The Big Questions: Physics (13 page)

Each of the supersymmetric Higgs particles will produce a different decay signature in the LHC’s detectors. Each signature
takes the form of a trail of particles that exist at certain energies then decay into other particles with particular properties. Though that might sound easier to spot than just one Higgs, the truth is that this complexity makes it easier to miss – or to get a false positive from the decay of other, non-Higgs particles.

The energies of these Higgs particles are such that there is also a chance they might be spotted in Fermilab’s Tevatron, the most powerful accelerator in the United States. They might even have been there in the data coming out of the LEP collider; maybe we just weren’t looking in the right place. It’s still an open question. But spotting any signature of supersymmetry will be a boon. In fact, CERN researchers expect the LHC to find evidence for supersymmetry – and it will be the first evidence for supersymmetry – before it finds the Higgs boson.

In the end, even if we find the Higgs boson, the root of mass will not be fully exposed; we still won’t understand why particles have the mass they do – why, for instance, the top quark has a million times the rest mass of an electron. The Higgs does reconcile the existence of mass with the way the weak force works, but why it gives so much mass to the quarks remains a mystery. What’s more, the mass of the quarks in a proton, added to the energy that holds the quarks together still doesn’t add up to the mass of the proton.

And there is yet more bad news for mass-hunters: we have no explanation at all for the electron’s mass. Whatever does eventually happen in our accelerators, it seems clear that the God particle is much less important to the future of physics than its name might lead one to believe. Seeing it – or something like it – will be thrilling, but we may well discover that our God particle has feet of clay.

The limits of our universe and the search for parallel worlds

If you have ever wondered what makes you you, or whether there is some unique, pre-determined purpose and path for your life, you have asked one of the biggest questions that physics could conceivably answer.

It is a question that writers play with all the time. Tales of other worlds, reachable from ours, abound in literature. The idea is a science fiction staple, but it also forms a central theme in books for children. There is the fictional world of Narnia in the C.S. Lewis series, for example, and the classic Lewis Carroll story of
Alice in Wonderland
.

But these books tend to assume uniqueness for their heroes and heroines, who reach a parallel world without losing time in their own world. This is no doubt to do with the limits on our consciousness, which tells us there is only one ‘me’: I can be in only one place at a time. But if we set aside the Zen-like problem of consciousness, and what ‘I’ is, the answer to our question is almost certainly a straightforward no: you are not unique. How we get to that answer, however, is far from straightforward.

There are three reasons why you might not be unique, and all of them are central to our view of the universe. One is to do with the physical extent of the universe, and whether it has an edge. The second has to do with something that Einstein called
the ‘biggest blunder’ of his life, and reaches out from the first moments of creation to raise questions about our infinite future. The third probes the essential nature of the quantum world. If‘am I unique?’ seemed like a silly question at first, it doesn’t now. The issue of whether there really is another you somewhere out there is actually the same as asking how much we know about the universe.

The simplest route to an answer is through an examination of the size of the universe. Here physicists have three possibilities to choose from. Perhaps the universe is infinite in extent. Or it could be finite but, like an ant on a tennis ball, we can never reach the edge. The third option is that the universe is finite, and its geometry is such that you could fall off an edge.

If the universe is infinite, then there is good reason to think that you are not unique. Though it would contain infinite numbers of worlds, and thus infinite numbers of worlds with Earth-like life, it seems that there are only so many ways a set of molecules can be configured to give a living being. That would mean that, somewhere, there is a carbon – pun intended – copy of you.

Of course, you will immediately counter this suggestion, saying that, even if all the molecular structures are identical, it wouldn’t make it
you
. There is the issue of memories and experience, but, apart from that, what is
you
, exactly? We are getting ahead of ourselves – at this stage we don’t even know if it’s a question that has to be faced. The question at hand is now, is the universe finite or infinite?

Scientists and philosophers have long pondered the size of the universe, but for most of history, it has been considered finite. Around
AD
140, Ptolemy conceived of the cosmos as a finite sphere centred on the Earth. Only in 1576 did anyone suggest otherwise. That was when the English astronomer Thomas Digges put forward the idea of an infinite universe populated by stars
similar to our sun. Digges was more fortunate than the Italian philosopher Giordano Bruno. When Bruno suggested something similar a few years later, he ignited the fury of the authorities of the Roman Catholic Church, who sent him to be burned at the stake.

We are still none the wiser as to the extent of the universe. Observations of the cosmic microwave background radiation, the echo of the Big Bang, seem to indicate that the universe might be finite. The most popular explanations of anomalies in the spectrum of this radiation suggest a limit to the size of the cosmos, but there are plenty of competing explanations. So, as we are unsure about whether the universe is infinite or not, we cannot say whether there is another you on a faraway world. Perhaps our second possibility, the one involving Einstein’s biggest blunder, can shed more light on the issue.

This possibility begins with something known as ‘eternal inflation theory’, which involves a succession of universes bubbling out then pinching off from one another. Though it sounds odd, there is quite some evidence for this as a natural, ongoing scenario. The idea was born with the discovery of an anomaly that haunted physics in the 1970s. A decade after the 1963 discovery of the cosmic microwave background radiation, few people doubted that the universe had begun with a ‘Big Bang’. The term had been coined by Fred Hoyle, one of the idea’s most strident critics, as a way of deriding the idea that the universe exploded into existence, but the evidence was good, the name catchy, and – probably most important – it fitted nicely with the dominant religious views of creation. There was one problem, though. The universe we saw couldn’t be explained by a big bang alone.

For a start, relativity tells us that space and time curve when in the presence of energy and matter (see
Why Does an Apple Fall?
). That will have a profound effect on our universe, altering its overall geometry. The consequences of this geometry change depend on how much matter and energy there is. In high
concentrations, space and time would curve catastrophically, closing up the universe. In low concentrations, the expanding power of the Big Bang would have dominated the shape of the early universe, throwing everything in it so far apart that stars and galaxies (and habitable planets) would never form. However, our universe was perfectly set up with a ‘flat’ geometry that allowed us to eventually exist. The question is, why should it be so perfect?

This ‘flatness problem’ is not the only tough question facing cosmologists. There is also the ‘horizon problem’. This stems from the fact that the temperature at opposite ends of the universe appears to be the same. The only way for that to happen is if heat has been distributed evenly through the universe, but we know that the universe is too big for that to have happened. Heat is carried by photons, which are particles of radiation. Even though photons travel at the speed of light, there has not been enough time for photons to move throughout the universe, carrying heat from one extreme to the other, so that the cosmos no longer has hot spots.

At the beginning of the 1980s, physicists solved these two problems with a single stroke. The solution was called ‘inflation’, and it suggested that, just after the Big Bang, the universe went through a period of super-fast expansion. Although no one knows how or why it might have happened, a period of inflation is still the best answer to the problems cosmologists have with the
Big Bang, explaining the spread of heat and the flatness of the universe. It also provides a path to a second you.

People have been playing around with possible mechanisms for inflation for nearly three decades now. The most popular ones suggest that inflation is a never-ending story. If a tiny point of space–time blew up once, it can do it again. According to these chaotic inflation theories, the fluctuating energy inherent in empty space can inflate a whole new universe from anywhere within our own space and time. In a process reminiscent of something in Willy Wonka’s chocolate factory, new universes are bubbling up from old ones all the time. The mouth of each one eventually pinches off, separating it from its parent for ever.

Though this does seem fantastical, the scenario got a big shot in the arm when string theorists seized on it as an idea that would solve their own set of problems. String theory is an attempt to create a ‘final’ theory of physics that unites Einstein’s relativity with the strangeness of the quantum world. The basic idea is that all matter is composed of tiny vibrating loops of energy; the frequency of the vibrations determines what kind of matter shows up. When string theorists tried to calculate the kind of universe that this would create, they were hoping they’d end up with one that looked and behaved rather similar to ours.

They didn’t. However hard they tried, they couldn’t create a single string universe that matched the one we live in. Instead, they created thousands, each one endowed with a different set of characteristics. The problem was compounded by the 1998 discovery that the expansion of the universe was speeding up. Although we expect the universe to be expanding still – the Big Bang’s influence is still strong – it should be slowing down as the gravitational pull of everything in the universe works against the expansion. If the expansion is speeding up, some unknown force is at work.

It didn’t take long for physicists to work out that the energy associated with this acceleration makes up approximately
70 per cent of the total mass and energy in the cosmos. Call it what you like – physicists call it dark energy – but that’s an awful lot of stuff to not know about.

The best answer to the dark energy mystery lay with a mathematical term that Einstein had crowbarred into his original equations describing the universe. Einstein didn’t know anything about a Big Bang, and thought the universe should be static, not expanding. Unfortunately, his equations created an unbalanced universe, so he inserted this term, known as the ‘cosmological constant’, to create a neat, static universe. After the discovery of the Big Bang, he called it his ‘biggest blunder’.

With the discovery of dark energy, however, the cosmological constant came right back into fashion. This term, it was thought, could explain why the universe was expanding ever faster. But it didn’t – in spectacular style. The calculated value for the constant came in at around 10
¹²⁰
times the measured value. That’s 1 with 120 zeroes behind it: even physicists have labelled it the most embarrassing mismatch between theory and experiment in the history of science.

But string theory has an answer. Don’t expect to understand why a universe is as it is; just glory in a multiplicity of diverse worlds. Chaotic inflation says they all exist, and so does string theory. Yes, we live in a universe with an inexplicably small cosmological constant, but why do we think we should be able to calculate the values of the constants of nature from scratch? They simply are what they are – and they are different in every one of the vast landscape of universes that string theory predicts to exist.

The current thinking at the frontiers of theoretical physics is that, rather than being a problem, the inexplicable value of the cosmological constant is proof that string theory is on the right track. It might seem like twisted logic, but if the string theorists are right, it does provide the route to another you. The vast landscape of universes bubbling out from each other via chaotic
eternal inflation has no end. Though their constants of nature are, effectively, random, some of them will be identical to ours. That means planets will form, stars will appear and cluster into galaxies, and elements such as carbon will be synthesized in the burning cores of those stars. Life will emerge and, in some cases, so will humans.