# Is it really superposition, or do we just not know its position?

• I
gjphys
TL;DR Summary
I have a basic question regarding quantum systems in their potential and defined states:

Do quantum systems exist in many possible states (i.e. defined as an uncollapsed wave function) merely because we don’t know what state it’s in (i.e. because we haven’t ‘observed’ or measured it, and collapsed the wave function), or because it truly (objectively) is in a state of all (or at least multiple) possibilities?

[Forgive me if I've butchered the language; I'm not a physicist -- just contemplating its philosophical ramifications.]

Homework Helper
Gold Member
2022 Award
A quantum state is like a vector. It may be known, but it is also a superposition of other states. E.g. a known energy state, where a measurement of the system's energy would result in a definite value is a superposition of infinitely many position states, where a measurement of position would yield different values with different probabilities.

gjphys
So you're saying there's an objective 'reality' to quantum states existing in superposition?

Ege_O
First of all, I should mention that the two options you provide are not the only understandings, so one does not have to accept any of them. For instance, in Copenhagen Interpretation, to talk about the position of a particle before the measurement is not meaningful. This is not saying that the particle has a position but we don't know what it is, no. CI says the concepts we use in macroscopic world such as having a definite position, momentum, and so on cannot be applied to the microscopic realm. Therefore, considering a quantum particle as a point-particle with definite trajectory is a wrong picture.

Secondly, there is a distinction that a quantum system is in superposition of two states and the particles are in either of two states but we do now know which one is which. The second one is called "mixed state". Let me give you an example:
Consider two boxes A and B, each contains 100 electrons. In the box A, all electrons are in the superposition spin-state of up and down along z direction. In the box B, %50 electrons are in the up spin-state and %50 electrons are in the down spin-state. In other words, the box A represents superposition and the box B represents our lack of knowledge.
Let's make an experiment, if you make a spin measurement on the electrons in two boxes along z direction. For both boxes, you will find around the spin of 50 electrons will be measured up, and the spin 50 electrons will be measured down. However, things get interesting when you make a spin-measurement along (let's say) x direction. In the box B, you will still get %50 of electrons have spin up and %50 of electrons have spin down state. However, this is not the case for the electrons in Box A. If they are in a certain state, you can measure %100 spin up or spin down along x. Therefore, there is experimentally difference between our lack of knowledge and quantum superposition.

Unfortunately, this example is not well-constructed because it requires math. For example, I should have written the exact state of the electrons in box A in order to justify the measurement results. However, the message is clear: even if quantum states represent knowledge (that's what Qbism says), they do not represent "classical knowledge".

Additionally, the experiments showed that Bell's inequality is violated, which means if particles have predetermined values, they (entangled particles) have non-local correlations. The other choice is accepting superdeterminism but I don't want to go into the details.

Jarvis323 and gjphys
Homework Helper
Gold Member
2022 Award
So you're saying there's an objective 'reality' to quantum states existing in superposition?
No, I'm saying a quantum state is a vector, in some cases of infinite dimension.

gjphys
Ege_O, are you saying there is no consensus on this among physicists, that it depends on the QM interpretation?

PeroK, are you saying it's simply the case that we haven't measured the vector, so we don't know it's exact state?

Mentor
there is no consensus on this among physicists, that it depends on the QM interpretation?
The physical meaning of the state vector depends on the interpretation.

t's simply the case that we haven't measured the vector, so we don't know it's exact state?
No. @PeroK is saying that whether or not any vector is a "superposition" depends on your choice of basis for the vector space. So "superposition" is not a good thing to be looking at if you want to understand the actual physics.

Homework Helper
Gold Member
2022 Award
PeroK, are you saying it's simply the case that we haven't measured the vector, so we don't know it's exact state?
I'm saying that even if we know the exact state that does not mean that the results of all possible measurements are well defined in advance.

The state is the QM equivalent of all you can know about a system.

That's what it means for QM to be fundamentally probabilistic.

gjphys
A blind physicist walks onto a beach, tasked with "defining the wave." Since he can't see its current state, he can only define it by the wave function that accounts for all possible forms within known constraints of ocean waves. But that's only because he can't see. The ocean is in a defined 'state' (represented by a collapsed wave function) at any time.

My question is whether a quantum beach is the same, or if the wave does in fact exist as a 'probability', until he steps onto the beach and measures it.

Homework Helper
Gold Member
2022 Award
A blind physicist walks onto a beach, tasked with "defining the wave." Since he can't see its current state, he can only define it by the wave function that accounts for all possible forms within known constraints of ocean waves. But that's only because he can't see. The ocean is in a defined 'state' (represented by a collapsed wave function) at any time.

My question is whether a quantum beach is the same, or if the wave does in fact exist as a 'probability', until he steps onto the beach and measures it.
There's no resemblance between QM.and a blind man on the beach!

Mentor
a defined 'state' (represented by a collapsed wave function)
There is no such thing as a "collapsed wave function". Wave functions are wave functions. "Collapse" in QM does not mean the system goes from a "non-collapsed wave function" to a "collapsed wave function".

Ege_O
Ege_O, are you saying there is no consensus on this among physicists, that it depends on the QM interpretation?

PeroK, are you saying it's simply the case that we haven't measured the vector, so we don't know it's exact state?
Yes.

Jarvis323
As far as I know, nobody knows if the particle is in a bunch of places at the same time or not, and what the true reality is behind the probability wave. QM is a mathematical model for making predictions, that's all. And it doesn't tell us what the reality is, only it gives us hints. Unfortunately those hints don't seem to be enough, are extraordinarily mind boggling, and lead to a wide range of possibilities, each one bizarre and unintuitive.

Last edited:
gjphys
Mentor
Do quantum systems exist in many possible states (i.e. defined as an uncollapsed wave function) merely because we don’t know what state it’s in (i.e. because we haven’t ‘observed’ or measured it, and collapsed the wave function), or because it truly (objectively) is in a state of all (or at least multiple) possibilities?
In some situations a quantum superposition of A and B ("it is neither A nor B until we measure it") behaves differently than a a system that is either A or B but but we haven't looked to see which yet. These differences can be tested experimentally, the experiments have been done (and steadily improved for almost the past half-century), and they agree with the quantum mechanical prediction.

Google for "Bell's Theorem", check out some of our many many threads on the subject, read this Scientific American article, and look for the web page maintained by our own @DrChinese.

Last edited:
Jarvis323 and PeroK
physika
As far as I know, nobody knows if the particle is in a bunch of places at the same time or not, and what the true reality is behind the probability wave. QM is a mathematical model for making predictions, that's all. And it doesn't tell us what the reality is, only it gives us hints.

Right, the Epistemic view.

Staff Emeritus