Why do we experience a ‘now’? Because all other ways of experiencing reality would lead to us starving
He has departed from this strange world a little ahead of me. That means
nothing. People like us, who believe in physics, know that the distinction
between past, present, and future is only a stubbornly persistent illusion.
Albert Einstein
I think we can all agree, the past is over.
George W. Bush, May 2000
Deep in the Amazon rainforest, a tree frog sits on a log watching a fly. A genetic fluke has furnished the frog with a brain that perpetually perceives its surroundings as they were one second before. When the fly comes within range, the frog lunges. But, with its out-of-date data, it misses. Weakened by hunger, it falls off the log and dies.
It is a heartbreaking story. However, if you think it is entirely fanciful, think again. A prominent American physicist believes it goes to the very heart of why we perceive the world the way we do. According to James Hartle of the University of California at Santa Barbara, there are a multitude of possible ways we could experience reality – even with a one-second time lag like the unfortunate Amazonian tree frog. ‘However, evolution by natural selection has ensured that people and frogs experience the world in the most effective way for their survival,’ he says. ‘A frog that uses the most recent data to calculate the trajectory of a fly eats; one that doesn’t, starves.’
Probably, this seems little more than common sense. However, Hartle, a scientific collaborator of Stephen Hawking’s, believes it addresses a major scientific puzzle: why we experience a past, present and future at all. ‘The peculiar thing is that none of these concepts is uniquely defined in our most fundamental description of physical reality,’ he says.
That fundamental description of reality is the special theory of relativity. According to the theory, proposed by Albert Einstein in 1905, space and time are not separate things, as they appear to us to be. Instead, they are blended together into a four-dimensional amalgam called space-time.*
In Einstein’s revolutionary picture, a person’s ‘time’ is defined by a particular direction in space-time whereas their ‘space’ is defined by the three perpendicular directions in space-time. The common-sense-defying thing is that there is no unique way of carving up space-time into space and time. Instead, there is an infinite number of possible ways, each of which corresponds to a different notion of time and space.
What determines the way in which someone’s space-time is divided up is the direction they are travelling in space-time. If you cannot quite visualise ‘direction in space-time’, do not worry. It is impossible. As lowly three-dimensional beings, we can directly perceive neither four-dimensional space-time nor ‘directions’ in four-dimensional space-time. Instead, two people travelling in different directions in space-time appear as two people travelling at different speeds.
How exactly will their notions of time and space differ? Well, the first thing to say is that the only meaningful way of measuring intervals of time is with a clock and intervals of space with a ruler. It follows that differences in the time and space of the two people travelling at different speeds will manifest themselves in the properties of their clocks and rulers. As Einstein was first to realise, ‘moving’ clocks run slow and ‘moving’ rulers shrink in the direction of motion. Consequently, each person will see the clock of the other running slow and the ruler of the other shrink!
At first sight, it may seem bizarre that each observer sees the same kind of thing when they look at the other. But that is the magic – and also the mind-bending peculiarity – of special relativity. One of the foundation stones of the theory is the recognition that it is impossible to tell who is moving and who is stationary. Imagine sitting on a train and watching another train go by. How can you tell whether it is your train or the other train that is moving? You cannot. As Einstein realised, relative motion alone has meaning. It is possible only to say that you are moving at such-and-such speed relative to someone else. And, since two observers moving with respect to each other each see the other moving with the same relative speed, each see the other’s clock slow and ruler shrink (which is just the same as saying that all ways of carving up space-time into time and space are equally valid).
The fact that the clocks of two observers moving with respect to each other run at different rates means that the two observers will fail to agree on a number of quite basic matters. For instance, they will not agree on whether two events – say, the firing of two widely separated cannons – occur at the same time. Though one observer may see the cannons go off simultaneously, the other will see them go off one after the other. And this is not all. The two observers will also not agree on ‘what is happening now’, ‘what happened ten minutes ago’, and so on. In fact, if the cannons are so far apart that a light ray cannot span the distance separating them in the time between the explosions, the two observers may not even agree on whether one cannon was fired before the other! While one observer sees the one cannon fire first, the other will see it fire second – ‘And, if the two observers cannot agree on such basic things, they will not be able to agree on what is past, present and future,’ says Hartle.
So why, contrary to special relativity, do we have such a strong feeling that the past, present and future are uniquely defined? The answer is that noticeable differences in the rate of clocks occur only if observers are moving relative to each other at speeds approaching that of light itself.* However, everything we do on Earth involves speeds much slower than that of light – even a passenger airplane flying through the air travels more than a million times slower than a light beam in empty space. ‘In everyday situations, it turns out the differences in our notions of time are too small to notice,’ says Hartle. ‘This is the ultimate reason we can all agree on a past, present and future.’
The fact that we are condemned to live our lives in the cosmic slow lane may explain why we experience a common past, present and future. However, it does not explain why the present has such a special immediacy to us. Why do we focus our attention on the most recent information we have gathered from our surroundings – what is happening right ‘now’? Why do we not focus on information gathered one second ago like the unfortunate tree frog? Or ten seconds ago? Or half an hour ago, for that matter?
This is a tough question to address. The human brain, after all, interacts with its surroundings in an extremely complex way. However, in the time-honoured manner of theoretical physicists, Hartle attempts to answer the question from the point of view of a more rudimentary entity than a human: a robot which ‘experiences’ and ‘reacts’ to reality in the simplest way imaginable.
The robot Hartle envisages is an ‘information gathering and utilising system’, or IGUS. This is an abstract entity first imagined by Nobel Prize-winning physicist Murray Gell-Mann of the California Institute of Technology in Pasadena. An IGUS has an ‘input register’ in which it records information from its environment. This could, for example, be an image of the surroundings. The storage capacity of the input register is limited, however. In order to make room for new information continually flooding in from the environment, the information is passed after a while to a ‘memory register’.
The robot might have many memory registers along which it continually shuffles past information. But it does not have infinite resources at its disposal. So, eventually, the registers come to an end and the information is unceremoniously dumped. However, simply dumping the information without first extracting anything useful from it is wasteful. Before the information is thrown away, therefore, it is passed on to other parts of the robot. And this is where the important stuff goes on.
Hartle’s robot, in addition to having storage registers, has a ‘schema’ and a ‘computer’. The schema is a simplified model of its environment plus a set of rules, culled from the robot’s past experience, which tell it how to behave in particular circumstances. The computer is, well, just a computer. However, it carries out two quite distinct types of computation. First, there are ‘unconscious’ computations, which update and improve the model of the environment stored in the schema. Then there are ‘conscious’ computations. These determine how the robot should respond to information pouring in from its surroundings, based on the rules stored in its schema.
The robot probably seems a bit on the complicated side. However, Hartle claims that a robot like this, carrying out unconscious and conscious computing, mediated by a schema, can imitate some of the key features of human perception. And, crucially, it has the advantage of being fantastically simpler to deal with than a human being.
For the robot to mimic a human being, says Hartle, it is necessary only for it to carry out its ‘conscious’ computation exclusively on the contents of its input register and its ‘unconscious’ computation on its memory registers. ‘This distinction is very important,’ says Hartle. ‘It ensures that – just like a person – the robot consciously “experiences” the present but only “remembers” the past.’
Hartle maintains that a robotic IGUS set up this way also experiences the world in other ways that are like a person’s. For instance, while the past in the registers is ‘remembered’, the future is ‘predicted’. The future is the result of computation – for instance, the computation that a car will hit you in a second’s time unless you take avoiding action. The past and future are therefore qualitatively different from each other, just as they are for us. ‘Even God cannot change the past,’ said the Greek dramatist Agathon. Furthermore, because such a robot focuses its attention on the input register – that is, on the information most recently acquired from its surroundings – the ‘now’ has a very special immediacy.
This may all seem terribly abstract. However, in practice, a human-mimic IGUS works quite simply. Say, for instance, an image of a cheeseburger appears in the first register. According to Hartle: ‘The computer consults the schema, which has abstracted rules from a previous experience – from previous visits to burger restaurants – and realises “Hey, I like cheeseburgers.” The robot therefore decides to buy a cheeseburger. Or perhaps the schema contains information on the fat content of burgers, which overrides the liking of burgers, so the computer decides not to buy a cheeseburger after all!’
Hartle claims this human-mimic IGUS sheds light on the long standing puzzle of why we seem to experience a ‘flow of time’, even though this cannot be so. ‘Something which flows must, by definition, change with time,’ says Hartle. ‘But how can time change with time? It’s a logical impossibility.’
The impossibility of a flow of time is in fact explicit in special relativity. According to Einstein, space-time is like a ‘map’ in which all the ‘events’ in the history of the Universe – from the birth of everything in the Big Bang way into the far future – are laid out, exactly as if they are pre-ordained. Nothing at all flows.
In the human-mimic IGUS, however, images pass from memory register to memory register until finally they are dumped, or erased – the robot’s equivalent of ‘forgetting’. It is this, says Hartle, which represents the ‘flow of time’. It is a flow of information. ‘Something like this flow of information from register to register must be happening in our brains,’ he says. ‘Ultimately, this is what we interpret as the flow of time.’ As Austin Dobson wrote in ‘The Paradox of Time’:
Alas, Time stays, we go.
All this IGUS stuff may seem a bit complicated in order to simply conclude that we are interpreting the flow of information from neurone to neurone in our brains as the flow of time. Fortunately, there is a pay-off. The great beauty of Gell-Mann’s IGUS is that it is extremely flexible. By wiring it up in different ways, it is possible to change the flow of information between its components. This enables a robotic IGUS to experience and react to reality in different ways – ways which can be very different from a human being.
This capability enables Hartle to ask a key question: ‘Are there other ways that creatures could organise their experience – ways that are different from ours but still consistent with the basic laws of physics?’
To try and answer the question, Hartle imagines several different types of robot, each of which experiences reality in a different way. The first robot focuses on not one but two times, ten seconds apart. In other words, it has two ‘presents’. A second kind, much like the Amazonian tree frog, is always behind, seeing the world as it was a few seconds ago. And a third type of robot has no ‘schema’. It therefore has no unconscious thought and no simplified model of its environment. Its next move must be computed from contents of all its registers – it has no way of focusing only on relevant information. ‘The question is: would any of these creatures be viable?’ asks Hartle.
According to Hartle, the two-time robot would waste valuable computing resources by consciously focusing on unessential information in the past. The always-behind robot would starve to death just like the tree frog. And the no-schema robot would squander even more precious computational resources than its two-time cousin. ‘Should creatures ever arise with any of these variant ways of organising their experience, they would be weeded out by natural selection,’ says Hartle. ‘Outcompeted by creatures who experience reality much like us, they would pretty quickly become extinct.’
Remarkably, Hartle concludes: ‘The way we experience time is determined as much by the laws of biology as the laws of physics.’ This turns out to have implications for any extraterrestrials we may one day meet. Hartle believes he can say categorically that they will experience the world in exactly the same way as us, sharing the concepts of past, present and future, and the idea of a flow of time.
There is, however, a way that creatures could have arisen that organise their experience differently from us, according to Hartle – if the laws of physics were different. ‘Say, the force on a massive body depended not on the force acting on it and the position of the body, as it does in the world we live in, but on its position now and 10 seconds ago,’ says Hartle. ‘In such circumstances, natural selection would clearly favour the evolution of a split-time creature with two presents.’
Evolution by natural selection may have made us the way we are and given us our concept of time. Nevertheless, we have the technology to build robots any way we like. This raises a bizarre possibility, which Hartle considers. ‘Might it be possible to build a robot which organises its experience back-to-front?’ he says. ‘A robot, in other words, that remembers the future and predicts the past?’
According to Hartle, it is not difficult to imagine a robot that is in a position to remember the future. Images would simply have to flow backwards along its registers – say, from right to left rather than from left to right. This would involve ‘unerasing’ information that had been dumped, or erased. Unerasing information is the equivalent of reassembling a piece of paper that has gone through a shredder. ‘It is difficult and it costs energy but it is not impossible,’ says Hartle. ‘It is perfectly possible to conceive of building a robot with a reverse-time sense.’
The really difficult problem, however, turns out to be giving the robot a future environment to remember! This is because of the difficulty in reversing the so-called arrow of time.
Large collections of atoms such as people and medieval castles grow old and crumble as time passes. These transformations are associated with a change from order to disorder. Physicists associate this increase in disorder, or ‘entropy’, with the so-called thermodynamic arrow of time. The direction of the arrow – from past to future – is associated with the direction in which things become disordered, in which people grow old and medieval castles crumble to dust. Consequently, if a robot is to remember the future rather than the past, the arrow of time must be reversed in its surroundings so that people grow young and medieval castles uncrumble.
It goes without saying that this is extremely difficult. ‘The past is a foreign country. They do things differently there,’ wrote L.P. Hartley in The Go-Between. According to Hartle, reversing the arrow of time would mean reversing the velocity of every particle of matter in the robot’s neighbourhood. Even doing this to the air molecules flying about a room would be unimaginably harder than unshredding a shredded piece of paper. ‘Also, all matter interacts with light, so simply to reverse the arrow of time for a day, it would be necessary to deal with every molecule and particle of light within the distance light travels in a day – a radius of 26 billion kilometres,’ says Hartle. ‘Perhaps a super-advanced civilisation might find it amusing to do. But it’s way beyond our capabilities.’
As noted earlier, we experience reality in the ultra-slow lane where the full effects of special relativity are not felt and it is possible to agree on a ‘common’ past, present and future. But what about the more general situation when the full effects of relativity are important and peculiar things happen to time? Well, it turns out that the satellites that make up the Global Positioning System can be considered as an IGUS set up to experience time in a peculiar way.
People using GPS rely on receiving signals from clocks on the different satellites. The receiver determines its position on Earth by comparing the different amounts of time the various signals take to reach it. The system depends on the clocks on the satellites all ticking at the same rate. However, they do not. As special relativity predicts, time slows down at high speed. In fact, it is worse than this. Einstein’s general theory of relativity, which incorporates special relativity, predicts that time also slows for bodies experiencing strong gravity. Since gravity is stronger closer to the Earth time slows down for a GPS satellite when it swoops close to the surface.
The ‘relativistic’ slowing of time experienced by the satellites is tiny – one clock may slow by only a few nanoseconds per second relative to another. However, measuring time delays to nanoseconds turns out to be crucial if a receiver is to determine its position on the surface of the Earth to an accuracy of a few metres.
So much for the details of GPS. The point, as far as the experiencing of time is concerned, is that all the satellite-borne clocks are running at slightly different rates. And this, in turn, means that they each have different perceptions of ‘now’. In fact, for the system to operate, the GPS engineers had to design a particular meaning of ‘now’, which is different from the everyday meaning. ‘In other words, they tinkered with the system’s perception of time,’ says Hartle.
But the familiar concepts of past, present and future not only break down at speeds comparable to light – for instance for the GPS satellites – they break down in other circumstances too. Imagine that it is possible to perceive things which happen in ultra-small intervals of time – say, in less than a billionth of a second. This would radically affect your perception of a friend you are talking with. Why? Because light takes about a billionth of a second to bring a picture of them to you.
Such an interval is ten million times shorter than any event that can be perceived by the human brain, so it is unnoticeable in normal circumstances. However, if you have ultra-fast perception, you do not hear what your friend is saying ‘now’ but what they said in your past. The two of you do not share a ‘now’. And people standing farther away from you appear even farther in the past. No longer is there a common present.
The other situation where the concept of past, present and future breaks down is when observers are separated by a large distance compared with the distance light can travel in the time it takes any events of interest to occur. This would be the case, for instance, for a galaxy-spanning empire, which, like a human being and like the GPS system, can be considered an IGUS. ‘There would be no point in defining a “now” on a planet at the centre of the Galaxy when light takes 60,000 years to take knowledge of it to a planet on the periphery,’ says Hartle. ‘Clearly, such a civilisation would need to organise its time differently from us.’
Time is often considered a slippery, elusive thing. ‘What, then, is time?’ St Augustine wrote in the fifth century. ‘If no one asks me, I know what it is. If I wish to explain what it is to him who asks me, I do not know.’ Hartle, however, disagrees that time is elusive at all. ‘All special relativity teaches us is that the idea there is “one” time is an illusion,’ he says. ‘Rather there are many.’ And, just because special relativity does not say anything about the past, present and future, does not mean they are illusions – they are real, strongly held properties, of an IGUS like a human being. ‘Newtonian physics doesn’t tell us how many continents there are on the Earth’, says Hartle. ‘But the continents are real nonetheless.’
Perhaps the final word should go to the nineteenth-century British essayist Charles Lamb: ‘Nothing troubles me more than time and space,’ he wrote. ‘And yet nothing troubles me less, as I never think about them!’
* Space-time is four-dimensional because it blends together one dimension of time and three of space. The time dimension extends of course in the direction past–future while the three dimensions of space are in the directions east–west, north–south and up–down.
* The second foundation stone of special relativity – in addition to the idea that only relative motion has meaning – is that the speed of light is always the same. In fact, the reason that time and space distort when relative speeds approach that of light is specifically so that all observers, no matter what their speed, measure exactly the same speed for a light beam. Space and time are but shifting sand; the speed of light turns out to be the bedrock on which the Universe is built.