# I What is in superposition?

#### thermia

Summary
Is it the partikel or the answer to the question?
When we say that a partikel - say a photon - is in superposition we assume the partikel itself is in superposition, but as I see it, the partikel is what it is. It is just that we don't know for the moment.
So, is the partikel itself in superposition, or is it the answer to the question about the position that is in superposition?

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#### DennisN

Summary: Is it the partikel or the answer to the question?

So, is the partikel itself in superposition, or is it the answer to the question about the position that is in superposition?
It doesn't have to be. Various properties/states of particles can be in superposition. For instance, the property spin (quantum spin) can be in a superposition of states.

#### thermia

So is the superposition only a matter of definition? Schrodinger's cat's life is in superposition but also our knowledge about it's life is in superposition, or am I taking it too far?

#### PeterDonis

Mentor
the property spin (quantum spin) can be in a superposition of states.
More precisely, it can be in a superposition of eigenstates of the spin operator in a particular direction. But whether or not a particular spin state is a superposition will depend on the direction you choose for the spin operator. Every spin state is an eigenstate of the spin operator in some direction, so every spin state is not a superposition for some choice of spin operator.

is the superposition only a matter of definition?
It's a matter of choice of operator. See above.

Schrodinger's cat's life is in superposition but also our knowledge about it's life is in superposition
This depends on which interpretation of QM you adopt. On a collapse interpretation, when you observe the cat, its state collapses to either "alive" or "dead". So even if the cat was in a superposition of alive and dead before you observed it, it isn't afterwards, so your knowledge of it is knowledge that it's alive or knowledge that it's dead, never a superposition of the two.

On a no collapse interpretation, such as many worlds, when you observe the cat, your state of knowledge (which is actually a physical state, the state of your brain) gets entangled with the state of the cat, so there are two "branches" or "worlds": one in which the cat is alive and you observe it to be alive, and one in which the cat is dead and you observe it to be dead. This is often described as a superposition, but "entangled" is a better term because the entanglement between the cat's state and your state of knowledge is independent of any choice of operator (whereas, as we saw above, whether or not a particular state of a quantum system is a superposition does depend on your choice of operator).

#### thermia

Thanks for your response.
As you may have got, my basic idea was if superposition refers only to physical objects or if immaterial objects such as "knowledge" also can be super positioned.
E.g. Your colleague opens the box and knows the state of the cat, but you still don't.

#### PeterDonis

Mentor
Your colleague opens the box and knows the state of the cat, but you still don't.
This isn't a case of quantum superposition. It's just straightforward classical ignorance--you don't know because you haven't interacted with the cat or your colleague so you have no information about their state.

#### thermia

So you say that immaterial objects can not be super positioned, or ... ?

#### PeterDonis

Mentor
So you say that immaterial objects can not be super positioned
What are "immaterial objects"? Physics isn't about "immaterial objects". It's about real, physical objects.

Sure

#### Mentz114

Gold Member
Have a look at this Insights article.

#### ftr

Summary: Is it the partikel or the answer to the question?

When we say that a partikel - say a photon - is in superposition we assume the partikel itself is in superposition, but as I see it, the partikel is what it is. It is just that we don't know for the moment.
So, is the partikel itself in superposition, or is it the answer to the question about the position that is in superposition?
Both, because the question links both the state of the material and our knowledge about it. We usually say the state is undefined until a measurement is performed, you can infer the linkage, right.

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