Shared universes and crossovers in television have been a staple of the medium almost since its creation.
Think of the entire Star Trek universe. Or the more current DC Universe with Green Arrow, Supergirl, & The Flash. All of those shows exist in their own fictional world.

Studios have long used one existing product as a way to introduce new characters and see how the public responds. That’s how Laverne & Shirley were spawned from Happy Days. And then Mork & Mindy. And then Joanie Loves Chachi. (And don’t forget the short-lived Out of the Blue and time-distorted Blansky’s Beauties.)
Even going back to The Danny Thomas Show in 1960. Danny got arrested in the tiny town of Mayberry for driving through a stop sign. Sheriff Andy Taylor and his son Opie made such a splash that they had their own show six months later.

But what if I told you that all of these smaller worlds, and many, many more, are also thought to exist in the same wider universe? Would you believe there is a theory that they are all connected? It’s true. But to get a look at the big picture, we need to travel back to Boston in the early-80’s.
St. Elsewhere was a medical drama that ran 137 episodes on NBC from 1982-1988. It was nominated for the Best Drama Emmy every year it was on the air and won numerous acting and writing awards. It broke new ground for medical dramas and set a template that would be followed years later by Chicago Hope and ER.

The cast was made up of a great blend of TV veterans and relative newcomers. Ed Flanders & Norman Floyd may not mean much to many of you, but how about William Daniels (Mr. Feeney on Boy Meets World as well as the voice of K.I.T.T. on Knight Rider)? Denzel Washington? Mark Harmon? Howie Mandel and Helen Hunt? Even Stephen Furst (Flounder from Animal House). They were all regulars on the show.
St. Elsewhere received critical praise not only for its realistic portrayal of the not-always-perfect field of medicine, but also for how it played with the medium. One episode saw Howie Mandel’s character get shot and took place in a version of Heaven, Hell & Purgatory. Another took some characters to visit family in small-town New Hampshire. Throughout the episode, one of the doctors would occasionally break the fourth wall and address the viewers directly, a’la Thornton Wilder’s play, Our Town.
And then there was the finale. Towards the end, Ed Flanders’ Dr. Westphall & his autistic son Tommy, are in another doctor’s office watching the snow fall outside. The scene moves to an exterior shot of the hospital blanketed by the falling snow. And then the whole shot begins to shake. Immediately, it cuts to Tommy sitting on the floor of a small apartment shaking a snow globe.

His father gets home from “work” and based on his attire and the dialogue, it’s clear that he does construction and not actually medicine. The Chief of Medicine is also there, but apparently he is now Tommy’s grandfather.
Father: “Hi Pop, how you doing?”
Grandfather: “Good. How was your day up on the building?”
Father: “Well, we finally topped off the 22nd story. And I’m beat. How’s he been? (referring to Tommy) He give you any trouble?”
Grandfather: “He’s been sitting there ever since you left this morning, just like he does every day. World of his own.”
Father: “I don’t understand this autism thing, Pop. Here’s my son, I talk to him, I don’t even know if he can hear me. He sits there, all day long, in his own world, staring at that toy. What’s he thinking about?”
The dad asks Tommy to wash his hands for dinner and Tommy hands him the snow globe. As the camera zooms in for a closeup, we see the snow globe holds a replica of St. Eligius – the setting for the show.

That one closeup is the basis for one of the greatest fan theories I’ve ever heard. The implication is that the entire show is purely a figment of Tommy’s imagination.
St. Elsewhere managed to take the idea of a dream and alter it just enough, putting it in the imagination of an autistic boy… remind[ing] viewers that the fiction they have watched for six years is actually fiction within a fiction, occupying a second level of unreality, one level beyond the space of illusion filled by all narrative television.
Author Cynthia Burkhead
That means that the entire six year run of the show never existed. By the properties of association, anything that was associated with the show also never happened. Keith Gow and Ash Crow began discussing these implications in 1999 and by 2000, the Tommy Westphall Theory was established thanks to a man named Munch.

Richard Belzer played Detective John Munch on Homicide: Life on the Street for six years before “moving on up” to New York’s Law & Order. Rather than just taking on a new role, they said his character had been transferred. The assumption of the Tommy Westphall Theory says that since two doctors from St. Elsewhere had appeared IN CHARACTER on Homicide, Munch must not really exist either. And since he went to a different show, the entire Law & Order universe is also a figment of Tommy’s imagination.
The “in character” part is the most important. It’s not just about actors playing different roles – they have to be the same character. Crossovers have existed in television for years, but Gow & Crow followed as many of these crossovers as they could to create this chart:

The light blue lines in the center indicate the primary point of crossover – when St. Elsewhere & Homicide connected. From there, all of the other shows can fall into place. As of August 2016, there were 441 shows that could be traced back to this original point. 441 shows that “theoretically” exist only in the imagination of one boy.
I know, I know. That’s ridiculous. That’s not realistic. But, just like all great fan theories, it’s a fun thing to contemplate and say, “What if…?”

We know for a fact that some of these shows use these characters and things exactly for that purpose – to tie them into a bit of nostalgia. To create an homage, if you will. Does that negate or perpetuate the theory? Maybe all of these are serving in some weird way as an homage to one another.
Like the Six Degrees of Kevin Bacon, just about everything can be connected in some way. It’s not just sci-fi shows or even dramas.

For example:
A few of the St. Elsewhere doctors visited Cheers. So Cheers is also part of Tommy’s imagination and doesn’t exist.

Cheers spun off Frasier, which is now imaginary.

One of the calls on Frasier‘s radio show was from The John Laroquette Show’s John Hemingway.

The John Laroquette Show referenced Yoyodyne, which was a client of Angel’s Wolfram & Hart, thereby wiping out Buffy the Vampire Slayer by association.
Another client of Wolfram & Hart is Weyland-Yutani, which made some of the weapons used by Malcolm Reynolds on Firefly.

In the Red Dwarf episode “Psirens”, the crew visit a Space ship graveyard which includes a Weyland-Yutani ship. Also in that graveyard – a Klingon Bird of Prey. Bye-bye, entire Star Trek Universe!
The Tardis from Doctor Who appears in an episode of Red Dwarf, which takes out decades worth of Who-related programming.

In Power Rangers: Lost Galaxy, they make a reference to Galifrey – which is Dr. Who’s home – rendering all versions of the Mighty Morphin’ Power Rangers as non-existent.

Here’s another example:
We’ve already established Frasier as not being “real”.
Frasier’s Daphne and Miles read Caroline in the City’s comic strip.

Caroline in the City crossed over with Friends.

Friends‘ Phoebe & Mad About You‘s Ursula are sisters.

Mad About You‘s Paul created a documentary narrated by Alan Brady from The Dick Van Dyke Show.

Buddy from Dick Van Dyke appeared on an episode of Make Room for Daddy, which was rebooted as Make Room for Granddaddy.

Make Room For Grandaddy featured a guest appearance by Lucy Carter from Here’s Lucy.

She and Mannix‘s Mannix were held hostage together on her show.

Mannix returned to TV on an episode of Diagnosis Murder.

Matlock appeared on Diagnosis Murder to help when Dr. Sloan was accused of murder.

These connections also extend to fictional places or companies as well. For example:
Matlock, All My Children, Miami Vice, Quincy ME and The Young and the Restless all feature characters traveling on the fictional Trans Global Airlines.

Young and the Restless crossed over with numerous other soap operas that are all now imaginary.
Emergency had a crossover with Adam-12 which, in turn, crossed with Dragnet.

All of them…. gone.

“Someone did the math once… and something like 90 percent of all [American] television took place in Tommy Westphall’s mind. God love him.”
St. Elsewhere writer Tom Fontana in 2003
Product placement also plays a large role in the theory, most specifically involving Morley cigarettes. They were the preferred brand of the X-Files‘ “Cigarette Smoking Man”. (X-Files ties in because Detective Munch once interviewed the Lone Gunmen on Homicide.)

The tie-in to Morley cigarettes eliminates CSI, American Horror Story, Mission Impossible, Malcolm in the Middle, Nash Bridges, Seinfeld, ER and many more.
One of the things that always impresses me about fan theories is the dedication put into it. Some theories are a single sentence and that’s it. This, however…. This is a whole new level of dedication. There have been Doctoral theses written about the possibilities and implications of this single snow globe.
And before you think this is just a free-for-all and any show can find a way to tie in, you need to understand that there are rules to the theory. For example, cartoons are not allowed. If they were, The Simpsons & Family Guy could tie into hundreds of shows on their own. Anthology series where the episodes/characters do not connect from week to week are also excluded.
Here’s one more to show how deep this rabbit hole can go:
The Bob Newhart Show‘s Elliot Carlin visited the psych ward at St. Eligius.
In the ultimate dream within a dream scenario, The Bob Newhart Show‘s Bob Hartley woke up after dreaming the entire series of Newhart.

Newhart’s Larry (and his brothers) Darryl & Darryl squatted at the home of Hayden & Christine on Coach.
Coach’s Luther and The Drew Carey Show‘s Drew gambled together on Grace Under Fire.

The Drew Carey Show‘s Mimi appeared on The Hughleys, which crossed over with The Parkers, a spinoff from Moesha.
Characters from Moesha attended the same prom as Cher from Clueless (the tv version.)
Sabrina, the Teenage Witch also visited Cher from Clueless.

Sabrina also crossed paths with Boy Meets World on several occasions.

In one of the classic TGIF-major crossover events, some Boy Meets World characters interacted with the family from Step By Step, who met Steve Urkel from Family Matters.

Family Matters was a spin-off from Perfect Strangers.

And finally…. Perfect Strangers featured a game show segment in which a toilet was donated by Cunningham Hardware in Milwaukee, WI – the same Cunningham Hardware owned by Howard Cunningham of Happy Days.

Whew…..

I highly recommend checking out the original site: https://thetommywestphall.wordpress.com/.
Why not take a few moments to see if some of your favorite shows have been rendered “non-existent” and appreciate all of the hard work and dedication that went into this?