Astronomy

What does Stephen Hawking mean by 'an infinite universe'?

What does Stephen Hawking mean by 'an infinite universe'?


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In the recent $100m search for extra terrestrial life project project, Stephen Hawking is quoted in the following way:

"We believe that life arose spontaneously on Earth," Hawking said at Monday's news conference, "So in an infinite universe, there must be other occurrences of life."

My understanding is that scientists believe that the universe isn't infinite - there there is a finite number of stars, it's continuously expanding but has a finite size at a given time.

What's Stephen Hawking refering to here?


Due to the finite speed of light, and the finite age of the Universe, only a portion of it is observable. When people talk about "the size of the Universe", "the number of stars in the Universe", etc., they usually refer to the observable Universe, i.e. the sphere in which we are centered, and which has a radius given by the distance light has been able to travel in the 13.8 billion years since the Big Bang. Note that since the Universe is expanding, this radius is more than 13.8 billion light-years. In fact it's 46.3 billion light-years.

Observations indicate that, on large scales (i.e. above roughly half a billion light-years), the Universe is homogeneous (the same everywhere) and isotropic (the same in all directions). Assuming that this is indeed true is known as the cosmological principle. If the rest of the Universe follows this principle, then there are three possible overall "versions" of universes that we can live in. We call these versions "flat", "closed", and "open". Whereas a globally closed universe would have a finite extent, globally flat or open universes must be infinitely large.

The observable Universe is, within measuring uncertainties but too a very high precision, flat (e.g. Planck Collaboration et al. 2016). Hence, we might think that the whole Universe is, in fact, infinite. But sort of like standing in a large forest with limited visibility doesn't tell you whether the forest is just larger than you can see, or if it's infinitely large, we can't with our current theories and observations know whether the Universe is finite or infinite.


A relativistic universe which is expanding faster than light (like ours) is effectively infinite for all practical purposes.

Also due to its relativistic nature and faster than light expansion, even if you assume it's not infinite at some given moment, it still doesn't have any edges or borders - for you. You're in some random place in it, the maximum speed of any possible interaction is speed of light, and the universe is expanding faster than light - then anything that happens at some (real or imaginary) "edge" is outside your realm of existence. It is effectively infinite - for you.

Give it enough time and it would grow as big as you want. Give it asymptotically infinite time and it will grow asymptotically infinite.

As to what the "edge" might be, see Eternal Inflation. This is a model in modern cosmology where local bubbles like ours have stopped inflating (they still expand, but not at the tremendous rate of the initial inflationary phase); however, inflation continues forever outside the bubbles and at the edges. Therefore the bubbles keep growing indefinitely at faster than light speeds. Because speed of light is a limit for any interaction inside the bubbles, for any internal observer each bubble is effectively infinite for any practical purpose.

Be aware that there is no proof that this is actually the case, but this is a model that fits well what we now observe.


EDIT: TBH, I'm not even sure this is a question for StackExchange. It's very open-ended, and we don't really have the conclusive answers. All we can say for now is that the universe appears to be infinite for all practical purposes, but we can't know for sure. So often scientists like Hawking just simplify the language and refer to the universe as "infinite" without any of the qualifiers that would be required in a strict context.

I don't think there's any single, final answer here.


I believe his meaning is being misinterpreted as a claim that the universe actually is infinite. If someone says "in an infinite universe", they are not asserting that the universe is indeed infinite (as no scientist could ever claim to know), they are simply saying "if the universe is infinite." It is merely a case from which to begin the discussion. For example, if you were having a discussion about whether to pass a seatbelt law, you might start with "in a world where people always make the safer choice in everything they do, we would not need seatbelt laws." That would not be a claim we live in such a world, it is merely setting a kind of baseline for going on to talk about what is the actual situation.


Stephen Hawking's Final Theory of the Universe Has Been Published

Stephen Hawking isn't done yet. The brilliant physicist, who died in March after one of the most extraordinary careers of the century, co-authored a new theory about the origins of the universe released by Cambridge today.

The theory, co-authored with Thomas Hertog of the Belgian university KU Leven, uses string theory develop an idea of a cosmic origin, a departure from Hawking's earlier ideas of a "no boundary" universe, or the idea that space existed without time before the Big Bang and therefore the universe has no beginning. The new theory, according to Hawking and Herzog, is that the universe is "finite and reasonably smooth."

&ldquoNow we&rsquore saying that there is a boundary in our past,&rdquo says Hertog in a press statement.

However, Hawking and Hertog's new theory is still skeptical of the Big Bang. The scientists were critical of an idea known as "eternal inflation." Eternal inflation is the theory that even though the universe isn't still growing at the same rate it did at the beginning of time, growth still occurs, starting from that boundary, and will continue infinitely.

Eternal inflation has been used to support the idea of a multiverse, or multiple universes beyond our own. If the energy of eternal inflation existed before the Big Bang, the theory goes, then it might not have been entirely spent in our universe alone.

But Hawking and Hertog have their own ideas about eternal inflation. Using string theory, the two theorize that there is a point where eternal inflation begins, and at that point it exists in a timeless state. &ldquoWhen we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning,&rdquo says Hertog.

Hawking has been critical of the theory of a multiverse for years. In a Cambridge interview from last year, he said, "I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can&rsquot be tested."

&ldquoWe are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes,&rdquo said Hawking.

The new theory is testable, Hertog says. There's a chance that primordial gravitational waves&mdashripples in spacetime&mdashcould be detected by a planned European space-based gravitational wave observatory, LISA. These gravitational waves from the beginning of the universe could help physicists study the amount of energy thought to be associated with eternal inflation. LISA is scheduled to be up and running by the 2030s, which isn't that long on the scale of the universe.

The paper, published in the Journal of High Energy Physics, has been made free to download.


What does Stephen Hawking mean by 'an infinite universe'? - Astronomy

This lecture is the intellectual property of Professor S.W.Hawking. You may not reproduce, edit, translate, distribute, publish or host this document in any way with out the permission of The Stephen Hawking Estate. Note that there may be incorrect spellings, punctuation and/or grammar in this document. This is to allow correct pronunciation and timing by a speech synthesiser.

'This lecture is about whether we can predict the future, or whether it is arbitrary and random. In ancient times, the world must have seemed pretty arbitrary. Disasters such as floods or diseases must have seemed to happen without warning, or apparent reason. Primitive people attributed such natural phenomena, to a pantheon of gods and goddesses, who behaved in a capricious and whimsical way. There was no way to predict what they would do, and the only hope was to win favour by gifts or actions. Many people still partially subscribe to this belief, and try to make a pact with fortune. They offer to do certain things, if only they can get an A-grade for a course, or pass their driving test.

Gradually however, people must have noticed certain regularities in the behaviour of nature. These regularities were most obvious, in the motion of the heavenly bodies across the sky. So astronomy was the first science to be developed. It was put on a firm mathematical basis by Newton, more than 300 years ago, and we still use his theory of gravity to predict the motion of almost all celestial bodies. Following the example of astronomy, it was found that other natural phenomena also obeyed definite scientific laws. This led to the idea of scientific determinism, which seems first to have been publicly expressed by the French scientist, Laplace. I thought I would like to quote you Laplace's actual words, so I asked a friend to track them down. They are in French of course, not that I expect that would be any problem with this audience. But the trouble is, Laplace was rather like Prewst, in that he wrote sentences of inordinate length and complexity. So I have decided to para-phrase the quotation. In effect what he said was, that if at one time, we knew the positions and speeds of all the particles in the universe, then we could calculate their behaviour at any other time, in the past or future. There is a probably apocryphal story, that when Laplace was asked by Napoleon, how God fitted into this system, he replied, 'Sire, I have not needed that hypothesis.' I don't think that Laplace was claiming that God didn't exist. It is just that He doesn't intervene, to break the laws of Science. That must be the position of every scientist. A scientific law, is not a scientific law, if it only holds when some supernatural being, decides to let things run, and not intervene.

The idea that the state of the universe at one time determines the state at all other times, has been a central tenet of science, ever since Laplace's time. It implies that we can predict the future, in principle at least. In practice, however, our ability to predict the future is severely limited by the complexity of the equations, and the fact that they often have a property called chaos. As those who have seen Jurassic Park will know, this means a tiny disturbance in one place, can cause a major change in another. A butterfly flapping its wings can cause rain in Central Park, New York. The trouble is, it is not repeatable. The next time the butterfly flaps its wings, a host of other things will be different, which will also influence the weather. That is why weather forecasts are so unreliable.

Despite these practical difficulties, scientific determinism, remained the official dogma throughout the 19th century. However, in the 20th century, there have been two developments that show that Laplace's vision, of a complete prediction of the future, can not be realised. The first of these developments was what is called, quantum mechanics. This was first put forward in 1900, by the German physicist, Max Planck, as an ad hoc hypothesis, to solve an outstanding paradox. According to the classical 19th century ideas, dating back to Laplace, a hot body, like a piece of red hot metal, should give off radiation. It would lose energy in radio waves, infra red, visible light, ultra violet, x-rays, and gamma rays, all at the same rate. Not only would this mean that we would all die of skin cancer, but also everything in the universe would be at the same temperature, which clearly it isn't. However, Planck showed one could avoid this disaster, if one gave up the idea that the amount of radiation could have just any value, and said instead that radiation came only in packets or quanta of a certain size. It is a bit like saying that you can't buy sugar loose in the supermarket, but only in kilogram bags. The energy in the packets or quanta, is higher for ultra violet and x-rays, than for infra red or visible light. This means that unless a body is very hot, like the Sun, it will not have enough energy, to give off even a single quantum of ultra violet or x-rays. That is why we don't get sunburn from a cup of coffee.

Planck regarded the idea of quanta, as just a mathematical trick, and not as having any physical reality, whatever that might mean. However, physicists began to find other behaviour, that could be explained only in terms of quantities having discrete, or quantised values, rather than continuously variable ones. For example, it was found that elementary particles behaved rather like little tops, spinning about an axis. But the amount of spin couldn't have just any value. It had to be some multiple of a basic unit. Because this unit is very small, one does not notice that a normal top really slows down in a rapid sequence of discrete steps, rather than as a continuous process. But for tops as small as atoms, the discrete nature of spin is very important.

It was some time before people realised the implications of this quantum behaviour for determinism. It was not until 1926, that Werner Heisenberg, another German physicist, pointed out that you couldn't measure both the position, and the speed, of a particle exactly. To see where a particle is, one has to shine light on it. But by Planck's work, one can't use an arbitrarily small amount of light. One has to use at least one quantum. This will disturb the particle, and change its speed in a way that can't be predicted. To measure the position of the particle accurately, you will have to use light of short wave length, like ultra violet, x-rays, or gamma rays. But again, by Planck's work, quanta of these forms of light have higher energies than those of visible light. So they will disturb the speed of the particle more. It is a no win situation: the more accurately you try to measure the position of the particle, the less accurately you can know the speed, and vice versa. This is summed up in the Uncertainty Principle that Heisenberg formulated the uncertainty in the position of a particle, times the uncertainty in its speed, is always greater than a quantity called Planck's constant, divided by the mass of the particle.

Laplace's vision, of scientific determinism, involved knowing the positions and speeds of the particles in the universe, at one instant of time. So it was seriously undermined by Heisenberg's Uncertainty principle. How could one predict the future, when one could not measure accurately both the positions, and the speeds, of particles at the present time? No matter how powerful a computer you have, if you put lousy data in, you will get lousy predictions out.

Einstein was very unhappy about this apparent randomness in nature. His views were summed up in his famous phrase, 'God does not play dice'. He seemed to have felt that the uncertainty was only provisional: but that there was an underlying reality, in which particles would have well defined positions and speeds, and would evolve according to deterministic laws, in the spirit of Laplace. This reality might be known to God, but the quantum nature of light would prevent us seeing it, except through a glass darkly.

Einstein's view was what would now be called, a hidden variable theory. Hidden variable theories might seem to be the most obvious way to incorporate the Uncertainty Principle into physics. They form the basis of the mental picture of the universe, held by many scientists, and almost all philosophers of science. But these hidden variable theories are wrong. The British physicist, John Bell, who died recently, devised an experimental test that would distinguish hidden variable theories. When the experiment was carried out carefully, the results were inconsistent with hidden variables. Thus it seems that even God is bound by the Uncertainty Principle, and can not know both the position, and the speed, of a particle. So God does play dice with the universe. All the evidence points to him being an inveterate gambler, who throws the dice on every possible occasion.

Other scientists were much more ready than Einstein to modify the classical 19th century view of determinism. A new theory, called quantum mechanics, was put forward by Heisenberg, the Austrian, Erwin Schroedinger, and the British physicist, Paul Dirac. Dirac was my predecessor but one, as the Lucasian Professor in Cambridge. Although quantum mechanics has been around for nearly 70 years, it is still not generally understood or appreciated, even by those that use it to do calculations. Yet it should concern us all, because it is a completely different picture of the physical universe, and of reality itself. In quantum mechanics, particles don't have well defined positions and speeds. Instead, they are represented by what is called a wave function. This is a number at each point of space. The size of the wave function gives the probability that the particle will be found in that position. The rate, at which the wave function varies from point to point, gives the speed of the particle. One can have a wave function that is very strongly peaked in a small region. This will mean that the uncertainty in the position is small. But the wave function will vary very rapidly near the peak, up on one side, and down on the other. Thus the uncertainty in the speed will be large. Similarly, one can have wave functions where the uncertainty in the speed is small, but the uncertainty in the position is large.

The wave function contains all that one can know of the particle, both its position, and its speed. If you know the wave function at one time, then its values at other times are determined by what is called the Schroedinger equation. Thus one still has a kind of determinism, but it is not the sort that Laplace envisaged. Instead of being able to predict the positions and speeds of particles, all we can predict is the wave function. This means that we can predict just half what we could, according to the classical 19th century view.

Although quantum mechanics leads to uncertainty, when we try to predict both the position and the speed, it still allows us to predict, with certainty, one combination of position and speed. However, even this degree of certainty, seems to be threatened by more recent developments. The problem arises because gravity can warp space-time so much, that there can be regions that we don't observe.

Interestingly enough, Laplace himself wrote a paper in 1799 on how some stars could have a gravitational field so strong that light could not escape, but would be dragged back onto the star. He even calculated that a star of the same density as the Sun, but two hundred and fifty times the size, would have this property. But although Laplace may not have realised it, the same idea had been put forward 16 years earlier by a Cambridge man, John Mitchell, in a paper in the Philosophical Transactions of the Royal Society. Both Mitchell and Laplace thought of light as consisting of particles, rather like cannon balls, that could be slowed down by gravity, and made to fall back on the star. But a famous experiment, carried out by two Americans, Michelson and Morley in 1887, showed that light always travelled at a speed of one hundred and eighty six thousand miles a second, no matter where it came from. How then could gravity slow down light, and make it fall back.

This was impossible, according to the then accepted ideas of space and time. But in 1915, Einstein put forward his revolutionary General Theory of Relativity. In this, space and time were no longer separate and independent entities. Instead, they were just different directions in a single object called space-time. This space-time was not flat, but was warped and curved by the matter and energy in it. In order to understand this, considered a sheet of rubber, with a weight placed on it, to represent a star. The weight will form a depression in the rubber, and will cause the sheet near the star to be curved, rather than flat. If one now rolls marbles on the rubber sheet, their paths will be curved, rather than being straight lines. In 1919, a British expedition to West Africa, looked at light from distant stars, that passed near the Sun during an eclipse. They found that the images of the stars were shifted slightly from their normal positions. This indicated that the paths of the light from the stars had been bent by the curved space-time near the Sun. General Relativity was confirmed.

Consider now placing heavier and heavier, and more and more concentrated weights on the rubber sheet. They will depress the sheet more and more. Eventually, at a critical weight and size, they will make a bottomless hole in the sheet, which particles can fall into, but nothing can get out of.

What happens in space-time according to General Relativity is rather similar. A star will curve and distort the space-time near it, more and more, the more massive and more compact the star is. If a massive star, which has burnt up its nuclear fuel, cools and shrinks below a critical size, it will quite literally make a bottomless hole in space-time, that light can't get out of. Such objects were given the name Black Holes, by the American physicist John Wheeler, who was one of the first to recognise their importance, and the problems they pose. The name caught on quickly. To Americans, it suggested something dark and mysterious, while to the British, there was the added resonance of the Black Hole of Calcutta. But the French, being French, saw a more risqué meaning. For years, they resisted the name, trou noir, claiming it was obscene. But that was a bit like trying to stand against le weekend, and other franglais. In the end, they had to give in. Who can resist a name that is such a winner?

We now have observations that point to black holes in a number of objects, from binary star systems, to the centre of galaxies. So it is now generally accepted that black holes exist. But, apart from their potential for science fiction, what is their significance for determinism. The answer lies in a bumper sticker that I used to have on the door of my office: Black Holes are Out of Sight. Not only do the particles and unlucky astronauts that fall into a black hole, never come out again, but also the information that they carry, is lost forever, at least from our region of the universe. You can throw television sets, diamond rings, or even your worst enemies into a black hole, and all the black hole will remember, is the total mass, and the state of rotation. John Wheeler called this, 'A Black Hole Has No Hair.' To the French, this just confirmed their suspicions.

As long as it was thought that black holes would continue to exist forever, this loss of information didn't seem to matter too much. One could say that the information still existed inside the black hole. It is just that one can't tell what it is, from the outside. However, the situation changed, when I discovered that black holes aren't completely black. Quantum mechanics causes them to send out particles and radiation at a steady rate. This result came as a total surprise to me, and everyone else. But with hindsight, it should have been obvious. What we think of as empty space is not really empty, but it is filled with pairs of particles and anti particles. These appear together at some point of space and time, move apart, and then come together and annihilate each other. These particles and anti particles occur because a field, such as the fields that carry light and gravity, can't be exactly zero. That would mean that the value of the field, would have both an exact position (at zero), and an exact speed or rate of change (also zero). This would be against the Uncertainty Principle, just as a particle can't have both an exact position, and an exact speed. So all fields must have what are called, vacuum fluctuations. Because of the quantum behaviour of nature, one can interpret these vacuum fluctuations, in terms of particles and anti particles, as I have described.

These pairs of particles occur for all varieties of elementary particles. They are called virtual particles, because they occur even in the vacuum, and they can't be directly measured by particle detectors. However, the indirect effects of virtual particles, or vacuum fluctuations, have been observed in a number of experiments, and their existence confirmed.

If there is a black hole around, one member of a particle anti particle pair may fall into the hole, leaving the other member without a partner, with which to annihilate. The forsaken particle may fall into the hole as well, but it may also escape to a large distance from the hole, where it will become a real particle, that can be measured by a particle detector. To someone a long way from the black hole, it will appear to have been emitted by the hole.
This explanation of how black holes ain't so black, makes it clear that the emission will depend on the size of the black hole, and the rate at which it is rotating. But because black holes have no hair, in Wheeler's phrase, the radiation will be otherwise independent of what went into the hole. It doesn't matter whether you throw television sets, diamond rings, or your worst enemies, into a black hole. What comes back out will be the same.

So what has all this to do with determinism, which is what this lecture is supposed to be about. What it shows is that there are many initial states, containing television sets, diamond rings, and even people, that evolve to the same final state, at least outside the black hole. But in Laplace's picture of determinism, there was a one to one correspondence between initial states, and final states. If you knew the state of the universe at some time in the past, you could predict it in the future. Similarly, if you knew it in the future, you could calculate what it must have been in the past. The advent of quantum theory in the 1920s reduced the amount one could predict by half, but it still left a one to one correspondence between the states of the universe at different times. If one knew the wave function at one time, one could calculate it at any other time.

With black holes, however, the situation is rather different. One will end up with the same state outside the hole, whatever one threw in, provided it has the same mass. Thus there is not a one to one correspondence between the initial state, and the final state outside the black hole. There will be a one to one correspondence between the initial state, and the final state both outside, and inside, the black hole. But the important point is that the emission of particles, and radiation by the black hole, will cause the hole to lose mass, and get smaller. Eventually, it seems the black hole will get down to zero mass, and will disappear altogether. What then will happen to all the objects that fell into the hole, and all the people that either jumped in, or were pushed? They can't come out again, because there isn't enough mass or energy left in the black hole, to send them out again. They may pass into another universe, but that is not something that will make any difference, to those of us prudent enough not to jump into a black hole. Even the information, about what fell into the hole, could not come out again when the hole finally disappears. Information can not be carried free, as those of you with phone bills will know. Information requires energy to carry it, and there won't be enough energy left when the black hole disappears.

What all this means is, that information will be lost from our region of the universe, when black holes are formed, and then evaporate. This loss of information will mean that we can predict even less than we thought, on the basis of quantum theory. In quantum theory, one may not be able to predict with certainty, both the position, and the speed of a particle. But there is still one combination of position and speed that can be predicted. In the case of a black hole, this definite prediction involves both members of a particle pair. But we can measure only the particle that comes out. There's no way even in principle that we can measure the particle that falls into the hole. So, for all we can tell, it could be in any state. This means we can not make any definite prediction, about the particle that escapes from the hole. We can calculate the probability that the particle has this or that position, or speed. But there's no combination of the position and speed of just one particle that we can definitely predict, because the speed and position will depend on the other particle, which we don't observe. Thus it seems Einstein was doubly wrong when he said, God does not play dice. Not only does God definitely play dice, but He sometimes confuses us by throwing them where they can't be seen.

Many scientists are like Einstein, in that they have a deep emotional attachment to determinism. Unlike Einstein, they have accepted the reduction in our ability to predict, that quantum theory brought about. But that was far enough. They didn't like the further reduction, which black holes seemed to imply. They have therefore claimed that information is not really lost down black holes. But they have not managed to find any mechanism that would return the information. It is just a pious hope that the universe is deterministic, in the way that Laplace thought. I feel these scientists have not learnt the lesson of history. The universe does not behave according to our pre-conceived ideas. It continues to surprise us.

One might not think it mattered very much, if determinism broke down near black holes. We are almost certainly at least a few light years, from a black hole of any size. But, the Uncertainty Principle implies that every region of space should be full of tiny virtual black holes, which appear and disappear again. One would think that particles and information could fall into these black holes, and be lost. Because these virtual black holes are so small, a hundred billion billion times smaller than the nucleus of an atom, the rate at which information would be lost would be very low. That is why the laws of science appear deterministic, to a very good approximation. But in extreme conditions, like in the early universe, or in high energy particle collisions, there could be significant loss of information. This would lead to unpredictability, in the evolution of the universe.

To sum up, what I have been talking about, is whether the universe evolves in an arbitrary way, or whether it is deterministic. The classical view, put forward by Laplace, was that the future motion of particles was completely determined, if one knew their positions and speeds at one time. This view had to be modified, when Heisenberg put forward his Uncertainty Principle, which said that one could not know both the position, and the speed, accurately. However, it was still possible to predict one combination of position and speed. But even this limited predictability disappeared, when the effects of black holes were taken into account. The loss of particles and information down black holes meant that the particles that came out were random. One could calculate probabilities, but one could not make any definite predictions. Thus, the future of the universe is not completely determined by the laws of science, and its present state, as Laplace thought. God still has a few tricks up his sleeve.


A brilliant mind

Hawking continued at Cambridge after his graduation, serving as a research fellow and later as a professional fellow. In 1974, he was inducted into the Royal Society, a worldwide fellowship of scientists. In 1979, he was appointed Lucasian Professor of Mathematics at Cambridge, the most famous academic chair in the world (the second holder was Sir Isaac Newton, also a member of the Royal Society).

Over the course of his career, Hawking studied the basic laws governing the universe. He proposed that, since the universe boasts a beginning &mdash the Big Bang &mdash it likely will have an ending. Working with fellow cosmologist Roger Penrose, he demonstrated that Albert Einstein's Theory of General Relativity suggests that space and time began at the birth of the universe and ends within black holes, which implies that Einstein's theory and quantum theory must be united.

Using the two theories together, Hawking also determined that black holes are not totally dark but instead emit radiation. He predicted that, following the Big Bang, black holes as tiny as protons were created, governed by both general relativity and quantum mechanics. [PHOTOS: Black Holes of the Universe]

In 2014, Hawking revised his theory, even writing that " there are no black holes" &mdash at least, in the way that cosmologists traditionally understand them. His theory removed the existence of an "event horizon," the point where nothing can escape. Instead, he proposed that there would be an "apparent horizon" that would alter according to quantum changes within the black hole. But the theory remains controversial. [Portrait of Genius: Stephen Hawking Exhibit Photos]

Hawking also proposed that the universe itself has no boundary, much like the Earth. Although the planet is finite, one can travel around it (and through the universe) infinitely, never encountering a wall that would be described as the "end."


Professor Stephen Hawking’s Final Theory on Origin of Universe

Professor Stephen Hawking’s theory about the Big Bang, which he worked on in collaboration with Katholieke Universiteit Leuven’s Professor Thomas Hertog, has been published this week in the Journal of High-Energy Physics (arXiv.org preprint).

Professor Stephen Hawking.

Modern theories of the Big Bang predict that our local Universe came into existence with a brief burst of inflation — in other words, a tiny fraction of a second after the Big Bang itself, the Universe expanded at an exponential rate.

It is widely believed, however, that once inflation starts, there are regions where it never stops. It is thought that quantum effects can keep inflation going forever in some regions of the Universe so that globally, inflation is eternal.

The observable part of our Universe would then be just a hospitable pocket universe, a region in which inflation has ended and stars and galaxies formed.

“The usual theory of eternal inflation predicts that globally our Universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean,” Professor Hawking explained in an interview last autumn.

“The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested.”

In their new paper, Professor Hawking and Professor Hertog say this account of eternal inflation as a theory of the Big Bang is wrong.

“The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats the quantum effects as small fluctuations around this,” Professor Hertog said.

“However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down in eternal inflation.”

“We predict that our Universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure,” Professor Hawking said.

The theory of eternal inflation that the team put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the Universe as tiny vibrating strings.

Their approach uses the string theory concept of holography, which postulates that the Universe is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.

Professor Hawking and Professor Hertog developed a variation of this concept of holography to project out the time dimension in eternal inflation. This enabled them to describe eternal inflation without having to rely on Einstein’ theory.

In the new theory, eternal inflation is reduced to a timeless state defined on a spatial surface at the beginning of time.

“When we trace the evolution of our Universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning,” Professor Hertog said.

Professor Hawking’s earlier ‘no boundary theory’ predicted that if you go back in time to the beginning of the Universe, the Universe shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work.

“Now we’re saying that there is a boundary in our past,” Professor Hertog said.

The physicists used their new theory to derive more reliable predictions about the global structure of the Universe.

They predicted the Universe that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.

Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm.

“We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes,” Professor Hawking said.

This makes the theory more predictive and testable.

Professor Hertog now plans to study the implications of the new theory on smaller scales that are within reach of our space telescopes.

He believes that primordial gravitational waves generated at the exit from eternal inflation constitute the most promising ‘smoking gun’ to test the model.

The expansion of our Universe since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors. But they might be heard by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.


What does Stephen Hawking mean by 'an infinite universe'? - Astronomy

This lecture is the intellectual property of Professor S.W.Hawking. You may not reproduce, edit, translate, distribute, publish or host this document in any way with out the permission of The Stephen Hawking Estate. Note that there may be incorrect spellings, punctuation and/or grammar in this document. This is to allow correct pronunciation and timing by a speech synthesiser.

'In this talk, I would like to speculate a little, on the development of life in the universe, and in particular, the development of intelligent life. I shall take this to include the human race, even though much of its behaviour through out history, has been pretty stupid, and not calculated to aid the survival of the species. Two questions I shall discuss are, 'What is the probability of life existing else where in the universe?' and, 'How may life develop in the future?'

It is a matter of common experience, that things get more disordered and chaotic with time. This observation can be elevated to the status of a law, the so-called Second Law of Thermodynamics. This says that the total amount of disorder, or entropy, in the universe, always increases with time. However, the Law refers only to the total amount of disorder. The order in one body can increase, provided that the amount of disorder in its surroundings increases by a greater amount. This is what happens in a living being. One can define Life to be an ordered system that can sustain itself against the tendency to disorder, and can reproduce itself. That is, it can make similar, but independent, ordered systems. To do these things, the system must convert energy in some ordered form, like food, sunlight, or electric power, into disordered energy, in the form of heat. In this way, the system can satisfy the requirement that the total amount of disorder increases, while, at the same time, increasing the order in itself and its offspring. A living being usually has two elements: a set of instructions that tell the system how to sustain and reproduce itself, and a mechanism to carry out the instructions. In biology, these two parts are called genes and metabolism. But it is worth emphasising that there need be nothing biological about them. For example, a computer virus is a program that will make copies of itself in the memory of a computer, and will transfer itself to other computers. Thus it fits the definition of a living system, that I have given. Like a biological virus, it is a rather degenerate form, because it contains only instructions or genes, and doesn't have any metabolism of its own. Instead, it reprograms the metabolism of the host computer, or cell. Some people have questioned whether viruses should count as life, because they are parasites, and can not exist independently of their hosts. But then most forms of life, ourselves included, are parasites, in that they feed off and depend for their survival on other forms of life. I think computer viruses should count as life. Maybe it says something about human nature, that the only form of life we have created so far is purely destructive. Talk about creating life in our own image. I shall return to electronic forms of life later on.

What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorous. One can speculate that one might have life with some other chemical basis, such as silicon, but carbon seems the most favourable case, because it has the richest chemistry. That carbon atoms should exist at all, with the properties that they have, requires a fine adjustment of physical constants, such as the QCD scale, the electric charge, and even the dimension of space-time. If these constants had significantly different values, either the nucleus of the carbon atom would not be stable, or the electrons would collapse in on the nucleus. At first sight, it seems remarkable that the universe is so finely tuned. Maybe this is evidence, that the universe was specially designed to produce the human race. However, one has to be careful about such arguments, because of what is known as the Anthropic Principle. This is based on the self-evident truth, that if the universe had not been suitable for life, we wouldn't be asking why it is so finely adjusted. One can apply the Anthropic Principle, in either its Strong, or Weak, versions. For the Strong Anthropic Principle, one supposes that there are many different universes, each with different values of the physical constants. In a small number, the values will allow the existence of objects like carbon atoms, which can act as the building blocks of living systems. Since we must live in one of these universes, we should not be surprised that the physical constants are finely tuned. If they weren't, we wouldn't be here. The strong form of the Anthropic Principle is not very satisfactory. What operational meaning can one give to the existence of all those other universes? And if they are separate from our own universe, how can what happens in them, affect our universe. Instead, I shall adopt what is known as the Weak Anthropic Principle. That is, I shall take the values of the physical constants, as given. But I shall see what conclusions can be drawn, from the fact that life exists on this planet, at this stage in the history of the universe.

There was no carbon, when the universe began in the Big Bang, about 15 billion years ago. It was so hot, that all the matter would have been in the form of particles, called protons and neutrons. There would initially have been equal numbers of protons and neutrons. However, as the universe expanded, it would have cooled. About a minute after the Big Bang, the temperature would have fallen to about a billion degrees, about a hundred times the temperature in the Sun. At this temperature, the neutrons will start to decay into more protons. If this had been all that happened, all the matter in the universe would have ended up as the simplest element, hydrogen, whose nucleus consists of a single proton. However, some of the neutrons collided with protons, and stuck together to form the next simplest element, helium, whose nucleus consists of two protons and two neutrons. But no heavier elements, like carbon or oxygen, would have been formed in the early universe. It is difficult to imagine that one could build a living system, out of just hydrogen and helium, and anyway the early universe was still far too hot for atoms to combine into molecules.

The universe would have continued to expand, and cool. But some regions would have had slightly higher densities than others. The gravitational attraction of the extra matter in those regions, would slow down their expansion, and eventually stop it. Instead, they would collapse to form galaxies and stars, starting from about two billion years after the Big Bang. Some of the early stars would have been more massive than our Sun. They would have been hotter than the Sun, and would have burnt the original hydrogen and helium, into heavier elements, such as carbon, oxygen, and iron. This could have taken only a few hundred million years. After that, some of the stars would have exploded as supernovas, and scattered the heavy elements back into space, to form the raw material for later generations of stars.

Other stars are too far away, for us to be able to see directly, if they have planets going round them. But certain stars, called pulsars, give off regular pulses of radio waves. We observe a slight variation in the rate of some pulsars, and this is interpreted as indicating that they are being disturbed, by having Earth sized planets going round them. Planets going round pulsars are unlikely to have life, because any living beings would have been killed, in the supernova explosion that led to the star becoming a pulsar. But, the fact that several pulsars are observed to have planets suggests that a reasonable fraction of the hundred billion stars in our galaxy may also have planets. The necessary planetary conditions for our form of life may therefore have existed from about four billion years after the Big Bang.

Our solar system was formed about four and a half billion years ago, or about ten billion years after the Big Bang, from gas contaminated with the remains of earlier stars. The Earth was formed largely out of the heavier elements, including carbon and oxygen. Somehow, some of these atoms came to be arranged in the form of molecules of DNA. This has the famous double helix form, discovered by Crick and Watson, in a hut on the New Museum site in Cambridge. Linking the two chains in the helix, are pairs of nucleic acids. There are four types of nucleic acid, adenine, cytosine, guanine, and thiamine. I'm afraid my speech synthesiser is not very good, at pronouncing their names. Obviously, it was not designed for molecular biologists. An adenine on one chain is always matched with a thiamine on the other chain, and a guanine with a cytosine. Thus the sequence of nucleic acids on one chain defines a unique, complementary sequence, on the other chain. The two chains can then separate and each act as templates to build further chains. Thus DNA molecules can reproduce the genetic information, coded in their sequences of nucleic acids. Sections of the sequence can also be used to make proteins and other chemicals, which can carry out the instructions, coded in the sequence, and assemble the raw material for DNA to reproduce itself.

We do not know how DNA molecules first appeared. The chances against a DNA molecule arising by random fluctuations are very small. Some people have therefore suggested that life came to Earth from elsewhere, and that there are seeds of life floating round in the galaxy. However, it seems unlikely that DNA could survive for long in the radiation in space. And even if it could, it would not really help explain the origin of life, because the time available since the formation of carbon is only just over double the age of the Earth.

One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely. However, in a universe with a very large, or infinite, number of stars, one would expect it to occur in a few stellar systems, but they would be very widely separated. The fact that life happened to occur on Earth, is not however surprising or unlikely. It is just an application of the Weak Anthropic Principle: if life had appeared instead on another planet, we would be asking why it had occurred there.

If the appearance of life on a given planet was very unlikely, one might have expected it to take a long time. More precisely, one might have expected life to appear just in time for the subsequent evolution to intelligent beings, like us, to have occurred before the cut off, provided by the life time of the Sun. This is about ten billion years, after which the Sun will swell up and engulf the Earth. An intelligent form of life, might have mastered space travel, and be able to escape to another star. But otherwise, life on Earth would be doomed.

There is fossil evidence, that there was some form of life on Earth, about three and a half billion years ago. This may have been only 500 million years after the Earth became stable and cool enough, for life to develop. But life could have taken 7 billion years to develop, and still have left time to evolve to beings like us, who could ask about the origin of life. If the probability of life developing on a given planet, is very small, why did it happen on Earth, in about one 14th of the time available.

The early appearance of life on Earth suggests that there's a good chance of the spontaneous generation of life, in suitable conditions. Maybe there was some simpler form of organisation, which built up DNA. Once DNA appeared, it would have been so successful, that it might have completely replaced the earlier forms. We don't know what these earlier forms would have been. One possibility is RNA. This is like DNA, but rather simpler, and without the double helix structure. Short lengths of RNA, could reproduce themselves like DNA, and might eventually build up to DNA. One can not make nucleic acids in the laboratory, from non-living material, let alone RNA. But given 500 million years, and oceans covering most of the Earth, there might be a reasonable probability of RNA, being made by chance.

As DNA reproduced itself, there would have been random errors. Many of these errors would have been harmful, and would have died out. Some would have been neutral. That is they would not have affected the function of the gene. Such errors would contribute to a gradual genetic drift, which seems to occur in all populations. And a few errors would have been favourable to the survival of the species. These would have been chosen by Darwinian natural selection.

The process of biological evolution was very slow at first. It took two and a half billion years, to evolve from the earliest cells to multi-cell animals, and another billion years to evolve through fish and reptiles, to mammals. But then evolution seemed to have speeded up. It only took about a hundred million years, to develop from the early mammals to us. The reason is, fish contain most of the important human organs, and mammals, essentially all of them. All that was required to evolve from early mammals, like lemurs, to humans, was a bit of fine-tuning.

But with the human race, evolution reached a critical stage, comparable in importance with the development of DNA. This was the development of language, and particularly written language. It meant that information can be passed on, from generation to generation, other than genetically, through DNA. There has been no detectable change in human DNA, brought about by biological evolution, in the ten thousand years of recorded history. But the amount of knowledge handed on from generation to generation has grown enormously. The DNA in human beings contains about three billion nucleic acids. However, much of the information coded in this sequence, is redundant, or is inactive. So the total amount of useful information in our genes, is probably something like a hundred million bits. One bit of information is the answer to a yes no question. By contrast, a paper back novel might contain two million bits of information. So a human is equivalent to 50 Mills and Boon romances. A major national library can contain about five million books, or about ten trillion bits. So the amount of information handed down in books, is a hundred thousand times as much as in DNA.

Even more important, is the fact that the information in books, can be changed, and updated, much more rapidly. It has taken us several million years to evolve from the apes. During that time, the useful information in our DNA, has probably changed by only a few million bits. So the rate of biological evolution in humans, is about a bit a year. By contrast, there are about 50,000 new books published in the English language each year, containing of the order of a hundred billion bits of information. Of course, the great majority of this information is garbage, and no use to any form of life. But, even so, the rate at which useful information can be added is millions, if not billions, higher than with DNA.

This has meant that we have entered a new phase of evolution. At first, evolution proceeded by natural selection, from random mutations. This Darwinian phase, lasted about three and a half billion years, and produced us, beings who developed language, to exchange information. But in the last ten thousand years or so, we have been in what might be called, an external transmission phase. In this, the internal record of information, handed down to succeeding generations in DNA, has not changed significantly. But the external record, in books, and other long lasting forms of storage, has grown enormously. Some people would use the term, evolution, only for the internally transmitted genetic material, and would object to it being applied to information handed down externally. But I think that is too narrow a view. We are more than just our genes. We may be no stronger, or inherently more intelligent, than our cave man ancestors. But what distinguishes us from them, is the knowledge that we have accumulated over the last ten thousand years, and particularly, over the last three hundred. I think it is legitimate to take a broader view, and include externally transmitted information, as well as DNA, in the evolution of the human race.

The time scale for evolution, in the external transmission period, is the time scale for accumulation of information. This used to be hundreds, or even thousands, of years. But now this time scale has shrunk to about 50 years, or less. On the other hand, the brains with which we process this information have evolved only on the Darwinian time scale, of hundreds of thousands of years. This is beginning to cause problems. In the 18th century, there was said to be a man who had read every book written. But nowadays, if you read one book a day, it would take you about 15,000 years to read through the books in a national Library. By which time, many more books would have been written.

This has meant that no one person can be the master of more than a small corner of human knowledge. People have to specialise, in narrower and narrower fields. This is likely to be a major limitation in the future. We certainly can not continue, for long, with the exponential rate of growth of knowledge that we have had in the last three hundred years. An even greater limitation and danger for future generations, is that we still have the instincts, and in particular, the aggressive impulses, that we had in cave man days. Aggression, in the form of subjugating or killing other men, and taking their women and food, has had definite survival advantage, up to the present time. But now it could destroy the entire human race, and much of the rest of life on Earth. A nuclear war, is still the most immediate danger, but there are others, such as the release of a genetically engineered virus. Or the green house effect becoming unstable.

There is no time, to wait for Darwinian evolution, to make us more intelligent, and better natured. But we are now entering a new phase, of what might be called, self designed evolution, in which we will be able to change and improve our DNA. There is a project now on, to map the entire sequence of human DNA. It will cost a few billion dollars, but that is chicken feed, for a project of this importance. Once we have read the book of life, we will start writing in corrections. At first, these changes will be confined to the repair of genetic defects, like cystic fibrosis, and muscular dystrophy. These are controlled by single genes, and so are fairly easy to identify, and correct. Other qualities, such as intelligence, are probably controlled by a large number of genes. It will be much more difficult to find them, and work out the relations between them. Nevertheless, I am sure that during the next century, people will discover how to modify both intelligence, and instincts like aggression.

Laws will be passed, against genetic engineering with humans. But some people won't be able to resist the temptation, to improve human characteristics, such as size of memory, resistance to disease, and length of life. Once such super humans appear, there are going to be major political problems, with the unimproved humans, who won't be able to compete. Presumably, they will die out, or become unimportant. Instead, there will be a race of self-designing beings, who are improving themselves at an ever-increasing rate.

If this race manages to redesign itself, to reduce or eliminate the risk of self-destruction, it will probably spread out, and colonise other planets and stars. However, long distance space travel, will be difficult for chemically based life forms, like DNA. The natural lifetime for such beings is short, compared to the travel time. According to the theory of relativity, nothing can travel faster than light. So the round trip to the nearest star would take at least 8 years, and to the centre of the galaxy, about a hundred thousand years. In science fiction, they overcome this difficulty, by space warps, or travel through extra dimensions. But I don't think these will ever be possible, no matter how intelligent life becomes. In the theory of relativity, if one can travel faster than light, one can also travel back in time. This would lead to problems with people going back, and changing the past. One would also expect to have seen large numbers of tourists from the future, curious to look at our quaint, old-fashioned ways.

It might be possible to use genetic engineering, to make DNA based life survive indefinitely, or at least for a hundred thousand years. But an easier way, which is almost within our capabilities already, would be to send machines. These could be designed to last long enough for interstellar travel. When they arrived at a new star, they could land on a suitable planet, and mine material to produce more machines, which could be sent on to yet more stars. These machines would be a new form of life, based on mechanical and electronic components, rather than macromolecules. They could eventually replace DNA based life, just as DNA may have replaced an earlier form of life.

This mechanical life could also be self-designing. Thus it seems that the external transmission period of evolution, will have been just a very short interlude, between the Darwinian phase, and a biological, or mechanical, self design phase. This is shown on this next diagram, which is not to scale, because there's no way one can show a period of ten thousand years, on the same scale as billions of years. How long the self-design phase will last is open to question. It may be unstable, and life may destroy itself, or get into a dead end. If it does not, it should be able to survive the death of the Sun, in about 5 billion years, by moving to planets around other stars. Most stars will have burnt out in another 15 billion years or so, and the universe will be approaching a state of complete disorder, according to the Second Law of Thermodynamics. But Freeman Dyson has shown that, despite this, life could adapt to the ever-decreasing supply of ordered energy, and therefore could, in principle, continue forever.

What are the chances that we will encounter some alien form of life, as we explore the galaxy. If the argument about the time scale for the appearance of life on Earth is correct, there ought to be many other stars, whose planets have life on them. Some of these stellar systems could have formed 5 billion years before the Earth. So why is the galaxy not crawling with self designing mechanical or biological life forms? Why hasn't the Earth been visited, and even colonised. I discount suggestions that UFO's contain beings from outer space. I think any visits by aliens, would be much more obvious, and probably also, much more unpleasant.

What is the explanation of why we have not been visited? One possibility is that the argument, about the appearance of life on Earth, is wrong. Maybe the probability of life spontaneously appearing is so low, that Earth is the only planet in the galaxy, or in the observable universe, in which it happened. Another possibility is that there was a reasonable probability of forming self reproducing systems, like cells, but that most of these forms of life did not evolve intelligence. We are used to thinking of intelligent life, as an inevitable consequence of evolution. But the Anthropic Principle should warn us to be wary of such arguments. It is more likely that evolution is a random process, with intelligence as only one of a large number of possible outcomes. It is not clear that intelligence has any long-term survival value. Bacteria, and other single cell organisms, will live on, if all other life on Earth is wiped out by our actions. There is support for the view that intelligence, was an unlikely development for life on Earth, from the chronology of evolution. It took a very long time, two and a half billion years, to go from single cells to multi-cell beings, which are a necessary precursor to intelligence. This is a good fraction of the total time available, before the Sun blows up. So it would be consistent with the hypothesis, that the probability for life to develop intelligence, is low. In this case, we might expect to find many other life forms in the galaxy, but we are unlikely to find intelligent life. Another way, in which life could fail to develop to an intelligent stage, would be if an asteroid or comet were to collide with the planet. We have just observed the collision of a comet, Schumacher-Levi, with Jupiter. It produced a series of enormous fireballs. It is thought the collision of a rather smaller body with the Earth, about 70 million years ago, was responsible for the extinction of the dinosaurs. A few small early mammals survived, but anything as large as a human, would have almost certainly been wiped out. It is difficult to say how often such collisions occur, but a reasonable guess might be every twenty million years, on average. If this figure is correct, it would mean that intelligent life on Earth has developed only because of the lucky chance that there have been no major collisions in the last 70 million years. Other planets in the galaxy, on which life has developed, may not have had a long enough collision free period to evolve intelligent beings.

A third possibility is that there is a reasonable probability for life to form, and to evolve to intelligent beings, in the external transmission phase. But at that point, the system becomes unstable, and the intelligent life destroys itself. This would be a very pessimistic conclusion. I very much hope it isn't true. I prefer a fourth possibility: there are other forms of intelligent life out there, but that we have been overlooked. There used to be a project called SETI, the search for extra-terrestrial intelligence. It involved scanning the radio frequencies, to see if we could pick up signals from alien civilisations. I thought this project was worth supporting, though it was cancelled due to a lack of funds. But we should have been wary of answering back, until we have develop a bit further. Meeting a more advanced civilisation, at our present stage, might be a bit like the original inhabitants of America meeting Columbus. I don't think they were better off for it.


Searching for new civilizations

Milner explained that the work of Frank Drake &mdash who helped to pioneer the first modern search for extraterrestrial intelligence (SETI) &mdash inspired him to become a physicist. At the news conference, Drake noted the key challenge in finding government funding for SETI: "We cannot tell you how much it will take to succeed until we succeed," said Drake, who will lead discussions on Breakthrough Message's technical aspects. "We simply have to explore in the dark and hope people like Yuri Milner will keep us going for however long it takes for us to succeed."

Breakthrough Listen will harness two of the world's largest telescopes &mdash the 100-meter (330 feet) Green Bank Telescope in West Virginia and the 64-meter (210 feet) Parkes Telescope in Australia &mdash covering 10 times more of the sky than previous SETI programs, scanning at least five times more of the radio spectrum and doing so 100 times faster.

"We will be examining something like 10 billion radio channels simultaneously," planet-hunting pioneer Geoffrey Marcy, an astronomer at the University of California, Berkeley, said at the news conference. "We're listening to a cosmic piano, and every time we listen with the telescopes, we'll be listening not to 88 keys, but 10 billion keys."

If a civilization based around one of the 1,000 nearest stars is transmitting at Earth with the power of a common aircraft radar, or if they are transmitting from the center of the Milky Way with more than a dozen times the output of the interplanetary radars that scientists on Earth use to probe the solar system, these radio telescopes can detect it.

"It's a huge gamble, of course," added Lord Martin Rees, Astronomer Royal of the United Kingdom. "No one would count on success, but the payoff would be so colossal on recognizing that there was life elsewhere that this investment is hugely worthwhile."


Taming the multiverse—Stephen Hawking's final theory about the big bang

Stephen Hawking. Credit: Andre Pattenden

Professor Stephen Hawking's final theory on the origin of the universe, which he worked on in collaboration with Professor Thomas Hertog from KU Leuven, has been published today in the Journal of High Energy Physics.

The theory, which was submitted for publication before Hawking's death earlier this year, is based on string theory and predicts the universe is finite and far simpler than many current theories about the big bang say.

Professor Hertog, whose work has been supported by the European Research Council, first announced the new theory at a conference at the University of Cambridge in July of last year, organised on the occasion of Professor Hawking's 75th birthday.

Modern theories of the big bang predict that our local universe came into existence with a brief burst of inflation – in other words, a tiny fraction of a second after the big bang itself, the universe expanded at an exponential rate. It is widely believed, however, that once inflation starts, there are regions where it never stops. It is thought that quantum effects can keep inflation going forever in some regions of the universe so that globally, inflation is eternal. The observable part of our universe would then be just a hospitable pocket universe, a region in which inflation has ended and stars and galaxies formed.

"The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean," said Hawking in an interview last autumn. "The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can't be tested. "

In their new paper, Hawking and Hertog say this account of eternal inflation as a theory of the big bang is wrong. "The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein's theory of general relativity and treats the quantum effects as small fluctuations around this," said Hertog. "However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein's theory breaks down in eternal inflation."

"We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure," said Hawking.

The theory of eternal inflation that Hawking and Hertog put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the universe as tiny vibrating strings. Their approach uses the string theory concept of holography, which postulates that the universe is a large and complex hologram: physical reality in certain 3-D spaces can be mathematically reduced to 2-D projections on a surface.

Hawking and Hertog developed a variation of this concept of holography to project out the time dimension in eternal inflation. This enabled them to describe eternal inflation without having to rely on Einstein' theory. In the new theory, eternal inflation is reduced to a timeless state defined on a spatial surface at the beginning of time.

"When we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning," said Hertog.

Hawking's earlier 'no boundary theory' predicted that if you go back in time to the beginning of the universe, the universe shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work. "Now we're saying that there is a boundary in our past," said Hertog.

Hertog and Hawking used their new theory to derive more reliable predictions about the global structure of the universe. They predicted the universe that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.

Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm. "We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes," said Hawking.

This makes the theory more predictive and testable.

Hertog now plans to study the implications of the new theory on smaller scales that are within reach of our space telescopes. He believes that primordial gravitational waves – ripples in spacetime – generated at the exit from eternal inflation constitute the most promising "smoking gun" to test the model. The expansion of our universe since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors. But they might be heard by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.


Space Beyond Space: The Divide Between Science and Modern Philosophy

One of the very greatest questions to confront humankind has to do with the origins of our universe, and whether there is indeed an “origin point”, or if the universe as we know it may indeed be timeless, at least in a sense.

Modern astrophysics seems to suggest that there was indeed some kind of a starting point, as evidenced by the observable expansion of the universe and all matter within it. Such observations form the foundation upon which the concept of a “big bang” rests, with the constant outward motion of stellar bodies evidence of a point in our distant past–the outset of the universe, as it were–during which all of space and time existed within a single, point of virtually infinite density (this is called a singularity), in which even the laws of Einstein’s ever-reliable theory of general relativity appear to break down.

Ever troubling to the scientist is the philosophical question of whether, if our universe had a point of origin, there might also have been (or could still be) something outside the boundaries of the universe as well… a “space beyond space”, as it were. Of this, physicist and Lucasian Professor of Mathematics at Cambridge University, Sir Stephen Hawking, argues that, since anything beyond (or in this case, also before) our universe exists outside of observable scientific laws, it is therefore a concept that science does not address. (I will note here that, as we are discussing the possibility of those things “beyond” our universe, if such a state is to be considered, we will draw the necessary distinction here between those things presumably “outside” our universe, in a physical sense, and such similar, but unrelated ideas as a multiverse).

Left to the philosophers for consideration, Hawking similarly expresses of such questions that philosophy has not kept up with science, citing Ludwig Wittgenstein’s fascination (and to an extent, perhaps preoccupation) with the relationship between concepts and human language as being evidence that language was all there is left for study by the philosopher of the 20th century. “Philosophers reduced the scope of their inquiries so much that Wittgenstein, the most famous philosopher of this century, said, ‘The sole remaining task for philosophy is the analysis of language.’ What a comedown from the great tradition of philosophy from Aristotle to Kant!”

“The people whose business it is to ask why, the philosophers, have not been able to keep up with the advance of scientific theories,” Hawking wrote in A Brief History of Time. Hawking succeeded in outlining the divide that has been steadily growing between philosophy and science (and of course, let us be clear in noting that all of this remains a matter of ongoing debate between philosophers and scientists). However, one might argue that Hawking’s estimation of Wittgenstein’s views toward language may have been taken slightly out of context.

To be brief, rather than saying that language was all that philosophers had left to study, one interpretation of Wittgenstein’s work may be that language formed a sort of barrier between concepts, and that by recognizing this, certain philosophical (and perhaps scientific) conundrums might be freed, as if releasing the proverbial fly from the bottle. In a sense, we might infer that Wittgenstein would have applied this same logic to all manner of observable things in our universe, and hence, it was not “merely language” that his work sought to address. Conceptually, his views toward reality, and knowledge capable of being obtained by humankind (as expressed solely through language, along with mathematics), had much deeper implications.

Much more could be said of the divide between scientific thought, where physicists from Hawking to Feynman have been famously critical of philosophers of science, while philosophers maintain that their fundamental questions about the universe, rather than hindering or “falling behind” science serve to balance scientific inquiry. But returning to the question over the universe’s origins, and whether it represents some kind of finite, or to the contrary, an infinite space, the debate between the Hawking and Wittgenstein viewpoints has led to some interesting conjecture regarding notions of what, if anything, might exist beyond or “outside” our known universe.

Writer Giles Humphrey presents an excellent essay on this subject at his website, titled “Wittgenstein Versus Hawking – The Impossibility of Infinity”, in which he addresses such concepts as the breakdown between the outlook of physicists such as Hawking, as it relates to concepts of infinity. I’ll note here that, although Humphrey is a writer and a journalist, rather than an astrophysicist or philosopher, he notes at his site’s page on philosophy that, “I am vain enough to believe that both the articles on this page are philosophically significant,” and I would agree that the one referenced here offers some rewarding concepts for the careful reader. Specifically, on this troubling notion of “space beyond space”, Humphrey includes the following passage, which is worthy of consideration here:

“Perhaps, following the “outer outer space” model someone might seek to show only that space stretches “infinitely” far ahead of us. One could propose a kind of pure space, a complete absence of anything, waiting to receive the expanding universe, that is an “outer outer” space. But this would be to confuse the absence of anything for the presence of something. If someone insisted on talking in this way, and declared this “pure space” to be “infinite”, then I could understand them only if I took them to mean that “infinity” = nothing. In either case I can’t see any way of proving that a flat universe is simply connected and thus “infinite”. An “infinite” plane is a mathematical concept, in practice one imagines a plane with fuzzy edges, a kind of note to self that the plane can always be extended. One can’t actually get to the edge of the plane and see it has no edge.”

While the passage above is conceptually relevant, I advise that those interested in the subject read Humphrey’s entire essay at his website, so that the passage can be taken properly into the broader context of its discussion about infinity, and whether such an idea is conceptually feasible.

More specific, perhaps, to the divide between Wittgenstein and Hawking’s outlook had been the latter’s own assertion of the primacy of the former. Dino Jakušić of the University of Warwick authored an insightful paper on this, titled “ Stephen Hawking and the death of Philosophy” (Dino’s essay can be read here). In it, he underscores the fundamental misunderstandings of Hawking’s derailment of philosophy, but with specific regard to Wittgenstein, addresses the question of why Hawking would ascribe such significance to he alone, in calling him “the most famous philosopher of this century”… and perhaps misinterpreting “fame” as being equivalent to influence:

Hawking’s claim about Wittgenstein being the most famous philosopher of 20th Century is questionable. Regardless of the question of fame, it is very contentious to take Wittgenstein as the representative of the whole of philosophy of the 20th Century. Firstly, both “Wittgensteins‟, one of the Tractatus and the other of the Philosophical Investigations can be seen as some sort of anti-philosophers. First makes his attempt in solving all the problems of philosophy by logical reductionism, the other claims that “philosophical problems arise when language goes on holiday.”

Physicist Richard Feynman, who was mentioned briefly a bit earlier in this post, was traditionally (if not famously) critical of philosophers of science, and to a somewhat lesser extent in recent years, Hawking has continued to promote this sort of position. Few would (nor could they) debate the importance of either of these gentlemen, let alone their numerable contributions to our understanding of the physical universe. But does this remove questions like those Ludwig Wittgenstein had addressed with his philosophical researches into the divide between language and conceptual thought, and what relationship they may have to the study of the physical sciences? In equal measure, if notions of anything “before” (or after) the big bang, and thus beyond the known universe aren’t questions that science seeks to address, does that rightly remove them from falling within the purview of modern philosophers?

Separate though they may be from the modern physicist in thought and approach, our philosophers are very much still relevant, and there are indeed rewards that philosophy can still offer the modern thinker. While we continue to deal with such questions about the nature of reality, as well as the origins, and ultimately, the fate of the cosmos, philosophy may be able to lend guidance, balance, and even a degree of hope for the deeper questions we have about the universe, and what role we play within it.


Big bang

These days, the big bang theory is the widely accepted scientific explanation of the origin of our universe, but there was a time when the idea seemed preposterous. Even British astronomer Fred Hoyle, who coined the term in 1949, didn't believe the theory.

In 1970, Hawking, along with fellow physicist Roger Penrose, suggested the universe began with a singularity, a location where space and time are indistinguishable. It's as if a black hole went in reverse. Their research supported the theory that the universe began with a big bang.


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The new model cuts the structure of the universe down to a ‘more manageable, smaller, smoother universe,’ Hertog said.

While it doesn’t do away with the concept of multiple universes entirely, it trims them down to finite entities.

‘We predict that our universe, on the largest scales, is reasonably smooth and globally finite,’ Hawking said.

‘So it is not a fractal structure.’

Doing this could help to bridge the gap between classical and quantum physics, as, according to Professor Hertog, ‘Einstein’s theory breaks down in eternal inflation.’

The new paper could upend everything we know about the events surrounding the Big Bang. Borrowing points from the holographic universe theory, Hawking and colleague Thomas Hertog argue that our universe and others are not the infinite structures some have proposed

The researchers pull from the string theory concept of holography, which argues that 3D reality – including time and space – can be projected on a 2D surface.

‘Our new theory is based on a technique which we have borrowed from string theory, and that seems to imply a much more manageable, global structure of the universe in which regions can differ from each other, but not at all as much as in the old theory,’ says Hertog, a professor at KU Leuven in Belgium.

‘I think the key point about our model is not so much that constant density surfaces in the universe are finite, but rather that the variation in the multiverse is restricted.

‘In other words, that the range of different pocket universes is much smaller. That makes the cosmology based on our new theory a lot more predictive, a lot stronger as a scientific theory.

‘And therefore, ultimately we hope, testable.’

By this model, it is essential to examine our own reality as a part of the vast, global cosmology and its history, the physicist explains.

‘The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean,’ Hawking said before his death. The Big Bang and subsequent events are illustrated above

‘We are not just somehow outside the system and looking at it – no we are part of reality,’ Hertog says.

‘This is very different from other scientific dissonance. It’s a very subtle point, with a lot of debate about it. And it’s extremely difficult to implement if the multiverse is truly infinite.’

According to Hawking and Hertog, the new model will allow for the concepts on the nature of the universe to be tested in a way that earlier theories did not.

And, the key to investigating it lies in gravitational waves that go far beyond those generated by black hole mergers.

WHAT DOES PROFESSOR STEPHEN HAWKING THINK HAPPENED BEFORE THE BIG BANG?

Professor Stephen Hawking believed that before the Big Bang 3.7 billion years ago, time and space as we know it did not exist.

In the past, the esteemed theoretical physicist predicted that as the universe goes back in time, it shrinks and closes off like a sphere.

His latest paper, however - submitted just weeks before his death - adds new constraints to the history of the universe that challenge his previous theories.

According to earlier 'no boundaries' theory, the universe was shrunk and condensed to an incredibly dense ball of heat and energy the size of a single atom.

Inside this speck, the laws of physics and time as we know them cease to function, and time as we understand it did not exist.

If we move back in time from the Big Bang, the 'arrow' of time shrinks infinitely as the universe becomes smaller, never reaching a clear starting point.

Professor Stephen Hawking believes that before the Big Bang 3.7 billion years ago, time and space as we know it did not exist

Hawking said in a recent interview that before the Big Bang, time was bent - 'It was always reaching closer to nothing but didn't become nothing.'

Essentially, 'there was never a Big Bang that produced something from nothing. It just seemed that way from mankind's point of perspective.'

In a lecture on the so-called no-boundary proposal, Hawking wrote: 'Events before the Big Bang are simply not defined, because there's no way one could measure what happened at them.

Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang.'

But, his new work, published posthumously, challenges this earlier ‘no boundary theory.'

Borrowing points from the string theory – the concept that the universe is a complex hologram – Hawking and colleague Thomas Hertog argue that our universe and other ‘pocket universes’ are not the infinite structures some have proposed.

Instead, Hawking says the universe is ‘reasonably smooth and globally finite,’ setting new boundaries in cosmological history that could ultimately allow the theory to be tested.

‘Now we’re saying that there is a boundary in our past,’ Hertog added.

The ‘most promising’ observable phenomena that could help to test the new model would be gravitational waves generated in the Big Bang itself, according to Hertog.

‘The creation of space and time goes together with the generation of gravitational waves in our theory, and perhaps the detailed pattern of those gravitational waves will give us a key signature of our model,’ Hertog says.

These ancient gravitational waves, however, are far too faint to be detected by current instruments such as the LIGO experiment.

The upcoming LISA observatory – the Laser Interferometer Space Antenna – ‘should be ideally suited to capture those gravitational waves from the Big Bang,’ the physicist says.


Watch the video: 20. Τι είναι το άπειρο; (January 2023).