length and the Quantum Theory of Gravity

    Will a Quantum Theory of Gravity help to explain what is going on within the Planck Length? Or what may have went on in the first 10^-43 seconds after the Big Bang? Will it credit or discredit the extra dimensions of Space-Time per String/M-Theory? Or do the Heisenberg and Quantum Uncertainty...
  3. E

    Entropy, microscopic quantum theory of space-time, lorentz invariance

    The existence of entropy in gravity implies that there are microscopic degrees of freedom in space that carries the entropy. This implies space is discrete. Discrete space breaks lorentz invariance, which has been strongly constrained by both FERMI and thought experiments. String theory...
  4. S

    Quantum Theory (physical chemistry)

    Homework Statement Consider the one dimensional space in which a particle can experience one of three potentials depending on its position. They are: V=0 for -\infty<x\leq0, 0, V=V_{2} for 0\leqx\leqL, and V=V_{3} for L\leqx<\infty. The particle wavefunction is to have both a component...
  5. L

    Weinberg: The Quantum Theory of Fields I, Eq. 2.5.14

    First off I am a physics instructor at a local school and am trying to learn QFT from books. So this is not homework for a class at all. I am trying to re-derive Eq. 2.5.14 from the above equation to it and it is in fact that "above" equation which is my hurdle. It is on pg. 66 for those of...
  6. K

    Quantum Theory Book Recommendations

    I wasn't really sure if here was where I should be posting such a question, but all the same; I'm currently taking a Modern Physics course and find myself struggling immensely with some of the problems and derivations, particularly when it comes to Schrödinger's equations. I was wondering...
  7. G

    Understanding Quantum Theory: A Beginner's Guide to Particle Physics

    I have a particle physics question. I know I'm in the wrong forum, but when I posted this to the quantum forum, it got lost because of all the students asking for help with their physics homework. Can anyone point me to a book or article or youtube video or some resource that could help me with...
  8. 0

    Exploring the Quantum Theory of the Past

    Does anyone know of a quantum theory of the past. If I measure an observable, this gives me certain information of the possible states it has been in before the wave function collapse. I can exclude those states without any overlap with the observed eigenstate. But it is more complicated...
  9. P

    Redshift, Quantum Theory, and the Standard Model

    I am stuck on a few cosmological points I could use some help on. The first being "redshift" as an indication of inflation. To me red shift just seems like it's a result of the degradation of light quanta over long distances. Since light slows down in environments close to absolute zero...
  10. I

    Quantum Theory vs. Relativity: Explained in Layman's Terms

    can someone tell me what to read in "layman " terms ,the main differences between quantum theory and relativity?
  11. P

    Workshop on Weinberg's Quantum Theory of Fields

    Hello, I would like to re-open a topic raised by Si in 2003. I agree with Si that Weinberg's vol.1 is a reference for setting the fundings of QFT, but it lacks explanations between the lines. I would then suggest to post here the questions everyone has on this book, so that people...
  12. M

    Old Quantum Theory and the Linear Harmonic Oscillator

    Homework Statement Calculate the quantized energy levels of a linear harmonic oscillator of angular frequency $\omega$ in the old quantum theory. Homework Equations \[ \oint p_i dq_i = n h \] The Attempt at a Solution This is supposed to be a simple "exercise" (the first in...
  13. B

    Is causality challenged by quantum theory's reliance on indeterminacy?

    Hello all. I am merely an inquirer into physics; not even an amateur. The question that has puzzled me with regard to quantum theory/physics/mechanics, is the status of causality. What is the status of causality in quantum theory? I would be grateful if anyone could explain this to...
  14. R

    Ideas on Quantum Theory Curriculum

    So I've been given the opportunity to create my own undergraduate honours program. I can take whatever courses from whatever faculty I want at almost any university (although I'm keeping it local). I can create from scratch courses that do not yet exist immediately prior to taking them. I am to...
  15. R

    What is the best book for learning quantum theory?

    I'm sure this question comes up a lot around here (I can see as much from the other threads on this page), but the flavour of this question is particular to me: I'm looking for books on quantum theory. Now, I say "quantum theory" because I'm not entirely sure wherein lies the subsets of...
  16. G

    Relativity Theory and Quantum Theory

    Does the reason for the incompatibility between Relativity Theory and Quantum Theory lie in how Relativistic equations for the space-time manifolds taken on a differential scale differ from Quantum Theory's equations (I don't even know if they differ or if there are such equations...I'm only...
  17. Fredrik

    Lectures on quantum theory, by Chris Isham

    This is an excellent book, so I'm surprised it's only been mentioned once before in the science book forum. It deserves a lot more attention than that, so I thought I'd at least post a recommendation. The full title is "Lectures on quantum theory: mathematical and structural foundations". If you...
  18. A

    Nature of quantum theory of gravity.

    Can anybody explain why the theory describing quantum gravity is expected to be discrete (rather than a continuum theory), nonlocal (rather than a local theory) and Lorentz violating (rather that a Lorentz invariant theory)?
  19. pellman

    What is the role of proper time in quantum theory

    What role does proper time play in quantum theory? Does proper time have any meaning in QM? By quantum theory I should perhaps say "relativistic quantum mechanics" since I don't know enought QFT to ask a proper question. If there were a proper time \tau parameter or dynamical variable...
  20. K

    Old Quantum Theory = Oscillations?

    Hello again, I am having this problem understanding Quantum Theory, when it comes to calculations and applications. I'll give you an example. When using the Bohr Sommerfeld quantization rule we use calculus in...what? Is it an oscillating particle?? That's what I can't seem to understand...
  21. G

    Discussions Regarding Quantum Theory , by David Bohm

    Discussions Regarding "Quantum Theory", by David Bohm Hi, I am an Electrical and computer engineer. I recently decided to start studying physics in my spare time in the futile attempt to understand the universe, or at least parts and interpretations of it. I decided to start with quantum...
  22. Q

    Exploring the Quantum Theory and Electron Orbital Distances

    can anyone please explain in detail what the quantum theory is and how it is different from normal physics?
  23. scupydog

    The Quantum Theory Behind Lasers and Masers

    Hi does anyone know where i can find an online version of the quantum mechanical theory behind lasers and masers. regards Dave.
  24. B

    How many of you guys accept quantum theory?

    By ways, I mean, how many of you guys accept that there is only meaninglessness behind "things we can't know anything about?" How many side with Dirac? Or, are you with Einstein, Podolsky and Rosen in at least thirsting for some tenable explanation for phenomena such as entanglement? Are you at...
  25. Peter Morgan

    Field theory approaches to understanding Quantum Theory

    Why do people who think that quantum theory is great go on and on that classical particle models are impossible? Why do they never mention fields? There are endless articles in Nature on the latest experiment and why classical particle models won't do. Give or take the de Broglie-Bohm approach...
  26. A

    Quantum Theory in Curved Spacetime

    I recently checked out Davies' and Birrell's book "Quantum fields in curved space" and I noticed that it was written in 1982. My question is, did the authors know about the anomalies and that QFT could not incorporate gravity at that time? The table of contents has nothing on anomalies. I am...
  27. D

    Is quantum theory really necessary?

    I often encounter statements to the effect that classical physics cannot describe processes at the atomic and subatomic level. I also understand fully well that no one ever has to date successfully described these quantum processes, even the most basic ones, using classical physics. But that...
  28. X

    A linear theory (quantum theory)?

    Some textbooks say that quantum mechanics is a linear theory. I don't know why? How can we decide a theory is a linear theory? Thank you!
  29. Hydr0matic

    Exploring Bell's Assumptions and Quantum Theory

    The other thread here made me read up a bit on Bell, EPR, Aspect's and so on.. An important part is ofcourse the assumptions discussed intensively in the other thread. vanesch expressed my view about this: Why isn't it obvious here that the mistaken assumption is within quantum theory? Is...
  30. N

    Commutation relations in relativistic quantum theory

    Given the Hamiltonian H = \vec{\alpha} \cdot \vec{p} c + \beta mc^2, How should one interpret the commutator [\vec{x}, H] which is supposedly related to the velocity of the Dirac particle? \vec{x} is a 3-vector whereas H is a vector so how do we commute them. Is some sort of tensor product in...
  31. Loren Booda

    Exploring Friction at the Quantum Level

    Recently, macroscopic friction has been explained at the quantum level. Can you give a simple synopsis of how atomic forces act to create such nonconservative systems?
  32. H

    Gravitomagnetism and quantum theory of gravity?

    I was reading this: http://www.esa.int/SPECIALS/GSP/SEM0L6OVGJE_0.html and apparently it seems that Martin Tajmar seem to have made a significant step towards the long sought quantum theory of gravity. and at the same time he proven wrong some aspect about general relativity. althought he...
  33. E

    So, the question is: Can hidden variables explain quantum behaviour?

    Please tell me what I'm thinking wrong. Take Flatland and imagine it on a piece of construction paper. We theorize that the world is curved, so for simplicity's sake, roll up the construction paper into a cylinder. Now take a pencil and poke it completely through. To the Flatlanders, they...
  34. hagopbul

    Exploring Atom's Mysteries: Bohr Model & Quantum Theory

    Atom But? I am studying atom and I reach the Bohr model & the quantum theory of atom for now. But I have some point that I don’t understand for example: 1st We assume that the positive charge (protons) and 99% of the mass (neutrons) are stable, (in most cases), in the center of the atom .And...
  35. C

    Does Quantum Theory by David Bohm Cover All Aspects of Quantum Mechanics?

    David Bohm's "Quantum Theory" i was wondering if "Quantum Theory" by David Bohm covers everything in quantum mechanics
  36. P

    Hydrogen energy levels (quantum theory)

    Hello, I really need help with this question. Continuum is when the proton/electrons exist in the continuous band of energy levels above the highest bound energy level of the atom. What can be said about the combined energies of the proton and electron is the hydrogen atom is in...
  37. M

    A relativistic quantum theory of gravity

    Hello, I would like to submit my own "crazy" theory of quantum gravity for discussion. The relevant paper was posted in the ArXiv recently E.V. Stefanovich "A relativistic quantum theory of gravity" http://www.arxiv.org/physics/0612019 Let me make sure I haven't ignored any of the...
  38. Q

    Understanding the Well-Defined Properties of Quantum Systems

    I want to start with my current understanding of QT before i get into my question: Quantum theory states that any physical system remains in a superposed state of all possibilities until it interacts with the mind of an observer. Is this essentially correct?
  39. Demystifier

    Is quantum field theory a genuine quantum theory?

    In my recent paper http://xxx.lanl.gov/abs/0705.3542 entitled "Is quantum field theory a genuine quantum theory? Foundational insights on particles and strings" I argue the following: Practically measurable quantities resulting from quantum field theory are not described by hermitian...
  40. D

    Two slit experiment - quantum theory - problem

    Hi, I'm new and I'm from Poland. I have problem with equation(average number of photons registered behind pinhole 1 in two slit experiment): \bar{n}_1=\langle n|a_{1}^{+}a_{1}|n\rangle=\frac{\langle 0|(a^{+})^{n}a_{1}^{+}a_{1}(a^{+})^{n}|0\rangle}{n!} using...
  41. V

    Quantum theory, consciousness and existence

    Hi everyone I'm currently readig Bruce Rosenblum & Fred Kuttner's book Quantum Enigma: Physics Encounters Consciousness and studying to be an NLP practitioner as well as practising tai chi. There are lot more parallels in these three areas than I imagined there would be. I have a few...
  42. J

    Ether before relativity in quantum theory also

    I think this all makes sense to me, but I've never heard of this in lectures or in books, so I'll check if I'm getting this right. In one dimension Galilean coordinate transformations are x'=x-ut and t'=t. Momentum transforms as p'=p-mu, and energy is E=p^2/(2m). With a quick calculation I get...
  43. C

    Why do we need a quantum theory of gravity? What would it do?

    Why do we need a quantum theory of gravity? What would it do?
  44. A

    Questions in the quantum theory of radiation

    I have several questions in quantum optics or the quantum theory of radiaition. I begin by the following three questions: How can a DC field ( static field) be constructed using photons ?? If the spin of photons is related to circular polarization of the constructing fields. How can...
  45. L

    Quantum Theory - The Photo-Electric Effect

    Hi, I got this question and I have spent a while trying to crack it but I'm still stuck. White light, with frequencies ranging from 4.00 x10^14 Hz to 7.85x10^14 Hz, is incident on a potassium surface. Given that the work function of potassium is 2.24 eV, find the following values. (a) the...
  46. P

    Quantum Theory makes me go crazy

    I am in a General Chemistry class but we are on the chapter: Quantum Theory and Electronic Structure of Atoms. I do not understand my teacher and want some kind of real human being to explain to me how to find an answer to the question I'm going to give. Please do not think that I'm cheating on...
  47. N

    Speed of universe and quantum theory

    I have just read slate.com which debunks string theory. What I am about to say will probably display my ignorance and lack of education however I am trying to understand QT. Here goes. I have read the universe is expanding. I, in my elementary way, understand QT is a condition where you...
  48. Q

    Copenhagen interpretation of Quantum Theory

    I found a website that says: "The still-dominant "Copenhagen interpretation" of Quantum Theory developed by Niels Bohr, Werner Heisenberg, Wolfgang Pauli, and others says two basic things: 1. Reality is identical with the totality of observed phenomena (which means reality does not exist...
  49. S

    Regarding the discovery of quantum theory

    Why did Max Planck think that energy ought to be radiated in quantas? What were the shortcomings of the earlier theories that made the Quantum Theory a necessity. For example, the shortcomings in Galileo's model of planetary motion was that it could not explain why did the planets revolve around...
  50. B

    Hyperspace engine (Heim's Quantum Theory)

    A hyperspace (faster then c) engine is being worked on by the U.S. It would work by going into another dimension based of of Heim's Quantum Theory. How realistic is this and could we go faster then c with enough energy like in the experiment. Obviously not relevant to us but in parallel? If you...
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