Questions tagged [causality]

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1answer
68 views

Can quantum particles really communicate in the past?

The quantum entanglement and delayed choice quantum eraser variation to the double slit experiment has given very strange results. So if a particle is detected then the pattern is changed. Does the ...
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1answer
44 views

Interpretation of Feynman propagator for massive scalar field in position space

I've always treated propagator in the momentum representation so when it diverges, we are on-shell. But what is the interpretation of light-cone divergences in position space? If it is something we ...
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2answers
237 views

What is causality? [closed]

I was looking a few videos on youtube and I came across a video named the speed of light is nothing related to light. In the video, it was said that speed of light is actually speed of causality so ...
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0answers
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Are causal sets based on set-theory?

There is a theoretical approach with the aim of formulating a quantum gravity theory which uses causal sets. Are these sets based on mathematical set theory? Are causal sets part or belong to the ...
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1answer
32 views

Determining global hyperbolicity

Given a spacetime metric, how can one show that the spacetime is globally hyperbolic? I know that a globally hyperbolic metric has a Cauchy surface, but how can we determine the existence of a Cauchy ...
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1answer
66 views

Changing the signature of the metric and consecuences in the causal structure of the spacetime

I was reading about the implications in the causal structure due to a change in the signature of the metric. I know that if you choose a spacetime $(M,g_{ab})$. With $g_{ab}$ a Lorentzian metric, of ...
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1answer
91 views

Gravity at light speed

my question in regards to light. Since gravity warps space time, and we have discovered gravitational waves. Would the light at the front of the wave, be traveling faster than lightspeed? Essentially ...
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1answer
36 views

Constraints of Contact Forces

Are contact forces limited to the speed of light like non-contact forces like gravity? If I took an infinitely long newton's cradle and placed in a vacuum with zero energy loss, would the time between ...
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0answers
103 views

Spacelike, timelike, lightlike vectors and the light-cone structure

Consider a semi-Riemannian manifold which of these statements is false: All vectors on the light-cone are light-like, all vectors in the interior of the light-cone are time-like and all vectors in ...
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3answers
48 views

How fast is electrostatic force? [closed]

Suppose we have a charge $Q$ in deep space (isolated), now suppose another charge $q$ appears instantly (no matter how). Obviously it will feel electrostatic force but the question is how quickly (...
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0answers
25 views

FTL communication and paradoxes with local causality

I have Yet Another Question about FTL communication, causality and sending a message into your past. I've read this article and this excellent answer and I understand why it's possible to find a ...
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0answers
19 views

Divergence of a vector which has explicit and implicit position dependence

I am doing EMT and I am trying to calculate the divergence of this current density given as, $$\vec{J}(\vec{r}', t_r) = \vec{J}(\vec{r}', t - \frac{|\vec{r}-\vec{r}'|}{c})$$ for $\vec{r} = (x,y,z)$ ...
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1answer
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What Ron Mallett doing for time machine here? [duplicate]

Here Ron Mallett pouring liquid chemical on array of lasers for progress of time machine. I didn't understand it. Source What exactly Ron Mallett doing for time machine here?
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1answer
77 views

Resolving the measurement problem with mathematical theorems? [closed]

The key debate, around the measurement problem is whether collapse should be interpreted as a physical process(Bohmian Mechanics) or as an immaterial process(e.g. Copenhagen Interpretation, ...
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0answers
45 views

Does effect of force travel faster than light? [duplicate]

Suppose force (sufficient to move a block & imagine it(the block of course) to be extremely long and rigid ) is applied at one end of the block and the Block starts moving. Now force must be ...
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0answers
65 views

How to realize explicitly an $SO^+(1,3)$ Lorentz transformation of $(x-y)$ to $(y-x)$ for space-like separation?

Consider two spacetime points $x, y$ which are space-like separated. How can one realize explicitly a proper, orthochronous Lorentz transformation between two frames such that $x-y$ becomes $y-x$? ...
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1answer
60 views

Is temporal causality broken by quantum entanglement?

When one particle of an entangled pair is measured, it forces the state of the other particle. This effect propagates faster than we can measure, certainly faster than light and possibly ...
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1answer
63 views

What happens when you remove the mass inside a black hole?

Imagine you could delete the sun from its existence at an instance. Information cannot travel faster than the speed of light so the earth will keep orbiting the sun for about 8 minutes without knowing ...
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0answers
48 views

Simultaneity and Tachyons

I open this question because the discussion on @Demigan's question did not come to a satisfactory conclusion. One of the best discussions on tachyon relativity that I’ve seen is by David Bohm in his ...
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0answers
52 views

Would Doubly Special Relativity or Mach's Principle allow faster-than-light travel to occur without violating causality?

Doubly Special Relativity posits that the Planck length is the same in all reference frames. Mach's Principle posits inertia is produced by the combined mass of the universe. From my limited ...
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2answers
56 views

Lorentz vectors orthogonality

I am reading through the book "Electrodynamics and Classical Theory of Fields and Particles" by A.O. Barut, and I'm still currently in the first chapter of the book in which he is discussing Lorentz ...
2
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1answer
76 views

What is the mathematical meaning of locality?

I have read on the Principle of locality Wikipedia page that: "The special theory of relativity limits the speed at which all such influences can travel to the speed of light, c. Therefore, the ...
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0answers
42 views

What is an equation which tells you if two points are time-like separated in curved spacetime?

I was thinking about how to tell if two points in a general spacetime are time-like separated. I think the only way of doing this is checking every path from $x$ to $y$ and seeing if one of the paths ...
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2answers
83 views

Given a metric what is the condition that 2 points are causally linked?

In Minkowski space-time two points $x$ and $y$ are causally linked if they are within each other's lightcones. $|x-y|^2<0$. Given a general metric $g^{\mu\nu}(x)$ with no time-loops or exotic ...
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3answers
143 views

Does the light-cone at every point plus a scale uniquely define a spacetime metric?

In general relativity, knowing the local light-cone shape at each point (i.e. the tilt, and angles of the light cone) doesn't seem enough to get the metric. Do we also need a scale as well at each ...
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0answers
64 views

What is the null vector for a metric?

I am studying the Robertson Walker metric. I now understand what the geodesic looks like but what is a null vector of the geodesic? Is it just the tangent vector to the geodesic curve?
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1answer
79 views

Particle picture in position space in quantum field theory

When I operate $a^{\dagger}_k$ on vacuum, $|0\rangle$, I get a particle created in momentum space with a 4-momentum equal to $(\omega_k, \vec{k})$ where $\omega_k=\sqrt{m^2+\vec{k}^2}$ here I'm only ...
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2answers
652 views

Do the forces between Earth and Sun point in the directions that the Earth and the Sun were 8 minutes ago?

No information can travel faster than light, so does the force between Earth and Sun point in the direction that the Earth was 8 minutes ago or in the direction that the Earth is now?
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2answers
53 views

Simulation of Feynman path integral in literature

Can someone provide me a numerical simulation of Feynman path integral? Where the contribution of each path is added individually so that I can understand how much do the paths outside the light cone ...
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2answers
59 views

If the method of FTL cannot be observed, does it still break causality [duplicate]

So, say we discover a method of FTL. However, anyone or anything that makes use of this FTL is unable to be observed by the outside world. For example. In the year 3000 a ship leaves Earth headed 6 ...
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0answers
150 views

Causality, branch cut choice and analytic continuation of Euclidean 2-pt. correlator in 2D CFT

In 2D CFT, the Euclidean two point correlator of a primary operator $\mathcal{O}$ with conformal weights $h$, $\bar{h}$ is given by $$ \begin{align} \langle\mathcal{O}(z,\bar{z})\mathcal{O}(0,0)\...
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3answers
78 views

Irreversible processes in special relativity

According to special relativity (as I understand it) simultaneity is no longer a universal concept in special relativity. Consider two events A and B which are simultaneous in reference frame 1; in ...
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1answer
69 views

Time Ordering and Causality in General Relativity

The time-ordering of two space-like separated events is undefined in special relativity. Is there an analogous concept in GR?
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1answer
36 views

Question regarding speed of causality and speed of light

Recently I came across a question regarding to why the speed of light and the speed of causality are the same. Link-Why is the speed of causality equal to the speed of light? I came up with an ...
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4answers
167 views

Why is the speed of causality equal to the speed of light?

I heard that the most fundament concept is Causality, and also that it is turn out that speed of light has nothing special in it so that It is ultimate speed limit but because it is happen to be ...
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4answers
214 views

What is the chain of cause(s) and effect(s) which does a butterfly's wing-flapping cause a hurricane 1000's of kilometers further away?

I once asked if the butterfly effect in the weather system is a real effect: a butterfly flaps her wings, which can cause a hurricane thousands of kilometers further. But how will this happen? To be ...
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1answer
48 views

Is there a physical interpretation of the wave equation with a third (or higher) order time derivative?

In physics we learn about the heat equation, $$ \frac{\partial u}{\partial t} = k \Delta u,$$ which describes how heat propagates in a material. We also learn about the wave equation, $$ \frac{\...
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1answer
27 views

Behavior of particles at very small scale of spacetime

If I act $\phi(x)$ on $|0\rangle$ I get a particle created at $x$. Now if I let my system evolve for some time and now I do the measurement on this system at $y$ and my $(x-y)^2$ is small let say of ...
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2answers
100 views

Is causality concurrent (ie. multi-threaded)? [closed]

There exist well-known problems in programming concurrency and multi-threading such as deadlocks and race conditions that can cause undefined behavior when running such programs. A race condition is ...
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0answers
101 views

Does quantum entanglement imply a universal “now”? [duplicate]

If a particle on earth is entangled with a partner particle in Andromeda, and the wave function collapses here on earth, the wave function would, as I understand it, collapse in Andromeda too. Does ...
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1answer
401 views

Can Increment / maximization of Entropy be the “Cause” behind any phenomena?

I understand that the increment/maximization of Entropy (of the universe) is "Accompanied" with all "Natural" phenomena we see. In many of the questions, I and others have asked on Stack Exchange, ...
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1answer
83 views

Black holes, and faster than light communication

Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other ...
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3answers
122 views

How does disregarding realism but maintaining locality explain the Bell Experiment?

If we conduct a simple Bell experiment, we can show that "hidden attributes" are inconsistent with the probabilisitic distribution of results that we get in an Alice/Bob type game played with ...
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2answers
78 views

How do we know the speed of gravity? [duplicate]

I have read things like if the sun disappeared, it would take 8 minutes or so before we knew this, cf. e.g. this Phys.SE post. However, in real life the sun will never disappear and if it exploded, ...
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5answers
94 views

Force can only come from force, right? [closed]

Force is the interaction between objects, so force can only come from force, right? Can force come from velocity? Is the cause of one force another?
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3answers
70 views

Information propagating at less than the speed of light

Which speed does information propagate at in a medium? For example, if we live in a pool which is one light-year cube and somebody dewaters of the pool from an observer which is at 1 light-year ...
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2answers
716 views

If you suddenly increase the force, is the reaction force equal to the force?

If you suddenly increase the force, is the reaction force equal to the force? Press a force on the object, and then suddenly increase the force. In this process, is the reaction force equal to the ...
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2answers
106 views

Why is electricity not faster than light if all the electrons are just shifted in the wire?

https://www.youtube.com/watch?v=jbi7gJTPSXk Please check 1:33. Many people said that all the electrons are just shifted at the same time when a current flows in the wire. So electricity can move ...
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1answer
98 views

Does exist a (truly) stochastic but causal physical process?

I wonder if it is possible for causality to be held, but not determinism. Does any real-world example exist? If yes, please explain how does that satisfy causality and not determinism. (for example, ...
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2answers
128 views

I could prove that Copenhagen QM and causality are incompatible; is it right? [closed]

A quantum system consists of some states. It is in all of the states but beCAUSE of the act of the measurement, the wavefunction collapses into one state. I am really confused because I can repeat the ...

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