# Quantum superposition

1. Apr 15, 2015

### TimeRip496

What caused particles to be in quantum superposition? Is it because they are very small or very light? If an object is very small or light, why will it quntum superpose?

Besides, when a particle undergoes quantum superposition, does all its states exist in the same world or each exist in their respective universe? If all of them exist in the same world, do all of them have their respective effects on the same world all of them reside in?

2. Apr 15, 2015

### jerromyjon

Things are in superposition when you have degrees of freedom as in probability. The "dead and alive" cat is a famous example. There only needs to be a possibility of change, so to summarize.

It really doesn't make a huge difference how you look at it. My preference is that when you make an observation or record information a definite state exists. Many worlds interpretation says the universe branches at every possible change. Consistent histories says many possible histories could have lead to every event... the result is just probabilities of occurrences. If you do it a thousand times and there are only a few possibilities, do you really need to think about which universe we reside in every repetition?

3. Apr 15, 2015

### Staff: Mentor

There is no cause. Superposition simply reflects the vector space structure of pure states.

The principle of superposition states given any two pure states |a> and |b> and any two complex numbers c1 and c2, c1*|a> + c2*|b> is another state. This also means given any state it is the superposition of many other states.

Thanks
Bill

4. Apr 15, 2015

### Staff: Mentor

A superposition is a single state, not multiple states.

For example, consider a spin-1/2 particle in the state spin-up. We say that's not a superposition, the particle is 100% spin up and we'd write its state as $|\psi\rangle = 1|\psi_{up}\rangle + 0|\psi_{down}\rangle = |\psi_{up}\rangle$... no superposition, and if we measure the spin on the vertical axis it'll be up every single time.

However, we can also write $|\psi_{up}\rangle = \frac{\sqrt{2}}{2}(|\psi_{left}\rangle + |\psi_{right}\rangle)$ where $|\psi_{left}\rangle$ and $|\psi_{right}\rangle$ are the states in which there is a 100% chance of finding the spin to be left or right if we measure along the horizontal axis (Google for "Pauli spin matrices" to see how I did that). This tells us that this 100% spin-up state is a superposition of spin-left and spin-right and that if we measure the spin in the horizontal direction we will get spin-left half the time and spin-right half the time.

So this state is a superposition of spin-left and spin-right, but also a 100% superposition-free spin-up, and we can describe it either way.