Noether's theorem -- Time inversion

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Discussion Overview

The discussion revolves around the relationship between Noether's theorem, time inversion, and the implications of symmetry in physics. Participants explore the nature of time symmetry, particularly in the context of conservation laws and the distinction between time translation and time reversal symmetries.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant asserts that Noether's theorem indicates that the law of conservation of energy arises from the homogeneity of time, questioning the connection to time inversion.
  • Another participant clarifies that Noether's theorem pertains to differential symmetries like time translation, rather than discrete symmetries such as time reversal.
  • A participant suggests that time reversal symmetry is contingent upon having symmetry under time translations.
  • Another participant questions whether the transformation ## t \rightarrow -t## can be considered a form of time translation.
  • A response points out that the function ##\cos(t)## is symmetric under time reversal but does not exhibit symmetry under time translations, providing an example to illustrate this distinction.
  • A later reply reiterates the example of ##\cos(t)##, emphasizing its symmetry under time reversal while not being symmetric under time translations.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between time translation and time reversal symmetries, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants do not fully resolve the implications of time inversion in relation to Noether's theorem, and there are unresolved distinctions between types of symmetries discussed.

LagrangeEuler
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Noether's theorem said that because of homogeneity in time the law of conservation of energy exists. I am bit of confused and I am not sure is also time inversion some consequence of this. For example in the case of free fall we have symmetry ## t \rightarrow -t##. I am sometimes confused of that. Symmetry ## t \rightarrow -t##. is valid when we have conservation of energy and conservation of energy we have because of homogeneity in time. Could please give me some more insight on this problem. Thanks.
 
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Noether's theorem applies to differential symmetries like time translation, and not to discrete symmetries like time reversal.
 
Yes but we will not have time reversal symmetry if we did not have symmetry under the time translations. Right?
 
Also when we have ## t \rightarrow -t## didn't we did translation of time ##t## of some kind?
 
No. Consider the function ##\cos (t)##. It is symmetric under time reversal ##\cos(t)=\cos(-t)## but not time translations ##\cos(t)\ne \cos(t+\delta t)##
 
DaleSpam said:
No. Consider the function ##\cos (t)##. It is symmetric under time reversal but not time translations.
Thanks Sir. Very nice example.
 

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