what do we mean by"minus" energy ??

[blue bloom]

in the classic physics the minus energy express the gain or loss of the particle ..but when we substitute in the quantum theory lows ,the electron has a minus energy ..I think the energy is not like the charge to be minus ..& I can not also imagine any particle even a small one have a minus energy ..!!!!

[Bob_for_short]

Right, all particles in QM have plus energy.

[Feldoh]

It just depends on how you define potential energy...

[xepma]

The energy is taken relative to some state (which is defined to have zero energy -- this sometimes happens implicitly). The reason is that in any particular process you only talk about energy differences anyway.

On top of this: usually the negative energy states refer to bounded states, while positive energy states refer to unbounded states.

[Vanadium 50]

On top of this: usually the negative energy states refer to bounded states, while positive energy states refer to unbounded states.

Exactly.

Right, all particles in QM have plus energy.

No, xepma has it right.

[blue bloom]

On top of this: usually the negative energy states refer to bounded states, while positive energy states refer to unbounded states.

I think this is the logistic reply ..
I asked a teacher about this ,he told me that if we considered the energy between the electron & the proton equal zero before gravitation ,then the attraction would lead to lose energy & that make the energy of electron always minus (we find that in Bohr's equation)
but I still don't know what are the results of these sign??