Posts tagged with spacetime
Geotagged photos (e.g. flickr) and text (e.g. twitter) associate data to a particular point on the globe × time. In other words, a fibre bundle over S²×T.
Imagine the position future historians will be in — if they can synthesise the petabytes of digital data we generate these days. I would love to have crowd-sourced pictures or live-tweeting bystanders’ microblogs of the Yan tie lun 鹽鐵論 (81 B.C.) — Rashomon effect be damned.
In the loop quantum gravity approach, space-time is quantized by a procedure that encodes it in a discretized structure, consisting of spin networks and spin foams.
A spin network consists of an oriented embedded graph in a 3-dimensional manifold with edges labelled by SU(2) representations and edges labelled by intertwiners between the representations attached to incoming and outgoing vertices. These representations relate to gravity in terms of holonomies of connections, and the formulation of Einstein’s equations in terms of vierbein, or tetrads, and dual co-tetrads.
Thus, to a spin networks, or the 1-skeleton of a triangulation by tetrahedra, one assigns operators of quantized area and volume, coming from counting intersection points of a surface, or 3-dimensional regions, with the edges or vertices of the spin network with a multiplicity given in terms of the spin representation attached to the edges and the intertwiners attached to the vertices.
SOURCE: Listening to Golem
Loop quantum gravity is one of a few general frameworks that may eventually form the basis of how physicists think of the smallest scales of time and distance.
(These frameworks are sometimes called Theories of Everything but they’re really just thoughts-on-the-way-to-theories of brief-and-tiny matter.)
GLOSSARY
I love randomly throwing out dense mathematical statements from theoretical physicists like this. Sometimes math people sound like wizards canting magical runes.
Here is the shortest way I can describe the difference between old-school (Newtonian) spacetime and modern, relativistic (Minkowskian) spacetime: in the old view, time was something separate and distinct from space:
Now, in the modern view, time is another kind of thing that’s just like space.
Read on if that doesn’t say it all.
You know how left-right and forward-back are only relative to some axis system? Like if you turn to the left, then left-right are different. Also if you lie down, then up-down are different. Well, sooner-later is the same kind of thing as left-right. If you go really fast (like 10% of the speed of light) then you notice this.
Going this fast relative to me, your time will be slower than my time — your watch will literally run slower. Just like, if we were facing the same direction, and I turned 10 degrees to the left, then our “forwards” would be different. Which means, walking at the same speed I would achieve less “forward” than you, according to your “forward”.
SOURCE: Sean Carroll
Branes, D-branes, M-theory, K-theory … news articles about theoretical physics often mention “manifolds”. Manifolds are also good tools for theoretical psychology and economics. Thinking about manifolds is guaranteed to make you sexy and interesting.
Fortunately, these fancy surfaces are already familiar to anyone who has played the original Star Fox—Super NES version.
In Star Fox, all of the interactive shapes are built up from polygons. Manifolds are built up the same way! You don’t have to use polygons per se, just stick flats together and you build up any surface you want, in the mathematical limit.
The point of doing it this way, is that you can use all the power of linear algebra and calculus on each of those flats, or “charts”. Then as long as you’re clear on how to transition from chart to chart (from polygon to polygon), you know the whole surface—to precise mathematical detail.
Regarding curvature: the charts don’t need the Euclidean metric. As long as distance is measured in a consistent way, the manifold is all good. So you could use hyperbolic, elliptical, or quasimetric distance. Just a few options.
Manifolds are relevant because according to general relativity, spacetime itself is curved. For example, a black hole or star or planet bends the “rigid rods” that Newton & Descartes supposed make up the fabric of space.
In fact, the same “curved-space” idea describes racism. Psychological experiments demonstrate that people are able to distinguish fine detail among their own ethnic group, whereas those outside the group are quickly & coarsely categorized as “other”.
This means a hyperbolic or other “negatively curved” metric, where the distance from 0 to 1 is less than the distance from 100 to 101. Imagine longitude & latitude lines tightly packed together around “0”, one’s own perspective — and spread out where the “others” stand. (I forget if this paradigm changes when kids are raised in multiracial environments.)
If you stitch together such non-Euclidean flats, you’ve again constructed a manifold.
Think about this: the pixel concept re-presents brush-stroke or natural images by a wall of sequential colored squares. You could extend it to 3-D, for example representing humans by little blocks—white for the bone, burgundy for the blood, pink for the fingernails, etc.
In a similar fashion, the manifold concept extends rectilinear reasoning familiar from grade-school math into the more exciting, less restrictive world of the squibbulous, the bubbulous, and the flipflopflegabbulous.
