We had a theoretical physicist break down our biggest question about the Netflix hit 'Stranger Things,' and it blew our minds

Warning: Spoilers ahead if you have not watched all of "Stranger Things."

"E.T." "The Goonies." "Game of Thrones." Roll them all into one and you've got "Stranger Things," Netflix's newest hit thriller series, which centers around a boy named Will Byers who disappears into a shadowy, dark copy of his own world that his friends call "the upside-down."

If you're as obsessed with the show as we are, you've probably got a few questions. Namely, What the heck is this "upside-down" place that Will disappears to and how come his friends can't just go there and rescue him?

We put the question to a theoretical physicist to find out, and his answer blew our minds.

In the upside-down, everything looks the same as it does in Will's "normal" world - almost. Will can walk along replicas of his middle school hallways, snuggle into the blankets in a version of his beloved backyard fort, and even have some (extremely limited contact) with his friends and family on the other side. But the other dimension is cold and dark, and Will is utterly alone in it. His only company is a vicious plant-like demon who appears to feed off of people.

Luckily, Will's friends happen to be obsessed with a Dungeons-and-Dragons-esque boardgame. The game has a lot in common with this other dimension. There's even a scene in the show where the boys turn the game board upside down in order to explain to each other how they might imagine the "upside-down" where Will is trapped.

But they still can't figure out how to get there, or how to rescue Will. So, in a meeting with Mr. Clarke, they ask him for some "purely theoretical" help: Namely, how they'd access another dimension, if one existed.

The acrobat and the tightrope

Mr. Clarke starts with an analogy. He asks them to picture their own dimension as a tightrope, and all the people in it - including Mr. Clarke and the boys - as acrobats on the tightrope. To an acrobat, the tightrope is flat - the acrobat can walk forward or backwards along the rope, but that's it. The acrobat can't turn upside down (thanks, gravity), or plunge past the rope.

Now, the teacher says, imagine a flea perched on that same tightrope. The flea isn't limited like the acrobat. The flea can go up and down. The flea can get to the other dimension.

Personally, this is where the show lost me. If the flea exists in the same three-dimensional world as the acrobat, why wouldn't the flea have to follow the same laws?, I asked myself. Why can't the acrobat be a flea? Why can't we all be fleas?!

Theoretical physicist Paul Steinhardt cleared it up beautifully for us. Go back to the acrobat-flea analogy, only this time, picture this:

The acrobat walking along the tightrope is huge compared to the thickness of the skinny rope! So, she sees the rope as a one-dimensional line; she can only move back and forth along this surface. She never walks around the circular direction of the rope, "partly because she'd fall off, but mostly because she's too big for it," Steinhardt said.

But a flea walking on that same rope could not only walk back and forth, but also around the rope. The flea could also crawl down the side of the rope, and even underneath it.

This analogy is an easy way to imagine string theory, which proposes that there are extra dimensions, curled up into little balls. Creatures much bigger than the balls - like you, me, and the acrobat - can only move through the regular three dimensions. But tiny, (tiny!) particles can move through the three dimensions and in the extra dimensions.

Unfortunately, that analogy doesn't completely fit in with the extra dimension we see in "Stranger Things," Steinhardt said.

Acrobat, meet sandwich

So, in order to better understand what's happening in the show, Steinhardt suggested imagining a sandwich instead of a tightrope. The sandwich is made of two flat surfaces, like pieces of bread, which represent two parallel universes. (These parallel universes are more accurately called "brane-worlds," but we'll get into that later.) Between the universes is spacetime, which we can picture being represented by a nice, thick slather of hummus.

Now picture a person shrunk down to the size of a flea. She can walk freely along the top or bottom of either of the two pieces of bread, but she can't go through the hummus - there'd be nothing to walk on. As you might know from Einstein's theory of general relativity, our spacetime (hummus) can wriggle and get warped. And just like a soggy hummus sandwich, this wriggling or warping can also affect the brane-worlds (bread slices) alongside it.

So how could our little person walking on one slice of our sandwich bread get to the other slice of bread (i.e. the other brane-world)?

The brane-worlds (bread slices) are pretty flexible. So to join the two, you'd need a massive amount of energy - like a black hole or a worm hole, which you can imagine as squishing the sandwich together in one spot using your finger. If a black hole formed between the two brane-worlds, its intense gravity could actually squeeze the two brane-worlds together, sort of like pushing the two sandwich slices together would. (Unfortunately, this would have the added side-effect of decimating anyone who tried to go from one world to the other). A worm hole, on the other hand, would create a tunnel-like connection between the bread, as if you stuck a toothpick in it.

Mr. Clarke tells the kids that one way to generate such a hole is to create an impossibly massive amount of energy.

As you probably remember, the boys live nearby the Hawkins Electric Plant, which likely generates a bunch of energy. (It almost certainly wouldn't be enough for the purposes of joining two brane-worlds, Steinhardt pointed out, but hey, we're also talking about parallel universes and monsters that eat people.) Now, keep in mind that one of the show's central characters, Jane ("Eleven"), has extraordinary abilities, one of which involves tapping into energy fields. That's how she turns off the noisy electric fan at the burger joint in episode one.

Eleven uses her powers to help the kids communicate with Will and assure him that they're coming to save him. And later, it's the hole she's ripped in the fabric of spacetime using her powers that allows the town sheriff (Hopper) and Will's mom to travel into the upside down and rescue Will.

Mind. Blown.