## 物理代写|量子场论代写Quantum field theory代考|PHYS8302

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Thermal Correlation Functions

The energies of excited states are encoded in the thermal correlation functions. These functions are expectation values of products of the position operator
$$\hat{q}{\mathrm{E}}(\tau)=\mathrm{e}^{\tau \hat{H} / \hbar} \hat{q} \mathrm{e}^{-\tau \hat{H} / \hbar}, \quad \hat{q}{\mathrm{E}}(0)=\hat{q}(0),$$
at different imaginary times in the canonical ensemble,
$$\left\langle\hat{q}{\mathrm{E}}\left(\tau{1}\right) \cdots \hat{q}{\mathrm{E}}\left(\tau{n}\right)\right\rangle_{\beta} \equiv \frac{1}{Z(\beta)} \operatorname{tr}\left(\mathrm{e}^{-\beta \hat{H}} \hat{q}{\mathrm{E}}\left(\tau{1}\right) \cdots \hat{q}{\mathrm{E}}\left(\tau{n}\right)\right)$$
The normalizing function $Z(\beta)$ is the partition function (2.56). From the thermal two-point function
\begin{aligned} \left\langle\hat{q}{\mathrm{E}}\left(\tau{1}\right) \hat{q}{\mathrm{E}}\left(\tau{2}\right)\right\rangle_{\beta} &=\frac{1}{Z(\beta)} \operatorname{tr}\left(\mathrm{e}^{-\beta \hat{H}} \hat{q}{\mathrm{E}}\left(\tau{1}\right) \hat{q}{\mathrm{E}}\left(\tau{2}\right)\right) \ &=\frac{1}{Z(\beta)} \operatorname{tr}\left(\mathrm{e}^{-\left(\beta-\tau_{1}\right) \hat{H}} \hat{q} \mathrm{e}^{-\left(\tau_{1}-\tau_{2}\right) \hat{H}} \hat{q} \mathrm{e}^{-\tau_{2} \hat{H}}\right) \end{aligned}
we can extract the energy gap between the ground state and the first excited state. For this purpose we use orthonormal energy eigenstates $|n\rangle$ to calculate the trace and in addition insert the resolution of the identity operator $\mathbb{1}=\sum|m\rangle\langle m|$. This yields
$$\langle\ldots\rangle_{\beta}=\frac{1}{Z(\beta)} \sum_{n, m} \mathrm{e}^{-\left(\beta-\tau_{1}+\tau_{2}\right) E_{n}} \mathrm{e}^{-\left(\tau_{1}-\tau_{2}\right) E_{\mathrm{m}}}\langle n|\hat{q}| m\rangle\langle m|\hat{q}| n\rangle$$
Note that in the sum over $n$ the contributions from the excited states are exponentially suppressed at low temperatures $\beta \rightarrow \infty$, implying that the thermal two-point function converges to the Schwinger function in this limit:
$$\left\langle\hat{q}{\mathrm{E}}\left(\tau{1}\right) \hat{q}{\mathrm{E}}\left(\tau{2}\right)\right\rangle_{\beta} \stackrel{\beta \rightarrow \infty}{\longrightarrow} \sum_{m>0} \mathrm{e}^{-\left(\tau_{1}-\tau_{2}\right)\left(E_{m}-E_{0}\right)}|\langle 0|\hat{q}| m\rangle|^{2}=\left\langle 0\left|\hat{q}{\mathrm{E}}\left(\tau{1}\right) \hat{q}{\mathrm{E}}\left(\tau{2}\right)\right| 0\right\rangle$$

## 物理代写|量子场论代写Quantum field theory代考|The Harmonic Oscillator

We wish to study the path integral for the Euclidean oscillator with discretized time. The oscillator is one of the few systems for which the path integral can be calculated explicitly. For more such system, the reader may consult the text [19]. But the results for the oscillator are particularly instructive with regard to lattice field theories considered later in this book. So let us discretize the Euclidean time interval $[0, \tau]$ with $n$ sampling points separated by a lattice constant $\varepsilon=\tau / n$. For the Lagrangian
$$L=\frac{m}{2} \dot{q}^{2}+\mu q^{2}$$
the discretized path integral over periodic paths reads
\begin{aligned} Z &=\int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n}\left(\frac{m}{2 \pi \varepsilon}\right)^{n / 2} \exp \left{-\varepsilon \sum_{j=0}^{n-1}\left(\frac{m}{2}\left(\frac{q_{j+1}-q_{j}}{\varepsilon}\right)^{2}+\mu q_{j}^{2}\right)\right} \ &=\left(\frac{m}{2 \pi \varepsilon}\right)^{n / 2} \int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n} \exp \left(-\frac{1}{2}(\boldsymbol{q}, \mathrm{A} q)\right) \end{aligned}
where we assumed $q_{0}=q_{n}$ and introduced the symmetric matrix
$$\mathrm{A}=\frac{m}{\varepsilon}\left(\begin{array}{cccccc} \alpha & -1 & 0 & \cdots & 0 & -1 \ -1 & \alpha & -1 & \cdots & 0 & 0 \ & & \ddots & & & \ & & & \ddots & & \ 0 & 0 & \cdots & -1 & \alpha & -1 \ -1 & 0 & \cdots & 0 & -1 & \alpha \end{array}\right), \quad \alpha=2\left(1+\frac{\mu}{m} \varepsilon^{2}\right)$$
This is a Toeplitz matrix in which each descending diagonal from left to right is constant. This property results from the invariance of the action under lattice translations. For the explicit calculation of $Z$, we consider the generating function
\begin{aligned} Z[j] &=\left(\frac{m}{2 \pi \varepsilon}\right)^{n / 2} \int \mathrm{d}^{n} q \exp \left{-\frac{1}{2}(\boldsymbol{q}, \mathrm{A} q)+(\boldsymbol{j}, \boldsymbol{q})\right} \ &=\frac{(m / \varepsilon)^{n / 2}}{\sqrt{\operatorname{det} \mathrm{A}}} \exp \left{\frac{1}{2}\left(j, \mathrm{~A}^{-1} \boldsymbol{j}\right)\right} \end{aligned}

## 物理代写|量子场论代写Quantum field theory代考|Problems

2.1 (Gaussian Integral) Show that
$$\int \mathrm{d} z_{1} \mathrm{~d} \bar{z}{1} \ldots \mathrm{d} z{n} \mathrm{~d} \bar{z}{n} \exp \left(-\sum{i j} \bar{z}{i} A{i j} z_{j}\right)=\pi^{n}(\operatorname{det} \mathrm{A})^{-1}$$
with A being a positive Hermitian $n \times n$ matrix and $z_{i}$ complex integration variables.
2.2 (Harmonic Oscillator) In (2.43) we quoted the result for the kernel $K_{\omega}\left(\tau, q^{\prime}, q\right)$ of the $d$-dimensional harmonic oscillator with Hamiltonian
$$\hat{H}=\frac{1}{2 m} \hat{p}^{2}+\frac{m \omega^{2}}{2} \hat{q}^{2}$$
at imaginary time $\tau$. Derive this formula.
Hint: Express the kernel in terms of the eigenfunctions of $\hat{H}$, which for $\hbar=m=$ $\omega=1$ are given by
$$\exp \left(-\xi^{2}-\eta^{2}\right) \sum_{n=0}^{\infty} \frac{\alpha^{n}}{2^{n} n !} H_{n}(\xi) H_{n}(\eta)=\frac{1}{\sqrt{1-\alpha^{2}}} \exp \left(\frac{-\left(\xi^{2}+\eta^{2}-2 \xi \eta \alpha\right)}{1-\alpha^{2}}\right)$$
The functions $H_{n}$ denote the Hermite polynomials.
Comment: This result also follows from the direct evaluation of the path integral.
2.3 (Free Particle on a Circle) A free particle moves on an interval and obeys periodic boundary conditions. Compute the time evolution kernel $K\left(t_{b}-t_{a}, q_{b}, q_{a}\right)=$ $\left\langle q_{b}, t_{b} \mid q_{a}, t_{a}\right\rangle$. Use the familiar formula for the kernel of the free particle (2.26) and enforce the periodic boundary conditions by a suitable sum over the evolution kernel for the particle on $\mathbb{R}$.

## 物理代写|量子场论代写Quantum field theory代考|Thermal Correlation Functions

q^和(τ)=和τH^/ℏq^和−τH^/ℏ,q^和(0)=q^(0),

⟨q^和(τ1)⋯q^和(τn)⟩b≡1从(b)tr⁡(和−bH^q^和(τ1)⋯q^和(τn))

⟨q^和(τ1)q^和(τ2)⟩b=1从(b)tr⁡(和−bH^q^和(τ1)q^和(τ2)) =1从(b)tr⁡(和−(b−τ1)H^q^和−(τ1−τ2)H^q^和−τ2H^)

⟨…⟩b=1从(b)∑n,米和−(b−τ1+τ2)和n和−(τ1−τ2)和米⟨n|q^|米⟩⟨米|q^|n⟩

⟨q^和(τ1)q^和(τ2)⟩b⟶b→∞∑米>0和−(τ1−τ2)(和米−和0)|⟨0|q^|米⟩|2=⟨0|q^和(τ1)q^和(τ2)|0⟩

## 物理代写|量子场论代写Quantum field theory代考|The Harmonic Oscillator

\begin{aligned} Z &=\int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n}\left(\frac{m}{2 \pi \varepsilon}\right)^{ n / 2} \exp \left{-\varepsilon \sum_{j=0}^{n-1}\left(\frac{m}{2}\left(\frac{q_{j+1}-q_ {j}}{\varepsilon}\right)^{2}+\mu q_{j}^{2}\right)\right} \ &=\left(\frac{m}{2 \pi \varepsilon} \right)^{n / 2} \int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n} \exp \left(-\frac{1}{2}(\boldsymbol{q }, \mathrm{A} q)\right) \end{对齐}\begin{aligned} Z &=\int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n}\left(\frac{m}{2 \pi \varepsilon}\right)^{ n / 2} \exp \left{-\varepsilon \sum_{j=0}^{n-1}\left(\frac{m}{2}\left(\frac{q_{j+1}-q_ {j}}{\varepsilon}\right)^{2}+\mu q_{j}^{2}\right)\right} \ &=\left(\frac{m}{2 \pi \varepsilon} \right)^{n / 2} \int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n} \exp \left(-\frac{1}{2}(\boldsymbol{q }, \mathrm{A} q)\right) \end{对齐}

\begin{对齐} Z[j] &=\left(\frac{m}{2 \pi \varepsilon}\right)^{n / 2} \int \mathrm{d}^{n} q \exp \左{-\frac{1}{2}(\boldsymbol{q}, \mathrm{A} q)+(\boldsymbol{j}, \boldsymbol{q})\right} \ &=\frac{(m / \varepsilon)^{n / 2}}{\sqrt{\operatorname{det} \mathrm{A}}} \exp \left{\frac{1}{2}\left(j, \mathrm{~A }^{-1} \boldsymbol{j}\right)\right} \end{aligned}\begin{对齐} Z[j] &=\left(\frac{m}{2 \pi \varepsilon}\right)^{n / 2} \int \mathrm{d}^{n} q \exp \左{-\frac{1}{2}(\boldsymbol{q}, \mathrm{A} q)+(\boldsymbol{j}, \boldsymbol{q})\right} \ &=\frac{(m / \varepsilon)^{n / 2}}{\sqrt{\operatorname{det} \mathrm{A}}} \exp \left{\frac{1}{2}\left(j, \mathrm{~A }^{-1} \boldsymbol{j}\right)\right} \end{aligned}

## 物理代写|量子场论代写Quantum field theory代考|Problems

2.1（高斯积分）证明

∫d和1 d和¯1…d和n d和¯n经验⁡(−∑一世j和¯一世一个一世j和j)=圆周率n(这⁡一个)−1
A 是正厄米特n×n矩阵和和一世复杂的积分变量。
2.2（谐波振荡器）在（2.43）中，我们引用了内核的结果ķω(τ,q′,q)的d具有哈密顿量的维谐振子

H^=12米p^2+米ω22q^2

2.3 （圆周上的自由粒子） 自由粒子在一个区间上移动并服从周期性边界条件。计算时间演化核ķ(吨b−吨一个,qb,q一个)= ⟨qb,吨b∣q一个,吨一个⟩. 对自由粒子的核使用熟悉的公式 (2.26)，并通过粒子在演化核上的适当总和来强制周期性边界条件R.

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYS3101

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Quantum Mechanics in Imaginary Time

The unitary time evolution operator has the spectral representation
$$\hat{K}(t)=\mathrm{e}^{-\mathrm{i} \hat{H} t}=\int \mathrm{e}^{-\mathrm{i} E t} \mathrm{~d} \hat{P}{\mathrm{E}},$$ where $\hat{P}{\mathrm{E}}$ is the spectral family of the Hamiltonian. If $\hat{H}$ has discrete spectrum, then $\hat{P}{\mathrm{E}}$ is the orthogonal projector onto the subspace of $\mathscr{H}$ spanned by all eigenfunctions with energies less than $E$. In the following we assume that the Hamiltonian operator is bounded from below. Then we can subtract its ground state energy to obtain a non-negative $\hat{H}$ for which the integration limits in (2.35) are 0 and $\infty$. With the substitution $t \rightarrow t-\mathrm{i} \tau$, we obtain $$\mathrm{e}^{-(\tau+\mathrm{i} t) \hat{H}}=\int{0}^{\infty} \mathrm{e}^{-E(\tau+\mathrm{i} t)} \mathrm{d} \hat{P}_{\mathrm{E}}$$
This defines a holomorphic semigroup in the lower complex half-plane
$${z=t-\mathrm{i} \tau \in \mathbb{C}, \tau \geq 0}$$
If the operator $(2.36)$ is known on the negative imaginary axis $(t=0, \tau \geq 0)$, one can perform an analytic continuation to the real axis $(t, \tau=0)$. The analytic continuation to complex time $t \rightarrow-\mathrm{i} \tau$ corresponds to a transition from the Minkowski metric $\mathrm{d} s^{2}=d t^{2}-\mathrm{d} x^{2}-\mathrm{d} y^{2}-\mathrm{d} z^{2}$ to a metric with Euclidean signature. Hence a theory with imaginary time is called Euclidean theory.

The time evolution operator $\hat{K}(t)$ exists for real time and defines a oneparametric unitary group. It fulfills the Schrödinger equation
$$\mathrm{i} \frac{\mathrm{d}}{\mathrm{d} t} \hat{K}(t)=\hat{H} \hat{K}(t)$$
with a complex and oscillating kernel $K\left(t, q^{\prime}, q\right)=\left\langle q^{\prime}|\hat{K}(t)| q\right\rangle$. For imaginary time we have a Hermitian (and not unitary) evolution operator
$$\hat{K}(\tau)=\mathrm{e}^{-\tau \hat{H}}$$
with positive spectrum. The $\hat{K}(\tau)$ exist for positive $\tau$ and form a semi-group only. For almost all initial data, evolution back into the “imaginary past” is impossible.
The evolution operator for imaginary time satisfies the heat equation
$$\frac{\mathrm{d}}{\mathrm{d} \tau} \hat{K}(\tau)=-\hat{H} \hat{K}(\tau)$$
instead of the Schrödinger equation and has kernel
$$K\left(\tau, q^{\prime}, q\right)=\left\langle q^{\prime}\left|\mathrm{e}^{-\tau \hat{H}}\right| q\right\rangle, \quad K\left(0, q^{\prime}, q\right)=\delta\left(q^{\prime}, q\right)$$
This kernel is real ${ }^{1}$ for a real Hamiltonian. Furthermore it is strictly positive:

## 物理代写|量子场论代写Quantum field theory代考|Imaginary Time Path Integral

To formulate the path integral for imaginary time, we employ the product formula $(2.28)$, which follows from the product formula (2.27) through the substitution of it by $\tau$. For such systems the analog of $(2.31)$ for Euclidean time $\tau$ is obtained by the substitution of $i \varepsilon$ by $\varepsilon$. Thus we tind
\begin{aligned} K\left(\tau, q^{\prime}, q\right) &=\left\langle\hat{q}^{\prime}\left|\mathrm{e}^{-\tau \hat{H} / \hbar}\right| \hat{q}\right\rangle \ &=\lim {n \rightarrow \infty} \int \mathrm{d} q{1} \cdots \mathrm{d} q_{n-1}\left(\frac{m}{2 \pi \hbar \varepsilon}\right)^{n / 2} \mathrm{e}^{-S_{\mathrm{E}}\left(q_{0}, q_{1}, \ldots, q_{n}\right) / \hbar} \ S_{\mathrm{E}}(\ldots) &=\varepsilon \sum_{j=0}^{n-1}\left{\frac{m}{2}\left(\frac{q_{j+1}-q_{j}}{\varepsilon}\right)^{2}+V\left(q_{j}\right)\right} \end{aligned}
where $q_{0}=q$ and $q_{n}=q^{\prime}$. The multidimensional integral represents the sum over all broken-line paths from $q$ to $q^{\prime}$. Interpreting $S_{\mathrm{E}}$ as Hamiltonian of a classical lattice model and $\hbar$ as temperature, it is (up to the fixed endpoints) the partition function of a one-dimensional lattice model on a lattice with $n+1$ sites. The realvalued variable $q_{j}$ defined on site $j$ enters the action $S_{\mathrm{E}}$ which contains interactions between the variables $q_{j}$ and $q_{j+1}$ at neighboring sites. The values of the lattice field
$${0,1, \ldots, n-1, n} \rightarrow\left{q_{0}, q_{1}, \ldots, q_{n-1}, q_{n}\right}$$
are prescribed at the endpoints $q_{0}=q$ and $q_{n}=q^{\prime}$. Note that the classical limit $\hbar \rightarrow 0$ corresponds to the low-temperature limit of the lattice system.

The multidimensional integral (2.52) corresponds to the summation over all path on the time lattice. What happens to the finite-dimensional integral when we take the continuum limit $n \rightarrow \infty$ ? Then we obtain the Euclidean path integral representation for the positive kernel
$$K\left(\tau, q^{\prime}, q\right)=\left\langle q^{\prime}\left|\mathrm{e}^{-\tau \hat{H} / h}\right| q\right\rangle=C \int_{q(0)=q}^{q(\tau)=q^{\prime}} \mathscr{D} q \mathrm{e}^{-S_{\mathrm{E}}[q] / h}$$
The integrand contains the Euclidean action
$$S_{\mathrm{E}}[q]=\int_{0}^{\tau} d \sigma\left{\frac{m}{2} \dot{q}^{2}+V(q(\sigma))\right}$$
which for many physical systems is bounded from below.

## 物理代写|量子场论代写Quantum field theory代考|Path Integral in Quantum Statistics

The Euclidean path integral formulation immediately leads to an interesting connection between quantum statistical mechanics and classical statistical physics. Indeed, if we set $\tau / \hbar \equiv \beta$ and integrate over $q=q^{\prime}$ in (2.53), then we end up with the path integral representation for the canonical partition function of a quantum system with Hamiltonian $\hat{H}$ at inverse temperature $\beta=1 / k_{B} T$. More precisely, setting $q=q^{\prime}$ and $\tau=\hbar \beta$ in the left-hand side of this formula, then the integral over $q$ yields the trace of $\exp (-\beta \hat{H})$, which is just the canonical partition function,
$$\int \mathrm{d} q K(\hbar \beta, q, q)=\operatorname{tr} \mathrm{e}^{-\beta \hat{H}}=Z(\beta)=\sum \mathrm{e}^{-\beta E_{n}} \quad \text { with } \quad \beta=\frac{1}{k_{B} T}$$
Setting $q=q^{\prime}$ in the Euclidean path integral in (2.53) means that we integrate over paths beginning and ending at $q$ during the imaginary time interval $[0, \hbar \beta]$. The final integral over $q$ leads to the path integral over all periodic paths with period $\hbar \beta$
$$Z(\beta)=C \oint \mathscr{D} q \mathrm{e}^{-S_{\mathrm{E}}[q] / \hbar}, \quad q(\hbar \beta)=q(0)$$
For example, the kernell of the harmonic oscillator in $(2.43)$ on the diagonal is
$$K_{\omega}(\beta, q, q)=\sqrt{\frac{m \omega}{2 \pi \sinh (\omega \beta)}} \exp \left{-m \omega \tanh (\omega \beta / 2) q^{2}\right}$$
where we used units with $\hbar=1$. The integral over $q$ yields the partition function
\begin{aligned} Z(\beta) &=\sqrt{\frac{m \omega}{2 \pi \sinh (\omega \beta)}} \int \mathrm{d} q \exp \left{-m \omega \tanh (\omega \beta / 2) q^{2}\right} \ &=\frac{1}{2 \sinh (\omega \beta / 2)}=\frac{\mathrm{e}^{-\omega \beta / 2}}{1-\mathrm{e}^{-\omega \beta}}=\mathrm{e}^{-\omega \beta / 2} \sum_{n=0}^{\infty} \mathrm{e}^{-n \omega \beta} \end{aligned}
where we used $\sinh x=2 \sinh x / 2 \cosh x / 2$. A comparison with the spectral sum over all energies in (2.55) yields the energies of the oscillator with (angular) frequency $\omega$,
$$E_{n}=\omega\left(n+\frac{1}{2}\right), \quad n=0,1,2, \ldots$$
For large values of $\omega \beta$, i.e., for very low temperature, the spectral sum is dominated by the contribution of the ground state energy. Thus for cold systems, the free energy converges to the ground state energy
$$F(\beta) \equiv-\frac{1}{\beta} \log Z(\beta) \stackrel{\omega \beta \rightarrow \infty}{\longrightarrow} E_{0}$$
One often is interested in the energies and wave functions of excited states. We now discuss an elegant method to extract this information from the path integral.

## 物理代写|量子场论代写Quantum field theory代考|Quantum Mechanics in Imaginary Time

ķ^(吨)=和−一世H^吨=∫和−一世和吨 d磷^和,在哪里磷^和是哈密顿量的谱族。如果H^有离散谱，则磷^和是在子空间上的正交投影H由能量小于的所有特征函数跨越和. 在下文中，我们假设哈密顿算子是从下面有界的。然后我们可以减去它的基态能量得到一个非负的H^(2.35) 中的积分限制为 0 并且∞. 随着替换吨→吨−一世τ， 我们获得

ķ^(τ)=和−τH^

ddτķ^(τ)=−H^ķ^(τ)

ķ(τ,q′,q)=⟨q′|和−τH^|q⟩,ķ(0,q′,q)=d(q′,q)

## 物理代写|量子场论代写Quantum field theory代考|Imaginary Time Path Integral

\begin{aligned} 左(\tau, q^{\prime}, q\right) &=\left\langle\hat{q}^{\prime}\left|\mathrm{e}^{- \tau \hat{H} / \hbar}\right| \hat{q}\right\rangle\&=\lim{n\rightarrow\infty}\int \mathrm{d}q{1}\cdots\mathrm{d}q_{n-1}\left(\frac {m}{2 \pi \hbar \value psilon}\right)^{n/2}\mathrm{e}^{-S_{\mathrm{E}}\left(q_{0}, q_{1} , \ldots, q_{n}\right) / \hbar}\S_{\mathrm{E}}(\ldots) &=\varepsilon \sum_{j=0}^{n-1}\left{\frac { m}{2}\left(\frac{q_{j+1}-q_{j}}{\valuepsilon}\right)^{2}+V\left(q_{j}\right)\right} \结束{对齐}\begin{aligned} 左(\tau, q^{\prime}, q\right) &=\left\langle\hat{q}^{\prime}\left|\mathrm{e}^{- \tau \hat{H} / \hbar}\right| \hat{q}\right\rangle\&=\lim{n\rightarrow\infty}\int \mathrm{d}q{1}\cdots\mathrm{d}q_{n-1}\left(\frac {m}{2 \pi \hbar \value psilon}\right)^{n/2}\mathrm{e}^{-S_{\mathrm{E}}\left(q_{0}, q_{1} , \ldots, q_{n}\right) / \hbar}\S_{\mathrm{E}}(\ldots) &=\varepsilon \sum_{j=0}^{n-1}\left{\frac { m}{2}\left(\frac{q_{j+1}-q_{j}}{\valuepsilon}\right)^{2}+V\left(q_{j}\right)\right} \结束{对齐}

{0,1, \ldots, n-1, n} \rightarrow\left{q_{0}, q_{1}, \ldots, q_{n-1}, q_{n}\right}{0,1, \ldots, n-1, n} \rightarrow\left{q_{0}, q_{1}, \ldots, q_{n-1}, q_{n}\right}

ķ(τ,q′,q)=⟨q′|和−τH^/H|q⟩=C∫q(0)=qq(τ)=q′Dq和−小号和[q]/H

S_{\mathrm{E}}[q]=\int_{0}^{\tau} d \sigma\left{\frac{m}{2} \dot{q}^{2}+V(q( \sigma))\对}S_{\mathrm{E}}[q]=\int_{0}^{\tau} d \sigma\left{\frac{m}{2} \dot{q}^{2}+V(q( \sigma))\对}

## 物理代写|量子场论代写Quantum field theory代考|Path Integral in Quantum Statistics

∫dqķ(ℏb,q,q)=tr⁡和−bH^=从(b)=∑和−b和n 和 b=1ķ乙吨

K_{\omega}(\beta, q, q)=\sqrt{\frac{m \omega}{2 \pi \sinh (\omega \beta)}} \exp \left{-m \omega \tanh ( \omega \beta / 2) q^{2}\right}K_{\omega}(\beta, q, q)=\sqrt{\frac{m \omega}{2 \pi \sinh (\omega \beta)}} \exp \left{-m \omega \tanh ( \omega \beta / 2) q^{2}\right}

\begin{对齐}Z(\beta)&=\sqrt{\frac{m\omega}{2\pi\sinh(\omega\beta)}}\int\mathrm{d}q\exp\left{- m\omega\tanh(\omega\beta/2)q^{2}\right}\&=\frac{1}2\sinh(\omega\beta/2)}=\mathrm{e}^{- \omega\beta/2}{1-\mathrm{e}^{-\omega\beta}}=\mathrm{e}^{-\omega\beta/2}\sum_{n=0}^{\ infty} \ mathrm {e} ^ {- n \ omega \ beta} \ end {对齐}\begin{对齐}Z(\beta)&=\sqrt{\frac{m\omega}{2\pi\sinh(\omega\beta)}}\int\mathrm{d}q\exp\left{- m\omega\tanh(\omega\beta/2)q^{2}\right}\&=\frac{1}2\sinh(\omega\beta/2)}=\mathrm{e}^{- \omega\beta/2}{1-\mathrm{e}^{-\omega\beta}}=\mathrm{e}^{-\omega\beta/2}\sum_{n=0}^{\ infty} \ mathrm {e} ^ {- n \ omega \ beta} \ end {对齐}

F(b)≡−1b日志⁡从(b)⟶ωb→∞和0

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYSICS 3544

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Path Integrals in Quantum and Statistical Mechanics

Already back in 1933 , Dirac asked himself whether the classical Lagrangian and action are as significant in quantum mechanics as they are in classical mechanics $[1,2]$. He observed that for simple systems, the probability amplitude
$$K\left(t, q^{\prime}, q\right)=\left\langle q^{\prime}\left|\mathrm{e}^{-\mathrm{i} \hat{A} t / h}\right| q\right\rangle$$
for the propagation from a point with coordinate $q$ to another point with coordinate $q^{\prime}$ in time $t$ is given by
$$K\left(t, q^{\prime}, q\right) \propto \mathrm{e}^{\mathrm{i} S\left[q_{\mathrm{cl}}\right] / h}$$
where $q_{\mathrm{cl}}$ denotes the classical trajectory from $q$ to $q^{\prime}$. In the exponent the action of this trajectory enters as a multiple of Planck’s reduced constant $h$. For a free particle with Lagrangian
$$L_{0}=\frac{m}{2} \dot{q}^{2}$$ the formula $(2.2)$ is verified easily: A free particle moves with constant velocity $\left(q^{\prime}-q\right) / t$ from $q$ to $q^{\prime}$ and the action of the classical trajectory is
$$S\left[q_{\mathrm{cl}}\right]=\int_{0}^{t} \mathrm{~d} s L_{0}\left[q_{\mathrm{cl}}(s)\right]=\frac{m}{2 t}\left(q^{\prime}-q\right)^{2}$$
The factor of proportionality in $(2.2)$ is then uniquely fixed by the condition $\mathrm{e}^{-\mathrm{i} \hat{H} t / \hbar} \longrightarrow 1$ for $t \rightarrow 0$ which in position space reads
$$\lim {t \rightarrow 0} K\left(t, q^{\prime}, q\right)=\delta\left(q^{\prime}, q\right)$$ Alternatively, it is fixed by the property $\mathrm{e}^{-\mathrm{i} \hat{H} t / h} \mathrm{e}^{-\mathrm{i} \hat{H} s / h}=\mathrm{e}^{-\mathrm{i} \hat{H}(t+s) / h}$ that takes the form $$\int \mathrm{d} u K\left(t, q^{\prime}, u\right) K(s, u, q)=K\left(t+s, q^{\prime}, q\right)$$ in position space. Thus, the correct free particle propagator on a line is given by $$K{0}\left(t, q^{\prime} \cdot q\right)=\left(\frac{m}{2 \pi \mathrm{i} \hbar t}\right)^{1 / 2} \mathrm{c}^{\mathrm{i} m\left(q^{\prime}-q\right)^{2} / 2 h t}$$
Similar results hold for the harmonic oscillator or systems for which $\langle\hat{q}(t)\rangle$ fulfills the classical equation of motion. For such systems $\left\langle V^{\prime}(\hat{q})\right\rangle=V^{\prime}(\langle\hat{q}\rangle)$ holds true. However, for general systems, the simple formula (2.2) must be extended, and it was Feynman who discovered this extension back in 1948. He realized that all paths from $q$ to $q^{\prime}$ (and not only the classical path) contribute to the propagator. This means that in quantum mechanics a particle can potentially move on any path $q(s)$ from the initial to the final destination,
$$q(0)=q \quad \text { and } \quad q(t)=q^{\prime}$$

## 物理代写|量子场论代写Quantum field theory代考|Recalling Quantum Mechanics

There are two well-established ways to quantize a classical system: canonical quantization and path integral quantization. For completeness and later use, we recall the main steps of canonical quantization both in Schrödinger’s wave mechanics and Heisenberg’s matrix mechanics.

A classical system is described by its coordinates $\left{q^{i}\right}$ and momenta $\left{p_{i}\right}$ on phase space $\Gamma$. An observable $O$ is a real-valued function on $\Gamma$. Examples are the coordinates on phase space and the energy $H(q, p)$. We assume that phase space comes along with a symplectic structure and has local coordinates with Poisson brackets
$$\left{q^{i}, p_{j}\right}=\delta_{j}^{i}$$
The brackets are extended to observables on through antisymmetry and the derivation rule ${O P, Q}=O{P, Q}+{O, Q} P$. The evolution in time of an observable is determined by
$$\dot{O}={O, H}, \quad \text { e.g. } \quad \dot{q}^{i}=\left{q^{i}, H\right} \quad \text { and } \quad \dot{p}{i}=\left{p{i}, H\right}$$
In the canonical quantization, functions on phase space are mapped to operators, and the Poisson brackets of two functions become commutators of the associated operators:
$$O(q, p) \rightarrow \hat{O}(\hat{q}, \hat{p}) \quad \text { and } \quad{O, P} \longrightarrow \frac{1}{\mathrm{i} \hbar}[\hat{O}, \hat{P}]$$

The time evolution of an (not explicitly time-dependent) observable is determined by Heisenberg’s equation
$$\frac{\mathrm{d} \hat{O}}{\mathrm{~d} t}=\frac{\mathrm{i}}{\hbar}[\hat{H}, \hat{O}]$$
In particular the phase space coordinates $\left(q^{l}, p_{i}\right)$ become operators with commutation relations $\left[\hat{q}^{i}, \hat{p}{j}\right]=\mathrm{i} \hbar \delta{j}^{i}$, and their time evolution is determined by
$$\frac{\mathrm{d} \hat{q}^{i}}{\mathrm{~d} t}=\frac{\mathrm{i}}{\hbar}\left[\hat{H}, \hat{q}^{i}\right] \quad \text { and } \quad \frac{\mathrm{d} \hat{p}{i}}{\mathrm{~d} t}=\frac{\mathrm{i}}{\hbar}\left[\hat{H}, \hat{p}{i}\right]$$
For a system of non-relativistic and spinless particles, the Hamiltonian reads
$$\hat{H}=\hat{H}{0}+\hat{V} \quad \text { with } \quad \hat{H}{0}=\frac{1}{2 m} \sum \hat{p}{i}^{2}$$ and one arrives at Heisenberg’s equations of motion $$\frac{\mathrm{d} \hat{q}^{i}}{\mathrm{~d} t}=\frac{\hat{p}{i}}{m} \quad \text { and } \quad \frac{\mathrm{d} \hat{p}{i}}{\mathrm{~d} t}=-\hat{V}{, i}$$
Observables are represented by Hermitian operators on a Hilbert space $\mathscr{H}$, whose elements characterize the states of the system:
$$\hat{O}(\hat{q}, \hat{p}): \mathcal{H} \longrightarrow \mathcal{H}$$
Consider a particle confined to an endless wire. Its Hilbert space is $\mathcal{H}=L_{2}(\mathbb{R})$, and its position and momentum operator are represented in position space as
$$(\hat{q} \psi)(q)=q \psi(q) \quad \text { and } \quad(\hat{p} \psi)(q)=\frac{\hbar}{i} \partial_{q} \psi(q)$$

## 物理代写|量子场论代写Quantum field theory代考|Feynman–Kac Formula

We shall derive Feynman’s path integral representation for the unitary time evolution operator $\exp (-\mathrm{i} \hat{H} t)$ as well as Kac’s path integral representation for the positive operator $\exp (-\hat{H} \tau)$. Thereby we shall utilize the product formula of Trotter. In case of matrices, this formula was already verified by Lie and has the form:
Theorem 2.1 (Lie’s Theorem) For two matrices $\mathrm{A}$ and $\mathrm{B}$
$$\mathrm{e}^{\mathrm{A}+\mathrm{B}}=\lim {n \rightarrow \infty}\left(\mathrm{e}^{\mathrm{A} / n} \mathrm{e}^{\mathrm{B} / n}\right)^{n}$$ To prove this theorem, we define for each $n$ the two matrices $\mathrm{S}{n}:=\exp (\mathrm{A} / n+\mathrm{B} / n)$ and $\mathrm{T}{n}:=\exp (\mathrm{A} / n) \exp (\mathrm{B} / n)$ and telescope the difference of their $n$ ‘th powers, $$\mathrm{S}{n}^{n}-\mathrm{T}{n}^{n}=\mathrm{S}{n}^{n-1}\left(\mathrm{~S}{n}-\mathrm{T}{n}\right)+\mathrm{S}{n}^{n-2}\left(\mathrm{~S}{n}-\mathrm{T}{n}\right) \mathrm{T}{n}+\cdots+\left(\mathrm{S}{n}-\mathrm{T}{n}\right) \mathrm{T}{n}^{n-1}$$ Now we choose any (sub-multiplicative) matrix norm, for example, the Frobenius norm. The triangle inequality together with $|X Y| \leq|X \mid| Y |$ imply the inequality $|\exp (X)| \leq \exp (|X|)$ such that $$\left|\mathrm{S}{n}\right|,\left|\mathrm{T}{n}\right| \leq a^{1 / n} \quad \text { with } \quad a=\mathrm{e}^{|\mathrm{A}|+|\mathrm{B}|}$$ Thus we conclude $$\left|\mathrm{S}{n}^{n}-\mathrm{T}{n}^{n}\right| \equiv\left|\mathrm{e}^{\mathrm{A}+B}-\left(\mathrm{e}^{\mathrm{A} / n} \mathrm{e}^{B / n}\right)^{n}\right| \leq n \times a^{(n-1) / n}\left|\mathrm{~S}{n}-\mathrm{T}{n}\right|$$ Finally, using $\mathrm{S}{n}-\mathrm{T}_{n}=-[\mathrm{A}, \mathrm{B}] / 2 n^{2}+O\left(1 / n^{3}\right)$, the product formula is verified for matrices. But the theorem also holds for self-adjoint operators.

Theorem $2.2$ (Trotter’s Theorem) If $\hat{A}$ and $\hat{B}$ are self-adjoint operators and $\hat{A}+$ $\hat{B}$ is essentially self-adjoint on the intersection $\mathscr{D}$ of their domains, then
$$\mathrm{e}^{-\mathrm{i} t(\hat{A}+\hat{B})}=s-\lim {n \rightarrow \infty}\left(\mathrm{e}^{-\mathrm{i} t \hat{A} / n} \mathrm{e}^{-\mathrm{i} t \hat{B} / n}\right)^{n}$$ If in addition $\hat{A}$ and $\hat{B}$ are bounded from below, then $$\mathrm{e}^{-\tau(\hat{A}+\hat{B})}=s-\lim {n \rightarrow \infty}\left(\mathrm{e}^{-\tau \hat{A} / n} \mathrm{e}^{-\tau \hat{B} / n}\right)^{n}$$
The operators need not be bounded and the convergence is with respect to the strong operator topology. For operators $\hat{A}{n}$ and $\hat{A}$ on a common domain $\mathscr{D}$ in the Hilbert space, we have s- $\lim {n \rightarrow \infty} \hat{A}{n}=\hat{A}$ iff $\left|\hat{A}{n} \psi-\hat{A} \psi\right| \rightarrow 0$ for all $\psi \in \mathscr{D}$. Formula (2.27) is used in quantum mechanics, and formula $(2.28)$ finds its application in statistical physics and the Euclidean formulation of quantum mechanics [16].

Let us assume that $\hat{H}$ can be written as $\hat{H}=\hat{H}{0}+\hat{V}$ and apply the product formula to the evolution kernel in (2.22). With $\varepsilon=t / n$ and $\hbar=1$, we obtain \begin{aligned} K\left(t, q^{\prime}, q\right) &=\lim {n \rightarrow \infty}\left\langle q^{\prime}\left|\left(\mathrm{e}^{-\mathrm{i} \varepsilon \hat{H}{0}} \mathrm{e}^{-\mathrm{i} \varepsilon \hat{V}}\right)^{n}\right| q\right\rangle \ &=\lim {n \rightarrow \infty} \int \mathrm{d} q_{1} \cdots \mathrm{d} q_{n-1} \prod_{j=0}^{j=n-1}\left|q_{j+1}\right| \mathrm{e}^{-\mathrm{i} \varepsilon \hat{H}{0}} \mathrm{e}^{-i \varepsilon \hat{V}}\left|q{j}\right\rangle \end{aligned}
where we repeatedly inserted the resolution of the identity $(2.21)$ and denoted the initial and final point by $q_{0}=q$ and $q_{n}=q^{\prime}$, respectively. The potential $\hat{V}$ is diagonal in position space such that
$$\left\langle q_{j+1}\left|\mathrm{e}^{-\mathrm{i} \varepsilon \hat{H}{0}} \mathrm{e}^{-\mathrm{i} \varepsilon \hat{V}}\right| q{j}\right\rangle=\left\langle q_{j+1}\left|\mathrm{e}^{-\mathrm{i} \varepsilon \hat{H}{0}}\right| q{j}\right\rangle \mathrm{e}^{-\mathrm{i} \varepsilon V\left(q_{j}\right)}$$

## 物理代写|量子场论代写Quantum field theory代考|Path Integrals in Quantum and Statistical Mechanics

ķ(吨,q′,q)=⟨q′|和−一世一个^吨/H|q⟩

ķ(吨,q′,q)∝和一世小号[qCl]/H

∫d在ķ(吨,q′,在)ķ(s,在,q)=ķ(吨+s,q′,q)在位置空间。因此，一条线上正确的自由粒子传播子由下式给出

ķ0(吨,q′⋅q)=(米2圆周率一世ℏ吨)1/2C一世米(q′−q)2/2H吨

q(0)=q 和 q(吨)=q′

## 物理代写|量子场论代写Quantum field theory代考|Recalling Quantum Mechanics

\left{q^{i}, p_{j}\right}=\delta_{j}^{i}\left{q^{i}, p_{j}\right}=\delta_{j}^{i}

○(q,p)→○^(q^,p^) 和 ○,磷⟶1一世ℏ[○^,磷^]

（不是明确的时间相关的）可观测的时间演化由海森堡方程确定

d○^ d吨=一世ℏ[H^,○^]

dq^一世 d吨=一世ℏ[H^,q^一世] 和 dp^一世 d吨=一世ℏ[H^,p^一世]

H^=H^0+在^ 和 H^0=12米∑p^一世2一个到达海森堡的运动方程

dq^一世 d吨=p^一世米 和 dp^一世 d吨=−在^,一世
Observables 由希尔伯特空间上的 Hermitian 算子表示H，其元素表征系统的状态：

○^(q^,p^):H⟶H

(q^ψ)(q)=qψ(q) 和 (p^ψ)(q)=ℏ一世∂qψ(q)

## 物理代写|量子场论代写Quantum field theory代考|Feynman–Kac Formula

Theorem 2.1 (Lie’s Theorem) 对于两个矩阵一个和乙

|小号n|,|吨n|≤一个1/n 和 一个=和|一个|+|乙|因此我们得出结论

|小号nn−吨nn|≡|和一个+乙−(和一个/n和乙/n)n|≤n×一个(n−1)/n| 小号n−吨n|最后，使用小号n−吨n=−[一个,乙]/2n2+○(1/n3)，乘积公式针对矩阵进行验证。但该定理也适用于自伴算子。

ķ(吨,q′,q)=林n→∞⟨q′|(和−一世eH^0和−一世e在^)n|q⟩ =林n→∞∫dq1⋯dqn−1∏j=0j=n−1|qj+1|和−一世eH^0和−一世e在^|qj⟩

⟨qj+1|和−一世eH^0和−一世e在^|qj⟩=⟨qj+1|和−一世eH^0|qj⟩和−一世e在(qj)

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYS8302

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Symmetric Tensors

There is a very important twist to the notion of tensor product when one considers systems composed of several, say $n$ identical particles. Identical particles are indistinguishable from each other, even in principle. If an electron in motion scatters on an electron at rest, two moving electrons come out of the experiment and there is no way telling which of them was the electron at rest (and it can be argued that the question may not even make sense). This has to be built in the model. To this aim, we consider a Hilbert space $\mathcal{H}$ with a basis $\left(e_{i}\right){i \geq 1}$, a given integer $n$ and we denote by $\mathcal{H}{n}$ the tensor product of $n$ copies of it, in the above sense. We observe that a permutation $\sigma$ of ${1,2, \ldots, n}$ induces a transformation of $\mathcal{H}{n}$, simply by transforming the basis element $\bigotimes{k \leq n} e_{i_{k}}$ into $\bigotimes_{k \leq n} e_{i_{\sigma(k)}}$. If an element $x$ of $\mathcal{H}_{n}$ describes the state of a system consisting of $n$ identical particles, its image under this transformation describes the same particle system, so that it must be of the type $\lambda x$ for

$\lambda \in \mathbb{C}$. Since the transform of $x$ has the same norm as $x$ then $|\lambda|=1$, that is the transform of $x$ differs from $x$ only by a phase. 10

In Part I of this book we consider the simplest case where the transform of each state $x$ is $x$ itself. Particles with this property are called bosons. 11 These are not the most common and interesting particles, but must be understood first. Later on, we will meet fermions, which comprise most of the important particles (and in particular electrons). Let us denote by $\mathrm{S}_{n}$ the group of permutations of ${1, \ldots, n}$.

## 物理代写|量子场论代写Quantum field theory代考|Creation and Annihilation Operators

We define and study the operators $A_{n}(\xi)$ and $A_{n}^{\dagger}(\eta)$, which will play a crucial role in the next section. It is convenient here to define $\mathcal{H}{0, s}:=\mathbb{C}$. For $n \geq 1$ and $\xi \in \mathcal{H}$ the operator $A{n}(\xi): \mathcal{H}{n, s} \rightarrow \mathcal{H}{n-1, s}$ transforms an $n$-particle state into an $(n-1)$-particle state, and for this reason is called an annihilation operator. For $n \geq 0$ and $\eta \in \mathcal{H}$ the operator $A_{n}^{\dagger}(\eta)$ : $\mathcal{H}{n, s} \rightarrow \mathcal{H}{n+1, s}$ transforms an $n$-particle state into an $(n+1)$-particle state, and is called a creation operator.

In order to avoid writing formulas which are too abstract, we pick an orthonormal basis $\left(e_{i}\right){i \geq 1}$ of $\mathcal{H}$, so that an element $\eta$ of $\mathcal{H}$ identifies with a sequence $\left(\eta{i}\right){i \geq 1}{ }^{13}$ Similarly we think of an element $\alpha$ of $\mathcal{H}{n, s}$ as a symmetric tensor $\left(\alpha_{i_{1}, \ldots, i_{n}}\right){i{1}, \ldots, i_{n} \geq 1}$.

Let us then introduce an important notation: Given a sequence $i_{1}, \ldots, i_{n+1}$ of length $n+1$ we denote by $i_{1}, \ldots, \hat{i}{k}, \ldots i{n+1}$ the sequence of length $n$ where the term $i_{k}$ is omitted.

## 物理代写|量子场论代写Quantum field theory代考|Boson Fock Space

A relativistically correct version of Quantum Mechanics must describe systems with a variable number of particles, because the equivalence of mass and energy allows creation and destruction of particles. Let us assume that the space $\mathcal{H}$ describes a single particle. We have constructed in (3.7) the space $\mathcal{H}_{n, s}$ which describes a collection of $n$ identical particles. The boson Fock space will simply be the direct sum of these spaces (in the sense of Hilbert space) as $n \geq 0$ and will describe collections of any number of identical particles. ${ }^{16}$ We do not yet incorporate any idea from Special Relativity. The construction of the boson Fock space is almost trivial. The non-trivial structure of importance is a special family of operators described in Theorem 3.4.2.

For $n=0$ we define $\mathcal{H}{0, s}=\mathbb{C}$, and we denote by $e{\emptyset}$ its basis element (e.g. the number 1 ). The element $e_{\emptyset}$ represents the state where no particles are present, that is, the vacuum. It is of course of fundamental importance. Then we define
$$\mathcal{B}{0}=\bigoplus{n \geq 0} \mathcal{H}{n, s},$$ the algebraic sum of the spaces $\mathcal{H}{n, s}$, where again $\mathcal{H}{n, s}$ is the space defined in (3.7). By definition of the algebraic sum, any element $\alpha$ of $\mathcal{B}{0}$ is a sequence $\alpha=(\alpha(n)){n \geq 0}$ with $\alpha(n) \in \mathcal{H}{n, s}$ and $\alpha(n)=0$ for $n$ large enough. Let us denote by $(\cdot, \cdot){n}$ the inner product on $\mathcal{H}{n, s}$. Consider $\alpha(n), \beta(n) \in \mathcal{H}{n, s}$ and $\alpha=(\alpha(n)){n \geq 0}, \beta=(\beta(n)){n \geq 0}$. We define $$(\alpha, \beta):=\sum{n \geq 0}(\alpha(n), \beta(n)){n} .$$ The boson Fock space $\mathcal{B}$ is the space of sequences $(\alpha(n)){n \geq 0}$ such that $\alpha(n) \in \mathcal{H}{n, s}$ and $$\left|(\alpha(n)){n \geq 0}\right|^{2}:=\sum_{n \geq 0}|\alpha(n)|^{2}<\infty,$$
where $|\alpha(n)|$ is the norm in $\mathcal{H}{n, s}$. We will hardly ever need to write down elements of $\mathcal{B}$ which are not in $\mathcal{B}{0}$.

We will somewhat abuse notation by considering each $\mathcal{H}{n, s}$, and in particular $\mathcal{H}=\mathcal{H}{1, s}$, as a subspace of $\mathcal{B}{0}$. Again, $\mathcal{H}{n, s}$ represents the $n$-particle states. Given $\xi, \eta$ in $\mathcal{H}$ we recall the operators $A_{n}(\xi)$ and $A_{n}^{\dagger}(\eta)$ of the previous section.

## 物理代写|量子场论代写Quantum field theory代考|Symmetric Tensors

λ∈C. 自从转型X具有相同的规范X然后|λ|=1，也就是变换X不同于X只是一个阶段。10

## 物理代写|量子场论代写Quantum field theory代考|Boson Fock Space

(一个,b):=∑n≥0(一个(n),b(n))n.玻色子福克空间乙是序列的空间(一个(n))n≥0这样一个(n)∈Hn,s和

|(一个(n))n≥0|2:=∑n≥0|一个(n)|2<∞,

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYS3101

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|A First Contact with Creation and Annihilation Operators

High-energy interacting particles create other particles, and a relativistic theory must consider multiparticle systems, where the number of particles may vary.

Let us describe what is probably the simplest example of a multiparticle system. ${ }^{67}$ The “particles” are as simple as possible. ${ }^{68}$

Consider a separable Hilbert space with an orthonormal basis $\left(e_{n}\right){n \geq 0}$. The idea is that the state of the system is described by $e{n}$ when the system consists of $n$ particles. The important structure consists of the operators $a$ and $a^{\dagger}$ defined on the domain
$$\mathcal{D}=\left{\sum_{n \geq 0} \alpha_{n} e_{n} ; \sum_{n \geq 0} n\left|\alpha_{n}\right|^{2}<\infty\right}$$
by
$$a\left(e_{n}\right)=\sqrt{n} e_{n-1} ; a^{\dagger}\left(e_{n}\right)=\sqrt{n+1} e_{n+1} .$$
The definition of $a\left(e_{n}\right)$ is to be understood as $a\left(e_{0}\right)=0$ when $n=0$. The reason for the factors $\sqrt{n}$ and $\sqrt{n+1}$ is not intuitive, and will become clear only gradually.
The notation is consistent, since for each $n, m$,
$$\left(e_{n}, a\left(e_{m}\right)\right)=\sqrt{m} \delta_{n}^{m-1}=\sqrt{m} \delta_{n+1}^{m}=\left(a^{\dagger}\left(e_{n}\right), e_{m}\right)$$
where $\delta_{n}^{m}$ is the Kronecker symbol (equal to 1 if $n=m$ and to 0 otherwise).
Exercise 2.17.1 Prove that $a^{\dagger}$ is the adjoint of $a$. Prove in particular that if $|(y, a(x))| \leq$ $C|x|$ for $x \in \mathcal{D}$ then $y \in \mathcal{D}$.

Exercise 2.17.2 Prove that for each $\lambda \in \mathbb{C}$ the operator $a$ has an eigenvector with eigenvalue $\lambda$. Can this happen for a symmetric operator?
It should be obvious from $(2.88)$ that
$$a^{\dagger} a\left(e_{n}\right)=n e_{n} ; a a^{\dagger}\left(e_{n}\right)=(n+1) e_{n} .$$
Let us then consider the self-adjoint operator ${ }^{69}$
$$N:=a^{\dagger} a .$$
Thus $N\left(e_{n}\right)=n e_{n}$. Since a system in state $e_{n}$ has $n$ particles, the observable corresponding to this operator is “the number of particles”. The operator $N$ is therefore called the number operator.
As another consequence of $(2.90)$,
$$\left[a, a^{\dagger}\right]\left(e_{n}\right)=e_{n t}$$

## 物理代写|量子场论代写Quantum field theory代考|The Harmonic Oscillator

The fundamental structure outlined in the previous section is connected to an equally fundamental system, the harmonic oscillator. A classical one-dimensional harmonic oscillator of angular frequency ${ }^{73} \omega$ consists of a point of mass $m$ on the real line which is pulled back to the origin with a force $m \omega^{2}$ times the distance to the origin. The quantum version of this system is the space $\mathcal{H}=L^{2}(\mathbb{R})$ with Hamiltonian
$$H:=\frac{1}{2 m}\left(P^{2}+\omega^{2} m^{2} X^{2}\right),$$
where $P$ and $X$ are respectively the momentum and the position operators of Section $2.5$. This is the Hamiltonian $(2.79)$ in the case where $V(x)=m \omega^{2} x^{2} / 2$. That this formula provides a quantized version of the classical harmonic oscillator is not obvious at all. ${ }^{74}$ We will explain in Section $6.6$ the systematic procedure of “canonical quantization” to discover formulas such as (2.77) or (2.95). This procedure is by no means a proof of anything, and the resulting formulas are justified only by the fact that they provide a fruitful model. So there is little harm to accept at this stage that the formula (2.95) is indeed fundamental. We have not proved yet that this formula defines a self-adjoint operator, but this is a consequence of the analysis below.

Exercise 2.18.1 Prove that a symmetric operator which admits an orthonormal basis of eigenvectors is self-adjoint. Hint: If we denote by $\left(e_{n}\right)$ an orthonormal basis of eigenvectors, and $\lambda_{n}$ the eigenvalue of $e_{n}$, the natural domain $\mathcal{D}$ of the operator is
$$\left{x=\sum_{n} x_{n} e_{n} ; \sum_{n}\left(1+\left|\lambda_{n}\right|^{2}\right)\left|x_{n}\right|^{2}<\infty\right}$$
The program for this section is first to find a basis of eigenvectors for the Hamiltonian (2.95), and then to examine how some classical quantities transform under quantization.

## 物理代写|量子场论代写Quantum field theory代考|Tensor Products

The present section is standard material, but our presentation attempts to balance rigor and readability.

Principle 6 If the states of two systems $\mathcal{S}{1}$ and $\mathcal{S}{2}$ are represented by the unitary rays in two Hilbert spaces $\mathcal{H}{1}$ and $\mathcal{H}{2}$ respectively, the appropriate Hilbert space to represent the system consisting of the union of $\mathcal{S}{1}$ and $\mathcal{S}{2}$ is the tensor product $\mathcal{H}{1} \otimes \mathcal{H}{2}$.

Our first task is to describe this space. ${ }^{2}$ A mathematician would love to see an “intrinsic” definition of this tensor product, a definition that does not use bases or a special representation of these Hilbert spaces. This can be done elegantly as in e.g. Dimock’s book [23]. We shall not enjoy this piece of abstraction and we shall go the ugly way.

If $\left(e_{n}\right){n \geq 1}$ is an orthonormal basis of $\mathcal{H}{1}$ and $\left(f_{n}\right){n \geq 1}$ is an orthonormal basis of $\mathcal{H}{2}$ then the vectors $e_{n} \otimes f_{m}$ constitute an orthonormal basis of $\mathcal{H}{1} \otimes \mathcal{H}{2}$, which is thus the set of vectors of the type $\sum_{n, m \geq 1} a_{n, m} e_{n} \otimes f_{m}$ where the complex numbers $a_{n, m}$ satisfy $\sum_{n, m \geq 1}\left|a_{n, m}\right|^{2}<\infty$. Here the quantity $e_{n} \otimes f_{m}$ is just a notation, which is motivated by the fact that for $x=\sum_{n \geq 1} \alpha_{n} e_{n} \in \mathcal{H}{1}$ and $y=\sum{n \geq 1} \beta_{n} f_{n} \in \mathcal{H}{2}$ one defines $x \otimes y \in$ $\mathcal{H}{1} \otimes \mathcal{H}{2}$ by $$x \otimes y=\sum{m, n \geq 1} \alpha_{n} \beta_{m} e_{n} \otimes f_{m} .$$
When either $\mathcal{H}{1}$ or $\mathcal{H}{2}$, or both, are finite-dimensional, the definition is modified in the obvious manner.

Exercise 3.1.1 When $\mathcal{H}{1}$ and $\mathcal{H}{2}$ are finite-dimensional, what is the dimension of $\mathcal{H}{1} \otimes \mathcal{H}{2}$ ? How does it compare with the dimension of the usual product $\mathcal{H}{1} \times \mathcal{H}{2}$ ?
When either $\mathcal{H}{1}$ or $\mathcal{H}{2}$ is infinite-dimensional, $\mathcal{H}{1} \otimes \mathcal{H}{2}$ is an infinite-dimensional Hilbert space. The important structure is the bilinear form from $\mathcal{H}{1} \times \mathcal{H}{2}$ into this space given by (3.1).

Recalling that $(x, y)$ denotes the inner product in a Hilbert space we observe the formula
$$\left(x \otimes y, x^{\prime} \otimes y^{\prime}\right)=\left(x, x^{\prime}\right)\left(y, y^{\prime}\right),$$
which is a straightforward consequence of the fact that the basis $e_{n} \otimes f_{m}$ is orthonormal. $^{3}$
The problem with our definition of the tensor product is that one is supposed to check that “it does not depend on the choice of the orthonormal basis”, a tedious task that joins similar tasks under the carpet. ${ }^{4}$ The good news is that all the identifications one may wish for are true. If both $\mathcal{H}{1}$ and $\mathcal{H}{2}$ are the space of square-integrable functions on $\mathbb{R}^{3}$, then $\mathcal{H}{1} \otimes \mathcal{H}{2}$ is the space of square-integrable functions on $\mathbb{R}^{6} .5$ This fits very well with the Dirac formalism: If $|x\rangle$ denotes the Dirac function at $\boldsymbol{x}$, (so that these generalized vectors provide a generalized basis of $\mathcal{H}{1}$, and similarly for $|\boldsymbol{y}\rangle$, then $|\boldsymbol{x}\rangle|\boldsymbol{y}\rangle$ denotes the Dirac function at the point $(\boldsymbol{x}, \boldsymbol{y}) \in \mathbb{R}^{6}$, and these generalized vectors provide a generalized basis of $\mathcal{H}{1} \otimes \mathcal{H}{2}$. Furthermore, if $f \in \mathcal{H}{1}$ and $g \in \mathcal{H}_{2}$ then $f \otimes g$ identifies with the function $(\boldsymbol{x}, \boldsymbol{y}) \mapsto$ $f(x) g(y)$ on $\mathbb{R}^{6}$.

## 物理代写|量子场论代写Quantum field theory代考|A First Contact with Creation and Annihilation Operators

\mathcal{D}=\left{\sum_{n \geq 0} \alpha_{n}e_{n} ; \sum_{n \geq 0} n\left|\alpha_{n}\right|^{2}<\infty\right}\mathcal{D}=\left{\sum_{n \geq 0} \alpha_{n}e_{n} ; \sum_{n \geq 0} n\left|\alpha_{n}\right|^{2}<\infty\right}

(和n,一个(和米))=米dn米−1=米dn+1米=(一个†(和n),和米)

ñ:=一个†一个.

[一个,一个†](和n)=和n吨

## 物理代写|量子场论代写Quantum field theory代考|The Harmonic Oscillator

H:=12米(磷2+ω2米2X2),

\left{x=\sum_{n} x_{n} e_{n} ; \sum_{n}\left(1+\left|\lambda_{n}\right|^{2}\right)\left|x_{n}\right|^{2}<\infty\right}\left{x=\sum_{n} x_{n} e_{n} ; \sum_{n}\left(1+\left|\lambda_{n}\right|^{2}\right)\left|x_{n}\right|^{2}<\infty\right}

## 物理代写|量子场论代写Quantum field theory代考|Tensor Products

X⊗是=∑米,n≥1一个nb米和n⊗F米.

(X⊗是,X′⊗是′)=(X,X′)(是,是′),

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYS4040

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|One-parameter Unitary Groups and Stone’s Theorem

A (strongly continuous) one-parameter unitary group is simply a (strongly continuous) unitary representation of $\mathbb{R}$, that is a map which associates to $t \in \mathbb{R}$ a unitary operator $U(t)$ on Hilbert space $\mathcal{H}$ in such a manner that
$$U(s) U(t)=U(s+t),$$
and (the continuity condition)
$$\forall x, y \in \mathcal{H}, \lim _{t \rightarrow 0}(x, U(t)(y))=(x, y) .$$

The archetypical example is the operator $U(t)$ on $L^{2}(\mathbb{R})$ given for $f \in L^{2}$ and $w \in \mathbb{R}$ by
$$U(t)(f)(w)=\exp (\mathrm{i} t w / h) f(w) .$$
This is simply the operator “multiplication by the function exp(it $\cdot / \hbar) . “$ Another example is the operator $V(t)$ on $L^{2}(\mathbb{R})$ given for $f \in L^{2}$ and $w \in \mathbb{R}$ by
$$V(t)(f)(w)=f(w+t) .$$
In both cases it is a nice exercise of elementary analysis to prove that these operators are strongly continuous. These one-parameter groups are closely related by the Fourier transform. Indeed,
$$\widehat{V(t)(f)}=U(t) \hat{f} .$$
Exercise 2.14.1 Make sure you understand every detail of the proof of the important formula (2.55)

Theorem 2.14.2 (Stone’s theorem) There is a one-to-one correspondence between the strongly continuous one-parameter unitary groups on a Hilbert space $\mathcal{H}$ and the self-adjoint operators on $\mathcal{H}$. Given the unitary group $U$, the corresponding self-adjoint operator $A$ is called the infinitesimal generator of U. It is defined by the formula ${ }^{58}$
$$A(x)=\lim _{t \rightarrow 0} \frac{h}{i}(U(t)(x)-x) .$$
and its domain $\mathcal{D}=\mathcal{D}(A)$ is the set of $x$ for which the previous limit exists.

## 物理代写|量子场论代写Quantum field theory代考|Time-evolution

Consider a physical system, the state of which is described by a vector in $\mathcal{H}$.
Principle 5 If the system does not change with time (not in the sense that it does not evolve, but in the sense that it is not subjected to variable external influences), its evolution between time $t_{0}$ and time $t_{1}$ is described by a unitary operator ${ }^{62} U\left(t_{1}, t_{0}\right)$.

This operator depends only on $t_{1}-t_{0}$, reflecting the fact that the laws of physics are believed not to change with time ${ }^{63}$

Please observe the notation: the evolution $U\left(t_{1}, t_{0}\right)$ is from $t_{0}$ to $t_{1}$. The reason for this notation is that the evolution of the system from $t_{0}$ to $t_{1}$ and then from $t_{1}$ to $t_{2}$ is represented by $U\left(t_{2}, t_{1}\right) U\left(t_{1}, t_{0}\right)$, which also represents the same evolution as $U\left(t_{2}, t_{0}\right)$ so that as in (2.45) these two operators should differ only by a phase, $U\left(t_{2}, t_{1}\right) U\left(t_{1}, t_{0}\right)=c U\left(t_{2}, t_{0}\right)$ for some $c$ of modulus 1. Thus $U\left(t_{2}-t_{1}, 0\right) U\left(t_{1}-t_{0}, 0\right)=c U\left(t_{2}-t_{0}, 0\right)$. This means that $U(t):=$ $U(t, 0)$ should be a projective representation of $\mathbb{R}$ in $\mathcal{H}$, and on physical grounds it should be continuous in some sense. As shown in Section $2.13$ this projective representation arises from a true representation, so we can as well assume that it already is a true representation, and Stone’s theorem describes these. Therefore, there exists a self-adjoint operator $H$ on $\mathcal{H}$ such that
$$U(t)=\exp (-\mathrm{i} t H / \hbar) .$$
Probably it is worth making explicit the following fundamental point:
The time-evolution of a quantum system is entirely deterministic.
The minus sign in (2.75) is conventional. The reason for this convention will appear in Section 9.7. Since $h$ has the dimension of an energy times a time, $H$ has the dimension of an energy. It is called the Hamiltonian of the system. Although this is certainly not obvious ${ }^{64}$
The Hamiltonian should be thought of as representing the energy of the system.
A consequence of (2.75) is that if $\psi$ belongs to the domain of $H$, then, by the second part of $(2.64), \psi(t):=U(t)(\psi)$ satisfies the equation
$$\psi^{\prime}(t)=-\frac{\mathrm{i}}{h} H(\psi(t)) \text {. }$$

## 物理代写|量子场论代写Quantum field theory代考|Schrödinger and Heisenberg Pictures

The previous description of an evolving state $\psi(t)$ and of time-independent operators is called the Schrödinger picture.

We have seen that we may be able to improve matters by re-shuffling the state space using a unitary transformation. A fundamental idea is to try this using a time-dependent unitary transformation $V(t)$, replacing the state $\psi$ by $V(t)(\psi)$, and replacing the operator $A$ by $A(t)=V(t) A V(t)^{-1}$. Here we use a first simple implementation, with $V(t)=$ $\exp ($ it $H / \hbar)=U(t)^{-1}$. This is called the Heisenberg picture. In the Heisenberg picture the state $\psi(t)$ is replaced by $V(t)(\psi(t))=U(t)^{-1} U(t)(\psi)=\psi$, so that states do not change with time. On the other hand, an operator $A$ is replaced by the operator
$$A(t)=U(t)^{-1} A U(t)=\exp (\mathrm{i} t H / \hbar) A \exp (-\mathrm{i} t H / \hbar) .$$
Suppose that at time $t=0$ the system is in state $\psi$. Then, in the Schrödinger picture, the average value of $A$ at time $t$ is given by
$$(\psi(t), A \psi(t))=(U(t) \psi, A U(t) \psi)=\left(\psi, U(t)^{-1} A U(t) \psi\right)=(\psi, A(t) \psi),$$
where the last expression is the same quantity in the Heisenberg picture. Thus these two pictures are fortunately consistent with each other.

We may wonder why there is anything to gain by moving from the Schrödinger picture, where the simpler objects, the states, evolve, but where the complicated objects, the operators, are constant, to the Heisenberg picture where the simpler objects are constant, but the complicated ones evolve. One reason is that while it is correct in principle that the states, which are simply vectors of $\mathcal{H}$ are simpler objects than operators, it will often happen that the operators of interest have a simple description, while the states have a very complicated one. Another reason will be given at the end of the present section.

## 物理代写|量子场论代写Quantum field theory代考|One-parameter Unitary Groups and Stone’s Theorem

∀X,是∈H,林吨→0(X,在(吨)(是))=(X,是).

## 物理代写|量子场论代写Quantum field theory代考|Time-evolution

(2.75) 中的减号是常规的。这种约定的原因将出现在第 9.7 节中。自从H具有能量乘以时间的维度，H具有能量的维度。它被称为系统的哈密顿量。虽然这肯定不是很明显64

(2.75) 的结果是，如果ψ属于的领域H，然后，由第二部分(2.64),ψ(吨):=在(吨)(ψ)满足方程

ψ′(吨)=−一世HH(ψ(吨)).

## 物理代写|量子场论代写Quantum field theory代考|Schrödinger and Heisenberg Pictures

(ψ(吨),一个ψ(吨))=(在(吨)ψ,一个在(吨)ψ)=(ψ,在(吨)−1一个在(吨)ψ)=(ψ,一个(吨)ψ),

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYS7076

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Projective versus True Unitary Representations

Let us start the discussion of the concepts involved in Definitions $2.10 .1$ and 2.10.3. The word “unitary” refers of course to the fact that each of the operators $U(a)$ is unitary. Unless mentioned otherwise, all representations are unitary, so that we shall nearly always omit the word “unitary”, and the expressions “representation” and “projective representations” have to be understood by default as “unitary representation” and “projective unitary representations”.

To insist that a representation satisfies $r(a, b)=1$ for all $a, b$ we will sometimes say true representation, even though throughout the book, the word “representation” means “true representation”. When we consider a representation that is only a projective representation we will always say so explicitly. It is most important to understand the relationship between representations and projective representations.

• The concept of “representation” is far more restrictive than the concept of “projective representation”.
• From the point of view of mathematics, the nice objects are representations. The study of group representations is a vast subject in mathematics.
• From the point of view of Quantum Mechanics, the natural objects are projective representations.

The following explains an important relationship between representations and projective representations.

Definition 2.11.1 Given a true representation $V$ of $G$, and for $a \in G$ a number $\lambda(a)$ of modulus 1 , the formula
$$U(a):=\lambda(a) V(a)$$
defines a projective representation, since (2.45) holds for the function
$$r(a, b)=\lambda(a b) /(\lambda(a) \lambda(b)) .$$
When this is the case we will say that the projective representation Uarises from the true representation $V$.

More generally, there is an important idea behind this definition: two projective representations $U, U^{\prime}$ such for each $a \in G$ one has $U(a)=\lambda(a) U^{\prime}(a)$ for some complex number $\lambda(a)$ with $|\lambda(a)|=1$ are to be thought of as “the same projective representation”.

## 物理代写|量子场论代写Quantum field theory代考|Mathematicians Look at Projective Representations

This material is not needed to follow the main story. It assumes that you know some very basic group theory. A map $U$ from $G$ to the group $\mathcal{U}(\mathcal{H})$ of unitary transformations of $\mathcal{H}$ is a true representation if and only if it is a group homomorphism. The group $\mathcal{U}(\mathcal{H}$ ) has a remarkable subgroup, the subgroup consisting of the transformations $\lambda 1$ with $|\lambda|=1$. Let us denote by $\mathcal{U}{p}(\mathcal{H})$ the quotient of $\mathcal{U}(\mathcal{H})$ by this subgroup, and by $\Phi$ the quotient map $\mathcal{U}(\mathcal{H}) \rightarrow \mathcal{U}{p}(\mathcal{H})$. Thus the elements of $\mathcal{U}{p}(\mathcal{H})$ are unitary operators “up to a phase”, i.e. up to a multiplicative constant of modulus 1 . It is immediate to check that a map $U$ from a group $G$ into $\mathcal{U}(\mathcal{H})$ is a projective representation in the sense of Definition $2.10 .1$ if and only $\Phi \circ U$ is a group homomorphism from $G$ to $\mathcal{U}{p}(\mathcal{H})$. The important object is thus the map $\Phi \circ U$. Accordingly, mathematicians define a projective representation as a group homomorphism from $G$ to $\mathcal{U}{p}(\mathcal{H})$. This formalizes the idea that two projective representations $U$ and $U^{\prime}$ such that $U(a)=\lambda(a) U^{\prime}(a)$ “are the same projective representation” (because this is the case if and only if $\Phi \circ U=\Phi \circ U^{\prime}$ ). Another benefit of this approach is that it becomes natural to define “continuous projective representations”, a topic which is investigated in Section A.2. In mathematical language, the fundamental question, is, given a projective representation $U$ of $G$, that is a group homomorphism from $G$ to $\mathcal{U}{p}(\mathcal{H})$, whether there exists a true representation $V$, that is a group homomorphism from $G$ to $\mathcal{U}(\mathcal{H})$, such that $U=\Phi \circ V$.

## 物理代写|量子场论代写Quantum field theory代考|Projective Representations of R

We do not investigate in detail how true and projective representations are related in general, but we examine this question in the centrally important case $G=\mathbb{R}$. However, we must first discuss a technical question. In the cases of greatest interest, $G$ is a topological group, and to avoid pathologies, one requires also a mild continuity assumption.

Definition 2.13.1 The map $a \mapsto U(a)$ which associates to each element $a$ of $G$ a unitary operator $U(a)$ is called strongly continuous if for each $x \in \mathcal{H}$ the map $a \mapsto U(a)(x)$ from $G$ to $\mathcal{H}$ is continuous.

The topology on $\mathcal{H}$ is the topology induced by its norm, so the condition of strong continuity means that for each $x \in \mathcal{H}$ the norm $\left|U(a)(x)-U\left(a_{0}\right)(x)\right|$ goes to 0 as $a \rightarrow a_{0}$. Despite the adjective “strong”, this condition is much weaker than the continuity of the map $a \mapsto U(a)$ in the operator norm.

A simple but instructive example of a representation is the case where $G=\mathbb{R}$, $\mathcal{H}=L^{2}(\mathbb{R})$ and $U(a)(f) \in L^{2}(\mathbb{R})$ is the function $w \mapsto f(w-a)$. The map $a \mapsto U(a)$ is not continuous when the space of unitary operators is provided with the topology induced by the operator norm but it is strongly continuous (as one sees by approximating $f$ with a continuous function of bounded support).

When the map $a \mapsto U(a)$ is strongly continuous, then for $x, y \in \mathcal{H}$ the map $a \mapsto(x, U(a)(y))$ is continuous. This apparently weaker condition is equivalent to strong continuity. To prove this, assume the weaker condition. Then as $a \rightarrow a_{0},\left(U(a)(x), U\left(a_{0}\right)(x)\right)$ tends to the square of the norm of $U\left(a_{0}\right)(x)$, and since both vectors $U(a)(x)$ and $U\left(a_{0}\right)(x)$ have the same norm they become close to each other (as follows from the relation $\left.|u-v|^{2}=|u|^{2}+|v|^{2}-2 \operatorname{Re}(u, v)\right)$

## 物理代写|量子场论代写Quantum field theory代考|Projective versus True Unitary Representations

• “表示”的概念比“投影表示”的概念要严格得多。
• 从数学的角度来看，好的对象是表示。群表示的研究是数学中的一门广泛的学科。
• 从量子力学的角度来看，自然物体是射影表示。

r(一个,b)=λ(一个b)/(λ(一个)λ(b)).

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYSICS 3544

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Dirac’s Formalism

It is regrettable that neither the position nor the momentum operator has a basis of eigenvectors, for this would indeed be very convenient. Paul Dirac invented a remarkable formalism to deal with this problem. It is used in almost every physics textbook, where it is typically considered as self-evident. We try to explain some of the basic features and meaning of this formalism here, striving as usual to explain what this means, but with no serious attempt to make matters rigorous.

Dirac’s formalism works beautifully. It allows a great economy of thought and notation. It is however unfriendly to mathematicians, and the mathematically inclined reader must brace for a kind of cold shower, as things are likely to look horrendously confusing at first. Let us stress that it is not necessary to master this formalism to follow most of the rest of this book. Still, we will use it at times, if only to formulate results in the same language as they are found in physics textbooks. The reader who finds the present section overwhelming is encouraged to move on and to come back when the need arises.

As a consequence of the use of Dirac’s formalism, if one looks at a physics textbook discussing (say) particles on the real line, one may find it difficult to recognize any of the previous material. First, one is likely to find very early the sentence “let $|x\rangle$ denote the state of a particle located at $x . . . “$. An element of the position state space $\mathcal{H}=L^{2}(\mathbb{R}$, $\mathrm{d} x)$ that could be said to be located at $x$ would have to be an eigenvector of the position operator $X$ with eigenvalue $x$ and these do not exist. The expression $|x\rangle$ can however be given a meaning in a kind of “distributional sense”. It makes sense only when integrated against a Schwartz function $f$, that is for such a function the integral $\int \mathrm{d} x|x\rangle f(x)$ makes sense as an element of $\mathcal{H}$. The value of this integral is simply the function $f$ seen as an element of $\mathcal{H}$. Quite naturally, we denote by
$|f\rangle$ the function $f \in \mathcal{S}$ seen as an element of $\mathcal{H}$,
so that
$$|f\rangle=\int \mathrm{d} x|x\rangle f(x) .$$
This should certainly remind us of a basis expansion $f=\sum_{i}\left(f, e_{i}\right) e_{i}=\sum_{i} e_{i}\left(f, e_{i}\right)$, and physicists think of $|x\rangle$ as a “continuous basis”. In this manner we have given a meaning to the quantity $|x\rangle$ as “an element of $\mathcal{H}$ in the distributional sense”. The principle at work here is important enough to be stated clearly: ${ }^{41}$

## 物理代写|量子场论代写Quantum field theory代考|Why Are Unitary Transformations Ubiquitous

Let us go back to the setting of a general Hilbert space, whose elements are denoted $x, y, z, \ldots$. To each observable is associated a self-adjoint operator. Conversely, to each self-adjoint operator is associated an observable (although it is another matter in concrete situations to design an experiment that actually measures it). We now describe a fundamental class of such observables. Given $x \in \mathcal{H}$ of norm $1,(x, x)=1$, the projector $P_{x}(y):=(x, y) x$ is Hermitian since
$$\left(z, P_{x}(y)\right)=(z, x)(x, y)=(x, z)^{*}(x, y)=((x, z) x, y)=\left(P_{x}(z), y\right)$$
Exercise 2.9.1 In Dirac’s formalism for a norm-1 vector $|\alpha\rangle$ one writes the projector $P_{\alpha}=|\alpha\rangle\langle\alpha|$. Why does it seem to require no proof that $P_{\alpha}$ is Hermitian?

Thus the operator $P_{x}$ corresponds to an observable $\mathcal{O}$. The possible values of $\mathcal{O}$ are the eigenvalues of $P$, namely 0 and 1 . We may describe $\mathcal{O}$ as asking the question: Is the state of the system equal to $x ?^{48}$ The average value of this operator in state $y$ is given by $\left(y, P_{x}(y)\right)=(y, x)(x, y)=|(x, y)|^{2}$ and is the probability to obtain the answer “yes” to your question. It is called the transition probability between $x$ and $y^{49}$ In physics, the inner product $(x, y)$ is often called an amplitude, so that the transition probability is the square of the modulus of the amplitude. ${ }^{50}$ The transition probability does not change if one multiplies $x$ and $y$ by complex numbers of modulus 1 , as expected from the fact that this multiplication does not change the state represented by either $x$ or $y$.

We should expect that any transformation ${ }^{51}$ which preserves the physical properties of a system preserves the transition probabilities. Transition probabilities are preserved by unitary transformations, since for such a transformation $|(U x, U y)|^{2}=|(x, y)|^{2}$. They are also preserved under anti-unitary transformations, i.e. anti-linear operators which preserve the inner product. Conversely, which are the transformations of $\mathcal{H}$ which preserve transition probabilities? A deep theorem of Eugene Wigner [92, appendix to Chapter 20] shows that this is the case only for unitary and anti-unitary transformations. ${ }^{52}$ A fundamental consequence is that any transformation which preserves the physics of the system corresponds to a unitary or anti-unitary transformation. As we explain in the next section, there are many of these, corresponding to the symmetries of Nature. The symmetries of Nature of a certain type naturally form a group, bringing us to group theory. Furthermore time-evolution will also be represented by a unitary transformation.

## 物理代写|量子场论代写Quantum field theory代考|Unitary Representations of Groups

Certain types of invariance in Nature are among the most important guiding principles in developing physical theories about the real world. This will be a recurring theme in this book. It forces us to choose models which satisfy certain symmetries and this implies extremely strict restrictions on the possible forms of physical theories.

In this section we start to use this principle in the simplest case, translation invariance. In physics each observer uses a reference frame to describe the positions of points in space (or in space-time). These reference frames need not have the same origin, may use different privileged directions and may even move with respect to each other. ${ }^{53}$ Here we just consider the situation of different origins. If you study the motion of an object using a different origin for your reference frame than mine, we may disagree on the coordinates of the object, but we should agree that it follows the same laws of physics. Mathematically, the space $\mathbb{R}^{3}$ acts on itself by translations, and we examine first the effect of these translations at a purely classical level. Suppose that the system you are studying is translated by a vector $a^{54}$ The object at position $\boldsymbol{x}$ that you were studying has now been moved to position $\boldsymbol{x}+\boldsymbol{a}$. Say that you use a function $f$ on $\mathbb{R}^{3}$ to measure e.g. the electrical potential at a point of space. Before you translate the system, the value $f(\boldsymbol{x})$ measures the potential at the point $\boldsymbol{x}$. After the translation this value of the potential occurs at the point $\boldsymbol{x}+\boldsymbol{a}$. Thus the value of the new function $U(a)(f)$ you use to measure the potential at this point $x+a$ equals the value of the old function $f$ at the point $x$ :
$$U(a)(f)(x+a)=f(x)$$
i.e.
$$U(a)(f)(x)=f(x-a) .$$
Observe the all-important minus sign and note the fundamental property $U(\boldsymbol{a}+\boldsymbol{b})=$ $U(\boldsymbol{a}) U(\boldsymbol{b})$

Suppose now that more generally we study a system whose state is described by a vector $x$ in a Hilbert space $\mathcal{H}$. If the system is translated by a vector $a$ we expect that the system will be described by a new state $U(a)(x)$. This new description should not change the physics. The transition probability between $x$ and $y$ should be the same as the transition probability between $U(a)(x)$ and $U(a)(y)$, i.e. $|(x, y)|^{2}=|(U(a)(x), U(a)(y))|^{2}$. According to the discussion of the previous section, $U(a)$ is either unitary or anti-unitary. Moreover, it is obvious that $U(0)$ should be the identity. What becomes very interesting is when we perform two such translations in succession, first by a vector $a$ and then by a vector $b$. The state $x$ is transformed first in $U(a)(x)$ and then in $U(b)(U(a)(x))$. This also amounts to perform the translation by the vector $\boldsymbol{a}+\boldsymbol{b}$, and this transforms the state $x$ into the state $U(\boldsymbol{a}+\boldsymbol{b})(x)$. Therefore $U(b) U(\boldsymbol{a})$ and $U(\boldsymbol{a}+\boldsymbol{b})$ should represent the same transformation of the system.

## 物理代写|量子场论代写Quantum field theory代考|Dirac’s Formalism

|F⟩功能F∈小号被视为一个元素H,

|F⟩=∫dX|X⟩F(X).

## 物理代写|量子场论代写Quantum field theory代考|Why Are Unitary Transformations Ubiquitous

(和,磷X(是))=(和,X)(X,是)=(X,和)∗(X,是)=((X,和)X,是)=(磷X(和),是)

IE

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYSICS 7013

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Position State Space for a Particle

In this section we analyze how the previous machinery works to describe a very simple system, a massive point that can be located anywhere on the real line. This provides a first concrete example, and at the same time allows us to discuss the intricacies of considering infinite-dimensional state spaces. Almost nothing of what we explained in the finite-dimensional case will carry on exactly the same, but a suitable infinite-dimensional reinterpretation of the concepts will basically suffice.

The state space $\mathcal{H}$ is the space $L^{2}=L^{2}(\mathbb{R}, \mathrm{d} x)$ of complex-valued square-integrable functions ${ }^{23}$ on the real line. ${ }^{24}$ An element of $\mathcal{H}$ is thus a complex-valued function ${ }^{25} f$ on $\mathbb{R}$. The traditional terminology is to call this function the wave function. A wave function of norm 1 therefore describes the possible state of a massive point, which for simplicity we will call a particle.

The basic idea is that the position of a particle in state $f$ is not really determined, but that the function $|f|^{2}$ represents the probability density to find this particle at a given location. This statement will eventually appear as the proper interpretation of (2.6) in the present “continuous case”. To develop this idea, consider an interval $I$ of $\mathbb{R}$, and the operator $1_{I}$ defined by $1_{l}(f)(x)=f(x)$ if $x \in I$ and $1_{l}(f)(x)=0$ if $x \notin I$. This operator is bounded since $\left|1_{I}(f)\right| \leq|f|$. After we develop the right generalization of Hermitian operators in infinite dimensions, it will become apparent that this operator corresponds to an observable, and the average value of this observable on the state $f$ is
$$\left(f, 1_{I}(f)\right)=\int_{I} \mathrm{~d} x|f(x)|^{2},$$
which is the probability to find the particle in the set $I$. It is worth repeating the fundamental fact: When you actually measure whether the particle is in $I$ or not, you get a yes/no answer. But you are certain to find the particle in $I$ only if its state vector $f$ is an eigenvector of $1_{I}$

of eigenvalue 1, i.e. $f(x)=0$ for $x \notin I$, and you are certain not to find it in $I$ only when $f$ is an eigenvector of $1_{l}$ of eigenvalue 0 , i.e. $f(x)=0$ for $x \in I .^{26}$

In the present setting, the position of the particle is an observable so that it corresponds to a “Hermitian operator” $X$, which will be called the position operator. It is not difficult to guess what the operator $X$ should be. If indeed $|f|^{2}$ represents the probability density that the particle is at a given location, its average position is given by
$$\int \mathrm{d} x x|f(x)|^{2}=\int \mathrm{d} x f(x)^{*}(x f(x))$$

## 物理代写|量子场论代写Quantum field theory代考|Unitary Operators

We introduce now unitary operators between Hilbert spaces. It is a fundamental notion in at least two respects:

• Mathematically it provides (as explained in this section) a way to recognize whether two different models “are in fact the same”.
• Physically, countless processes are represented by unitary operators. Why this is the case is explained at the beginning of Section 2.10.

Definition 2.6.1 A linear operator $U$ between Hilbert spaces is called unitary if it is one-to-one, onto, and preserves the norm, $|U(x)|=|x|$.

Unitary transformations are in a sense the “natural class of isomorphisms between Hilbert spaces”. The “polarization identity” $|x+y|^{2}=|x|^{2}+|y|^{2}+2 \operatorname{Re}(x, y)$ shows that a unitary operator preserves the inner product, ${ }^{38}$
$$(U(x), U(y))=(x, y) .$$
It is almost obvious that the set $\mathcal{U}(\mathcal{H})$ of unitary operators on a Hilbert space $\mathcal{H}$ forms a group.

Next we reformulate the condition that an operator on a Hilbert space $\mathcal{H}$ is unitary using the notion of adjoint operator. As already noted, for a bounded operator $A$ one has $\mathcal{D}(A)=$ $\mathcal{D}\left(A^{\dagger}\right)=\mathcal{H}$ and
$$\forall x, y \in \mathcal{H} ;\left(A^{\dagger}(x), y\right)=(x, A(y)) \text {. }$$
A unitary operator on a Hilbert space is bounded, so its adjoint is defined everywhere. Furthermore by $(2.21)$ one has $\left(U^{\dagger} U(x), y\right)=(U(x), U(y))$ and $(2.20)$ is equivalent to

$U^{\dagger} U=1$. Consequently, an operator on a complex Hilbert space is unitary if and only if it is invertible and
$$U^{-1}=U^{\dagger}$$
Thus a unitary operator also satisfies $U U^{\dagger}=1$.
The following trivial fact is stressed because of its considerable importance.

## 物理代写|量子场论代写Quantum field theory代考|Momentum State Space for a Particle

To put the ideas of the previous section to use we go back to the model of a massive particle on the line which we studied in Section $2.5$. Let us consider $\mathcal{H}^{\prime}=L^{2}(\mathbb{R}, \mathrm{d} p /(2 \pi h))$, where the notation $\mathrm{d} p /(2 \pi \hbar)$ means that we include the factor $1 /(2 \pi \hbar)$ whenever we integrate in p. Consider the Fourier transform $U: f \mapsto \hat{f}$ of $(1.30)$ from $\mathcal{H}$ to $\mathcal{H}^{\prime}$. It is a unitary operator since it preserves the scalar product by (1.32) and since it has an inverse, the inverse Fourier transform $\varphi \mapsto \check{\varphi}$. The state which was represented by $f \in \mathcal{H}$ is now represented by $\varphi=\hat{f} \in \mathcal{H}^{\prime}$. As in Lemma 2.6.2 the Fourier transform $U$ transports an operator $A$ on $\mathcal{H}$ to the operator $A^{\prime}=U A U^{-1}$ on $\mathcal{H}^{\prime}$ given by $A^{\prime}(\varphi)=\widehat{A(\breve{\varphi})}$. Using (1.33) for $f=\breve{\varphi}$ yields (provided $\varphi$ is well behaved)
$$P^{\prime}(\varphi)(p)=p \varphi(p)$$
and, similarly,
$$X^{\prime}(\varphi)=\mathrm{i} \hbar \frac{\mathrm{d} \varphi}{\mathrm{d} p} .$$
(The plus sign here is not surprising since (2.19) implies that $\left[X^{\prime}, P^{\prime}\right]=\mathrm{i} \hbar 1$.) It is now the momentum operator which looks simple and the position operator which looks complicated. Just as in position state space $|f|^{2}$ represented the probability density of the location of the particle in state $f$, we can now argue that $|\varphi|^{2}$ represents the probability density of the momentum of the particle (when the basic measure is $\mathrm{d} p / 2 \pi h$ ). For this reason we will call this space $\mathcal{H}^{\prime}$ the momentum state space. Taking the Fourier transform is the standard way to analyze a wave. Thus it can be said that using momentum state space amounts to thinking of a particle as wave. This is the wave-particle duality. ${ }^{39}$

## 物理代写|量子场论代写Quantum field theory代考|Position State Space for a Particle

(F,1我(F))=∫我 dX|F(X)|2,

∫dXX|F(X)|2=∫dXF(X)∗(XF(X))

## 物理代写|量子场论代写Quantum field theory代考|Unitary Operators

• 从数学上讲，它提供了（如本节所述）一种识别两个不同模型是否“实际上相同”的方法。
• 在物理上，无数的过程由单一运算符表示。为什么会出现这种情况在 2.10 节的开头进行了解释。

(在(X),在(是))=(X,是).

∀X,是∈H;(一个†(X),是)=(X,一个(是)).

## 物理代写|量子场论代写Quantum field theory代考|Momentum State Space for a Particle

X′(披)=一世⁇d披dp.
（这里的加号并不奇怪，因为（2.19）意味着[X′,磷′]=一世⁇1.) 现在是动量算子看起来很简单，而位置算子看起来很复杂。就像在位置状态空间中一样|F|2表示状态中粒子位置的概率密度F，我们现在可以说|披|2表示粒子动量的概率密度（当基本度量为dp/2圆周率H）。出于这个原因，我们将这个空间称为H′动量状态空间。傅里叶变换是分析波的标准方法。因此可以说，使用动量态空间相当于将粒子视为波。这就是波粒二象性。39

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。

## 物理代写|量子场论代写Quantum field theory代考|PHYC90008

statistics-lab™ 为您的留学生涯保驾护航 在代写量子场论Quantum field theory方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写量子场论Quantum field theory代写方面经验极为丰富，各种代写量子场论Quantum field theory相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|量子场论代写Quantum field theory代考|Measuring Two Different Observables on the Same System

Suppose now that we consider a second observable $\mathcal{O}^{\prime}$ with a corresponding Hermitian operator $B$. Then if a system in state $|\alpha\rangle$ is such that it will always yield the same value
14 One should say that the problem here is formulated in a rather outrageous way to make the point clear. The universe seems to have done fine prior to our existing! The problem is nonetheless real. On the one hand, one may argue that Quantum Mechanics applies only to the microscopic world. Then a measurement process will be anything that interacts with a macroscopic object such as a photosensitive chemical in a photographic emulsion and has nothing to do with a conscious observer. On the other hand, if one refuses this arbitrary and ill-defined boundary between macroscopic and microscopic worlds, the quantum realm extends all the way to the consciousness of the observer. It is then very difficult to escape the conclusion that this consciousness plays a role, and matters become very murky. The theory of decoherence tries to address these issues.
15 Including cases where the mathematical justification is not ironclad.
16 The measured value of the observable $O$ in state $|\alpha\rangle$ is a random variable, and as such has an expected value. When we repeat the experiment many times and average the corresponding measurements, the quantity we obtain is near this expected value. This is why both names are used.

when $\mathcal{O}$ is measured, and also when $\mathcal{O}^{\prime}$ is measured, then $|\alpha\rangle$ must be an eigenvector of both $A$ and $B$. Then $[A, B]|\alpha\rangle=0$. When no such $|\alpha\rangle$ exists, ${ }^{17}$ there does not exist a state for which both $\mathcal{O}$ and $\mathcal{O}^{\prime}$ can be measured with certainty. It is simply impossible to ever know at the same time the values of both $\mathcal{O}$ and $\mathcal{O}^{\prime}$ for any state of the system. It is fallacious to think that to know them both one just has to measure $\mathcal{O}$ and then $\mathcal{O}^{\prime}$. After the measurement of $\mathcal{O}^{\prime}$ has taken place you no longer know the value of $\mathcal{O}$. If you measure $\mathcal{O}$, then measure $\mathcal{O}^{\prime}$, and “immediately after” measure $\mathcal{O}$ again, the result of this second measurement of $\mathcal{O}$ will sometimes be different from the result of the first measurement. This is because, as we explained earlier, the measurement of $\mathcal{O}^{\prime}$ has changed the state of the system. Right after the measurement of $\mathcal{O}^{\prime}$, the state of the system is an eigenvector of $B$, and by hypothesis, this eigenvector of $B$ is not an eigenvector of $A$, so that in this state of the system, the result of the measurement of $A$ cannot be predicted with certainty.

## 物理代写|量子场论代写Quantum field theory代考|Uncertainty

Heisenberg’s uncertainty principle, which we will study in the present section, is related to but different from the phenomenon of the previous section. If the system is in state $x$ with $(x,[A, B] x) \neq 0$, one cannot measure both observables $\mathcal{O}$ and $\mathcal{O}^{\prime}$ with arbitrary accuracy. We are not talking here of successive measurements on the same experiment, where the first measurement changes the state of the system. We are talking of measurements on different experiments. One repeats the experiment many times, each time measuring either $\mathcal{O}$ or $\mathcal{O}^{\prime}$. If the results of measuring $\mathcal{O}$ are concentrated in a small interval, then the results of measuring $\mathcal{O}^{\prime}$ must spread out. In the important special case where $[A, B]$ is a multiple of the

identity, whatever the state of the system, you can never measure both $\mathcal{O}$ and $\mathcal{O}^{\prime}$ with arbitrary accuracy. A quantitative version of this statement is given in Proposition 2.3.2.
Definition 2.3.1 Consider an observable $\mathcal{O}$ with associated Hermitian operator $A$. The uncertainty $\Delta_{x} A \geq 0$ of $\mathcal{O}$ in the state $x \in \mathcal{H}$ is given by
$$\left(\Delta_{x} A\right)^{2}:=\left(x, A^{2} x\right)-(x, A x)^{2} .$$
For instance, in the case of the basic example, the uncertainty of the position in state $x$ is $\sqrt{\sum_{i \leq n} \lambda_{i}^{2}\left|x_{i}\right|^{2}-\left(\sum_{i \leq n} \lambda_{i}\left|x_{i}\right|^{2}\right)^{2}}$. To make sense of $(2.9)$, one may observe that $\left(\Delta_{x} A\right)^{2}$ is just the variance of the probability distribution of Principle 4 , or in other words, $\Delta_{x} A$ is the standard deviation of this probability distribution. The physical content of this definition should be stressed. When you make a measurement of the observable $\mathcal{O}$ for a system in state $x$, you get a random result, and $\Delta_{x} A$ is the standard deviation of this random result. ${ }^{18}$
To explain (2.9) in more mathematical terms, this quantity measures “the squaredeviation of $\mathcal{O}$ from its average in state $x “$. Indeed, denoting by 1 the identity operator of $\mathcal{H}$ (and since $x$ is of norm 1 ),
$$\left(\Delta_{x} A\right)^{2}=\left(x, A^{\prime 2} x\right)$$
where the Hermitian operator $A^{\prime}:=A-(x, A x) 1$ is “the deviation of $\mathcal{O}$ from its average in state $x^{\prime \prime}$. When $x$ is an eigenvector of $A$, obviously $\left(\Delta_{x} A\right)^{2}=0$. Conversely, when $\left(\Delta_{x} A\right)^{2}=0$, since
$$\left(x, A^{\prime 2} x\right)=\left(A^{\prime} x, A^{\prime} x\right)=\left|A^{\prime} x\right|^{2},$$
then $A^{\prime} x=0$ so that $x$ is an eigenvector of $A$. Therefore, $\left(\Delta_{x} A\right)^{2}=0$ if and only if $x$ is an eigenvector of $A$, i.e. if and only if the measurement of $\mathcal{O}$ in state $x$ offers no uncertainty, in accord with our calling $\Delta_{x} A$ the “uncertainty of $\mathcal{O}$ in state $x$ “.
Observe finally that $\left(x, A^{\prime} x\right)=0$, so that $(2.10)$ means
$$\left(\Delta_{x} A\right)^{2}=\left(\Delta_{x} A^{\prime}\right)^{2} .$$

## 物理代写|量子场论代写Quantum field theory代考|Finite versus Continuous Models

Mathematicians are trained to think of physical space as $\mathbb{R}^{3}$. But our continuous model of physical space as $\mathbb{R}^{3}$ is of course an idealization, both at the scale of the very large and at the scale of the very small. This idealization has proved to be very powerful, but in the case of Quantum Field Theory, it creates multiple problems, and in particular the infamous infinities (in the form of diverging integrals).

Can we dispense with continuous models and their analytical problems? A physical measurement is made through a device with finite accuracy, and this measurement is no different from the same measurement rounded to the last significant digit. The result of the measurement is also bounded, ${ }^{19}$ so it may yield only finitely many possible values, and we might be able to study physics using only finite-dimensional Hilbert spaces (of huge dimension).
There is a fundamental reason why we stubbornly keep infinite models. Probably the most important guiding principle in finding good models is that a proper theory should be Lorentz invariant, ${ }^{20}$ reflecting the fact that physics should be the same for all inertial observers ${ }^{21}$ (who undergo no acceleration). There is no way this can be implemented in a finite model, say one which replaces the continuous model of physical space by a finite grid. Lorentz invariance can be recovered from a finite model only “in the infinite limit”. Further, there is no canonical choice for such a finite model, so that one has to show that the results obtained by a finite approximation are indeed essentially independent of how this approximation is performed. Heuristically, this is plausible but it is quite another matter to really prove independence. In fact, it can be argued that settling this question is of the same order of difficulty as constructing a continuous model which in some sense would be the limit of the finite model as the grid becomes finer. In the case of Quantum Field Theory, this is a highly non-trivial task. Most importantly, considering finite models does not really solve anything. The infinities reappear in the guise of quantities that blow up as the grid becomes finer, and it is very hard to make sense of this behavior.

## 物理代写|量子场论代写Quantum field theory代考|Measuring Two Different Observables on the Same System

14 应该说，这里的问题是以一种相当离谱的方式表述的，以明确这一点。在我们存在之前，宇宙似乎做得很好！问题仍然是真实的。一方面，有人可能会争辩说，量子力学只适用于微观世界。那么测量过程将是与宏观物体相互作用的任何东西，例如照相乳剂中的感光化学物质，与有意识的观察者无关。另一方面，如果拒绝这种在宏观和微观世界之间任意且不明确的界限，那么量子领域就会一直延伸到观察者的意识中。那么很难摆脱这个意识起作用的结论，事情变得非常模糊。退相干理论试图解决这些问题。
15 包括数学论证不明确的情况。
16 可观测的测量值○处于状态|一个⟩是一个随机变量，因此具有期望值。当我们多次重复实验并平均相应的测量值时，我们得到的数量接近这个预期值。这就是使用这两个名称的原因。

## 物理代写|量子场论代写Quantum field theory代考|Uncertainty

(ΔX一个)2:=(X,一个2X)−(X,一个X)2.

(ΔX一个)2=(X,一个′2X)
Hermitian 算子在哪里一个′:=一个−(X,一个X)1是“的偏差○从它的平均状态X′′. 什么时候X是一个特征向量一个， 明显地(ΔX一个)2=0. 相反，当(ΔX一个)2=0， 自从

(X,一个′2X)=(一个′X,一个′X)=|一个′X|2,

(ΔX一个)2=(ΔX一个′)2.

## 有限元方法代写

tatistics-lab作为专业的留学生服务机构，多年来已为美国、英国、加拿大、澳洲等留学热门地的学生提供专业的学术服务，包括但不限于Essay代写，Assignment代写，Dissertation代写，Report代写，小组作业代写，Proposal代写，Paper代写，Presentation代写，计算机作业代写，论文修改和润色，网课代做，exam代考等等。写作范围涵盖高中，本科，研究生等海外留学全阶段，辐射金融，经济学，会计学，审计学，管理学等全球99%专业科目。写作团队既有专业英语母语作者，也有海外名校硕博留学生，每位写作老师都拥有过硬的语言能力，专业的学科背景和学术写作经验。我们承诺100%原创，100%专业，100%准时，100%满意。

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。