In the
mathematical theory
A mathematical theory is a mathematical model of a branch of mathematics that is based on a set of axioms. It can also simultaneously be a body of knowledge (e.g., based on known axioms and definitions), and so in this sense can refer to an area o ...
of
Riemannian geometry
Riemannian geometry is the branch of differential geometry that studies Riemannian manifolds, smooth manifolds with a ''Riemannian metric'', i.e. with an inner product on the tangent space at each point that varies smoothly from point to poin ...
, there are two uses of the term Fermi coordinates.
In one use they are local coordinates that are adapted to a
geodesic
In geometry, a geodesic () is a curve representing in some sense the shortest path ( arc) between two points in a surface, or more generally in a Riemannian manifold. The term also has meaning in any differentiable manifold with a connection. ...
. In a second, more general one, they are local coordinates that are adapted to any
world line
The world line (or worldline) of an object is the path that an object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics.
The concept of a "world line" is distinguished from con ...
, even not geodesical.
Take a future-directed timelike curve
,
being the proper time along
in the spacetime
.
Assume that
is the initial point of
.
Fermi coordinates adapted to
are constructed this way.
Consider an orthonormal basis of
with
parallel to
.
Transport the basis
along
making use of
Fermi-Walker's transport.
The basis
at each point
is still orthonormal with
parallel to
and is non-rotated (in a precise sense related to the decomposition of Lorentz transformations into pure transformations and rotations) with respect to the initial basis, this is the physical meaning of Fermi-Walker's transport.
Finally construct a coordinate system in an open tube
, a neighbourhood of
, emitting all spacelike geodesics through
with initial tangent vector
, for every
.
A point
has coordinates
where
is the only vector whose associated geodesic reaches
for the value of its parameter
and
is the only time along
for that this geodesic reaching
exists.
If
itself is a geodesic, then Fermi-Walker's transport becomes the standard parallel transport and Fermi's coordinates become standard Riemannian coordinates adapted to
.
In this case, using these coordinates in a neighbourhood
of
, we have
, all
Christoffel symbol
In mathematics and physics, the Christoffel symbols are an array of numbers describing a metric connection. The metric connection is a specialization of the affine connection to surfaces or other manifolds endowed with a metric, allowing distan ...
s vanish exactly on
. This property is not valid for Fermi's coordinates however when
is not a geodesic.
Such coordinates are called Fermi coordinates and are named after the Italian physicist
Enrico Fermi
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...
. The above properties are only valid on the geodesic. The Fermi-Coordinates adapted to a null geodesic is provided by Mattias Blau, Denis Frank, and Sebastian Weiss.
Notice that, if all Christoffel symbols vanish near
, then the manifold is
flat
Flat or flats may refer to:
Architecture
* Flat (housing), an apartment in the United Kingdom, Ireland, Australia and other Commonwealth countries
Arts and entertainment
* Flat (music), a symbol () which denotes a lower pitch
* Flat (soldier), ...
near
.
See also
*
Proper reference frame (flat spacetime)#Proper coordinates or Fermi coordinates
*
Geodesic normal coordinates
In differential geometry, normal coordinates at a point ''p'' in a differentiable manifold equipped with a symmetric affine connection are a local coordinate system in a neighborhood of ''p'' obtained by applying the exponential map to the tang ...
*
Fermi-Walker transport
*
Christoffel symbols
In mathematics and physics, the Christoffel symbols are an array of numbers describing a metric connection. The metric connection is a specialization of the affine connection to surfaces or other manifolds endowed with a metric, allowing distance ...
*
Isothermal coordinates In mathematics, specifically in differential geometry, isothermal coordinates on a Riemannian manifold are local coordinates where the metric is conformal to the Euclidean metric. This means that in isothermal coordinates, the Riemannian metric loc ...
References
{{DEFAULTSORT:Fermi Coordinates
Riemannian geometry
Coordinate systems in differential geometry