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^{mu}or the stress energy tensor?

It suddenly stuck me that if we are to be so inclined by theory as we are by empirical evidence that energy is a conserved quantity, then there should be an equation that describes it in four dimensions.

Rather than bring in general relativity all at once where energy is not well defined (I have reservations), the stress energy tensor is still a tensor in special relativity, and so special relativity might be the better place to start.