# Questions tagged [causality]

The tag has no usage guidance.

642 questions
Filter by
Sorted by
Tagged with
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 ...
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 ...
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 ...
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 ...
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, ...
141 views

28 views

218 views

### How are Penrose diagrams derived? [duplicate]

How are Penrose diagrams derived?  (source: wikimedia.org)
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 ...
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 ...
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 ...
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 \...
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 ...
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 ...
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....
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 ...
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? ...
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)}}...
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, ...
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-...
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 ...
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,...
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 ...
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 ...
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. ...
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 ...