Wigner's Friend shows the difference between Observer and Measurement

[Quantum Alchemy]

You often hear this debate about the role of the observer in Quantum Mechanics. How you view this role is usually dictates the interpretation you prefer. If it's Copenhagen, then the observer is more robust and plays a crucial role in wave function collapse. If it's Many Worlds, then the observer is no different than a rock as Sean Carroll says and there's no wave function collapse.

I think the recent Wigner's Friend experiment clarified the difference between observer and measurement. Here's the experiment.

Experimental test of local observer independence

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics the objectivity of observations is not so clear, most markedly exposed in Wigner’s eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers’ narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner’s friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.

https://advances.sciencemag.org/content/5/9/eaaw9832

This is important when looking at the role of measurement vs. the role of the observer.

If you have a measurement, it looks like this:

|↑⟩⟹(|→⟩+|←⟩)/2

If you add in an observer, it looks like this:

|↑⟩|obs⟩⟹(|→⟩+|←⟩)|obs⟩/2

The observer can tell whether you're in state |→⟩ or |←⟩ a measurement can't.

Take a measuring device in the double slit experiment. It doesn't know what state it's in. It has stored information about which slit the particle went through but it takes a conscious observer to know the difference |→⟩ or |←⟩ or which slit the particle went through or not.

People who support MWI can't say consciousness has nothing to do with QM because this experiment shows that it does. How you define consciousness is another debate but a measuring device can just store information. A brain is a measuring device that stores information but consciousness can look at that information in an abstract way. It knows we're in one measured state and not the other. It can publish papers and write books about it as well as think about what it means.

In Wigner's Friend, it shows that you can have a measurement but still have two different wave functions. Wigner's Friend knows a measurement took place. He recorded the results. Wigner outside of the lab can look at the same particle and see interference and conclude that no measurement has occurred.

When Wigner's Friend calls him and says I carried out a measurement and this is the result, then Bayesian type updating occurs for Wigner's wave function and now it's in sync with his friends.

So a measurement collapses the wave function so to speak but a measurement can't tell which state it's in. Without observers to resolve this issue, you could have different measurements all over the place.

This reminds me of Wheeler drawing the universe as a Big U. At one end is the Big Bang and at the other end is Us(Consciousness).

All of these measurements would evolve over billions of years until consciousness evolved. At that point, Bayesian updating would occur and one history of the universe or a singular measured history would be realized.

 

[PeterDonis]

If you add in an observer, it looks like this

No, it doesn't, at least not if the observer observes the qubit (and if the observer doesn't observe the qubit, no measurement has been made and there's nothing to discuss). If the observer observers the qubit, you have this:

$$
| \uparrow \rangle | \text{obs} \rangle \rightarrow \frac{1}{\sqrt{2}} \left( | \leftarrow \rangle | \text{obs} \leftarrow \rangle + | \rightarrow \rangle | \text{obs} \rightarrow \rangle \right)
$$

The rest of your post simply carries forward the same error.

 

[Quantum Alchemy]

|↑⟩|obs⟩→1√2(|←⟩|obs←⟩+|→⟩|obs→⟩)

I agree but the only different term you have is 1√2. That doesn't change anything about the difference between measurement and the observer that was said in the post.

Measurements can't tell what state they're in. Wigner's Friend experiment shows the wave function of observers can have different outcomes until information about the measured state is shared between observers. Say you have a particle that goes through a slight and is recorded by a measuring device. How can that measuring apparatus relay to another measuring apparatus what measurement has occurred without consciousness?

Here's more from the paper:

Before we describe our experiment in which we test and indeed violate inequality (2), let us first clarify our notion of an observer. Formally, an observation is the act of extracting and storing information about an observed system. Accordingly, we define an observer as any physical system that can extract information from another system by means of some interaction, and store that information in a physical memory.

Such an observer can establish “facts”, to which we assign the value recorded in their memory. Notably, the formalism of quantum mechanics does not make a distinction between large (even conscious) and small physical systems, which is sometimes referred to as universality. Hence, our definition covers human observers, as well as more commonly used non-conscious observers such as (classical or quantum) computers and other measurement devices—even the simplest possible ones, as long as they satisfy the above requirements. We note that the no-go theorem formulated in [5] requires observers to be “agents”, who “use” quantum theory to make predictions based on the measurement outcomes. In contrast, for the no-go theorem we tested here [4] it is sufficient that they perform a measurement and record the outcome. The enhanced capabilities required of agents were recently discussed in [13].

https://arxiv.org/pdf/1902.05080.pdf

This goes to my point about human observers.

The human brain can extract information about a system and store it in it's memory but the difference is, we can look at that stored information in abstract ways. We can relay the outcomes of measurements to other observers and we update their wave function.

Without consciousness, how can this information be transmitted to different observers? Without this Bayesian updating, two observers can measure different outcomes for the same event.

 

[PeterDonis]

I agree but the only different term you have is 1√2.

Um, no, I also have the observer entangled with the observed system. You didn't.

For the rest of your post, I strongly suggest that you get the math right first, before trying to make claims about what it means. A lot of what you're saying looks like nonsense.

 

[PeterDonis]

Wigner's Friend experiment

There has been plenty of previous discussion of experiments of this type here at PF, including a discussion of the very paper you link to:

https://www.physicsforums.com/threa...-basic-wigners-friend-type-experiment.968181/

Please read that thread before trying to speculate on your own.

This thread is closed.