## From NOVA: “Are Space and Time Discrete or Continuous?”

NOVA

01 Oct 2015
Sabine Hossenfelder

Split a mile in half, you get half a mile. Split the half mile, you get a quarter, and on and on, until you’ve carved out a length far smaller than the diameter of an atom. Can this slicing continue indefinitely, or will you eventually reach a limit: a smallest hatch mark on the universal ruler?

The success of some contemporary theories of quantum gravity may hinge on the answer to this question. But the puzzle goes back at least 2500 years, to the paradoxes thought up by the Greek philosopher Zeno of Elea, which remained mysterious from the 5th century BC until the early 1800s. Though the paradoxes have now been solved, the question they posed—is there a smallest unit of length, beyond which you can’t divide any further?—persists.

The most famous of Zeno’s paradoxes is that of Achilles and the Tortoise in a race. The tortoise gets a head start on the faster-running Achilles. Achilles should quickly catch up—at least that’s what would happen in a real-world footrace. But Zeno argued that Achilles will never pass over the tortoise, because in the time it takes for Achilles to reach the tortoise’s starting point, the tortoise too will have moved forward. While Achilles pursues the tortoise to cover this additional distance, the tortoise moves yet another bit. Try as he might, Achilles only ever reaches the tortoise’s position after the animal has already left it, and he never catches up.

Obviously, in real life, Achilles wins the race. So, Zeno argued, the assumptions underlying the scenario must be wrong. Specifically, Zeno believed that space is not indefinitely divisible but has a smallest possible unit of length. This allows Achilles to make a final step surpassing the distance to the tortoise, thereby resolving the paradox.

It took more than two thousand years to develop the necessary mathematics, but today we know that Zeno’s argument was plainly wrong. After mathematicians understood how to sum an infinite number of progressively smaller steps, they calculated the exact moment Achilles surpasses the tortoise, proving that it does not take forever, even if space is indefinitely divisible.

Zeno’s paradox is solved, but the question of whether there is a smallest unit of length hasn’t gone away. Today, some physicists think that the existence of an absolute minimum length could help avoid another kind of logical nonsense; the infinities that arise when physicists make attempts at a quantum version of [Albert]Einstein’s General Relativity, that is, a theory of “quantum gravity.” When physicists attempted to calculate probabilities in the new theory, the integrals just returned infinity, a result that couldn’t be more useless. In this case, the infinities were not mistakes but demonstrably a consequence of applying the rules of quantum theory to gravity. But by positing a smallest unit of length, just like Zeno did, theorists can reduce the infinities to manageable finite numbers. And one way to get a finite length is to chop up space and time into chunks, thereby making it discrete: Zeno would be pleased.

He would also be confused. While almost all approaches to quantum gravity bring in a minimal length one way or the other, not all approaches do so by means of “discretization”—that is, by “chunking” space and time. In some theories of quantum gravity, the minimal length emerges from a “resolution limit,” without the need of discreteness. Think of studying samples with a microscope, for example. Magnify too much, and you encounter a resolution-limit beyond which images remain blurry. And if you zoom into a digital photo, you eventually see single pixels: further zooming will not reveal any more detail. In both cases there is a limit to resolution, but only in the latter case is it due to discretization.

In these examples the limits could be overcome with better imaging technology; they are not fundamental. But a resolution-limit due to quantum behavior of space-time would be fundamental. It could not be overcome with better technology.

So, a resolution-limit seems necessary to avoid the problem with infinities in the development of quantum gravity. But does space-time remain smooth and continuous even on the shortest distance scales, or does it become coarse and grainy? Researchers cannot agree.

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.
Date 18 May 2008
Source http://www.nasa.gov/mission_pages/gpb/gpb_012.html
Author NASA

In string theory, for example, resolution is limited by the extension of the strings (roughly speaking, the size of the ball that you could fit the string inside), not because there is anything discrete. In a competing theory called loop quantum gravity, on the other hand, space and time are broken into discrete blocks, which gives rise to a smallest possible length (expressed in units of the Planck length, about 10-35 meters), area and volume of space-time—the fundamental building blocks of our universe. Another approach to quantum gravity, “asymptotically safe gravity,” has a resolution-limit but no discretization. Yet another approach, “causal sets,” explicitly relies on discretization.

And that’s not all. Einstein taught us that space and time are joined in one entity: space-time. Most physicists honor Einstein’s insight, and so most approaches to quantum gravity take space and time to either both be continuous or both be discrete. But some dissidents argue that only space or only time should be discrete.

So how can physicists find out whether space-time is discrete or continuous? Directly measuring the discrete structure is impossible because it is too tiny. But according to some models, the discreteness should affect how particles move through space. It is a miniscule effect, but it adds up for particles that travel over very long distances. If true, this would distort images from far-away stellar objects, either by smearing out the image or by tearing apart the arrival times of particles that were emitted simultaneously and would otherwise arrive on Earth simultaneously. Astrophysicists have looked for both of these signals, but they haven’t found the slightest evidence for graininess.

Even if the direct effects on particle motion are unmeasurable, defects in the discrete structure could still be observable. Think of space-time like a diamond. Even rare imperfections in atomic lattices spoil a crystal’s ability to transport light in an orderly way, which will ruin a diamond’s clarity. And if the price tags at your jewelry store tell you one thing, it’s that perfection is exceedingly rare. It’s the same with space-time. If space-time is discrete, there should be imperfections. And even if rare, these imperfections will affect the passage of light through space. No one has looked for this yet, and I’m planning to start such a search in the coming months.

Next to guiding the development of a theory of quantum gravity, finding evidence for space-time discreteness—or ruling it out!—would also be a big step towards solving a modern-day paradox: the black hole information loss problem, posed by Stephen Hawking in 1974. We know that black holes can only store so much information, which is another indication for a resolution-limit. But we do not know exactly how black holes encode the information of what fell inside. A discrete structure would provide us with elementary storage units.

Black hole information loss is a vexing paradox that Zeno would have appreciated. Let us hope we will not have to wait 2000 years for a solution.

Editor and author’s picks for further reading

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NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

## From Max Tegmark on Nautilus: “Life Is a Braid in Spacetime”

Nautilus

October 1, 2015

By Max Tegmark

But that is not how we usually think about it. Our language reveals how differently we think of space and time: The first as a static stage, and the second as something flowing. Despite our intuition, however, the flow of time is an illusion. [Albert] Einstein taught us that there are two equivalent ways of thinking about our physical reality: Either as a three-dimensional place called space, where things change over time, or as a four-dimensional place called spacetime that simply exists, unchanging, never created, and never destroyed.

I think of the two viewpoints as the different perspectives on reality that a frog and a bird might take. The bird surveys the landscape of reality from high “above,” akin to a physicist studying the mathematical structure of spacetime as described by the equations of physics. The frog, on the other hand, lives inside the landscape surveyed by the bird. Looking up at the moon over time, the frog sees something like the right panel in the figure, The Moon’s Orbit: Five snapshots of space with the Moon in different positions each time. But the bird sees an unchanging spiral shape in spacetime, as shown in the left panel.

The Moon’s Orbit: We can equivalently think of the moon as a position in space that changes over time (right), or as an unchanging spiral shape in spacetime (left), corresponding to a mathematical structure. The snapshots of space (right) are simply horizontal slices of spacetime (left). To keep things legible, I’ve drawn the orbit much smaller than to scale and made several simplifications. To get snapshots of space (right) from spacetime (left), you simply make horizontal slices through spacetime at the times you’re interested in. Max Tegmark

For the bird—and the physicist—there is no objective definition of past or future. As Einstein put it, “The distinction between past, present, and future is only a stubbornly persistent illusion.” When we think about the present, we mean the time slice through spacetime corresponding to the time when we’re having that thought. We refer to the future and past as the parts of spacetime above and below this slice.

This is analogous to your use of the terms here, in front of me, and behind me to refer to different parts of spacetime relative to your present position. The part that’s in front of you is clearly no less real than the part behind you—indeed, if you’re walking forward, some of what’s presently in front of you will be behind you in the future, and is presently behind various other people. Analogously, in spacetime, the future is just as real as the past—parts of spacetime that are presently in your future will, in your future, be in your past. Since spacetime is static and unchanging, no parts of it can change their reality status, and all parts must be equally real.

The idea of spacetime does more than teach us to rethink the meaning of past and future. It also introduces us to the idea of a mathematical universe. Spacetime is a purely mathematical structure in the sense that it has no properties at all except mathematical properties, for example the number four, its number of dimensions. In my book Our Mathematical Universe, I argue that not only spacetime, but indeed our entire external physical reality, is a mathematical structure, which is by definition an abstract, immutable entity existing outside of space and time.

What does this actually mean? It means, for one thing, a universe that can be beautifully described by mathematics. That this is true for our universe has become increasingly clear over the centuries, with evidence piling up ever more rapidly. The latest triumph in this area is the discovery of the Higgs boson, which, just like the planet Neptune and the radio wave, was first predicted with a pencil, using mathematical equations.

That our universe is approximately described by mathematics means that some but not all of its properties are mathematical. That it is mathematical means that all of its properties are mathematical; that it has no properties at all except mathematical ones. If I’m right and this is true, then it’s good news for physics, because all properties of our universe can in principle be understood if we are intelligent and creative enough. It also implies that our reality is vastly larger than we thought, containing a diverse collection of universes obeying all mathematically possible laws of physics.

This novel way of viewing both spacetime and the stuff in it implies a novel way of viewing ourselves. Our thoughts, our emotions, our self-awareness, and that deep existential feeling “I am”—none of this feels the least bit mathematical to me. Yet we too are made of the same kinds of elementary particles that make up everything else in our physical world, which I’ve argued is purely mathematical. How can we reconcile these two perspectives?

The first step is to consider how we look as a spacetime structure. The cosmology pioneer George Gamow entitled his autobiography My World Line, a phrase also used by Einstein to refer to paths through spacetime. However, your own world line strictly speaking isn’t a line: It has a non-zero thickness and it’s not straight. The roughly 1029 elementary particles (quarks and electrons) that your body is made of form a tube-like shape through spacetime, analogous to the spiral shape of the Moon’s orbit (“The Moon’s Orbit”) but more complicated. If you’re swimming laps in a pool, that part of your spacetime tube has a zig-zag shape, and if you’re using a playground swing, that part of your spacetime tube has a serpentine shape.

However, the most interesting property of your spacetime tube isn’t its bulk shape, but its internal structure, which is remarkably complex. Whereas the particles that constitute the Moon are stuck together in a rather static arrangement, many of your particles are in constant motion relative to one another. Consider, for example, the particles that make up your red blood cells. As your blood circulates through your body to deliver the oxygen you need, each red blood cell traces out its own unique tube shape through spacetime, corresponding to a complex itinerary though your arteries, capillaries, and veins with regular returns to your heart and lungs. These spacetime tubes of different red blood cells are intertwined to form a braid pattern as seen in the figure “Complexity and Life” which is more elaborate than anything you’ll ever see in a hair salon: Whereas a classic braid consists of three strands with perhaps thirty thousand hairs each, intertwined in a simple repeating pattern, this spacetime braid consists of trillions of strands (one for each red blood cell), each composed of trillions of hair-like elementary-particle trajectories, intertwined in a complex pattern that never repeats. In other words, if you imagine spending a year giving a friend a truly crazy hairdo, braiding the hair by separately intertwining all their individual hairs, the pattern you’d get would still be very simple in comparison.

Complexity and Life: The motion of an object corresponds to a pattern in spacetime. An inanimate clump of 10 accelerating particles constitutes a simple pattern (left), while the particles that make up a living organism constitute a complex pattern (middle), corresponding to the complex motions that accomplish information processing and other vital processes. When a living organism dies, it eventually disintegrates and its particles separate from each other (right). These crude illustrations show merely 10 particles; your own spacetime pattern involves about 1029 particles and is mind-blowingly complex. Max Tegmark

Yet the complexity of all this pales in comparison to the patterns of information processing in your brain. Your roughly 100 billion neurons are constantly generating electrical signals (“firing”), which involves shuffling around billions of trillions of atoms, notably sodium, potassium, and calcium ions. The trajectories of these atoms form an extremely elaborate braid through spacetime, whose complex intertwining corresponds to storing and processing information in a way that somehow gives rise to our familiar sensation of self-awareness. There’s broad consensus in the scientific community that we still don’t understand how this works, so it’s fair to say that we humans don’t yet fully understand what we are. However, in broad brush, we might say this: You’re a pattern in spacetime. A mathematical pattern. Specifically, you’re a braid in spacetime—indeed, one of the most elaborate braids known.

Some people find it emotionally displeasing to think of themselves as a collection of particles. I got a good laugh back in my 20s when my friend Emil addressed my friend Mats as an “atomhög,” Swedish for “atom heap,” in an attempt to insult him. However, if someone says “I can’t believe I’m just a heap of atoms!’’ I object to the use of the word “just”: the elaborate spacetime braid that corresponds to their mind is hands down the most beautifully complex type of pattern we’ve ever encountered in our universe. The world’s fastest computer, the Grand Canyon or even the Sun—their spacetime patterns are all simple in comparison.

At both ends of your spacetime braid, corresponding to your birth and death, all the threads gradually separate, corresponding to all your particles joining, interacting and finally going their own separate ways (As seen in the right panel of “Complexity and Life”). This makes the spacetime structure of your entire life resemble a tree: At the bottom, corresponding to early times, is an elaborate system of roots corresponding to the spacetime trajectories of many particles, which gradually merge into thicker strands and culminate in a single tube-like trunk corresponding to your current body (with a remarkable braid-like pattern inside as we described above). At the top, corresponding to late times, the trunk splits into ever finer branches, corresponding to your particles going their own separate ways once your life is over. In other words, the pattern of life has only a finite extent along the time dimension, with the braid coming apart into frizz at both ends.1

This view of ourselves as mathematical braid patterns in spacetime challenges the assumption that we can never understand consciousness. It optimistically suggests that consciousness can one day be understood as a form of matter, a derivative of the most beautifully complex spacetime structure in our universe. Such understanding would enlighten our approaches to animals, unresponsive patients, and future ultra-intelligent machines, with wide-ranging ethical, legal, and technological implications.

This is how I see it. However, although this idea of an unchanging reality is venerable and dates back to Einstein, it remains controversial and subject to vibrant scientific debate, with scientists I greatly respect expressing a spectrum of views. For example, in his book The Hidden Reality, Brian Greene expresses unease toward letting go of the notions change and creation as fundamental, writing “I’m partial to there being a process, however tentative […] that we can imagine generating the multiverse.” Lee Smolin goes further in his book Time Reborn, arguing that not only is change real, but that time may be the only thing that’s real. At the other end of the spectrum, Julian Barbour argues in his book The End of Time not only that change is illusory, but that one can even describe physical reality without introducing the concept of time at all.

If we discover the ultimate nature of time, this will answer many of the most exciting open questions facing physics today. Did time have some sort of beginning before our Big Bang? Will it ultimately end? Did it emerge out of some sort of timeless quantum fuzz into which it will eventually dissolve? We physicists haven’t found the mathematical theory of quantum gravity required to convincingly answer these questions, but whatever this “theory of everything” turns out to be, time will be the key to unlocking its mysteries.

Max Tegmark is an MIT physics professor who has authored more than 200 technical papers. Known as “Mad Max” for his unorthodox ideas and passion for adventure, his scientific interests range from precision cosmology to the ultimate nature of reality, all explored in his new popular science book Our Mathematical Universe.

Stem Education Coalition

Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

## From New Scientist: “Breaking relativity: Celestial signals defy Einstein” January 2014 but Very Important

New Scientist

02 January 2014
Stuart Clark

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.

This is the landscape of space-time: the underlying fabric of the physical universe, perhaps of reality itself.

Spacetime

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.

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.

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. MAGIC – the Major Atmospheric Gamma-ray Imaging Cherenkov telescope – 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.

MAGIC

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 John Ellis, 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 Robert Wagner 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, HESS – the High Energy Stereoscopic System based in the Namibian outback – caught sight of another giant galactic flare in July 2006, it was the perfect opportunity to test the theory.

HESS

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.

PKS 2155-304 obtained in R band at ESO-NTT.

ESO/NTT

But… nothing. “We saw no hint of a time delay,” says Agnieszka Jacholkowska of Pierre and Marie Curie University in Paris, France, one of the team analysing the signals. 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 NASA’s Fermi telescope on 27 April 2013 – known prosaically as GRB130427A – was eye-popping.

NASA/Fermi

GRB130427A – The maps in this animation show how the sky looks at gamma-ray energies above 100 million electron volts (MeV) with a view centered on the north galactic pole. The first frame shows the sky during a three-hour interval prior to GRB 130427A. The second frame shows a three-hour interval starting 2.5 hours before the burst, and ending 30 minutes into the event. The Fermi team chose this interval to demonstrate how bright the burst was relative to the rest of the gamma-ray sky. This burst was bright enough that Fermi autonomously left its normal surveying mode to give the LAT instrument a better view, so the three-hour exposure following the burst does not cover the whole sky in the usual way.

LAT

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 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 (arxiv.org/1305.2626v2). “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 (arxiv.org/abs/1305.3463).

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 [LMC] some 170,000 light years away. This is still too close for its neutrinos to manifest any measurable time lag.

This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the centre of the remnant. Hubble (in green) and Chandra (in blue) data show the expanding shock wave.
6 January 2014

ESO/NRAO/NAOJ/ALMA

NASA/ESA Hubble

NASA/Chandra

LMC

Decisive help could now be at hand. IceCube is a neutrino detector buried in a cubic kilometre of Antarctic ice that came fully on stream in 2011.

IceCube

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, Dan Hooper 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.

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 Cherenkov Telescope Array [CTA] would be 10 times as sensitive, and capable of seeing between 10 and 20 active galaxy flare-ups every year.

CTA

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.

Stuart Clark is a New Scientist consultant and the author of The Sensorium of God (Polygon), which dramatises Newton’s struggle to find the meaning of space and time

Stem Education Coalition

## From PBS Nova: “The Fabric of the Cosmos: What Is Space

I found this on YouTube, so, you know, all’s fair.

Brian Greene takes us on an incredible journey to learn about Space. This video was produced in 2011. Among the subjects are: Albert Einstein & Relativity; Space-Time; Quantum Mechanics; CERN & the LHC; Peter Higgs, the Higgs Boson and the Higgs Field; Nobel Laureate Saul Perlmutter and the expansion of the universe; Dark Energy; Black Holes. There is a really brief cameo by ESO’s VLT. Missing from the video are Fermilab, Penzias & Wilson, Dark Matter, Stephen Hawking, Brane Theory, String Theory.

Depiction of Space-Time with Gravity

I hope you enjoy this.

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