It’s not possible to build a time machine.
If it was, we would already have one.
From the moment a time machine exists, people will use it infinite times. If time-travel is used endless times, sooner or later, by intention or accident, the means for such will end up in “our” hands. Or those of our ancestors.
The other possibility is: from the moment one goes back in time, one creates a new universe. Which means that, for the practical purposes of ordinary 4-dimensional mortals like ourselves, who inhabit this universe exclusively… There is not and will never be a time machine.
The third possibility is that the time machine, and they who use it, do not move through time in the same plane of existence as us, are not perceivable by us in any way.
More on this in an upcoming essay …