Questions tagged [causality]

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1answer
69 views

What is the mathematical meaning of locality?

I have read on the Principle of locality Wikipedia page that: "The special theory of relativity limits the speed at which all such influences can travel to the speed of light, c. Therefore, the ...
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0answers
40 views

What is an equation which tells you if two points are time-like separated in curved spacetime?

I was thinking about how to tell if two points in a general spacetime are time-like separated. I think the only way of doing this is checking every path from $x$ to $y$ and seeing if one of the paths ...
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2answers
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Given a metric what is the condition that 2 points are causally linked?

In Minkowski space-time two points $x$ and $y$ are causally linked if they are within each other's lightcones. $|x-y|^2<0$. Given a general metric $g^{\mu\nu}(x)$ with no time-loops or exotic ...
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3answers
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Does the light-cone at every point plus a scale uniquely define a spacetime metric?

In general relativity, knowing the local light-cone shape at each point (i.e. the tilt, and angles of the light cone) doesn't seem enough to get the metric. Do we also need a scale as well at each ...
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1answer
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Particle picture in position space in quantum field theory

When I operate $a^{\dagger}_k$ on vacuum, $|0\rangle$, I get a particle created in momentum space with a 4-momentum equal to $(\omega_k, \vec{k})$ where $\omega_k=\sqrt{m^2+\vec{k}^2}$ here I'm only ...
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2answers
647 views

Do the forces between Earth and Sun point in the directions that the Earth and the Sun were 8 minutes ago?

No information can travel faster than light, so does the force between Earth and Sun point in the direction that the Earth was 8 minutes ago or in the direction that the Earth is now?
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2answers
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Simulation of Feynman path integral in literature

Can someone provide me a numerical simulation of Feynman path integral? Where the contribution of each path is added individually so that I can understand how much do the paths outside the light cone ...
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2answers
58 views

If the method of FTL cannot be observed, does it still break causality [duplicate]

So, say we discover a method of FTL. However, anyone or anything that makes use of this FTL is unable to be observed by the outside world. For example. In the year 3000 a ship leaves Earth headed 6 ...
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0answers
135 views

Causality, branch cut choice and analytic continuation of Euclidean 2-pt. correlator in 2D CFT

In 2D CFT, the Euclidean two point correlator of a primary operator $\mathcal{O}$ with conformal weights $h$, $\bar{h}$ is given by $$ \begin{align} \langle\mathcal{O}(z,\bar{z})\mathcal{O}(0,0)\...
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3answers
76 views

Irreversible processes in special relativity

According to special relativity (as I understand it) simultaneity is no longer a universal concept in special relativity. Consider two events A and B which are simultaneous in reference frame 1; in ...
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1answer
66 views

Time Ordering and Causality in General Relativity

The time-ordering of two space-like separated events is undefined in special relativity. Is there an analogous concept in GR?
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1answer
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Question regarding speed of causality and speed of light

Recently I came across a question regarding to why the speed of light and the speed of causality are the same. Link-Why is the speed of causality equal to the speed of light? I came up with an ...
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4answers
158 views

Why is the speed of causality equal to the speed of light?

I heard that the most fundament concept is Causality, and also that it is turn out that speed of light has nothing special in it so that It is ultimate speed limit but because it is happen to be ...
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4answers
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What is the chain of cause(s) and effect(s) which does a butterfly's wing-flapping cause a hurricane 1000's of kilometers further away?

I once asked if the butterfly effect in the weather system is a real effect: a butterfly flaps her wings, which can cause a hurricane thousands of kilometers further. But how will this happen? To be ...
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1answer
44 views

Is there a physical interpretation of the wave equation with a third (or higher) order time derivative?

In physics we learn about the heat equation, $$ \frac{\partial u}{\partial t} = k \Delta u,$$ which describes how heat propagates in a material. We also learn about the wave equation, $$ \frac{\...
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1answer
25 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
96 views

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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0answers
101 views

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
399 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
78 views

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
115 views

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
93 views

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
70 views

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
713 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
93 views

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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2answers
124 views

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
259 views

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
84 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
223 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 ...
2
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1answer
144 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
60 views

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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0answers
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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
38 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
39 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
153 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
102 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
34 views

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
68 views

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
786 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
102 views

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
57 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
183 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
90 views

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
256 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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26 views

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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