As far as the universe can be said to exist at all, it probably has 10 dimensions. That’s the only way string theory makes sense. And physicists want string theory to make sense because it could be the Theory of Everything, uniting the otherwise incompatible theories of general relativity (for very big things) and quantum mechanics (for very small things) into one internally consistent package.

We don’t see the additional six, they say, because they’re just too small to notice, curled up or “compactified” in the form of Calabi-Yau manifolds. And it may be just as well, since they get pretty weird as they go on.

We’ll get to them later, but first let’s start with the basics.

## 10. The First Dimension

The first dimension is pretty simple: a straight line connecting two points, or a length without width or depth. But as Rob Bryanton (on whose work this list is based) points out, within that apparent simplicity there’s a great deal of complexity. After all, the first dimension isn’t bound by any two points that define its trajectory. It stretches to infinity beyond them.

Nor does any 1D line exist in isolation. None of the dimensions do. So a single one-dimensional line passing through any two points in your living room also passes through every other point along the same trajectory, taking in distant planets, stars, galaxies, neighboring universes even – whatever lies in its path. In other words, a line has the potential to contain an infinite stream of information within its one-dimensional confines.

## 9. The Second Dimension

The second dimension arises from the first with the addition of two extra dimensions, allowing us to move not just forwards and backwards but left and right as well. With two dimensions, we can define a plane — a flat surface with length and width but no depth – extending infinitely like the first dimension in every direction available to it.

In the 1800s, theologian Edwin Abbott gave some thought to what “life” might be like in 2D, inspiring him to write his book *Flatland*. The inhabitants (squares, triangles, pentagons, and so on) would only see each other as lines, he said, since all they could see would be the sides. And even 3D visitors to Flatland would appear this way, since only once slice of them could ever be visible.

But that 3D visitor would be able to do all kinds of all seemingly miraculous things. Entering from above or below they would appear to have materialized out of thin air. And, since safes built to withstand 2D thieves would only need to be outlines (in fact *could* only ever be outlines), a 3D thief would have no trouble getting inside.

And if, as in Abbot’s story, a Flatlander were to be plucked up and out of their world by a (3D) “Spacelander,” they’d be astounded to see the exposed insides of other Flatlanders’ bodies.

## 8. The Third Dimension

Actually, Abbott used more poetic license in imagining Flatland than even he may have been aware of. Not only would Flatlanders require a third dimension just to see each other as lines, they would also require a fourth, duration. And the same is true for us in the third dimension.

That’s because it takes time for the light rebounding off objects to reach our eyes, even when things are right in front of us. In fact, for other beings even to exist to see in the first place, they need to have some kind of duration, a substance or thickness in time.

Without duration, three dimensions of space are just a static, conceptual snapshot, a timeless image. Physicists call this a Planck frame, which, defined by the speed of light and Planck’s constant, represents the smallest possible distance and duration.

What does all this mean for us? It means that, far from living in a three-dimensional world, we’ve never even seen one. What we’re really seeing at any given moment is a two-dimensional visual image of a three-dimensional frame of a four-dimensional reality.

## 7. The Fourth Dimension

Just as the first dimension is a line stringing multiple zero-dimensional points together, the fourth dimension may be seen as a line stringing multiple Planck frames together.

There’s another way to look at this: just as Flatlanders could only see a one-dimensional slice (a line) of three-dimensional beings, so can we only see, at any given moment, a two-dimensional slice (the image of a Planck frame) of four-dimensional beings, including each other.

So what do we really look like?

Bryanton imagines each of us as a “long, undulating snake, with birth at one end and death at the other.” All we ever see, though, is one slice at a time – a “moment” in other words – but in quick succession, one after the other. This is what gives the impression of gradual, one-way aging, sort of like a flipbook animation.

In reality, every moment, both past and future, is happening all of the time – just like a 3D being standing in a 2D world also exists above and below it. As our fourth-dimensional forms move through the third dimension, we appear to grow from birth, diminish with age, disintegrate in the grave, and ultimately disappear entirely. Yet all the while we’re whole and complete in 4D.

And the same is true of anything – a tree from seed to decay, a city through all of its eras, the universe from big bang to big crunch. In the fourth dimension, objects are complete in time as well as space.

So as a fourth-dimensional being, you’d be able to see everything that ever took place in a room, and everything that ever will. In the movie *Interstellar*, that’s pretty much how Cooper sees his daughter’s bedroom while inside the tesseract (itself a four-dimensional hypercube).

## 6. The Fifth Dimension

The previous entry throws up an essential question: If everything you’ve ever done and ever will do is already played out, how can you have free will?

The answer, according to string theory, is you don’t – at least, not as you know it.

While everything you’ve ever done and ever will do is already played out, so is everything you could have done and could do next. In the fifth dimension, every possible path from every possible moment exists as equally real. Whatever you’ve done, whatever you didn’t do, there’s some other version of you who did otherwise.

It might not be free will, but then again what is? After all, not one single choice remains unmade in the fifth dimension – including the choice not to choose.

And the *real you *takes all of them, because there isn’t just one real you, they’re all real – every possible iteration. Every outcome is just as real as any other. In fact, from moment to moment (every 5.4 x 10 to the power of -44 seconds or so) you sort of cease to exist entirely, exploding into countless new replacements. You’re not some quintessential “you” from which the other yous branch off, you are the branches.

So next time you narrowly avoid a collision with another car, you might want to spare a thought for the people you actually hit. And instead of cheering for your favorite football team, you might as well save your breath; they’re bound to win and lose anyway.

## 5. The Sixth Dimension

Of course, there’s still only one of you per timeline and no way of accessing the others. From the point of the split, each timeline becomes permanently inaccessible from the others because of the same entropy-driven laws that stop us going back in time.

And the same is true from the very beginning of the universe – from the big bang – the point at which all possible timelines with those initial conditions diverged. From that point on, each timeline becomes practically a self-contained universe. So you can’t just slip into the timeline where dinosaurs never went extinct, Bryanton says, because there’s no causal connection to your own. In other words, the events leading up to the present moment on your timeline have no relationship to the events leading up to the equivalent present moment on the timeline where dinosaurs are around in 2017. Looking out the window at a passing brontosaurus just isn’t something that could happen next.

Unless of course you’re in the sixth dimension.

A being capable of traveling through the sixth dimension could freely move between one timeline and another, regardless of whether they’re causally connected.

Thinking of the fourth, fifth, and sixth dimensions in the same way as the first, second, and third makes this a little more comprehensible.

The fourth is the “one-dimensional” “forwards-backwards” (time)line you’re on, and the fifth is the “plane” on which that timeline branches “left and right” into others based on probabilistic outcomes from moment to moment.

Since each set of outcomes branch from those that come before, you need an additional degree of freedom to “jump” from one to another – sort of like moving “up and down,” except this time in what’s called a sixth dimensional “phase space.”

## 4. The Seventh Dimension

So far we’ve been thinking about a universe of possibilities with the same initial conditions, the same big bang. But what if the starting conditions were different? Such a universe couldn’t feature in the sixth-dimensional phase space of this universe, so there must be another dimension.

In the seventh dimension, our universe, with all its myriad possibilities and probabilities, is effectively reduced to a single point. And a phase space containing all the possibilities that could occur from different starting conditions in another universe would be a second point. Join the two and you’ve got a line in the seventh dimension.

And, just like a line in the first, it would contain not just the two points that define it but an infinite number of others, an infinite number of other universes in this case – a multiverse.

So if the second point is a universe with different laws of gravity to our own, the line between us would contain infinite other universes with infinite different values for gravity, including universes rendered physically impossible because of them.

Imagine, for instance, a universe where planets zoom in and out of stars instead of round and round. Or where they simply stop moving after just one orbit. What about a universe where planets sprout wings and fly away from eachother, or where stars are really just lightbulbs? All of these exist along the seventh dimension – as do universes where the constants are constantly changing.

## 3. The Eighth Dimension

Why do we need an eighth dimension? Because there will always be universes that are not included along any seventh-dimensional line we can draw.

A line on which the laws of gravity change from universe to universe while every other law remains the same, for instance, will have no universe that has the same gravity as our own but a different speed of light.

And the same is true regardless of what physical laws change from universe to universe, even if they all do — since in that case there would be no line on which only one law changes and the others stay the same. Remember, the seventh dimension, like the first and fourth, can only define a line – one line – and never two simultaneously. The eighth dimension, like the second and fifth before it, defines a plane on which lines can co-exist.

It’s also important to remember that none of these dimensions exist in isolation. It’s only a way of visualizing. Another way, perhaps closer to the reality, is to think of the eighth dimension as a plane rolled up on itself to form a tube. Viewed from far away enough, it would look like a one-dimensional line, which in this case would be the seventh dimension.

In other words, the eighth dimension is the true or ultimate multiverse, containing every single possibility, at every single moment, in every single universe both imaginable and unimaginable. Conveniently, this also explains why we exist – because there has to be a universe in which we do – and why, anyway, there’s something rather than nothing – because the multiverse represents both.

## 2. The Ninth Dimension

Still, you’re going to have trouble getting to any of those other universes in anything less than a ninth-dimensional space ship. Travel from any one universe to another without having to pass through the infinitely diverse intermediate universes to get there requires an additional degree of freedom.

Enter the ninth dimension.

If the eighth, the ultimate multiverse, is a plane rolled up on itself to form a tube, the ninth is the space inside. This is beyond physical reality, says Bryanton. It’s a realm of organizing patterns, “a seething foam of possibilities.”

Like the third and sixth dimensions, the ninth introduces a new kind of “up and down” freedom of movement. But in this case it allows the 9D traveler to “fly around” inside the eighth-dimension. They could pick from a truly infinite array of universes and visit in an instant.

## 1. The Tenth Dimension

Take every possibility, no matter how unlikely, happening to every single thing, no matter how tiny, in every moment at any time, occurring all of the time, in every universe of an infinite and unthinkably varied multiverse, and reduce it all to a single point.

This is how Bryanton begins thinking about every other dimension – with a point – to find another point beyond it and draw a line in the dimension above. But, as Bryanton points out, from the tenth dimension there’s nowhere left to go. There’s nothing left to imagine.

So what we’re left with is effectively the zeroth dimension – a dimensionless dot without another. And in fact that’s exactly what Bryanton thinks the tenth dimension could be – absolutely nothing, no dimensions, “just infinite vectors within one underlying fabric which is dimensionless.”

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