Questions tagged [causality]
The influence one event, process, or state, has on another event, process, or state, whereby the latter is at least partly dependent on the former.
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Interpretation of real scalar field, its propagator and causality [duplicate]
It seems this has been discussed here previously (e.g this and this post), but I still feel uneasy. Essentially, I think it all boils down to the popular interpretation of the scalar field operator in ...
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What exactly is different about the slice of space and lightcone concept in special relativity?
In Sean Carroll's book on GR, in the very first chapter about SR, he mentions how the difference between Newtonian concepts of space and time, and the view put forward in SR is how there is an "...
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How to define an effective light cone or null direction from discrete arrival times on a lattice?
In several discrete or lattice-based approaches to spacetime (causal sets, Regge-like discretizations, lattice field theory, numerical GR, fast-marching/eikonal methods), one often works with a ...
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Is every symmetric causal isomorphism between distinguishing spacetimes a conformal diffeomorphism?
For this question let us consider spacetimes which are time-oriented and distinguishing (or stronger).
Some bijections between such spacetimes are known to be conformal diffeomorphisms because they ...
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What’s a spacelike geodesic? [duplicate]
Timelike geodesics are the paths taken by particles not subject to gravitational forces. So what’s a spacelike geodesic? You can’t send a particle along a spacelike path.
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In the Twin Paradox, can one twin travel backwards in time?
In the Twin Paradox, if the travelling twin, Betty, instead of returning to her brother Albert at home, continues her journey after a period of rest, why is causality not violated due to her brother, ...
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Entanglement and light cones
Please consider the following thought experiment. The conclusion surprises me, I don't know if it is correct.
A and B are particles with entangled spin: Measure the up-down spin of A and B then the ...
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Does General Relativity allow sudden gravitational source changes?
When I was taught General Relativity as a student, many years ago, I was told that there was no way to create a gravitational "surprise", where the source strength of some system changed ...
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Ampere-Maxwell law seems to contradict causality? [duplicate]
Let us take the Ampere-Maxwell law
$$\nabla \times \mathbf{B} = \mu_0\,\mathbf{J}+\frac{1}{c^2}\frac{\partial \mathbf{E}}{\partial t}.\tag{1}$$
Assume we produce a spark that is so fast that the $\...
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Microcausality in Fermions
We can show that for the Dirac field, the anti-commutator between the field and its adjoint vanishes for space-like separated points. However, for causality we need to show that the commutator instead ...
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Why null geodesic enter chronological future or past of a surface?
Following is a theorem 8.3.7 in wald at page#205
If $\Sigma$ is a closed, achronal, edgeless set, then $\Sigma$ is a Cauchy surface if and only if every inextendible null geodesic intersects $\Sigma$ ...
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Can you see same event at two times?
It may happen that my setup is based on a misunderstanding of GR:
Imagine you have a clock on your wrist - your proper time device upto an initial $\tau_0$, and you live in the spacetime $(M,g)$ and ...
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Can a photon cross the visible horizon and come back if some galaxies across the visible horizon curved it's path significantly?
Can photons cross the visible horizon of the universe and the gravity of galaxies across the horizon make them come back to our side and would we be able to tell from their redshift?
user550784
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Why deforming a null geodesic leads to timelike geodesic?
Wald at page #232 wrote
For timelike geodesics conjugate points signal when a timelike geodesic can be varied to yield a curve of greater length between two points $p$ and $q$.
He then said we can ...
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Prove surface is compact in Hawking and Ellis
I am trying to prove proposition 9.2.1 from text book "The large scale structure of spacetime" by Hawking and Ellis at page # 311, last line and got stuck in a topological/geometrical ...
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