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#1 2007-06-05 04:59:45

Laterally Speaking
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Basic Quantum Physics

I told somebody (I can't remember who) that I'd post some stuff on quantum physics here.

An odd example of the odd world of quantum physics is the follow:

If you take a lamp, a piece of thick cardboard with TWO parallel slits cut in it, and a screen, and put them in that order, lined up, you would expect to see TWO lines on the screen corresponding to the slits in the shader, right? WRONG. What you get is a venetian blind pattern, with strips of light, shadow, and medium lighting alternating. Why?

The explanation lies with the wavelike characteristics of light. The light waves going through the two slits interfere with each other, creating the pattern.

This is where it gets weird:
If you use a light source that emits one photon at a time, one photon per minute (that's incredibly dim), you would expect that you wouldn't get the same pattern, because there's nothing to interfere with the one photon per minute, right? WRONG. You get the same pattern. Why?

Some may know the Uncertainty Principle. The most common example is this: if you put a cat and a fragile vial of cyanide in a box, close the box do not observe the inside in ANY WAY, you will have no way of knowing whether the cat is dead or alive. The way the physicists explain what happens is that the cat is, until you observe the actual state of it, BOTH DEAD AND ALIVE at the same time, or that the cat enters two parallel universes, one in which it's dead, the other alive. (Note that this example wasn't originally supposed to be one, apparently).

Back to the double-slit experiment.
The theory proposed is that the one photon per minute, since it's going UNOBSERVABLY
fast (the speed of light), it's going through BOTH slits at the same time, thus interfering with itself, creating the interference pattern. This is just the start of the weirdness of quantum physics.


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#2 2007-06-05 05:06:28

Laterally Speaking
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Re: Basic Quantum Physics

I'm not sure as to the details of this part, but I think it's something like this:

If you do the experiment while using equipment to record the behavior of the particles, you don't even actually see the same pattern, while if you record the information and delete it immediately, you do see the pattern. If you try to record the wave behavior of light, you get a different pattern, and if you delete that information instantly, you get the venetian blind pattern.

The conclusion that scientists have made is the following: Information in the present can influence perception in the past.

This is why most people struggle with the subject of quantum physics... It doesn't follow the rules that we can observe at the non-quantum level (the visible level).


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#3 2007-06-05 05:39:33

Ricky
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Re: Basic Quantum Physics

If you take a lamp...

No, it has to be a monochromatic light.

a piece of thick cardboard with TWO parallel slits cut in it

For anyone who is thinking of trying this at home, these slits must be very small, precise, and a short distance apart.  Furthermore, there must be a way to allow no other light sources from entering these slits.

Some may know the Uncertainty Principle. The most common example is this: if you put a cat and a fragile vial of cyanide in a box, close the box do not observe the inside in ANY WAY, you will have no way of knowing whether the cat is dead or alive. The way the physicists explain what happens is that the cat is, until you observe the actual state of it, BOTH DEAD AND ALIVE at the same time, or that the cat enters two parallel universes, one in which it's dead, the other alive. (Note that this example wasn't originally supposed to be one, apparently).

Where to start?  It's not the uncertainty principle.  It's simply the wavelike properties of a wave.  They have nothing to do with each other to my knowledge.  But as for the Uncertainty Principle, it is simply that you can't know velocity and momentum of an electron at one time.  The more you know of one, the less you know of the other.  This is partially because the only way we know how to measure electrons is by bombarding them with other electrons.  But I am also told that it is not just an effect of how we measure, that we can never know both no matter what measurement technique we use.  I don't fully believe this, but I take it a bit on faith since quantum physistics typically know what they are talking about.

But what you speak of above is known as Schroedinger's cat.  The way it works is as follow.  There is vial of cyanide, and a release mechanism which responds to a quantum even such as the radioactive decay of an isotope.  We have no way to predict when exactly there will be a radioactive decay thanks to the uncertainty principle.  And so since we can't tell which state the isotope is in, we can't tell what state the vial is in, and we can't tell what state the cat was in.  Schroedinger used this as an example of how ludicrous quantum physics is.  With much egg on his face, it became a standard example of how non-intuitive quantum physics is.

BOTH DEAD AND ALIVE at the same time, or that the cat enters two parallel universes, one in which it's dead, the other alive.

Both of these misconceptions are the result of quantum physicists and their like for science fiction.  They are simple "what if..." scenarios and not meant to be taken seriously.  We know that quantum properties don't apply to macroscopic matter, such as a cat.

If you do the experiment while using equipment to record the behavior of the particles, you don't even actually see the same pattern, while if you record the information and delete it immediately, you do see the pattern. If you try to record the wave behavior of light, you get a different pattern, and if you delete that information instantly, you get the venetian blind pattern.

I've never heard of anything like this, do you have a source for it?


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#4 2007-06-05 05:55:41

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Re: Basic Quantum Physics

But as for the Uncertainty Principle, it is simply that you can't know velocity and momentum of an electron at one time.  The more you know of one, the less you know of the other.  This is partially because the only way we know how to measure electrons is by bombarding them with other electrons.  But I am also told that it is not just an effect of how we measure, that we can never know both no matter what measurement technique we use.  I don't fully believe this, but I take it a bit on faith since quantum physistics typically know what they are talking about.

I read up a bit more on this and I think I have a better understanding now.  We use photons (not electrons... sheesh) to measure the position of electrons, bombarding them like a blind mind would throw tennis balls to see where a car is.  If it travels in the expect path and hits the ground... no car there.  If it bounces off of something, then we found it (assuming nothing else is around).

But we aren't throwing tennis balls at cars.  Its more like throwing cars at cars.  When we hit a car with another car, we move the car we were looking for.  The faster we throw the car, the more we move it.  So in comparison, the higher energy (the more photos) that we use to find an electron, the more they move it.  Because of quantum physics, each energy level is discrete and there is a lowest energy level for photos... one at a time.  We can't do 1/2 a photon.  But we can still decrease the energy.

But Planck's law allows us to do something tricky.  Planck's law says that the energy of a single photon is proportional to the frequency.  So by lowering the frequency, we are moving the electron less and less and we get a better idea of it's velocity.  But the catch is that our margin of error for determining the electrons position is inversely proportional to the frequency.  So as we use less and less energy, we get worse and worse readings for where the electron is.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#5 2007-06-05 07:40:55

MathsIsFun
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Re: Basic Quantum Physics

Ricky wrote:

... so by lowering the frequency, we are moving the electron less and less and we get a better idea of it's velocity.  But the catch is that our margin of error for determining the electrons position is inversely proportional to the frequency.  So as we use less and less energy, we get worse and worse readings for where the electron is.

And this tells me that the method has problems (and I would look for another method), but physicists say that the whole world has this problem and call it the Uncertainty Principle.


"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman
 

#6 2007-06-05 13:19:06

George,Y
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Re: Basic Quantum Physics

I don't understand the one photon at one time experiment. Really weird. The Photon splits into two parts of itself before it sees two slits?? Really weird.

But can anyone explain why it won't work when two slits are wide or distant from each other?


X'(y-Xβ)=0
 

#7 2007-06-05 14:08:11

Ricky
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Re: Basic Quantum Physics

I don't understand the one photon at one time experiment. Really weird. The Photon splits into two parts of itself before it sees two slits?? Really weird.

No body really understands, even quantum physicists.  Sure, we can represent it mathematically and make incredibly accurate predictions, as well as analogies as to why, but to say what is really going on is beyond us.  It's simply because the quantum world is nothing like our macrsopic world that we can't even begin to picture how things work.

For example, Feynman came up with an alternate model.  He said imagine that a photon takes every possible path imaginable.  Then allow certain pairs of paths to cancel each other out.  What you will be left with is one path, and you only know with a percentage of chance as to which path this will be, creating the interference pattern we see.  And this theory describes all observed evidence just as well as wave functions.  They are two completely different ideas which agree with all the evidence.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#8 2007-06-05 14:11:13

Ricky
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Re: Basic Quantum Physics

But can anyone explain why it won't work when two slits are wide or distant from each other?

Perhaps I was wrong on this point.  I can't seem to find any source saying the limit of the distance of the slits.  But I would imagine once the slits are far enough apart that the effect would be non-existent.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#9 2007-06-05 14:39:56

George,Y
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Re: Basic Quantum Physics

Err, I like the cancelling out theory.


X'(y-Xβ)=0
 

#10 2007-06-06 01:27:09

Laterally Speaking
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Re: Basic Quantum Physics

Like you said, Ricky, no-one really gets this, not even those who claim to be specialists in the matter. (no pun intended)


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#11 2007-06-06 01:39:45

Laterally Speaking
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Re: Basic Quantum Physics

The main thing you've gotta understand is that we can't be absolutely sure of anything.

Here's something to think about: how many variables lead to any given instant in history?

The answer: the number of particles in the universe multiplied by the time of existence of the universe before that instant. Assuming that time can be divided into infinitely small increments, and that particles may have that same characteristic (it probably doesn't, as far as we can tell), then there are an infinite amount of variables. This may seem exaggerated, but look at it this way: if 10000 years before our observed point, there was some tiny abnormality in one of the testicles of the ancestor of the person who is now observed, the descendant of that ancestor may not be exactly the same as without this tiny change, thus causing this person's descendants to be slightly more different each generation, the end result being that the person who does the event that we're observing doesn't do it quite the same, if at all.

Last edited by Laterally Speaking (2007-06-06 02:03:05)


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#12 2007-06-06 02:01:54

Laterally Speaking
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Re: Basic Quantum Physics

Sorry, that's not really quantum physics, but it's not the clearest thing to see either. If I'm not mistaken, that's Chaos Theory, which is loosely related to the quantum world, though slightly easier to understand.


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#13 2007-06-06 02:35:08

Ricky
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Re: Basic Quantum Physics

Here's something to think about: how many variables lead to any given instant in history?

The answer: the number of particles in the universe multiplied by the time of existence of the universe before that instant.

That's classical physics thinking.  On the another hand, quantum physics says you can take two exact (and I mean exact) same systems and end up with entirely different results, since the results are based not only upon what's in the system, but chance as well.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#14 2007-06-06 08:23:24

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Re: Basic Quantum Physics

And I would argue that without chance the universe is fully predictable, and so would (in a sense) be static.


"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman
 

#15 2007-06-06 09:15:17

Ricky
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Re: Basic Quantum Physics

MathsIsFun wrote:

And I would argue that without chance the universe is fully predictable, and so would (in a sense) be static.

Predicable and static are two completely different things.  Static implies that nothing changes, predictable just means that you know what's going to change and how it's going to change.

When we get to the macroscopic world, all the probabilities (sort of) cancel each other out and things become very predictable.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#16 2007-06-06 12:03:58

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Re: Basic Quantum Physics

I know it is a funny thing to say, and I can't think of the right way of saying it, but if you treat time as (say) the words in a book, then the book is a static thing, even though you can dynamically read it.

So a "no-chance" universe would be predictable form end-to-end and would be no better than an already-written book, and hence "static".

If you get my drift maybe you could word it better smile


"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman
 

#17 2007-06-06 12:06:15

Ricky
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Re: Basic Quantum Physics

By static, are you trying to say that some jerk flipped to the end of the book and told you how it ends?


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#18 2007-06-06 21:50:48

Laterally Speaking
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Re: Basic Quantum Physics

I think I should have named the thread something like "Debates about stuff nobody really understands"...

I think what he means is that, if everything is predictable, it doesn't really change in the sense that your predictions are based upon something that doesn't change and you know this, otherwise, there's always a chance that your predictions might be off.

Sorry if this made things any more complicated than before, which is pretty hard.


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

#19 2007-06-06 22:15:32

Identity
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Re: Basic Quantum Physics

When you are firing the photon at the screen, doesn't it matter in which direction you fire it? Or are you talking about the case where the photon is fired directly at the space between the two slits?

 

#20 2007-06-06 23:17:19

JaneFairfax
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Re: Basic Quantum Physics

Ricky wrote:

By static, are you trying to say that some jerk flipped to the end of the book and told you how it ends?

I think a better comparison would be to get inside the author's mind and “know” the story for yourself. tongue


Q: Who wrote the novels Mrs Dalloway and To the Lighthouse?

A: Click here for answer.
 

#21 2007-06-07 00:46:23

Ricky
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Re: Basic Quantum Physics

JaneFairfax wrote:

Ricky wrote:

By static, are you trying to say that some jerk flipped to the end of the book and told you how it ends?

I think a better comparison would be to get inside the author's mind and “know” the story for yourself. tongue

That's ludicrous Jane.  How would you fit?  I mean, maybe if you're a midget and the author has a really really big head, just maybe.  But otherwise, that's crazy. tongue

When you are firing the photon at the screen, doesn't it matter in which direction you fire it? Or are you talking about the case where the photon is fired directly at the space between the two slits?

I don't believe photon firing is an exact science (heh), and we don't have precise control over what direction the photon goes in.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#22 2007-06-07 01:42:06

John E. Franklin
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Re: Basic Quantum Physics

Richard P. Feynman used to go out to a bar when he got a chance to leave Los Alamos for an evening and he would write Physics on napkins in the bar.  He also drummed for fun.  He also could compute answers faster than calculators of the day!


igloo myrtilles fourmis
 

#23 2007-06-07 02:58:22

Ricky
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Re: Basic Quantum Physics

He also could compute answers faster than calculators of the day!

I doubt that.


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."
 

#24 2007-06-14 04:46:27

Laterally Speaking
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Re: Basic Quantum Physics

Well, actually, the exact phrasing would be " he could calculate stuff faster than people could enter it into a calculator at the time".


"Knowledge is directly proportional to the amount of equipment ruined."
"This woman painted a picture of me; she was clearly a psychopath"
 

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