Solution

ID: past-exam-of-the-symmetry-and-relativity-course-of-the-university-of-oxford/2021/3/b/solution

Use the electromagnetic field tensor convention stated above, and lower indices with the Minkowski metric . Lowering a single time index changes its sign. Summing both orders of each antisymmetric index pair gives the Lorentz scalar
For , and . Direct substitution in the printed electromagnetic stress-energy tensor yields
For a spatial index , and . Hence
Thus the full top row is
Here is the electromagnetic energy density and the Poynting vector, using . By the symmetry , these components describe both energy flux divided by and times the electromagnetic momentum density . Keeping the factor is essential because the time coordinate is .

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