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/sci/ - Science & Math

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4239137 No.4239137 [Reply] [Original]

I've been reading into quantum computing, and that led me to the strange world of quantum physics. A quantum particle can exist in multiple places at the same time - am I getting this right?

Could somebody explain it to me?

>> No.4239139

>>4239137
>Could somebody explain it to me?

no. it's one of those retarded subjects that only makes sense to the person who came up with it.

>> No.4239154

>>4239137
Particles behave like particles and waves, depending on whether or not they are being observed.

>> No.4239156

Morgan Freeman.

http://www.youtube.com/watch?v=DZGINaRUEkU

>> No.4239159

>>4239154
When it looks like a particle, it is a particle. When it looks like a wave, it is a wave.

>> No.4239178

>people actually think that quantum theory is science and not philosophy

>> No.4239187

wow, 6 posts and we're already overrun with misinformation

people don't get to understand quantum physics, our brains aren't suited to it

the best way to have a vague understanding of it is to know about the schrodinger equation and the wavefunction

when you observe something (read 'any interaction', not some faggy omg human thought changes da world shit) there is 0 possibility of it being somewhere else, so the wavefunction becomes a delta function at the place you observe it, simple enough. If you don't look at it for a while the wave function spreads out because there is now a probability of it being somewhere other than where you just observed it.
the same thing is true for momentum but in the momentum basis (you need maths for the basis part)

you can't observe both momentum and position because the operators cannot be simultaneously diagonalised (yet again, you need maths) because they don't commute

there is no 'the particle is in 2 places at once' or whatever else
there is a 'the particle could be anywhere with a finite possibility' however

the particle like behavior comes from the dirac delta functions we get when we observe the thing
the wave like behavoir comes from the spread out wave function we get when we're not observing something

>> No.4239192

>>4239187
>there is a 'the particle could be anywhere with a finite possibility' however
so a particle could be anywhere it wants to be but it only decides where it is when I look at it?

>> No.4239200

>>4239192
yes
except there are places it can't be if there's potential energies involved, it still can't disobey conservation of energy
if there's a potential wall it can tunnel through, but it cannot appear inside the potential well (when I say it I mean the wave function, the wave function can be non zero on either side of the wall)

and there's no 'want' the wave function isn't sentient

>> No.4239204

>quantum computing
the crazy thing is that the actually made this work, and you can solve shit with it, optimisation type problems, like the travelling salesman and all that, it's fucking magic

>> No.4239205
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4239205

"If you think you understand quantum mechanics, you don't understand quantum mechanics."

>> No.4239209

>>4239205
I don't know what makes Feynman greater the fact he was an intellectual god or the fact he hated authority and honors.
Sorry for the side track.

>> No.4239215

>>4239178

people actually think that quantum theory is science and not philosophy

>Not science

>Philsophy

>0/10

>> No.4239217

>>4239209

^This.

>> No.4239224 [DELETED] 

i thibnk its much more intuative to think about it in the history over paths formulation. a particle travels all posabkle ways between point A and B, but in such a way that highly improbable paths cancel out. so at some time you can say that its every ware between A and B, but not really.

>> No.4239226

>>4239217
Bongo time!
http://www.youtube.com/watch?v=HKTSaezB4p8

>> No.4239236
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4239236

People here make it seem impossible to understand, but there is no justification for that assumption.

And I totally disagree with that feynman quote where he basically impyies that.

>>4239209
well he said he didn't like it, I don't really believe him when he says he's rather not won the nobel price.

>> No.4239241

>>4239224

Are you slow? Why can't you spell everywhere correctly?

>> No.4239247

>>4239187
>>4239187
this broseph knows his shit. listen to him.

>> No.4239279

>>4239236
Go right ahead and explain your profound understanding to us without involving anything external of quantum theory itself.

You can't. Beyond the mathematics it's just interpretations.

>> No.4239282

>>4239236
I can't speak for him nor would I want to but I believe his reason for not rejecting it (as far as he tells) is that refusing it would of caused an even bigger media shitstorm.
FEYNMAN REFUSES NOBEL PRIZE! *speculation speculation*
*reporters* etc.. etc..
by the sounds of some of the interviews it looks as if something psychologically bothers him about honors but again he could be trolling you never know he's like the worlds biggest troll.

>> No.4239294

>>4239236

It's like trying to understand Planck length. Ya can't. You can have a vague notion founded on comparison, but otherwise the number 10^-35 is nonsensical to us.

>> No.4239303

>>4239294
most people also think that the plank length is something that it isn't
it's simply a unit of length that plots out once you set c=hbar=1

>> No.4239314

Ok I've taken a quantum physics class, I'll do the best I can to explain.

Suppose you have a vector in 3D space (it could be a bowling ball's position, velocity, acceleration, force, etc.). To convey the information about this vector, it must be projected onto some arbitrary axis. So first you point around and say "this way is x, this way is y, this way is z" in some arbitrary directions, and only then can you give the components of each vector and thus information.

A quantum state is a lot like a vector. To convey the information about the state you have to project it onto an axis. In QM the "axis" is an observable (anything you can measure, such as position, momentum, energy, spin, etc.), and the components along each "direction" are the probabilities of making certain measurements. For example suppose you knew an electron could only be in four positions, x1, x2, x3, and x4. You could "project" the state onto the "position" observable and get four probabilities, such as 1/4, 0, 1/2, and 1/4. But you could also project the state onto the energy axis. It could be the case that there are only three possible energies, with probabilities 1/3, 2/3, and 0 for example. I'm making up what the possible measurements are just to demonstrate, a real quantum system might have an infinite number of energies or a continuum of positions, but the concept is the same.

>> No.4239317

>>4239303

That is incorrect.

>> No.4239321

>>4239314

>Taken one quantum physics class

>Theoretical physicist

>> No.4239322

So what is a measurement, exactly? A measurement means you are forcing the state to choose a state. So in the example above if you measured energy you might get E1, you would probably get E2, but you certainly wouldn't get E3. Suppose you get E1. Now the state projected onto energy looks like 1, 0, 0. This would also change the position projection. How it actually changes depends on the system, and you can figure it all out with schrodinger's equation.

You're probably thinking "how do we know it was actually in a superposition of probabilities? Maybe it was in E1 the whole time?" There are ways of telling. Maybe you've heard of the double slit experiment. If you fire a single electron through a double slit, it will interfere with itself. After firing a large number of isolated electrons you see an interference pattern. This shouldn't be too shocking. Electrons are a probability wave in position projection. This wave goes through both slits, and like water waves, interferes with itself. Then at the detector we "measure" position, forcing it to choose a position. (I hate the word "measure" since it implies the particle was there before the measurement, which is not the case. That is why I prefer the word "collapse." But the physics is all the same regardless).

>> No.4239336

>>4239317
hbar=G=c=1

I honestly can't remember which combination it is and it really doesn't matter, the plank units are nothing more than the system of units that set certain physical constants equal to 1

>> No.4239337 [DELETED] 

>>4239317
no he is correct, the plank length is of absolutely no physical significance, its just a unit of length that doesn't need a conversion factor.

>> No.4239342

>>4239337
aw wow, that's the second time you've backed me up today EK

>> No.4239355

>>4239337
>>4239342

Are you arguing it's arbitrarily set? I don't understand.

http://en.wikipedia.org/wiki/Plank_length

>> No.4239382 [DELETED] 
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4239382

>>4239342
as long as you are correct, ill continue to back you up.

>> No.4239401

bump for quantum computing reading materials

>> No.4239408

>that feel when you have a masters in CS&E and wish you would have gone for physics

Well, at least there's the slight possibility that I make it big on my own, allowing me to study physics instead of slacking off..

>> No.4239419

>>4239204
>like the travelling salesman and all that
Travelling salesman is NPC, and without knowing jack shit about quantum computing, I can fairly safely tell you that they don't solve problems in NPC. That would imply that a quantum computer can take an arbitrary set of choices and make the right one every time.

>> No.4239422

>>4239187
>>4239187
>>4239187
>>4239187
>>4239187

The best quick explanation of anything I have ever read on /sci/. Congrats bro. OP, read this shit.

>> No.4239454

>>4239355
I'm not arguing, I'm stating

the plank units are the units you get when you set certain physical constants equal to 1 you (or the guy that said I was incorrect) seems to think that this is not the case

>> No.4239509

Contrary to popular belief Quantum Mechanics is actually not that strange at all when you think about it the right way.

Often people word the problems in a way that is almost correct but makes QM seem more mysterious and complicated than it actually is.

The correct way to view it is that the particle is actually in only a single definite state described by the wavefunction.
It is NOT in multiple states at once, and there is nothing uncertain about it. The wavefunction is actually completely DETERMINISTIC and evolves in a predictable way according to the Schrodinger equation.

The ONLY aspect of QM that has indeterminism happens when a measurement is made.

It's like a vector at a 45deg angle.
Is it correct to say that this vector is both horizontal and vertical at the same time?
Well it can be described by a combination of horizontal and vertical vectors right? So if we use the same popular logic most people apply to quantum mechanics (like schrodinger's cat for example) they would have to conclude that a vector at 45deg is both horizontal and vertical at the same time.
The reason that this seems to be confusing is because IT DOESN'T MAKE SENSE!
Guess what, it doesn't make sense in quantum mechanics either, and is just downright incorrect.

The vecotor is neither horizontal nor vertical (in general), it is something else entirely.

The wavefunction is the same way. Take Schrodinger's cat for example. Everyone likes to say it is alive and dead at the same time, but this is again bad language.
Like our 45deg angle vector, it is actually in a SINGLE state that can be described by a combination of alive and dead vectors but it is something else entirely.
So the correct wording is actually to say that Schrodinger's cat is NEITHER alive nor dead, it is simply in a state which can be DESCRIBED by a superposition of both.

>> No.4239540

I'd like to know more about this "wave." In two dimensions we can imagine a sinoid, in three we can imagine something like the surface of the sea. But neither of these would explain where something might be in a room. Am I missing something or is this wave hyperdimensional?

>> No.4239578

>>4239540
It's hard to say. The full wavefunction describes the entire state of the particle (position, momentum, energy, spin, etc..).
So yes, it is multidimensional. In fact it exists in an infinite dimensional space.

However, we can get some idea of what it looks like by projecting it into configuration space.
http://farm4.static.flickr.com/3418/3362309031_5daf2e001d.jpg

on the left is the wavefunction projected into configuration space, and on the right is the probability of finding the particle at a point when measuring its position.

>> No.4239590

Quantum computers run on something known as qubits. While normal bits can only be I or O, a qubit can be either I, or O, or both at the same time, which is called a superposition. Two bits can represent one of four different states, while a qubyte (4 qubits) can represent 256 states at the SAME time. The capability of quantum computing is massive.

However! There is a major problem with quantum computing, and one that will likely limit its use forever. As soon as you try to measure the quantum bits, the wavefunction collapses and destroys the superposition, as well as almost all the information it was carrying, and because of the very nature of quantum computers, this information cannot be stored in memory like a classical computer can. This means quantum computers will likely never replace classical computing; they will only have very specific applications like code-breaking.

Quantum computing has successfully been tested, successfully factoring the number 15. While this is not impressive in itself, the important thing is that it proves quantum computing is possible.

Hope this all made sense.

>> No.4239630

The founders of quantum mechanics debated the role of the observer, and of them, Wolfgang Pauli and Werner Heisenberg believed that it was the observer that produced collapse