let's start with the basics you might have heard of the famous double slit experiment it's where Photon that's particles of light are fired at a barrier with two slits If You observe which slit the photon goes through it behaves like a particle but if you don't observe it behaves like a wave creating an interference pattern sounds crazy right Wheeler's genius twist to this experiment was to ask what if we change our mind halfway through what if we decide whether to observe or not only after their Photon has passed the slits but before it hits the detector this is where things get really interesting in Wheeler's delayed Choice experiment we set up the experiment so that the photon doesn't know whether it will be observed or not until the very last moment the results still follow the rules of the double slit experiment if we choose to measure the photon acts like a particle if we don't it acts like a wave it's as if the photon somehow knows our decision in advance even though was made after it passed the slits now that we've wrapped our heads around Wheeler's delayed Choice experiment let's explore another fascinating tool used to investigate the behavior of photons the ma Zender interferometer imagine an apparatus where a single photon is emitted into an entry Port it then encounters a beam splitter which gives the photon equal probabilities of being transmitted or reflected this sets up two potential paths for the photon one leading to a detector at the top and the other to a detector on the right in the first scenario without any additional beam Splitters the photon behaves like a particle randomly hitting one of the detectors but never both however if we add a second beam splitter at the upper right corner something extraordinary happens the photon now traveling as a wave recombines at the second beam splitter showing interference patterns this means it always hits the detector on the right demonstrating its wave nature to understand this consider that the photon travels both paths simultaneously like a wave when the photon waves recombine they interfere with each other in the path leading to the top detector the waves cancel out due to destructive interference in the path leading to the right detector the waves reinforce each other through constructive interference ensuring the photon always hits this detector this intriguing Behavior raises a question when does the photon decide to act as a wave or a particle wheeler proposed an experiment to find out what if we let the photon pass through the first beam splitter without the second one in place making it decide to travel as a particle and then quickly introduce the second beam splitter would the photon continue as a particle or would it switch to wave behavior and show interference experiments have shown that the photon always adapts to the final setup it encounters if the second beam splitter is introduced mid-flight the Photon exhibits wave characteristics if removed it behaves like a particle hitting one detector without interference these findings suggest that the presence or absence of the second beam splitter retroactively influences the photon's behavior wheeler however rejected the idea of retroactive decision making instead he emphasized that the final experimental setup determines the photon's manifestation as a wave or particle imagine a cosmic version of this experiment proposed by wheeler himself in this thought experiment wheeler asks what would happen if light from a quazar or another galaxy millions or billions of light years away passes around an intervening Galaxy or a cluster of galaxies that acts as a gravitational lens at this point the photon must decide whether to travel as a particle taking one path around the lensing Galaxy or as a wave traveling both paths simultaneously when the photon finally arrives at an astronomical Observatory on Earth it creates two images of the same quazer one to the left of the lensing Galaxy and one to the right if the photon traveled as a particle it would only be detected by a telescope aimed at one of these images either left or right indicating it made a decision millions of years ago but what if the experimenters decide to combine the outputs of the two telescopes into a beam splitter they find that one output is bright indicating positive interference while the other is essentially zero indicating destructive interference this suggests that the photon traveled as a wave even though it had to decide millions of years ago wheeler provocatively suggests that the insertion of the beam splitter retroactively influenced the photon's decision as if the photon decided to travel as a wave millions of years ago based on a future event this concept challenges are traditional Notions of cause and effect several real experiments have been conducted to test Wheeler's idea and they support his conclusion the final experimental setup determines whether the photon displays wave or particle characteristics retro causality remains a fascinating yet elusive Concept in our next exploration we delve into another iconic experiment the double slit experiment but with a retrocausal Twist this variant is akin to Wheeler's delayed Choice experiment and adds a layer of complexity to our understanding of quantum Behavior imagine a photon Source emitting single photons toward w a double slit diaphragm on the far side of the slits a lens causes the paths from each slit to diverge slightly after they cross each other near the lens here the wave functions of each Photon are in superposition creating an interference pattern on a detection screen placed within this region in this setup no single Photon can be definitively traced back to one slit or the other reinforcing the wave nature of light however what happens if we remove the detection screen the wave functions continue continue to diverge and their combined probability amplitudes diminish as they travel further when telescopes are positioned to intercept the paths there's an equal probability about 50% that a photon will appear in one of them detecting a photon in telescope 1 suggests it emerged from the lower slit while detection in telescope 2 implies it came from the upper slit portraying the photon as a particle here's where it gets intriguing theorists propos that inserting or removing the detection screen mid experiment could retroactively alter the photon's behavior if the screen is inserted interference patterns emerge indicating the photon traveled as a wave through both slits conversely removing the screen suggests the photon decided to travel through one slit as a particle wheeler however did not accept this retrocausal interpretation he posited that the delayed choice between wav like or particle likee behavor avior does not alter the experimental outcomes instead it highlights the complimentarity principle our choice of measurement apparatus at the final stage determines the observed phenomena as Neils bour suggested in conclusion while Wheeler's delayed Choice experiment challenges our understanding of quantum mechanics it also opens up new questions does the Observer truly influence the outcome or is there a deeper hidden layer of reality we have yet to uncover to explore this further let's consider the role of the observer in quantum mechanics is it possible that our conscious observation collapses the wave function bringing a particular reality into existence this idea often referred to as the observer effect suggests that the act of measurement itself plays a fundamental role in determining the state of a Quantum system however not everyone agrees with this interpretation some physicists argue for a more deterministic view where all possible outcomes EX exist simultaneously in a Multiverse of parallel realities according to this perspective our observation merely selects one of these pre-existing realities rather than creating it a new this brings us to the heart of the quantum observers Paradox can we ever truly know whether our observations shape reality or are we simply uncovering a predetermined path the answer May lie in future experiments