Unitarity Definition and 36 Threads

The usual justification for why the evolution of physical systems is unitary in quantum mechanics involves arguments like "probabilities must sum to 1" or similar arguments that apply equally to any CPTP map. I'm just curious what justifications people here would use for selecting out unitary...

Does unitarity have to be obeyed in a full quantum gravity model?

Hello, I recently saw ##U|v\rangle= e^{ia}|v\rangle, \, a \in \mathbb{R}## and am wondering how to come up with this or how to show this. My first thought is based on the definition of unitary operators (##UU^\dagger = I##), I would show it something like this: ##(U|v\rangle)^\dagger =...

Does unitarity of the evolution of wavefunction get rid of the need for a "conscious observer", and does collapse in contrast demand a "conscious observer"? For with unitarity there are is no requirement for such an observer, and collapse can't be explained without such an observer. The...

It is usually said that unitarity is necessary for the consistent probabilistic interpretation. But is that really so? Suppose that ##|\psi(t)\rangle## does not evolve unitarily with time, so that ##\langle\psi(t)|\psi(t)\rangle## changes with time. Even then one can propose that probability...

Hi all... As far as I understand, the concept of "unitarity" is pretty close to that of perturbation; in that it tells you that your amplitudes are finite with energy scale (one solution the Higgs gave in e.g. the Vector Boson Scattering). However, since an EFT comes with a natural cut-off, why...

Cutkosky rule states that: $$2Im \big(A_{ab}\big)=(2\pi)^4\sum_c \delta\Big(\sum_c p^{\mu}_{c}-\sum_a p^{\mu}_{a}\Big)|A_{cb}|^2\hspace{0.5cm} (1)$$ putting ##a=b=p## in Cutkosky rule we deduce the Optical Theorem for ##pp## scattering: $$2Im \big(A_{pp}\big)=(2\pi)^4\sum_c \delta\Big(\sum_c...

Homework Statement I want to proof for $$V_{us}V^{*}_{ub}+V_{cs}V^{*}_{cb}+V_{ts}V^{*}_{tb}=0$$ unitarity triangle that left angle is $$\pi-\gamma$$ (see below picture from my lecture notes). Homework Equations [/B] $$\gamma \approx - arg(V_{ub})$$ $$\beta_s \approx arg(V_{ts})+\pi$$...

What do we mean when we say that a system can't change (in time) because its evolution is unitary?

Quote lifted from a thread in the cosmology forum. What does it mean to know the exact state of a QM system? QM predicts probabilities that particles will be in one of multiple states when the particles are observed, and when observed, not all properties of a particle are simultaneously...

Hi, I'm looking at the unitarity constraints for the Two-Higgs Doublet Model and I'm trying to follow what they do in the attached article, which can also be found here: https://arxiv.org/pdf/hep-ph/0312374v1.pdf. However I do not know how to get the scattering matrices in eq. (7). They say...

Hi all, I'm little confused about the unitarity and perturbativity constrains which imposed on a potential's parameters, like 2HD potential. Look for example: [arXiv:1507.03618v3 [hep-ph]] First, I'd like to know what is most essential ? I mean if unitarity constraind ## \lambda## to say less...

In "The Theoretical Minimum" of Susskind (p.98) it says that if we take any two basisvectors |i \rangle and |j \rangle of any orthonormal basis, and we take any linear time-development operator U, that the inner product between U(t)|i \rangle and U(t)|j \rangle should be 1 if |i \rangle=|j...

Has anyone analyzed the process of Neutron formation and decay from the perspective of "information conservation"? Does anyone have any thoughts on what the results of such an analysis would be? What is the status of the observed evidence of reality in respect of whether one should conclude...

Not quite. But it necessarily has to be described by a different quantum model than unitary dynamics if it is an open system and the rest of the universe is not explicitly modeled. For convenience, physicists often want to describe a small quantum system in terms of only its Hilbert space, when...

A paper I'm reading says that the 2nd Law of Thermodynamics is related to unitarity. And it references: S. Weinberg, "The Quantum theory of fields. Vol. 1: Foundations" Does anyone here know what this might mean?

I am taking my first semester of QM so excuse my question if it is way off mark, totally wrong, or very well known. As I understand it, one of the postulates of QM are that states evolve unitarily, a consequence (but not THE defining feature) of unitary transformations is that they are...

From a physical perspective, is the reason why one requires that the norm of a state vector (of an isolated quantum system) is conserved under time evolution to do with the fact that the state vector contains all information about the state of the system at each given time (i.e. the...

I have a question regarding a derivation in Peskin and Schroeder's QFT book. On page 298, he is discussing a method for defining a gauge invariant S matrix. He does this by defining projection operators ##P_0## that project general particle states into gauge invariant states, and then defining...

I've read in a set of lecture notes that the requirement of locality and unitarity in QFT imply that the vacuum must have a non-zero energy associated with it (http://arxiv.org/pdf/1502.05296v1.pdf , top of page 3 under heading "What is the problem?"). My question is, why does the locality and...

Lenoard Susskind's video courses on Clasical Mechanics and Quantum Mechanics, often mention convervation of information. Susskind likes to call it "the minus first law." In classical physics, it is Liouville's Theorum which tells us that the number of states is conserved in time evolutions...

my question is this: you know than in feynman path integra, you integrate eiS/hbar along all the fields. you also know that S is real and that it is the integral of local functions (fields and derivatives of fields). you also know that path integral generates an unitary and local...

What it means "the theory violates unitarity" Hello, I know what unitary transformation is, but what does it mean that the theory does or does not violate unitarity? For example in some textbooks on QFT one can read that the Fermi theory of beta decay, which is not renormalizable, also violates...

Hi, I'm confused by a sentence in a set of lecture notes I have on quantum mechanics. In it, it is assumed there is some representation \pi of SO(3) on a Hilbert space. This representation is assumed to be irreducible and unitary. It is then said that the operators J_i, which are said to...

I am reading a quantum mechanics book. I did not clearly understand one particular idea. When the book talks about the time-evolution operator U(t,t_0), it says that one very important property is the unitary requirement for U(t,t_0) that follows from probability conservation. My question is...

I doing some reading on why the wave-function is complex. From what I can tell, it's due to its evolution by unitary operators. But unitary operators seem to have something to do with information conservation. So I wonder if these idea have been developed somewhere in a concise fashion that...

In canonical LQG, unitarity is presumably guaranteed by the canonical formalism. How does one check for unitarity in the spin foam (path integral) formalism? Do the new spin foams pass the necessary tests?

Hey folks, I've been stumbeling recently about new unitarity methods to obtain one-loop amplitudes by cutting them in all possible channels thereby reducing the full amplitude to products of tree amplitudes (pioneered by Bern, Dixon, Kosower, Dunbar). From what I understand from my QFT...

Hi, Okay, I thought I had created this thread but looks like I didn't hit submit or something. Anyway, my question is, why is the PMNS matrix which is used to go between the mass eigenstates and flavour eigenstates of neutrinos unitary? Is it an assumption (if so, what is the motivation...

Given that in GR there's no gauge invariant choice of "equal-time slices", how is unitarity formulated in quantum gravity? I guess the problem may be absent if spacetime is asymptotically flat. But what happens in other cases? In AdS/CFT, the notion of unitarity comes from the CFT side. Is this...

What is the reasoning for saying that the scattering matrix in quantum field theory is unitary? Take the initial state to be an electron and a positron. All sorts of crazy products can result in the final state, from photons to Z's to Higgs to an electron/positron with different momenta, to...

Let V be a finite dimensional complex inner product space with inner product < , >. Let U be unitary with respect to this inner product. If ( , ) is another inner product, is U also unitary with respect to ( , )? The definition of unitary I'm working with is the one that says: U is unitary if...

I read a statement that there is a breakdown of unitarity in the Standard Model at 1 TeV. What exactly is meant by that?

This is a known fact that CKM matrix, a matrix that is used to connect the weak interaction eigenstates to mass eigenstates is unitary. I have studied that this is due to the conservation of probability. i.e. an up type quark will decay into exactly three type of d quarks, nothing more.

Partial Wave Unitarity ?? Is there anyone who deals with "Partial Wave Unitarity" in Quantum Mechanics before??

A stupid couple of questions... In quantum computations, one typically starts with some initial quantum state on which an operator is applied. This operator must be unitary , right? (I guess that otherwise, it would not corerspond to an actual physical quantum setup). And this implies...