# Need an Exact Explanation of the Double Slit Experiment

1. Sep 1, 2011

All I know about the Double Slit Experiment is that some particles are shot at a pair of slits, and the particle apparently splits, goes through both slits, and interferes with itself, creating an interference pattern, but if the particles are observed, they collapse and give the results you'd expect from particles. However, I need details.
1. What particles are used in the experiment? I've heard conflicting stories. Some people say electrons, others say we don't have the tech to use electrons, so we use neutrons. Which is it?
2. How are the particles observed? If the answer to this is what I'm thinking, I'll have another question.

2. Sep 1, 2011

### Fyzix

Come on man, you are of the google generation, use it.
This is perhaps the single most famous experiment in all of science history, there are infinite explanations who will give you all the answers regarding the details on how the experiment is conducted.
Just type in "double slit" in google and you'll find infinite sources.

3. Sep 2, 2011

### Staff: Mentor

4. Sep 2, 2011

Google is how I learned what I know. If the information wasn't vague or contradictory, I wouldn't be asking.

5. Sep 2, 2011

### ibysaiyan

What particle? That shouldn't be hard to guess. Young's double split experiment was done back in the 20th-19th century where modern technology was just nurturing. Photon, is the only candidate which comes to my mind.

Here's how it all works:

We eject a no. of photons (or any particle with which quantum phenomenon is observable) at a screen . In between the laser/gun, wall we have two slit. The following observations we will notice:
1)If a given no. of photons are fired at the wall over a certain period, we should notice a point,a dot on the screen.This happens when one of the slit is covered.Showing us that light has particle-like attributes.

2)If a given no. of photons (or as above ^ *) are fired at the wall over a certain period, we notice that this time an interference pattern is observed.Indicating wave-like property of light.This happens when both the slits are wide open and since we don't know which slit the photon/s went through i.e we are uncertain over it's position .

Technically, QM proposes that when we know the exact path of a given particle,we bring it into a definite position/spatial co-ordinate. In other words, we collapse the wave-function of the photon (wave-function is the probability distribution which consists of high and low outcomes ,you can liken it to frequency of existing at state, A,B,C,X,Y,Z,)

There are various Q. Mechanical interpretations.. for CI: read on Schrodinger's 'cat in a box experiment' then there's MWI and so on.. something which I am not familiar with,yet.

-ibysaiyan

6. Sep 2, 2011

### edpell

Young experiment in 1799 was done using photons.

7. Sep 2, 2011

Wait, I'm talking about modern versions of the experiment, which use particles like neutrons or electrons. Also, you guys are only answering half of my question. I also want to know how the wave function collapse is caused. Everything I've read has said that observing the particle as it enters a slit will cause the wave function to collapse so that instead of giving an interference pattern, it gives the expected result for particles. I want to know how they perform this observation.

8. Sep 3, 2011

### ibysaiyan

If that was so obvious and into everyone's knowledge then we wouldn't have this many interpretations of QM. I'll be honest I don't know other then what I have said... HUP is applied but I don't know what causes the collapse. Having read same threads on this forum the general consensus is that consciousness doesn't cause the collapse.
-ibysaiyan

9. Sep 3, 2011

### edpell

Since no measurement can tell us if the wave function is collapsed or not collapsed why is it even considered a part of the physical world?

10. Sep 3, 2011

Ok, it's starting to make more sense then. I guess I'll go ahead and post my idea then. Maybe it's just interactions in general that cause the collapse? This seems like an obvious idea, but I haven't seen any interpretations that suggest this.

11. Sep 4, 2011

### Dmitry67

The self-interactions and heat readiation from macroscopic bodies in fact create quantum decoherence in matter of few picoseconds. Quantum decoherence is what replaces the old fashioned "collapse", which is out of favor.

12. Sep 4, 2011

### ibysaiyan

I see. I think I understand what you mean by 'heat radiation' being a factor for this but what about self-interaction ? Are you saying that it's the mere presence of quantum bodies which irrevocably produces this decoherence , but isn't that what wave-function is defined as ?

- ibysaiyan

13. Sep 4, 2011

### Dmitry67

The self-interaction is irrelevant because no information is leaked outside. Hence, the evolution of the envronment would be 'compatible' with any outcome of any experiment made inside. For example, self interaction inside C60 molecules used in 2slit experiment does not change the interference pattern.

Regarding the "what wave-function is defined as", different interpretations of QM dont even agree if wavefunction is real or not. What is your favourite interpretation?

Last edited: Sep 4, 2011