# Has this retrocausality experiment been done?

The delayed choice quantum eraser experiment implies that the order of events is violated when a photon has chosen it's path, i.e. retrocausality (see http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser). I was wondering if an experiment has ever been performed to check if the path of photons are time independent.

I have an experiment where BBO crystal emits an entangled pair of photons. The idler photon passes through the optical switch and follows path 1. The signal photon is sent to a photo-multiplier which triggers a relay switch to its ON position. The relay switch will turn the optical switch on to potentially change the idler's path to path 2 at time t, after it has passed through the switch and before it is observed. After the photon is detected at either D1 or D2 the relay switch is set to its OFF position and the optical switch is set to route the next photon back to path 1.

If a photon's path was truly time independent (in support of the retrocausality theory), the photon should take the newly introduced path and be detected at D2 instead of the path it took when it entered the optical switch (path 1). If the photon is detected at D1, this would be evidence against retrocausality in the delayed choice quantum eraser experiment.

Here is a diagram of an experiment of what I am asking about:

• What kind of state does the entangler produce ? Is there spin entanglement and or measurement in this experiment ? – agemO Jan 21 '15 at 18:43
• "The delayed choice quantum eraser experiment implies that the order of events is violated when a photon has chosen it's path, i.e. retrocausality" -- Not really. You can predict the results perfectly well by assuming that whichever photon is detected first, it collapses the wavefunction of the combined system in a way that gives the other photon the correct probabilities of ending up at different detector locations. For example, if the idler is detected first at detector D3 in the original setup, the signal photon becomes more likely to hit D0 at the peaks of the D0/D3 interference pattern. – Hypnosifl Jan 21 '15 at 18:47
• @agemO - I was thinking an entangled pair emitted from a BBO crystal – user71265 Jan 21 '15 at 18:57
• @Hypnosifl - I was referring to the section "Possibility of retrocausality" in en.wikipedia.org/wiki/Delayed_choice_quantum_eraser – user71265 Jan 21 '15 at 19:01
• @user71265 - Well, that's why you shouldn't trust any wikipedia claim that isn't backed up with a reference--that section just appears to reflect the incomplete understanding of some random editor(s). But getting back to your own question, can you clarify what you mean by "time independent"? Are you talking about the probabilities of detecting the photons at different combinations of locations, or something else? And why do you say time independence implies "the photon should take the newly introduced path (end up at D2)"? – Hypnosifl Jan 21 '15 at 19:23

No retro-influence in this case.

The signal and idler are emitted together.

The wave-packets generated in down-conversion, i.e. the signal and idler, are short. Their "coherence-time* (it's a concept whose rigorous meaning you'll learn in quantum optics) is ~ than 100 picosec. For the needs of the present experiment you can take it as the duration in time of these wave-packets.

Now, I don't know the dimensions in your diagram, i.e. the distances from the BBO crystal that emits the pair to the optical switch and to the photomultiplier (PM). But even if the path difference between them is 10cm, the idler will reach the switch and leave it, before the signal comes to the PM. 100picosec x 3x$10^10$ = 3cm. As the signal travels to the PM, all the idler wave-packet passes through the switch and left it.

And, after the switch, the idler doesn't return back to wait for the effect of the PM on the switch.

• That actually is the point of the experiment. The idler photon would be past the switch when the signal photon caused the optical switch to route on to path 2. The question is is it possible now that the switch is on, that the photon would be present on path 2 assuming Feynman's interpretation of light taking all pathways? – user71265 Jan 21 '15 at 23:15
• To clarify, the entanglement between the signal and idler photons is just needed so that the optical switch can be thrown after the idler has passed through. After that the signal photon doesn't figure into the experiment. – user71265 Jan 21 '15 at 23:28

Your diagram or this diagram is missing something very important, the Bose-Einstien condensate along its path, to "hold" the photon "down" to NOT have the wavefunction collapse... There is an amateur on youtube who had an idea, which is rather ingenious, who explains this type of "retrocausality" being able to actually happen, he came up with the idea by putting together a group of different and tried experiments. He also said he came up with a certain "double slit experiment" processor or something...

The title of the video on youtube is "TIME TRAVEL DISCOVERY with retrocausality through QUANTUM PHYSICS "

He published it 5 months ago and he goes by the name "J Pares"

Some people are calling him a nut others are seeing him as a genius.