The Big Questions: Physics (24 page)

It has to be said, there is no experimental evidence to support this idea, just a deductive argument: that inflation, though devoid of explicit experimental support of its own, is the best explanation for the features of our universe, and thus might be
applicable again and again. Worse still, there can be no evidence – at least in the scientific terms of falsification put forward by the philosopher Karl Popper.

The standard idea in science is that you make a hypothesis and see if experiments can falsify it. Hypotheses that withstand attempts at falsification gain support, and can eventually be developed into theories. The string theory idea of the landscape of universes cannot be falsified in these terms. There is no way to make any prediction about the properties of our universe compared to another – the other universes are simply not accessible to our experiments.

It is possible to make a virtue of this; string theorists have made much of the observation that the expansion of our universe is accelerating, for example. There is no good explanation for why this should be, and the string theorists have jumped on the lack of explanation as a kind of backhanded proof: maybe there is no explanation, they say: maybe it is an example of how our universe is just one possibility. In other universes, the laws of physics work to keep the expansion constant, and yet others have a decelerating expansion. Diversity is the only law. Whether this makes string theory a white elephant to science is an ongoing debate amongst physicists. But the fact remains that we don’t have a better way forward at the moment.

There are other attempts at building a theory of everything. Perhaps the most advanced is ‘loop quantum gravity’ or LQG. This suggests that space is ultimately composed of indivisible quanta that are around 10
^–35
metres in size. A network of links between these quantum nodes – imagine an airline route map – creates the space–time we live in. The particles that come together to create our familiar world of atoms and molecules are created when quantum fluctuations induce knots and tangles in this space–time.

Or that’s the idea. LQG is not yet a well-defined answer to the problem of unifying quantum theory and relativity. In fact, there are probably only a hundred or so researchers working on it
worldwide. Which means string theory, with its workforce of thousands, is maintaining its dominance. Eventually, though, the plan is to replace it with another theory: M-theory.

Rather surprisingly, no one is sure what the M stands for. Whatever its true provenance, however, the M of M-theory has come to be associated with membranes. To make the mathematics work, string theorists have postulated that the 11 dimensions of string theory are populated by surfaces called ‘branes’ (short for membranes) as well as the strings. These branes can have up to nine dimensions.

Though they add to the richness of string theory, wrapping around compactified dimensions, providing an anchor point for wandering strings and allowing new kinds of universe that might exist, their most famous role might be in establishing what – according to string theory – came before the Big Bang. The idea is that our universe came about through a collision between two four-dimensional branes. The enormous kinetic energy of the colliding branes creates a vast amount of heat: the Big Bang fireball and, crucially, the standard zoo of particles known to physics. This scenario is known as the ‘ekpyrotic’ universe, taken from the Greek phrase ‘born out of fire’.

Interestingly, the ekpyrotic universe does away with the need for inflation because it is created homogeneous. Doing away with inflation undermines the idea of an infinite landscape of varied universes. And that means we don’t have to give up on creating falsifiable hypotheses about why our universe is as it is. Having said all that, only a minority of string theorists subscribe to the ekpyrotic view of the universe, and it may be that only a minority of physicists have any faith in string theory’s power to explain the universe. So where will this go? Can we at least test string theory? This is another contentious question. As yet, after four decades of work, we have yet to find a way to properly test the string idea. But there are some possibilities.

One of the hopes has been that we will see hints of the hidden extra dimensions. Such a hint could be an anomaly in gravity as we examine its effects on ever-smaller scales. Gravity is an ‘inverse square’ law: double the distance between the two objects under test and the force between them drops by a factor of four. Triple the distance, and it drops by a factor of nine. But with a tiny rolled-up dimension in play, that inverse square law may not describe exactly what is going on. Gravity may work slightly differently between objects that are less than a millimetre apart, for example.

So far we have seen no evidence of this. Tests of the inverse square law down to less than six-tenths of a millimetre have shown up no such anomaly. Perhaps we shouldn’t be surprised, though. Strings themselves are tiny, after all – less than a trillionth of a trillionth of the diameter of an atom. How could we detect such an incredibly small thing? One hope is that some of them have grown because of the expansion of the universe. As the cosmos has grown, some cosmic strings might have expanded into ‘superstrings’ that might be scattered through space. It is possible that we could detect their presence through their effect on light travelling to us across the universe: the high mass of the superstrings would bend the light as it passed, creating an optical illusion known as gravitational lensing.

Then there’s the idea that in the standard, non-ekpyrotic universe scenario, inflation would have created ripples in the gravitational field of the early universe. These ‘gravitational waves’ should have been preserved in the cosmic microwave background (CMB) radiation, the echo of the Big Bang, but string theory places limits on how strong those ripples should be. If they were large, they would have unfurled some of the compactified dimensions, and we would have more than the three dimensions of space we currently experience. So string theorists are hoping for no gravitational waves in the CMB. Again, it’s hardly a conclusive test, though. As yet, there are no ‘direct hit’ experiments that will give us a definitive yes or no to the theory. Is the universe made of strings? It’s a definite maybe.

The Big Bang, antimatter and the mystery of our existence

Could there be a bigger question? Why is it that we, the galaxy, the universe, everything, exist at all? To understand the answer, we have to go back to the beginning of everything – if, that is, we can find one.

In many cultures, there is no such thing as a beginning. The ancient Greeks, for example, revered the concept of the circle, and everything essential to the universe, including the universe itself, existed in ever-repeating cycles.

Until the beginning of the 20th century, the consensus amongst astronomers was very much the same: our universe had existed for all eternity and it made no sense to talk about a beginning. Which, of course, made the church authorities slightly uncomfortable.

The Book of Genesis begins with a beginning: a something was created out of nothing. Perhaps that is why a young Belgian priest called Georges Lemaître decided that astronomy needed to give consideration to a point of genesis for the universe. Lemaître, a professor of physics and an accomplished astronomer, was the first to suggest the idea that came to be known as the ‘Big Bang’. His hypothesis was that everything arose from a ‘primeval atom’, which split apart to produce all the matter in the universe. Starting with Einstein’s equations of general relativity, which describe the
dimensions of the universe, he showed that its radius could change – the universe, in other words, could expand.

It was more than a theoretical consideration: there was evidence for it too. Astronomical observations gathered by Lemaître and others showed that most galaxies were moving away from ours. An intriguing implication was clear to Lemaître. Perhaps the galaxies were moving away because Einstein’s space– time was expanding? Lemaître’s resulting paper suggested that we live in an expanding universe, spawned by the explosion of what he called ‘the cosmic egg’.

The Pope was thrilled with Lemaître’s work; astronomers less so. The idea that the Pope would approve of a scientific theory – that their data and theories were supporting the doctrine of creation
ex nihilo
– sat uncomfortably. Nevertheless, within a few years the English astronomer Edwin Hubble had propelled the idea of a beginning to the universe to the very forefront of cosmology. Hubble took Lemaître’s work forward, gathering together data from many different astronomers, and supplementing it with his own. He showed definitively that almost all the galaxies were flying away from us at enormous speeds, and that the universe must be expanding.

The idea that the universe hadn’t always been as we see it remained a subject of intense debate for decades, however. It was only in 1963 that the conclusive evidence – the cosmic microwave background radiation, sometimes known as the ‘echo of the Big Bang’, was found. At that point, almost all opponents of the Big Bang became convinced it was indeed our best explanation for cosmic history. With the advent of Big Bang cosmology came an answer for why there is something rather than nothing. But it was only a partial answer. The idea raises obvious questions: ‘What caused the Big Bang?’, and ‘What banged?’

Physicists have taken diverse paths here. Some say the questions are meaningless because time came into existence at the
moment of the Big Bang; the notion of ‘before’ therefore makes no sense. It is, they say, like asking what lies north of the North Pole. Others make some attempts at an answer, but the answers are little more than untestable speculations. They invoke quantum phenomena such as the Heisenberg uncertainty principle, which says that nothing can have an exact amount of energy, and that includes a universe with zero energy. Quantum fluctuations, then, will give rise to a universe with some amount of energy, and there are processes that can amplify that to create a Big Bang.

There are some physicists – notably Stephen Hawking – who say the Big Bang was not the beginning of everything, but resulted from processes occurring in other dimensions (see
Can I Change the Universe?
). Others take this further and suggest that we are in a ‘cyclic universe’ that is caught in a never-ending cycle of creation and destruction as objects known as ‘branes’, which exist in these other dimensions, repeatedly crash together and move apart (see
Is String Theory Really About Strings?
). Such reasoning is satisfying to those who want to believe there is no need for a divine hand in creation, but unconvincing to those who don’t. These issues, it seems, may lie beyond the reach of science.

But even given the problems of describing how a Big Bang arose, there is another, later issue that ought to have made sure that, very soon after the something was created, there was nothing again. Before Edwin Hubble had laid the foundations of Big Bang theory as an explanation for our existence, another Englishman, Paul Dirac, was undermining them. The issue is blown wide open thanks to Dirac’s greatest contribution to physics: antimatter.

Dirac was a strange, quiet man with few social skills. One much-reported conversation sums him up rather neatly: during a formal dinner at Cambridge, Dirac sat next to the equally reticent E.M. Forster. Their entire conversation through the multiple courses consisted of one exchange. In reference to a scene in Forster’s novel
A Passage to India
, Dirac asked, ‘What happened in the cave?’ Forster, much later in the meal, replied, ‘I don’t know.’

The pair evidently whiled away the hours in their heads. In Dirac’s case, that was certainly productive. The existence of antimatter, now known to be an essential part of the zoo of subatomic particles, was not suggested because of an experimental result in need of a theory. It arose directly through Dirac’s considerations of the governing equation of quantum theory: the Schrödinger equation (see
What Happened to Schrödinger’s Cat?
).

When describing the energy of a quantum particle, Schrödinger’s equation threw out something that was, at first glance, an impossible puzzle. The energy of a fast-moving particle, it said, involved two numbers. These two numbers, when multiplied together, would give a result of 0. Each one multiplied by itself, however, had to give the answer 1.

In any normal mathematics, this simply cannot be done. But using arrays of numbers known as matrices, Dirac did it. The only price to pay was that the energy of the quantum particle could be negative as well as positive. And, through a tortuous chain of reasoning, Dirac showed that the negative energy particles could manifest in our world. They would look like familiar particles, but with some strange adjustments.

In 1928, in a series of talks, Dirac proposed the existence of an antielectron. It would look exactly like an electron, but would have a positive charge. He was ridiculed: physicists of the time considered matter to be made from negatively charged electrons and positively charged protons, and nothing else (the neutron was still four years from discovery). Undaunted, Dirac published his theory three years later. The antielectron, he said, would be ‘a new kind of particle, unknown to experimental physics’. When it met an electron, there would be an explosive annihilation, Dirac predicted. And the same would be true of any particle: each and every one had an antimatter nemesis.