What is Quantum theory: Definition and 225 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.
Usually states and observables are treated as fundamentally different entities in quantum theory. But are they really different? A state can always be represented by a density "matrix", which is really a hermitian (or self-adjoint) operator. Since observables are also hermitian (or self-adjoint)...
In interpretations of quantum mechanics there are two types of physicists: those who care about ontology and those who don't. The ontologists, or realists, want to know what is the world made of. The non-realists, on the other hand, think that this question is not relevant to physics.
Usually...
hello i would to get some help with my homework.
1. true
2. i dont know
3. true
4. i dont know
5, false
6. i dont know
about 2,4,6 i really have know idea what to think I really appreciate help
My understanding of quantum theory and information theory is that, given complete information on the state of the universe at present, it is possible to predict its state at all times in the future and past. 3 questions: 1: is this true? 2: how are quantum-probabilistic outcomes accounted for...
Heisenberg is regarded as a major figure of the Copenhagen interpretation. In a certain degree, he is influenced by Logical positivism.
Despite of that, has he any ontological commitments regarding the Quantum theory?
Or, he considers the Quantum theory merely as calculation procedure for...
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.040403
In QM, I was taught that the imaginary unit ##i## in wave functions is merely a mathematical tool. It has no physical meaning. We can always take the real part of the complex wave functions. Therefore, there should be some...
It came to my attention yesterday this, from my ignorant point of view, amazing paper that describes what it looks as another Theory of Everything: https://arxiv.org/abs/2110.02062
If I didnt understand incorrectly, from first principles / a pre quantum theory (Trace Dynamics, 8D octonionic...
Is it possible to have some kind of General Relativistic Quantum Theory without passing through the stage of Quantum Field Theory (where Quantum Theory is married to Special Relativity)?
Einstein attached primary significance to the concept of general covariance as shown in this letter in 1954...
Every second the universe branches into 5000 universes and each of those 5000 universes branches into 5000 more after one more second.
Now, consider an 80 year old person, he has lived close to 80*365*24*60*60 seconds, which is 2.5 Billion seconds. So, in his life time, universe has branched...
Hello!
I am a 'mature' learner and am fascinated by all kinds of physics and math ideas. Learning is the key to enjoying science and keeping an open mind. I must admit, I am not very sharp on my physics skills and my calculus is pretty rusty now (I don't work in the science field, per se) so I...
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...
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...
Summary: Is "free will", as it is usually interpreted, a foundational assumption of QM?
I've done a search using the term "free will" and have done a ctrl+f "free will" to go through those threads but I feel that the best way to find definitive clarification on my question is to ask specific...
[Edit 23.12.2019: A much extended, polished version of my contributions to this thread can be found in my paper Born's rule and measurement (arXiv:1912.09906).]
Well, it is simpler than to introduce in full generality Born's rule.
Everything can be motivated and introduced nicely for a qubit...
Quantum theory is widely thought to be a theory of the fundamental microscopic constituents of matter. It is supposed to tell us something about how matter behaves at the fundamental microscopic level, from which the classical macroscopic behavior should somehow emerge as an approximation based...
Have there been any recent developments in the attempt to unify the standard model of quantum theory with General Relativity? It appears the no progress has been made recently in string theory or loop quantum gravity.
From here:
From here:
Peres writes on p.11:
And on p.58:
Note that Peres says that these issues are not yet fully understood!
On p.63, Peres writes:
On p.424:
And on the next page:
The footnote quoted by Peres says:
And on p.25, where Peres introduces ensembles, he says (like Gibbs...
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' ...
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...
Square law?
i raise this question because of recently reading some QM, and realizing that for significantly short periods of time, it becomes hard to detect the mathematical patterns. E.g. in the double slit experiment, the standard pattern doesn’t appear after just a few photons. It takes...
Earlier this morning I came across this article -- https://phys.org/news/2018-09-errors-quantum-world.html -- which is about an open for all article in Nature Communications -- https://www.nature.com/articles/s41467-018-05739-8
I've yet to fully work through the paper, but if this thread is...
Hi all,
I'm trying to find a mathematical way of showing that given a complete set $$\left |a_i\right \rangle_{i=1}^{i=dim(H)}∈H$$ together with the usual property of $$\left |\psi\right \rangle = ∑_i \left \langle a_i\right|\left |\psi\right \rangle\left |a_i\right \rangle ∀ \left...
Homework Statement
Approximately 4% of the intensity of light is reﬂected at a glass-air boundary. Classically one expects roughly 8% of light to be reﬂected from a thin glass plate (4% at the front and back boundary). Outline brieﬂy what quantum theory predicts for a single photon instead of...
In the following there is a proof, for positive values of ##a## only, of (8.18) of Kaku, reference 1, I quote'
$$\int_{-\infty}^\infty~\mathrm{d}p~e^{iap^2+ibp}=\sqrt \frac{i\pi}{a}e^{-ib^2/4a}~~~~~~~~~~~~~(8.18)$$
'. Kaku says this result can be proved by completing the square.
$$iap^2+ibp =...
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...
I have found a general result for certain exponential integrals that may be of interest to those involved with using path integrals. I am not certain that I am applying it correctly but it appears to work, and I can reproduce results quoted in various textbooks , using it. This may however be...
I believe this could be interesting to many people here who are interested in quantum theory but are not (yet) professional physicists:
http://lanl.arxiv.org/abs/1803.07098
Hi. Maybe you can help me clarify the ideas a little. I've put in google the following.
rebuilding.quantum
It seems that there are scientists interested in founding quantum theory on bases that harmonize with simple criteria.
After more than 100 years of elaborating and debugging quantum...
Hi,
I was watching a really interesting old video from Richard Feynman who was talking about how precise/accurate the predictions of quantum theory agree with observation. The number of decimal places it agrees he said is astounding. He likened it to measuring the circumference of the Earth to...
This is probably extremely wrong I just want to know how. If photons don't experience time doesn't that mean they are every where at once, and if that is true doesn't part of quantum physics say before a particle is observed its in all the states it can be in? So doesn't that apply in that sense...
I've always read that these two theories are incompatible, and how General Relativity works for large scales while Quantum Theory represent reality in extra-small cases.
So my question is where is the limit where General relativity ceases to exist and Quantum Theory gives a better portrait of...
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
Why there must be a way of including gravity into a quantum theory? What leads scientists to believe that ultimately all the interactions must be subjected to quantisation?
Would that be because we describe all other interactions in quantum theory and so we must include gravity on this...
Hello,
I have read somewhere that the next major step in physics will be unifying general relativity with the quantum theory. Does anyone know about such? I wanted to know a bit more.
I have read about such in the book by Stephen Hawking "A Brief History of Time".
Thanks,
I.
P.S. My apologies...
Homework Statement
In analogy to the Bohr theory of the hydrogen atom, develop a quantum theory of Earth satellites, obtaining expressions for the orbit radius, r_n, and the energy, E_n in terms of the quantum number n and other relevant parameters. Use the "Old" Quantum Theory. A satellite of...
Homework Statement
Question:
To find/ explain why there exists a continuous lorentz transformation that flips the sign for space-like separation but not time-like.
Homework Equations
Signature ## (-,+,+...) ##
Definition of lorentz transformation:
##x^u=\lambda^u_v x^v ##...
Homework Statement
Action attached:
To find the EoM of ##\phi ## / ##\phi^* ##
Homework Equations
The Attempt at a Solution
[/B]
Without deriving from first principles, using E-L equations I have:
## \partial_{u}\frac{\partial L}{\partial_u \phi} - \frac{\partial L}{\partial \phi} =0 ##...
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...
Hi , in the classical course " nuclear reactor theory we treated neutrons as particles. can we treat neutrons as waves when it comes to diffusion or transport equation ? neutrons flux/ cross-sections ? I googled it and I didn't find enough info. Like there a lot of common terms between quantum...
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...
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...
Homework Statement
[/B]
Use Max Planck's quantum theory to explain the following behavior of photoelectrons.
i) Low-intensity light does not release any photoelectrons. What will happen if the light is made brighter? Explain your reasoning.
ii) Low-intensity light releases photoelectrons...
This was off-topic in the thread on vacuum fluctuations where the quote appeared, so I opened a new one.
I didn't know the paper before, so first need to read it...
Homework Statement
Hi,
Please see attached.
I am trying to show the second equality , expressing all as a total derivative (I can then show that ##\delta S = ##)
Homework Equations
See above
The Attempt at a Solution
So the ## m ## term is pretty obvious, simply using the chain rule.
It...
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...