Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.
I would like to address the reverse question: Is the *past* determined?
In other words, is the past fixed once we pass it?
Another interpretation of the problem, possible a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?
When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.
So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?
Reality might be able to be adjusted after events have occurred. this according to the results of the delayed-choice quantum-eraser experiment.
Figure 1. Setup of the delayed-choice quantum-eraser experiment of Kim
et al. Detector D0 is movable
Figure 2. Simulated recordings of photons jointly detected between
D0 and
D1,
D2,
D3,
D4(
R01,
R02,
R03,
R04)
The experimental setup of the earliest performed DCQE involved an argon laser that shot 351.1 nm photons which went through a double-slit apparatus. After an individual photon went through one (or both) of the 2 slits, a Beta Barium Borate Crystal converted the photon into 2 identical entangled photons at half the original photon's frequency. The paths followed by each of the entangled photons were caused to become diverged by a Glan-Thompson Prism. One of these 702.2 nm photons (the signal photon) then traveled on a path from the Glan-Thompson Prism to a lens and then to a detector designated as D0. This point was scanned along its X-axis. A plot of the "signal photon counts" recorded at D0 versus X were examined to determine if the cumulative signal formed an interference pattern. The other entangled photon (the idler photon) went from the Glan-Thompson Prism to another prism where the idler photon was then deflected along a divergent path, depending upon which slit the photon went through. Beyond this path split, the idler photons encountered beam splitters that gave the idler photon a 50% chance of passing through and a 50% chance of being reflected by a mirror. The beam splitters and mirrors directed the idler photons towards detectors which were designated as D1,D2,D3 and D4. This experiment was setup so if an idler photon was recorded at D1 or D2, then this detected photon could have passed through either slit. If an idler photon were recorded at D3, then it must have passed through the one slit designated as Slit B. If an idler photon were recorded at D4, then it must have only passed though the one slit designated as Slit A. The optical pathway from slit to D1,D2,D3 and D4 was 2.5m longer than the pathway length from slit to D0. Thus, information acquired from an idler photon would occur 8ns later than information acquired from the corresponding entangled signal photon. The idler photon recorded at D3 or D4 provided a delayed "which-path" indication of whether the signal photon with which it was entangled had gone through Slit A or B. Whereas, the idler photon recorded at D1 or D2 provided a delayed indication that such "which-path" information was not available for its entangled signal photon. The experiment used a coincidence counter to isolate the entangled signal from photo-noise, recording only events where both signal and idler photons had been detected. ( after compensating for the 8ns delay ) When signal photons whose entangled idler photons were recorded at D1 or D2, the experimenters detected an interference pattern. When signal photons whose entangled idler photons were recorded at D3 or D4, the experimenters detected a simple diffraction patterns with no interference.
Phys. Rev. Lett. 84, 1 (2000) - Delayed ``Choice'' Quantum Eraser
Reference: Delayed “Choice” Quantum Eraser Yoon-Ho Kim, Rong Yu, Sergei P. Kulik, Yanhua Shih, and Marlan O. Scully Phys. Rev. Lett. 84, 1 – Published 3 January 2000 Issue
Vol. 84, Iss. 1 — 3 January 2000
The DCQE is unlike the classic double-slit experiment, in that the choice to preserve or obfuscate the which-path information of the idler photon was not done until 8ns after the position of its corresponding signal photon had already been measured at D0.
Although, an idler photon was unobserved until after its corresponding entangled signal photon arrived at D0, interference at D0 was determined by whether a signal photon's entangled idler photon was recorded at D1 or D2 which was on a pathway where the photon's "which-path" information had been obfuscated, or at D3 or D4 which was on a pathway where the photon's "which-path" information was preserved.
Does the DCQE indicate that the delayed choice to observe or not observe the idler photon's path affect the outcome of a past event?
Does Relativity reveal that if quantum entanglement influences are able to travel faster than light, then they must also be able to travel backward in time and influence the past (which they do in this experiment)?
I suppose that'd be all fine and dandy, so long as there'd be no immediately decode-able information transfer FTL into the past. So then, you couldn't go into the past and kill off your great great great grandparents, and create a paradox.
Does the universe permit anything that doesn't make paradoxes, including FTL and backwards time travel of certain quantum influences (which are intertwined in relativity)?
Perhaps you can reach back into time, so long as you preserve causality. Your changes would have to look like noise at the time, and could only have been seen to be otherwise when it's too late to make any difference (or light has had time to travel that far anyway, in the case of FTL)