5 ms·

> What’s more, there is an energy associated with any given volume of the universe. If that volume increases, the inescapable conclusion is that this energy mus

> What’s more, there is an energy associated with any given volume of the universe. If that volume increases, the inescapable conclusion is that this energy must increase as well. And yet physicists generally think that energy creation is forbidden. It's important to remember that energy conservation is not an axiom of basic physics. It is a consequence of Noether's theorem and temporal symmetry in the laws of physics. In the framework of GR, this means that any static metric will have some conserved quantity that corresponds to energy (though, in general, it will not be the same as the Newtonian energy). But if the metric is not static (as in the case of an expanding universe), there is no need for energy to be conserved.

rotorblade 12y ago
Indeed. I commented on this earlier: https://news.ycombinator.com/item?id=8996190 https://news.ycombinator.com/item?id=8996190
jessriedel 12y ago
Right, this is all well understood. Instead of saying that, we're told: "And yet ask them about it and they shuffle their feet and stare at the ground. Clearly, any theorist who can solve this paradox will have a bright future in cosmology." So the author is either badly informed or willfully deceitful.
api 12y ago
Tangential side question: Must energy conservation be local? Would it be "legal" for -- say -- energy to be spent/dissipated (entropy increase) at one location leading to an apparent over-unity effect ("free" entropy decrease) somewhere else? I'm not talking about energy transmission through wires or EM radiation... I mean something apparently disconnected yet coupled. I ask because I've encountered such ideas in sci-fi before and couldn't think of a reason why they're forbidden -- though I have no idea how you would do such a thing.
- nitid_name 12y ago
  If it's coupled, wouldn't it cross the boundary of the system, and then be included in normal entropy calculations as a flow across the boundary?
  - api 12y ago
    Exactly... just wondering if energy had to be conserved locally or globally? Let me give you an example scenario: Let's say (ignore the mechanism for now) that it's possible to send a small amount of information from point A to B. This information takes a tremendous amount of energy to generate at point A, and when it's received at point B it can be used to (somehow) unlock a tremendous release of energy. If you only looked at point B, it would look like you had an "over-unity device." But in reality the energy spewing out at point B is paid for at point A, and the information transfer by occurring at the speed of light does not violate causality. Not claiming it's possible, just asking if it's expressly forbidden by fundamental physics or not. (If it were possible it would of course open up all kinds of very amazing things like remotely powered space propulsion, a whole next level of power grid technology, or orbital solar power that actually worked, but I know of no effect that accomplishes anything like this so it's total sci-fi even if not forbidden by any known law.) Edit: according to another response this likely is not possible. Oh well. :)
- antognini 12y ago
  Noether's theorem works locally as well as globally. So it would not be possible for energy to disappear in one location and spontaneously appear in another.
mcguire 12y ago
From the linked ArXiv paper: "The first puzzling feature of the space expansion physics is that the Friedmann’s equations Eq. (6, 5) in terms of the metric distance r(t) = S(t) · χ get the exact Newtonian form: ... "where M g (r) = ... is the gravitating mass of a ball with radius r(t). So according to general relativity the dynamics of the whole universe is determined by the exact Newtonian acceleration and Newtonian kinetic plus potential energy conservation (here velocity of light c does not change the Newtonian character of the equations)." Page 7, I believe. http://arxiv.org/abs/1501.01919 http://arxiv.org/abs/1501.01919
snowwrestler 12y ago
Isn't the idea of "zero point" vacuum energy that even though the vaccuum energy level is not actually zero, it is still the lowest state, so no work can be done with it? So from the perspective of energy conservation, why would it matter if new space has a non-zero amount of energy? It still does not increase the amount of energy available to do anything.

rotorblade 12y ago

Indeed. I commented on this earlier: https://news.ycombinator.com/item?id=8996190 https://news.ycombinator.com/item?id=8996190

jessriedel 12y ago

Right, this is all well understood. Instead of saying that, we're told: "And yet ask them about it and they shuffle their feet and stare at the ground. Clearly, any theorist who can solve this paradox will have a bright future in cosmology." So the author is either badly informed or willfully deceitful.

api 12y ago

Tangential side question: Must energy conservation be local? Would it be "legal" for -- say -- energy to be spent/dissipated (entropy increase) at one location leading to an apparent over-unity effect ("free" entropy decrease) somewhere else? I'm not talking about energy transmission through wires or EM radiation... I mean something apparently disconnected yet coupled. I ask because I've encountered such ideas in sci-fi before and couldn't think of a reason why they're forbidden -- though I have no idea how you would do such a thing.

nitid_name 12y ago
If it's coupled, wouldn't it cross the boundary of the system, and then be included in normal entropy calculations as a flow across the boundary?
- api 12y ago
  Exactly... just wondering if energy had to be conserved locally or globally? Let me give you an example scenario: Let's say (ignore the mechanism for now) that it's possible to send a small amount of information from point A to B. This information takes a tremendous amount of energy to generate at point A, and when it's received at point B it can be used to (somehow) unlock a tremendous release of energy. If you only looked at point B, it would look like you had an "over-unity device." But in reality the energy spewing out at point B is paid for at point A, and the information transfer by occurring at the speed of light does not violate causality. Not claiming it's possible, just asking if it's expressly forbidden by fundamental physics or not. (If it were possible it would of course open up all kinds of very amazing things like remotely powered space propulsion, a whole next level of power grid technology, or orbital solar power that actually worked, but I know of no effect that accomplishes anything like this so it's total sci-fi even if not forbidden by any known law.) Edit: according to another response this likely is not possible. Oh well. :)
antognini 12y ago
Noether's theorem works locally as well as globally. So it would not be possible for energy to disappear in one location and spontaneously appear in another.

mcguire 12y ago

From the linked ArXiv paper: "The first puzzling feature of the space expansion physics is that the Friedmann’s equations Eq. (6, 5) in terms of the metric distance r(t) = S(t) · χ get the exact Newtonian form: ... "where M g (r) = ... is the gravitating mass of a ball with radius r(t). So according to general relativity the dynamics of the whole universe is determined by the exact Newtonian acceleration and Newtonian kinetic plus potential energy conservation (here velocity of light c does not change the Newtonian character of the equations)." Page 7, I believe. http://arxiv.org/abs/1501.01919 http://arxiv.org/abs/1501.01919

snowwrestler 12y ago

Isn't the idea of "zero point" vacuum energy that even though the vaccuum energy level is not actually zero, it is still the lowest state, so no work can be done with it? So from the perspective of energy conservation, why would it matter if new space has a non-zero amount of energy? It still does not increase the amount of energy available to do anything.