The Time Machine
The defining rule break of this world is the existence of time machines. These work according to some very specific rules, which in turn result in all of the divergences from our world (it’s otherwise the same as the real world). I should warn beforehand that this is a fairly large amount of text.
The time machine takes the entire area inside it and transports it back into the time and location that you specify. Whatever was originally in that location is completely overwritten and erased from existence. The original timeline will still exist, and the time machine does not move through time – think of it like a microwave oven rather than an elevator.
Because the existing time machines are in almost constant use, the standard model of the universe is now a tree of timelines that cause each other. To a normal person, this is seamless – you don’t notice yourself hurtling in different directions – but the effects of it are widely felt, because information (and less commonly, objects) get sent down from the future all the time. In this world, all of the time machines are controlled by a single company, but I believe the implications would be roughly the same even without that wrinkle.
Political Implications: Scandals come out much more freely, since any scandal that happens in the future with sufficient documentation can be used as condemnation of a public official. Additionally, the effectiveness of any proposed program can be known ahead of time, so changes are made much quicker and problems fixed far faster. However, basic research is far less funded, because people would rather get information from the future instead of paying for it in the present (tragedy of the commons). Game theory prospers because of how common equilibrium states arise. People from the future start out as non-citizens, and are usually segregated from the general population until they meet some criteria for citizenship. There has been some significant population drain from people leaving for the past, but no one is sure what’s to be done about that.
Media Implications: Commercial mass media, with the exception of news, is dead. Live entertainment flourishes, but it’s nearly impossible to make money on pure information entertainment, because as soon as you start a novel it’s likely that a copy of it will come back in time, making your work on it redundant. There is a deluge of content, which is free because of pirates and the inability to make a copyright claim on something that you haven’t written. Most of the money remaining in media is from news (which needs constant revisions), people who own pre-time-travel copyrights for exploitation, people who sell physical things like sculptures, and those that have alternative income streams (patronage or merchandising).
Personal Implications: More than half of new relationships begin with people finding their mates through foreknowledge. It is fairly common to send messages back in time to yourself, usually through one of the co-op messaging companies. Recursive knowledge is very helpful here, and among other things, helps people to be happier in their choices, turns them on to experiences that they enjoy, and generally give themselves warnings and fix their lives. This is in addition to the near-elimination of preventable diseases, major accidents, and all that comes with that. There has also been a big increase of interest in the past, with many people choosing to travel there for various reasons (set right what once went wrong, live in a simpler time, fame and fortune through foreknowledge, etc.). Secrets are much harder to keep.
Economic Implications: The stock market exists in a radically altered form, as now everyone has foreknowledge about how the stock market might behave. Market losers lose more faster, while market winners win more faster, causing some extremes due to the feedback loops. There is still some risk involved, as equilibrium is not easily arrived at with the multitude of factors going on. Future technology somewhat eliminates the need for R&D, which in turn stifles R&D, which ensures that the march of technology is about as fast as it was prior to the invention of time travel. R&D has also slowed down because of the complete shattering of copyright/patents, which can no longer be protected by the government. The economy is losing some population and skilled workers, who are traveling into the past, which is slightly offset by people coming in from the future and going through immigration. The invention of time travel caused a depression, but the economy is now in a slow recovery.
Security Implications: Because the bit of spacetime that you transport backwards completely erases whatever was there before, the time machines can be used as a weapon to kill people with near impunity. This is slightly alleviated by the regulation that demands a nearly indestructible identifying plastic strip be sent back with every transport post-2000, but that’s still quite easy to get around. If you’re someone who is at high risk for assassination, you will likely want to keep your location secret all the time, or randomized, so that would-be future assassins can’t disappear your head by sending a cube of air back in time. (There is no direct benefit from assassinating someone in this matter, since the person who sent it stays in the original timeline, but there are game theoric reasons.)