杏吧原创

IMAGINE a place far from here, deep in the emptiness of space. This point is light years from Earth, vastly distant from any nebula, star or lonely atom. We have many words for what you would find in such a place: a void, a vacuum, a lacuna. In fact, this nothingness is a sea of activity.

According to quantum theory, empty space is filled with virtual particles. They are always there, keeping reality ticking over smoothly. They are also completely undetectable 鈥� unless, that is, you have an incredibly powerful searchlight. 鈥淯sually when people talk about a vacuum, they mean something that鈥檚 empty,鈥� says theorist at the Chalmers University of Technology in Gothenburg, Sweden. 鈥淏ut a laser can show you the vacuum鈥檚 secrets.鈥�

To expose virtual particles, to transform them into something tangible, takes one serious laser. But that is exactly what physicists are putting the finishing touches to in Romania. Switched on for the first time a few months ago, this machine could not only reveal the truth about empty space, but also teach us about another big mystery: dark energy, the unknown entity accelerating the expansion of the cosmos. It is time to rip nothingness apart and see what is inside.

The notion that nothingness is full of virtual particles might sound fanciful. After all, no astronaut swims in a virtual sea, no satellite is hindered by virtual drag. Virtual particles just aren鈥檛 tangible. The reason we believe they exist goes back to the foundations of quantum electrodynamics (QED), the branch of quantum theory used to calculate what happens when photons, particles of light, interact with electrons.

When physicists developed QED in the 1930s, their calculations only worked properly if they took into account all ways in which the particles could approach and ricochet off each other, including routes that normal particles couldn鈥檛 take because they break the laws of physics. The travellers of these impossible routes can鈥檛 have been strictly real, then. There has been a lingering question over how to interpret things that are mathematically necessary, yet not entirely there, ever since (see 鈥�What is a virtual particle?鈥�).

QED also left open an alluring possibility. If you had an electrical field strong enough, you could 鈥渂reak the vacuum鈥� and make the virtual particles real. Virtual electrons always exist alongside their antimatter equivalent, virtual positrons, and the two usually annihilate on contact. But create a monumental electrical field and the virtual particles could escape one another and become particles proper, ones we could detect.

The energy threshold where this should happen is known as the Schwinger limit, named after the QED theorist and Nobel laureate Julian Schwinger, though actually predicted by others before him. At this limit, the vacuum鈥檚 defining property, its emptiness, is broken. 鈥淭he vacuum ceases to be a void,鈥� says theorist Sergei Bulanov, based in Doln铆 Breany in the Czech Republic.

鈥淲ith enough energy, a vacuum ceases to be a void. Its emptiness is broken鈥�

Getting to the Schwinger limit would require bombarding virtual particles with an incredible number of photons in order to transfer the requisite energy. How much energy would you need? It is equivalent to cramming a billion times the output of all the world鈥檚 power plants into a space not much bigger than an atom. Not a likely proposal, unless you can gradually store up energy and then emit it all in one enormous flash.

Which is where lasers come in. Inside them, a chain reaction produces many photons of the same frequency, before releasing them in a narrow beam that can be powerful enough to cut though steel. It took a while to get to that stage. The intensity of early lasers was limited while we struggled to find materials for their innards that didn鈥檛 waste away in such blinding conditions. Then in 1985, physicists G茅rard Mourou and Donna Strickland, while at the University of Rochester in New York, came up with a turbo boost. They found a way of pre-stretching and thus pre-weakening a laser pulse. It could then be amplified without burning through the material inside the laser鈥檚 chamber before being compressed to its original form, intensifying the energy it had gained in the amplification stage. It was this that earned Mourou and Strickland the 2018 Nobel prize in physics.

Even then, however, Mourou was setting his sights on a more ambitious target. 鈥淭he laser power was at gigawatts, then terawatts, and then even petawatts were a possibility,鈥� he says. 鈥淪o naturally we thought, could we actually break the vacuum?鈥�

In 2005, Mourou began to conceive of a laser behemoth that could, by the sheer intensity of its light, generate electrical fields at the Schwinger limit. His idea became the , and within a few years, 40 laboratories from 13 European countries were on board, with a European Union-backed budget of more than 鈧�850 million. The project now spans three sites. These include the ELI Nuclear Physics facility near Bucharest, Romania, which houses two 10-petawatt lasers that, when working at peak strength, will be the most intense in the world. Tests are being performed at gradually higher intensities.

The lasers could do more than just break the vacuum. Transforming virtual particles into real ones may also tell us about dark energy, perhaps the biggest mystery in cosmology. We know something is driving the expansion of the universe faster and faster, but what? Some suspect that the mysterious factor could be the energy inherent in ever-present virtual particles.

That probably isn鈥檛 the whole story 鈥� add up the energy of those particles and you would expect the universe to be expanding much faster. But probing virtual particles directly could shed light on the mystery.

ELI could also help us understand the fast, bright bursts of gamma rays and radio waves that astronomers keep seeing in the night sky. These sometimes explode with thousands of times more energy than the sun produces in a year. No one knows how these explosions happen, only that they probably involve the generation of plasmas made of electrons and positrons, which twist and gyrate violently, throwing off photons as they do so. ELI鈥檚 lasers could create a cloud of electron-positron plasma, letting us study the material that makes these cosmic flashes so bright.

The biggest prize would be finding out what lies beyond the limits of QED. In everyday conditions, the theory successfully predicts experimental outcomes involving electrons and photons to extreme levels of precision. A little beyond the Schwinger limit, however, the methodology breaks down, as the routes taken by virtual particles get more and more convoluted, until the calculations become meaningless. 鈥淲hat happens is an open question,鈥� says Bulanov.

With all this potential, it is no surprise ELI鈥檚 experiments are hotly anticipated. Yet despite Mourou鈥檚 grand vision, none of the lasers will be able to reach the Schwinger limit on their own. Even a mooted fourth ELI laser facility with an intensity 10 times greater than any of the other three would be 10,000 times too dim. 鈥淣othing being built currently gets there,鈥� says Michael Donovan, manager of the Texas Petawatt Laser in Austin.

Fortunately, that doesn鈥檛 rule out any magic. For one thing, the Schwinger limit is thought to be less a cliff face than a steep ascent. In other words, there is a chance virtual particles could start transforming into real ones at lower laser intensities (see 鈥�diagram鈥�).

Fatal avalanche?

That would be quite a discovery, although there could be a sting in the tail. In 2010 Mourou, now at the 脡cole Polytechnique in Paris, and his colleagues worked out that these newly materialised pairs of virtual particles would spiral around the laser beam, flinging off photons that subsequently transform into more electron-positron pairs, until there is an electron-positron avalanche.

This might be welcome for astrophysicists wanting to study electron-positron plasmas, but the avalanche would mask the transformation of the original virtual particle pair. Worse, it would , preventing it from reaching the true Schwinger limit. 鈥淚t might be one of those instances where we say, OK, we can鈥檛 reach this limit,鈥� says Jonathan Wheeler, a collaborator of Mourou鈥檚 at the 脡cole Polytechnique. 鈥淏ut in not reaching it, we鈥檝e learned something.鈥�

Time to give up then? Not yet. Shortly after the publication of Mourou鈥檚 paper, Bulanov and his colleagues realised that the avalanche effect should only occur if the laser beam is circularly polarised, with its electrical field rotating in a corkscrew fashion as it travels along the beam. They calculated that, if the laser is instead linearly polarised, so the electrons and positrons zigzag along the beam, they generate far fewer photons, containing the avalanche. 鈥淚n fact, we can reach the Schwinger limit,鈥� says Bulanov.

鈥淥ne outlandish idea to up laser intensity is a mirror flying at the speed of light鈥�

He isn鈥檛 alone in his optimism. To get over that thousandfold deficit in ELI鈥檚 intensity, laser theorists have begun to get creative. One option is to cross two or more laser beams, to double, or more, the laser intensity at the intersection. This Ghostbusters-style approach sounds easy, but Wheeler and others believe the practical details quickly get complicated.

A better option might be something more outlandish: a mirror flying near the speed of light. If a laser beam reflects off such a mirror, its wavelength is compressed, allowing it to be focused on a smaller spot. The smaller the spot, the more intense the light. First proposed by Bulanov back in 2003, the concept couldn鈥檛 employ a regular bathroom mirror 鈥� the energy required to accelerate it would be unthinkable. But Bulanov says a mirror consisting of a wave in an electron plasma would reflect the light just as well.

Five years ago, Bulanov reported the first results of an . He now leads ELI鈥檚 High Field Initiative, which explores ways of maximising laser intensity. 鈥淚鈥檓 absolutely sure that my idea works,鈥� says Bulanov. In the meantime, Wheeler and Mourou are probing similar concepts to break the Schwinger limit. 鈥淲e will find a way,鈥� says Mourou.

Some are boosting the intensity of lasers already, by other means, and seeing strange effects. In February 2018, an international group led by at Imperial College London directed a beam from the Gemini laser at the UK鈥檚 Central Laser Facility straight into an oncoming electron beam. Just as a car crashing into oncoming traffic makes a bigger bang than a car crashing into a wall, colliding the two beams boosted the intensity.

The team saw the electrons emitting photons, and . For this to happen, the electrons in the experiment had to absorb a barrage of photons in a complex process nearing the edge of QED鈥檚 descriptive powers. 鈥淚t is physics on the way to the Schwinger limit,鈥� says Mangles.

Meanwhile, other laser facilities with similar powers to ELI are in development, including the Exawatt Center for Extreme Light Studies in Russia, and the Station of Extreme Light in Shanghai. David Reis, a laser physicist at Stanford University in California, says the Shanghai facility is being built near another big laser, perhaps allowing one beam to be collided into the other. 鈥淭hat would really be spectacular,鈥� he says.

The future is looking bright for laser physics. Mourou鈥檚 desire to see another surge in laser intensity has been boosted by the Nobel he shared with Strickland. Wheeler recalls hearing the laureate鈥檚 prediction for exceeding the Schwinger limit at a recent conference. 鈥淢ourou said it would be within five years, and I laughed nervously,鈥� he says. 鈥淏ut let鈥檚 just say the next few years should be a very exciting time.鈥�