The tag has no usage guidance.

learn more… | top users | synonyms

-1
votes
1answer
25 views

Global Hyperbolicity in spacetime Manifold [on hold]

If space time is timelike or null geodesically incomplete but cannot be embedded in a larger spacetime then we say that it has singularity. What does incompleteness means here?
2
votes
1answer
30 views

Is Entropy Maximized, or Just Increased?

Do different real processes involve different rates of change of entropy? (Is the rate of change of entropy constant with time throughout all regions of space, or perhaps in other words, constant with ...
4
votes
4answers
3k views

Space-like and time-like: where do the names come from?

Space-like separated events are events that, in a well-chosen reference frame, can take place at the same time but never happen at the same location. On the other hand for time-like events, one can ...
10
votes
2answers
1k views

Can't quantum teleportation be superluminal some percentage of times?

I apologize if this is a really silly question. In the (textbook) quantum teleportation algorithm, in the step right after Alice has measured her system but before she has sent her classical ...
5
votes
2answers
404 views

Why does the Dopfer EPR experiment require coincidence counting?

Dopfer Momentum-EPR experiment (1998) seems to provide a interesting tweak in the EPR experiment. To read more details on this experiment, see: Page 3 (labelled S290) of 'Experiment and the ...
4
votes
1answer
235 views

Closed timelike curves in the spin-2 gravity formalism

Let's say we take some topologically trivial CTC spacetime, like the Gödel metric: $$ds^2 = -dt^2 - 2e^{\sqrt{2}\Omega y} dt dx - \frac{1}{2}e^{\sqrt{2}\Omega y} dx^2 + dy^2 + dz^2$$ And then I ...
12
votes
1answer
187 views

Explaining causal completion axiom in Haag-Kastler axioms?

There are several variants of the Haag-Kastler axioms for algebraic quantum field theory. Usually one associates an algebra $\mathcal{A}(O)$ to each open region $O$ of spacetime. An often-suggested ...
4
votes
1answer
144 views

Locality defined in terms of the Lagrangian density

I've been reading through Matthew Schwartz's book "Quantum Field Theory and the Standard Model" and in chapter 24 there is a section on locality (section 24.4). In it he defines locality in terms of ...
8
votes
4answers
5k views

What spacelike, timelike and lightlike really mean?

Suppose we have two events $(x_1,y_1,z_1,t_1)$ and $(x_2,y_2,z_2,t_2)$, then we can define $$\Delta s^2 = -(c\Delta t)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2$$ which is called the spacetime ...
8
votes
1answer
222 views

Delocalization in the square root version of Klein-Gordon equation

In this Wikipedia article a relativistic wave equation is derived using the Hamiltonian $$H=\sqrt{\textbf{p}^2 c^2 + m^2 c^4}$$ Substituting this into the Schrödinger equation gives the square root ...
5
votes
1answer
107 views

Physical interpretation of the retarded vs. Feynman propagators?

We calculate the real-space propagator $\Delta(x)$ for a free real scalar field $\varphi(x)$ with mass $m$ by performing the Fourier transform (using sign convention +---) $$\Delta(x) = \int ...
3
votes
0answers
103 views

Do gravitational waves propagate backwards in time?

Gravitational waves are spacetime waves, which stretch and squeeze both space and time. Since relativity puts space and time (almost) on an equal footing, it seems to me that since gravitational waves ...
1
vote
1answer
184 views

If a point r lies in the boundary of the chronological future of another point p, why does the chronological future of r belong to that of p?

I am studying the global causality of the spacetime. Here, I come across a problem. Suppose a point $r\in \partial I^+(p)$. $I^+(p)$ is the chronological future of a different point $p$ in ...
1
vote
0answers
64 views

Path integral (sum over paths where $v>c$) [closed]

The path integral formalism is used to get for example the propagator of particles. In this formalism we integrate over all mathematically possible paths (and weight them with the non-relativistic ...
26
votes
2answers
3k views

Definitions: 'locality' vs 'causality'

I'm having trouble unambiguously interpreting many answers here due to the fact that the terms locality and causality are sometimes used interchangeably, while other times seem to mean very different ...
9
votes
4answers
853 views

Where does movement come from? [closed]

When you put a body in motion, where does that movement come from? Ok, you will say things like "acceleration", but where does that acceleration come from? Then you might say "a force is creating the ...
9
votes
6answers
1k views

What is the physical definition of causality?

Maxwell's equations give a physical relationship between the electric and magnetic fields $\vec E$, $\vec B$ at the same time, which some interpret as changes in one causes changes in the other etc. I ...
0
votes
1answer
11 views

A media in which the electrical displacement vector is not causal

I recently did an electrodynamics homework problem in which we showed that in a certain model (Lorentz-Drude), where the permittivity of free space $\epsilon$ was dependent upon the angular frequency ...
6
votes
5answers
139 views

Can the mass within the event horizon of a black hole interact gravitationally with the mass outside the event horizon?

If so, gravitons and their fields, unlike photons, must be able to cross the event horizon freely in both directions. If not, the observed mass of a black hole must depend only on the particles ...
4
votes
1answer
48 views

anticausal group velocity?

https://en.wikipedia.org/wiki/Anomalous_dispersion claims: Recently, it has become possible to create gases in which the group velocity is not only larger than the speed of light, but even ...
1
vote
3answers
213 views

How can an action be dependent on both its past and future?

Is it true that whenever an action takes place it is dependent on both its past and future? I mean if we already know that whatever we are doing is dependent on future as much as it is dependent on ...
1
vote
3answers
121 views

What would happen if you went back in time to get a random number?

For example, you go to a website that generates a random number. You get the number 8. What would happen if you went back in time a few minutes, and repeated the same actions. Would you get the same ...
2
votes
1answer
100 views

Particle here at a given time, in another galaxy a second later… Really?

I read "The Quantum Universe (Cox & Forshaw)" that a particle can be measured at a given position at a given time, and in another galaxy one second later. The probability of such event may be ...
20
votes
2answers
2k views

What do physicists mean by “information”?

On the question why certain velocities (i.e. phase velocity) can be greater than the speed of light, people will say something like: since no matter or "information" is transferred, therefore the ...
4
votes
2answers
157 views

When does causal separation imply no spacelike separation?

(See here for notation.) In Minkowski space, if $p\prec q$, then there is no spacelike curve $c:[0,1]\to \mathbb{R}^{n-1,1}$ with $c(0)=p$ and $c(1)=q$. This is obvious from a spacetime diagram. Here ...
4
votes
0answers
205 views

How does one determine if a spacetime is globally hyperbolic?

A spacetime $M$ is said to be globally hyperbolic if it is strongly causal and if the sets $J^+(p)\cap J^-(q)$, for all $p,q\in M$, are compact. (For more information, see the Wiki article on causal ...
87
votes
9answers
10k views

How fast does gravity propagate?

A thought experiment: Imagine the Sun is suddenly removed. We wouldn't notice a difference for 8 minutes, because that's how long light takes to get from the Sun's surface to Earth. However, what ...
3
votes
1answer
455 views

Are some gravitational wavelengths forbidden by causality?

Consider a gravitational wave in linearized gravity $d_{\mu \nu}(X_{\eta}) = D_{\mu \nu} e^{i X_{\eta} K^{\eta}}$ with $K^{\eta} = (-\omega t, \textbf{k})$. Let $d=| \textbf{D}|$ the scalar maximum ...
19
votes
3answers
3k views

In superluminal phase velocities, what is it that is traveling faster than light?

I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but ...
10
votes
4answers
2k views

The Lagrangian in Scalar Field Theory

This is perhaps a naive question, but why do we write down the Lagrangian $$\mathcal{L}=\frac{1}{2}\eta^{\mu\nu}\partial_{\mu}\phi\partial_{\nu}\phi - \frac{1}{2}m^2\phi^2$$ as the simplest ...
7
votes
3answers
199 views
3
votes
1answer
85 views

Signal travels with speed greater than light breaks causality

Signal can't travel at speed greater than light speed in vacuum which is a assumption of special relativity. But if a signal travels at speed greater than $c$ then it will violate causality. I tried ...
0
votes
0answers
54 views

How to get anti-commuting rule from the view of field?

I was reading the 1951 Lectures on Advanced Quantum Mechanics and I found something really disturbing. That's the anti-commuting rule mentioned on Page 40 at last. Though it was named as Quantum ...
1
vote
2answers
112 views

How is locality preserved in quantum mechanics?

I was reading this post: http://motls.blogspot.com/2015/06/locality-nonlocality-and-anti-quantum.html Specifically here: "There is no nonlocality. There is no action at a distance. There is no doubt ...
66
votes
10answers
8k views

Quantum Entanglement - What's the big deal?

Bearing in mind I am a layman - with no background in physics - please could someone explain what the "big deal" is with quantum entanglement? I used to think I understood it - that 2 particles, say ...
-1
votes
1answer
88 views

Is causality in quantum physics also always valid?

For different observers the laws of causality are the same. So the cause and the effect are clear for all observers in any space or time. But is this still valid in quantum mechanics?
0
votes
1answer
73 views

Time Travel possibility and Paradoxes of The Past [duplicate]

can one travel back in time and if not so, what laws prevent time travel to the past. this is quite a challenge to understand.
0
votes
0answers
26 views

Linkal causality in special relativity

I have a slight variation of the barn / ladder paradox, where there is a ladder too long to fit into a barn at rest, but when moving at a sufficient speed, it is length contracted with regards to the ...
0
votes
1answer
41 views

Precise definition of “Observable Universe” and its alternatives

The Observable Universe is generally said to contain all space that could "in principle" have had a causal impact on Earth, but the exact limits of the "in principle" causal interaction go ...
1
vote
1answer
154 views

What is the meaning of the particle horizon in conformal diagrams?

I'm reading "Physical Foundations of Cosmology" (Mukhanov) and in Chapter 2.3 conformal diagrams get introduced. They seem to be a (graphical) tool to understand the causal structure of the universe. ...
1
vote
1answer
91 views

Relationship between locality, causality, and free theories

This text on QFT defines a free theory as that in which dynamics of the field for each degree of freedom evolves independently from all the other. In principle we have an infinite degrees of freedom, ...
1
vote
1answer
56 views

response function and Fourier transform

A response function defined as the kernel of the following integral: $\rho(t) = \int_{-\infty}^t \chi(t,t') E(t')dt'$ (1), where $\chi(t,t')$ is the response function. Physically, it relates ...
1
vote
3answers
283 views

Does quantum mechanics break causality? [duplicate]

If quantum mechanics is probabilistic, there is no reason for a particle to be in one place and not the other, but particles do make up their minds... but how?
0
votes
1answer
161 views

Intuitive meaning of Globally Hyperbolic

I am been studying differential geometry and spacetime and I keep coming across the term globally hyperbolic. I am having a hard time coming up with an intuitive understanding of this idea. What is an ...
5
votes
4answers
542 views

How can we know, today, that there's something from 100 light-years from here?

In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
3
votes
1answer
63 views

Are closed timelike curves generic feature of ANEC-violating stress-energy tensor?

Kip Thorne has shown that in order to create closed timelike curves (CTCs), one needs stress-energy tensor $T^{\mu\nu}$ that violates averaged null energy condition (ANEC). Will $T^{\mu\nu}$ with ...
1
vote
0answers
27 views

Can a solid object inside the black hole event horizon avoid the singularity? [duplicate]

I will ask this as a simple lay man. Let's consider an astronomical black hole that was creating by in falling matter on a neutron star. As neutron star get heavier it gets smaller and a Schwarzschild ...
6
votes
2answers
2k views

Is time going backwards beyond the event horizon of a black hole?

For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside. Is this the case? This question is a followup ...
22
votes
1answer
1k views

Causality and how it fits in with relativity

I was talking to my teacher the other day about Einstein's spacetime and there's one thing he couldn't explain about the nature of Cause. I may be being stupid or just unable to comprehend, thanks for ...
0
votes
2answers
120 views

Is causality a total order?

I've read that it is physically not possible to violate causality defined as a total order on the spacetime graph. So I was wondering if at least causality can be broked down to a partial order and if ...