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 USSR 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.).

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. The date is further confused by certain residents of Megaton in Fallout 3: occasionally you can hear the phrase, "Don't let them fool you with their hippie crap." While the term 'hippie' existed as early as 1945, it didn't enter the popular lexicon until the 1960s in our reality. However, it could be possible that some events remained the same in the timelines (hippies protesting against the China and United States conflict rather than the Vietnam War).

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.

A demonization of communism, which was common during the Second Red Scare of the 1950s, is apparently also a part of everyday life. For example Liberty Prime in Take it Back! proclaims, "Death is a preferable alternative to communism!", or "Better dead than Red!" 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.

Washington, D.C.
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 Washington Monument is made of a wooden skeleton instead of masonry stones, the Capitol Building never had a pond built in front of it, and its design is slightly different (stairs leading directly up the building, instead of two separate sets of stairs), the Jefferson Memorial is farther away from the Mall than in our reality, 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 area is dominated by overhead highways, which stand in ruins.

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 may have never been developed at all. As a result, the computers in Fallout are all of the old reel-to-reel tape type 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 user interface operating systems. Their small size and use of one personal computer on every desk in some of the buildings suggests that there had been progress in semiconductors at least just before the Great War and that even though there was a lack of development into the semiconductors, the networking allowed companies to either have simple personal computers available on desks for employees, or most likely had several terminals networked to a mainframe computer or server. Consumer television sets and radios also failed to evolve past the early 1960s level. Post war, it could be conceived that the microprocessor was indeed invented, as androids would undoubtedly require miniaturized electronics in order to function. Whether this technology was limited to the Institute and possibly the Commonwealth is currently unknown.

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 and equipment
All three Fallout games use a combination of fictional weapons and weapons familiar or identical to real-world examples. Fallout 2 included large numbers of real-life weapons, which should be disregarded in the divergent timeline. 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). You may also notice the R91 Assault Rifle appears to have a resemblance to the Heckler & Koch G3 and FN FAL. 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 frightened civilians and hungry armies alike.

The Fat Man is another interesting example, as it is apparently based on either the WWII PIAT or the Davy Crockett Recoilless 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. Unless used from a distance, where there is hardly any use, as its accuracy is really rather bad. Also you would need to have large open spaces, like in the wasteland where there is hardly any targets worth hitting with a mini nuke.

Nuclear weapons as a whole also differed from our universe, in that they always remained in bomb form, instead of being developed into missiles. (There is one exception: the Minuteman ICBM, launchable from Fort Constantine.) Research into missile borne nuclear weapons was conducted, and it is suggested that nuclear capable countries, such as the USA and China had begun converting their nuclear stockpiles to missile form by the Great War. Nuclear weapons took the form of old 40's/50's style bombs similar to the Fat Man dropped on Nagasaki in World War II. Although miniaturization was accomplished in the Fallout universe, nuclear bombs still retained the Fat Man-esque shape. The damage they do is also different. Washington DC was said to be bombarded by nuclear bombs, yet the city has standing ruins and only the White House is a crater, and the underground infrastructure is mostly intact.

Aircraft are not frequently found in the Fallout universe but there is evidence their designs have not changed remarkably since WWII. The Chinese bombers glimpsed in Operation: Anchorage appear even more anachronistic. They appear to be a development of the 1933 Soviet Kalinin K-7. This aircraft flew as a prototype only but in the world it seems the design was passed from the Soviets to their Communist neighbours in China who developed it further into an effective bomber. Aircraft from the 1950s such as the P-80 Shooting Star, long retired in our timeline, continued to serve in the US Air Force with few modifications until the Great War. Interestingly the P-80 seems to have been "navalised" for use on aircraft carriers as folding winged versions are seen on the flight deck of Rivet City. A plane, which is clearly a P-51 Mustang, can be found in the Capitol Preservation Society in Rivet City, whether this is recovered from a museum or not is not clear. Jet propulsion has been researched and implemented, as the bombers seen are jet powered. In Vernon Square there is what appears to be a crashed rocket plane but whether this is a manned vehicle or a winged missile is unclear. It closely resembles the German A4b Intermediate range boost-glide missile developed during WWII from the V2 ballistic missile.

Another, albeit rather minor, difference in the Fallout universe is the approach to plastic polymer use. Syringes are glass and reusable, Stimpaks essentially come in a glass vial inside a metal casing etc. Although water seems to come in PET bottles, the shape and size of the bottles themselves suggest they are mainly part of laboratory equipment, which in turn would suggest plastic was not considered a viable alternative to glass. However, the military has widely employed polymers - combat armor is made of advanced defensive polymers, as is the T-51b Power Armor and later models in the line - although shortages in material, due to the war with China, forced the US industry to cut corners and certain series of armors were produced out of steel.

Physics in a different world
Scientific and physical factors of the Fallout world are bent to reflect the science of 1950s science fiction. 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 fantasy "Science!", rather than mathematical physical science, is dominant. In our world, we know that exposure to ionizing radiation merely causes disruptions to cell mitosis (brought on by faulty DNA replication because of the shifting of nucleotides (mainly thymine) by radiation particles), causing cancer, death, and other bizarre conditions. 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 portrayed in 1950s pop fiction. For instance, in the Fallout universe, slight irradiation functions like a preservative 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!). 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.