# Double-slit questions

1. Jun 3, 2005

### underworld

I have some questions about the so-called double-slit experiment which exploits the wave-like characteristics of light.

1) why are the interference patterns two-dimensional? i.e. they are always vertical bands....
2) does the pattern change if the slits are vertically arranged instead of horizontal?
3) why is it assumed that the only explanation is that each photon interacts with itself? isn't it possible that the interference could be caused by a "residual effect" of the previous photon with the current photon? or am i missing something?

i have some other questions - but these are the ones that are most nagging at the moment....

thanks...

2. Jun 3, 2005

### Edgardo

Hello underworld,

The interference bands are vertical, because of the slits' geometry and their orientation.
As you pointed out correctly in question 2 the pattern changes if you change the slits orientation. Horizontal slits yield horizontal stripes and vertical slits yield vertical bands.

By the way, stripes are not the only interference patterns. There are lots of other patterns, like circles.

You can even calculate how the pattern will look like for a certain slit geometry (the physicists perform the calculation in the so-called "Fraunhofer approximation").
There's an optics book by Eugene Hecht (maybe your library has it) that contains pictures of such interference patterns.

It is important to understand that the double-slit experiment shows that a photon can "interact itself" and that a photon does NOT require any other photon. You can clearly see this in the experiment. The particles (photons or electrons) get to the detector screen ONE by ONE. And if you wait long enough you'll get the interference pattern. That is what lets a photon's behaviour appear so strange, but isn't that wonderful?

Here a website that also shows a movie with electrons in a double slit experiment (Save it with rightclick from the "movie" icon)
http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm [Broken]
(Listen to what the commentator says, very interesting)

There's also a quantum mechanical calculation where you assume that the photon only "interacts with itself". That calculation (by a physicist named Marcello) indicates that the assumption above is reasonable.

Last edited by a moderator: May 2, 2017
3. Jun 3, 2005

### underworld

thank you for the insight - and i will look at those references...
i still don't follow the above completely though. what exactly are you waiting for in the above quote? for more electrons to go through so that the pattern emerges? in other words, if you can demonstrate that a single particle yields an interference pattern - than that supports the concept of the superposition quite well. if, on the other hand, the demonstration requires multiple particles to produce the pattern, then I think conceptually the door is still open to other possibilities (despite the mathematics). for example - i can conceptualize a scenario where the electrons (or particles) leave an "imprint" of their path and the follow-on particles interact with the imprint to create the interference. my conceptual model is kind of like the einstein model of gravity - with a difference. in my mind, the "space" or whatever that is bending to create gravity in his model doesn't return quickly to it's "stable" or idle form. instead, the distortions linger and slowly return to idle (like the tempur pedic foam used in beds). then when another particle comes along, it (or some part of its essence, i.e. it's wave) interacts with the lingering imprint.

am i way off here or what?

4. Jun 4, 2005

### inha

What do you base that on? If you were correct one would get different diffraction patterns with different electron launching rates which certainly is not the case.

5. Jun 4, 2005

### ZapperZ

Staff Emeritus
Do you see a "pattern" on a graph when only 1 point is plotted? What about 2? Or 3 points?

Or look at an image but with only 3 pixels visible. Can you recognize the picture?

You do not see a pattern when there's insufficient 'dots'. This is what is meant.

You may also want to do a search on PF about this. This topic has been discussed ad nauseum, with several credible references being cited already. The interference pattern need not be described by invoking "wave" behavior. It can be sufficiently described by the photon description via the superposition of path. In fact, ALL so-called wavelike properties exhibited by photons, electrons, neutrons, buckyballs, etc. are the manifestation of such superposition.

Zz.

6. Jun 4, 2005

### James Jackson

Elgardo mention the Fraunhofer Approximation, this is where you assume all the waves are planar (ie you've got an infinite distance between source and slit and slit and screen). This is easily realised with optics.

A beautiful result of Fourier optics is that (roughly) the diffraction pattern is given by the Fourier transform of the electric field strengh over the aparture. To get a pattern from a number of slits, you compute the fourier tranform of the convoltion of the individual slit pattern with an array of Dirac delta functions at the centre of each slit position. By convolution theory, this is equal to the FT of the slit x the FT of the array. I think that's a lovely result.

7. Jun 4, 2005

### underworld

i guess my question is not really clear here. what i'm asking is why the superposition and its associated mathematics are assumed to be both the correct and only answer/explanation for the double slit experiment.

in other words ... newtons laws and mathematics were a great explanation of the effects of gravity. however, einstein provided a nice refinement to show that, while not exactly wrong, newton was not exactly right. some of newton's assumptions were not correct.

my question is similar about the superposition principle and the double-slit experiment. although the concept and the math match the experiment - that doesn't mean it's the right or full answer. what i'm asking is - are there other explanations for the double-slit experiment that are, perhaps, less popular? my problem is that in most explanations, the weirdness is explained as only possible through superposition. since superposition in itself is pretty weird, i'm wondering if there are alternate solutions (maybe more, maybe less weird).... -- since there's still this overarching question of granularity between QM and einstein.

8. Jun 4, 2005

### James Jackson

David Deutsch (rather well regarded physisict, and one of the 'founding fathers' of quantum computation) explains it from a multiverse point of view. His book The Fabric Of Realist is a very readable book detailing it - I haven't read it in a while, but as far as I remember the photon travels as a particle through each slit, but in two different universes (i.e. in one universe the photon goes through one slip, and in another universe the photo goes through the other slit). There is a weak correlation between the two 'new' photons in each universe, hence the interference pattern.

9. Jun 4, 2005

### ppnl2

There are many alternate explanations for quantum phenomena but none of them are less weird. Worse, you really can't tell them apart because for the most part they all give the same predictions.

10. Jun 4, 2005

### ZapperZ

Staff Emeritus
The problem here is that you are using a "personal preference" to argue against something, i.e. it is "weird". You will, I'm sure, agree that such a criteria is very weak. There are many things that you may find weird that the someone else does not.

Superposition doesn't look weird to me. If I consider the Delft and Stony Brook experiments that CLEARLY show the effects of superposition on 10^11 particles, and all the other effects that can only be described via such principle (i.e. hydrogen molecule bonding-antibonding bands), then I no longer find such things weird. Moreover, maybe it is OUR concepts that we are forcing into the QM world that no longer fit. Maybe our classical idea of position and path and momentum, etc. have very vague meanings at the QM scale. So you are trying to force something that works well in one world to also work as well in another scale. You should consider the possibility that you are trying to force a square object through a round hole. When it doesn't fit, you blame the hole when in fact, the object itself it at fault.

I've mentioned this before in other threads, but I truly believe that QM is very difficult to comprehend with one doesn't understand or start from it's formal formulation. See my journal entry

[05-01-2005 04:22 PM] - Why is Quantum Mechanics SO difficult?

This is because one has NOTHING to build one's understanding on from classical understanding. There are every indication that what we call "weird" is the norm at the QM scale IF we insist on using the same classical terms.

Zz.