Young's Double Slit

[blike]

If anything comes closest to complete wierdness in physics, it would have to be modifications of this classic experiment.

 

I'll give a little backround on the experiment before I continue.

 

In the early 1800's, the debate was raging on whether light was a particle or a wave. An English physicist named Thomas Young devised a plan to test these theories. Today we call it the "Double Slit Experiment".

 

Thomas young thought that if light was a particle, it would travel in a straight line from the source, through the two slits, and form two stripes on a photosensitive screen behind the slits.

 

However, if light was a wave, it would travel through the slits and create an interference pattern typical of waves on the screen.

 

 

Young performed the experiment, and found that light created an interference pattern, typical of a wave. This seem to put to rest the debate over the wave or particle properties of light for the time being.

 

Double slit experiments with electrons

 

Fast foward a hundred years or so and we find electrons have been discovered and can easily be isolated. Physicists decide to run the double slit experiment, except this time using electrons. The setup is essentially the same, except the decide to fire they electrons through the slits one-by-one.

 

Firing them one by one would not allow for them to interact with each other like waves. To physicists astonishment, the electrons still left an interference pattern, even though they couldn't interfere with each other because they were each fired individually! No matter how long the duration between electron shots, an interference pattern still was recorded.

 

Thats only the tip of the iceburg.

 

Now physicists added electron detectors in each slit to determine which slit each electron travelled through. They ran the experiment again and recorded the results. Suprisingly, this time there was no interference pattern, only two stripes left on the photosheet!! Physicists thought perhaps the electron detectors altered the experiment or motion of the electrons. So they ran various combinations of the experiment to determine the problem.

 

Turn off the electron detectors at the slits:

The scientists left the electron detectors at the slits, but turned them off. The results? They found an interference pattern. This meant that electron detectors on the slits do not alter the electrons paths.

 

Leave the electron detectors on, but don't gather the information:

This time, they will leave the electron detectors ON, but not look at or record the results of the electron detectors in any way. They will not obtain results from the fully functioning electron detectors. The results? They found an interference pattern. This meant that fully functioning electron detectors that are turned on do not effect the results so long as the results are not observed.

 

Record the measurements at the slits, but then erase it before analyzing the results at the back wall:

 

Everything is the same as the above, except the results from the electron detectors at the slits were analyzed and erase the recorded data. Remember, that the experiment has already been carried out by the time they choose to keep or erase the data before checking the photoplate for an interference pattern, or a dual slit pattern. When they erase the data, they find an interference pattern! This seemingly changes the results of a completed experiment.

 

If you are not bowled over by this fact, you have either heard of this experiment, or you need to re-read it (or I need to be clearer )

 

Physicists found that it wasn't the electron detectors that changed the experiment; it was the fact that they had observed the data. Deciding to observe the data after the experiment had been performed--electrons had already left their mark on the plate--changed the pattern of the electrons!

 

source: "The Elegant Universe" by Brian Greene.

 

Am I the only one who finds this extremely wierd?

[Radical Edward]

Is that the only place you have seen it? does the article reference any papers I can look at, as I don't hav the book.

[blike]

Heres a research paper on "A Delayed Choice Quantum Eraser".

 

http://xxx.lanl.gov/pdf/quant-ph/9903047

 

It gets pretty detailed, but you can get a good idea of whats going on. There are also other good papers referenced from that one.

 

You can find TONS of information on the classic electron double slit experiment by searching google.com for electron double slit experiment.

[Radical Edward]

thanks. I know the classic double slit experiment. I will ask one of my friends about this today, he has been working on QM for a while...

[aman]

An explanation might be related to a postulation of mine. I postulated that the particle that entered the universe to create the big bang effect never became larger but incrementally increased it's travels over and over at C+. One particle repeatedly creating the hologram type universe we experience.

At quantum levels if you try to measure it with an instrument then it creates the instrument that is trying to measure it.

Since it cannot measure itself without being a particle to be measured then it changes to particle results. When not measured it shows wave since that is the easiest propogation mode for energy in all directions.

It seems to show our consciousness is important to the universe.

Just for thought

Just aman

[blike]

Aman, you should make a thread explaining your particle theory; sounds interesting.

[aman]

Getting started on it. I'll post it with my buddy Zarkov and I'll be ready for any critisism and appreciate it. Coming soon. It'll be fun. :bravo: :bravo: :bravo:

I'm old but I'm wise cus grumpy old farts start to question.

Just aman

[fafalone]

You really want to declare your theories in advance to be associated with Zarkov's?

 

 

On another note, an interesting conjecture I've heard that given previous states and trajectories, there could only be one possible outcome for each particle. This would suggest that there is no consciousness, no thought as we know it. This also raises the issue of quantum behaviour, where if there are two equal probabilities, both occur.

[blike]

On another note, an interesting conjecture I've heard that given previous states and trajectories, there could only be one possible outcome for each particle.

 

I thought quantum mechanics finally put to rest the deterministic view..

[quantumdream]

I propose that any quantum, such as a photon, has a phase space inherent to itself, reciprocal (through Planck's constant) to the conventional phase space surrounding it. It follows that the inner and outer phase spaces may interfere at this "h" surface. "Reciprocal phase space infers to the individual photon the information needed for interference with surrounding conventional phase space. Correlated at some past event, these phase spaces together enable single particles to 'self interfere' where [action eigennumber equals reciprocal action eigennumber]." Any mutation of Young's experiment may be cast as phase space interference local to the quantum's spacetime. More detail may be found at the first article of my website, http://www.quantumdream.net.

 

blike, does not the quantum eraser you mention depend whether or not the detector records the events macroscopically? I. e., I cannot chalk up an event on a slate, then erase it, and expect the interference pattern to change.

 

Scientific American has had some good articles on quantum logic in the past few years.

[aman]

There's a lot to digest. It might be a little while to get back to you on this one. Thanks for the effort.

Just aman

[fafalone]

Just found this today.

http://physicsweb.org/article/world/15/9/1

[arivero]

http://arxiv.org/abs/quant-ph/9903047 is the right format to quote a XXX preprint. This article is a proposal of an experiment based on

 

http://cornell.mirror.aps.org/abstract/PRA/v25/i4/p2208_1

 

which is also a proposal. Press "Citations" in that URL to get more up-to-date work and even some real experiments.

 

If I understand well, all these "eraser" effects are about quantum entaglement, where the "recorder" is also a quantum device, not to be confused with an macroscopic non-quantum recorder.

 

If Greene's book actually speaks of an experiment done with a classical recorder, it should be really really surprising.

[blike]

Originally posted by arivero

If Greene's book actually speaks of an experiment done with a classical recorder, it should be really really surprising.

 

Perhaps I misread it, I'll go back and double check it this evening.

[arivero]

Well, even the quantum version is interesting, but then it is just another version of Aspect's experiments, the ones on the EPR paradox.

[Raider]

It seems to me that uncertainty is not fundamental. Both position and momentum can be measured precisely (not at the same time of course). Our devices for measuring such properties affect the properties themselves, thus making it impossible right now to get both measurements precisely at once.

 

However, beyond that, i've seen no evidence that the certainty of the principles breaks down in reality--only that we can not be certain today. It seems an illogical leap to assume that things we can acquire individually really do not exist because we can't get them all at once due to imperfect measuring devices.

[Guest Don]

The interference patterns in the electron double slit experiments are due to the electric field in the slits and the charged particles in the material surrounding the slit. The pattern would be similar for a positron. For a neutrino and larger charged and uncharged particles the patterns would be much less dispersed and would result primarily from interactions with the material surrounding the slit. An explanation of the appearing/disappearing patterns with variation in detection requires more detail about the nature of the detection devices and the pattern achieved when there is nothing between the generator and the target or when there is no slit in the material between the generator and the target.

[Radical Edward]

Originally posted by Don

The interference patterns in the electron double slit experiments are due to the electric field in the slits and the charged particles in the material surrounding the slit.

 

actually no, the electron double slit interference experiments are done in a slightly different way, and the results are because of the wave nature of electrons, rather than electric field interactions. this can be demonstrated mathematically since we have a good understanding of both the quantum mechanical nature of the system, and the field structure too.

[blike]

Originally posted by Radical Edward

actually no, the electron double slit interference experiments are done in a slightly different way, and the results are because of the wave nature of electrons, rather than electric field interactions. this can be demonstrated mathematically since we have a good understanding of both the quantum mechanical nature of the system, and the field structure too.

 

What I don't understand is how the electrons create an interference pattern if fired one at a time? There is no other electrons to interfere with, how does this work?

[Kylon]

We need to find a way to harness this kind of power! I mean, if this works, then we could erase data from a far part of the galaxy to create alterations in some other part of the galaxy = METHOD OF COMMUNICATION MUAHAHHAAHAHAHAHAHHAHAHA

 

EHEHEHEHEHEHEHE

 

 

[Radical Edward]

Originally posted by blike

What I don't understand is how the electrons create an interference pattern if fired one at a time? There is no other electrons to interfere with, how does this work?

 

wave particle duality; the "particles" have both particle like, and wave like properties. you cannot apply classical thought like that in the quantum realm.

[blike]

Originally posted by Radical Edward

wave particle duality; the "particles" have both particle like, and wave like properties. you cannot apply classical thought like that in the quantum realm.

 

(maybe I'm applying classical thought here again)....but for there to be an interference pattern, there must be waves (electrons) interfering with each other right.... If electrons are fired one by one, how do they "know" how they are affected by electrons that came before it to form an interference pattern...

[Radical Edward]

you can fire photons one by one and get the same effect.... the problem is that you are thinking classically, and in these cases you can't think of an electron as a particle alone, only when it interacts with one thing.

[Martin]

http://arxiv.org/abs/quant-ph/9903047 is the right format to quote a XXX preprint. This article is a proposal of an experiment based on

 

http://cornell.mirror.aps.org/abstract/PRA/v25/i4/p2208_1

 

which is also a proposal. Press "Citations" in that URL to get more up-to-date work and even some real experiments.

 

If I understand well' date=' all these "eraser" effects are about quantum entaglement, where the "recorder" is also a quantum device, not to be confused with an macroscopic non-quantum recorder.

 

If Greene's book actually speaks of an experiment done with a classical recorder, it should be really really surprising.[/quote']

 

Perhaps I misread it, I'll go back and double check it this evening.

 

Well, even the quantum version is interesting, but then it is just another version of Aspect's experiments, the ones on the EPR paradox.

 

 

this conversation was back in April 2003. we didnt actually hear the end

(I dont have Greene's book so i cant check arivero's guess either)

[swansont]

(maybe I'm applying classical thought here again)....but for there to be an interference pattern, there must be waves (electrons) interfering with each other right.... If electrons are fired one by one, how do they "know" how they are affected by electrons that came before it to form an interference pattern...

 

The electron (or photon) interferes with itself. It's a wave so it goes through both slits. If you do anything that lets you know that it went through a particular slit the interference pattern goes away.