# Is my understanding of the double-slit experiment correct?

I'm no quantum scientist. I'm just a software engineer with a healthy (?) fascination with quantum mechanics and knowledge gained from Googling. :) I've read many different articles about the double-slit experiment over many years. One I just read made something "click" in my head and I want confirmation, or at least discussion about it. The article (Are electrons waves or particles?) stated:

"The device interacts with the electron sufficiently to determine which slit, so the electron collapses down to a particle and goes through only one slit. Once past the barrier, the electron, freed from interaction, reverts to its wavy state. Upon interaction with the detection screen, it again collapses down to a particle and lands as a tiny localized dot."

For the first time it occurred to me that quantum particles are ALWAYS in a wave state except at the very moment they're interacted with.

So, in the double-slot experiment, the quantum particle is fired and travels from the "gun" to the slots in a wave state, spreading out from the gun, and goes through both slots, emerges from the slots as two waves, interferes with itself and then collapses to a particle when it interacts with the detection screen. In my mind, I see the "wave" instantly vanishing (it's spread out quite a ways in many directions by this point) and the particle "popping" into existence. I've heard the wave pattern described as a "probability" wave before, so in my mind, I see the interference wave pattern having higher "probability" in stripes (due to the interference) strongest directly across from the slots and getting weaker further from the slots. So the quantum particle "collapses" to a physical particle mostly where the probability in its wave pattern is strongest (directly across from the slot), but also (less frequently) sometimes in the less strong stripes, and mostly not in the areas between the stripes where the probability is the absolute lowest.

Add the observer at the slots to detect which slot it goes through. Again, the quantum particle is fired from the "gun" toward the two slots. It travels from the gun to the slots, but this time it's interacted with by the detectors. It instantly collapses to a physical particle at whichever slot its probability wave is strongest at? I'm guessing here. This is a point that I've never read described in a way that makes it clear to me or fits my "internal" mental model of how this works (which may be very wrong). At this point, the entire wave (which has been spreading out from the "gun") completely vanishes (stops propagating as if it never existed) and a physical particle appears at one of the slots. But the interaction that collapsed it is instantaneous (or at least a very small portion of time) and the particle immediately reverts back to a wave; but with a new "point of origin", the slot it was just forced to collapse at momentarily. Since now the wave is only originating from one of the slots, there's no interference with itself and so when it hits the detection screen, its point of strongest "probability" is directly across from the slot it originated from (after its momentary collapse and re-conversion back to a wave) and so it mostly hits directly across from the slots.

Am I way off base? Does this sound crazy? Or am I actually starting to get a better-than-normal grasp on this? If I'm on the right track, why have I never read it described this way before? There are areas I'm still unclear on, such as when the "wave" leaves the "gun" and the "waves" leave the "detectors" at the slots, they're both waves with probability. I think of some "interaction" happening at the slots that collapses the "gun" fired wave, but in reality (no pun intended), both waves are collapsing at some point. Since they're both just waves of probability, where does this interaction happen? Is it simply because their combined "probability" at the slot is highest so that's where they both mostly collapse?

Also, if I'm right, then if the detectors were placed a bit BEFORE the slots (between the "gun" and the slots), when the quantum particle (the one fired toward the slots) collapses and then re-converts back to a wave, wouldn't the wave have enough time to spread out again to the other slot (maybe weaker at the other slot?) and still interfere with itself? Maybe producing weaker stripes between and to either side of the slots on the detection screen than if the detectors weren't there?

Thanks for hearing me out. I had all this in my head after reading that article and having the epiphany that maybe quantum particles are ALWAYS in the wave state except for the brief moment of "interaction". This also has me thinking that everything I see is mostly in a wave state except for the brief moments a photon hits a particle in it. When that rare photon that hits a particle, collapsing it momentarily, and bouncing off it, hits my eye, long before my brain interprets that photon as "yellow", that particle has re-converted back to a wave... and probably collapsed and re-converted to a wave MANY times from other photons "interacting" with it. My mind is reeling with this possible new way to think of reality. I love it! Even if I'm totally wrong, it's so fun to think about it. :)

• The first professor I had for quantum mechanics liked to say that asking whether an electron was a particle of a wave was like asking whether an ant was a fly or a lobster. The idea was that an electron has similarities to both particles and waves, as we encounter them macroscopically, but it is not the same kind of thing as either a classical particle or a classical wave.
– Buzz
Commented Dec 24, 2021 at 6:04
• There are no "quantum particles". There are only quanta of energy. From that alone it follows that basically everything you have ever heard about the double slit is based on poor mental models. Technically the double slit is not even a quantum experiment. For light it's a classical wave optics experiment and for electrons, etc. it's, at most, a wave mechanics experiment. Commented May 31, 2023 at 13:35
• @FlatterMann All particles exhibit behaviour typical to the quantum realm. Commented Jun 7, 2023 at 4:54
• @DukeWilliam That's the point: there are entire libraries filled with quantum effects like "chemistry", "astrophysics", "nuclear physics", "solid state physics" and "biology" and absolutely nothing of value about "wave particle duality". That term is merely an old and useless false dichotomy fallacy that needs to be removed from science. We aren't in antiquity and the middle ages any longer where scientific reductionism took on absurd forms like the four humors hypothesis in medicine. We know much better now. Commented Jun 7, 2023 at 20:55
• Does this answer your question? Is the wave-particle duality a real duality? Commented Oct 20, 2023 at 1:51

You're doing fine.

People will quibble with some of your language choices. They will point out that your way of thinking is not as useful as some other ways of thinking for one or another specific purpose. (Some of those people will confuse the concept of "useful for me" with the concept of "likely to be useful for you".) But unless you are trying to do research in quantum field theory, none of that matters. You have a perfectly good way of thinking that serves plenty of people perfectly well, and at this level there is no alternative way of thinking that is likely to serve you better.

As a software engineer, you might want to study some quantum information theory, where the wave functions (which form an infinite dimensional vector space) are replaced by elements of finite dimensional vector spaces, and much of quantum mechanics --- including all of the phenomena you're thinking about --- come down to linear algebra. If you're comfortable with linear algebra, that study will sharpen the vision you've already got, but won't change it substantially.

If only a single edge is needed to produce the wavelike intensity distribution on the screen, isn't this the most basic experiment from which to draw our conclusions?

If one sees a wavelike intensity distribution on a screen, does this mean that the objects hitting the screen are waves?

If the measuring instrument acts on the object with a force that sensitively disturbs its trajectory, should we be surprised that the wavelike intensity distribution on the screen changes?

Furthermore in physics polaritons are introduced. They are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation.

Plasmons have also been proposed as a means of high-resolution lithography and microscopy due to their extremely small wavelengths; both of these applications have seen successful demonstrations in the lab environment.
Finally, surface plasmons have the unique capacity to confine light to very small dimensions, which could enable many new applications.

Based on the above considerations, it would be good to reconsider an interpretation Young made 220 years ago: the intensity pattern on the screen is the result of the interaction of objects with the disturbed surface electrons of the obstacle.

The only criterion for rejecting this interpretation are calculations that rule it out. Only a studied physicist can do this calculation. Unfortunately, however, trained physicists are so attached to their training that they reject the new interpretation without investigation.

• Actually, the calculation of the diffraction pattern seen behind the two slits is not so difficult. Anybody with an understanding of Fourier transforms can do it. The implication is that we do not need to use very sophisticated understandings of the interaction between the light or electron passing through the slits and the screen to predict the diffraction pattern. Commented Oct 20, 2023 at 2:49

I'm also questioning my understanding of this experiment and how popular science content creators describe it.

I haven't found much information about how they detect from which slit a particle goes through. What are the tools and methods for this?

I've read somewhere that the way they do this is just to cover one of the slits so we know which slit it goes through. And there's never 2 parallel strips as a result of this experiment. Any insights about that?

• Hi, welcome to Physics SE. Unlike forums, SE websites follow a question exclusively with answers to it. I recommend you post a question of your own.
– J.G.
Commented Oct 17, 2023 at 20:27

You ask "am I way off base?" and I would say yes! .... but not that far off. All particles, even baseballs, have wave properties in theory ... but for a large mass the wavelength is so small it is impractical to measure, you would need a photon of such short wavelength that it could not exist.

Per famous physicists Dirac and Feynman they stated that every particle determines its own path. It is still a particle with wave properties, the particles NEVER splits into 2 pieces.

All of these slit (not slot) experiments are performed with particles under the influence of the EM field. The EM field acts 1) virtually (virtual photons) which is forces (like static electricity) and 2) with real photons that can transfer energy across space.

There are many virtual forces occurring before an electron or a photon even leaves its starting position, these forces can divide over the 2 slits (or many). As a result of these forces the particle or photon or electron will take a path to the screen.

For a photon we have complete collapse, the photon no longer exists, the screen (or your eyeball) gets the energy. For an electron experiment the screen gets a negative charge and increased mass, but the electron has lost its wave properties and gains new wave properties in an orbital of an atom/molecule.

• Dave, you state very explicitly that the particle never splits into 2 pieces. Why then do some assume that an interference pattern means the particle travels through both slits at the same time; why doesn't the central section of the double-slit divide a particle? Commented Mar 23 at 13:01
• The EM field of the electron field sees the 2 slits, the electron is not in control of its path.... the EM field rules all the paths taken ... the photon or electron are just along for the ride. In an apparatus/experiment the EM field sees all the charges/electrons of all the atoms ... excited electrons have a bigger effect. An electron can be exited in an atom for along time .... Commented Mar 23 at 16:17

maybe quantum particles are ALWAYS in the wave state except the brief moment of "interaction".

It is misleading and not according to the mathematics of quantum mechanics to assign a wave nature to a single particle in space. The particle's wave nature appears in the probabilily distributions of many particles under the same boundary conditions, and that is why it appears in a double slit experiment.

The probability distribution for the results of a given experiment is given by the $$Ψ^*Ψ$$ value where $$Ψ$$ is the wave function modeling the experiment. It needs many electrons with the same boundary conditions to measure a probability distribution and check with the quantum mechanical prediction. What is waving is the wavefunction, not the electron, which has a probability,( as the point particle it is in the standard model), of being at (x,y,z,t), probability given by $$Ψ^*Ψ$$.

Here is a description of the famous Tonomura double slit experiment one electron at a time

The electrons leave the footprint of a particle, one at a time. The accumulations of single hits from frame a at frame e show the wave nature of the accumulated electron footprints.

The point particle electron passes either through one slit or through the other , never a single electron from both. The probability of its going through one or the other is given by the wavefunction , a complex number solution, $$Ψ$$ of the differential equation and the potential/boundaries "electron scattering through two slits a given width a given distance apart". The probability is given by $$Ψ^*Ψ$$. A detector at the slit will change the boundary conditions and a new $$Ψ$$ will describe the electron, incoherent in probability space with the original , so there can be no interference in probability values of the exit from the two slits.

that maybe quantum particles are ALWAYS in the wave state except the brief moment of "interaction".

Yes they are, but in probability space , the (x,y,z,t) for single particles is not calculable, only the probability of appearing at (x,y,z,t) has a wave nature.

Here is a simulation of the interference pattern associated with same energy/momentum single electrons '

The experiments are consistent with this.

• Nope. Quantum particles very definitely go through both slits. We know this as follows Cover one slit and get a pattern. Then uncover that slit and cover the other and get a patttern. Then uncover both and get a pattern, and note that the result is not the same as the sum of the two single-slit patterns. There is very definitely interference of each slit with the other.
– Dan
Commented Dec 24, 2021 at 5:34
• @Dan you are as misled. The patterns you see are probability distributions and as I clearly state in my answer they are the ones showing interverence. A single particle shows no interference or wave footprint on the screens. Unfortunately popularization explanations lead to this misunderstanding for people who do not know mathematics or equally people who do not understand experiments and their modeling by quantum mechanics. Commented Dec 24, 2021 at 7:35
• It seems to me that if you only fire one particle at a time, eventually they will form the interference wave pattern on the detection screen. Doesn’t this indicate that individual particles are indeed “showing interference pattern”? Why do none (or at least very few) collapse in between the stripes? Why do some collapse in the weaker stripes between or to the outsides of the stripes? That seems to indicate to me that individual particles are governed by the interference of the waves going through both slots.
– Lee
Commented Dec 24, 2021 at 7:42
• @Lee our present mainstream physic model is quantum mechanical.Yes the mathematical interference comes from waves, but they are probability waves. They do not collapse in between because the probability function that models the scattering gives small probability to fall there. By the way, collapse is also a misleading term, the probability is not a balloon, What happens is when the electron hits the screen a new wavefunction is necessary to describe "electron-screen" interaction. Commented Dec 24, 2021 at 7:59
• There are theorists working on deterministic underlayers for quantum mechanics, but are not successful up to now. Commented Dec 24, 2021 at 18:19