FIRSTHAND KNOWLEDGE GRAVITATIONAL PHYSICS A Universe of Bubbling Loops overcome this, the most significant plastic film, but textured like terry quantum gravity were developed to- ones being string theory and loop cloth. The same also applies to time: tally isolated from each other. Then quantum gravity. it consists of tiny steps whose fur- the two theories slowly converged, String theory assumes that all ther breakdown is neither possible but even today, they are still funda- types of elementary particles, from nor practical. mentally incompatible. Nevertheless, leptons to electrons to quarks, which Why didn’t we ever notice this scientists take both very seriously, make up protons and neutrons, structure before? Because it is ex- and an increasing number – current- manifest themselves as different tremely fine. The smallest unit, a ly around 300 worldwide – are work- states of excitement of a single type loop, is approximately 10-33 cen- ing on loop theory and similar ap- of object: strings. These strings vi- timeters long. This is called the proaches to quantum gravity. brate like the strings on a guitar Planck length, calculated from three GRAVITY NEEDS (hence the name) in a space that has natural constants: the speed of light, NO FIXED TIME 10 or 11 dimensions. Planck’s constant and the gravita- Loop quantum gravity does not tional constant. Similarly, a time Along with the Perimeter Institute in need this multidimensionality. It re- quantum is the time a particle trav- Waterloo, Canada and the Institute mains within the 4 familiar dimen- eling at the speed of light would take for Gravitational Physics and Geom- sions of space and time and postu- to move through a space quantum. etry in the US, the Max Planck Insti- lates that they are not continuous, In other words, the surface of an A4 tute for Gravitational Physics in but consist of quanta (the smallest sheet of paper would be made up of Potsdam is one of the centers for this indivisible units) – the loops. Using 1068 – or 1 followed by 68 zeros – of research. Around 150 loop theoreti- this imagery, it is possible to picture these loops. cians, including all the leaders in the the world as having a web-like For more than 10 years, from 1984 field, met last October at the Loops structure: space is not smooth like a to 1996, string theory and loop 05 conference in Potsdam to share their latest ideas and progress re- ports. They also discussed how the This image, as well as that on the left above, shows an artist’s impression of theory might be put to the test in ex- quantized space. They are snapshots of a dynamic geometry consisting of loops. periments. Matter can be present only where the geometry is activated. Of the four known forces – elec- IBRARY tromagnetic, weak and strong forces L and gravity – gravity is the least un- A theory of everything is the Holy Grail of physicists. Yet, in the search for this theory that unites HOTO P derstood. Unlike the other forces, it all forces, they are brought up short by the limits of what the human mind can actually grasp. CIENCE resists quantization. Quantization of : S HOTO the other forces has been described Today, loop quantum gravity is considered a promising candidate for the solution to this problem. P by theories such as quantum electro- THOMAS THIEMANN from the MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS in dynamics, quantum flavor dynamics and quantum chromodynamics. Each Potsdam is one of its leading proponents worldwide. In the following article he reports on his work. of these theories, summarized in the Standard Model of matter, is subject to physical predictions that ince the beginning of the 20th and particles. Today, we have physi- everything, one that takes into ac- have been measured very precisely HYSICS Scentury, physics has celebrated cal theories that correctly describe count and explains every force and P and confirmed by experiments in some brilliant successes. Not only natural processes over a huge range every particle. The biggest names in accelerators, such as CERN, DESY has it developed quantum theory of distances – from the diameter of a physics have tried, but none of them RAVITATIONAL and SLAC. There is, therefore, no G and the theory of relativity, it has al- proton to the expanse of the visible has yet had any success. Particularly FOR doubt that quantum theory is the so demonstrated their validity in nu- universe. And all of that with mathe- the attempt to reconcile the phe- correct description for these forces. : MPI merous experiments. Gradually, the- matical formulas that fit on a sheet nomena of the fourth interaction, HOTO So-called interaction mediators – the ories were developed that describe of paper! namely gravity, with quantum theo- P particles that carry the forces – play the electromagnetic force, the nu- However, despite these great feats, ry always comes up against an un- a part in it. clear force and weak interaction, as physics remains essentially a patch- yielding mathematical limit. Today What is so different about gravity well as their connection with fields work. There is still no theory of there are various approaches to that it resists quantization? Does it 48 M AXP LANCKR ESEARCH 2/2006 2/2006 M AXP LANCKR ESEARCH 49 FIRSTHAND KNOWLEDGE GRAVITATIONAL PHYSICS seething mass of tiny loops. Similar taken to its logical limits. This guar- ÖRING HYSICS D THE LEGO BRICKS to the way electrons incessantly or- antees that either the right theory is P OF LOOP THEORY RAFIK bit the nucleus of an atom, here, the constructed or, in the course of In loop quantum gravity, the basic structure of geometry of space dances and analysis, it is systematically deter- RAVITATIONAL surges. mined which new structures are re- G spacetime turns out to be discrete: the values of ar- FOR eas and volumes can be changed in (naturally tiny) The edges between the crystal sur- quired, rather than having to guess : MPI steps and there are minimum areas and space vol- . 20, 2005/F.A.Z.-G CT faces are particularly important; like at them. umes below which the space cannot be further di- , O RAPHIC the surfaces themselves, they are la- We assess a philosophy of this na- G vided – in the same way that it is not possible to Such images illustrate the construction of quantized space. On the left, a spin build an object with a Lego set that is smaller than network of polyhedrons, on the right, the development of spins over time. The knots beled with quantum numbers. As the ture, which might look like a highly loops are all linked together, every- intelligent game, based on whether it the smallest Lego brick available. The basic structure in the left-hand image can split open and create a kind of foam structure. ONNTAGSZEITUNG S of space is a “spin network” of linked knots as shown thing is vastly interconnected – so progresses the description of nature in the illustration. The smallest conceivable volume, a space nonlinear. Nevertheless, the theorists beyond the existing theories – and quantum, contains one knot. Any volume, therefore, consists of a certain number of knots. LLGEMEINE even need to be quantized? The an- tromagnetic fields, the interesting A use only those mathematical tools loop theory does just that. It re- If another knot is added, the space content increases by one characteristic space quantum. swer can be found by carefully con- quantities are electrical and magnetic with which they are already familiar. quires, as I already mentioned, only sidering Einstein’s theory of relativi- fluxes. For example, magnetic flux RANKFURTER They exploit mathematics to the log- our four dimensions, and it also : F ty. According to this theory, gravity can be shown as an integral over a ical extreme. They also use every- manages to avoid a shortcoming of Quantization of space helps to nucleus would therefore fall into the is not a force in the traditional sense, surface or as a linear integral over the thing that the quantum field theo- the general theory of relativity: ap- avoid such singularities. This exam- nucleus after a short time. The atoms LLUSTRATIONS but is equivalent to geometry. A edge of this surface. This is how the I rists have come up with in the last plying its equations logically gives ple from atomic physics is an ana- would be unstable and we would planet does not orbit a star because image of loops is created – they are century. Even when they put some- rise to bizarre physical situations, logy: according to the classical positively burn up. Quantum theory interaction-carrying particles (gravi- the edges of the flux surfaces. These thing new together, they use new va- known as spacetime singularities. Maxwell theory of electromagne- explains why this does not happen: tons) facilitate an exchange of forces quantities are interesting as they are rieties of familiar constructions. These are the points in space where tism, atoms should not exist at all. the permitted energy levels in the between them; rather, the planet or- invariant (that is, unchanging) in the Accordingly, gravity creates the the curvature, and thus the concen- This is because electrons, which atom are discrete, that is, quantized. bits the star because the star bends face of movement of any kind. space in which everything happens. tration, of matter is infinitely large. move in an orbit, normally emit Experiments also allow this to be the geometry of spacetime. In the In the Maxwell theory, for exam- The theorists also want to bring mat- Black holes are such singularities, as bremsstrahlung, thus losing energy.