Science of "The Upside Down"

Why does everyone on this poster look like Winona Ryder???

Stranger Things Have Happened
    I've recently been enjoying the third season of the Netflix original series Stranger Things, directed by Matt and Ross Duffer.  If you've not run across it, Stranger Things is a nostalgia smoothie of 1980s pop-culture, homaging the sci-fi/horror works of Stephen King, John Carpenter, Steven Speilberg, Tobe Hooper and George A. Romero, with a few Weird Al Yankovic songs sprinkled in for good measure. 
    It's mostly harmless cotton-candy fun, blending coming-of-age drama with gross-out-horror, and does a nice job of honouring Generation X Hollywood without ripping it off.  As someone whose adolescence included a healthy diet of these movies/novels/comics, I've enjoyed all three seasons so although I didn't grow up in 1980s small-town America I can enjoy it for the flashy, splashy, trashy homage-athon it is.
    The central conciet of Stranger Things is that shady government forces have accidentally opened a portal to a parallel universe (oops) through which horrible beasties come crawling, and only a group of bicycle-riding pre-teen nerds can save the world from annihilation.  Oh, and one of them is telekenitc.  As I say, I didn't grow up in 1980s America, so I can only asssume this was a pretty common occurence.
    In the show, this alternate dark-dimension is neatly called "The Upside Down", a reference to flipping a Dungeons and Dragons board upside down to get to the dark side of existence.  Alternate realities have been a staple of fantasy and myth for centuries, but the idea was formalised in science fiction in the 1934 short story Sideways in Time by Murray Leinster, in which humanity learns there are parallel versions of Earth occupying the same space and time, only on a different frequency, the same way radio station signals exist in the same location but can only be picked up one at a time.
    Surprisingly, and rather awesomely, in the last few decades theoretical physics has started taking the idea of parallel realities seriously because they may be necessary to explain some of the most puzzling phenomena about the world around us.  By my count, there are three main places in modern Science where parallel universes are talked about, so let's take a look at what physics says about the Strangest Things of all...the laws of nature.

Hop in, kids!

1. The Many Worlds Hypothesis - Quantum Mechanics
    I don't want to give too much away on this one, because I've got a book out in less than a month which has a whole chapter on the topic (click here to pre-order Fundamental).  I'll whizz through the basics however.
    One of the many peculiarities of quantum mechanics is that particles are seemingly able to do more than one thing at a time, even things which are contradictory.  Consider the humble light bulb, a device which can be switched either on or off, but never both simultaneously.  This seems like an inviolable law of logic: something cannot be in two mutually exclusive states at the same time.  Individual particles however do precisely that - an electron can choose to spit out a photon (a particle of light) and simultaneously not spit it out, meaning the electron is both giving out light and not.  Although the everyday world seems to follow the laws of logic, quantum particles have no interest in them.  Take that, Aristotle!
    Unfortunately, we have no way of really understanding how this is possible.  There are a few ways of tackling the idea to make it more palatable for our feeble brains however, the most popular of which was the view promoted by Werner Heisenberg (of Breaking Bad fame) and Niels Bohr (of I Hate Einstein fame).  In their way of looking at things, you just sort of shrug your shoulders and decide nature doesn't have to make sense to us.  If particles want to do contradictory things we have no choice but to let them.  But not everybody is happy with that approach.
    Probably the most talked-about alternative is the one suggested by Hugh Everett III, who pointed out that although we know particles can apparently do opposite things at the same time, when we observe them we only see one outcome.  We have never actually detected the light bulb being on or off together, even though we can infer it must be happening, so Everett suggested both states exist in different Universes, only one of which we can see.
    That, according to Everett, explains the bizarre dual nature of particles.  There are two overlapping Universes and particles can take entirely different choices/paths in each one.  We can infer and calculate that both are taking place at the same time, but we can only ever witness one reality - the one we are in.  It's a crazy idea but large numbers of physicists, including Richard Feynman and Stephen Hawking, felt it was the only way to make head and tail of the mystery.
    This means there could be a vast number of parallel Universes in which a countless number of events have taken place.  The laws of physics would basically be the same, but the way particles choose to operate within those laws could be different everywhere.  There are Universes where the particles in your brain have done different things, meaning you have made different choices and lived a different life entirely.

There could be a Universe where Steve's hair looks like this!

2. The Multiverse Hypothesis - Cosmology
    The Universe exists (spoiler alert) and within it, there are physical laws which allow it to do so sesnibly.  Things like the law of gravity, electricity and magnetism, nuclear decay, the behaviour of atomic nuclei and so on.  If any one of these laws were changed a little bit, the Universe would look very different to the point of being unrecognisable.
    For instance, take gravity.  A Universe in which gravity did not exist would be totally unfamiliar.  Not only would the apple never fall in front of Newton, it would not have existed in the first place because there would be no solar systems, no planets and no objects on their surfaces. 
    At the start of the Universe there were just a bunch of particles free-floating with little to do with each other.  Gradually these particles started sucking themselves inwards until they were crushed into hot balls of plasma - suns - and the only way for this to happen is through gravitational attraction.  No gravity, no stars.  And, if there are no stars, there is no way for heavier elements to get formed via the fusion process, no heavier elements means no chemistry, and therefore no planets either.  Without gravity, the Universe would be one big, boring cloud of hydrogen and helium.  In a very real sense, you owe your existence to gravity.
    Or let's instead suppose all the laws of physics did exist but in different ratios.  What I mean by that is that the strengths of the various laws could be totally different to what they are for us.  For instance, there is a force inside the core of a nucleus called the "Strong Force" because it holds protons and neutrons together strongly - gotta hand it to physicists for creative nomenclature. 
    If the Strong Force were not quite so strong, protons and neutrons would not stick together, meaning atoms would not exist.  Forget a big cloud of hydrogen and helium, without this force being strong enough, there would be no freaking atoms at all!
    In fact, we are susprisingly lucky that the fundamental forces of nature interact the way they do.  Tweak them just a little and physics looks very different.  So...how come we're lucky?  Is there some reason the laws of physics just happened to fall into just at the right strengths to allow the beauty we take for granted to exist?  There are a number of possible explanations.
    The first is that it's due to random chance and that we were simply fortunate.  Another explanation is that the Universe was arranged this way by a benevolent science-loving entity, which some people call God.  Another explanation is that there are trillions of Universes out there all existing in different regions of space like bubbles in a foam.  Inside each Universe the laws of physics are slightly different, and we just happen to be in one of the more interesting ones.  This idea is called the Multiverse hypothesis and explains why the Universe is so conveniently put together.  It's not that the Universe is special, it's just that there are so many Unvierses anything is possible in at least one of them.       

There could be a Universe where Eleven plays Ewan MacGregor in Trainspotting

3. The Bulk Hypothesis - String Theory
    The Many Worlds and Multiverse hypotheses are quite similar.  They both propose many Universes with alternate timelines and everything distributed at random with us experiencing just one of them.  The physicist Leonard Susskind has even published a paper arguing  that these two hypotheses could be the same thing.  He proposes that random fluctuations during the big bang meant the laws of physics chose different identities in different realities before branching off.  In this view, the Multiverse idea is just what you get as a consequence of applying the Many Worlds approach to the big bang itself.  The Bulk Hypothesis, which we get from String Theory (which Susskind co-invented) is very different though. 
    String Theory is a hell of a subject and I plan to write about it in more detail in the future.  For this blog however, we only need to focus on one small aspect of the theory: stacking branes.
    In String Theory there are no such things as particles.  Instead, all the laws of physics can be explained using a buffet of different objects which interact with each other in complicated ways.  One of these objects is called a string...duh...but more relevant to our purposes are objects called Branes (short for Membranes).  Membranes are surfaces which can be layered together like pages in a book, each one entirely separate to the one adjacent and the pile of membranes is referred to as a "bulk". 
    Now imagine an Atlas which has a 2D image of the world on every page.  These 2D worlds would be stacked together in a 3D bulk and to any 2D creatures living on the page they would have no idea there was another world right next to them.  Now all you have to do is imagine the whole thing in higher dimensions...easier said than done of course. 
    In The Bulk Hypothesis of String Theory, our Universe could be a 3D membrane, stacked alongside other Universes in a 4D bulk.  We just can't see these other Universes.  The same way a 2D being could not perceive a third dimension, we cannot perceive a fourth, but there is no reason such a dimension could not exist.  In this view, there could be countless parallel worlds all around us, separated along a dimensional axis we cannot see.

There could be a Universe where we finally see Madonna's true form

Which Is The Upside Down?
   The Upside Down in Stranger Things is the classic "dark dimension" where evil psychic tentacle monsters exist, and there are no Ikeas.  The scientists in the show are able to access it using a bunch of hand-wavy Universe-penetrating machines in order to exploit a place where the Upside Down Universe and ours are so close the barrier between them is thin.  And this actually gives us a major clue about which alternative Universe The Upside Down has to be.
    One of the key features of the Many Worlds hypothesis from quantum mechanics is that you can never observe the other reality where particles are doing opposite things.  In fact, when two overlapping Universes take different routes they are said to "decohere" from each other and we can only detect them indirectly through the mathematics of our experiments.  The many worlds of quantum mechanics are completely inaccessible.  Which rules them out.
    The same can also be said of the Multiverse hypothesis.  In this one, the different Universes are either separated by enormous distances in space, or enormous stretches in time and we have no way of getting to any of them by conventional means.  We could potentially create a wormhole between the dimensions but statisically we would be far more likely to find a Universe where the laws of physics are completely different.  In Stranger Things, the Upside Down is different to our reality but is still pretty close; everything is made from atoms, carbon-based life-forms exist, there is a weather system, light and electricity behave in a similar fashion etc., so it is very unlikely The Upside Down is part of the Multiverse.
    But The Bulk Hypothesis works nicely.  If we were to connect to one of the other membranes of the bulk, it would probably be somewhat different to our own Universe, but close enough to be recognisable.  The same basic laws of physics would apply, just with a few minor discrepancies - which is what we see in the show.  In fact, the show makes a big deal of objects and locations in our world corresponding to objects and locations in the other one e.g. we don't open a portal to the Upside Down and find ourselves in the middle of empty space - there's a planet the same size as ours on the other side, with the same gravity and roughly the same geography.
    What's more, the other membranes of the bulk are the only ones we might actually have a way of "reaching".  Whereas the many worlds and multiverse realities are completely separated from ours, in String Theory, there are hypothetical particles called gravitons which can move from one membrane universe to another, allowing two universes to talk.  Perhaps if we found a way of controlling a graviton beam (which the scientists in the show seem to have done) we might be able to send a pulse from our Universe to the next one along, opening a channel across which information and maybe even matter might be exchanged.  What's more, once this rift in the bulk had been opened it would be very difficult to close...leading to huge problems and lots of useful plot developments.  So there you have it, Elven and the gang are early pioneers of String Theory! 
    Personally, I find it pretty cool that the ideas of a speculative TV show like Stranger Things are actually matched by real developments in theoeritcal physics.  Sometimes sci-fi shows have no regard for real science, which is fine of course (it's entertainment not education), but I always find it rewarding when there's a plausible way to justify the fun.
    The only other show to feature String Theory in any detail was NBC's dubious sitcom The Big Bang Theory, which I have written about here.  That show also attempts to mash-up a bunch of nerdy pop culture references but the main difference is that in The Big Bang Theory the nerd characters just spend their time arguing over movies and sex, whereas in Stranger Things the nerds get to save the world.

Image Credits:
Poster: KyleLambert
Bandwagon: Winesthatdelliver
Steven's Hair: popbuzz
Eleven's Hair: Goldberry
Demogorgon: Dictionary