Misner space

Misner space is an abstract mathematical spacetime,^[1] first described by Charles W. Misner.^[2] It is also known as the Lorentzian orbifold $\mathbb {R} ^{1,1}/{\text{boost}}$ . It is a simplified, two-dimensional version of the Taub–NUT spacetime. It contains a non-curvature singularity and is an important counterexample to various hypotheses in general relativity.

Michio Kaku develops the following analogy for understanding the concept: "Misner space is an idealized space in which a room, for example, becomes the entire universe. For example, every point on the left wall of the room is identical to the corresponding point on the right wall, such that if you were to walk toward the left wall you will walk through the wall and appear frm the right wall. This suggests that the left and right wall are joined, in some sense, as in a cylinder. The opposite walls are thus all identified with each other, and the ceiling is likewise identified with the floor. Misner space is often studied because it has the same topology as a wormhole but is much simpler to handle mathematically. If the walls move, then time travel might be possible within the Misner universe."^[3]

Metric

The simplest description of Misner space is to consider two-dimensional Minkowski space with the metric

ds^{2}=-dt^{2}+dx^{2},

with the identification of every pair of spacetime points by a constant boost

(t,x)\to (t\cosh(\pi )+x\sinh(\pi ),x\cosh(\pi )+t\sinh(\pi )).

It can also be defined directly on the cylinder manifold $\mathbb {R} \times S$ with coordinates $(t',\varphi )$ by the metric

ds^{2}=-2dt'd\varphi +t'd\varphi ^{2},

The two coordinates are related by the map

t=2{\sqrt {-t'}}\cosh \left({\frac {\varphi }{2}}\right)

x=2{\sqrt {-t'}}\sinh \left({\frac {\varphi }{2}}\right)

and

t'={\frac {1}{4}}(x^{2}-t^{2})

\phi =2\tanh ^{-1}\left({\frac {x}{t}}\right)

Causality

Misner space is a standard example for the study of causality since it contains both closed timelike curves and a compactly generated Cauchy horizon, while still being flat (since it is just Minkowski space). With the coordinates $(t',\varphi )$ , the loop defined by $t=0,\varphi =\lambda$ , with tangent vector $X=(0,1)$ , has the norm $g(X,X)=0$ , making it a closed null curve. This is the chronology horizon : there are no closed timelike curves in the region $t<0$ , while every point admits a closed timelike curve through it in the region $t>0$ .

This is due to the tipping of the light cones which, for $t<0$ , remains above lines of constant $t$ but will open beyond that line for $t>0$ , causing any loop of constant $t$ to be a closed timelike curve.

Chronology protection

Misner space was the first spacetime where the notion of chronology protection was used for quantum fields,^[4] by showing that in the semiclassical approximation, the expectation value of the stress-energy tensor for the vacuum $\langle T_{\mu \nu }\rangle _{\Omega }$ is divergent.

References

^ Hawking, S.; Ellis, G. (1973). The Large Scale Structure of Space-Time. Cambridge University Press. p. 171. ISBN 0-521-20016-4.
^ Misner, C. W. (1967). "Taub-NUT space as a counterexample to almost anything". In Ehlers, J. (ed.). Relativity Theory and Astrophysics I: Relativity and Cosmology. Lectures in Applied Mathematics. Vol. 8. American Mathematical Society. pp. 160–169.
^ Kaku, Michio (28 December 2004). Parallel Worlds: The Science Of Alternative Universes And Our Future In The Cosmos. Penguin. pp. 136–138.
^ Hawking, S. W. (1992-07-15). "Chronology protection conjecture". Physical Review D. 46 (2). American Physical Society (APS): 603–611. Bibcode:1992PhRvD..46..603H. doi:10.1103/physrevd.46.603. ISSN 0556-2821. PMID 10014972.

Time travel
General terms and concepts	Chronology protection conjecture Closed timelike curve Novikov self-consistency principle Self-fulfilling prophecy Quantum mechanics of time travel
Time travel in fiction	Timelines in fiction in science fiction Time loop in film
Temporal paradoxes	Grandfather paradox Causal loop
Parallel timelines	Alternative future Alternate history Many-worlds interpretation Multiverse Parallel universes in fiction
Philosophy of space and time	Butterfly effect Determinism Eternalism Fatalism Free will Predestination
Spacetimes in general relativity that can contain closed timelike curves	Alcubierre metric BTZ black hole Gödel metric Kerr metric Krasnikov tube Misner space Tipler cylinder van Stockum dust Traversable wormholes

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