Questions tagged [causality]

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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
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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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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
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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
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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
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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
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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
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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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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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1answer
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Is general relativity compatible with an arrow of time?

The spacetime of Special Relativity is Minkowski's "Block Universe" where time is treated as a dimension. Does General Relativity allow for an arrow of time or "flow" of time? If yes, can this be ...
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1answer
98 views

What is a Past Event Horizon?

I've recently been informed about a few things from a reliable source: 1) Wormholes can't form via gravitational collapse 2) The event horizons in an ER bridge can't collide I understand what a ...
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4answers
247 views

Is wave function collapse non-local or local?

Imagine a particle in a very large box which takes years to travel from one end to the other. Alice and Bob are outside the box, on opposing ends. Each can remove their side of the box to check if the ...
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1answer
151 views

What do I see looking forward inside the black hole? [closed]

I have read this question: Future light cones inside black hole What happens to light that falls into a black hole? What would be the view like from inside a black hole looking towards the event ...
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2answers
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Why do we need 3 variables to parametrize $\scr{I}^\pm$ in a Penrose diagram?

In the figure we can see the Penrose diagram for Minkowski space If I understand correctly, $i^-$ and $\scr{I}^-$ have coordinates $r=\infty$ and $t=-\infty$ while $i^+$ and $\scr{I}^+$ have ...
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Time ordering and energy in Feynman diagrams

Given a topologically valid Feynman diagram, can one always sum over all momenta without regard to the sign of the $k_0$ on internal edges ? In other words is it true that the requirement that only ...
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1answer
44 views

Initial in time conditions and Lagrangian approach in classical mechanics

When we derive Euler-Lagrange equations in classical mechanics following the Lagrangian approach we introduce Boundary conditions at the starting- and end-points of the path in the configuration space....
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1answer
49 views

Let $M$ denote a connected time-oriented Lorentzian manifold. If $M$ is compact then $M$ must have a closed-curve

Problem Let $M$ denote a connected time-oriented Lorentzian manifold. If $M$ is compact then $M$ must have a closed-curve. I don't know how to use this information that M is compact to obtain a ...
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0answers
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Axiomatization of SR: can we replace light rays with timelike world-lines?

If I eliminate a lot of details and just sketch the general ideas, then a common way of presenting SR is this: Axiom 1: Clocks exist. Axiom 2: Light rays exist. This is the approach followed in, e.g....
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2answers
175 views

Is spacetime really not physical? [closed]

This question has nothing to do with the expansion of space or the speed of the expansion. I do understand that space expansion is not constrained by the speed of light limit, but I am not asking ...
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1answer
106 views

Does the speed of universal expansion lead to time going backwards? [closed]

After reading that time stops at the speed of light and at event horizons I wondered if when the speed of light is exceeded does time loop backwards ... The thought struck me too that the term space-...
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0answers
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Time-like null-like and space-like killing vector fields

If the Lie derivative of a metric tensor with respect to some vector field is zero then the vector is called killing vector (KV). The KV can be time-like, null-like and space-like. What is the ...
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2answers
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Why is the field commutator $[\phi(\vec x, t), \phi(\vec y, t)]$ related to causality?

It's well known that $$ \langle 0| \phi(\vec x, t) \phi(\vec y, t) |0 \rangle \neq \delta(\vec x - \vec y) . $$ It is then regularly argued that this is not a big problem since the commutator $$ \...
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3answers
793 views

Why are observers/reference frames able to see themselves moving through time but not through space?

All observers are stationary in their own reference frames. That is, their space coordinates are constant at all times (in their frame). However, they can see themselves moving through time. What ...
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2answers
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Is dynamics in GR unique?

Quote: "... a map $h$ of an open set $\Theta$ of a Bnach space $B_1$ into a Banach space $B_2$ is Lipschitz in $\Theta$ if there exists $k\in \mathbb{R}$ such that $||h(a)-h(b)||\leq k||a-b||$ for ...
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0answers
59 views

What would the eternal black hole look like?

The white hole and black hole regions in a Kruskal diagram are said to be actually two different locations. Given the problems with white holes it might be a silly question but, hypothetically, what ...
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3answers
192 views

Does uncertainty principle imply that the past history of universe is also undetermined?

I read the page here: Is the Uncertainty Principle valid for information about the past?, but I am still somewhat confused. If you measure the momentum/position (with uncertainty) of a particle, what ...
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0answers
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Imaginary term solution in the limit $v\ll c$ of relativistic Lagrangian

The relativistic action is $$ S=- m \int_a^b d s. $$ With metric $ds^2=dx^2 - dt^2$, we get: $$ \begin{align} S&=\pm m \int_a^b \sqrt{dx^2-dt^2}\\ &=\pm mc\int_a^b dt\sqrt{\left(\frac{dx}{...
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2answers
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Geodesics in general relativity changing from timelike to spacelike?

Is it possible for geodesic in general relativity to be time-like, and at some point cross null-plane and become space-like?
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1answer
259 views

Are black holes bound to merge once their event horizons touch? [duplicate]

Even if they move toward each other at relativistic velocities and not head-on? I looked at questions Black hole collision and the event horizon and Dynamics of Event Horizons between head-on ...
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0answers
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Question about operator spreading in QFT

It is a well-known effect that generic localized operators spread out under evolution by lattice Hamiltonians. For example , $e^{iHt} \sigma^x _j e^{-iHt}$ will in general not be supported only at ...
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0answers
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How to define the number of causally disconnected patches on the sky?

I'm considering the spatially flat geometry ($k = 0$) of the general FLRW universe: $$\tag{1} ds^2 = dt^2 - a^2(t) (\, dr^2 + r^2 \, d\vartheta^2 + r^2 \sin^2{\! \vartheta} \: d\varphi^2). $$ The ...
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1answer
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Horizon problem: Angular size of the causality patches on the CMB surface

I'm having difficulties in calculating the angular size of the causally connected regions on the cosmic microwave background (CMB), as seen from Earth today. I read in several documents that this ...
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5answers
301 views

Is it theoretically possible to escape a classical black hole with force?

A stone being thrown from the surface must have a velocity of $11.2$ km/sec if it wants to escape Earth's gravity. However, a stone that has a constant force being applied on it need not have an ...
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1answer
87 views

Circumstances of a superluminal causality violation [closed]

In the case of the Roman ring, a traveler is able to go through a wormhole mouth and emerge at some point in the past without violating causality, so long as the time traveled back is less than the ...
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2answers
81 views

In the most trivial spacetimes, is the existence of a null geodesic equivalent to horismos relations?

Take a globally hyperbolic topologically trivial spacetime $M \cong \mathbb{R} \times \Sigma$, $\Sigma \cong \mathbb{R}^{(n-1)}$. Given $p, q \in M$, such that there exists a future-directed null ...
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0answers
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Example of a metric of the form $ds^2 = g_{vv}(v,x)dv^2+2g_{vx}(v,x)dv\,dx+r^2(v,x)d\Omega^2$ with trapped region

Is there a known example of a spherically symmetric metric, especially one describing a spacetime containing trapped region/black hole, which takes the form $$ds^2 = g_{vv}(v,x)dv^2+2g_{vx}(v,x)dv\,dx+...
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1answer
167 views

Is the Schwarzschild wormhole (Einstein-Rosen Bridge) time dependent?

In Kruskal-Szekeres Coordinates for the Schwarzschild blackhole, we know that the wormhole "opens up" joining regions I and III between Kruskal times $T = -1$ (vertex of the green hyperbola below) and ...
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0answers
32 views

Singularities of conformal coefficient

It is known that the conformal transformation saves the causal structure of the spacetime. Then, what is the physical interpretation of the singularities of the conformal coefficient? I know that ...
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2answers
149 views

Physics near null infinity

The concept of null infinity $\mathscr{I}$ is standard in general relativity, and more recently in the analysis of infrared structure of gravity (see e.g. the article by Strominger). I am curious ...
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1answer
57 views

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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2answers
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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
100 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
619 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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2answers
420 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
82 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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