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. The Fallout world is more or less what Americans of the 1950's thought things would be like in a future decimated by nuclear war, and if the mindset of the 1950's never vanished.

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 fission engines. Computers are still giant banks of machines and use reel-to-reel tape 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, rather than televison, 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 American lexicon until the 1960's in our reality. Clearly, in the Fallout universe, something similar to the rise of anti-war 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 compact nuclear fusion power generators and an enhanced and miniaturized form of nuclear fission, as well as more advanced robotics, cybernetics and genetic engineering than we currently possess in our universe. This meant that things like Power Armor and laser weaponry could be built, as well as the large number of housekeeping robots used by many Americans before the Great War. 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 North 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 skyscrapers that define Arlington, VA in our reality do not exist; buildings such as the Smithsonian National Air and Space Museum are replaced with 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, invented in our world in 1947, was not developed in the Fallout universe 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 that take up large amounts of room. The mixed vacuum tube/transistor personal computers used on desktops are very large and bulky, while displays are small monochromatic cathode ray tubes rather than the liquid crystal flat panel displays now common in our own universe. Data is stored on holotapes or holodisks which electromagnetically store information as three-dimensional digital images. 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 (GUI) 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. Another example of technology failing to evolve can also be found in cameras. Cameras in the Fallout universe are big and bulky with large flash bulbs like those used in the late 50s and 60s.

Androids like those found in the Capital Wasteland would undoubtedly require miniaturized electronics in order to function. Whether this technology was limited to the possession of the scientists working at the organization known only as the Institute and 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 of the Fallout games use a combination of fictional weapons and weapons similar or identical to real-world examples. The games vary in their faithfulness to canon and logic in their choices of which weapons are included as well as in how the weapons' characteristics have been modified from their real-world counterparts.

Nuclear weapons
The development of nuclear weapons in the Fallout world differed from our universe in that the majority of nuclear weapons remained in aircraft-delivered bomb form, while 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 in Fort Constantine. Other areas, such as Fort Bannister and the Wheaton Armory, have missile silo doors that are similar to the one found at Fort Constantine but have no way to launch missiles. It is suggested that nuclear-capable countries, such as the U.S. 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 dropped on Nagasaki in 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.

Firearms
Assault rifles retained designs of weapons that originated in the 1940s, such as the AK-47 or G3. Handguns also retained similar designs (the Chinese pistol being based on a Chinese copy of the Mauser C96, a pistol introduced at the end of the 19th century) though the N99 10mm pistol has similarities to the IMI Desert Eagle, which wasn't introduced until the 1970s of our timeline (and the Desert Eagle itself appears in the first two games as a rather common weapon, suggesting it was produced in much larger quantities than in real life). Heavy weapons such as missile launchers and the Fat Man, had unique designs such as side mounted foregrips and pneumatic ammunition loading systems. Weapons that seem impractical in our timeline, such as nuclear catapults and man portable miniguns, were extensively developed in the Fallout timeline and issued to frontline troops. The development of miniguns likely led to the decreased dependance on LMG squad support weapons, as the miniguns were of equivalent weight. LMGs are not common in the Fallout timeline. Ammunition calibers that are not common in our timeline, such as the .32 caliber and 10mm, are widely used in the Fallout era, while common ammunition in our timeline, such as 9mm Parabellum, .357 Magnum, 7.62x39mm, .45 ACP, etc are rare or nonexistent.

Aircraft
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. However, in the Fallout universe it seems the design was passed from the Soviets to their Communist neighbors 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 U.S. Air Force, with some modifications, until the time of the Great War. Interestingly the Shooting Star seems to have been "navalized" for use on American aircraft carriers, as folding-wing versions are seen on the flight deck of Rivet City. Helicopters have been fully developed in the Fallout universe, as the helicopters seen are all jet powered. Interestingly the helicopter that is used by the Enclave are based on the V-22 Osprey tiltrotor aircraft designed in the 1980's.

Plastic
Another 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 come in a glass vial inside a metal casing, etc. Although water seems to come in plastic Polyethylene Terephthalate (PET or PETE) bottles, the shape and size of the bottles themselves suggest they are mainly part of laboratory equipment. 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.

Laser weapons
Lasers and plasma guns exist in the Fallout world and are capable of burning targets to a pile of ash or into a liquefied puddle--the famed "death ray" of 1950's pulp fiction and B-movies.

Physics in a different universe
The laws of physics in the Fallout universe are fundamentally different from our own, bent to reflect the Science! of 1950's pulp science fiction rather than actual 20th and 21st century science. In our world, we know that exposure to high levels of ionizing radiation leads to radiation sickness, cancer, and other deadly conditions. In the Fallout world, however, severe radiation exposure is not always fatal, and it occasionally produces 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 Attack of The 50 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.

However, as the full extent of mutations created by ionizing radiation is as of yet not totally understood, one could argue that the effects of it in the Fallout universe could somewhat mimic the effects in ours. Generally the effects of ionizing radiation is indeed creating mutations in the genome of whatever living organisms it hits. Because in the Fallout universe a lot of products use ionizing radiation to some extent and nature eventually finds a way to adapt to radiation, it could very well be possible that prolonged effects of radiation cause mutations we're currently unaware of.