Questions tagged [causality]

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What do spacelike and timelike intervals represent? [duplicate]

I've understood that with timelike intervals all observers agree on the order of events and with spacelike intervals the order of the events can be changed with the reference frame. However, I wanted ...
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Graviton propagator, and Gauss-Bonnet gravity

Let's say we consider Einstein's Lagrangian from GR. In linearized gravity, we would expand the Ricci scalar to quadratic order in the perturbation parameter to find the propagator. My question is as ...
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What does the time reversibility of the laws of physics mean for causality?

Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
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1answer
157 views

Wave function collapse violates causality?

I have read this question: Does photon interference violate causality? where S.Mcgrew says: Edit 8/10/18 Bottom line: you're right that wave function collapse violates the principle of ...
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Why using time-ordering causality? and what is Difference between Locality and Causality?

In QFT, the time-ordering causality is generally used. There are 4 ways to bypassing the pole called time-ordering, anti-time-ordering, retarded and advanced. But in many case only time-ordered ...
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2answers
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Is spacetime in special relativity hyperbolic?

I thought the spacetime diagrams were usual flat planes just with a new formula for distance. But the source I was reading suddenly started talking about hyperbolic angles in triangles ( I don't know ...
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Raychaudhuri equation for black holes

Since the Raychaudhuri equation is defined only for timeline and null geodesic congruences, is it valid to use his equation to describe the null generators of the event horizon of a black hole? ...
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3answers
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Does the Doppler effect happen instantly?

Assume that a far star sends light toward a receiver. If we move this antenna such that it accelerate first for a moment, and then it moves with constant speed, we can see that the frequency of the ...
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Timelike, spacelike etc. for higher-order tensors

Vectors $V^\mu$ in relativity can be classified into those which are timelike, spacelike and null. A similar classification is available for tensors: A tensor $$T^{\mu_1\mu_2...\mu_p}_{\phantom{\mu_1\...
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1answer
166 views

Uniqueness constraint(s) on spacetime

What additional constraint(s), if any, must be used with the gravitational field equations $$R_{\mu\nu}=\kappa \left( T_{\mu\nu}-\frac{1}{2}Tg_{\mu\nu} \right)$$ to uniquely determine the Christoffel ...
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Light-like normal vectors

Can someone please show me how to mathematically establish that the normal vector to the event horizon of a Kerr Black Hole is light-like?
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Constraints vs Boundary Conditions

I have a very broad question about how the mathematical framework that classical theories of physics utilize to solve problems. The question is: What are the intrinsic differences between the ...
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1answer
116 views

Rotating Black Holes

All stars rotate. And the more they contract the faster the rotation, so is there such a thing as a non-rotating black hole? And as gravity is less at the equator of a rotating star, assuming that ...
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How can time go in different directions in the Universe?

This is not a duplicate, I am not asking about any kind of time dilation caused by a BH. My question is about the direction of time (and if it is possible to have different directions for time, other ...
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2answers
130 views

What is meant by globally hyperbolic spacetime? [duplicate]

Can somebody explain what is meant by globally hyperbolic spacetime? What other kinds can there be?
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On causality in physics: is the concept of cause an operational concept in physics? [duplicate]

This is a broad question motivated by philosophical interests. I hope it will not be considered as off topic. What I am looking for is a technical answer reflecting the standard conception of ...
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2answers
118 views

Difference between an entangled pair of photon and two magnets in a box? [duplicate]

I try to understand quantum entanglement and especially what it’s called « Action at a distance » from my understanding, if you have a pair of entangled photon, after measuring the polarization of ...
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Antiparticles travelling backward through time [duplicate]

Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time? I have read about their idea of an antiparticle behaving as a particle travelling ...
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2answers
114 views

What is the formal difference between the light cone and a black hole?

A black hole can be loosely defined as a spatial closed surface from which nothing, not even light, can leave. The light cone of special relativity is in some sense similar to a black hole because by ...
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35 views

Penrose diagram and coordinate transformation

I am looking at the Minkowski line element in spherical coordinates $$ \mathrm{d}s^2 = - \mathrm{d}t^2 + \mathrm{d}r^2 + r^2 \mathrm{d}\theta^2 + r^2 \sin^2(\theta) \mathrm{d}\phi^2$$ and want to ...
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1answer
348 views

Is a vector space automatically spacelike if it has a basis of spacelike vectors?

I am studying Kerr Spacetime and I am not sure about something used in a proof I am trying to understand. I am wondering, if you consider a 4-dimensional Lorentzian manifold $\mathcal{M}$ and $X_i \...
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Rindler's trap door experiment with a rod (or a tank, or a pile of fragile matter) of finite hight

About 60 years ago a thought experiment was proposed which is widely referenced, e.g. as "Man falling into grate variation" of "The ladder paradox" or, perhaps more poignantly, "The tank paradox": ...
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Are there theories or experiments involving multi variable particles?

Every hidden variable/entanglement experiment I’ve ever heard described involves ONLY one variable of either polarization or spin. Therefore Venn calculations backed by diagrams highlight the ...
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958 views

How does the Penrose diagram for a spinning black hole differ in realistic scenarios (formed by stellar collapse)?

The Penrose diagram for a non-spinning Schwarzschild black hole is Notably, there is a second universe "on the other side" of the black hole. However, actual black holes form by stellar collapse, and ...
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2answers
137 views

How Maxwell theory of electromagnetism solved action at a distance problem?

According to wikipedia Maxwell's equations were an essential inspiration for Einstein's development of special relativity. Possibly the most important aspect was their denial of instantaneous action ...
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Hausdorff property in Minkowski spacetime

In the 4-dimensional Minkowski spacetime, for a given point $x = (x^0,x^1,x^2,x^3)$, its timelike future/past set is defined as, $$ I^{\pm}(x) = \{y =(y^0,...,y^3) \in \mathbb{R}^4 : \eta_{\mu \nu}(y-...
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Microcausality for Dirac's current

I`m supposed to show as an exercises that for the Dirac field's associated current: $$j^\mu=\bar{\Psi}\gamma^\mu\Psi$$ The microcausality relation holds: $$ [j^\mu(x),j^\nu(y)]=0 \text{ for } (x-y)^...
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Does the state of curl of the $E$-Field at a point adjust itself instantaneously as soon as $B$ begins changing at a fixed rate, or is there delay? [duplicate]

Faraday’s Law () states that a time-changing magnetic field vector induces a curl of the Electric field around that point. However it does not specify how quickly the necessary spatial gradient is ...
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How quickly is the desired state of curl of the Electric Field around a point achieved where a Magnetic Field Vector has just begun changing?

My question and context with explanation are given below. Thank you in advance. Faraday’s Law () states that a time-changing magnetic field vector induces a curl of the Electric field around that ...
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1answer
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Difficulty in understanding the three classes of events of light-cone

Recently I'm reading Chapter 2: Space and Time of A Brief History of Time. The paragraph that follows the light cone says: Given an event P, one can divide the other events in the universe into ...
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2answers
403 views

Minkowski space

In Minkowski space, coordinates which satisfy $x^2 = t^2 - X^2 > 0$ are in the region of spacetime that is time-like. If it's $x^2 = t^2 - X^2 < 0$ the region is space-like. But if $x^2 = ...
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Aging while traveling faster than speed of light? [closed]

theoretically, if organic matter was able to travel faster than the speed of light, without becoming "pure energy", and since theoretically traveling faster then the speed of light would travel ...
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Locality and relativity - a paradox?

The equations of nature are supposed to exhibit locality in the sense that the action depends on fields and their derivatives. i.e. comparing the values of fields at local points. But two points on a ...
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2answers
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Is a finite lightspeed necessary for cause and effect to exist? [duplicate]

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
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Will an infinite speed of light stop things from moving and can that be reason that the speed of light is actually finite? [closed]

I edited this question thoroughly, after reading it again and realizing which was the reason why it was closed. Assume the speed of light would be infinite. Of course, one can wonder how it can be ...
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1answer
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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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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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3answers
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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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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
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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
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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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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
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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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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
373 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
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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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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
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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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391 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
1k views

The dynamics of a cornering wheel

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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