# Questions tagged [causality]

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### CTC, determinism and valuedness of Riemann tensor

What I understood is that By the math, the Riemann tensor is obtained by parallel-transporting a vector along a closed curve in the considered space, then apply Stoke's theorem. Now if physics is ...
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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### (Anti)commutators at different times

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 ...
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### What conclusions can we draw from Thomas Breuer self-reference theorem?

This question is based on the another question of mine. Can we conclude one of the following statements from the results of Breuer? (1) The universe constantly receives information (influence) from ...
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### Does the notion of blurriness of lightcone appear in any theory of quantum gravity?

In the above diagram, black lines are $x=\pm t$ and the highlighted space is what I meant by the blurriness of the lightcone. Personally I have 2 arguments and both seems to suggest there shouldn't be ...
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### If an object is pulled at one end, how fast would an impulse travel to the other end in a realistic scenario?

In SR, there are a couple of paradoxes such as the bug-rivet paradox that require us to take into account a finite speed of impulse. Suppose I have a rod at rest, and I yank one end to some speed. I ...
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### Alternative to the “eraser” in the Quantum Eraser experiment?

I haven't seen any literature on the difference between erasing the photon's which-slit information vs just hiding it forever. Erasing quantum-information is effectively just a perpetual quarantine ...
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### Are following statements valid for quantum gravity?

In the concluding section of this post user Chiral Anomaly states following: On the other hand, since any stable marriage of quantum theory and gravity (in the sense of general relativity, not just ...
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### Force creates the motion or Motion creates the force?

Motion Creates Force ?: When you move your hand (to push a box for example), it creates a force. When electron moves through a wire it creates electrical force. Or Force Creates Motion ?: Gravity ...
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### How can we prove that a non-linear equation of motion for a classical scalar field satisfies causality?

Let $\phi$ be a classical scalar field in $1+D$-dimensional spacetime with coordinates $(t,\vec x)$, and consder the equation of motion  \newcommand{\pl}{\partial} (\pl_t^2-\nabla^2)\phi+m^2\phi+ g\...
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### Why doesn't finite field propagation speed contradict Gauss's law? [duplicate]

 Not sure why this was closed. The answers there do not answer my question, and are not even correct... Imagine a charge sitting in space. It causes an electric field everywhere, with magnitude ...
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### Variation of the pole-barn or train-tunnel paradox ( does it break causality )

I know that in the standard train tunnel paradox, the train shrinks in the tunnel frame and the tunnel shrinks in the train frame. Paradox is resolved by the fact that even though both doors of ...
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### Why nobody is revising Newton law of universal gravitation to show that the force don't instantly act on the object? [closed]

I think Sir Newton was aware of the serious flaw when he formulated the law of gravity. We all know time is an important component in doing science, but I don't remember that anyone tried to revise ...
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### What would prevent object from experiencing an infinitely powerful jerk in an instant?

Unlike speed which is capped for anything with rest mass at speed of light in a vacuum, what would prevent an object to undergo infinite acceleration in an instant? I assume in theory if we can apply ...
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### Is causality violated in QFT?

I'm studying QFT, and Peskin is his book takes a couple of paragraphs to talk about causality in QFT, using the Klein-Gordon field as an example. The book says on p. 28: To really discuss causality, ...
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### Difference between chronological future and domain of dependence

Given the following standard definitions of the two concepts, I fail to see how the chronological future differs from the future domain of dependence? Chronological future: The chronological future of ...
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### Quantum: Is it possible to determine the past knowing the present?

Due to quantum randomicity, it's impossible to determine the future knowing the present. But is it possible to determine the past knowing the present? As far as I understand, it is impossible because ...
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### Is the probabilistic nature of QM simply just randomness (does it exclude causality)? [closed]

I have read this question: What is the reason that Quantum Mechanics is random? where Puk says in a comment: I see. I would call both "random", with the degree of "randomness" ...