Causality Definition and 185 Threads

I have read that if two photons are entangled, and one photon is sent through a 2-slotted paper and is unobserved, then the second is sent through and is observed, the earlier photon is found to have behaved as though it were observed right along with the second one, seemingly defying...

This is pretty theoretical, so I don't know whether it would better belong in the "other physics" section. As I understand it, a pair of pants situation of topology change where one universe splits in two is described by a global wick-rotated riemannian metric so as to avoid the causality...

When I tap my left hand and my right hand against the floor 3 feet apart, at the same time, I know that I’m not “really stretching 3 feet” at the same time, my brain is reading the nerves of each hand at a speed actually even slower than c but no matter how my circuits were designed it would...

Hi, I was following some discussions here about delayed-choice entanglement swapping (DCES) and its implications regarding causality. I'm recently came across the following paper where the authors provides a forward-in-time analysis that seems to avoid any backwards influence due to the BSM and...

The conventional picture We often describe two perspectives of infalling matter: Outside observers see the object asymptotically approach but never cross the horizon, with signals becoming increasingly redshifted The infalling object supposedly experiences a finite proper time to cross the...

We want to check whether specially treated bitter gourd is effective, marketed as BitterHeal, in lowering blood sugar levels in diabetics. They take a random sample of diabetics and assign them randomly to ##3## groups: Group A: Are given BitterHeal Group B: Are given untreated bitter gourd...

The graph below shows a very clear, but not perfect, correlation between the red line and the blue line. That fact says nothing whatsoever about causality. BUT ... if you add in the additional fact that there is absolutely zero chance that the blue events cause the red events and a quite...

I wanted to ask about a potential difference between general relativity and quantum mechanics phenomena - that we are observing them at different moments in time. Because causality has a speed limit (##c##), every point in space where you observe it from will be the closest to the present...

These papers claim to present a realistic stochastic interpretation of quantum mechanics that obeys a stochastic form of local causality. (A lecture I recently attended mentioned these papers). It also claims the Born rule as a natural consequence rather than an assumption. This appears to me to...

In Star Wars, they move FTL by accessing hyperspace which is a short-cut of sorts or like a wormhole that allows them to travel very fast without breaking causality as they are technically not in this dimension. FTL signals are also sent through this same dimension (called hyperwave...

Hi, I want to try to solve this puzzle in my head. They say that faster than c travel would break causality. And yet particles can travel through a medium faster than light can in that medium. But surely if that can happen then a particle can arrive at a place faster than information about the...

I have read that if causality exists, then the cause must exist in the past light cone because causes can't go faster than light. However, as an implication of special relativity, space and time are interdependent, leading to a four-dimensional continuum. Since reality is a block universe (block...

A few years ago I was studying special relativity and came across a thought experiment that explored a faster-than-light causality-flip. It consisted of an observer on the track-side firing a bullet at a speeding train, with the bullet entering the back of a carriage and exiting the front. There...

There is something I don't understand that I want to ask quantum physics experts here: Suppose the happening of event X results logically speaking in the happening of event A. So we could for instance have the following logical implication ##X.happens \rightarrow A.happens##. If this is...

I was told that PI controller is a causal filter, and has frequency response represented by H(w) = Ki/(iw)+ Kp. I was also told that causal filter should satisfy this relationship H(w) = G(w) -i G_hat(w) where G_hat(w) is the Hilbert transform of G(w). Does this mean that we cannot freely...

Suppose someone entangled 2 particles many years ago and kept particle E here on Earth and sent particle S a light year away from Earth. So the observation of particle E on Earth would fix the state of particle S a light year away. But they did it in such a way that observing particle E would...

A couple of the sites that say that the vanishing commutator defines causality: https://www.physics.umass.edu/events/2020-02-07-causal-uncertainty-quantum-gravity https://phys.org/news/2019-11-aspects-causality-quantum-field-theory.html Is "define" too strong? Or, to put it another way: cannot...

Prove that if a system's transfer function is not proper (order of numerator greater than order of denominator) then the system is not causal.

If I use the expression ##(|A\rangle+|B\rangle)|M\rangle \rightarrow |A\rangle|M_A\rangle+|B\rangle|M_B\rangle## for decoherence, does that mean that we can infer that, IF the measured value is ##A## that THEN we will measure ##M_A##, OR that IF we measure ##M_A## THEN the measured value must...

In the wikipedia article of the observable universe I have read the following: "Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct...

Eq 4.44 in Peskin and Schroeder. My question is: Does causality imply that time coordinates of z (internal vertex over which we doing the integration) should lie between the time coordinates of field phi(x) and phi(y)?

I hope my thread won't be closed (too soon) and some forummers can shed their light over my question. Let's suppose a deterministic universe. I assume all events have a cause, that everything is cause and effect. Then, given that the state of the universe at some instant is X, and this leads to...

Suppose the system under examination is fully deterministic. Does that imply that effects follow causes and not precede them? For instance, if in this system Alice would respond to event X with A, but, if instead of X event Y would have happened, with B, does that mean she has no choice between...

If two events A and B are spacelike separated, is it then true that there can be no causal relationship between the two?

In the mid-1990's, an electrical engineer/computer scientist by the name of Judea Pearl started to change the world by greatly improving our understanding of causality. He brought together many strands of thought that had gone before him, then synthesized them into an integrated whole, with many...

Hi All I am a bit exasperated right now. On another forum a person claimed Bell's second theorem proved QM was not local. I carefully explained what local causality was, and what the theorem states: There exist quantum phenomena for which there is no theory satisfying local causality. It...

causality is a property that follows from QFT (or maybe even from the Dirac equation?) the simplest example is a scalar field with Lagranagian ##\mathcal {L}=\frac 1 2 \partial^\mu \phi \partial_\mu \phi - \frac 1 2 m^2 \phi^2## The fact causality is preserved follows by that [ϕ(x),ϕ(y)]=0...

Summary: Born's rule for the QED electron violates causality. [Since the thread where some of this material was presented was closed for further discussion, I summarize here the main content relevant for the above topic.] The free QED electron can be described in terms of a non-local...

[Moderator's note: Spin off from a previous thread due to topic change.] Actually, the form of the Hamiltonian does matter. Hegerfeldt admits that his results are not correct for the Dirac Hamiltonian unless one considers only positive energy solutions. And why should we do that? It is clear...

I have been reading some fairly mind bending stuff about the principle or least time (and those of least action) raising questions about causality and free will. Can anyone explain this to me? Is this total 'woo woo' psuedo science, or are these philosophical questions widely accepted? Thanks...

Hello everyone! I have a question regarding the causality in QFT. If I take into consideration a real scalar field and I calculate: $$[\phi(x),\phi(y)] = 0 \space \space \space \space \space \text{if (x-y)}^2 < 0$$ Thanks to this relation we state that causality in QFT is preserved: a...

Are timelike entanglement and experiments demonstrating causal non-separability by quantum superposition of causal orders an indication that causal principles may not be applicable to quantum mechanics?

What is currently the common opinion on Chronology Protection manifested by Hawking almost 30 years ago? There does not seem to be any fully accepted no-go result for Thornian time machines. Energy conditions can be violated in QFT, semi-classical results suffer from counter-examples, Novikov's...

Let me present what I think is the understanding of a particular situation in quantum mechanics, and ask people to tell me whether I am right or wrong. To say that everything happens randomly in QM would be misleading at best. We get at least statistical prediction. But discussions such as the...

Hi everyone! Sorry for the bad English and the silly question! My mind isn't very Sharp, so I just want to make sure I understood it right... I've read the faq and I found this info amazing! If I understood correctly hahahaha! So, there's two main interpretations of the findings of...

Or does ontological probability exist? I was reading an article that came up in my google searches ( https://breakingthefreewillillusion.com/ontic-probability-doesnt-exist/ ) ignore the free will philosophy stuff. But the author makes the claim that ontological probability simply does not...

[Moderator's note: spun off from another thread.] And yet modern physicists accept non-locality which is far worse :) Do you know where Newton expressed his misgivings? It would be interesting to know exactly what he thought.

Hello everyone ! I try to find the expression of the time derivative of the entropy for the CMB (photon gas) but I am stuck with the calculations. We are in the matter-domination area and at the present time (Ro=1). No radiation and no vacuum, only the curvature. The different equations are...

can we find causality in quantum mechanic like the classical physics ?

Proposition: Consider an ##n + 1##-dimensional metric with the following product structure: $$ g=\underbrace{g_{rr}(t,r)\mathrm{d}r^2+2g_{rt}(t,r)\mathrm{d}t\mathrm{d}r+g_{tt}(t,r)\mathrm{d}t^2}_{:=^2g}+\underbrace{h_{AB}(t,r,x^A)\mathrm{d}x^A\mathrm{d}x^B}_{:=h} $$ where ##h## is a Riemannian...

According to General Relativity, everything must be causal, something from the future cannot effect the past, and spacetime geometry is this way. By this logic, does this mean that if we were to ever time travel, via any means, arriving in the past is a violation of causality, as you are...

Hello, I am studying control theory. And I have encountered something I have never considered or thought about. Consider a system with y as the output differential equation and u as the input. any(n) + ... + a1y(1) + a0y = bmu(m) + ... + b1u(1) + b0u Here, the subscripts indicate...

I have previously posted Preserving local realism in the EPR experiment . I have since given up on simulating local realism since I now understand it is impossible. However I have not given up on causality. Attached is code that simulates the EPR experiment and gives the same result as what...

I've never understood the problem with a violation of causality. Obviously we are talking about in the context of FTL travel, probably one of the most asked about topics there is. Everyone wants it to be a thing. While I see the benefits of it, I don't care about if it could be possible or not...

Hi everyone! Sorry for my bad English! I read old posts in this forum, googled it and still can't figure out one thing: What other explanation could there be, other than a random event in the future determined if the photon behaved as a particle or wave in the first detector? Thanks a lot! I...

Forgive my novice question; but, how does one explain the fact that decoherence doesn't contradict the evolution of the wavefunction in every world? Meaning, how is causality preserved in each world and what concept of time is professed wrt. to each world in the MWI? In other words, it seems...

Hi everyone! Sorry for my bad English! Please, suppose you have a subject A that opens his arms like a "T", in each hand he holds a laser and shoots the light at the same time. There are 2 targets at the same distance and, to A, the light hits both targets simultaneously. I Know that in some...

I just wanted to share this idea with other people who may be interested: I watched a PBS Space Time episode about the speed of light more accurately described as the speed of causality. And I Submit! That "time" is simply a interaction between atoms. And though an interaction can be...

In quantum field theory (QFT), the requirement that physics is always causal is implemented by the microcausality condition on commutators of observables ##\mathcal{O}(x)## and ##\mathcal{O}'(y)##, $$\left[\mathcal{O}(x),\mathcal{O}'(y)\right]=0$$ for spacelike separations. Intuitively, I've...

[Moderator's note: this was originally the second part of a post in another thread; it has been moved to this separate thread.] PLEASE BE ADVISED: I'm a complete ignorant with regards to the details of what I'm about to say and I've never looked at the mathematics of the subject, let alone...