# Questions tagged [causality]

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### Methods to study the causal/light cone structure of General Relativity

I've been looking into causal set theory as an approach for quantum gravity, and my research has gotten me interested in the causal structure of spacetime. If I understand correctly, if one has ...
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### Would a tachyon be able to escape a black hole?

Or at least escape from a portion of the hole inside the photon horizon?
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### Is the effect of friction force transferred INSTANTLY in this case?

I read on a website that if the sun is removed from its position then it would take some time for the earth to feel the effect i.e. flying off tangentially from the orbit. In a slightly different case,...
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### Killing vectors in Minkowsky Metric

So I was in the process to find the Killing vectors for the Minkowsky Metric and I stumbbled into a material that does a different procedure at the very end of the process, in comparisson to usual ...
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### Is This Hypothesis Correct?

I have constructed a thought experiment regarding whether the future is deterministic or not. It goes like this: Let’s take an Observer 100 light years away from earth. Now presently, if he looks at ...
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### Null vector space in Minkowski space

Let us consider a Minkowski space of the form: $$ds^2 = -dt^2 + dx^2 + dy^2 +dz^2.$$ What would the linearly independent null vectors of this space be? I am aware this is a trivial question but is ...
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### 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 ...
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### What does a 4+1D wave look like at the light cone?

I need help making sense of a few comments from under this answer. I think it’s best if I reproduce the comments below: “The Green's function for the wave equation in even spatial dimensions is ...
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### Are wormholes evidence for traversal of a higher dimension?

Warning, pop science coming.. please correct what I’m getting wrong. Einstein’s equations of relativity showed the potential for existence of wormholes that can connect different points in space time....
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### Wouldn't a negative mass be going faster than $c$ according to our current models of relativity?

I heard that there are some physicists trying to figure out, out least hypothetically, how things with positive and negative mass may interact with each other. I'm really confused about how this can ...
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### Do tachyons move faster than light?

I am trying to understand whether or not tachyons travel faster than light. The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing. For instance, the first ...
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### Causality: Why can't things move backwards in time, within past lightcone?

My question Why can't effects propagate backwards in time, within the backwards light cone of a cause? For example, when I turn on a flashlight, why doesn't the light travel backwards in time just ...
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### What laws prevent massive particles to travel at the speed of light?

I and a few of my friends have come across an interesting question. Jackson talks about the case where photon has non-zero mass. By adding an extra term to the Lagrangian, he shows how Maxwell's ...
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### Will an infinite speed of gravity stop things from moving and can that be reason that the speed of light is actually finite? [closed]

Again, I edited, after reading a related question question asked yesterday (that wasn't closed). It's easier to read now too, but the content hasn't changed, basically. In mainstream physics, it is ...
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### How to show time evolution operator obeys causality?

If we are given a time evolution function $K_t(\phi,\phi')$ which give the amplitude for a field starting in confiruation $\phi$ to go to configuration $\phi'$ after time t. What is the condition that ...
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### Why is the anti-commutation relation $\lbrace \psi_a(x), \bar{\psi}_b(y) \rbrace = 0$ enough to ensure causality?

In quantum field theory, it is crutial that two experiments can not effect each other at space-like seperation. Thus $[\mathcal{O}_1(x), \mathcal{O}_2(y)] = 0$ if $(x-y)^2 < 0$. For the Klein-...
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### Causality and processes in QFT

We have virtual particles in quantum field theory (QFT). In general, they don't have the need to obey causality. My question is: Do the processes in QFT (electron self-energy, photon self-energy, ...
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### Action at a distance in Quantum Field Theory

Definitely, I don't mean entanglement here: Suppose we have an electron and proton situating some distance apart, there is an electrostatic force between them, and this force is mediated by virtual ...
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### Escape Velocity, Misattribution, and Black Holes

Escape velocity is the ballistic speed required to escape from a gravitational field to infinity, ignoring any third body dynamics. The operative word here being ballistic, meaning unpowered. ...
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### Is a still object in 4D spacetime lightlike?

Every particle in the universe is moving in spacetime: a massive "still" one (in the 3D sense) is moving in a purely timelike direction, a massive "moving" one in a direction with ...
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### Entanglement & QFT

Quantum Field Theory (QFT) is essentially a local theory, which I take to be simply : "cause preceeds effect" (as far as I know). It is possible to calculate entanglement entropy in QFTs. So ...
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### What is a 'spacelike surface' in relativity?

I am studying Noether's theorem in field theory and I am not understanding what spacelike-surfaces mean. I will reproduce the bit of the argument below that contains the term "spacelike-sufaces&...
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### Another form of Black Hole Information Paradox?

Consider the Penrose Diagram of Collapsing Gravitational matter :                           Any radial light ray (say P) originating from $\mathscr{I}^{-}$ is bound to end up in the Black Hole. The ...
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### Can Increment / maximization of Entropy be the “Cause” behind any phenomena?

I understand that the increment/maximization of Entropy (of the universe) is "Accompanied" with all "Natural" phenomena we see. In many of the questions, I and others have asked on Stack Exchange, ...
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### Representing bound geodesics in Penrose diagrams

I recently started reading about conformal (Penrose) diagrams and have since been faced with a couple of conceptual doubts. Based on the coordinate transformations, null curves in Penrose diagrams are ...
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### Quantum Fluctuations of Light-Cones

We typically draw light-cones to study causal relation between two space-time points. Source : Wikipedia Space-Time is manifest as the metric in various calculations. If one considers that something ...
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### Is general relativity the simplest possible theory of gravitation?

I can't find this, but i've seen that GR is the only possible theory of gravity if you assume causality and principle of equivalence?
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### 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} \...
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### How superhorizon power spectrum would have looked like if there was no inflation?

As it is well known, one of the main motivations for the idea of inflation is the explanation of superhorizon CMB correlations: we observe the existence of perturbations with sizes larger than the ...
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### Is wave function collapse non-local or local?

Imagine a particle in a very large box which takes years to travel from one end to the other. Alice and Bob are outside the box, on opposing ends. Each can remove their side of the box to check if the ...
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### What kind of causality would be broken if black hole singularities would be real?

In his article "The Universe as a Whole" 1, physicist Dennis Sciama said We therefore face a crisis in theoretical physics. Either classical general relativity breaks down, or effectively ...
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### In what sense does the “speed limit” $c$ prevent violations of causality? Could they be prevented some other way? [closed]

I have seen it said that if we want to avoid violations of causality (I can see why we would want to avoid that!), we need the information-transfer "speed limit" of c. I presume(?) this ...
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### Very long Newton cradle as a comparision to the invariance of the speed of light regarding the movement of the source?

If we place a Newton cradle 1 km long along the road and now passing on a bicycle hit the first element (sphere) with a hand the speed of the disturbance would be the same no matter how fast the ...
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### What is time travel and why is the grandfathers Paradox a Paradox?

While watching a talk given by Neil deGrasse 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 ...
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### Why would FTL imply time travel? [duplicate]

Edit: Thank you all for the explanations, but I think until I fully grok special relativity, Lorentz transformations and relativity of simultaneity, the answers won't make any sense to me. Maybe the ...
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### Does it contradict special relativity that an electron beam in a television picture tube can move across the screen faster than the speed of light?

While looking at some exercises in my physics textbook, I came across the following problem which I thought was quite interesting: It is possible for the electron beam in a television picture tube ...
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### Will Coulomb's law be valid always?

We know about the general theory of relativity which is modern theory of gravitation. Einstein formed it by stating no action can be observed including gravitational force faster than speed of light. ...
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### 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 ...
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### Can gravity act through time? [closed]

I know, that gravity acts through time and space, as it propagates at the speed of light, through space. I mean to discuss whether gravity has an effect in spacetime, along the time axis. Effects of ...
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### Can you fall into a black hole, and then back out again?

There is a innumerable number of questions on this site about black hole event horizons, the possibility of falling in and back out again, but none of them answer my question specifically. I have read ...
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### What is a naked singularity?

What is a naked singularity? Would a naked singularity be one that that the event horizon is so small that it is the same size as the singularity? what could make one that small?
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### What's wrong with this “proof” that QFT violates causality?

In An Introduction to Quantum Field Theory, by Peskin and Schroeder, when discussing the quantized real Klein-Gordon field ($\phi=\phi^\dagger$), they show the commutator $[\phi(x),\phi(y)]$ vanishes ...
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### Does relativistic quantum mechanics really violate causality?

The Hamiltonian $H=\sqrt{p^2+m^2}$ defines a one-particle quantum mechanics in the usual way. Let us call this theory RQM for short. Peskin and Schroeder claim that RQM violates causality because ...
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### How would wormhole-based FTL violate causality?

We already have an answer why physically traveling faster than light would violate causality (the clock on board our hypothetical FTL spaceship would tick backwards to some outside observers). ...
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### Causuality-effect relation and time order

We know that from view of special relative theory, there is no Lorentz transformations that converts or reverses the time order in causuality-effect relation. Now my question is: if there is no ...
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### Is general relativity about Lorentzian manifolds only?

General relativity is often used in context of Lorentzian manifolds. But the texts which describe Einstein Field Equations discuss them in context of general pseudo-Riemannian manifolds. It seems ...
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### Difference between action-at-a-distance and a field according to Maxwell?

My question is more on a historical note that involves Maxwell’s equations. Besides the information that I have obtained from textbooks, I am mainly getting it from History of Maxwell's Equations and ...
Why does the commutator of two operators evaluated at different times vanish? Take for instance a fermonic field $\psi_\sigma (\vec{x},t)$, which satisfies the well known anti-commutation relations ...