Then what does that bit about in both directions mean?
The second time travel is invented, it instantaneously begins to exist everywhere in the future as well as everywhere in the past, because it allows anyone to travel to any of those points and, given enough time, they will.
So, I invent time travel on tuesday. And then, on thursday I decide to travel back to 1980 and invest in microsoft stock or something. Great, now timetravel technology exists in 1980, 34 years before it was invented. So now causality is fucked. So I trade the details of my time machine to some guy for a hotdog and he time travels into the future, meanwhile, someone else who bought the time machine I invented in present time travels even farther into the past and uses his technology to rule a preindustrial tribe as a god. After he dies of orgy overdose, one of them get in and travel far into the future, beyond a nuclear war that would have otherwise wiped out the time travel tech.
Now, what this means is that, the second I created the time machine it began to exist in several points in time both in the past and future.
Now imagine we have 50 time machines. Imagine they're all popping in and out of time, changing things. Now nothing is stable and the past and future are altering uncontrollably and physicists are weeping.
EVERYTHING IS FUCKED FOREVER.
I DON'T EVEN SEE ANY SUGGESTION THAT HAS MORE THEN ONE VOTE! TRY HARDER FOOLS!