NEWS FEATURE NATURE|Vol 446|1 March 2007 EXTREME LIGHT Physicists are planning powerful enough to rip apart the fabric of space and time. Ed Gerstner is impressed.

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lmost 100 million times more pow- something altogether more exotic, unpredict- High-power lasers achieve their awesome erful than its earliest ancestor, the able and nonlinear. intensities by squeezing moderate amounts of

Large Hadron Collider (LHC) is Pump in enough energy, and the paired vir- energy into very short-lived bursts, thus driv- C. DARKIN Athe latest triumph in a century of tual particles become real, separated at birth by ing up the power — which is energy divided astounding physics. Officially inaugurated the extraordinarily strong fields involved. This by time. So although the power of NIF’s lasers later this year, the €3.7-billion (US$4.9-bil- energy level, currently thought to require fields sounds incredible, they actually use relatively lion) accelerator at CERN, Europe’s particle- of a little more than 8×1018 volts per metre, is small amounts of energy. A single pulse con- physics laboratory, will next year be fully up to known as the Schwinger limit, and it is the tains just 2,000 kilojoules — roughly half a speed in its search for exotic phenomena, such point at which the vacuum sea begins to boil. kilowatt hour. It’s just that all that energy is as the Higgs boson, that can only be discovered “When I give talks to general audiences, delivered in a few billionths of a second. at the extreme energies it makes available. Yet that’s the thing that they really seem to get even before it is turned on, the particle-physics drawn in by,” says Tom Katsouleas, an extreme- Power trip community is already looking beyond it, with enthusiast at the University of Southern The ELI takes the same principle further. By plans for an even more powerful machine, the California in Los Angeles. The problem is that generating pulses a million times shorter than International Linear Collider (ILC), which will the Schwinger limit is a long way away. For those of NIF — five femtoseconds — the ELI cost some $6.7 billion. fields of 8×1018 V m−1 you need a laser with an should fairly quickly be able to generate peak At about the time that the LHC hopes to intensity of more than 1030 W cm−2 — a thou- powers of more than a petawatt (1015 watts) be homing in on the Higgs, construction of sand trillion times more intense than NIF. from just a few joules of energy. This radically a much less feted multibillion-dollar physics Given NIF’s multibillion-dollar pricetag, that reduced need for energy makes things much research facility, the National Ignition Facility seems an overly ambitious target. easier than they are at NIF. By shortening the (NIF) at Lawrence Livermore National Labo- But a team led by Gérard Mourou, director pulse lengths by a factor of a hundred more, ratory in California, will be reaching comple- of the Laboratory of Applied Optics near Paris, down to tens of attoseconds (10−18 seconds), tion. At a cost of about US$4 billion, NIF is believes it can meet this target for a relatively the ELI’s proponents hope to reach peak inten- an assembly of 192 lasers that, for a billionth moderate price. And the researchers predict all sities of more than 100 petawatts. of a second at a time, can pump out energy at manner of wonders on the way to their even- The ELI’s extraordinarily short pulses will be more than 50 times the rate that it is generated tual goal. Throughout their made possible by a technique in all of Earth’s power stations put together. history, lasers have excited “I am always called chirped-pulse amplifi- Its aim is to ignite a fusion reaction that turns physicists by opening up new cation (CPA), which Mourou a tiny pellet of hydrogen at the lasers’ focus possibilities with light. In the astonished at what developed at the University into helium. 1960s, the fact that early lasers experimentalists can of Rochester, New York, in Science at NIF will bring astrophysics into were powerful enough to actually do if they the mid-1980s. CPA works by the laboratory by aping stars in microcosm, change the refractive index of decomposing the light in a laser and it could conceivably provide the basis for the medium through which put their minds to it.” pulse using a diffraction grat- future energy generation. But the main ration- they travelled opened up fresh — William Unruh ing known as a stretcher, which ale for NIF, and the reason it has been able to vistas in ‘nonlinear’ optics. acts like a prism. Having been command the budget that it has, is to help the Today the frontier buzzword stretched, the pulse’s compo- United States assure the operability and safety is ‘relativistic optics’ — systems in which the nents, now spread out in space and time, are of its nuclear arsenal. Similar motivations lie fields associated with the laser light can accel- fed individually through an , behind the French Laser MegaJoule (LMJ) erate every in the medium the light is before a similar grating designed for the oppo- facility, which is expected to achieve ignition passing through close to the . site effect — a ‘compressor’ — re unites them a few years after NIF. into a pulse far shorter and more intense than The lasers of the LMJ and NIF don’t come Need for speed the original. close to an accelerator such as the LHC in Mourou’s proposal — the Extreme Light Infra- These stretchers and compressors are cur- terms of generating excitement among physi- structure (ELI) — would up the ante further, rently used on almost all of the world’s most cists. At least, not yet. But laser beams even moving into ‘ultrarelativistic systems’ in which powerful lasers except those, such as NIF and more intense than NIF’s — and far cheaper to not only but also the ions from which the LMJ, that need pulses that are relatively generate — might in the next decades begin to they have been stripped move close to the long (of the order of nanoseconds). Osaka Uni- take over from particle accelerators in explor- speed of light. Mourou notes that the nonlin- versity and the at Ruther- ing the outermost frontiers of the physical ear effects of lasers revealed in the 1960s far ford Appleton Laboratory in Didcot, UK, both world. The world may never see conventional exceeded expectations. “Only the tip of the have CPA lasers that can generate a petawatt, particle accelerators much more powerful iceberg was predicted,” he says. He is similarly and the University of Rochester is also build- than the LHC and ILC. But lasers a million optimistic about what the new energies avail- ing one, as are other institutions. Mourou and times more intense than NIF are already to be able at the ELI could deliver. others feel that the technique has, as yet, no found in the presentations of physicists look- On 15 February, the French government obvious limitations; pulses can go on getting ing for funding. announced that it had bought into the vision shorter and shorter. enough to pay for a new laser beamline at the In 2006, the ELI was one of 35 projects Let there be light Laboratory of Applied Optics to show that the short-listed for consideration under the Euro- The guiding inspiration for extreme lasers lies ELI could work. If it goes ahead, the full facility pean Roadmap for Research Infrastructures, in the way light interacts with the vacuum. would provide unprecedented opportunities a programme that will provide money to help Quantum field theory sees the vacuum as a for scientists to pursue fundamental, curiosity- develop proposals for international projects. strange sea of possibilities, where pairs of ‘vir- driven science, says Mourou. Perhaps more Also on the shortlist is another, costlier tual’ particles and antiparticles ceaselessly pop importantly, it would be relatively cheap, cost- project that plans to use a CPA-enabled laser. in and out of existence. When light is bright ing €138 million to build and €6 million per A consortium led by the Central Laser Facil- enough, the electromagnetic fields it is com- year to run — considerably less than the £380- ity wants to build HiPER — the High Power posed of begin to interact with that sea in unu- million (US$746-million) Diamond synchro- Laser Energy Research facility — as a civilian sual ways. The vacuum no longer behaves as tron X-ray source recently opened in Britain, equivalent to NIF and the LMJ, but pursuing a simple, predictable medium — it becomes for example. a subtly different path to fusion. Whereas NIF

17 NEWS FEATURE NATURE|Vol 446|1 March 2007 and the LMJ use megajoule beams to hitch a ride on the wake left behind by crush their targets into fusion, the an intense pulse of laser light blasting €855-million HiPER would com- through a . press the target comparatively gently “If you look at the progress that has and then ignite it with a much shorter gone on in laser wakefield accelera- high-power pulse. tors,” says Katsouleas, “and extrapolate One advantage of this approach is that to the kinds of powers that peo- that the laser could be fired far more ple are talking about for the ELI, then frequently than NIF. With more pulses, it becomes possible to think about LAB. APPLETON RUTHERFORD CCLRC and freed from the demands of weap- accelerating particles to [ILC ener- ons research, HiPER should offer gies] in just a short section of plasma.” physicists far greater scope for non- But even if the vast engineering chal- fusion research — NIF’s non-fusion lenges of such an accelerator could be work is effectively limited to about one met, it would hardly be a replacement pulse a week. And the power would be for what the current and planned greater. With further developments accelerators do, because the average of CPA, Mike Dunne, director of the power would be far lower: big accel- Central Laser Facility, predicts that erators store large amounts of energy amalgamating HiPER’s beams could in their beams. push the machine’s limits beyond “To get the luminosity one needs petawatts to exawatts (1018 watts). “If A researcher examines one of the two diffraction gratings used by for doing high-energy physics,” says — and it’s a big if — such beamlines petawatt lasers to achieve their extreme power. Katsouleas, “and for events to be could be coherently combined,” Dunne detectable on a reasonable timescale, says, “then we believe that a beam of tational field and an acceleration are indis- the average power of the laser isn’t up to 2 exawatts is feasible. Although horribly tinguishable. So, as theorist William Unruh sufficient. It’s about three orders of magnitude difficult in practice.” pointed out in the 1970s, when a particle is away. But there are already many ways you accelerated at a sufficient rate, it should see could think about doing this.” Curiouser and curiouser and be affected by Hawking-like radiation in In fundamental physics terms, energies at the its own frame of reference — if, that is, certain Follow the light exawatt level would offer some intriguing pos- assumptions about the curvature and structure Barry Barish, director of the global design sibilities for producing strangeness from the of space-time are correct. effort for the ILC, is intrigued by the possi- vacuum, and so could allow physicists to study The accelerations involved in Unruh radia- bility of using lasers to accelerate particles, phenomena unreachable any other way, such tion are far too extreme for a traditional par- and agrees that it might be one of the future as those found at the edges of ticle accelerator, but perhaps technologies that keeps the field going. “It’s black holes. In the 1970s, Stephen not for lasers on the scale of the certainly a very promising avenue to pursue, Hawking predicted that when a ELI. “Nothing generates fields because up front there doesn’t seem to be any virtual particle–antiparticle pair even close to those produced obvious limitation to it, and it could be that is created just ‘outside’ a black by an ultra-high-intensity laser it will help in the long term,” he says. But he hole, the warping of the local — except perhaps a black hole,” doesn’t see it as the only game in town, or as vacuum by the hole’s gravity will says Bingham. “Some of the a sure thing. “It’s just hard to know where the be strong enough to tear the pair lasers at the Rutherford Apple- show-stoppers will be. But at the level where it asunder, with one disappearing ton Laboratory will produce a won’t require enormous resources, this is the into the black hole and the other field of about 3 trillion volts per sort of thing that needs to be pursued.” surviving as matter outside it. centimetre. Nothing else comes Mourou, on this as on much else, is bullish. The effect is analogous to the “We’re going to close to that!” “Probably within 20 years!” he says of laser- production of electron–posi- change the index Using their lasers to acceler- driven follow-ons to the ILC, before back- tron pairs by electric fields at the ate electrons fast enough to feel pedalling, at least a little. “The time constant Schwinger limit. “The vacuum of refraction of Unruh effects could be a tan- to design it and to build it is ten years. And really doesn’t care if it’s an electric the vacuum, and talizing problem for the ELI, you always build these things with the previ- field, a , a gravita- produce new HiPER or other extreme lasers ous technology. So I have to be very careful tional field, or even if it’s a weak to tackle. “It seems to me to be when I say 20 years. But in 10 to 20 years we nuclear field or a strong nuclear particles.” a very hard experiment — creat- will have this technology, and then it would field,” says Bob Bingham of the — Gérard Mourou ing such strong and short intense take another 10 years to build.” Beyond that, Rutherford Appleton Laboratory. pulses,” says Unruh, now at the he and his colleagues say, there are even more “If you can pack enough energy University of British Columbia remarkable technologies: gamma–gamma in, you can excite particles out of the vacuum.” in Vancouver. “But I am always astonished at colliders that can reach energies millions of The attraction of Hawking radiation is that what experimentalists can actually do if they times higher than today’s accelerators; and its dependence on a gravitational field means put their minds to it.” ‘relativistic mirrors’ that can push light to the that its subtleties will depend on interactions This is not the only way in which lasers could Schwinger limit and beyond. between quantum field theory and general rel- beat accelerators at their own game — or help “We’re going to change the index of refrac- ativity, the sort of thing that could throw light them to greater heights. By definition, relativis- tion of the vacuum,” enthuses Mourou, on quantum theories of gravity. tic optics requires that electrons be accelerated evoking the ultimate fulfilment of the laser’s The laser-builders don’t want to create close to the speed of light. Get them very close original promise. “And we’re going to produce black holes with which to look for Hawking and researchers might be able to do particle new particles — the vacuum is the mother of radiation — but they don’t have to. General physics beyond the reach of the LHC and ILC. all particles. And I’m sure we’re going to dis- relativity’s equivalence principle means that to One approach to this would build on the idea cover more.” ■ something experiencing one of them, a gravi- of a ‘wakefield’ accelerator, in which electrons Ed Gerstner is a senior editor on Nature Physics.

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