Spacetime expansion and conservation of energy

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Discussion Overview

The discussion revolves around the implications of spacetime expansion on the conservation of energy, exploring how energy conservation is perceived in the context of general relativity and cosmological models. Participants examine the relationship between energy, mass, and the changing geometry of spacetime, raising questions about the nature of energy conservation in an expanding universe.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant questions how spacetime expansion can occur without violating the law of conservation of energy.
  • Another participant suggests that spacetime deformation due to mass in general relativity must be considered when discussing energy conservation.
  • There is a mention of the stretching of photon wavelengths in the cosmic microwave background (CMB) while the number of photons remains constant.
  • A participant notes that the concept of energy and what it means to be conserved can be complex and context-dependent.
  • Links to a blog post by cosmologist Sean Carroll are shared, which discusses the non-conservation of energy in cosmological contexts.
  • One participant provides an analogy involving a truck and a bike to illustrate changing frames of reference and its implications for perceived kinetic energy.
  • Another participant questions whether changing frames of reference requires energy, suggesting that this might relate to the energy conservation issue.
  • A further response elaborates on the analogy, emphasizing that the perceived kinetic energy changes depending on the observer's frame of reference.
  • One participant cautions that no analogy will perfectly capture the complexities of spacetime expansion and encourages further study of the underlying physics.

Areas of Agreement / Disagreement

Participants express differing views on the implications of spacetime expansion for energy conservation. While some acknowledge the complexities and nuances involved, there is no consensus on how these concepts reconcile with traditional notions of energy conservation.

Contextual Notes

Participants highlight the dependence on definitions of "energy" and "conserved," indicating that the discussion may be limited by varying interpretations of these terms. The conversation also reflects a lack of resolution regarding the mathematical and theoretical underpinnings of energy conservation in an expanding universe.

EasterEggs123
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I came across this issue a while ago, when spacetime expands, then energy doesn' seem to be conserved? But does not that violate the law of conservation of energy? I don't get it, how can spacetime expansion happen without energy issues? Thanks in advance
 
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Could you be more specific about why you think this is a contradiction? You must take into account that space-time is deformed according to general relativity due to the presence of mass, and therefore it involves energy.
 
PabloAMC said:
Could you be more specific about why you think this is a contradiction? You must take into account that space-time is deformed according to general relativity due to the presence of mass, and therefore it involves energy.
Well the wavelengths of photons (CMB) get stretched but the number of them is the same.
 
This is actually a complicated issue and depends on what you mean by "energy" and "conserved". There's a good link that explains it that I'll try to find.
 
A huge truck that whizzes by me has loads of kinetic energy. I get on my bike and pedal so furiously that the truck has no relative velocity with respect to me, and thus zero kinetic energy. Where did the energy go? Nowhere, I changed frames of reference. Using a non-rigorous analogy, we continuously change frames of reference as the universe expands.

Mass-energy is, however, always conserved locally in general relativity. More technically, the 4-dvergence of the stress-energy tensor is zero.
 
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George Jones said:
A huge truck that whizzes by me has loads of kinetic energy. I get on my bike and pedal so furiously that the truck has no relative velocity with respect to me, and thus zero kinetic energy. Where did the energy go? Nowhere, I changed frames of reference. Using a non-rigorous analogy, we continuously change frames of reference as the universe expands.

Mass-energy is, however, always conserved locally in general relativity. More technically, the 4-dvergence of the stress-energy tensor is zero.

Apologize if I sound like a high schooler (because I am) but would not the acceleration of our bike be equal to usage of energy? Would it not take energy to change the frame of reference, which is supposed to explain the energy issue?
 
Because of this possible confusion, I carefully chose the two vehicles. Suppose that initially you and I are are stationary with respect to the ground. You remain stationary. After I start pedaling, you see: 1) the truck and me moving with the same speed; 2) the kinetic energy of the truck to be much larger than the kinetic energy of the bike/me system bike, because the mass of the truck is so much larger. The gain that you see for my kinetic energy is much smaller that the loss of kinetic energy that I see for the truck.
 
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Unfortunately no analogy will be perfect. You simply have to accept that the analogy is only vaguely like expansion and try to increase your knowledge of physics and math until you can learn the real theory.
 
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