FORMER US Defense Secretary Donald Rumsfeld famously distinguished between âknown knownsâ, âknown unknownsâ and âunknown unknownsâ. Itâs a distinction that should ring clear bells with Harvard physicist Howard Georgi, because heâs choosing to make the same distinction between different types of matter.
Most of us are familiar with the âknown knownsâ of matter â ordinary stuff, such as tables, chairs, quarks and electrons. Many physicists spend their days hunting for âknown unknownsâ, like the Higgs boson and the particles that make up dark matter. But Georgi has gone a step further. He is dabbling in the world of the âunknown unknownsâ by proposing the existence of an entirely new type of matter unlike anything we have encountered before. He calls it the unparticle.
Unparticles are slippery customers. Breaking all the rules that constrain normal particles, they can shift identity and masquerade as fractions of particles. âItâs very difficult to even find the words to describe what unparticles are, because they are so unlike anything we are familiar with,â says Georgi. If his ideas are correct, unparticles exist all around us. They could even exude their own âungravityâ force, which could replace dark matter as a way of explaining some of the universeâs mysteries.
Advertisement
âUnparticles could even exude their own âungravityâ force, replacing dark matterâ
Unparticles challenge our sensibilities, yet Georgi is worth taking seriously. He was among the first theorists to suggest that three of the four forces in nature might be united in a single super-force described by a âgrand unified theoryâ. He pioneered supersymmetry, a theory he proposed in 1981 with Savas Dimopoulos at Stanford University, California, in which every known particle has a heavier, as-yet-unseen âsuperpartnerâ. He was also among the architects of quantum chromodynamics, the theory that describes the strong force that acts inside nuclei.
Other physicists certainly listen to him. When Georgi made his public last March, he provoked a flood of research activity. More than 95 papers have now been written investigating the peculiar facets of unparticles â a level of research on a subject that has not been seen since 1998, when theorists including Dimopoulos proposed that extra dimensions could be large enough to be detected (). âUnparticles are such a novel idea,â says Pran Nath at Northeastern University in Boston, Massachusetts. âThey excite people because they are completely different from anything anyone has seen before.â
We might even spot unparticles soon. According to Georgi, they could make a surprise appearance in collisions at the (LHC) particle accelerator, due to turn on later this year at the CERN laboratory near Geneva, Switzerland. âTo my mind, unparticles would be a much more striking discovery than supersymmetry or extra dimensions,â says Georgi. âUnparticle stuff would astonish us immediately.â
Unforeseen
Georgi certainly did not intend to come up with a new form of matter. He was wondering whether anything might show up at the LHC that would be dramatically different from what has been predicted so far. One of the great hopes for the LHC is that it might come up with some discoveries that will help us update or replace our best description of the particles that make up matter and the forces that glue them together. Known as the standard model, the theory has worked very well, yet it has plenty of shortcomings too, and this has led researchers to concoct a number of theoretical frameworks, including supersymmetry, as possible replacements. Georgi wanted to know what discoveries each theory would be likely to produce at the LHC. âMany theories have been studied for a long time in terms of their mathematics,â says Georgi, âbut no one had looked at the consequences.â
Georgi was inspired by work carried out by Cornell University physicist Kenneth Wilson, who is an expert on describing the multitude of atoms inside solids and their interactions using quantum mechanical fields. Wilson showed that certain phenomena in magnetic materials can be explained by a particular type of quantum field theory where the interactions between atoms have the same strength at all scales. This insight won him the 1982 Nobel prize for physics.
Quarks, electrons and the like are all described by quantum fields too. So, asked Georgi, could the same trick Wilson used be turned on its head to predict something new in particle physics?
Unmistakable
At first sight, the answer is no. According to the standard model, the forces that govern particle interactions are different at different scales. For example, zoom inside an atomic nucleus, and your world will be governed by the strong nuclear force that binds quarks together. Gravity will be irrelevant. Zoom out again, though, and the strong nuclear force pales in comparison to the force of gravity holding you to the floor. This is one of the main stumbling blocks with the standard model, because it means that it cannot unify the four main forces into a single field.
Yet who says weâre stuck with just the standard model? Physicists have shown that in modified versions of the standard model, including some supersymmetric theories, forces might have the same strength at different scales, at least over some distance ranges. When Georgi looked at the properties of the fields in these theories, he was taken aback. He discovered that the fields described entities that looked nothing like normal particles. In fact, they behaved in ways he had never seen before, so he christened them unparticles.
At first, Georgi didnât believe his own findings. âI did think I was crazy,â he recalls. âIt was such an interesting result. I thought I was either wrong or that everyone had seen it before.â Other physicists, however, confirmed that he wasnât having delusions: unparticles were indeed something entirely new.
So what makes unparticles a new kind of matter? For one thing, they have no definite mass. We know where we are with ordinary particles because we know how heavy they should be. Einstein dented our feelings of security about this with his special theory of relativity, which showed that the mass of an object depends on its motion and becomes greater the nearer its velocity gets to the speed of light. But by and large we still know how to tell electrons apart from protons and quarks thanks to the fact that they have a definite mass when they are standing still. Even the advent of quantum mechanics, which told us particles can sometimes impersonate waves, wasnât enough to shake the conviction that particles can ultimately be recognised by their mass.
Unparticles are something entirely different. An unparticle does not have one particular mass, but can take on any possible mass or have all possible masses at the same time, depending on how you look at it. That sounds weird, but anyone who has ever tried to measure the length of a coastline has seen a similar effect. If you are trying to find out how long a coastline is, you will get a different answer depending on how closely you look at it and the size of the measuring stick you use. Zooming in on a jagged coastline reveals finer and finer detail, first the bays and harbours, and then smaller coves, nooks and crannies. Each time you try and take all this extra detail into account, the coastlineâs overall length appears to increase.
The reason that coastlines, with their jagged edges, donât have a definite length is because they are examples of fractals. It doesnât matter how much you magnify a fractal, you always see pretty much the same pattern â in the case of a coastline, another jagged edge. The pattern is the same whatever the scale â fractals are âscale invariantâ.
Under scale invariance, matter loses all sense of perspective, and distance is no longer important, says Francesco Sannino of the Niels Bohr Institute in Copenhagen, Denmark, who has also studied unparticles.
This has a remarkable effect on the strength of the force holding unparticles together. Sannino likens it to being able to hear a quiet whisper as easily as a loud yell. âIn the normal world, if two people are sitting in different countries and they want to have a conversation, they must use a phone,â he says. âBut in a scale-invariant world, even if they were separated by 2000 kilometres they could speak to each other as if separated by just 2 metres.â
There may be different types of unparticle species, just as there are protons and electrons. However, within each species all unparticles are inextricably tied together because of their undiminishing interaction. Think of an unparticle as an infinite chain or ever-growing tower of particles with different masses, says Georgi. âBut you can never pull one of these particles out of the tower.â
That wreaks havoc with the standard model and Einsteinâs special theory of relativity, encapsulated in his famous equation E = mc2, which says a particleâs speed is limited by the amount of energy it has. Unparticles do not have to obey this rule. For every mass you can think of, there is a piece of the unparticle tower that looks like a normal particle with any energy that you want, Georgi says. This bizarre property means that unparticles could manifest in odd ways at the LHC.
âUnparticles wreak havoc with Einsteinâs special theory of relativityâ
How will we know? Unparticles interact very weakly with ordinary matter, so they would leave no trace in a detector. Yet particle physicists already have ways to spot signs of invisible particles produced in particle collisions at accelerators. For example, they can pick out the presence of other ghostly particles called neutrinos because they carry away energy and momentum. When researchers tally up the energy and momentum measured in their detectors and find some has gone missing, they usually attribute it to the presence of neutrinos.
Unorthodox
Things will look even stranger if unparticles are produced. When researchers sit down and do their sums, says Georgi, their calculations will reveal a momentum and energy imbalance that comes from something disguised as a fractional number of particles. This will be a sure sign that the normal relationship between energy, mass and velocity doesnât hold.
Georgi knew he had stumbled on a type of matter with extremely peculiar properties, but he was not sure exactly what the existence of unparticles meant for the universe. âI didnât know all that they could imply,â he says. âI just knew they should be investigated further.â
Other physicists have done this in spades, checking to see how unparticles might affect everything from the orbit of Mercury to the production of black holes at the LHC (see Diagram). Among the most intriguing suggestions is work carried out by Nath and his colleague Haim Goldberg. They have turned their attention to the force that unparticles exert on normal particles.
In standard physics, forces are carried by particles. For instance, the electromagnetic force arises when particles exchange photons between them. In an upcoming paper in Physical Review Letters, Nath and Goldberg say another force arises when ordinary particles start playing catch with unparticles ().
âItâs an oddball thing, but the mathematics is surprisingly simple to work out,â says Goldberg. The new force acts as an extra attractive force, making gravity appear a little bit stronger than you would expect for standard physics. Effects of the new force, which Nath and Goldberg have dubbed ungravity, would be too small for us to perceive in everyday life, but precise tests of gravity that will be carried out in space by the French and European space agenciesâ MICROSCOPE mission, the Italian Galileo Galilei probe and Stanfordâs STEP satellite could pick up signs of it.
We may already have encountered ungravity in another guise: dark matter. This invisible form of matter was introduced to account for the fact that stars orbiting the centres of spiral galaxies like our own, and galaxies orbiting within galaxy clusters, appear to be moving too fast, as if in the grip of stronger-than-expected gravity. However, no one has ever detected dark-matter particles, which has led to the suspicion that something could be amiss with Newtonâs law of gravity.
Modifying Newtonâs law, making it stronger in places to mimic the effects of dark matter, is an old trick first proposed in the 1980s by Mordehai Milgrom of the Weizmann Institute in Rehovot, Israel. Still, no one could explain why gravity should be tweaked in this way. Ungravity might be the answer.
Things arenât that simple, though. Before explaining away dark matter, physicists have to be sure that the existence of unparticles does not have other knock-on effects in cosmology that would have shown up already, says Hooman Davoudiasl at the Brookhaven National Laboratory in Upton, New York. âUnparticles have so much potential, but we must be careful not to let our imaginations run wild without checking the ramifications for the rest of cosmology,â he says.
For example, in a recent paper, Davoudiasl shows that if too many unparticles were created in the early universe, they would have disrupted the delicate energy balance that allowed helium, lithium and other light elements to be forged minutes after the big bang (). That puts a limit on how many unparticles could have existed back then, he says.
If unparticles behaved like normal particles, the story could end there. The limit on early unparticle numbers could scupper its hopes for mimicking dark matter today, since the current unparticle concentration would be too small to produce the effects that we see. But Georgi has shown that unparticles are far from normal, with no set relationship between the amount of energy they have, their mass and the speed they can move at. Because of this, even a small number of unparticles in the early universe could have a surprisingly large effect today, says Shao-Long Chen at the National Taiwan University in Taipei.
Chen and his colleagues examined what happens to the energy of unparticles as their temperature is ramped up and down. They assumed that the universe started out with a small number of unparticles compared to the number of photons â the requirement needed so that the early unparticles would not affect the production of light elements. As the universe expanded and cooled, they found that things became interesting. While the photons, particles and unparticles all slowed down, unparticles did not lose energy as quickly as their more conventional counterparts. âIn an expanding universe, the behaviour of unparticle energy is dramatically different than that for photons,â says Chen.
Today, even relatively few unparticles could make up a significant part of the energy budget in the universe â enough to act as dark matter, says Chen (). âIf this really happened, it would impact significantly on our standard picture of cosmology,â he says.
Georgi reserves judgement on whether his unparticles really could be the key to solving the dark matter problem until more work is done, but heâs pleased that people are investigating the possibility. âAll I knew was that I had found something cool and I wanted other people to take a look and see what kinds of weird things they might be capable of doing â what mysteries they might solve,â he says. âIâm happy because thatâs exactly what people are now doing.â
Cosmology â Keep up with the latest ideas in our .
