Causality Definition and 188 Threads

Hello! I am reading Peskin's book on QFT and in chapter one he shows that ##[\phi(x), \phi(y)] = D(x-y) - D(y-x)##, with ##D(x-y)## being the propagator from ##x## to ##y##. He says that if ##(x-y)^2<0## we can do a Lorentz transformation such that ##(x-y) \to -(x-y)## and hence the commutator...

So I'm not a professional scientist, but rather an avid physics fan. This is one question that no matter how much I research I can't find a clear answer... So for the sake of argument, let's assume that you could actually build something like the Alcubierre Drive and go FTL (I know a lot of...

http://advances.sciencemag.org/content/2/8/e1600162.full They claimed to have shown that non local causaulity models of QM do not work. But does that include the Transactional interpretation, which is non local and retrocausal?

So in GR, for a classical black hole, if A is approaching the event horizon, to an observer far away, let say B, B would never observe A crossing the event horizon as B would observe A's time slow down in the limit to 0 and A's length contract in the limit to 0. In fact, according to B, A never...

An inductor and resistor are arranged in parallel to a constant voltage source. There is a switch connected to a terminal on the inductor that can create a closed loop that includes either the voltage source, or the resistor. The switch is left connecting the source and inductor for a long...

Experimental test of nonlocal causality http://advances.sciencemag.org/content/2/8/e1600162 Quantum mechanics trumps nonlocal causality http://physicsworld.com/cws/article/news/2016/aug/18/quantum-mechanics-trumps-nonlocal-causality Having just read the full paper it seems to that it doesn't...

My intuition tells me that it does not given that physical phenomena don't obey the principle of sufficient reason under quantum mechanics (a dogma many still hold certain). A lucid definition of the PoSR can be found here. Meaning, that some events are non-localized and the distinction between...

So I have heard it mentioned that there are causally separate regions in the CMB. For instance a point A and point B that we can see here on earth, but are outside of each other's light cones. My question is then, how far apart are these points A and B at minimum to be causally separate in this...

I take an interest in physics in my spare time (in IT during the day). I have a query regarding the 'problem' of information loss at a black hole, and would be grateful for some enlightenment. It seems generally regarded that any information going past the horizon of a black hole is...

There is something that has been puzzling me about the way that light cones are drawn, when we consider causality diagrams. Lines representing a substantial point (as Minkowski referred to them) that is traveling as close to the speed of light, that is virtually at the speed of light, is drawn...

Dear PF Forum, I've tried to googled how causality can happen by FTL communcation (I use communication, because if it "travels", I'm having trouble calculating ##\gamma## imaginer). But from all examples that I've found it can violate causality if one of the party is moving relative to the...

Hi! Let ##T^{ik}## be the stress-energy-tensor, and ##v_k## some future-pointing, time-like four vector. How can I see that the object ##T^{ik}v_k## is future-pointing and not space-like? Thank you for your help!

In superluminal light pulse experiments, as shown in Fig. 4 of the Letter [Nature 406 (2000) 277], the whole pulse intensity profile observed is advanced by 62 ns nearly without any distortions, and the light pulse energy must be transported faster than the speed of light in vacuum...

I am wondering about the meaning of 'preservation of causality' in quantum mechanics. Is there causality in QM? And does it act back in time? I have some ideas of myself, but want to learn about the general accepted view first.

This relates to z-transform causality, but I'll try to phrase it as a complex analysis question. Suppose I have a function ##X(z)## whose poles are all inside the unit circle, and which has the property \lim_{|z|\to\infty} \frac{X(z)}{z} = 0 Is that sufficient to guarantee that \frac{1}{2\pi...

I am having a tough time getting my head around this. I get that traveling faster then c in itself has an effect on the the flow of time from the reference of external observation and the person doing it (isn't actual FTL velocity causing backwards time?). But in a "warp bubble" for lack of a...

Suppose I have an apparatus A that is entangled with apparatus B. In my reference frame, I observe apparatus A, which simultaneously causes apparatus B to do its thing. However, because there exists a reference frame where apparatus B does its thing before apparatus A, it follows that there...

This has been causing some confusion to me as the two concepts seem very similar, if not the same (especially when taking special relativity into account). As far as I understand, even in classical physics (i.e. even before considering QFT and the like), one requires that interactions are...

Consider the wave equation ##\nabla^{2} f - \frac{1}{c^{2}} \frac{\partial ^{2} f}{\partial t^{2}} = \delta(r) \delta(t) ## where there is no wave before ##t=0## The solution will be something up to a constants like ##f=\frac{\delta(r-ct)}{r}##. So we have a dirac delta function that...

Time-like and light like separations between events make sense, because a particle or a light flash can travel between them. However, how can you have a space-like interval when even light cannot travel such an interval (because its velocity is not greater than c). Obviously the first event...

As many, I wondered why FTL communication has problems with causality. Went over many posts here in PF, and wasn't satisfied. And then I read this explanation, by Richard Baker: (2015-4-2, link is still active) http://www.theculture.org/rich/sharpblue/archives/000089.html His explanation is a...

I found the thread "SR, LET, FTL & Causality Violation", looked through through all of it and read large parts. I believe I found my answer there, but to make sure, I'm asking the question here: Suppose we had some "magical" black-boxes that allow us to communicate information in both...

I have heard some philosophers state that cause and effect can be simultaneous with each other. Kant for example apparently claimed that when a bowling ball rests on a pillow creating a depression that the cause and the effect are simultaneous. It seems to me this is forbidden in relativity...

I have been wondering for some time now why causality is a prerequisite for every "good theory" all the way from classical mechanics, to QM, even in QFT the correlators for spacelike separated interactions cancel out. Now, since we usually take make a general theory and then usually simplify...

This type of integration is a special case of something that occurs over and over in QM and QFT (it's everywhere in Peskin and Schroeder), but I am having a bit of trouble working out the details. Set \hbar=1 and consider the propagation amplitude for a free, nonrelativistic particle to move...

I have a quick question about what is going on with the following scenario: There are three planets: A, B, and C. They are arranged in the following manner: A is 4 light years away from B and 2 light years from C; the distance between B and C is 3 light years. Now suppose that there are two...

I understand that the questions behind causality and FTL have been brought up numerous times, and I have read several of the old threads. My question isn't with how is causality violated, it is always assumed that any signal sent would appear to travel backwards in time from one point of...

Hello everybody. I would like to kindly ask your help with a hypothetical hairy question about which I think a lot recently. It is known fact, that it is not possible to construct a wormhole without exotic mass that violates the weak energy condition. It is also known that many quantum fields...

In a 1940 article in the Physical Review Wolfgang Pauli provides a definition for quantum causality: it is 'implemented microscopically by the requirement that observables commute at spacelike separations'. I find this confusing. Doesn't spacelike separation by definition exclude causal...

I have a quick question regarding causality in SR and some specific examples. Collision between two bodies is one of the examples when one event causes another, let's say that a body travels towards another which is at rest and hits it. Now my question may seem strange, but what is really the...

Hello, As written, I am wondering if the causality principle gets violated in QM. Do virtual particles violate the causality principles? Do instantaneous events violate the causality principle? Thank you in advance.

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality? Say, we make warp speed possible and get on it and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there...

I will bare in mind that causes may simply not be known today, however I would like to enquire about current non classic causality examples such as uncaused events; more specifically outcomes of experiments or matter to appear or change in an uncaused manner.

A seemingly good way to understand the overshoot and decay (ringing) of a square wave on a scope is that it is the result of bandwidth limiting. In that case, the Fourier series of a square wave \Pi(t) = \frac{1}{2 \pi} \sum_{n=-\infty}^\infty \frac{\sin(n \omega/2)}{n \omega/2} \exp(i n \omega...

excuse my lack of understanding! Is there anything in physics which directly suggests that if FLT was possible, you would travel back in time? does anything forbid the possibility that instead you gradually change your time reference to keep causality? With the example of a very fast train, a...

hi i read some text about causality and determinism, but i can't exactly distinguish between them. what's really difference between them? does not quantum mechanics respect one of them? i read this phrase in article of S.Carlip about quantum gravity "Quantum field theory includes...

I have been asked to write a couple of papers with a mathematician who has made extensive use of casual theory in his career and done pretty well. All i will be providing is neuroscience knowhow but obviously as a co-author i need to accept some responsibility for his product. Without...

Also how can local causality be violated?

By Bell’s theorem, an interpretation of QM must break either locality or causality. Linked below is the only interpretation I know of that chooses to maintain locality. (I don't know many.) Do you know of any others? CAUSALLY SYMMETRIC BOHM MODEL...

We are usually told in an introduction to general relativity that when special relativity was completed, there was a contradiction with Newtonian physics. The Newtonian gravitational force of M on m is F_g = -G \frac{M m}{r^2} \hat{r} where \vec{r} is the spatial vector from M to m. This...

I've seen other posts about this, but would appreciate an explanation if anyone knows. I've read that traveling faster than the speed of light could affect causality. Example: item I traveling in a tube could be seen arriving at end B before it ever left end A. A=======I=B Is this an...

First of all, I am fairly new to relativity, but not clueless. I am not saying that FTL is possible. I am not denying relativity principles. I am stating that FTL may be plausible. Relativity gives flexibility to how you can synchronize clocks and that does not affect outcomes of most...

How is causality usually proven when it's impossible to change the independent variable? Also, why is correlation often treated as a very reliable hint to causation, even in various respected journals? Here's an example...

Hi. I'm reading David Tong's notes (Causality at page 36: http://www.damtp.cam.ac.uk/user/tong/qft/two.pdf ) on QFT and I'm currently trying to understand the causality requirement that [O_1(x), O_2(y)] = 0 \ \ \forall \ \ (x-y)^2 < 0. For two operators O1 and O2. He then states that this...

I'm wondering if causality is enough to derive the Minkowski metric and the Lorentz transformations. It seems to me that in order to ensure that some set of events maintain a causal relationship to each other under a transformation to a moving frame of reference, that there must be some...

I wanted to post a small thought experiment that has been really bugging me. Let's say you could build a Newton's cradle the length of 1 AU, all the balls are elastic, there is no loss of energy due to friction, and you are standing at one end and your friend is standing at the other. At the...

"Quantum measurement predictions are consistent with relativity for macroscopic observations, but there is no consensus on how to explain this consistency in fundamental terms. The prevailing assumption is that the relativistic structure of spacetime should provide the framework for any...

It is well known that traveling at superluminal speeds violates causality, this can be shown by a space time diagram, where two frames would disagree on which event came after the other. My question is, if such particle were to exist (called tachyons), what would they see in their frame? Since...

Assume u:R\rightarrow C^n and define shift operator S(\tau) with S(\tau)u(t)=u(t-\tau) and truncation operator P(\tau) with P(\tau)u(t)=u(t) for t\leq\tau and 0 for t>\tau Then P(\tau)S(\tau)=S(\tau)P(0) for every \tau>=0. Can someone please prove last statement..

There are numerous posts on (attempting) FLT information via QE and whether causality is violated. I believe that there are definite answers to them and are buried (out there somewhere) in the gazillion posts/replies in the forum. FTL = faster than light, QE = quantum entanglement I hoping...