### 物理代写|量子力学代写quantum mechanics代考|PHYS3034

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• 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|量子力学代写quantum mechanics代考|Limits Between Different Theories

Quite often in the literature we can find a usual comparison between einsteinian Classical Mechanics and galilean Classical Mechanics by taking the limit $c \rightarrow \infty$ and between Quantum Mechanics and Classical Mechanics by taking the limit $\hbar \rightarrow 0$.

Indeed, such limits of scaled quantities are heuristically useful, but cannot be taken too seriously, as their true physical meaning is much more questionable than it might appear at a first insight.

In fact, the basic mathematical settings of the above theories are rather “rigid”, as there is no true continuous transformation which maps one into another one. For instance, there is no observer independent continuous transformation which maps a metric with signature $(-+++)$ into a metric with signature $(0+++)$. Indeed, there is a jump between these two metrics.

Moreover, just as an example, let us consider an equation, in a lorentzian framework, which involves the electric field $E$, the magnetic field $B$ and the speed of light c. From a pure mathematical viewpoint, it might be possible to parametrise such an equation by substituting the fixed value $c$ with a parametrised value $\lambda c$ and compute the limit of the equation for $\lambda \rightarrow \infty$. But, while we change the value of $\lambda$, the physical meaning of $E$ and $B$ pursues to be achieved in the lorentzian framework. So, at the limit $\lambda \rightarrow \infty$, we cannot say that the electric field $E$ and the magnetic field $B$ are the corresponding classical fields; in fact, in the galilean framework they are physically defined in a rather different way.

So, in the present book, we do not pay great attention to the limits $c \rightarrow \infty$ and $\hbar \rightarrow 0$, but we give more credit to a comparison of the structural differences of the different frameworks.

## 物理代写|量子力学代写quantum mechanics代考|Scales

A characteristic feature of the mathematical language of the present book (and of further related literature on Covariant Quantum Mechanics, as well) is the systematic explicit use of “scale spaces” representing the units of measurement.

We stress that the world “scales” used in the present book has a conventional meaning, which should not be confused with other meanings used in the standard engineering literature.

Indeed, we stress that the literature dealing with units of measurements, under different perspectives, is very huge; here, we would like to quote, for instance [32, $175,232,239,296,389,398]$
So, let us explain what we mean.
In standard literature, one usually represents many physical objects as tensors. For instance, just to fix the ideas, the metric and electromagnetic fields, are usually represented by tensors of the type $g: \boldsymbol{M} \rightarrow T^* \boldsymbol{M} \otimes T^* \boldsymbol{M}$ and $F: \boldsymbol{M} \rightarrow \Lambda^2 T^* \boldsymbol{M}$.
However, to be more precise, such representations depend on the choice of units of measurement. In fact, if we change the units of measurement, then the above tensors change by a numerical factor determined by the ratio of those units of measurement. For instance, the scalar product of two vectors should be regarded as a number multiplied by the square of the unit of measurement of lengths.

So, it would be more appropriate to introduce the above tensors as a “scaled tensors” of the type $g: M \rightarrow \mathbb{L}^2 \otimes\left(T^* M \otimes T^* \boldsymbol{M}\right)$ and $F: \boldsymbol{M} \rightarrow \mathbb{F} \otimes \Lambda^2 T^* \boldsymbol{M}$, where $\mathbb{L}$ is suitable “scale space” representing the space of lengths and $\mathbb{F}$ a suitable “scale space” associated with the electromagnetic field.

We stress that such kind of considerations apply to many other classical objects, such as volumes, velocities, accelerations, forces, and so on (see Proposition 3.2.4, Definitions 2.4.1, 7.1.3, and 5.7.1, and so on). And also to quantum objects, such as the hermitian quantum metric. In fact, the quantum metric is not properly valued in $\mathbb{C}$, because it yields objects which should be integrated, hence should have the scale dimension of a volume (see Proposition 14.3.1).

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