Note: While a narrator is used in much of this documentary, the words should be imagined as being spoken by Hawking himself.

SH: Hello, my name is Stephen Hawking, physicist, cosmologist and something of a dreamer. Although I cannot move and I have to speak from a computer, in my mind, I’m free.

Narrator: Free to explore the great questions of the universe, such as, is there a meaning to life? Is there a reason that we exist here on our pale precious world? Finding out delves deep into what it isto be alive, to think, to be a human being, right to the limits of reality itself.

SH: Check it out! We humans are a curious species; we wonder, we seek answers. So can we answer the greatest question of all? Is there a meaning to life? You might think it is a philosophical question but I think philosophy is dead. I believe science holds the key.

Narrator: Science has changed everything, not just the world around us, but how we see ourselves. It’s hard to overstate how profound these discoveries are. For a start, they force us to leave our common sense behind. When we look at the human race clearly and objectively, we see is a pretty amazing creature. We live and love and enjoy ourselves. We sometimes break the rules or behave badly. We all have hopes and dreams and desires. But the first thing we must accept as we go searching for the meaning of life is that all this is nothing more than physics.

You see, the entire universe works according to the laws of nature such as gravity. These laws control everything from the inner workings of atoms to the collisions of colossal galaxies. I see no reason why we tiny humans should be the exception to the rule. After all, we are made of the exact same materials, operating to the very same principles. So the challenge is to explain what humans really are and how we small and insignificant beings relate to the enormous, ancient and rather beautiful universe that produced us. Only then, I think, can we discover if there is a meaning to our lives and perhaps, even what that meaning is.

The first person to make any real headway with this thorny question was a man by the name of Renee Descartes. You may know Descartes as the father of modern philosophy but I consider him to be a pioneering forefather of science. Descartes proposed that humans are made of two distinct components – the body and the mind. He made careful anatomical drawings of the body. He saw it as a complex biological machine. But he was certain the mind was different. He proved this with a simple thought experiment. He tried to imagine that he had no physical body, as if he was floating around like a ghost. That was easy to do even though it’s a little strange. Then he tried to imagine having no mind but he couldn’t. After all, with no mind, how could you imagine anything? He summed it up rather neatly with the phrase, ‘I think, therefore I am.’

He believed that the mind and the body are fundamentally different kinds of things. Working out how they interconnect is the next step for us to find a scientific basis for the meaning of life. Here too, Descartes was well ahead of his time. He suggested the mind was connected to the body at the pineal gland, a small lobe at the top of the spine. Although he wasn’t entirely right, Descartes was pretty close.

We now know that the conscious mind is created by the brain as a whole; an organ of such stupendous complexity that I find it mindboggling. The human brain is far more intricate than you may realise, although you would not be able to realise anything if it weren’t. It contains as many brain cells as there are stars in the milky way – one hundred billion, give or take a few. These cells are coupled together creating more connections than there are galaxies in the known universe. It may seem that studying the brain is for neuroscience but since the brain is governed by fundamental forces, forces like electromagnetism, then thinking itself ultimately boils down to physics – fast, complex physics.

SH: As a physicist, I see the human mind as one of the universe’s more wonderful creations. It’s understanding of the mind as aware of that universe that will lead us to finding out of whether there is meaning at all.

The ancient Greeks were among the first people to wonder if the mind was subject to the laws of nature. That was such an unsettling idea, it was swept under the rug for nearly twenty centuries.

SH: After all, if we are just biological clockwork perhaps there is no meaning to life, perhaps no meaning at all. Well, let’s not be too hasty.

Narrator: Take a typical human scene here in Cambridge: three people having a pleasant day out on the River Cam which winds its way between the colleges. These people, whose bodies are controlled by their brains, can interact with each other; appreciate each other and their surroundings. They might even decide to play a song or fall in love. Their world is not without meaning. Quite the opposite; their world is full of meaning. To them, even a simple glance can be laden with meaning, so much so, that it’s easy to get carried away. So finding out where the meaning is, is a problem for science and it means delving into why we have consciousness at all.

Enter one of the greatest theories in science – evolution. We know all life on Earth evolved from complex molecules called amino acids. These molecules collided randomly to create the first simple living things. Over billions of years, these life forms became ever more sophisticated until eventually complex multi-cellular creatures, animals with brains, arrived.

Complex animals need brains in order to process large amounts of information. They need to be able to react to the world around them and even plan ahead. The more aware an animal is of its environment, the more successful it will be. Eventually awareness became so sophisticated that one animal became aware of itself. And that is what we are – self-aware animals; animals that evolution has equipped with the ability to be conscious. But how is this possible? How can a biological structure possess the ability to think, to feel and to assign meaning to things? Not easy questions to answer. But there are theories about how consciousness can arise.

Back in the 1970s an unexpected breakthrough was made by a mathematician named John Conway here in Cambridge. He devised something called ‘The Game of Life’; a simple simulation that shows how a complex thing like the mind might come about from a basic set of rules. The simulation consists of a grid, a bit like a chessboard, extending infinitely in all directions. Each square of the grid can either be lit up, which he called ‘alive’ or dark, which he called ‘dead’. Whether a given square is dead or alive depends on what is happening in the eight other squares around it, for example, if a living square, like this one, has no living squares nearby, the rules say it will die of loneliness. If a living square is surrounded by more than three living squares, this square will also die of overcrowding. But if a dead square is surrounded by three living squares, it becomes lit or is born. Once you set an initial state of living squares and let the simulation run, these simple laws determine what happens in the future. The results are surprising. As the program progresses, shapes appear and disappear spontaneously. Collections of shapes move across the grid bouncing off one another. There are whole kinds of objects, species that interact. Some can even reproduce just as life does in the real world. These complex properties emerge from simple laws that contain no concepts like movement or reproduction. It’s possible to imagine that something like ‘The Game of Life’, with only a few basic laws might produce highly complex features, perhaps even intelligence. It might take a grid with many billions of squares but that’s not surprising. We have many hundreds of billions of cells in our brains. So I think the human mind and the meaning it creates arise from a large, complex system operating to fairly simple rules which means Descartes was right. The body and mind are different. The body and the brain are made out of physical matter. The mind is a product of the ever changing state of this matter.

SH: Our bodies are the hardware; our minds are the software – just like the software that allows me to speak these words. But this does pose a problem: the problem of free will.

SH: When I was a young man, my father wanted me to become a doctor like him but I chose to study physics instead. Looking back, I’m pleased, because as things worked out, I would have made a pretty useless doctor.

Narrator: So I made the right choice or did I choose at all? Perhaps I am deluded about my own free will. After all if my mind follows the strict rules of nature maybe the path I chose was pre-determined. In fact, scientists have already discovered that our decisions to do something can be affected by many things, not least, electricity.

Now I do hope you’re not squeamish, but let’s imagine watching a surgical procedure called ‘awake brain surgery’. It’s used to treat neurological disorders. The brain is exposed and stimulated with electrical probes. The urge to move a foot, hand or face can be artificially triggered by electrically exciting the appropriate regions of the brain. All it takes is about 3.5 volts to move a face. The patient may think he’s made a choice but, in fact, the surgeon made it for him. We can imagine a future where advanced technology could allow the doctor to control someone’s thoughts, perhaps even make them fall in love. The unfortunate subject would believe he was acting out of free will but the opposite would be true. It’s all just physics in the brain. For many people this idea is a horrific thought, for it seems to deny our basic humanity and would turn us, not only into machines, but machines that could be controlled conceivably for evil means.

So perhaps I had no choice when I decided between physics and biology. Perhaps the laws of physics predetermined my career. Well, not necessarily. Predictability isn’t always a consequence of the laws of nature. It’s hard to predict an individual roll of the dice even though it’s pure physics. Scale that up to a really complex system, and predictability becomes impossible. To see such a system, you only have to look outside.

Now, as you may know, we English are slightly obsessed by our ever changing weather, especially when planning a summer bbq. Wouldn’t it be nice if we could be sure of a sunny day before inviting our friends? Predicting a nice day should be relatively simple; after all, we understand pretty well how the atmosphere reacts to heat and pressure to form clouds and thunder storms. But no matter how hard we try to compute all the details, it is impossible to say exactly what the weather will be at any particular place and time. Instead we make weather forecasts using simplified models that don’t take into account every single tiny variable. But tiny variables can have big consequences. So if a butterfly flaps its wings too hard in the Amazon, the bbq might be a washout. It’s a complex system.

I think our brains are just another complex system. Like the Earth’s atmosphere, we abide by the physical laws, yet are impossible to fully predict. The mind is like weather inside our heads. Free will is simply what we call the process when this vastly complex system faces a choice. Let me try and explain. Imagine a man wakes up at night and feels thirsty. Let’s say I’m right about the human mind and this man’s brain is working according to the laws of nature. Where exactly might free will come into it? Let’s give him a choice to make: either orange juice or apple juice to quench his thirst. As he smells the apple juice a storm of neurons fire and a memory kicks in. He is reminded of a special moment in an apple orchard and his mind is made up. So his choice is not surprising. He had to make a decision; he chose, and that’s what we call free will. But it’s all still a matter of physics.

SH: Looking back I certainly made the choice to be a physicist and I certainly feel like I have free will because free will is what we call the complex physics that happens when we decide.

Narrator: But if our choices are just physics does it mean that we are deluding ourselves and there is no meaning to life? To find the answer we have to go even deeper and question the very nature of reality itself.

Reality! Most of us share the same common sense view of what reality is. The world around us exists independently of us. It consists of real things that are really there. But science is unravelling this basic assumption which happily leads us to the meaning of life. Take this little girl here walking around through a busy, bustling market in Monza, Italy. Her reality is a riot of sound, colour, taste and smell, all based in the many bits of information her senses are feeding her brain. But once we accept that the mind is like a weather system in the brain, governed by physics yet unpredictable, reality itself begins to break down. It becomes subjective. My reality is different than yours or this fish’s. Its reality appears distorted by the round fishbowl. Everything is warped and curved. In Monza, they thought it was so cruel to keep fish in such a distorted reality, they actually banned round fish bowls. Speaking as a scientist, I think such a law is unfounded. Just because the goldfish’s view isn’t the same as ours, it doesn’t mean that it is living in a distorted reality. Imagine that this goldfish is something of a genius. Even though he sees the world differently than we do, he can still work out the laws of nature. The mathematics would be more complicated to account for the distorted perspective but the physics would be the same. If this clever fish could work out how fast the policeman’s motorcycle was moving, he could calculate the right trajectory and the right moment to escape.

I don’t think that one reality is more valid that another and that means that reality itself is in the mind of the beholder. When you think about it, even our point of view is far from perfect. It may seem that human eyes, for example, are pretty good at seeing the world around them, but in fact, they’re not so great. Our eyes only see a small area with good resolution, an area the width of your thumb when held at arm’s length. The eyes then send electrical signals to the brain down the optic nerve. The area where this nerve connects to our eyes means we have two blind spots in our vision.

But we don’t perceive a blurry world with two black holes. This is because of the incredible organ that is our brain. Our brains fill in the gaps, transforming the crude signals from the eyes into a three dimensional model of the outside world. It is these mental models that each of us call reality. Even when we sleep and our brains are starved of information from our senses, we can build whole realities where anything is possible. It’s only when we wake up that we realise that we were dreaming.

So how does this realisation bring us closer to the meaning of life? Well, at first it would seem to be bad news. If reality itself is just a model in each individual brain where can the meaning be? What’s more, perhaps there is no real reality out there at all!

SH: It may seem crazy to doubt that our concept of reality is true but I think to find the meaning of life we must answer the question: is there an independent reality or not?

Narrator: Imagine a scenario that is straight from a science fiction movie. The world around you is actually nothing more than an elaborate fabrication of some unknown superior intelligence. A giant super-computer provides you with all your senses from what you see, to what you smell, hear and touch. But in fact you have no senses. Your body does not exist. You are just a brain in a jar. It may sound bizarre but this is the genuine scientific hypothesis called ‘the simulation theory’. For all we know, every one of our perceived realities is simply fed to us by some all-powerful super-computer and the simulation is so perfect that we never even notice. But here’s the crux: it doesn’t actually matter! It’s as Descartes said, ‘we think therefore we are’. The hamburger could be nothing more than a piece of computer code but our desire to eat it is still our own desire. We still feel hunger. Our minds still exist even if we are in a simulation, so doubting the true nature of reality serves no purpose. It’s simpler to just accept that there are fundamental limits to what we can know.

Take this table for example. How do you know if a table still exists when you go out of a room and can no longer see it? For all you know the table could pack up and disappear out of the window. It could take a visit to the international space station, perhaps even fly to the moon, all this before returning to the exact same spot an instant before you re-enter the room. This of course, is a pretty unlikely scenario, but one we can’t rule out. It is much simpler to assume the table stays put when we are not there. It is our ‘best fit’ model of reality. This is essentially what we do in science. We create best fit models of how we believe the universe actually works.

The ancient Greeks were the first to build such scientific models. They suggested that the Earth was a large sphere, motionless and fixed at the centre of the universe. But later, pioneering scientists like Copernicus and Galileo found a much simpler and completely revolutionary model to describe the same observations. They proposed that the Earth itself was spinning and orbiting the sun at the same time along with all the other planets. But neither can be said to actually be true because they, like all models, are just models in our heads that best fit the reality we perceive. In fact, physicists are forever creating ever more sophisticated models and the truth of those models is impossible to establish.

A good example of this came in the 1960s when physicists devised the theory of really tiny bits of matter called quarks. These quarks were proposed to be the building blocks of the sub-atomic particle called a proton. The theory or model suggested that these quarks were held together by a force that got stronger as you tried to separate them, as if the quarks were bound by tiny rubber bands. This model also implied that there was no way one could ever see a single isolated quark. At first some people were sceptical. If something, by its very definition can never be seen, can it be said to exist? Does it make sense to say that quarks are real or not? In vast particle accelerators like this one at Cern in Switzerland, scientists are on the hunt for quarks and other sub-atomic particles. By smashing protons together at incredible speed, we can study the behaviour of the tiniest particles in nature and, although we haven’t been able to directly observe quarks, we have seen evidence of particle behaviour predicted by the quark model. So do quarks exist?

SH: The answer is, they exist only as far as they are a model that works. That is as far as we can go. This is called the concept of model dependent reality and I believe it leads us directly to the meaning of life.

Narrator: To my mind, science has taught us something pretty remarkable. We humans are highly complex biological machines behaving in accordance with the laws of nature. Our brains create and sustain our conscious minds through an extraordinary network of interacting neurons. That consciousness creates a three dimensional model of the outside world – a best fit model - that we call reality. This reality is much more than what we can see around us in our everyday lives. A vast array of ground and space telescopes have extended our senses allowing us to see deep into space and build a much bigger model than ever before. As we peer further and further into the cosmos, our reality has grown bigger and bigger still. Where once we saw chinks in heaven’s floor we now see distant stars like our own sun, many with their own planets and moons. Then we discovered distant galaxies, home to billions more stars. We have peered back in time all the way to the birth of the universe itself. All this, the entire 13.7 billion year history of the universe, exists as a model inside our minds.

So where does this leave us with finding a meaning to life? The answer I think is pretty clear. Meaning itself is simply another piece of the model of reality that we each build inside our own brain. Take this mother and child. They each create their own little bubbles of reality in their conscious minds. The youngster can create a detailed mental model of his surroundings even though he may not fully appreciate the fact that he is on the fifth floor. The mother’s reality is also produced by her mind and, for her, her love for her boy is as real as the telephone in her hand. In short, the brain is responsible for not only the reality we perceive but for our emotions and meaning too. Love and honour, right and wrong are part of the universe we create in our minds, just as a table, a planet or a galaxy. It’s pretty remarkable to think that our brains, which are essentially a collection of particles working to the laws of physics, have this wonderful ability to not only perceive reality but to give it meaning too. The meaning of life is what you choose it to be. Personally, I like to think that it is every one of us that gives meaning to the universe. We are, as cosmologist Carl Sagan once said, the universe contemplating itself.

SH: Meaning can only ever exist within the confines of the human mind and in this way the meaning of life is not somewhere out there but right between our ears. In many ways, this makes us the lords of creation.