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Questions tagged [causality]

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Can the future affect the past in general relativity?

Hypothetically, let us suppose that a black hole were to suddenly appear at time $t=1$ at position $x$. Can the effect of the black hole be felt at time $t = 1-\epsilon$ near $x$ due to the bending of ...
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2answers
49 views

Spacelike and timelike intervals confusion

I'm confused about this, specifically the spacetime interval. A timelike interval is one in which 2 events can be related to each other in a given reference frame within its light cone, that is, it ...
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1answer
59 views

The physics of a cornering wheel and its centripetal force

There is something wrong with the way we describe cornering vehicle wheels. My question is: can anyone else see this? Centripetal force is a force always orthogonal to the motion of a body. When a ...
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0answers
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Is causality a universal law? [on hold]

Sorry, I'm not very confident on my understanding of anything at a quantum level and superpositions have me wondering....
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1answer
43 views

Does quantum forbid duplication or copy of states?

Suppose a beam of particles or a single particle, or a single particle. Is it possible to make a duplication of the beam or a single particle without collapse the states of the original particles? I ...
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1answer
86 views

Does light know the future? [duplicate]

As I understand it, if light is traveling at the speed of light, then from it's point of view space is fully compressed in its direction of travel. Does that mean that from it's point of view, light ...
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39 views

Are vacuum-fluctuations a consequence of causality?

I'n new to QFT, and recently lerned about the propagator of a free scalar field theory in Minkowski-space, which according to our lecture notes looks like $$G(p, q) = \frac{1}{q^\mu q_\mu + M^2} \...
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2answers
161 views

Why can't we insert gravity in the special relativistic lagrangian?

I am a math student and I have taken four-five lessons about special relativity in a course about Lagrangian and Hamiltonian mechanics, so be patient with me if my question is stupid. My teacher says ...
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0answers
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Causal structure of spacetime. A pathological example of spacetime?

Consider a spacetime $(M,g)$ and $p \in M$. It is known that locally, the boundary of the chronological future of p is just the set of points along (future-directed) null geodesics starting at p. ...
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4answers
108 views

Exceeding the speed of light [closed]

I understand that the speed of light c is derived from the self-interaction between elections/photons, and is thus the maximum speed of anything composed of electrons/photons. Suppose that there is a ...
2
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4answers
128 views

Understanding the difference between timelike and spacelike separations

From Woodhouse's General Relativity: If $A$ is the origin and $B$ is a nearby event with coordinates $dt, dx, dy, dz$, then, $$ds^2 = dt^2 - dx^2 - dy^2 - dz^2$$ is the same in all local inertial ...
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4answers
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No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
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“Speed” of gravity? [duplicate]

This is probably going to be a poorly written question, since I know really a little of physics. Imagine a object of small mass, let's say a book, is floating in the universe in absence of any ...
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1answer
66 views

Why does my thought experiment involving entanglement appear to violate the speed of light?

I have a thought experiment that "seems" to allow messages passed at faster than the speed of light. Since that is impossible I'd like to learn what is wrong with my experiment/reasoning. Start with ...
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1answer
92 views

Do light cones “tilt” towards black holes?

In some diagrams, light cones becomes "thinner" near black holes. Meaning the light has trouble moving nearer or further away. As in this picture. I presume this corresponds to someone observing that ...
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2answers
87 views

Does basic QM allow for superluminal “particle movement” during wavefunction collapse?

Can particles move superluminally away from their "expected values" using basic quantum theory? Here's an example: The eigenstates of a harmonic oscillator are defined from $(-\infty, \infty)$. This ...
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9answers
2k views

What precisely does it mean for “information to not travel faster than the speed of light”?

This is something that's been bothering me for a while. The way we usually first hear about causality is that "nothing travels faster than $c$". But then you learn that phase velocities can ...
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0answers
28 views

In Causal Set Theory what constitutes a final state?

Causal Set Theory consists of a set of events and a partial time ordering. But what would constitute a final state? If you took as a final state simply a set of events which have no time-ordering, ...
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2answers
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Infinite Speed of Light

I recently watched a video that stated that Newtonian Mechanics assumed an infinite speed of light. That same video, "PBS Space Time: The Speed of Light is Not About Light", stated that if the speed ...
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2answers
57 views

Does retro-causality imply unpredictability?

In some interpretations of Quantum mechanics (e.g. transactional interpretation), the future affects present. Is this a source of unpredictability in such interpretations, which makes them have the ...
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1answer
72 views

What is the difference between space-like four-momentum and time-like four-momentum? [closed]

I was reading a wiki page on Tachyon, came across these terms? What i need is bit of a mathematical description to understand these terms?
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0answers
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How does one find the Vaidya black hole event horizon?

As for a definition, there are quite precise ones for what an event horizon is. One can define it as the boundary of the causal past of future null infinity, i.e., $\mathcal{H}=\partial J^-(\mathscr{I}...
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1answer
58 views

Are there timelike 3D surfaces in special relativity

I am reading Scharf's 'finite QED' and I am puzzled at the beginning. He first introduces Minkowski space with $(+,-,-,-)$ signature, and here is a definition I find difficult: A three-...
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0answers
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Feynman propagator for Dirac fields and $i\epsilon$ prescription for analytic continuation

The analytic continuation from Euclidean space to Minkowski spacetime is perturbatively well (uniquely) defined to all orders for the Feynman propagator for Dirac fields with the so called "$i\epsilon$...
2
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4answers
171 views

When electron moves constantly, it's electric field moves with it instantly?

When electron accelerates, there occurs a propagated ripple on it's electric field. But when it moves constantly, does the field "follow it", i.e. changes instantly? How does it deals with the fact ...
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3answers
96 views

Will the electron affected by another electron via Coulomb's force affect this electron instantly? [duplicate]

Coulomb's law is strict: $F=k\dfrac{q_1*q_2}{r^2}$ that means that between two charges occurs force. I.e. occurs force on $q_1$ and on $q_2$. If there are two electrons in vacuum with 300 00 000 ...
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0answers
60 views

At what critical Reynolds number does vortex shedding begin?

In: "Fluid Dynamics", Chapter 3 (Turbulence), Section 26, Landau and Lifchitz analyze the problem of the stability of a steady flow past a body of finite size. The fluid is assumed to be ...
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0answers
42 views

Dirac field local observables

This is actually a continuation of calculation I've been working on. It is well known that, in the case of Dirac fields $\psi(x)$, they satisfy anticommutatation relationships since they're fermionic ...
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4answers
161 views

Metric of the Universe

As some of you know, there is a fundamental flat space-time metric that describes our universe without any energy or matter in it. Correct me if I am wrong, but this metric and existance of the ...
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1answer
60 views

Chronology protection: current status

I am looking for some fresh references on the Chronology Protection Conjecture. I am aware of this question, but the answer there seems to resort to energy conditions. But, weren't they shown violated ...
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1answer
66 views

Faster than light in quantum gravity?

Imagine there's two objects a light second apart in a space with a certain metric. So no signal can reach the other under a second. But in quantum gravity where we sum over metrics, there may be a ...
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0answers
19 views

Switching between advanced and retarded solutions mid-integral?

In wave mechanics, an advanced solution can be thought of as a wave that propagates until it is "caught" and stopped by the forcing function, and a retarded solution can be thought of as a wave that ...
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2answers
198 views

Why doesn't Faster Than Sound information transfer imply time travel or violate causality? [closed]

I am struggling to understand the answer to What are some scenarios where FTL information transfer would violate causality? In particular, I have not seen a satisfactory rebuttal to the comment from ...
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0answers
67 views

Question on affine parameter of null world-lines versus light-like world-lines

I consider Minkowski space $M$ in this question. My question is about the following. For lightlike worldlines we can define the geodesic equation as follows: $$\nabla_TT=0,$$ where $$T = \gamma^\...
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3answers
95 views

Could you escape a black hole - using a black hole? [duplicate]

I'm pretty sure the answer to the question in title is "No". But why? Below is a naive Newtonian simulation I made. You can see that in the animation, both black hole's horizons seem to "recede" for ...
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2answers
83 views

What is a light cone? [closed]

What is a light cone? Why we can't escape the light cone? Why the speed of light being the limit for us to escape the cone the future and the past events of the light cone is that governs the future? ...
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1answer
92 views

Propagator Causality with commutators all the way

We know that two fields commute - by locality and causality - iff there is spacelike separation $\left[\phi_l^k(x) , \phi_m^{k'}(y)\right] = 0$ for $(x-y)^2<0$ In the canonical quantization of ...
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2answers
36 views

Why do we locate stars based on the light we see from earth?

Mapping space from our milky-way to Laniakea and the CMB, we always put a star or galaxies position from the light we see in that 3d coordinates of the universe. But really if a star or galaxy we look ...
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0answers
46 views

Is the Taub-NUT solution stationary?

The Wikipedia article about the Taub-NUT spacetime says that it was a first attempt in finding the Kerr solution. Since the Kerr spacetime is a stationary solution, meaning that it admits an ...
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0answers
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Is simulating the interaction of multiple particles possible in $0$ spacelike and $3$ timelike dimensions?

In multiple time like dimensions it is possible for particles to effect it's own past, and two particles can both be effected by each others future. I tried to simulate the interaction of multiple ...
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2answers
119 views

What is a 'spacelike surface' in relativity?

I am studying Noether's theorem in field theory and I am not understanding what spacelike-surfaces mean. I will reproduce the bit of the argument below that contains the term "spacelike-sufaces" in ...
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1answer
77 views

Understanding causality violation of tachyons in space-time diagrams [duplicate]

I have a hard time understanding how the world lines of tachyons in a space-time diagram imply the violation of causality. Can you explain why that is?
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1answer
60 views

If the path integral formulation includes future events, why doesn't that imply retrocausality?

I know that such events would cancel out in the math, but if an extreme event were to happen in the future (say a black hole forming or something on that par), would a particle in the present react to ...
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1answer
64 views

About the (retro)causality of an anti-atom

It is commonly said that one can view anti-matter as 'matter going back in time'. I'm trying to figure out how far this analogy goes. For an atom, it is well-known how that atom excites absorbing a ...
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1answer
44 views

Maximum velocity of interactions

In chapter 1, Section 1, para 7, of Landau & Lifshitz, Classical Theory of Fields, they argue that if a body moves faster than maximum velocity $V_m$ of interactions, that implies we can have an ...
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3answers
121 views

Faster-than-light communication using Taylor's theorem? [closed]

I was thinking about Taylor's theorem and how if a function $f(x)$ is analytic at a point $a$ and one can measure all the derivatives at $a$, $f^{(n)}(a)$ then one knows the complete behaviour of the ...
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1answer
50 views

In a globally-hyperbolic spacetime, does every pair of elements have overlapping light cones?

Suppose we have a spacetime $(M,g)$, and denote by $J^+(p)$ the set of points that lie in the causal future of $p$, i.e. $x \in J^+(p)$ iff there is a future-directed timelike curve $\gamma: [0,1]\...
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5answers
180 views

Does photon interference violate causality?

Suppose a hydrogen atom jumps down an energy level and emits a photon and that photon later goes through a double slit and gets absorbed by one atom of a screen making it jump up an energy level. ...
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1answer
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Why do wormholes necessarily alter time? Why don't they just make two places causally linked?

I've heard many-an-explanation of wormholes. From as a kid with the hole punch through paper, to the sci-fi something something magic, to popular YouTube science channels talking about how it could ...
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0answers
75 views

Analytic cotinuation between Minkowskian and Euclidean space, and causality

We can flip between Minkowkian and Euclidean signature by Wick rotation, and it is a well defined operation, provided there are no non - trivial singularities. Now, Unitarity in Minkowskian space ...