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Physics LibreTexts

15.15: Derivatives

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We’ll pause here and establish a few derivatives just for reference and in case we need them later.

We recall that the Lorentz relations are

x=γ(x+νt)

and

t=γ(t+βxc)

From these we immediately find that

(xx)t=γ;(xt)x=γν;(tx)t=βγc;(tt)x=γ.

We shall need these in future sections.

Caution

It is not impossible to make a mistake with some of these derivatives if one allows one’s attention to wander. For example, one might suppose that, since xx=γ then “obviously” xx=1γ - and indeed this is correct if t is being held constant. However, we have to be sure that this is really what we want. The difficulty is likely to arise if, when writing a partial derivative, we neglect to specify what variables are being held constant, and no great harm would be done by insisting that these always be specified when writing a partial derivative. If you want the inverses rather than the reciprocals of Equations 15.15.3a,b,c,d the rule, as ever, is: Interchange the primed and unprimed symbols and change the sign of ν or β. For example, the reciprocal of (xx)t is (xx)t, while its inverse is (xx)t. For completeness, and reference, then, I write down all the possibilities:

(xx)t=1γ;(tx)x=1γν;(xt)t=cβγ;(tt)x=1γ.

(xx)t=γ;(xt)x=γν;(tx)t=βγc;(tt)x=γ.

(xx)t=1γ;(tx)x=1γν;(xt)t=cβγ;(tt)x=1γ.

Now let’s suppose that ψ=ψ(x,t) where x and t are in turn functions (Equations ??? and ???) of x and t. Then

ψx=xxψt+txψt=γψx+βγcψt

and

ψt=xtψx+ttψt=γνψx+γψt.

The reader will doubtless notice that I have here ignored my own advice and I have not indicated which variables are to be held constant. It would be worth spending a moment here thinking about this.

We can write Equations ??? and ??? as equivalent operators:

x=γ(x+βct)

and

t=γ(νx+t).

We can also, if we wish, find the second derivatives. Thus

2ψx2=γ2(2x2+2βc2xt+β2c22t2).

In a similar manner we obtain

2xt=γ2(ν2x2+(1+β2)2xt+βc2t2)

and

2t2=γ2(ν22x2+2ν2xt+2t2).

The inverses of all of these relations are to be found by interchanging the primed and unprimed coordinates and changing the signs of ν and β.


This page titled 15.15: Derivatives is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.

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