Concerning Dark Matters

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Séadna

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Interpretations of QM:

So Complementarity blocks attempts to move beyond direct experimental or sense data. It does this by banning an extremely common aspect of human thought, using logic or reason to compose data gathered in different situations into a single mental model. It even prevents us from imagining the data we obtain in one experiment still exists but is unobserved when we make a different observation. As above if you chose to see colour from light it's invalid to even think the light has photons, since that's not how you chose to observe it.

Obviously this is a massive shift in the scope of physics which toward the end of the 19th century was aiming toward a complete fundamental description of reality. Now physics is a description of experiments or observations that can actually be performed, what to expect from them and nothing more. A particle is a type of click in our devices, not a "thing" which objectively exists. To quote Aage Bohr (son of Niels Bohr):
Thus the clicks can be classified as electron clicks, neutron clicks, etc., although there are no electrons and neutrons
The reality of atoms and the content given to this notion have remained issues ever since the concept of an atom was introduced. This question obtains a new content by the identification of reality with the world of experience and the recognition that, accordingly, there is no world of atoms underlying the macroscopic world.
Vladimir Fock, one of the great Soviet physicists, on the common error or erroneous abstraction we tend to make that QM bans:
Neglect of these considerations leads to an abstraction that we may call the absolutization of physical processes. If we accept this abstraction, it becomes possible to consider physical processes as occurring by themselves regardless of whether there is a real possibility of their observation....All of classical physics is based on the absolutization of physical processes.

This is the understanding of quantum theory arrived at by the original inventors of the theory: Bohr, Heisenberg, Dirac, Pauli, Schrodinger, Born, Jordan, etc and is the one given in textbooks today. I'll give it its standard name: The Copenhagen Interpretation. However today we sometimes use the phrase "Participatory Realism" for reasons I'll explain in the next post.
The above post about Complementarity was quite long, but I wanted to make it because online essays and many popular books give a completely bizarre rendering of the Copenhagen Interpretation, taking about "waves" and "collapses" and stuff that has no real relation to it.

In the spoiler below I explain what the other Interpretations are and why I'm not going to use them to answer your question. It can be skipped if you're fine with ignoring them. I will stick to the Copenhagen Interpretation and explore the open questions within it today.

Unsurprisingly not everybody likes the Copenhagen Interpretation. The original generation considered these conclusions obvious from the experimental data and Heisenberg discovered quantum theory by removing from the mathematics of classical mechanics the assumption of some objective description. However starting in the 1950s the new generation had physicists who wanted to find an old fashioned "objective" theory of physics that described what things are actually like, not just observations and experiences. This is what led to the alternate "interpretations" of quantum mechanics.

I have "interpretations" in scare quotes because it's a big mistake I see repeated online that all these different interpretations "give the same predictions" and thus are all equivalent. As I'll explain that's not the case.

There are in two types of alternative interpretations of quantum theory:
  1. Hidden Variable theories. These attempt to come up with a new theory that replaces quantum theory and isn't restricted to observations.

  2. Modal theories. These attempt to rewrite quantum theory itself, i.e. take a subset of the same mathematics and redevelop it in a way that can be understood in a classical objective sense.
Today the first type, Hidden Variables, has been scientifically eliminated. None of these theories have managed to correctly model most atomic phenomena like nuclear decay where as quantum theory continues to expand its ability to explain into further and further regimes. Even more extreme than that every year a new experiment is run that rules out larger and larger classes of these theories. Not just that they don't currently model decay etc, but that they could never be repaired/salvaged to do so. We know enough today to say that no working Hidden Variable theory will ever exist. This is the standard scientific conclusion about these models.

The second type, the modal theories that attempt to rewrite and redevelop quantum theory itself were mostly proven not to work by their own practitioners in the 1990s. This was really commendable as they had been working on these ideas for twenty years, but managed to refute their own work. The only Modal type theory still discussed today is the "Many Worlds Interpretation" as in the video TristramEvans TristramEvans posted above. The reason this wasn't refuted in the 1990s is that it's not well-defined what the idea actually is. Unlike most of these ideas it has never gotten to the point of being able to replicate even a small set of experiments.

For this reason I will set aside all of these alternative views, since they have never been able to replicate the predictions of quantum theory and the ones that managed to match some data are today experimentally ruled out.
 
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Séadna

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The Cat Redux:

So back to the Cat experiment, but we'll replace it with a human as you did.
So we have somebody in a sealed container, they observe a radioactive source decay or not by hearing a click of a Geiger counter. Then later somebody outside goes into the lab and checks the Geiger counter.

The question is when does the decay of the atom get set. When the first observer hears the counter click or is it only set for each individual observer when they personally check the click or what?

Again Complementarity is what sets limits on when you can imagine something to be objectively true. To again repeat an example you chose to learn a either colour or photon count of light. Since no device can do both Complementarity bans imagining either to have an objective truth value. So the question here is whether there's any experiment the outside observer can conduct on the lab that would prevent them ever checking whether the counter clicked or not as a matter of principle.

The original form of the Copenhagen Interpretation due to Bohr at al said that once things get large and complex enough they are no longer subject to Complementarity. So your lost keys are somewhere in your house even if you personally haven't seen them, unlike an atom which is nowhere and in fact has no existence at all if you haven't seen it.

I won't go into the calculational details of how you check which things are subject to Complementarity or not since it's technical mathematics, but most calculations confirm Bohr's conclusion. In short the atom decays when the cat sees it and the cat is either dead or alive long before you open the box, because sufficiently "large" things have objective observation independent properties.

The question of where exactly the dividing line, known as the Heisenberg cut, is located is a complex one and unsettled. Bohr thought the division was set by complex discussions about human inter-communication, Heiseneberg thought it corresponded to a German philosophical notion called "Den Anschauliche" the realm of human direct perception, John Wheeler thought its nature could be directly calculated mathematically.

Like most physicists I'm happy enough with Roland Omnès's calculations from the 80s showing that where ever the line is everything in our everyday life is well above it. So the world of the microscopic is intrinsically observer dependent and with no objective description, but everyday life is not like that, i.e. your keys are somewhere. The details of the realm between these two extremes is a marginal technical concern.

Now I should say there are those who disagree like Carlo Rovelli* and Chris Fuchs who think that fundamentally that even large scale things have no objective properties, since there is still enough wiggle room in the details of calculating where the Heisenberg cut is to argue this.

*His recent popular book "Helgoland" discusses this where everything only has properties relative to things that have actually observed it.

I hope that helps, but it's mental stuff so no worries if all of this made little sense! :thumbsup:
 

Klibbix!

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The Cat Redux:

So back to the Cat experiment, but we'll replace it with a human as you did.
So we have somebody in a sealed container, they observe a radioactive source decay or not by hearing a click of a Geiger counter. Then later somebody outside goes into the lab and checks the Geiger counter.

The question is when does the decay of the atom get set. When the first observer hears the counter click or is it only set for each individual observer when they personally check the click or what?

Again Complementarity is what sets limits on when you can imagine something to be objectively true. To again repeat an example you chose to learn a either colour or photon count of light. Since no device can do both Complementarity bans imagining either to have an objective truth value. So the question here is whether there's any experiment the outside observer can conduct on the lab that would prevent them ever checking whether the counter clicked or not as a matter of principle.

The original form of the Copenhagen Interpretation due to Bohr at al said that once things get large and complex enough they are no longer subject to Complementarity. So your lost keys are somewhere in your house even if you personally haven't seen them, unlike an atom which is nowhere and in fact has no existence at all if you haven't seen it.

I won't go into the calculational details of how you check which things are subject to Complementarity or not since it's technical mathematics, but most calculations confirm Bohr's conclusion. In short the atom decays when the cat sees it and the cat is either dead or alive long before you open the box, because sufficiently "large" things have objective observation independent properties.

The question of where exactly the dividing line, known as the Heisenberg cut, is located is a complex one and unsettled. Bohr thought the division was set by complex discussions about human inter-communication, Heiseneberg thought it corresponded to a German philosophical notion called "Den Anschauliche" the realm of human direct perception, John Wheeler thought its nature could be directly calculated mathematically.

Like most physicists I'm happy enough with Roland Omnès's calculations from the 80s showing that where ever the line is everything in our everyday life is well above it. So the world of the microscopic is intrinsically observer dependent and with no objective description, but everyday life is not like that, i.e. your keys are somewhere. The details of the realm between these two extremes is a marginal technical concern.

Now I should say there are those who disagree like Carlo Rovelli* and Chris Fuchs who think that fundamentally that even large scale things have no objective properties, since there is still enough wiggle room in the details of calculating where the Heisenberg cut is to argue this.

*His recent popular book "Helgoland" discusses this where everything only has properties relative to things that have actually observed it.

I hope that helps, but it's mental stuff so no worries if all of this made little sense! :thumbsup:

Your response and explanation are very appreciated! I can't say that I've understood everything since I'm still trying to absorb it all but what I have understood is pretty mind-blowing. I also went and looked up the Aspect experiments and that was an illuminating read as well. Thanks to Robert Anton Wilson I've heard of Bell's Inequalities/non-locality/etc etc before so it was helpful to go over it again.

I just want to clarify something though as I may have missed it. Does Complementarity, as you've described it, only relate to the realm of the very small? It seems to me, a non-scientist, that there likely must be 'known' properties of reality-as-we-know-it that have been determined by connecting the data revealed by separate experiments solely with logic and reason and not with single experiments that determine something in one go. I apologize if I've misunderstood something here.
 

Séadna

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there likely must be 'known' properties of reality-as-we-know-it that have been determined by connecting the data revealed by separate experiments solely with logic and reason
Can you give an example? Just to make sure I am answering correctly.
 

Klibbix!

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Can you give an example? Just to make sure I am answering correctly.

I think I’m just assuming situations like this exist, but I meant a situation in which an individual or group does an experiment to prove something and then another group takes the results of that experiment as true without performing it themselves and then uses it as an axiom to logical prove something else.

I’m doing a terrible job of explaining what I’m trying to get at. I’m going to recalibrate and come back!
 

Séadna

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I think I’m just assuming situations like this exist, but I meant a situation in which an individual or group does an experiment to prove something and then another group takes the results of that experiment as true without performing it themselves and then uses it as an axiom to logical prove something else.

I’m doing a terrible job of explaining what I’m trying to get at. I’m going to recalibrate and come back!
No that's fine, I think I get what you mean. Say (as a dumb example) somebody goes out watches how elephants migrate and writes a report, then another group takes that report and takes a single elephant in captivity and tests how its digestive system works and then writes a further paper going "Since they process food in way X and in the wild they migrate like Y, then doing Z to plants in certain area will change the migration pattern" i.e. they combine the original report and their own tests to make some new conclusion.

Is that the kind of thing you mean?
 

Klibbix!

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No that's fine, I think I get what you mean. Say (as a dumb example) somebody goes out watches how elephants migrate and writes a report, then another group takes that report and takes a single elephant in captivity and tests how its digestive system works and then writes a further paper going "Since they process food in way X and in the wild they migrate like Y, then doing Z to plants in certain area will change the migration pattern" i.e. they combine the original report and their own tests to make some new conclusion.

Is that the kind of thing you mean?

Yes, that’s exactly what I mean!
In that case, if the researchers who have taken the previous information as a given and not done a, ‘big picture’ experiment, does it render their eventual results meaningless?
 

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Yes, that’s exactly what I mean!
In that case, if the researchers who have taken the previous information as a given and not done a, ‘big picture’ experiment, does it render their eventual results meaningless?
Ah perfect.

So their results are fine because somebody could have done the big picture experiment. Nobody actually did and in most cases we don't in real scientific experiments, but in principle somebody could have. As long as somebody could have (regardless of whether they actually do) combining conclusions/data is fine.

This is not like the atomic realm where different types of experiments can mutually exclude each other and a "big picture" experiment is impossible. As mentioned before nothing can be both an eye and a photodetector even in principle, thus combining the ideas of colour and photons is meaningless. The error of classical thought was to think combining data in these situations still had meaning, since the fact that somebody couldn't learn two things at the same time was considered just a practical detail about gathering information not a fundamental limit on logic.

Another way of phrasing Complementarity is If nobody in principle can actually learn two types of information at the same time, it turns out neither type exists until somebody chooses to learn one type or the other with only the one chosen gaining a well-defined value.

Here's a very extreme case though called "Horizon Complementarity". Our visible universe has an edge, the cosmological horizon. Past that horizon things retreat from us so fast due to the expansion of the universe that we can never observe beyond it.
Thus nobody could observe our Earth and the stuff beyond that horizon at the same time. Complementarity would then say sentences like "There's probably planets like Earth beyond the cosmological horizon" have no meaning since "Earth" can only be seen by people on this side it cannot be combined with statements about the other side.

I'd say it's easy to see a statement like "There's probably planets like Earth beyond the cosmological horizon" as something we might never know the truth about. That would be the classical idea of statements being either true or false independent of observation, even if you never know whether it's true or false. It's much harder to understand it's a meaningless sentence you should never say.
 

Klibbix!

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Ah perfect.

So their results are fine because somebody could have done the big picture experiment. Nobody actually did and in most cases we don't in real scientific experiments, but in principle somebody could have. As long as somebody could have (regardless of whether they actually do) combining conclusions/data is fine.

This is not like the atomic realm where different types of experiments can mutually exclude each other and a "big picture" experiment is impossible. As mentioned before nothing can be both an eye and a photodetector even in principle, thus combining the ideas of colour and photons is meaningless. The error of classical thought was to think combining data in these situations still had meaning, since the fact that somebody couldn't learn two things at the same time was considered just a practical detail about gathering information not a fundamental limit on logic.

Another way of phrasing Complementarity is If nobody in principle can actually learn two types of information at the same time, it turns out neither type exists until somebody chooses to learn one type or the other with only the one chosen gaining a well-defined value.

Here's a very extreme case though called "Horizon Complementarity". Our visible universe has an edge, the cosmological horizon. Past that horizon things retreat from us so fast due to the expansion of the universe that we can never observe beyond it.
Thus nobody could observe our Earth and the stuff beyond that horizon at the same time. Complementarity would then say sentences like "There's probably planets like Earth beyond the cosmological horizon" have no meaning since "Earth" can only be seen by people on this side it cannot be combined with statements about the other side.

I'd say it's easy to see a statement like "There's probably planets like Earth beyond the cosmological horizon" as something we might never know the truth about. That would be the classical idea of statements being either true or false independent of observation, even if you never know whether it's true or false. It's much harder to understand it's a meaningless sentence you should never say.

Thank you for your answer! That makes sense to me, even though I’m sure I’m missing some of the finer points and implications. It really does get mind boggling when things get very small.
 

Séadna

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Interesting, I would think teleporting was impossible because of the no-cloning theorem, but I know Sabine thinks QM is wrong so she's arguing from a more general perspective. Cool video.
 
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Séadna

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I'll give it its standard name: The Copenhagen Interpretation. However today we sometimes use the phrase "Participatory Realism" for reasons I'll explain in the next post.
Sorry Klibbix! Klibbix! I realised I never explained this.

Basically as I said above in the Copenhagen interpretation a physical quantity or even a physical object like a particle only has a well defined value or existence if you choose to observe it, thus you "participate" in making something "real", hence "Participatory realism".

However Bohr himself said more beyond this basic aspect. So although most physicists agree with this basic aspect that doesn't mean they agree with everything he said. So often we say Participatory Realism or "Copenhagenish" for any view that includes this essential part and reserve "Copenhagen" for what Bohr specifically said. My posts above don't deal with the subtle differences between these views because they're mainly mathematical.

The phrase "Participatory Realism" was first given by John Wheeler with this image:

3-Figure1-1.png
 

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Sorry Klibbix! Klibbix! I realised I never explained this.

Basically as I said above in the Copenhagen interpretation a physical quantity or even a physical object like a particle only has a well defined value or existence if you choose to observe it, thus you "participate" in making something "real", hence "Participatory realism".

However Bohr himself said more beyond this basic aspect. So although most physicists agree with this basic aspect that doesn't mean they agree with everything he said. So often we say Participatory Realism or "Copenhagenish" for any view that includes this essential part and reserve "Copenhagen" for what Bohr specifically said. My posts above don't deal with the subtle differences between these views because they're mainly mathematical.

The phrase "Participatory Realism" was first given by John Wheeler with this image:

View attachment 37202

Thank you for the explanation. Are there any entry-level books you would recommend for someone dipping their toe into the realm of the very small?
 

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Thank you for the explanation. Are there any entry-level books you would recommend for someone dipping their toe into the realm of the very small?
Hans von Bayer's QBism
Jauch's "Are Quanta real?"
Jeffrey Bub's "Totally Random" graphic novel

The last one some people find obtuse, but with others it really clicks. von Bayer's book is the only popular book I think that just explains the theory directly in prose with no fluff. Jauch's book teaches it through a fictional argument between three men and focuses explicitly on the experimental evidence that makes it very hard to consider particles as objective "things", sort of trying to place you in the shoes of Bohr and Heisenberg.

All are pretty cheap.

Rovelli's Helgoland is also a very good recent book in terms of accuracy, although he mixes in plenty of general philosophising which some don't like but I think you might like it.
 
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Klibbix!

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Hans von Bayer's QBism
Jauch's "Are Quanta real?"
Jeffrey Bub's "Totally Random" graphic novel

The last one some people find obtuse, but with others it really clicks. von Bayer's book is the only popular book I think that just explains the theory directly in prose with no fluff. Jauch's book teaches it through a fictional argument between three men and focuses explicitly on the experimental evidence that makes it very hard to consider particles as objective "things", sort of trying to place you in the shoes of Bohr and Heisenberg.

All are pretty cheap.

Rovelli's Helgoland is also very good recent book in terms of accuracy, although he mixes in plenty of general philosophising which some don't like but I think you might like it.

I think I remember you recommending the graphic novel on these forums. I suggested it for a reading group I was a part of but the group fell apart before we got to it. I think I’ll try it myself.

They all sound good but Helgoland does indeed look up my alley. Thank you for the list!
 

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An image of one of my favourite experiments. The neutron Cheshire Cat interferometer in Vienna:

2019-10-24-257655_DSC3251-copy.jpg

Basically neutrons are created in the central chamber and one at a time fired down a tube which eventually forks into two separate tunnels. The tunnels have detectors which can detect a neutron's mass or its spin. Only one detector can be inserted into a tunnel at a given time.

The interesting thing is that when a spin detector is put in the tunnels they always detect the spin in the upper tunnel, but when mass detectors are placed in they detect the mass in the lower tunnel. So the spin and the mass properties of the neutron are found in two different tunnels, like the Cheshire cat separated from its grin*.

*Of course the usual explanation is that its a mistake to think there is an object, a neutron, having a mass and spin rather than there just being spin-clicks and mass-clicks.
 
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