# Double Slit Interference.

## Main Question or Discussion Point

This might sound very basic but...
when the electron passes through the two slits and we see the 'pretty' interference pattern on the opposite side what causes the dark fringes to be seen,
how does an electron, after acting as a wave and then as an electron when it is recieved, interfere with itself.
What happens in these dark fringes, where there is destructive interference, if one electron is fired and interferes with itself, will it be detected??
does that make sense?

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Mentz114
Gold Member
When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. If you continue firing electrons at the slits, the dots will build up into an interference pattern.

Think of the dark bands as places where no ( or few ) electrons end up.

neu
For each electron there is a probability of going through each slit. After many passes of single electrons through the apparatus, you begin to see the probability distribution i.e the frequency (number of occurances) of each possible path the lectron can take.

experimentally the observated distribution turns out to be equivalent to that made by a wave.

(t's been a while since I did this so I may have got this wrong.)

The experiment is easy to understand, the implications are what makes it interesting. Ie the fact that a single warticle (wave/particle) can interfere with itself, but if the warticle is measured after it passes through the slits then no interference pattern builds and a pattern on the back screen appears as it would if a particle had travelled through either the top or bottom slit, with a 1/1 distribution. If you can get your head round why that is you're laughing.

When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern.
Are you sure?

JesseM
Are you sure?
Do you think Mentz's statement is not correct? The interference pattern is in the probability distribution for finding a given electron at a single definite location, but you do always find it at a single definite location when you measure its position. Only by looking at the distribution of large numbers of electrons can you actually see an interference pattern.

Do you think Mentz's statement is not correct? The interference pattern is in the probability distribution for finding a given electron at a single definite location, but you do always find it at a single definite location when you measure its position. Only by looking at the distribution of large numbers of electrons can you actually see an interference pattern.
So are you saying that a single electron does not interfere with itself?

JesseM
So are you saying that a single electron does not interfere with itself?
No, why would you think I was saying that? As I said, the probability distribution does show interference when both slits are open and you don't measure which slit the electron went through. But you can't see a probability distribution if you only measure a single electron--you only find it at one single location, with the probability of finding it at one location vs. another location given by the probability distribution.

As I said, the probability distribution does show interference when both slits are open and you don't measure which slit the electron went through.
As you said? Where?

At any rate, if you now say so you clearly must disagree with:

"When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. "

JesseM
As you said? Where?
In post #6, where I said "The interference pattern is in the probability distribution for finding a given electron at a single definite location"
MeJennifer said:
At any rate, if you now say so you clearly must disagree with:

"When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. "
Your logic is completely inscrutable. Why would I "disagree" with this, when I just said exactly the same thing in my last post? i.e.:
As I said, the probability distribution does show interference when both slits are open and you don't measure which slit the electron went through. But you can't see a probability distribution if you only measure a single electron--you only find it at one single location, with the probability of finding it at one location vs. another location given by the probability distribution.

What part of ""When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. " talks about measuring which slit the electron went through? Did you read the OP? Where does it question the case when a measurement at the slits take place?

JesseM
What part of ""When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern. " talks about measuring which slit the electron went through?
It doesn't, but the statement is true regardless of whether you measure which slit it went through or not. Whether you measure or not just changes the probability distribution (the probability distribution is an interference pattern if you don't measure, a non-interference pattern if you do), but you never see the probability distribution experimentally in a measurement of a single electron, you deduce the probability distribution either by sending a lot of electrons through with the same experimental setup, or by doing a theoretical calculation using the equations of QM. In a measurement of a single electron, all you see experimentally is the electron being detected at a single definite location on the screen.

Anyway, I really don't understand why you're asking me that question. Are you implying that I have claimed the statement does talk about measuring which slit the electron went through? If so you have misunderstood me.
MeJenniver said:
Did you read the OP? Where does it question the case when a measurement at the slits take place?
It doesn't, it seems to just be talking about the normal version of the experiment where there is no measurement at the slits. Again, I don't understand the point of this question.

Patently JesseM a lot of people can't get their heads round what is happening in a consistent manner. And you really do often need a visual representation to get this across. The number of times this thread comes up demonstrates this more than adequately. But if they want an answer try the FAQ.

I think it might need a rewrite to include a good "visual" model but here it is:

IS LIGHT A WAVE OR A PARTICLE?

Contributed by Marlon and ZapperZ.

In our ordinary world, “wave” and “particle” behavior are two different and opposite characteristics. It is difficult for us to think that they can be one of the same. Is light a particle or a wave? The simple, naïve answer to that is “both” or “neither”.

Light, or photon, was never defined as a “particle” the way we normally define a particle. Light is not defined to have a definite boundary in space like a ping-pong ball, or a grain of sand. Instead, light is defined as having quanta of energy. So the discreteness is not defined as discrete object in space, but rather in the energy it can carry. Already, this is not your regular “particle”, and should not be confused as such.

Secondly, in quantum mechanics, the description and properties of light has only ONE, single, consistent formulation, not two. This formulation (be it via the ordinary Schrodinger equation, or the more complex Quantum Electrodynamics or QED), describes ALL characteristics of light – both the wave-like behavior and the particle-like behavior. Unlike classical physics, quantum mechanics does not need to switch gears to describe the wave-like and particle-like observations. This is all accomplished by one consistent theory.

So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.
And here a good "visual" model is, if only copyright laws weren't such a beast.

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html" [Broken]

Don't worry it does photons and electrons although the issues are the same for all intents and purposes here.

Warticle, not particle not wave, not neither not never.

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Mentz114
Gold Member
Originally Posted by Mentz114
When a single electron goes through the slits, you only see one spot on your screen, not an interference pattern.
Are you sure?
Yes. The electron will be detected at one place and time and make one spot. Are you suggesting that the electron splits ?

Here's a link that shows a pic of the DeB-B trajectories in the double-slit experiment -

www.math.rutgers.edu/~oldstein/quote.html

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Chances

is there a chance that with the first electron traveling through this hole that it interferes with itself and ends up on the furthest left point on a light fringe not straight through the hole, or does this interference only work with multiple electrons?

do any of the electrons that are fired when they become waves interfere and dissapear and do some hit the space between the two slits before travelling through(that sounds basic i am just tryin to build up a picture in my mind)

After my little knowledge I really do think there is some photons that hit the wall inbetween the slits.

I suggest you check out deBrogile bohm's pilot wave explanation of the double slit experiment.
Really layman easy to understand

Electrons don't go from being particles to waves and vice versa. Pick a description (both are correct/equivalent), and stick with it. The electron goes through the slits, however it does that, and will excite the detector at exactly one point. That point can be anywhere on the detector, so it's not good to talk about "interfering with itself".

However, if you performed the experiment over and over with many electrons, one at a time, then fewer electrons will end up in some places, and more will end up in other places. The interference pattern you see resembles that of a wave, and so we postulate the existence of a wavefunction that governs the electron's probability amplitude. Here's a nice picture: http://en.wikipedia.org/wiki/Image:Double-slit_experiment_results_Tanamura_2.jpg

Also, be ware of explanations that are "easy to understand" but which are either wrong, kludgy or both. The postulates of quantum mechanics may not be easy to understand the first time around, but they are very elegant and very simple. Stuff like pilot waves might sound appealing now but they're god-awful when you want to do quantum field theory, and you end up having to sweep a lot of ugliness under the rug.

oh wow i think i get it now, thanks guys!

To me, all of the following descriptions of the interference patterns are basically the same and tries to explain the wave-nature of photon/electron or other particles used in the experiment:

- Photon/Electron interfered with itself.
- Probability Wave interfered.
- Pilot Wave interfered.

Also, the moment we make an observation to detect which slit the particle went through, the wave function "collapses." I'm not exactly sure what this means, but it seems to imply that the source of interference are no longer compatible to cause the interference (due to the act of measurement), so for all intents and purposes, the Photon/Electron can now be considered a Particle.

One question I have is, are the Probability Waves and/or Pilot Waves real or are they simply a proxy of something that the scientists think may exist in an effort to explain these phenomenons?

One question I have is, are the Probability Waves and/or Pilot Waves real or are they simply a proxy of something that the scientists think may exist in an effort to explain these phenomenons?
I don't really agree with the other statements but I'll try to answer this one. According to Bell's theorem, the wavefunction is "as real as it gets". But there is a difference between "real" and "observable", and we can't observe/measure the wavefunction, only the probability.

They are as real as they get.
However, what the pilot wave is made of, or if it exist isn't proven.

Here's a great interview with Antony Valentini who explains the pilot wave very nicely:

http://www.nyu.edu/classes/neimark/valentini.html [Broken]

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Thanks for that link, confusedashell (I should be using that screen name... :) )

It's an interesting theory, but that faster-than-light requirement is a bit of stretch, isn't it? To say that faster-than-light interactions used to be visible, but is no longer visible is rather too convenient to make the theory fit the existing models. Regardless, I think it's a theory that can shed light on the true makeup of the universe/matter.

Mentz114
Gold Member
Thanks for that link, confusedashell (I should be using that screen name... :) )

It's an interesting theory, but that faster-than-light requirement is a bit of stretch, isn't it? To say that faster-than-light interactions used to be visible, but is no longer visible is rather too convenient to make the theory fit the existing models. Regardless, I think it's a theory that can shed light on the true makeup of the universe/matter.
The FTL (non-local) element in Bohmian mechanics is also part of other QM paradigms. Apparently a local theory could not reproduce QM.

Well said Mentz114,

JinChang Im by no means a cosmologist or a Quantum phycisist but the thing is 3 choices:

#1 Nonlocality - FTL
#2 Observation cause the universe to exist, like you are God
#3 The universe magically splits every split nanosecond without you noticng and there exist infinite universes

Nonlocal quantum nature seems very very much more likely than the two other options.
Who said the speed of light was the speed limit of the universe? except Einstein.

If he was alive I'm sure he'd either be stuck searching desperately to find a local hidden variable theory which is proven impossible.
Or he'd join his long time partner and good friend David Bohm and accept nonlocality.
Ofcourse never talk about a dead mans perspective, but just after what I've read about einstein, out of those 3 options we know for sure he HATED copenhagen (observeration creates the universe)
and I doubt he'd go with Everetts many worlds, but who knows, doesnt matter, einstein wasn't "The Old One".

And as for the "before we could see" sure, I can agree, but pondering the same **** I came up with the same conclusion: how can QM as it stands now been in the early universe ?