quantum physics vid.

[gen2teggy]

http://video.google.com/videoplay?docid=-4237751840526284618&q=quantum

[snowboard]

thats intresting.

[handsomebassman]

very cool video!:D

[djayz]

whoa
the first part made sense where there was interference with waves and what not

but then with the marbles and the observer wth went on?!

[Lo)2enz0]

good old physics. the only part i don't get was the observation part at the end. electrons are just particals, matter, no intellegence so how could they preform how they did when they were being watched. this baffels me and wouldn't mind a runner up video to explain, unless someone could.

[Unknown303]

I think thats the whole thing behind that vid is that the particles act different under different conditions.

[Lo)2enz0]

ya but whats the difference when you shoot electrons in a forward direction and you have something off to the side. how would this effect the electrons?

[legendboy]

Neato, i love physics.

if you guys want to watch a really good movie watch this

http://www.pbs.org/wgbh/nova/elegant/program.html

its about 2 hours long tho

[Lo)2enz0]

sweet something to do today.....

[tulit]

Alright I'll try explaining this.

You have to understand on the quantum level, you can never know all the properties of a particle, simply a probability distribution of what a property is.

In the case of the double split experiment, we want to know its position (and hence then be able to determine the pattern you see on the screen).

When you fire a photon, its exact position where it ends up is unknown. There is always a small amount of true randomness in the universe. Hence, why in the normal single slit experiment, firing a bunch of photons at the same slit (with the same initial conditions) creates a long line on the screen rather than a single dot (i.e. there is some variation)

Also observation is a bad term. It should really be any interaction with the universe.

In the case of this experiment, what is happening is that you aren't really firing a single particle proton (like the balls they had in the first half) but rather whats known as a wavefunction. A wavefunction in its simplicity is just a probability distribution curve (which you can treat as a normal wave). It says that the probabiltiy of the particle existing at position X is a, position Y is b, position Z is c and so on.

In the first experiment, we fire these wave functions at the double slits, the wave function splits and they interfere with each other on the other side just like normal waves. This creates highs and lows in the probability distributions. When the system is observed (in this case, proton hitting the screen), we say that the wave function collapses and the particle must then choose which point on the screen it will be based on the current probability distribution. Fire a whole bunch of these, and you start to see how the original interference pattern is generated.

Now, by measuring which slit the particle goes through at the double slits, you are effectively collapsing the wave function much earlier. The particle is set to "choose" one of the of two slits at this point with 100% certainty. It either goes through the left one or the right one. Hence, there is no wave passing through both slits and therefore can be no interference on the other side of the slits (since something only went through one of them). What you end up with is the two single lines.

Its really hard to get your head around the first time.

[gen2teggy]

i dont get physics.

[A790]

That was neat. INteresting how it works out- I wonder who that video is aimed at.

[Khyron]

A simplier concept is that the smaller something gets, the more it gets interferred with by simply being measured or observed. Just you watching something involves light bouncing off the subject and back into your eye. Staring at a car or a model - this has no concequence because it's big, but on something really small, that light can actually alter what it would have done if you hadn't interferred with it in the first place.

If you like this sort of stuff, check out the string theory series on PBS, I think it's downloadable. I think I got lost on the last episode.

Khyron

[RX-7_TWINTURBO]

Originally posted by A790
That was neat. INteresting how it works out- I wonder who that video is aimed at.
It was from the movie "What the #$*! Do We (K)now!? " very interesting movie on quantum physics.. its a little dry in places but still interesting none the less.

[tulit]

Originally posted by Khyron
A simplier concept is that the smaller something gets, the more it gets interferred with by simply being measured or observed. Just you watching something involves light bouncing off the subject and back into your eye. Staring at a car or a model - this has no concequence because it's big, but on something really small, that light can actually alter what it would have done if you hadn't interferred with it in the first place.

If you like this sort of stuff, check out the string theory series on PBS, I think it's downloadable. I think I got lost on the last episode.

Khyron

That doesn't make any sense. By you looking at something, your eye isn't emitting light and receiving what bounced off of what you are looking at. Your eye simply receives light (from some other source, the sun, a lightbulb, etc) that is bouncing off of the object regardless if you are looking at it or not.

The explanation I gave is the way it is (or at least to human's best understanding so far). At the quantum level all particles behave like a wave until their position is precisely known, at which point they behave as a particle.

[Drew]

^^^ To add a little more clarity if someone doesn't understand the "wavefuntion" part, in math terms its an equation that carries with it all the information relative to the particle, what it is doing at what time. And you cannot measure it because the second you try to, you change the wavefunction. And the probability part is simply saying we never actually know where the particle is, we only know the most likely place it is going to be found. BUT if we try and find it, then we just screwed with it and altered its path. Think about it like this, were playing with the smallest things out there so how do we measure it without affecting it.....


good explanation tulit, was just maybe clearing up some terms for people. However what do I know, I thought the damn plane would stay on the ground...hahaha DAMN YOU SLIPPING!!!

[Khyron]

Originally posted by tulit
That doesn't make any sense. By you looking at something, your eye isn't emitting light and receiving what bounced off of what you are looking at. Your eye simply receives light (from some other source, the sun, a lightbulb, etc) that is bouncing off of the object regardless if you are looking at it or not.


I over-simplified. I was referring to the observer effect, which is what I got from the video in explaining the last bit. The act of observing or measuring interferes with the thing you are observing in the first place. Obviously you can't just look at an electron, but again, trying to keep it simple. Been more than a few years since physics though.

Khyron

[tulit]

Originally posted by Khyron


I over-simplified. I was referring to the observer effect, which is what I got from the video in explaining the last bit. The act of observing or measuring interferes with the thing you are observing in the first place. Obviously you can't just look at an electron, but again, trying to keep it simple. Been more than a few years since physics though.

Khyron

The term "observing" is really misleading. Its more along the lines of a interaction with anything in the universe. Any matters position is not known until you measure it. Before measuring, we can only infer probabilities of where its located (i.e. its most likely to be here, here or here, but we don't know for sure where precisely).

Any interaction with ANYTHING that forces it to be in just one position causes a collapse of the wave function (which represents the probability), and hence it becomes (and acts) as a particle, in which it can and does only exist at one position. The key-- Observing doesn't create some mystical force that forces the particle to go one way or the other.

Take for example the double slit example. Pretend that we put a camera on one slit, but the camera can't see the other slit. Infact, we'll put the slits so far apart (opposite sides of the universe) so that what happens at one slit can't possibly have any net effect on the other. Now, we fire a proton at the slits. On the first firing, the proton passes the slit with our camera, and we see it --- great we know which slit it went through with certainty. However, on the next firing, the proton travels through the other slit. Even though the camera didn't see the proton and had absolutely no interaction with it, we still know with 100% certainty which slit the proton passed through. Its absence at our camera'd slit tells us it must be existing at the other slit, hence the wavefunction collapses and we end up again with two solid strips on the screen, not the interference pattern.

I know this is really hard to grasp since it goes against what we normally see in daily life.

Theoretical physics is based on finding explanations of things we observe. This explanation just happens to fit these observations (and incidently, any other experiment scientists have come up with). No one has devised a experiment that shows otherwise, hence why this is the current THEORY. No one understands what precisely causes the phenomenom, just that this appears to be what is happening.

[Goblin]

Neat video, reminds me of physics in school :rolleyes:

[iceburns288]

Schrodinger's Cat pwns you. That is all.

[Phy]

Thanks for the link to the Nova program, legendboy, I've been wanting to see that for years.

[TimG]

Originally posted by tulit


That doesn't make any sense. By you looking at something, your eye isn't emitting light and receiving what bounced off of what you are looking at. Your eye simply receives light (from some other source, the sun, a lightbulb, etc) that is bouncing off of the object regardless if you are looking at it or not.

The explanation I gave is the way it is (or at least to human's best understanding so far). At the quantum level all particles behave like a wave until their position is precisely known, at which point they behave as a particle.

I think what Khyron was meaning to say is that when the dimension of your probe approaches the dimesions of the sample that you want to probe quantum effects become more and more apparent.

Here is another somewhat relevant analogy:

you have a road that passes through a tunnel under a mountain. Rumour has it that the tunnel might have collapsed and you need to find out. For some odd reason, the only way that you can check is be sending a car speeding down the road and wait at the tunnel opening and listen. If you don't hear a crash, then you know that the tunnel is clear, but if you do hear the crash, you'll know that the tunnel is blocked.

Before you send the cars down the road, there is a superposition of states of the tunnel - blocked and unblocked. After the experiment, the tunnel is known to be either blocked or unblocked.

Tulit, in a deterministic situation, yes, an observation forces the system into an eigenmode. If, however, you have a non-deterministic case and apply Everett's Many-Worlds forumulation, you end up with a linear combination of eigenmodes as your observable.

I almost miss taking quantum theory courses :p