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00:00Hello. My name is Stephen Hawking. Physicist, cosmologist, and something of a dreamer.
00:14Although I cannot move, and I have to speak through a computer, in my mind, I'm free.
00:23Free to explore the great questions of the universe.
00:31Such as, is there a meaning to life?
00:37Is there a reason that we exist here on our pale, precious world?
00:43Finding out delves deep into what it is to be alive.
00:48To think.
00:51To be a human being.
00:55Right to the limits of reality itself.
01:00Check it out.
01:12We humans are a curious species.
01:16We wonder.
01:18We seek answers.
01:20So can we answer the greatest question of all?
01:25Is there a meaning to life?
01:28You might think it is a philosophical question.
01:32But I think philosophy is dead.
01:35I believe science holds the key.
01:44Science has changed everything.
01:49Not just the world around us, but how we see ourselves.
02:00It's hard to overstate how profound these discoveries are.
02:03For a start, they force us to leave our common sense behind.
02:09When we look at the human race clearly and objectively.
02:15What we see is a pretty extraordinary creature.
02:24We live and love and enjoy ourselves.
02:30We sometimes break the law or behave badly.
02:35We all have hopes.
02:37And dreams.
02:39And desires.
02:41But the first thing we must accept as we go searching for the meaning of life.
02:47Is that all this is nothing more than physics.
02:53You see, the entire universe works according to the laws of nature, such as gravity.
03:08These laws control everything.
03:11From the inner workings of atoms.
03:14To the collisions of colossal galaxies.
03:18I see no reason why we tiny humans should be the exception to the rule.
03:30After all, we are made of the exact same materials.
03:34Operating to the very same principles.
03:39So the challenge is to explain what humans really are.
03:43And how we, small, insignificant beings, relate to the enormous, ancient, and rather beautiful universe that produced us.
03:53Only then, I think, can we discover if there is a meaning to our lives.
03:58And perhaps even what that meaning is.
04:02The first person to make any real headway with this thorny question.
04:09Was a man by the name of René Descartes.
04:14You may know Descartes as the father of modern philosophy.
04:20But I consider him to be a pioneering forefather of science.
04:25Descartes proposed that humans are made of two distinct components.
04:32The body and the mind.
04:37He made careful anatomical drawings of the body.
04:41He saw it as a complex biological machine.
04:47But he was certain the mind was different.
04:53He proved this with a simple thought experiment.
05:00He tried to imagine that he had no physical body.
05:04As if he was floating around like a ghost.
05:13That was easy to do.
05:15Even though it's a little strange.
05:18But he couldn't.
05:19After all, with no mind, how can you imagine anything?
05:36He summed it up rather neatly with the phrase,
05:40I think, therefore, I am.
05:45He believed that the mind and the body
05:48are fundamentally different kinds of things.
05:53Working out how they interconnect is the next step for us
05:57to find a scientific basis for the meaning of life.
06:01Here, too, Descartes was well ahead of his time.
06:11He suggested the mind was connected to the body at the pineal gland,
06:16a small lobe at the top of the spine.
06:20Although he wasn't entirely right, Descartes was pretty close.
06:26We now know that the conscious mind is created by the brain as a whole.
06:30An organ of such stupendous complexity that I find it mind-boggling.
06:47The human brain is far more intricate than you may realize.
06:51Although you would not be able to realize anything if it weren't.
06:55It contains as many brain cells as there are stars in the Milky Way.
07:05One hundred billion, give or take a few.
07:09These cells are coupled together, creating more connections than there are galaxies in the known universe.
07:21It may seem that studying the brain is a job for neuroscience.
07:25But since the brain is governed by fundamental forces, forces like electromagnetism,
07:31then thinking itself ultimately boils down to physics.
07:37Fast, complex physics.
07:40As a physicist, I see the human mind as one of the universe's more wonderful creations.
07:52It's understanding how the mind is aware of that universe that will lead us to finding out whether there is meaning to it all.
08:03The ancient Greeks were among the first people to wonder if the mind was subject to the laws of nature.
08:16That was such an unsettling idea.
08:18It was swept under the rug for nearly twenty centuries.
08:22After all, if we are just biological clockwork, perhaps there is no meaning to life.
08:32Perhaps no meaning at all.
08:35Well, let's not be too hasty.
08:41Take a typical human scene here in Cambridge.
08:50Three people having a pleasant day out on the river Cam, which winds its way between the colleges.
08:57These people, whose bodies are controlled by their brains, can interact with each other, appreciate each other, and their surroundings.
09:14They might even decide to play a song, or fall in love.
09:24Their world is not without meaning.
09:27Quite the opposite.
09:29Their world is full of meaning.
09:32To them, even a simple glance can be laden with meaning.
09:39So much so that it's easy to get carried away.
09:47So finding out where the meaning is, is a problem for science.
10:03And it means delving into why we have consciousness at all.
10:10Enter one of the greatest theories in science.
10:14Evolution.
10:17We know all life on Earth evolved from complex molecules called amino acids.
10:24These molecules collided randomly to create the first simple living things.
10:31Over billions of years, these life forms became ever more sophisticated.
10:36Until eventually, complex, multicellular creatures, animals with brains, arrived.
10:44complex animals need brains in order to process large amounts of information.
10:57They need to be able to react to the world around them, and even plan ahead.
11:03The more aware an animal is of its environment, the more successful it will be.
11:17Eventually, awareness became so sophisticated that one animal became aware of itself.
11:26And that is what we are.
11:28Self-aware animals.
11:30Animals that evolution has equipped with the ability to be conscious.
11:39But how is this possible?
11:43How can a biological structure possess the ability to think, to feel, and to assign meaning to things?
11:55Not easy questions to answer.
11:59But there are theories about how consciousness could arise.
12:14Back in the 1970s, an unexpected breakthrough was made by a mathematician named John Conway, here in Cambridge.
12:24He devised something called the game of life.
12:34A simple simulation that shows how a complex thing like the mind might come about from a basic set of rules.
12:42The simulation consists of a grid, a bit like a chessboard, extending infinitely in all directions.
12:57Each square of the grid can either be lit up, which he called alive, or dark, which he called dead.
13:11Whether a given square is dead or alive depends on what is happening in the eight other squares that surround it.
13:26For example, if a living square like this one has no living squares nearby, the rules say it will die of loneliness.
13:39If a living square is surrounded by more than three other living squares, the square will also die of overcrowding.
13:52But if a dead square is surrounded by three living squares, it becomes lit, or is born.
14:07Once you set an initial state of living squares and let the simulation run, these simple laws determine what happens in the future.
14:18The results are surprising.
14:31As the program progresses, shapes appear and disappear spontaneously.
14:36Collections of shapes move across the grid, bouncing off one another.
14:44There are whole kinds of objects, species, that interact.
14:50Some can even reproduce, just as life does in the real world.
14:56These complex properties emerge from simple laws that contain no concepts like movement or reproduction.
15:13It's possible to imagine that something like the game of life with only a few basic laws might produce highly complex features.
15:23Perhaps even intelligence.
15:27It might take a grid with many billions of squares, but that's not surprising.
15:32We have many hundreds of billions of cells in our brains.
15:40So I think the human mind and the meaning it creates arise from a large, complex system, operating to fairly simple rules.
15:51Which means Descartes was right.
15:55The body and mind are different.
15:58The body and the brain are made out of physical matter.
16:03The mind is a product of the ever-changing state of this matter.
16:17Our bodies are the hardware.
16:21Our minds are the software.
16:23Just like the software that allows me to speak these words.
16:32But this does pose a problem.
16:35The problem of free will.
16:44When I was a young man, my father wanted me to become a doctor like him.
16:49But I chose to study physics instead.
16:53Looking back, I'm pleased.
16:56Because as things worked out, I would have made a pretty useless doctor.
17:04So I made the right choice.
17:07Or did I choose at all?
17:12Perhaps I am deluded about my own free will.
17:15After all, if my mind follows the strict rules of nature, maybe the path I chose was predetermined.
17:29In fact, scientists have already discovered that our decisions to do something can be affected by many things.
17:35Not least, electricity.
17:38Now, I do hope you're not squeamish.
17:42But let's imagine watching a surgical procedure called awake brain surgery.
17:49It's used to treat neurological disorders.
17:53The brain is exposed and stimulated with electrical probes.
18:04The urge to move a foot, hand or face can be artificially triggered by electrically exciting appropriate regions of the brain.
18:17All it takes is about 3.5 volts in the right place.
18:26The patient may think he has made a choice.
18:31But in fact, the surgeon made it for him.
18:34We can imagine a future where advanced technology could allow the doctor to control someone's thoughts.
18:39Perhaps even make them fall in love.
18:50The unfortunate subject would believe he was acting out of free will.
18:55But the opposite would be true.
18:58It's all just physics in the brain.
19:01For many people, this idea is a horrific thought.
19:08For it seems to deny our basic humanity.
19:12And would turn us not only into machines.
19:15But machines that can be controlled, conceivably, for evil means.
19:20So perhaps I had no choice when I decided between physics and biology.
19:47Perhaps the laws of physics predetermined my career.
19:53Well, not necessarily.
19:58Predictability isn't always a consequence of the laws of nature.
20:04It's hard to predict an individual roll of the dice, even though it's pure physics.
20:11Scale that up to a really complex system, and predictability becomes impossible.
20:27To see such a system, you only have to look outside.
20:31Now, as you may know, we English are slightly obsessed by our ever-changing weather.
20:44Especially when planning a summer barbecue.
20:49Wouldn't it be nice if we could be sure of a sunny day before inviting our friends?
20:54Predicting a nice day should be relatively simple.
21:00After all, we understand pretty well how the atmosphere reacts to heat and pressure to form clouds and thunderstorms.
21:06But 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.
21:22Instead, we make weather forecasts using simplified models that don't take into account every single tiny variable.
21:43But tiny variables can have big consequences.
21:46So, if a butterfly flaps its wings too hard in the Amazon, the barbecue might be a washout.
22:03It's a complex system.
22:04I think our brains are just another complex system, like the Earth's atmosphere.
22:22We abide by the physical laws, yet are impossible to fully predict.
22:29The mind is like weather inside our heads.
22:32Free will is simply what we call the process that happens when this vastly complex system faces a choice.
22:44Let me try and explain.
22:51Imagine a man wakes up at night and feels thirsty.
22:55Let's say I'm right about the human mind.
23:02And this man's brain is working according to the laws of nature.
23:08Where exactly might free will come into it?
23:11Let's give him a choice to make.
23:16Either orange juice or apple juice to quench his thirst.
23:21As he smells the apple juice, a storm of neurons fire, and a memory kicks in.
23:30He is reminded of a special moment in an apple orchard.
23:35And his mind is made up.
23:47So his choice is not surprising.
23:50He had to make a decision.
23:52He chose.
23:53He chose.
23:54And that is what we call free will.
23:58But it's all still a matter of physics.
24:03Looking back, I certainly made the choice to be a physicist.
24:09And I certainly feel like I had free will.
24:13Because free will is what we call the complex physics that happens when we decide.
24:19But if our choices are just physics, does it mean we are deluding ourselves and there is no meaning to life?
24:30To find the answer, we have to go even deeper and question the very nature of reality itself.
24:37Reality.
24:43Most of us share the same common sense view of what reality is.
24:49The world around us exists independently from us.
24:54It's full of real things that are really there.
24:57But science is unraveling this basic assumption, which handily leads us to the meaning of life.
25:11Take this little girl here, walking round through a busy bustling market in Monza, Italy.
25:16Her reality is a riot of sound, color, taste, and smell.
25:30All based on the many bits of information her senses are feeding her brain.
25:34But once we accept that the mind is like a weather system in the brain, governed by physics yet unpredictable,
25:49reality itself begins to break down.
25:54It becomes subjective.
25:56My reality is different than yours, or this fish's.
26:08Its reality appears distorted by the round fishbowl.
26:13Everything is warped and curved.
26:15In Monza, they thought it was so cruel to keep fish in such a distorted reality,
26:25that they actually banned round fishbowls.
26:38Speaking as a scientist, I think such a law is unfounded.
26:41Just because the goldfish's view isn't the same as ours,
26:48it doesn't mean that it is living in a distorted reality.
26:54Imagine that this goldfish is something of a genius.
27:00Even though he sees the world differently than we do,
27:03he could still work out the laws of nature.
27:05The mathematics would be more complicated to account for the distorted perspective,
27:13but the physics would be the same.
27:20If this clever fish could work out how fast the policeman's motorcycle was moving,
27:27he could calculate the right trajectory and the right moment to escape.
27:35I don't think that one reality is more valid than another.
27:45And that means that reality itself is in the mind of the world.
27:49the beholder.
27:50When you think about it, even our point of view is far from perfect.
27:59It may seem that human eyes, for example, are pretty good at seeing the world around them.
28:06But in fact, they're not so great.
28:09Our eyes only see a small area with good resolution, an area the width of your thumb when held at arm's length.
28:23The eyes then send electrical signals to the brain down the optic nerve down the optic nerve.
28:46The area where this nerve connects to our eyes means we have two blind spots in our vision.
28:52But we don't perceive a blurry world with two black holes.
29:00This is because of the incredible organ that is our brain.
29:05Our brains fill in the gaps, transforming the crude signals from the eyes into a three-dimensional model of the outside world.
29:15It is these mental models that each of us call reality.
29:24So how does this realization bring us closer to the meaning of life?
29:40Well, at first it would seem to be bad news.
29:46If reality itself is just a model in each individual brain, where can the meaning be?
29:53What's more, perhaps there is no real reality out there at all.
30:04It may seem crazy to doubt that our concept of reality is true.
30:09But I think to find the meaning of life, we must answer the question.
30:15Is there an independent reality or not?
30:19Imagine a scenario that is straight from a science fiction movie.
30:32The world around you is actually nothing more than an elaborate fabrication of some unknown, superior intelligence.
30:46A giant supercomputer provides you with all your senses.
30:53From what you see, to what you smell, hear and touch.
31:00But in fact, you have no senses.
31:05Your body does not exist.
31:07You are just a brain in a jar.
31:16It may sound bizarre, but this is a genuine scientific hypothesis called the simulation theory.
31:26For all we know, every one of our perceived realities is simply fed to us by some all-powerful supercomputer.
31:34And the simulation is so perfect that we never even notice.
31:45But here's the crux.
31:47It doesn't actually matter.
31:51It's as Descartes said,
31:53we think, therefore we are.
31:56The hamburger could be nothing more than a piece of computer code.
32:08But our desire to eat it is still our own desire.
32:15We still feel hunger.
32:17Our minds still exist.
32:20Even if we are in a simulation.
32:22So doubting the true nature of reality serves no purpose.
32:30It's simpler to just accept that there are fundamental limits to what we can know.
32:35Take this table, for example.
32:45How do you know if a table still exists if you go out of a room and can no longer see it?
32:51For all you know, the table could pack up and disappear out of the window.
33:10It could take a visit to the International Space Station.
33:13Perhaps even fly under the moon.
33:27All this before returning to the exact same spot an instant before you re-enter the room.
33:33This, of course, is a pretty unlikely scenario, but one we can't rule out.
33:46It is much simpler to assume the table stays put when we are not there.
33:51It is our best fit model of reality.
33:55This is essentially what we do in science.
34:03We create best fit models of how we believe the universe actually works.
34:11The ancient Greeks were the first to build such scientific models.
34:15They suggested that the Earth was a large sphere, motionless and fixed at the center of the universe.
34:26But later, pioneering scientists like Copernicus and Galileo found a much simpler and completely revolutionary model to describe the same observations.
34:38They proposed that the Earth itself was spinning and orbiting the Sun at the same time.
34:45along with all the other planets.
34:52But neither can be said to actually be true.
34:56Because they, like all models, are just models in our heads that best fit the reality we perceive.
35:08In fact, physicists are forever creating ever more sophisticated models.
35:12And the truth of those models is impossible to establish.
35:21A good example of this came in the 1960s, when physicists devised the theory of really tiny bits of matter, called quarks.
35:30These quarks were proposed to be the building blocks of the subatomic particle called a proton.
35:42The theory, or model, suggested that these quarks were held together by a force that got stronger as you tried to separate them.
35:49As if the quarks were bound by tiny rubber bands.
35:56This model also implied that there is no way one can ever see a single, isolated quark.
36:02At first, some people were sceptical.
36:07If something by its very definition can never be seen, can it be said to exist?
36:15Does it make sense to say that quarks are real or not?
36:20In vast particle accelerators, like this one at CERN in Switzerland, scientists are on the hunt for quarks and other subatomic particles.
36:36By smashing protons together at incredible speeds, we can study the behaviour of the tiniest particles in nature.
36:43And although we haven't been able to directly observe quarks, we have seen evidence of particle behaviour predicted by the quark model.
36:59So do quarks exist?
37:04The answer is, they exist only in as far as they are a model that works.
37:09That is, as far as we can go.
37:15This is called the concept of model-dependent reality.
37:19And I believe it leads us directly to the meaning of life.
37:28To my mind, science has taught us something pretty remarkable.
37:32We humans are highly complex biological machines behaving in accordance with the laws of nature.
37:41Our brains create and sustain our conscious minds through an extraordinary network of interacting neurons.
37:48That consciousness creates a three-dimensional model of the outside world, a best-fit model that we call reality.
38:03This reality is much more than what we see around us in our everyday lives.
38:08A 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.
38:25As we peer further and further into the cosmos, our reality has grown bigger and bigger still.
38:38Where once we saw chinks in heaven's floor, we now see distant stars like our sun.
38:45Many with their own planets and moons.
38:51Then we discovered distant galaxies.
38:55Home to billions more stars.
39:00We have peered back in time, all the way to the birth of the universe itself.
39:06All this, the entire 13.7 billion year history of the universe, exists as a model inside our minds.
39:26So where does this leave us with finding a meaning to life?
39:37The answer, I think, is pretty clear.
39:42Meaning itself is simply another piece of the model of reality that we each build inside our own brain.
39:50Take this mother and child.
40:00They each create their own little bubbles of reality in their conscious minds.
40:07The youngster can create a detailed mental model of his surroundings.
40:11Even though he may not fully appreciate the fact, he's on the fifth floor.
40:26The mother's reality is also produced by her mind.
40:30And for her, her love for her boy is as real as the telephone in her hand.
40:35In short, the brain is responsible for not only the reality we perceive.
40:44But for our emotions and meaning too.
40:54Love and honor.
40:56Right and wrong.
40:57are part of the universe we create in our minds, just as a table, a planet, or a galaxy.
41:07It's pretty remarkable to think that our brains,
41:13which are essentially a collection of particles working to the laws of physics,
41:17have this wonderful ability to not only perceive reality, but to give it meaning too.
41:34The meaning of life is what you choose it to be.
41:37Personally, I like to think that it is every one of us that gives meaning to the universe.
41:51We are, as cosmologist Carl Sagan once said,
41:56the universe contemplating itself.
41:59Meaning can only ever exist within the confines of the human mind.
42:14And in this way, the meaning of life is not somewhere out there,
42:19but right between our ears.
42:21In many ways, this makes us the lords of creation.
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