I How does observation affect reality

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bob012345

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The aim of the natural sciences is not to "explain" anything but to "describe" quantitatively and as accurately as possible phenomena in Nature and to compare the "descriptions" to quantitative and accurate observations.
If that were true there would be little enthusiasm for the push for more basic unification theories. Separate theories adequately describe nature right now if that's all we want. Epicycles worked very well for describing celestial motion. But Copernicus, Kepler and Newton gave us more insight.
 
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Separate theories adequately describe nature right now if that's all we want.
Only if we assume we will never see any evidence that contradicts them. Physicists anticipate that we will: that at some point, when we are able to do sensitive enough experiments, we will see evidence that, for example, General Relativity is not exactly correct at arbitrarily large spacetime curvatures. That is why they are trying to get ahead of the game by working out possible theories that might be testable in such a regime.

Epicycles worked very well for describing celestial motion.
Not once Tycho Brahe got accurate enough observations. That's why Kepler was forced to develop his theory using ellipses: because Tycho's observations convinced him that there was simply no way to make an accurate enough model using circles alone.
 

bob012345

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Only if we assume we will never see any evidence that contradicts them. Physicists anticipate that we will: that at some point, when we are able to do sensitive enough experiments, we will see evidence that, for example, General Relativity is not exactly correct at arbitrarily large spacetime curvatures. That is why they are trying to get ahead of the game by working out possible theories that might be testable in such a regime.



Not once Tycho Brahe got accurate enough observations. That's why Kepler was forced to develop his theory using ellipses: because Tycho's observations convinced him that there was simply no way to make an accurate enough model using circles alone.
Good points! Glad you agree that the theories need to be testable. I assume you mean beyond computer 'experiments'.
 

atyy

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Of course, I agree with that. All models and theories are always preliminary. One day, there might be better and better theories. The Balmer series was an empirical law, i.e., it was found by looking at data and fitting a mathematical formula to it. Nevertheless it was a very important step towards building a model, because it provided a clear evidence for a systematic pattern. Bohr's model was an ad-hoc description, and many physicists tinkered around with it (most famously the Sommerfeld school) using a wealth of spectroscopic data for many different atoms becoming less and less satisfied by it and finally discovering modern QT. Whether modern QT or not is the "final word" is not known yet. Maybe there's a better/more comprehensive theory one day (including the quantum theory of gravitation perhaps).

Still, it's not the aim of all this progress in theory building to "explain" nature but to "describe" it with as consistent as possible mathematical models. The very fact, why this works at all, is not explainable. It's just an empirical fact that it works with an astonishing success!
You miss the point about explaining nature. The measurement problem is not about explaining nature, but describing nature. The observer is also a part of nature, yet the quantum state cannot describe the observer and the system together.
 
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vanhees71

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Well, since I don't see any measurement problem beyond the challenge for the experimentalists to invent ever better devices, I can't follow this argument. The apparent measurement problems you are talking about are, in my opinion, outside of the realm of the natural sciences but rather in the realm of the philosophy of science.

The one mind-boggling theoretical problem in contemporary fundamental physics, in my opinion, is to find a consistent description of quantum gravity. Maybe related to it is the enigma of the value of the cosmological constant ("dark energy") or the question, whether there's really "dark matter" or whether one needs a new theory of gravitation.
 
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"We do not belong to this material world that science constructs for us. We are not in it; we are outside. We are only spectators. The reason why we believe that we are in it, that we belong to the picture, is that our bodies are in the picture. Our bodies belong to it. Not only my own body, but those of my friends, also of my dog and cat and horse, and of all the other people and animals. And this is my only means of communicating with them."

https://www.amazon.com/dp/1107431832/?tag=pfamazon01-20 page 96

http://www.azquotes.com/author/13142-Erwin_Schrodinger
 
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bob012345

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It's the modulus squared of the wave function which gives probability distributions, nicely real and positive.

The point is, you cannot observe anything without exchanging energy between the measurement device and the system you measure. While for macroscopic systems you can make the influence of the measurement device negligibly small, that's not possible for microscopic objects like an electron. To probe it you need other particles or em. waves to scatter at it, and there's nothing "smaller" than an elemetary particle as the electron itself to make the influence of the measurement device arbitrarily small. That's why observation and measurement are so much more emphasized in QT vs. in classical theory.
It seems to me that you are describing the 'Observer Effect' which is not the same as quantum limits to measurment due to the HUP. Can you elucidate your view on the matter (sorry for the pun!)? Thanks.
 

vanhees71

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The HUP is no limitation for measurements but for preparation. We have discussed this very often in this forum. Just use the search function!
 

vanhees71

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"We do not belong to this material world that science constructs for us. We are not in it; we are outside. We are only spectators. The reason why we believe that we are in it, that we belong to the picture, is that our bodies are in the picture. Our bodies belong to it. Not only my own body, but those of my friends, also of my dog and cat and horse, and of all the other people and animals. And this is my only means of communicating with them."

https://www.amazon.com/dp/1107431832/?tag=pfamazon01-20 page 96

http://www.azquotes.com/author/13142-Erwin_Schrodinger
Hm, poor Schrödinger...
 
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"We do not belong to this material world that science constructs for us.
We are just as much part of it as a chair, car or whatever.

Please, things have moved on a lot since the days of the early pioneers, its not wise to take on board their writings, instead study a modern text like Ballentine. Observation these days can be defined quite easily without observers in the sense Schrodinger etc were thinking of - indeed Von-Nemannn fell into the same trap.

Thanks
Bill
 
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Feynman's explanation of how mirrors work is a delight. (As I remember it, in Six Not So Easy Pieces, but please correct me.) But back to the topic, I claim observation cannot possibly affect reality. Only the transfer of energy from one place to another affects reality. There you have it, no observer effect whatsoever. The Universe seems to work fine unobserved. When we look millenia later, it seems to have got on fine without us. There's a probability I understand the magnitude of the wave equation, but the real and imaginary components phase me :-)
Oh dear. Please don't use words like reality - they are very ill defined even amongst experts. You should see what Penrose thinks reality is - if you haven't read it please do and you might come to understand its a word, while not to be banished from physics, is to be used with great caution.

As to energy transfer I would first become aquainted with what energy is in a modern sense using Noethers Theorem. Its surprising subtle even defining it little alone its realation to affecting reality, whatever your conception of it is.

Thanks
Bill
 
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There's a probability I understand the magnitude of the wave equation, but the real and imaginary components phase me :-)
There is a deep reason from the mathematical theory of generalized probability models. The simplest generalized probability model is just good old probability theory. But it can be generalized further and the next most complex one is - wait for it - QM:
https://arxiv.org/pdf/1402.6562.pdf

The difference has to do with what are called pure states. If you want to allow continuous transformations between them then one must use QM - ordinary probability theory will not allow it. So in going from one pure state to another, physically, we would expect it to go through some other state while doing it. It turns out that's where complex numbers come in:
http://www.scottaaronson.com/democritus/lec9.html

Thanks
Bill
 
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Everyone, please bear in mind that this is a physics forum, not a history forum. Some recent posts about history have been deleted as they are off topic.
 

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