# B Trying to understand the double-slit experiment

1. May 2, 2017

### Bruce Barron

Has anyone ever thought that the wave pattern on the screen is not because the particle is a wave. There is no reason that the wave appearance could not be due to a photon being distributed in this fashion and still be a particle while both slits are open. You have a different distribution with a single slit open. For instance, the photon may be simply interacting with a different geometrical pattern when both slits are open. This is really a question.

2. May 2, 2017

### Bruce Barron

With the single slit one pattern is seen using a single particle. Why is it a wave just because a wave like distribution is seen.^ That is to say, when both slits are open we no longer have a particle but a wave because?..ìWhy? Just because a wave distribution is seen the particle must now be a wave? Perhaps the particle remains a particle and gives this distribution not becase it is a wave but because the particle interacts with the geometrical pattern of two open slits and gives this new pattern as a particle. Just because you have a wave pattern does not prove the particle is now itself a wave. The possibility of a particle being in two places at the same time is impossible

3. May 2, 2017

### Staff: Mentor

That's not far off of what's really going on, although you wouldn't know it if you've trying to understand QM from pop-sci sources. (Everything you've been told about how we have a wave with interference when two slits are open and a particle with no interference when one slit is open is somewhere between seriously misleading and just plain wrong. Try to unlearn it).

Here's what we observe when we do the double slit experiment with individual photons:

No matter what geometrical pattern of slits we have, each individual photon leaves a single dot on the screen. It doesn't matter whether we have one slit open or one thousand slits open, each photon makes a single dot where it lands so we never see anything wavelike about it. Now suppose we do the experiment repeatedly. We might send one photon a second for a day (86400 seconds) or we might send a single burst of 86400 photons in a few microseconds, but it makes no difference; either way we get 86400 dots on the screen where 86400 particles landed. As these dots appear, they gradually form some pattern.

However, something interesting happens: the pattern formed by the dots is not the tight clump behind each slit you'd expect if we were shooting bullets through the slits. Instead, it is the pattern that you'd expect if the slit or slits were being illuminated by a wave and the dots were more likely to appear in the areas of the screen where the wave's intensity after passing through the slits was greater. If you have just one slit open the dots will build up the interference pattern you'd expect from a wave passing through the simple geometrical arrangement of a single slit; if you have more slits open you'll get the more complicated patterns you'd expect from a wave passing throughout that more complicated multiple-slit geometrical arrangements.

Last edited: May 2, 2017
4. May 2, 2017

### SWB123

Speaking as someone with considerably LESS, than a 'beautiful mind', I have pondered this particular question; but, I have produced nothing I can claim as being a definative answer. However, I do have a thought on one aspect of the behaviour of photons; of which, I would appreciate any explination, be it 'pro', or 'con'.

I had wondered if it is possible, that photons are indeed waves from the start, and throughout their travel, UNTIL they encounter something with physical mass! At that instant, their electromagnetic field interacts with an opposite polar field/spin within the mass. At that instant, the photon's energy is imparted to the mass, and 'THAT INTERACTION' is felt by the mass, as though it had been hit by a partical.

So! What say ye? Has there been ANY findings of photons displaying the characteristics of a partical, WITHOUT it having physically interacted with a mass?

AND, I have no idea if the resuts of 'these interations', on it's own is even a factor in the double slit experiment.

I do have some questions myself about the double slit experiment:
1) Is there a physical size to, or distance between the slits, which alters/eliminates the findings, which we all find so amazing?
2) If so, how exactly do those factors change the behaviour of the photon?
3) Can knowing the results of those variations help in figuring out the original iteration of the experimant?

5. May 2, 2017

### Staff: Mentor

They are neither particles or waves. They are quantum objects whose behavior is described by the laws of quantum mechanics (actually, quantum electrodynamics). "Wave/particle duality", the idea that sometimes photons are particles and sometimes they are waves, is not part of the modern understanding of quantum mechanics. It's something that was conjectured early last century when physicists first encountered quantum phenomena, and was abandoned when the modern form of the theory was discovered.
The size and position of the slits definitely matters. Generally it will be towards the end of the second year of a bachelor's degree program that you'll learn how to calculate what pattern is built up on the screen when you have slits of a given size and position, and there's a fair amount of mathematical heavy lifting involved. However, Feynman's fun and layman-friendly math-free book "QED: The strange theory of light and matter" will give you a pretty good feel for how it works.

6. May 3, 2017

### Ddddx

wave-particle duality was never about photons being "sometimes waves and sometimes particles". The idea of wave-particle duality is that the particle picture and the wave picture are both essential, and the experimental data has aspects which can only be described by a combined use of both. This idea is very much part of the modern understanding of quantum mechanics, and it is easy to see in the basic equations. For instance, the schrodinger wave equation is

Hψ = ih/2π ∂ψ/∂t

where H is a differential operator which is obtained by making substitutions of the form p = h/2πi ∂/∂x in the classical hamiltonian. The classical hamiltonian refers directly to the particle picture.

7. May 3, 2017

### ueit

Why would you expect the particles to behave like bullets? An electron is a charged particle and it interacts with a barrier composed of many charged particles in motion (electrons and nuclei). It is a difficult problem to solve in classical electrodynamics and it is not obvious to me why do you find the "tight clump" solution an obvious one.

If you really like the "bullets" analogy try to use charged bullets (having the enormous charge/mass ratio of an electron) and a barrier formed by an array of dipoles. Again I'm not sure what pattern to expect, but I bet it's not a "tight clump".

Andrei

8. May 3, 2017

### Staff: Mentor

OK, maybe should have said "charged bullets"...... although this thread is about photons, which have no charge....

However, the key point for purposes of this thread is that the pattern you get is not the pattern that would be produced by small solid objects (which is how most non-specialists understand the word "particle") moving under the influence of any force or combination of forces. You get the pattern that is produced by a wave interacting with the entire surface of the barrier, slits and all.

9. May 3, 2017

### dlgoff

here the particles are electrons

10. May 3, 2017

### lightandmatter

A question regarding the double slit experiment, if you were to replace the detector with a second double slit placed at the location of the expected central peak of the interference pattern, and the detector goes at the usual position for the second double slit what subsequent pattern if any would you get at the final detector? Would you get the usual pattern eventually.

11. May 4, 2017

### Happy_Jack

12. May 4, 2017

### ueit

I did not use photons in my example because classically they are not particles at all, but EM waves. So, to say that you would expect photons to behave like bullets seems bizarre. Also, the interaction between those photons and the barrier is more difficult to understand as compared to the electrons. So, I decided to refer to electrons.

I disagree that an interference pattern cannot be obtained by "small solid objects moving under the influence of any force or combination of forces". On what evidence do you base this assertion? It seems to me possible that the EM interaction between the incoming electrons and the electrons and nuclei in the barrier could in principle account for the observed pattern.

Andrei

13. May 4, 2017

### Staff: Mentor

This is simply personal speculation and goes against mainstream physics.

This thread has veered off course and it is time to close it.