Backwards time travel is not possible.

Two points to be made against backwards timetravel.
POINT 1

A law of physics states matter can't be created.

This means there is a determined amount of particles in the universe.

Any time travel in reverse would add particles ( our intrepid time traveler and any matter he takes with him) into those space/time slices thus violating the first law of thermodynamics for the universe' space time for then forward.

When you go back in time the particles of your body or ship or whatever could have been a rock, tree or some maiden 500 years ago.

So if you time travel into the past, you just added particles into that timeline, unless you take them away from their rightful timeline owner.

A side note is that by the time we live a normal lifespan we don't have any of the original cells or particles we were born with.

We move thru the environment all the while exchanging particles.

You could actually share particles with every human ever born. You could be made up of Henry the 8th’s piss and spit.

POINT 2

If our traveler could re-arrange particles from the past to re-constitute himself and any matter he brings back rather than duplicate or add particles (which is impossible), he would change the past by “taking particles away from the rightful owners in that timeline.

It would be like travelling back in time and the particles that are you now would have to be taken away from those legitimate “timeliners”, thus affecting the past just by showing up.