Tagged Questions
3
votes
2answers
160 views
What is the Andromeda Paradox?
I have been studying causality (specifically why there is no such thing as a simultaneous instant of time across all observers) recently and I keep hearing references to the Andromeda paradox. Can ...
5
votes
2answers
205 views
Causality and Quantum Field Theory
I have a problem with proof of causality in Peskin & Schroeder, An Introduction to QFT, page 28. To avoid confusion I use three vectors notation, rewriting the Eq. (2.53) for $y=0$ as follows:
...
8
votes
2answers
236 views
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
$$\phi(x)\phi^\dagger(y)\ne0.$$
...
3
votes
2answers
108 views
Extended Rigid Bodies in Special Relativity
I was reading Landau & Lifhsitz's Classical Field Theory and I noticed that they mention that an extended rigid body isn't "relativistically correct".
For example, if you consider a rigid rod ...
9
votes
1answer
294 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 ...
3
votes
2answers
222 views
The order of seeing event in different spacetimes
Assume this question:
Three events A, B, C are seen by observer O to occur in the order ABC. Another observer O$^\prime$ sees the events to occur in the order CBA. Is it possible that a third ...
5
votes
2answers
284 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 ...
6
votes
3answers
234 views
Is the commutation of all possible operators sufficient to identify a spacelike interval?
It has been claimed (e.g. here) and apparently already been established, that the interval $x - y$ being (called) "spacelike" implies that $\bigl[\hat O (x),\, \hat O' (y)\bigr]=0$ for any two (not ...
4
votes
1answer
109 views
Causality without light
Related to my prior question: Would a species insensitive to light have developed special relativity?
Imagine a species insensate to light and other em radiation. They are very attuned to sound ...
7
votes
8answers
481 views
Is “Causality” the equivalent of a claim that the future is predictable based on the present and the past?
In classical (Newtonian) mechanics, every observer had the same past and the same future and if you had perfect knowledge about the current state of all particles in the universe, you could ...
14
votes
2answers
649 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 ...
5
votes
3answers
88 views
Analyticity and Causality in Relativity
A few weeks ago at a conference a speaker I was listening to made a comment to the effect that a function (let's say scalar) cannot be analytic because otherwise it would violate causality. He didn't ...
5
votes
3answers
136 views
How can we know, today, that there's something from 100 light-years from here?
In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
7
votes
9answers
823 views
Is causality a formalised concept in physics?
I have never seen a “causality operator” in physics. When people invoke the informal concept of causality aren’t they really talking about consistency (perhaps in a temporal context)?
For example, if ...
3
votes
3answers
2k views
Example of space-like intervals in spacetime
From wikipedia:
When a space-like interval separates
two events, not enough time passes
between their occurrences for there to
exist a causal relationship crossing
the spatial distance ...
7
votes
3answers
600 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 ...
