Did the Particle Go Through the Two Slits, or Did the Wave Function?
30 comments
·March 13, 2025Willingham
analog31
Perhaps the closest thing would be some nuclear decays that spit out two gamma rays of equal energy in opposite directions. I'm struggling to remember which isotope does this.
out_of_protocol
Double slit experiment did happen and totally reproducible even then photons/electrons are sent by one at a time.
"two photons come out" part makes no sense though. On a target side, there's always single hit after single photon/electron, but distribution of theses hits as if said electron got through both slits and interfered with itself
P.S. the funny thing is - this works on any small thingy, measured up to 2000 atoms-big, as if it's the property of the universe itself
bad_haircut72
I would love to try this experiment with something basketball sized out in space. Like we build an enormous basketball detector behind a double slit inside an unobservable black box. If thr basketball started acting like a wave I would be sooo freaked out
whatshisface
The largest double-slit projectile I know of is C-60, a soccer-ball shaped molecule of sixty atoms.
https://iopscience.iop.org/article/10.1088/2058-7058/12/11/4
judofyr
Are you referring to the double-slit experiment? If so, yes: It has always been an experiment. The experiment came before any theory explaining the behavior AFAIK. https://en.m.wikipedia.org/wiki/Double-slit_experiment
somat
I think the problem is in insisting on referring to the photon as a particle.
In fact the photon may not actually exist. and I have questions as to what "single photon experiments" are actually measuring. let me explain.
The EM field is not quantized, or at least not quantized at the level of a photon, what we call a photon is the interaction of the EM field with matter, or more precisely with the electron shell of matter. it is the sound of the wave breaking on the shore, not the wave.
Now none of this actually matters as the only method we have of interacting with the EM field is through matter(electrons really) so we can only measure it in photon sized increments.
null
shagie
The double slit experiment has been replicated even with fairly hefty molecules.
https://www.nature.com/articles/s41567-019-0663-9
> Here, we report interference of a molecular library of functionalized oligoporphyrins with masses beyond 25,000 Da and consisting of up to 2,000 atoms, by far the heaviest objects shown to exhibit matter-wave interference to date.
It would be awkward to say that the 2000 atom molecule comes out of both sides... but it does, until you look.
The double slit experiment is not a duplication cheat of reality... it's weirder than that.
marcosdumay
What does "one photon hits a slit and on the other side two photons come out" mean?
There is no photon multiplication happening on the double slit.
Etheryte
The experiment came first, all the rest of it came after to try and figure out why the results are as weird as they are.
ryoshu
Live experiments have been done and they get really weird: https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
SiempreViernes
I'm not sure very many people will actually be helped by reading the linked discussion, which appears both too technical to be clear for newcomers to Quantum mechanics while also not providing any interesting detail for the more experienced reader.
This seems to be entire argument:
> But the wave function is a wave in the space of possibilities, and not in physical space.
Which is fair enough as an initial claim, but it doesn't really get motivated further, or at least not before I got bored reading and started skimming.
aap_
For a single particle they are easy to confuse. A wave function ψ(t,x) for a single particle gives a probability amplitude to find the particle at coordinate x at time t. In this case one can imagine an amplitude at each point in space and time, like a field. This interpretation however completely breaks down once you introduce a second particle: the wave function ψ(t,x1,x2) gives a probability amplitude to find particle 1 at x1 and particle 2 at x2 at time t. This no longer admits an interpretation of assigning some value to locations in space. Intuitively one might think you get one amplitude for each particle at some location but that's not how QM works, so we shouldn't think of the wave function as living in physical space.
whatshisface
That's true, but it's also true of the classical probability distribution p(t,x1,x2).
criddell
Considering a particle is an excitation of a quantum field, the space of possibilities could be seen as the only space there is. At least that’s what I think (but don’t know for sure) that the mathematical universe hypothesis people posit.
baq
Isn't it one of the 'does it matter if you didn't interact with it?' questions, and keep in mind 'observation' at quantum scales is to a good approximation synonymous to 'interaction'.
eptcyka
Approximation?
One can only measure by interacting, there is no other way.
falcor84
I'm a bit confused by the argument posed here:
> Figure 4: The wrong wave function! Even though it appears as though this wave function shows two particles, one trailing the other, similar to Fig. 3, it instead shows a single particle with definite speed but a superposition of two different locations (i.e. here OR there.)
I understand that if treat the act of adding two particles' wave functions as creating a new wave function for one particle, then we have this problem, essentially by definition. But it got me thinking - would it not make sense to treat the result as an expected value, such that we could then measure how many particles are likely to be to the right of the door at each point in time?
gitfan86
Particles don't exist. We just perceive waves with high decoherence rates as particles. Things we call objects effectively have a 100% decoherence rate. Things we call waves like light have low decoherence rates.
But underneath it is all quantum mechanics.
polishdude20
What's a decoherence rate?
whatshisface
Decoherence is the process that makes it impractically difficult for an experiment to be designed that makes your observations the two interfering possibilities in some kind of double-slit experiment.
Interpreting this in the many-particle case is more difficult, but the basic idea is that due to single-particle uncertainty, you can't have a definite number of particles indexed by momentum and a definite number of particles indexed by position at the same time. If I had 100 particles that were definitely at x=0, in terms of momentum they'd be spread out over the range of possibilities unpredictably.
punnerud
Rather than viewing wave functions as abstract mathematical objects in possibility space, we might understand them as describing the probabilistic nature of fundamental spinning energy entities whose rotational states generate wave-like behavior in measurement outcomes. Strassler's "possibilities" could be reinterpreted as different rotational configurations of spinning energy, with interference patterns emerging not from physical objects passing through slits but from how these spinning states evolve when constrained by sequential measurements.
waveletsyo
I’m tired of metaphorical discussions about this experiment.
Stop with the “wave particle duality”.
Stop with the “until it’s measured”.
Explain the experimental setup in grosse detail.
What do you mean by “a particle is emitted?”. What do you mean by “a particle is measured?”.
Even within the bounds of self described “double slit experiment”s there are numerous variations on how it is designed, constructed, and conducted.
Stop explaining the abstract notion of the experiment through a lens of your preconceived interpretation.
Show me data.
Show me numerical analysis.
mrdlm
[dead]
mplewis9z
Yes.
aeblyve
Two words, Bohmian Mechanics
Three words, pilot wave theory
To quote Cockshott, the Copenhagen Interpretation is an idealist recapitulation of Russian Machism/Bishop Berkley. The statement "nothing /is/ until it is observed" is not necessarily a Weird Quantum formulation but just a solipsistic attitude applicable towards all scientific observation in general.
nathan_compton
If one tries to formulate QFT theory with Bohmian Mechanics the results are less than satisfying. Regular Quantum Mechanics in a Bohmian mode is, in addition to failing to be invariant, also pretty paltry if pressed to really serve, primarily in that it appears to be the case (for both theories) that one has quite a lot of freedom with respect to what precisely lives with the particle and what lives with the pilot wave function.
In another sense Bohmian mechanics just kicks the can down the road - we may decide to associate the specific thing we observe with a particle situated on the pilot wave, but in fact, as far as the theory goes, the particle can live at any point in the pilot wave it wishes and nothing about the dynamics of the pilot wave changes at all. Thus we simply place the non-determinism in the past rather than in the present.
Furthermore, Bohmian mechanics seems to break Newton's First Law, since the pilot particle, as hinted above, is influenced by the pilot wave but not vice versa. The appeal of Bohmian mechanics is obvious, but superficial. It does not dispense with the can of worms, just opens it from the other side, in my opinion.
I’ve always wondered, has there ever been a definitive experiment where one photon hits a slit and on the other side two photons come out, but then when you add a photon observer, it immediately only comes out on one side? Or has the proof always been mathematical rather than a live experiment?
Edit: Thank you all for the responses, it has been very educational. It appears I was misunderstanding the most important aspect of the double slit experiment. A photon is a wave function when unobserved, it literally goes through both slits and creates an interference pattern like how waves in water would. However, when observed at the slit, or at the detector screen, the wave function collapses and only one photon(billiard like particle) will be detected.