4 ms·

It's hard to make time symmetry (and thus conservation of energy) "disappear", though.

myWindoonn 8y ago
There probably isn't a plain time symmetry, but maybe there is CPT symmetry. Lucky 10000: https://en.wikipedia.org/wiki/CPT_symmetry https://en.wikipedia.org/wiki/CPT_symmetry
- wnoise 8y ago
  He's talking about time-translation symmetry, not time-reversal symmetry, which is the T in CPT.
  - stcredzero 8y ago
    Good point!
  - tobmlt 8y ago
    A quick check makes me think that one has fallen quite recently(?): https://arxiv.org/pdf/1202.2539v2.pdf https://arxiv.org/pdf/1202.2539v2.pdf https://en.wikipedia.org/wiki/Time_translation_symmetry https://en.wikipedia.org/wiki/Time_translation_symmetry https://en.wikipedia.org/wiki/Time_crystal https://en.wikipedia.org/wiki/Time_crystal Edit: Also: https://www.reddit.com/r/askscience/comments/52has4/what_is_timetranslation_symmetry_and_how_do/ https://www.reddit.com/r/askscience/comments/52has4/what_is_... https://arxiv.org/pdf/1603.08001v4.pdf https://arxiv.org/pdf/1603.08001v4.pdf Periodic, discrete symmetry holds, continuous is toast. Interesting.. Maybe there should be a rule of thumb (XKCD?) that says for any intuition one may hold, some researcher somewhere is, at ``this'' moment (in some measuring system) constructing a contrived situation that will break it via some contrivance that is strange in inverse proportion to the homeliness of the intuition? ....And some talky-site-poster (me in this case) is preparing to make it yak about it?
    - wnoise 8y ago
      These don't break fundamental time translation symmetry, anymore than regular crystals break fundamental space translation symmetry (which is equivalent to conservation of momentum). The fundamental versions of these symmetries are that the laws of physics are unchanged under such changes. These are merely systems such that the system itself lacks such a symmetry. This will indeed allow a change in the "effective" laws of physics (like electrons having different effective mass in metals or semiconductors than in free space, and quantized momenta), and give something interacting with the system the illusion that energy or momentum is not conserved. But this is only an illusion: momentum and energy of something interacting with the system are exchanged with the system. This shouldn't be mysterious though -- a time changing potential already didn't preserve the energy of what it's interacting with, and a crystal can absorb momentum as a whole without any local disturbances that could be seen by a particle in it (e.g. Mossbauer effect). But the upshot is that what counts as a symmetry depends on what's fixed under your analysis vs what's part of the state space.
      - tobmlt 8y ago
        Thanks for the details and big picture. Very nicely put.
danbruc 8y ago
Energy is not conserved in general relativity in the general case due to the expansion of the universe.
stcredzero 8y ago
Physicists have already shown that Time symmetry isn't absolute. https://www.youtube.com/watch?v=yArprk0q9eE https://www.youtube.com/watch?v=yArprk0q9eE We already know that conservation of energy isn't absolute. It is, however, a very powerful analysis tool, just as Euclidian geometry is a very powerful powerful analysis tool. It's very accurate on certain scales, however, that's not how the universe works if you look closely enough.

There probably isn't a plain time symmetry, but maybe there is CPT symmetry. Lucky 10000: https://en.wikipedia.org/wiki/CPT_symmetry https://en.wikipedia.org/wiki/CPT_symmetry

wnoise 8y ago
He's talking about time-translation symmetry, not time-reversal symmetry, which is the T in CPT.
- stcredzero 8y ago
  Good point!
- tobmlt 8y ago
  A quick check makes me think that one has fallen quite recently(?): https://arxiv.org/pdf/1202.2539v2.pdf https://arxiv.org/pdf/1202.2539v2.pdf https://en.wikipedia.org/wiki/Time_translation_symmetry https://en.wikipedia.org/wiki/Time_translation_symmetry https://en.wikipedia.org/wiki/Time_crystal https://en.wikipedia.org/wiki/Time_crystal Edit: Also: https://www.reddit.com/r/askscience/comments/52has4/what_is_timetranslation_symmetry_and_how_do/ https://www.reddit.com/r/askscience/comments/52has4/what_is_... https://arxiv.org/pdf/1603.08001v4.pdf https://arxiv.org/pdf/1603.08001v4.pdf Periodic, discrete symmetry holds, continuous is toast. Interesting.. Maybe there should be a rule of thumb (XKCD?) that says for any intuition one may hold, some researcher somewhere is, at ``this'' moment (in some measuring system) constructing a contrived situation that will break it via some contrivance that is strange in inverse proportion to the homeliness of the intuition? ....And some talky-site-poster (me in this case) is preparing to make it yak about it?
  - wnoise 8y ago
    These don't break fundamental time translation symmetry, anymore than regular crystals break fundamental space translation symmetry (which is equivalent to conservation of momentum). The fundamental versions of these symmetries are that the laws of physics are unchanged under such changes. These are merely systems such that the system itself lacks such a symmetry. This will indeed allow a change in the "effective" laws of physics (like electrons having different effective mass in metals or semiconductors than in free space, and quantized momenta), and give something interacting with the system the illusion that energy or momentum is not conserved. But this is only an illusion: momentum and energy of something interacting with the system are exchanged with the system. This shouldn't be mysterious though -- a time changing potential already didn't preserve the energy of what it's interacting with, and a crystal can absorb momentum as a whole without any local disturbances that could be seen by a particle in it (e.g. Mossbauer effect). But the upshot is that what counts as a symmetry depends on what's fixed under your analysis vs what's part of the state space.
    - tobmlt 8y ago
      Thanks for the details and big picture. Very nicely put.

danbruc 8y ago

Energy is not conserved in general relativity in the general case due to the expansion of the universe.

stcredzero 8y ago

Physicists have already shown that Time symmetry isn't absolute. https://www.youtube.com/watch?v=yArprk0q9eE https://www.youtube.com/watch?v=yArprk0q9eE We already know that conservation of energy isn't absolute. It is, however, a very powerful analysis tool, just as Euclidian geometry is a very powerful powerful analysis tool. It's very accurate on certain scales, however, that's not how the universe works if you look closely enough.