# Double Slit Experiment: Wave-Particle Duality

• Sunil.Iyengar
In summary, the double slit experiment demonstrates that electrons behave like both waves and particles. When the distance between the slits is smaller than the radius of the wave field, the wave field passes through each slit and then on coming out they interact with one another forming the diffraction pattern. The frequency and amplitude of the resultant wave field will be the same even after splitting due to the slits.

#### Sunil.Iyengar

Here is a noobie thought.

We all know the double slit experiment (say for electrons) and the explanation that electrons behave like both wave and a particle. So they pass through both the slits forming the interference pattern. But I think a quanta (electron in this case) behaves like a particle by default. But it also and always has a wave function/property associated with it. Each quanta has a "wave field" around it permeating through space. So, this wave function/property (probably a subtle property for quantas) gets exposed based on what one is doing and the distance between the slits. If the distance between the slits is less than the radius of the wave field, the wave field passes through each slit and then on coming out they interact with one another forming the diffraction pattern. The frequency and amplitude of the resultant wave field will be same even after splitting due to the slits which is why there is a regular pattern.

In the double slit experiment the electrons still go through just one of the slits (since they are particles). But the wave field (being energy) associated with each quanta goes into both the slits (if the distance between slits is smaller than the radius of the wave field) producing interference and a diffraction pattern.

Sunil.Iyengar said:
Here is a noobie thought.

We all know the double slit experiment (say for electrons) and the explanation that electrons behave like both wave and a particle. So they pass through both the slits forming the interference pattern. But I think a quanta (electron in this case) behaves like a particle by default. But it also and always has a wave function/property associated with it. Each quanta has a "wave field" around it permeating through space. So, this wave function/property (probably a subtle property for quantas) gets exposed based on what one is doing and the distance between the slits. If the distance between the slits is less than the radius of the wave field, the wave field passes through each slit and then on coming out they interact with one another forming the diffraction pattern. The frequency and amplitude of the resultant wave field will be same even after splitting due to the slits which is why there is a regular pattern.

In the double slit experiment the electrons still go through just one of the slits (since they are particles). But the wave field (being energy) associated with each quanta goes into both the slits (if the distance between slits is smaller than the radius of the wave field) producing interference and a diffraction pattern.

The wave represents the probability of finding the particle at a particular location. If the wave goes through both slits then the probability distribution for the location of the particle goes through both slits. There is no definite position for the particle until it is detected, all we have is the probability distribution.

If we talk about the particle taking an exact position, between the times that its location is measured then we're talking about a mechanism that doesn't contribute to any physical process. Furthermore, the path that the particle must take, must match the probability distribution.

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Well the assumption here is that "finding a particle at a particular location". How do we know that the resultant diffraction on the plate/board is due to the particle falling on it or due to the resultant wave energy interacting with the plate/board causing the pattern or even both?

I think the particle goes in just one of the slits based on how close it is to a slit. There could be some particles that hit in between the slits and hence do not even reach the other side but their wave field could.

Sunil.Iyengar said:
Well the assumption here is that "finding a particle at a particular location". How do we know that the resultant diffraction on the plate/board is due to the particle falling on it or due to the resultant wave energy interacting with the plate/board causing the pattern or even both?

I think the particle goes in just one of the slits based on how close it is to a slit. There could be some particles that hit in between the slits and hence do not even reach the other side but their wave field does.

We can perform experiments that demonstrate the wave energy must arrive in quanta on screen.

The nature of the interference pattern demonstrates that the wave interferes with itself.

Particles do indeed hit between the slits. The probablily of finding a particle beyond the slits, increases at the size of the slits is increased.

Let me give you a simpler example that should make this a little clearer. If we take 2 laser beams of the same frequency and fire them at each other in anti-phase, with a very slight angle offset. In the region where they cross, we can find no photons. According to your description, where do these particles go? How do they get to the other side?

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Sunil.Iyengar said:
How do we know that the resultant diffraction on the plate/board is due to the particle falling on it or due to the resultant wave energy interacting with the plate/board causing the pattern or even both?
These are exactly the same. They are two equivalent descriptions of the same thing.

Sunil.Iyengar said:
I think the particle goes in just one of the slits based on how close it is to a slit. There could be some particles that hit in between the slits and hence do not even reach the other side but their wave field could.
You seem to be thinking that the particle and its wavefunction are two separate things, which they are not. At any point, the wavefunction gives the probability of finding the particle at that point. You cannot have the particle going in one direction and the wavefunction going in another!

Sunil.Iyengar said:
Here is a noobie thought.

We all know the double slit experiment (say for electrons) and the explanation that electrons behave like both wave and a particle. So they pass through both the slits forming the interference pattern. But I think a quanta (electron in this case) behaves like a particle by default. But it also and always has a wave function/property associated with it. Each quanta has a "wave field" around it permeating through space. So, this wave function/property (probably a subtle property for quantas) gets exposed based on what one is doing and the distance between the slits. If the distance between the slits is less than the radius of the wave field, the wave field passes through each slit and then on coming out they interact with one another forming the diffraction pattern. The frequency and amplitude of the resultant wave field will be same even after splitting due to the slits which is why there is a regular pattern.

In the double slit experiment the electrons still go through just one of the slits (since they are particles). But the wave field (being energy) associated with each quanta goes into both the slits (if the distance between slits is smaller than the radius of the wave field) producing interference and a diffraction pattern.

Welcome to PhysicsForums, Sunil.Iyengar!

There is an interpretation of Quantum Mechanics which is somewhat similar. It is called Bohmian Mechanics. In it, there is a point particle as well as something usually called a guide wave or pilot wave. The pilot goes through both, the particle through just one.

The catch to the Bohmian view, if you want to call it that, is that the pilot wave is non-local. That means relativity is not strictly respected. There are other usual aspects to it. However, most scientists consider Bohmian Mechanics to be one of several viable interpretations.

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I think the assumption here is that the particle is following the path of wave nodes. The wave energy is released from the particle on passing through the slit (provided the wave field of the particle is larger than the slit) thereby slowing down the particles on the other side obeying the laws of conservation. The slit is what causes the particle to release its wave energy in the first place. The particle that does not hit the edges of the slit continues to follow its path. Those that hit the edges or hit the middle of the slit follow based on the angle they hit. So, basically once the wave energy is released on passing through the slits, the particle and wave energy independently travel. I think it is mostly the wave that cause the diffraction pattern and not always the particle (or particle may not even be the cause of the pattern on the plate).

Sunil.Iyengar said:
I think the assumption here is that the particle is following the path of wave nodes. The wave energy is released from the particle on passing through the slit (provided the wave field of the particle is larger than the slit) thereby slowing down the particles on the other side obeying the laws of conservation. The slit is what causes the particle to release its wave energy in the first place. The particle that does not hit the edges of the slit continues to follow its path. Those that hit the edges or hit the middle of the slit follow based on the angle they hit. So, basically once the wave energy is released on passing through the slits, the particle and wave energy independently travel. I think it is mostly the wave that cause the diffraction pattern and not always the particle (or particle may not even be the cause of the pattern on the plate).

When we detect photons that have traveled through slits, they have the same energy and speed as they started with.

We can detect individual particles building up an interference pattern. We can count them coming in, 1 by 1.

What do you mean by "wave nodes"?

Due to lack of a better term that I am aware of, "wave node" meaning the trough and the crests of the wave. Again when we count "them" coming in 1 by 1, how do we know it is the particle and not the crest of the wave? I am not saying the particles do not hit the plate but the particle will pass through only one slit and hit the plate only at one location based on its original path. It is the crest of the waves that hit the plate forming the bright interference pattern. The crest will have the peak energy and trough the least (even zero). Extending on the thought, if we look for the surrounding space around the plate we may find these particles floating in space. Also the waves hitting the plate will bounce back due to reflection. Similarly the particle hitting the plate will release more wave energy on hitting the plate resulting in reflected wave energy.

Sunil.Iyengar said:
Due to lack of a better term that I am aware of, "wave node" meaning the trough and the crests of the wave. Again when we count "them" coming in 1 by 1, how do we know it is the particle and not the crest of the wave? I am not saying the particles do not hit the plate but the particle will pass through only one slit and hit the plate only at one location based on its original path. It is the crest of the waves that hit the plate forming the bright interference pattern. The crest will have the peak energy and trough the least (even zero). Extending on the thought, if we look for the surrounding space around the plate we may find these particles floating in space. Also the waves hitting the plate will bounce back due to reflection. Similarly the particle hitting the plate will release more wave energy on hitting the plate resulting in reflected wave energy.

How far a wave is though it's crest and trough cycle, is called its phase. The phase affects inteference, but it is the amplitude that determines the probability of finding a particle. The amplitude of a wave in independent of it's phase. Even the parts half way between a crest and a trough can have the same amplitude as at the crest and trough.

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Agreed as per the current theory. But the problem with this model is that the particle has to split into as many crests in the resultant wave forming the diffraction pattern on the screen. The question is why should a particle "adapts" itself as a wave when passing through the slit and suddenly become a particle when it hits the screen. There does not seem to a logical reasoning behind this. However, with the model I suggested the wave energy exists independent of the particle (but can bind with particles to form a field around it) and when it comes in contact with material particle, the particle interacts with it.

Sunil.Iyengar said:
Agreed as per the current theory. But the problem with this model is that the particle has to split into as many crests in the resultant wave forming the diffraction pattern on the screen. The question is why should a particle "adapts" itself as a wave when passing through the slit and suddenly become a particle when it hits the screen. There does not seem to a logical reasoning behind this. However, with the model I suggested the wave energy exists independent of the particle (but can bind with particles to form a field around it) and when it comes in contact with material particle, the particle interacts with it.

You still misunderstand the role of the peaks and troughs of the wave. They're only really there for the superposition. The particle is no more likely to be found at a peak, a trough or anywhere in between.

If you want to create your own theory of particles, you're going to need to understand Quantum Mechanics first, because any theory you create needs to be able to replicate it.

Sunil.Iyengar said:
Agreed as per the current theory. But the problem with this model is ...

The results are predictable by current theory, which maps the outcome probabilities. That is what QM is, a useful theory that accurately predicts many phenomena. It is all well and good to imagine things look like this or that in your own mind. There are numerous models, called interpretations, as I mentioned, that can help if you like.

A new model MUST be as useful as previous ones to gain traction. That is the way of science. You cannot really say "my model does everything the other ones do, plus X" - unless of course you can prove it. The second you propose a new model, you MUST be aware of existing ones and their minute details.

You don't yet have the knowledge to do that, I hope you will continue and learn more!

And this is not the place for new models anyway.

Fully agree. Maybe proposing a model is an incorrect terminology. But this is the thought process and IMO the best way to understand something is to challenge it with one's own understanding. What is wrong with the thought process that I am saying is not clear to me? The current model is not be entirely incorrect but the interpretation looks flawed to me. After all knowing and understanding the truth is the objective rather than a dedicated love for something that may not be fully true.

Sunil.Iyengar said:
Fully agree. Maybe proposing a model is an incorrect terminology. But this is the thought process and IMO the best way to understand something is to challenge it with one's own understanding. What is wrong with the thought process that I am saying is not clear to me? The current model is not be entirely incorrect but the interpretation looks flawed to me. After all knowing and understanding the truth is the objective rather than a dedicated love for something that may not be fully true.

I think everyone who has ever learned Quantum Mechanics know's where you're coming from, including the most famous physicists. I don't think I'm exaggerating when I say that it is the most counterintuitive theory every conceived. Hold onto your scepticism, but learn the phenomena and how Quantum Mechanics explains them. You'll realize that many of the things that you don't trust are very real and necessary aspects of the theory, but in the process of doing so you'll find more things to be sceptical about.

This is a very good, up to date, non-mathematical introduction to the subject:
http://www.amazon.com/dp/1780223951/?tag=pfamazon01-20

Once you're happy with that, I'd recommend moving on to an undergrad teaching text, but be sure that you've studied enough maths, particularly calculus and complex numbers. An understanding of classical vibrations and waves and optics, will be very helpful too.

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Fair enough and point well taken and thanks for the link. But if the two counter intuitive thoughts interprets differently the simplest one must be more true (occam's razor). I feel the way I interpret it is much more simpler unless of course there is a serious flaw in what I interpret resulting in a different result. No one has shown it though in this thread. That is the last thing I want is to interpret something wrongly. In fact, I will be the happiest person to see myself wrong. That is because realizing the mistake will put me in the right track closer to the truth.

Sunil.Iyengar said:
Fair enough and point well taken and thanks for the link. But if the two counter intuitive thoughts interprets differently the simplest one must be more true (occam's razor). I feel the way I interpret it is much more simpler unless of course there is a serious flaw in what I interpret resulting in a different result. No one has shown it though in this thread. That is the last thing I want is to interpret something wrongly. In fact, I will be the happiest person to see myself wrong. That is because realizing the mistake will put me in the right track closer to the truth.

I think it has been shown to you but you missed it.

Try answering this, that I posted for you earlier:

craigi said:
Let me give you a simpler example that should make this a little clearer. If we take 2 laser beams of the same frequency and fire them at each other in anti-phase, with a very slight angle offset. In the region where they cross, we can find no photons. According to your description, where do these particles go? How do they get to the other side?

My apologies...I could not visualize it fully although I answered assuming that you are talking about waves crossing each other and after crossing the slit.

"The wave energy is released from the particle on passing through the slit (provided the wave field of the particle is larger than the slit) thereby slowing down the particles on the other side obeying the laws of conservation. The slit is what causes the particle to release its wave energy in the first place. The particle that does not hit the edges of the slit continues to follow its path. Those that hit the edges or hit the middle of the slit follow based on the angle they hit. So, basically once the wave energy is released on passing through the slits, the particle and wave energy independently travel. I think it is mostly the wave that cause the diffraction pattern and not always the particle"

But to be clear let me ask you anyways. Are you saying the waves crossing each other or the laser beams. Does this happen before the slit or after crossing the slit? What is the purpose of the angle offset?

There is no slit in this example. Just 2 lasers. Nothing else.

They're fired at each other, with a very slight angle offset and the waves are in anti-phase (half a wavelength offset).

Where do you expect to find photons?

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Sunil.Iyengar said:
"The wave energy is released from the particle on passing through the slit (provided the wave field of the particle is larger than the slit) thereby slowing down the particles on the other side obeying the laws of conservation. The slit is what causes the particle to release its wave energy in the first place. The particle that does not hit the edges of the slit continues to follow its path. Those that hit the edges or hit the middle of the slit follow based on the angle they hit. So, basically once the wave energy is released on passing through the slits, the particle and wave energy independently travel. I think it is mostly the wave that cause the diffraction pattern and not always the particle"

OK, I don't know what any of this is quoted from but it is pretty ridiculous. The interference has nothing whatsoever to do with this.

You mentioned that no one has shown you why you are incorrect in this thread. Please note that after you know something about QM, you will see how silly that statement is. No one proved a lot of things in this thread. You are bordering on speculation. That is inappropriate here. Ask a specific question.

DrChinese said:
OK, I don't know what any of this is quoted from but it is pretty ridiculous. The interference has nothing whatsoever to do with this.

You mentioned that no one has shown you why you are incorrect in this thread. Please note that after you know something about QM, you will see how silly that statement is. No one proved a lot of things in this thread. You are bordering on speculation. That is inappropriate here. Ask a specific question.

He's quoting himself. I've already shown him 3 simple ways that his model doesn't describe reality, but he hasn't acknowledged them yet.

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Sunil,

As I mentioned, you need to learn more. There are dozens of ways to create interference, and the double slit is just one. Interference is a quantum effect and is not describable by classical analogies. Before you start speculating with ideas off the top of your head which bear no resemblance to actual experiments, please read more.

For example: polarizers can be placed in front of each of the 2 slits. When aligned in certain ways, interference is produced. When aligned differently, there is no interference. Can you explain when the interference appears and when it does not? Standard QM can exactly explain this.

My point being that your model is what is known as "ad hoc" which is a nice term for saying you made it up off the top of your head. Those are not to be discussed here. This is a forum to discuss accepted science.

DrChinese said:
Sunil,

A good book might be "The Quantum Challenge" - https://www.amazon.com/dp/076372470X/?tag=pfamazon01-20

or even "Sneaking a Look at God's Cards" by GianCarlo Ghirardi,

or even "The Quantum Universe" by Brian Cox and Jeff Forshaw.

DrChinese said:
The results are predictable by current theory, which maps the outcome probabilities. That is what QM is, a useful theory that accurately predicts many phenomena. It is all well and good to imagine things look like this or that in your own mind. There are numerous models, called interpretations, as I mentioned, that can help if you like.

Hi DrChinese,

I was wondering whether the double-slit experimental results take into account photons that instead of going through both slits, tunnel through the barrier between the source and end screen, and make it to that end screen?

StevieTNZ said:
Hi DrChinese,

I was wondering whether the double-slit experimental results take into account photons that instead of going through both slits, tunnel through the barrier between the source and end screen, and make it to that end screen?

Although tunelling is a quantum phenomena, it's not relevant to the double slit experiment.

The probability for a photon tunnelling through an opaque barrier is so low that it is easily neglected. In an actual experimental setup, the contribution from other background effects are many orders of magnitude greater.

You can easily test this for yourself. Hold up an opaque object to a light souce. How much light gets through? Nothing noticable? Well, only a tiny fraction of that unnoticable amount is due to tunelling. Virtually nothing of virtually nothing.

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I gather that in Feynman's approach to calculating the paths to the screen, paths that include the particle tunneling through barrier would also be accounted for. When calculating the probability for the particle to land at position x on the end screen, that includes all paths to get to x (going through both slits, going to the end of the universe and back), including the tunneled paths, which replicates the Schrodinger equation (QM) prediction as well?