Entangled in space ánd time

Entanglement in space

Quantum entanglement is generally presented as the effect that if two (or more) quantum objects have a common history that they are quantum entangled until one of them is measured. Common history means that they have had physical interaction in the past. That is, the objects have one common quantum wave and, if one of the objects is measured, its collapse has an immediate influence on the properties of the other entangled objects. This has been demonstrated time and again in numerous Bell experiments that seem to convincingly demonstrate effects that are contrary to the laws of relativity. These say that the maximum speed of information transfer is the speed of light. For more on entanglement see this page on this site.

Entanglement in time

In 2013, an experiment was conducted – Entanglement Swapping between Photons that have Never Coexisted – that showed that photons can become entangled even when they have never been together. What does that mean? A somewhat clearer way of describing the experiment is ‘entanglement in time instead of space’. See the diagram below from the publication of the experiment.

Four photons exhibit a common quantum wave that starts with the creation of the first photon entangled with a second photon because they were created as pair. The first photon is measured, irrevocably thereby ending its existence. The second photon is then entangled with one of a second pair of photons that are created later in time than the destruction of the first photon by its detection. Finally, photon four, the second of the second pair, is measured. The measurements of photons one and four have been shown to be correlated. Which means that all photons were indeed entangled.

Their entanglement, a common history, happened via entanglement of photon two and three. This was done by a Bell projection where these two photons were detetected after passing through a Bell type polarizer with two inputs and two output channels. I can’t explain this further here, because I’m not really familiar with this technique. I therefore refer quantum physicists wanting to know more to the publication of the experiment. The conclusion the authors draw from the outcome is that photon one and four were entangled while the existence of photon one ended before photon four was created. So, they evoked and observed entanglement in time, not in space.

Explanation of the diagram:

  • I: The birth of entangled photons one and two at t=0.
  • II: The destruction of photon one by its detection.
  • III: Birth of entangled photons three and four.
  • IV: Bell projection of photons two and three. This will make these photons to become entangled. Because photon three has become entangled with photon four, all four photons have become entangled, even though photon one no longer exists.
  • V: Detection of photon four.
  • VI: Observation of the result by observer!


The results of the detections of photon one and four are of course only viewed after photon four has been detected. Only then is the result ‘experienced’ in the experimenter’s awareness. That’s important in this regard.

This is strongly reminiscent of the retrocausal effects that occur in the delayed choice experiments and about which heated discussions can be found on the internet. As I note at the end of the post Schrödinger’s stopwatch, the Copenhagen interpretation of quantum mechanics implies that observation (measurement) causes the spatial manifestation of the quantum object through the collapse of the quantum wave. Our looking inside the box not only manifests its contents in space, the radioactive atom, the stopwatch and the Geiger counter, but also manifests the stopwatch with the clock pointer indicating a specific moment in the past. This implies that these three observed entangled objects are not only manifested in space but also in time.

But that is, in my opinion, only apparent retrocausality. I think, it is the literal creation of history through the observation of an observer carried out at a later moment in time. It has always been clear that any experience of ours always lags behind what the experience tells us now. In most cases, the distance to the interpreted past is small, otherwise we would never be able to respond adequately to the world. But however small that distance may be, our experience is always an interpretation of a past event. We are always lagging after the PRESENT. We live however in the past.

Consensus in a virtual reality

Observation thus records an event in the past. After that, the result cannot be altered any more. The past, after observation, is fixed. In my book I make a case for the view that our experienced world is only an experience that takes place entirely within the mind. A virtual experience. But then the question inevitably arises how it is that we can usually agree with each other about our experiences. This is ensured within this virtual reality by recording everything that is observed as fixed history which can no longer be changed. The experimenter who first looks at the results of the experiment irrevocably records what happened. After that, the others can only confirm this. That is not true retrocausality but the creation of history. Creative accounting.

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