Space-time is the fabric of the universe, perhaps of reality itself. But what is it? (Image: Sam Chivers)
Strange signals picked up from black holes and distant supernovae suggest there’s more to space-time than Einstein believed
WE LIVE in an invisible landscape: a landscape that, although we cannot perceive it directly, determines everything that we see and do. Every object there is, from a planet orbiting the sun to a rocket coasting to the moon or a pencil dropped carelessly on the floor, follows its imperceptible contours. We battle against them each time we labour up a hill or staircase.
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This is the landscape of space-time: the underlying fabric of the physical universe, perhaps of reality itself. Although we don’t see its ups and downs, we feel them as the force we call gravity. Developed by the physicist Hermann Minkowski in the 20th century, and used by Albert Einstein in his general theory of relativity, space-time has become one of the most powerful concepts in all of physics.
“Space-time is the fabric of the universe, perhaps of reality itself. But no one knows what it is”
There is just one nagging problem: no one knows what it is. Einstein envisaged space-time as a perfectly smooth surface warped by the mass of stars, planets and galaxies to produce gravity. Now signals from a variety of celestial objects are hinting at something different. If the observations are confirmed – and they are controversial – they suggest that the landscape of reality is altogether more rugged than Einstein thought. That would mean his isn’t the last word on space-time, or gravity – and change our perception of the universe fundamentally.
Read more: “Sorry, Albert: Physics that challenges Einstein“
Before Einstein, space and time were thought to be separate properties of the universe. For Isaac Newton, they were a rigid framework of creation, and perhaps even some sort of embodiment of God – a “sensorium” through which He viewed the world – with gravity and movement the Almighty’s will made manifest. For many, this strayed too far into the realms of maverick theology, and Newton’s religious interpretations were soon sidelined. But few questioned the underlying science.
Only in the mid 19th century did it become clear that Newton’s dynamics couldn’t explain the subtleties of Mercury’s orbital motion around the sun. Einstein’s relativity could, but only by melding space and time into one mathematically indistinguishable whole, in which what happened to one also affected the other: the space-time continuum.
But although the mathematics of relativity describes space-time’s properties very well, it is silent on its underlying nature. We are left to scratch around for observational clues. Everything in the universe, from the largest galaxy to the smallest particle, the dullest radio wave to the brightest ray of light, is immersed in space-time and so presumably must interact with it in some way. The question becomes whether those interactions imprint any signature that we might measure and interpret, and so see the true physical guise of space-time. “This is a beautiful question, and we are at the beginning of answering it,” says Giovanni Amelino-Camelia of the University of Rome La Sapienza in Italy.
In 2005, we seemed to have glimpsed an answer. – is a series of giant receivers on La Palma in Spain’s Canary Islands tuned to detect cosmic light of the highest energy: gamma rays. On the night of 30 June, the array detected a burst of gamma radiation from a giant black hole at the heart of Markarian 501, a galaxy some 500 million light years away. This wasn’t so unexpected. Our theories predict that every time something falls into such a black hole, a flare of radiation will be given off. But those large enough to be caught by an earthbound telescope, even a mighty receiver like MAGIC, are few and far between, and the Markarian flare was pretty much the first of its type to be seen.
Quantum foam
And detailed analysis revealed something decidedly unusual about the burst: the lower energy radiation seemed to have arrived up to 4 minutes before the higher energy radiation. This is a big no-no if space-time behaves according to Einstein’s relativity. In relativity’s smooth space-time, all light travels at the same speed regardless of its energy. But the effect was entirely compatible with other, rival theories that attempt to characterise space-time in terms of quantum mechanics – the theory entirely separate to, and incompatible with, general relativity that explains how everything besides gravity works.
In quantum theory, nothing is static or certain. Particles and energy can fluctuate and pop in and out of existence on the briefest of time scales. Many theories of quantum gravity – the yearned-for “theories of everything” that will unify our descriptions of space-time and gravity with quantum mechanics – suggest something similar is true of space-time: instead of a smooth continuum, it is a turbulent quantum foam with no clearly defined surface. Einstein’s undulating landscape becomes more like a choppy seascape through which particles and radiation must fight their way. Lower-energy light with its longer wavelengths would be akin to an ocean liner, gliding through the foamy quantum sea largely undisturbed. Light of higher energy and shorter wavelengths, on the other hand, would be more like a small dinghy battling through the waves.
In 1998, Amelino-Camelia and , then at CERN near Geneva, Switzerland, had proposed that high-energy light from distant, active galaxies could be used to check for this effect. The huge distance would allow for even subtle effects to build into a detectable time lag. On the face of it, this was exactly what MAGIC had seen.
Things are seldom that simple in physics, and the MAGIC observations have generated lively discussion. “This has become quite a musical,” says of the Max Planck Institute for Physics in Munich, Germany, part of the team that made the initial observation. When a similar gamma-ray telescope, based in the Namibian outback – caught sight of another giant galactic flare in July 2006, it was the perfect opportunity to test the theory. The galaxy in question, PKS 2155-304, is four times as far away from Earth as Markarian 501, so the time delay should have been even bigger.
But… nothing. “We saw no hint of a time delay,” says Agnieszka Jacholkowska of Pierre and Marie Curie University in Paris, France, . If we assume that space-time, whatever it may be, is probably the same everywhere, this suggests that the original time delay was something intrinsic to the source of the gamma rays in Markarian 501. It is conceivable, for example, that particles were accelerated along magnetic fields near the centre of the galaxy, which would naturally result in the emission of lower-energy gamma rays first. But since no one quite knows what processes take place in these dark galactic hearts, there was still plenty of room for debate.
And so things remained until last year, when the most energetic gamma rays ever seen in our short history of observations hit Earth.
It was a gamma-ray burst (GRB): a short, intense flash of radiation not from the heart of an active galaxy but from the explosive death of a hypergiant star. GRBs are so bright that modern telescopes can see them across the entire universe, meaning that their light has travelled through space-time for several billions of years.
Even so, the one observed by on 27 April 2013 – known prosaically as GRB130427A – was eye-popping. It showered Earth with 10 times as many high-energy gamma rays as a run-of-the-mill burst, and included one gamma-ray photon that carried 35 billion times more energy than a visible photon. Automatic alerts were sent out to observatories across the world and within hours a battery of telescopes was scrutinising the burst’s aftermath. One of the scientists alerted was Amelino-Camelia.
“In April last year, Earth was hit by the most eye-poppingly powerful flash of gamma rays ever observed”
In May, he and his colleagues circulated a paper claiming to see a time lag of hundreds of seconds between the burst’s lower- and higher-energy gamma rays (). “The numbers work out remarkably well. This is the first time there is robust evidence of this feature,” says Amelino-Camelia.
Robust, because unlike the Markarian 501 observations, it was possible to match the arrival times of photons of various energies with those predicted by a simple equation. This relationship is pleasing to the mathematical eye and might also help us to see what lies beyond relativity if it is indeed broken: different variants of quantum gravity sketch different pictures of space-time and might have different effects on light.
In string theory, for instance, quantum space-time is a tangle of six extra dimensions of space, in addition to the usual three of space and one of time. Photons of different energies will propagate through this arrangement in quite a different way than is predicted in loop quantum gravity, another popular theory that imagines space-time as a form of chain mail composed of interwoven loops.
For the time being, Amelino-Camelia has banned his team from investigating which, if any, of these competing theories is closest to the mark. “For the moment, I think it’s important to keep separate how we wish nature was in theory and how nature really is from the facts we have,” he says.
Instead, the next stage is to see what predictions the time-delay equation makes about time lags in bursts of radiation from other sources. In their paper, Amelino-Camelia and his team report four other GRBs whose behaviour was consistent with the equation, although not conclusively in support.
Others find no such evidence. Just days after Amelino-Camelia’s paper came out, Jacholkowska and her colleagues published their analysis of four other, less energetic GRBs observed by the Fermi telescope. They found no hint of time lags ().
In Jacholkowska’s view we cannot draw any firm conclusions, because Amelino-Camelia’s interpretation assumes, like the Markarian 501 analysis before it, that the gamma rays were emitted simultaneously regardless of their energy. This is always going to be a problem as long as interpretations are based on single observations of one type of source, Ellis says. “If you found an effect that was similar in two, you’d really begin to think you had found something,” he says.
One test that might clear things up involves neutrinos. These ghostly particles travel at virtually the speed of light, interacting with hardly anything. Because they carry energy, however, they should interact with space-time, and, if Amelino-Camelia is correct, suffer an energy-dependent time lag – although one that is only measurable if we can find neutrinos that have travelled far enough.
That was always a problem. Nuclear fusion reactions make the sun such a prodigious neutrino factory that it washes out almost all signals from further away. Besides solar neutrinos, the only cosmic neutrinos ever seen have been from the supernova SN1987A – a star that just happened to explode in our cosmic backyard, in the Large Magellanic Cloud some 170,000 light years away. This is still too close for its neutrinos to manifest any measurable time lag.
Decisive help could now be at hand. is a neutrino detector buried in a cubic kilometre of Antarctic ice that came fully on stream in 2011. In April 2012, it found two neutrinos that set scientific tongues wagging. Called in a fit of scientific whimsy Bert and Ernie, after two characters from the TV show Sesame Street, they were far more energetic than those generated by the sun. For that reason alone, of Fermilab in Batavia, Illinois, thinks it’s likely that they come from a gamma-ray burst. “There aren’t that many things that can make that amount of energy in a single particle. GRBs top the list,” he says. Just recently, IceCube announced the discovery of a further 26 neutrinos whose energies possibly betrayed an extragalactic source.
“The two neutrinos, nicknamed Bert and Ernie, were far more energetic than those from the sun”
Amelino-Camelia thinks he has found three more in earlier IceCube data – ones that perfectly fit the idea of quantum space-time effects taking place. They all arrived from the general direction of three independently verified GRBs – but, if they are indeed associated with the bursts, got to Earth thousands of seconds earlier than the gamma rays.
Neutrinos are expected to escape from a collapsing star sooner than the light of a GRB because they don’t interact, whereas the visible blast has to fight its way through the collapsing gas before speeding through space. But even taking this into account, Amelino-Camelia maintains that the huge size of the gap between the neutrinos and gamma-ray light is consistent with the different effects of a space-time interaction on them.
Ellis remains sceptical. “Every once in a while, somebody gets a little bit excited but I don’t think there’s any statistically solid evidence yet,” he says. “One of the problems is that extraordinary claims require extraordinary proof, so you have to do something that is really convincing.”
That will inevitably require larger telescopes capable of spotting more gamma rays and neutrinos more quickly. Wagner is involved in an international collaboration of more than 1000 researchers from 23 countries that is aiming to build a giant successor to MAGIC and HESS. The would be 10 times as sensitive, and capable of seeing between 10 and 20 active galaxy flare-ups every year. After three years developing technology and looking at possible locations, with funding so far mainly from the governments of Germany, Spain and the UK, the collaboration will now be looking for the €200 million needed to turn the telescope into a reality.
Will it finally open our eyes to the landscape around us? Those involved hope so. “There is no reason to be pessimistic,” says Wagner. To find any kind of structure in space-time would be a revolution to rival Einstein’s, and could show the way forward when physics is struggling to see its next step. “It would be hard to overstate how important that would be,” says Hooper.
This article appeared in print under the headline “Warning light”
