# I Would this experiment disprove that consciousness causes collapse?

#### john taylor

A double slit experiment, is taking place. There are detectors, placed inside both of the slits. On the first run if a particle travels through one of the slits, the detector registers, that it has detected a particle, but doesn't specify which slit it has travelled through(to the conscious experimenter). Moreover, the potential for having knowledge as to which slit the particle went through is totally removed since the detector, is set up in such a manor that it is only possible to know that a particle was detected and not which slit it went through. On the second run of the experiment, however, the potential for the experimenter to know which slit it went through is given, by the detector revealing which slit it went through(which would in turn give the experimenter, the potential to know which slit). If there is no interference pattern there on the second run, but on the first run, this would imply, that consciousness causes collapse, since it shows when the potential for human knowledge is there the particle collapses to an eigenstate, as opposed to not collapsing to an eigenstate, on the first run, when a measurement is being made but without the potential for a conscious observer to know the result. Would this prove or disprove(in principle) that consciousness causes collapse?

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#### Mentz114

Gold Member
The detector, if it gives enough information for location, will cause decoherence which inhibits the interference.
No consciousness required.

#### DrChinese

Gold Member
On the first run if a particle travels through one of the slits, the detector registers, that it has detected a particle, but doesn't specify which slit it has travelled through(to the conscious experimenter). Moreover, the potential for having knowledge as to which slit the particle went through is totally removed since the detector, is set up in such a manor that it is only possible to know that a particle was detected and not which slit it went through.
Actually, there are versions of the double slit in which this occurs. There is NO interference pattern, essentially proving that human consciousness does NOT cause collapse.

"An apparatus for a double-slit interference experiment in the single-photon regime is described. The apparatus includes a which-path marker that destroys the interference... "

The markers are never examined to determine which slit the particle traversed, and yet the interference disappears. When the marker is eliminated, interference is restored. However, the apparatus itself is the same in both versions; the marker might be considered "virtual".

#### StevieTNZ

Would this prove or disprove(in principle) that consciousness causes collapse?
No.

#### atyy

Would this prove or disprove(in principle) that consciousness causes collapse?
No, as @StevieTNZ said.

#### atyy

Actually, there are versions of the double slit in which this occurs. There is NO interference pattern, essentially proving that human consciousness does NOT cause collapse.

"An apparatus for a double-slit interference experiment in the single-photon regime is described. The apparatus includes a which-path marker that destroys the interference... "

The markers are never examined to determine which slit the particle traversed, and yet the interference disappears. When the marker is eliminated, interference is restored. However, the apparatus itself is the same in both versions; the marker might be considered "virtual".
This is wrong, because there is a "collapse" even if there is no interference pattern.

Or to be more strictly correct, there is no collapse in either version of the experiment, whether there is an interference pattern or not.

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#### DrChinese

Gold Member
This is wrong, because there is a "collapse" even if there is no interference pattern.

Or to be more strictly correct, there is no collapse in either version of the experiment, whether there is an interference pattern or not.
Not really sure what you mean here. Perhaps it is nomenclature. Usually, in a double slit setup, collapse refers to there being a single slit being traversed unambiguously. Obviously, there is also a degree of collapse when the particle is detected on the screen as well.

#### atyy

Not really sure what you mean here. Perhaps it is nomenclature. Usually, in a double slit setup, collapse refers to there being a single slit being traversed unambiguously. Obviously, there is also a degree of collapse when the particle is detected on the screen as well.
In the standard interpretation (with respect to which the OP question makes sense, since in this interpretation measurement has a special status and collapse is defined) collapse is used when describing the probabilities of measurement outcomes when there are successive measurements. A measurement is when there is a definite outcome; equivalently it is used when one applies the Born rule.

I should also say I am not fond of the terminology of "which path" experiments. In the standard interpretation, particles are not assigned paths. In the Bohmian interpretation, they are - but the "which path" language does not use the Bohmian interpretation. Instead it uses an influential "which path" terminology developed by Scully and colleagues, which is not part of the standard interpretation, and is flawed.

"In this paper we have shown that the claim that we can meaningfully talk about particle trajectories in an interferometer such as the one shown in figure 1 within quantum mechanics made by ESSW2 [8] and by Scully [9] does not follow from the standard (Copenhagen) interpretation. An additional assumption must be made, namely, that the cavity and the atom can only exchange energy when the atom actually passes through the cavity. Here the position of the particle becomes an additional parameter, which supplements the wave function and therefore is not part of the orthodox interpretation. Furthermore we have shown that this way of introducing the position coordinate leads to a contradiction as we are forced to conclude that although the atom follows one path, it behaves as if it went down both paths."

Also nice: https://arxiv.org/abs/1707.07884

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#### vanhees71

Gold Member
This paper by Fankhauser is funny ;-)). He envokes the collapse hypothesis in a way just to prove his proposition (in my opinion nevertheless very valid point that nothing is retrocausal in the quantum-eraser experiments). This just solidifies my prejudice against any kind of collapse interpretation. Since it's an unnecessary unsharply defined esoterical happening in the minds of philsophers, which is not of course not described nor describable mathematically within the formalism, you can massage it in any way you like.

In the minimal interpretation there's no problem whatsoever, of course. It's just a wise advice by the author to stop reading at the end of Sect. 2. Then you are condemned as an "instrumentalist", but so what? It's the only consistent interpretation without adhoc esoterics like collapses or Bohmian paths, where the Bohmian theory cannot even consistently formulated (namely for photons which are massless and thus as much relativistic as anything can get and have no well-defined position observable to begin with).

All there is, is quantum entanglement describing correlations due to state preparation, which are not possible in any local deterministic hidden variable theory (according to Bell). Since the correlations are due to state preparation, i.e., before the measurement procedure on the so prepared object starts, there's neither an instantaneous action at a distance nor any possibility for retrocausation.

As the author correctly states, the selection of whether you want to realize the "which-way-information" setup or the "interference-pattern setup" can be made after the experiment has done and all photons are gone by just choosing the corresponding subensembles of photon pairs according to the appropriate detection events as fixed in the measurement protocol. There's nothing mysterious in this within the minimal interpretation: One just has to accept that quantum theory has taught us more details about how nature works than our "common sense" built solely from our every-day experience with macroscopic objects which almost always seeem to follow the rules of classical physics (except the quantum specific fact that there's stable matter at all ;-)), which they do because we don't (and often cannot) look close enough but only look at coarse-grained macroscopic observables usually not resolving even the quantum and and even the much larger thermal fluctuations.

#### atyy

This paper by Fankhauser is funny ;-)). He envokes the collapse hypothesis in a way just to prove his proposition (in my opinion nevertheless very valid point that nothing is retrocausal in the quantum-eraser experiments).
The collapse interpretation is instrumentalist - everything is the same whether one takes collapse to be real or not real - but it is a required mathematical operation. He simply shows that there is nothing retrocausal in the standard instrumentalist interpretation.

#### vanhees71

Gold Member
Ok, I can accept this, but I still don't get why you always insist on the collapse. What's done in this eraser exp. doesn't anywhere need a collapse:

(a) Preparation (defining the state of the two photons): laser light excites an appropriate birefringent crystal which fluoresces a pair of polarization (and momentum) entangled photons (parametric down conversion). Nowhere has anything collapsed.

(b) You register photons at D0 either

(b.1) coincidently with D3 or D4, either of which implies that you have which-way information, and no interference pattern is found at D0

or

(b.2) coincidently with D1 or D2, either of which implies that there's no which-way information, and the double-slit interference pattern is found at D0

Just looking at D0 does never produce an interference pattern, and just the choice of the partial ensemble according to (b.1) or (b.2) lead to either which-way information and no interference pattern or no which-way information and an interference pattern. All photons at D0 together don't give the interference pattern.

There has nothing collapsed either (except when the photons are registered, where you might call the absorption of a photon in the detector a "collapse", but it's nothing else than the interaction of photons with (the electrons of) the matter making up the detector, which is not outside of QT either).

The fact that you can choose whether you have which-way information about the photon registered at D0 or not via manipulations of the other photon is not caused by any interactions of either photon with the equipment but by the preparation of this photon pair by parametric down conversion in the crystal, and the correlations needed are due to these correlations.

You could even just make a measurement protocol, where you consider all events at registering a photon at D0 and also marking which of the Detectors D1-D4 has clicked coincidently (and you only register such events). Then you can reconstruct the interference pattern of photons registered by just looking at events where the 2nd photon hit D1 (or D2) or get which-way information by just looking at events where the 2nd photon hit D3 (or D4).

It is also clear that the detection of the one photon at D1-D4 doesn't do anything causally with the other photon registered at D0. You can, e.g., put the part of manipulating and detecting the 2nd photon as far away from D0 as you want. As long you make sure that nothing disturbes the photon on its way to the equipment, i.e., no decoherence happens, you still can "post-select" whether you want which-way information or an interference pattern by the above defined coincidence measurements.

So there's no collapse needed to explain the results of the experiment and the possibility of post-selection or erasure of which-way information. It's just the simple fact that there are correlations which are described by the entanglement of the two photons.

#### DrChinese

Gold Member
Skipping the intermediate discussion, I would repeat: the Rueckner/Peidle reference (#3) demonstrates clearly that human consciousness is not required to cause the double slit interference to disappear. The relative settings of the polarizers is the only thing that is varied to create or eliminate the interference.

#### atyy

Ok, I can accept this, but I still don't get why you always insist on the collapse. What's done in this eraser exp. doesn't anywhere need a collapse:

(a) Preparation (defining the state of the two photons): laser light excites an appropriate birefringent crystal which fluoresces a pair of polarization (and momentum) entangled photons (parametric down conversion). Nowhere has anything collapsed.

(b) You register photons at D0 either

(b.1) coincidently with D3 or D4, either of which implies that you have which-way information, and no interference pattern is found at D0

or

(b.2) coincidently with D1 or D2, either of which implies that there's no which-way information, and the double-slit interference pattern is found at D0
Yes, when the registrations are coincident, there is no need for collapse. But if the registrations are not coincident, then the first measurement will collapse the state. It's exactly the same as in spacelike separated Bell tests. Because of the spacelike separation, there is a frame in which Alice and Bob measure at the same time - in this frame there is no collapse. In other frames, the first measurement collapses the state. So collapse is frame-dependent.

#### vanhees71

Gold Member
Why is there need for collapse if you simply perform another measurement, i.e., not a coincidence measurement? Then there's even nothing to be surprised about, because then you simply measure all photons arriving at D0, and as QT predicts, there's no interference pattern there.

Your entire posting contradicts itself: Indeed if the registration events at A and B's sites are space-like separated there cannot by construction be any causal connection between these events. You explained this yourself by the fact that you can always find reference frames, where the causal order is the opposite from that in the original one and also one where both events are at the same coordinate time of this frame. This clearly shows that the events cannot be causally connected.

Now also the usual relativistic QFT (of which QED is the paradigmatic example) without the unnecessary additional collapse postulates does not contradict this causality structure built in in the relativistic spacetime model (Minkowski space). The QFT is tailored such that this cannot happen, because any local observables (i.e., a operators $\hat{O}_k(x)=\hat{O}_k(t,\vec{x})$ built from the fundamental field operators) commute at spacelike separations of their arguments. Particularly that's the case for the interaction-Hamilton density between photons and charged particles and thus the detector material. Thus this interaction cannot cause anything at space-like distances and thus no causal effect spreads larger than the speed of light. This is by construction!

Thus the collapse hypothesis bluntly contradicts the theory it is supposed to interpret. This makes no sense, and it is not necessary here: The correlations described by the entangled state are due to a (also local!) interaction preparing the entangled two-photon state in the very beginning of the experiment, i.e., before any of the detector clicks, and here indeed before means that the "preparation event" and all detector-click events are really time-like separated, and this is a true cause for the observed correlations, i.e., that the two photons were entangled before any measurement was done on them by A and B at their sites. So there's no need to explain through a mysterious collapse the findings of the corresponding correlations in coincidence experiments, and you can only find them by doing coincidence experiments.

#### atyy

Now also the usual relativistic QFT (of which QED is the paradigmatic example) without the unnecessary additional collapse postulates does not contradict this causality structure built in in the relativistic spacetime model (Minkowski space). The QFT is tailored such that this cannot happen, because any local observables (i.e., a operators $\hat{O}_k(x)=\hat{O}_k(t,\vec{x})$ built from the fundamental field operators) commute at spacelike separations of their arguments. Particularly that's the case for the interaction-Hamilton density between photons and charged particles and thus the detector material. Thus this interaction cannot cause anything at space-like distances and thus no causal effect spreads larger than the speed of light. This is by construction!
The commutation of spacelike separated observables means that no information can be transmitted faster than light. The collapse is consistent with this, as it does not allow information to be transmitted faster than light.

#### DrChinese

Gold Member
But if the registrations are not coincident, then the first measurement will collapse the state. It's exactly the same as in spacelike separated Bell tests. Because of the spacelike separation, there is a frame in which Alice and Bob measure at the same time - in this frame there is no collapse. In other frames, the first measurement collapses the state. So collapse is frame-dependent.
I'm gonna disagree here. There is no evidence that the first measurement causes collapse, as opposed to the second measurement. Such is strictly by assumption and nothing more. It makes equal "sense" to say the second measurement causes collapse. Your statement is not a part of the predictive science. (It is relevant to some interpretations.)

Further, such experiments CAN be performed in the same reference frame, so that there is no ambiguity that one occurs first. There is no frame dependence for the results, nor on the interpretation of the results, OTHER than by assumption, hypothesis or interpretation.

#### vanhees71

Gold Member
The commutation of spacelike separated observables means that no information can be transmitted faster than light. The collapse is consistent with this, as it does not allow information to be transmitted faster than light.
The collapse is not consistent with this, because it's assumed that the state collapses instantaneously. You need a lot of handwaving to make collapse consistent with QFT, if there's any convincing formulation at all (I've seen none so far). The good thing is, that the collapse assumption is not needed at all.

#### vanhees71

Gold Member
I'm gonna disagree here. There is no evidence that the first measurement causes collapse, as opposed to the second measurement. Such is strictly by assumption and nothing more. It makes equal "sense" to say the second measurement causes collapse. Your statement is not a part of the predictive science. (It is relevant to some interpretations.)

Further, such experiments CAN be performed in the same reference frame, so that there is no ambiguity that one occurs first. There is no frame dependence for the results, nor on the interpretation of the results, OTHER than by assumption, hypothesis or interpretation.
Yes, indeed, and that's why you just give up the unnecessary collapse assumptions. It's nowhere needed to understand quantum experiments, including those of this kind with entangled parts of quantum systems (like the two photons in this example).

#### atyy

The collapse is not consistent with this, because it's assumed that the state collapses instantaneously. You need a lot of handwaving to make collapse consistent with QFT, if there's any convincing formulation at all (I've seen none so far). The good thing is, that the collapse assumption is not needed at all.
The collapse is consistent with relativity, eg. see Figure 1 of https://arxiv.org/abs/0706.1232.

This is not available free online, but you might be able to access it: https://link.springer.com/chapter/10.1007/BFb0104397.

#### keithdow

You should look up Wigner's friend. Simply stated, put someone inside a space suit next to Schrodingers cat. It won't affect the outcome of the experiment. For the external observer, the wave function only collapses when he looks. For the friend, the observation in continuous.

#### vanhees71

Gold Member
I've no clue what you want to tell me. If you believe in a collapse (which in my opinion is pretty absurd) then the presence of the 2nd observer for the external observer is (let's call them Alice and Bob for convenience) that if Bob knows that the cat is continuously watched by Alice "collapsing" the poor creature to a definite state "alive" or "dead", he'll use the stat op
$$\hat{\rho}=p_{\text{alive}} |\text{alive} \rangle \langle{\text{alive}} + p_{\text{dead}} |\text{dead} \rangle \langle \text{dead}|.$$
That's because then he knows that the cat is no longer entangled with the state of the unstable particle ("still there" vs. "decayed").

In the other case, i.e., if Alice is not present he'd use the pure state
$$\hat{\rho}_{\text{particle}+\text{cat}}=|\Psi \rangle \langle \Psi | \quad \text{with} \quad |\Psi \rangle=\frac{1}{\sqrt{2}} |\text{particle there},\text{cat alive} \rangle + |\text{particle decayed},\text{cat dead} \rangle.$$
Now the funny thing is, ignoring the particle, Bob will also assign the state
$$\hat{\rho}_{\text{cat}}'=\mathrm{Tr}_{\text{particle}} \hat{\rho}_{\text{particle}+\text{cat}} = \hat{\rho}.$$
So definitely nothing has changed for Bob. It's just different ways to describe the cat's state given the knowledge Bob has due to the knowledge about the setup of Schrödingers devilish experiment.

Even if you believe in collapse Alice's presence has no effect on the assignment of a state to the cat by Bob, because that't the description Bob has to choose given the setup of the experiment (or in formalistic terms the preparation of the particle and the cat).

#### bob012345

Gold Member
The whole cat thing started out as an obviously absurd parody but unfortunately morphed into a rather serious teaching concept that frankly just makes physics look silly to the public.

#### StevieTNZ

The whole cat thing started out as an obviously absurd parody but unfortunately morphed into a rather serious teaching concept that frankly just makes physics look silly to the public.
So are you denying quantum theory applies to macroscopic objects, or ...?

#### keithdow

"I've no clue what you want to tell me."

The wave function collapses when a measurement is made. For the external observer, it collapses when he checks to see if the cat is dead. For the internal observer it is collapsing all the time because the internal observer is making a continuous measurement. Do you get it now?

#### atyy

I've no clue what you want to tell me. If you believe in a collapse (which in my opinion is pretty absurd) then the presence of the 2nd observer for the external observer is (let's call them Alice and Bob for convenience) that if Bob knows that the cat is continuously watched by Alice "collapsing" the poor creature to a definite state "alive" or "dead", he'll use the stat op
$$\hat{\rho}=p_{\text{alive}} |\text{alive} \rangle \langle{\text{alive}} + p_{\text{dead}} |\text{dead} \rangle \langle \text{dead}|.$$
That's because then he knows that the cat is no longer entangled with the state of the unstable particle ("still there" vs. "decayed").

In the other case, i.e., if Alice is not present he'd use the pure state
$$\hat{\rho}_{\text{particle}+\text{cat}}=|\Psi \rangle \langle \Psi | \quad \text{with} \quad |\Psi \rangle=\frac{1}{\sqrt{2}} |\text{particle there},\text{cat alive} \rangle + |\text{particle decayed},\text{cat dead} \rangle.$$
Now the funny thing is, ignoring the particle, Bob will also assign the state
$$\hat{\rho}_{\text{cat}}'=\mathrm{Tr}_{\text{particle}} \hat{\rho}_{\text{particle}+\text{cat}} = \hat{\rho}.$$
So definitely nothing has changed for Bob. It's just different ways to describe the cat's state given the knowledge Bob has due to the knowledge about the setup of Schrödingers devilish experiment.

Even if you believe in collapse Alice's presence has no effect on the assignment of a state to the cat by Bob, because that't the description Bob has to choose given the setup of the experiment (or in formalistic terms the preparation of the particle and the cat).
So collapse is subjective?

"Would this experiment disprove that consciousness causes collapse?"

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