In a previous post, about the book ‘The Idea of the World‘ by Bernardo Kastrup I wrote:
'A universe with only matter offers no explanation whatsoever for the fact that the detection of the slit that was passed, has an effect back in time. That is because the ultimate cause of the disappearance of interference – the manifestation of the photon in one of the slits – must have occurred before the moment of detection of the slit passage.'
A reader stumbled upon this piece of text and rightly so. In my reply to her message I promised to pay extensive attention to retrocausality, cause and effect, as manifested in delayed double-slit experiments. So here’s my attempt to clear things up.
Interference and the double slit
First, let’s look at the common double slit. Whether photons, electrons or buckeyballs of 64 carbon atoms are fired at it, the result is always interference. That’s because these objects pass, in the form of a quantum wave of a certain frequency and wavelength, through the double slit on their way to detection. In both slits, a separate but synchronous wave source is created for each passing object. Those synchronous waves coming out of those two slits will amplify or cancel each other out in certain places.
In Figure 1, the two waves will amplify each other along the dotted lines. The mathematical interpretation of a quantum wave is that those maxima represent the locations of the highest probability that the object will be found there during measurement. On the screen behind the double slits we observe a pattern of light and dark bands. This is not the result of one particle. To get such a pattern, you have to fire at least thousands of particles, where all of them have the same wavelength, at the double slit. This a pattern is the result of interference.
Observing the slit
The wave therefore always passes through both slits. If we now set up the experiment in such a way that we can observe through which of the two slits each object passes, something remarkable happens. Each particle wave then adapts in such a way that it only appears in one of the two slits. The probability of finding the object in that slit on measurement has become now apparently 100% at the location of that slit. The wave will proceed now beyond the slit. A wave coming out of one slit cannot interfere with itself. Figure 2 show the result when we measure through which slit the object passes. In figure 2 the object passes through the left slit. But the probability of passing through the right slit is of course equal. Only one single wave for each individual object will now leave one of the two slits. The result on the screen is now a spread out spot right behind the middle between the two slits because the individual objects pass the slits alternately. What we see actually is the superposition of two spread out spots of light.
Entangled photon pairs with shared information
Observing the photons at the slit is done by first entangling two photons and then sending one, the signal photon, through the double slit. I describe this experiment in my book in chapter 7 – The delayed choice experiments. Because of this two-photon entanglement, the state wave of the other photon, the idler, has information about the slit through which the signal photon passes. The idler state wave thus possesses information about which slit is being passed. When that information is irretrievably erased the result is interference fringes as in figure 1. If that information is not erased the result is a single spread out spot as in figure 2.
The quantum information eraser
Whether or not information is erased is done by sending the idler photon through a semi-transparent mirror. Passing or reflecting is a fundamentally unpredictable quantum process with a 50/50 probability distribution. When passing, the information is preserved, when reflecting, the information is erased. In the first case, information preservation, the experimental result of a beam of signal photons is indeed a spread out spot, in the second case, information erasure, we see a clear interference pattern.
So far, it’s already an important and hopefully now better understandable quantum experiment. Whether or not information is erased determines the pattern that appears on the screen behind the slits. The real interesting thing now is that we can place the semi-transparent mirror – the information eraser – so far away that the signal photon has already passed through the double slit long and wide, at the moment the idler hits the semi-transparent mirror where randomly is decided to pass (keep information) or reflect (delete information). Even in this set-up, the experimentally measured result is that the interference fringes either do or don’t appear, when the information is respectively either erased or not. This is even true if this random erasure happens in time after the wave of the signal photon has already passed the double slit. The causation of the interference pattern, the manifestation of two synchronous waves or of a single one, happens therefore in time after the slit passage.
Retrocausality? Or an observer effect?
This therefore appears to be an effect with a retroactive effect in time, retrocausality. Study the timeline in Figure 3. Another interpretation, which is the one I prefer, is that the quantum wave of the photons becomes entangled with the measurement setup and that the real quantum collapse, the manifestation of the measured object, only happens when the observer sees the view results. See figure 3 again and consider what it is implicating.
This experiment, Random Delayed-Choice Quantum Eraser via Two-Photon Imaging, was done and published in 2007. The results confirm the apparent retrocausality. However, what I did not find in the description of the experiment is the idea of moving the information-erasing semi-transparent mirror further away so that the signal photon has already been detected as the idler hits the mirror. The event of the photon hitting the detector, conform either the interference pattern or the spread out light spot, would already exist before the idler hitted the mirror. That would confirm even more convincingly that the quantum collapse is ultimately an observer effect and that it is not an effect of the measurement set-up. A missed opportunity.
Cause, effect and time thus become something created by the observer.
I hope this has made the cryptic text at the head of this blog text a lot more understandable. Comments are always welcome, they are the source of clearer texts.
A reaction on a reader comment.
What is information? What is observation?
Those are the hard questions in quantum physics:
What does it mean to observe? When is something observed? What is an observer?
What is information? When do we ‘have’ information?
They seem simple words used and understood by everyone. Apparently they are not.
As far as I’m concerned, everything that enters my consciousness as experience is an observation. Whether I do that directly with my physical senses or whether I use on the other hand a giant instrument like the Large Hadron Collider in Geneva for my measurements, in both cases I receive information about the world. And ultimately always through my physical senses. Only when that information manifests itself in my consciousness can I say that I have been given information and that I understand what it means. In the same process, history is recorded, and time.
In the case of the described experiment above, the information about the result will be stored on a hard drive in a computer. These bits are processed by a computer program so that it can be displayed on a screen. The experimenter observes the result on his screen as little dots of light. Or it can be printed on paper, after which the experimenter views the results. In both cases, only then the information does enter the consciousness of the experimenter and becomes history that can be shared with other observers.
When is information irretrievably lost?
Now what does it mean when we say that the information is lost? If that information has already been observed, then as far as I am concerned, it has not been lost, even when the information has been erased from the hard disk after being observed. In this type of experiment it is a requirement that the information present in the entangled and unmanifested quantum wave is so irretrievably lost that the probablity that it can ever reach an observer somewhere in the future is absolutely zero.
In all the experiments I’ve read about it, the information is lost before the quantum wave will reach the detector. A semi-transparent mirror is very suitable information erasure device. It can be set up in such a way that:
- only when the quantum wave passes it, the wave will reach the detector.
- when reflected the information, that was contained in the unmanifested quantum wave, gets lost, erased.
The erased information can then never reach the observer. If, on the other hand, the wave passes the semi-transparent mirror, the information is still contained in the entangled wave. This wave reaches the detector, which in fact also consists of a complex of quantum waves. So, the detector and the quantum wave become entangled. That entanglement then extends to the computer to which the detector is connected and only ends with the observation by the experimenter. Only then will the information contained in the – now with the instruments entangled – quantum wave enter consciousness as an experience of the experiment. This is in fact John van Neumann’s projection postulate that – despite its inherent mind-matter duality – I still find the most plausible explanation for the so-called quantum collapse. Apart from the idealistic interpretation of quantum physics.
If we want to know for sure that it is by the information that eventually reaches the observer that the quantum collapse occurs, irrevocably destroying it can of course also be done by ensuring that it does not end up on the hard disk of the computer. Or immediately and irretrievably deleted. That seems also pretty irrevocable to me. I describe such an experiment in my book Chapter 13, Falsifiability of the Consciousness Model, section ‘Adapted Quantum Eraser’. Or look on this website: ‘A true quantum information eraser‘.