A First Course in String Theory

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(b) Show that with spatial dimensions, the potential due to a point charge is given by

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Eq. (3.74):

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— Me@2023-05-17 09:11:07 AM

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2023.05.17 Wednesday (c) All rights reserved by ACHK

A First Course in String Theory

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(b) Show that with spatial dimensions, the potential due to a point charge is given by

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Eq. (3.74):

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— Me@2023-05-17 09:11:07 AM

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2023.05.17 Wednesday (c) All rights reserved by ACHK

A First Course in String Theory

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(a) Show that for time-independent fields, the Maxwell equation implies that . Explain why this condition is satisfied by the ansatz .

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Eq. (3.23):

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— Me@2023-03-18 11:08:24 AM

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2023.03.18 Saturday (c) All rights reserved by ACHK

A First Course in String Theory

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The generating function is an infinite product:

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To evaluate the infinite product, you can use `Mathematica`

(or its official free version `Wolfram Engine`

) with the following commands:

TeXForm[ HoldForm[ (1/x)*Product[ (1+x^(r-1/2))^32/(1-x^r)^8, {r, 1, Infinity}]]] f[x_]:=(1/x)*Product[ (1+x^(r-1/2))^32/(1-x^r)^8, {r, 1, Infinity}] Print[f[x]] TeXForm[f[x]] TeXForm[Series[f[x], {x,0,3}]]1 32 QPochhammer[-(-------), x] Sqrt[x] ------------------------------------ 1 32 8 (1 + -------) x QPochhammer[x, x] Sqrt[x]

— Me@2022-11-23 04:40:28 PM

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2022.11.23 Wednesday (c) All rights reserved by ACHK

A First Course in String Theory

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(b) Repeat the analysis of three-dimensional electromagnetism starting with the Lorentz covariant formulation. Take , examine , the Maxwell equations (3.34), and the relativistic form of the force law derived in Problem 3.1.

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Eq. (3.20):

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Eq. (3.33):

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Eq. (3.34):

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P. (3.1):

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— Me@2022-11-08 03:46:01 PM

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2022.11.10 Thursday (c) All rights reserved by ACHK

A First Course in String Theory

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(a) Find the reduced Maxwell equations in three dimensions by starting with Maxwell’s equations and the force law in four dimensions, using the ansatz (3.11), and assuming that no field can depend on the direction.

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Eq. (3.11):

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— Me@2022-10-22 04:17:10 PM

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2022.10.22 Saturday (c) All rights reserved by ACHK

A First Course in String Theory

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(b) Show explicitly that the Maxwell equations with sources emerge from (3.34).

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Eq. (3.34):

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— Me@2022.10.02 11:49:59 AM

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2022.10.02 Sunday (c) All rights reserved by ACHK

A First Course in String Theory

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(a) Show explicitly that the source-free Maxwell equations emerge from .

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— Me@2022.09.11 01:40:50 PM

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2022.09.12 Monday (c) All rights reserved by ACHK

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Is gauge invariant?

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… the defining property of a Lorentz transformation, :

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… 4-vectors and (Lorentz)-tensors are transformed like this:

and

where we have used the conventional notation

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Let us then take your equation and apply on both sides (recall this Lorentz transformation does not depend on ), and try rewriting everything in terms of prime quantities:

This “game” can always be done with contracted indices, …

— answered Jul 7, 2020 at 15:06

— ohneVal

— Lorentz invariance of the Lorentz force law

— Physics StackExchange

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How come ?

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Note that

does not mean that

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Also

does not mean that

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At the first glance, it seems to be unlikely that

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because while in , for any , ‘s have visible negative terms; in , only ‘s do.

Without additional mathematical properties among those physical quantities, the identity is impossible to prove.

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Just for future reference:

From the invariance of the spacetime interval it follows

— Wikipedia on

Lorentz transformation.

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Inserting the identity into the expression:

This path does not work. Also, the formula is plain wrong!

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Inserting the identity into the expression *before* (actually *without*) lowering the index:

— Me@2022-08-23 12:03:54 PM

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2022.08.26 Friday (c) All rights reserved by ACHK

A First Course in String Theory

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The Lorentz force equation (3.5) can be written relativistically as

,

where is the four-momentum.

(a) Check explicitly that this equation reproduces (3.5) when is a spatial index.

(b) What does (1) gives when ? Does it make sense?

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Eq. (3.5):

Eq. (2.20):

if

Eq. (2.21):

The spacetime interval is Lorentz invariant. If , we have Eq. (2.27) and (2.28):

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…

— Me@2022-08-04 04:17:59 PM

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2022.08.06 Saturday (c) All rights reserved by ACHK

A First Course in String Theory

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Since has the same unit in all dimensions,

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Eq. (3.104):

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— Me@2022-07-17 04:23:42 PM

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2022.07.17 Sunday (c) All rights reserved by ACHK

A First Course in String Theory

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Equation (3.96) has content: if you move a particle along a closed loop in a static gravitational field, the net work that you do against the gravitational field is zero.

Prove the above statement.

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Eq. (3.96):

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By the **gradient theorem** (aka the **fundamental theorem of calculus for line integrals**):

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— Me@2022-07-03 04:27:19 PM

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2022.07.03 Sunday (c) All rights reserved by ACHK

A First Course in String Theory

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Show that this condition fixes uniquely , and , thus reproducing the result in (3.90).

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Eq. (3.93):

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var('a b c') solve([3*a+b+2*c==1, -a+c==0, -2*a-b-c==0], a, b, c)

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Eq. (3.90):

— Me@2022-06-23 10:46:22 AM

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2022.06.23 Thursday (c) All rights reserved by ACHK

A First Course in String Theory

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The force on a test charge in an electric field is . What are the units of charge in various dimensions?

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Eq. (3.74):

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— Me@2022-06-08 11:09:27 AM

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**Lorentz–Heaviside units** (or **Heaviside–Lorentz units**) constitute a system of units (particularly electromagnetic units) within CGS, named for Hendrik Antoon Lorentz and Oliver Heaviside. They share with CGS-Gaussian units the property that the electric constant and magnetic constant do not appear, having been incorporated implicitly into the electromagnetic quantities by the way they are defined. Heaviside-Lorentz units may be regarded as normalizing and , while at the same time revising Maxwell’s equations to use the speed of light instead.

Heaviside–Lorentz units, like SI units but unlike Gaussian units, are *rationalized*, meaning that there are no factors of appearing explicitly in Maxwell’s equations. That these units are rationalized partly explains their appeal in quantum field theory: the Lagrangian underlying the theory does not have any factors of in these units. Consequently, Heaviside-Lorentz units differ by factors of in the definitions of the electric and magnetic fields and of electric charge. They are often used in relativistic calculations, and are used in particle physics. They are particularly convenient when performing calculations in spatial dimensions greater than three such as in string theory.

— Wikipedia on *Lorentz–Heaviside units*

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2022.06.08 Wednesday (c) All rights reserved by ACHK

A First Course in String Theory

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Verify that for equation (3.74) coincides with (3.67).

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Eq. (3.67):

Eq. (3.74):

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When ,

— Me@2022-05-30 01:15:00 PM

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2022.05.30 Monday (c) All rights reserved by ACHK

A First Course in String Theory

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Show that

~~~

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**Geometric proof**

The relations and and thus the volumes of -balls and areas of -spheres can also be derived geometrically. As noted above, because a ball of radius is obtained from a unit ball by rescaling all directions in times, is proportional to , which implies .

Also, because a ball is a union of concentric spheres and increasing radius by corresponds to a shell of thickness . Thus, ; equivalently, .

— Wikipedia on *Volume of an -ball*

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— Me@2022-05-18 09:08:11 AM

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2022.05.19 Thursday (c) All rights reserved by ACHK

A First Course in String Theory

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Show that

~~~

Eq. (3.15):

Eq. (3.29):

Eq. (3.16):

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— Me@2022.05.05 07:49 PM

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2022.05.06 Friday (c) All rights reserved by ACHK

A First Course in String Theory

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Verify the equations in (3.26).

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Eq. (3.26):

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— Me@2022-04-15 05:10:09 PM

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2022.04.16 Saturday (c) All rights reserved by ACHK

A First Course in String Theory

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Verify that the gauge transformation (3.10) are correctly summarized by (3.21).

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Eq. (3.21):

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Eq. (3.10):

— Me@2022-04-07 07:05:29 PM

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2022.04.07 Thursday (c) All rights reserved by ACHK

A First Course in String Theory

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Verify that , as given in (3.8), is invariant under the gauge transformation (3.10).

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Eq. (3.8):

Eq. (3.10):

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— Me@2022-04-01 03:34:28 PM

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2022.04.01 Friday (c) All rights reserved by ACHK

A First Course in String Theory

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(b) Assume that in such a way that …

…

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[guess]

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[guess]

— Me@2022-03-10 10:56:10 AM

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2022.03.10 Thursday (c) All rights reserved by ACHK

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