9 ms·

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.
    - maxerickson 10y ago
      Pedantic needling is always a relevant response to internet point outrage.
- Nomentatus 10y ago
  Not gravitational waves (you're thinking caused by solar flares moving at the surface of the sun) - but the particle radiation from the sun will continue to slow planets over time.
  - fmihaila 10y ago
    > Not gravitational waves. Yes, gravitational waves. You are correct that in the case of the Earth, other causes will lead to its demise long before said waves would amount to any significant effect. But even in an ideal case where a single planet like the Earth revolved around, say, a black hole with the same mass as the Sun, which would emit no particles, it would still not orbit indefinitely due to the energy lost to gravitational waves emitted as a result of its revolution around another body. The amount of energy lost in one revolution is tiny, but is not zero (according to this calculation [0], for the Sun-Earth system it amounts to about 200W). [0] https://en.wikipedia.org/wiki/Gravitational_wave#Power_radiated_by_orbiting_bodies https://en.wikipedia.org/wiki/Gravitational_wave#Power_radia...
    - Nomentatus 10y ago
      Yes, two bodies create a gravitational wave that affects other bodies - but I don't see how those variations represent work - and they aren't observable on the two bodies themselves, so one wouldn't expect any effect upon those two bodies. Throw in the other planets and you might have a better argument, for example that Jupiter-Sun waves affect earth. Still, gravity "waves" aren't like water waves, as far as we yet know they don't represent (changes in) the impact of particles. However if you wish to argue that Higg's Bosons mediate gravity (some have, I believe) this objection of mine might be overcome. In other words, though I might be wrong, I just don't think we're at the point where we can deduce friction from gravitational waves, and we certainly aren't at the point where we can empirically measure such friction (as opposed to the "wave" - variation in gravitational field.) PS, sorry: there should have been a question mark in my previous post asking whether you were thinking about flares.
      - fmihaila 10y ago
jacquesm 10y ago
Yes and yes. Any other softball questions?
- 2_listerine_pls 10y ago
  Doesn't inertia guarantee the possibility of linear perpetual motion?
  - jacquesm 10y ago
    No because there is such a thing as the heat death of the universe, which with the knowledge we have at present seems to be the most likely end-game. In other words: planets and comets will eventually completely disintegrate because their component atoms and even the components of those atoms will possibly disintegrate. You're looking at 10^30 years or so, so don't wait up for it, but it is still substantially shorter than 'forever'.
    - Dylan16807 10y ago
      That is such a pedantic and irrelevant answer.
      - jacquesm 10y ago
        Given the context (planets and comets) it actually isn't. Planets are just like clocks, slowly winding down, especially the ones with moons through tidal friction. And even that weren't the case the long run will still get you. The problem with anybody proposing some kind of perpetual motion device is that perpetual really means 'forever', so no cheating. If you want to change the scope you can do so but you'll have to drop the 'perpetual' bit.
        Dylan16807 10y ago
        If it lasts as long as the universe as we know it, I'm happy to call it 'perpetual'.
coding123 10y ago
I'm willing to bet that existence of perpetually travelling energy or mass has been around for an infinite amount of time.

> 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.
  - maxerickson 10y ago
    Pedantic needling is always a relevant response to internet point outrage.
Nomentatus 10y ago
Not gravitational waves (you're thinking caused by solar flares moving at the surface of the sun) - but the particle radiation from the sun will continue to slow planets over time.
- fmihaila 10y ago
  > Not gravitational waves. Yes, gravitational waves. You are correct that in the case of the Earth, other causes will lead to its demise long before said waves would amount to any significant effect. But even in an ideal case where a single planet like the Earth revolved around, say, a black hole with the same mass as the Sun, which would emit no particles, it would still not orbit indefinitely due to the energy lost to gravitational waves emitted as a result of its revolution around another body. The amount of energy lost in one revolution is tiny, but is not zero (according to this calculation [0], for the Sun-Earth system it amounts to about 200W). [0] https://en.wikipedia.org/wiki/Gravitational_wave#Power_radiated_by_orbiting_bodies https://en.wikipedia.org/wiki/Gravitational_wave#Power_radia...
  - Nomentatus 10y ago
    Yes, two bodies create a gravitational wave that affects other bodies - but I don't see how those variations represent work - and they aren't observable on the two bodies themselves, so one wouldn't expect any effect upon those two bodies. Throw in the other planets and you might have a better argument, for example that Jupiter-Sun waves affect earth. Still, gravity "waves" aren't like water waves, as far as we yet know they don't represent (changes in) the impact of particles. However if you wish to argue that Higg's Bosons mediate gravity (some have, I believe) this objection of mine might be overcome. In other words, though I might be wrong, I just don't think we're at the point where we can deduce friction from gravitational waves, and we certainly aren't at the point where we can empirically measure such friction (as opposed to the "wave" - variation in gravitational field.) PS, sorry: there should have been a question mark in my previous post asking whether you were thinking about flares.
    - fmihaila 10y ago

jacquesm 10y ago

Yes and yes. Any other softball questions?

2_listerine_pls 10y ago
Doesn't inertia guarantee the possibility of linear perpetual motion?
- jacquesm 10y ago
  No because there is such a thing as the heat death of the universe, which with the knowledge we have at present seems to be the most likely end-game. In other words: planets and comets will eventually completely disintegrate because their component atoms and even the components of those atoms will possibly disintegrate. You're looking at 10^30 years or so, so don't wait up for it, but it is still substantially shorter than 'forever'.
  - Dylan16807 10y ago
    That is such a pedantic and irrelevant answer.
    - jacquesm 10y ago
      Given the context (planets and comets) it actually isn't. Planets are just like clocks, slowly winding down, especially the ones with moons through tidal friction. And even that weren't the case the long run will still get you. The problem with anybody proposing some kind of perpetual motion device is that perpetual really means 'forever', so no cheating. If you want to change the scope you can do so but you'll have to drop the 'perpetual' bit.
      - Dylan16807 10y ago
        If it lasts as long as the universe as we know it, I'm happy to call it 'perpetual'.

coding123 10y ago

I'm willing to bet that existence of perpetually travelling energy or mass has been around for an infinite amount of time.