With its fiery rains, speedy magnetic flips and an atmosphere that defies the laws of physics, our home star is as weird as it gets, says Rebecca Boyle.
Billions and billions of stars fill our galaxy. Many burn bright, destined to become supernovae, while others are dim burnouts. They come alone and in pairs; with or without planetary companions. We have searched the far reaches of the universe in the hope of understanding the stars, but ultimately everything we know is based on our sole reference point, the sun. Yet our home star remains plenty mysterious.
‘It’s expected that it’s understood, because it’s right there, it’s so close and dominant in the sky,’ says astrophysicist Eamon Scullion from Trinity College, Dublin. ‘How are we going to understand any other aspect of space if we can’t get to grips with the nearest star?’ While we may have to go back to square one, there are things we do know about our sun. It is made of plasma – gas that has been ionised, or highly charged. It fuses hydrogen in its core. It blasts us with radiation and, crucially, its life-giving light. As stars go, it is roughly middle-aged, having been around for 4.6 billion years. And it probably has 5 or so billion more to go before it swells into a red giant that consumes Mercury, Venus and Earth. Yet strange solar phenomena abound, and here are some of the strangest.
We know the sun affects weather on Earth and in space, but it has its own dramatic weather phenomena, too. ‘People have this image of a giant ball of gas that’s on fire, and everything is streaming away from it at thousands of kilometres per second,’ says Scullion. In fact, the sun’s plasma can fall back to the surface as rain.
Though this so-called coronal rain was predicted about 40 years ago, we couldn’t see or study it until our telescopes became powerful enough to spot it happening. It works a bit like the water cycle on Earth – where vapour warms, rises, forms clouds, cools enough to condense into a liquid and falls back to the ground as precipitation. The big difference is that the plasma doesn’t change from gas to liquid, it simply cools enough to fall back down to the solar surface.
This all happens very quickly and on a gargantuan scale, with ‘droplets’ the size of countries plunging from heights of 63,000 kilometres – about one-sixth the distance from Earth to the moon. ‘You basically generate something the size of Ireland in 10 minutes, and drop it out of the sky at a rate of 200,000 kilometres an hour,’ Scullion says.
Solar tornadoes also form in a familiar fashion. Swirling solar plasma creates a vortex, which causes magnetic fields to twist and spiral around into a super-tornado that reaches from the surface into the upper atmosphere. Here they transfer energy and help to heat it, or so scientists believe.
The sun may be on its lonesome now – its closest neighbour is 4.2 light years away – but that wasn’t always the case. Once upon a time it had close family. After their birth in the same cloud of dust and gas that formed our solar system, these solar siblings scattered hundreds of light years apart in the Milky Way. In May 2014, astronomers reported the first one: a star called HD 162826.
‘It looks like the sun, but a little bit bluer,’ says Ivan Ramirez at the University of Texas at Austin, who led the study. It’s also warmer than the sun and 15 per cent more massive. The star is about 110 light years away, and you can see it with the aid of a pair of binoculars in the left arm of the constellation Hercules.
To find its family ties, Ramirez’s team combed through galactic archaeology studies, which model the motions of the Milky Way. These predictions laid out where sibling stars would be now if they had formed in the same place as the sun. Though they spread out in different directions, their positions still give away their birthplace, Ramirez says.
He narrowed down the search area to 30 stars, and then looked at them closely to find a family resemblance. Only HD 162826 had a similar chemical make-up to the sun. A separate team led by Eric Mamajek at the University of Rochester in New York also studied the star and found it is the same age as the sun, as would be expected for two stars born together. Even more tantalising, HD 162826 is already in a catalogue of stars that might harbour planets.
More than a dozen solar siblings have been identified since then. Locating them could tell astronomers more about the birth of our solar system, including what conditions were like when the sun and planets formed. But beyond scientific curiosity, Ramirez just wanted to find a member of the sun’s nuclear family. ‘It’s a cool thing to do,’ he says.
He plans to keep looking for more of our sun’s lost littermates. Most are probably red dwarf stars, which are the most common stars in the galaxy. They are smaller and cooler than the sun, so they are much harder to find. But the Gaia telescope, launched in 2013, may help locate more solar siblings, as it will observe a billion stars to make the first 3D map of the Milky Way.
Our planet’s calendar is well known: it takes 24 hours to spin once on its axis – a day – and 365 days to travel around the sun – a year. Yet the sun’s schedule is nothing like ours. Different parts of the sun spin at different rates. So while a day at the equator lasts 25 days, regions close to the poles take a few days longer to make a complete rotation. This uneven spin leads to distortion in the sun’s magnetic field, which has knock-on effects. As the equator spins, it drags the magnetic field that connects the sun’s poles, says Alex Young at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. This results in another strange calendar phenomenon: solar maxima and minima.
As the sun’s magnetic field gets wound up by the spin ‘it starts to build tension and pressure, much like when you twist a rubber band and it knots up’, Young says. Something has to give, so the magnetic fields snap and release energy in the form of heat, either as solar flares or furious clouds of energy called coronal mass ejections (CMEs).
This cycle, from magnetic twisting to energy releasing, happens over roughly 11 Earth years – giving the sun its own calendar. During what’s called a solar minimum, flares are few and so are dark patches called sunspots that appear on the sun’s surface due to intense magnetic fields.
In solar maxima, more sunspots burst over the surface where they spew more flares and CMEs. Torrents of charged particles also stream through gaps in the sun’s atmosphere and across the entire solar system. This can affect us, causing blackouts on Earth and damaging satellites. But each solar cycle varies, and we don’t understand why, which makes them and their effects unpredictable.
The current cycle is unusually calm and has been one of the weakest since records began in 1755. This is in spite of some major solar storms, together with a colossal solar flare in 2012, which would have packed some punch had it hit Earth.
Predictions just a couple of years earlier suggested the cycle would be a scorcher, which shows just how little we understand solar cycles, says Todd Hoeksema, a solar physicist at Stanford University in California. ‘It’s like predicting the stock market. Past performance is no guarantee,’ he says.
Also roughly every eleven years, the sun undergoes yet another calendar change: its magnetic field reverses. North becomes south, and vice versa. Earth does this, too, but only every 300,000 years or so (we are long overdue one). The sun’s polarity last reversed in 2013, though the flip took scientists many months of analysis to confirm.
‘Why is it 11 and 22 years and not 15 and 30? We don’t know the answer to that yet,’ Young says. ‘When you think about it, it’s such a short amount of time, given that the sun has been around for 4.6 billion years.’
As the sun follows its 11-year solar cycle, it changes, altering its output of solar wind, X-rays, ultraviolet and visible light. This has the knock-on effect of changing the size of the huge magnetic bubble of charged particles, called the heliosphere, that the sun blows around itself to way out beyond Pluto.
These changes affect everything from Earth’s climate to the Voyager 1 spacecraft, which finally entered interstellar space in 2013.
The sun provides nearly all the energy that drives Earth’s climate – 2500 times as much as all other sources combined, according to Greg Kopp, a solar physicist at the University of Colorado’s Laboratory for Atmospheric and Space Physics. In past epochs, solar cycles were partly responsible for warm periods and mini ice ages. Low solar activity drives cold winters in northern Europe and the US, and mild winters over southern Europe – although global warming means globally averaged temperatures are on the rise.
We now understand what’s going on a little better thanks to a space-borne instrument called TIM, launched by NASA in 2003. TIM keeps tabs on the spectrum of energy the sun emits, and detects subtle changes in energy output so scientists can distinguish between human causes of climate change and purely natural causes we can’t control.
Changes in the sun’s output affect much more than just our climate, however. During a solar minimum, the solar wind streams from the poles at a much faster speed, so there’s more pressure pushing against material from interstellar space. During solar maxima, the sun’s magnetic fields are more knotted up and not as much wind escapes, so the heliosphere contracts. ‘There’s sort of an 11-year breathing,’ says Hoeksema.
The solar wind has been 20 to 40 per cent weaker than expected this cycle, he says. This shallower breath is one reason why Voyager 1 left the heliosphere earlier than scientists expected.
Solar tornadoes are bizarre enough on their own, but they might help explain one of the sun’s weirdest characteristics: its atmosphere is hotter than its surface. At 5700 kelvin, the sun’s surface is scarcely cold, but it is frigid compared to the corona. The highest part of the sun’s atmosphere, more than 1 million kilometres above the surface, can reach temperatures of several million kelvin.
Generally, an object cools as it moves away from a heat source; a marshmallow will toast faster when it’s closer to a campfire flame than further away. But the sun’s atmosphere does the opposite. Energy must be flowing into the corona, heating it up – but no one knows where this energy comes from. ‘We don’t fully understand the physics of what’s going on,’ Scullion says.
Computer visualisations might paint a clearer picture of this process – and quite artistically, too. In one simulation, NASA Goddard astrophysicist Nicholeen Viall added colour to data coming in from NASA’s Solar Dynamics Observatory (SDO), which observed the sun’s coronal plasma in 10 different wavelengths that each correspond to a temperature. The result is a swirling movie reminiscent of a Van Gogh painting. But Viall’s visualisation suggested the atmospheric plasma was cooling, not heating. This may be because the heating is happening faster than SDO can detect.
Much of the energy that heats the corona appears to come from the so-called transition region – the area between the sun’s corona and the next atmospheric layer down. Tornadoes, rain, magnetic braids, plasma jets and strange phenomena called ‘spicules’ are all thought to play a role in this heating process, bringing energy from the lower regions of the sun and depositing it higher up. But no one knows exactly how. NASA’s Interface Region Imaging Spectrograph mission has been observing this region since 2013, and physicists like Scullion try to simulate these energy exchanges using models in the hope that they will yield clues that scientists can look for on the real thing.
To truly understand all these solar conundrums, we need to get as close to the sun as possible. That’s not as simple as flying straight there, as the operators of two new spacecraft that will fly closer to the sun than ever before are finding.
Solar Orbiter is a European Space Agency mission launching in 2018, aiming to fly within 45 million kilometres of the sun. It will photograph the sun’s poles for the first time, which should help scientists understand how the sun generates its magnetic field, and may even give insights into why its magnetic polarity flips so frequently. By getting a close-up view, the probe will also be able to sniff the pristine solar wind, before it has reached Earth. The main goal is understanding how the sun interacts with the environment around it, says Tim Horbury, a physicist at Imperial College London and the principal investigator on the Solar Orbiter’s magnetometer. ‘The basic physics is understood, but a lot of the detail is not,’ he says.
NASA’s Parker Solar Probe mission is set to launch shortly before ESA’s mission and come even closer, just 6 million kilometres from the sun’s surface. To get there, it will approach in a looping, circuitous route, like a matador approaching a wary bull. The slow approach is partly for safety’s sake: as the probe gets closer, scientists can carefully monitor any threats from radiation or heat and adjust the approach if anything goes awry.
The Parker Solar Probe will lap around Venus seven times to put it on the right trajectory and also to build up speed and momentum to slingshot closer to the sun – at its closest approach, it’ll zip past the sun at 200 kilometres per second.
Shielding a spacecraft from solar radiation is one of the most important jobs in space flight, but it’s even harder when you are sidling up to the source. The technology to do it hasn’t existed until now, Horbury says. Both craft will have beefy heat shields to protect their sensitive instruments from searing temperatures.
Both spacecraft will try to answer questions, including how the atmosphere is heated and how the sun generates its wind. But they will still be far from answering everything there is to know about our star, says Young. ‘The problem is that you don’t know what you don’t know,’ he says.
Black holes suck – but do they have mirror twins that blow? A far-flung space telescope is peering into galactic nuclei to spot one for the first time and may offer a gateway to parallel universes, writes Katia Moskvitch.
Physics is full of opposites. For every action, there’s a reaction; every positive charge has a negative; every magnetic north pole has a south pole. Matter’s opposite number is antimatter. And for black holes, meet white holes.
Black holes are notorious objects that suck in everything around them. Famously, not even light can escape their awesome gravity. White holes, in contrast, blow out a constant stream of matter and light – so much so that nothing can enter them. So why have so few people heard of them?
One reason is that white holes are exotic creatures whose existence is speculated by theorists, but believed by few because no one has ever seen one. Now Nikolai Kardashev and his colleagues at the Astro Space Centre in Moscow are hoping to change that using a vast radio telescope with a view equivalent to that of a dish about 30 times wider than Earth. They are aiming to identify what lies at the heart of many galaxies. If they confirm the existence of white holes, they will cast into doubt our current notion of what lies at the centre of galaxies – including our own. It would also be vindication at last for physicist Igor Novikov, who was the first to theorise their existence in 1964. Back then, black holes were called frozen stars, and were even less well understood than they are today. Novikov did what theoretical physicists do when confronted with situations that are impossible to test in the laboratory: he used pure reasoning to ask what would happen to a black hole if time were to flow backwards. His thought experiment yielded a new kind of object that spewed matter and light continually: a white hole.
Others ran with the idea. What if a black hole was attached to a wormhole, a shortcut through space–time that connects two regions of our universe, or maybe even two different universes? The black hole would draw in matter, while at the other end of the wormhole there would be a white hole emitting it.
Many physicists, though, have found the notion of a white hole hard to swallow. After all, black holes are thought to form when a massive star collapses under its own gravity; the collapsing matter results in a singularity at its core. This is the heart of a black hole, where all physical quantities diverge to infinity and all the known laws of physics break down.
But in the time-reversed version, ‘a white hole existed in the past, and somehow exploded outward’, says Novikov. Even he concedes the fundamental problem: ‘Researchers accepted that, from a mathematical and theoretical standpoint, white holes could exist. But there were questions about how such an object could actually form.’
Wormholes offered a way, but there were theoretical problems with them, too. They seemed to collapse as soon as they formed, shutting down the white hole too. Novikov himself outlined this instability problem in the 1970s. A decade later, however, theoretical physicist Kip Thorne of the California Institute of Technology showed that wormholes could indeed be stable, which gave the white hole theory a boost. In 2014, Carlo Rovelli and Hal Haggard at Aix-Marseille University in France showed that quantum theory can transform a collapsing black hole into an expanding white hole.
Perhaps the fact that we have found no signs of a white hole, despite peering ever deeper into space, is a more fundamental problem. Enter a space telescope called RadioAstron, whose wildly elongated orbit takes it out to a distance of 350,000 kilometres – nearly as far as the moon and 30 times wider than Earth’s diameter. Launched from Kazakhstan’s Baikonur Cosmodrome in 2011, its dish is only 10 metres across. But when its signals are combined with those from radio telescopes on Earth, the resulting images are as sharp as those from a dish 350,000 kilometres wide.
Right now, RadioAstron’s resolution is 20 times better than that of any telescope on the ground. It is so good that it can pick out objects covering an angle of 27 microarcseconds – the size a snooker ball would appear on the moon as viewed from Earth. Kardashev and his colleagues have used RadioAstron to survey 100 active galactic nuclei, the compact regions at the centre of galaxies that are much brighter than expected. Many astronomers think that these owe their brilliance to supermassive black holes. As the black hole sucks in gas, the unlucky matter is sent swirling around and gets hot enough to sparkle before plunging into oblivion.
But could some of these dazzling displays instead be due to matter and light streaming out of a supermassive white hole? Novikov thinks so: ‘Certain active galactic nuclei are not black holes, as most researchers suggest, but exist in the form of white holes, linking our universe to another universe.’
If RadioAstron can make a detailed enough image, then it should be easy to tell black holes and white holes apart, says Kardashev. ‘If it’s a black hole, then in the middle there should be a dark spot on the image,’ he says. ‘And if it’s a white hole, then there should be a bright spot in the centre.’
But perhaps the reason we haven’t seen a white hole is that we’ve been looking in the wrong place at the wrong time. Alon Retter, an astrophysicist who now works for Israel Aerospace Industries in Tel Aviv, thinks so. What’s more, he believes that we may already have caught one flickering into existence.
In 2006, NASA’s Swift satellite detected a gamma-ray burst called GRB 060614. Such bursts are usually associated with supernovae or regions of high star formation, but GRB 060614 was neither. Retter believes that it may instead have been a white hole. He argues that white holes will appear as temporary flashes, rather than shining continuously, because all that matter and light coming out will collapse under its own gravity into a black hole.
Kardashev and Novikov agree with Retter’s ideas. ‘The nature of these flashes in the sky is still unclear,’ says Kardashev. ‘So once we spot a gigantic powerful gamma-ray burst with a lot of radio radiation, we will take a close look with RadioAstron and try to determine its shape and size for the first time.’ That could provide important clues about its source. ‘It may be a white hole or a wormhole. Maybe the flashes are coming from another universe.’
Retter calls his idea a ‘small bang’ – a spontaneous emergence of a white hole. If we extrapolate this thought, he says, we could assume that our entire universe is the result of a white hole that emerged as the big bang.
Hardly anyone is hunting for white holes these days, but hopefully that will change. With each passing day, RadioAstron is beaming back more observations of fine structures in active galactic nuclei. ‘It is not theoretically excluded that the central engine in active galactic nuclei is something more interesting than a supermassive black hole,’ says Konstantin Postnov, an astrophysicist at Moscow State University. ‘So let’s keep our eyes open and not discard even very exotic possibilities.’
White holes aren’t the only things breaking the laws of physics. Stuart Clark examines five cosmic impossibilities that just might turn out to be true.
The speed of light in a vacuum is the ultimate cosmic speed limit. Just getting close to it causes problems: the weird distortions of Einstein’s relativity kick in, so time slows down, lengths go up, masses balloon and everything you thought was fixed changes. Only things that have no mass in the first place can reach light speed – photons of light being the classic example. Absolutely nothing can exceed this cosmic max.
We have known about the special nature of light speed since an experiment by US physicists Albert Michelson and Edward Morley in the 1880s. They set two beams of light racing off, one parallel and one at right angles to the direction of Earth’s rotation, assuming the different relative motions would mean the light beams would travel at different speeds – only to find the speed was always the same.
Light’s constant, finite speed is a brake on our ambitions of interstellar colonisation. Our galaxy is 100,000 light years across, and it is more than four years’ light travelling time even to Proxima Centauri, the closest star to the sun and home, possibly, to a habitable planet rather like Earth.
Then again, if the speed of light were infinite, massless particles and the information they carry would move from A to B instantaneously, cause would sit on top of effect and everything would happen at once. The universe would have no history and no future, and time as we understand it would disappear. We wouldn’t like a universe like that.
But don’t put the brakes on just yet. The fact is, a larger light speed would solve one of the biggest problems in cosmology: that the universe’s temperature is more or less the same everywhere, even though there hasn’t been enough time since the big bang for this thermal equalisation to have taken place.
Standard cosmology solves this problem with inflation, a period in the very early universe when space itself suddenly inflated faster than light speed (something Einstein’s relativity does allow), carrying an equalised temperature to far-flung climes. But no one can find a plausible way for space to behave like this. Models of inflation have to be made flexible so they can retroactively fit just about any observation thrown at them.
You could achieve the same effect as inflation, however, if cosmic light speed started out infinite (or at least a lot larger) at the big bang and has been getting slower ever since as space has expanded. Initially, the speed fell precipitously. These days, it creeps downwards imperceptibly, explaining why we measure it as a constant.
That sounds wacky, but in 2016 Niayesh Afshordi at the University of Waterloo, Canada, and João Magueijo of Imperial College London proposed ways to test for a variable light speed in galaxy surveys or in fluctuations of the cosmic microwave background, the leftover radiation from the big bang. ‘The idea that the speed of light could be variable was radical when first proposed, but with a numerical prediction, it becomes something physicists can actually test,’ says Magueijo. ‘If true, it would mean that the laws of nature were not always the same as they are today.’
And we should soon have answers. The HETDEX experiment at the recently upgraded Hobby–Eberly Telescope in Fort Davis, Texas, should soon start to provide data on the distribution of distant galaxies, as could the Dark Energy Spectroscopic Instrument experiment under construction at the Kitt Peak National Observatory in Arizona. Failing that, the next-generation CMB-S4 experiment should scrutinise the microwave background to the required accuracy. This alternative universe might not be too alternative at all.
Imagine a world where, if you and I had once met, my missing the bus to work would automatically make you late too. Or where, if I put on odd socks, yours would be odd too. A great excuse, maybe – but also deeply weird.
The classical world we live in isn’t like that. I do X and Y happens, and what Z is doing over there generally has little influence on that. But these clear relationships disappear when we enter the quantum world, the world of subatomic particles that are the building blocks of the universe – and encounter the phenomenon of entanglement.
Described by Einstein in 1935, this is a kind of particle telepathy that defies complete characterisation even today. Particles can become entangled when they interact, and once they do, no matter how far apart they are, measuring the properties of one automatically fixes the properties of the other – changes its socks, as it were.
Einstein decried this ‘spooky action at a distance’, yet many experiments have shown it is an essential ingredient of our world. ‘Without quantum entanglement, we could not have quantum theory as we know it, and quantum theory is the basis of chemistry, our semiconductor industry, even life,’ says Caslav Brukner of the Institute for Quantum Optics and Quantum Information in Vienna.
But here’s the really weird thing. There’s nothing stopping the quantum world having different levels of underlying correlation – largely uncorrelated worlds are possible within the broad sweep of the theory, as are ones that are far more connected. But only a universe with the exact level of weirdness that corresponds to entanglement produces the rich tapestry of phenomena, including life, that ours does.
So we probably shouldn’t wish for any level of weirdness other than our own – but it would still be nice to know why things are as they are. Finding out how would probably mean deriving quantum theory from underlying principles like the constant speed of light, which is the foundation of Einstein’s relativity. But the sheer universality of quantum theory makes this a far-off prospect, says Brukner. ‘I’m not even sure that this goal can be achieved.’
According to quantum physicist Sandu Popescu of the University of Bristol, we may have to accept that such questions are not physical, but philosophical. ‘We can predict exactly what will happen, but to say why it happens, we don’t have a clue,’ he says. ‘It happens because nature is quantum mechanical – that is probably the best answer you will ever get.’
If there’s one thing that eats up time, it’s working out what time is. It pops up in physical laws all over the place – but never quite as we expect it. In quantum theory, a ‘master clock’ ticks away somewhere in the universe, measuring out all processes. But in Einstein’s relativity, time is distorted by motion and gravity, so clocks don’t necessarily agree on how it is passing – meaning any master clock must, somewhat implausibly, be outside the universe.
Even odder, neither theory seems to place any restriction on time going backwards. The familiar one-way flow of time is expressed in only one area of physics: thermodynamics. If time flowed both ways, sometimes your coffee would warm up while sitting forgotten on your desk. Dropped eggs might spontaneously reassemble and leap from the floor into your hand. The dead might return to life and live backwards to birth, Benjamin Button style.
The culprit is entropy, essentially the thermodynamic measure of a system’s disorder. When the universe was born, matter was randomly distributed throughout its tiny dimensions and it was the same temperature everywhere. Then gravity kicked in, pulling together matter and heating it up to form galaxies, stars, planets and other ordered imperfections. Thermodynamics has been trying to re-establish disorder, increasing entropy every which way it can.
At a local level, entropy increase seems to be associated with information loss. Broken eggs do not reassemble because information about the former ordering is lost to us in the smash. You don’t have all the information needed to put Humpty Dumpty together again – and that amounts to a barrier to travelling back in time.
Or does it? ‘When the story is told like this it appears compelling, but the moment you start looking into more detail, it becomes more convoluted,’ says Popescu. In classical physics, you could in principle reverse a thermodynamic process if you preserved the information by measuring the trajectories and velocities of all the components of a breaking egg – suggesting that we could reverse time.
So why can’t we? One possibility, Popescu thinks, is an information gap intrinsic to the way the quantum world works. Here we are back with the phenomenon of entanglement. When a cup of coffee cools, Popescu believes, continual interactions between molecules of air and coffee increase their quantum entanglement. Although you can know what states an entangled particle pair contains, you can’t definitively know which one has which state – leading to a continuous sapping of information from the world.
It’s still just an idea, Popescu admits. ‘Quantum mechanics is consistent with our macroscopic phenomenon being driven by quantum rules, but we cannot prove it,’ he says. And there is a huge sting in the tail: if he’s right, in some sense, time may be capable of flowing backwards after all.
That’s because in a classical physics calculation, in theory all you need is a system’s initial state and the laws of mechanics to work out what will happen for the rest of time. But in quantum mechanics, where a system’s evolution is probabilistic, you can specify conditions for the initial state and final states of the system, and both of these conditions will influence the evolution. Apply this idea to the universe as a whole and ‘information could be coming from plus infinity and propagating back through time’, says Popescu.
There’s no evidence of any of this so far, Popescu cheerfully admits. ‘No one yet has investigated it seriously,’ he says. ‘It is speculative.’ But if in the future physics shows that time really can travel backwards, well – in some sense we must already know.
We’re accustomed to living in a three-dimensional universe. Well, four dimensions – time is a dimension too, albeit an oddly unidirectional one. But we’ve long thought there might be more large-scale spatial dimensions than the up-down, left-right, in-out we are all used to.
In the late nineteenth century, British mathematician Charles Howard Hinton suggested that what we perceive as different objects moving in relation to one another could be thought of as single, solid objects in a four-dimensional space passing through our three-dimensional universe. To get a sense of what that means, imagine what a spherical ball looks like observed as it passes through a two-dimensional sheet – as a circle whose radius expands and then contracts in time.
Adding extra dimensions to the universe is easy enough, on paper at least: you just need additional terms in your coordinate system. The question becomes how we perceive them. Einstein slipped in an additional space-like dimension to his equations of general relativity to explain how mass warps space–time. We don’t perceive this dimension directly, but experience it as an acceleration and explain it as the force of gravity.
Some physicists are adamant that more physical dimensions must exist beyond those we can see. In string theory – still most physicists’ chosen route to a unifying theory that combines gravity and the forces of the quantum world – the number of spatial dimensions is at least 10. This gives physicists enough wiggle room to try to explain all of the forces of nature together – but doesn’t explain where these extra dimensions are.
Extra dimensions have some odd consequences, too – implying, for example, a multiverse of distinct universes next to one another. Not everyone likes that. ‘I’m not a fan of the multiverse picture,’ says physicist Erik Verlinde of the University of Amsterdam. ‘Universes that we cannot communicate with are not that interesting to talk about. I think that we should be happy if we can explain the universe that we live in.’
Verlinde has been developing a quantum description of space and gravity to replace Einstein’s smooth space–time ‘continuum’. In his picture, minuscule building blocks made of quantum information become increasingly quantum entangled and create the seemingly continuous three dimensions of space.
But why three? That question remains open. ‘A lot of these ideas can be implemented in two, three, four or higher dimensions, so I don’t have an immediate reason why there should be three dimensions,’ Verlinde says. And until someone can find one, tales of dimensions beyond those we can see might not be so wacky after all.
Antimatter has always been full of surprises. The first was that it existed. The second was that it didn’t.
First things first. In the 1920s, British physicist Paul Dirac managed to marry quantum theory with Einstein’s special relativity to explain how tiny, fast-moving fundamental particles such as electrons work. But his austerely beautiful unifying equation, honoured with a plaque in London’s Westminster Abbey, had an unwanted consequence. For every matter particle like an electron, it predicted the existence of a second particle that was the same, but opposite in things like electric charge.
Dirac initially brushed this under the carpet – out of ‘pure cowardice’ he later said – but three years on, the antimatter version of the electron, the positron, was discovered in cosmic rays. Since then, as the standard model of particle physics was built on the foundation that Dirac and others laid, a very different problem has emerged. Antimatter shouldn’t just exist, it should be abundant: every time a matter particle is made, an antimatter particle should also be conjured from the void. ‘We should have a universe half full of antimatter,’ says Michael Capell, an astroparticle physicist at the Massachusetts Institute of Technology. So where are these particles?
They can’t be near us because matter and antimatter mutually ‘annihilate’ whenever they meet, and we would notice the flash of X-ray energy produced when they do. Various small-scale particle behaviours might allow there to be slightly more matter than antimatter, but none of these effects is nearly big enough to explain the size of the discrepancy we see.
Perhaps, then, the missing antimatter is elsewhere – in stars and galaxies made exclusively of the stuff, much as our sun and Milky Way are made solely of matter. Stars made of antimatter would give out the same light as ordinary stars, but also a wind of antiparticles, just as our sun gives out matter particles. When these antiparticles come into contact with ordinary matter outside their galaxy, they should produce X-rays that would again be visible across the universe.
We are yet to see anything of that ilk either, but the Alpha Magnetic Spectrometer (AMS) is performing a more direct test. This giant particle detector, lofted onto the International Space Station in 2011, can sort matter from antimatter in passing cosmic rays.
Positrons and antiprotons can be made relatively easily in today’s universe, for example when high-energy particles collide in the strong magnetic fields around dead stars. The real prize would be something bigger. Most helium was made in the first few minutes of the universe’s existence, so to find anti-helium could mean that the same process created the expected large quantity of antimatter. Stars are the only places where carbon and heavier nuclei can be made, so a single anti-carbon nucleus would confirm that there is an antimatter star somewhere in our universe.
It’s like looking for a needle in a haystack – you would expect one complex antiparticle for every billion or so matter particles AMS detects, says Capell, who works on the project. The experiment has just about collected enough events to start saying something meaningful, but it is a race against time. The hunt has to be conducted in space, because antiparticles annihilate on contact with our atmosphere, but space is harsh on technology. ‘AMS has been working like a champ but we can see that it is ageing,’ says Capell. In 2014, one of its four cooling pumps stopped working – a worrying development for an experiment designed to last until 2024.
So fingers crossed for something soon to overturn the evidence of our eyes – that we live in an entirely matter-dominated universe.
If a substance could resist gravity, it would rewrite physics textbooks. Amazingly fiddly experiments to test whether antimatter can are kicking off. Joshua Howgego meets the physicists turning the cosmos upside down.
On 11 November 2016, a small birthday party was held in an apparently unremarkable hangar on the outskirts of Geneva. Nothing too fancy, just a few people gathered around a cake. The honourees were there. Well, sort of – they were still locked in the cage where they had spent their first year. But then again, there is no other way to treat a brood of antimatter particles.
The antimatter realm is so bizarre as to be almost unbelievable: a mirror world of particles that destroy themselves and normal matter whenever the two come into contact. But it’s real enough. Cosmic rays containing antiparticles constantly bombard Earth. A banana blurts out an anti-electron every hour or so. Thunderstorms produce beams of the stuff above the planet.
Making and manipulating antimatter ourselves is a different kettle of fish. Hence that birthday party held at the particle physics centre CERN, celebrating on behalf of a quadruplet of antiprotons. There’s a lot we would like to learn from these caged beasts and their ilk, not least this: do they fall up?
Cards on the table, few physicists believe that such ‘antigravity’ effects exist – that if you released one of those antiprotons and somehow ensured it free passage through the hostile world of matter, it would magically float up. But the recalcitrant nature of antimatter means we’ve never done the experiments, and until we do, we simply don’t know. ‘Progress is often made by asking the questions we think we already know the answer to,’ says Daniel Kaplan of the Illinois Institute of Technology in Chicago.
The scepticism about all forms of antigravity dates back to the 1950s, when the physicist Hermann Bondi was pondering the implications of general relativity, Einstein’s theory of how gravity arises from warping the fabric of the universe. Gravity is an odd sort of force, not least because it only ever works one way. With electromagnetism, say, there are positive and negative charges that attract and repel. With gravity, however, there are only positive masses that always attract.
Bondi showed what a bizarre world it would be if this were not the case, demonstrating how negative mass would end up pursuing positive mass across the universe. This sort of ‘runaway motion’ does not appear to exist – but we should be careful about what we draw from that, says Sabine Hossenfelder of the Frankfurt Institute for Advanced Studies. ‘People who speak of the runaway problem often jump to conclusions from Bondi’s argument and conclude that anti-gravitation itself is inconsistent,’ she says. ‘But it merely requires a modification of general relativity.’
And here’s the thing: general relativity is probably due a modification. The theory is incompatible with quantum mechanics, the other great pillar of modern physics, and if we are to find a way to make a unified description of the universe, that must change. Then everything is up for grabs.
So in a few labs around the world, the search for negative mass and its associated effects goes on. Antimatter is a particularly promising place to look. It is just like normal matter but with the opposite electric charge and a few other mirrored quantum properties. There’s no reason to think it has the opposite mass and anti-gravitates, and some good reasons to think it can’t have.
But if antimatter did anti-gravitate, that might help with another of its central mysteries: where most of it is. Our theories say matter and antimatter should have been created in equal proportions in the big bang, and yet we live in a matter-dominated world.
Explaining this glaring inconsistency has largely been a case of trying to find asymmetries in the processes of particle physics that favour normal matter. Such asymmetries do exist – but they are about a trillionth of the size needed to explain matter’s supremacy. ‘People have been trying to make it work – and it doesn’t work,’ says Kaplan.
Antigravity could provide a better explanation. A repulsive gravitational interaction could have driven matter and antimatter away from each other so they never had the chance to annihilate in the early universe. Since then, the ongoing expansion of the universe would have driven the twain ever farther apart – and the antimatter might eventually have created its own galaxies in other corners of the universe. ‘Then the missing antimatter would be hiding in plain sight,’ says Kaplan’s colleague Thomas Phillips.
Add to that the technological possibilities that levitating matter away from Earth’s surface might bring, and even the US air force wants in – it has given millions of dollars to antimatter researchers over the years. Unfortunately, doing the experiments turns out to be quite an ask.
The problems start with needing a home for antimatter that is almost entirely free of normal matter. That requires some of the emptiest boxes on Earth, containing just hundreds of gas molecules per litre (there are about 1022 in a typical litre of air). But even these boxes have sides. To stop the antimatter banging into them and instantly annihilating, you must slow it down by cooling it to within a few degrees of absolute zero and then catch it in a vortex of electromagnetic fields. Little by little, we’ve been perfecting these arts, holding antimatter particles for seconds, minutes, days – and for a year, as celebrated at the November 2016 party.
That milestone was reached by CERN’s Baryon Antibaryon Asymmetry Experiment (BASE), one of six experiments competing to measure antimatter’s fundamental properties that are all housed in CERN’s vast Antimatter Deceleration Hall. Inside, past a sign marked ‘Antimatter factory’, the most noticeable things are the bright yellow cranes, swinging around the vats of liquid nitrogen required for cooling. Somewhere below, a beam of particles from CERN’s Proton Synchrotron accelerator smashes into a block of metal, creating a plethora of particles. A system of magnets selects the antiprotons and funnels them into a ring of more magnets that keep them on course as they are decelerated for trapping.
Experiments have been running here since the 1990s, studying whether antimatter and matter particles truly are as close to identical as we think. In 2015, by measuring how antiprotons danced around in a magnetic enclosure known as a Penning trap, BASE produced the most precise measurement yet of their mass-to-charge ratio. They showed it was the same as a proton’s, to about 69 parts per trillion, four times more precise than the previous best value. In late 2016, the neighbouring ASACUSA experiment produced the most accurate measurement yet of the antiproton’s mass, finding no evidence of a different value from the proton’s.
The same value – but is the mass positive or negative? That is the multimillion dollar question, and it takes the experiments to a new level of fiddliness. Gravity is weak and easily overwhelmed by the electromagnetic force, so using charged particles such as antiprotons and controlling them with magnetic fields won’t do. You could try getting an antiproton in position and shutting off the magnets to see which way it falls, but the antimatter’s electrostatic interactions with its surroundings would overwhelm any gravitational push or pull it might feel.
A better bet is neutral atoms of antimatter, such as antihydrogen. Making these is no cakewalk, but they have a tiny electric polarity that makes it worth going the distance – their electrostatic interaction isn’t strong enough to swamp gravity, but very strong magnetic fields will still hold them in place. CERN’s Antihydrogen Laser Physics Apparatus (ALPHA) experiment has been doing this since 2005, and now routinely traps and holds bunches of antihydrogen atoms for about 15 minutes. ‘Just the other day we trapped 350,’ says Jeff Hangst, head of ALPHA.
In 2013, ALPHA published a proof of principle measurement, briefly collecting a cloud of 434 antiatoms, turning off the magnets and tracking their subsequent motion by where they annihilated. It was a crude test, and inconclusive – the final answer was compatible with the antiparticles having either negative or positive gravitational mass.
Work on a souped-up version that gives the particles more space to fall started in 2018. ‘We’re going to knock out a wall and build a vertical version of the experiment next door,’ says Hangst. Getting the necessary accuracy won’t be easy, because the antiatoms ALPHA uses are relatively hot and so jiggle around, which clouds the issue. But large enough numbers of antiatoms should help us answer the central question. ‘Up or down – that should be possible,’ says Hangst.
A further CERN experiment, AEGIS, also aims to perform tests within a few years. Kaplan is planning experiments with muons, heavier cousins of the electron, and a team led by David Cassidy of University College London is planning to use positronium, an ‘atom’ consisting of an electron and its antimatter partner, a positron, orbiting one another.
Back at CERN, the Gravitational Behaviour of Antimatter at Rest, or GBAR, experiment intends to tackle the question using a single antihydrogen ion, a combination of one antiproton and two positrons. In theory, it should be easy to hold this charged speck in place with magnetic fields and cool it with lasers. The idea is then to knock off a positron using another laser, making the antiatom neutral. At this point it would cease to feel the effect of the trapping field and fall – up or down. GBAR’s head, Patrice Perez, says they expect to make measurements sensitive to detect even a 1 per cent deviation from the gravity felt by normal matter.
Construction of the experiment requires new lasers and an extra antiproton decelerator called ELENA. Hangst is confident of beating the upstart to the punch. ‘I view GBAR as a case of five miracles happen and then it works,’ he says. One telling fact is that GBAR plans on using only one detector, below the trap. ‘We really do not expect antimatter to fall up,’ says Perez.
Even if it falls at all differently, however, that would still be hugely interesting. ‘In all the descriptions I know, antimatter cannot antigravitate,’ says Sergey Sibiryakov of CERN. What’s more plausible, he thinks, is that there might be other forces that modify gravity whose effects cancel out on normal matter, but not on antimatter. In that case, antimatter might not fall up – just less down. ‘Now, that’s not natural, but it is logically possible,’ he says. Similar gravity-modifying effects might be produced if the graviton, a quantum particle proposed to carry the force of gravity, has a small mass, rather than being massless as is usually assumed.
Even so, we probably shouldn’t be holding our breath for amazing self-levitating machines any time soon. A more immediately practicable way of using antimatter to beat gravity might be to harness the energy released when it annihilates. One firm, Positron Dynamics in Livermore, California, has been developing the idea with financial support from PayPal co-founder Peter Thiel, among others.
Positron-fuelled rockets could power spacecraft much further and faster than is currently possible, according to Positron Dynamics co-founder Ryan Weed. ‘Our vision is to create technology that allows humanity to venture outside of our solar system,’ he says. The company’s patented system involves harvesting positrons from radioactive sodium-22 and using these to start off a nuclear fusion reaction that generates thrust. Weed says the team is set to test the device in a lab and wants to test it in orbit in the next few years.
But experience makes Stefan Ulmer, the head of the BASE experiment, cheerfully sceptical of immediate progress. Antimatter won’t be easily tamed. ‘In the whole history of the CERN Antimatter Deceleration Hall, we’ve produced about enough to heat up a cup of water by about 5 degrees,’ he says. Not even enough, in other words, to make a pot of tea to wash down that birthday cake.
The loss of countless tiny drops of energy since the start of the universe might be behind the rising tide of dark energy accelerating the cosmos’s expansion, writes Joshua Sokol.
If physicists went in for commandments, the first would surely be: thou shalt not get something from nothing. Also known as the principle of energy conservation, this universal accounting law makes it impossible for energy to be magicked either into or out of existence.
Whenever a suspicious transaction seems to take place in physics, a careful audit with the principle of energy conservation usually reveals the source of the error – some overlooked entry in the ledger that, once taken into account, helps balance the books.
This time-honoured technique has allowed us to predict planets and discover particles. But now it appears to be under attack. Look out into the depths of the universe today, and you see a vast quantity of energy. It is so vast, in fact, that it accounts for over two-thirds of all the energy there is. And this mysterious stash is growing continuously – energy laundering on the grandest of cosmic scales.
Working out where this so-called dark energy comes from is probably the biggest problem in physics. We have long been frustrated in finding a solution, but now two groups of physicists think they have it. If they are right, we may have found dark energy’s source in the imperfect joins of the universe where different theories of reality meet. Follow the trail back, and we could even arrive at a better theory of reality.
We’ve known about the invisible elephant in the universe for some time. In 1998, astronomers observing distant supernovae noticed that they were even dimmer than expected. We expected their light to fade as it travelled towards us across an expanding universe, but these new results suggested there was a foot on the accelerator.
Dark energy is the mysterious substance conjured up to explain what is pushing the universe apart ever faster. And as the latest data from sources like the Planck space telescope reveal, it is spread evenly throughout the universe at a density equivalent to around half a dozen protons in every cubic metre of space.
The simplest way to explain this all-pervasive energy is to think of empty space as not being empty after all. On very small scales, quantum mechanics says that any vacuum is filled with the wriggling of quantum fields. But calculations following that approach give us a dark energy density that is 120 orders of magnitude larger than the one astronomers measure from the accelerating expansion of the universe. Almost laughably wrong.
Some researchers, however, haven’t given up on making these two numbers square. According to a new paper by Qingdi Wang, a student of theoretical physicist Bill Unruh at the University of British Columbia, these jiggling fields would tend to cancel each other out on larger scales, drastically deflating the prediction.
But frustration at the inability to make progress has now led some to suggest that it’s all down to a cosmic accounting error. The idea is that dark energy is not actually a substance held in the universe’s vaults – it’s something that appears on the books purely because there’s something else we’ve overlooked.
Energy conservation is such a basic principle that any apparent violation should give pause for thought. The seminal work of mathematician Emmy Noether in the early twentieth century showed that energy conservation was an expression of something even more fundamental: the idea that the laws of physics are immutable over time. And indeed, it is a principle that has paved the way for centuries of discovery.
When it comes to dark energy, cosmologists already had a vague idea where the accounting error might lie. According to Einstein’s equations of general relativity, energy is absorbed and released all the time by the bending and stretching of the fabric of space–time. When photons seem to lose energy as they travel across an expanding universe, for example, that energy is all assumed to go into the universe’s geometry. On the scale of the cosmos as a whole, energy is always appearing to be either created or destroyed.
Similarly, dark energy isn’t adding or subtracting anything from the universe’s overall budget. From afar, cosmologists are confident that everything balances out between the universe’s stuff and the warped space–time holding it. But up close, the exact nature of the transaction bestowing space with extra energy remains mysterious. ‘The question is “Where is it coming from?”’ says Spiros Michalakis at the California Institute of Technology.
Such a grainy structure would have repercussions for the objects that inhabit it. Relativity dictates that particles with mass bloat or compress the space around them depending on how much mass they have. The process is often equated to a taut sheet bending under the influence of a bowling ball rolling around on top of it. But what if up close, the sheet’s surface was stippled?
In such a situation, Josset and his colleagues argue, particles are likely to feel that graininess as a form of friction, shedding energy into the stitching of space. If their model holds, the matter in the universe has been losing energy continuously since a fraction of a second after the big bang.
Adding up the little losses of energy between then and now gives an estimate for dark energy’s strength closer to reality than the 120 orders of magnitude overestimate, although still quite a way off. ‘We are only seven orders of magnitude away,’ says Josset’s colleague Alejandro Perez of Aix-Marseille University, noting that they plan to keep refining their estimate.
For Thibaut Josset of Aix-Marseille University, the processes responsible for that transaction lie in the jagged edges where quantum mechanics and general relativity meet. For decades, we have been looking for a unified theory of quantum gravity, one capable of explaining microscopic quantum processes alongside the large-scale workings of gravity. Thus far, no such theory exists.
One key difference between general relativity and quantum mechanics lies in the way they see the universe’s fundamental structure. In Einstein’s view, which works perfectly for objects on the scale of planets, stars and galaxies, the four dimensions of space and time are smooth and continuous. But quantum mechanics, which seems to govern reality at small scales, implies that deep down, space, like everything else, must be made up of discrete units that we still don’t know how to describe.
‘The magic of this thing is that very tiny violations of energy conservation, that are very, very hard to detect in normal, local experiments, build up during the very long history of the universe,’ says Perez. Add them all up, and you could have enough to explain away dark energy. In other words, it is the tiniest drip-drip of energy – the smallest of leaks in space–time – that is causing this biggest of problems to accumulate.
The leak would have to be so small as to have gone unnoticed so far. At the Large Hadron Collider and elsewhere, experimental physicists are on the lookout for apparent violations of conservation of energy, as spotting one might indicate the existence of new particles. So far they haven’t found any good leads, and the chances are they won’t with current particle colliders. But the amount of energy non-conservation these experiments allow is still enough to hide the observed strength of dark energy, Perez says, something like the mass of a proton going missing every year from a cube of water 10 kilometres across.
While physicists have long known about general relativity’s ability to transfer energy in and out of the space–time curvature on a grand scale, they have struggled to make it work on the scales Josset describes. What has stood in their way, says Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies, is that Einstein’s equations are ruthless about energy conservation when you zoom in on small regions of space. Any quantum jiggery-pokery would invalidate the mathematics. That is, until Josset’s colleagues suggested using a less restricted view that Einstein himself had worked on. This workaround allowed Josset to relax restrictions on energy conservation. ‘I’m annoyed I didn’t think of it earlier,’ says Hossenfelder.
But one of Josset’s assumptions remains contentious. The idea that space–time is ultimately made up of grains, while popular, is far from proven. Identifying the source of dark energy in the interplay between quantum theory and general relativity may require a different approach. Natacha Altamirano of the Perimeter Institute in Waterloo, Canada, and her colleagues have come at the problem from a different angle. Or rather from the largest possible scale, to examine how quantum mechanics and general relativity play off each other across the entirety of the universe.
Altamirano’s work considers what would happen to a particle traversing the smooth hills and valleys described by Einstein’s theory, but within a universe itself following the fuzzy rules of quantum mechanics. Considering a quantum universe is a familiar gambit in theories of quantum cosmology, which try to explain the universe’s earliest instants back when it was still tiny and ruled by wild fluctuations. If the whole universe was quantum, then, much like an electron orbiting an atom, the cosmos could theoretically exist as a superposition of many different possible sizes and states at once.
In practice, the universe’s choices are a lot more limited. The reason lies in Heisenberg’s uncertainty principle, which governs the precision with which we can know the value of any quantum variable. Measure the position of a particle very accurately, for example, and you can’t closely measure its momentum, and vice versa. As photons travel from one galaxy to another, they lose energy. And in the language of the uncertainty principle, that’s a lot like taking a cosmological measurement.
All those unintentional measurements of the universe force the quantum uncertainty to go somewhere else. And one of the ways that can manifest itself is in the form of information loss elsewhere: a little more uncertainty in the rate the universe is accelerating, for example. That change, in turn, would have consequences for all other variables that depend on that rate, further changing the acceleration of the universe in a perpetual feedback loop.
Unless you accounted for it, all that noise would add up to a mysterious dark energy-like term popping out of the void. ‘If I decide to describe my universe with a theory of general relativity that conserves energy, I would see this extra fluid,’ says Altamirano. As to whether the dark energy density that emerges from such a model might turn out to match reality in a way that rivals Josset’s, Altamirano is still working on generating such a figure. ‘I can’t tell how possible that is,’ she says.
Josset’s and Altamirano’s approaches come at a time when dark energy has repeatedly foiled theorists’ attempts to nail it down. Not everyone is convinced the pair are barking up the right tree, though. For Antonio Padilla at the University of Nottingham, the sheer difference in scales makes it unlikely that quantum gravity effects on the smallest imaginable sizes can explain dark energy, which is manifest across billions of light years.
They can’t both be right, either. Because the two approaches use different mathematical language to discuss how gravity and the quantum world interact, they produce different answers for what happens to cosmology. Altamirano’s model produces something like dark energy, but it’s a kinder, gentler version. As the universe expands, it dilutes in space, whereas the dark energy density predicted by Josset’s model remains a constant, in keeping with observations.
Ironing out such wrinkles to everyone’s satisfaction would probably need a fully formed theory of quantum gravity – or at least some as-yet unimagined experimental test that would allow us to look at the universe’s very earliest moments. Until then, dark energy will continue to accumulate interest in the distant reaches of the cosmos – a silent rebuke to the idea that we have our cosmic accounting practices in hand.
Signatures of extra dimensions that don’t normally affect the four dimensions we can observe could show up in the way they warp ripples in space–time, writes Leah Crane.
Hidden dimensions could cause ripples through reality by modifying gravitational waves – and spotting such signatures of extra dimensions could help solve some of the biggest mysteries of the universe.
Physicists have long wondered why gravity is so weak compared with the other fundamental forces. This may be because some of it is leaking away into extra dimensions beyond the three spatial dimensions we experience.
Some theories that seek to explain how gravity and quantum effects mesh together, including string theory, require extra dimensions, often with gravity propagating through them. Finding evidence of such exotic dimensions could therefore help to characterise gravity, or find a way to unite gravity and quantum mechanics – it could also hint at an explanation for why the universe’s expansion is accelerating. But detecting extra dimensions is a challenge. Any that exist would have to be very small in order to avoid obvious effects on our everyday lives. Hopes were high (and still are) that they would show up at the Large Hadron Collider, but it has yet to see any sign of physics beyond our four dimensions.
In the last few years, though, a new hope has emerged. Gravitational waves, ripples in space–time caused by the motion of massive objects, were detected for the first time in 2015. Since gravity is likely to occupy all the dimensions that exist, its waves are an especially promising way to detect any dimensions beyond the ones we know.
‘If there are extra dimensions in the universe, then gravitational waves can walk along any dimension, even the extra dimensions,’ says Gustavo Lucena Gómez at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. Lucena Gómez and his colleague David Andriot set out to calculate how potential extra dimensions would affect the gravitational waves that we are able to observe. They found two peculiar effects: extra waves at high frequencies, and a modification of how gravitational waves stretch space.
As gravitational waves propagate through a tiny extra dimension, the team found, they should generate a ‘tower’ of extra gravitational waves with high frequencies following a regular distribution. But current observatories cannot detect frequencies that high, and most of the planned observatories also focus on lower frequencies. So while these extra waves may be everywhere, they will be hard to spot.
The second effect of extra dimensions might be more detectable, since it modifies the ‘normal’ gravitational waves that we observe rather than adding an extra signal. ‘If extra dimensions are in our universe, this would stretch or shrink space–time in a different way that standard gravitational waves would never do,’ says Lucena Gómez.
As gravitational waves ripple through the universe, they stretch and squish space in a very specific way. It’s like pulling on a rubber band: the ellipse formed by the band gets longer in one direction and shorter in the other, and then goes back to its original shape when you release it.
But extra dimensions add another way for gravitational waves to make space shape-shift, called a breathing mode. Like your lungs as you breathe, space expands and contracts as gravitational waves pass through, in addition to stretching and squishing. ‘With more detectors we will be able to see whether this breathing mode is happening,’ says Lucena Gómez.
‘Extra dimensions have been discussed for a long time from different points of view,’ says Emilian Dudas at the École Polytechnique in France. ‘Gravitational waves could be a new twist on looking for extra dimensions.’ But there is a trade-off: while detecting a tower of high-frequency gravitational waves would point fairly conclusively to extra dimensions, a breathing mode could be explained by a number of other non-standard theories of gravity. ‘It’s probably not a unique signature,’ says Dudas. ‘But it would be a very exciting thing.’