Double Slits with Single Atoms
physicsworld.com
physicsworld.com
Just saying "wave-particle duality explains it" is false. It was a shock for me to realize that everything taught in physics textbooks was so misleading. (For a good explanation see MWI)
"Why do some stars make loop-de-loops as they traverse the sky?"
"Because entities on the celestial scale don't behave in ways directly analogous to anything humans are intuitively familiar with."
"Oh..."
Sure. But, AFAIK, newtonian physics covers most of the behavior of the planets; that GR affects the timing rather than the paths. Or am I substantially wrong?
It's not just "not an explanation", it's declaring that there _is no_ correct and easily understandable mathfree explanation or analogy. The only way you can properly understand things like this is within the context of the experimental measurements we've made and the mathematical models we've developed as a result. There's no way to explain these to a five year old in a way that's not significantly incorrect (unless perhaps they happen to be uncommonly good at linear algebra) .
The current layperson explanation is so significantly incorrect that I think it's actually detrimental. Photons aren't "particles" like marbles, and they're not also "waves" like you see on the ocean. They're sure as hell not both at the same time in some sort of religiously inspired "don't question it" union.
What they are are entities, abstract things that we can create and play with and measure. We can make somewhat complicated mathematical models to predict certain aspects of their (and similar entity's) behavior. They at times exhibit behavior one would expect from a particle. In other cases they exhibit behavior one would expect from a wave. They're neither though, they're their own thing that behave by their own rules. That's it.
It absolutely is an explanation. The explanation is not just "they don't behave as neutonian macro objects" - that's the a beginning of it. The explanation itself comes after when physicists describe in exact ways how these quantum objects behave, explained so specifically that it can be calculated upon with specific equations.
The fact that those explanations don't fit in your current set of intuitions about world and objects in it is more of a "you-problem", not "physics-problem". The universe has no obligation to make sense to you, it is more of your job to create those new intuitions in your mind and those new models of the world. While the physicists have provided you with all the needed tools, it is still up to you to build them into your mind.
It's easy to explain.
The particles aren't interfering with each other. A wave is interfering with itself due to the two slits 'splitting' the wave. The presence of other particles has no baring on this.
> And how does covering up one slit destroy the interference pattern when there is only one particle going though one of the slits in the first place?
With one slit you don't split the wave so there is no interference. The particle doesn't go through one of the slits, it goes through both simultaneously in 'wave form', this is the mystery.
Edit: If you try to actually define "wave function collapse" and "observer" you are back into the same mumbo-jumbo as "wave-particle duality". They are just placeholders that obscure the mystery.
Edit: I think I understand what you're saying. The field isn't separate from the particle - the particle is the quantized field itself.
This lecture is long but goes into the everything is fields idea https://youtu.be/gEKSpZPByD0
As strange as it sounds, it does make some sense. Think of elementary particles; we never observe them as such, we only observe them when they're interacting with something.
Yes. A particle is nothing but a very localized wave. Aka. "everything is fields."
(At least that's may layman's understanding having read quite a bit about this in recent years.)
(In Ukrainian) https://www.facebook.com/groups/194031297610629/permalink/10...
The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.
I think it's a much simpler interpretation than the Copenhagen interpretation.
Here's an awesome discussion/debate between Carroll and a major MWI skeptic, David Albert: https://youtu.be/AglOFx6eySE
In this discussion, they cover what they consider "silly" and "important" criticisms of MWI. Respectfully, some of what you said in your comment is placed by them in the "silly" box.
To me it seems there can be two purposes for creating a good interpretation: 1) make it more in line with most people's intuitive thinking about the world, to make them understand the theories better and faster and 2) make it easier to produce new scientific results, or predict other things.
So far I don't see how the MWI manages either of those better than the other models.
Many worlds isn't the universe forking. It is more straightforward to think of the other worlds as probability spaces. The probability space simply divides as we call each division a "world".
On the contrary. When you look at the specifics it's all very obvious. Look at the wavefunction, not the TV-show verbal explanation, and the answer to your first two questions are very obvious, as is the ill-formedness of your final question.
It's a pretty simple concept. If you create a random hypothetical universe that follows all the laws of physics what reason is there for it to not exist? What constraints beyond the laws of physics ensure that our universe is the only one that can possibly exist?
Whether this is simpler than wave collapse or hidden variable interpretations is an almost purely subjective opinion.
From the perspective of the photon the time stands still, it arrives instantly at the destination.
You can see this in that the quantum eraser experiment only works with photons and not particles with mass like electrons.
It works with even larger particles as well such as molecular ions or buckyballs.
So my thought was maybe the particle in the two-slit experiment was entangled with itself? Like I said, idle thoughts of a layman so not sure how right, wrong or not-even-wrong this is but hey, always fun to think about QM :)
That out of the way -
The "thing" is neither a particle or a wave, it's a propagating probability amplitude.
With one slit, you get a simple probability distribution. With two slits, you get a complex probability distribution, where the amplitudes are summed (and since they can be negative, sometimes sum to zero)
The detector adds constraints to the probability amplitudes, zeroing part of the complex probability of the double slit, thus reducing it to a non-complex distribution.
The expectation was that the electrostatic forces would create "bubbles" of empty around the ions, which (the bubbles, not the ions) would then be detected when they reached the bottom.
They found those bubbles. They also found many differently sized bubbles, all smaller, in a size distribution correlating to the probability that the ion would pierce the surface tension of the liquid.
The interpretation was that the smaller bubbles were the "pieces" of the waveform that traversed the surface tension. This was taken as evidence the waveform is an actual thing that actually exists.
I really wish I could find it...
https://en.m.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect
But you can't look at a single photon/electron/whatever and get this contextual information out of what we know about the physical structure of the object itself. There is no physical marker we can read "inside" a photon that says it's entangled, or that it's passed through two slits by "interfering with itself".
So the information - and probably the process itself - is contextual. We don't know where this information lives. But it has to exist somewhere, because we can see the effects of its operation very easily.
So it's physical in the sense that we can see physical effects. Photons etc know what they're doing, and we can make excellent predictions about that behaviour. But the exact nature of the entities and relationships that makes this process work, and the way it maps to observable events and properties, remains a mystery.
The obvious implication is that what we see isn't really fundamental. Neither field excitations nor the fields of QFT are the ground truth, and there's another layer of reality that generates them - and probably spacetime too. But that layer has some very unusual properties from our POV, and we're going to need some new metaphors to understand it.
If you were modelling the universe and everything in it, say via a simulation, fancy probability distributions would be a good way of calculating behaviour. What if the "stuff of reality" we're looking for (and trying to map the math to), doesn't actually exist? What if the math is all there is? What if behaviour is all that matters?
In primitive cultures, spirits were hypothesised as the animating force behind behaviour of animals, stars, weird people etc. For a long time, it proved a reasonable model until it was supplanted by our more scientific notions of the observable universe. But what if quarks, leptons, photons, are just "spirits" i.e. a good enough approximation given what we know? What if it's "spirits" all the way down? i.e. the only fundamental is the math and everything else is a visualisation (of sorts)?
BTW, that kind of speculative question (and an awesome high school physics teacher), are what made me study physics at university. Shame (in one way) I was more interested in rocking out and drinking beer.
EDITED TO ADD: don't take anything I post too seriously. If you find yourself rolling your eyes, just downvote me and move on.
Behavior, observation, accurate prediction and empirical evidence is all that matters in science, yes. What difference does it make if you call it "particle" or "just math"? What practical difference does it make?
If none then that is not a question of science. It might be an interesting question of philosophy or spirituality, but not for science. Science deals with falsifiable hypothesis and accurate predictions and making functional machines.
If you measure before/at the slit you affect the experiment. You affect how it travels through the slits, removing the interference pattern. So yes it’s not split but that has no bearing on when the detector is not at the slit. They’re different situations and have different outcomes.
So in reverse then, when absorbed the wave could be cancelled out and it is forced into particle behaviour?
"that we can see". That's exactly the difference. The observation (measurement) affects the state of the system. That's like quantum mechanics 101.
This is to me the most baffling aspect of QM.
With a TIRF microscope you use the 'Evanescent Wave' to image fluorescent molecules in macro objects like neurons or diamonds.
Like, you can see, even more clearly, the yeast that makes your beer.
https://en.wikipedia.org/wiki/Total_internal_reflection_fluo...
https://lightmicroscopy.ucdenver.edu/img/seminars/June17.png
Particles are like boats on a lake - they both cause waves when they move and are moved by relfections of their own and other boats' wakes.
Interference is simply waves generated by a boat interfering with other waves from the boat.
Given that quantum mechanic is non local (quantum teleportation), I don't think that this is the big issue. Why QM is non-local yet cannot send data faster than light, that's much more unexplained.
Relativity showed us that our assumptions about space and time being constant were wrong and instead light is a constant.
Similarly, QFT shows us that our measurements do not have single outcomes and never did. Instead reality is made up of quantized waves whose amplitudes are complex numbers and can interact constructive or destructively as long as they remain coherent.
Can I setup my experiment so that quantum decay decides if I emit an electron or not - and then wait for no electron, but I somehow detect the electron that was released in a another universe?
MWI also doesn't explain why the individual electrons would take different paths and interfere with each other - the universes are identical, so why are the electrons taking different paths?
See "decoherence": https://en.m.wikipedia.org/wiki/Quantum_decoherence
MWI is a local theory. Changes spread out at the speed of light, they just spread out (and back from) adjacent universes as well. https://www.hedweb.com/manworld.htm#local
> the universes are identical...
Think of a 3D volume of space with a plane wave travelling through it, such as a beam of coherent laser light. An alternative way of thinking about this is that each point emits a spherical wave and the plane wave is just constructive interference of a continuum of spherical waves.
In many worlds theories, you can think of each point as also spreading out into non-spatial dimensions that are directions towards other universes. Similarly, aggregate behaviour is the constructive interference of not just the points in "one" universe, but also the points in a neighbouring "volume" of universes.
If the neighbouring universes are identical, or nearly so, it's like the coherent laser scenario. If they're all different, it's more like white light -- it's unpredictable and "random", but otherwise it's still just ordinary light.
Very slight deviations would result in various interference effects, such as the double-slit scenario.
The universes aren’t identical, of course. There’s a universe for every path of the electron that is physically possible.
V^2 + P^2 + C^2 = 1
I'm just a layman so have no idea how profound this is, but I found it interesting.
[1]: https://www.osapublishing.org/optica/abstract.cfm?uri=optica... (open access)
There are lots of videos about the various interpretations of quantum weirdness through a robust scientific lens, while still being entertaining enough to watch even if you don't grasp all the details.
In this model, both the wave and the particle are simultaneously real. the particle is being moved around by the wave. When we get near the double slit, the wave naturally travels through both slits and forms an interference pattern.The particle randomly moves through one of the slits, and is them carried further by the interference pattern, the highest probability being that it will be carried to one of the local maxima.
The nicest part is that this exact behavior can be seen by bouncing a droplet of oil or silicone on a vibrating bath of the same material: https://www.youtube.com/watch?v=nsaUX48t0w8 .
Of course, this being a consistent model for QM, it doesn't get away from the Bell inequalities, it doesn't have local realism. It does preserve realism, but it it is a nonlocal theory (some effects travel instantaneously).
We can clearly label particle (droplet), wave (around droplet), interference caused by wave with itself (when it goes trough both slits), and quanta (single wave, distance between two valleys).
One poor universe, however, sees no evidence of wave-like behaviour at all!
There's a paper out there about positing a series of non-quantum universes (particles that aren't also waves), except where each universe exerts a force on all other universes to not be identical. Apparently, under this framework, for N such universes you get an N-step approximation of the wave form (reaching the wave form as n->infinity).
Which is pretty cool in an of itself, _and then_...
We observe the wave form. Does that then mean we're observing across all of those universes?
There is a reason why almost nobody is taking it seriously.
> A real experiment along these lines appears feasible, as all components of the apparatus have been realized separately; for details consult Appendix A. The purpose of the proposed experiment is to verify the predicted quantum theoretical correlations: Whenever an atom hits the screen in the lower region A, the upper detector says "yes" - and likewise for the upper region B and the lower detector. Since we know that (some of) the retrodicted Böhm trajectories pass through the other detector, this verification suffices to demonstrate the asserted metaphysical character of the Böhm trajectories.
Now all of that said, a group actually did the experiment and showed that they only way someone would incorrectly conclude the trajectories were surrealistic would be if they mistakenly ignored the non-local nature of their behavior. They actually performed an experiment and demonstrated the results.
https://advances.sciencemag.org/content/2/2/e1501466.full
> We have verified the effect pointed out by ESSW that for a WWM with a delayed readout, Bohmian trajectories originating at the lower slit may be accompanied by WWM results associated with either the upper or the lower slit. However, this surreal behavior is merely the flip side of the nonlocality we also demonstrated. In Fig. 3, we showed that the trajectory of photon 1 depends on the choice of measurement (polarization basis) for photon 2. In Fig. 4, we see that the polarization of photon 2 depends on the choice of when (that is, at what point along the trajectory) to measure the position of photon 1. This nonlocality is due to the entanglement of the two photons, which, in Bohmian mechanics, makes their evolution inseparable even when the photons themselves are separated. Because entanglement is necessary for the delayed measurement scenario of ESSW, this nonlocal behavior is to be expected and is the reason for the surreal behavior they identify. Indeed, our observation of the change in polarization of a free space photon, as a function of the time of measurement of a distant photon (along one reconstructed trajectory), is an exceptionally compelling visualization of the nonlocality inherent in any realistic interpretation of quantum mechanics.
Essentially the claim in the paper you linked was incorrect, and experimentally demonstrated to be so.
https://blogs.scientificamerican.com/critical-opalescence/ho...
My guess is that he could use his sputtering device or electrochemical deposition to develop a thin film for the target, electron microscope to mill and validate the slits in the film, then either the electron microscope modified or mass spectrometer almost as as-is to actually run the experiment as long as it's OK to bend the electron's path with a magnetic field after the slit and scan the interference pattern across a fixed point sensor. The mass spectrometer seems like it's basically set up to do the atomic interference patterns already (again assuming steering is OK).
If you're not allowed to manipulate the path of the electron he'd probably have to come up with a mechanism to detect the interference pattern.
In Galilei's time not many had access to high quality optics, precise enough clocks and a high enough and sufficiently inclined towers;-) I also believe that most people at his time didn't grasp the importance of his findings. Nowadays everyone can replicate his experiments and repeat his observations and that is good, but do people really understand the implications of the insights we gained in the last 100 years? I think most people outside of physics (and a few select areas of tech) see this modern experiments as some weird outliers instead of profound discoveries about our world. In my opinion the best, and maybe only, way to change this to have experiments like the one you described - modern physics in the garage.
Schools are also important: In the same way we let children do Galileis experiments we should be able to let them do modern experiments as well. My dream would be showing CHSH inequality in a fourth grade classroom.
Image you're tracking someone across the country. You have some data that they were in 2 towns 100 miles apart with the hour. They must be in a car!
But then, you catch up to them and find their physical tracks. It's a single line. No car could drive like that. They must be on a bicycle.
You sit there scratching your head wondering how they could possibly be going by car and bicycle at the same time.
The above situation looks impossible if you don't know about the existence of motorcycles. I always thought we were dealing with things that were not waves, not particles but something that shares some of the properties of both.
I'd love anyone with a deeper knowledge of physics to correct me/add to my thinking.
“2000 atoms in two places at once: A new record in quantum superposition“
https://phys.org/news/2019-09-atoms-quantum-superposition.ht...
It's purely classical, based on the wave equation however.
https://en.wikipedia.org/wiki/Hydrodynamic_quantum_analogs#S...
-particles have variable density and gravity. -particles rotate. -slits create multiple paths. -particle rotation + variable density and gravity makes particles choose multiple paths.
It's an explanation which is totally based on macro observations, as if particles where balls with a small heavy object in them, thrown out in multiple tracks. The initial configuration is random, the balls will roll into specific tracks, but since all outcomes are equally possible, over time the distribution of the balls will be equal for all tracks.
https://www.livescience.com/19268-quantum-double-slit-experi...