Causality, as per Wiki

Is the relation between an event (the cause) and a second event (the effect), where the first event is understood to be responsible for the second.

For this relationship to hold true, it is necessary that the first event must precede the second one.

Causality is a staple of contemporary philosophy. And it is something that we take for granted; of course what happens now will affect the future, but not vice versa!

However, in delayed choice quantum eraser experiment,

If a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on earth on how to observe a photon could alter a decision made millions or even billions of years ago.

In other words, what is happening now will be determined by what will happen in the future, in direct contradiction with the principle of causality that we know so well! A principle that was almost true to the tautological sense is now not always true.

Is the delayed choice quantum eraser a refutation of principle of causality? How does contemporary philosophy make sense and incorporates this latest scientific finding?


One of the fundamental principles of quantum mechanics is the principle of superposition. Its most simple application reads: If two paths exist to move from state A to state B, then the transition function (psi-function) develops to the final state as the sum of the two separate transition functions.

The double split experiment has two different paths from A to B. Hence there are two transition functions which have to be considered. The experiment can be executed with individual photons, one photon after the other is sent through the double split. If the experiment performs undisturbed then both transitions functions of a photon are coherent and interfere. In the experimental setting the double split is substituted by a beam splitter which produces a left and a right beam.

For the theoretical proposal, the experimental setting and the interpretation of the result of the delayed-choice quantum eraser see

  • Brian Greene: The Fabric of the Cosmos. 2004. p. 101ff
  • Aharonov, Yakir; Zubairy, Suhail: Time and the Quantum: Erasing the Past and Impacting the Future. Science, Vol. 307, 2005, p. 875-879

Scully and Drühl in 1982 propose a means to tag photons differently after leaving the beam splitter. Tagging adds to each transition function a different which-path information. As expected, this tagging destroys the coherence. The result does not show any interference.

Secondly, they propose a quantum eraser which removes the tagging just before the final detection of the photon. As expected, the two transition functions with erased which-path information interfere.

As a third step, Scully and Drühl propose a delayed-choice quantum eraser. Behind the beam splitter each photon is down-converted to a pair of entangled photons of half frequency. One photon of the pair is named the signal photon, the other the idler photon. The idler photon is tagged with which-path information.

Now, the transition functions of the signal photons are treated as before. But the idler photons, each carrying the which-path information of it and its signal partner, are either observed separately and their which-path information is read off. Or they match and loose the which-path information.

The astonishing result of the experiment emerges when separating the paths of the signal photons from the paths of the idler photons by a far distance, e.g. 10 light years. Assume that today the signal photons terminate their path. One observes non-interference of the signal photons, because their which-path information still exists, namely contained in the transition function of their idler partners.

10 year laters also the idler photons terminate their much longer path. They are detected either after matching or they are detected in separation. Now one can single out those signal photons whose idler partners have matched and thereby erased their which-path information. The subset of the corresponding signal photons provides an interference pattern.

Greene draws the following conclusion:

Again, let me emphasize that the future measurements do not change anything at all about things that took place in your experiment today; the future measurements do not in any way change the data you collected today.

Hence the answer to your original question is: The delayed-choice quantum eraser is no refutation of the principle of causality. The future event does not change the past event.

Green continues:

But the future measurements do influence the kinds of details you can invoke when you describe what happened today. Before you have the results of the idler photon measurements, you really can’t say anything at all about the which-path history of any given signal photon. However, once you have the results, you conclude that signal photons whose idler partner were successfully used to ascertain which-path information can be described as having – years later – traveled either left or right. You also conclude that signal photons whose idler partners had their which-path information erased cannot be described as having – years earlier – definitely gone one way or the other […]. We thus see that the future helps to shape the story you tell of the past.

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    You are right. I clarified the issue. – Jo Wehler Sep 28 '15 at 14:04

Delayed choices undermines past to future causality only if one assumes that the initial state of a system must directly determine measurement results, and that wave functions represent our ignorance of an underlying state which is already determined and only revealed by measurements, but there are other interpretations.

In sum, the situation is as follows: after a measurement, in retrospective, we can come up with a nice classical story of what happened, but the story would have been different (including for the initial state) had we decided to measure something else. But before the measurement, we cannot know the relevant aspects of the initial state that would tell us which kind of "classical story" will occur. All "classical stories" are still possible, so it's not as if the measurement changes something we knew for sure about the system, that suddenly became false. Measurement only affects something we would know if we had measured the system differently, so it's a problem only if one assumes that this "something we would know" must already be determined in some way from the start of the experiment or in other words, if we insist in having classical stories of experiments.

Here are some more details. Quantum mechanics puts some constraints on causality but things are a bit complicated. All depends on the notion of causality.

Some authors, e.g. Lewis, attempt to reduce causal talk to counterfactuals, correlations and the like (A cause B iff A had not occurred, B wouldn't have occurred, or if A rises the probability of B...). Reichenbach's principle can be invoked to that effect (if two events are correlated, either one causes the other, or the two have a common cause). Causality would be reducible to correlation talk + temporal relations.

Having causal relations of this sort in an experiment is what I called above having a "classical story" of the experiment (the technical term is counterfactual definiteness).

With such an account, you'd have to admit that there are non-local causal influences in quantum mechanics because we find non-local correlations. Since which correlations are displayed depends on the way a system is measured, either the way a system is measured has retrocausal influence on the initial state, or it has instantaneous non-local influence. In the latter case, since according to relativity there is no absolute ordering in time of distant events, it seems that one cannot say anymore whether the cause precedes its effect or not (unless one introduces a privileged frame of reference, which is at odd with relativity). So in any case under this conception of causality we have a problem with time ordering of causes and effects.

However there are other accounts of causality: causality as production (A causes B iff it is possible to produce A to make B occur) or energy or information transfer. Under these accounts there is never a non local influence because there is no way an agent could influence a remote measurement as s/he wishes only by deciding to measure the system in a way or another at his position. This is because in any case, the measurement result will be random. Then the correlation at a distance could be explained not by a causal influence from one place to another, or by a common cause, but for example because the properties we measure pertain to a non-local, holistic system.

So although some aspects will remain puzzling, there are ways to understand causality in such a way that no retrocausal influence is entailed by quantum mechanics.


Your question is about the eraser experiment specifically. In this case you have a double slit experiment, where the particles are entangled with others which are used at a later time so as to extract an interference pattern from the first experiment. This extraction involves "erasing" the information on which slit the particles went through.

Note however that the pattern initially measured is not changed by future actions: it shows no interference, we only extract an interference pattern from it by selecting some particles (the ones that go through the eraser). Which particles will go through the eraser is unpredictable. So although it seems natural to view the eraser as causing the interference pattern retrospectively, one could as well say that the interference pattern was already there and influenced which particles would go through the eraser or not.

What this means is that adopting a view of causality as correlation, the eraser experiment can be explained by non local causal influences without recourse to retrocausal influences because the pattern is actually extracted (not created) in the future.

The quantum eraser experiment is particularly challenging only if you have a classical picture in mind (it exacerbates how quantum mechanics violates our intuitions) but in effect, it's just a normal quantum experiment where a wave function gets measured in various ways and correlations are observed. In any case no past measurement result ever change and there can only be a problem if one assumes that there must be something out there that determines these results according to a "classical" story.

  • I don't see how nonlocality interpretation ( even if it's true) helps causality here. In some cases, nonlocality doesn't violate causality, but in this case it's completely different. If "quantum entanglement" is entangling one "object" ( proton or electron) over the space ( bell inequality experiments), then no causality problem. But in this case, the entanglement is over the time, and it does it in such a way that the future determines the past, then it will be a problem for causality, isn't it? – Graviton Sep 27 '15 at 10:38
  • Ok I see that your question is about the quantum eraser experiment, not delayed choices in general. I'll add a paragraph to my answer because it's a bit too long for a comment. – Quentin Ruyant Sep 27 '15 at 10:48
  • I think you might have misunderstand this eraser experiment and my question. The whole point is that the change of measurement apparatus can affect a photon's behavior prior to the change of the apparatus, which is a violation of causality no matter at how you look at it. Or is there anything that I miss? – Graviton Sep 27 '15 at 11:24
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    @Graviton my point is that there is a "trick" in the eraser experiments which is often overlooked, which is that you can never know for sure that a photon will enter the eraser (it's random). But if you select only the photons that enter the eraser, you'll observe that they formed a pattern in the past experiment. So the view that putting the eraser affects the behaviour of photons of the past (that it "makes them form a pattern") is a matter of interpretation. You can always say that actually the behaviour of photons in the past affects the fact that they will enter the eraser or not. – Quentin Ruyant Sep 27 '15 at 12:08
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    @Graviton Your argument shows that imagining photon as switching between a wave and a particle is inconsistent with no retrocausality. But this interpretation is incoherent for many other reasons as well. Quantum objects are neither, "wave-particle duality" is an unfortunate historical misnomer that simply refers to the fact that occasionally they exhibit those types of properties. There is no point asking in this way what and how went through the slits. – Conifold Sep 29 '15 at 2:59

If the OP cared to read a little further down on the linked wiki article, he would have discovered:

if the photon is interpreted in flight as a super-position of states, ie as having the potentiality to manifest itself as a wave or particle, but in flight it is neither then there is no time paradox.

They add:

This is the standard view and recent experiments have supported it.

To which they quote the paper on a quantum erasure experiment by Ma, Zeilenger et al:

Our results demonstrate that the viewpoint that the system photon behaves either definitely as a photon or a wave would require faster than light communication. Since this would be in string tension with Special Relativity, we believe this viewpoint should be given up entirely.

Interestingly, I recall reading in one of his popular books that Feynman worked on Wheelers theory of back-reaction but abandoned it because he couldn't get it to work.

Causality is a strong constraint on physical thinking; and counter-factual or speculative thinking on this is generally only to understand this better: for example the SEP distinguishes between temporality and causality.

As an additional remark on this, causal set theory is an alternative approach to QG which takes this structure as basic and is motivated by a theorem that shows the geometry of GR follows from its causal structure.


In simple terms, that would only violate Relativity. By going outside the so-called Light Cone. What I mean is, one could as easily regard it as a failure of the way of mathematically representing so-called space-time. In other words, "time" is here understood relative to a thought experiment which has been formalized and is in many cases functional, i.e., it does stuff/predicts things.

addendum explanatory:

The notion of "curved space", e.g., is not sufficient for philosophy, diological discussion. Since it implies a "rectilinear" space, as a limit state, out of which the so-called curve is defined. It simply hangs like an axiom in mid air, and asks, do I give results? It doesn't connect to the ground of human sense. So, it is rather like a complex game, and not like a true place from which one can say, here is time as such.


Thanks to all the posters for their views on the delayed choice quantum eraser experiment. In the end, we seem to have only two explanations for the observed results. Either the idler photon that is observed later then the signal photon is effecting the signal photon's pattern at an earlier time, retro-causality, or the signal photon's pattern is affecting which path the idler photon is taking, which could be explained with non-locality at a later time, perhaps even years later in the case of the cosmic scale experiment. I suppose most people would prefer to accept non-locality at a later time, even to the point of changing the path of a photon in flight or effecting what should be a purely random event like the reflection or transmission of a photon through a half silvered mirror, than accept retro-causality. I suppose either explanation can be made to fit, but non-locality at a later time at least preserves causality. I personally don't think we should go to heroic efforts to preserve our classical notions of causality, but I do agree that non-local action at a later time (influencing the idler photon's actions to be erased or not) is the explanation that is most consistent with known physics. Of course, if we accept the explanation that this is simply the result of a delayed non-local action, we'll have to assume some sort of axiom or law of nature that drills all the way down the reflection or transmission of that individual idler photon at that particular moment in order to keep all the observations consistent, even when they are separated by an arbitrary amount of time or space. Fascinating. Thanks again to all.

  • Are you the original user who posted the question? The thanks and the name suggest so, but this is a new user. – Frank Hubeny Mar 23 at 22:36
  • No, he is not me – Graviton Mar 23 at 23:51
  • To the answerer, the problem with non locaility is that it contradicts GR, which is a bigger problem to fix – Graviton Mar 24 at 7:07

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