Susskind and 't Hoof t's remarkable idea was motivated by g round-brea king work on black holes by Jacob B ekenstein of the Hebrew University of Jerusalem in Israel and Stephen Hawking at the University of Cambridge. In the mid-1970s, Hawking showed that black holes are in fact not entirely "black" but instead slowly emit radiation, w hich causes them to evaporate and eventu ally disappear. This pose s a puzzle, because Hawking ra diation does not convey any information about the interior of a black hole. When the black hole has gone , all the information a bout the star that collapsed to form t he black hole has van ished, which contradicts the widely aff irmed principle that information cannot be destroyed . This is known as the black hole information paradox . Bekenstein's work provided an important clue in resolving the paradox. He disc overed that a black hole's entropy - which is synonymous with i ts information content - is proportional to the surface area of its event horizon. This is the theoretical surface that cloaks the black hole and marks the po int of no return for infalling matter or light. Theorists have since shown tha t microsc opic quant um ripples at the eve nt horizon can encode the information inside the black hole, so there is no mysterious information loss as the black hole evaporates. Crucially , this provides a deep physical insight: the 3D information about a precursor star can be complete ly encoded in the 2D horizon of t he subsequent black hole - not unli ke the 3D image of an object b eing encode d in a 2D hologram. Susskind and 't Hooft extended the insight to the universe as a whole on the basis that the cosm os has a horizon too - the bo unda ry from b eyond which light ha s not had time to reach us in the 13 .7-billion- year lifespan of the univ erse. W hat's more, work by se veral string theorists, most notably Juan Maldacena at the Institute for Advanced Study in Princeton, has confirmed that th e ide a is on the right track. He showed that the physics inside a hypothetical universe with five dimensions and shaped like a Pringle is the same as the physics takin g place on the fo ur-dimensional boundary. Acco rding to Hogan , the holograph ic principle radically cha nges our picture of sp ace-time. Theoretical physicists have long believed that quantum effects will cause space-time to convulse wildly on the tiniest scales. At this magnification, the fabric of space-time be come s grainy a nd is ultimately ma de of tiny unit s rather like pixels, b ut a hun dred b illion billion times smaller than a proton. This d istance is known as the Planck length, a mere 10

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metres. The P lanck len gth is far beyond the reach of any conceivable experiment, so nobody dared dream that the graininess of space-time might be discernable. That is, not until H ogan realised that the holographic principle changes everything. If s pace-time is a grain y hologram, then you can th ink of the universe as a sp here whose outer surface is papered in Planck length- sized s quares, each containin g one b it of information. The holographic principle says that the amount of information paperin g the outside mus t match the nu mber of bits co ntain ed inside the volume of the universe. Since the volume of the s pherical uni verse is m uch bigger than it s outer surface, how co uld this be true? Hogan reali sed that in order to hav e the s ame number of bits inside the universe as o n the boundary , the world inside must be made up of grains bigger than the Planck length. "Or, to put it anoth er way, a holographic univer se is blurry," says Hogan. This is goo d news for anyone tryin g to p robe the smallest unit of s pace-time. "Contrary to all expectations, it brings its micros copic quan tum structure with in reach of c urrent experiments," says Hogan. So wh ile the Planck length i s too small for expe riments to detect , the holographi c "p rojection" of that grai niness c ould be much, much larger, at around 10

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metres. "If you lived inside a hologram, you could tell by m easurin g the blurring," he says. When Hogan first realised this, he wondered if any experiment might be able to de tect the holographic blurriness of space-time. That's where GEO600 comes in. Gravit ational w ave detecto rs like GEO 600 are esse ntia lly fantastically sensitive rulers. The idea is that if a gravita tional wa ve pass es t hrough GEO600, it will alt ernately stretch space in one direc tion and squeeze it in an other. To me asure this, the GEO600 te am fires a single laser through a ha lf-silvered mirror called a beam s plitter. This divides the light into two beams, which pass down the instrument's 600-metre p erpendicular arms and bounce back again. The returni ng light beams me rge toget her at the