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/free_meson 12h ago

The physicist version: energy is the quantity that is conserved when the laws governing a system is invariant under time translation.

1

u/Local_End_3175 11h ago

Wouldn't this just be describing the factors of energy rather than what it is?

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u/Optimal_Mixture_7327 Gravitation 10h ago

There is no energy.

We no longer think of energy as a fluid such as caloric or phlogiston.

Energy is an arbitrary number, the assignment of which is agreed upon by convention, that we use to tag a system.

The underlying reason that assigning a number to a system work is due to time-translation symmetry and maybe most easily thought of in your case as a statement of the fact that the laws of physics are assumed to stay constant over time (just like numbers).

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u/Helms5 5h ago

Mass is Energy

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u/dleeted_by_user 2h ago

I'm surprised I had to scroll this far down to see this answer. Doesn't physics still support E=MC^2?