[shahar2k]

or perhaps anything about it?
it's come up in several discussions here, and I'm not sure I, or anyone talking about truly knows what it's about.
so I'd highly apreciate anyone trying to explain it here, but please, if you aren't sure it's true, please state that you aren't sure.

[Nex]

the basics of quantum theory are best explained with a small thought experiment.

I guess you know basic physics so here we go:



we have the experimental setup of an interferometer. This basically means that you have a lightsource, 2 detectors and 4 mirrors. 2 of them two way mirrors. They are ideally 50% transparent, 50% reflective.

If you send a lightbeam (Amplitude=1) through this setup it will split up at the first mirror and join again at the 4th. on one side of this setup (lets say detector 1) the lightwaves will extinct each other and have an amplitude of 0 (thats the setup of an interferometer) and on the other detector (detector 1) the two waves will interfere and add up to amplitude=1 again.

Okay this is not spectacular or very special at all.. its a setup use to measure very small sizes (by moving one of the mirrors and recording the change of the amplitude on detector 1.)

But now we go one step further.
Instead of sending a beam of light through this setup we send a single quantum particle, a photon from the lightsource.
Same setup, just a single particle instead of a beam. And now something very strange happens. All particles still go to detector 2 even though they have no other particles to interfere with. Strange no?

They seem to know what the path before them looks like and they react according to how a beam would interfere etc. in that path of light. Even though its only a SINGLE Particle!!!

Generally the wave we are talking about now is called a probability wave. It tells us how probable it is that a particle is at a certain point of the wave. in our example the probability of a particle being in detector 1 is 0%.

Thats all I can explain so far.
This stuff is very hard to imagine and to explain and for me its even harder to explain first because its a long time ago that I learned it and second because I have to translate it to english.

I hope this gets your attention and stirs your curiousity to research further into that area.. its really really interresting.



[This message has been edited by Nex (edited November 17, 2000).]

[Nex]

oops .. double post.

If you have any additional Q's feel free to ask.. however I cannot guarantee that I know the Answer

[This message has been edited by Nex (edited November 17, 2000).]

[Ragnarok]

I read a book about this subject some months ago, but I just can't remember the details.
I remember the cat experiment: You leave in a sealed box a cat and some arsenic (As, in case it's spelled wrong). When the scientist comes back after an hour he can found the cat dead or alive, but until he actually sees inside of the box the cat is semi-alive, constantly going from almost-dead to almost-alive.
What quantum physics added to the physics is the importance of the observator.

[Ragnarok]

Shit, doble posting, sorry.
And I will look for something related with this topic, it's very interesting...

[This message has been edited by Ragnarok (edited November 17, 2000).]

[shahar2k]

two double posts... on a a quantom theory post.... weird... coincidence? probably

anyways I think I understand why it is so disputed, I still don't understand the first example (how would the wave cancel out in one direction and stay in another) but I think I should start reading up on this

byt the way, Ragnarock, I believe the cat in the box is the "uncertainty theory" it says that you can either tell the speed or the location of a particle with 100% accuracy, but not both (if you measure speed, it's allready moved, if you measuer position, you don't know the speed it was moving there

[Ragnarok]

"We can't see any subatomic happening without modifying it."
The cat experiment is from Schr�dinger. In reallity I was wrong. The experiment is as follows:
There is a cat in a sealed room where there is a closed glass with prusic acid and hanging from a rope there is a hammer which falls whenever a Geiger counter on the next room detects any radiation. We put a stuff that is radioactive next to the counter, and we now that there is 50% possibilities the stuff "drops" a radiation quantum. When the scientist comes after an hour, is the cat alive or dead? It's state is defined by a wave probability so the cat won't die or survive until the scientist looks at it.
Some quantum physics go further and say that it depends of what the scientist expects.
There is another interpretation called "the interpretation of the many worlds" by Everett, Wheeler and Graham. When the scientist goes to the room the worlds divides by two, so there's one where the cat is alive and on the other the cat is dead. This implies that every subatomic movement makes two worlds.

Bell's theorem says that every subatomic particles that have been in contact once keep affecting each other whatever distance there is between them. This is called the "cosmic glue" (funny name ) but this implies that information can travel faster than light and this goes against relativity theory.

Physics Nobel Niels Bohr said: "Those who haven't been impressed by quantum theories when they have first meet them, it's impossible they have understand it."

-Ragnarok

PS: I've been translating from spanish and sure there are some mistakes.

[Nex]

*whistles X-Files melody*
they KNOW about us, and THEY were the cause for our double posts!!

The thing with the waves is like this:

If a wave with
Frequency f
wavelength l
Amplitude a/2
phase offset = 0 (sine wave)

interferes with a wave with
Frequency f
wavelength l
Amplitude a/2
phase offset = pi

then the resulting wave has:
Frequency f
wavelength l
Amplitude a/2 - a/2 = 0
phase offset = -

so the waves extinct each other when they meet (this phenomenon can be heard when you have your stereo boxes far from each other in your room and you walk around listening closely.. at one point you will hear a change of sound, thats the extinction of two identical waves (the sound waves) with a different phase.

I will draw you some diagrams when I have time (maybe tomorrow if i survive my bands gig tonight )


[This message has been edited by Nex (edited November 18, 2000).]

[shahar2k]

heh don't worry about translation, I speak with enough people who barely spell a word of english to understand no matter how distorted it is, and you are pretty good compared to some of my other friends

anyways, that split worlds thing is what nex(and I) were refering to in the infinite possibilities

I've heard of that "cosmic glue" thing, it's one of the things they plan on using for quantom computing

heh moving on, you heard of string theory?

[Ragnarok]

Yep, but I don't remember it now. I'm sure you could post some interesting stuff so we keep debating a year or so

-Ragnarok

[Catfish]

Ohh goody - quantum theory again.
The way I look at the cat thing is:

Teeny tiny particles (electrons & smaller) exist in a fuzzy state - their wave property.
The exact location of the particle is not determined until an observer 'looks' at it. A few people think an observer has to be human, and so infer lots of strange, weird & wonderful things from that, like humans have to exist for the universe to work....anyway.
An observer is anything that will transfer information from that particle to the rest of the universe.
If a photon hits it - it has interacted with our particle, and rebounds, carrying information about the particle. In its simplest form, we could technically look at a photons position & velocity, and trace backwards to find where our particle is. The photon has then transfered information from the particle to us.

But a photon is a very small thing - there is no guarantee that once it has interacted with our quantum particle it will transfer meaningful information to the rest of the universe. Hence our particle is still in a pretty fuzzy state. Not quite as fuzzy as before, but still pretty damn uncertain as to where it is.
However, scale our particle up to cat size, & suddenly there are millions of photons hitting it. The giant particle is sending out information all over the place, and there is an extremely high possibility that that info will be transfered. We can be virtually certain that that particle is exactly where we think it is - the particle loses its wave properties & becomes more like a particle, & is distinctly unfuzzy.

Sooo - obviously, Schrodingers cat is a thought experiment. Quantum physics only has these sort of effects on small particles, that aren't going to interact with the universe a great deal....

Ummmm - thats quite enough thought for now
Catfish

[This message has been edited by Catfish (edited November 22, 2000).]

[Chris]

quote

Quote:

When the scientist goes to the room the worlds divides by two, so there's one where the cat is alive and on the other the cat is dead. This implies that every subatomic movement makes two worlds.

That sounds like 'denial' to me heh

I don't get what you are all saying. Maybe I am reading wrong, so something doesn't happen unless someone sees it happens? Isn't that false analogy? Cause it happens weather someone sees it done or not. But I don't know, just curious.

[Nex]

Chris:

Schr�dinger (he was from my country btw hence the "-er" ending and the "�" in his name ) said once that he would have been happy never to have discover Quantum theory after he found what derived from it..

The thing with Schr�dingers cat was as follows:

Put a cat into a black box. "Noo!!" leave your kitty alone, its just a thought experiment.
So IMAGINE you put a cat into a black box(soundproof, non transparent). In addition to the cat you put a small device in there.
The device consists of a Radioactive particle that has a probability of 50% to collapse (or whatever you call it in english) after a defined time .. lets say 10 Minutes. Additionally the device contains a vial with poisonous gas that is released if the radioactive particle collapses and kills the cat instantly.
So put all thin into a box, then wait 10 Minutes. Now, is the cat alive or dead?

the answer: she is 100% alive and.. AND 100% dead.

That is because the paricle can have 2 different states at exactly the same time. it is both collapsed and not collapsed because the probability allows both.

Whis state the particle actually "chooses" depends on if it is observed. As long as it is not observed it has all of the possible states at the same time.

The problem in this thought experiment is that the cat actually would be an observer and so the experiment could not function this way. It only works with particles the size of quantum particles.

It is however a common way to explain the probability wave of quantum particles so I brought it here.

The theory of schr�dinger(s cat) was later proved (not with a cat but with Na-Atoms) in experiment.

[Chris]

quote

Quote:

The theory of schr�dinger(s cat) was later proved (not with a cat but with Na-Atoms) in experiment.

Where is the results of this documented..I sort of understand, but this is mind blowing.

So the probability of quantum particles depends on the observation brought upon those particles, and if no observation is on these particles then the probability is in both states (in the cat example) both dead and alive. Correct?

What if it was 99% probability that it would callapse. Would it still be both states depending on no observation was on the particles?

[Chris]

Just got done reading about quantum computers. This is some really cool stuff.

[shahar2k]

thing is, from what I understand here, quantum theory isn't really a theory, it's more like an admission of ignorance...
it's like saying "I don't know where my car is unless I can see it in the parking lot, unless that is only it's image" it's more philosophical than scientific.

[Snakebyte]

Yea, I know Quantum theory, he lives down the street, short, fat, never made any since. Never liked him too much.

[Nex]

What if it was 99% probability that it would callapse. Would it still be both states depending on no observation was on the particles?

it would still be in both states but with different probabilities (100% alive at 99% probability and 100% dead at 1% probability)

I'll see if I find this proof with the Na-Atoms.

[edit] did not find the link any more.. (the proof was made in January 2000 as far as I remember and it involved a setup that is used to show interference patterns that are caused by photons normally.)

however if you are insterrested in all that here is a link with extensive and not too mathematical information:
http://plato.stanford.edu/entries/qt-measurement/


[This message has been edited by Nex (edited November 22, 2000).]

[Nex]

thing is, from what I understand here, quantum theory isn't really a theory, it's more like an admission of ignorance...

its not like schr�dinger once decided "Hey- lets just say the car over there isn't there when I don't look at it".

He came to his theories by solving equations and making experiments. His work would not have been accepted worldwide (and nowadways almost all new physical research is done using this theory) if it would not have been plausible.

That a quantum has a defined state only when you 'measure' it is not easy to accept, but is it easy to accept that time goes slower in a fast moving vehicle or that it gets heavier (mass increases)?

The fact that there is a lot of philosophiy on that question should not hide the fact that the Quantum theory is not a product of philosohpy but one of science.

well.. i hope they ship me my quantum computer soon, imagine how fast I could play Project IGI on that thing

[shahar2k]

oh yah, I know that the theory (I hate spelling it every time ) has been thuroughly proven, I'm sure it would be thrown out if it wasnt, it's just that hard to accept when you don't know all the proofs and experiements...
as for quantum computers, well that's just weird, I mean you could give it a 640 MB space, and it will come up with every single working piece of software that could ever be stored on a CD. it's like programming for dummies, FOR DUMMIES....

[kig]

About objective observation. To objectively observe would mean that the observation doesn't change the state of anything, ie. the observation didn't happen, hence all observation is subjective, since observation handles information, passing it through a filter, which in turn changes the space-time parameters of the filter - enter the butterfly effect and observing changed the state of the observed.

AGHFGHGNGH gotta shower these filthy scientific thoughts off my mind now.