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Questions tagged [causality]

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2
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2answers
71 views

How can time go in different directions in the Universe?

This is not a duplicate, I am not asking about any kind of time dilation caused by a BH. My question is about the direction of time (and if it is possible to have different directions for time, other ...
0
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2answers
67 views

What is meant by globally hyperbolic spacetime? [duplicate]

Can somebody explain what is meant by globally hyperbolic spacetime? What other kinds can there be?
1
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0answers
28 views

On causality in physics: is the concept of cause an operational concept in physics? [duplicate]

This is a broad question motivated by philosophical interests. I hope it will not be considered as off topic. What I am looking for is a technical answer reflecting the standard conception of ...
1
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3answers
97 views

Difference between an entangled pair of photon and two magnets in a box? [duplicate]

I try to understand quantum entanglement and especially what it’s called « Action at a distance » from my understanding, if you have a pair of entangled photon, after measuring the polarization of ...
0
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0answers
35 views

Antiparticles travelling backward through time [duplicate]

Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time? I have read about their idea of an antiparticle behaving as a particle travelling ...
5
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2answers
85 views

What is the formal difference between the light cone and a black hole?

A black hole can be loosely defined as a spatial closed surface from which nothing, not even light, can leave. The light cone of special relativity is in some sense similar to a black hole because by ...
2
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0answers
23 views

Penrose diagram and coordinate transformation

I am looking at the Minkowski line element in spherical coordinates $$ \mathrm{d}s^2 = - \mathrm{d}t^2 + \mathrm{d}r^2 + r^2 \mathrm{d}\theta^2 + r^2 \sin^2(\theta) \mathrm{d}\phi^2$$ and want to ...
4
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1answer
265 views

Is a vector space automatically spacelike if it has a basis of spacelike vectors?

I am studying Kerr Spacetime and I am not sure about something used in a proof I am trying to understand. I am wondering, if you consider a 4-dimensional Lorentzian manifold $\mathcal{M}$ and $X_i \...
2
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0answers
27 views

Rindler's trap door experiment with a rod (or a tank, or a pile of fragile matter) of finite hight

About 60 years ago a thought experiment was proposed which is widely referenced, e.g. as "Man falling into grate variation" of "The ladder paradox" or, perhaps more poignantly, "The tank paradox": ...
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0answers
16 views

Are there theories or experiments involving multi variable particles?

Every hidden variable/entanglement experiment I’ve ever heard described involves ONLY one variable of either polarization or spin. Therefore Venn calculations backed by diagrams highlight the ...
11
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2answers
217 views

How does the Penrose diagram for a spinning black hole differ in realistic scenarios (formed by stellar collapse)?

The Penrose diagram for a non-spinning Schwarzschild black hole is Notably, there is a second universe "on the other side" of the black hole. However, actual black holes form by stellar collapse, and ...
2
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2answers
88 views

How Maxwell theory of electromagnetism solved action at a distance problem?

According to wikipedia Maxwell's equations were an essential inspiration for Einstein's development of special relativity. Possibly the most important aspect was their denial of instantaneous action ...
2
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0answers
47 views

Hausdorff property in Minkowski spacetime

In the 4-dimensional Minkowski spacetime, for a given point $x = (x^0,x^1,x^2,x^3)$, its timelike future/past set is defined as, $$ I^{\pm}(x) = \{y =(y^0,...,y^3) \in \mathbb{R}^4 : \eta_{\mu \nu}(y-...
0
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1answer
61 views

Microcausality for Dirac's current

I`m supposed to show as an exercises that for the Dirac field's associated current: $$j^\mu=\bar{\Psi}\gamma^\mu\Psi$$ The microcausality relation holds: $$ [j^\mu(x),j^\nu(y)]=0 \text{ for } (x-y)^...
-1
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1answer
27 views

Does the state of curl of the $E$-Field at a point adjust itself instantaneously as soon as $B$ begins changing at a fixed rate, or is there delay? [duplicate]

Faraday’s Law () states that a time-changing magnetic field vector induces a curl of the Electric field around that point. However it does not specify how quickly the necessary spatial gradient is ...
-1
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1answer
78 views

How quickly is the desired state of curl of the Electric Field around a point achieved where a Magnetic Field Vector has just begun changing?

My question and context with explanation are given below. Thank you in advance. Faraday’s Law () states that a time-changing magnetic field vector induces a curl of the Electric field around that ...
0
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1answer
44 views

Difficulty in understanding the three classes of events of light-cone

Recently I'm reading Chapter 2: Space and Time of A Brief History of Time. The paragraph that follows the light cone says: Given an event P, one can divide the other events in the universe into ...
2
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2answers
360 views

Minkowski space

In Minkowski space, coordinates which satisfy $x^2 = t^2 - X^2 > 0$ are in the region of spacetime that is time-like. If it's $x^2 = t^2 - X^2 < 0$ the region is space-like. But if $x^2 = ...
-3
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1answer
34 views

Aging while traveling faster than speed of light? [closed]

theoretically, if organic matter was able to travel faster than the speed of light, without becoming "pure energy", and since theoretically traveling faster then the speed of light would travel ...
2
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0answers
89 views

Locality and relativity - a paradox?

The equations of nature are supposed to exhibit locality in the sense that the action depends on fields and their derivatives. i.e. comparing the values of fields at local points. But two points on a ...
3
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2answers
107 views

Is a finite lightspeed necessary for cause and effect to exist? [duplicate]

In this question I asked in somewhat unclear (unclear enough to be closed) fashion about the relationship between an infinite speed of light and the existence of cause and effect. In thís question, I ...
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4answers
2k views

Are cause and effect the same as in our Universe in a non-relativistic, Newtonian Universe in which the speed of light is infinite? [closed]

Suppose the Universe was non-relativistic so time and space would be independent of each other. In other words, both of them separately would be absolute and independent of an observer's motion (...
0
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1answer
52 views

Vanishing of a solution of Dirac equation

Let $\psi(x,t)$ be a solution of the free Dirac equation. Assume that $$\psi(\vec x,0)=\delta^{(3)}(\vec x) u,$$ where u is a fixed spinor. (In other words $\psi(\vec x,0)$ is assumed to be supported ...
0
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1answer
35 views

Hubble radius and communication between two observers

According to Dodelson, Modern Cosmology (p.146) There is a subtle distinction between the comoving horizon $\eta$ and the comoving Hubble radius $(aH)^{-1}$. If particles are separated by ...
39
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4answers
9k views

Is there a frame of reference in which I was born before I was conceived?

I'm struggling to understand the relativity of simultaneity and position. If my conception and birth are separated by time but not space, a frame of reference in which my birth and conception are ...
1
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1answer
44 views

Physics of time running backwards

Although it would seem weird to analyze physical phenomena when time runs backwards, it seems to have a logical sense, at least for me: Entropy would tend to decrease: two balls having energy ...
3
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2answers
58 views

Paths of least action and loops in time

In the book Quantum Field Theory for the Gifted Amateur link: https://books.google.ca/books?hl=en&lr=&id=nIk6AwAAQBAJ&oi=fnd&pg=PP1&ots=JZjwG_qDt5&sig=...
2
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1answer
77 views

What is time-like path in GR?

My understanding: Given a metric, at each point of spacetime, there is a tangent vector u that maximize the quantity $g_{ab}u^au^b$, which is the proper-time length. Does it mean at each point, there ...
0
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2answers
43 views

Time direction in the BH parallel exterior region

In the III region of the maximally extended Schwarzschild solution, described in the Kruskal coordinates , Sch. time runs in the reversed direction as compared to the I - event horizon III-II of the ...
4
votes
4answers
107 views

Does Electromagnetic wave propagation really use $E$ to create $B$

I was studying how electromagnetic waves (EM waves) were generated and propagated. I was shown a dipole antenna with an AC source sloshing charges back and forth. To my understanding, when the ...
0
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2answers
47 views

If some matter slows light going through it, is actual causality also slowed? [duplicate]

tldr; If some forms of matter change the that light travels through that it (like refraction in water or glass), does that mean the speed of causality is going slower through that matter? I've heard ...
4
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4answers
298 views

Are there null geodesics inside null infinity?

Looking at a Penrose diagram for Minkowski space, you would think that you could draw a null geodesic running from $i^0$, along $\mathscr{I}^+$, and ending up on $i^+$. In fact there would be many ...
2
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1answer
107 views

How to make sense of $\mathcal{I}^-$ as a Cauchy surface rigorously?

In some references, like Hawking's derivation of black hole radiation, one considers that $\mathcal{I}^-$ is a Cauchy surface. One recent reference with such a claim is the paper "Soft Hair on Black ...
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1answer
37 views

Looking back in past [closed]

Can faster than light communication give us the ability to see in the past, that is to say, if we can imagine a ultra huge highly advance telescope 60 light minutes away from earth and we want to look ...
1
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2answers
95 views

Can the future affect the past in general relativity? [closed]

Hypothetically, let us suppose that a black hole were to suddenly appear at time $t=1$ at position $x$. Can the effect of the black hole be felt at time $t = 1-\epsilon$ near $x$ due to the bending of ...
0
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2answers
96 views

Spacelike and timelike intervals confusion

I'm confused about this, specifically the spacetime interval. A timelike interval is one in which 2 events can be related to each other in a given reference frame within its light cone, that is, it ...
2
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1answer
227 views

The physics of a cornering wheel and the centripetal force

Can a rolling wheel create a side force without first rotating on a vertical axis? There is something wrong with the way we describe how a cornering vehicle wheel creates a cornering force. I think ...
0
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1answer
49 views

Does quantum forbid duplication or copy of states?

Suppose a beam of particles or a single particle, or a single particle. Is it possible to make a duplication of the beam or a single particle without collapse the states of the original particles? I ...
12
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6answers
3k views

Why does a sign difference between space and time lead to time that only flows forward?

Ever since special relativity we've had this equation that puts time and space on an equal footing: $$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2.$$ But they're obviously not equivalent, because there's a ...
0
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1answer
94 views

Does light know the future? [duplicate]

As I understand it, if light is traveling at the speed of light, then from it's point of view space is fully compressed in its direction of travel. Does that mean that from it's point of view, light ...
0
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0answers
44 views

Are vacuum-fluctuations a consequence of causality?

I'n new to QFT, and recently lerned about the propagator of a free scalar field theory in Minkowski-space, which according to our lecture notes looks like $$G(p, q) = \frac{1}{q^\mu q_\mu + M^2} \...
4
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2answers
175 views

Why can't we insert gravity in the special relativistic lagrangian?

I am a math student and I have taken four-five lessons about special relativity in a course about Lagrangian and Hamiltonian mechanics, so be patient with me if my question is stupid. My teacher says ...
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4answers
126 views

Exceeding the speed of light [closed]

I understand that the speed of light c is derived from the self-interaction between elections/photons, and is thus the maximum speed of anything composed of electrons/photons. Suppose that there is a ...
2
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4answers
191 views

Understanding the difference between timelike and spacelike separations

From Woodhouse's General Relativity: If $A$ is the origin and $B$ is a nearby event with coordinates $dt, dx, dy, dz$, then, $$ds^2 = dt^2 - dx^2 - dy^2 - dz^2$$ is the same in all local inertial ...
8
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4answers
2k views

No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
0
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0answers
28 views

“Speed” of gravity? [duplicate]

This is probably going to be a poorly written question, since I know really a little of physics. Imagine a object of small mass, let's say a book, is floating in the universe in absence of any ...
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1answer
70 views

Why does my thought experiment involving entanglement appear to violate the speed of light?

I have a thought experiment that "seems" to allow messages passed at faster than the speed of light. Since that is impossible I'd like to learn what is wrong with my experiment/reasoning. Start with ...
3
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1answer
129 views

Do light cones “tilt” towards black holes?

In some diagrams, light cones becomes "thinner" near black holes. Meaning the light has trouble moving nearer or further away. As in this picture. I presume this corresponds to someone observing that ...
1
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2answers
94 views

Does basic QM allow for superluminal “particle movement” during wavefunction collapse?

Can particles move superluminally away from their "expected values" using basic quantum theory? Here's an example: The eigenstates of a harmonic oscillator are defined from $(-\infty, \infty)$. This ...
29
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9answers
3k views

What precisely does it mean for “information to not travel faster than the speed of light”?

This is something that's been bothering me for a while. The way we usually first hear about causality is that "nothing travels faster than $c$". But then you learn that phase velocities can ...