The causality tag has no wiki summary.
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What's wrong with this QFT thought experiment?
In quantum field theory, the propagator $D(x-y)$ doesn't vanish for space-like separation. In Zee's book, he claims that this means a particle can leak out of the light-cone. Feynman also gives this ...
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2answers
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Which causal structures are absent from any “nice” patch of Minkowski space?
Which "causal separation structures" (or "interval structures") can not be found among the events in "any nice patch ($P$) of Minkowski space"?,
where "causal separation structure" ($s$) should be ...
4
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2answers
170 views
Extended Rigid Bodies in Special Relativity
I was reading Landau & Lifhsitz's Classical Field Theory and I noticed that they mention that an extended rigid body isn't "relativistically correct".
For example, if you consider a rigid rod ...
3
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1answer
69 views
Can the vanishing of the Riemann tensor be determined from causal relations?
Given a Lorentzian manifold and metric tensor, "$( M, g )$", the corresponding causal relations between its elements (events) may be derived; i.e. for every pair (in general) of distinct events in set ...
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1answer
42 views
Is this hypo-theoretical model of future prediction feasible? [closed]
First let me state that I am not, nor ever have I been, a physics student. I am working on an idea for a book I'm writing.
This is a thought experiment that posits the existence of a computer system ...
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2answers
486 views
Does a Weak Energy Condition Violation Typically Lead to Causality Violation?
In the answer to this question: ergosphere treadmills Lubos Motl suggested a straightforward argument, based on the special theory of relativity, to argue that light passing through a strong ...
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1answer
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Causal structure of the inflationary multiverse
In the multiverse as it is described by eternal inflation, it is not clear to me what is its causal structure and in particular if the bubble-universes are causally connected. We start from a ...
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2answers
109 views
Superluminal particles with causality
What kind of CLASSICAL theories would allow to true (non-apparent) superluminal particles (beyond speed of light, BSOL) agreeing with causality to exist? I mean, are causal superluminal classical ...
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2answers
178 views
What is the Andromeda Paradox?
I have been studying causality (specifically why there is no such thing as a simultaneous instant of time across all observers) recently and I keep hearing references to the Andromeda paradox. Can ...
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1answer
81 views
Casuality between ordinary people's language and physisct [closed]
in every day word people use casuality principle , for example fire causes the paper to burn . What is physisc view is it fire causing the paper to burn or laws of physics makes the fire burn the ...
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8answers
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Why quantum entanglement is considered to be active link between particles?
From everything I've read about quantum mechanics and quantum entanglement phenomena it's unobvious for me, why quantum entanglement is considered to be active link. I.e. it's stated every time that ...
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Gravity versus light [duplicate]
I've read this problem somewhere but don't remember where I saw it, but anyways...
So Earth revolves around the sun, and let's hypothetically remove the sun (make it disappear) would Earth just ...
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2answers
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Solar Catastrophe [duplicate]
Consider all of sudden the sun vanishes. What would happen to planetary motion. Will it continue to move in elliptical path or move in a tangential to the orbit immediately after sun vanishes or move ...
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42 views
Do all the 4 forces of nature act at the same speed? [duplicate]
It is believed that gravity, the weakest of the four forces propagates at the speed of light, cf. e.g. this Phys.SE post. One would expect (perhaps erroneously) that the other, stronger, forces acted ...
4
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1answer
86 views
How soon that a force affect another object?
Imagine this scenario: I have 2 objects in vacuum without any force exerted upon them not even a possible gravitational force between them.
Now if one of them gets a gravitational or magnetic force, ...
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6answers
1k views
The speed of gravity?
Sorry for the layman question, but it's not my field.
Suppose this thought experiment is performed. Light takes 8 minutes to go from the surface of the Sun to Earth. Imagine the Sun is suddenly ...
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0answers
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Going through a ring of black holes
Mathematician here with a speculative physical question -- feel free to boot me if the level isn't right.
Suppose one finds, or builds, a constellation of several black holes arranged in a circle. ...
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1answer
84 views
Grandfather Paradox [closed]
This question related to the Grandfather Paradox.
Assume that time travel to the past is a reality. What experiment/test could the time traveler perform in order to determine if he is in his own ...
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2answers
211 views
Causality and Quantum Field Theory
I have a problem with proof of causality in Peskin & Schroeder, An Introduction to QFT, page 28. To avoid confusion I use three vectors notation, rewriting the Eq. (2.53) for $y=0$ as follows:
...
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3answers
407 views
How does faster than light travel violate causality?
Let's say I have two planets that are one hundred thousand lightyears away from each other. I and my immortal friend on the other planet want to communicate, with a strong laser and a tachyon ...
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1answer
105 views
How is the direction of time determined in general relativity?
In special relativity every frame has its own unique time axis, represented in Minkowski diagrams by a fan-out of time vectors that grows infinitely dense as you approach the surface of the light cone ...
5
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3answers
144 views
How can we know, today, that there's something from 100 light-years from here?
In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
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2answers
252 views
In QFT, why does a vanishing commutator ensure causality?
In relativistic quantum field theories (QFT),
$$[\phi(x),\phi^\dagger(y)] = 0 \;\;\mathrm{if}\;\; (x-y)^2<0$$
On the other hand, even for space-like separation
$$\phi(x)\phi^\dagger(y)\ne0.$$
...
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5answers
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The transactional interpretation of quantum mechanics
John Cramer’s transactional interpretation of quantum mechanics (TIQM) is billed as resolving the fuzzy agnosticism of the Copenhagen interpretation while avoiding the alleged ontological excesses of ...
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3answers
162 views
What was wrong with action a distance?
It is usually said that the idea of fields was introduced (electric and magnetic fields) in electricity and magnetism after Coulomb's law to cure the conceptual problems of action at a distance.
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3answers
93 views
Analyticity and Causality in Relativity
A few weeks ago at a conference a speaker I was listening to made a comment to the effect that a function (let's say scalar) cannot be analytic because otherwise it would violate causality. He didn't ...
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1answer
99 views
An issue about the compactness and the existence of CTCs
There is a well known fact that a compact spacetime necessarily contains a closed timelike curve (CTC). Proof can be found in several books on GR (e.g. Hawking, Ellis, Proposition 6.4.2), and in ...
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1answer
151 views
Theories with non-vanishing commutators outside the lightcone
I'm reading Weinberg's new book on Quantum Mechanics, and in Chapter 8.7 "Time-Dependent Perturbation Theory" he derives the usual Dyson series for the $S$ matrix when the interaction Hamiltonian ...
5
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1answer
188 views
Tachyonic antitelephone vs messaging through a wormhole
From the wikipedia article on tachyons:
Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.[3][5] If such particles ...
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4answers
705 views
In superluminal phase velocities, what is it that is traveling faster than light?
I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but ...
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1answer
310 views
How is quantum mechanics compatible with the speed of light limit?
Consider a free electron in space. Let us suppose we measure its position to be at point A with a high degree of accuracy at time 0. If I recall my QM correctly, as time passes the wave function ...
5
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1answer
64 views
Quantum causal structure
We take causal structure to be some relation defined over elements which are understood to be morphisms of some category. An example of such a relation is a domain, another is a directed acyclic ...
8
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1answer
87 views
Hamilton operator in absence of causal order?
I hope, this question isn't too broad or vague.
In a recent paper, Ognyan Oreshkov et al. worked out a theory of quantum correlations in absence of any causal order, dropping the assumptions of a ...
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2answers
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Causality in a gedanken experiment on the hydrogen atom
Consider a gedanken(=thought) experiment where I am tracking the motion of the electron in a hydrogen atom with a time resolution of (say) $\Delta t = 10^{-20}$ seconds. Further assume (for ...
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Is eternal inflation and the multiverse compatible with causal patch complementarity?
The argument for eternal inflation is we have some patch of metastable vacuum with positive cosmological constant, and so it expands exponentially a la de Sitter. Most of the patch decays to something ...
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5answers
705 views
Is there such a thing as “Action at a distance”?
What ever happened to "action at a distance" in entangled quantum states, i.e. the Einstein-Rosen-Podolsky (EPR) paradox? I thought they argued that in principle one could communicate faster than ...
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1answer
153 views
Information faster-than-light and GR vs. QM
What is meant by the statement that information cannot travel faster than light?
If I write down something on a paper, isn't there according to QM a non-zero probability that an identical paper can ...
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2answers
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Could we get rid of explicit fields derivatives in Quantum Field Theories?
For instance, if we choose the following scalar field Lagrangian, which is (I hope) Lorentz-invariant, where $l$ is a a length scale, and with a $(-1,1,1,1)$ metric:
$$ \mathfrak{L}(x) \sim ...
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8answers
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What criteria distinguishes causality from retrocausality?
The brilliant philosopher David Hume remarked that if two events are always found to be correlated to each other with one event happening prior to the other, we call the earlier event the cause and ...
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2answers
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The order of seeing event in different spacetimes
Assume this question:
Three events A, B, C are seen by observer O to occur in the order ABC. Another observer O$^\prime$ sees the events to occur in the order CBA. Is it possible that a third ...
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2answers
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The Lagrangian in Scalar Field Theory
This is perhaps a naive question, but why do we write down the Lagrangian
$$\mathcal{L}=\frac{1}{2}\eta^{\mu\nu}\partial_{\mu}\phi\partial_{\nu}\phi - \frac{1}{2}m^2\phi^2$$
as the simplest ...
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2answers
700 views
Definitions: 'locality' vs 'causality'
I'm having trouble unambiguously interpreting many answers here due to the fact that the terms locality and causality are sometimes used interchangeably, while other times seem to mean very different ...
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1answer
113 views
What would it take for a physical phenomenon to be telekinetic?
I've just watched an episode by MinutePhysics called "Real World Telekinesis". In it, Neil Turok (I wonder if that is his actual name; I remember playing a game called "Turok: Dinosaur Hunter" on ...
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3answers
175 views
Acausality in solving time-domain inhomogeneous differential equations with Fourier transforms?
I was always wondering about the acausal nature of solutions obtained by Fourier transforms in the case of inhomogeneous equations. The solution usually revolves around the integration of the ...
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3answers
312 views
Using quantum entanglement to send messages back to the past [duplicate]
Possible Duplicate:
Entanglement in time
I heard that there is an experiment that uses quantum entanglement to try to send messages back to the past. I am having a hard time understanding ...
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8answers
488 views
Is “Causality” the equivalent of a claim that the future is predictable based on the present and the past?
In classical (Newtonian) mechanics, every observer had the same past and the same future and if you had perfect knowledge about the current state of all particles in the universe, you could ...
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3answers
244 views
Is the commutation of all possible operators sufficient to identify a spacelike interval?
It has been claimed (e.g. here) and apparently already been established, that the interval $x - y$ being (called) "spacelike" implies that $\bigl[\hat O (x),\, \hat O' (y)\bigr]=0$ for any two (not ...
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1answer
111 views
Causality without light
Related to my prior question: Would a species insensitive to light have developed special relativity?
Imagine a species insensate to light and other em radiation. They are very attuned to sound ...
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How can one know if a theory allow action at a distance effects or not?
1-In general, if a theory has action at a distance effects, where can that appear exactly in the theory?
2-Does it appear in the dynamical law of the theory? (does it appear in Newton's 2nd law? ...
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experiment proposal to validate microcausality
I've been wondering about microcausality for some time now (a recent question of mine regarding the topic) and i'm wondering if its possible to devise an experiment to detect potential violations
I ...
