'Lovely, lovely, lovely.'

'Pwaw, bad eggs! Let's try somewhere else.'

The Second Doctor and Jamie Macrimmon, The Krotons (19681969)

The contrast makes us laugh. If we've ever experienced the stench of bad eggs, our memory will remind us how horrible it was. Even if we don't have that direct experience, Jamie's horrified reaction will make us think, 'Yuck!' an emotional response because our sense of smell is linked to our emotions. But this is a trick: television can only convey sound and images, not smells. We react emotionally to something we're not actually sensing, which makes this alien planet seem more vibrant and real.

The End of the World (2005) does something similar with smell, but the other way round. At the end of the episode, the Doctor reveals his great secret to Rose and to us watching at home: his planet has been destroyed and he is the sole survivor of his people. It's a big moment whether or not we have memories of the Doctor's home planet from previous adventures.

But the Doctor shares his secret on an ordinary street on Earth in the present day. We don't see anything of the great Time War that destroyed the Doctor's planet until The End of Time (2010) and then only briefly. We'll discuss the Time War in more detail in Chapter 9, but the name suggests something epic and complex which might have been difficult for the people making Doctor Who to convey on screen. Yet the first time we hear of the Time War, it's made to seem vibrant and real using the same trick as in The Krotons.

'I'm a Time Lord. I'm the last of the Time Lords. They're all gone. I'm the only survivor, I'm left travelling on my own, cos there's no one else.'

'There's me.'

'You've seen how dangerous it is. D'you want to go home?'

'I don't know. I want... Can you smell chips?'

'Yeah.'

'I want chips.'

'Me, too.'

The Ninth Doctor and Rose Tyler, The End of the World (2005)

Watching at home, we can't smell those chips, but it's a recognisable smell and one likely to spark warm, happy memories of times when we've had chips. Again, our memory evokes an emotional reaction. More than that, because chips seem normal and real, so does all this talk of the Time War, even though we don't see it.

We've talked in previous chapters about the different ways scientists can measure and test things, using ever more sophisticated technologies to make more accurate measurements. But there's a basic problem: human beings don't experience the universe directly. As we've already discussed, our brains are supplied with data by our five senses. We then process that data into something we think of as meaningful.

That means our perception can be subjective and our senses can be fooled. Television is a series of still images like photographs, run together at speed to make your brain think that it's watching movement. Look closely at the TV screen and you'll see the picture is made up of tiny blobs of just three colours: specific combinations of blue, green and red. At a distance, our brains can't differentiate these separate blobs, so we see a seamless picture.

Another good example is our sense of time pa.s.sing. Explaining his theory of relativity, Einstein is said to have used the a.n.a.logy that a minute spent doing something we don't enjoy can feel like an hour, while an hour spent with someone we really like can feel like only a minute has pa.s.sed. Of course, this has nothing to do with the physics of relativity, but Einstein was making the point that our experience of time is a slippery thing and one we shouldn't take for granted. There's a related phenomenon, where people who've been in accidents or faced great danger speak of time seeming to slow down.

'Let me tell you about scared. Your heart is beating so hard, I can feel it through your hands. There's so much blood and oxygen pumping through your brain, it's like rocket fuel. Right now, you could run faster and you could fight harder, you could jump higher than ever in your life. And you are so alert, it's like you can slow down time. What's wrong with scared? Scared is a superpower.'

The Twelfth Doctor, Listen (2014)

Is it true that when we're stressed our brains work faster, effectively meaning that we experience everything else around us pa.s.sing more slowly? To find out, in 2007, the American neuroscientist David Eagleman conducted an experiment that was as brilliant and ridiculous as anything the Doctor might think of: he dropped people including himself off the top of a very tall building.

The falling people were caught by a net, but the fall itself was still scary. Did that fear make people's brains work faster? While they were falling, the people in the experiment had to look at a special wrist.w.a.tch they'd been given to wear, on which two numbers were displayed. The numbers alternated very quickly faster than could be read in normal conditions. The question was whether people could read the numbers clearly while they were falling and scared.

The experiment showed that they couldn't, which proved that stress doesn't make the brain work more quickly and doesn't slow down time. But Eagleman had a second experiment, and after people had fallen and been caught in the net, he gave them a stopwatch. They were asked to let the stopwatch run for the same amount of time that they had fallen for. People who had watched the fall but not fallen themselves were also given stopwatches and asked to estimate the length in time of the fall. The results showed that people who had fallen thought about a third more time had pa.s.sed than those who didn't fall. The fallen people felt that time had pa.s.sed more slowly as they fell, even though their brains hadn't been working any more quickly.

There are different theories about why that might be. In a stressful situation, our perception seems to focus on ways to escape or solve the problem facing us. We stop taking in everything else around us so the world seems less busy and complex. That might affect our sense of time.

Another idea which might happen in tandem with the first is that there's a difference between how we experience time as it's happening and how we look back at time that has pa.s.sed. As we've seen, our emotions and sense of smell are linked to the way we make memories. Strong emotions seem to etch memories more strongly into our minds. A key way in which, looking back, we feel how much time has pa.s.sed is through the number of new memories made in that period. Say we usually create a strong new memory once a day. In a stressful situation, we create several strong memories in a few hours. Looking back, it will feel like several days' worth of time happened in those hours.

If that is right, it explains why, as we get older, time seems to pa.s.s more quickly. Individual days still feel the same length, but it seems as if Christmas comes round faster than it used to. The reason would be that as we get older we experience fewer new things, we get into routines and maybe have better control of our emotions. All that means that we lay down fewer strong new memories, so when Christmas arrives we look back to the last one and feel little time has pa.s.sed.

The Doctor and his companions are constantly having new experiences and dangerous adventures so perhaps they wouldn't feel Christmas coming round any more quickly. More than that, they might be healthier, because our subjective sense of time has been linked to various diseases such as depression and dementia. There's good evidence that having lots of new experiences and laying down new memories helps with 'cognitive reserve' that is, the way the brain resists damage and these kinds of diseases.

Our bodies also keep time in another way, called the circadian clock. We seem to unconsciously keep track of levels of daylight all the time. The suprachiasmatic nucleus of the brain uses this information to control a number of things, such as production of melatonin a hormone that makes us sleepy. The hormone cortisol, which affects levels of blood sugar and so affects whether we feel hungry, also seems to be governed by this internal clock, as are body temperature and women's menstrual hormones. In animals, hibernation seems to be controlled by the same inner clocks.

Our inner clocks also synchronise our senses. If someone touches your foot and your shoulder at the same time, you feel both sensations at once but the sensation in your foot has much further to travel to your brain than the sensation in your shoulder. It's thought that your brain leaves a tiny amount of time between what's happening and your perception of it, effectively a buffer that allows the sensation from your foot to catch up with the sensation from your shoulder. That buffer means the world as we experience it is always slightly behind the world as it is happening so we are always living slightly in the past. It's even thought that the taller you are and the further sensations have to travel to reach your brain the longer the delay you experience, so the more in the past you are living.

It also seems that we continually use our senses to check our inner clocks are in sync with the world around us. There's an experiment that gets you to press a b.u.t.ton which turns on a light. You keep pressing the b.u.t.ton, but the light starts coming on fractionally later say by 200 milliseconds. You continue pressing the b.u.t.ton long enough that your brain puts the two things in sync, then we stop the delay. Now you press the b.u.t.ton again and the light comes on immediately but your brain, synchronised to the tiny delay makes you think the light came on before you pressed the b.u.t.ton. It feels as if the light turning on has travelled backwards in time.

Because our bodies depend on the workings of this inner clock, when that clock gets out of sync it can make us feel very peculiar. If we travel a long distance round the Earth, we cross many time zones. If we travel relatively slowly say, on a ship our bodies can keep in sync with the changing time. But if we cross time zones more quickly such as by flying in a jet plane it can make us ill, with a condition called desynchronosis or 'jet lag'.

The Doctor's companions don't seem to experience jet lag when they travel in the TARDIS. In The Bells of Saint John (2013), the Doctor and Clara jump ahead a few hours as a weapon against their enemies.

'Why did we travel to the morning? What's the point in that?'

'Whoever's after us spent the whole night looking for us. Are you tired?'

'Yes.'

'What? Then imagine how they feel.

They came the long way round.'

Clara Oswald and the Eleventh Doctor, The Bells of Saint John (2013)

Clara's not tired because they're jumped forward from night to day, but because she's already had quite an adventure. If the jump in time caused jet lag, she'd be feeling sick. Perhaps the TARDIS compensates for jet lag somehow.

We can get over jet lag in a few days as our inner clocks adjust to the time zone we've moved to. But people who regularly work or party into the night can be at risk of more serious harm. They might sleep longer into the next day, but their bodies' sense of time will still be out of sync. It's been shown that shift workers have a greater chance of heart disease, stomach illness and problems sleeping. What's more, our inner clocks seem to explain seasonal affective disorder a kind of depression that usually strikes in winter, when there is less daylight and may be related to diseases including type 2 diabetes, obesity and cancer. It's even thought that our inner clocks decide how long human beings can live for as we'll see in Chapter 14.

How do you know what happened to you in the past? And how do you know that you know it? In Listen, the Doctor and Clara meet a small boy called Rupert Pink who will grow up to be Clara's boyfriend, Danny. Clara is worried about the consequences of the encounter.

'Will he remember any of that?'

'Scrambled his memory. Gave him a big old dream about being Dan the soldier man.'

Clara Oswald and the Twelfth Doctor, Listen (2014)

The suggestion is that Rupert won't remember Clara and the Doctor explicitly, but something of their adventure together will remain with him and shape his personality.

There seem to be two kinds of long-term memory: explicit and implicit. Explicit memory is conscious and intentional: we remember that a new episode of Doctor Who will be on at a specific hour, and we remember to watch it. Implicit memory is different: we're not conscious of the way that past experience affects our behaviour. When you tie your shoes or ride a bike, you do it without (consciously) thinking.

In young children, the ability to make implicit memories develops before the explicit memory. Babies recognise things that make them feel safe or scared and develop responses when those things happen again.