# Quantum theory Definition and 20 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. ### Application of Quantum Theory to Cognition, Decision Making and Finances - Andrei Khrennikov

Source: http://emqm17.org/presentations/
2. ### A Cluster Decomposition.Vanishing of the connected part of the S matrix.

Im following Weinberg's QFT volume I and I am tying to show that the following equation vanishes at large spatial distance of the possible particle clusters (pg 181 eq 4.3.8): S_{x_1'x_2'... , x_1 x_2}^C = \int d^3p_1' d^3p_2'...d^3p_1d^3p_2...S_{p_1'p_2'... , p_1 p_2}^C \times e^{i p_1' ...
3. ### I QFT - Confusion about Fermi's Golden Rule & Cross-Sections

Hey there! I've recently been looking at calculating amplitudes, densities of states and scattering cross sections in QFT, but am having a little bit of trouble with the exact form of the cross section - particularly with factors of ##2E## for the energies of the incoming and outgoing particles...
4. ### Insights The Quantum Mystery of Wigner's Friend - Comments

Greg Bernhardt submitted a new blog post Wigner's Friend Continue reading the Original Blog Post.
5. ### Insights The Unreasonable Effectiveness of the Popescu-Rohrlich Correlations - Comments

Greg Bernhardt submitted a new blog post The Unreasonable Effectiveness of the Popescu-Rohrlich Correlations Continue reading the Original Blog Post.

9. ### B What is the underlying reason for stimulated emission?

I learned that stimulated absorption, spontaneous emission and stimulated emission are three fundamental concepts in the process of radiation. Among these three concepts, I found stimulated emission really hard to comprehend, it says when atom in its excited state is stimulated by external...
10. ### I Physical interpretation of a Hamiltonian with a constraint

Dear physics forums, What is the physical interpretation of imposing the following constrain on a Hamiltonian: Tr(\hat H^2)=2\omega ^2 where \omega is a given constant. I am not very familiar with why is the trace of the hamiltonian there. Thanks in advance, Alex
11. ### Quantum Thoughts on Sakurai's Quantum Mechanics textbook

I have taken a couple of graduate level courses in quantum mechanics and I have decided to read Sakurai's Modern Quantum Mechanics from cover to cover, partly because I liked his style and I have heard a lot of people talk highly of it. But I recently realized that the second edition of the same...
12. ### Show that d^4k is Lorentz invariant

Homework Statement Show that ##d^4k## is Lorentz Invariant Homework Equations [/B] Under a lorentz transformation the vector ##k^u## transforms as ##k'^u=\Lambda^u_v k^v## where ##\Lambda^u_v## satisfies ##\eta_{uv}\Lambda^{u}_{p}\Lambda^v_{o}=\eta_{po}## , ##\eta_{uv}## (2) the Minkowski...
13. ### Strings, Virasoro Operators&constraints, commutator algrebra

Homework Statement [/B] Question: (With the following definitions here: ) - Consider ##L_0|x>=0## to show that ##m^2=\frac{1}{\alpha'}## - Consider ##L_1|x>=0 ## to conclude that ## 1+A-2B=0## - where ##d## is the dimension of the space ##d=\eta^{uv}\eta_{uv}## For the L1 operator I am...
14. ### 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.
15. ### 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...
16. ### 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...
17. ### Quantum Theory definition

What are the branches that we can say are categorized under [Quantum Theory](http://en.wikipedia.org/wiki/Quantum_theory)? [Broken] 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...
18. ### 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...
19. ### 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. ### How does anything change?

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.