Divergence

The Fallout world is an anachronistic setting historically divergent from our own and fundamentally different from our universe in how the laws of science work. The base concept for the setting is a 1950's World of Tomorrow, a future as envisioned through the lens of the Atomic and Jet Ages. In 2277, the Fallout world is more or less what the people of the 1950's thought things would be like in a future decimated by nuclear war.

World of tomorrow
The Fallout world is home to hovering housecleaning robots, and the use of laser guns is the norm. Automobiles look like Motorama concept vehicles from the 1950's: massive tail-finned and chromed behemoths, yet powered by nuclear fusion engines. Computers are still giant banks of machines and use tape for storage. Clothing styles and building interiors and furnishings remain very much stuck in the culture of the American 1950's. Posters and signage also largely hearken 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 historical details of the Divergence and the exact moment when it occurred are unknown. What is known is that it happened at some point after the end of World War II in 1945. The date is hard to pin down, because, even after the Divergence, the two timelines are not entirely different. For example, certain residents of Megaton in Fallout 3 occasionally utter 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. Clearly, in the Fallout world, something similar to the rise of hippie culture in our world also occurred.

Instead of working to develop supercomputers and miniaturized electronics, post-World War II humanity in the Fallout universe 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, common to both worlds during the 1950's, remains 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!", and "Better dead than Red!"

Washington, D.C.
City design in the Fallout universe differs from that in ours. Washington, DC, for example, looks similar to the American capital city of our world in terms of the placement of signature buildings and overall urban design, but has some noticeable changes. Much of the pre-Great War contemporary architecture is 1940's/'50's art deco & 1950's/'60's modernist. The National Mall is smaller and narrower; the Capitol Building doesn't have a pond 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; the skyscrapers that define Arlington, VA in our reality do not exist; the National Archives building is located farther from the National Mall; buildings such as the Smithsonian National Air and Space Museum are replaced with others (in this case the Museum of Technology); and busts of persons apparently famous in the Fallout world while unknown in ours are located on many buildings. Many buildings and memorials built since the 1950's and 1960's in our timeline (such as the Vietnam War Memorial, the World War II Memorial, Nationals' Park, the Kennedy Center and the Newseum) either were never built or were completely destroyed and totally forgotten. Factories remain fairly common, as was the case during the American industrial economy of the 1940's-1960's, and, while heavily automated with robotics, are still quite primitive by our present-day standards.

Computers that fit in a single room!
One of the major divergences from our own history is that, in the Fallout world, the rapid miniaturization of digital 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. The mixed vacuum tube/transistor computers are 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.

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 and other highly advanced digital technologies were indeed invented, as androids like those found in the Capital Wasteland would undoubtedly require miniaturized electronics in order to function. Whether this technology was limited to the scientists working at the organization known only as the Institute and possibly its location in the Commonwealth, is unknown.

Various references to uploading and downloading, as well as to e-mail and networked communications, also demonstrate that though the Fallout universe lacks our mastery of microprocessor technology, other aspects of computer science proceeded unhindered, such as robotics, the development of the Internet, and orbital communications satellites.

Arms and Equipment
All three Fallout games use a combination of fictional weapons and weapons similar 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 of 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 similar to those in the Wasteland where there are hardly any targets worth hitting with a miniature nuclear weapon.

The development of nuclear weapons in the Fallout world also differed from our universe, in that the majority of nuclear weapons remained in aircraft-delivered bomb form, and only a minority were miniaturized into warheads and placed atop ballistic missiles. (There is only one example of a nuclear missile in-game: the Minuteman ICBM, launchable from Fort Constantine in the Capital Wasteland.) 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. Aircraft-delivered nuclear weapons took the form of the 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 House has been reduced to a crater. 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. Another explanation could be 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 is unclear. Jet propulsion has been fully developed in the Fallout universe, as the Chinese bombers seen are all jet powered. In Vernon Square in the DC ruins there is what appears to be a crashed rocket plane, but whether this was a manned vehicle or a winged cruise 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 in consumer products. However, the American military has widely employed plastic polymers - military combat armor is made of advanced defensive polymers, as is the T-51b Power Armor and later power armor 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 Universe
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.