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. M

    [Quantum Theory] Quality Texts

    Hello all, I am new to the forums here. My background is biochemistry and I currently work in a medical laboratory. What brings me here is a new found interest in physics, in particular, quantum theory. I would like to learn much more ranging from the basics to M theory etc.. I would...
  2. A

    Hermitization and a problem from Bohm's quantum Theory

    Hi, Bohm discusses several ambiguities is the hermitization of operators that consist of products op p's and x's (page 184 and further) He reaches the conclusion that an operator p^m x^n should be replaced by \frac{p^m x^n + x^n p^m}{2} on the grounds that this way the expectation...
  3. A

    Derivation in bohm's Quantum Theory

    halfway page 41 Bohm obtains for the action variable J = 2\int_{a(E)}^{b(E)} dq \sqrt{2m[E-V(q)]} Then he obtians the partial derivative to E "by a well-known theorem of the calculus": \frac{\partial J}{\partial E} = 2\left\{ \sqrt{2m\left[E-V(q)\right]} \right\}_{q=b} \frac{\partial...
  4. S

    A question about <The Quantum Theory of Fields> P.120

    The Quantum Theory of Fields, Steven Weinberg, P.120 above 3.3.24, it says, If the matrix elements of W between H0 - eigenstates are sufficiently smooth functions of energy, then matrix elements of W(t) between smooth super-positions of energy eigenstates vanish for t→±∞ I can't get...
  5. G

    No logically consistent relativistic quantum theory?

    I just read this as I was skimming over the preface of a relativistic QM book. My question is this; is it the case that there is not yet a logically consistent and complete relativistic quantum theory or is this statement just because the book was published in 1980 and there has since been...
  6. D

    About eq. (5.7.23) in Weinberg's The quantum theory of fields vol. I

    On page 237, Weinberg checked eq. (5.7.23) with an example when \mathbf p is along the three direction. Below that equation the phase factor \exp([-a + b - \tilde{a} + \tilde{b}]\theta)=\exp([2\tilde b-2a]\theta). Under the transformation p^0\rightarrow -p^0;\mathbf p\rightarrow -\mathbf p, the...
  7. T

    Exploring the Many Worlds Interpretation of Quantum Theory

    I have a question regarding the Many Worlds Interpretation(MWI) of quantum theory. Is there any proposed thought experiment that can/does provide some kind of solid/good proof for the MWI? I have an idea, an experiment and I am very curious about it's validity/soundness and the...
  8. J

    Layman question about quantum theory.

    Hi folks, I've been watching the Teaching Company videos on quantum mechanics, and I have a few questions regarding energy quantization. It seems he is saying that energy can only appear in the universe in discrete quantities, which explains blackbody radiation and the heat capacity...
  9. askhetan

    On the meaning of indeterminism of Quantum Theory

    On the meaning of "indeterminism" of Quantum Theory I was reading more into the details about the successes and failures(apparently none) of quantum theory. I have some very basic questions that do not require a mathematical answers. When quantum theory describes the indeterministic nature of...
  10. Z

    Can someone explain to me the wave theory and quantum theory.

    Basically I have looked up both out of curiosity and it just flew right over my head. I have a general understanding of the Quantum Theory (I need some understanding on the wave theory) but I can't figure out how they correspond with each other. Is the Quantum theory just a way of explaining...
  11. O

    Is It Possible to Find a Romney Voter in an Excited State?

    Has anyone seen this yet? It's really funny! Quantum theory of Mitt Romney.
  12. Demystifier

    What is quantum theory about? From quanta to M-theory

    https://www.physicsforums.com/blog.php?b=3873 What do you think? :-p Can you shed some light on that mess? Or even better, can you further confuse it?
  13. G

    What is the conflict between General theory of relativity and Quantum theory?

    General relativity and Quantum theory are two pillars of physics... The unified theory concept trying to combine 4 basic forces of nature. I don't know what is the actual conflict between gravitational force and other 3 forces?
  14. H

    Is there a quantum theory of magnetism?

    After asking my teachers the same question, I came away with the impression that the classical field model was as far as magnetism went, as if after Maxwell's macroscopic laws there had been no development into a quantum theory of magnetism. Were they wrong? Additionally, if they were to some...
  15. E

    Question about Quantum Theory regarding hydrogen atom and photon

    Assume the universe consists of a single photon, a single atom of hydrogen and a spherical detector (like an encompassing shell) with a semi-infinite radius. The photon gets "absorbed" by the hydrogen atom. Moments after the photon is emitted. My question is the following: from the time...
  16. G

    How fundamental are complex numbers in quantum theory?

    The initial development of QM inherited the use of complex numbers from Fourier analysis. Had Hartley analysis been invented first, is it possible that QM might have been formulated in terms of real-valued quantities instead, or are complex numbers in some sense natural or necessary when...
  17. W

    Questions in Weinberg The Quantum Theory of Fields Vol 1

    I have a few questions about the contents in Weinberg, the Quantum Theory of fields Vol1. First one : At the end of page 77(Sec 2.6), we got -ζ_σ=ζ_σ±1. From this, we could refer that ζ_σ should be proportional to (-1)^σ. But, in the next page, Weinberg concluded that ζ_σ =ζ(-1)^(j-σ). As I...
  18. S

    Quantum Theory of Metal- Mean Free Path

    Homework Statement I'm off by a factor of 106 and I have no idea why. "Silver has a density of 10.5*103 kg/m3 and a resistivity of 1.60*10-8 Ω*m at room temperature. Assume that each silver atom contributes one electron to the electron gas. Assume that EF = 5.48 eV" What's the mean free...
  19. S

    How Quantum Theory Explains Human Consciousness

    Hi, After some (limited) reading I've written an article on the captioned subject : http://3fathom.dyndns.org/QM-Consciousness.htm I post it here for feedback. Intellectual stimulation, please! Thanks, Stewart
  20. Physics Monkey

    Quantum theory of fluctuating space?

    Hi all, This is partially motivated by the thread on Ashtekar's new review and by old results on emergent spacetime in condensed matter models. Has any progress been made on the problem of emergent space (not emergent spacetime) in LQG or allied approaches? By this I mean problems that...
  21. R

    What is the difference between the Copenhagen Interpretation and Quantum Theory?

    They say Copenhagen Interpretation is not Shut Up and Calculate. So what really is Copenhagen Interpretation. What is its belief and statements? And how many percentage of physicists just focus on Quantum Theory and not care about interpretations and how many of them are into Copenhagen?
  22. A

    My personal criticism to Quantum theory and Human conscience

    my personal criticism to "Quantum theory and Human conscience" I have to do a couple of premises: 1. There are some previous posts on this arguments, among them I found interesting this one: https://www.physicsforums.com/showthread.php?t=334994 2.This topic requires a deep knowledge in several...
  23. B

    Quantum mechanics or quantum theory?

    Hi all, Can someone please explain to me how/when to properly use the terms quantum mechanics, quantum theory, and quantum physics? They aren't exactly interchangeable are they? Thanks,
  24. T

    Basic Question About Quantum Theory

    I want to preface this by stating that I have only a rudimentary knowledge of this subject. I've only began reading about quantum physics casually and on my own time. With that being said. Here's what I don't fully understand. Is the above quote actually stating that matter only exists...
  25. G

    Planck's quantum theory question

    electromagnetic energy is quantized (it only comes in discrete units related to the wave function)... my exam krackers book says: "if we transfer energy from one point to another via an electromagnetic wave, and we wish to increase the amount of energy that we are transferring without...
  26. E

    Integral's Results Challenge Quantum Theory: GR vs. QM

    http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html "ESA’s Integral gamma-ray observatory has provided results that will dramatically affect the search for physics beyond Einstein. It has shown that any underlying quantum ‘graininess’ of space must be at much smaller scales than previously...
  27. E

    Park's Introduction to Quantum Theory (Thoughts? I'm not a fan but I'm still reading)

    I own a copy of Griffith's Quantum Mechanics and I like how it is written very much but while skimming through Griffith's Particle book I saw a reference that said "at the level of Park" and I decided to investigate. I started to read through it and the text is structured the way most books...
  28. DrChinese

    Experimental Test Limits Predictive Power Beyond Quantum Theory

    As an example of how far things have come post Bell/Aspect, here is a just announced experimental test which limits any future expectations one might have on "completing" QM: An experimental test of all theories with predictive power beyond quantum theory, Terence E. Stuart, Joshua A. Slater...
  29. P

    Exploring the Quantum Theory of Magnetism: Callen's Result & the P_S Function

    \langle\hat{S}^z\rangle=\hbar\frac{(S-P_S)(1+P_S)^{2S+1}+(1+S+P_S)P_S^{2S+1}}{(1+P_S)^{2S+1}-P_S^{2S+1}} Can you tell me something more about Callen result? And this function P_S? In book Nolting "Quantum theory of magnetism" this relation is get from diferential equation...
  30. marcus

    Reconstruction of Quantum Theory by Lucien Hardy

    Lucien Hardy is a prominent foundations of QM guy at Perimeter and he just came out with this paper http://arxiv.org/abs/1104.2066 Reformulating and Reconstructing Quantum Theory Lucien Hardy 159 pages. Many pictures (Submitted on 11 Apr 2011) "We provide a reformulation of finite...
  31. J

    Quantum theory has affected technological advances?

    I keep hearing that we wouldn't have the same micro-electric circuts if not for the "advancments" of the quantum theory. Can someone explain this? Also, I don't believe the transistor is due to quantum physics... As far as I know, someone took some silicon "or any semi-conductive...
  32. S

    Use of potentials in Quantum theory

    In quantum mechanics, we rarely talk in terms of forces and acceleration. Instead, we work with potentials and energy, and we restrict ourselves to conservative systems where the energy is constant. Why? There are several alternative formulations of classical mechanics, all of which are...
  33. S

    Doubt regarding Planck's Quantum Theory

    Hello We all see this statement everywhere : "Planck's quantum theory states that the radiant energy emitted or absorbed by a body is in the form of discreet packets of energy called Quanta, and the energy of the quantum depends on the frequency by the formula E = hv" Now, my question is...
  34. G

    Quantum Theory: Physics Equations Book

    I was wondering if there was a book that listed and described the majority of physics equations, especially the ones pertaining to quantum theory?
  35. G

    Discover the Best Introductory Books for Quantum Theory | Expert Recommendations

    What are some good introductory books to Quantum theory.
  36. S

    Which Photon Energy Can Overcome a 3.00 eV Work Function?

    Homework Statement Of the following energies for photons, which is the least energy that could result in photoeletron production if the work function is 3.00 eV? a. 1.50 eV b. 2.90 eV c. 3.50 eV d. 6.01 eV Homework Equations KE = E - W The Attempt at a Solution I don't know...
  37. N

    Was the relation between E and f known BEFORE quantum theory?

    Hello, In classical EM, would one have said that red light carries less energy than blue light? (if both have the same intensity)
  38. rpt

    Why time is not an observable in quantum theory?

    Why "time" is not an observable in quantum theory? Why "time" is not considered as an observable as any other quantity in quantum theory? Is it because we cannot imagine anything in the absence of time?
  39. H

    How Do Electron Wavelengths Influence Diffraction Patterns?

    Homework Statement An electron is traveling at the "non-relativistic" velocity of 0.05 times the speed of light. Find its de Broglie wavelength. Explain what size an object needs to be in order to cause a stream of such electrons to form a significant diffraction pattern. Homework...
  40. N

    Exploring Quantum Theory: Schrödinger's Cat & the Many-Worlds Interpretation

    Hi I was wondering if someone could better explain this. I do not quite understand the implications. Thanks for any input. "Unlike the Schrödinger's cat thought experiment which used poison gas and a radioactive decay trigger, this version involves a lethal weapon and a device that...
  41. N

    A Few Clear Questions on the Nature of Quantum Theory

    Hello, I'm adding one or two words in bold in front of each question so you can browse for questions that interest you, please don't get turned off by the amount of text, I'll be very grateful even if you can answer just one. These are questions that popped up whilst following an introductory...
  42. W

    Einstein against Quantum theory

    Im currently reading Entanglement by Amir D. Aczel. It explains the arguments for and against quantum theory at the Solvay Conference in 1930. At the conference Einstein told Bohr that he could refute the uncertainty principle for energy and time. Einstein said he designed a box with an opening...
  43. E

    What is the rationale behind quantum theory and the afterlife?

    This really bugs me as people like Dinesh D'souza, Deepak Chopra and many other "spiritual scientists" claim that using quantum theory, they have evidence or proof of the afterlife. I really would like you guys' insight into this theory. I am personally agnostic but would rather not have this...
  44. W

    Shortcomings of quantum theory

    What do members think of the idea that the philosophy which began quantum theory in the first place was inadequate to explain and describe reality in a sufficiently deep way, so that we now have a theory which in many ways fails to describe the reality which we experience in the world?
  45. N

    Calculating Material Bandgap Using Quantum Theory

    Quantum theory says that the energy, E , (in eV) of a photon of wavelength λ is E = hc/eλ, where h is Planck's Constant, c is the velocity of light and e is the charge on an electron. Calculate the bandgap of a material which absorbs electrons shorter than 500nm.
  46. M

    What are the minimal conditions for a non-relativistic quantum theory?

    What are the minimal conditions to have a non-relativistic quantum theory like Schrödinger theory?
  47. 0

    Concerning Planck's quantum theory

    Considering Planck's assumption of quantized energy, was this idea proved later in quantum electrodynamics or any other theory of electromagnetic radiation? I have seen at places that Max Planck had an intuition about this idea, along with extensive research on the problem. Is intuition thing...
  48. C

    Revisiting the Completeness of Quantum Theory: A Scientist's Perspective

    I am sorry if this issue has been already addressed previously in this forums, but I have been looking for old threads on the subject and I haven't found any specific to the matter. If anyone knows about any, please let me know and sorry again. Otherwise, this is my question: "Is Quantum...
  49. U

    Google's most relevant names in Quantum Theory

    For fun only, put +"name" +"quantum theory" into google search, note approx number of matches :smile: 1. Einstein 407,000 2. God 268,000 3. Bohr 147,000 4. Planck 138,000 5. Heisenberg 136,000 6. Dirac 133,000 7. Weinberg 120,000 8. Feynman 114,000 9. Fermi 91,500 10. Schrödinger...
  50. O

    Second route to a relativistic quantum theory

    Hey everyone, from Srednicki's QFT textbook I've read that there has been work on the following attempt to obtain a relativistic quantum theory: instead of treating space as a label like time, promote time to an operator, such that you get 4-position and -momentum operators. Now I know that...
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