Atom (2007) s01e03 Episode Script

The Illusion of the Reality

In 1912, in a hot air balloon about three miles above the ground, an Austrian scientist called Victor Hess made one of the most astonishing discoveries in science.
Up here Hess found that incredibly mysterious rays of energy were pouring in from outer space and streaming through the Earth.
They were incredibly powerful, yet unlike anything seen before.
They were called cosmic rays.
At the same time in laboratories down below, scientists were studying equally mysterious and powerful energy rays pouring out from the interior of atoms - known as radioactivity.
Mysterious rays from the vast emptiness of space and mysterious rays from deep within the atom, the tiniest building block.
No one really understood what they were or if they might be connected.
Then an incredible story unfolded.
Cosmic rays and radioactivity turned out to be connected in a way so shocking that it beggars belief.
The discovery of this connection would force us to rethink the nature of reality itself.
The world we think we know, the solid, reassuring world of our senses, is just a tiny sliver of an infinitely weirder universe than we could ever conceive of.
Our reality is just an illusion.
In the years up to the mid-1920s, the atom revealed its strange secrets to us at a prodigious rate, as it produced one scientific revolution after another.
In 1897, Marie Curie studied strange rays pouring out of some rare metals.
She called them radioactivity.
Then, in 1905, Albert Einstein conclusively proved the existence and size of an atom by studying the way pollen moves in water.
A few years later, the New Zealander Ernest Rutherford performed an experiment in Manchester that revealed to him the shape of the interior of an atom.
Scientists were shocked to discover that the atom is almost entirely empty space.
The question then became, "How could this empty atom possibly make the solid world around us?" The answer to that was worked out by a group of revolutionary physicists in Denmark.
They proposed that the world of the atom ran on principles which were completely different to any mankind had ever seen before.
It meant that the atom, the basic building block of everything in the universe, was unique.
And perhaps outside human comprehension.
Then a scientist explored the nucleus, the tiny heart of the atom.
They found it bursting with powerful energy.
This discovery gave them the potential to bring about the destruction of the Earth, but in a shocking turnaround, it also gave them a fundamental understanding of how the universe was created.
And yet, despite this, the journey to understand the strange and capricious atom had only just started.
In 1927, a young man was studying at the Mathematics Department of Cambridge University.
Shy, awkward, clumsy and frighteningly brilliant, his name was Paul Adrien Maurice Dirac.
It's probably fair to say that Paul Dirac isn't a household name.
But it should be.
He was recently voted, by other physicists, as the second-greatest English physicist of all time, second only to Newton.
And he deserves the accolade.
All the brilliant minds that pioneered atomic physics were left trailing by Dirac, aghast at the sheer boldness and lateral thinking in his work.
When Einstein read a paper by the then 24-year-old Dirac, he said, "I have trouble with Dirac.
"This balancing on the dizzying path between genius and madness is awful.
" In 1927, for reasons no one has ever really fathomed, Paul Dirac set himself a task that was monumental in its scope - to unify science.
To bring its scattered parts into one beautiful entity.
And what this meant, above all, was to unite the two most difficult and counter-intuitive ideas in history.
Here's what Dirac was trying to reconcile.
First there's quantum mechanics, mathematical equations describing the atom and its component parts.
Then there's Einstein's Special Theory of Relativity, which at first seems unrelated.
It deals with loftier matters like the nature of space and time.
One of its consequences is that objects behave very differently when they travel close to the speed of light.
The first thing you might ask is why would anyone want to reconcile two such different theories? Well, by the late 1920s, the equations of quantum mechanics were consistently getting the wrong answers when describing electrons, one of the constituents of atoms, as they move at very high speed.
But for Dirac there was a much more esoteric motivation.
He was once quoted as saying, "A physical theory must have mathematical beauty".
So for him, the fact that quantum mechanics and relativity weren't reconciled wasn't just inconvenient, it was downright ugly.
So around 1925, in Cambridge, Dirac put his extraordinary mind, a mind that even Einstein had trouble keeping up with, to work.
This is Room A4, New Court.
It was Dirac's original study.
The original fireplace has been boarded up, but it was here that Dirac tried to understand and bring together the two new ideas of physics.
Word is Dirac would sit here in front of his blazing fireplace and try to understand and bring together these two different theories into one unified picture, one single equation.
For three frustrating years, he laboured alone on the problem.
Then, one evening in early 1928, he had an amazing revelation.
The only way I can explain what happened is to say that the equations of quantum mechanics and special relativity coalesced inside Dirac's mind.
Einstein's description of space and time somehow stretched and squeezed the existing equations of the atom.
They bent and twisted them into new weird and wonderful shapes.
Then, guided by his unshakeable belief that nature's laws must be beautiful, Dirac homed in on one equation, an entirely new description of what goes on inside the atom.
Dirac knew it was right because it had mathematical beauty.
Here it is, the Dirac equation.
Don't try to understand it.
Just look at it and marvel.
As human achievements go, it's up there with King Lear, Beethoven's Fifth or The Origin of the Species.
Hidden in these symbols is the perfect description of how reality works at a fundamental level.
It's the key to nature's secret code.
With perfect mathematical elegance, Dirac's equation describes an atomic particle travelling at any speed, right up to the speed of light.
That much Dirac was expecting to achieve, but when he looked at his own equation more carefully, he noticed something breathtakingly revolutionary about it.
He later said his equation knew more than he did.
In essence, Dirac's equation was telling him there's another universe we've never noticed before.
That's because instead of his equation having one answer, it has two.
The first describes the universe we know, made of the atoms we're familiar with.
The second describes a mirror image to our universe, made of atoms whose properties are somehow reversed.
Science fiction fans will know what's coming.
As well as matter, Dirac's equation predicts the existence of antimatter.
Dirac's theory seemed to say that for everything in our known world, for every part of an atom, every particle, there can exist a corresponding anti-particle with the same mass, but exactly opposite in every other way.
And just like a world in a mirror, the universe made of antimatter atoms would look and work just like ours.
It would be perfectly possible for me to be made out of antimatter.
Anti-me would look and behave exactly the same as original me.
And it's possible that out there in the vast expanses of the cosmos, there are stars and planets and even living beings made of antimatter.
There's one final prediction of the Dirac equation.
It states that matter and antimatter must never come into contact.
If they do, they will annihilate each other in a conflagration of pure energy.
The combined mass of matter and antimatter would convert completely into energy, according to Einstein's famous equation, E=MC2.
So if I ever do meet my doppelganger, we would explode with an energy equivalent to a million Hiroshima-sized atom bombs.
All this sounds like science fiction and the idea of antimatter has inspired huge swathes of it.
But the truth is antimatter, particularly antimatter electrons, called positrons are made routinely now in laboratories.
Positrons are used in sophisticated medical imaging devices called PET scanners that can see through our skulls and accurately map pictures of our brains.
But back in the 1920s, the initial reaction to Dirac's equation among physicists was deeply sceptical.
Even Dirac had trouble believing his own results.
Antimatter seemed such a preposterous concept.
Then came resounding confirmation of the Dirac equation and all its paradoxical implications and it came from the most unexpected place - outer space.
In 1932, physicist Carl Anderson was working here at Caltech in Los Angeles when he made an amazing discovery.
He'd been studying cosmic rays.
These are high-energy subatomic particles that continuously bombard the Earth from outer space.
To do this, he used a device called a cloud chamber.
This is basically a vessel filled with a fine mist of water vapour.
This shows up the tracks of the particles as they stream down through the vapour.
Placed inside a magnetic field, these tracks are deflected one way or the other, depending on the electric charge of the particle.
Positive tracks go one way, negative the other.
Anderson found evidence of particles that look exactly like electrons, but which are deflected in the opposite direction.
He had discovered Dirac's anti-electrons, particles of antimatter.
The Dirac equation is an impressive achievement.
Its prediction of the existence of antimatter, using abstract mathematics alone, would be enough to make it a significant milestone in the history of human thought.
But within just a few years of publication, first Dirac and then others sensed that his new equation was telling them something profound, something completely new about nature.
And they were right.
But the revelation hidden within Dirac's equation would take the best efforts of the greatest minds 30 years to uncover.
The problem with Dirac's equation was this - although it was incredibly powerful and led to the discovery of antimatter, ultimately it could only describe a single electron.
It fails completely to explain what happens when there is more than one electron present.
What was needed was a new theory to explain how electrons interact with each other.
And that turned out to be the most difficult question of the mid-20th Century, but when an answer came, it was to bring with it an unexpected revelation.
This office in Caltech used to belong to the great Richard Feynman.
In our story of so many geniuses of science, Feynman stands, in my view, second only to Einstein in the list of greatest 20th Century physicists.
Feynman wasn't just a common or garden genius.
Many referred to him as a magician, he was so smart, such an innovative thinker.
Like Einstein, he became this mythical figure, a household name.
Feynman was a larger-than-life character with a huge personality.
He loved cultivating and telling anecdotes about himself.
He used to frequent strip clubs, he had affairs with his students and was rumoured to go to orgies, but his greatest contribution to physics was the part he played in developing the next phase of quantum mechanics.
Feynman and his contemporaries were attempting to pick up the atomic torch from Paul Dirac and develop a theory that took our understanding of the atom literally a quantum leap further.
Like Dirac's antimatter equation before, the intention of the new theory was unification.
They wanted to understand how electrons affect each other.
In other words, it aimed to explain how everything works together through the electromagnetic field.
They called their unification project quantum electrodynamics or QED.
The project was a formidable challenge, but the end result was magnificent - nothing less than the most far-reaching and accurate scientific theory ever conceived.
For instance, it predicts a certain property of the electron called its magnetic moment to have a value of Experiments measure precisely the same number.
That's an agreement between theory and experiment to one part in ten billion.
It's an unprecedented level of agreement.
It's like measuring the distance between London and New York to within the thickness of a hair.
The phenomenal accuracy of quantum electrodynamics shows it to underpin almost everything we experience in the physical world.
It's as close to a theory of everything as we have ever come.
It defies the laws of nature - the atomic scale.
It explains shape, colour, texture and the way almost everything interacts and fits together.
It encompasses everything from the biochemistry of life to why we don't fall through the floor.
So what does QED actually say? Well, this is where the going gets very tough.
It may be a wonderful scientific description of nature, but trying to understand what Feynman was doing with his theory is almost impossible.
This is what he himself said when he introduced his theory: "It is my task to convince you not to turn away because you don't understand it.
"My physics students don't understand it.
That's because I don't understand it.
Nobody does.
" If the inventor of the theory doesn't understand, what possible hope is there for the rest of us? With that disclaimer, I'm going to try to explain anyway.
First, you have to abandon your most basic intuition about nature.
You have to give up the notion that empty space is empty.
Let me try to explain.
If I were to suck out all the air from this jar, you'd quite rightly say that having removed all the atoms, I'm left with a vacuum, a volume of pure emptiness.
Quantum electrodynamics flies in the face of this idea by saying that the vacuum is NOT, I repeat not, a place where nothing exists and nothing happens.
Instead, it's full of stuff.
And it's heaving with activity.
How can this possibly be true? Well, let's imagine one tiny point in the emptiness.
Common sense tells us that there's nothing there, but quantum physics tells us there's only nothing there ON AVERAGE.
This forces us to rethink our understanding of reality.
Think of empty space like a bank account, which on average has nothing in it.
This is a concept I'm familiar with! Some days it might be £100 in credit, others £100 overdrawn.
But on average it has a zero balance.
Empty space turns out to have similar accounting skills, but it can borrow energy rather than money and this is literally borrowed from the future, provided it's paid back very quickly.
In practice this means the borrowed energy can be used to create a particle and an anti-particle, which are spontaneously formed from the void, provided that a fraction of a second later they annihilate each other and disappear.
Energy is borrowed out of nowhere.
It's turned into matter.
The matter then self-destructs back into energy.
And this happens in an instant all over the void.
In fact, in a stunning confirmation of Dirac's antimatter theory, the vacuum seethes with huge numbers of matter and antimatter particles, continually being created and annihilated.
Down at the smallest scale, space is a constant storm of creation and destruction.
Physicists call it the quantum foam.
The particles in the quantum foam come and go so quickly, we're completely unaware of them.
We refer to them as virtual particles, but if we could slow time down almost to a standstill, we'd be able to see this seething activity, this constant creation and annihilation of matter and energy that's the fabric of reality itself.
From this comes the most jaw-dropping idea of all.
Quantum electrodynamics says that the matter we think of as the stuff that makes up the everyday world, the world that we see and feel, is basically just a kind of leftover from all the feverish activity that virtual particles get up to in the void.
So you, me, the Earth, the stars, everything, is basically just a part of a deeper, infinitely more complex reality than we ever imagined.
Of course, when Feynman first started to develop his revolutionary ideas in Caltech in the mid '40s, his contemporaries were horrified because at that time the general opinion was that the quantum electrodynamics project was an unmitigated disaster.
The theory couldn't be solved.
The equations had no sensible solutions.
The mathematics had spiralled out of control.
But Feynman believed that he could see a way through the mathematical complexity to a new truth.
What Feynman did, with all the arrogance and confidence of youth, was slash through the insanely complicated maths.
Feynman developed a new series or revolutionary, but almost childlike, diagrams to explain his new ideas.
Their elegant simplicity flew in the face of the complex maths of traditional quantum mechanics.
Conflict seemed inevitable.
Then, in 1948, at the age of 30, Richard Feynman decided to unveil his controversial version of quantum electrodynamics with his idiosyncratic diagrams to the physics world.
And he chose the most important science conference of the American calendar.
Set on the coast of Pennsylvania, the Shelter Island Conference was a physics celebrity circus.
Present were Niels Bohr, so-called "father of atomic physics", the discoverer of antimatter Paul Dirac and the man behind America's atom bomb, Robert Oppenheimer.
The atmosphere at the start of the conference was grim.
Confidence in quantum electrodynamics was at rock bottom.
It seemed a hopeless mess.
One after another, the physicists stood up and droned on despairingly about failing to find a solution.
Then it was the turn of Richard Feynman.
Barely 30 years old, he stood up and took his place in front of the world's most illustrious scientists and started to unveil his new diagrams and equations.
What happened next was astonishing.
A row broke out, not over Feynman's weird description of reality - What happened next was astonishing.
A row broke out, not over Feynman's weird description of reality - physicists were used to weird - but because he dared to visualise what was going on.
Instead of using arcane, complicated mathematics, Feynman was describing what all his virtual particles were up to, using his simple pictures.
There was uproar.
Niels Bohr, the father of quantum mechanics, leapt from his chair in disgust.
He hated Feynman's diagrams because they went completely against everything he'd devoted his life to.
He believed that atomic particles could not be visualised under any circumstances.
Feynman defended his new theory, trying to explain that the diagrams were simply a tool to help visualise his new equations.
But the rest of the scientists, including Dirac, wouldn't hear it, calling him an idiot who understood nothing about quantum mechanics.
Feynman ended his lecture bruised, but unrepentant.
He knew that his diagrams and equations were correct.
If only he could convince the others.
That evening, Feynman met another young physicist called Julian Schwinger.
He was the same age as Feynman and had been identified as a child prodigy at the age of 12.
Although he and Feynman had been working independently and approached the problem very differently, they'd reached identical conclusions.
With their new equations, they could solve quantum electrodynamics and with Feynman's diagrams they produced a theory of awesome power.
Together now as allies, they planned a full-frontal attack on Niels Bohr and the conservatives.
By the end of the conference, the mood in the Pennsylvanian roadhouse had changed from one of frustrated hopelessness to one of excitement and idealism.
Over the next few years, their theory was fleshed out and rapidly became the most accurate and powerful theory mankind had ever had.
Despite finally being tamed, quantum electrodynamics' talk of empty space seething with energy we can't feel and virtual particles we can't see does make many people, including physicists, a little suspicious.
And many sceptics might say these ghostly objects that allegedly fill the vacuum aren't actually real.
Yes, the complicated mathematical equations seem to require them, but that doesn't itself mean they exist.
They might just be mathematical fantasies with no basis in reality.
Well, I have bad news for the sceptics.
Since the late 1950s, direct evidence that empty space isn't in the slightest bit empty but is, in fact, seething with activity has been observed time and time again in laboratories.
And what's wonderful about the proof that emptiness isn't empty is that the first clue came from a jar of mayonnaise.
In 1948, a physicist called Hendrik Casimir was working at the Philips Research Laboratories in Holland on the seemingly obscure problem of colloidal solutions.
This is just a fancy name for substances like paint and mayonnaise which consist of tiny solid particles suspended in a liquid.
You see, no one knew why mayonnaise wasn't runny.
Why doesn't it behave like a normal liquid? It's as if some strange force holds the molecules of mayonnaise together, making it viscous.
And that got Casimir thinking.
In an astonishing insight, Casimir realised that the mysterious force that attracts molecules of mayonnaise together is related to the mysterious virtual particles in empty space.
And even better, he came up with an experiment that would reveal these particles for all to see.
It took another ten years of tinkering in labs to carry out Casimir's experiment, but in essence it's quite simple.
You suspend two metal plates very close to each other in a vacuum.
These plates aren't magnetic or electrically charged, so you'd expect them to sit there immobile, unaffected by each other.
In fact, over time, they start to move towards each other due to a tiny force that pushes them together.
And this force doing the pushing, Casimir showed, was caused by the virtual particles that fill the vacuum.
Like wind pushing the sail of a boat at sea, the stuff that emptiness is made of pushes the plates together.
The fact that nothingness, pure emptiness, could exert a small, but real mechanical force is surely one of nature's greatest magic tricks.
In their more fanciful moments, physicists speculate that this so-called vacuum energy might one day be harnessed.
They imagine it powering intergalactic spaceships carrying humans across the cosmos.
Who knows if this will ever come to pass, but that mayonnaise might lead to space travel is a connection Douglas Adams would be proud of.
Quantum electrodynamics is, by any measure, a truly magnificent discovery.
It's one great pinnacle of our story, a glorious conclusion to five amazing decades of science.
In quantum electrodynamics, the atom had given us a theory that explains much of our universe with stunning accuracy.
But since quantum electrodynamics' triumphant arrival in the late '40s, our story becomes rather messy and awkward.
As a result of quantum electrodynamics, scientists were convinced that the vast majority of everything in the universe consisted of essentially just two things - atoms and light.
Light was made out of tiny particles called photons.
And atoms were made out of three components - the electron, the proton and the neutron.
And because of antimatter, there were anti-protons, anti-neutrons and positrons - a bit strange, but pleasingly symmetrical.
Everything in the physics garden was rosy thanks to the rules of quantum electrodynamics, but then, much to the profound irritation of every working physicist, a load of new and exotic particles suddenly appeared like party gatecrashers to spoil the fun.
Exotic entities that didn't fit in to any known theories were appearing in physics labs with such frequency that scientists couldn't keep up with naming them all.
The neutrino, the positive pion, the negative pion, the kaon, the lambda, the delta And each of these had their antimatter counterparts.
When one new particle, the muon, was discovered, a physicist quipped, "Who ordered that?" The whole thing was a mess and physicists despairingly refer to it as the particle zoo.
It began to seem as though every time scientists solved one of nature's mysteries, the atom would present them with something even more weird.
Within just a few years, atomic physics had gone from a position of quiet confidence to total chaos.
And, of course, to make some sense of this new mystery would require - yes, you've guessed it - another scientific revolution.
The third genius in our story is Murray Gell-Mann.
Gell-Mann was a child prodigy.
By 15, he'd already started at Yale to study Physics and finished his PhD by his early 20s.
His incredible intelligence terrified those around him.
He spoke many languages and seemed to have a deep knowledge of any subject you threw at him.
Like Richard Feynman, whom he joined here at Caltech in the early '60s, he seemed to have this ability to see beyond the mathematics to the underlying secrets of nature below.
Together, Gell-Mann and Feynman made an awesome duo.
This office, Number 456, used to belong to Feynman.
What's great is that just two doors along the corridor was the office of Murray Gell-Mann.
There was an intense academic rivalry between these two giants, but they fed off the creativity.
They were very different.
Feynman played the buffoon, Gell-Mann the cultured elitist.
Gell-Mann used to get upset by Feynman's loud voice.
Feynman enjoyed winding him up.
But during the 1960s and '70s, these two geniuses here at Caltech dominated the world of particle physics.
Their bitter rivalry pushed them both to the very limits of their imaginations and Gell-Mann especially was desperate to prove himself over Feynman by bringing order to the particle zoo.
Within the feverishly intellectual atmosphere of Caltech, Gell-Mann's mind did something very strange.
He started working with a different kind of mathematics to deal with the preponderance of subatomic particles.
He used an obscure form of maths called group theory.
As its name suggests, this is a theory that analyses groups of numbers and symbols and tries to organise them into simple patterns.
It's like working with an abstract form of origami.
Using this technique, Gell-Mann started working all known particles into an organised system, which he called the Eightfold Way, after a Buddhist poem.
But then he had his most awesome revelation.
Gell-Mann realised that his group theory pointed to a deeper underlying mathematical truth, with the potential to rewrite the atomic rule book.
What Gell-Mann's mathematics revealed to him was that in order to make coherent patterns of all the new particles in his Eightfold Way, he had to acknowledge a deeper, underlying, fundamental reality.
Once again, it turned out that things were not at all as they seemed.
Physicists had been comfortable with the notion that atoms have three different kinds of particles - electrons orbiting around the outside of a nucleus made up of proton and neutrons.
Gell-Mann had the temerity to suggest that protons and neutrons were themselves composed of more elementary particles, particles that he called quarks.
Murray Gell-Mann was cultured and arrogant, but at heart lacked confidence.
He knew that for his colleagues, even those used to the strangeness of the atom, quarks were a step too far.
And, in any case, there'd been no evidence of anything remotely like a quark.
He was convinced his new theory would be declared outlandish or just wrong, so Gell-Mann sat on his revelation and one of the greatest ideas in science was almost lost forever.
Then something extraordinary turned up, just a few hundred miles north of his office.
This is the Stanford Linear Accelerator, south of San Francisco.
What you can see is one end of what is basically a giant electron gun.
A beam of high-energy electrons is fired through a tunnel that starts off over two miles away in the hills, travels under the freeway and comes out here where it enters the experimental area.
The grey building is End Station A, where one of the most important discoveries in physics was made.
It was built during the 1960s, when it was - and still is today - the longest single building on Earth.
Although 40 years old, there's construction work going on, and it's still being used for fundamental research today.
I'm now inside the two-mile-long linear accelerator building.
The red objects on your right are called klystrons and they provide the power that boosts the electron beam 20 feet beneath us.
Such is the acceleration, these electrons will, within the first few metres, have reached 99% the speed of light.
Let me put it another way.
If these electrons were to start off their journey at the same time as you fire a bullet from a gun, they would have covered the full two-mile distance before the bullet has left the barrel.
The electron beam now travelling at almost the speed of light would have arrived at the target area.
There would have been, in 1968, where I'm standing now, a large tank of hydrogen - basically, protons.
The electrons would smash into the protons and scatter off through tubes to be picked up by huge detectors that filled the hall outside.
And as they did this, physicists got their biggest ever confirmation that there might be a deeper set of rules underpinning the particle zoo.
What they had discovered from the way the electrons scattered with their extremely high energy was conclusive proof that protons had internal structure.
In other words, protons were made of more elementary particles.
Here were Gell-Mann's quarks.
This was an astonishing moment.
For decades, people were confident that the components of the atomic nucleus - the proton and neutron - were absolutely fundamental.
And now, for the first time, there was evidence of something deeper.
The quark is a tricky and elusive beast.
There are six different kinds or flavours of quark - up, down, strange, charm, top and bottom.
Also, quarks never exist in isolation, only in combination with other quarks.
This makes them impossible to see directly.
We can only infer their presence.
Despite these caveats, the quark brought some semblance of order to the particle zoo.
In recent years, it's allowed us to concoct a simple, yet powerful description of how the universe is built up.
Basically, everything in the universe made of atoms is built up from just quarks and electrons.
That's it.
This now brings us pretty well up to date.
The discovery of the quark in 1967 was the last significant experimental discovery of a new type of fundamental particle.
Some say we may yet discover the quark is made of something even stranger.
And it's possible.
But for now it's as good as it gets.
Our journey from Einstein's proof of the existence of atoms in 1905 until now has been extraordinary.
We've learnt so much about the atomic world, from the size and shape of the atom to how its centre holds the secret of the universe itself.
From how it reveals an unknown world of antimatter to how empty space is far from empty.
From what we thought was a basic building block of the universe to the discovery of something even more fundamental inside it.
And yet, despite all the powerful science which we've uncovered, something doesn't quite add up.
There are two startling and worrying anomalies.
The first of these is now at the forefront of theoretical physics across the world and it concerns one of the oldest scientific principles there is.
Gravity.
It's been thoroughly understood since Einstein, but never really been part of atomic theory, until now.
Suddenly there's a glimmer of hope from ideas that sound almost too crazy to be possible.
Some of these are called string theories, that regard all subatomic particles as tiny vibrating strings that have higher dimensions of space trapped inside them.
Some, called brain theories, suggest that our entire space and time is just a membrane floating through the multiverse.
Another, called quantum loop gravity, suggests that nothing really exists at all and everything is ultimately made up of tiny loops in space and time themselves.
But despite gravity's unwillingness to fit in with quantum theory, I believe there's something worse lurking in the quantum shadows, something truly nightmarish.
Late into the night at physics conferences all over the world, over drinks at the bar when we huddle together to debate and discuss our strangest ideas, there are still things that really, really bother us.
Chief among these are the quantum mechanical laws that atoms obey.
In particular, one aspect of them.
Something called the measurement problem.
If you want to see fear in a quantum physicist's eyes, just say "the measurement problem".
The measurement problem is this - an atom only appears in a particular place if you measure it.
In other words, an atom is spread out all over the place until a conscious observer decides to look at it.
So the act of measurement, or observation, creates the entire universe.
Just to show how mad this idea is, I'm going to explain one of the most famous hypothetical experiments in the whole of science.
It's called the Schrodinger's Cat Experiment.
Erwin Schrodinger was a founding father of atomic theory.
In the mid-1930s he devised a thought experiment to highlight the absurdity of quantum mechanics.
He suggested you take a box in which you place an unopened container of cyanide, connected to a radiation detector and some radioactive material.
If an atom in the material emits a particle, this is picked up by the detector, which releases the cyanide.
Next you take Schrodinger's cat, which in this case is a lovely Norwegian forest cat called Dawkins.
I should point out that this isn't real cyanide.
You place the cat in the box, you close the lidand wait.
Here's the conundrum - according to traditional quantum mechanics, known as the Copenhagen Interpretation, all the time the box is closed, the radioactive atom inside has yet to make up its mind whether it has decayed and spat out a particle.
So we have to describe it as having both decayed and not decayed at the same time.
Think about what this means.
Since the radioactive particle triggers the release of the poison, the cat is both poisoned AND not poisoned.
So until we open the lid to check on the fate of the cat, what's called making a measurement, it's not just that we don't know, but that the cat is literally both dead and alive at the same time.
This is clearly a paradox.
Or is it? The paradox of Schrodinger's cat and the contradictory nature of the measurement problem really does force us to accept that tiny objects down at the atomic scale obey their own set of profoundly strange rules.
But at larger scales, those of everyday experience, those rules vanish and an utterly new set of nice, intuitive rules take over.
How can this be? Some argue that, in fact, the quantum weirdness of the atom might actually be writ large across the cosmos, that we may have to rethink everything known about the universe.
Welcome to the many worlds interpretation of quantum mechanics and its chief adherent, David Deutsch, of Oxford University.
Deutsch proposes that reality itself is profoundly misunderstood.
He says that what quantum mechanics actually describes is not one universe, but an infinite number of parallel universes.
He calls it a multiverse, in which every possible quantum mechanical outcome for each and every atom in the universe exists somewhere.
So an atom and its electron are multiversal objects.
And that multiversal object is what quantum mechanics is describing.
Now that means that the parallel universe aspect of reality as described by quantum theory must apply to objects of all sizes - humans, stars, galaxies, everything.
And that's why we call it the parallel universe theory rather than just parallel electrons theory.
Because we are made of atoms.
That's right.
The same theory that says the atoms exist in more than one place says that we humans also exist in more than one place in different universes.
And there are some universes where you and I don't exist at all.
'The highly-respected author and physicist, Paul Davies, has an even more bizarre idea.
'He suggests that the strangeness of the measurement problem explains how the universe came into being.
' The experimenter today in the lab can make a measurement that affects the nature of reality as it was five billion years ago.
There's a sort of feedback loop between the existence of living organisms and observers and the laws and conditions which have given rise to them.
Otherwise it just seems a bit miraculous that the universe happens to have started out with the right laws and conditions that lead to observers like ourselves who can make measurements and make sense of it all.
But quantum mechanics provides just such a feedback through time.
It allows this backwards in time effect.
Not causation.
It's not that we here now can change the past to fix it so that we exist, but we have an influence on the past through quantum measurements we make.
But there's a pragmatic side to the debate, too.
Other scientists are worried that these bizarre and metaphysical speculations leave the world of measurement and laboratory experiment far behind.
Professor Andrew Jackson of the Niels Bohr Institute in Copenhagen says that ultimately we shouldn't worry about the interpretation or the measurement problem or the cat.
He says we shouldn't be concerned about the so-called true nature of reality.
It's enough that the theory works.
All of the things we can measure give us questions we can answer from quantum mechanics.
So the quantum mechanics itself, without the need for interpretation, provides us with answers or predictions regarding the result of every experiment we can do.
So I don't know That's enough for me.
Yeah.
So you don't think quantum mechanics needs a unique interpretation because it doesn't add anything? It doesn't add anything and I don't think it will lead us to the next step.
An interpretation doesn't change the results or the rules and that's why it's not testable.
The whole purpose of the last 200 years in physics, this incredible leap forward that we've made, has come because experiment confronted theory and led to new theory when theory broke down and back and forth and back and forth.
Interpretations don't do that.
Interpretations only give us some kind of a way of believing we understand what quantum mechanics tells us, but that's a fixed point.
There's no new content to it.
Quantum mechanics is counterintuitive and goes against common sense.
What do you say to people who insist on wanting to know what an atom is doing when you're not looking at it? I'm not sure I'm quoting, Feynman or Dirac, but the answer is, "Shut up and calculate".
So shut up and use the maths.
Right.
"Shut up and calculate"? Is this really scientific pragmatism or just dogmatic fundamentalism? The reason that this is unacceptable, philosophically, can I think be best understood by comparing it with an earlier episode in the history of physics, namely the Inquisition's attitude several hundred years ago to the idea that the Earth goes round the Sun, not the Sun round the Earth.
They wanted to promote a compromise with Galileo where they would admit that the positions of the stars and planets and Sun in the sky are exactly as predicted by that theory, but that it was presumptuous of humans to purport to be able to describe the underlying reality - why the stars appeared there.
The same thing happened with quantum mechanics.
A group of people who didn't like the implications of the theory about reality realised you could use it in practice by just using its predictions.
That is a move you can always make with any scientific theory.
You can always deny it describes reality.
You can't be proved wrong by experiment, but as a philosophical position it's a dead end.
Sterile.
I think it's fair to say that most physicists use quantum mechanics to describe the subatomic world without worrying too much about the interpretation.
Personally, I'm not in favour of this view.
I don't have a preferred interpretation, but believe nature must behave in a particular way.
So only one of the interpretations can be correct and, to be quite honest, we probably haven't found the final answer yet, but I think it's only a matter of time.
I certainly don't subscribe to "Shut up and calculate".
I prefer the "Shut up WHILE you calculate" view.
I'm happy to do my calculations to study atoms, but when I'm away from my work, I still worry about what it all means.
In the last 100 years we have peered deep inside the atom, the basic building block of the universe, and inside this tiny object we found a strange, new world governed by exotic laws that at times seemed to defy reason.
Atoms present us with dizzying contradictions.
They can behave both as particles or waves, they appear to be in more than one place, they force us to rethink what we mean by past and future, by cause and effect, and they tell us strange things about where the universe came from and where it's going.
Pretty amazing stuff for something a millionth of a millimetre across.
That's why Niels Bohr, the father of atomic physics, once said that when it comes to atoms language can only be used as poetry.
What's fascinating to me is that although we've learnt an incredible amount about atoms, our scientific journey has only just begun.
Although we know how a single atom or just a few atoms behave, the way trillions of them come together in concert to create the world around us is still largely a mystery.
To give you one dramatic example - the atoms that make up my body are identical to the atoms in the rocks, the trees, the air, even the stars.
And yet they come together to create a conscious being who can ask the question, "What is an atom?" Explaining all that is surely the next great challenge in science.