Questions tagged [causality]

The influence one event, process, or state, has on another event, process, or state, whereby the latter is at least partly dependent on the former.

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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 ...
Stefano Borini's user avatar
134 votes
11 answers
102k views

Is anti-matter matter going backwards in time?

Some sources describe antimatter as just like normal matter, but "going backwards in time". What does that really mean? Is that a good analogy in general, and can it be made mathematically precise? ...
Gerard's user avatar
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Why is quantum entanglement considered to be an active link between particles?

From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every ...
Andrey Tatarinov's user avatar
103 votes
7 answers
123k views

What do spacelike, timelike and lightlike spacetime interval 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 ...
Gold's user avatar
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103 votes
12 answers
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Why are differential equations for fields in physics of order two?

What is the reason for the observation that across the board fields in physics are generally governed by second order (partial) differential equations? If someone on the street would flat out ask me ...
Nikolaj-K's user avatar
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95 votes
10 answers
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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 ...
Pete Oakey's user avatar
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80 votes
2 answers
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Why did Feynman's thesis almost work?

A bit of background helps frame this question. The question itself is in the last sentence. For his PhD thesis, Richard Feynman and his thesis adviser John Archibald Wheeler devised an astonishingly ...
Terry Bollinger's user avatar
78 votes
7 answers
51k views

How does faster than light travel violate causality?

Let's say I have two planets that are one hundred thousand lightyears away from each other. I and my immortal friend on the other planet want to communicate, with a strong laser and a tachyon ...
markovchain's user avatar
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76 votes
9 answers
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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 ...
Zaibis's user avatar
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54 votes
2 answers
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In QFT, why does a vanishing commutator ensure causality?

In relativistic quantum field theories (QFT), $$[\phi(x),\phi^\dagger(y)] = 0 \;\;\mathrm{if}\;\; (x-y)^2<0\,.$$ On the other hand, even for space-like separation $$\phi(x)\phi^\dagger(y)\ne0\,.$...
hwlin's user avatar
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49 votes
5 answers
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Could I, within my lifetime, reach any star I wanted if I went fast enough?

Disclamer: I'm not talking about FTL travel here. I'm also not talking about any weird space warping mechanics like wormholes and such. I've always thought that if a star was 4 light years away, then ...
Augs's user avatar
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38 votes
10 answers
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Is there a notion of causality in physical laws?

I was reading "A Few Useful Things to Know about Machine Learning" by Pedro Domingos and towards the end of the paper he makes this statement: "Many researchers believe that causality is only a ...
covfefe's user avatar
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37 votes
3 answers
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 ...
IchVerlore's user avatar
37 votes
7 answers
15k views

How does "warp drive" not violate Special Relativity causality constraints?

I'm talking about this nonsense: http://www.washingtonpost.com/news/post-nation/wp/2014/06/11/this-is-the-amazing-design-for-nasas-star-trek-style-space-ship-the-ixs-enterprise/ Now, I'm aware that ...
Larry Gritz's user avatar
36 votes
2 answers
7k 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 ...
user1247's user avatar
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34 votes
3 answers
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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 ...
Approximist's user avatar
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33 votes
4 answers
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How do electrons know which path to take in a circuit?

The current is maximum through those segments of a circuit that offer the least resistance. But how do electrons know beforehand that which path will resist their drift the least?
Swapnanil Saha's user avatar
33 votes
9 answers
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 ...
Yly's user avatar
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32 votes
1 answer
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Does the heat equation violate causality?

I've ran across the idea that, besides simply writing partial differential equations in covariant form, they need to be hyperbolic with all characteristic speeds less than the speed of light. A ...
octonion's user avatar
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31 votes
6 answers
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Why can't you escape a black hole?

I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is $c$. But it's also true that you don't have to go escape velocity to escape an ...
Carson Myers's user avatar
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29 votes
9 answers
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Why do we care about black hole interiors' physics?

Whatever happens in there is not falsifiable nor provable to the outside. If for (amusing) example the interior consisted of 10^100 Beatles clones playing "Number Nine" backwards, do we know how to ...
Andrew Palfreyman's user avatar
28 votes
5 answers
14k 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 ...
Edward Hughes's user avatar
28 votes
4 answers
6k views

What are some scenarios where FTL information transfer would violate causality?

I've always heard people saying, "Faster than light information transfer can't happen because it would violate causality! Effects can precede their causes!" However, I'm trying to think of a ...
Justin L.'s user avatar
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27 votes
4 answers
2k views

Does relativistic quantum mechanics (RQM) 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 ...
Sam Gralla's user avatar
27 votes
2 answers
1k views

Can one classify partial differential equations according to the causality properties of their solutions (and if yes, then how)?

Recently, I bumped into this interesting comment by Valter Moretti which made me wonder about the following, more general question (to which I suspect the answer is affirmative): Can we easily tell, ...
Danu's user avatar
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26 votes
2 answers
5k 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 ...
Fraïssé's user avatar
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24 votes
14 answers
7k views

Does the collapse of the wave function happen immediately everywhere?

It is usually taught that when we measure some measurable value the wave function collapses immediately everywhere. This idea sounds like a simplification of some more complicated mechanism. Are ...
ziv's user avatar
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24 votes
1 answer
2k 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 ...
Oliver Cohen's user avatar
24 votes
5 answers
5k views

Doesn't Veritasium's Recent Video About Circuits Violate The Speed Of Light?

A recent Veritasium video discusses the following circuit: ...
David's user avatar
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24 votes
3 answers
3k views

How is quantum mechanics compatible with the speed of light limit?

Consider a free electron in space. Let us suppose we measure its position to be at point A with a high degree of accuracy at time 0. If I recall my QM correctly, as time passes the wave function ...
Elliotte Rusty Harold's user avatar
23 votes
4 answers
6k views

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 ...
A.R.K's user avatar
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23 votes
3 answers
10k views

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). ...
vsz's user avatar
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22 votes
6 answers
4k views

Quantum entanglement and spooky action at a distance

When quantum entanglement is explained in "layman's terms", it seems (to me) that the first premise, that we have to accept on faith, is that a particle doesn't have a certain property (the particle ...
DWalker's user avatar
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20 votes
10 answers
2k views

Cause-effect definition of fictitious forces

I'm currently teaching a general-physics-for-engineers class, and we approached fictitious forces. As I was explaining them, students asked me how to discriminate "real" forces from ...
Mauro Giliberti's user avatar
20 votes
6 answers
2k views

Is there such a thing as "Action at a distance"?

What ever happened to "action at a distance" in entangled quantum states, i.e. the Einstein-Rosen-Podolsky (EPR) paradox? I thought they argued that in principle one could communicate faster than ...
Jeremy's user avatar
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20 votes
3 answers
5k 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 ...
user1379857's user avatar
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20 votes
5 answers
3k views

Event horizons without singularities

Someone answered this question by saying that black hole entropy conditions and no-hair theorems are asymptotic in nature -- the equations give an ideal solution which is approached quickly but never ...
spraff's user avatar
  • 5,084
19 votes
5 answers
5k views

Why does the speed of light totally prevent instantaneous information exchange?

Based on the classical light-cone approach it's easy to see you can't transmit information faster than $c$ but why does the speed of light (as far as I know) treat information transmission in this way ...
Mr. Boy's user avatar
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19 votes
6 answers
5k views

The transactional interpretation of quantum mechanics

John Cramer’s transactional interpretation of quantum mechanics (TIQM) is billed as resolving the fuzzy agnosticism of the Copenhagen interpretation while avoiding the alleged ontological excesses of ...
Nigel Seel's user avatar
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19 votes
1 answer
6k views

Physics behind this neutrino-related joke

In the comment section of a newspaper article reporting on the 2015 Nobel Prize for Physics, which was awarded for work on neutrino oscillation, I found the following joke: "I'm sorry, we do not ...
user avatar
19 votes
8 answers
9k views

Flaws of Broglie–Bohm pilot wave theory?

I recently learned about an oil drop experiment that showed how a classical object can produce quantum like behavior because its assisted by a pilot wave. How has this not gained more attention? What ...
Snerd's user avatar
  • 425
17 votes
4 answers
3k views

Is momentum space "less physical" than position space?

In quantum mechanics and quantum field theory it is specially common to work in both position and momentum space. Passing the theory to momentum space is sometimes crucial, as one usually finds that ...
MathMath's user avatar
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17 votes
2 answers
3k views

A question about causality and Quantum Field Theory from improper Lorentz transformation

Related post Causality and Quantum Field Theory In Peskin and Schroeder's QFT p28, the authors tried to show causality is preserved in scalar field theory. Consider commutator $$ [ \phi(x), \phi(y) ]...
user26143's user avatar
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17 votes
4 answers
926 views

Why do we take two different approaches to the "meaning of causality" in quantum mechanics and quantum field theory?

In non-relativistic quantum mechanics, causality is violated by saying that the amplitude of propagation of a particle $$A=\langle \textbf{x}|\exp{\Big(\frac{-i\textbf{p}^2t}{2m\hslash}}\Big)|\textbf{...
SRS's user avatar
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16 votes
6 answers
11k views

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 ...
wrongusername's user avatar
16 votes
2 answers
2k views

Why does gravity travel at the speed of light?

In electromagnetism, Maxwell's equations predict that electromagnetic disturbances travel at the speed $$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}.$$ Does general relativity predict that gravitational ...
jim's user avatar
  • 3,855
16 votes
3 answers
2k views

Reversibility = non-causality. Can this be right?

I read yesterday the Norton Dome's paper, which shows that some Newtonian systems can be non-causal, based on specific solutions of Newton's laws. The author justifies the solutions in very nice, ...
The Quantum Physicist's user avatar
16 votes
4 answers
6k views

What is 'past null infinity'?

For example, in the sentence "there is no incoming radiation at past null infinity".
user12345's user avatar
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16 votes
1 answer
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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^...
tparker's user avatar
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15 votes
7 answers
5k views

Is force due to electric field instantaneous? If not then why?

Suppose a charge $q$ is experiencing a force due to charge $Q$. Suppose we move the charge $Q$ very slowly (no acceleration) what's the instantaneous impact on the charge $q$? How will the $q$ react?
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