[yor_on]

It's a weird concept Bill. Semantically a zero balance of energy seems to implicate 'different types' of energy?
That doesn't make sense if 'energy' is a coin of exchange?

If it isn't?

then 'energy' is something in itself, and very tentatively can have different values
I don't think that is right though

[Bill S (OP)]

It's a weird concept Bill. Semantically a zero balance of energy seems to implicate 'different types' of energy?
That doesn't make sense if 'energy' is a coin of exchange?

Are you saying that there cannot be negative and positive energy?

[geordief]

I’ve tried on a number of occasions to get an answer to this, including in a recent thread.  Perhaps separating it from other material might do the trick.

Consider the statement: “The total amount of energy in the universe is zero.”
Are there two ways in which this could be interpreted? 

1.  There is no energy in the Universe.
2.  There is a specific amount of positive energy in the Universe.  There is also a specific amount of negative energy in the Universe.  These amounts are equal, therefore, the balance is zero.

In the case of our Universe, there is, manifestly, an energy content, therefore 1 cannot apply.

If 2 is the appropriate interpretation, although the positive and negative energies cancel each other, they cannot destroy each other; thus, there must be a difference between “zero energy” and “a zero balance of energy”. 

I’m prepared to acknowledge that my reasoning could be wrong, but I would appreciate knowing how/where.

Are  both (1 and 2) just mathematical abstractions?

If you take the first then we are talking about a situation vanishingly close to no relative motion between any object in the Universe (cannot be  "none" ) and the second case ,similarly applies (presumably) to our present situation where energy measurement balance out  almost exactly  ( such a measurement could not be made to any degree of accuracy bearing in mind that an unknowable(?) amount of the universe is unobservable.

[PmbPhy]

There can never be just zero energy. Otherwise nothing exists.

There can always be zero energy even in classical physics. The universe may have started out with zero energy wherein the creation of matter consisted of positive energy the associated gravitational energy was negative making the total energy  zero.

Seems like people ignored me on that one (or I forgot to mention it). This is explained by Guth in his book on inflation.

[yor_on]

Well, one argument for negative energy is, as Pete points out, gravity. To get out of a gravitational 'field/hole' you need to expend energy. And the energy spent then becomes a opposite, a so called 'positive energy' (rocket f.ex). It's a difficult definition though to accept, as 'energy', to me then, doesn't seem to be anything specific in itself
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Then again. if you look at transformations there must be some sort of loss aka going from useful energy to unusable. one main idea is that nothing is ever lost in this universe, it just transforms. But if something goes from being able to do work in some way, to lose that ability? It's a difference, and even if nothing measurable is 'lost' something definitely has disappeared, hasn't it? The ability to do further work

Now let's take that reasoning one step further, I think we both would agree on this rocket expending/losing energy climbing the gravity well. But can the same be said for the gravity well? I don't think so, it stays the same, loosely speaking neither losing nor gaining anything by the rocket leaving. The mass of that rocket could be said to be lost to this gravity well but I don't see that balancing out the energy spent from the rockets point of view. Just think of it leaving a neutron star.
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the main point being that if you have a definition split into opposites, positive and negative, they should act the same although from different sides of the fence (balance out). So positive energy transforms and lose the ability to do work, but this negative energy represented by gravity?

You can also think of it in terms of 'virtual particle pairs' spontaneously appearing, are they consisting of one 'positive' and one 'negative'? Aren't those particles both, if able to stay, 'positive', then meaning able to do work?
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There is one more hurdle to it from my perspective. The way I think of 'gravity' is as geodesics, there is no 'force' acting on a thrown ball, it's just following its geodesic. Now, I'm not sure how to see this 'negative energy' but it can't be a force if it is to fit that.