The Day Time Began

Don't ask me what came before the big bang,says physicist Paul Davies.Time and space only popped into existence at that instant,so the question doesn't apply.But what made it happen and where did the laws of physics come from?

CAN science explain how the Universe began? Even suggestions to that effect have provoked an angry and passionate response from many quarters. Religious people tend to see the claim as a move to finally abolish God the Creator. Atheists are equally alarmed, because the notion of the Universe coming into being from nothing looks suspiciously like the creation, ex nihilo, of Christianity. The general sense of indignation was well expressed by writer Fay Weldon. " Who cares about half a second after the big bang," she railed in 1991 in a scathing newspaper attack on scientific cosmology. "What about the half a second before?" What indeed. The simple answer is that, in the standard picture of the cosmic origin, there was no such moment as "half a second before."
To see why, we need to examine this standard picture in more detail. The first point to address is why anyone believes the Universe began at a finite moment in time. How do we know that it hasn't simply been around for ever? Most cosmologists reject this alternative because of the severe problem of the second law of thermodynamics. Applied to the Universe as a whole, this law states that the cosmos is on a one-way slide towards a state of maximum disorder, or entropy. Irreversible changes, such as the gradual consumption of fuel by the Sun and stars, ensure that the Universe must eventually "run down" and exhaust its supplies of useful energy. It follows that the Universe cannot have been drawing on this finite stock of useful energy for all eternity.

'The Universe cannot have been drawing on this finite stock of useful energy for all eternity'

Body of evidence
Direct evidence for a cosmic origin in a big bang comes from three observations. The first, and most direct, is that the Universe is still expanding today. The second is the existence of a pervasive heat radiation that is neatly explained as the fading afterglow of the primeval fire that accompanied the big bang. The third strand of evidence is the relative abundances of the chemical elements, which can be correctly accounted for in terms of nuclear processes in the hot dense phase that followed the big bang.
But what caused the big bang to happen? Where is the centre of the explosion? Where is the edge of the Universe? Why didn't the big bang turn into a black hole? These are some of the questions that bemused members of the audience always ask whenever I lecture on this topic. Though they seem pertinent, they are in fact based on an entirely false picture of the big bang. To understand the correct picture, it is first necessary to have a clear idea of what the expansion of the Universe entails. Contrary to popular belief, it is not the explosive dispersal of galaxies from a common centre into the depths of a limitless void. The best way of viewing it is to imagine the space between the galaxies expanding or swelling.
The idea that space can stretch, or be warped, is a central prediction of Einstein's general theory of relativity, and has been well enough tested by observation for all professional cosmologists to accept it. According to general relativity, space-time is not a static arena, but an aspect of the gravitational field. This field manifests itself as a warping, or curvature, of space-time geometry, and when it comes to the large scale structure of the Universe, such a warping occurs in the form of space being stretched with time.
A helpful, albeit two-dimensional, analogy for the expanding Universe is a balloon with paper spots stuck to the surface. As the balloon is inflated, so the spots, which play the role of galaxies, move apart from each other. Note that it is the surface of the balloon, not the volume within, that represents the three-dimensional Universe.
Now, imagine playing the cosmic movie backwards, so that the balloon shrinks rather than expands. If the balloon were perfectly spherical (and the rubber sheet infinitely thin), at a certain time in the past the entire balloon would shrivel to a speck. This is the beginning.
Translated into statements about the real Universe, I am describing an origin in which space itself comes into existence at the big bang and expands from nothing to form a larger and larger volume. The matter and energy content of the Universe likewise originates at or near the beginning, and populates the Universe everywhere at all times. Again, I must stress that the speck from which space emerges is not located in anything. It is not an object surrounded by emptiness. It is the origin of space itself, infinitely compressed. Note that the speck does not sit there for an infinite duration. It appears instantaneously from nothing and immediately expands. This is why the question of why it does not collapse to a black hole is irrelevant. Indeed, according to the theory of relativity, there is no possibility of the speck existing through time because time itself begins at this point.
This is perhaps the most crucial and most difficult aspect of the big bang theory. The notion that the physical Universe came into existence with time and not in time has a long history, dating back to St Augustine in the fifth century. But it took Einstein's theory of relativity to give the idea scientific respectability. The key feature of the theory of relativity is that space and time are part of the physical Universe, and not merely an unexplained background arena in which the Universe happens. Hence the origin of the physical Universe must involve the origin of space and time too.
But where could we look for such an origin? Well, the theory of relativity permits space and time to possess a variety of boundaries or edges, technically known as singularities. One type of singularity exists in the centre of a black hole. Another corresponds to a past boundary of space and time at the big bang. The idea is that, as you move backwards in time, the Universe becomes more and more compressed and the curvature or warping of space-time escalates without limit, until it becomes infinite at a singularity. Very roughly, it resembles the apex of a cone, where the fabric of the cone tapers to an infinitely sharp point and ceases. It is here that space and time begin.
Once this idea is accepted it is immediately obvious that the question "What happened before the big bang?" is meaningless. There was no such epoch as "before the big bang", because time began with the big bang. Unfortunately, the question is often answered with the bald statement "There was nothing before the big bang", and this has caused yet more misunderstandings. Many people interpret "nothing" in this context to mean empty space, but as I have been at pains to point out, space did not exist either prior to the big bang.

A horizontal slice through each of these cones represents space at a given moment. In the classical picture, the big bang is a sharp point at the apex of the cone. In the quantum view, the separate identities of time and space are smeared near the apex by quantum uncertainty.

'Quantum fluctuations are genuinely spontaneous and intrinsic to nature at its deepest level. There is no deeper reason, no underlying causes that explain when a nucleus will decay. It just happens'

Absolutely nothing
Perhaps "nothing" here means something more subtle, like pre-space, or some abstract state from which space emerges? But again, this is not what is intended by the word. As Stephen Hawking has remarked, the question "What lies north of the North Pole?" can also be answered by "nothing", not because there is some mysterious Land of Nothing there, but because the region referred to simply does not exist. It is not merely physically, but also logically, non-existent. So too with the epoch before the big bang.
In my experience, people get very upset when told this. They think they have been tricked, verbally or logically. They suspect that scientists can't explain the ultimate origin of the Universe and are resorting to obscure and dubious concepts like the origin of time merely to befuddle their detractors. The mind-set behind such outraged objection is understandable: our brains are hardwired for us to think in terms of cause and effect. Because normal physical causation takes place within time, with effect following cause, there is a natural tendency to envisage a chain of causation stretching back in time, either without any beginning, or else terminating in a metaphysical First Cause, or Uncaused Caused, or Prime Mover. But cosmologists now invite us to contemplate the origin of the Universe as having no prior cause in the normal sense, not because it has an abnormal or supernatural prior cause, but because there is simply no prior epoch in which a preceding causative agency-natural or supernatural-can operate.
Nevertheless cosmologists have not explained the origin of the Universe by the simple expedient of abolishing any preceding epoch. After all why should time and space have suddenly "switched on"? The latest thinking is that this spontaneous origination of time and space is a natural consequence of quantum mechanics. Quantum mechanics is the branch of physics that applies to atoms and subatomic particles, and it is characterised by Heisenberg's uncertainty principle, according to which sudden and unpredictable fluctuations occur in all observable quantities. Quantum fluctuations are not caused by anything-they are genuinely spontaneous and intrinsic to nature at its deepest level.

'Time does not switch on in Hartle and Hawking's theory, it emerges continuously from space. There is no first moment at which time starts, but neither does it extend backwards for all eternity'

Impossible predictions
For example, take a collection of uranium atoms suffering radioactive decay due to quantum processes in their nuclei. There will be a definite time period, the half-life, after which half of the nuclei present should have decayed. But according to Heisenberg it is not possible, even in principle, to predict when a given nucleus will decay. If you ask, having seen a particular nucleus decay, why the decay event happened at that moment rather than some other, there is no deeper reason, no underlying set of causes, that explains it. It just happens.

The key step for cosmogenesis is to apply this same idea not just to matter, but to space and time as well. Because space-time is an aspect of gravitation, this entails applying quantum theory to the gravitational field of the Universe. The application of quantum mechanics to a field is fairly routine for physicists, though it has to be said that there are special technical problems associated with the gravitational case that have yet to be satisfactorily resolved ("Can gravity take a quantum leap?", 10 September 1994, p 28). The quantum theory of the origin of the Universe therefore rests on shaky ground.
In spite of these technical obstacles, one may say quite generally that once space and time are made subject to quantum principles, the possibility immediately arises of space and time "switching on", or popping into existence, without the need for prior causation, entirely in accordance with the laws of quantum physics.
The details of this process remain both subtle and contentious, and depend to some extent on the interrelationship between space and time. Einstein showed that space and time are closely interwoven, but in the theory of relativity they are still distinct. Quantum physics introduces the new feature that the separate identities of space and time can be "smeared" or "blurred" on an ultramicroscopic scale. In a theory proposed in 1982 by Hawking and American physicist Jim Hartle, this smearing implies that, closer and closer to the origin, time is more and more likely to adopt the properties of a space dimension, and less and less likely to have the properties of time. This transition is not sudden, but blurred by the uncertainty of quantum physics. Thus time does not switch on abruptly in Hartle and Hawking's theory, but it emerges continuously from space. There is no specific first moment at which time starts, but neither does time extend backwards for all eternity (see Diagram p 34).
Unfortunately, the topic of the quantum origin of the Universe is fraught with confusion because of the publicity given to a preliminary, and in my view wholly unsatisfactory theory of the big bang based on an instability of the quantum vacuum. According to this alternative theory, first mooted by Edward Tryon in 1973, space and time are eternal, but matter is not. It suddenly appears in a pre-existing and unexplained void due to quantum vacuum fluctuations. In such a theory, it would indeed involve a serious misnomer to claim that the Universe originated from nothing: a quantum vacuum in a background space-time is certainly not nothing.
Law unto itself
However, if there is a finite probability of an explosive appearance of matter, it should have occurred an infinite time ago. In effect, Tryon's theory and others like it run into the same problem of the second law of thermodynamics as most models of an infinitely old Universe.
It will be obvious from what I have said that the attempt to explain the origin of the Universe is based on an application of the laws of physics. This is normal in science: one takes the underlying laws of the Universe as given. But when tangling with ultimate questions, it is only natural that we should also ask about the status of these laws. One must resist the temptation to imagine that the laws of physics, and the quantum state that represents the Universe, somehow exist before the Universe. They don't-any more than they exist north of the North Pole. In fact, the laws of physics don't exist in space and time at all. They describe the world, they are not "in" it. However, this does not mean that the laws of physics came into existence with the Universe. If they did-if the entire package of physical Universe plus laws just popped into being from nothing-then we cannot appeal to the laws to explain the origin of the Universe. So to have any chance of understanding scientifically how the Universe came into existence, we have to assume that the laws have an abstract, eternal character. The alternative is to shroud the origin in mystery and give up.
It might be objected that we haven't finished the job by baldly taking the laws of physics as given. Where did those laws come from? And why those laws rather than some other set? This is a valid objection. I have argued - that we must eschew the traditional causal chain and focus instead on an explanatory chain, but inevitably we now confront the logical equivalent of the First Cause -the beginning of the chain of explanation.
In my view it is the job of physics to explain the world based on lawlike principles. Scientists adopt differing attitudes to the metaphysical problem of how to explain the principles themselves. Some simply shrug and say we must just accept the laws as a brute fact. Others suggest that the laws must be what they are from logical necessity. Yet others propose that there exist many worlds, each with differing laws, and that only a small subset of these universes possess the rather special laws needed if life and reflective beings like ourselves are to emerge. Some sceptics rubbish the entire discussion by claiming that the laws of physics have no real existence anyway-they are merely human inventions designed to help us make sense of the physical world. It is hard to see how the origin of the Universe could ever be explained with a view like this.
In my experience, almost all physicists who work on fundamental problems accept that the laws of physics have some kind of independent reality. With that view, it is possible to argue that the laws of physics are logically prior to the Universe they describe. That is, the laws of physics stand at the base of a rational explanatory chain, in the same way that the axioms of Euclid stand at the base of the logical scheme we call geometry. Of course, one cannot prove that the laws of physics have to be the starting point of an explanatory scheme, but any attempt to explain the world rationally has to have some starting point, and for most scientists the laws of physics seem a very satisfactory one. In the same way, one need not accept Euclid's axioms as the starting point of geometry; a set of theorems like Pythagoras's would do equally well. But the purpose of science (and mathematics) is to explain the world in as simple and economic a fashion as possible, and Euclid's axioms and the laws of physics are attempts to do just that.
In fact, it is possible to quantify the degree of compactness and utility of these explanatory schemes using a branch of mathematics called algorithmic information theory. Obviously, a law of physics is a more compact description of the world than the phenomena that it describes. For example, compare the succinctness of Newton's laws with the complexity of a set of astronomical tables for the positions of the planets. Although as a consequence of Gödel's famous incompleteness theorem of logic, one cannot prove a given set of laws, or mathematical axioms, to be the most compact set possible, one can investigate mathematically whether other logically self-consistent sets of laws exist. One can also determine whether there is anything unusual or special about the set that characterises the observed Universe as opposed to other possible universes. Perhaps the observed laws are in some sense an optimal set, producing maximal richness and diversity of physical forms. It may even be that the existence of life or mind relates in some way to this specialness. These are open questions, but I believe they form a more fruitful meeting ground for science and theology than dwelling on the discredited notion of what happened before the big bang.

World without End

How sure can we be that the Universe began with the big bang? Was there only one big bang, or have there been many? Could the Universe really have begun as a quantum blip? Some of the world's leading cosmologists give their views

TRY asking a bunch of cosmologists about the origin of the Universe, and it's hard to get a clear answer. "The Universe didn't start. It's infinite," says British cosmologist Fred Hoyle. "It's an open question," says Steven Weinberg, Nobel prizewinning particle physicist from the University of Texas. "It's up in the air," says Paul Steinhardt from Pennsylvania State University, co-developer in the I 980s of a key theory about the early Universe. "It must have had a beginning," says cosmologist Alexander Vilenkin of Crufts University in Massachusetts. The standard big bang model is agreed, says Oxford mathematician Roger Penrose, and everything else is "embellishments and flights of fancy". So what gives? Well, Hoyle is convinced that the big bang is a myth, and that the Universe is eternal, with matter continuously created at the centres of galaxies. But virtually everyone else is happy with the big bang model, at least as far back as the early stages of the Universe. Says Weinberg, "We are in an expanding Universe which at one time before any of the stars or galaxies formed-was very hot and dense. I don't think there's any serious argument that in that sense there was a big bang and the part of the Universe that we live in had a start. But beyond that we really don't know."
To try to trace the history of the Universe back to its origin, cosmologists picture the expansion running backwards to a point where the Universe was almost unimaginably small and dense. The first problem they meet, when they do this, is that the concept of time comes apart in their hands. The reason is that at the so-called Planck scale (a mere 10-35 metres), two theories begin to clash. Einstein's smooth, large-scale, classical theory of gravity makes no provision for the fuzzy, indeterminate quantum theory of tiny particles, and all bets are off. "Questions about what happened before what begin to lose meaning," says Steinhardt. "Before only makes sense if there's a sensible time ordering to things, and that notion breaks down at the Planck scale." Weinberg agrees: "Any description that tries to go to earlier times has to give up the idea of time. It's no longer a meaningful concept."

'When they do this, the concept of time comes apart in their hands'

Glimmers of hope for reconciling relativity and quantum theory come from an idea called superstrings-in which all matter is made up of tiny 10-dimensional strings. Although we appear to live in a Universe with just four dimensions, three for space and one for time, the theory goes that the other six dimensions present are curled up so tightly that we can't detect them directly. But this causes even greater problems, because at the Planck scale the tightly curled extra dimensions become significant. "You go back in time and it looks like you're heading towards a singularity and all of a sudden-wham-physics changes because all those extra dimensions that you weren't aware of suddenly come into play," says Steinhardt.
It is usually easy to tell time and space apart. But, says Steinhardt, "When you unwrap the extra dimensions,you don't know what they'll be like. It may be that you even have two time-like coordinates, or more. The idea of before and after would then be even shakier.
How the Universe could appear from nothing in the first place? In 1982, Vilenkin came up with the idea that the Universe literally tunnelled its way into existence, something allowed by quantum theory but impossible on an everyday large scale. In the classical world, if you have a heavy oblect lying in a dip it will need a push to climb over the edge and roll down the other side. But in the quantum world, there is a small, but nonzero probability that the object can simply tunnel to the other side of the dip without any outside help. The only condition is that it does not gain any energy in the process.
So how does this relate to the Universe? Well, say you start with nothing at all-not even space or time. Presumably the total energy of this system would be zero. Is it possible to make a Universe of space, time and matter whose total energy is still zero? The answer is yes. "You can't create something out of nothing," says Vilenkin. "But the Universe is an exception. Gravitational energy is negative and matter energy is positive. In a closed Universe-one where if you keep going in one direction you come back to the same point-the negative energy of gravity exactly cancels the positive energy of matter, so the total energy is zero.

'It's likely that our own personal big bang is actually a pretty insignificant one'

In the classical picture, the Universe cannot appear out of nothing because it is forbidden to adopt a certain range of sizes. But in quantum theory, the Universe can tunnel through this size barrier, and appear spontaneously with a size greater than the critical value.
Can we ever know if the Universe began at a single point, or has simply been going on for ever? There is yet another complication, which may make the whole question academic. It stems from an idea called inflation, first developed in the early 1980s to solve some vexing problems with the standard big bang model. In its earliest versions, inflation theory stipulated that, immediately after the big bang, the Universe suddenly ballooned, increasing its diameter by more than a trillion trillion times in just a tiny fraction of a second. After this, the Universe switched to a noninflationary phase, and expanded at a more sedate rate. But in the mid-1980s, cosmologist Andrei Linde at Stanford University realised that such a system would be self-replicating. Once you kicked it off with a big bang,it would go on forever.
Even when most of the Universe had moved out of the inflationary phase, Linde reasoned, tiny fluctuating regions would still be capable of undergoing inflation. These would then go from being infinitesimal regions to sizeable chunks of Universe in a split second, and would themselves go on to spawn new patches of Universe and so on. In each case, once inflation was over, the patch would evolve according to standard big bang theory.
If this is true, the whole Universe could be made up of a huge number of expanding patches, which could be quite different from our own. The problem is that we can never know. "We are removed by a tremendous distance from regions that underwent a different history," says Steinhardt "Inflation casts a pall on things because it makes the part of the Universe we see so infinitesimal compared to the entire Universe, and perhaps not even representative. We will never be able to see the edge of the patch we live in, and this puts us beyond the ability to be able to probe things through observations."
What's more, an eternal, self-replicating Universe may not even need a big bang. Vilenkin says he has proved in a theorem that the inflationary Universe must have had an origin, but Linde is skeptical. He thinks it likely but not proved that there was an initial big bang from which all of the "pretty big bangs" came. However, he adds that the question is so far removed from our experience that it is irrelevant: "Say if you have an infinite number of bubbles, all producing new ones. You live in one of these bubbles and you look at the point the bubble was formed. For all practical purposes that's the beginning of your Universe." Because there are infinitely many such bubbles, we have no reason to believe that ours is the first, or even the hundredth. It's more likely, says Linde, that our own personal big bang is actually a pretty insignificant one, way down the line from the one that set the whole Universe going.
Gabrielle Walker

Countdown to oblivion: Why time itself could end

20:31 28 September 2010 by Rachel Courtland, Cambridge, Massachusetts Magazine issue 2780.

"We could run into the end of time," Ben Freivogel tells a seminar at the Massachusetts Institute of Technology in Cambridge. Several colleagues seem nonplussed, and one Nobel laureate looks downright exasperated. "I'm aware that this sounds like a crazy conclusion," Freivogel admits, generating a round of what sounds like relieved laughter. But perhaps their relief is short-lived. The nature of time, our perception of it and even whether it exists at all are hot topics for both physicists and philosophers. But Freivogel isn't pushing a strange new concept of time. His idea is arguably even more baffling. He thinks that time, as described by Einstein's theory of general relativity, could simply end in our universe, taking us with it. He gives us another 5 billion years or so before the axe falls (see "Five billion years to go", below). This unsettling idea arises from a popular theory called eternal inflation. In this theory, different parts of space can undergo dramatic growth spurts, essentially ballooning into separate universes with their own physical properties. The process happens an infinite number of times, creating an infinite number of universes, called the multiverse.

Measure problem
The infinities involved mean that anything that can happen does happen – an infinite number of times. That makes it difficult to figure out how common a universe like ours is. "It sort of pulls the rug out from underneath your intuition of how to define probabilities," says Freivogel's colleague Raphael Bousso of the University of California, Berkeley. To get around this problem, physicists take a "cut-off" of the multiverse, cutting out a finite patch of space-time and counting the universes within it to get a representative sample. However, doing this inevitably slices through individual universes on the edge of the sample. This leads to incorrect probabilities of experimental outcomes in the multiverse – unless, Freivogel and his team argue, the mathematical cut-offs somehow have real and dire consequences for the places they intersect. Time would end there, they say, causing everything present to disappear. "The world, including you, would just cease to exist," says Bousso.

Predictive power
The idea is more than strange, not least because it is not clear how these mathematical constructs could impinge on the real world. But the team say the cut-offs have to be considered real if they are to be used to calculate probabilities in the multiverse. These are key to making cosmological predictions about properties in our own universe such as the strength of dark energy. The alternative, they say, is that applying cut-offs is simply not the right way to calculate probabilities in eternal inflation. "We're stuck between a rock and a hard place," says Bousso. "If you don't like the cut-off, then you have no way of making predictions and deciding what's probable in eternal inflation." Ken Olum of Tufts University in Medford, Massachusetts, who was not involved in the work, agrees that there is a problem. "The whole issue is one that needs resolution," he says. "If we don't take the cut-off seriously, then we better figure out how to do calculations that are consistent with each other." The trouble began last year, with a thought experiment raised over breakfast at a conference by Alan Guth of MIT and Vitaly Vanchurin of Stanford University in California. They imagined a scenario in which someone flips a coin and sets an alarm depending on the outcome. Heads and the alarm is set to wake you up after 1 minute, tails and you get 1 billion years' sleep. Before going to sleep, the chance of waking up in 1 minute or 1 billion years is 50:50.

Changing odds
Now imagine that the experiment happens in an infinite number of universes. If a cut-off is taken to study a subset of these universes, far more people in that subset – wherever the cut-off is made – will have woken up after a short nap than a long one. So the odds are no longer 50:50. How can the probability change once you get up? The team argue that the only way to make sense of the changing odds is if the cut-off is physical. If the cut-off is real, then many of the people who got tails – and went to sleep for 1 billion years – would hit the end of time before their alarm could go off ( "If we do have the end of time, then that's a strange situation, but at least it solves this paradox," says Olum. The radical idea "certainly is one way to resolve the paradox", says Guth. But he adds that it is hard to lend much credence to the suggestion, since it is far from clear what physical mechanism could cause time to vanish.

No physical meaning
Instead, Guth suggests the paradox could just be an artefact of the measurement technique, since the exponential nature of eternal inflation means that newer universes will always be more common than older ones. He suspects cut-offs have no physical meaning: people who don't wake up before the cut-off will still wake up, just after it. The cut-off "is the end of the data set. It doesn't necessarily mean it's the end of the world." However, Bousso says we have to take the cut-off seriously, since it's the only good way physicists have of calculating probabilities. "In current approaches to understanding eternal inflation, the cut-off and only the cut-off defines what is possible, likely, unlikely and impossible," he says. For the moment, Guth says he is comfortable with not fully understanding how probabilities work in eternal inflation. "We are dealing with something that's exponentially expanding and expanding forever," he says. "It is conceivable that will introduce new problems that ordinary actuaries have never encountered."

Five billion years to go
To calculate probabilities in the infinite multiverse, physicists have devised "geometric cut-offs", ways of slicing off finite samples of space-time. This lets them count finite numbers of events and extrapolate out to the whole multiverse by taking larger and larger samples. But doing this inevitably slices through some universes that lie on the edge of whatever cut-off is used, a process that Ben Freivogel at MIT and colleagues say could end time there (see main story). So could this happen to our universe? Sadly, yes. They say several methods of taking cut-offs suggest a universe the same age as ours, 13.7 billion years old, is likely to reach the end of time in 5 billion years or so. "The point is that the way people treat eternal inflation, time can end, no matter whether we understand precisely how time would actually do that," says team member Raphael Bousso of the University of California, Berkeley. "Is there a way of thinking about the end of time that makes it seem less weird?" he asks. One of the cut-off methods offers a way to visualise the process. It slices the multiverse by taking a single "causal patch" – a region of space beyond which light has not had time to reach since that region's big bang. From this viewpoint, you can think of our 5-billion-year expiration date as the average time needed for a galaxy located anywhere inside our 13.7-billion-light-year causal patch to reach the edge of the region. What would it mean for time to end in this way? The team speculate that reaching the edge of a causal patch might be like encountering the event horizon of a black hole, the boundary beyond which nothing that falls in can escape. So just as someone watching an object fall into a black hole will see the object burn up, someone inside a given universe might see an object hitting the edge of the cut-off – where time ends – incinerate on contact.


Paul Davies is professor of theoretical physics at the University of Adelaide.

Further Reading

His book About Time is published by Viking. A New Scientist newsletter is available containing all the letters received recently about the origin of the Universe. Write/fax to "big bang newsletter" or e-mail





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