quantum physics

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quantum physics
« on: July 07, 2006, 10:43:23 PM »
so can anyone explain this to me.
i dont understand how a particle can be in two places at once.

?

Erasmus

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quantum physics
« Reply #1 on: July 08, 2006, 12:24:05 AM »
Firstly, no, I certainly cannot explain it.

However I can try.

The notion is that of "superposition".  You can describe the "state" of any particle by listing out its properties -- where it is, where it's going, and how it's spinning are usually the interesting features.  We can just focus on the where-it-is bit.  It can be over here, or over there.  But if we don't check, we don't know.  However, what we do know is how likely it is that when we look, we'll find it over here.  Maybe there's a 25% chance of finding it here.  If there are only two possibilities of where it can be -- here and there -- then the probability of finding it over there is 75%.

So when we describe where the particle is (before we look for it), we say it's "25% over  here and 75% over there".  That's known as a "superposition of states", the states in question being "over here" and "over there".
Why did the chicken cross the Möbius strip?

quantum physics
« Reply #2 on: July 08, 2006, 12:35:50 AM »
Erasmus:

Is that also in reference to the uncertainty principal?

or is it a totally different theory, as it is something i dont particularly understand, and would like to understand better.   I noticed your mention of quantum numbers, and i just would like to make sure
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TheEngineer

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quantum physics
« Reply #3 on: July 08, 2006, 12:41:01 AM »
Quantum mechanics is very counterintuitive and difficult to understand but I will do my best to give you an answer to your question.

Quote
i dont understand how a particle can be in two places at once.

It's not actually the particle that is in two places, but it's infinite wave functions.  I know you are thinking 'what waves?'.  

One of the basis of QM is the wave-particle duality of matter.  It has been observed through the double slit experiment that light will interfere with itself as the waves pass through each slit in a wall( board, etc).  This interference will produce bands of light and darkness on a receptive screen on the far side of the wall.  We also can see this behavior with water waves using the same set up.  When one of the slits is covered up, the light just shines normally through the open slit.  

Now, this same experiment was conducted using electrons.  We all know that electrons are particles.  So when this experiment was conducted, guess what was found-the electrons interfered with each other!  This had to mean that the electrons exibit a wave like behavior as well as particle behavior.  This all goes back to the probability of finding electrons in a particular place in an orbital.

This concept is further expanded to include ALL matter.  Even us and everything around us.  However, we don't notice this duality because the wavelength of matter waves is proportional to Planck's constant, which is extremely small.

Now that we have some background, let's address the question.  The electron/double slit experiment was then conducted using ONE ELECTRON AT A TIME.  Each electron was shot at the wall with the slits individually.  To the surprise of everyone, the interference pattern showed up again!  How could one electron interfere with itself?  Remember, electrons = probablility.

Feynman proclaimed that each electron that makes it to the screen actually goes through BOTH slits!  Even more astounding, he said that the electron actually traverses every possible path at the same time.  The electron goes straight through the left slit.  It also goes straight through the right slit, starts off going to the left then changes and goes into the right slit, and even goes around the moon and then enters the right slit.  He assigned each one of these paths numbers so that the combined average was the same as the probability of the wave function.  This is referred to as the 'sum over paths'.  The paths cancel out and leave just one remaining.

This is how a particle (which is also a wave) can be everywhere at once.  Remember when I said that even people are waves?  Well probability says that a person can walk through a wall to get to another room because he takes all paths at once.  Sooner or later, the one wave function that is not cancelled out will be the one that passes through the wall!

If all this sounds absurd, Feynman said it best: "[Quantum mechanics] describes nature as absurd from the point of view of common sense. And yet it fully agrees with experiment.  So I hope you can accept nature as She is-absurd."


"I haven't been wrong since 1961, when I thought I made a mistake."
        -- Bob Hudson

quantum physics
« Reply #4 on: July 08, 2006, 02:22:56 AM »
i should add that engineer is not liable for you knocking yourself out by running into your wall trying to appear at the other side.

quantum physics
« Reply #5 on: July 08, 2006, 02:23:41 AM »
Anyway chances are that if you did get to the point where you started to quantum tunnel through the wall you would probably stop halfway though it. that would be painful.


Also Erasmus the Superposition principe is simple that if A is a solution of the Wave Equation, and B is a solution of the Wave Equation, then A+B is a solution of the wave equation. It is merely an extension of the wave partcile duiality. (the superposition principle originates from wave theory after all).


Also a particle can never be in two places at once. On travelling a quantum partiucle will act like a wave on detection it will act like a particle. Therefore it is not possible to actually detect an electron in two places at once. You can consider it to be in two places at once in the double slit experiment, but since it is a wave, you cannot define it with a position.

quantum physics
« Reply #6 on: July 08, 2006, 07:40:45 AM »
Quote from: "TheEngineer"
Quantum mechanics is very counterintuitive and difficult to understand but I will do my best to give you an answer to your question.

Quote
i dont understand how a particle can be in two places at once.

It's not actually the particle that is in two places, but it's infinite wave functions.  I know you are thinking 'what waves?'.  

One of the basis of QM is the wave-particle duality of matter.  It has been observed through the double slit experiment that light will interfere with itself as the waves pass through each slit in a wall( board, etc).  This interference will produce bands of light and darkness on a receptive screen on the far side of the wall.  We also can see this behavior with water waves using the same set up.  When one of the slits is covered up, the light just shines normally through the open slit.  

Now, this same experiment was conducted using electrons.  We all know that electrons are particles.  So when this experiment was conducted, guess what was found-the electrons interfered with each other!  This had to mean that the electrons exibit a wave like behavior as well as particle behavior.  This all goes back to the probability of finding electrons in a particular place in an orbital.

This concept is further expanded to include ALL matter.  Even us and everything around us.  However, we don't notice this duality because the wavelength of matter waves is proportional to Planck's constant, which is extremely small.

Now that we have some background, let's address the question.  The electron/double slit experiment was then conducted using ONE ELECTRON AT A TIME.  Each electron was shot at the wall with the slits individually.  To the surprise of everyone, the interference pattern showed up again!  How could one electron interfere with itself?  Remember, electrons = probablility.

Feynman proclaimed that each electron that makes it to the screen actually goes through BOTH slits!  Even more astounding, he said that the electron actually traverses every possible path at the same time.  The electron goes straight through the left slit.  It also goes straight through the right slit, starts off going to the left then changes and goes into the right slit, and even goes around the moon and then enters the right slit.  He assigned each one of these paths numbers so that the combined average was the same as the probability of the wave function.  This is referred to as the 'sum over paths'.  The paths cancel out and leave just one remaining.

This is how a particle (which is also a wave) can be everywhere at once.  Remember when I said that even people are waves?  Well probability says that a person can walk through a wall to get to another room because he takes all paths at once.  Sooner or later, the one wave function that is not cancelled out will be the one that passes through the wall!

If all this sounds absurd, Feynman said it best: "[Quantum mechanics] describes nature as absurd from the point of view of common sense. And yet it fully agrees with experiment.  So I hope you can accept nature as She is-absurd."


Any person(idiot) should know about this experiment. http://www.whatthebleep.com/trailer/DS_sm2.wmv

quantum physics
« Reply #7 on: July 08, 2006, 07:54:41 AM »
There is a theory called coheirence that attempts to explain why we don't see quantum effects in the actual world. Since all of these experiments that produce quantum effects occur in pristine lab conditions, we never see them in the real world. In the real world particles are always bouncing off each other and being bombarded with light and other waves, yes, even empty space is filled with light and radiation. Meaning the wave function of particles will remain collapsed.

There is also an experiment using quantum effects that eilminates the notion of linear time. Meh i'll talk about it later, i'm busy.

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TheEngineer

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quantum physics
« Reply #8 on: July 08, 2006, 09:01:41 AM »
Quote
Any person(idiot) should know about this experiment.

I seriously doubt that everyone has heard of this experiment.


"I haven't been wrong since 1961, when I thought I made a mistake."
        -- Bob Hudson