This entry is a by-special-request blog, which Derek Wise invited me to write for the blog associated with the International Loop Quantum Gravity Seminar, and it will appear over there as well. The ILQGS is a long-running regular seminar which runs as a teleconference, with people joining in from various countries, on various topics which are more or less closely related to Loop Quantum Gravity and the interests of people who work on it. The custom is that when someone gives a talk, someone else writes up a description of the talk for the ILQGS blog, and Derek invited me to write up a description of his talk. The audio file of the talk itself is available in .aiff and .wav formats, and the slides are here.
The talk that Derek gave was based on a project of his and Steffen Gielen’s, which has taken written form in a few papers (two shorter ones, “Spontaneously broken Lorentz symmetry for Hamiltonian gravity“, “Linking Covariant and Canonical General Relativity via Local Observers“, and a new, longer one called “Lifting General Relativity to Observer Space“).
The key idea behind this project is the notion of “observer space”, which is exactly what it sounds like: a space of all observers in a given universe. This is easiest to picture when one has a spacetime – a manifold with a Lorentzian metric, – to begin with. Then an observer can be specified by choosing a particular point in spacetime, as well as a unit future-directed timelike vector . This vector is a tangent to the observer’s worldline at . The observer space is therefore a bundle over , the “future unit tangent bundle”. However, using the notion of a “Cartan geometry”, one can give a general definition of observer space which makes sense even when there is no underlying .
The result is a surprising, relatively new physical intuition is that “spacetime” is a local and observer-dependent notion, which in some special cases can be extended so that all observers see the same spacetime. This is somewhat related to the relativity of locality, which I’ve blogged about previously. Geometrically, it is similar to the fact that a slicing of spacetime into space and time is not unique, and not respected by the full symmetries of the theory of Relativity, even for flat spacetime (much less for the case of General Relativity). Similarly, we will see a notion of “observer space”, which can sometimes be turned into a bundle over an objective spacetime , but not in all cases.
So, how is this described mathematically? In particular, what did I mean up there by saying that spacetime becomes observer-dependent?
The answer uses Cartan geometry, which is a framework for differential geometry that is slightly broader than what is commonly used in physics. Roughly, one can say “Cartan geometry is to Klein geometry as Riemannian geometry is to Euclidean geometry”. The more familiar direction of generalization here is the fact that, like Riemannian geometry, Cartan is concerned with manifolds which have local models in terms of simple, “flat” geometries, but which have curvature, and fail to be homogeneous. First let’s remember how Klein geometry works.
Klein’s Erlangen Program, carried out in the mid-19th-century, systematically brought abstract algebra, and specifically the theory of Lie groups, into geometry, by placing the idea of symmetry in the leading role. It describes “homogeneous spaces”, which are geometries in which every point is indistinguishable from every other point. This is expressed by the existence of a transitive action of some Lie group of all symmetries on an underlying space. Any given point will be fixed by some symmetries, and not others, so one also has a subgroup . This is the “stabilizer subgroup”, consisting of all symmetries which fix . That the space is homogeneous means that for any two points , the subgroups and are conjugate (by a symmetry taking to ). Then the homogeneous space, or Klein geometry, associated to is, up to isomorphism, just the same as the quotient space of the obvious action of on .
The advantage of this program is that it has a great many examples, but the most relevant ones for now are:
- -dimensional Euclidean space. the Euclidean group is precisely the group of transformations that leave the data of Euclidean geometry, lengths and angles, invariant. It acts transitively on . Any point will be fixed by the group of rotations centred at that point, which is a subgroup of isomorphic to . Klein’s insight is to reverse this: we may define Euclidean space by .
- -dimensional Minkowski space. Similarly, we can define this space to be . The Euclidean group has been replaced by the Poincaré group, and rotations by the Lorentz group (of rotations and boosts), but otherwise the situation is essentially the same.
- de Sitter space. As a Klein geometry, this is the quotient . That is, the stabilizer of any point is the Lorentz group – so things look locally rather similar to Minkowski space around any given point. But the global symmetries of de Sitter space are different. Even more, it looks like Minkowski space locally in the sense that the Lie algebras give representations and are identical, seen as representations of . It’s natural to identify them with the tangent space at a point. de Sitter space as a whole is easiest to visualize as a 4D hyperboloid in . This is supposed to be seen as a local model of spacetime in a theory in which there is a cosmological constant that gives empty space a constant negative curvature.
- anti-de Sitter space. This is similar, but now the quotient is – in fact, this whole theory goes through for any of the last three examples: Minkowski; de Sitter; and anti-de Sitter, each of which acts as a “local model” for spacetime in General Relativity with the cosmological constant, respectively: zero; positive; and negative.
Now, what does it mean to say that a Cartan geometry has a local model? Well, just as a Lorentzian or Riemannian manifold is “locally modelled” by Minkowski or Euclidean space, a Cartan geometry is locally modelled by some Klein geometry. This is best described in terms of a connection on a principal -bundle, and the associated -bundle, over some manifold . The crucial bundle in a Riemannian or Lorenztian geometry is the frame bundle: the fibre over each point consists of all the ways to isometrically embed a standard Euclidean or Minkowski space into the tangent space. A connection on this bundle specifies how this embedding should transform as one moves along a path. It’s determined by a 1-form on , valued in the Lie algebra of .
Given a parametrized path, one can apply this form to the tangent vector at each point, and get a Lie algebra-valued answer. Integrating along the path, we get a path in the Lie group (which is independent of the parametrization). This is called a “development” of the path, and by applying the -values to the model space , we see that the connection tells us how to move through a copy of as we move along the path. The image this suggests is of “rolling without slipping” – think of the case where the model space is a sphere. The connection describes how the model space “rolls” over the surface of the manifold . Curvature of the connection measures the failure to commute of the processes of rolling in two different directions. A connection with zero curvature describes a space which (locally at least) looks exactly like the model space: picture a sphere rolling against its mirror image. Transporting the sphere-shaped fibre around any closed curve always brings it back to its starting position. Now, curvature is defined in terms of transports of these Klein-geometry fibres. If curvature is measured by the development of curves, we can think of each homogeneous space as a flat Cartan geometry with itself as a local model.
This idea, that the curvature of a manifold depends on the model geometry being used to measure it, shows up in the way we apply this geometry to physics.
Gravity and Cartan Geometry
MacDowell-Mansouri gravity can be understood as a theory in which General Relativity is modelled by a Cartan geometry. Of course, a standard way of presenting GR is in terms of the geometry of a Lorentzian manifold. In the Palatini formalism, the basic fields are a connection and a vierbein (coframe field) called , with dynamics encoded in the Palatini action, which is the integral over of , where is the curvature 2-form for .
This can be derived from a Cartan geometry, whose model geometry is de Sitter space . Then MacDowell-Mansouri gravity gets and by splitting the Lie algebra as . This “breaks the full symmetry” at each point. Then one has a fairly natural action on the -connection:
Here, is the part of the curvature of the big connection. The splitting of the connection means that , and the action above is rewritten, up to a normalization, as the Palatini action for General Relativity (plus a topological term, which has no effect on the equations of motion we get from the action). So General Relativity can be written as the theory of a Cartan geometry modelled on de Sitter space.
The cosmological constant in GR shows up because a “flat” connection for a Cartan geometry based on de Sitter space will look (if measured by Minkowski space) as if it has constant curvature which is exactly that of the model Klein geometry. The way to think of this is to take the fibre bundle of homogeneous model spaces as a replacement for the tangent bundle to the manifold. The fibre at each point describes the local appearance of spacetime. If empty spacetime is flat, this local model is Minkowski space, , and one can really speak of tangent “vectors”. The tangent homogeneous space is not linear. In these first cases, the fibres are not vector spaces, precisely because the large group of symmetries doesn’t contain a group of translations, but they are Klein geometries constructed in just the same way as Minkowski space. Thus, the local description of the connection in terms of -valued forms can be treated in the same way, regardless of which Klein geometry occurs in the fibres. In particular, General Relativity, formulated in terms of Cartan geometry, always says that, in the absence of matter, the geometry of space is flat, and the cosmological constant is included naturally by the choice of which Klein geometry is the local model of spacetime.
The idea in defining an observer space is to combine two symmetry reductions into one. The reduction from to gives de Sitter space, as a model Klein geometry, which reflects the “symmetry breaking” that happens when choosing one particular point in spacetime, or event. Then, the reduction of to similarly reflects the symmetry breaking that occurs when one chooses a specific time direction (a future-directed unit timelike vector). These are the tangent vectors to the worldline of an observer at the chosen point, so the model Klein geometry, is the space of such possible observers. The stabilizer subgroup for a point in this space consists of just the rotations of space around the corresponding observer – the boosts in translate between observers. So locally, choosing an observer amounts to a splitting of the model spacetime at the point into a product of space and time. If we combine both reductions at once, we get the 7-dimensional Klein geometry . This is just the future unit tangent bundle of de Sitter space, which we think of as a homogeneous model for the “space of observers”
A general observer space , however, is just a Cartan geometry modelled on . This is a 7-dimensional manifold, equipped with the structure of a Cartan geometry. One class of examples are exactly the future unit tangent bundles to 4-dimensional Lorentzian spacetimes. In these cases, observer space is naturally a contact manifold: that is, it’s an odd-dimensional manifold equipped with a 1-form , the contact form, which is such that the top-dimensional form is nowhere zero. This is the odd-dimensional analog of a symplectic manifold. Contact manifolds are, intuitively, configuration spaces of systems which involve “rolling without slipping” – for instance, a sphere rolling on a plane. In this case, it’s better to think of the local space of observers which “rolls without slipping” on a spacetime manifold .
Now, Minkowski space has a slicing into space and time – in fact, one for each observer, who defines the time direction, but the time coordinate does not transform in any meaningful way under the symmetries of the theory, and different observers will choose different ones. In just the same way, the homogeneous model of observer space can naturally be written as a bundle . But a general observer space may or may not be a bundle over an ordinary spacetime manifold, . Every Cartan geometry gives rise to an observer space as the bundle of future-directed timelike vectors, but not every Cartan geometry is of this form, in any natural way. Indeed, without a further condition, we can’t even reconstruct observer space as such a bundle in an open neighborhood of a given observer.
This may be intuitively surprising: it gives a perfectly concrete geometric model in which “spacetime” is relative and observer-dependent, and perhaps only locally meaningful, in just the same way as the distinction between “space” and “time” in General Relativity. It may be impossible, that is, to determine objectively whether two observers are located at the same base event or not. This is a kind of “Relativity of Locality” which is geometrically much like the by-now more familiar Relativity of Simultaneity. Each observer will reach certain conclusions as to which observers share the same base event, but different observers may not agree. The coincident observers according to a given observer are those reached by a good class of geodesics in moving only in directions that observer sees as boosts.
When one can reconstruct , two observers will agree whether or not they are coincident. This extra condition which makes this possible is an integrability constraint on the action of the Lie algebra (in our main example, ) on the observer space . In this case, the fibres of the bundle are the orbits of this action, and we have the familiar world of Relativity, where simultaneity may be relative, but locality is absolute.
Lifting Gravity to Observer Space
Apart from describing this model of relative spacetime, another motivation for describing observer space is that one can formulate canonical (Hamiltonian) GR locally near each point in such an observer space. The goal is to make a link between covariant and canonical quantization of gravity. Covariant quantization treats the geometry of spacetime all at once, by means of a Lagrangian action functional. This is mathematically appealing, since it respects the symmetry of General Relativity, namely its diffeomorphism-invariance. On the other hand, it is remote from the canonical (Hamiltonian) approach to quantization of physical systems, in which the concept of time is fundamental. In the canonical approach, one gets a Hilbert space by quantizing the space of states of a system at a given point in time, and the Hamiltonian for the theory describes its evolution. This is problematic for diffeomorphism-, or even Lorentz-invariance, since coordinate time depends on a choice of observer. The point of observer space is that we consider all these choices at once. Describing GR in is both covariant, and based on (local) choices of time direction.
This is easiest to describe in the case of a bundle . Then a “field of observers” to be a section of the bundle: a choice, at each base event in , of an observer based at that event. A field of observers may or may not correspond to a particular decomposition of spacetime into space evolving in time, but locally, at each point in , it always looks like one. The resulting theory describes the dynamics of space-geometry over time, as seen locally by a given observer. In this case, a Cartan connection on observer space is described by to a -valued form. This decomposes into four Lie-algebra valued forms, interpreted as infinitesimal transformations of the model observer by: (1) spatial rotations; (2) boosts; (3) spatial translations; (4) time translation. The four-fold division is based on two distinctions: first, between the base event at which the observer lives, and the choice of observer (i.e. the reduction of to , which symmetry breaking entails choosing a point); and second, between space and time (i.e. the reduction of to , which symmetry breaking entails choosing a time direction).
This splitting, along the same lines as the one in MacDowell-Mansouri gravity described above, suggests that one could lift GR to a theory on an observer space . This amount to describing fields on and an action functional, so that the splitting of the fields gives back the usual fields of GR on spacetime, and the action gives back the usual action. This part of the project is still under development, but this lifting has been described. In the case when there is no “objective” spacetime, the result includes some surprising new fields which it’s not clear how to deal with, but when there is an objective spacetime, the resulting theory looks just like GR.