Divergence

One of the most common misconceptions about the Fallout world is that it is just the future of our own world and that it works just like ours. Another misinterpretation is that the Fallout world is the result of a nuclear war between the USA and Russia that occurred in the 1950s of our timeline. Both of these ideas are entirely false, but could arise partly due to game artwork.

The Fallout world is an anachronistic setting historically divergent from our own, and also is fundamentally different in terms of how science works. The base concept for the setting is a 1950s World of Tomorrow, decimated by a global atomic war. This means that before the war, the Fallout world was more or less what the people of the 1950s thought things would be like in 2077, a future as envisioned through the lens of the Atomic Age and Jet Age. This means hovering housecleaning robots and laser guns were the norm, and automobiles looked like Motorama concept vehicles from the 1950s: massive tail-finned and chromed behemoths but with nuclear fusion engines. At the same time, clothing styles and building interiors and furnishings apparently remained very much stuck in the 1950s. Posters and signage also largely harken to this decade. Radio remains the most common mass media, and food products are based on those popularized in the TV dinner era (boxed macaroni and cheese, canned meat, Salisbury steak TV dinners, etc.).

City design also differed. Washington, DC, for example, looks similar in terms of placement and design, but has some noticeable changes. Much of the pre-war contemporary architecture is 1940s/'50s art deco & 1950s/'60s modernist. The National Mall is smaller and narrower, the skyscrapers that define Arlington in our reality do not exist here, the National Archives is located farther from the Mall than in our reality, buildings such as the Air and Space Museum are replaced with others (in this case the Museum of Technology), and Randian busts of unknown persons are located on many buildings. Many buildings and memorials built since the '50s and '60s in our timeline (such as the Vietnam War Memorial, the WWII Memorial, Nationals Park, Kennedy Center and the Newseum) either were never built or (less likely, given how well many of the other buildings survived) were both completely destroyed and totally forgotten by all. Factories remain fairly common, as was the case during the smokestack economy of the '40s-'60s, and while heavily automated are still quite primitive by today's standards.

The Divergence of the Timelines
The exact historical details of the divergence and even the exact moment when it occurred are unknown, but it is known that it happened at some point after World War 2. The sparse evidence scattered throughout the games suggests that the break occurred between 1947 and 1955, although this remains speculative.Further speculation is added to the exact date when you talk to megaton settlers in fallout 3. they can say "don't let them fool you with your hippie crap" as the split from our reality and there's supposidly happened in the 50's when the hippies were a 60's movement.

Instead of working to develop supercomputers (in the process creating the semiconductor, as was the case in our universe), humanity invested its technological efforts in further harnessing the atom, inventing fusion power. Fusion power allowed the Fallout world a clean, renewable, plentiful and portable source of power. This meant that things like power armor and energy weapons could be built, as well as all the housekeeping robots. Many such power sources continue to function hundreds of years after their construction.

In addition, a demonization of communism, which was common during the Second Red Scare of the 1950s, is apparently also a part of everyday life, with blame for unfortunate events being aimed at China or communism and with the patriotic robot in Take it Back! proclaiming "Death is a preferable alternative to communism!" among other humorous and memorable propaganda. This is apparently largely due to a massive propaganda campaign launched by the U.S. government during the Resource Wars, referred to in a recovered memo found on a computer in The Citadel.

Computers that fit in a single room!
One of the major divergences is that in the Fallout world, the rapid miniaturization of computers and electronics never occurred. The transistor was not developed until just before the war, while its successor the semiconducting microprocessor chip was never developed at all. As a result, the computers in Fallout are all of the old reel-to-reel tape drive and mixed vacuum tube/transistor type and are generally very large and bulky, while displays are small monochromatic cathode ray tubes. These computers are very advanced in their processing power, indicating that progress continued in this field albeit at a slower rate than in our universe, but the technology to make them smaller never emerged, nor did user-friendly icon-based graphical interface operating systems. The Pip-Boy is a strong indication of this, as it represents 2070's state-of-the-art in miniaturized electronics, yet lacks the functionality of even an obsolete PDA or cell phone. Consumer television sets and radios also failed to evolve past the early 1960s level.

Various references to uploading and downloading, as well as E-mail and networked communications also demonstrate that though the Fallout universe lacks our mastery of microprocessor technology (in the same way we lack their mastery of nuclear fission and fusion), other aspects of computer science proceeded unhindered, such as robotics, the Internet, and satellites.

Arms & Equipment
All three Fallout games use a combination of fictional weapons and weapons familiar or identical to real-world examples. The advantage with using fictional weapons, beyond simply respecting the timeline divergence, is that it allows designers to create the weapons they need to fit the game. The benefit of having an alternate reality is that these familiar weapons can be used, tolerating inconsistencies with "real-life" firearms. For instance, in Fallout 3, there is no practical use for firing automatic weapons in controlled bursts, since all weapons have a predetermined spread which affects even the first shot, so engaging in distanced combat is not the same as modern warfare.

Given that the historical divergence occurred soon after World War II, it is possible that war-era weapons would exist in Fallout. That said, given that a weapon manufactured in 1941 would be 220 years old by the time Fallout began, finding a functioning wartime weapon is unlikely, unless a wartime-designed weapon was manufactured after the Great War due to simplicity of production (as with Tommy Guns). The similarity between the Chinese Assault Rifle and the Russian AK-47 is more than aesthetic; there is no reason to doubt the simple and robust design is obvious to the alternate-future Chinese. You may also notice the standard US Assault Rifle appears to have a Heckler & Koch-styled barrel and sights. Also, the reliable .32, .44, 5.56, and .308 cartridges present today can easily be expected to last well into the future, and there is no doubt of the massive number of cartridges that pre-war armories churned out to frighten civilians and hungry armies alike.

The Fat Man is another interesting example, as it is apparently based on either the WWII PIAT or the Davey Crockett Recoiless Tactical Nuclear Rifle, a piece of field artillery which was built and tested but abandoned because of an obvious flaw. This is apparent to anyone who has used it in game: the user tends to be only slightly less irradiated than the recipient.

Physics in a Different World
The Fallout world does not merely diverge historically. The laws of physics in the Fallout universe are fundamentally different. The "World of Tomorrow" theme is not limited to what technologies exist and how history unfolded; it also applies to the laws of physics, where Science!, not science, is dominant. In our world, we know that exposure to ionizing radiation merely causes disruptions to cell mitosis (resulting in faulty DNA replication), causing cancer and death. In the Fallout world, however, severe radiation isn't always fatal, and it occasionally produces unlikely or impossible mutations including increased size and, in the case of ghouls, extremely long life coupled with a decaying body. Classic movies like Them or The Fifty Foot Woman, in which freak nuclear accidents caused giant ants or people to appear, are good examples of the Fallout universe's take on scientific principles.

All science behaves the way it was popularly thought to behave in the 1950s. For instance, slight irradiation functions to keep pre-war foodstuffs edible and unspoiled for hundreds of years. A new beverage product, Nuka-Cola Quantum, deliberately contains a Strontium isotope for lift and kick (and to create an appealing lavender glow!). Similarly, this means that there was likely no nuclear winter in the Fallout universe, since that theory was put forward in our 1970s and only concretized by Carl Sagan and a team of researchers in 1983.

Finally, functional compact directed energy weapons exist, with a nod to Nikola Tesla's research, and such are capable of burning targets to a pile of ash. Robots are capable of hovering about using tiny jet thrusters that never run out of fuel, presumably due to some sort of internal fuel generating system powered by fusion cells.