Re: Time symmetric physical laws

Edward Green wrote:

> Is it as simple as "invariant in form under the change of variables
> t = - t?

Yes, at least for Lagrangian systems ...

Time is isotropic if the Lagrangian is unchanged under
a time reversal (t replaced by -t). In that case the
equations of motion are also unchanged, and if a given
motion is possible in a system, then so is the reverse
motion (that is, the motion in which the system passes
the same states in the reverse order).

In classical mechanics, the usual form of the Lagrangian,
L = T - U, guarantees time isotropy.
See Landau-Lifshitz, Mechanics, page 8-9.

/mel
.

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