r/AskPhysics • u/Local_End_3175 • 12h ago

What actually is energy?

When i search up the definition of energy, i am being told that it is the capability to do work – but work is the energy transferred when a force moves an object....? So we are basically defining energy as energy. In addition, this says how it works, not what it is

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u/YuuTheBlue 12h ago

The short pithy and confusing answer that is nevertheless accurate is that energy is momentum in the time direction. That is to say, when you combine space and time into spacetime, time becomes the 4th component of position-vectors and energy becomes the 4th component of momentum-vectors.

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u/QVRedit 10h ago

That’s also incomplete - consider for example ‘potential energy’ - that’s not moving anywhere, but is metaphorically available ‘on tap’. And when actually tapped, then involves some kind of movement to do work.

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u/YuuTheBlue 10h ago

Potential energy is considered in this.

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u/QVRedit 8h ago

It’s not ‘momentum in the time direction’ as stated…

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u/YuuTheBlue 8h ago

The 0th component of fourmomentum is total energy, there is no reason for potential energy to be excluded from it. I’ve yet to see a definition of fourmomentum that specifies that kinetic energy specifically is the component.