Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Harri on 31/10/2021 16:37:06
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As a non scientist I get all my information from popular science books and websites. And as such I take these as good places to get factual scientific information. I often read that particles can be in two separate locations at once. Of course particles in the plural suggests more than one particle so the fact that they can be in two separate locations at once isn't too surprising. It is when I read that 'a particle' can be in two separate locations at once I wonder if the statement is true and if it is at all helpful to describe it as such when discussing superposition? My initial reaction is to be in awe of such a possibility. But then if I think about it a bit more I ask would this be possible in spacetime? How could one particle occupy two places in spacetime?
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I often read that particles can be in two separate locations at once.
This has certainly never been demonstrated, and while not denying it, I am unaware of a quantum interpretation which goes to far as to assert this.
So it sounds to me like poorly worded pop literature. I've no doubt that such statements are out there.
I might say that it is incoherent to talk about a particle being at a location (let alone more than one) at all. A particle is measured at a location, or it isn't. That's all we know for sure.
It is when I read that 'a particle' can be in two separate locations at once I wonder if the statement is true and if it is at all helpful to describe it as such when discussing superposition?
A cat being in superposition of dead and alive is not the same as saying it is both dead and alive. I think that's the disconnect not spelled out well in statements that word it otherwise.
How could one particle occupy two places in spacetime?
Spacetime is a different story. My hand is down. My hand is raised. Both these states are in spacetime, but in only one state at a particular time, where time is a chosen cross section of spacetime. That's the same as a highway 80 being in Chicago and New York at the same time. It's not a contradiction, it's just in those places at different locations along its length. A particle is similarly not a point (an event) in spacetime, but rather a worldline in spacetime. Being a line, it doesn't exist all in one place. This has nothing to do with superposition. It's a classic concept.
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I often read that particles can be in two separate locations at once.
This has certainly never been demonstrated, and while not denying it, I am unaware of a quantum interpretation which goes to far as to assert this.
I’m not aware of one either. I think this is a misinterpretation of the solution to some wave equations where there is equal probability of 2 or even 4 solutions; that doesn’t mean those solutions exist simultaneously.
It is when I read that 'a particle' can be in two separate locations at once I wonder if the statement is true and if it is at all helpful to describe it as such when discussing superposition?
A cat being in superposition of dead and alive is not the same as saying it is both dead and alive. I think that's the disconnect not spelled out well in statements that word it otherwise.
Agreed, it’s a common misunderstanding of the Schrödinger’s thought experiment.
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It is when I read that 'a particle' can be in two separate locations at once
Perhaps you are referring to the 2 slit experiments, where an electron passes through both slits? This is not a particle being in 2 places at on time, this is a demonstration of the wave nature of an electron. An electron is not a particle in the classical sense, so the 2 slit experiment does not say a particle is in 2 places at the same time.
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A particle, by definition , can only be in one place at any instant. Problem is that the more accurately you know where it is, the less accurately you know how fast it is travelling or where it will be next. That's simple continuum physics leading to Heisenberg's indeterminacy principle.
Not to be confused with the Schrodinger interpretation which states that eg for an electron, all we know is the probability density distribution of finding it anywhere.
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A particle, by definition , can only be in one place at any instant. Problem is that the more accurately you know where it is, the less accurately you know how fast it is travelling or where it will be next. That's simple continuum physics leading to Heisenberg's indeterminacy principle.
Is that an "existential **" situation(I mean ,is that just the way things behave) ?
...or is it a question of a limit of the powers of perception?
I am going to "guess" it is the former and not the latter as I have unconsciously assumed until now.
** bad choice of word,but I want to contrast the ways things actually work with the way we perceive them to do.
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A particle, by definition , can only be in one place at any instant. Problem is that the more accurately you know where it is, the less accurately you know how fast it is travelling or where it will be next. That's simple continuum physics leading to Heisenberg's indeterminacy principle.
I want to contrast the ways things actually work with the way we perceive them to do.
The difference between science and philosophy is exactly this distinction. Science deals with what is measured, 'how we perceive them' so to speak. How things actually work, or 'what actually is' is metaphysics, which is philosophy: open to interpretation but impossible to know.
So a scientific statement would say that a particle can be measured at only one location at a given time. To assert that it can only be at one location at a time, or that it is even meaningful to discuss the location of a particle unmeasured, would be a metaphysical interpretation of the observations. I can for instance find at least two quantum interpretations that assign meaning to a particle having position despite lack of measurement, and the rest (dozen?) not. Those two interpretations (and not the others) would say that the particle definitely has some factual location and momentum, and Heisenberg's uncertainty is just an epistemological limit, as alan has worded it.
Not to be confused with the Schrodinger interpretation
There's such a thing as a 'Schrodinger interpretation'?
which states that eg for an electron, all we know is the probability density distribution of finding it anywhere.
That sounds like quantum theory, a scientific thing, not some metaphysical interpretation. It references what we can expect to measure instead of describing what is. That makes it a theory, not an interpretation.
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The simple derivation of Heisenberg's indeterminacy principle is that if it were possible for the position and momentum of an electron to be defined simultaneously, the most likely place to find any electron would be glued to a proton, so all atoms would have diameters about 105 times smaller than they do. Hence the Bohr model (orbiting electrons) and deBroglie wavelengths, which consist with Δp.Δx = h. Note that this is an inherent indeterminacy, nothing to do with the uncertainty of a measurement - a persistent mistranslation that has baffled many a journalist.
Problem then arises that an orbiting charged particle loses energy in the form of electromagnetic radiation, but this is not observed, hence Schrodinger's fuzzy "probability cloud", which is consistent with observation. I probably used "interpretation" loosely - apologies.
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If I look down onto a football field and see a player, I can tell where his position is on the field and from his momentum I can see which direction he is going. If I look at the position of a particle in a 'field' why is it not possible to see which direction it is going due to its momentum?
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If I look down onto a football field and see a player, I can tell where his position is on the field and from his momentum I can see which direction he is going.
Yes, a player is a classical thing and such things have position and momentum that can be simultaneously measured to considerable accuracy.
The classical properties start to drop off as you go smaller and smaller, to the point where intuitive things (like having an objective state, unmeasured, having a mass-density, etc.) vanish.
If I look at the position of a particle in a 'field' why is it not possible to see which direction it is going due to its momentum?
It's more like you know its momentum because you've measured its movement. And no, this cannot be done, because to measure its position accurately, you need to interact with it with a significant energy. It might as well have been kicked by a mule; it goes skittering off somewhere, momentum completely unmeasured.
The trick is to compromise. A given proton in a cold diamond lattice is practically a classic thing with its position confined to the small wiggle-room of the elements of the crystal, and its average velocity is the same of that of the diamond. Not sure what the mean deviation from that velocity would for a proton in a carbon nucleus were you to measure it. It's not like anybody has actually measured the atom, let along a given proton. Only the diamond, so all measurements are indirect.
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I've never liked the "bouncing photon measurement" approach because it tends towards "uncertainty" rather than "indeterminacy".
The simple fact is that to determine the player's momentum, which is a vector, he has to move so you know the direction of the vector. But now he has moved, he isn't where he was when he had that momentum, so you can't know both to an infinitesimal precision at any instant. Heisenberg proposed that the limit had a single value, and that turned out to be consistent with all our observations.
As with relativity, the derivation given by the inventor is a lot simpler than most of the "explanations" in the textbooks!
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If I look down onto a football field and see a player, I can tell where his position is on the field and from his momentum I can see which direction he is going. If I look at the position of a particle in a 'field' why is it not possible to see which direction it is going due to its momentum?
Different example to add to what @Halc is saying:
Let’s say your friend is coming to visit you, gives a call as he’s leaving. You know it takes about half an hour, so after 15min he should be half way, but you don’t know. Your model has to include some randomness because of traffic, roadworks etc.
It’s similar with quantum objects, we can model them and we can predict where they probably are, but unless we measure we can’t be sure, and because we can’t see them directly there are problems with the measurements. As @Halc says a measurement can disturb the position of the particle unless we can confine it. There are ways of detecting the electric field of atoms so that researchers can build up some pretty amazing images of the structures and atomic bonds and take measurements from these, however the boundaries of the atoms are fuzzy due to the nature of the electron field.
(Some overlap with @alancalverd I see).
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If I look down onto a football field and see a player, I can tell where his position is on the field and from his momentum I can see which direction he is going. If I look at the position of a particle in a 'field' why is it not possible to see which direction it is going due to its momentum?
Different example to add to what @Halc is saying:
Let’s say your friend is coming to visit you, gives a call as he’s leaving. You know it takes about half an hour, so after 15min he should be half way, but you don’t know. Your model has to include some randomness because of traffic, roadworks etc.
It’s similar with quantum objects, we can model them and we can predict where they probably are, but unless we measure we can’t be sure, and because we can’t see them directly there are problems with the measurements. As @Halc says a measurement can disturb the position of the particle unless we can confine it. There are ways of detecting the electric field of atoms so that researchers can build up some pretty amazing images of the structures and atomic bonds and take measurements from these, however the boundaries of the atoms are fuzzy due to the nature of the electron field.
(Some overlap with @alancalverd I see).
You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the hip
Suppose there was a new particle discovered that was millions of times smaller than the electron and it was able to be manipulated would this allow us the disentangle the electron's momentum from its position or are these two properties simply two sides of the same coin?
Is there forever and intrinsically a limit to how close an approximation there can be to a separation of these two descriptions?
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It's open for interpretations, superposition. A entanglement can be seen as a superposition too and so can a two slit experiment. It also relates to whether you take HUP seriously or just assume that it is a result of us not 'knowing ' all parameters beforehand. It opens for a lot of interpretations. Myself I take HUP seriously and when it comes to particles it's about their 'wave nature'. If the same particles can exist in 'two places spatially'? I don't know, you have to measure/probe them directly to do that and once you've done it they no longer will be 'entangled', nor will they be 'super positioned'.
https://scitechdaily.com/quantum-superposition-record-2000-atoms-in-two-places-at-once/
the more interesting question is if you get 'work' out of it before measured, somehow? I don't think that's possible but I don't know.
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You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the hip
Quite the opposite. If you know the position of a particle to infinitesimal precision, you have no information as to its momentum. People look different when they are running compared with standing still, but if you photograph a car with a very short flash, you can't tell whether it is moving forwards, backwards, or stationary Cameras have advanced to the point that you can now get "propellor disc blur" software so that photos of classic aircraft in flight look different from stationary models, but a true "snapshot" gives you no clue as to its speed.
So far, so intuitive. But intuition breaks down if you know that an electron is absolutely stationary, Heisenberg says in that case, you can have no idea where it is!
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You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the hip
Quite the opposite. If you know the position of a particle to infinitesimal precision, you have no information as to its momentum. People look different when they are running compared with standing still, but if you photograph a car with a very short flash, you can't tell whether it is moving forwards, backwards, or stationary Cameras have advanced to the point that you can now get "propellor disc blur" software so that photos of classic aircraft in flight look different from stationary models, but a true "snapshot" gives you no clue as to its speed.
So far, so intuitive. But intuition breaks down if you know that an electron is absolutely stationary, Heisenberg says in that case, you can have no idea where it is!
Oh,it kind of "blinks" out of the realm of information when it is stationary wrt the observer?
Or does it "fadeout" rather the more relatively stationary it becomes?
And I suppose that applies to any object ,and not especially to an electron....
There is no principle involved , is there such that at the quantum level it is impossible for any two objects to be completely at rest wrt each other?
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You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the hip
No, I’m not saying that, as Alan has explained very clearly.
What I’m saying is that the analogy @Harri was giving is inappropriate. There are two factors at work here, one is our state of knowledge of a particle because we can’t see it in the same way as looking at a player on a field. We see the player on the field because photons bounce off him and hit our eyes, those photons don’t move him or alter his momentum significantly, but they do if the player is an electron. One way we can detect an electron is to have it hit a detector eg phosphor screen, but then it’s stopped moving. Even if we use a speed gun on the player s/he has to move in order to get a doppler reading. These are the physical and practical problems.
The second problem is answered by Alan.
Reread Alan’s reply https://www.thenakedscientists.com/forum/index.php?topic=83459.msg659621#msg659621
There is an intrinsic limit to how accurately we can know these 2 properties at the same time and is quite different from the measurement problems already described. This is a limit set by the way the universe works. Reread both posts by Alan as he has put down very clearly what is an endless source of confusion to most non physicists, and unfortunately a few physicists!
For a large object like a ball kicked by a player the difference is so small that it is irrelevant around 10-30m.
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A cat being in superposition of dead and alive is not the same as saying it is both dead and alive. I think that's the disconnect not spelled out well in statements that word it otherwise.
But that is what it means to me Halc. But the cat is singular, in the same place
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A cat being in superposition of dead and alive is not the same as saying it is both dead and alive. I think that's the disconnect not spelled out well in statements that word it otherwise.
But that is what it means to me Halc. But the cat is singular, in the same place
Why does it mean that to you? It didn’t to Schrödinger who intended it as an example of the absurdity of such an interpretation. The problem is that many people have misunderstood what he was trying to say.
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A cat being in superposition of dead and alive is not the same as saying it is both dead and alive. I think that's the disconnect not spelled out well in statements that word it otherwise.
But that is what it means to me Halc. But the cat is singular, in the same place
Why does it mean that to you? It didn’t to Schrödinger who intended it as an example of the absurdity of such an interpretation. The problem is that many people have misunderstood what he was trying to say.
Because our actual actions of seeking definition render either answer if I am right in my understanding of it. It can be both alive and dead, 0 and 1.
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It didn’t to Schrödinger who intended it as an example of the absurdity of such an interpretation. The problem is that many people have misunderstood what he was trying to say.
But it only seemed absurd back then in the early days of QM when they were still hoping for a classic interpretation of things. In principle, there's nothing contradictory about the cat being in such a superposition. In practice, there's no conceivable way to put it in a box and actually not measure it, except if the box was so large that it took significant time for the measurement to be taken. The have done this to macroscopic objects (large enough to see unaided), but the conditions utilized to 'put it in a box' would have killed any cat.
Because our actual actions of seeking definition render either answer if I am right in my understanding of it. It can be both alive and dead, 0 and 1.
This seems to be a classic assumption, and while intuitive, it's wrong. It is in superposition of 0 and 1 until measured, at which point it collapses to one state or the other, as described by an interpretation with collapse. Other interpretations deny collapse, in which case the observer simply becomes entangled with the cat.
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This seems to be a classic assumption, and while intuitive, it's wrong. It is in superposition of 0 and 1 until measured, at which point it collapses to one state or the other, as described by an interpretation with collapse. Other interpretations deny collapse, in which case the observer simply becomes entangled with the cat.
OK so this presumably applies to any quantum object,?
For the purposes of us making any determination about it we have to take every probability regarding its properties and take those probabilities(why are there more than one I don't know) and fix them to actually represent the object.
When that object interacts with its environment the probabilities are rearranged instantly (?) and there is s new object based on a new set (again why so many superpositioned probabilities?) of probabilities
Is that anywhere close?
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OK so this presumably applies to any quantum object,?
All objects are quantum objects. It's just really hard not to interact with any nearby thing with any significant size.
For the purposes of us making any determination about it we have to take every probability regarding its properties and take those probabilities(why are there more than one I don't know) and fix them to actually represent the object.
If I read that correctly, it sounds like every object has a wave function via which one can in principle compute probabilities of measurements not yet taken. If that's what you mean, I agree.
When that object interacts with its environment the probabilities are rearranged instantly (?) and there is s new object based on a new set (again why so many superpositioned probabilities?) of probabilities
That sounds like collapse. Yes, that's a way of describing collapse, for an interpretation that supports collapse.
Is that anywhere close?
I didn't disagree with any of it.
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It is in superposition of 0 and 1 until measured, at which point it collapses to one state or the other,
Which clearly cannot be true as no molecule inside the cat knows that it is not being observed.
Usual problem of confusing a mathematical model with reality. The model gives us an idea of what we are most likely to observe - it is a probability function - but doesn't explain the mechanism.
An accurate model of roulette, superposing your chosen number and the 37 other numbers and predicting the outcome of the probability function collapse, tells you that you will almost certainly lose, but the ball and the wheel are visibly solid throughout the losing process (one could hardly call it a "game"). It's more interesting than Schrodinger's cat because there are three outcomes: you win everyone else's money (very unlikely) , somebody else wins yours (very likely), the house wins everyone's (twice as likely as you winning).
Hence many predictive "Monte Carlo" algorithms, leading to controlled nuclear fission and a whole bunch of other stuff.
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It didn’t to Schrödinger who intended it as an example of the absurdity of such an interpretation. The problem is that many people have misunderstood what he was trying to say.
But it only seemed absurd back then in the early days of QM when they were still hoping for a classic interpretation of things. In principle, there's nothing contradictory about the cat being in such a superposition.
I agree. I have no problem with the cat being described as in a superposition of states, just with the cat being dead and alive at the same time
For the purposes of us making any determination about it we have to take every probability regarding its properties and take those probabilities(why are there more than one I don't know) and fix them to actually represent the object.
It’s the nature of probabilities, by which I’m assuming you mean the probability of various outcomes. For a coin toss there are 2 outcomes, for the top card of a deck 52, so we can calculate the probability of these outcomes. The probability given by the wave equation of say the hydrogen atom gives us the probability of finding the electron at different positions around the nucleus - Alan’s fuzzy electron cloud. This is quite similar to the example of a friend driving to your home, where we can create a probability distribution of the probability of finding your friend at a certain distance from your home.
As Alan says we can use probabilities to model many practical situations including supermarket checkout queues, telling us how many staff and checkouts are needed.
When that object interacts with its environment the probabilities are rearranged instantly (?) and there is s new object based on a new set (again why so many superpositioned probabilities?) of probabilities
Let’s take a practical example of an electron inside a tunnel diode. We have a wave function describing the electron from which we derive the probabilities that the electron will be at certain distances from the barrier. Inside, highest probability, outside lowest - you should be able to find the actual distribution online. If the electron crosses the barrier its probability of being outside is 1 and the probability of it being inside is 0, we say the wave function has collapsed to a definite value. The electron is not a new object, but we do need a new wave function to describe the probability that it will cross back over the barrier or travel on its way through the copper lattice. How you describe this process depends, you could say wave function collapse + new wave function or wave function update. A lot of us are really considering updates as a good way of describing what happens and you can see this is the state changes eg in the hydrogen atom, when the atom absorbs a photon and the wave function of the electron changes giving a different probability distribution for position.
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The measurement problem in quantum physics is a can of snakes.
https://physics.mq.edu.au/~jcresser/Phys301/Chapters/Chapter13.pdf
and https://plato.stanford.edu/entries/qt-issues/
spelling, again
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There are three different parameters here. One is the measurement, another is the observer (observer effect) and the third is HUP.
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Take two atoms 'colliding/bumping' without any measurement. Would that be different from us making them bump while probing? You can't do this one, the first is more or less impossible to test, other than very indirectly. You can use statistics for it, experiences of how it usually falls out of course but for a singular case?
It depends on how strict you want to be possibly? But HUP do set a, so far indisputable, limit to what is knowable.
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https://www.thoughtco.com/what-is-schrodingers-cat-2699362 is about the observer effect. At least if viewed from the Copenhagen interpretation.
" Still, in some strict views of the Copenhagen interpretation, it is actually an observation by a conscious entity which is required. This strict form of the interpretation is generally the minority view among physicists today, although there remains some intriguing argument that the collapse of the quantum wavefunctions may be linked to consciousness "
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And that one leads me to Bells theorem
http://scholarpedia.org/article/Bell%27s_theorem
which leads me to decoherence as an explanation. Your reason for existing, decoherence. And here's one try to explain it :)
https://www.theatlantic.com/science/archive/2018/10/beyond-weird-decoherence-quantum-weirdness-schrodingers-cat/573448/
which all is good and clean, Understandable in some wavy way. But then we have HUP :)
Don't read me wrong here. I totally agree with this.
" We don’t need a conscious mind to “look” in order to “collapse the wave function.” All we need is for the environment to disperse the quantum coherence. We obtain classical uniqueness from quantum multiplicity when decoherence has taken its toll. "
It's what I say too. It must be that way. Eh, the first part I mean " We don’t need a conscious mind to “look” in order to “collapse the wave function.” Whether the rest of it is the most correct explanation I don't know.
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You could look at it this way. Assume that the 'strict' Copenhagen interpretation is correct. You need to look before the moon will exist. What that state though is that the moon must 'know' where to 'exist' each time you search for it. It would either make you the whole universe, or make that moon 'sentient' in some mean.
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You could look at it this way. Assume that the 'strict' Copenhagen interpretation is correct. You need to look before the moon will exist. What that state though is that the moon must 'know' where to 'exist' each time you search for it. It would either make you the whole universe, or make that moon 'sentient' in some mean
That would mean the whole universe's existence depended on a particular sentient observer's perception. (solipsism?)
That is a reductio ad absurdissimum.
It is the other way round,of course.
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You could look at it this way. Assume that the 'strict' Copenhagen interpretation is correct. You need to look before the moon will exist. What that state though is that the moon must 'know' where to 'exist' each time you search for it. It would either make you the whole universe, or make that moon 'sentient' in some mean
You seem to describe the Wigner interpretation. Copenhagen (as a metaphysical interpretation) does not put any special significance on humans or consciousness.
That would mean the whole universe's existence depended on a particular sentient observer's perception. (solipsism?)
Indeed, even Wigner himself abandoned the interpretation bearing his name because of exactly this reason.
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I'd just like to say thanks to everyone who's taken the time to contribute to this post. It's given me a really interesting insight into the subject.
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Ahh geordief, but it do open for some very interesting philosophy
Just imagine. Where ever you are you will be the center of the universe, and so will I.
It holds astronomically too :)
No matter where you go you will see the same universe around you, as defined large scale. and the same physics, laws and properties. Those are fundamentals for most mainstream physics today that I've seen.
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actually, and this is even more peculiar, it also holds for the definition of locality in relativity. At least the way I interpret it, because everything is 'observer dependent' there making you, and your definition of the universe, distances and time, a complementary unique experience. and if that doesn't blow your mind you're tougher than me.
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Halc, you might find this interesting :) in a somewhat rambling way.
https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics_(Walet)/13%3A_Miscellaneous_Quantum_Mechanics_Topics/13.03%3A_Complementarity_and_Copenhagen_Interpretation
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Halc, you might find this interesting :) in a somewhat rambling way.
I don’t think this is telling @Halc anything he doesn’t know.
However, it does remind me that I owe you a reply. Been busy.
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It all depends on how you read it Collin. What Bohr called 'complementary'. The most easily accepted version is the one in where every interaction can be seen as a 'observation', the process 'observing itself'. But as far as I remember the other school exist too, the one that connects it to consciousness. As mentioned in https://www.thoughtco.com/what-is-schrodingers-cat-2699362
I normally prefer the one where every 'interaction observe itself', but I'm not totally sure of it.
This one argues for it. https://www3.nd.edu/~dhoward1/Copenhagen%20Myth%20A.pdf
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I don't know? You might be able to connect it to decoherence stating that it is a question of the scale of what you observe, or of it 'interacting'', thinking of 'inanimate objects' interacting (not needing any 'conscious observation')
thinking some more of it. Assume you're at some relative motion, as defined from your origin. According to relativity your distance in the direction of motion shrinks. You have presumably 'zero point energy' existing intrinsic to the vacuum consisting of that shrunk distance, or we may call it 'waves'. You now go out to measure the energy density of the vacuum in front of you. Would you expect it to differ from some other observer doing the same while not at your relative motion? Does it matter what relative motion you define to yourself? It's just a thought experiment, although you might be able to use the Casimir effect to measure it by? https://en.wikipedia.org/wiki/Casimir_effect