Unscrambling the question of time

十二月 24, 1999

Neil Johnson plans to attend one of his own lectures - as a traveller from 2050. Alison Goddard talks to this year's Royal Institution lecturer about the possibilities of time.

Neil Johnson, a 37-year-old theoretical physicist with boyish looks, is being aged 50 years by a make-up artist. Earlier in the day, he placed an atomic clock on board a flight to Shanghai to demonstrate how, on its return, it will be a fraction of a second behind an identical clock that remained in London. Filming for the Christmas lectures at the Royal Institution in London - to be broadcast by the BBC - is in full swing.

Appropriately, the Royal Institution has chosen the subject of time to celebrate its bicentenary and the millennium. From Boxing Day onwards, Johnson, a physics lecturer at the University of Oxford, will take a five-part look at the physical concept of time, how time is measured in our everyday lives, and whether time travel is possible.

This is why he is being made up to look 87 years old. He will attend one of his own Christmas lectures as a time traveller from the year 2050, which should delight the audience of children and parents who come every year to the Royal Institution to try to understand the latest academic theories. "There are no physical reasons for why time travel is impossible," he says. But then, he admits, there is no evidence of anything travelling in time; we are scarcely overrun by tourists from 2050. Perhaps, Johnson suggests, it is because "you wouldn't be able to travel back in time past the point at which the time machine was invented".

OK, but that theory does not get round the problem of "the grandfather paradox" - whereby a time traveller kills her grandfather before her father has been conceived. How does Johnson explain that? One possibility, he says, is to think of the universe dividing into parallel universes at each point a decision has to be made. In one universe, the paradox would not arise, because the time-traveller's attempt to kill her grandfather would fail, although it would succeed in a parallel world.

The lectures raise questions about the very nature of time. "Isaac Newton said that time and space were absolute and that speeds are relative to one another - for example, if you run after a car it appears to be travelling more slowly than it would if you were stationary. Albert Einstein turned that theory on its head and said that while the speed of light was absolute, time and space were relative," Johnson says.

Certainly, at the quantum level time does not seem to be a crucial physical quantity. Just as there is a fundamental limit on space at the quantum level - the Planck scale - there appears to be a fundamental limit on time. "It's as though time doesn't want to be pinned down any further," he says.

To explain why time always appears to move in one direction - forwards and not backwards - Johnson talks about the natural progression of the universe from order into disorder - the second law of thermodynamics. An egg can easily be broken and scrambled; to reverse the move from order to disorder; to separate out the white and the yolk, and rebuild the shell - would require a lot more energy.

Another physical measure that changes with time is how much information something carries, Johnson says. New interest has recently emerged in Fisher information, named after Cambridge statistician Ronald Fisher, who developed the theory in the 1920s.

"There is a new school of thought in physics - that the building blocks of physics are not subatomic particles but the information that these particles carry. Roy Frieden (a professor of optics at the University of Arizona) has analysed the laws of physics in terms of Fisher information, which is a measure of how much information something carries. He uses it to derive most known physics," Johnson says.

"It may be that physics is just a branch of information theory, that the world is just some information storage device that processes answers according to the question asked. The way in which you ask the question determines nature's answer," he adds.

The concept of Fisher information turns out to have striking parallels with Heisenberg's uncertainty principle, which puts a physical limit on the knowledge we can have of the world. At the quantum scale, if we know the speed at which a fundamental particle is moving, there is a fundamental physical limit on how well we can know its position. Likewise, if we know its position, there is a fundamental physical limit on how well we can know its speed. There is an information gap between what nature knows and what it is prepared to reveal.

The processing of information is a topic that fascinates Johnson. His research interests involve complex dynamic systems, including the development of traffic jams and the pricing of financial options. One of his lectures examines order at the edge of chaos, looking at how organised behaviour emerges over time, even when individuals are acting in their own selfish interests.

"For example, it has been shown that traffic jams on the M25 will evolve to the most efficient situation that can occur (given the number of cars and where they join the road). The traffic system evolves in time so as to optimise some global pay-off even though the individual drivers are each acting selfishly," he says.

"There are a lot of similar ideas - I don't know whether they are related but it would be interesting to find out. Physics looks at the collective behaviour of groups of objects. In physics, the objects don't care if it is Friday night or Saturday morning. Humans, however, do. Sociologists approach problems by using a model called the prisoner's dilemma - two suspects are separated, and each needs to decide whether to collaborate by sticking to a jointly agreed story or whether to betray their colleague."

Humans modify their behaviour to optimise their chances of success. Johnson is planning an experiment with a top trader from Bank One. He has developed a computer program in which software agents have rules to play by. His agents will play a version of the financial options market, competing against or collaborating with a human trader. If they lose, they can discard the old rules in favour of new ones. Johnson calls his work "the physics of lucky streaks".

A jazz saxophonist in his gap year - his first job was playing a jingle for Radio Caroline - Johnson found that music did not pay the rent. Now his lectures use music to demonstrate how well the human body can judge time. "I first got interested in physics through music. I play the sax and it always amazed me that small changes to the size of the instrument or even to the reed could have a big effect," he says.

"Physics is really creative. You have to learn the rules, as in music, but then you can throw them away, the way you can with jazz. You have got to be rigorous and remember what other people have done, but you have also got to let your mind wander."

So instead of opting for a career in music, Johnson went to the University of Cambridge to read physics before gaining his PhD from Harvard University. While at Harvard, he met his wife, Maria. They spent 18 months in her native Colombia - he as a professor of physics at the Universidad de Los Andes in Bogota - before coming to the United Kingdom in 1992. They have two children, Daniela aged five and Nicholas aged three, who will be watching the television programmes.

Did Johnson watch the Christmas lectures as a child? "Yes. I always really enjoyed them - though I don't know if I always understood them," he confesses.

The first of the Royal Institution Christmas lectures will be transmitted on Boxing Day, BBC2, 11.20am.



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