The Star Trek episode also brought him to the attention, far more than his books had, of young people with various forms and degrees of disability. A Time magazine article in September 1993 described him speaking in Seattle for more than an hour to a rapt, ‘completely focused’ audience of teenagers in wheelchairs.12 After the talk, they crowded around him, asking questions most of which had to do with the practicalities of living with disability and political issues pertaining to the disabled, rather than with science or the cosmos. ‘As they wait for Hawking to tap out his answers, they can’t stop grinning. Here’s a famous scientist, a best-selling author, a Star Trek star – and he’s disabled, just like they are.’13 The truth of Hawking’s statement that his fame, while being a mixed blessing, ‘enables me to help other disabled people’,14 was evident.
He would help in other ways. In the summer of 1995, he lectured to a capacity crowd at London’s Royal Albert Hall, no mean amount of crowd-drawing power for a venue seating 5,000. The proceeds went to an ALS charity. He helped to publicize an exhibit of technological aids for disabled people, called ‘Speak to Me’, at London’s Science Museum. His presence or sponsorship could assure a sellout crowd nearly anywhere in the world. A January 1993 article in Newsweek described the public and media frenzy at lectures in Berkeley, California, where many of the audience showed up more than three hours early to get seats.15 As he rolled to centre stage, photographers jostled one another for good positions and there was a blizzard of flash bulbs. Harnessing that sort of excitement for the good of disabled people was well worth doing.
Over a decade Hawking had become a master manipulator of his public … or was it just an accident that he always seemed to come up with attention-getting statements whenever public and media attention appeared to require a boost. As one of his personal assistants once commented to me, ‘He isn’t stupid, you know.’ Whichever it was, it proved an advantage not only for himself but for advocates for the disabled and for science in general.
A Challenge for the ‘Prime Directive’
An uncharacteristically sour pronouncement at the Macworld Expo in Boston in August 1994 made the news all over the world: ‘Maybe it says something about human nature that the only form of life we have created so far is purely destructive. We’ve created life in our own image.’16 Hawking was talking about computer viruses.
Are computer viruses a form of life? Hawking thought they should ‘count as life’ and, with those words, initiated an uproar. In a recent episode of Star Trek, Captain Picard, confronting a super-intelligent virus, had negotiated with it rather than destroy it, to avoid violating the Star Fleet ‘prime directive’ that forbids interfering in the internal development or social order of any alien society. Destruction of the virus, in this case, would have constituted such a violation. Clearly the writers of Star Trek agreed with Hawking. There were plenty of fans of both Star Trek and Hawking ready to join the debate, on one side or the other.
‘A living being usually has two elements,’ Hawking argued. ‘First, an internal set of instructions that tell it how to sustain and reproduce itself. Second, a mechanism to carry out the instructions.’ Life as we know it is biological life, and these two elements are the genes and the metabolism. But ‘it is worth emphasizing that there need be nothing biological about them’. A computer virus copies itself as it moves into different computers and infects linked systems. Though it does not have a metabolism in the usual sense, it uses the metabolism of its hosts, like a parasite. ‘Most forms of life, ourselves included, are parasites in that they feed off and depend for their survival on other forms of life.’
Because in biology it is by no means a settled matter what is life and what is not, biologists asked to comment were unwilling to say that Hawking was wrong. Computer viruses do certainly fit some definitions of life.
Hawking closed his speech with yet another startling suggestion for what ‘life’ might include. Human life spans are too short for long-distance interstellar and intergalactic travel, even at the speed of light. However, the necessary longevity would not be difficult for mechanical spaceships that could land on distant planets, mine their resources and then produce new spaceships. The voyage could go on for ever. ‘These machines would be a new form of life based on mechanical and electronic components rather than macro molecules [like biological life],’ said Hawking. A bleak prophecy!
With everything else he was managing to fit into his schedule, Hawking found time in 1993 to co-edit a volume of technical papers on Euclidean quantum gravity with Gary Gibbons.17 Hawking wrote or co-wrote sixteen of the thirty-seven papers himself. That same year he also published a collection of his own papers on black holes and the big bang.18
Arrows of Time
Another subject Hawking was lecturing on in his public lectures in the early 1990s was far less disturbing than viruses as a form of life. It was something that had intrigued him for many years: ‘arrows of time’. The increase of entropy (disorder) and the human perception of past and future seem to be linked with the expansion of the universe. Why should this be so? As a doctoral student he had considered writing his thesis on this mysterious topic but decided he wanted something ‘more definite and less airy-fairy’. Singularity theorems were ‘a lot easier’.19 However, when he and Jim Hartle were developing their no-boundary proposal, Hawking had recognized that this work had interesting implications for arrows of time. He returned to the subject in a paper he wrote in 1985 and intended to work on it more at CERN that summer when he instead ended up so disastrously in hospital.
In the early nineties, with an increasing demand for public lectures, Hawking found that this was indeed a topic that interested his non-expert audiences and that he could explain fairly simply and succinctly. It was also a subject that allowed him to show that eminent scientists are capable of changing their minds and admitting mistakes.
With very few exceptions, the laws of science make no distinction between forward and backward directions of time. The laws are symmetrical with respect to time. You could make a film of most physical interactions and reverse the direction of the film and no one who saw it could say which way it ought to run. Strange, then, that this is not our experience at the level of the everyday world. We have a well-defined future and past. We nearly always can tell if a film is running backwards. It would be difficult to mistake one direction for another. How this ‘symmetry-breaking’ occurs is still one of the great mysteries, but we do know that in the universe as we experience it, our perception of the passage of time seems to be linked with the fact that, in any closed system, disorder (or entropy) always increases with time. The road from order to disorder is a one-way street. Broken pottery does not pull its scattered pieces together and hop back on to the shelf. Entropy, disorder, never decreases.
There are three ‘arrows of time’: the ‘thermodynamic arrow’ (the direction in which disorder, or entropy, increases); the ‘psychological’ or ‘subjective’ arrow (the way human beings experience time passing); and the ‘cosmological arrow’ (the direction of time in which the universe is expanding, not contracting). The question that interested Hawking was why these three arrows exist at all, why they are so well defined and why they point in the same direction. Disorder increases, and we experience the passage of time from past to future, while the universe expands. He suspected the answer lay in the no-boundary condition for the universe, with some help from the anthropic principle.
The thermodynamic arrow (having to do with the increase of disorder or entropy) and the psychological arrow (our everyday perception of time) do point, always, in the same direction. It is common experience that as time moves forward, disorder, or entropy, increases. Hawking admitted that this is a tautology, concluding that ‘entropy increases with time because we define the direction of time to be that in which entropy increases’20 – but he was satisfied that the psychological arrow and the thermodynamic arrow are essentially the same arrow.
Why then does it point in the same direc
tion as the cosmological arrow of time, with the expansion of the universe? Must it? Enter the no-boundary proposal. Recall that in the classical theory of general relativity all physical laws break down at the Big Bang singularity. It is impossible to predict whether or not the beginning of time would have been orderly or a situation of complete disorder in which there was no possibility of disorder increasing. However, if Hawking’s and Hartle’s no-boundary proposal is correct, the beginning was ‘a regular, smooth point of spacetime and the universe would have begun its expansion in a very smooth and ordered state’.21 As the universe expanded, the gradual development of all the structure we observe today – galaxy clusters, galaxies, star systems, stars, planets, you and me – represented a continuous, enormous increase in disorder, and this trend continues. Hence, in the universe as we know it, the thermodynamic arrow, the psychological arrow and the cosmological arrow all point in the same direction.
But consider what might happen if Friedmann’s first model of the universe is correct (see Figure 6.1), the model in which the universe eventually stops expanding and begins to contract. When expansion changes to contraction, the cosmological arrow of time reverses direction. The big question was, would the thermodynamic and psychological arrows of time also reverse direction? Would disorder start to decrease? Hawking thought there were all sorts of interesting possibilities for science fiction writers here, but he also pointed out that it was ‘a bit academic to worry about what would happen when the universe collapses again, as it will not start to contract for at least another ten thousand million years.’22
Nevertheless, the no-boundary condition did seem to mean that disorder would decrease in the collapsing universe, and Hawking at first concluded that when the universe stopped expanding and started to collapse, not only the cosmological arrow but all three arrows would reverse direction and all three continue to point in the same direction as one another. Time would be reversed and people would live their lives backwards, ‘youthening’, as the magician Merlin did in T. H. White’s Arthurian novel The Once and Future King. Broken teacups would reassemble.
Don Page, by then in the physics faculty at Pennsylvania State University, begged to differ. In a paper that eventually appeared in the same issue of Physical Review as Hawking’s arrow of time paper, Page argued that the no-boundary condition did not mean that all three arrows would have to be reversed when the universe was in its contracting phase.23 Raymond LaFlamme, one of Hawking’s students, found a more complicated model, and the three argued and sent calculations back and forth. Page, with more experience of working with Hawking, suggested to LaFlamme that it would be best not to tell Hawking their conclusion but first to lay out all their assumptions in such a way that Stephen would arrive at the same result without their having told him what it was.24 They finally convinced their mentor that he had been wrong. Though the cosmological arrow of time would reverse when the universe stopped expanding and began to contract, the thermodynamic and psychological arrows would not. It was too late to change Hawking’s paper, but he was able to insert a note admitting, ‘I think that Page may well be right in his suggestion.’25
What, then, is the answer to the question, why do we observe the thermodynamic, psychological, and cosmological arrows pointing in the same direction? Because, even though we would not find ourselves ‘youthening’, we could not survive in the universe when it is collapsing, when the cosmological arrow of time has reversed. At that distance in the future, the universe will be in a state of nearly total disorder, all the stars burned out, the protons and neutrons in them decayed into light particles and radiation. There will no longer be a strong thermodynamic arrow of time at all. We couldn’t survive the death of our sun, but even if we could, we also require a strong thermodynamic arrow of time in order to exist. For one thing, human beings have to eat. Food is a relatively ordered form of energy. The heat into which our bodies convert food is more disordered. Hawking had concluded that the psychological and thermodynamic arrows of time are for all intents and purposes the same arrow, and if one fizzles out, so does the other. In the contracting phase of the universe there could be no intelligent life. The answer to the question of why we observe the thermodynamic, psychological and cosmological arrows pointing in the same direction is: because, if things were different, there would be no one around to ask those questions. If that sounds familiar, it is none other than the anthropic principle. As time (in all three senses) passed, Hawking was thinking less and less of the anthropic principle as a cop-out, ‘a negation of all our hopes of understanding the underlying order of the universe’, and increasingly regarding it as a powerful principle indeed.
More Conjuring at the Event Horizon
Hawking had suspected in 1981 that Leonard Susskind was ‘the only one in the room who fully appreciated the implications of what I had said’, in Werner Erhard’s attic. In the years since then, Susskind had never been able to leave the information paradox problem alone. ‘Just about everything I have thought about since 1980 has in one way or another been a response to [Hawking’s] profoundly insightful question about the fate of information that falls into a black hole. While I firmly believe his answer was wrong, the question and his insistence on a convincing answer have forced us to rethink the foundations of physics.’26 In 1993, referring back to work Hawking had done in the 1970s, Susskind came up with a new way of dealing with a contradiction that defied common sense at the event horizon of a black hole.
It will come as no news to anyone who has read even the most rudimentary book about black holes that if someone (let us call her Miranda) falls into one, the experience for her will be radically different from what it appears to be from the vantage point of someone (let us call him Owen) who is watching from a spaceship at a distance outside the black hole. Einstein showed that if two people are moving rapidly relative to one other, each one sees the other’s clock slow down and sees the other being flattened out in the direction of motion. Also a clock that is in the vicinity of a massive object (and a black hole is a very massive object) will run more slowly compared with one that is not.
The upshot is that from the vantage point of distant observer Owen, Miranda as she falls towards the black hole seems to be falling more and more slowly with her body squashed to a thinner and thinner pancake. Finally, when Miranda reaches the event horizon, Owen sees her come to a stop. He never sees her fall through the horizon, in fact, never sees her quite get there. Meanwhile, Miranda’s own experience is that she falls through the event horizon intact. From Owen’s point of view, she is stuck and flattened; from Miranda’s point of view, she is still falling.
Susskind had set himself to find out how both could be true and pointed out that although he and you and I, who are neither falling in nor watching from a distance, can agree that both scenarios in our example have occurred, and be troubled by the contradiction, none of us is actually on the spot. Suppose instead that you and I are part of the action. This time I will be the observer at a distance. You fall into the black hole. The crux of the matter is that in a real-life playing out of this story, neither I, the observer at a distance, nor you, who fall into the black hole, ever observes or experiences the contradiction. And you who have experienced an uneventful fall through the event horizon are absolutely incapable of going back and comparing notes with me or of sending me a message. If I happen to fall in later (this possibility stumped Susskind for a while), you would still be so far ahead on the way to the singularity that I would never catch up. It would be impossible for either of us, ever, to know about the version of the story that contradicted our own.
Susskind, and colleagues Lárus Thorlacius and John Uglum, called this principle that neither observer ever sees a violation of the laws of nature ‘horizon complementarity’.
Take a moment to recall what ‘complementarity’ means. It is using two different, perhaps mutually exclusive descriptions in order to gain a better understanding than either description alone provides. Early in the twentieth cen
tury it was physicist Niels Bohr’s way of addressing a problem in physics known as wave-particle duality. People experimenting with the way light propagates (the way it travels) found that it acts as though it were waves. The description of it as particles is ruled out. However, when they studied the way light interacts with matter, they found that it acts as though it must be particles. The model that describes it as waves is ruled out. By 1920 it was clear that light could be conceived of either in terms of waves or in terms of particles, but that neither model by itself could explain the experimental data, and this odd situation could not be resolved by saying that light is sometimes particles and sometimes waves, or that light is both particles and waves. The problem applies to matter as well as to radiation. Bohr wrote to Einstein in 1927, concluding that it was possible to live with what looked like a contradiction ‘as long as we don’t allow our intuitive feeling that matter and radiation must be either wave or particle to “lead us into temptation”’.27 The descriptions were incompatible but both necessary, and both correct.
The same could be said in the case of horizon complementarity. As Susskind summed it up, ‘The paradox of information being at two places at the same time is apparent and yet a careful analysis shows that no real contradictions arise. But there is a weirdness to it,’ he admits.28 Gerard ’t Hooft of Utrecht had in 1993 introduced what he called ‘dimensional reduction’. Susskind rechristened it the ‘holographic principle’.
Go back to thinking about Miranda falling towards the event horizon, as viewed by Owen, the distant observer in the spaceship. From the spaceship, because of time dilation, Miranda appeared to freeze and spread out at the event horizon. Susskind points out that, by the same token, Owen will also see everything else that originally went into the formation of the black hole, and everything that has fallen into it, likewise frozen at the horizon. ‘The black hole consists of an immense junkyard of flattened matter at its horizon,’ says Susskind.
Stephen Hawking, His Life and Work Page 24