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4
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1answer
257 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 ...
0
votes
1answer
53 views

Action at a Distance [duplicate]

Since photons move at $c$, do they experience time or distance? If they don't, doesn't this explain action at a distance? From the point of view of the photons, there is no time, so the action at a ...
6
votes
3answers
737 views

Feynman's path integral formulation and faster-than-light paths

In Brian Greene's book "The elegant Universe", he talks about the double slit experiment and Feynman's interpretation of Quantum Mechanics. According to the book, Feynman said that one vaild ...
2
votes
3answers
89 views

Bell inequality violations evidence for 1935 EPR claims?

Is it possible that Bell inequality tests provide experimental evidence in support for the EPR claims in their 1935 paper titled "Can Quantum-Mechanical Description of Physical Reality Be Considered ...
2
votes
1answer
71 views

How does a photon 'know' when to reflect in case of refraction? [closed]

In thin-film interference and in glass fibers refraction is involved. Depending on the angle of which a light ray enters the material it will be reflected or transmitted. The refraction depends on the ...
1
vote
3answers
1k views

Are random quantum phenomena happening without a cause?

In everyday life, most of us assumes every event and object has a cause in some sense. I am wondering if the same is true for quantum physics. Does the random nature of quantum phenomena mean they ...
2
votes
1answer
326 views

In QFT, why do fermions have to anticommute in order to insure causality?

I have seen this question and I believe I understand the answer to it. However, AFAIK, only for bosons the causality condition is a vanishing commutator. For fermions we expect the anticommutator $[\...
5
votes
1answer
69 views

Does Microcausality follow from Lorentz Invariance?

In a Lorentz Invariant theory, does microcausality automatically hold? In a free theory this is obvious. In an interacting theory I found some 'proof's in this paper: http://arxiv.org/abs/0709.1483 ...
3
votes
1answer
52 views

How do non-contact forces work?

One question has been bugging me ever since I started learning physics, the textbooks successfully describe the empirical observations but haven't given any reasoning behind it. What I want to ...
1
vote
1answer
245 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. ...
2
votes
1answer
191 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 spacetime....
92
votes
9answers
11k 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 ...
6
votes
2answers
2k views

What does “causally connected” or “causes” really mean?

In a different thread, a user stated the following about events preceding or following other events: However, if the two events are causally connected ("event A causes event B"), the causal order ...
12
votes
1answer
196 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 ...
5
votes
3answers
178 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 ...
0
votes
1answer
48 views

How can I send a message to myself in the past? [duplicate]

It is easy to send a message to myself in the future: I can put a letter in the post and will receive it tomorrow. Can you think about a way to send a message to myself in the past; it does not need ...
4
votes
1answer
163 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 ...
53
votes
8answers
12k views

If the speed of light is constant, why can't it escape a black hole?

When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole? It may take a long time to happen but shouldn't there be some light ...
7
votes
5answers
860 views

The relativistic principle of causality

From Wikipedia: "The relativistic principle of causality says that the cause must precede its effect according to all inertial observers" What exactly does this mean? Also, is it an ...
2
votes
1answer
87 views

What is the latest science on closed timelike curves? [closed]

In Scientific American (Sept 2014), Lee Billings writes: Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,”...
2
votes
2answers
46 views

What's behind the moment of inertia and other “body-global” properties of bodies?

I'm an electrical engineer currently doing some (computational) mechanics stuff. In introductory literature about mechanics, you can read plenty about the moment of inertia and how you use it in ...
-1
votes
1answer
81 views

Global Hyperbolicity in spacetime Manifold [closed]

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
36 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 ...
5
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
422 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 ...
9
votes
4answers
6k 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
229 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
149 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 \frac{d^...
1
vote
0answers
70 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
868 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
146 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
50 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
214 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
127 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
101 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
167 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
213 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 ...
3
votes
1answer
462 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
4k 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 ...
11
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 ...
8
votes
3answers
228 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
115 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 ...