A showcase of Future affecting the Past

7 min readSep 5, 2019

We spend our days with regular activities without knowing the elements that make our reality at the most basic level so strange that if we start to learn about them, it will be extremely hard to believe. I am going to explain and show such behavior which is one of the weirdest things in Science and a mystery that no one has solved yet. That thing is Delayed Choice Quantum Eraser and it’s believed to have the power which enables the Future to affect the Past !! Quite counterintuitive, right? Let’s see how it’s happening.

Quantum Physics is the branch of physics that describes how the smallest part of our us, plants, rocks, rivers and the universe behave, communicate and make things: visible & invisible. It seems the particles like electron, proton, neutron, quarks, photon, etc, the inhabitants this Quantum world don’t obey the rules of our day-to-day world. For example, an electron can be at multiple places at once, it even doesn’t have a specific form. It can be a wave at any time and when observed becomes a particle. This has been verified experimentally hundreds of times and found to be true. These things inspired me to study Quantum Mechanics and like a love affair, I became obsessed with the subject!

One day I came across one experiment, which was so amazing that I couldn’t sleep for 2 days at night thinking about the possibilities, yet it happens everywhere, all the time with all the objects without grabbing anybody’s attention. Yes, as I mentioned this event/concept is called “Delayed Choice Quantum Eraser”. I will try to explain in as simple words as possible so that I can share those amazements with you. This experiment was designed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully in 1999.

**Original Delayed Choice Quantum Eraser Experiment**

What the experiment shows as the output is that “A photon when emitted from a source after traveling from two different paths with its entangled partner can change its starting path even after reaching the destination”.What this means is before entering one of the two holes/paths the photon knows from where it will enter and.

I will go step by step from beginning to the actual experiment in a simple way. Below is the original Double slit experiment which shows that when light, a photon (or electron) is allowed to pass through both slits, it will create an interference pattern as if a wave is passed and this happens when the single-photon passes through both slits simultaneously and interfere with itself creating an interference pattern.

**Young’s Double Slit Experiment showing Interference**

This happens when no one is observing at the slits and observing at the final output i.e. the screen. But the moment an observation is made ( camera or human) the interference pattern is gone and only 2 lines are visible. This is called Zeno effect where observation in a quantum world/experiment destroys the original output.

This experimented has been done hundreds of times and the same output is made. Although this experiment was conducted in 1801 AD by Thomas Young, it remains the same and matter of study for the next 198 years when Kim, Kulik, Shih & Scully try to find out what happens if the observation is made after the photons have reached the destination.

For that, they started with standard double-slit set up and after the slits, they put a BBO Crystal adjacent to it. This crystal creates an entangled pair from a single photon and this helps to measure one part of the pair without touching the other.

The Lens that is shown near Detector 1 is there to combine the output from both top & bottom slits ( in wooden color in Photon emitter) so that when the detector-1 detects we won’t be able to know from which slit the single-photon came through. Hence Lens is called ‘Quantum Eraser’ since it erases its path and history.

Next, we will take several measurements for the beam/beams (beams: since there may be a chance the single-photon may come out of both slits simultaneously) coming out of the slits by setting more detectors and beam-splitters.

Below I have mentioned each component, showing photon that went through the slit, the beam-splitters, and all detectors.

As shown in the picture on the left side, the beam-splitters create a scenario where the photon has a chance of 50% to go through the splitter and 50% chance of being reflected towards detector-4. We do this for the full time and for both Top slit and Bottom slit separately.

When the beam is diverted downwards, the two beams from the top slit and bottom slit combined such that the which-way information can’t be known. In other words if the detector-4 detects the full time(all the time), it will not have the which-way information and hence we can’t know from which (of Top or Bottom) slit the photon passed through. So here “combining” of both beams in Detector-4 acts as an eraser and erases the information of its trajectory.

Now let’s see what happens if the photon/particle moves through the beam-splitter and reaches detector-2 (for detecting path via Top slit) or detector-3 (for detecting path via Bottom slit) instead of being reflected and going to detector-4. As we can see in the image below if detector-2 lights up than we know for SURE that the photon has passed through the TOP slit. The same is for detector-3, i.e. if it lights up we know for sure that the photon has passed through the BOTTOM slit. So we can say that if any of detector-2 or detector-3 light up, we have the ‘which-way’ information.

The result of this arrangement and experiment starts becoming mind-bending. This is why: when detector-2 senses a photon, detector-1 senses its entangled photon- shows that it is a particle, not a wave by showing the patterns as only straight lines instead of an interference pattern (as seen in case of Youngs double-slit experiment). (below fig-1)

fig-1

As we discussed above when detector-4 senses the photon, all the information about its traveled path is lost and we can’t determine from which slit the photon/particle came. Also when this detector senses the photon it shows a wave pattern (interference pattern), detector-1 senses the photon as wave and shows wave pattern too. This is because path information is not known to it too. (as below fig-2)

fig-2

So far so good. Now the shocking part comes.. !! If we observe closely to this arrangement of experiment we find that the path from slits (& BBO) to detector-1 is shorter than path to detector-2, detector-3 & detector-4. So, the photon is ALWAYS detected 1st at detector-1 than other detectors.

Now we have arrived at the important moment of this discussion, the delayed choice part. An interference pattern formes at detector-1, even though which-way information corresponding to the photons that are diverted is erased much later in time at detector-4. (fig-3)

Similarly double-line pattern ( occurs for single photon/particle) occurs at detector-1 before the same ascertained at detectors-2 or detector-3. This is happening because as we saw the path to detector-1 is shortest and hence the photon reaches there first.

This means the photon at detector-1 knows ahead of time which detector its entangled pair will end up in the future. So, it seems so that the future state of the photon at detectors 2,3 & 4 affects the past state of its entangled state at detector-1. Reading all this please remember that all these photons at different detectors are the same photon i.e. this is a single photon or electron starting from Photon emitter.

This experiment and the above results have been demonstrated over and over again many times. But, what is the reason behind this no one knows yet. If anyone or even you can explain the reason for this, there is a Nobel Prize waiting for you!

So the take away from this reading is that Measurement of the particle NOW affects its properties in the PAST. So, a decision made at present can influence something in the past. This is the weirdness in subatomic particles. They somehow don’t obey the space-time rule like us. Unlike us who can go in one direction in Time, they can go in both directions. In this way, they affect our reality - past, present &future.

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A showcase of Future affecting the Past

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Written by Siddharth Saraf

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