i will try and explain Delayed choice quantum eraser in the simplest way possible. Which questions the deterministic nature of the universe and also somehow proves why probably there exists alternate realities. A thread.

We’ll start with simple double slit. Assume you have a tank filled with water. You drop a stone and it will create ripples, those are waves. If you put a slab with two holes in it, the waves will do wavy-things and make an inference pattern. Something like the gif

Now, you say okay that’s how waves work right? Let’s try particles. You start shooting bullets through those holes and you found that the bullets make a clump pattern. Which makes sense. Kinda Like this. Plain and simple.

Now you decide to do that with electrons which in fact are particles. So you fire a jet of electrons through the slits and to your surprise they also form an interference pattern. Just like the water waves. Now you’re angry and decided to look at the electron from where it went

Surely for an electron, which is a particle like a bullet, it is impossible thatbit goes through both slit and when you observe it, it goes through one slit at a time and the clump pattern is shown again. Which means your observation made it change to particle again.

You thought this can’t get any weirder but it is quantum mechanics so yeah it can. You decided to look after the electron have passed through the slit. So there are two possiblities: 1. Electron went as a wave through slit we observe it just before they hit the screen

2. Electrons traveled as particle. In both case we’d know. But in fact when you do it. The electrons are particles. And they were particles all along. And if you closed your eyes, they were waves. And displayed interference pattern. Your act of observation is altering the reality

It is as though you open your eyes and electrons said “oh shit the human knows we came from which slit, better go back and behave as particles all along.” And yeah, this indeed sounds like traveling back in time

But we try to out smart the universe so technology made it possible to design such experiment in which we have a twin electron now. And we never will observe the original electron so it will never know that it has been observed. And we will measure the twin way after the original

So original experiment had 8 nano seconds delay for the twin electron to be observed after the original electron hit the screen. But still and still if you measure the twin one, it not only communicates with the original that “we are measured, lets behave as particles”

But also 8ns apart. Like it tells the twin after 8ns that what to do in the past?!? This is completely mind boggling to think. And this effect has been verified many times by different experiments. This is the infamous delayed choice.

Now the quantum eraser is the extension to this experiment in which we simply add two more detectors to measure the twin electrons and in such a way that we never know from what slit they came from. Meaning we deleted our info about the electron.

And guess what? The electron will behave like wave again. Meaning knowledge is the key here, knowing about the reality of electrons made them lose their wave nature and made particles. And being unknown made them wave again.

What it tells about the reality? The choice! When you chose to measure, you were also entangled with the system so the universe made a copy where you made a selection. Meaning the universe splits into two 1. Where u measured 2. Where you don’t

And since we’re all made from atoms and electrons. This, in theory, happens all the time. Whenever you make a choice by going to a concert or not. The universe splits where one ver of you always going and one not.

If this does not blow your mind idk what will. Hope it was understandable.