BY CHRIS LASSIG
Everything you know is wrong. Everything you see is an illusion. Nothing is real.
You expect to hear unsettling messages like those from cult leaders and TV magicians, but reassuringly they usually turn out to be some sort of trick. All is good, reality is real. At least, it feels real.
Because that’s how you tell, isn’t it? You can’t trust the evidence of your eyes, but if you can touch it, if it’s solid, if it has depth and texture, then you’re pretty sure it’s real. The cool, smooth, roundness of an apple. The velvety, soft, teardrop of a ripe fig. The vicious, spiky end of a pineapple and the bumpy, also kind of spiky other bit of a pineapple. In other words, fruit is real, magic tricks and cult manifestos are not.
The trouble is, the more scientists learn about the universe, the more they’re finding that this solid feeling is just an illusion too.
Consider the atom. Just over a hundred years ago, New Zealander Ernest Rutherford figured out an atom consists of electrons whirling about a nucleus. But although the nucleus is the heaviest bit, it’s only about one hundred thousandth the size of the whole atom. Most of the atom—and so most of everything, including you—is empty space. 
What stops you falling through your chair as you read this fact is that those orbiting electrons all have a negative electric charge, and negative charges repel each other. The electrons in your bum are repelled by the electrons in the chair, and that’s all that’s holding you up.
But, as if the ground we walk on wasn’t shaky/non-existent enough, now some physicists want to take away even the empty space. The three dimensions we see—up-down, left-right, forwards-backwards—may themselves be an illusion, and reality may actually only have two dimensions.
The first person to suggest this was Dutch physicist Gerardus t’Hooft, in 1993. A clever chap, t’Hooft later shared the Nobel Prize for some work he did in the 1970s on the quantum behaviour of fundamental particles.  However, in the era of grunge he was more interested in what would happen if the entire universe was swallowed by a black hole.
Now, before you get too concerned, be aware that this is merely a 'thought experiment.' In German, these are called Gedankenexperiments, which possibly translates as 'thank you for not actually doing the experiment, you saved us a lot of money.' And in the case of dropping the entire universe into a black hole, I believe we can all be thankful.
The reason he’d want to not-actually-do-this-but-think-about-it is because black holes are great for testing your theoretical ideas. Even though we’ve never seen one—they’re rather black, after all, as is much of space—we have learned a lot over the years by pondering what effects their extreme gravity may cause.
One thing we’re pretty sure of is that anything that falls into a black hole is permanently cut off from everything outside. But this concept bothered t’Hooft, because it would mean that any information dropped into a black hole would be lost forever. And he’s not the kind of person who tolerates that sort of thing; nor is the universe, under normal operating conditions.
So instead, t’Hooft figured out a loophole where the information doesn’t actually disappear but is recorded in some way on the black hole’s surface. Not in a readable form, mind you—more like a damaged hard disk, from which the computer guy says you can never get your files back, even though you know they’re still in there.
But the key point is that all the information embodied in whatever falls in, which could be the entire universe if you want to push this to its ultimate conclusion, can be recorded on the two-dimensional surface of the black hole. This means you don’t need three dimensions to describe everything, you can get by perfectly well with only two.
This is called the 'holographic principle', named after holograms, which are also two-dimensional objects that produce a three-dimensional image. 
A hologram works a bit like a special window that records all the light coming through from another room. When you remove this window and shine another light on it, it exactly reconstructs the light that it had recorded, as if the room was still there; you can even look at different angles to see around objects. But again, the surface of the window has only two dimensions, yet it’s recording all the three dimensions of the room.
It sounds a bit like a parlour trick—especially if the room you recorded was a parlour—but when you apply the concept to the whole universe it challenges the whole notion of reality. Is our three-dimensional world really real or is it itself just a holographic illusion?
This is the point where it starts to sound like one of those meaningless arguments that do your head in when you’re a kid but when you’re older you’d rather just go to bed,  but it actually has practical consequences. These were worked out in 1997 by an Argentinean physicist, Juan Maldacena, who’s one of the most brilliant scientists you’ve never heard of. 
Maldacena’s father was an elevator engineer in Buenos Aires, and he says that was what got him interested in finding out how things work, reading books on electricity and radio and television, etc. But he long ago left such everyday concerns behind for the quest to find the fabled Theory of Everything.
The main sticking point for a Theory of Everything is finding a way to combine gravity and quantum mechanics.  Gravity is described by Einstein’s general theory of relativity, and it governs how stars and galaxies move. Quantum mechanics, on the other hand, describes the nature of atoms, nuclei and their constituents.
One of the problems is that general relativity relies on space being smooth and well-organised, whereas quantum mechanics is inherently messy and random: they’re the Felix Unger and Oscar Madison of scientific theories. 
Maldacena found that instead of forcing this Odd Couple to live together hilariously in an apartment, he could keep them in separate rooms and use the holographic principle to switch between them.
By projecting everything onto a surface right at the very edge of the universe, Maldacena was able to establish what he called a 'duality' with an equivalent, two-dimensional world in which gravity didn’t exist. He could do his quantum calculations in this flat world with ease, then bring gravity back by recreating the three-dimensional hologram, with his calculations still valid.
Despite the casual shattering of existence this implied, the scientific world was impressed: physicists danced in the streets. Well, not literally in the streets, just on the campus of the University of California in Santa Barbara. But they literally danced, to the tune of the Macarena, only with the words changed to 'Maldacena'. 
Of course, there are some caveats. The 'universe' Maldacena used wasn’t exactly our universe. For a start, his actually had an edge on which to project, and as far as we know, ours goes on forever. Also, his had negative curvature, which is really too hard to explain, except to say it kind of looks like curly lettuce, whereas our universe is flat, which is more like what I think they call butter lettuce. 
Still, the basic concept seems to work, and it may just be a matter of finding a suitably large surface we can use to make a hologram of our universe. But for the rest of us, who aren’t trying to make quantum meet gravity, the bigger question is: hang on, is this true? Wouldn’t we know if we were living inside a hologram?
Maldacena would say no. After all, in his model the universe and its projection are equivalent—that’s what 'duality' means. Gravity may not exist in the two-dimensional version, so you can’t say it’s fundamentally, deeply, really, truly real. However, you’ll still get bruised when you trip over. When you’re in a movie, you can’t see the screen.
To put it another way, think of Plato’s Cave. The ancient Greek philosopher’s famous allegory involves prisoners chained in an underground dungeon, with all they can see being shadows cast by firelight on the wall of their cave.
Then, one of them is temporarily released to visit the surface, where he encounters a world of solid objects lit by bright sunlight. But when he returns to the cave, his fellow prisoners dismiss his tales as fantasy. To them, the shadows are the only reality. 
This is usually taken to mean that somewhere there is a world that is more real and more solid than our own, with brighter colours and more authentic experiences. But perhaps we’ve got it wrong; perhaps the prisoners’ view is just as valid. Maybe the shadows are real after all.
Still, if it feels real to you, does it really matter?
Chris Lassig is a science writer, broadcaster and web professional. Find him on Twitter @astrocave.
- About 99.99999999 99995% empty space, based on hydrogen, which is the most common atom in our bodies, and indeed the universe. If the hydrogen nucleus had a volume of 1 mL, the entire atom would be 88,390 Olympic swimming pools, or about half a Sydney Harbour.
- Not, as you might have thought, for innovations in spelling and pronunciation.
- You may be familiar with holograms from Star Wars, where R2-D2 projects a tiny Princess Leia in front of her horny twin brother. In our time period, we mostly just use them to make credit cards and drivers’ licences harder to copy. We really do have a long way to go.
- For example, 'is an orange called an orange because it’s orange, or is orange called orange because it’s the colour of oranges?' (The second one.)
- There’s this thing called the 'Einstein Index, which aims to find the modern equivalent of Albert Einstein, defined as the physicist whose top three papers have been cited the most times. Maldacena tops the list, mostly due to his work on the holographic principle: that’s how big a deal it is.
- At least, the current sticking point; come on, it’s a Theory of Everything, there are bound to be more obstacles on the way.
- Or, if you want a more contemporary reference, they are Paula Abdul and MC Skat Kat.
- Actual words (Jeffrey A. Harvey, Strings ’98 Conference): You start with the brane/ and the brane is B.P.S./ Then you go near the brane/ and the space is A.d.S./ Who knows what it means/ I don't, I confess/ Ehhhh! Maldacena!
- But without an edge, remember. I told you it was hard to explain.
- Plato intended this allegory in part to demonstrate how much smarter and underappreciated philosophers are: they’re represented by the prisoner who sees the 'real' world. Clearly, physicists aren’t going to let them get away with that.