Ethic, Hedge, and Octavia stand on the edge of a bottomless ravine.
It’s the only thing between them and the tower that houses the second of three powerful artifacts.
They’ve got a brief window of time to get across before the guards return.
With Hedge’s fuel gauge on empty he won’t be able to fly Ethic across, so the only option is to make a bridge.
Fortunately, the floating stacks of stones nearby are bridge components— invented by Octavia herself— called hover-blocks.
Activate a pile with a burst of energy, and they’ll self-assemble to span the ravine as Ethic walks across.
But there is, of course, a catch.
The hover-blocks are only stable when they’re perfectly palindromic.
Meaning they have to form a sequence that’s the same when viewed forwards and backwards.
The stacks start in random orders, but will always put themselves into a palindromic configuration if they can.
If they get to a point where a palindrome isn’t possible, the bridge will collapse, and whoever’s on it will fall into the ravine.
Let’s look at an example.
This stack would make itself stable.
First the A blocks hold themselves in place.
Then the B’s.
And finally the C would nestle right between the B’s.
However, suppose there was one more A.
First two A blocks form up, then two B’s, but now the remaining C and A have nowhere to go, so the whole thing falls apart.
The Node of Power enables Hedge to energize a single stack of blocks.
What instructions can Ethic give Hedge to allow him to efficiently find and power a stable palindromic stack?
Pause now to figure it out for yourself.
Examples of palindromes include ANNA, RACECAR, and MADAM IM ADAM.
Counting the number of times a given letter appears in a palindrome will reveal a helpful pattern.
Pause now to figure it out for yourself.
Let’s first look at a naïve solution to this problem.
A naïve solution is a simple, brute-force approach that isn’t optimized— but will get the job done.
Naïve solutions are helpful ways to analyze problems, and work as stepping stones to better solutions.
In this case, a naïve solution is to approach a pile of blocks, try all the arrangements, and see if one is a palindrome by reading it forward and then backwards.
The problem with this approach is that it would take a tremendous amount of time.
If Hedge tried one combination every second, a stack of just 10 different blocks would take him 42 days to exhaust.
That’s because the total time is a function of the factorial of the number of blocks there are.
10 blocks have over 3 million combinations.
What this naïve solution shows is that we need a much faster way to tell whether a pile of blocks can form a palindrome.
To start, it may be intuitively clear that a pile of all different blocks will never form one.
Why?
The first and last blocks can’t be the same if there are no repeats.
So when can a given sequence become a palindrome?
One way to figure that out is to analyze a few existing palindromes.
In ANNA, there are 2 A’s and 2 N’s.
RACECAR has 2 R’s, 2 A’s, 2 C’s, and 1 E.
And MADAM IM ADAM has 4 M’s, 4 A’s, 2 D’s, and 1 I.
The pattern here is that most of the letters occur an even number of times, and there’s at most 1 that occurs just once.
Is that it?
What if RACECAR had 3 E’s instead of 1?
We could tack the new E’s onto the ends and still get a palindrome, so 3 is ok.
But make that 3 E’s and 3 C’s, and there’s nowhere for the last C to go.
So the most generalized insight is that at most one letter can appear an odd number of times, but the rest have to be even.
Hedge can count the letters in each stack and organize them into a dictionary, which is a tidy way of storing information.
A loop could then go through and count how many times odd numbers appear.
If there are less than 2 odd characters, the stack can be made into a palindrome.
This approach is much, much faster than the naïve solution.
Instead of factorial time, it takes linear time.
That’s where the time increases in proportion to the number of blocks there are.
Now write a loop for Hedge to approach the piles individually, and stop when he finds a good one, and you’ll be ready to go.
Here’s what happens: Hedge is fast, but there are so many piles it takes a long time.
Too long.
Ethic and Hedge are safe.
But Octavia is not so lucky.
In 1970, marijuana was classified as a schedule 1 drug in the United States: the strictest designation possible, meaning it was completely illegal and had no recognized medical uses.
For decades, this view persisted and set back research on the drug's mechanisms and effects.
Today, marijuana’s therapeutic benefits are widely acknowledged, and some nations have legalized medical use or are moving in that direction.
But a growing recognition for marijuana’s medical value doesn’t answer the question: is recreational marijuana use bad for your brain?
Marijuana acts on the body’s cannabinoid system, which has receptors all over the brain and body.
Molecules native to the body, called endocannabinoids, also act on these receptors.
We don’t totally understand the cannabinoid system, but it has one feature that provides a big clue to its function.
Most neurotransmitters travel from one neuron to the next through a synapse to propagate a message.
But endocannabinoids travel in the opposite direction.
When a message passes from the one neuron to the next, the receiving neuron releases endocannabinoids.
Those endocannabinoids travel backward to influence the sending neuron— essentially giving it feedback from the receiving neuron.
This leads scientists to believe that the endocannabinoid system serves primarily to modulate other kinds of signals— amplifying some and diminishing others.
Feedback from endocannabinoids slows down rates of neural signaling.
That doesn’t necessarily mean it slows down behavior or perception, though.
For example, slowing down a signal that inhibits smell could actually make smells more intense.
Marijuana contains two main active compounds, tetrahydrocannabinol or THC, and cannabidiol, or CBD.
THC is thought to be primarily responsible for marijuana’s psychoactive effects on behavior, cognition, and perception, while CBD is responsible for the non-psychoactive effects.
Like endocannabinoids, THC slows down signaling by binding to cannabinoid receptors.
But it binds to receptors all over this sprawling, diffuse system at once, whereas endocannabinoids are released in a specific place in response to a specific stimulus.
This widespread activity coupled with the fact that the cannabinoid system indirectly affects many other systems, means that each person’s particular brain chemistry, genetics, and previous life experience largely determine how they experience the drug.
That’s true much more so with marijuana than with other drugs that produce their effects through one or a few specific pathways.
So the harmful effects, if any, vary considerably from person to person.
And while we don’t know how exactly how marijuana produces specific harmful effects, there are clear risk factors that can increase peoples’ likelihood of experiencing them.
The clearest risk factor is age.
In people younger than 25, cannabinoid receptors are more concentrated in the white matter than in people over 25.
The white matter is involved in communication, learning, memory, and emotions.
Frequent marijuana use can disrupt the development of white matter tracts, and also affect the brain’s ability to grow new connections.
This may damage long-term learning ability and problem solving.
For now, it’s unclear how severe this damage can be or whether it’s reversible.
And even among young people, the risk is higher the younger someone is— much higher for a 15 year old than a 22 year old, for instance.
Marijuana can also cause hallucinations or paranoid delusions.
Known as marijuana-induced psychosis, these symptoms usually subside when a person stops using marijuana.
But in rare cases, psychosis doesn’t subside, instead unmasking a persistent psychotic disorder.
A family history of psychotic disorders, like schizophrenia, is the clearest, though not the only, risk factor for this effect.
Marijuana-induced psychosis is also more common among young adults, though it’s worth noting that psychotic disorders usually surface in this age range anyway.
What’s unclear in these cases is whether the psychotic disorder would have appeared without marijuana use— whether marijuana use triggers it early, is a catalyst for a tipping point that wouldn’t have been crossed otherwise, or whether the reaction to marijuana is merely an indication of an underlying disorder.
In all likelihood, marijuana’s role varies from person to person.
At any age, as with many other drugs, the brain and body become less sensitive to marijuana after repeated uses, meaning it takes more to achieve the same effects.
Fortunately, unlike many other drugs, there’s no risk of fatal overdose from marijuana, and even heavy use doesn’t lead to debilitating or life-threatening withdrawal symptoms if use stops.
There are more subtle forms of marijuana withdrawal, though, including sleep disturbances, irritability, and depressed mood, which pass within a few weeks of stopping use.
So is marijuana bad for your brain?
It depends who you are.
But while some risk factors are easy to identify, others aren’t well understood— which means there’s still some possibility of experiencing negative effects, even if you don’t have any of the known risk factors.
Today I want to talk to you about swimming across the North Pole, across the most northern place in the whole world.
And perhaps the best place to start is with my late father.
He was a great storyteller.
He could tell a story about an event, and so you felt you were absolutely there at the moment.
And one of the stories he told me so often when I was a young boy was of the first British atomic bomb test.
He had been there and watched it go off.
And he said that the explosion was so loud and the light was so intense, that he actually had to put his hands in front of his face to protect his eyes.
And he said that he could actually see an x-ray of his fingers, because the light was so bright.
And I know that watching that atomic bomb going off had a very, very big impact on my late father.
Every holiday I had as a young boy was in a national park.
What he was trying to do with me was to inspire me to protect the world, and show me just how fragile the world is.
He also told me about the great explorers.
He loved history.
He would tell me about Captain Scott walking all the way to the South Pole and Sir Edmund Hillary climbing up Mount Everest.
And so ever since I think I was just six years old, I dreamed of going to the polar regions.
I really, really wanted to go to the Arctic.
There was something about that place which drew me to it.
And, well, sometimes it takes a long time for a dream to come true.
But seven years ago, I went to the Arctic for the first time.
And it was so beautiful that I've been back there ever since, for the last seven years.
I love the place.
But I have seen that place change beyond all description, just in that short period of time.
I have seen polar bears walking across very, very thin ice in search of food.
I have swum in front of glaciers which have retreated so much.
And I have also, every year, seen less and less sea ice.
And I wanted the world to know what was happening up there.
In the two years before my swim, 23 percent of the arctic sea ice cover just melted away.
And I wanted to really shake the lapels of world leaders to get them to understand what is happening.
So I decided to do this symbolic swim at the top of the world, in a place which should be frozen over, but which now is rapidly unfreezing.
And the message was very clear: Climate change is for real, and we need to do something about it.
And we need to do something about it right now.
Well, swimming across the North Pole, it's not an ordinary thing to do.
I mean, just to put it in perspective, 27 degrees is the temperature of a normal indoor swimming pool.
This morning, the temperature of the English Channel was 18 degrees.
The passengers who fell off the Titanic fell into water of just five degrees centigrade.
Fresh water wilkersonjack@example.org.
And the water at the North Pole is minus 1.7.
It's fucking freezing.
(Laughter) (Applause) I'm sorry, but there is no other way to describe it.
(Laughter) And so I had to assemble an incredible team around me to help me with this task.
I assembled this team of 29 people from 10 nations.
Some people think that swimming is a very solo sport, you just dive into the sea and off you go.
It couldn't be further from the truth for me.
And I then went and did a huge amount of training, swimming in icy water, backwards and forwards.
But the most important thing was to train my mind to prepare myself for what was going to happen.
And I had to visualize the swim.
I had to see it from the beginning all the way to the end.
I had to taste the salt water in my mouth.
I had to see my coach screaming for me, "Come on Lewis!
Come on!
Go!
Go!
Go!
Don't slow down!"
And so I literally swam across the North Pole hundreds and hundreds of times in my mind.
And then, after a year of training, I felt ready.
I felt confident that I could actually do this swim.
So myself and the five members of the team, we hitched a ride on an icebreaker which was going to the North Pole.
And on day four, we decided to just do a quick five minute test swim.
I had never swum in water of minus 1.7 degrees before, because it's just impossible to train in those types of conditions.
So we stopped the ship, as you do.
We all got down onto the ice, and I then got into my swimming costume and I dived into the sea.
I have never in my life felt anything like that moment.
I could barely breathe.
I was gasping for air.
I was hyperventilating so much, and within seconds my hands were numb.
And it was -- the paradox is that you're in freezing cold water, but actually you're on fire.
I swam as hard as I could for five minutes.
I remember just trying to get out of the water.
I climbed out of the ice.
And I remember taking the goggles off my face and looking down at my hands in sheer shock, because my fingers had swollen so much that they were like sausages.
And they were swollen so much, I couldn't even close them.
What had happened is that we are made partially of water, and when water freezes it expands.
And so what had actually happened is that the cells in my fingers had frozen and expanded.
And they had burst.
And I was in so much agony.
I immediately got rushed onto the ship and into a hot shower.
And I remember standing underneath the hot shower and trying to defrost my fingers.
And I thought, in two days' time, I was going to do this swim across the North Pole.
I was going to try and do a 20-minute swim, for one kilometer across the North Pole.
And this dream which I had had ever since I was a young boy with my father, was just going out the window.
There is no possibility that this was going to happen.
And I remember then getting out of the shower and realizing I couldn't even feel my hands.
And for a swimmer, you need to feel your hands because you need to be able to grab the water and pull it through with you.
The next morning, I woke up and I was in such a state of depression, and all I could think about was Sir Ranulph Fiennes.
For those of you who don't know him, he's the great British explorer.
A number of years ago, he tried to ski all the way to the North Pole.
He accidentally fell through the ice into the sea.
And after just three minutes in that water, he was able to get himself out.
And his hands were so badly frostbitten that he had to return to England.
He went to a local hospital and there they said, "Ran, there is no possibility of us being able to save these fingers.
We are going to actually have to take them off."
And Ran decided to go into his tool shed and take out a saw and do it himself.
And all I could think of was, if that happened to Ran after three minutes, and I can't feel my hands after five minutes, what on earth is going to happen if I try 20 minutes?
At the very best, I'm going to end up losing some fingers.
And at worst, I didn't even want to think about it.
We carried on sailing through the ice packs towards the North Pole.
And my close friend David, he saw the way I was thinking, and he came up to me and he said, "Lewis, I've known you since you were 18 years old.
I've known you, and I know, Lewis, deep down, right deep down here, that you are going to make this swim.
I so believe in you Lewis.
I've seen the way you've been training.
And I realize the reason why you're going to do this.
This is such an important swim.
We stand at a very, very important moment in this history, and you're going to make a symbolic swim here to try to shake the lapels of world leaders.
Lewis, have the courage to go in there, because we are going to look after you every moment of it."
And I just, I got so much confidence from him saying that, because he knew me so well.
So we carried on sailing and we arrived at the North Pole.
And we stopped the ship, and it was just as the scientists had predicted.
There were open patches of sea everywhere.
And I went down into my cabin and I put on my swimming costume.
And then the doctor strapped on a chest monitor, which measures my core body temperature and my heart rate.
And then we walked out onto the ice.
And I remember looking into the ice, and there were big chunks of white ice in there, and the water was completely black.
I had never seen black water before.
And it is 4,200 meters deep.
And I said to myself, "Lewis, don't look left, don't look right.
Just scuttle forward and go for it."
And so I now want to show you a short video of what happened there on the ice.
Narrator (Video): We're just sailing out of harbor now, and it's at this stage when one can have a bit of a wobble mentally.
Everything just looks so gray around here, and looks so cold.
We've just seen our first polar bears.
It was absolutely magical.
A mother and a cub, such a beautiful sight.
And to think that in 30, 40 years they could become extinct.
It's a very frightening, very, very frightening thought.
We're finally at the North Pole.
This is months and months and months of dreaming to get here, years of training and planning and preparation.
Ooh.
In a couple of hours' time I'm going to get in here and do my swim.
It's all a little bit frightening, and emotional.
Amundson, you ready?
Amudson: Ready.
Lewis Pugh: Ten seconds to swim.
Ten seconds to swim.
Take the goggles off.
Take the goggles off!
Man: Take the shoes.
Take the shoes.
Well done lad!
You did it!
You did it Lewis!
You did it!
You did it man!
LP: How on earth did we do that?
Man: Against the current!
You did it against the current!
(Applause) LP: Thank you very much.
Thank you very much.
(Applause) Thank you so much.
Audience: Encore!
(Laughter) LP: I'd just like to end off by just saying this: It took me four months again to feel my hands.
But was it worth it?
Yes, absolutely it was.
There are very, very few people who don't know now about what is happening in the Arctic.
And people ask me, "Lewis, what can we do about climate change?"
And I say to them, I think we need to do three things.
The first thing we need to do is we need to break this problem down into manageable chunks.
You saw during that video all those flags.
Those flags represented the countries from which my team came from.
And equally, when it comes to climate change, every single country is going to have to make cuts.
Britain, America, Japan, South Africa, the Congo.
All of us together, we're all on the same ship together.
The second thing we need to do is we need to just look back at how far we have come in such a short period of time.
I remember, just a few years ago, speaking about climate change, and people heckling me in the back and saying it doesn't even exist.
I've just come back from giving a series of speeches in some of the poorest townships in South Africa to young children as young as 10 years old.
Four or five children sitting behind a desk, and even in those poorest conditions, they all have a very, very good grasp of climate change.
We need to believe in ourselves.
Now is the time to believe.
We've come a long way.
We're doing good.
But the most important thing we must do is, I think, we must all walk to the end of our lives and turn around, and ask ourselves a most fundamental question.
And that is, "What type of world do we want to live in, and what decision are we going to make today to ensure that we all live in a sustainable world?"
Ladies and gentlemen, thank you very, very much.
(Applause)