Importance of the observer in double slit experiment and entanglement

[Quarlep]

Hi

In double slit experiment our observe can change the past and in the entanglement our observe affact other
particle pair. How could such a thing possible ?

 

[phinds]

I don't think the observer changes the past in the double slit experiment, but in any case, both things are just quantum reality. That is, they are observed phenomena.

In entanglement, the observer is not actually changing the "other" particle, he/she/it is defining the combined state of the particle-pair. What's weird is that it happens FTL, but no useful information is transmitted.

 

[DrChinese]

How could such a thing possible ?

Welcome to PhysicsForms, Quarlep! A few comments to add to what phinds said.

There are several interpretations of Quantum Mechanics which are intended to account for these. And each has its own drawbacks as well. A few examples are: Many Worlds, dBB/Bohmian Mechanics, Time Symmetric, etc.

No one truly knows what is occurring. And yet, the current understanding is good enough that many experiments can be conceived, predicted and explained.

 

[Quarlep]

look this http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

 

[DrChinese]

look this http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

Here is a variant which has been performed. Particles are entangled AFTER they are detected, which seems crazy. However, directly predicted by Quantum Mechanics.

http://arxiv.org/abs/quant-ph/0201134

 

[Quarlep]

It was good so it is evindence of my idea isn't it

 

[Cthugha]

In the delayed choice quantum eraser you do not actually change the past. In a nutshell you can pick at a later point in time whether you want to perform a measurement which allows you to gather some more information about a second measurement done in the past or a measurement which does not allow you to do that.

One can give a more detailed description, but it involves quite some math. I am not sure you want the details.

 

[Quarlep]

I wiil glad if you share it

 

[Quarlep]

I didnt understand it. Can you explain it more simple.
I will be glad if you show me the math.And this idea is proved or its just
a mathematical explanation of this if its I want to learn other ideas.

 

[Cthugha]

I posted the in-a-nutshell version here:

https://www.physicsforums.com/showpost.php?p=2241460&postcount=8

This is aimed at a version of the delayed choice quantum eraser done in the version by Kim et al.

The double slit version used by Walborn and others is maybe best explained by Walborn himself in the following paper: http://arxiv.org/abs/1010.1236.

You will find the important results around page 52. Equation 96 is especially important. It gives you coincidence count rates. They explain how to get conditional fringes and you can play around with that equation for yourself to check the influence of detector size or other important parameters. The equation is rather lengthy, though.

 

[Quarlep]

Thank you very much