The term "matter" here implies all things in the universe . Anything the occupies space and has mass.
So, questioning yourself why you could not be a baby again when you walk back or even you move in extremely high speed or in a speed of light, it is because you could never return back the state of matter that have already been done. Or return back the cell divisions in your body in reverse motion until it reaches its younger cells. No, it would never be possible. There's no such thing as reverse construction. What has been done is done and continue to move in forward motion. Even if you repeat the work, still its moving in forward motion. It is just like a loop or a wheel which continues rotating but time is moving forward and never backward.
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Raymond say: A theory is a proposed explanation for something that you cannot go check in person.
A proof is some evidence that you can use to show something is true.
You come home and there is a warm cherry pie on the kitchen table. How did it get there?
Different possible explanations.
1. Mom baked a pie this afternoon.
2. A neighbor brought a pie as a gift.
3. Aliens materialized a pie by magic.
all of them are "possible" explanations, even though number 3 has a lower probability than number 1.
If you cannot find out which one really happened, you will use Lex Parcimoniae (a.k.a. Occam's Razor) and go with number 1, until you find evidence proving it is false.
Finally Mom comes home and you ask her.
"No, darling, I did not bake a pie, as I was away at work all day"
Now THAT is proof that number 1 does not hold. You now have to turn to another explanation.
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Einstein's theory is a proposed explanation for the behavior of objects in different relative frames of reference, and for the mathematics to work (the equations), time must be transformed into a spatial coordinate... AS IF it were space. However, that does not mean you can travel along the time axis in any direction, or at a selected rate.