Cause, effect and time

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

Double slit interference. © Joerg Enderlein

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.

Figure 1 – Origin of the interference pattern behind the double slit.

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.

Figure 2 – Observing the slit – no interference. The particle manifests itself in one of the two slits. The pattern on the screen is the summation of the light received from the two slits.

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.

Figure 3 – Time line for the two-photon quantum information erasure experiment. Information is erased in time after the moment of having either two waves or a single particle having manifested in the slits. Retrocausality?

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.

Missed opportunity?

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‘.

Beyond Weird & The Quantum Handshake

To keep up to date with the subjects on my website I have to read quite a bit. And a lot of highly interesting material on quantum physics is being written and published. But occasionally I come across something that impresses me particularly and seems worth of special attention. Especially when it considerably broadens or clarifies my view on quantum physics and its interpretations. Therefore highly recommended stuff for visitors of my website. So, I’ll discuss two books here. The first one I want to discuss is: “Beyond Weird – Why Everything You Thought About Quantum Physics is .. different” by Philip Ball.

Beyond Weird

I am grateful to the student who put this book in my hands. Philip Ball is a science journalist who has been writing about this topic in Nature for many years. You don’t need to be able to solve exotic Schrödinger equations to follow his fascinating and utterly clear explanation of the quantum world and the riddles it presents. Also, he clears some misunderstandings up about this subject. Such as the word quantum, which is actually not the fundamental thing in quantum physics but rather an emerging phenomenon. The state wave is not quantized but fundamentally very continuous. He desctibes how quantum physics in its character and history deviates from all previous physical theories. It is a theory that is not built by extrapolation on the older theories. You can’t imagine what happens in the quantum world as you can do with, for example, gravity, electric currents, gas molecules, etc. The mathematical basis of quantum physics, quantum mechanics was not created by starting from fundamental principles but was the result of particularly happy intuitions that worked well but whose creators could not fundamentally explain what they were based on. Examples are: The matrix mechanics of Heisenberg, the Schrödinger equation, the idea of ​​Born that the state function gives you the probability of finding the particle at a certain place when measured. It was all inspired intuitive guesswork that laid the foundation for an incredibly successful theory we still don’t really understand how and why it works. Ball makes presents a good case for the idea that quantum mechanics seems to be about information. It is a pity, in my opinion, that he ultimately appears to adhere to the decoherence hypothesis. That is the point in his book where the critical reader will notice that what was until then comparably good to follow step by step suddenly loses its strict consistency and that from there one has to do with imperfect metaphors. His account remains interesting but isn’t that convincing anymore. Despite that, the book is highly recommended for anyone who wants to understand more about the quantum world and especially about quantum computers.

The Quantum Handshake

A completely different type of book is “The Quantum Handshake – Entanglement, Nonlocality and Transactions” by John Cramer. His interpretation of quantum physics seems, in my opinion incorrectly, not to be placed on the long list of serious quantum interpretations. Not a big group of supporters. In any case, I had never heard of his interpretation until it was brought forward by someone at a presentation about consilience I attended a short time ago. The subject made me curious because the state wave seems to stretch out backward and forward in time as I see it. Cramers’ hypothesis is that the state wave can also travel back in time, creating a kind of ‘handshake’ between the primary departing state wave and the secondary backwards in time reflected state wave. The reflected state wave traveling back in time arrives at the source thus exactly at the time of departure of the primary wave. This handshake between both waves effects the transfer of energy without the need for the so-called quantum collapse. The measurement problem where the continuous state wave instantaneously changes into an energy-matter transfer would then be explained as the result of a energy transfer by the handshaking state waves. However, in order to finally be able to complete that energy-matter transfer from source to measurement device, Cramer has to assume that the state wave is “somewhat” material-physical. This ephemeral quality of the state wave is considered as a severe weakness in his interpretation. Nevertheless the book provides worthwhile reading for those who want to delve into the various interpretations of quantum physics, also and especially because of Cramer’s discussion of a large number of experiments with amazing implications such as, for example, quantum erasers and delayed choice experiments where retro causality appears to occur. His idea of ​​a state wave that is traveling back in time – which is not forbidden in the formulations of quantum mechanics – remains a fascinating possibility.

Quantum physics and time

From Wikipedia: Vlatko Vedral is a Serbian-born (and naturalised British citizen) physicist and Professor of Physics at the University of Oxford and CQT (Centre for Quantum Technologies) at the National University of Singapore and a Fellow of Wolfson College. He is known for his research on the theory of Entanglement and Quantum Information Theory. As of 2017 he has published over 280 research papers in quantum mechanics and quantum information and was awarded the Royal Society Wolfson Research Merit Award in 2007. He has held a Lectureship and Readership at Imperial College, a Professorship at Leeds and visiting professorships in Vienna, Singapore (NUS) and at Perimeter Institute in Canada. As of 2017, there were over 18,000 citations to Vlatko Vedral’s research papers. He is the author of several books, including Decoding Reality.

Watch this movie “Living in a quantum world” from Vlatko Vedral on YouTube: At the end of his presentation a question from the audience about time and quantum physics is asked (at about 1: 10) and in his answer he describes the behavior of a super-accurate clock and what happens to the last digits when you lift that clock half a meter in the gravitational field. And then he wonders what it means when you imagine that clock to be in a quantum superposition at the two different heights in the gravitational field. A superposition of two different timelines. Fascinating.

By the way, the first part of his presentation – about 45 minutes – is actually a very compact version of my quantum physics book. Everything is presented in an almost blazing speed: interference, the Mach-Zehnder interferometer, Schrödinger’s cat, the Copenhagen interpretation against the multiverse interpretation, delayed choice experiments, interference with very large molecules shot through double slits, the orientation of our robin on the earth’s magnetic field in its annual migration, the 100% efficiency of chlorophyll. Highly recommended.

A definitive test of only-matter quantum hypotheses

There are a number of hypotheses that have as a common element the elimination of the influence of the observer’s consciousness. Those are decoherence – very popular with builders of quantum computers, collapse by macro instruments – the followers of Bohr and Heisenberg and not very deep thinking, super selection – nature doesn’t allow anything but don’t explain why not and finally spontaneous collapse – that doesn’t seem to realize that no system in the universe is truly isolated from the rest. We are not talking about the multiverse here.

The real quantum information eraser experiment. © Paul J. van Leeuwen

There is an experiment – ​​based on an already almost classical experiment, the delayed quantum eraser, which should be feasible in any well-equipped optical laboratory and at a relatively low cost. That experiment will be able to show whether the state wave does indeed collapse at the measuring instrument used, so that the quantum object manifests itself there, or whether it is much more fundamentally about information. In this experiment, that measuring instrument is a photon detector. For a full description I refer you to another page – a real quantum information eraser – on this website.