Double-slit experiment and time

[Enlanda]

TL;DR Summary

I was interested in whether anyone had conducted experiments that would reveal unusual properties of the particles being experimented on from the point of view of time.

Talking to my AI assistant about the Double-slit experiments that have been conducted many times, I was interested in whether anyone had conducted experiments that would reveal unusual properties of the particles being experimented on from the point of view of time.

For example, place one slit directly opposite the particle projector so that the electron would get there at a right angle along the shortest distance, and the other slit very far from the projector, say to the right or left at an acute angle. At such a distance that it would be possible to detect the difference in time with which the observed particles passed through these two differently located slits.

In order to confirm or refute the assumption that a particle in a quantum state will pass through both slits at the same time or will go to the far slit faster than to the near one.

The AI answered my question in the negative. It had no information about such experiments.

I described only one example that came to my mind off the top of my head. But I would like to know about any experiments studying entanglement in terms of time anomalies, rather than space or energy, that have already been confirmed.

 

[Nugatory]

In order to confirm or refute the assumption that a particle in a quantum state will pass through both slits at the same time or will go to the far slit faster than to the near one.

You may have been misled by non-technical descriptions in the popular press, because the double-slit experiment is not like you are thinking.

There is no “in a quantum state” or not, the particle is always in a quantum state and the “wave-particle duality” you may have read about is a misconception abandoned almost a century ago. It makes no sense to talk about the particle passing through one or both slits at the same or different times, that’s just not how quantum mechanics works.

What is true, and has been tested many many times, is that interference only happens when the lengths of the paths through the two slits differ by less than a certain amount (the “coherence length”, a property of the incoming particles).

You might give Giancarlo Ghirardi’s book “Sneaking a look at god’s cards” a try. It’s a layman-friendly explanation of quantum mechanics that avoids some of the worst popular misunderstandings.
(And be very cautious about anything an AI says. They do some things very well, but separating good information from bad is not one of them - which is how the AI didn’t tell you that everything you had heard about the experiment was wrong. Indeed, they are so unreliable for this purpose that the forum rules do not recognize anything they say as a valid reference).

 

[PeroK]

I described only one example that came to my mind off the top of my head. But I would like to know about any experiments studying entanglement in terms of time anomalies, rather than space or energy, that have already been confirmed.

In practice, the double-slit experiment requires that the slits be close together. If they are too far apart, then either you get have two separate single-slit experiments; and/or, most of the source is absorbed by the barrier between the slits and doesn't get through, so you see nothing.

It usually requires a delicate configuration of the experimental apparatus in order to see the underlying QM interference. Otherwise, the experiment becomes what is technically known as incoherent, and the interference effects tend to get washed out.

 

[Enlanda]

In practice, the double-slit experiment requires that the slits be close together. If they are too far apart, then either you get have two separate single-slit experiments; and/or, most of the source is absorbed by the barrier between the slits and doesn't get through, so you see nothing.

It usually requires a delicate configuration of the experimental apparatus in order to see the underlying QM interference. Otherwise, the experiment becomes what is technically known as incoherent, and the interference effects tend to get washed out.

This is very interesting. Thanks for the explanation.
And what if instead of moving the slits at different distances from the projector. We put not one, but two screens in the path of what comes out of the different slits? So that what comes out of one slit gets on the near screen and what comes out of the second slit gets on the far away one.
Is it possible to conduct an experiment with the properties of time in this way?

 

[PeroK]

This is very interesting. Thanks for the explanation.
And what if instead of moving the slits at different distances from the projector. We put not one, but two screens in the path of what comes out of the different slits? So that what comes out of one slit gets on the near screen and what comes out of the second slit gets on the far away one.
Is it possible to conduct an experiment with the properties of time in this way?

QM is quite logical once you use the mathematics of the wavefunction. The detector screen has to be far enough away from the slits for the interference to be practically detectable.

If you close one slit, you get single slit diffraction. The further the detector screen, the wider the diffraction pattern.

That said, if the slits are close together, then there is no much room to have two detector screens.

 

[PeroK]

PS note that a lot of popular sources give the impression that the light or particles behave differently when there are two slits, than when there is only one. Even Jim al-Khalili in his lecture at the Royal Institution presented this quite explicitly.

The reality is that if each slit is too wide to produce significant diffraction, then two wides slits will produce little or no interference. It will just be two bands of light. Like you get through window blinds, for example.

What happens when there are two narrow slits is that you get diffaction from each and the diffraction patterns overlap. And the overlapping patterns produce a different interference pattern. There is no mystery here when you use a classical wave theory. There is also no mystery when you use a QM wave-function - which explains why an electron diffracts and interferes with itself; and why ultra-low-intensity light (one photon at a time) diffracts and produces an interference pattern.

The mysteries are sustained, IMO, by a half-hearted acceptance of QM, which still aims to explain things in terms of classical concepts of wave and particle, somehow scrambled by QM. Whereas, the classical concepts of wave and particle emerge from a single QM model. If you start with QM, there is no mystery. It's only when you insist on continuing to think classically that you get tangled up in apparanetly mysterious behaviour.

 

[Enlanda]

Yes. I think I understand what the problem is with both of my proposals.
Did I understand correctly that when an electron in the form of a wave penetrates two closely spaced slits, we see approximately the same picture that we would observe in a calm sea. We would have a breakwater with two narrow slits and a wave, say a small tsunami, would go to the shore, meeting the breakwater on its way?

A tsunami wave, although expressed and visible by means of water molecules, still has a clear energy born thanks to, say, an underwater earthquake. And this pure invisible energy gives itself away by moving water molecules.

 

[Enlanda]

You might give Giancarlo Ghirardi’s book “Sneaking a look at god’s cards” a try. It’s a layman-friendly explanation of quantum mechanics that avoids some of the worst popular misunderstandings.
(And be very cautious about anything an AI says. They do some things very well, but separating good information from bad is not one of them - which is how the AI didn’t tell you that everything you had heard about the experiment was wrong. Indeed, they are so unreliable for this purpose that the forum rules do not recognize anything they say as a valid reference).

Thank you very much for this explanation. Yes, I myself have encountered the unreliability of AI when it comes to information. Sometimes it slipped me its own calculations when it could not find links to real ones. I will try to read this book.

So in practice there is nothing super unusual in either quantum entanglement or quantum superposition?

P.S. Still, by sending me to this forum, AI did me a good deed. :)

 

[PeroK]

Did I understand correctly that when an electron in the form of a wave penetrates two closely spaced slits, we see approximately the same picture that we would observe in a calm sea.

The first thing to understand is that at a certain level of precision, the electron appears to be a classical point particle with a well-defined trajectory. If you look more closely, the electron has an uncertainty in position and cannot be described as a point particle beyond a certain level of precision. When it passes through a slit (or a crystal that acts as a diffraction grating), the more precise QM model is needed. There is the concept in QM of a minimal-uncertainty wave packet. This is a state of a single particle that has effectively as little uncertainty as possible. Once the scale of the slits is on the scale of this minimal uncertainty, then the probabilistic wave-packet that describes the electron extends across both slits. And then the phenomena such as diffraction and double-slit interference are fully observed. But, the electron always was described by a probabilistic wave packet. If the slits are too far apart, then the wave-packet can only practically extend over one slit, not both. And, if the slits are too wide, the wave-packet only extends across some of the slit and effectively passes through like a particle would.

This explains the experimentally observed wave-particle duality, without the electron inherently having any such duality.

This is the only way to understand QM. That the probabilistic wave-function/wave-packet exhibits both classical particle-like and classical wave-like behaviour, depending on the characteristic of the experiment. In the context of a free electron fired at a screen, it always is a probabilistic wave-packet - it's not a classical wave and it's not a classical particle. It's a quantum object.

 

[Enlanda]

The first thing to understand is that at a certain level of precision, the electron appears to be a classical point particle with a well-defined trajectory. If you look more closely, the electron has an uncertainty in position and cannot be described as a point particle beyond a certain level of precision. When it passes through a slit (or a crystal that acts as a diffraction grating), the more precise QM model is needed. There is the concept in QM of a minimal-uncertainty wave packet. This is a state of a single particle that has effectively as little uncertainty as possible. Once the scale of the slits is on the scale of this minimal uncertainty, then the probabilistic wave-packet that describes the electron extends across both slits. And then the phenomena such as diffraction and double-slit interference are fully observed. But, the electron always was described by a probabilistic wave packet. If the slits are too far apart, then the wave-packet can only practically extend over one slit, not both. And, if the slits are too wide, the wave-packet only extends across some of the slit and effectively passes through like a particle would.

This explains the experimentally observed wave-particle duality, without the electron inherently having any such duality.

This is the only way to understand QM. That the probabilistic wave-function/wave-packet exhibits both classical particle-like and classical wave-like behaviour, depending on the characteristic of the experiment. In the context of a free electron fired at a screen, it always is a probabilistic wave-packet - it's not a classical wave and it's not a classical particle. It's a quantum object.

Did I understand correctly that a wave packet is the width, height and depth of a wave that describes all possible positions of an electron, thus creating a "wave packet"?

P.S. In other words, an electron is not a point but a sphere?

 

[PeterDonis]

the path of what comes out of the different slits?

There is no such thing as "the" path for what comes out of each slit.

 

[PeterDonis]

an electron is not a point but a sphere?

No. It's neither. It's a quantum object.

 

[PeroK]

Did I understand correctly that a wave packet is the width, height and depth of a wave that describes all possible positions of an electron, thus creating a "wave packet"?

P.S. In other words, an electron is not a point but a sphere?

It's not a sphere. It's important to recognise that a wave-packet is a mathematical description of an electron in terms of a complex-valued wave-function. Technically it describes a complex probability amplitude; not even an actual real probablity.

This is the intellectual step that understanding modern physics demands: that objects at the sub-atomic scale are described by mathematics only. Technically, a wave-packet extends over all of space, but it can be highly localised. The probability of an electron in a localised state interacting with a slit even a few centimetres away is effectively zero. This is why the slits cannot practically be far apart: the probability (amplitude) of interaction with a slit becomes almost zero.

For example,

https://en.wikipedia.org/wiki/Wave_packet#Gaussian_wave_packets_in_quantum_mechanics

To go further, you need to start to grasp the mathematics.

 

[Enlanda]

For both PeroK and PeterDonis.
I'll make one last attempt to understand it on my own, then I'll go read Giancarlo Ghirardi's book. :)
A quantum object is information. Something that is the same fundamental basis of the universe as time, energy, space.

 

[PeterDonis]

A quantum object is information.

No, it's a quantum object.

Something that is the same fundamental basis of the universe as time, energy, space.

I don't know where you're getting this from.

 

[Enlanda]

No, it's a quantum object.


I don't know where you're getting this from.

I see.Thank you. Maybe the book will help me. :)

 

[Nugatory]

So in practice there is nothing super unusual in either quantum entanglement or quantum superposition?

It depends on what you consider "super unusual". The behavior of quantum objects is different from anything we have experienced in a lifetime spent with classical objects, so our common sense about how things should be just doesn't work. For example: things have positions? Nope. If a particle is emitted at point A and detected at point it must have present somewhere in the space between A and B? Nope. The number of left-handed men in a room cannot be greater than the number of left-handed non-smokers of either gender plus the number of men who smoke? That logic works for classical objects but not quantum objects.... and many more things that you might consider super unusual, weird, counterintuitive.

However, these are all cleanly described by the math - at small scales the world works the way the math says it does, it's just not what we expect. Analogies like "wave", "particle", "goes through two slits", "position is spread out" all fail because they are referring to classical concepts and quantum behavior isn't like that, but when we apply the math it's all clear.

This is why in your other thread @PeroK said "Physics deals with those, largely through the use of mathematics". Math is the language of quantum mechanics, and studying QM without the math is like studying art through written descriptions instead of looking at the paintings.