I am beginning to study Special Relativity and Classical Field Theory - The Theoretical Minimum by Leonard Susskind and Art Friedman.

The book's notation for a 4-Vector is X superscript mu. Its components are X superscripts 0, 1, 2 and 3 which, in sequence, refer to t(ime), x, y and z.

Proper time, tau, is defined using these vector components in that sequence.

The book's notation for 4-Velocity is U superscript mu with components U superscripts 0 thru 3. U superscript zero is defined to be the derivative of X superscript zero with respect to tau i.e. dt/dtau and the remaining U components follow this pattern.

As a beginner in this area, which Maple packages/commands would most easily and clearly implement this notation in a Maple worksheet? 

Why does solve not find the value of xp = sqrt(1-v^2) in this worksheet?

SpecialRelativity.mw

Reference: Special Relativity and Classical Field Theory. Authors: Leonard Susskind and Art Friedman

This solve command produces both unconditional and conditional solutions.

Is there a way to have it only output unconditional solutions, or, failing that, is there a way to select the unconditional solutions from its output?

solve({2*x+y+z = 3, x^2+y^2+z^2 = 3}, [x, y, z], real, parametric)

The linked worksheet displays the geodesic between two points on a surface z(x,y).

Surfacepath.mw

Gradient descent will find a stepped approximate path of fastest descent between the points, but is there a way to find a function defining an exact i.e. smooth path of fastest descent? If so, how can a precise time of fastest descent be determined?

The following procedure describes the path of a projectile fired from the origin with a launch angle of alpha and a launch velocity of u.

z := proc (x) options operator, arrow; tan(alpha)*x-(1/2)*g*x^2/(u^2*cos(alpha)^2) end proc

The projectile's path must pass through the point [a, h], both values greater than zero.

Can an expression be found for the minimum value of u and its related value for alpha?