## Quantum Entanglement

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danpilon54
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### Re: Quantum Entanglement

Technical Ben wrote:Still cannot comprehend why they just don't have opposite spins anyway. That's a whole lot simpler than all this Quantum entanglement. (some phrase about a razor springs to mind ) Why is the spin undetermined at 50/50 up or down. Why can it not be "we have made a particle with up" spin for example? The pair have a 50/50 of up/down or down/up instead of each particle having a 50/50 of up/down?

Because the states of the two particles before they are observed are a superposition of up and down spins. One cannot say which it is (as it is both and neither at the same time). Only when one particle is observed does its wavefunction collapse into one of the two states, up or down. Since angular momentum is conserved, if the system the two particles came from had no angular momentum, then the second particle will have opposite spin. As many have said, however, this cannot be used to transmit information. You cannot force the observed particle to be an up spin without first breaking the entanglement. Tricky question: If two people observed both particles at the exact same time (which we can talk about because the other particle is said to be determined instantaneously), then who forced the other particle to be the opposite spin?
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Technical Ben
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### Re: Quantum Entanglement

Sorry, your post does not add an further information. I'm not saying your wrong. I'm just saying you have repeated what I said I do not understand, or need more clarification of. Saying it is a wave function, and not already defined up/down does not explain why it is. I still see QM as a calculation of the event, not necessarily the actual event itself.
Why can it not be the measuring equipment that is tilted produces a slanted result (non linear as the results are non linear). If I follow through with device that produces "up/down" boxes in opposites, would this not give the same result? If those opposites are on reflective mirrors, that I shine a laser beam at an angle, and get a different reading (from it reflecting at different angles, would this not be the same?

Just explaining my picture of it, so it can be corrected/disputed or show how it really works.
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ThomasS
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### Re: Quantum Entanglement

Technical Ben wrote:Why can it not be the measuring equipment that is tilted produces a slanted result (non linear as the results are non linear). If I follow through with device that produces "up/down" boxes in opposites, would this not give the same result? If those opposites are on reflective mirrors, that I shine a laser beam at an angle, and get a different reading (from it reflecting at different angles, would this not be the same?

The Copenhagen interpretation of quantum mechanics states that each particle is a wave function, and any quantity which we might measure is a differential operator. When the measurement device is applied to the wave function, the wave function is forced (collapsed) to be an eigenfunction of the differential operator. Typically, the operators are such that the set of eigenfunctions form a countable basis of the function space. Given a measurement device, any wave-function can be seen as a linear combination of these eigenfunctions. The collapsing is such that the probability of collapsing into a particular state is proportional the magnitude of the state in the original combination. (The terms eigenfunctions and operator used by physicists here are mostly the same as a mathematician uses when studying functional analysis. They are similar to the terms eigenvectors and linear operator used in linear algebra. Let me know your math background if you'd like me to try to explain further.)

This leads to some oddness. Spin measurement, or linear polarization measurement, forces the particle into an angle dependent state. However, the possible states depend on the orientation of the measurement apparatus. So if you have a horizontal polarizing filter followed by a vertical polarizing filter, no light makes it through. But if you place a 45 degree polarizer in between, some light starts to make it through. This makes perfect sense if you see light as a wave, but a photon should only be absorbed by a polarizing filter (or not absorbed) so as you throw photon by photon at the assembly it is a bit "odd" that adding a filter increases the number which make it through. You can do similar experiments with spin traps, but it has been just long enough that the details are fuzzy to me.

Anyway, strictly speaking "entanglement" means that the eigenfunctions of multiple particles are connected and the collapse (measurement) of one triggers the collapse of the other. For example a device might emit two photons with identical (but uncertain) polarization, or if two electrons collide measurement of the momentum of one is a measurement of the momentum of the other. What seems interesting to me isn't that things can be entangled, but the mechanism by which they might be not entangled.

Ingolifs
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### Re: Quantum Entanglement

Quantum entanglement may not be able to send information at faster than light speed, but i'm wondering how effective it is at sending informationat light speed. More specifically, i'm wondering if you could send information between two places an arbitrary distance apart and not experience any loss of information. If so, QE would be a valueable form of communication between earth and any space probes sent out to explore stuff.
I belong to the tautologist's school of thought, that science is by definition, science.

Yakk
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### Re: Quantum Entanglement

A barrier was found to 'entanglement' like communication, above which you could reduce the amount of conventional bits down to a constant number to communicate an arbitrary amount of information.

It was found that QM entanglement doesn't cross that boarder, IIRC.

In fact, there is buffer room between the QM entanglement and the border above which information transfer is constant cost.

People are apparently looking for ways to justify why QM entanglement is bounded further away from such information transfer.

The above is something I know no more than at a the pop-sci level.
One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision - BR

Last edited by JHVH on Fri Oct 23, 4004 BCE 6:17 pm, edited 6 times in total.

doogly
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### Re: Quantum Entanglement

The Holevo bound! It's a really cool effect, I just heard about it two weeks ago. It's pretty neat. It takes 2^n numbers to encode an n-qubit, but you can only use it to send n numbers.
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Ingolifs
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### Re: Quantum Entanglement

Yakk wrote:A barrier was found to 'entanglement' like communication, above which you could reduce the amount of conventional bits down to a constant number to communicate an arbitrary amount of information.

It was found that QM entanglement doesn't cross that boarder, IIRC.

In fact, there is buffer room between the QM entanglement and the border above which information transfer is constant cost.

People are apparently looking for ways to justify why QM entanglement is bounded further away from such information transfer.

The above is something I know no more than at a the pop-sci level.

I belong to the tautologist's school of thought, that science is by definition, science.

Yakk
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### Re: Quantum Entanglement

So in order for QE to send bits, you need to augment it with light-speed transported bits.

You can paramatarize 'how related' two things are. If things are too related (past the The Holevo bound), you can 'prime the pump' between two people using that communication channel, then with a fixed and finite number of bits decode an arbitrary amount of information communicated via the 'prime the pump' phase.

This would be the "we use QE to send the message, then at light speed we unlock the message" hope. But QE is below the Holevo bound, so it doesn't work. You cannot send a huge message via QE, then decode it using an arbitraryly small light-speed 'real' communication.

This kind of thought experiment is used to find a 'justification' for the limits of QE "communication" abilities. The interesting thing is not just that QE is below the bound, but that there is a _further_ gap between QE and the bound. Maybe there is something weird in there that could happen if multiple QE particles where that related to each other in observed behaviour.

It is sort of like how fact that faster than light travel -> time travel -> causality is broken is interesting in the light of it taking an infinite amount of energy to speed matter up to the speed of light. Or the math showing that if you use wormholes to travel faster than light, you get a vacuum energy flux that destroys the wormhole faster than you could use it, etc. Or the hope that naked singularities are impossible due to some other properties of the universe.
One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision - BR

Last edited by JHVH on Fri Oct 23, 4004 BCE 6:17 pm, edited 6 times in total.

PM 2Ring
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### Re: Quantum Entanglement

Yakk wrote:It is sort of like how fact that faster than light travel -> time travel -> causality is broken is interesting in the light of it taking an infinite amount of energy to speed matter up to the speed of light. Or the math showing that if you use wormholes to travel faster than light, you get a vacuum energy flux that destroys the wormhole faster than you could use it, etc. Or the hope that naked singularities are impossible due to some other properties of the universe.

The universe doesn't like it when one tries to be a smartarse and exploit apparent loopholes like these. Fortunately, it's got a good sense of humour. Usually.

The way I comprehend an entangled system is that from the entangled component's POV, they are in some sense always local to one another, even though they may not seem to be local from an outside observer's POV. And that analyzing any isolated quantum system is only an approximation: the exact solution requires including the observer and the rest of the universe. Luckily, we can pretend that systems are isolated and we get (almost) the right answers when we do the calculations. But doing things this way opens the door to the quantum spookiness that perturbed Albert.

ThomasS
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### Re: Quantum Entanglement

PM 2Ring wrote: the exact solution requires including the observer and the rest of the universe.

This is (perhaps) easy to say. However, nothing in conventional quantum mechanics tells us how to identify an observer. Experimentally when you expose a small quantum system to a "macroscopic collection of particles" through electromagnetic or other interactions, the system collapses. So the cat is an observer, and its fate is sealed before the box is opened.

But what is a macroscopic collection of particles and why does the interaction change from entanglement to collapse of the wave function as you add particles? I mean, Copenhagen is pretty binary in pedagogy, either it is collapsed or it isn't. So what is the specific condition which determines if it occurs? Put one particle in a box and it adds to the entanglement, add a mole and it does the opposite and settles the system. There is a lot of uncertainty between the two extremes, no? How can you, even in principle, explain this in the context of Copenhagen?

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### Re: Quantum Entanglement

Decoherence has something to do with it. This is, for example, the usual way Schroedinger's Cat is resolved. Basically, a quantum system loses its quantum behavior when there a sufficiently large number of particles involved, as happens when the system in question interacts with a measurement apparatus.

You can avoid decoherence by having particles that interact extremely weakly, or are extremely cold, which is how Bose-Einstein condensates are achieved.

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ThomasS
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### Re: Quantum Entanglement

thoughtfully wrote:Decoherence has something to do with it. This is, for example, the usual way Schroedinger's Cat is resolved

Well yes, and it is interesting that they make a point of saying that it "approximates" wave function collapse. Also interesting is that it is attributed to Bohm and his hidden variable theory. Check out the "In interpretations of quantum mechanics" section of the wiki, near the end. Also note that they seem to tacitly imply that if you take measures to avoid this approximate collapse, you don't get a real collapse either.

I sort of suspect that people might be taking decoherence theory seriously because they need to to analyze quantum computers and to make predictions about when entanglement stops. Like science itself, they use it because it works. But, either the wave function collapse of spin correlation experiments is a "real" and fundamental wave function collapse, or it is not. If so, then we still don't know how to determine when a collapse occurs. If not, then the world is looking rather non-local in precisely the way that scares people who believe that causality and free will matter to electrons.

ThinkerEmeritus
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### Re: Quantum Entanglement

You know, I think that discussions of measurement theory would be a lot more productive if everyone agreed on some term other than "wave function collapse" for the information gained by a measurement. The term, even though it is almost always used, is misleading. Wave functions are governed by the Schroedinger equation and they don't suddenly change just because we know something. "State collapse" would be a much better term.

Certainly the measurement process can be described by the Schroedinger equation if we know enough. We usually don't, and we approximate the result by going ahead and doing something not specified in detail and saying "this time the spin was up," and the next time maybe "this time the spin was down." The process of determining that the spin was "down" or "up" in a given case may be complicated theoretically so that we can't tell what happened to change the wave function.

What might be useful to do is to choose a measurement process that is simple enough to handle mathematically, see what happens, and try to do our philosophy from there. I think that this is what decoherence theory is doing, but I don't know enough about it to be sure. A single spin would be a good case to examine because it is much simpler and we could turn the mathematics into words more easily. I'll work on that and see what I get. Well, to be fair I have started working on that and I'll see if it works. The physics of determining the spin is easy but the philosophical interpretation is not. We'll see.
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ThomasS
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### Re: Quantum Entanglement

ThinkerEmeritus wrote:You know, I think that discussions of measurement theory would be a lot more productive if everyone agreed on some term other than "wave function collapse" for the information gained by a measurement. The term, even though it is almost always used, is misleading. Wave functions are governed by the Schroedinger equation and they don't suddenly change just because we know something. "State collapse" would be a much better term.

Oh but they do. At least, you'll be hard pressed to find a well accepted theory or interpretation in which they do not collapse in the sense of a sudden discontinuous change in the wave function. To see what is odd in a continuous evolution theory, consider electrons passing through two slits and forming a diffraction pattern on a mesh of detectors at the far end. In the Copenhagen interpretation, the electron's wave function collapses and it ends up stuck in a ccd cell or bound to a phosphor or otherwise interacting with one particular detector. Now suppose we claim that quantum electrodynamic interactions between the electron and the detector somehow causes the wave function to continuously deform into the known end state. Poof, no wave function collapse, everything is fine, Schrodinger's equation to the rescue!

But when does this deformation start to occur? This continuous deformation has some speed no? In order for this electron to find and reshape itself to one specific detector, it must start changing before it reaches the detector. And if the detector is moving, it has to change according to how the detector mesh will be aligned before it gets there. The fact that it somehow shrinks down far enough to interact with and potentially push around a tightly bound particle is exactly why this is normally an example of wave particle duality.

Now I think Feynman-Wheeler advanced potentials are just dandy and probably have a place in quantum electrodynamics, but you might be amazed at how hard it is to convince people to accept the loss of microscopic causality that comes with them.
Certainly the measurement process can be described by the Schroedinger equation if we know enough. We usually don't, and we approximate the result by going ahead and doing something not specified in detail and saying "this time the spin was up," and the next time maybe "this time the spin was down." The process of determining that the spin was "down" or "up" in a given case may be complicated theoretically so that we can't tell what happened to change the wave function.

Certainly this statement that "the measurement process can be described by the Schroedinger equation if we know enough." is not well accepted by many quantum physicists. In fact, claiming such a thing has oft been considered to be a sign of crack-pottery. Furthermore, they bring us back to a point made above. Because of the "entanglement at a distance" inherent in spin correlation and other experiments any such theory would be non-local. As in, violates Bell's-inequality-and-means-that-photons-and-electrons-know-where-they-are-going-before-they-get-there-and-doesn't-that-make-mincemeat-of-free-will non-local.

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### Re: Quantum Entanglement

ThomasS wrote:photons-and-electrons-know-where-they-are-going-before-they-get-there

I may be wrong on this, but it's my understand that since photons travel at c, they experience no passage of time, and hence from the perspective of the photon there is no "before". That is, the photon experiences all points on its path as a single point (no distance) and registers no temporal advance. So, if I am understanding correctly, the photon definitely "knows" where it is "going" as soon as it comes into existence. Because, to itself, it "comes into existence" in one spot that is local to its "start point" and its "end point" and every point it "travels" in between, and it never "moves". All of what we see as "space" along its "path" is, to the photon, compressed to zero length. Of course, I could be wrong.
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### Re: Quantum Entanglement

Qaanol wrote:
ThomasS wrote:photons-and-electrons-know-where-they-are-going-before-they-get-there

I may be wrong on this, but it's my understand that since photons travel at c, they experience no passage of time, and hence from the perspective of the photon there is no "before". That is, the photon experiences all points on its path as a single point (no distance) and registers no temporal advance. So, if I am understanding correctly, the photon definitely "knows" where it is "going" as soon as it comes into existence. Because, to itself, it "comes into existence" in one spot that is local to its "start point" and its "end point" and every point it "travels" in between, and it never "moves". All of what we see as "space" along its "path" is, to the photon, compressed to zero length. Of course, I could be wrong.

No. You are not. That is basically correct.

Charlie!
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### Re: Quantum Entanglement

Tass wrote:
Qaanol wrote:
ThomasS wrote:photons-and-electrons-know-where-they-are-going-before-they-get-there

I may be wrong on this, but it's my understand that since photons travel at c, they experience no passage of time, and hence from the perspective of the photon there is no "before". That is, the photon experiences all points on its path as a single point (no distance) and registers no temporal advance. So, if I am understanding correctly, the photon definitely "knows" where it is "going" as soon as it comes into existence. Because, to itself, it "comes into existence" in one spot that is local to its "start point" and its "end point" and every point it "travels" in between, and it never "moves". All of what we see as "space" along its "path" is, to the photon, compressed to zero length. Of course, I could be wrong.

No. You are not. That is basically correct.

Well... talking about the photon's path being a 100% compressed line is okay if it was a simple particle, but once it's though of as a wavicle (wave+particle = wavicle) that's only partly doing any one thing, I don't know that the picture from the photon's point of view is as cool.
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ThomasS
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### Re: Quantum Entanglement

Charlie! wrote:
Tass wrote:No. You are not. That is basically correct.

Well... talking about the photon's path being a 100% compressed line is okay if it was a simple particle, but once it's though of as a wavicle (wave+particle = wavicle) that's only partly doing any one thing, I don't know that the picture from the photon's point of view is as cool.

If you really believe that the path is 100% compressed then the photon is a non-local phenomenon, Bell's inequality is irrelevant, and many many currently accepted books and papers on quantum mechanics are simply wrong. I believe that you might be able to avoid the need for "wavicles" because its awareness of its future (from our point of view) destination might explain things like the photoelectric effect. Let it be a wave, or rather an interaction between waves, and let the PDEs do their thing. Advanced potentials, here we come!

I think this interpretation is nice mathematically, and have mentioned it here before. It resolves certain quantum oddness in a manner compatible with relativity, and could open the door to a UFT. However there are a lot of respected quantum theorists who do not respect this viewpoint and their arguments against it tend to boil down to waving around Bell's inequality and an uncomfortableness with the resulting loss of causality.

Suppose you have an emitter and detector next to each other on earth, and you manage to bounce a photon off of a mirror mounted on the moon. Then from our point of view a photon is emitted at [imath]t_1[/imath] and received at [imath]t_2[/imath], several seconds later. If you accept this sense of instantaneous, there is no problem with the details of the evolution of the emitter wave function at [imath]t_1[/imath] being affected by the details of the detector wave function at [imath]t_2[/imath]. This tends to weird people out, and whether you continue to talk about how easy and obvious this interpretation is, I think it is important to recognize why people are bothered by it.

PM 2Ring
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### Re: Quantum Entanglement

Advanced potentials are nice mathematically, but weird philosophically. OTOH, instantaneous & spontaneous wave collapse is pretty weird, too. As I've mentioned before, I'm a bit of a fan of the Transactional Interpretation of QM. Under this interpretation, a photon cannot be emitted unless it will eventually be absorbed, which may seem a bit weird at first, but it is a very neat picture.

I alluded to it earlier in this thread, but maybe it bears repeating: there's no point in trying to imagine what the universe looks like from a photon's rest frame, since photons don't have a rest frame. Even comparing different photons is problematic: how can you meaningfully compare null vectors? (Ok, Minkowski spacetime is not a metric space, but lightlike paths are still vectors of zero length.)

Technical Ben
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### Re: Quantum Entanglement

PM 2Ring wrote: As I've mentioned before, I'm a bit of a fan of the Transactional Interpretation of QM. Under this interpretation, a photon cannot be emitted unless it will eventually be absorbed, which may seem a bit weird at first, but it is a very neat picture.

Wow. I was thinking of this just yesterday. Wondering if a photon can only be emitted if it has a "contact" point. Similar to how you only get a flow of electricity through a wire if it has a circuit. However, this would need some connecting factor? And it still messes my mind up when relativity and supposed* breaks in causality.

*as I do not understand the concepts of QM and the rules it breaks yet.
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ThomasS
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### Re: Quantum Entanglement

Technical Ben wrote:*as I do not understand the concepts of QM and the rules it breaks yet.

Feynman wrote:I think I can safely say that nobody understands quantum mechanics.

There is definitely a lack of causality at the microscopic scale if the photon is only emitted when it has an absorber. Consider my experiment involving bouncing a photon of the mirror on the moon, or have the detector and emitter be at opposite ends of a mirrored tube/wave guide (bent into a loop if you want the emitter and detector to be right next to each other)

Now quantum mechanics more or less assumes that wave function collapse is instant. There is a standard long bit of arguing to show that wave function collapse can't actually be used to transfer information. Now whether this is an improvement over the definite lack described above is as philosophical a question as whether free will is "real". Also, figuring out what instant means when you add in relatively is one of the things which makes UFTs hard.

Charlie!
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### Re: Quantum Entanglement

ThomasS wrote:
Charlie! wrote:
Tass wrote:No. You are not. That is basically correct.

Well... talking about the photon's path being a 100% compressed line is okay if it was a simple particle, but once it's though of as a wavicle (wave+particle = wavicle) that's only partly doing any one thing, I don't know that the picture from the photon's point of view is as cool.

If you really believe that the path is 100% compressed then the photon is a non-local phenomenon, Bell's inequality is irrelevant, and many many currently accepted books and papers on quantum mechanics are simply wrong. I believe that you might be able to avoid the need for "wavicles" because its awareness of its future (from our point of view) destination might explain things like the photoelectric effect. Let it be a wave, or rather an interaction between waves, and let the PDEs do their thing. Advanced potentials, here we come!

I think this interpretation is nice mathematically, and have mentioned it here before. It resolves certain quantum oddness in a manner compatible with relativity, and could open the door to a UFT. However there are a lot of respected quantum theorists who do not respect this viewpoint and their arguments against it tend to boil down to waving around Bell's inequality and an uncomfortableness with the resulting loss of causality.

Suppose you have an emitter and detector next to each other on earth, and you manage to bounce a photon off of a mirror mounted on the moon. Then from our point of view a photon is emitted at [imath]t_1[/imath] and received at [imath]t_2[/imath], several seconds later. If you accept this sense of instantaneous, there is no problem with the details of the evolution of the emitter wave function at [imath]t_1[/imath] being affected by the details of the detector wave function at [imath]t_2[/imath]. This tends to weird people out, and whether you continue to talk about how easy and obvious this interpretation is, I think it is important to recognize why people are bothered by it.

Hm. Either it's a particle and the "it's a point" thing helps justify thinking of it non-locally (and you can't explain its wavelike properties), or it's a wavicle and that justification is a significantly larger stretch (since you can't define a reference frame that follow a wave, even according to one observer).

I'm not sure that this would solve more problems than it creates, either. "Why does the universe appear to be causal?" maybe.

And lastly... Okay, so how would you test it?
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Technical Ben
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### Re: Quantum Entanglement

ThomasS wrote:Now quantum mechanics more or less assumes that wave function collapse is instant. There is a standard long bit of arguing to show that wave function collapse can't actually be used to transfer information. Now whether this is an improvement over the definite lack described above is as philosophical a question as whether free will is "real". Also, figuring out what instant means when you add in relatively is one of the things which makes UFTs hard.

I am just making mental pictures of the systems/processes being described. So "instant" or "emitting only when there is an absorber" were only ways of me thinking about how it works. In your example, the mirror is the absorber no?
And I agree with you on "instant". But as we don't have a full picture on QM yet, we shouldn't jump to the conclusion that the universe is 100% predetermined or 100% chaotic (or free will does/does not exist) just on our current theories.
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ThomasS
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### Re: Quantum Entanglement

Technical Ben wrote:I am just making mental pictures of the systems/processes being described. So "instant" or "emitting only when there is an absorber" were only ways of me thinking about how it works. In your example, the mirror is the absorber no?

Perfect mirrors don't absorb, or at least, to the extent that they do, they re-emit immediately. Does anybody know what they make the walls of quantum tunneling experiments out of? I always assumed that they use something reflective because otherwise you'd have the walls absorbing photons and making trouble. If you prefer to think about using a waveguide instead of a long route with a mirror, that is fine, but ofc a waveguide is more or less just a tube with mirrored walls.

If you put an excited atom in a sufficiently well mirrored small chamber it won't really emit because there is no place for the photon to go. You can argue about standing waves and virtual photons leaving returning, but at the ends of the day the atom is stuck in its state. What happens when the chamber is larger/long is a more interesting question.

Technical Ben
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### Re: Quantum Entanglement

Then is it not from the photons point of view a straight journey? And just "space" that has been bent? (sorry to mix things up and be argumentative )
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.