Physics

Physics · modern-physics

Problem

(a) The homogeneity of space and time imply that the laws of physics are the same no matter where in space and time you are. In other words, they do not depend on a choice of origin for coordinates

x

and

t

. Use this fact to show that

\frac{\partial X}{\partial x}

is independent of the position

x

and

\frac{\partial T}{\partial t}

is independent of the time

t

. (Hint: Recall the definition of the partial derivative.) Given constants: 4. Lorentz Boost In Newtonian kinematics, inertial frames moving relatively to each other are related by the following transformations called Galilean boosts:

\begin{aligned} x^{\prime} & =x-v t \\ t^{\prime} & =t \end{aligned}

In relativistic kinematics, inertial frames are similarly related by the Lorentz boosts:

\begin{aligned} x^{\prime} & =\frac{1}{\sqrt{1-v^{2} / c^{2}}}(x-v t) \\ t^{\prime} & =\frac{1}{\sqrt{1-v^{2} / c^{2}}}\left(t-\frac{v}{c^{2}} x\right) \end{aligned}

In this problem you will derive the Lorentz transformations from a minimal set of postulates: the homogeneity of space and time, the isotropy of space, and the principle of relativity. You will show that these assumptions about the structure of space-time imply either (a) there is a universal "speed limit" which is frame invariant, which results in the Lorentz boost, or (b) there is no universal "speed limit," which results in the Galilean boost. For simplicity, consider a one-dimensional problem only. Let two frames

F

and

F^{\prime}

be such that the frame

F^{\prime}

moves at relative velocity

v

in the positive-

x

direction compared to frame

F

. Denote the coordinates of

F

(x, t)

and the coordinates of

F^{\prime}

\left(x^{\prime}, t^{\prime}\right)

. The most general coordinate transformations between

F

and

F^{\prime}

are given by functions

X, T

\begin{aligned} x^{\prime} & =X(x, t, v) \\ t^{\prime} & =T(x, t, v) \end{aligned}

which we will refer to as the generalized boost.

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