One example is that in an expanding universe (like ours) total energy is not even defined. Also note that the dark energy component of whatever can possibly be defined as energy increases with time in an expanding universe. And if some day we manage to convert it into a usable energy source, we’ll have something like a perpetuum mobile. A silly example: connect two receding galaxies to an electric motor in the middle with really long and strong ropes and use the relative pull to spin the motor.
What is conserved, however, according to general relativity, anyway, is the local stress-energy-momentum tensor field at each point in spacetime.
When isn’t it? (This is another Stupid Question.)
One example is that in an expanding universe (like ours) total energy is not even defined. Also note that the dark energy component of whatever can possibly be defined as energy increases with time in an expanding universe. And if some day we manage to convert it into a usable energy source, we’ll have something like a perpetuum mobile. A silly example: connect two receding galaxies to an electric motor in the middle with really long and strong ropes and use the relative pull to spin the motor.
What is conserved, however, according to general relativity, anyway, is the local stress-energy-momentum tensor field at each point in spacetime.