It's just a question of timing

If the notion of time is relative, why are we so sure that we experience a past, present and future? Marcus Chown reports on the perception that is fundamental to our - and all animals' - survival

On the plains of the Serengeti, a dangerously emaciated cheetah stalks an antelope. A genetic accident has given the cheetah a brain that perpetually perceives its surroundings as they were a few seconds ago. When the antelope comes within range, the cheetah leaps from cover, but the antelope is already long gone. Weakened by hunger, the cheetah lies down and dies.

On the plains of the Serengeti, a dangerously emaciated cheetah stalks an antelope. A genetic accident has given the cheetah a brain that perpetually perceives its surroundings as they were a few seconds ago. When the antelope comes within range, the cheetah leaps from cover, but the antelope is already long gone. Weakened by hunger, the cheetah lies down and dies.

It's a heart-breaking story. But, according to a prominent American physicist, it goes to the very heart of why we perceive the world the way we do. "Animals have been equipped by natural selection to experience the world in the most effective way for their survival," says James Hartle of the University of California at Santa Barbara. "An animal that hunts using the most recent data from its surroundings, eats; one that doesn't, starves."

Probably, this seem no more than common sense. However, Professor Hartle, a scientific collaborator of Professor Stephen Hawking, believes it addresses a major scientific conundrum: why we experience a past, present and future at all. "The odd thing is that none of these concepts is uniquely defined in our most fundamental description of physical reality," he says.

That description is the special theory of relativity, proposed by Albert Einstein in 1905. According to relativity, space and time are not separate but blended together into a four-dimensional amalgam called space-time. Crucially, space-time can be divided up into space and time not just in one way but in many different ways. A clock carried by one "observer" defines their "time" - a particular direction in space-time - while the perpendicular direction defines their "space".

However, an observer moving in a different direction in space-time will have a different way of dividing up the four directions of space-time into space and time, and consequently a different notion of time and a different notion of space.

Because of their different notions of time, observers moving with respect to each other will not agree on whether two events occur at the same time. They also will not agree on "what is happening now", "what happened 10 minutes ago", and so on. In fact, they may not agree even on whether one thing happened before another. "That's why fundamental physics supplies no unique notion of past, present, and future," says Professor Hartle.

Why then do we have such a strong feeling that such things exists? According to Professor Hartle, it is because significant differences in observers' notions of time occur only if they are moving relative to each other at speeds close to that of light. Since, on Earth, we are moving much slower than light - which travels more than a million times faster than a passenger airliner - we are all moving more or less in the same direction in space-time. "Differences in our notions of time are therefore too small to notice and we can agree on a past, present and future," says Professor Hartle.

But just because we can experience a "common" past, present and future, it does not explain why the present has a special immediacy - why we focus our attention on information gathered most recently from our surroundings and not several seconds ago, like the unfortunate cheetah. To try to make sense of this, Professor Hartle has made use of an abstract entity capable of experiencing and reacting to reality in the simplest way imaginable.

Such an entity, dubbed an information-gathering and utilising system, or IGUS, was actually concocted by the Nobel-prizewinning physicist Murray Gell-Mann. An IGUS records information from its environment - perhaps an image of its surroundings - in an "input register". The register, however, has a limited capacity and, as more information comes in from the environment, it is passed to a "memory register" to clear space for new input. An IGUS might have many memory registers along which it can shuffle the past information. Eventually, however, it will have to be dumped.

Of course, dumping this past information without using it would be a waste. Before this happens, therefore, it is passed to other parts of the IGUS.

In addition to the registers, it has a "schema", a simplified model of its environment with rules culled from past experience, which tells it how to behave in particular circumstances. It also has a computer that works out how it should react to its surroundings, based on the rules stored in the schema. The computer carries out two distinct types of computation: C (conscious), which makes decisions; and U (unconscious), which updates the schema.

According to Professor Hartle, some of the key features of human perception can be mimicked by this simple set-up if C is made to focus on the input register and U on the other registers. This distinction is important. It means that the IGUS consciously "experiences" the present but "remembers" the past, just like we do.

Also, the past and future are qualitatively different in that the past in the registers is "remembered" while the future is "predicted" as the output of computation. Furthermore, because the IGUS focuses its attention on the most recently acquired image, "now" has a special immediacy, just like it has to us.

The human-mimic IGUS works in the following way. Suppose an image of a chocolate doughnut appears in the first register. "The computer consults the schema, which has abstracted rules from a previous experience - from previous visits to cake shops - and realises 'Hey, I like chocolate doughnuts'," says Professor Hartle. "The IGUS therefore decides to buy a chocolate doughnut. Or perhaps the schema contains information on the fat and sugar content of doughnuts, which overrides the liking of doughnuts, so the computer decides not to buy a chocolate doughnut."

According to Professor Hartle, this human-mimic IGUS hints at why we experience a "flow of time". This is actually a puzzle because even a little thought reveals that such a concept is a nonsense. "Something which flows, changes with time," says Professor Hartle. "But how can time change with time? It's a logical impossibility." This impossibility is made explicit in special relativity where space-time is essentially a "map" in which all the "events" in the history of the Universe - from the birth of everything in the big bang into the far future - are laid out, exactly as if they are pre-ordained. Nothing at all flows.

In the IGUS, the flow of time is represented by the passage of images to the right until they are erased from the row of registers and are "forgotten". "This is what we mistake for the flow of time," says Professor Hartle. "Something analogous to the flow of information from register to register must happen in our brains. This is what ultimately gives us our sense of time passing."

If all this IGUS stuff seems a bit complicated simply to confirm that the flow of time is a flow of information, there is a pay-off. The beauty of Gell-Mann's concept of an IGUS is that it is flexible and so can be wired up in many different ways. There are many different ways in which information can pass between its components, corresponding to very different ways of experiencing and reacting to reality. Because of this, it is possible for Professor Hartle to ask: "Are there other ways in which creatures could organise their experience that are different from ours but which are still consistent with the basic laws of physics?"

Professor Hartle imagines several different typed of IGUS, each of which experiences reality in a different way. The first focuses on not one but two times, 10 seconds apart, so it has two "presents". A second kind, like the cheetah, is always behind, seeing the world as it was 10 seconds ago. And a third has no "schema" so that it must calculate its next move from the contents of all its registers because all possible moments are equally accessible. "Would any of these be viable?" asks Professor Hartle.

According to Professor Hartle, the two-time IGUS would waste valuable conscious focus on inessential information in the past. The always-behind IGUS would starve to death like the cheetah. And the no-schema IGUS would waste precious computational resources. Such variant ways of organising experience, should they ever arise, says Professor Hartle, would be promptly weeded out by natural selection. Professor Hartle concludes: "Our time sense is determined biology as well as physics."

If Professor Hartle is right, it means that any extraterrestrials we one day meet will experience the world in the same way as we do, sharing concepts of past, present and future, and the idea of a flow of time. Perhaps George W Bush was wiser than everyone thought when he said: "I think we can all agree, the past is over."

Marcus Chown is the author of 'The Universe Next Door' (Headline)