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 both timelines (hippies protesting against the Sino-American conflict rather than the Vietnam War).

Instead of working to develop supercomputers and miniaturized electronics (in the process creating the first semiconductor, the transistor, in 1947), post-World War II humanity in the Fallout universe instead invested its technological efforts in further harnessing the atom, inventing nuclear fusion power and an enhanced form of nuclear fission. 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 1950's, is apparently also a part of everyday American life in the Fallout universe. 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 the decade just before the Great War in 2077, while its successor, the semiconducting microprocessor chip, may have never been developed at all. As a result, the digital 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 rather than the liquid crystal displays now common in our own universe. These computers are very advanced in their processing power, indicating that progress continued in computer science 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, which first appeared commercially in our world in the 1980's. Their small size and use of one personal computer on every desk in some of the buildings suggests that there had been progress in the development of semiconductors like the transistor at least just before the Great War and that even though there was a lack of development in miniaturizing 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 larger mainframe computer or server. Consumer television sets and radios also failed to evolve past the early 1960's level and television in the Fallout universe remained in the same monochromatic hues as its computer screens. Post-Great 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, nuclear fusion, robotics and directed energy weapons), other aspects of computer science proceeded unhindered, such as robotics, the Internet, and communications 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 weapons of that era would exist in Fallout. That said, given that a weapon manufactured in 1941 would be 220 years old by the time Fallout began in the mid-twenty-second century, finding a functioning World War II-era weapon is unlikely, unless such a weapon was manufactured after the Great War due to iys greater 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-Great 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 in our universe during the 1950's 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 utility, as its accuracy is poor. Also you would need to have large open spaces, as in the Wasteland where there are hardly any targets worth hitting with a miniature nuclear weapon.

Nuclear weapons as a whole also differed from our universe, in that they remained overwhelmingly in aircraft-delivered bomb form, instead of being placed atop ballistic missiles. (There is one exception in-game: the Minuteman ICBM, launchable from Fort Constantine.) Research into missile-borne nuclear weapons was certainly conducted as part of the United States' and other nations' space programs, and it is suggested that nuclear-capable countries, such as the USA and China, had begun converting their nuclear stockpiles to ballistic missile form by the time of the Great War in 2077. Nuclear weapons took the form of old style bombs used in the 1940's and 1950's, similar to the Fat Man (approximately a 20 kiloton yield) dropped on the Japanese city of Nagasaki in August 1945 during World War II. Although electronic miniaturization was accomplished in the Fallout universe, nuclear bombs of the late twenty-first century still retained the Fat Man-esque shape. The physical damage they do is also different. Washington, DC was said to be bombarded by multiple Chinese nuclear bombs, yet the city has standing ruins and only the White Househas been reduced to a crater, and the subterranean infrastructure of the city is mostly intact, indicating that the nuclear bombs dropped on the city had a very low yield, in the 15-20 kiloton range, if not even smaller or that the weapons were calibrated to produce mostly radiation rather than thermal shock, similar to the neutron bomb developed by the United States in our own universe in the late 1970's.

Aircraft are not frequently found in the Fallout universe but there is evidence their designs have not changed remarkably since the immediate post-World War II era of the late 1940's and early 1950's. 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 only as a prototype and then was discarded as impractical at the time but in the Fallout universe it seems the design was passed from the Soviets to their Communist neighbours in China who developed it further into an effective jet-powered strategic bomber by 2077. Combat aircraft from the 1950's such as the P-80 Shooting Star, or at least jet fighter aircraft with that distinctive shape, long retired in our own timeline, continued to serve in the US Air Force, with some modifications, until the time of the Great War. Interestingly the P-80 seems to have been "navalised" for use on American aircraft carriers as folding-wing 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, though whether this was recovered from a museum like the Museum of Technology or not is not clear. Jet propulsion has been researched and fully implemented, as the Chinese bombers seen are jet powered. In Vernon Square in the DC ruins 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 World War II from the V2 ballistic missile.

Another, albeit rather minor, technological difference between the Fallout universe and our own is the approach taken to plastic polymer use. In the Fallout universe, plastic is far less prevalent than in our own, with glass and metal alloys being the materials of choice. Syringes are glass and reusable, stimpaks essentially come in a glass vial inside a metal casing, etc. Although water seems to come in plastic 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 American military has widely employed plastic 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 American defense industry to cut corners and certain series of armors were produced out of steel rather than the advanced polymers.

Physics in a different world
The physical laws of the Fallout universe are bent to reflect the science of 1950's pulp science fiction rather than actual twenty-first century science. The laws of physics in the Fallout universe are fundamentally different from our own. The "World of Tomorrow" theme is not limited to what technologies exist and how the history of the world unfolded differently in the twentieth and twenty-first centuries; 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 high levels of 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), that leads to radiation sickness, cancer, and other usually deadly conditions. In the Fallout world, however, severe radiation exposure is not always fatal, and it occasionally produces unlikely or impossible mutations including increased size and, in the case of ghouls, extremely long life span and increased physical durability, coupled with an externally decaying body. Classic 1950's horror 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 whimsical take on basic scientific principles. 

All science behaves the way it was popularly portrayed in 1950's popular fiction. For instance, in the Fallout universe, slight irradiation functions like a preservative to keep pre-Great 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 are capable of burning targets to a pile of ash--the famed "death ray" of 1950's pulp fiction and B-films. Robots are capable of hovering about using tiny jet thrusters that never run out of fuel, presumably due to some sort of direct thrust generated by microfusion power cells.