5 ms·

Well, perpetual motion is not impossible, obtaining energy from it impossible

by tonmoy 10y ago

fmihaila 10y ago
> Well, perpetual motion is not impossible, obtaining energy from it impossible This is incorrect. From Wikipedia [0]: "A perpetual motion machine of the third kind is usually (but not always)[10] defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). [...] It is impossible to make such a machine,[11][12] as dissipation can never be completely eliminated in a mechanical system, no matter how close a system gets to this ideal (see examples in the Low Friction section). " [0] https://en.wikipedia.org/wiki/Perpetual_motion https://en.wikipedia.org/wiki/Perpetual_motion
- Jach 10y ago
  Parent is probably referencing https://en.wikipedia.org/wiki/Time_crystal https://en.wikipedia.org/wiki/Time_crystal Because a time crystal is a driven (i.e. open) quantum system that is in perpetual motion, it does not violate the laws of thermodynamics... does not produce work...does not spontaneously convert thermal energy into mechanical work...cannot serve as a perpetual store of work... A time crystal has been said to be a perpetuum mobile of the fourth kind: it does not produce work and it cannot serve as a perpetual energy storage. But it rotates perpetually.
  - fmihaila 10y ago
    > Parent is probably referencing https://en.wikipedia.org/wiki/Time_crysta https://en.wikipedia.org/wiki/Time_crysta Possibly, but that is not the meaning intended in the article, nor the usual meaning of the expression, so it's not relevant.
  - roywiggins 10y ago
    In theory I think a closed system undergoing completely reversible processes could be considered perpetual motion (being reversible, entropy never increases). The second law states that those are the only sorts of isolated systems that don't gain entropy over time. https://en.wikipedia.org/wiki/Second_law_of_thermodynamics https://en.wikipedia.org/wiki/Second_law_of_thermodynamics But time crystals aren't perpetual motion just by dint of not being closed systems at all, so they're kind of neither here nor there.
    - deleted 10y ago
      [deleted]
    - greghatch 10y ago
      > But time crystals aren't perpetual motion just by dint of not being closed systems at all, so they're kind of neither here nor there. Literally!
  - jacobush 10y ago
    That was the weirdest shit I've seen in a good while.
- lutorm 10y ago
  A superconductor is sort of a perpetual motion machine for electrons, isn't it?
  - fmihaila 10y ago
    > A superconductor is sort of a perpetual motion machine for electrons, isn't it? It may seem like it, but no. As this link [0] explains, "A prime example is the superconductive metals, whose electrical resistance disappears completely at low temperature, usually somewhere around 20 K. Unfortunately, the energy required to maintain the low temperature exceeds the work that results from the superconductive flow." [0] https://www.britannica.com/science/perpetual-motion https://www.britannica.com/science/perpetual-motion
    - lutorm 10y ago
      Yeah I didn't mean to imply you could get work out of it, just that it's a system with zero dissipation, so it would seem to be a perpetual machine "of the third kind" as referenced by the post above. (If you don't count the work required to maintain low temperature, of course, which I don't think you should since you could place the device way out in intergalactic space where it would attain a temperature near 2.7K without you having to do any work to maintain it.
      - Dylan16807 10y ago
        It's a misuse of the word 'machine' to use it on something incapable of performing any kind of work.
- 2_listerine_pls 10y ago
  He said perpetual motion, not 'perpetual motion machine'. Do planets ever stop rotating?
  - fmihaila 10y ago
    > Do planets ever stop rotating? Of course they do, on an appropriately long time scale. First they stop rotating around their own axis due to tidal forces, after which they lose the remaining kinetic energy to the gravitational waves emitted by their rotation around the star (again, on a very long time scale, but not infinite). 2nd EDIT: <removed gratuitous reference to moderation>.
    - maxerickson 10y ago
      BTW, I find it amusing that people take internet points away because they think the laws of thermodynamics apply only in certain circumstances. They do only apply to closed systems. (which information we are unable to ascertain about our universe)
      - jacquesm 10y ago
        It's better than that: the fact that the universe even exists implies that at some point there was either a violation of this law or it is part of a larger closed system within which the law still holds.
        roywiggins 10y ago
        Or, there is no minimum entropy, and no matter how far back you look there's more entropy. Maybe the Big Bang is an illusion and before the big bang, there was even less entropy, and on and on infinitely into the past.
      - dandelany 10y ago
        Which is like saying "I can swing forever as long as someone keeps pushing me." Clearly true, but pedantic and irrelevant to the discussion.

Well, perpetual motion is not impossible, obtaining energy from it impossible

by tonmoy 10y ago

Well, perpetual motion is not impossible, obtaining energy from it impossible

> Well, perpetual motion is not impossible, obtaining energy from it impossible This is incorrect. From Wikipedia [0]: "A perpetual motion machine of the third kind is usually (but not always)[10] defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever (due to its mass inertia). [...] It is impossible to make such a machine,[11][12] as dissipation can never be completely eliminated in a mechanical system, no matter how close a system gets to this ideal (see examples in the Low Friction section). " [0] https://en.wikipedia.org/wiki/Perpetual_motion https://en.wikipedia.org/wiki/Perpetual_motion

Jach 10y ago
Parent is probably referencing https://en.wikipedia.org/wiki/Time_crystal https://en.wikipedia.org/wiki/Time_crystal Because a time crystal is a driven (i.e. open) quantum system that is in perpetual motion, it does not violate the laws of thermodynamics... does not produce work...does not spontaneously convert thermal energy into mechanical work...cannot serve as a perpetual store of work... A time crystal has been said to be a perpetuum mobile of the fourth kind: it does not produce work and it cannot serve as a perpetual energy storage. But it rotates perpetually.
- fmihaila 10y ago
  > Parent is probably referencing https://en.wikipedia.org/wiki/Time_crysta https://en.wikipedia.org/wiki/Time_crysta Possibly, but that is not the meaning intended in the article, nor the usual meaning of the expression, so it's not relevant.
- roywiggins 10y ago
  In theory I think a closed system undergoing completely reversible processes could be considered perpetual motion (being reversible, entropy never increases). The second law states that those are the only sorts of isolated systems that don't gain entropy over time. https://en.wikipedia.org/wiki/Second_law_of_thermodynamics https://en.wikipedia.org/wiki/Second_law_of_thermodynamics But time crystals aren't perpetual motion just by dint of not being closed systems at all, so they're kind of neither here nor there.
  - deleted 10y ago
    [deleted]
  - greghatch 10y ago
    > But time crystals aren't perpetual motion just by dint of not being closed systems at all, so they're kind of neither here nor there. Literally!
- jacobush 10y ago
  That was the weirdest shit I've seen in a good while.
lutorm 10y ago
A superconductor is sort of a perpetual motion machine for electrons, isn't it?
- fmihaila 10y ago
  > A superconductor is sort of a perpetual motion machine for electrons, isn't it? It may seem like it, but no. As this link [0] explains, "A prime example is the superconductive metals, whose electrical resistance disappears completely at low temperature, usually somewhere around 20 K. Unfortunately, the energy required to maintain the low temperature exceeds the work that results from the superconductive flow." [0] https://www.britannica.com/science/perpetual-motion https://www.britannica.com/science/perpetual-motion
  - lutorm 10y ago
    Yeah I didn't mean to imply you could get work out of it, just that it's a system with zero dissipation, so it would seem to be a perpetual machine "of the third kind" as referenced by the post above. (If you don't count the work required to maintain low temperature, of course, which I don't think you should since you could place the device way out in intergalactic space where it would attain a temperature near 2.7K without you having to do any work to maintain it.
    - Dylan16807 10y ago
      It's a misuse of the word 'machine' to use it on something incapable of performing any kind of work.
2_listerine_pls 10y ago
He said perpetual motion, not 'perpetual motion machine'. Do planets ever stop rotating?
- fmihaila 10y ago
  > Do planets ever stop rotating? Of course they do, on an appropriately long time scale. First they stop rotating around their own axis due to tidal forces, after which they lose the remaining kinetic energy to the gravitational waves emitted by their rotation around the star (again, on a very long time scale, but not infinite). 2nd EDIT: <removed gratuitous reference to moderation>.
  - maxerickson 10y ago
    BTW, I find it amusing that people take internet points away because they think the laws of thermodynamics apply only in certain circumstances. They do only apply to closed systems. (which information we are unable to ascertain about our universe)
    - jacquesm 10y ago
      It's better than that: the fact that the universe even exists implies that at some point there was either a violation of this law or it is part of a larger closed system within which the law still holds.
      - roywiggins 10y ago
        Or, there is no minimum entropy, and no matter how far back you look there's more entropy. Maybe the Big Bang is an illusion and before the big bang, there was even less entropy, and on and on infinitely into the past.
    - dandelany 10y ago
      Which is like saying "I can swing forever as long as someone keeps pushing me." Clearly true, but pedantic and irrelevant to the discussion.