# Does retrocausality follow from logic?

• B
Gold Member
I can't remember where this subject came forward in my topics, so I created a new topic.

Suppose that:
1. If X happens, we observe A, and:
2. If Y happens, we observe B.
Could we then say:
1. If we observe A, Y did not happen, and:
2. If we observe B, X did not happen,
if we apply this to quantummechanical events {X,Y} and observations {A,B} thereof?

{X,Y} are exclusive, {A,B} are exclusive.
In case of binary events: X=¬Y and A=¬B.

And if we could manipulate the observation, would it then (logically) be possible to manipulate which event happened by doing so?

A way to manipulate the observation could perhaps be choice of measurement basis.

This is mainly logic applied to quantum mechanics.

fresh_42
Mentor
I can't remember where this subject came forward in my topics, so I created a new topic.

Suppose that:
1. If X happens, we observe A, and:
2. If Y happens, we observe B.
Could we then say:
1. If we observe A, Y did not happen, and:
2. If we observe B, X did not happen,
No, we can not. O.k. we can, but it is wrong. You imply a dependence and continue to argue based on this false assumption, hence everything below is wrong.
... if we apply this to quantummechanical events {X,Y} and observations {A,B} thereof?

{X,Y} are exclusive, {A,B} are exclusive.
In case of binary events: X=¬Y and A=¬B.

And if we could manipulate the observation, would it then (logically) be possible to manipulate which event happened by doing so?

A way to manipulate the observation could perhaps be choice of measurement basis.

This is mainly logic applied to quantum mechanics.

entropy1
Gold Member
In my view, there is an issue with retrocausality:

Suppose A excerts a retrocausal influence on history H. Then, if this influence is excerted by free will in the present, then H will change, and history is subjective (because it adapts to the free will of A).

If H is objective it is as it is, and if A has excerted influence on it, A would have no free will, for H is objective and will not change.

If H is objective and A has free will, then there cannot be retrocausal influence.

So we have three properties that cannot hold all three: objectivity, free will and retrocausality.

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DrChinese
Gold Member
Suppose A excerts a retrocausal influence on history H. Then, if this influence is excerted by free will in the present, then H will change, and history is subjective (because it adapts to the free will of A).

Retrocausal interpretations need not look like what you describe. The quantum future can be a monogamous participant in measurement context (one which consists of a component in the present, and one or more components in the future). That way, there are no paradoxes to consider (like killing your father - you cannot change the macroscopic past).

An example is entanglement swapping. You can entangle pairs of particles before or after they are detected. Because the results are quantum random, there is nothing that changes (to the eye anyway) which you switch from entangling before detection vs. after detection (maintaining what you call objectivity). You have full free will (as far as anyone knows).

Gold Member
A different approach occurred to me:

Suppose we have a possible event ##A## at ##t_0##: ##A[t_0]##, and a possible event ##X## at ##t_1##: ##X[t_1]##. Suppose that the absence of event ##A## at ##t_0## implies the absence of event ##X## at ##t_1##. We write:

##NOT(A[t_0]) \rightarrow NOT(X[t_1])##.

So, following modus tollens we should have:

##X[t_1] \rightarrow A[t_0]##.

This is not even physics, it is more fundamental logic. So in what way is implication something else than causation?

Nugatory
Mentor
So in what way is implication something else than causation?
Causal relationships in physical systems can suggest statements of implication, but statements of implication do not necessarily require an underlying causal relationship.

Gold Member
Causal relationships in physical systems can suggest statements of implication, but statements of implication do not necessarily require an underlying causal relationship.
Does that have to do with the following: I can imagine that A is a cause and X an effect because we can control A. But if we can't control X, the reverse is not possible. Then what if it is in principle possible to control both A and X? Then the logical statement only seems to put constraints on the outcomes of A and X.

Halc
Gold Member
Suppose A excerts a retrocausal influence on history H. Then, if this influence is excerted by free will in the present, then H will change, and history is subjective (because it adapts to the free will of A).

If H is objective it is as it is,
What do you mean by objective? That it cannot be influenced by free will? So my free choice to stab Bob makes Bob conditionally dead, where he wouldn't be had I chose otherwise, thus Bob dying this way is not objective fact, instead being a function of my free will. Notice that I put H in the future of the choice here. It seems to make no difference in the formulation of your argument. If Bob's death this way is completely determined, then yes, my choice to do it must be equally determined. I agree that free will (defined as undetermined choice) is not compatible with hard determinism, but trivially by definition. Causality or retrocausality has nothing to add to that.

Past history H being objective seems to imply that there is in fact this or that that happened in the past of some subsequent measurement.
So say there there is a ball placed under one of 3 shells, and you exercise free will and always guess wrong. Thus your choice of shells influences the past history of which shell objectively contains the ball, which is retrocausality. The shell game actually kind of works this way, and kind of depends on the lack of free will, or at least the natural reluctance to use it.

If on the other hand the ball in not under any particular shell until you peek under one of them, then there's no retrocausality since your choice is what determines only where the ball is now, but not where it was in the past.

They've done quantum experiments trying to demonstrate that there is in fact an objective state of an unmeasured system. It has never been successful.

Gold Member
What do you mean by objective? That it cannot be influenced by free will?
By "objective" I mean that one observer cannot change history for all observers. So if there is retrocausal influence by an observer, the outcome is known, and the free will of the observer is constrained, or his retrocausal influence is constrained, or his retrausal influencence exactly matches what is already layed out in history, in which case the outcome in history could be the result of his retrocausal influence, but this outcome has already established in, or projected into, the past.

And of course there could also only be a correlation between the present and the past, in which case the problem of causation gets circumvented altogether.

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pbuk
Gold Member
Suppose we have a possible event ##A## at ##t_0##: ##A[t_0]##, and a possible event ##X## at ##t_1##: ##X[t_1]##. Suppose that the absence of event ##A## at ##t_0## implies the absence of event ##X## at ##t_1##. We write:

##NOT(A[t_0]) \rightarrow NOT(X[t_1])##.

So, following modus tollens we should have:

##X[t_1] \rightarrow A[t_0]##.

This is not even physics, it is more fundamental logic.
All of this is true, but it has nothing to do with causation.

So in what way is implication something else than causation?
Where did causation come from - you may as well ask "in what way is implication something else than basketball?"

fresh_42
Mentor
Temporarily closed for moderation.

berkeman
Mentor
After some thread cleanup, the thread will remain closed. Thanks everyone for trying to help the OP.