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My question is not about the nature or reality of quantum superposition, superposition is a fact and no argue about that.

My question is: What is the purpose of superposition and why does the universe need it?

Every law of physics has its necessity, and makes perfect sense that the universe cannot work without it. But when thinking about superposition, it seems physics laws will work fine with or without superposition!

Edit: I am not questioning superposition itself, but the wisdom behind it. Imagine a world without superposition, where every state is determined from initial conditions, what will change in the universe?

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    $\begingroup$ Your question is unclear. You say superposition is a fact, then you say it is unnecessary. You don't explain why you consider superposition unnecessary. Since you haven't explained your problem, nobody can address it. $\endgroup$
    – alanf
    Commented Feb 20, 2017 at 10:32
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    $\begingroup$ I don't think the question should be closed on language alone. The strong force holds the Nucleus together. The electromagnetic force holds atoms together. Most quantum laws provide, for lack of a better word, structure to the universe. Take away gravity and stars fly apart for example and the atmosphere leaves earth. It wasn't a good day when the gravity switch was turned off. So, if you make superposition go away, what happens? What blows up? That's how I read the question anyway. $\endgroup$
    – userLTK
    Commented Feb 20, 2017 at 10:47
  • $\begingroup$ @userLTK exactly! $\endgroup$
    – DeepBlue
    Commented Feb 20, 2017 at 12:15
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    $\begingroup$ This question might be better posed on Philosophy.SE. The universe doesn't "need" anything, and it has no "wisdom" according to science. As for imagining a world without superposition, it is likely completely and utterly unrelated to our world in every way if you include the big bang. If you only include modern life (as in this hypothetical universe spawns in its current state to be analyzed), all electronics stop working because modern electronics depends on quantum effects for things like Zener diodes. $\endgroup$
    – Cort Ammon
    Commented Feb 20, 2017 at 17:21
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    $\begingroup$ Philosophy also may be able to help you tease apart an ontological dilemma you've put yourself in: Our universe does not have quantum superposition. More correctly phrased, our universe is modeled by scientists using models which use quantum superposition to describe what they see. By focusing on the models rather than reality, it's easier to see why it's so easy to imagine a different world -- all you're imagining is a simpler model! $\endgroup$
    – Cort Ammon
    Commented Feb 20, 2017 at 17:25

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Your claim that

when thinking about superposition, it seems physics laws will work fine with or without superposition

is pretty much completely incorrect, and there are plenty of experiments that can only be given a consistent interpretation using quantum superpositions. You can take your pick from just about all of quantum mechanics; my favourite is the experiment in the second half of this answer, but you can equally well consider Mandel dips, atomic clocks, spin echoes, every single experiment where entanglement plays a role, the structures of every atom and molecule and indeed the basis of all of chemistry, and a myriad others.

If you want a list of all the experiments that need superpositions to be explained, though, this is not the venue for it.

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  • $\begingroup$ Verifying superposition through experiments is not my question. My question is deeper than that. Imagine a world without superposition were every state is determined from initial condition, physics laws will work just fine! $\endgroup$
    – DeepBlue
    Commented Feb 20, 2017 at 12:13
  • $\begingroup$ You mean, a world where one of the basic postulates of quantum mechanics is fundamentally altered? What is it you understand as "the laws of physics", then? $\endgroup$
    – Emilio Pisanty
    Commented Feb 20, 2017 at 12:15
  • $\begingroup$ You understand my question but you are avoiding answering it directly. read @userLTK comment on my question. $\endgroup$
    – DeepBlue
    Commented Feb 20, 2017 at 12:32
  • $\begingroup$ @DeepBlue you are talking of why a deterministic system would not describe the data and Emilio is giving you the list which makes quantum mechanical superposition necessary. $\endgroup$
    – anna v
    Commented Feb 20, 2017 at 12:53
  • $\begingroup$ @DeepBlue What part of "without superpositions there would be no atoms, no molecules, and no chemistry" isn't clear? $\endgroup$
    – Emilio Pisanty
    Commented Feb 20, 2017 at 13:36
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Why does the universe need quantum superposition?

Because of the wave nature of matter. Check out the Wikipedia quantum supposition article:

"Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution".

Note that the Schrödinger equation is a wave equation. We can diffract electrons because of the wave nature of matter. And neutrons too. And protons. Check out Direct Measurement of the Quantum Wavefunction by Jeff Lundeen and others. He reckons wavefunction is something real that can be measured in the lab.

My question is not about the nature or reality of quantum superposition, superposition is a fact and no argue about that. My question is: What is the purpose of superposition and why does the universe need it?

It doesn't need it per se. And it doesn't have a purpose per se. That's just how the world is. Everything is fields and waves. That's what quantum physics is all about. These waves can "ride over each other" or intersect or combine, whereupon they're in superposition.

Every law of physics has its necessity, and makes perfect sense that the universe cannot work without it. But when thinking about superposition, it seems physics laws will work fine with or without superposition!

These laws don't have any real existence in their own right. We observe things and take note of evidence, then we come up with theories that include "laws" that describe how the universe works.

Edit: I am not questioning superposition itself, but the wisdom behind it. Imagine a world without superposition

I'm afraid I can't. Because I can't imagine a world without waves. A world without waves isn't much of a world at all.

where every state is determined from initial conditions, what will change in the universe?

I can't say that I know for sure. But I fear the answer is nothing. A world without waves isn't much of a world at all, and nor is a world without change.

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  • $\begingroup$ But in QM we are not talking about real waves when describing the quantum states of a particle, but more about probabilities waves. $\endgroup$
    – DeepBlue
    Commented Feb 20, 2017 at 19:13
  • $\begingroup$ @DeepBlue : Jeff Lundeen and others are. Have a look at this: "...physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, we demonstrated a method to directly measure the wavefunction so that its real and imaginary components appear straight on our measurement apparatus". $\endgroup$
    – John Duffield
    Commented Feb 20, 2017 at 19:19
  • $\begingroup$ So your short answer to my question "It doesn't need it per se. And it doesn't have a purpose per se". For me I can't imagine the assistance of any physical phenomenon for no purpose, there must be a strong reason why superposition is a reality! $\endgroup$
    – DeepBlue
    Commented Feb 22, 2017 at 8:50
  • $\begingroup$ @DeepBlue : the short answer is superposition happens because at some fundamental level everything is waves in space. As for purpose, I'm afraid I can't say. $\endgroup$
    – John Duffield
    Commented Feb 22, 2017 at 13:12
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There are many physical phenomena which cannot be explained without superposition. But I won't go into that since it is pretty obvious. I would like to state another rather abstract reason to this. First of all, obviously, the question is a bit too vague it would be really helpful if you were to state it eloquently.

Anyway, so, most (I think it is quite safe to say 'all of') of the fundamental laws of physics are described by differential equations. And until we are talking under small angle approximation we see that if an equation has a solution x1 and another solution x2 it immediately follows that x1 + x2 is also a solution of this equation, this latter solution is known as superposition of the first two solutions. So, now, my argument establishes that superposition is indeed something which follows from the fundamental laws of physics. (For example, Newton's Laws of Motion and Schrödinger Equation in Quantum Mechanics; both of these are second order linear differential equations) Also you wouldn't see this happening in Nonlinear Systems or when you're out of the small angle approximation. Now, I would also like to add something philosophical about the term 'superposition' you see superposition is not something physical like an Electromagnetic Wave or an Eectron or the Earth or Force or Energy etc. It is not even a phenomenon but it is a description of a certain kind of state of a system.

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