This was recently discussed here:
https://www.physicsforums.com/showthread.php?t=285100
It's just a feature of the always-attractive nature of the force (that is, any two test masses are always attracted to one another). I honestly don't know what more there is to say about this, as there is no fundamental requirement that energy be positive. Perhaps one must recognize that energy isn't such a simple thing that it can always be thought of as the kinetic energy plus rest energy (mass) of a particle.Thanks, but his thread wanders off into philosophical issues. My question is how regarding gravity as negative energy impacts our understanding of physical theory.
In physics, the law of conservation of energy states that the total amount of energy in an isolated system remains constant. A consequence of this law is that energy cannot be created or destroyed. The only thing that can happen with energy in an isolated system is that it can change form, that is to say for instance kinetic energy can become thermal energy. Another consequence of this law is that perpetual motion machines can only work if they deliver no energy to their surroundings, and that devices that produce more energy than is put into them are impossible.
Bear in mind that conservation of energy isn't absolute. Conservation of energy only holds when certain properties of the system in question are independent of time. There is, therefore, no requirement that energy always be conserved.
Bear in mind that conservation of energy isn't absolute. Conservation of energy only holds when certain properties of the system in question are independent of time. There is, therefore, no requirement that energy always be conserved.
Now, that said, it is probably always going to be possible to re-formulate your system so that the rules that govern its behavior are independent of time, which means that it is probably always possible to write things in a way that energy is conserved. But there are many entirely accurate ways of writing down the behavior of a system where the conservation of energy simply does not hold.
Look up [URL [Broken] Theorem[/url] for some more detail on where the conservation of energy comes from.
Actually, it is, because this statement only appears in the context of a closed universe described with the Hamiltonian formalism. It does not appear in the more common formulation of gravity, where the energy in the gravitational fields is not considered.OK. Matter-energy may be or may not be fully conserved, but that's a long way from saying that the total (net) energy of the universe is zero. This is, I think, a very profound statement, not a mere issue of nomenclature.
Well, the difficulty is that there are many different ways of describing the exact same physics. They're all correct, however, so it is perfectly valid to state that the energy of a closed universe is indeed zero, provided you qualify what you mean by that. It's also equally valid to merely state that the energy of an expanding universe is not conserved, due to the conservation of the stress-energy tensor, provided you qualify what you mean by that. These two scenarios are just two different ways of looking at the same physics.It seems to fly in the face of any conservation law, and invalidate most of, if not all of physics. I say "seems" because I'm not a physicist. Nevertheless the suggestion is that zero must be "something"; "something" being in some sense observable and/or testable, at least in principle if we're talking about anything scientific.
Well, that just depends upon what you mean by "energy".if by that you mean that 'negative energy' is just a way of saying that energy isn't 'really' conserved in gravitational interactions then I tend to agree.
if by that you mean that 'negative energy' is just a way of saying that energy isn't 'really' conserved in gravitational interactions then I tend to agree.