Questions tagged [causality]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
1
vote
0answers
72 views

Can someone review this mathematical derivation that in Special Relativity FTL implies Time Travel

I am looking to show mathematically (i.e. with derivations and algebraic relationships) that FTL implies time travel. Here's what I got: Suppose we have 2 ships $O_1, O_2$ traveling at $0.9c$ in ...
11
votes
2answers
256 views

Causal explanation of Vaidman's group “Asking photons where they have been” experiment?

There is this surprising "Asking photons where they have been" PRL 2013 article about experiment where they are able to "ask photons" which mirrors they have visited: by vibrating each mirror with ...
7
votes
2answers
5k views

Can the increase in thickness of a piece of paper which can be folded infinitely surpass the speed of light? [closed]

Supposing I have a piece of paper that can be folded infinitely. In the first $5 \, \mathrm{s}$, I fold it to twice its thickness. In the next $5 \, \mathrm{s}$, I fold it to 4 times. If I fold it to ...
9
votes
2answers
420 views

Simplest mathematical model of a causal loop

Is there a simple mathematical model for causal loops? The physics seems pretty involved, but I'm wondering if I can understand just the math of the final answer (similar to how one can understand ...
0
votes
1answer
132 views

What are the uses of Penrose diagrams?

Are Penrose diagrams just used for a nice visual representation of compactified space and time? Are there any other applications? I figured out how to make my own Penrose diagram, built individually, ...
0
votes
2answers
141 views

Application of the principle of causality to classical electrodynamics

I am reading Griffiths' Introduction to Electrodynamics in which he shows that the retarded and advanced potentials, e.g. the retarded scalar potential $$ V(\mathbf{r},t) = \frac{1}{4\pi\epsilon_0} \...
0
votes
2answers
261 views

Shor's algorithm with adversary - can (delayed) erasure of value qubits cripple calculations?

Let's look at quantum subroutine of Shor's algorithm: Hadamard gates create superposition of all (exponential number) values for input qubits. Then we perform a classical function on them, which is ...
0
votes
1answer
108 views

What is time travel and why is the grandfathers Paradox a Paradox?

While watching a talk given by Neil DeGrase Tyson, he says that if something happens on earth, it would take light 2 million years to get to Andromeda. In theory, if you could generate a wormhole and ...
3
votes
2answers
131 views

How to deduce quantum non-locality from indeterminism?

In the paper ''Quantum non-locality as an axiom'' (Popescu, S. & Rohrlich, D. Found Phys (1994) 24: 379. https://doi.org/10.1007/BF02058098), it is stated that the conventional formalism of ...
1
vote
1answer
92 views

Is *Conservation of Distinction* a true conservation law in mainstream physics?

Both Leonard Susskind and Francis Heylighen have written about the Conservation of Distinction but it seems Susskind more closely connects this (law?) with unitarity in quantum mechanics. Heylighen ...
9
votes
4answers
263 views

What physically determines the point-set topology of a spacetime manifold?

Like any manifold, the pseudo-Riemannian manifold of spacetime in special or general relativity is a topological space, so there is a notion of open sets (or equivalently, neighborhoods) that allows ...
1
vote
0answers
31 views

Doubt regarding Fermat's principle [duplicate]

Which two points are we talking about in Fermat's principle? Are those points decided by light or decided by us? Can we take any two points?
1
vote
1answer
85 views

Pulling a Metal Bar in Space [duplicate]

Let's say I have a metal bar (which does not expand or contract at all) that's 1 light-second long which is connected to a block at the end. I am in space holding the other end of the bar. If I pull ...
2
votes
1answer
596 views

Cauchy Surface and its determination

A closed achronal surface is Cauchy if its domain of dependence is entire manifold. From information given on a Cauchy surface, we can predict what happens throughout all of spacetime. How do we ...
1
vote
1answer
151 views

Are all time-like and light like separated events causally connected?

It is well known that all causally connected pairs of events are either time-like or light-like separated. Is the inverse -'All time-like or light like separated events are causally connected'- also ...
1
vote
1answer
51 views

Is there a criterion for precisely identifying causality?

Can one consider the fact that the partial order among a set of time-like separated events is the same in all inertial frames of reference as a criterion to precisely identify causally connected ...
1
vote
0answers
106 views

Causality in Quantum mechanics

If we consider a quantum preparation procedure on a system followed by a projection measurement as constituting two sequential events in space-time (which seems plausible ,given that the Copenhagen ...
2
votes
0answers
71 views

Causality violation of naive RQM [closed]

So I am starting to read QFT from P&S, and in the very beginning they set out to clarify why a naive approach of writing a single particle relativistic hamiltonian would lead to propagation ...
1
vote
1answer
299 views

Definition of trapped surface

The definition of a trapped surface in Sean Carroll's "Spacetime and Geometry" is as follows. "A compact spacelike, two dimensional submanifold with the property that outgoing future directed light ...
1
vote
1answer
79 views

Special relativity and causality, which event actually happens?

In Revolutions in Twentieth Century Physics, David Griffith's introduces the relativity of simultaneity with a thought experiment akin to the "Train and platform" problem wherein the outcome is that ...
3
votes
2answers
110 views

Is it a coincidence that non-relativistic quantum mechanics prevents superluminal communication?

So non-relativistic quantum mechanics prevents superluminal communication. Given a bipartite state $\rho_{AB}$, the reduced density operators for systems $A$ and $B$ are given by the partial traces $$...
1
vote
0answers
28 views

Theoretical constraints of relativity on dispersion relations

A wave has a dispersion relation $\omega=\omega(\vec{k})$. (The function has been made single valued usinng initial conditions.) A delta function in the origin at $t=0$ ( $\psi(\vec{x},0)=\delta(\vec{...
3
votes
1answer
298 views

Why group speed can contain information but phase speed can't?

I was watching sixty symbol on YouTube about reflective index and learnt that it is the sum of all the speed of light or phase speed that appears slower than speed of light in the medium, and also the ...
1
vote
0answers
64 views

When is a Cauchy surface non-compact?

If we have a spacetime $(M,g)$ admitting some Cauchy surface $\mathcal{S}$, we know from Geroch's splitting theorem that $M = \mathcal{S}\times \mathbb{R}$. A standard result from topology is that the ...
-1
votes
2answers
945 views

Is the future predetermined and fixed? [closed]

I just watched a YouTube video which explained that life as a sequence of events is a geometric object in the four-dimensional spacetime and that the future is not only predetermined, but it already ...
2
votes
1answer
100 views

Causal ordering on spacetimes

On any spacetime $(M,g)$ we can form the causal ordering $\leq$, where for any two points $a,b \in M$ we have that $a \leq b$ iff there exists some future directed, non-spacelike curve from $a$ to $b$....
4
votes
1answer
266 views

Why do we experience a slice of time, but not a slice of space? [closed]

I (and every one I guess) perceive space as an extended region where things happen. In contrast, time is perceived as a lucky of slice. Also in space I can choose in which direction I want to move, ...
4
votes
0answers
67 views

The point of holedness

In general relativity there is a vaguely defined notion of a spacetime having "holes", with many different definitions. Many of those were cast off when it was realized that Minkowski space also has ...
1
vote
0answers
90 views

Cauchy Surface and topological structure

I heard in passing a result that if a connected spacetime without boundary admits a cauchy surface $\mathcal{S}$, then the manifold is topologically tubular, i.e. $\mathcal{M} \cong \mathcal{S} \times ...
1
vote
0answers
218 views

How are Penrose diagrams derived? [duplicate]

How are Penrose diagrams derived?  (source: wikimedia.org)
0
votes
2answers
148 views

Does a randomizer break the law of causality?

I am reading this book why does $E=mc^2$ and I learned that causality is what proves that there is a cozmic speed limit-I'm the only one here who is astonished by the looks of it ;-). Anyway. Would a ...
1
vote
4answers
908 views

Causes and Effects in the quantum world [closed]

I am debating with a person about questions of causes and effects in the quantum world, and some questions came into our minds, hence I would be very happy to know what is "your opinion" that is, what ...
10
votes
2answers
1k views

Where is the Feynman Green's function in quantum mechanics?

In quantum field theory, the Feynman/time ordered Green's function takes the form $$D_F(p) \sim \frac{1}{p^2 - m^2 + i \epsilon}$$ and the $i \epsilon$ reflects the fact that the Green's function is ...
10
votes
3answers
558 views

What are the details behind causality and Maxwell's equations?

Maxwell's equations are \begin{align} \nabla\cdot\mathbf{E} & = \frac{\rho}{\epsilon_0} & \nabla\times\mathbf{B} &= \mu_0\epsilon_0 \frac{\partial\mathbf{E}}{\partial t} + \mu_0 \...
1
vote
1answer
120 views

Reverse entropy, reverse causality and physical laws

I do not have any scientific education and I am far away from understanding relativity, quantum mechanics and such. I am just wondering about entropy, causality and if we could reverse these in a ...
1
vote
1answer
76 views

Basic question about local algebras in AQFT

AQFT (algebraic quantum field theory) assigns "local algebras of observables" to bounded regions of spacetime, in particular to double-cone ("diamond") regions. These algebras' projection operators ...
1
vote
2answers
153 views

Does time matter?

Aside from the second law of thermodynamics it seems physics just doesn't care about the direction of time. So does time really matter? What if instead of 3 spacial and one time dimension. 3+0 or 3+2....
0
votes
1answer
317 views

Can someone explain this causality argument about the speed of light from Special Relativity?

I was watching this Yale lecture by R. Shankar. From 8:29 to 15:05, he gives an argument for why theory of relativity "demands" that it should be impossible for events to influence other events ...
-1
votes
1answer
158 views

Can an a distance in Minkowski space, based on a Euclidean plane, be time-like?

In a Minkowski space diagram of ct vs x, with 2 stationary events in the same plane of spacetime (1 event located along a worldline, 1 not on a worldline) is this still considered euclidean space? ...
2
votes
1answer
739 views

$i\epsilon$ in the expression of Feynman Propagator

In Peskin, the Feynman's propagator for a real scalar field is first presented in a form without $i\epsilon$ \begin{equation} D_F(x-y)=\int\frac{dp^3}{(2\pi)^3}\int\frac{dp^0}{2\pi}\frac{ie^{-i(x-y)}}...
4
votes
3answers
173 views

Why strings have to be spacelike? What does it mean?

I don't understand the following statement from Zwiebach's book: "We want strings to be spacelike objects. More precisely, the interval between any two points on a string should be spacelike, ...
1
vote
2answers
257 views

Does the commutator of two quantum fields $\phi(x)$ and $\phi(y)$ vanish for $(x-y)^2<0$ and $m^2<0$?

The commutator of two free scalar fields at two spacetime points $x$ and $y$ given by $$[\phi(x),\phi(y)]=\int\frac{d^3p}{(2\pi)^32E_{\textbf{p}}}[e^{-ip\cdot (x-y)}-e^{ip\cdot (x-y)}]$$ is Lorentz-...
3
votes
2answers
635 views

Time dilation due to space expansion

As we observe a remote galaxy, we see it with a redshift. The most distant galaxy discovered to date is GN-z11 visible with the redshift of $z=11.09$. For simplicity, let's assume no gravitational ...
3
votes
1answer
80 views

Are Lorentz transformations a direct consequence of finiteness of signal speed?

I have this silly doubt in my head and it's bugging me for a real long time now. Let us consider the Galilean transformation $x=x'+vt$ for two frames measuring coordinates $x$ and $x'$. For simplicity,...
9
votes
2answers
512 views

Why does gravity travel at the speed of light?

In electromagnetism, Maxwell's equations predict that electromagnetic disturbances travel at the speed $$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}.$$ Does general relativity predict that gravitational ...
5
votes
2answers
1k views

Is space becoming time-like inside an event horizon a consequence of our coordinate system?

Is the fact that "space behaves like time" inside an event horizon a consequences of our particular coordinate system? Or is it a universal fact? I am asking based on the statement in this video How ...
0
votes
1answer
91 views

Is the curvature of spacetime in General Relativity instantaneous? [duplicate]

Assume one initially has perfectly flat Minkowski vacuum. At some point in this Gedankenexperiment a point mass spontaneously appears at some location in Minkowski creating a Schwarzschild geometry. ...
8
votes
2answers
1k views

Shor's algorithm - why doesn't the final collapse of the auxiliary qubits cripple the computation?

Let's look at quantum subroutine of Shor's algorithm (image source): Hadamard gates create superposition of all (exponential number) values for input qubits. Then we perform a classical function ...
2
votes
0answers
254 views

Causality and wick rotation

What is the connection between causality and wick rotation? I came across implication of this connection multiple times but can't find a rigorous explanation. For example in the answer to Wick ...
1
vote
0answers
228 views

Causal propagator relation from canonical field commutator

In Minkowski spacetime, the commutator of the Klein-Gordon field operator with itself at different spacetime points evaluates to the advanced minus retarded Green's function (i.e., the "causal ...

1
3 4
5
6 7
13