# 2 questions about the double slit experiments

1. Apr 6, 2007

### The_Thinker

I Have two questions in my mind about the double slit experiment about the electrons, i would be glad if you guys could clear it up.

1)When we try to observe which slit the electron is going through, how exactly do we go about making the measurement? what kind of "device" or "technique" do we use?

and

2) If suppose we were to observe only 1 slit instead of the two, then what would happen? Cause if we observe only one slit then we will know which slit exactly the electron is going through, (cause if we don't detect it in the slit we observe, then its naturally going in through the other one) Then since we do not directly "observe" the second slit, what happens then? do we get an intereferance pattern? or what exactly do we get?

Thx...

2. Apr 7, 2007

### DrChinese

The easiest way to learn which slit the electron is going through is to block off one completely. This is often done in an alternating manner of some kind in order to build up a suitable pattern to demonstrate the result.

Experiements have also been done - as confirmation - in which a beam of photons is used to determine which slit the electron goes through. In this scenario, the light beam is directed across the direction of travel of the electron.

3. Apr 7, 2007

### RandallB

Exactly the point!
1st - As DrC said - you use a beam of light in front of just one slit to detect the shadow of any electron going through that slit.
2nd - Now just as you proposed - they can count only those individual electrons that hit the screen when no shadow of an electron is detected. After all we can not expect an electron that has been hit by enough photons to stay on its same original path.
3rd - This way we can view just the pattern caused by only those electrons, that we now for sure "which way" they went, AND we also know we did not touch or disturb them.

So now what have they observed when they carefully look at the pattern of only these undisturbed electrons going though just one of two slits?

Of all the many times, (many, many, many times – there do this a lot because it is confusing) this experiment is done they always get the same result – a single slit pattern i.e. With No Interference!

Confusing?
Well yah, that’s why it takes a ‘confusing’ Non-local theory to explain or understand it.
Lots of choices there – OQM, MWI, BM-guide waves, String Multi Dimensions, superposition/entanglement, etc

4. Apr 7, 2007

### Robert J. Grave

So, do I have it right? If only one slit is used you get a single dot opposite the slit, no intereferance. If you use both slits, one at a time, you get the same intereferance pattern as if electrons were going through both slits at the same time. Right? -Robert

5. Apr 7, 2007

### RandallB

NO
A single slit gives you a wide random patten, not a single dot. you must send many electrons to collect data. one electron by itself cannot create any kind of pattern.

And as I said the double slit pattern for just the electrons you have figure out which way thay have gone looks the same as this single slit random pattern - no interferance bars.

6. Apr 7, 2007

### JesseM

I think by "one at a time", he meant using a large number of electrons but having one of the slits blocked off at each moment, alternating which slit was blocked, as DrChinese suggested. In this case you should be able to reconstruct which slit each electron went through, so you'd see no interference pattern--one of the strange things about QM is that interference vs. non-interference seems to depend on the information that the external environment has about which slit the electron went through, not on the particular method used to infer the electron's path. It's possible that if the slits were blocked and unblocked very rapidly, faster than the time needed for each electron to get from the emitter to the detector, then this would cause the which path-information to be at least partially lost and therefore interference would be partially restored--I'm not sure about this though, does anyone know of any references on this sort of situation?

Last edited: Apr 7, 2007
7. Apr 8, 2007

### Robert J. Grave

Yes, sorry I wasn't clear. If you block only one of two slits at a time alternately while sending many electrons, one at a time through the open slit, you would get an intereferance pattern just as though both slits were open at the same time. By the way the experiment has been done with atoms also with the same result. I read this in a book called "Schrodinger's Kittens and the Search for Reality" by John Gribbin, very good reading. -Robert

8. Apr 8, 2007

### Staff: Mentor

Just the opposite, I would think. As discussed in this thread, if one slit is blocked, any electron traveling through the open slit would contribute to a single slit pattern, not a double slit interference pattern. And if you alternate the blocking, the accumulated pattern will be that of two single slit patterns superposed, not a double slit interference pattern.
I suspect you misread the description.

9. Apr 8, 2007

### cybercrypt13

Actually I disagree with the comments about having a scatter pattern when you only have 1 slit versus two. I posted another thread on this a few minutes ago before I noticed this one. I'm not one to take other peoples word for things so I've actually built my own experiment to test this subject. In 100% of my tests, I get an interference pattern not only with one slit, but even if I have no slits and just block half the pathway with something solid.

I don't see strangeness in my experiments and I believe I can actually explain what is happening but would love to hear some opinions before I offer mine. The fact is, I take a laser beam, or a hand held flash light from walmart and shine it past a book sitting on the table, I get a perfect set of lines on the back wall. No slits at all.

By the way, can anyone explain the bending of light? Because I think I can using the results of my experiments and was wondering if we already knew in detail why it bends.

Thanks,

glenn

10. Apr 8, 2007

### JesseM

You will get a diffraction pattern if you shine a laser throught a single slit--see single slit diffraction pattern of light. But this is different from an interference pattern where the waves from one slit are interfering with the waves from the other--when two slits are open at the same time, the result is not simply a sum of two diffraction patterns from single slits, you actually will get regions where less light hits that region when both slits are open than when only one is, thanks to destructive interference.

Also, to see a visible interference pattern, the distance between slits has to be pretty small, you might need to use a diffraction grating--http://www.sasked.gov.sk.ca/docs/physics/u6c12phy.html [Broken] says that the relationship between the light's wavelength $$\lambda$$ and the distance between interference bands $$\Delta x$$ is $$\lambda = \Delta x (d/L)$$, where d is the separation between the slits and L is the distance between the slits and the screen. So if you use a red laser with a wavelength of around 680 nanometers, and L is 1 meter, then in order to have the bands be 1 millimeter apart, the distance between slits would have to be 0.68 millimeters (and if you want the bands to be twice as far apart, you have to either divide the distance between slits by 2, or multiply L by 2).

Last edited by a moderator: May 2, 2017
11. Apr 9, 2007

### The_Thinker

Hey.. thx for replying to the thread guys... appreciate it... well... you cleared up my doubts... but i guess its gonna be awhile before someone clears up the the whole thing though...

12. Apr 9, 2007

### Robert J. Grave

The experiment from John Gribbin's book, page 5, goes like this. There are TWO slits open at all times. Photons are put through the experiment one at a time. After millions of photons have passed through and accumulated on a photo plate an intereferance pattern forms. This was actually done in the mid 80's. In the 90,s it was done with atoms. Thanks for the correction. I hope I got it right this time. -Robert

13. Apr 9, 2007

### Staff: Mentor

Yes, now you've got it right.

14. Apr 9, 2007

### Staff: Mentor

This is well known and has been analyzed thoroughly in classical wave optics, as far back as the 1800s. The results carry over to quantum optics with the addition of particle-like features, just as with two-slit interference.

Diffraction by Opaque Barrier

15. Apr 9, 2007

### cybercrypt13

Thank you very much for the information. A couple more questions though:
1) It states that the distance between the slit and the back surface needs to be substantial, but I can duplicate my results with only a 2 foot seperation. I can also duplicate simply by breaking the light in half, not with a slit. Is 2 feet a large enough distance to see what they are talking about?

Should a slit actually be required to see the banded pattern?

Thanks,

glenn

Last edited by a moderator: May 2, 2017
16. Apr 9, 2007

### Staff: Mentor

The reason for the substantial distance is to make it easier to derive the equations that describe the interference pattern. With longer distances you can use certain approximations to simplify the math. This is called Fraunhofer diffraction. With shorter distances you have to use more complicated math to get the details right, and the result is called Fresnel diffraction. The site that I linked to in my previous post discusses both kinds of diffraction.

Any time you obstruct the original beam with an obstacle (or obstacles), of any shape or size, you get some kind of interference pattern. A slit is just one kind of obstacle. Or rather, the material surrounding the slit is the obstacle.

17. Apr 9, 2007

### JesseM

Not sure what you mean by "breaking the light in half", but in terms of the seperation needed, the first link on single-slit diffraction gives the equation y = L*I/a, where y is the distance between the center and the first diffraction minimum (black area), L is the distance from the slit to the screen, I is the wavelength, and a is the width of the slit. 2 feet is about 0.6 meters, and the wavelength of a red laser is about 680 nanometers, so to get a distance of 1 mm between the center and the first dark band, you'd need a very thin slit about 0.4 mm wide (edit: but I'm not sure how well the equation would work at this separation, see jtbell's post). But if you're not actually using a slit this equation wouldn't really be applicable, I'm also not sure how things work if you only block the beam on one side rather than both sides as with a slit.

Last edited: Apr 9, 2007
18. Apr 16, 2007

### DeepQ

I think it is great that you built your own experiment to gain your own perspective on the results. Personal confirmation is a good way to internalize concepts.

BTW, I am curious where you found the plans? I would like to build my own to play with. (Long ago, at the university, I established the laser defraction lab, but these days it would be much cheaper to do so, given the advances in technology. So I am looking for some plans.)

Yes, regarding the book and flashlight - you are seeing standard defraction, and you can spend hours experimenting with just that. In fact, with a laser printer and some transparent film sheets, you can create a wide range of your own defraction gratings for a substantial collection of interesting experiments. (So much easier that what we had to do back in the '70's).

You mentioned that you had your own explanation of the results. I am curious what you think, can you describe it?

19. Apr 17, 2007

### cybercrypt13

HAHA, Don't think I'll offer an opinion at this point after all the beating I've already taken... :-) As for where I found plans for the experiment, I've found none. This was another part of the process I found confusing, since there are many many references to its results, I had figured it would be more obvious. Of course, the actual experiment is not so complicated once you get all the pieces figured out so maybe thats why, but who knows.

I'd love to discuss my opinion of what I'm seeing, however, I think I'll continue to study and see if something comes to light that changes my opinion on it. I'll let you know if that happens.

Thanks,

glenn

20. Apr 17, 2007

### DeepQ

Try to ignore the beating. That's just the way of the physics community. It's been like that for a while. They beat down everyone who questions anything fundamental. I've seen it happen a hundred times. Don't feel like they singled you out. Keep questioning.

Yes, I've wanted to observe the dual slit experiment or read an original account of it for more than 30 years. I've never read the actual published research papers or even a decent detailed account of it. The informal descriptions of the experiment (as found in most textbooks) often violate the very rules of QM, such as when making statements or drawing conclusions about effects of single electrons.

Maybe what we need is a new website for those who want to perform QM experimentation and those who seek a greater understanding. Or, maybe one already exists?