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Questions tagged [causality]

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Is a finite lightspeed necessary for cause and effect to exist?

In this question I asked in somewhat unclear (unclear enough to be closed) fashion about the relationship between an infinite speed of light and the existence of cause and effect. In thís question, I ...
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4answers
2k views

Are cause and effect the same as in our Universe in a non-relativistic, Newtonian Universe in which the speed of light is infinite? [on hold]

Suppose the Universe was non-relativistic so time and space would be independent of each other. In other words, both of them separately would be absolute and independent of an observer's motion (...
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1answer
46 views

Vanishing of a solution of Dirac equation

Let $\psi(x,t)$ be a solution of the free Dirac equation. Assume that $$\psi(\vec x,0)=\delta^{(3)}(\vec x) u,$$ where u is a fixed spinor. (In other words $\psi(\vec x,0)$ is assumed to be supported ...
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1answer
34 views

Hubble radius and communication between two observers

According to Dodelson, Modern Cosmology (p.146) There is a subtle distinction between the comoving horizon $\eta$ and the comoving Hubble radius $(aH)^{-1}$. If particles are separated by ...
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4answers
9k views

Is there a frame of reference in which I was born before I was conceived?

I'm struggling to understand the relativity of simultaneity and position. If my conception and birth are separated by time but not space, a frame of reference in which my birth and conception are ...
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1answer
40 views

Physics of time running backwards

Although it would seem weird to analyze physical phenomena when time runs backwards, it seems to have a logical sense, at least for me: Entropy would tend to decrease: two balls having energy ...
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2answers
55 views

Paths of least action and loops in time

In the book Quantum Field Theory for the Gifted Amateur link: https://books.google.ca/books?hl=en&lr=&id=nIk6AwAAQBAJ&oi=fnd&pg=PP1&ots=JZjwG_qDt5&sig=...
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1answer
61 views

What is time-like path in GR?

My understanding: Given a metric, at each point of spacetime, there is a tangent vector u that maximize the quantity $g_{ab}u^au^b$, which is the proper-time length. Does it mean at each point, there ...
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2answers
37 views

Time direction in the BH parallel exterior region

In the III region of the maximally extended Schwarzschild solution, described in the Kruskal coordinates , Sch. time runs in the reversed direction as compared to the I - event horizon III-II of the ...
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4answers
100 views

Does Electromagnetic wave propagation really use $E$ to create $B$

I was studying how electromagnetic waves (EM waves) were generated and propagated. I was shown a dipole antenna with an AC source sloshing charges back and forth. To my understanding, when the ...
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2answers
46 views

If some matter slows light going through it, is actual causality also slowed? [duplicate]

tldr; If some forms of matter change the that light travels through that it (like refraction in water or glass), does that mean the speed of causality is going slower through that matter? I've heard ...
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4answers
269 views

Are there null geodesics inside null infinity?

Looking at a Penrose diagram for Minkowski space, you would think that you could draw a null geodesic running from $i^0$, along $\mathscr{I}^+$, and ending up on $i^+$. In fact there would be many ...
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1answer
100 views

How to make sense of $\mathcal{I}^-$ as a Cauchy surface rigorously?

In some references, like Hawking's derivation of black hole radiation, one considers that $\mathcal{I}^-$ is a Cauchy surface. One recent reference with such a claim is the paper "Soft Hair on Black ...
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1answer
37 views

Looking back in past [closed]

Can faster than light communication give us the ability to see in the past, that is to say, if we can imagine a ultra huge highly advance telescope 60 light minutes away from earth and we want to look ...
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2answers
92 views

Can the future affect the past in general relativity? [closed]

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
70 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
162 views

The physics of a cornering wheel and its centripetal force

Can a rolling wheel create a side force without first rotating on a vertical axis? There is something wrong with the way we describe how a cornering vehicle wheel creates a cornering force. I think ...
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1answer
48 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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6answers
3k views

Why does a sign difference between space and time lead to time that only flows forward?

Ever since special relativity we've had this equation that puts time and space on an equal footing: $$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2.$$ But they're obviously not equivalent, because there's a ...
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1answer
90 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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0answers
41 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
168 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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4answers
118 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 ...
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4answers
149 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
2k views

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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0answers
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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
70 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 ...
3
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1answer
104 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
90 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
3k 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
113 views

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
69 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
94 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
99 views

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
63 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
136 views

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 "$i\epsilon$...
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4answers
175 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
100 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
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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
64 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
172 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
62 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
67 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
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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
204 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
93 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
111 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
163 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
122 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 ...