# Is the potential energy in a compressed spring a Lorentz invariant?

The total energy of an object comes from the time part of the four-momentum, and so isn't a Lorentz invariant. On the other hand, is the potential energy of a compressed spring a Lorentz invariant?

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When a spring is compressed its mass increases by a tiny amount; so as far as energy is concerned a compressed spring is equivalent to an uncompressed but a little heavier spring. – user10001 Jul 15 '12 at 2:40
The density of potential energy is frequently a Lorentz scalar even though this is not a necessary condition for a relativistic theory. – drake Sep 13 '12 at 4:17

It depends of what potential energy you are using. If you refer to a potential energy that depends only on position variables, $V=V(x)$, this is not Lorentz invariant. If you refer to potential energy that depends both on position and time variables, this can be Lorentz invariant. Some examples of Lorentz invariant potential energies are given in the textbook by Schieve [1]