# B Explaining to middle and high schoolers the alternatives to retrocausality

1. Sep 2, 2016

### whitsona2

I run a maker space and it has taken a dive into physics recently. I have 2 students doing a quantum entanglement experiment from SciAm and the Spooky Action at a Distance website. And, we have done a simple quantum eraser with polarized light.

All I can tell them is that "physicists say" that this looks like retrocausality, but it probably isn't.

Can you help me with what that alternative explanation is?

2. Sep 2, 2016

### andrewkirk

Can you give an indication of what the experiment was?

There are a number of different quantum eraser experiments, and explanations of what is going on vary. For instance, in the Kim (delayed choice) experiment the explanation is that nothing is erased. The pattern that is formed by signal photons is a superposition of a non-interference pattern and an interference pattern, and the thing that we call the 'erased pattern' ie the non-interference pattern that arises from photons whose idler went through a path that enabled detection of 'which-path' info, is only visible by correlating the observations of the signal photons with the observations of the idler photons.

If it was the one where you start with a double slit that gives an interference pattern, and then you put linear polarising filters in front of the slits, with the angle between the polarisation direction of the two being 90 degrees, then there's a derivation of why that works here. The linked post doesn't contain the full proof because there's a last step missing, which is what the thread is asking about. The last step is supplied in post 4 of that thread. As I recall, one doesn't need any fancy quantum mechanics for the proof, only electromagnetic wave equations. So it might be accessible to maths-savvy school students. The simple 'words-only' explanation is that the polarising plates change the waves passing through the two slits so that they do not interfere with one another.

3. Sep 3, 2016

### whitsona2

Yes! It was the polarized light one. Thanks. I'll try to grok this and the Kim paper reference today. Than you! I hope B was the right destination for "high school help needed" on the phone app and can't find the key that tells which is which.

4. Sep 3, 2016