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Each time I look video in regards to quantum physics there is almost always one point where they claim small matter can be represented by a wave function (can be everywhere but the item in question has an higher % of being at a specific location than far away) until it is observed (now seen as a particle).

In practice, what is considered as an "Observer"?

Is consciousness essential to the observer role?

Example1: Let's say X is not observed (so in the wave function state), then a device is placed to determine the location of X but the result is never looked at by a conscious being (assuming a human) .

Is X considered observed so now represented by a particle?

Example2: Same example as 1 but now a conscious being looks at the result of the device.

I assume X is now being considered observed therefore it is now represented as a particle.

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  • $\begingroup$ Does this answer your question? If nothing is "objectively real" prior to "measurement", what exactly is a "measurement"? $\endgroup$ – BioPhysicist Feb 12 at 17:40
  • $\begingroup$ No but it helps me understand that what qualifies as an observer is still unclear. I would like to learn more about other observer dependent experiments. $\endgroup$ – Greg7000 Feb 12 at 18:51
  • $\begingroup$ If you want a full answer to this question, you're going to have to give your definition of "consciousness". Keep in mind that the definition you're currently thinking of may not be the same one that some, or even most, of us would assume. $\endgroup$ – probably_someone Feb 12 at 19:23
  • $\begingroup$ That is a good question. Let's say, anything that can experience making a decision or has the illusion it can take a decision in regards to the world around it. $\endgroup$ – Greg7000 Feb 12 at 19:30
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    $\begingroup$ Nobody knows what counts as a measurement (which is the same thing as saying that nobody knows what qualifies as an observer). This is the measurement problem. $\endgroup$ – Stéphane Rollandin Feb 12 at 20:56
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I would say that observer of a system A is any physical system B whose state remains unchanged through interaction with system A, or whose state always changes in the same way regardless of the state of system A. In other words it's a system that doesn't get any information about system A even when interacting with it.

If an interaction only changes the state of system A, but it doesn't change the state of system B, then system B is not an observer. Examples are: mirror reflecting a photon, a plate with two slits in Young experiment, alight-splitting plate. They definitely interact with our experimental system A, but does not change their own state. Thus they are not observers.

Systems that do change their state when interacting with system A are observers. The examples are: a particle detector, a soalr panel, a conscious person. All of them upon interaction change their state, and that change can be later further observed.

Of course no system truly remains unchanged in the interaction; so it can be argued that any real system interacting with system A is its observer. However, it can be noticed that sometimes system B observeres only some qualities of system A, and does not observe others. For example let's take the mirror reflecting a photon. It does not change it's state depending on the polarization of the reflected photon, so it does not observe the polarization of the photon; at the same time, it does change it's momentum accordingly to the momentum of the reflected photon, and thus it observes the momentum of the photon.

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Is consciousness essential to the observer role?

No. What is usually meant by an 'observer' could be better described as 'something that interacts with the system.

Consider a particle (perhaps an electron) floating around in space, and let's call it X. X does not have a well defined position in space, instead it is described by a wavefunction. If it collides with a photon flying around you could still say X was 'observed by the photon', in that the photon interacted with the system of X. But photons don't have consciousness.

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  • $\begingroup$ I like your answer, maybe we could debate on "photons don't have consciousness" but this is out of scope with my initial question so I will avoid that ;). Does this mean that for my Example 1 and 2 both result would be valid (we could either observe the item as a wave function or a particle)? Also, when I check videos regarding the double slit experiment, they often say that when they "add an observer" they witness different result, what do they physically add? A captor? What kind of captor? Something that throws photon? What if the item which we try to observe is light? $\endgroup$ – Greg7000 Feb 12 at 19:17
  • $\begingroup$ @Greg7000 If you want to talk about consciousness, you really need to define what you mean by the word. Otherwise you'll just be talking past people who assume a different definition than you do. $\endgroup$ – probably_someone Feb 12 at 19:26
  • $\begingroup$ answered in the question comment. $\endgroup$ – Greg7000 Feb 12 at 19:33
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    $\begingroup$ I don't agree that everything that interacts with a particle automatically becomes an observer. Consider a plate with two slits from the Young's experiment. It definitely interacts with the photon, limiting the ways it can move. But it does not observe the the photon, that's why the interference can still be observed later. $\endgroup$ – Adam Latosiński Feb 12 at 20:00
  • $\begingroup$ @Adam Latosiński You're correct, maybe instead I should have said 'Anything that causes wavefunction collapse', or 'Anything that interacts with the system to such an extent that decoherence takes place'. But the point I tried to illustrate is that there is no implied requirement for consciousness in physicists' meaning of the word observer. $\endgroup$ – Jesse Feb 12 at 20:17

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