# Tag Info

34

There is a consistent definition, but it involves a couple of arbitrary thresholds, so I doubt you'd consider it rigorous. The construction $X \gg Y$ means that the ratio $\frac{Y}{X}$ is small enough that subleading terms in the series expansion for $f\bigl(\frac{Y}{X}\bigr) - f(0)$ can be neglected, where $f$ is some relevant function involved in the ...

18

The statement "$A$ happens given that $B$" is equivalent to "If $B$, then $A$", which is symbolically represented as an implication $$B \Rightarrow A$$ or, if you want to preserve the order of $A$ and $B$ in the original statement $$A \Leftarrow B$$

14

IMHO, the notation $\int_a^b\mathrm{d}x\,f(x)$ is much cleaner than $\int_a^b f(x)\,\mathrm{d}x$, because the integration variable ($x$) and its associated integral range $(\int_a^b$) are kept together. This is particularly important in lengthy and multi-dimensional integrals. Consider $$\Upsilon_{pq}(k)= \int_0^\infty\mathrm{d}x ... 12 It's not just QFT literature. Physicists, especially adult research physicists, find this notation sensible and popular – even though it may be more popular among particle physicists than elsewhere. Formally, dx\,f(x) is a product of two factors and \int is a form of a sum. Because product is commutative, it doesn't hurt when the order is interchanged. ... 10 What exactly does \Delta_{r_e} mean ? Your wave function isn't a field in space, it is a field on configuration space, i.e. it assigns complex numbers to a configuration. If your electron is at (x_e,y_e,z_e) and your proton is at (x_p,y_p,z_p) then the configuration is the point (x_e,y_e,z_e,x_p,y_p,z_p) in a 6d space. A point in that 6d space ... 9 \mathfrak{R}e: real part. A^*: complex conjugate of probability amplitude A. 9 If you want to make statements over (discrete) time, linear temporal logic may be worth looking at. For instance, \qquad\displaystyle \Box (A \implies B) means that whenever A holds, B has to hold at the same time. \qquad\displaystyle \Box (A \implies \Diamond B) means that whenever A holds, B will hold at some point in the future. Or yet ... 8 Besides the reasons listed in Lubos Motl's answer, here is another reason for the \int \!dx ~f(x) notation: By writing the integral sign \int_a^b and dx next to each other in multiple nested integrations, it becomes more easy to trace which limits belong to which integration. This becomes particularly handy when changing the orders of integration. ... 7 Those Greek letters are indices indexing the components of g. Generally if one expresses a rank-2 tensor like g as a matrix, the first index indexes the rows, the second the columns. In your example, we have g_{rr} \equiv g_{11} = 1, g_{\theta\theta} \equiv g_{22} = r^2, g_{r\theta} \equiv g_{12} = 0, etc. As you can see, we sometimes use numbers ... 7 (Q\cdot Q)_{ij}=Q_{im}Q_{mj} (Q^T\cdot Q)_{ij}=(Q^T)_{im}Q_{mj}=Q_{mi}Q_{mj} where we use that (Q^T)_{im}=Q_{mi} 6 Given the way that you've presented your table, I would personally put a "-" rather than a 1 in the units column. This to me would signify that units such as "g, km, s, A" etc. do not apply here. In terms of your symbols, in many branches of physics it is common to use a "hat", "tilde" or "star" notation above a symbol to indicate that it is a ... 6 Force is indeed a vector. Technically you should write |\overrightarrow{F}| = 30N, however there is usually context given that let you omit this. If you are working in one dimension, then the vector-like direction is all encapsulated in the sign once you've defined your coordinate system (e.g. -30N is 30N downwards.) Beyond that, it is typically just a ... 6 The answer is already on page 2 of your link above: "Among the large number of radionuclides of medical interest, Sc-44 is promising for PET imaging. Either the ground-state Sc-44g or the metastable-state Sc-44m can be used for such applications, depending on the moleculeused as vector." So the metastable state Sc-44m decays to the ground state Sc-44g. 6 Dirac notation is ill-suited for non-self-adjoint operators. Here's why: Let (-,-) be the inner product on our Hilbert space. The expectation value of AB is then$$ \langle AB \rangle_\psi = (\psi,AB\psi)$$by definition, and Dirac notation writes \langle \psi \vert AB \vert \psi \rangle. for this. But, in this notation, it is no longer clear to which ... 5 These states represent intermediate coupling schemes that are halfway between the usual LS coupling and the more extreme jj coupling that happens in heavier atoms where relativistic effects mean that the spin-orbit coupling for each individual electron can match or exceed the orbit-orbit coupling between different electrons. The intermediate coupling ... 5 I think the answer is no. It generally precedes some approximation method with a bounded error, but there are so many approximations methods in physics -- some rigorous, some nonrigorous -- that it's way too presumptuous to give it a rigorous definition. Generally, it means one of several things: If a\ll b, expanding in powers of \frac{a}{b} is ... 5 Comments to the question (v5): In this quantum case the overline/bar notation \bar{A}=\langle A\rangle is borrowed from statistics and it denotes a quantum expectation value of a quantity A. See also Ehrenfest theorem. The problem from Ref. 1 considers a harmonic oscillator with Hamiltonian operator$$\tag{A} H~=~\frac{p^2}{2m} ...

5

Use: $$u_{\alpha}= g_{\alpha\beta}u^{\beta}$$ where $g_{\alpha\beta}$ is the metric tensor.

5

Two conventions. First - use a capital M - make sure you make it big and pointy, so it cannot be confused with lower case: When it is right next to the lower case 'm', the difference should stand out clearly. Second - some people use the "computer short hand" E6: 1.7E6 m This is generally understood to mean (but quicker to write than) $1.7\cdot ... 5 Either. It's context dependent. Chemists generally mean the whole atoms, nuclear physicists usually mean the nucleus, and people not in those categories could mean either. And there are exception to all those rules or thumb. And the distinctions is important when people start throwing masses around because the mass of an electron is almost on the same ... 5 You're getting tripped up by summation notation. Whenever you have a repeated index, this means that that index is to be summed from 1 to 3: $$\delta_{ij} \delta_{ik} \equiv \sum_{i=1}^3 \delta_{ij} \delta_{ik}.$$ You're right that there are two terms in this sum where$i \neq j$, and so the contribution to the sum from these terms is zero. But the ... 5 The index of the Laplacian tells you which of the coordinates it acts on, that is, if you write$r = (r_x,r_y,r_z)^T$and$R = (R_x,R_y,R_z)^Tas Cartesian coordinates, then \begin{align} \Delta_r & := \frac{\partial^2}{\partial {r_x}^2} + \frac{\partial^2}{\partial {r_y}^2} + \frac{\partial^2}{\partial {r_z}^2} \\ \Delta_R & := ... 5 Conventionally we use1$for dimensionless quantities, although it may cause some confusions. In additon, The International Committee for Weights and Measures contemplated defining the unit of 1 as the 'uno', but the idea was dropped. --https://en.wikipedia.org/wiki/Dimensionless_quantity 4 This is a covariant derivative along a world line (if you would not consider a world line the proper time$\tau$would not make any sense). So you consider a curve in space time parametrized in dependence of the proper time$x^\mu(\tau)$. Then you have: $$\frac{DA^\mu}{d\tau} = \frac{\partial A^{\mu}\big(x(\tau)\big)}{\partial \tau} + ... 4 The equation you phrase as$$|l,m\rangle=\int_\text{all space}\psi_{lm}(r,\theta,\phi)\,\left|r,\theta,\phi\right\rangle r^2\,\mathrm dr\,\mathrm d\Omega$$is, and must be, wrong. The reason is that |l,m⟩ inhabits the orbital part of Hilbert space, \mathcal H_\Omega, and the right-hand side is a vector in the full Hilbert space \mathcal H, which is the ... 4 The LHS is an inner product while the RHS is the evaluation of a function from an L^2 space at the point x. To somehow link the two you need to be able to write the RHS as an integral, so you need a "function" \delta_x such that$$\langle x|\psi\rangle = \int\overline{\delta_x(s)}\psi(s)\text ds = \psi(x).$$There is no such function, but the map ... 4 As Qmechanic pointed out in the comments, you're mixing Einstein and abstract index notation a bit. To make things absolutely clear, we will use early Latin indices for abstract indices (abc) and Greek indices for component indices (\mu\nu\rho) and will always indicate Einstein summation explicitly. First and foremost, an abstract index is nothing more ... 4 Just a coincidence. There are too many quantities and not enough letters. It probably does make a difference that the fields in which these two equations exist (material science and electromagnetism) are well enough separated that you typically won't see them both in the same papers or textbooks; if that weren't the case, people would start using different ... 4 It is a symbol and an idea used in mathematics too. But the important part is just that B is 'ignorable' relative to A. This depends on the level of precision that is being used experimentally. If you're working to a precision of 1 part in 100, then B should not effect the answer to that level of precision. If you're working to 1 part in a million, ... 4 It is common to write$$ \partial_i = \frac{\partial}{\partial x^i}$$for the derivative with respect to the$i$-th coordinate. Since time is customarily written as the$0$-th coordinate,$\partial_0\$ is the time derivative.

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