Sometimes a thing is discovered or invented by several people at the same time. It happened to calculus, and it happened to the telephone. A theory says that those inventions or discoveries happen when the right set of circumstances align and the time is ripe. The time machine is another example of this: it was invented simultaneously in the years 8583, 6383, 4725, 3174, 1997, and 47 B.C.
It is a common fear that making small changes in the past may result in enormous changes in the present. As it turns out, the timeline is more resistant to change than people credit it for. Contrary to popular belief, crushing an ant in the Cretaceous Age will not cause the total extinction of humankind. And it is not for lack of people who tried.
This doesn’t mean that it’s impossible to alter a single person’s timeline. The prime example of this is Gerald P. Zhou, a time travel pioneer who did a lot of research and experimentation and established the rules that allow time travel to occur safely. During the early days, while he was engaged in an experiment, an accident triggered a complicated chain of temporal paradoxes that caused him to be his own aunt Marie.
Nevertheless, thanks to modern protocols, time travel is very safe, and its even possible to visit and interact with one’s ancestors without fear of repercussions, though to visit your grandfather you are legally mandated to sign an affidavit that you are not going to try to kill him.
There have been many attempts to avoid or shorten World War 2. There were many failed plans like kidnapping baby Adolf and giving him in adoption to the Hitler family, making him fight in WW1 in the hopes he’d be killed, or getting him interested in art so that he’d forget about politics. The only plan that worked was the one with the guy who convinced the Führer to invade Russia, thus shortening the war by 6 years. However he is not credited with that since his original intention was to kill Stalin and avoid the Cold War.
Unsurprisingly, one of the first applications of time travel was tourism. Initially, time travel was extremely expensive, but eventually new companies appeared that lowered prices dramatically with a low-cost, no-frills approach. This means that now everyone can afford to travel all along the timeline for very cheap, assuming they can deal with some discomforts. For example, some companies charge extra for checked baggage. Other tactic those companies use is to advertise that they take you to popular dates but they really take you to a few years before and then you have to wait the rest of the time.
An unfortunate result of the democratization of time travel was that the most popular dates soon became swamped with tourists, and regulations limiting access to them had to be enacted. In an egregious case, so many people travelled to Bethlehem for the birth of Jesus that, when Mary and Joseph arrived, all the inns were full and they had to lodge in a cave.
Nowadays time travel is a regulated but flourishing industry that gives jobs to hundreds of thousands of people around the world. It will be shut down next year because someone will travel to the past and make big bets on all the winning teams, ruining it for everyone else.