# What does Quantum entanglement tell about Causality?

#### Soumya_M

Entangled particles seem to have 'causal impact' on each other 'instantly' violating the allowed speed-limit of causal influence i.e. 'c'. When one of the entangled particles is measured it has an impact on the other instantly. These two events (i.e. 1. the measurement of the first particle and 2. the impact on the other particle) can be said to be causally related ('1.' being the cause and '2.' being the effect). But if that is true (and it certainly is), it means 'a cause' and 'its effect' can occur at the same instance of time. So what does that tell about causality? How can it be so? A cause and its effect must be clearly separated by time. Otherwise it would be impossible to say which is the cause and which is the effect.

Related Quantum Physics News on Phys.org

#### DrChinese

Gold Member
Entangled particles seem to have 'causal impact' on each other 'instantly' violating the allowed speed-limit of causal influence i.e. 'c'. When one of the entangled particles is measured it has an impact on the other instantly. These two events (i.e. 1. the measurement of the first particle and 2. the impact on the other particle) can be said to be causally related ('1.' being the cause and '2.' being the effect). But if that is true (and it certainly is), it means 'a cause' and 'its effect' can occur at the same instance of time. So what does that tell about causality? How can it be so? A cause and its effect must be clearly separated by time. Otherwise it would be impossible to say which is the cause and which is the effect.
You cannot really say that the first "caused" the second. The logic is just as accurate if you say the second caused the first.

#### StevieTNZ

Could one say there is no causality in the Quantum world?

#### Soumya_M

Could one say there is no causality in the Quantum world?
If that is true, it will have immense implications on our understanding and way of reasoning. If there is no causality, it would mean that you could no longer say that the effect is dependent on the cause. It could just be the other way round, i.e. the cause depends on the effect. That would mean the cause was designed for the effect. And since every cause had a cause they were all designed for some purpose. And that would also mean that the universe and all particles in it were following a pre-fixed path to some ultimate destiny.

And if that were true, it would be difficult for us (sentient beings) to think of ourselves as responsible for our thoughts and actions. Indeed that way we could no longer distinguish or judge whether an action was good or bad. Indeed there would be no good or bad, only a blind destiny which we must follow mechanically.

OMG!!! That is too much for Theoritical Physics! Philosophers are you dead!!!

#### StevieTNZ

I guess it really depends if consciousness (thoughts, actions) is independent from the Quantum world (i.e. atoms, etc.), then causality might hold for consciouness.

I don't think Quantum Mechanics describes what happens in the world. Only possible realities for what could happen in the world.

#### ThomasT

Soumya M said:
Entangled particles seem to have 'causal impact' on each other 'instantly' violating the allowed speed-limit of causal influence i.e. 'c'.
Where did you get this idea from? Certainly not from qm. Quantum entanglement is acausal.

Soumya M said:
When one of the entangled particles is measured it has an impact on the other instantly.
No. Via particular preparations, and given certain other prerequisites, you might be able to infer something about what will be detected at B given a detection at A.

Soumya M said:
These two events (i.e. 1. the measurement of the first particle and 2. the impact on the other particle) can be said to be causally related ('1.' being the cause and '2.' being the effect).
No. If two events are related, and you have some idea how they're related via preparation, then you might be able to deduce, if you know the global measurement parameter, and only for certain global measurement parameters, what will be detected at B given a detection at A.

Soumya M said:
But if that is true (and it certainly is) ...
But what you're saying is almost certainly not true.

Soumya M said:
So what does that tell about causality?
Because your premises are wrong, and as per DrC, and QM, it doesn't tell anything about causality.

#### yoda jedi

Could one say there is no causality in the Quantum world?
there is causality, it depend on the frame (relativity).

Quantum teleportation and entanglement swapping viewed from different moving frames

"It is shown that quantum teleportation and entanglement swapping can be easily derived from Einstein, Podolsky and Rosen correlations and special relativity (SR). A recent proposal of entanglement swapping via delayed-choice experiment is discussed by means of a concrete example."

-------------
from other paper:
http://arxiv.org/PS_cache/quant-ph/pdf/0303/0303082v1.pdf

..."This seems paradoxical because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured", in other words, "This means that the physical interpretation of his (Bob’s) results depends on Alice’s later decision".Still quoting from their paper:"Therefore, this result indicates that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [2]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."
Naturally, firm belief is not the most appropriate way to try to solve a paradox. Actually, the reason why such a paradox does not occur is quite simple and has already been explained [7]. The point is that whenever Bob performs his measurement first, photons 1 and 2 are projected into well-defined polarization states. As a consequence, Alice will perform an experiment similar to the one performed by Hong, Ou, and Mandel [8], but in which the photons that arrive together at the beam splitter can have different polarizations. Since the probability of coincident detection depends on the relative polarization of the photons, by selecting the events in which the detectors click in coincidence, Alice obtains a subset that behaves as if it consisted of entangled pairs of distant particles. Naturally, there is nothing paradoxical in this result. The point is that for Alice the probability of coincident detection will depend on Bob’s experimental outcome. Therefore, no influence of future actions on past events needs to be invoked."...

Last edited:

#### zaybu

A...................................←●O●→....................................B

You have two particles that are released at O in opposite directions with opposite spins, moving toward A and B, where observers are stationed.

When A makes a measurement of the particle spin, say, it is an up spin. Then he can conclude that B will measure the spin of his particle to be down. Einstein had argued that this would contradict relativity since A knows the spin at B instantaneously, and no signal can travel faster than the speed of light. This is known as the EPR paradox. Note: a paradox is not a contradiction, but an apparent contradition.

The paradox is resolved when you look at the setup prior to the measurement of the spins. The observer at A had to know that another particle was sent to B. How does he know? Well the person at O releasing the two particles in opposite direction had to communicate with A and told him that a particle was coming his way, and another was moving in the opposite direction with an opposite spin towards B. So nowhere in this experiment is a signal moving faster than the speed of light. A happens to know the spin at B because he was told ahead of what O did in preparing the entangled particles.

The real mystery is why the internet is flooded with so many sites that contain the most obvious misconceptions about quantum entanglement. There is no spooky action at a distance. There is no need of hidden parameters. And no signal was sent faster than the speed of light. It simply means that if two particles are prepared in a given quantum state, unless there is an interaction, they will continue to stay in that quantum state.

The whole argument about causality is bogus.

#### Krokos

Can you combine a Bose--Einstein condensate with Quantum Teleportation of entangled photos to achieve Retro causality ?

Split an entangled beam of photos. Half the beam is directed into Bose--Einstein condensate. At some future point in time, the entangled photos that were captured in the Bose--Einstein condensate are manipulated into a quantum state that reflects the answer to a predetermined yes- no question.
The half of the beam that has not been captured in the Bose--Einstein condensate is observed and measured immediately after it is generated to determine its quantum state thus revealing the answer to a predetermined, yes- no question that was answered in the future. Because both particles are entangled, they both must have the same quantum state and thus allowing the information to travel backward in time.

#### DrChinese

Gold Member
Can you combine a Bose--Einstein condensate with Quantum Teleportation of entangled photos to achieve Retro causality ?

Split an entangled beam of photos. Half the beam is directed into Bose--Einstein condensate. At some future point in time, the entangled photos that were captured in the Bose--Einstein condensate are manipulated into a quantum state that reflects the answer to a predetermined yes- no question.
The half of the beam that has not been captured in the Bose--Einstein condensate is observed and measured immediately after it is generated to determine its quantum state thus revealing the answer to a predetermined, yes- no question that was answered in the future. Because both particles are entangled, they both must have the same quantum state and thus allowing the information to travel backward in time.
The following experiment demonstrates what you would probably define as retro-causality:

http://arxiv.org/abs/quant-ph/0201134

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger
(Submitted on 29 Jan 2002)

Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation.

From the paper:

"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because
Alice’s measurement projects photons 0 and 3 into an entangled state after they have been
measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably,
Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results
either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and
3. This means that the physical interpretation of his results depends on Alice’s later decision.
Such a delayed-choice experiment was performed by including two 10 m optical fiber
delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed
by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving