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# Quantum superposition Tweet

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1. Quantum superposition
I don't understand this concept at all. I've tried many Google searches but everything seems either illogical or too difficult to understand.

My questions are:

1. How can particles physically be in two places at one time?
2. How do physicists "observe" superposition, if observing it amounts to measuring the particle?

Help me out, please
2. Re: Quantum superposition
Before i start, QM is very very counter-intuitive, and if you don't understand something - you're not alone

Before a particle is observed it exists in all possible location with various probabilities associated with each one - upon observation the particle will then take a position. Until then it exists in all possible positions.

Look up the double-slit experiment - it was one of the key experiments leading to QM development. When we fire electrons through both slits they appear to interfere with each other, the way we would expect a wave to which is travelling through both holes.

But when we observe one of the holes (determining which hole each electron goes through) the particles stop behaving like waves, and go back to behaving as particles.

WEIRRRDDDD
3. Re: Quantum superposition
(Original post by CallMeASafeBet)
I don't understand this concept at all. I've tried many Google searches but everything seems either illogical or too difficult to understand.

My questions are:

1. How can particles physically be in two places at one time?
2. How do physicists "observe" superposition, if observing it amounts to measuring the particle?

Help me out, please
1)It's not clear if if a particle actually is at every point, this is a massive philosophical can of worms. What certainly can be said is that the wavefunction of a particle is everywhere.
2) They don't really. They measure observables and the results are consistent with a theory involving superposition.
4. Re: Quantum superposition
(Original post by theandyguthrie)
Before i start, QM is very very counter-intuitive, and if you don't understand something - you're not alone

Before a particle is observed it exists in all possible location with various probabilities associated with each one - upon observation the particle will then take a position. Until then it exists in all possible positions.

Look up the double-slit experiment - it was one of the key experiments leading to QM development. When we fire electrons through both slits they appear to interfere with each other, the way we would expect a wave to which is travelling through both holes.

But when we observe one of the holes (determining which hole each electron goes through) the particles stop behaving like waves, and go back to behaving as particles.

WEIRRRDDDD
Is Schrödinger cat an example? A cat is either dead or alive at a certain point and when humans 'observe' an outcome is 'created'/must be produced?
5. Re: Quantum superposition
(Original post by themancunian)
Is Schrödinger cat an example? A cat is either dead or alive at a certain point and when humans 'observe' an outcome is 'created'/must be produced?
There are many interpretations of QM, and some would argue in Schrödingers cat that the detector and the cat are both observers and break the superposition.

But some people like to use QM to argue how humans are special.
6. Re: Quantum superposition
(Original post by theandyguthrie)
There are many interpretations of QM, and some would argue in Schrödingers cat that the detector and the cat are both observers and break the superposition.

But some people like to use QM to argue how humans are special.
But we are not; we are still animals though. Animals with the power of imagination.
7. Re: Quantum superposition
(Original post by themancunian)
But we are not; we are still animals though. Animals with the power of imagination.
Exactly, but people try to use QM to claim the conciousness causes the wave collapse and that the world wouldn't exist without us.
8. Re: Quantum superposition
I appreciate the help people, thanks
9. Re: Quantum superposition
(Original post by CallMeASafeBet)
I don't understand this concept at all. I've tried many Google searches but everything seems either illogical or too difficult to understand.

My questions are:

1. How can particles physically be in two places at one time?
2. How do physicists "observe" superposition, if observing it amounts to measuring the particle?

Help me out, please
1. it might be easier (and possibly more accurate) to consider a particle having no well-defined position, only to have a certain probability of being found at a given point upon making a measurement. For one thing, this is true, this is how particles behave when they are measured repeatedly, but also (I think) it means the particle can't be in all places at once because that would require it to have a probability of 1 to be found in more than one place, which isn't allowed.

2. we don't really observe superposition directly as it is lost during a measurement (ie. an observation), some use the term "wavefunction collapse" to describe how a repeated measurement would yield one of the possible allowed results (or, "basis states") in the superposition, weighted according to the probability of observing it in any given measurement.
Last edited by SnoochToTheBooch; 13-07-2012 at 20:28.
10. Re: Quantum superposition
(Original post by SnoochToTheBooch)
1. it might be easier (and possibly more accurate) to consider a particle to have a certain probability of being found at a given point upon making a measurement. For one thing, this is true, this is how particles behave when they are measured repeatedly, but also it means the particle can't be in all places at once because that would require it to have a probability of 1 to be found in more than one place, which isn't allowed.

2. we don't really observe superposition as it is lost during a measurement (ie. an observation), some use the term "wavefunction collapse" to describe how a repeated measurement would yield one of the possible allowed results (or, "eigenstates of the operator associated with that observable quantity) in the superposition, weighted according to the probability of observing it in any given measurement.
Interesting. I read though, that in the double slit experiment, a single photon will interfere with itself. If the particles position is a probability thing, and not a physical thing, how is the interference possible?
11. Re: Quantum superposition
(Original post by CallMeASafeBet)
Interesting. I read though, that in the double slit experiment, a single photon will interfere with itself. If the particles position is a probability thing, and not a physical thing, how is the interference possible?
well interference is a wave phenomenon isn't it? so we aren't considering any point-like particle at all there, we're dealing with a wave, a spatially/temporaly (if that's even a word, it doesn't look right somehow - I'm high right now, lol) extended disturbance. We don't get the particle behaviour until it's measured as a flash somewhere on the screen in some probabilistic manner. It stays spatially extended until that measurement. I think it's fair to say that probability only matters when dealing with a measurement.

If you were going to measure which slit it went through by examining both slits at exactly the same time (maybe by placing a wire loop behind each slit and looking for a current spike as the particle passes through or something) you wouldn't observe the particle triggering both sensors, it'd be one or the other, as the particle isn't really "at both places at once" (I don't think anyway, maybe I'm wrong but this seems reasonable to me). I think this is where the true "weirdness" that is synomymous with QM comes from, it appears to act as a particle upon measurement (ie. when interacting) but propagate as a wave before observation. It's confusing and uncomfortable because it's so unfamiliar. I don't know whether people ever truly come to understand it or just become familiar with it, assuming I've got my story straight.
Last edited by SnoochToTheBooch; 13-07-2012 at 21:00.