Is State 1 the Cause of State 2 or Vice Versa?

[Ambitiousteen]

Suppose that one put all the apparatus for the double-slit experiment in a room, with a detector at one of the slits. Then, you record the the "which path" information that the detector makes in the computer. After however many min. it takes to make a pattern, the computer deletes the which path information. It is vital that through all this time there are no people looking at the computer, or the double-slit apparatus. After the computer deletes the the which path information, a person checks if there is an interference pattern or not. If there is interference, then the computer must have changed the past by deleting the which path information. In fact, it would have to change the pattern that had already been created. But if there is no interference, then the photons have been "fooled". But I feel certain that if the detector made a buzzer go off whenever a photon went through the left slit, then there would've been interference. I believe that this could have huge repercussions.

 

[cesiumfrog]

In this experiment there would be no interference, ever.

Go read the original papers on the delayed choice quantum eraser, and notice that there in fact isn't ever any raw visible interference there either. But more importantly, the "deletion" process is more sophisticated - the information can't just be erased on a computer, it actually needs to be combined with the data from the screen.

 

[Cane_Toad]

Would it be possible to create a sticky thread dealing with misconceptions that arise repeatedly, i.e. "there is no observer effect per se, whether a photon is watched by a human is meaningless; if a photon hits a detector, or a brick, the effect is the same"

 

[cesiumfrog]

Don't you love how everytime someone first meets the DCQE, they try to fool it into violating causality? That even includes me

 

[Ambitiousteen]

But there is no longer any which path information. So there shouldn't be any interference. Who says that the cause can't follow the effect? But then again, I don't really know what I'm talking about.

 

[ueit]

Who says that the cause can't follow the effect?

The definition of the word says so. By definition, the cause precedes the effect.

 

[Ambitiousteen]

No. The cause makes the effect happen. It does not have to precede the effect.

 

[ueit]

No. The cause makes the effect happen. It does not have to precede the effect.

You have state 1 (ball A traveling at some speed and ball B stationary). Ball A colides with B and after that A remains stationary and B starts moving (state 2).
What do you think, is state 1 the cause of state 2 or the other way around?

The laws of physics are time symmetric. If two states can be shown to be correlated (we rule out pure chance) we simply say that the past state caused the future state (or they were both caused by another state in their common past), not the other way around. This is the common usage of the word "cause". After all, it is a matter of convention. Putting the effect in the past and the cause in the future confuses things and doesn't help you better understand the phenomena.