On the outskirts of Hamburg, Germany, pulses from one of the world’s biggest X-ray lasers race underground towards a stainless-steel chamber. Directly above, another huge laser waits. Soon, its beam will be sent down to meet the X-rays head on, bringing the two extraordinarily powerful beams of light together in an almighty burst of energy.
All very exciting, to be sure. But there is a serious side to these laser theatrics: the scientists behind them are hoping to see the vacuum. Not stray material in the chamber, or leftover gas, or particles of any kind, but the vacuum itself – what most people regard as pure emptiness, but modern physics says is brimming with quantum fields. Usually, these fields are perfectly undetectable, and the vacuum looks like nothing. But with enough light, even nothing can start to look like something.
“The hope is that, if it exists according to theory, we’ll be able to see it,” says , an experimentalist at the Helmholtz-Zentrum Dresden-Rossendorf laboratory in Germany. “If it’s not there, there’s a problem.”
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Seeing the vacuum doesn’t merely require massive lasers, but also some of the most sensitive detection equipment ever devised. But the payoff could be great. The results will put our best theories of the vacuum to the test, and potentially give us a better handle on the dark matter that seems to occupy most of the universe. There is also an outside chance they could reveal that empty space is even stranger than quantum theory predicts.
The modern concept of the vacuum can be traced to the work of theorist Paul Dirac in the late 1920s. At that time, physicists already believed that the electromagnetic field filled all of space, but they still thought of matter as discrete particles, such as electrons and protons. Dirac saw that electrons could also be understood as energetic blips in an underlying field. Over a decade or so, his idea blossomed into quantum electrodynamics, or QED for short – the first successful quantum field theory.
By the 1970s, a more elaborate quantum field theory, the “standard model” of particle physics, had grown to encompass QED and account for all known matter and fundamental forces, besides gravity. When space is empty, these fields remain, but they aren’t totally placid. The uncertainty built into quantum theory predicts that a field can never be guaranteed to have any particular strength, even zero: there must always be the possibility of something a little more, the chance of a fluctuation.
This isn’t just abstract physics. As theorist Hendrik Casimir predicted in the late 1940s, quantum fluctuations can exert a measurable force. Put two metal plates extremely close together in a vacuum and the gap restricts the wavelengths of the fluctuating electromagnetic field that can fit between them. Outside the plates, the quantum fluctuations pile up, and the resulting imbalance draws the plates together. Today, engineers battle with this effect whenever they build nano-devices for smartphones, game controllers and other modern technology.
Still, no one has actually seen the quantum vacuum itself. Normally, seeing something – even something transparent – involves it changing the light in some way. Take water in a cup, for instance: light passes through it, reflecting and refracting off the electrons surrounding the water’s atoms. The distinctive optical pattern reaching our eyes is what tells us that the cup contains water, rather than nothing.
But when light passes through a vacuum, we don’t see a response from its fields at all, even if they are rife with fluctuations. The fields are perfectly invisible – they might as well not be there.
Except, maybe we haven’t been looking hard enough. In the mid-1930s, when QED was still in its infancy, the theorists Werner Heisenberg and Hans Euler showed that if light is strong enough, it should begin to strain the vacuum fields, such that they affect the passage of any more light passing through – an effect known as .
“One must attribute certain properties to empty space itself,” remarked theorist Victor Weisskopf, who furthered the analysis a year later. If they were right, the vacuum fields ought to be visible, after all – almost like some incredibly rarefied crystal.
Trouble is, no one had light bright enough to explore this uncharted realm of QED. In those days, the brightest light sources were arc lamps, commonly found in black-and-white cinema projectors, operating at a few kilowatts. But technology moved on. Once lasers arrived in the 1960s, maximum intensities ramped up to megawatts, then gigawatts and beyond.
More recently, “chirped pulse amplification” – recognised by the 2018 Nobel prize in physics – has opened the gateway to petawatt lasers, powerful enough to vaporise anything placed in their focus. Meanwhile, advances in particle accelerators have led to the creation of gargantuan lasers that emit X-rays, rather than visible or infrared light.
None of these instruments was invented to probe the quantum vacuum. High-power optical lasers were developed mainly to study how light interacts with matter, and to develop new methods of nuclear fusion. The scientists behind X-ray lasers mostly wanted to obtain the molecular structures of important proteins, and to watch chemical reactions in real time. Still, “you get a new hammer, and you look for a new nail to hit with it”, says , a theorist at the University of Plymouth, UK.
Two very powerful lasers
Two years ago, Heinzl was one of more than 50 authors who published a to use one of the world’s most powerful X-ray lasers, the European X-Ray Free-Electron Laser (XFEL) in Germany, to test Heisenberg and Euler’s predictions of vacuum birefringence.
The idea, which had been brewing for , involved crossing the XFEL’s pulsed X-ray beam with that from a separate, 300-terawatt optical laser. The optical laser would strain the electron field, such that when the X-rays passed through, their orientation, or polarisation, would be very slightly rotated: the signature of the vacuum itself. “This is QED in an uncharted regime,” says Heinzl. “No one has tested it before.”
On the face of it, this could sound like juvenile antics – firing two obscenely powerful lasers at each other to see what happens. But the predicted effect is anything but dramatic. A single pulse of the XFEL, which lasts around a hundred-trillionth of a second, contains 100 billion X-ray photons. at the Helmholtz Institute Jena in Germany suggested that, out of all those photons, would have its polarisation flipped by the vacuum. No detector on Earth could hope to pan for that kind of signal. A trick was needed.
The potential answer came from several members of the experiment’s research team, including Cowan. Known as a dark-field technique, it involves placing a small, circular object in the middle of the X-ray beam, just after it passes the light-strained region. Most of the X-rays are blanked out, but not those that have been affected by the vacuum: in the process of having their polarisations flipped, they are simultaneously diverted into a dark halo surrounding the beam, where a detector can spot them more easily.
“It’s like looking at the sun during an eclipse,” says Cowan. “Thanks to the moon’s shadow, you can see the sun’s corona because it’s no longer overwhelmed.”
Last year, the team conducted a proof-of-principle experiment at the XFEL’s Helmholtz International Beamline for Extreme Fields (HIBEF) research facility to test the dark-field concept, and found that it did . “Our goal wasn’t to do the measurement in that run,” says , who leads research on matter under extreme conditions at XFEL. “Really we just learned what we have to do.” The researchers hope to attempt their first proper observation next year.
If they are successful, they won’t actually be the first to claim to have detected vacuum birefringence. Ten years ago, astrophysicists using the European Southern Observatory’s Very Large Telescope (VLT) in Chile detected polarisation in the faint light coming from a neutron star 400 light years away. Neutron stars are the dense remnants of massive stars with extreme magnetic fields.
The researchers argued that if their star’s light had started off polarised, the magnetic fields ought to have scrambled it – so the fact that there was polarisation meant it could only have been generated by the surrounding vacuum. “The [results] that we measured with the VLT can’t be easily explained by our models unless the vacuum birefringence effects predicted by QED are included,” said at the National Institute for Astrophysics in Milan, Italy, at the time.
But almost immediately after the team’s claim was published, independent theorists , saying that the models depended too heavily on assumptions about the neutron star and its magnetic field. Earlier this year, results from NASA’s Imaging X-ray Polarimetry Explorer for vacuum birefringence, but the analysis still depends on various model assumptions about neutron star behaviour.
By contrast, the polarisations of the X-ray photons at the HIBEF experiment will be clearly known, before and after. If any are detected in a different polarisation, there will be little debate that it was the vacuum that flipped them.
Of course, this depends on the QED predictions being correct – which, to be fair, they always have been. No other theory has ever proved so successful as QED in the history of science. Famously, one particle property, the magnetic strength of the electron, can be correctly predicted by QED to a relative accuracy of around 1 part in 1 trillion. “If you talk to a particle physicist and say you want to test QED, they’ll start yawning,” says Heinzl. “They’ll say that’s already been done decades ago.” In this view, a successful observation of vacuum birefringence would just be the icing on the cake.
But, as Heinzl points out, past predictions of QED have all involved relatively weak light fields, and the addition of numerous corrections or “perturbations” to get the right answer, while avoiding any infinities cropping up in the calculations.
For strong light fields, this type of maths almost always doesn’t work – the calculations sum to infinity, whatever you do. The Heisenberg-Euler prediction of vacuum birefringence is a rare, lucky case in which a strong-field QED effect can be calculated without resorting to perturbations, and without any infinities. It is quite possible that this strong-field regime of QED fails to live up to the success of what has been tested before, and that some other, yet-unknown theory governs strong-field behaviour. “That’s why we get excited,” says Heinzl.
Uncharted territory
In such unexplored terrain, one new thing that could show up is the signature of a new particle related to dark matter. In the universe as a whole, dark matter is believed to account for 85 per cent of all matter, yet no one knows what it is. One of the leading candidates is a hypothetical, electrically neutral particle known as an axion, which barely interacts with ordinary matter. Current experiments involve looking for photons that might have been generated by axions in the presence of strong magnetic fields. But so far, no luck.
As these searches continue, theorists have begun to consider broader possibilities of axion-like particles – even a rich “dark sector” of various new particles and fields interacting with one another. In this case, an axion-like particle could be too short-lived to be detected by existing experiments, but could make its fleeting appearance known in a stronger-than-expected birefringence effect at HIBEF.
It’s a long shot, says theorist at the DESY particle accelerator lab in Germany, but a unique window nevertheless. “I think the experiment should be done,” he says. “It’s very well motivated. QED in this regime has never been tested in the laboratory before.”
There is even the question of whether QED itself is the whole story of how strong light affects the vacuum. Since the 1930s, physicists have explored other theories of electromagnetism, in which empty space responds differently. Some predict vacuum birefringence much like QED does, but not all, at least not to the same extent. HIBEF itself won’t be able to distinguish between these theories, but it will crack open the window to a new experimental testing ground that could potentially be explored more in future experiments.
As laser powers increase, physicists are looking not only to see the vacuum, but to force its quantum fields to erupt into real particles and antiparticles – the Schwinger effect, named after QED theorist Julian Schwinger, who described it (see “The vacuum volcano”, below). “The wonderful thing about physics is that, as you probe new experimental regimes, there’s always the possibility of surprises,” says at the University of Cambridge.
In truth, though, any deviation from strict QED predictions wouldn’t immediately be cause to throw away the theory. For over 30 years, the PVLAS (“polarisation of the vacuum with a laser”) experiment at the National Institute of Nuclear Physics in Ferrara, Italy, has been trying – and failing – to pick out vacuum birefringence from background noise by reflecting a dim laser beam back and forth though a magnetic field hundreds of thousands of times. In 2006, its team , which some theorists took as evidence for axions.
However, two years later, the team rebuilt the experiment, including replacing the laser, and . Likewise, any signal measured by HIBEF will need to be scrutinised for experimental artefacts.
And even with the dark-field technique and lasers, seeing the vacuum will be difficult. According to calculations by Karbstein, they can expect the vacuum to flip just two photons per hour. They can’t simply let the experiment run forever, as XFEL beamtime is sparingly distributed – most experiments are given just half an hour or so. In a limited time window, success isn’t guaranteed.
“It’s all about controlling background noise,” says Heinzl. “We might get lucky, we might not.” Yet, if just a handful of photons emerge from the blinding laser collision with their polarisation altered, then, for the first time, physicists will have seen empty space itself.
The vacuum volcano
With sufficiently powerful lasers, the vacuum should become visible, twisting the paths of passing photons (see main story). But push the vacuum even harder and it should produce particles of its own – an eruption of electrons and their antimatter partners, positrons. The phenomenon was first predicted by theorist Julian Schwinger in 1951, and requires immensely powerful laser fields – above 1 quintillion (10¹⁸) volts per metre.
Researchers at facilities such as the Extreme Light Infrastructure in the Czech Republic, Hungary and Romania are hoping they might still break the Schwinger limit by crossing several beams at one spot, or reflecting a laser from a plasma “mirror” travelling at near the speed of light. So far, though, these all remain blueprints.
