For one thing, if someone from the present or future can physically go to the past, then it’s had to have already happened. Here’s why: say John Doe exists in the year 2020 and develops a time machine. He goes back to the year 1865. Well, the year 1865 has already happened, so by present day’s time, John Doe has already been in the year 1865. Whatever he did to alter 1865 has already happened, thus not truly altering it.
Here’s another reason why the past cannot truly be altered: if it is truly altered, then by the time the future exists, the one(s) who went back to alter the past would have no reason to go back to alter it because they would have no knowledge of what was to be altered. Example: John Doe goes back in time to prevent Abraham Lincoln’s assassination. If he were successful, then Lincoln’s assassination would not have happened. This alteration of history would cause John Doe to have no knowledge of the assassination when the future comes around, so he would not then go back to the past to prevent it. The assassination would have to have happened because John Doe would no longer be going back to prevent it.
So, if John Doe goes back in time, whatever he does there has to be predestined to have already happened.
"Crook, do you know I believe there are men who want to take my life? And I have no doubt they will do it... I know no one could do it and escape alive. But if it is to be done, it is impossible to prevent it."
~Abraham Lincoln to bodyguard, William H. Crook, on April 14, 1865.
How true that is.