The Fallout world is an anachronistic setting that diverged from our own sometime after World War II. The base concept for the setting is a 1950s 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 1950s thought things would be like in a future decimated by nuclear war. As such, science also works a little bit different in the setting.
 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 1950s: massive tail-finned and chromed behemoths, yet powered by nuclear fusion engines. While there are many desktop computers similar to our own (albeit on a par with those which became available to us in the 1980s, with monochromatic screens and apparently tiny memory capacities), the major computers are still giant banks of machines and use reel-to-reel tape storage. However, the pinnacle models somehow are more powerful than our universe's supercomputers. Clothing styles, architecture, building interiors and furnishings remained heavily influenced by the culture of the American 1950s, and popular styles of this period such as art-deco, googie, and futurism remained prevalent. Posters and signage also largely hearken to this decade. Radio, rather than television, 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.).
Instead of working to develop miniaturized electronics, post-World War II humanity in the Fallout world invested its technological efforts in massive supercomputers (e.g., ZAX supercomputers), 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 and McCarthyism, common to both worlds during the 1950s, remains a part of everyday North American life in the Fallout world. Prominently, Liberty Prime during Take it Back! proclaims, "Better dead than Red!" and many other pro-democracy and anti-communism phrases.
 Historical 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 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," and graffiti on the outside of the Hidden Valley bunker in Fallout: New Vegas contains anti-nuclear, pro-peace slogans reminiscent of those used by the '60s counterculture. While the term 'hippie' existed as early as 1945, it didn't enter the popular American lexicon until the 1960s in our reality. Clearly, in the Fallout world, something similar to the rise of anti-war hippie culture in our world also occurred.
There are several notable differences between the politics in the Fallout world and our own.
 United States politics
The United States of America itself changed drastically after the divergence of the timelines, evidenced by the American flag. Sometime between 1945 and 1969, the 50 states were consolidated (And in California's case, split up) into 13 commonwealths, although it appears that the states themselves retained some degree of sovereignty, judging by the Nevada state flag outside of Doc Mitchell's home (As well as the various license plates that are adorned with state names rather than commonwealth names).
 World politics
The Union of Soviet Socialist Republics is still a political entity by the time of the Great War. In our world, the U.S.S.R. was dissolved in 1991, with Russia and the various Soviet republics becoming independent nations once more.
The People's Republic of China still resembles the China of our world during Chairman Mao's rule, and it seems that the country never experienced the liberalization and free market reforms that it did in our world after the United States reproached the Beijing government in the 1970s.
 City design and architecture
City design in the Fallout world differs from that in ours. Washington, D.C., 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 1940s/'50s art deco & 1950s/'60s 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 Smithsonian 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 1950s and 1960s 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 destroyed and totally forgotten. Factories remain fairly common, as was the case during the American industrial economy of the 1940s-1960s, 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 1980s. UIs remained fairly basic command-line affairs, and less frequently advanced voice interfaces (as per computers in 1950s science fiction) were developed for computer systems which made a GUI redundant on such systems.
However, even though computers remained rather large and clunky, complex Artificial Intelligences were developed. The AIs that have appeared in Fallout games have shown to be rather advanced and possessing creativity, desire, and, in many cases, emotion.
Television sets and radios also failed to evolve past the early 1960s 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. Interestingly enough, however, the CCTV cameras seen in Fallout games are just as compact as those in the real world.
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.
 Harnessing the Power of the Atom
In the Fallout world, nuclear power was not only used for atomic bombs, it was also harnessed in nuclear reactors, which became a prominent source of energy. Large scale fission reactors that powered whole towns as in our world existed, but were a lot more common. These power plants were smaller and they often existed underneath towns and cities, such as the one in New York that almost went into meltdown and the powerworks beneath Olney. But similar to our world where more priority was put towards making electronics smaller, more priority was put towards making nuclear reactors smaller in the Fallout universe.
They were reduced in size to the extent that they could be used in roles more typically occupied by internal combustion engines in our world, such as car engines and small electrical generators, or even in fission batteries. Controlled nuclear fusion, a source of energy which is yet to be harnessed by scientists of our world, was developed. Fusion reactors were used to power vehicles and, like fission reactors, were reduced to very small sizes for use in power cells which were the standard for powering medium energy weapons in the military. Many of these pre-War power sources are still functioning around the time of Fallout 3 and Fallout: New Vegas. One such example of functioning mini reactors are the ones in radios across the wasteland. It is seen in the intro of Fallout 3, that there is a mini reactor still pumping out energy in a destroyed bus radio. This also explains the lack of power cords on the radios and televisions in the wastes.
This nuclear obsession of the Fallout world saw nuclear energy and, to a lesser extent, radioactive materials, being utilized wherever possible; even seeing radioactive isotopes added to a type of cola at one point despite the inevitable health risks of ionizing radiation. Their world's obsession with nuclear energy ultimately ended in the destruction of pre-War society.
Along with this proliferation of nuclear technologies came the risk of radiation poisoning during accidental radiation exposure. In response to this threat, radiation treatment and inoculation technologies were developed in the Fallout universe. Such technologies have yet to be realized in our timeline beyond very early experiments.
In the Fallout world, robotics had advanced way beyond our world's understanding. So much so in fact that robots had been constructed for many purposes from simple helping hands and butlers to construction and full-fledged military combat robots, and many examples survived long enough to be a factor in the Fallout world.
Typically, robots are equipped with a basic programming structure that dictates behavior and priorities. In most cases, these rules are effectively set in stone. However, robots can also develop personalities or come pre-equipped with and AI. Some robots, such as the Nuka-Cola shipping foreman appear to have rather harsh personalities present from their first days of activation, while others, such as the protectron series seem to have little personality at all. At times, this personality seems to operate at odds with the fundamental core programming of the robot. In fact, Combat inhibitors are fitted on almost all robots to stop them from acting violently on humans as most robots who have a personality seem to an adverse attitude and urge kill any humans and anything living. With this in mind it can be safe to say that robots of the Fallout universe do not follow Asimov's three laws of robotics, or, if they do, very loosely.
Androids also exist in the Fallout universe, but did not exist before the Great War and were solely the invention of the Commonwealth. Unlike robots, Androids have advanced personalities able to display a wide range of emotions and feelings without being unwarrantly violent like robots. Androids can also learn and keep personal memories and feel pain unlike robots.
 Military technologies
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 are a few examples 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 feature no apparent way to launch missiles. This suggests that the missiles in these facilities were launched during the Great War. In The Divide, there are a large number of unlaunched ICBM's, suggesting they were prepped for launch, but never got the call during the short period of mass destruction. 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 1940s and 1950s, 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. However, in New Vegas, Mr. House mentions having defended the city from 77 nuclear missiles, suggesting that the Chinese at least had access to substantial numbers of warheads in missile form. Also, information on the computer terminals found in the Washington, D.C. Museum of Technology would suggest that the American military recommissioned the Delta IX space rockets as nuclear warhead vectors.
 Projectile firearms
Assault rifles retained designs of weapons that originated in the 1940s and 1950s, such as the AK-47 or G3. There is evidence that, despite the diverging timelines, weapons development in the Fallout universe followed a similar path to ours. Picatinny rails, an equipment system that is only recently been implemented in our universe, as well as weapons such as the China Lake grenade launchers and M79 grenade launchers, which were developed around the 1960s for the Vietnam War in our timeline. Handguns also retained similar designs to those of the early 20th century with notable exceptions, such as SIG-Sauer pistols. 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. 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 .50 BMG, .45 ACP, etc. are rare or nonexistent.
 Energy weapons
We often develop technology not because it's great immediately, but because developing that technology helps us move toward its potential. We've had various forms of hybrid vehicles (gasoline combustion engine + ???) around for a long time. Most of them were pretty bad and impractical. We've had biofuels around for a while, but most of those are STILL bad and/or impractical. We saw tanks developed in WW1 that were absolute garbage.
All of those things were kind of crummy for a while, but if we hadn't gone through the stage of "Yeah... this is... okay, I guess," we would never have reached the subsequent stages. Coil/rail gun technology used to be completely impractical. Now it's reached the stage where maybe/sorta we could mount an enormous one on a destroyer and blast through a bunker with a huge slug from miles away. We're probably not going to have Eraser- or Fallout-style Gauss rifles for a while, but we see the potential.
In the Fallout universe, I think that the military appeal of weaponry that uses a small number of more-or-less universal ammunition types would be great. Today, we have NATO standards so that allies armies can share ammunition. But what if you could use the same ammunition type for powering a sniper rifle that you'd use for a devastating close-range weapon (e.g. a Microfusion Cell powering a Laser Rifle or a Plasma Rifle)? For a military force in the field, the flexibility of that would be immense.
Anyway, I considered the EWs in F:NV to have reached the point where they were starting to replace conventional weapons, but had not yet completely eclipsed them -- sort of like the early days of firearms, when they were still being used concurrently with bows.”— J.E. Sawyer on energy weapons in the Fallout universe
Unlike in our world, handheld lasers, plasma, and pulse weapons exist in the Fallout world and are capable of burning targets to a pile of ash or into a liquefied puddle. Laser and plasma weapons in the Fallout world are far more advanced then anything our universe has made where the most advanced laser weapons made either never worked, are non-lethal crowd control, or too large to be used as anything other than defense. Also, because of the emphasis to advance laser and plasma weapons, miniaturization of laser and plasma weapons occurred. There is also evidence of alien energy based weapons such as alien blasters or the famed "death ray" of 1950s pulp fiction and B-movies.
 Orbital weapons
The Fallout universe has three prime examples of orbital weapons. An orbital missile platform in the Capital Wasteland which can be used via a terminal at a satellite station; Bradley-Hercules, a high explosive missile based orbital platform that destroys Liberty Prime in Fallout 3's expansion pack Broken Steel; and the laser based Archimedes II in Fallout: New Vegas. They both target a position on the surface and can take out any threat.
An example of a real-world orbital weapon system was the Soviet 8K69 Fractional Orbital Bombardment System, which placed a nuclear missile re-entry vehicle into low-earth orbit for an indefinite period of time and range, able to launch a strike with unprecedented speed similar to the Enclave's fictional Bradley-Hercules platform. FOBS was phased out in 1982 in compliance with the now-defunct SALT II treaty, which forbade deploying WMDs into Earth orbit.
 Other Technologies
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 1940s and early 1950s. Jet propulsion has been fully developed in the Fallout universe, as the Chinese bombers and American fighters are jet powered, and large jet engines may be spotted in Megaton's construction. Despite the development of jet power, some planes still use propellers, as evidenced by the aircraft at Camp McCarran in New Vegas.
Prosthesis in the Fallout universe is fairly uncommon, but evidence of its existence has been in every Fallout. From what can be seen from shown prosthetics in the Fallout universe, the science of prosthesis has advanced well beyond our world's understanding able to mechanically replicate the human hand that appears to have articulate joints able of a high freedom of movement. Other achievements that have yet to be achieved by our world are the entire enhancement of the human body with Implants which can accelerate muscular growth and enhance agility to improving cognition and accelerate cell growth to replace damaged tissue.
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.
 Physics in a different universe
The laws of physics in the Fallout universe are different in certain aspects from our own, bent to reflect the Science! of 1950s 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 1950s 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
- Fallout Bible 6 Questions I will not answer: "3. What was U.S./world history like before the timeline included in previous Fallout updates?"
"No one has asked this yet, but I thought I would cut this question off at the pass. Fallout takes place on a future earth, in an alternate timeline. I will not be including any information on how and when it diverged - it will remain one of the mysteries of the setting. Just let it be known that it diverged after WW2, and leave it at that."
- Etymology of Hippie on Wikipedia
- Experimental Treatments for Radiation Poisoning