• #### Gravity and non reversibility of time

From Ray Tomes@21:1/5 to All on Sat Apr 4 20:42:45 2020
[Moderator's note: A few obvious minor typos corrected. -P.H.]

There have recently been a number of articles in magazines concerning
the job reversibility of time for gravitational systems. For example:
Forbes: Ask Ethan: Why Can't Time Be Reversed For Three-Body Systems?. https://www.forbes.com/sites/startswithabang/2020/04/04/ask-ethan-why-cant-time-be-reversed-for-three-body-systems/

In some of these articles they mention that gravity is not reversible
for three body systems.

I would suggest that gravity is not reversible for two body systems.
That I'd because of the "speed of gravity" a topic that has been a
source of confusion for many. Gravity appears to come from the present
position of a body, not its position a light (or gravity) time ago. We
can understand this by considering a truck with people on the back
throwing balls in all directions. The balls share the velocity of the
truck and so not seen to come from where the truck was when they were
thrown.

Not that is not the end of the matter. They do not share the
acceleration of the truck or plane or star. It is this decongestant
order component that comes into play. The position that gravity seems to
come from its the position that the body was a light time ago combined
with its velocity at that time for a lift time. But not including its acceleration then.

When we time reverse the acceleration a light time ago is different
slightly. It is a small effect in most cases, but it is not time
reversible.

[Moderator's note: Not the whole answer but relevant is a paper by Steve Carlip: Physics Letters A, 267, 81 (2000) -P.H.]

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