Time's Corrective Mechanism
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A lot of people ask me things like "Why can't you just bring back some lottery numbers?" or "Why not just invest in the stock market?"
Well, a lot of people have tried. There's an automatic corrective mechanism in place in the universe that keeps things from getting too out of balance.
I've known of some people who tried playing the stock market and they lost a bunch of money attempting it. The info they had was good, but something just changed retroactively, and they lost their shirts.
Some have tried smuggling things like books and music from the future, but whenever they try to use them for their own gain, something happens, even if it's just a string of "bad luck" that stops them.
Whenever you bring a foreign element from the future into the past, it creates an imbalance. Even if it's as small as a single dollar, it creates a ripple effect. Those ripples extend far beyond in ways that you wouldn't think of.
You spend that dollar at a store, and that single dollar could be the tipping point needed for the owner to be able to buy more stock of an item at a bulk discount, enabling them to make more profit. Then more people are able to buy more things from that store. Suddenly, the store owner has more money than they normally would have had, and starts spending it elsewhere at other stores.
Now those stores have more money than they should have had, and the cycle begins again, only on a much larger scale. Money that shouldn't have been there, snowballing into a huge mess that keeps expanding. All started with a single dollar.
That's just one example of something that could happen, but what often actually happens are angles that aren't immediately apparent until something goes horribly wrong way down the timeline, like lives being lost.
Once that imbalance is introduced, those loose end ripples must be tied up. It's not always difficult, but it's extremely time consuming and tedious. The cost of the personnel required, and the energy used to power the time machines to correct those ripples, far exceed a single dollar by a long shot.
So imagine winning the lottery and having millions upon millions of those dollars, creating insane amounts of ripples all over the place. It might never end, and you'd spend all your time chasing and fixing those ripples instead of doing the things you want to do instead, while the expense of the resources to fix it all vastly exceeds the lottery winnings.
It's also the reason why memory compartmentalization was required, and years of training needed so travelers don't affect the past inadvertently.
The past can indeed be changed, it just has to be done carefully so you don't make a mess of things.