# B The double slit experiment

1. Mar 1, 2016

From my shallow understanding, when we shoot a small amount of electrons through the slits while observing, the observation interfere with the electrons and the quantum system and thus collapse the wave function, making the electrons behave like a particle and form a 2-slit pattern on the screen. However, why would we still see an interference pattern aka electrons acting like a wave when we shoot a whole beam through the slits like we did in highschool? Aren't we still observing the beam going through the slit? Why would the system not collapse?

2. Mar 1, 2016

### drvrm

"observation interfere with the electrons and the quantum system " pl. explain clearly

"making the electrons behave like a particle and form a 2-slit pattern on the screen"
i understand that wave nature leads to interference.

3. Mar 1, 2016

### nasu

"Observing" here meaning to measure or detect through which slit the electron went.
If you do this (by putting detectors at each slit, for example) you destroy the interference pattern. The fact that someone is there "observing" the experiment is irrelevant. You cannot tell through what slit a specific electron goes. It does not matter if you keep your eyes open or closed. :)

4. Mar 1, 2016

### drvrm

Is it not logical as 'waves' interfere and give rise to a pattern -e.g. photons the light waves produce interference-
and as soon as you destroy the wave character by measuring 'electrons' as particles - must have disturbed the waves so -no interference pattern is there;
i am just asking why you wish to know 'through which slit' an electron went through?
If you know it then you must have used some device to gain information and the 'interaction" with the device must have changed the state of electrons!

5. Mar 1, 2016

### nasu

I did not say that you destroy the wave character. Whatever that means.
Yes, this is what I said, you must use a device.
I don't really understand what are you asking and why. I was replying to the question in OP.

6. Mar 1, 2016

### drvrm

If you refer to electron beam in which a stream of electrons thousands per sec going through the slits you get an interference pattern- i think if undisturbed electrons behave like waves - using de broglie relation you can calculate the wavelength also which depends on the momentum of electrons- so this interference is due to wave nature of electrons.-
your earlier question is why the wave nature is lost when you catch the electrons and label them individually that 'this one went through the slit-1 and this other one went through slit -2... by doing such measurements the particle nature comes to the foreground and wave nature gets lost.
if you have some intro in QM-you can analyse it further using uncertainty principle.

7. Mar 1, 2016

Sorry for the ambiguity in my question. I underdtand the observer effect. What I really was asking in my initial post is that why do the double slit experiment we did in highschool - remember when we shot a laser beam through the slits and we could see a clear interference pattern forming on the wall - not correspond with the observer effect? Like technically when we were doing those rudimentary experiemnts we were still observing the whole process right? Why do we still see an interference pattern on the wall rather than a 2 slit pattern?

8. Mar 1, 2016

### Staff: Mentor

Because you aren't observing the photons in a way that tells you which slit they went through. You're simply seeing them after they reflect from the wall or screen after the slits.

9. Mar 1, 2016