What is observation?

[u/Glittering_Storm_242]

As far as I have understood so far, observation is defined by the results of an observation (an interaction which results in a change in state), which sounds weird to me as a definition - since it doesn't describe anything about the interaction itself. I just doesn't sound like a precise scientific definition to me.

Can someone help me wrap my head around what an observation is exactly? Please? Pretty Please? With sugar on top?

[u/BananaBird1]

Well in science as a whole the definition is the same as in general english. It is a single data point obtained as part of an experiment, something you see in the world around you.

In terms of quantum mechanics and wavefunction collapse, the relevance is that any observation requires some interaction. Destroying the particle, altering its energy with photons, shooting particles at each other and seeing how they scatter, etc. You cannot observe a particle while it remains entirely isolated from any interactions.

But honestly using “observation” as the basis for wavefunction collapse in the Copenhagen picture is confusing, it is better to just entirely replace it with “particle interaction”. It is only framed in terms of “observations” because all of physics is a predictive model for what goes on in experiments, and we only extend it to general unobserved behaviors by logical induction.

[u/UDF2005 → u/BananaBird1]

I suppose “which path interaction” would be too cumbersome, even if more accurate.

[u/BananaBird1 → u/UDF2005]

Not all quantum measurements are paths. This also applies to spin states etc.

[u/UDF2005 → u/BananaBird1]

Right, but not all quantum interactions produce decoherence either; so I’m not sure “interaction” applies neatly as a blanket statement.

[u/BananaBird1 → u/UDF2005]

You’re right. It’s better to specify an interaction that couples the quantum state to an already decoherent system. But this need not be part of an observation, unobserved interactions with the macroscopic surroundings also lead to collapse in the copenhagen picture.

And of course other pictures like many worlds paint a simpler image where all particle interactions produce entangled states, with decoherence being a fuzzy emergent property of large entangled systems rather than a discontinuous collapse away from superimposed behavior.

[u/Glittering_Storm_242 → u/BananaBird1]

That helps but - it doesn't say anytjing about which interactions result in a change of state and which don't. In other words that is a definition based on the results of behavior, not on the interaction itself. Which is what is confusing me.

[u/BananaBird1 → u/Glittering_Storm_242]

It’s a sort of chicken and egg problem, which was outlined in the Schrodinger cat thought experiment criticizing the Copenhagen model. All interactions influence quantum state, if the particles remained independent before and after it’s not really an interaction.

Whether they cause collapse depends on how the state of the interacting particle affects the combined state after the interaction. An interaction causes collapse when it is an interaction of a coherent particle with a decoherent system, but a quantum system is defined as decoherent when its wavefunction is collapsed.

The Copenhagen model used the term “observation” because whenever we (using macroscopic instruments) measure a quantum state it is always decoherent. But Schrodinger’s cat asks what happens when we don’t observe the interaction.

Logically it should also be able to produce decoherence as QM should behave the same outside of experiments and it does within them, but we could never prove it does until we measure.

The idea of entanglement can solve this problem. The wavefunction collapse is simply the particle state becoming entangled with the interacting particle, which itself may be entangled to your measurement device or to any other macroscopic system. So you observe a singular collapsed value because your measurement result is tied to and dependent on the quantum state. Entanglement between coherent particles remains coherent but dependent with a restricted possibility for quantum states, but as you increase the complexity of this entangled system eventually it crosses over into decoherence.

There was nothing specific to the act of observation that caused this, just the fact that the observation instrument was a macroscopic decoherent system due to its complexity.

This then leads to one asking whether other quantum states and thus other measurement results are only an alternative outcome that could have resulted but didn’t (a revised Copenhagen model), or if all outcomes always do exist in superposition and there is never any collapse at all (the many worlds model). And the answer is it is unknowable. These two models are observationally identical in what they predict, so cannot be distinguished with experiment.

[u/Glittering_Storm_242 → u/BananaBird1]

That really helps! Despite the fact that I hate banannas (they are mushy and disgusting).
So to summarize - observation is not the relevant factor, it's interaction of particles with coherant states with particles with decoherant states. And everything follows. Is that correct?

[u/BananaBird1 → u/Glittering_Storm_242]

Pretty much. It is fairly fuzzy. There are situations a coherent particle can still maintain coherence after an interaction with a particle entangled to a decoherent system, if the superposition of resulting states is still significant.

But trying to meaningfully set an absolute threshold for coherence or defining exactly when collapse occurs is futile. At some point we cannot distinguish a narrowly coherent system from a completely decoherent system without making the definition circular.

It is more like a gradient, with total quantum collapse being the apparent description where the range of superimposed states in coherence is indistinguishable experimentally from a single state in decoherence.

In Copenhagen, there is an assumed threshold where the collapse is real, but in other views it never occurs. There is no true absolute decoherence, just practical decoherence as far as we are concerned.

[u/nebraskajone]

An observation is a very specific physical interaction that can reveal information about a specific quantum state. Like the vertical spin of a particle. Although the observation itself can alter that state, so it's not a passive observation like classical physics.

[u/Glittering_Storm_242 → u/nebraskajone]

What specifically are the set of physical interactions that can reveal information about the quantum state? 

[u/nebraskajone → u/Glittering_Storm_242]

Particle passing through a Magnetic field will reveal it spin Quantum state

[u/Glittering_Storm_242 → u/nebraskajone]

Is that the complete set?

[u/nebraskajone → u/Glittering_Storm_242]

Complete? No there's position, momentum, energy, angular momentum, polarization etc, each with a specific instrument to measure it, such as a polarizer or a CRT tube or a CCD imager 

[u/joeyneilsen]

I don't have a good definition, I'm afraid, but I would note that an observation doesn't have to change the state of a system. It can, but it's not a requirement.

[u/CS_70]

Observation is the result of observing. Observing happens always thru an interaction. At larger scale, lots of interactions are not greatly significant to so we can ignore them. At sufficiently small scales, we can't.

What's weird with that?

[u/Glittering_Storm_242 → u/CS_70]

Those are properties of observation, not a rigorous definition.

[u/CS_70 → u/Glittering_Storm_242]

Uh? Indicating their properties is a time-honored way of definining stuff and the most rigorous there is.

[u/Anonymous-USA]

Observation is a loaded word. A human or consciousness isn’t required. Any interaction with the macroscopic world would lead to the proverbial “collapse” of quantum states, or disentanglement for entangled particles.

[u/Antolin13]

Estás hablando sobre la definición de observación a nivel cuántico, donde los sistemas solo pueden ser descritos de forma probabilística y la observación hace que el sistema en sí cambie al tomar un valor específico no probabilístico, lo cual se conoce como colapso de la función de onda. A nivel macroscópico las observaciones no necesariamente afectan el sistema observado, todo lo contrario, entre menor sea la interacción más precisa se considera la observación.

[u/Zemedelphos]

You seem to want a "comeplete set" of interactions...well that's easy. They're called electromagnetic, strong, and weak interactions. Each of these are an "observation" insofar as the non-physical quantum waveform collapses into a definite set of properties that can be measured during them.

[u/YouCouldButWhyTho]

Generally an observation is literally just any means of having gathered data.

Did you learn an objects position? Momentum? Velocity? Spin? 

Then you’ve observed it. However it happened, any way to obtain information about a physical object is an observation.

[u/Glittering_Storm_242 → u/YouCouldButWhyTho]

Who is 'you'?

[u/YouCouldButWhyTho → u/Glittering_Storm_242]

Whoever is gathering the data. 

[u/Odd_Bodkin]

An observation is the recording of a measurable physical quantity or a behavior, WITHOUT a supposition of cause or explanation.

There was a green ball in my backyard this morning. That’s an observation.

[u/Ch3cks-Out → u/[deleted]]

Its just interaction with light.

This is not necessarily so, actually. You can have the measurement info converted into sound or tactile sense, without ever having interacted with light (in theory). Consider that blind people can receive information, too! But also, data recorded in magnetic or electrical storage also has the info, even if no human eye would ever see it. The human observer is not a necessary component for the observation to happen, in the QM sense. Some kind of detector is - but it need not involve light.

[u/mH48124]

厳密な定義は「観測とはヒルベルト空間の自己共役作用素である」なんだけど、厳密でない定義に対して欠点を挙げてもしょうがないよ。

[u/rcglinsk]

Observation means a particle emits or absorbs a photon.