Questions tagged [causality]

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54 views

What are three predictions of quantum mechanics? [closed]

Specifically discussing the phenomenon of photons traveling through polarizers, what predictions are made by quantum mechanics?Bell’s theorem says “No physical theory of local Hidden Variables can ...
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1answer
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Define event horizon using only the notions of events and causality

Does this work? Consider a set $B$ of events which satisfies If $x$ belongs to $B$ and $x$ causes $y$ then $y$ belongs to $B$. The event horizon of $B$ is the set of events that are not in $B$ but ...
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Causality under relativity [duplicate]

If simultaneity is relative; meaning that for each any events A and B and we can find a reference frame in which two events A and B occur in inverse order, how does our notion of 'causality' withholds?...
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1answer
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Gravitational field of sun [closed]

Let there be a solar system without Earth. Now what happen if we place the Earth suddenly on its actual position does the gravitational force of sun acts on it immediately because of its pre ...
4
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1answer
91 views

Are extra dimensions timelike or spacelike?

In special relativity there is a clear difference between spatial and temporal dimensions of spacetime due to the Minkowski metric diag(-1,1,1,1). In higher dimensional theories (10- and 26-...
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4answers
570 views

Why radial coordinate of a particle must decrease continuously once it is inside the Schwarzschild radius?

Suppose we are inside the Schwarzschild radius of a black hole and throw a ball radially outward. It is said that the ball has no possibility to increase its radial coordinate. It must continuously ...
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3answers
348 views

How do we know we cannot escape the event horizon?

A doubt assails me: deepening the Schwarzschild and Kerr space-times I found a beautiful article of the University of Rome, which provides a method to identify an event horizon based on the analysis ...
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1answer
35 views

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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63 views

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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107 views

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
79 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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1answer
54 views

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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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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Proof that the boundary of the causal past of a Cauchy surface is the Cauchy surface

Let $(M,g)$ be a globally hyperbolic spacetime. Let $\Sigma$ be a Cauchy surface in $(M,g)$. In this paper, page 9, Lemma A.1, the author says that if we take $D = J^{-}(\Sigma)\setminus \Sigma$ then $...
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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
153 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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1answer
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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
105 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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2answers
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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
84 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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3answers
112 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
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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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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
269 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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0answers
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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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1answer
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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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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
104 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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1answer
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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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1answer
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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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1answer
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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
377 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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1answer
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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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Are cause and effect the same as in our Universe in a non-relativistic, Newtonian Universe in which the speed of light is infinite? [closed]

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
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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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1answer
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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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4answers
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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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1answer
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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 ...