is-quantum-communication-faster-than-the-speed-of-light?

dcbingaman

Joined Jun 30, 2021
1,065
True but the randomness of the decay event timing doesn't violate the physics meaning of causality irt FTL communications or modify a past timeline. There's no easy way to predict what number comes up when rolling dice, does that break physics causality? "Caused" isn't restricted to deterministic processes. A probabilistic component in the process doesn't mean that it is "uncaused".
I agree. The probabilistic component in quantum decay event or a electron emitting a photon and transitioning to another state does not mean it is 'uncaused'. But at the same time it does not mean it has a 'cause'. We simply don't know why quantum events happen. Our common sense wants us to believe that such things have a cause. But when it comes to quantum mechanics, we have to throw out common sense. I am not going to be to dogmatic about it. I have been wrong way to many times in my life to state something without mentioning I could be completely off base. :) I understand your point. Thanks for sharing.
 

nsaspook

Joined Aug 27, 2009
13,553
I mainly agree but IMO science and common sense intuitions about reality are completely compatible. Without common sense, preposterous things like 'free energy', the 'EMdrive' and a host of other pop-sci favorites become just things that might be possible if we break physics a bit. "Quantum gravity" theories of tiny strings, or loops, or membranes, in a 1000 dimensions, with "Bizarro World" parallel universes become perfectly acceptable scientifically with no possibility of scientific falsification.

Common sense is to ask for actual evidence instead of a non-physical math framework for scores of different grand unified theories.
 

dcbingaman

Joined Jun 30, 2021
1,065
I mainly agree but IMO science and common sense intuitions about reality are completely compatible. Without common sense, preposterous things like 'free energy', the 'EMdrive' and a host of other pop-sci favorites become just things that might be possible if we break physics a bit. "Quantum gravity" theories of tiny strings, or loops, or membranes, in a 1000 dimensions, with "Bizarro World" parallel universes become perfectly acceptable scientifically with no possibility of scientific falsification.

Common sense is to ask for actual evidence instead of a non-physical math framework for scores of different grand unified theories.
True but consider what 'common sense' was telling us say 300 years ago. First time was 'absolute' we could all agree on it. Along came Einstein with Special and General relativity and blew that out of the water. The idea that the speed of light is the same regardless of you relative motion defies all common sense but has turned out to be true. The fact that in one relative frame of reference event A could proceed event B and in another frame of reference event B could precede event A and both are correct. Our common sense would say no way. But it turned out to be true via scientific experimentation. Who knows what other 'common sense' ideas will be blown out of the water from future scientific enquiry as we learn more about the sub-atomic world.
200-300 years ago the idea that the 'speed of light' would be the same for all observers as long as they are in a constant velocity frame work would appear to them as preposterous, because it defied their common sense. Nevertheless that turned out to be the case.
 

nsaspook

Joined Aug 27, 2009
13,553
True but consider what 'common sense' was telling us say 300 years ago. First time was 'absolute' we could all agree on it. Along came Einstein with Special and General relativity and blew that out of the water. The idea that the speed of light is the same regardless of you relative motion defies all common sense but has turned out to be true. The fact that in one relative frame of reference event A could proceed event B and in another frame of reference event B could precede event A and both are correct. Our common sense would say no way. But it turned out to be true via scientific experimentation. Who knows what other 'common sense' ideas will be blown out of the water from future scientific enquiry as we learn more about the sub-atomic world.
200-300 years ago the idea that the 'speed of light' would be the same for all observers as long as they are in a constant velocity frame work would appear to them as preposterous, because it defied their common sense. Nevertheless that turned out to be the case.
I just think that lots of modern science reporting is filled with flim·flam we shouldn't take at face value. Serious researchers and science experts use their hard earned scientific common-sense to blast wild ideas to the curb all the time.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,476
Speaking for myself and a lot of other people, I suspect a lot of folks would love to find a loophole in the FTL laws. There are a lot of theoretical work-arounds, maybe someday one will come pan out. There may yet be other laws we have not discovered yet
 

nsaspook

Joined Aug 27, 2009
13,553
Speaking for myself and a lot of other people, I suspect a lot of folks would love to find a loophole in the FTL laws. There are a lot of theoretical work-arounds, maybe someday one will come pan out. There may yet be other laws we have not discovered yet
I'd love it but I'd also love free apple pie for life. Every theoretical work-around involves non-physical elements like negative energy/mass. Time dilation, length contraction, and mass increase are direct consequences (each experimentally verified to high levels of accuracy) of a universal speed limit. A FTL loophole is a lot more than just a faster speed, we would need different rules for the universe in some very fundamental ways. Things like the Alcubierre metric use Einstein's equations but in a manner that's inconsistent with reality and causality.

https://en.wikipedia.org/wiki/Alcubierre_drive
Causality violation and semiclassical instability[edit]
Calculations by physicist Allen Everett show that warp bubbles could be used to create closed timelike curves in general relativity, meaning that the theory predicts that they could be used for backwards time travel.[41] While it is possible that the fundamental laws of physics might allow closed timelike curves, the chronology protection conjecture hypothesizes that in all cases where the classical theory of general relativity allows them, quantum effects would intervene to eliminate the possibility, making these spacetimes impossible to realize. A possible type of effect that would accomplish this is a buildup of vacuum fluctuations on the border of the region of spacetime where time travel would first become possible, causing the energy density to become high enough to destroy the system that would otherwise become a time machine. Some results in semiclassical gravity appear to support the conjecture, including a calculation dealing specifically with quantum effects in warp-drive spacetimes that suggested that warp bubbles would be semiclassically unstable,[4][42] but ultimately the conjecture can only be decided by a full theory of quantum gravity.[43]

Alcubierre briefly discusses some of these issues in a series of lecture slides posted online,[44] where he writes: "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)". In the next slide, he brings up the chronology protection conjecture and writes: "The conjecture has not been proven (it wouldn't be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. The conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole."
https://www.google.com/books/editio...ON2aCwAAQBAJ?hl=en&gbpv=1&printsec=frontcover

1652480856611.png
 
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nsaspook

Joined Aug 27, 2009
13,553
Quantum Tunnelling and the Alpha Particle Paradox

How QM allows us to calculate radioactive delay rates.

 
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nsaspook

Joined Aug 27, 2009
13,553
I just think that lots of modern science reporting is filled with flim·flam we shouldn't take at face value. Serious researchers and science experts use their hard earned scientific common-sense to blast wild ideas to the curb all the time.
Here is a classic example of a perfectly valid scientific paper being turned into flim·flam:
https://www.tomshardware.com/news/q...es-the-way-for-galaxy-spanning-communications
Qubit Teleportation Paves the Way for Galaxy-Spanning Communications
This development, in turn, could enable a veritable quantum internet, where information only needs to be updated in a single physical location - such as a specific ship manufacturer's headquarters - with instantaneous propagation across the network of entangled qubits. Wouldn't that be handy for your firmware update antics above the skies of Gliese 667CC?
The actual paper from nature makes so such claim because the statement of instantaneous propagation across the network of entangled qubits for Galaxy-Spanning superluminal Communications shows an obvious misunderstanding of the entanglement property of quantum physics and information.

https://arxiv.org/pdf/2101.04427.pdf
Quantum Internet- Applications, Functionalities, Enabling Technologies, Challenges, and Research Directions
Using entanglement for communication in technical applications is not clear, because entanglement itself does not allow information transmission. However, it can help to build a virtual secure channel or a cryptographic channel.
https://en.wikipedia.org/wiki/Quantum_teleportation
Quantum teleportation is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. The sender does not have to know the particular quantum state being transferred. Moreover, the location of the recipient can be unknown, but classical information needs to be sent from sender to receiver to complete the teleportation. Because classical information needs to be sent, teleportation can not occur faster than the speed of light.
 
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MrAl

Joined Jun 17, 2014
11,715
The article is classic BS marketing hype for a maybe product that hints (with not Science, Physics or Math explanation) it can do the impossible. It's no wonder they bailed on building and selling the first quantum network in the U.S.
A link from the article: https://www.forbes.com/sites/chador...ter-than-light-communication/?sh=7094760a3a1e

https://www.forbes.com/sites/starts...-speed-of-light-nothing-does/?sh=18eb9e887c3c

No, no , no. FTL communications would break causality in a thousand horrible ways. Causality is more elemental than time or the speed of light.
https://arxiv.org/pdf/quant-ph/0310131.pdf
Just saw this.

Well we have to keep in mind that the word "causality" is a man made word, and the universe does not take orders from man it's the other way around.
Could it all be hype? With the experiments i have seen i cant believe that. What i can believe is that quantum physics is not physics it should not be called that, it should be called Elemental Physics or something like that, or maybe even dont include the word "physics" at all because it gives the reader the impression that things like classical mechanics still work at such tiny scales.

Here is my interpretation...
The things i have read in the past say that if you have two particles and you entangle them and then take one off to another city and leave one behind at home, if you then decide to measure something like the 'spin' of the particle at home and it is 'clockwise' then the spin of the far off particle is 'counter clockwise'. That's faster than light speed, but there may be a catch. I dont think we can keep constantly measuring the far off particle waiting for it to change and then say that we got it immediately (or after some extremely fast time period which might be currently being thought about) because once we measure the far off particle 'spin' that causes the at home particle to fall permanently into the opposite spin. So although we can measure the particle at home at any time, how do we know the far off particle changed if we cant measure it. Now if someone calls the location of the far off particle and tells them they measured the at home particle, the fastest that can happen is at the speed of light. So where is the gain.

However, quantum physics is still a mysterious phenomenon so we cant yet rule anything out. If one far off particle can 'collapse' into one permanent state just because the other one is measured, then there may be a way to exploit this. I think there may be a way because if there is no way to exploit this they would have never been able to tell it was actually happening either instantaneously or at some extremely fast speed faster than light.

There are going to be a lot of technical issues to weed through out i think. These tiny things have very fragile states so almost anything can mess it up. That means it may take a lot of particles to get just one bit transmitted successfully.
 

nsaspook

Joined Aug 27, 2009
13,553
Just saw this.

Well we have to keep in mind that the word "causality" is a man made word, and the universe does not take orders from man it's the other way around.
Could it all be hype? With the experiments i have seen i cant believe that. What i can believe is that quantum physics is not physics it should not be called that, it should be called Elemental Physics or something like that, or maybe even dont include the word "physics" at all because it gives the reader the impression that things like classical mechanics still work at such tiny scales.

Here is my interpretation...
The things i have read in the past say that if you have two particles and you them and then take one off to another city and leave one behind at home, if you then decide to measure something like the 'spin' of the particle at home and it is 'clockwise' then the spin of the far off particle is 'counter clockwise'. That's faster than light speed, but there may be a catch. I dont think we can keep constantly measuring the far off particle waiting for it to change and then say that we got it immediately (or after some extremely fast time period which might be currently being thought about) because once we measure the far off particle 'spin' that causes the at home particle to fall permanently into the opposite spin. So although we can measure the particle at home at any time, how do we know the far off particle changed if we cant measure it. Now if someone calls the location of the far off particle and tells them they measured the at home particle, the fastest that can happen is at the speed of light. So where is the gain.

However, quantum physics is still a mysterious phenomenon so we cant yet rule anything out. If one far off particle can 'collapse' into one permanent state just because the other one is measured, then there may be a way to exploit this. I think there may be a way because if there is no way to exploit this they would have never been able to tell it was actually happening either instantaneously or at some extremely fast speed faster than light.

There are going to be a lot of technical issues to weed through out i think. These tiny things have very fragile states so almost anything can mess it up. That means it may take a lot of particles to get just one bit transmitted successfully.
It's not all hype. If you drop a gold ring into a toilet of crap that bowl of crap now has value. The problem is not science or QM, the problem is POP-SCI stories about it.

Correct, "causality" is a man made word for something that keeps the universe running. Without something that we call physics "causality" the universe as we know it would not exist. There is no exchange of causal information in Quantum entanglement at any speed. The information exchange always requires a classical channel.

Viktor T. Toth
, IT pro, part-time physicist
Answered Sep 9, 2019 · Upvoted by
Frederic Rachford
, PhD Physics, Case Western Reserve University (1975) and
Balu Sreedhar
, Integrated MSc Physics, Pondicherry University (2019)



No, quantum physics does not disprove causality.

On the contrary, our best working quantum theory to date, quantum field theory, quite properly respects causality both on the macroscopic and on the microscopic level. Acausal (faster-than-light, backwards-in-time) influences are explicitly and precisely canceled out in the theory. This, in fact, is one of the major motivations behind quantum field theory.
Some confusion arises because of misunderstandings surrounding the obvious nonlocality of the quantum theory, in particular the nonlocality of quantum entanglement. Therefore, it is important to reiterate that although entanglement may be counterintuitive, “spooky” even, it does not imply the transmission of information, the transmission of energy or momentum or other influences. It is simply a nonlocal manifestation of conservation laws, but no acausal signaling is involved.
 

MrAl

Joined Jun 17, 2014
11,715
It's not all hype. If you drop a gold ring into a toilet of crap that bowl of crap now has value. The problem is not science or QM, the problem is POP-SCI stories about it.

Correct, "causality" is a man made word for something that keeps the universe running. Without something that we call physics "causality" the universe as we know it would not exist. There is no exchange of causal information in Quantum entanglement at any speed. The information exchange always requires a classical channel.
Hello,

I guess this gets pretty deep. One of the persons quoted said "Acausal signaling" is not involved, but certainly something is happening that appears to be different than anything we have seen in the past just like with the two slit experiment.
I guess we will just have to wait to see what is really going on.
 

nsaspook

Joined Aug 27, 2009
13,553
Hello,

I guess this gets pretty deep. One of the persons quoted said "Acausal signaling" is not involved, but certainly something is happening that appears to be different than anything we have seen in the past just like with the two slit experiment.
I guess we will just have to wait to see what is really going on.
That's not totally true (appears to be different than anything we have seen in the past) as we have group velocities of EM waves than can be FTL but not carry information. I see quantum entanglement 'speeds' as being much the same thing (Group Velocities).


https://www.nature.com/articles/223597a0.pdf

Do Faster-than-Light Group Velocities imply Violation of Causality ?
DiscussroN of the properties of hypothetical systems 1- 6
in which group velocities exceed the speed of light has
considered chiefly whether such systems would violate
the axioms of special relativity. In particular they appear
at first sight to violate causality.
https://plus.maths.org/content/faster-light
Scientists at the NEC Research Institute in Princeton have carried out an experiment in which a pulse of light appeared to emerge from a cloud of gas before it even entered.

If our fundamental understanding of cause and effect is not to be contradicted, the word "appeared" is crucial. Indeed, it's generally thought that information can't travel faster than a fixed speed, the speed of light in vacuum. (We know from Einstein's laws that no particles can travel faster than this speed.) So what exactly is going on here?
 

MrAl

Joined Jun 17, 2014
11,715
That's not totally true (appears to be different than anything we have seen in the past) as we have group velocities of EM waves than can be FTL but not carry information. I see quantum entanglement 'speeds' as being much the same thing (Group Velocities).


https://www.nature.com/articles/223597a0.pdf

Do Faster-than-Light Group Velocities imply Violation of Causality ?


https://plus.maths.org/content/faster-light
Hi,

Whatever you want to call it.
But sometimes things come up that allow something to happen or at least be possible that we never thought would be possible.
For example, in classical physics FTL is not possible, but then we have worm holes and expansion of the universe that kind of side steps the rule.
 

cmartinez

Joined Jan 17, 2007
8,285
Neither worm holes (if they exist) nor the expansion of the universe violate the FTL restriction. This because it is spacetime itself the thing that moves, and not something traveling through it.
 

nsaspook

Joined Aug 27, 2009
13,553
You opened a can of worms! :D

Worm hole transversal (if possible) might violate Causality even within the FTL restriction (if your spacetime has closed timelike curves (CTCs)) for the traveler by becoming a time machine to the past.
https://theconversation.com/time-tr...-you-have-an-object-with-infinite-mass-107063
The general conclusion that has emerged from previous research, including Thorne’s and Hawking’s, is that nature forbids time loops. This is perhaps best explained in Hawking’s “Chronology Protection Conjecture,” which essentially says that nature doesn’t allow for changes to its past history, thus sparing us from the paradoxes that can emerge if time travel were possible.

Perhaps the most well-known amongst these paradoxes that emerge due to time travel into the past is the so-called “grandfather paradox” in which a traveler goes back into the past and murders his own grandfather. This alters the course of history in a way that a contradiction emerges: The traveler was never born and therefore cannot exist. There have been many movie and novel plots based on the paradoxes that result from time travel – perhaps some of the most popular ones being the “Back to the Future” movies and “Groundhog Day.”
The time-cops are there to prevent this.
https://www.polygon.com/22537327/loki-tva-time-travel-cops


https://authors.library.caltech.edu/6469/
Billiard balls in wormhole spacetimes with closed timelike curves: Classical theory
The effects of self-interaction in classical physics, in the presence of closed timelike curves, are probed by means of a simple model problem: The motion and self-collisions of a nonrelativistic, classical billiard ball in a space endowed with a wormhole that takes the ball backward in time. The central question asked is whether the Cauchy problem is well posed for this model problem, in the following sense: We define the multiplicity of an initial trajectory for the ball to be the number of self-consistent solutions of the ball’s equations of motion, which begin with that trajectory. For the Cauchy problem to be well posed, all initial trajectories must have multiplicity one. A simple analog of the science-fiction scenario of going back in time and killing oneself is an initial trajectory which is dangerous in this sense:


https://authors.library.caltech.edu/9262/1/MORprl88.pdf
Wormholes, Time Machines, and the Weak Energy Condition
ABSTRACT
It is argued that, if the laws of physics permit an advanced civilization to create and maintain a wormhole in space for interstellar travel, then that wormhole can be converted into a time machine with which causality might be violatable. Whether wormholes can be created and maintained entails deep, ill-understood issues about cosmic censorship, quantum gravity, and quantum field theory, including the question of whether field theory enforces an averaged version of the weak energy condition.
AUTHORS & AFFILIATIONS
Michael S. Morris, Kip S. Thorne, and Ulvi Yurtsever
  • Theoretical Astrophysics, California Institute of Technology, Pasadena, California 91125
 
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MrAl

Joined Jun 17, 2014
11,715
Neither worm holes (if they exist) nor the expansion of the universe violate the FTL restriction. This because it is spacetime itself the thing that moves, and not something traveling through it.
I was not trying to imply that expansion of the universe violated LOCAL faster than light travel. But give this some thought....

Relative to you an me, right at this instant in time, or the time you are reading this, there could be stars way out there, unseen to us, that could be traveling faster than the speed of light RELATIVE to you and me. That does not mean that the light they are emitting never gets emitted because they actually beat the light to the punch, because locally that should not happen.

Another thing to think about which may be a side topic though. If the multiverse is possible, then how fast is our universe moving relative to some other universe? How fast is that second universe moving relative to a third universe? Can we even say they are moving, i think so because they talk about collisions, and collisions cant happen without movement, as far as i know.

Thanks for the reply i think this gets us all either thinking or refreshed :)
 
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