Crossing over to another cinematic universe seems impossible today. Even if in the comics, superheroes from different publishers have often teamed up or even fought each other, on the screen – given the complicated ownership of intellectual property – even heroes from the same comic book universe (but at different studios) teaming up is a big thing. For now, exploring shared cinematic universes remains a dream for most of us – but how about exploring parallel universes? These things physicists tell us may exist will likely be forever inaccessible to us – unless we use our imagination. Some great screenwriters have, and the result was amazing at times. Here are some of the best.

Mirror Universe Star Trek

Pete’s World – Doctor Who

During the first season of the David Tennant era, Doctor Who visited a parallel universe – by accident, mostly. It was a world where the Hindenburg didn’t explode, leaving the skies of London filled with zeppelins. But these were not the only major differences (even though they were the most visible): Great Britain is a People’s Republic instead of a monarchy, complete with its president, South America is unified into a single country, and most of Europe is contained in the country of New Germany, suggesting the Nazis won World War II. This parallel Earth has a more advanced technology, too – communication is handled by the EarPods, similar to our Bluetooth headsets but transmitting calls and information directly into the brain. These can be used by magnate John Lumic to control people remotely.

And, apparently, this universe doesn’t have its Doctor.

The Other Side – Fringe

Another alternate universe where the Hindenburg didn’t explode is depicted in Fringe, a series created by J. J. Abrams, Alex Kurtzman, and Roberto Orci. It is a universe that reveals many differences compared to our own, from minor things like Eric Stoltz being cast in “Back to the Future” instead of Michael J. Fox and Andrew Jackson not being president, to major discrepancies like the World Trade Center being untouched by the September 11th terrorist attacks. Once again, this parallel universe is more advanced (technologically) than our own, with them having pocket-sized cell phones in the 1980s, and by the time the series’ events take place, they have daily flights to the Moon, self-flying helicopters, and rapid healing techniques have replaced traditional medicine in hospitals.

The Mirror Universe – Star Trek

There are many examples of the Mirror Universe showing up in many Star Trek series, including the latest – Discovery. Let’s take a look at its occurrences chronologically.

In Enterprise, we get to take the first peek into the Terran Empire, where Earth is a bloodthirsty conqueror of worlds and species. The Vulcans are depicted as second-class citizens, everything reeks of intrigue and backstabbing, and paranoia runs rampant. Archer stages a mutiny to recover an Earth ship – the USS Defiant, see below – that is far more advanced than anything available to the Terrans at this point in history. The mission is successful even though the Enterprise is destroyed. Archer and the crew use the Defiant to win the war and install a new Emperor – at which they succeed but not without a twist.

In Star Trek: Discovery, the ship ends up in the Mirror Universe as the result of an accident. They need to retrieve the records of the same USS Defiant described above in order to be able to return to their own universe. Michael, disguised as her Mirror Universe counterpart, is sent to complete the mission. During this time, she gets to meet the Mirror Universe counterpart of Captain Georgiou, who is now the Emperor of the Terran Empire.

In Star Trek: The Original Series, four crew members of the Enterprise are switched with their Mirror Universe counterparts. They have to avoid being discovered by acting as if they would belong there. In Season 3 of the series, the USS Discovery disappears trapped in the Interphase in Tholian space. It is the same ship that shows up in the later series repeatedly.

In Deep Space Nine, we get to know the Mirror Universe counterpart of the Bajoran area, 100 years after Captain Kirk’s adventure there. By now, the Terran Empire is defeated, replaced by the Alliance of Klingons as the ruling military force. This time, the humans are treated as slaves and second-class citizens until a rebellion – with the help of Captain Sisko’s crew and some other individuals (like the Ferengi Quark and his brother Rom) – manages to take the upper hand.

The idea of the Mirror Universe didn’t show up in iconic series like Star Trek: Voyager and The Next Generation – perhaps this is a task for later series in the same universe – but it was extensively explored in books, comic books, and video games.