Can't see the forest for the trees?

"Particle physicists have a problem, he says. They are an anthropocentric bunch, too preoccupied with the particles and forces that impinge on humanity. They have spent so much time unravelling mysteries such as the structure of atoms and why the sun shines that they have neglected other avenues of inquiry. They need to broaden their horizons...."

"The hunt for the God particle"

We have all heard of 'dark matter'. But what about dark galaxies, dark planets - even dark people? Ian Sample reports on the new holy grail of physics

by

Ian Sample

June 21st, 2010

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Durham, northern England, December 2009. The largest meeting of particle physicists in the country is underway and James Wells, a leading theorist at Cern, the European nuclear research organisation near Geneva, is beguiling his audience with an idea that has all the makings of the next great revolution in science.

Wells, a tall, softly-spoken 44-year-old from Tampa Bay, Florida, begins with an uncomfortable home truth. Particle physicists have a problem, he says. They are an anthropocentric bunch, too preoccupied with the particles and forces that impinge on humanity. They have spent so much time unravelling mysteries such as the structure of atoms and why the sun shines that they have neglected other avenues of inquiry. They need to broaden their horizons, Wells says. To think beyond the world we see and touch.

If that was the stick, next came the carrot. Our knowledge of the cosmos tells us that the stuff around us, from plants and people to stars and planets, is made from just a handful of elementary particles. On top of these, there is a small number of forces that make nature run smoothly, doing things like keeping planets in their orbits and ensuring everyday objects don't suddenly collapse into a pile of atoms. But how do we know, asks Wells, that there isn't much more going on than this? Our knowledge of nature and how it works is based on observations. What if we can't see everything? What might we be missing out on? There could be a "hidden world" out there, Wells says, where particles and forces are busily at work, all around us, but beyond the realm of our senses.

The phrase "hidden world" sounds like a science-fiction cliche, but it simply means that there may be more particles and forces at work in the world – and the cosmos at large – than those we see when we look around. They are so aloof, so hidden from our daily experience, that they go completely unnoticed.

"It would be strange if we were so special that we could feel and observe everything that is going on out there," says Wells, who is one of a growing number of physicists working on the hidden worlds idea. "We are lumps of clay swirling on a little blue marble in an overwhelming vastness of universe. We have to envision that there is more going on. There really should be additional particles and forces," he says.

Six months after his Durham lecture, Wells is back in his office at Cern. For hundreds of scientists like him, June is turning out to be a hectic month. One of the most important meetings in the academic calendar, the International Conference on High Energy Physics in Paris, is only weeks away and this year is the first time that physicists at Cern will unveil results from their shiny new machine, the $6bn Large Hadron Collider (LHC). People are furiously writing up papers and cross-checking data. Heads are down; blood pressure is up.

While many of his colleagues are busy writing up results from the LHC's first few months of running, Wells is preparing another lecture, this time on using the LHC to find evidence for a hidden world. The LHC, it turns out, is perfectly placed to be the first instrument in history that could shed light on whether a hidden world exists.

The LHC is aptly named. The machine sits in a giant circular tunnel with a five-mile diameter that crosses the French-Swiss border 100m beneath the Cern campus. Inside the machine, subatomic particles, protons, are whipped up to within a whisker of the speed of light and slammed together in head-on collisions. These orchestrated acts of violence recreate conditions that prevailed in the first moments of the big bang.

Physicists have a lengthy shopping list of new phenomena they want the LHC to find, but most prominent is the Higgs boson, an elusive particle dreamt up in the 1960s that is believed to give mass to other particles. The Higgs boson is a glittering prize in its own right, but to Wells and many other physicists, it has an added appeal. The Higgs particle should be influenced by what happens in the hidden world. As such, it could act as a kind of bridge or window into the unknown world.

"The LHC will likely be the first collider in history to be able to see the Higgs boson and so illuminate this bridge," Wells says. "We may be on the brink of discovering new worlds by means of it."

The idea of a hidden world might sound absurd, but physicists have good reason to believe it exists. Even with today's most advanced telescopes, astronomers can see only 4% of what makes up our cosmic neighbourhood. The rest is invisible to us, revealing itself only by the effects it has on the galaxies we can see. Around 70% of the unseen universe is labelled as "dark energy", a mysterious force that drives the expansion of the universe, making galaxies race away from us. The remaining quarter is chalked up as "dark matter", an obscure substance that clings to galaxies and exerts an unmistakable gravitational pull on them. The word "dark" means we cannot see it, but it also means scientists haven't the faintest clue what it is. Last week, British scientists reported a new analysis that suggests dark matter and dark energy might not even exist, though other researchers reject the findings. Charles Bennett at Johns Hopkins University in Baltimore has worked on both. "We unequivocally stand by our results," he says.

As the Milky Way spins on its axis, our planet passes through vast stretches of dark matter – if it does exist – without us even noticing. And though dark matter is part of the hidden world, it is only a part. "The likely existence of dark matter suggests that there is more stuff out there that we do not know than we do know," says Wells.

Ask physicists to speculate about a hidden world – and that is half the fun of theoretical physics – and the possibilities of what might be lurking beyond the reach of our senses are endless. "Once you start considering these ideas actively, there's no theoretical reason to rule out a very interesting, dynamic and diverse dark or hidden world," says Neal Weiner, a physicist at New York University. "It leads to all sorts of conversations about the possibilities of dark people and dark planets. Now that is extremely unlikely, but it's something to think about. Once you open the box, it's not obvious where it will end."

What is more likely, according to physicists working in the field, is that the hidden world is filled with a wispy fog of dark matter and puny dark forces that are incapable of forming dark planets and more exotic objects like dark life. When normal planets form, cosmic matter has to cool down and coalesce into enormous lumps of rock, but it can only do this by losing heat. As far as we know, dark matter doesn't cool down: if it did, we would see the heat if gives off. It would glow.

Other particles might flit in and out of existence in the hidden world, just as they do in ours. Of all the particles physicists have found in nature – often in cosmic rays and particle colliders like the LHC – only a tiny fraction are stable enough to form long-lasting objects. The rest decay immediately, into lighter, more durable particles.

The uncertainty over what exists in the hidden world has done nothing to dampen physicists' enthusiasm for the idea. John March-Russell, a theoretical physicist at Oxford University, says proof of a hidden world could become the central plank of a scientific revolution that rivals any in history. When Copernicus put the sun at the centre of the solar system in the 16th century, and when Charles Darwin described evolution in the 19th century, they both knocked humans down a peg or two. The discovery of a hidden world would force us to reassess our place once more. The cosmos as we know it – with all its stars and planets – might turn out to be nothing more than a mediocre microcosm of a far richer and more complicated universe.

"Just as the Copernican revolution told us that the Earth isn't special, the same could be true for everything that we've so far discovered," says March-Russell. "All of this stuff around us, the stuff of our reality, is it the dominant and most complex part of the universe? It might not be."

It's a view that Weiner shares. "If evidence for a hidden world started showing up in experiments, you would unleash a huge amount of experimental creativity on the problem. If we find dark forces it would be a sea-change. I don't think it's hyperbole to say it would be one of the most important discoveries in particle physics."

Frank Wilczek is a theoretical physicist at Massachusetts Institute of Technology and considered one of the most brilliant minds in physics. At the age of 21, he developed a theory about the so-called "strong force" that holds the innards of atoms together. The work was so groundbreaking he was awarded the Nobel prize in physics for it in 2004. Two years after receiving the award, Wilczek and his student at MIT, Brian Patt, coined the phrase "Higgs portal" in a theoretical paper that fleshed out how the Higgs boson could be used to study hidden worlds. Wilczek forgets how they came by the name, but it means the same thing as the "bridge" Wells described earlier. "The Higgs particle is special because it is more open to influence from the hidden world," says Wilczek. "It might be that the Higgs decays into particles that are invisible, in which case it will look as though it has just disappeared." This would not leave physicists as stuck as it might seem. The LHC would register that some energy – that wrapped up in the Higgs particle - had gone missing. The vanishing act could be intriguing evidence, at least, that a hidden world is real.

Another possibility is that the Higgs boson collapses into particles from the hidden world, which themselves decay back into real-world particles we are more familiar with. This would really give scientists at the LHC something to think about. Their detectors would flash with bursts of particles that seem to come out of nowhere. The crucial point is that by studying how the Higgs boson behaves in the LHC, physicists should be able to build up a picture of the particles and perhaps even forces at work in the hidden world.

One of the most compelling aspects of the hidden world idea is that it doesn't require physicists to tear up all the work they have already done in describing how the universe works. "Physics has advanced so far that it's not easy to take things on in a way that is consistent with what we already know. The hidden world idea at least passes that test. It's easy to add all of this stuff into our existing theoretical framework," says Wilczek.

So when is the LHC going to find this thing? The short answer is that nobody expects the Higgs boson to be discovered any time soon. To find it physicists need a collider that has enough energy to make the particle, but how much is enough is not clear. They then need to find the telltale signature of the Higgs particle among the subatomic detritus spewed out by collisions in the machine, which is a formidable task. The last major collider at Cern, which shut down in 2000, came up empty-handed despite a lengthy search for the particle. Another atom smasher, the Tevatron at Fermilab near Chicago, has been hunting the Higgs particle for a while, but is due to close within a year or so. Many physicists believe the LHC is guaranteed to find the Higgs boson, but not for three or four years. In 1993, the American Nobel prizewinning physicist Leon Lederman gave the Higgs boson a nickname: the God particle, because he considered it critical to our understanding of matter. Considering the wait, a more appropriate nickname might be the Godot particle.

Finding the Higgs boson will end one of the greatest hunts in modern physics, but as that chapter closes, a new one will open. Wrapping up his talk in Durham in December last year, Wells issued a rallying call. The Higgs particle could help them get over their anthropocentric ways and open up vast new territories of hidden worlds. "And that would only be the beginning," says Wells.

[This article was amended on 22 June 2010. The original referred to Copernicus putting the Earth at the centre of the solar system. This has been corrected.]