What is an observer, defined by QM?

In summary, the conversation discusses questions about the quantum mechanical definition of an observer and the role of measurement in the double slit experiment. The discussion touches on the different interpretations of quantum mechanics, including the Copenhagen interpretation, decoherence, and the role of the human mind in collapsing the wave function. The conversation concludes with a mention of Von Neumann's proof and the importance of understanding the concept of entanglement in understanding quantum mechanics.
  • #1
=entropy
3
0
Couple noob questions.

1.What is quantum mechanical definition of an observer?

2.Why is looking at the double slit experiment with your eyes while the electron goes through any different than a camera looking? Each one is looking, one can just see better. Is it the fact that the outcome can be known with the camera, but not with your eyes that collapses the wave function? So does the electron makes a distinction between resolution capabilities?

I read some were that you have to accept the fact that qm is all about probabilities. So the electron leaves as a particle, becomes a probability wave and goes through the slits interfering with itself. Now is the probability wave actually manifesting? What ACTUALLY happens to the mass of the electron. Since matter cannot be created or destroyed, and if the particle doesn't actually manifest as a wave then were does the mass/energy from the electron go while its a "probability" while going through the slits.
 
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  • #2
2) IMO you're being too literal, and may be helped by not putting so much stock in the particle viewpoint. It's only very special situations where it's best to describe the position of a particle by a delta function in space such that you can really call it a particle. Consider it as a wave and you'll almost never be far off and will be much less confused.
 
  • #3
You might find my post from another thread interesting:
lugita15 said:
The reason there is still disagreement as to what constitutes measurement is that it makes no experimental difference according to quantum mechanics. The way QM works under the Copenhagen interpretation is that you have to split the world into two parts, the “observer” or measurement device, and the “observed” or the particles you’re measuring.

The measurement device is assumed to behave classically. The particles in the observed system are in a superposition of states described by the wave function which keeps evolving until it interacts with the classical measurement device. The question is where to draw the line. You could consider a photon to be the observed system and an atom to be the measuring device, but you can also consider the photon-and-atom system as in a superposition of states, and take a Geiger counter to be the measurement device. So there is this von-Neumann chain, going from elementary particles to Geiger counters to human beings, and we have to decide where to cut it off.

Von Neumann proved in his famous "Bible" of QM that regardless of where you cut the chain, you would get the same experimental results. But he argued that wherever you cut the chain you have things made out of particles on each side of the cut, so there’s no principled way to place the cut in the middle. So he decided that you should place the cut between the human mind and the human body, because he believed that the mind is non-physical. Hence "consciousness causes collapse" was born. Nowadays, the most popular view is decoherence, where there is no real collapse, it's just that when you have a large number of particles in the environment interacting with the system, the wave function becomes smeared out and looks like it has collapsed. So decoherence gives us a reasonable place to cut the chain, when the number of particles involved reaches a critical number so that interference effect become negligible.
 
  • #4
=entropy said:
Couple noob questions.

Why is looking at the double slit experiment with your eyes while the electron goes through any different than a camera looking? Each one is looking, one can just see better. Is it the fact that the outcome can be known with the camera, but not with your eyes that collapses the wave function? So does the electron makes a distinction between resolution capabilities?

there is no difference between a camera and the (human/life) eye.

Both collapse the wave function.
 
  • #5
San K said:
there is no difference between a camera and the (human/life) eye.

Both collapse the wave function.
As I said above, that's not the only possible interpretation. You can instead construct the wavefunction of the electron-and-camera system, which remains in a superposition of states until some third object collapses this wave function. Or you can construct a wavefunction of the system that includes electron, the camera, and all other objects of the universe, so that you can consider the whole universe to be in a superposition of states which never collapses. Or you can take the view that it's the human mind that collapses the wavefunction. The thing is, it makes absolutely no experimental difference at what stage you assume the wave function collapsed at.
 
  • #6
lugita15 said:
The thing is, it makes absolutely no experimental difference at what stage you assume the wave function collapsed at.

it does make a difference -- if you assume a different stage of collapse of the wave function.

the observations, the maths, the logic/explanation, the interference pattern etc won't match up...
 
  • #7
San K said:
it does make a difference -- if you assume a different stage of collapse of the wave function.

the observations, the maths, the logic/explanation, the interference pattern etc won't match up...
Yes they will. If you don't believe me, read Von Neumann's proof in his Foundations of Quantum Mechanics.
 
  • #8
lugita15 said:
Yes they will. If you don't believe me, read Von Neumann's proof in his Foundations of Quantum Mechanics.

if you assume, for example/one, that the wave function collapses well after detection...

then the whole concept of wave function falls apart...
 
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  • #9
San K said:
if you assume, for example/one, that the wave function collapses after detection...

then the whole concept of wave function falls apart...
No, because as I said, you can construct the wavefunction of the particle-and-detector system, and then you'll need a third object to collapse that wave function. Anyway, you can read Von Neumann's proof for yourself.
 
  • #10
lugita15 said:
No, because as I said, you can construct the wavefunction of the particle-and-detector system, and then you'll need a third object to collapse that wave function. Anyway, you can read Von Neumann's proof for yourself.

i see your logic about entanglement...will go through Neumann's literature
 
  • #11
lugita15 said:
No, because as I said, you can construct the wavefunction of the particle-and-detector system, and then you'll need a third object to collapse that wave function. Anyway, you can read Von Neumann's proof for yourself.

What Lugita has been saying is, I believe, known as the Many-Worlds interpretation of quantum mechanics.

Truth is, there is no collapse, there's just the appearance of collapse due to decoherence, which is caused by the fact that when talking about Quantum Mechanics you can no longer separate the measuring apparatus from the measurement. There is no distinction, everything in the end is made of particles (or, if you will be more accurate, amplitude distributions), and physics doesn't care that humans call different things electron, camera, eye or mind. It's all the same thing for physics, and any measuring apparatus used to detect the "actual" state of a quantum system will simply join the superposed state, and then you have a camera that's superposed too.

And since however you put it physics doesn't seem to care what you think, or even for the fact that you think, once you look into the camera, you become superposed, too, and then there's more than one of you, and each of those versions of you has seen a different result, all as real as you, not a single one of them being "youer" than the other.
 
  • #12
JamesOrland said:
What Lugita has been saying is, I believe, known as the Many-Worlds interpretation of quantum mechanics.

Truth is, there is no collapse, there's just the appearance of collapse due to decoherence, which is caused by the fact that when talking about Quantum Mechanics you can no longer separate the measuring apparatus from the measurement. There is no distinction, everything in the end is made of particles (or, if you will be more accurate, amplitude distributions), and physics doesn't care that humans call different things electron, camera, eye or mind. It's all the same thing for physics, and any measuring apparatus used to detect the "actual" state of a quantum system will simply join the superposed state, and then you have a camera that's superposed too.

And since however you put it physics doesn't seem to care what you think, or even for the fact that you think, once you look into the camera, you become superposed, too, and then there's more than one of you, and each of those versions of you has seen a different result, all as real as you, not a single one of them being "youer" than the other.

Wow. Good post.
 
  • #13
JamesOrland said:
What Lugita has been saying is, I believe, known as the Many-Worlds interpretation of quantum mechanics.
No, I was talking about the Copenhagen interpretation, in which we can arbitrarily specify the stage in which wave function collapse occurs. See post #3.
 
  • #14
lugita15 said:
No, I was talking about the Copenhagen interpretation, in which we can arbitrarily specify the stage in which wave function collapse occurs. See post #3.

Um... you did mention that if you specify no stage you have a superposed Universe, and that's pretty much all Many-Worlds is about.

Besides, the Copenhagen assumes the collapse occurs somewhere during measuring, it never says anything about the measuring apparatus and/or measurer becoming superposed too. The ideas of superposed apparatus and observer are the ideas that sort of define Many-Worlds.
 
  • #15
JamesOrland said:
Um... you did mention that if you specify no stage you have a superposed Universe, and that's pretty much all Many-Worlds is about.
Yes, if you do not cut the Von Neumann Chain anywhere, i.e. you make the whole universe the system under observation, then you have no wave function collapse at all and thus you're in the Many World interpretation.
Besides, the Copenhagen assumes the collapse occurs somewhere during measuring, it never says anything about the measuring apparatus and/or measurer becoming superposed too. The ideas of superposed apparatus and observer are the ideas that sort of define Many-Worlds.
In the Copenhagen interpretation, you're allowed to cut the Von Neumann chain anywhere. In other words, you're allowed to divide the world into observer and observed any way you like, and thus you can include measuring devices and whatever else you want in the observed system. So for instance, if a photon interacts with an atom and the atom interacts with a Geiger counter, Copenhagen says that you can treat the photon-and-atom-and-Geiger-counter system as being in a superposition of states, and that the wave function of this system is only collapsed when some other object, say a human, goes and measure the state of the Geiger counter.
 
  • #16
lugita15 said:
In the Copenhagen interpretation, you're allowed to cut the Von Neumann chain anywhere. In other words, you're allowed to divide the world into observer and observed any way you like, and thus you can include measuring devices and whatever else you want in the observed system. So for instance, if a photon interacts with an atom and the atom interacts with a Geiger counter, Copenhagen says that you can treat the photon-and-atom-and-Geiger-counter system as being in a superposition of states, and that the wave function of this system is only collapsed when some other object, say a human, goes and measure the state of the Geiger counter.

Huh. Huh, huh, huh. I had been under the impression that the Copenhagen Interpretation, or at least the one that is most common, assumes collapse as a fact of nature which happens somewhen/somewhere during observation.
 
  • #17
JamesOrland said:
Huh. Huh, huh, huh. I had been under the impression that the Copenhagen Interpretation, or at least the one that is most common, assumes collapse as a fact of nature which happens somewhen/somewhere during observation.
Yes, but in the Copenhagen interpretation it's up to you what you consider to be a mere interaction (described by the Schrodinger equation) and an actual observation that collapses the wave function.
 
  • #18
lugita15 said:
Yes, but in the Copenhagen interpretation it's up to you what you consider to be a mere interaction (described by the Schrodinger equation) and an actual observation that collapses the wave function.

So one could say that, basically, Many-Worlds is the objective description of the Universe (the "bird perspective") and Copenhagen is the subjective one (the "frog perspective"). That is very interesting.
 

FAQ: What is an observer, defined by QM?

What is an observer in quantum mechanics?

An observer in quantum mechanics is any physical entity that interacts with a quantum system and causes its state to collapse into a measurable state. This can include humans, measuring devices, or any other macroscopic object that can interact with the quantum system.

How does an observer affect a quantum system?

An observer affects a quantum system by collapsing its wave function into a definite state. This means that the observer's interaction with the system causes it to have a definite position, momentum, or other measurable property, rather than existing in a superposition of multiple states.

Can an observer be a conscious being or does it have to be a physical device?

In quantum mechanics, an observer can be either a conscious being or a physical device. As long as the entity is capable of interacting with the quantum system and causing its wave function to collapse, it can be considered an observer.

Is the observer effect the same as the uncertainty principle?

No, the observer effect and the uncertainty principle are two different concepts in quantum mechanics. The observer effect refers to the act of observing a quantum system and causing its state to collapse, while the uncertainty principle states that the more precisely we know a particle's position, the less we can know about its momentum, and vice versa.

Can an observer change the outcome of a quantum experiment?

In quantum mechanics, an observer's interaction with a quantum system can affect the outcome of an experiment. This is because the act of observation causes the system's state to collapse into a definite state, and the outcome of the experiment depends on the state of the system. However, the exact role of the observer and the extent of their influence on the outcome is still a topic of debate among physicists.

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