Overunity! Not really, but...

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Some years ago i reasoned that the gravitational field must have a negative energy content. Apparently that was not a very original though.
I was also speculating whether the total energy (gravitational energy + the mass_energy it originates from) would be negative, equal to zero, or positive. My guess was that it must zero but i did not know how to make a logic argument for that, let alone make calculation with that. The best answer that i could find was that is was arbitrary, but lately i stumbled upon a lecture of Lawrence Krauss in which he also reasons that the total energy must be zero (although interesting, that is not what i wanted to discuss here. He also wrote a book: "a universe from nothing", explaining this).

Then another though came up: when this is correct (mass_energy + gravitational energy = 0), then using the first law of thermodynamics as argument against creating energy from nothing no longer holds: creating energy would immediately also create the negative gravitational energy resulting in a total of zero energy added to the universe (to be clear: i do not know how to do this ) .

Or would there be other fundamental principles or nature laws that would forbid creating energy from nothing?

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2 hours ago, hu?? said:

Some years ago i reasoned that the gravitational field must have a negative energy content. Apparently that was not a very original though.
I was also speculating whether the total energy (gravitational energy + the mass_energy it originates from) would be negative, equal to zero, or positive. My guess was that it must zero but i did not know how to make a logic argument for that, let alone make calculation with that. The best answer that i could find was that is was arbitrary, but lately i stumbled upon a lecture of Lawrence Krauss in which he also reasons that the total energy must be zero (although interesting, that is not what i wanted to discuss here. He also wrote a book: "a universe from nothing", explaining this).

Then another though came up: when this is correct (mass_energy + gravitational energy = 0), then using the first law of thermodynamics as argument against creating energy from nothing no longer holds: creating energy would immediately also create the negative gravitational energy resulting in a total of zero energy added to the universe (to be clear: i do not know how to do this ) .

Or would there be other fundamental principles or nature laws that would forbid creating energy from nothing?

I think this idea - that the sum of gravitational,  mass energy and other energy in the cosmos equates to zero - has been around for quite a long time. I seem to recall reading about it over ten years ago.

The thing I always struggle to keep in mind is that energy is not an entity, but merely a property of a physical system of some kind. So if you have no system, there can't be any energy. Presumably in this model the physical system of the cosmos came into existence - somehow or other - with a net zero of energy, which then became divided between -ve gravitational energy and +ve energy in radiation, rest mass of sub-atomic QM entities such as quarks, etc.  I'm not sure that applying the laws of thermodynamics back to the first instants of the cosmos is valid, though. My understanding of QM is that energy is only conserved on average, due to quantum uncertainty - of which I can imagine there might have been a lot in the beginning. Then again, I'm not sure we can even assume that any of the laws of nature were in operation!

But no doubt there are better qualified people than I on this forum to comment on all this.

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4 hours ago, hu?? said:

Or would there be other fundamental principles or nature laws that would forbid creating energy from nothing?

Energy of what ?

'Energy' is not a substance.

What is the 'energy' of a single point mass in an otherwise empty universe ?
So the question arises "How do you treat the energy of the very first point mass to be 'created out of nothing' ?"

Gravitational 'energy' is potential energy or the energy of configuration (of a system).
Some of this is released to other forms as a system is reconfigured.
This happens through the action of forces (if your view is Newtonian) or GR field equations (if your view is relativistic).

Makes one feel like the first chicken contemplating the first egg.

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8 hours ago, studiot said:

Energy of what ?

'Energy' is not a substance.

What is the 'energy' of a single point mass in an otherwise empty universe ?
So the question arises "How do you treat the energy of the very first point mass to be 'created out of nothing' ?"

Gravitational 'energy' is potential energy or the energy of configuration (of a system).
Some of this is released to other forms as a system is reconfigured.
This happens through the action of forces (if your view is Newtonian) or GR field equations (if your view is relativistic).

Makes one feel like the first chicken contemplating the first egg.

I just mean (for as far as i know) that all energy gravitates: It doesn't matter whether it is stored as mass (e=mc^2) or not.

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24 minutes ago, hu?? said:

I just mean (for as far as i know) that all energy gravitates: It doesn't matter whether it is stored as mass (e=mc^2) or not.

I have no idea what you mean as a reply to my comments.

[ ] head scratching icon with puzzled look on face [/]

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14 hours ago, hu?? said:

Or would there be other fundamental principles or nature laws that would forbid creating energy from nothing?

Conservation of energy stems from the time translation symmetry of the laws of physics. But one has to remember that conservation only applies to a single inertial frame of reference.

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8 hours ago, swansont said:

Conservation of energy stems from the time translation symmetry of the laws of physics. But one has to remember that conservation only applies to a single inertial frame of reference.

I don't know what "time translation symmetry" means. I'll look it up. Thanks.

That said, i thought that conservation laws applied to closed systems in the widest meaning possible. But note that  a closed system is a pure hypothetical construct: e.g. anything accelerating within a closed system will cause gravitational waves leaking out of the 'closed' system.

10 hours ago, studiot said:

I have no idea what you mean as a reply to my comments.

[ ] head scratching icon with puzzled look on face [/]

Perhaps it's the same for me not understanding your comment? Bit confused here. What has a single point mass in an empty universe  to do with, eh, with what? Looks like kind of a philosophical question to me: in that case the point-mass is the universe. 🤔

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6 hours ago, hu?? said:

I don't know what "time translation symmetry" means. I'll look it up. Thanks.

Things (in this case, laws of physics) don't change over time. They look the same at time T, and time T+t

6 hours ago, hu?? said:

That said, i thought that conservation laws applied to closed systems in the widest meaning possible. But note that  a closed system is a pure hypothetical construct: e.g. anything accelerating within a closed system will cause gravitational waves leaking out of the 'closed' system.

A closed system is a thermodynamic term, and unless you're doing cosmology you're probably tacitly assuming an inertial frame. An accelerating frame is not inertial. (and gravitational waves are emitted only under some accelerations)

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I recognise (at least) three (thermodynamic) system types.

Isolated   :   Neither matter (mass) nor energy may pass across the system boundary to or from the surroundings.

Closed    :    Energy but not matter (mass) may pass across the system boundary to or from the surroundings.

Open      :    Both energy and matter (mass) may pass across the system boundary to or from the surroundings.

Some rather parsimonious authors only detail two of these and say closed when they really mean isolated.

9 hours ago, hu?? said:

Perhaps it's the same for me not understanding your comment? Bit confused here. What has a single point mass in an empty universe  to do with, eh, with what? Looks like kind of a philosophical question to me: in that case the point-mass is the universe.

"in that case the point-mass is the universe."

Yes indeed so that is the point.

But such a universe is isolated so how do you determine its energy, whether kinetic or potential. ?

It is also worth noting that both the First and Second Laws of Thermodynamics refer to processes acting across the system boundary.

That, of course, presupposes there is a system boundary.
And, of course, that there is available energy on the other side of it whatever that means in terms of the universe.

So I am saying we have run smack into the perennial problem of trying to extend properties of a finite system to a (perhaps) infinite one.

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Posted (edited)
15 hours ago, studiot said:

Some rather parsimonious authors only detail two of these and say closed when they really mean isolated.

As a none native speaker i had to look up 'parsimonious'. Not very flattering... But ok, in this case it more or less fits me: I did not know there was a distinction between closed and isolated, so thank you for the free education.
With the thought experiment i proposed i think closed-system is still appropriate though: energy created from 'nothing', will actually be supplied by the negative energy of the expanding gravitational field originating from the created energy. Here you have the chicken and egg problem... 😁

15 hours ago, studiot said:

So I am saying we have run smack into the perennial problem of trying to extend properties of a finite system to a (perhaps) infinite one.

This seem to be the thing: my simple thought experiment is not so simple.

So we did find a new physics law preventing creating energy from nothing: an egg shall be made by a chicken and a chicken shall come from an egg.  🤣

Edited by hu??
typo correction
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On 4/18/2021 at 8:06 AM, hu?? said:

As a none native speaker i had to look up 'parsimonious'. Not very flattering... But ok, in this case it more or less fits me: I did not know there was a distinction between closed and isolated, so thank you for the free education.
With the thought experiment i proposed i think closed-system is still appropriate though: energy created from 'nothing', will actually be supplied by the negative energy of the expanding gravitational field originating from the created energy. Here you have the chicken and egg problem... 😁

This seem to be the thing: my simple thought experiment is not so simple.

So we did find a new physics law preventing creating energy from nothing: an egg shall be made by a chicken and a chicken shall come from an egg.  🤣

I am sorry for any comment that could have been taken as offensive. Thank you for not being offended by my coment.
I assure you no offence was intended.

There is one more type of systems , that is really a variant of an open system.

Flow systems are often modelled by the use of a 'control volume' , where the mass flow in is exactly equal to the mass flow out.
Such a system is called quasi-static since the composition within the control does not change by energy and momentum flow through the box.

The boundary is more problematic.

Take for instance conformal mapping of flows or fields.
2D maps have sources and sinks which draw on materials or field lines from another dimension perpendicular to the plane.
2D manifolds may have not boundary, yet be finite eg the surface of a sphere.

3D manifolds may be finite in 3D and unbounded, yet flows or fields can require activity in fourth dimension.

So the logic of the question "where does the energy come from ?" remains unanswered.

The issue of working in X dimensions and borrowing from another introduces many wierd effects in Maths, some which spill over into theoretical Physics.

You may have heard of Gabriel's Horn.

An infinite surface that bounds a finite volume.

This is one reason why the Lebesgue integral and measure theory was introduced to extend ordinary Riemann integrals.

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