• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Two-Slit Experiment Questions

DallasDad

Graduate Poster
D
Joined
Sep 7, 2009
Messages
1,161
I have only a popular science understanding of quantum physics, so pardon me if my questions are misphrased.

In the famous two-slit experiment (let's say using electrons), a stream of particles are aimed at the barrier between two vertical slits. The characteristic interference pattern shows up on the screen behind, thus proving a wave-like property despite other tests that prove a particle-like property.

If we magnify the scale to billiard balls, I would expect all of the billiard balls to bounce off the barrier between the slits and none to pass through. So, Question #1: Why doesn't this happen at the sub-atomic level? Is the aim of the electron particle source wobbly, or is the position of each electron in the stream occupying a field larger than the width of the central barrier? If neither of these, how do the electrons get through at all?

In the popular science YouTubes and books, the Two-Slit Experiment gets weirder when they fire a single electron (or photon) at a time. The resulting dot on the screen appears to have a random position, but if you leave the experiment running, the characteristic interference pattern emerges out of the seemingly-random hits. The conclusion, confusingly, appears that each electron interferes with itself as it passes through both slits. This self-interference disappears if a particle detector is positioned beside either of the two slits. When measured at the point of passing through the slits, the particle never goes through both, but only one or the other.

Question #2: How do you fire a single electron? I mean, what's the physical setup that produces one, and only one, electron at a time? In order to know, wouldn't you have to place a particle detector beside the emission source? If so, wouldn't there be interference of some sort similar to measuring at one of the slits? Ditto for using photons instead of electrons. Without a detector, how do we know two particles don't slip through, interfering with each other (rather than one interfering with itself)?

Question #3: Is there a specified width of the slits, and a specified distance apart to produce the effects? That is, if the central barrier were a meter wide, and the slits on each side only an Angstrom wide, would the Two-Slit experiment show the same results? Does it work with any size slit at any distance apart? What happens to the interference pattern if the slits are wildly different; i.e., the left-hand slit is a millimeter wide, and the right-hand slit is a meter wide?

Question #4: Does a collimated light source, such as a good laser, produce the same weird effects? What if it were a purely theoretical "perfectly collimated" source?

Question #5: The slits can be very well-machined, but they can't be perfect. How do we know that the particles aren't interacting with the rough edges of the slits as they pass?

And last, Question #6: Does the Two-Slit experiment produce the same results if performed (a) in a vacuum, (b) at normal air pressure, (c) underwater?
 
Quick partial response.
Q1: many of the particles will hit the barrier and not go through the slits.

Q6: electrons will be stopped by air or water, you can't do electron experiment in either. But you can do photon version in air and water. Doesn't change much. Interference pattern will shift slightly since wavelength changes, but that's it.
 
Q1: it does happen all the way down. That's why electron "orbits" are funky clouds instead of neat orbits, for example. Or why an electron can be shared all around a benzene ring.

Q2: Just about any process will fire one electron at a time. Just the time between them will be very short, basically. But if you want to have very few of them, some beta radiation source will work just fine, for example.

Q3: Yes, there are all sorts of considerations for slit width. For a start it should be smaller than what you expect the separation of lines observed to be, or basically it will smear the pattern all over the place. You also need the slit to be small enough for diffraction to allow the light or electrons from both to hit the same spot.

Q4: It can't be perfectly collimated, because of diffraction, basically. So basically yes it does.

Q5: You don't, really. But if it didn't bother you (or rather, your experiment) that the slit was, say, 20 microns wide and you don't know which side of it the photon came through, then it being +/- 1 microns due to imperfections won't either.

But basically the more general answer is that you EXPECT all sorts of errors in any science experiment ever. Your experiment being a lot less than absolute perfection is the EXPECTED outcome. That's why you always calculate the error bar and all.

Q6: What Ziggurat said. In fact, doing the two slit experiment in air is very much the normal way to do it for photons. I don't think any high school wants to pay for a vacuum chamber just for that one :p
 
Last edited:
Is the aim of the electron particle source wobbly, or is the position of each electron in the stream occupying a field larger than the width of the central barrier? If neither of these, how do the electrons get through at all?
Click to expand...

This part of the question has not been answered. The answer has nothing to do with the aim of the gun. In fact you could do the same experiment with the electrons going though several very small holes that are arranged in a straight line.

What is happening is that any one electron is not in any one fixed position at any one time. It is spread out over an area. The area is wide enough that it covers both slits so all electrons go through both holes and in the barrier at the same time. Once this electron has gone through the holes its position is spread out again. Now there are two areas, one for each hole. These two areas interfere with each, hence the pattern it generates. Remember this works even if an electron cannot interfere with any other electron.

One strange way to get the pattern to disappear is to have a detector that detector which hole the electron goes through. The detector will say the electron will go through one hole and thus not interfere with itself.



Key word probability wave
 
Question 4: light will do this as well. You don't need a laser, you can use an ordinary lightbulb and collimate it with a single slit at a distance from the double slit. Different wavelengths will produce different interference patterns, so you can get a rainbow sort of effect. You can enhance this further by using more than one slit. You can even do the same thing with thin mirrors instead of slits, to form diffraction gratings.
https://en.m.wikipedia.org/wiki/Diffraction_grating
Same principles at work.
 
Question 3:

In principle, any combination will produce some result. But the pattern depends on the parameters used. Generally, the larger the dimensions of the slit setup, the smaller the dimensions of the interference pattern (ie, distance between fringes). Go too big, and the interference pattern features will get smaller than your detection resolution, and so you won't see anything. That's part of why you can't do the experiment with, say, basketballs.
 
Thanks for the responses. There are still some things I don't quite grasp, but I'll do more research before asking for more help.
 
Just to add a small wrinkle ...

Electrons are charged, so analysis of what happens depends to some extent on the composition of the plates the slits are in, conductor or insulator (to make this binary), say.

Also, if one slit is above (at a higher elevation) the other, the interference pattern will be different than if they at the same elevation ... General Relativity rules, again! :)
 
JeanTate said:
Also, if one slit is above (at a higher elevation) the other, the interference pattern will be different than if they at the same elevation ... General Relativity rules, again! :)
Click to expand...
Hmmn, what if the experiment were performed vertically? Wouldn't this reduce the situation to an inertial frame?

To take it one step further, go back to the standard slit arrangement, but put the entire thing (emitter, slits, screen) on a very rapidly rotating platform. Pretend you can accelerate to a substantial fraction of the speed of light. How would relativity affect the interference pattern?
 
DallasDad said:
Hmmn, what if the experiment were performed vertically? Wouldn't this reduce the situation to an inertial frame?

To take it one step further, go back to the standard slit arrangement, but put the entire thing (emitter, slits, screen) on a very rapidly rotating platform. Pretend you can accelerate to a substantial fraction of the speed of light. How would relativity affect the interference pattern?
Click to expand...

While I’m mildly interested in this general topic, I don’t follow the literature, even reviews (OK, Living Reviews in Relativity, but they’re mostly irrelevant to this thread). I think experiments at least somewhat like what you describe have been done (not rotating at up to even 0.1c of course). And IIRC nothing inconsistent with QM or GR (let alone SR) was found ... I’m sure we’d have heard about it otherwise! :D
 
Ziggurat said:
Question 4: light will do this as well. You don't need a laser, you can use an ordinary lightbulb and collimate it with a single slit at a distance from the double slit. Different wavelengths will produce different interference patterns, so you can get a rainbow sort of effect. You can enhance this further by using more than one slit. You can even do the same thing with thin mirrors instead of slits, to form diffraction gratings.
https://en.m.wikipedia.org/wiki/Diffraction_grating
Same principles at work.
Click to expand...
My college had a variation in that, alongside the likes of cloud chambers. It's good what you can do at that sort of level to demonstrate these effects.
 
Well, probably the closest to what is proposed -- albeit not by rotating the frame of reference -- are the various setups to measure gravity waves by interferometry, the most ambitious being the LISA. And AFAIK the biggest working setup so far being the LIGO.

It's a bit different than the two slit experiment, but ultimately boils down to interference between two beams, so basically the same idea. And it's perfectly capable of detecting space-time distortions, in the form of gravity waves, of FAR lesser magnitude than the big gravity well of Earth.

Anyway, I'd say it's ample confirmation that the curvature of space, aka gravity, affects interference. Since, you know, that's why it works at all.
 
It's hard to detect gravitational effects on light in the lab, since it goes so fast. It's also hard to do with electrons. If the electrons go slow, you have to worry about electric field interactions messing things up. So you need something that can go slow, still have observable wavelengths, and not interact too strongly with other stuff.

And that's easiest to do with neutrons. I can't look up links right now, but there have been neutron interferometry experiments which observe the effects of gravity on the wave properties on neutrons.
 
You must log in or register to reply here.

Back
Top Bottom