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Guess the Result of the Experiment of the Day

A classic double slit experiment is done with electrons emitted toward two slits followed by an image screen. An interference pattern is seen on the screen. Now the electron source is slowed to emit a single electron at a time, so that only a single electron goes through the experiment at a time. What pattern appears on the image screen?

The initial poll, where spoilers were not allowed, can be found here: http://asterisk.apod.com/vie ... 30&t=19521 . If you are new to this GRED and want to ponder this question without seeing spoilers, please go there now instead of scrolling down.

This GRED and poll are now open for comments below. If you think you know the answer, please do tell.

I have now posted the answer below. A direct link to this answer is here:
http://asterisk.apod.com/vie ... 43#p122388

I will keep comments open for others to expand and better explain this great experiment.

Hi, RJN

If I remember correctly, I Think it is both, An interference pattern and No interference pattern. And the simple act of watching the experiment makes all the difference. But I do not understand why.

Mark

no interference with single particle would make this experiment too boring to be famous.

Interference pattern if there is no observation of the electron traveling through the slit (if it follows suit with the photon experiment)
The idea is, in laymen terms, that there are essentially two electrons that are traveling when shooting one at a time and we note it as "interfering with itself".

However when we have outside observations, we get the non interference pattern for if we are to observe this, then the "other" electron/photon must interact with the observer and thus not be able to interact with itself

I know the right answer to "how many angels can dance on the head of a pin?" is - as many as want to.

But how can we call it an experiment when to get the predicted result requires us to NOT take any data? If "not observing" means we simply make up our own answer, then I say we get an interference pattern shaped exactly like the Invisible Pink Unicorn.

A related question:

How many double sheets could a sheet slitter slit If a sheet slitter could slit sheets?

I've always wondered if this experiment has been conducted correctly. If the material with a slit in it has a thickness greater than zero, and the source of the electrons, or photons, is a single source between the two slits. I would expect some of them to hit the sides of the slit and bounce to form the pattern. No interference needed. Just particles or waves bouncing of the slit walls. If you fired an electron through the slit and it was aimed just right as to not be influenced by either side of the slit then there should be just one point on the target where they hit. This also assumes there is no gas in the experiment chamber to cause collisions and random bouncing. The results are influenced by the experiment setup, size of slits, alignment of beam, and other particles between the source and target. Because these parameters are not mentioned, the question is ambiguous.

I'm guessing uncertainty plays a huge role. Since we would know precisely where the electron would be, between the slits, we would NOT know where the electron was going--so, the electron would not necessarily have to move in a straight line once passing through the slits!

My guess!

Sblair

I thought photons traveled in wave patterns but electrons weren't subject to wave pattern propagations.

biddie67 wrote:I thought photons traveled in wave patterns but electrons weren't subject to wave pattern propagations.

How does one "travel in wave patterns" ?

(( grin )) ... maybe by losing a lot of weight and receiving a high-energy boot in the rear, one is free to merrily move on outwards in a grand freeform manner ??

My understanding is that when you slow down the experiment, you see only one electron on the screen. But because of the dual property of the electron (wave/particle) it will hit the screen with a different probability and this probability law is the interference pattern that you would see when you have collected a lot of events. I do not think that observing will change the result.

The interference pattern is the result of a (resonant) disturbance to the quantum vacuum?
...Lorenzo

The answer is: "An Interference pattern." This result has been hailed as one of the most profound in all of science. It is absolutely amazing to me that single electrons (photons, etc.) can build up an interference pattern. One reason I like it is that it gets right at the root of the misconception that particles primarily interfere with each other. Dirac is famous for saying something like "particles interfere only with themselves".

The video linked above is well done and correct in principle, but I was bit bothered by the lack of overlap of the two bands in the "no-interference" case. If there is no overlap, how can there be interference? Perhaps I am too nit-picky.

Another good discussion is given by Wikipedia here:
http://en.wikipedia.org/wiki/Double-slit_experiment
and in particular here:
http://en.wikipedia.org/wiki/Double-sli ... y_emission

I will keep comments open for others to expand and better explain this great experiment.

RJN wrote: Dirac is famous for saying something like "particles interfere only with themselves".

Particles also interfere with their own "Siamese twins" with which are quantum mechanically entangled:

http://asterisk.apod.com/vie ... 1t#p121771
http://asterisk.apod.com/vie ... nt#p120763

Found the quote:

Each photon interferes only with itself. Interference between two different photons never occurs. - P. A. M. Dirac

It occurs in his book "The Principles of Quantum Mechanics" published in 1958.

Here is a paper published in 2006 that refers to it:
http://www.ensmp.fr/aflb/AFLB-314/aflb314m435.pdf
This is an interesting paper that does discuss the "one or more particle" point. It quotes a generalization of Dirac's rule as

A system interferes only with itself.

In the conclusion, the paper goes on to say

The interference effects found in the previous section are explained in terms of the existence of common modes. The detector cannot distinguish if the common modes belong to one or other of the particles. In presence of alternatives that cannot be distinguished quantum mechanics leads to interference effects.

I just watched the little video several times and have some questions:

Why don't you get some kind of dispersal pattern with a single slot if an electron has some wave properties?
So what happens if the two slits are moved further apart?
Or if a 3rd slit is introduced in parallel to the first two?
Or if a 3rd slit is introduced that is perpendicular to the first two?
Or if there are just 2 slits that are perpendicular to each other?
Or is timing of the electrons significant - do you get different patterns if they are emitted in a timing sequence such that the first one has not reached the tracking surface before the 2nd is emitted as compared to a large enough time gap between any two electrons so that there is only one electron in transit at a time?

There just doesn't seem like there is enough info in this video to really lock in the described effect.

It just seems possible that it isn't the electron, by itself, that is causing the apparent interference pattern but something else, while the electron is in transit, is affecting the electron that is causing it to seem to have an interference pattern ....

(( sorry guys - I know that I'm probably getting to be a pain-in-the-butt ))

It just seems possible that it isn't the electron, by itself, that is causing the apparent interference pattern but something else, while the electron is in transit, is affecting the electron that is causing it to seem to have an interference pattern ....

something else like what? evil ghosts?

there's really not much of a problem picturing an electon passing through both slits, as soon as you step away from "point particle" abstraction. everything that has any space extent can perfectly be in two places at the same time.

(( grin )) oh my!! The little jpg above is an example of a declaration from a single slot situation - it doesn't help me. And for "evil ghosts" - who knows what unknown forces still lurk in the quantum world?

.... step away from "point particle" abstraction. everything that has any space extent can perfectly be in two places at the same time ....

That is an interesting statement - could you write a little more about it?

biddie67 wrote:I just watched the little video several times and have some questions:

Why don't you get some kind of dispersal pattern with a single slot if an electron has some wave properties?

You do.

The dispersal pattern is in the probability pattern of where the electron shows up after interfering with itself.

It's just that you can't really visualize that pattern with just one photon.

But one can be sure that that single photon will NOT show up at the pattern minima.

So what happens if the two slits are moved further apart?
The interference pattern is basically the same but becomes narrower.
(However, the falling off of the cyclical pattern with distance is fixed by the width of the slits.)

Or if a 3rd slit is introduced in parallel to the first two?
Then every other maximum is enhanced to correspond the narrowest slit spacing.

If a thousand slits are introduced then every 999th maximum is enhanced
to correspond the narrowest slit spacing and the interference pattern collapses from being
simple sinusoidal to a periodic set of narrow sharp peaks suitable for spectrometry.

Or if a 3rd slit is introduced that is perpendicular to the first two?
You'll have to be more specific. (Note: the little cartoon video
hasn't really treated the up down dimension properly in the first place.)

Or if there are just 2 slits that are perpendicular to each other?
A single slit should produce a single line interference pattern
perpendicular to the slit (unlike in the cartoon),

The basic pattern of 2 slits perpendicular to each other would be an
interference pattern of 2 lines perpendicular to each other with obvious
interference effects only noticeable where the lines intersect each other.

Or is timing of the electrons significant - do you get different patterns if they are emitted in a timing sequence such that the first one has not reached the tracking surface before the 2nd is emitted as compared to a large enough time gap between any two electrons so that there is only one electron in transit at a time?
As noted previously the photons are really only interfering with themselves such that timing is NOT an issue.

biddie67 wrote:There just doesn't seem like there is enough info in this video to really lock in the described effect.

It just seems possible that it isn't the electron, by itself, that is causing the apparent interference pattern but something else, while the electron is in transit, is affecting the electron that is causing it to seem to have an interference pattern ....

Nobody really understands what is causing the interference pattern; we just have
a miraculous method to accurately predict what the probabilities are going to be.

No one yet has come along with an alternative explanation that can
accurately predict what these probabilities are going to be
much less to predict what is actually going to take place.

biddie67 wrote:The little jpg above is an example of a declaration from a single slot situation - it doesn't help me.

How about this one: This fella is in both slits in the same time.

biddie67 wrote:

.... step away from "point particle" abstraction. everything that has any space extent can perfectly be in two places at the same time ....

That is an interesting statement - could you write a little more about it?

Like what? For some reason things tend to occupy some space. That dude in a picture is in both slits, a thread tying your clothes together is in thousands of holes at the same time, anything that has positive diameter can be found in multiple locations within some area - on the other hand, "point particle" has zero diameter, so it can only be found in one very precise location and nowhere else. Why our mental picture of electron has to be so different from other things in our experience that we have to restrict it to zero volume and single location of "point particle"?