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IN May 2001 Giovanni Amelino-Camelia quietly started a revolution. It didn鈥檛 look like much: just an eight-page paper published in Physics Letters B. But it was scientific dynamite.

Einstein鈥檚 special theory of relativity, which describes the behaviour of space, and time and bound them together as 鈥渟pace-time鈥�, has been passed down the generations as an immutable fact. It is supported by a wealth of experimental evidence. But, says Amelino-Camelia, Einstein may have had only half the story.

It hardly needs to be said that this is a very controversial claim. Yet since publishing his paper, Amelino-Camelia has become more confident of it. Theory and experiment offering tempting hints that Einstein鈥檚 theory might be due for a make-over, he says. And he鈥檚 not alone. Other physicists are gathering behind his banner and joining the crusade against the reverent acceptance of special relativity. 鈥淭he religion of special relativity is dead,鈥� he declares.

Based at La Sapienza University in Rome, Amelino-Camelia studies quantum gravity 鈥� the attempt to mesh relativity with quantum theory and thus produce one consistent description of the Universe. Researchers in this field began to feel a certain discomfort with relativity in its present form long ago: it is proving extremely difficult to tie together with quantum mechanics. The offending experimental evidence comes in the form of cosmic rays detected in Japan with such high energies that special relativity says they shouldn鈥檛 exist. It was a step too far for Amelino-Camelia. 鈥淚 guess the cosmic ray data put me in the position of questioning the dogma of special relativity,鈥� he says.

This was a risky path to take, but it has led him to a new form of relativity. Not only does this provide a new route to unifying the laws of physics, it鈥檚 also consistent with all available observations and can be directly tested in the near future.

It starts with just one idea: that there must be some scale at which space and time can no longer be described according to familiar classical rules, and start behaving in a quantum manner. This transition marks a border where the rules of play change, and can manifest as a particular amount of energy or a particular length. To physicists, length and energy are linked: the energy of a photon, for example, dictates the wavelength of its light.

It doesn鈥檛 sound like much. But this seemingly innocuous idea stands in direct conflict with special relativity. Take the idea of a length threshold, for example. According to Einstein鈥檚 theory, the length of something depends on who is measuring it. If you were to stand still and measure the length of a particle zooming past you at close to the speed of light, you would find its length to be much shorter than if it were simply idling by. This is called the Lorentz contraction, and depends on how the observer is moving relative to the object: someone else walking past you just as the particle zoomed by would measure a different length again.

The conflict becomes clear when you imagine the particle under observation to be slightly longer than the threshold length. According to Einstein, observers moving one way would see it contracted below the threshold length, while to others, moving differently, its length would remain above the threshold. When observers work out how the particle behaves, they come up with different answers because some would see the particle moving in classical space-time, and others would see it moving in a quantum version. In other words, the laws of physics would appear different for different people. And, if we鈥檙e to have a proper description of the way the Universe behaves, that鈥檚 simply not possible.

There鈥檚 a further problem: in which frame of reference do you define the threshold length in the first place? If one person says, 鈥渋t鈥檚 this long鈥� and points to a given length, special relativity says that length will look different to observers in moving frames of reference.

Amelino-Camelia鈥檚 solution to all this is to make the threshold length or energy 鈥渋nvariant鈥� 鈥� it looks the same to all observers. In that way, all observers will agree on whether or not a particle has energy and length above or below the threshold.

Combining constants

So where might this threshold lie? For physicists, the most obvious scale to investigate is the one determined by combining the fundamental constants of relativity and quantum theory: Planck鈥檚 constant h, the gravitational constant G and the speed of light c. Combine them in one way, and you get a length: the Planck length. Do it another way and they produce the Planck energy. These scales are the natural place to expect relativity and quantum theory to mesh.

Amelino-Camelia鈥檚 paper in Physics Letters B (vol 510, p 255) showed that this approach could be developed into a new theory that was consistent with all the requirements of quantum gravity. He has called this revolution 鈥渄oubly special relativity鈥� (DSR). Where Einstein鈥檚 theory had one impassable threshold 鈥� no particles with mass could accelerate beyond the speed of light 鈥� Amelino-Camelia鈥檚 had two: the speed of light and the new impassable threshold of length or energy.

Soon after the paper appeared, a couple of other researchers began to realise that Amelino-Camelia might have inadvertently begun to solve their problems too. Jo茫o Magueijo, a theorist working at Imperial College, London, had been formulating an explanation of the evolution of the Universe. It offered an alternative to 鈥渋nflation鈥�, the idea that the Universe underwent a period of very rapid expansion shortly after the big bang. But there was a heavy price to pay: Magueijo鈥檚 idea hinged on the speed of light being variable 鈥� a direct contradiction of Einstein鈥檚 theory. He was suggesting that the speed of light has been slowing ever since the big bang.

Magueijo admits that, although this has since been found to fit with some otherwise inexplicable astronomical data, this variable speed of light was conjured from thin air. Fortunately, however, Amelino-Camelia鈥檚 doubly special relativity provided an explanation.

Introducing a threshold energy to relativity produces a strange twist: in DSR the speed of light depends on the energy of the photons involved. In the searing heat of the early Universe particles had very high energies and so, according to DSR, the typical speed of light was then somewhat higher than the one we observe in our present experiments.

It was just what Magueijo had been looking for. And, by coincidence, a colleague working in the same building was also getting excited about Amelino-Camelia鈥檚 ideas. Lee Smolin, now at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, had spent 10 years developing a theory of 鈥渓oop quantum gravity鈥�, where the very fabric of space-time is composed of tiny packets, or quanta, bound together in a kind of foam. The theory was progressing well, but Smolin was still having some problems: the results from a few of his calculations were in direct conflict with the requirements of special relativity. 鈥淔or about 10 years I was confused,鈥� he says.

Changing the view

It had occurred to Smolin that the answer might be to suppose that the Planck length looks the same to all observers, but he admits he just didn鈥檛 know how to work up a relativity theory that could handle it. It was Magueijo who persuaded him that it could 鈥� and should 鈥� be done. 鈥淚 didn鈥檛 have the idea, or the courage, to try to formulate this concretely before the work with Jo茫o,鈥� Smolin says.

Smolin and Magueijo soon developed a DSR of their own 鈥� indeed, they showed that various different types of DSR theories could be constructed (Physical Review Letters, vol 88, p 190403). There are now a number of DSR variations, each amending Einstein鈥檚 work, and even offering its own version of his famous E = mc² relation. Magueijo, Smolin and Amelino-Camelia are now working together on this revolutionary approach.

So what does this newborn theory say about our Universe? Einstein鈥檚 basic principle 鈥� that we keep an invariant speed 鈥� remains. But all experiments available to Einstein concerned light with a long wavelength and low energy. And so the invariant speed which Einstein interpreted generically as 鈥渢he speed of light鈥� is described in DSR as the speed of low-energy light particles. Light particles of high energy are allowed to travel slightly faster. And just as objects have different speeds for different observers in special relativity, in DSR anything that travels at less than the invariant speed can appear to change speed.

Also, where special relativity allows photons moving at the speed of light to appear to be different colours to different observers, DSR goes a little further. Different colours travel at different speeds. That鈥檚 because different wavelengths correspond to different energies 鈥� and in DSR, this affects the speed of the light.

These features alone could change the face of standard cosmology. For over 20 years, cosmologists have been building the inflation model of the Universe鈥檚 history. It was put forward to explain why the Universe is so uniform in temperature and density. With a finite speed of light, there is a limit to the rate at which particles and radiation could spread throughout something the size of the Universe, so how could it have reached the relative equilibrium we observe? If it was all compacted (and thus in physical contact) before expanding suddenly, the uniformity is not a problem.

But although inflation explains some of the features and history of the Universe, it only goes so far. DSR, on the other hand, explains everything that inflation does, and more. In DSR, the high energies of the early Universe meant that light travelled faster on average then than now. Parts of the Universe that are now out of contact with each other might once have been able to communicate because light travelled faster then.

DSR also accounts for our Universe鈥檚 accelerating expansion, which inflation can鈥檛 explain. Physicists sometimes ascribe this expansion to 鈥渄ark energy鈥�, but no one knows exactly what this is or how it works. A variety of mechanisms have been proposed. One way to think about dark energy is to say it is equivalent to space being slightly curved rather than flat: this sets up a kind of elastic tension that forces it to expand outwards.

Laura Mersini, Mar Bastero-Gil and Panagiota Kanti of the Scuola Normale Superiore in Pisa and Italy鈥檚 National Institute of Nuclear Physics in Rome recently showed that modifications to Einstein鈥檚 E = mc² relation lead to just such a curvature of space (Physical Review D, vol 64, p 043508). In other words, the modifications DSR makes to relativity also explain 鈥渄ark energy鈥�.

Perhaps most importantly, DSR explains the otherwise inexplicable: the cosmic ray data from Japan. For more than a decade, physicists working at the University of Tokyo鈥檚 Akeno Giant Air Shower Array have been seeing cosmic rays that special relativity says shouldn鈥檛 exist.

Cosmic rays are particles produced in violent events such as supernova explosions, which then travel at enormous speeds through space. If they are of high enough energy, relativity dictates that any collision with the abundant low-energy photons in the Universe will destroy them. By the time they reach our detectors on Earth, there shouldn鈥檛 be any cosmic ray particles left with energies higher than 5 脳 10¹⁹ electronvolts. This is known as the Greisen-Zatsepin-Kuzmin limit. Over the past decade, however, the Akeno Giant Air Shower Array has detected several cosmic rays above the GZK limit, seemingly violating special relativity.

Raising the limit

In DSR, however, there鈥檚 no problem: the theory modifies the energies at which particles are created and destroyed. By adopting special relativity unquestioningly, we may have underestimated the GZK limit, Amelino-Camelia says. He has shown that there is a class of DSR theories that could solve the cosmic-ray puzzle (), but it is not yet clear which DSR theory in that class would be best.

And therein lies DSR鈥檚 Achilles鈥� heel 鈥� there isn鈥檛 yet any proof positive. All DSR has going for it is that it doesn鈥檛 contradict the evidence that is starting to emerge from cosmic ray observations.

For some people, that鈥檚 not enough. John Ellis, a theoretical physicist based at CERN in Geneva, doesn鈥檛 think that the cosmic-ray data is a good reason for modifying Einstein鈥檚 equation. Harvard cosmologist and Nobel laureate Sheldon Glashow agrees: he sees no experimental or theoretical evidence to make him think special relativity needs modifying.

The DSR proponents are themselves quick to admit that the cosmic-ray evidence could be flawed. The number of cosmic ray events that contradict the GZK limit is small and some physicists, Amelino-Camelia included, have suggested that misinterpretation of the data may explain the anomalies. So is this is just a passing fad? 鈥淣ot even that,鈥� says Glashow.

Dismissive as the critics are, Magueijo believes that there鈥檚 a showdown approaching. Since the DSR framework can answer any question put to special relativity 鈥� and in many cases give a subtly different answer 鈥� he feels there will come a point where people have to face up to the possibility that Einstein needs a tweak. 鈥淭he fact that the answers are usually slightly different means that you have a way of placing relativity in the courtroom of experiment,鈥� he says.

Many of DSR鈥檚 experimental implications seem to be limited to scales that our experiments can鈥檛 yet probe, however. The difference between the maximum speed in the universe and the speed of light, for instance, should be extremely small, and lies beyond the precision of any equipment we have. It is only when particles have extremely high energies, those approaching the invariant energy of DSR, that the difference becomes appreciable.

And while DSR also changes the way lengths change with an observer鈥檚 motion, just as special relativity does, the difference between DSR and special relativity only shows up at around the Planck length: 10^鈭�35 metres. That鈥檚 10^鈭�20 times the size of a proton. This length, and its corresponding energy scale, are not even close to accessible in the world鈥檚 most powerful particle accelerators.

More experimental evidence may come with the 2006 launch of the Gamma Ray Large Area Telescope (GLAST), a NASA satellite designed to detect ultra-high-energy gamma ray bursts from distant galaxies. Experiments to be carried out on this telescope will measure the way particles move, with a sensitivity that can discriminate between the predictions of DSR and special relativity. By then, more extensive cosmic-ray data will also be available from the Pierre Auger Observatory, which is under construction in Argentina.

But since DSR is only a couple of years old, many more implications are likely to emerge. According to Amelino-Camelia, physicists could find accessible and definitive tests for the new theory at any time. 鈥淥nly a relatively small number of contexts have been analysed in the necessary detail,鈥� he says.

Magueijo is confident that DSR will eventually spread far and wide: all of physics could fit in a DSR framework 鈥渁nd it might be easier than we think鈥�, he says. This could prove extremely uncomfortable. 鈥淒oubly special relativity will surely require us to give up some other concept which we presently hold as obviously correct,鈥� Amelino-Camelia cautions. If these physicists are right, then Einstein鈥檚 reign is coming to an end. When the edifice of 20th-century physics starts crumbling around your ears, don鈥檛 say you weren鈥檛 warned.