# What is Hilbert: Definition and 302 Discussions

David Hilbert (; German: [ˈdaːvɪt ˈhɪlbɐt]; 23 January 1862 – 14 February 1943) was a German mathematician and one of the most influential mathematicians of the 19th and early 20th centuries. Hilbert discovered and developed a broad range of fundamental ideas in many areas, including invariant theory, the calculus of variations, commutative algebra, algebraic number theory, the foundations of geometry, spectral theory of operators and its application to integral equations, mathematical physics, and the foundations of mathematics (particularly proof theory).
Hilbert adopted and defended Georg Cantor's set theory and transfinite numbers. In 1900, he presented a collection of problems that set the course for much of the mathematical research of the 20th century.Hilbert and his students contributed significantly to establishing rigor and developed important tools used in modern mathematical physics. Hilbert is known as one of the founders of proof theory and mathematical logic.

View More On Wikipedia.org
1. ### A X measurement modeled in non-separable Hilbert space

Am reading a book (Ballentine, "Quantum Mechanics: A modern development) which I have found very helpful. Am now puzzled by section 3.4, where the position operator satisfies Q|x> = x |x> (I have simplified from 3 dims to 1 dim). Here, x is any real number. There are, thus, uncountably many...
2. ### POTW Orthonormal Bases on Hilbert Spaces

Let ##H## be a Hilbert space with an orthonormal basis ##\{x_n\}_{n\in \mathbb{N}}##. Suppose ##\{y_n\}_{n\in \mathbb{N}}## is an orthonormal set in ##H## such that $$\sum_{n = 1}^\infty \|x_n - y_n\|^2 < \infty$$ Show that ##\{y_n\}_{n\in \mathbb{N}}## must also be an orthonormal basis.
3. ### I Operators in finite dimension Hilbert space

I have a question about operators in finite dimension Hilbert space. I will describe the context before asking the question. Assume we have two quantum states | \Psi_{1} \rangle and | \Psi_{2} \rangle . Both of the quantum states are elements of the Hilbert space, thus | \Psi_{1} \rangle , |...
4. ### Hilbert Transform, Causality, PI Controller

I was told that PI controller is a causal filter, and has frequency response represented by H(w) = Ki/(iw)+ Kp. I was also told that causal filter should satisfy this relationship H(w) = G(w) -i G_hat(w) where G_hat(w) is the Hilbert transform of G(w). Does this mean that we cannot freely...
5. ### A POVMs for Infinite Dimensional Hilbert Spaces

After reading up on some of the discussion in the Quantum Interpretations forums, I became interested in learning more about POVMs. However, most of the examples are from the finite dimensional setting. If I wanted to model a POVM that approximately measures position and momentum...
6. ### Mathematica Einstein Hilbert Action syntax

Hello , I was trying to vary Einstein Hilbert action in Mathematica , but the syntax failed me badly. I have derived the result by hand , but I want to present the topic with nb.file . Nevertheless, as I said, the syntax is my major concern now. any help will be appreciated! thank you
7. ### A An ab initio Hilbert space formulation of Lagrangian mechanics

I want to share my recent results on the foundation of classical mechanics. Te abstract readWe construct an operational formulation of classical mechanics without presupposing previous results from analytical mechanics. In doing so, several concepts from analytical mechanics will be rediscovered...
8. ### Engineering I/Q of Signals and Hilbert Transform

Hello, would anyone be willing to provide help to the following problem? I can find the Fourier Transform of the complex envelope of s(t) and the I/Q can be found by taking the Real and imaginary parts of that complex envelope, but how can I approach the actual question of finding the carrier...
9. ### A About the rigged Hilbert space in QM

In Quantum Mechanics, how can you justify the use of distributions like the delta functional without introducing a rigged Hilbert space? I see that some texts do not make any reference to this subject.
10. ### Bounded operators on Hilbert spaces

I have to show that for two bounded operators on Hilbert spaces ##H,K##, i.e. ##T \in B(H)## and ##S \in B(K)## that the formula ##(T \bigoplus S) (\alpha, \gamma) = (T \alpha, S \gamma)##, defined by the linear map ##T \bigoplus S: H \bigoplus K \rightarrow H \bigoplus K ## is bounded...
11. ### A Fundamental reality: Hilbert space

What do you guys think of this soberly elegant proposal by Sean Carroll? Reality as a Vector in Hilbert Space Fundamental reality lives in Hilbert space and everything else (space, fields, particles...) is emergent. Seems to me a step in the right conceptual direction.
12. ### A Hilbert spaces and kets "over" manifolds

Background: One can construct a Hilbert space "over" ##\mathbb{R}^{3}## by considering the set of square integrable functions ##\int_{\mathbb{R}^{3}}\left|\psi(\mathbf{r})\right|^{2}<\infty##. That's what is done in QM, and there, even if they are not normalizable, to every...
13. ### A Do we really need the Hilbert space for Quantum Mechanics?

Let's play this game, let's assume the infinite Hilbert Space, the operators and all the modern machinery introduced by Von Neuman were not allowed. How would be the formalism? Thanks
14. ### I Functor between the category of Hilbert Space and the category of sets

I have a question that is related to categories and physics. I was reading this paper by John Baez in which he describes a TQFT as a functor from the category nCob (n-dimensional cobordisms) to Vector spaces. https://arxiv.org/pdf/quant-ph/0404040.pdf. At the beginning of the paper @john baez...
15. ### B Are subspaces of Hilbert space real?

When orthogonal states of a quantum system is projected into subspaces A and B are A and B real spaces?
16. ### I Completeness relations in a tensor product Hilbert space

Hello, Throughout my undergrad I have gotten maybe too comfortable with using Dirac notation without much second thought, and I am feeling that now in grad school I am seeing some holes in my knowledge. The specific context where I am encountering this issue currently is in scattering theory...
17. ### A Integrability along a Hilbert space?

Suppose we have an infinite dimensional Hilbert-like space but that is incomplete, such as if a subspace isomorphic to ##\mathbb{R}## had countably many discontinuities and we extended it to an isomorphism of ##\mathbb{R}^{\infty}##. Is there a measure of integrating along any closed subset of...
18. ### I Dynamical System & Hilbert Space: Analyzing the Relationship

Is there any relation between dynamical system and Hilbert space(functional analysis)?
19. ### B My argument why Hilbert's Hotel is not a veridical Paradox

Hello there, I had another similar post, where asking for proof for Hilbert’s Hotel. After rethinking this topic, I want to show you a new example. It tries to show why that the sentence, every guest moves into the next room, hides the fact, that we don’t understand what will happen in this...
20. ### Quantum Hilbert spaces and quantum operators being infinite dimensional matrices

I just realized quantum operators X and P aren't actually just generalizations of matrices in infinite dimensions that you can naively play with as if they're usual matrices. Then I learned that the space of quantum states is not actually a Hilbert space but a "rigged" Hilbert space. It all...
21. ### I Quantum logic based on closed Hilbert space subspaces

One proposal that I have read (but cannot re-find the source, sorry) was to identify a truth value for a proposition (event) with the collection of closed subspaces in which the event had a probability of 1. But as I understand it, a Hilbert space is a framework which, unless trivial, keeps...
22. ### B Hilbert's Hotel: new Guest arrives (Infinite number of Guests)

Hilberts Hotel has infinity numbers of rooms and in every room is exactly one guest. On Wikipedia Hilberts Hotel gets described as well: Suppose a new guest arrives and wishes to be accommodated in the hotel. We can (simultaneously) move the guest currently in room 1 to room 2, the guest...
23. ### Dimension of Hilbert spaces for identical particles

My thoughts are: a) it should just be N^2 b) just N since they're identical c) due to Pauli exclusion would it be N^2 - N since they have to be different states?
24. ### A Equivalence Relation to define the tensor product of Hilbert spaces

I'm following this video on how to establish an equivalence relation to define the tensor product space of Hilbert spaces: ##\mathcal{H1} \otimes\mathcal{H2}={T}\big/{\sim}## The definition for the equivalence relation is given in the lecture vidoe as ##(\sum_{j=1}^{J}c_j\psi_j...
25. ### I On-shell virtual particles and 'physical' Hilbert spaces....

Hi all, Just a clarification question as I'm learning. It's possible to have Feynman diagrams where the internal lines (virtual particles) are in fact on shell. 'On shell' would imply 'observable,' (maybe?) but as noted in @A. Neumaier's great FAQ, only sets of Feynman diagrams have predictive...
26. ### I Rigged Hilbert Space X: Eq (1) and (2)

X=e+or-kx (1) <X(x)|Φ(x)>=∫-∞∞X*(x)Φ(x)dx (2) where Φ(x) satisfies the following. ∫-∞∞|Φ(x)|2(1+|x|)ndx is finte if n=0, 1, 2,...
27. ### I Question regarding a Free particle and Hilbert space (QM)

In quantum mechanics, the Eigenfunction resulting from the Hamiltonian of a free particle in 1D system is $$\phi = \frac{e^{ikx} }{\sqrt{2\pi} }$$ We know that a function $$f(x)$$ belongs to Hilbert space if it satisfies $$\int_{-\infty}^{+\infty} |f(x)|^2 dx < \infty$$ But since the...
28. ### A Recent paper on QED using finite-dimensional Hilbert space - validity?

I've been struggling with a somewhat-recent paper by Charles Francis, "A construction of full QED using finite dimensional Hilbert space," available here: https://arxiv.org/pdf/gr-qc/0605127.pdf But also published in...
29. ### A Reproducing Kernel Hilbert Spaces

I have been reading a lot about Reproducing Kernel Hilbert Spaces mainly because of their application in machine learning. I do not have a formal background in topology, took linear algebra as an undergrad but mainly have encountered things such as, inner product, norm, vector space...
30. ### Creating a vectorized statement in MatLab to output a 5x5 Hilbert matrix

My first attempt was: V=zeros(5,5) a=1; i=1:5; j=1:5; V(i:j)=a./(i+j-1) I figured to create a 5x5 with zeros and then to return and replace those values with updated values derived from the Hilbert equation as we move through i and j. This failed with an error of : Unable to perform assignment...
31. M

35. ### B Infinite dimensional Hilbert Space

Could someone tell me in what sense the following photo of Hilbert is a infinite dimensional Hilbert Space? It's shown in a pdf I'm reading. Perhaps I'm putting the chariot in front of the horses as one would say here in our country, by considering infinite as infinite dimensional?
36. M

### A Why Must the Constant in Hilbert Space Function B[f(x)] Be Defined as Shown?

Hi PF! Given a function ##B## defined as $$B[f(x)]\equiv f''(x) + f(x) + const.$$ Evidently in order for this function to be in the real Hilbert space ##H## we know $$const. = -\frac{1}{x_1-x_0}\int_{x_0}^{x_1} (f''(x) + f(x))\,dx.$$ Can someone please explain why? I can elaborate further if...
37. ### 2D subspace of a Hilbert space

Homework Statement Have to read a paper and somewhere along the line it claims that for any distinct ## \ket{\phi_{0}}## and ##\ket{\phi_{1}}## we can choose a basis s.t. ## \ket{\phi_{0}}= \cos\frac{\theta}{2}\ket{0} + \sin\frac{\theta}{2}\ket{1}, \hspace{0.5cm} \ket{\phi_{1}}=...
38. M

### A How to determine constant to be in Hilbert space

Hi PF! I'm trying to solve an ODE through the Ritz method, which is to say approximate the solution through a series $$\Phi = \sum_{i=1}^N a_if_i,\\ f_i = \phi_i-d_i.$$ Here ##a_i## are constants to be determined and ##f_i## are prescribed functions, where ##\phi_i## is a function and ##d_i##...
39. ### Insights Hilbert Spaces And Their Relatives - Part II - Comments

Greg Bernhardt submitted a new PF Insights post Hilbert Spaces And Their Relatives - Part II Continue reading the Original PF Insights Post.
40. ### I Understanding Hilbert Vector Spaces

Hello, I think I understand what a vector space is. It is inhabited by objects called vectors that satisfy a certain number of properties. The vectors can be functions whose integral is not infinite, converging sequences, etc. The vector space can be finite dimensional or infinite dimensional...
41. M

### I Why do these functions form complete orthogonal systems in the Hilbert space?

Hi PF! A text states that the following two functions $$\psi^o_k = \sin(\pi(k-1/2)x)\cosh(\pi(k-1/2)(z+h)): k\in\mathbb{N},\\ \psi^e_k = \cos(\pi kx)\cosh(\pi k(z+h)): k\in\mathbb{N}$$ each form complete orthogonal systems in two mutually orthogonal subspaces, which compose the Hilbert...
42. ### I Do we need a reference frame in Quantum Hilbert space?

Entangled states are only separable relative to certain basis states. So does that mean that reference frames have importance beyond those in spacetime?
43. ### Insights Hilbert Spaces and Their Relatives - Comments

Greg Bernhardt submitted a new PF Insights post Hilbert Spaces and Their Relatives Continue reading the Original PF Insights Post.
44. M

### I Understanding Spin States in Hilbert Space

Hello In our Quantum Mechanics lecture we have been discussing a simplified model of the Stern-Gerlach experiment. Let ##|+>## and ##|->## denote an electron that is "spin up" and "spin down" (with respect to ##\hat{z}##), respectively. Our professor then asserted that ##|+>## and ##|->## acted...

Hi, while reading a comment by Dr Du, I looked up the definition of Hilbert adjoint operator, and it appears as the same as Hermitian operator: https://en.wikipedia.org/wiki/Hermitian_adjoint This is ok, as it implies that ##T^{*}T=TT^{*}##, however, it appears that self-adjointness is...
46. ### A What separates Hilbert space from other spaces?

Hi, I have the impression that the special thing about Hilbert space for Quantum Mechanics is that it is simply an infinite space, which allows for infinitively integration and derivation of its elements, f(x), g(x), their linear combination, or any other complex function, given that the main...
47. ### I Exploring the Equivalence of Different Representations in Quantum Mechanics

Hi, I found this article very interesting, given the loads of question I have posted in this regard in the last months. I cannot recall where I got the link from, and if it came from Bill Hobba in some discussion, thanks Bill! If not, thanks anyway for your answers and contributions. Here is...
48. ### I Hilbert space and conjugate of a wave function

Take a wavefunction ##\psi## and let this wavefunction be a solution of Schroedinger equation,such that: ##i \hbar \frac{\partial \psi}{\partial t}=H\psi## The complex conjugate of this wavefunction will satisfy the "wrong-sign Schrodinger equation" and not the schrodinger equation,such that ##i...
49. S

### A Eigenvectors and matrix inner product

Hi, I am trying to prove that the eigevalues, elements, eigenfunctions or/and eigenvectors of a matrix A form a Hilbert space. Can one apply the inner product formula : $$\int x(t)\overline y(t) dt$$ on the x and y coordinates of the eigenvectors [x_1,y_1] and...
50. S

### A Operator mapping in Hilbert space

Hi, I have an operator given by the expression: L = (d/dx +ia) where a is some constant. Applying this on x, gives a result in the subspace C and R. Can I safely conclude that the operator L can be given as: $$L: \mathcal{H} \rightarrow \mathcal{H}$$ where H is...