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What makes the Universe tick?

We鈥檝e reached the point where we can鈥檛 go any further with many of the other really interesting phenomena and questions in physics until this problem is solved. To understand topics such as the origin of the Universe, the ultimate fate of black holes and the possibility of time travel, we need to understand how the Universe works.

We now have a good idea what the basic building blocks of matter might be. Physics in the 20th century was built on the twin revolutions of quantum mechanics (a theory of matter) and Einstein鈥檚 theory of space, time and gravitation known as relativity. But it鈥檚 extremely unsatisfying to find two ultimate descriptions of reality when you鈥檙e looking for just one.

Trying to unify the two theories presents formidable technical and conceptual obstacles that have challenged some of the finest theoretical physicists for decades. For example, because gravitation manifests itself as a warping of a four-dimensional environment called space-time, applying quantum theory to gravity causes problems. For one thing, it means bringing Heisenberg鈥檚 uncertainty principle to bear on space-time itself, which is decidedly problematic.

But maybe that means there鈥檚 a problem with this approach: perhaps we shouldn鈥檛 be trying to quantise gravity alone. Most recent attempts at unification subsume the problem of quantising gravity into a broader approach that aims to bring all the forces of nature, as well as all subatomic particles, into one theoretical framework. This is the idea that some physicists call a 鈥渢heory of everything鈥�.

One current approach is superstring theory, which posits tiny loops of string (rather than point-like particles) as the basis of all matter. Another approach, M theory, is still more abstract and can be pictured as membranes moving in higher space dimensions. But perhaps the level of progress in these ideas is best summed up by the fact that no one quite remembers what the 鈥淢鈥� in M theory is supposed to stand for. There鈥檚 a long road ahead.

Was Einstein鈥檚 antigravity really a mistake?

Einstein called it his biggest blunder. But he may have been right after all to include a type of antigravity called the cosmological term to his general theory of relativity.

The extra term gives space a repulsive property: it pushes itself apart, making it expand faster and faster. Einstein added this fudge factor because the Universe was thought to be static, so something was needed to balance the gravitational pull of matter to prevent the Universe collapsing. But in the 1920s, Edwin Hubble found that the Universe is actually expanding, so Einstein abandoned the 鈥渃osmological constant鈥� in dismay.

But the idea has refused to die. The quantum theory of fields predicts that even empty space is seething with energy, the gravitational effect of which precisely mimics Einstein鈥檚 antigravity force (this is the dark energy referred to in question 2). The theory is vague about the actual strength of the repulsion, but puts a guesstimate value on it.

About five years ago, however, astronomers found that the expansion rate of the Universe seems to be picking up and put their own 鈥渆xperimental鈥� value on the strength of the antigravity force. To the theorists鈥� bafflement, the astronomers made it about 120 powers of 10 smaller than the theoretical guess.

It鈥檚 an exasperating result. If the constant were zero, a profound law of nature might account for it, but a non-zero number that is so small compared to theory is very hard to explain. To make matters worse, cosmologists like the idea of a very strong cosmic repulsion during the first split second after the big bang, because this underpins the popular scenario of an inflationary universe. According to this theory, the Universe abruptly jumped in size by an enormous factor just after it was born, driven by a pulse of intense antigravity.

So if we want to keep inflation and account for today鈥檚 accelerating expansion, we need a theory that explains why antigravity was once intense, then dropped precipitously, and after that hovered at just above zero. In other words, we want to know why the antigravity force was almost but not quite totally eliminated in the primeval phase of the Universe.

One possibility is that the force fades with time. Another is that it varies in space, so that far beyond the limit of our telescopes it may be much bigger. If it were, the matter in that region would have flown apart too fast for galaxies and stars to form, so it鈥檚 unlikely there would be any observers around to measure the force. This explanation assumes that the cosmological term in our part of the Universe is small purely by accident.

What we need is a theory that derives the strength of the antigravity force as part of a unified description of all the forces of nature. Unfortunately existing candidate theories, such as superstrings and M theory, do not seem to pin down this particular number, and its tiny value remains mysterious. So we鈥檙e back to question 1.

Why do we live in three dimensions?

Is it just a fluke that space has three dimensions, or is there a deeper explanation? Some theorists speculate that space emerged from the big bang with three dimensions just by chance, and that there may be other regions of the Universe with a different number.

Logically there is no reason why the Universe should not have, say, only two dimensions. A hundred years ago, Edwin Abbott wrote Flatland, an account of a two-dimensional world in which beings lived their lives confined to a surface. But the physics of a 2D world would probably be very different from ours. For example, waves wouldn鈥檛 propagate cleanly as they do in 3D, raising all sorts of problems about signalling and information transfer. And since conscious life depends on accurate information processing, these differences might be enough to rule out our observing such a region.

Going above three dimensions brings different problems. For example, planetary systems would be impossible because the inverse square law of gravity becomes an inverse law of higher powers. So it seems that a three-dimensional world might be the only one in which physicists could exist to write about the subject.

But there are hints that this question is based on a false assumption. Maybe space is not three-dimensional at all, but only appears that way to us. It could have 9 or 10 dimensions, maybe more. Some theories aiming to unify the forces of nature, such as superstring theory, invoke the existence of more dimensions than those we see.

They do this because the equations describing what is going on often work out better when they are given a higher number of dimensions in which to operate. It鈥檚 not exactly a fudge: extra dimensions have a history of solving the most pressing problems in physics. Einstein needed a fourth dimension, time, to correctly describe gravity, for example. And Theodor Kaluza added another space dimension in an attempt to unify gravity with Maxwell鈥檚 equations for electromagnetism.

Of course, we can鈥檛 see the extra dimensions, but there may be a reason for that. They could be rolled up extremely small. Imagine viewing a hosepipe from afar: it looks like a wiggly line. On closer inspection, the line is revealed as a tube, and what was taken to be a point is really a little circle going around the circumference of the tube. Similarly, what we take to be a point in three-dimensional space could be a tiny circle going around a fourth space dimension that鈥檚 too small to detect.

It is possible to conceal any number of extra dimensions this way. Unfortunately, however, superstring theory does not yet predict three unrolled dimensions, so it cannot offer a convincing explanation for our experience of the Universe.

But there is another way to conceal a higher dimension. Suppose physical forces restrict light and matter to a three-dimensional sheet or 鈥渕embrane鈥�, while allowing some physical effects to penetrate into the fourth dimension. The inhabitants of Flatland perceived three-dimensional objects as two-dimensional projections into their plane: a sphere, for example, looked like a circle. In the same way, although we see only three dimensions, what we do see could be a mere slice or section from higher dimensions.

Our 鈥渢hree-membrane鈥� space need not be alone in four dimensions. There could be other membranes out there similar to our three-dimensional membrane, but sitting in a four-dimensional space. It will take as yet untried experiments to confirm the existence of a fourth spatial dimension, but it was recently suggested that the collision of two such membranes might explain the big bang. So the fact that we鈥檙e here at all might eventually be considered evidence that space isn鈥檛 really three-dimensional.

Is time travel possible?

Maybe this should be question 1. Forget dark matter and quantum gravity, this is the question everyone would love to answer.

Time travel has been a favourite science fiction theme ever since H.G. Wells鈥檚 trailblazing novel The Time Machine. But not everything it describes is science fiction: travelling forwards in time, for example, is a proven fact. Einstein鈥檚 theory of relativity predicts that an observer moving relative to Earth can leap into Earth鈥檚 future, and the effect has been confirmed using atomic clocks. Dramatic time warps require speeds close to that of light, which is possible in principle but would take a major feat of engineering, not to mention a lot of money.

Going back in time is far more problematic. Relativity does not rule out an observer being able to make a journey through space-time and return to their past. But all scenarios so far discussed require exotic circumstances.

One way to go back in time is to use a wormhole in space. Theorists speculate that such a tunnel鈥� or star gate鈥� linking two points in space-time, really might exist. Find one and you could jump through it, coming out moments later in another part of the Universe. They also suggest that if wormholes do exist, then one could be adapted to form a time machine. You could go through it and exit not only somewhere else, but 鈥渟omewhen鈥� else too. And that could be in the future or the past.

If it were possible to visit the past then all sorts of paradoxes ensue, of course, such as the conundrum of the time traveller who goes back and murders his mother when she was still a baby. Paradoxes can be avoided by insisting that nothing can defy the principle of cause and effect, but two-way time travel is still very weird.

It certainly seems too anti-rational for some physicists. Stephen Hawking suggested a 鈥渃hronology protection conjecture鈥�, surmising that something would intervene to prevent physical objects or influences looping back in time. This may occur because of fundamental physical obstacles to constructing a time machine; for example, quantum vacuum energy might surge without limit near the entrance to the wormhole, in effect blocking it.

The conjecture remains unsolved, but it鈥檚 not a problem that many people can devote time and effort to. As Hawking has noted, it鈥檚 hard to get funding for time travel research. So it is likely that a proof or disproof will have to wait for solutions to more general problems, such as producing a tractable theory of quantum gravity.

Are we living in a cosmic colander?

Familiar though black holes may now be, they could still spring a few nasty surprises on theoretical physicists. A black hole may form when a large star burns out. The core collapses in a split second, crushed by its own enormous gravity. If the material were exactly spherical, by symmetry all the matter would fall radially towards a point at the geometrical centre of the core, so the density and gravitational field there would escalate to infinity. Since gravity manifests itself as a warp in the geometry of space-time, the curvature of space-time would also become infinite, creating a sort of edge or boundary to space and/or time. Mathematicians call this a singularity.

No one knows what to make of singularities. Does space-time really end there, or do singularities merely signal a breakdown of our theory? If space-time did have a boundary, then it would be impossible to predict what might come out of it. Since prediction and determinism form the basis for any rational scientific picture of the world, singularities would mark a line beyond which science can鈥檛 set foot.

With a black hole enveloping it, though, at least the singularities are veiled, and not quite such a threat. In 1967, Roger Penrose proposed a 鈥渃osmic censorship hypothesis鈥�, saying that all singularities formed by gravitational collapse should be decently clothed by a black hole and thus rendered unobservable to us. The alternative鈥� the existence of a 鈥渘aked鈥� singularity that could bring about events that have no rational cause鈥� was regarded as abhorrent.

Then, a few years later, Stephen Hawking provided a new twist on the problem. He discovered that black holes would emit heat radiation and slowly evaporate away. Theorists puzzled over what becomes of them in the end: does this evaporation eventually expose the singularity at the black hole鈥檚 heart?

The issue has been rephrased in the language of information theory. When a star collapses to form a black hole, the information content of the star鈥� how many particles it contains of each type, say鈥� becomes inaccessible to an external observer. So when the black hole evaporates, is that information given back, encoded somehow in the Hawking radiation? Or does it go down the singularity plughole and disappear for good? Black holes appear to be pretty much ubiquitous in our Universe. If a singularity marks a hole in space-time, is the Universe leaking information like a cosmic colander? And if so, where does it all go?

How come I can ask these questions?

Where does consciousness come from? Why do some swirling electrical patterns, such as those in a brain, have thoughts and sensations attached, whereas others, such as those in the national grid, presumably do not? Conversely, how does something as insubstantial as a thought or desire move electrons and ions around in brains to trigger physical movement? Or are these questions themselves a meaningless muddle of concepts? Are these even questions for physicists to answer?

Some think they are for physicists to answer鈥� myself among them. Relating the mental and physical worlds is something most physicists avoid, but if physics claims to be a universal discipline then it must eventually incorporate a description of consciousness.

Quantum mechanics has been cited as the key, largely because the observer plays a central role in the description of quantum systems. But it is far from clear whether quantum effects can ever amount to much on the scale of neurons.

Perhaps the real key is to go back to descriptions of life. Nobody knows how, or precisely when or where, life began. Somehow a mixture of lifeless chemicals became a primitive living thing. This is unlikely to have happened in a single dramatic leap; doubtless there was a long and complicated sequence of physical processes. But it is not even clear that this biogenesis is a problem of physics per se.

It is sometimes claimed that life is written into the laws of physics. Although it is true that life would probably be impossible if the laws had been slightly different, there is nothing in the known laws to compel matter to organise into life. If a 鈥渓ife principle鈥� exists in nature, it will be found not in basic physical laws but in areas such as complexity and information theory. After all, the living cell is not some sort of magic matter, but a highly complex information processing and replicating system.

The principles governing information and complexity are still being worked out. At some level, quantum mechanics must play a part in the story of life, as Erwin Schr枚dinger surmised in the 1940s. Since the rules for quantum information processing differ dramatically from those for classical systems, perhaps that will provide the key to solving this puzzle.