# Any loophole tactics to cheat locality with entanglement?

1. Aug 12, 2013

### QuestionMarks

I understand that relativity at least philosophically excludes the ability to use entanglement to instantaneously send information, but as I understand it, it seems only philosophically, and the hard problem is that you break the entanglement by measurement.

(Before I go on, I'm sure that ^^ needs some logic correcting)

Anyways, it got me thinking that, if you weak measurements were possible, that perhaps we could trick the system. What I mean is that instead of fully decohering the entanglement, Alice and Bob make weak measurements that infer on a more probabilistic matter rather than factual. They then make decisions on the "most likely" state of the entangled system. The "most likely" state could very well not at all be the real one, but based on having a defined convention of use, we might could take advantage of the scenario. Is this at all theoretically possible, and if not what directly stands in its way?

Have any other scenarios been proposed (that are somewhat legitimate) which insinuate loopholes as such, or is this a "No. Period." issue?

2. Aug 12, 2013

### Simon Bridge

You mean as in the complimentarity vs locality stuff that's been coming out more recently?
i.e.

The point of doing the thought experiments in a complete entanglement picture is to show the paradoxical nature absolutely - in principle. There is a proposed resolution that suggests that causality is local - the disjoint is visible only to an omnicognisant obsever ... as soon as you try to figure how Alice and Bob could possible get to compare notes you see that there is no paradox locally.

I'd expect allowing weaker measurements would not provide an inconsistent view to either observer because the fully entangled version doesn't. That is to say, the models will tell us that Alice and Bob get a different picture, but they can never show that difference to the other.

But it will be fun to do the math ... a bit like the exercise where one twin in the twin's paradox thing takes a time-stamped photo of the other twin's clock, then posts it back to them.

Last edited by a moderator: Sep 25, 2014
3. Aug 13, 2013

### San K

Roughly speaking --

the increase in probability (which-slit) will reduce the interference pattern by the same measure, hence information cannot be transmitted ftl that way.

When you are less sure of the slit it went through the clearer the pattern and vice versa.

A vague analogy could be a see-saw - you just cannot have the seats, at both/opposite ends, down at the same time. You cannot even bend the arms/poles of the see-saw by a small amount.

I.e. Which slit (probability) AND interference pattern (clarity)

Last edited: Aug 13, 2013
4. Aug 13, 2013

### Simon Bridge

I have a feeling that is what QuestionMarks is talking about - by weakening the measurement, each observer may get to see a weak interference effect between Alice and Bob's POVs. Did you see the vid? Still waiting for QuestionMark to get back to me on it's relevance. FTL communication is not at issue here but any information at all.

5. Aug 13, 2013

### StevieTNZ

I do believe information can still be sent from one particle to the other, especially in light of the measurement problem. its all well saying each photon is detected, but whether it assumes one state over the other is another matter. There is still time for a signal, not going faster than light, to get to the other detector if measurement is not actually over. According to QM, it never is.

6. Aug 14, 2013

### QuestionMarks

I'll try and watch that video tomorrow but stopping in for a thought at the moment, but Simon I think you feel my question. But perhaps another way to put it is, as our measurement is still (to even a low degree) uncertain, have we violated locality if we are able to ascertain the "type" of uncertainty (i.e. a wave function allowing the probability distribution we see)? I then propose we "cheat" by simply assuming the most likely outcome of that function based on a previous agreement of Alice and Bob to do so (agreed upon and specified in a perfectly normal local way).

7. Aug 15, 2013

### Simon Bridge

The short answer is that nobody knows for sure - it is an area of active investigation.
The favorite, if I were to bet, would be that you cannot use a trick like this to "cheat" things.
The video explains in more detail. The reason maximal entanglement is used in the thought experiment is that this gives the maximum opportunity that the proposed paradox would be revealed.
According to the speaker (iirc) - the black hole geometry enforces causal locality in a special way.
Each observer still sees non-local effects, but they see a picture that is locally consistent.

It's subtle and I only recently heard of it myself.