What do the goatee-sporting Spock of the Terran Empire, the blood-thirsty Demogorgon of the Upside Down and the Cybermen who battled David Tennant’s Doctor have in common? Apart from being some of the most ruthless villains in sci-fi, that is?


All of them, of course, are characters spawned from an alternate universe, a world similar to our own but with a few sinister alterations.

Yet there’s something else they all share: all have roots in a major scientific mix-up. And it’s not a mistake restricted to the likes of Star Trek, Stranger Things or Doctor Who, either. Delve into the majority of alternative reality stories and you’ll find two giant ideas mistakenly melded together: alternate universes and alternate dimensions.

For instance, while explaining the mysterious Upside Down via teacher Mr Clarke, Stranger Things labels the realm both as a “parallel universe” and “another dimension”. Captain Kirk does exactly the same in Star Trek when explaining the Mirror Universe to his crew. And The Doctor? In series two finale Doomsday, even Gallifrey’s finest says the Cybermen originate from a “different dimension” and “different universe”.

But ‘dimension’ and ‘universe’ aren’t interchangeable terms. And as much as we’d hate to call out our favourite Time Lords, Starfleet captains or, um, AV club leaders, getting the distinction wrong could be holding back sci-fi from something really special.

Okay, what is the difference between an alternate universe and dimension?

As many sci-fi fans will know, parallel universes aren’t only found in fiction. They’re a concept, at least theoretically, supported by experts in cosmology.

“The remarkable thing, I suppose, that emerges about modern physics is that the whole idea of alternative universes other than our own is not that crazy. There are lots of signs that there could be other universes out there,” says Dr Andrew Pontzen from University College London’s Astrophysics Group.

“In quantum mechanics, for instance, it seems like it’s possible for particles to be in multiple places at once, for histories to play out at the same time. In some sense, when you zoom down to very small things there are parallel universes there.

“Parallel universes could be playing out in front of us the entire time, yet somehow we can’t become aware of these multiple histories. Our consciousness is just somehow conditioned to focus on one history at a time.”

Granted, the streams of equations needed to support these ideas don’t make for the most exciting (or understandable) reading, but what they come down to is this: parallel universes aren’t completely out the question. There really might be an evil goatee-wearing version of yourself out there.

It’s not likely, though. “In most pictures, when we talk about alternative universes, we would expect them to have fundamentally different laws of physics. That means if you could somehow teleport yourself into one of those universes, you’d cease to exist because the basic laws of physics that dictate the material we’re made out of would change,” says Pontzen.

“You’d dissolve the moment you’d get there. It’ll probably be quite a nice way to die, actually. I don’t think you’d be aware of it at all.”

So, on the downside, a parallel universe is likely to entirely wipe you from existence instantaneously. But at least it makes sense as a concept. ‘Alternate dimensions’? Not so much. That’s because the term itself misunderstands what dimensions are: they’re not a physical universe, but a tool to define a point inside that universe.

All of us know about two dimensions: left and right, and up and down – like a simple two-axis graph or sheet of paper. And the idea of three dimensions is a no-brainer too: everyone is familiar with height, width and length (think a cube, or snazzy x,y,z graph).

However, most Einstein aficionados will also point out another dimension often ignored: time. Although we can’t move back and forth through it like other planes (although time travel isn’t completely ruled out by the maths of space-time), it’s still a dimension you can define and measure in our reality.

In this way, time is just another coordinate we use to pinpoint something in our world. Just think what happens when you meet a friend for dinner: not only do you arrange a physical position to meet (effectively giving your coordinates in space – the left/right, up/down, backwards/forwards), but you also agree on a position in time to eat, whether midday or 1pm.

Yet although we talk about it, this temporal plane is always overlooked. For example, when you pay to see a 2D film you’re actually served a 3D screening for the same price and without the need to wear those awkward glasses: not only are you watching a screen that has two dimensions (up/down, left/right), but you’re actually consuming a film reliant on the dimension of time too.

“Imagine if you took a film and chopped it up into individual frames and stacked them up into a 3D block,” explains Pontzen. “Two dimensions would represent each individual frame and the height – a third dimension – would be time.”

This misunderstanding of cosmology isn’t only found underselling cinema tickets, but in the dialogue of Doctor Who too.

True, scientific concepts boldly materialise throughout the franchises best stories (for example, the “quantum-locked” Weeping Angels embody the very real Quantum Zeno Effect whereby uranium doesn’t decay while being observed), but dimensions are often mishandled.

Take Flatline, the Peter Capaldi-era episode where the Doctor and companion Clara Oswald defend the earth from “two-dimensional” monsters dubbed “The Boneless”. These flat aliens who crawl across walls and ceilings, flattening all who cross them, certainly make for an interesting enemy. But a monster with a scientifically sound origin? Not really.

Although The Doctor claims dimensions “are kind of [my] thing” in the episode, he makes no reference at all to time, describing our world as “three-dimensional”. Of course, there’s logic to this omission: some may say ignoring the temporal plane and focusing on the three spatial ones make the story easier to understand. And, let’s face it, in this case, monsters melting people into dimensional doodles makes for better TV than a lecture about space-time.

But The Doctor still falls into a physics faux pas when claiming The Boneless are both from “another dimension” and “another universe”. This doesn’t make a lot of sense because although the baddies are said to be from a two-dimensional realm, they move through some of the same dimensions as The Doctor, not alternative ones.

In the same way, a sci-fi show portraying an “alternate dimension” implies it’s a place where one of our usual dimensions has been switched with a new plane. But, in almost all cases, left/right, up/down, forward/backwards and time all still exist in these alternate realities. A dimension hasn’t been replaced.

So, what’s happened here? Why do sci-fi writers get their dimensions and universes muddled? There may be a reasonable answer: “What some might mean is that there’s an extra dimension that lets you get to an alternative universe,” says Pontzen. “What you really want is to add to the number of dimensions – not swap them out.

“Imagine if you were walking on the surface of the earth straight on level ground,” says Pontzen. “You’re only moving through a few dimensions here: forwards and backwards, left and right, and, if you want to include it, time.

“Suppose you want to get to A to B, but there’s a huge mountain in the way. Obviously if you remain on the surface of the earth then you’ve got no choice but to go around the mountain and that might actually be near impossible.

“But then suppose somebody comes along and says ‘I can build a tunnel under the mountain’. In effect, what they’re doing is using another dimension: height. And by using this additional dimension, it becomes clear A and B aren’t so far apart.”

Yes, it really is possible. We might actually be able to tunnel to another universe if we came across another dimension – something we haven’t detected so far. And owing to a life trapped in our four dimensions, it’s a reality we’re not equipped to visualise.

Well, for the moment, anyway…

Why it’s so important to get this distinction right

We get it. Calling out the difference between two closely-related concepts seems like nit-picking. But it is important nit-picking. Because if we drop the trope that a dimension is just a copy of our world with more baddies then we create room to think about the real-life science of dimensions. And that’s where we get to ask some seriously interesting questions.

Firstly, how can we even imagine an extra dimension on screen? How could science visualise a five-dimensional universe – something that’s outside our understanding of known space and time? How could TV or film portray a reality beyond us that, as Pontzen says, can’t be pictured by its very nature?

It’s when science fiction takes on these mind-boggling questions that we’re treated to mind-boggling answers.

Interstellar, a seminal work of sci-fi from Chris Nolan, did just this with the help of Nobel Prize-winning theoretical physicist Kip Thorne. At the film’s end [SPOILERS MATERIALISING], it plunged Matthew McConaughey through a black hole and into a higher dimension, one where he was able to move through all of space and time as if was a physical plane.

And that’s just scratching the surface of extra-dimensional theories left largely untouched by popular sci-fi. “One of the first signs that we should think about extra dimensions beyond three of space and one of time is something called Kaluza–Klein theory,” says Pontzen. “It’s basically the idea that electric and magnetic forces – ones which are basically used to power all technology – could be described in terms of dimensions.

“It’s quite a subtle mathematical thing, but you can find an imprint of another dimension there. It’s not proof, but if correct it could show there are extra dimensions there that you don’t realise.”

Sure, a thriller about electromagnetism doesn’t sound like the easiest sell, but the idea that hidden dimensions exist all around us? Doesn’t that seem like a concept ripe for science fiction on screen?

And then there’s string theory, the idea that our reality contains 10 dimensions (three of regular space, one time and six hyperspace) and bosonic string theory, which supposes space-time is 26-dimensional.

Yes, they’re scary-sounding theories taught at the highest University level, but both boil down to an idea begging for a big-screen mass-audience conversion: “They embody the idea that the laws of physics that we see are one of a possible set of laws that could have been and maybe do exist in these other universes,” explains Pontzen.

Tackling these dimensions and inventing entirely new laws of physics may sound like an impossible task for any writer, but it’s precisely when presented with such challenges that sci-fi truly thrives. The best stories of the genre come when it breaks down huge scientific ideas into powerful – and understandable – drama.

Case in point: Doctor Who’s series 10 episode World Enough and Time, a story which saw a 400-mile-long ship stranded above a giant black-hole. Thanks to time dilation – a concept also seen in Interstellar, where objects with large gravitational fields like black holes can bend the plane of time – a few seconds on the lower decks on this ship equaled years in upper levels. This real science gifted a fantastic plot device where an entire generation of terrifying cloth-faced Cybermen seemingly evolved in a few minutes.

And with its central Gallifreyan travelling in a TARDIS (Time And Relative Dimension In Space), Doctor Who is in a prime position to delve into these dimensional dilemmas too. And the show’s done a lot right already: in Classic Who the idea that the Tardis interior and exterior exist in different planes is briefly mentioned, most significantly with Tom Baker’s doctor brief explanation of it being “dimensionally transcendental”. But it hasn’t been fully explored in a story yet, with “dimension” now often reduced to a timey-wimey buzzword.

Why can’t sci-fi add more meaning to the word ‘dimension’? What’s stopping a new brand of fiction that invents new planes of existence and laws of physics – mind-boggling stories that can act as an epiphany for viewers who, in turn, could be inspired to create even more ambitious stories that propel the genre as a whole?


For the current popular understanding of the topic, maybe that’s too big a leap. But getting the difference between universes and dimensions correct? That’s an easy step in the right direction.