Comments about the "News & Views" article in Nature: Death by experiment for local realism

Following is a discussion about this article in Nature Vol 526 29 October 2015, by Howard Wiseman
For the "Letter" article about the same subject read: nature 29 Oct 2015 page 682.htm In the last paragraph I explain my own opinion.


The world is made up of real stuff, changing only through local interactions - this local realism hypothesis is about the most intuitive scientific postulate imaginable.
What this sentence "claims" is that all changes in the physical parameters of our world propagate at a finite speed.
What is not mentioned here is that the speed of light is also relevant.
But Quantum mechanics implies that it is false, as has been known for known for more than 50 years.
This sentence tells us that Quantum mechanics 50 years ago predicted that this is not true.
Hensen et all report the first violation of a constraint called a Bell inequality etc
and therefore rigourously reject local realism for the first time.
The Bell inequality is a statistical property. The issue why do you need such a property to demonstrate that it is possible that "communication" is possible faster than the speed of light.
Bell's original formulation, which states it as the conjugation of two other hypotheses: realism essential meaning that measurements reveal preexisting physical properties of the world; and locality roughly meaning that any change enacted at one place cannot have an effect at another place unless there would have been time for a light signal to get from the first place to the second.
Two comments:
  1. A measurement is nothing more than a action (a process) to make something visible to an observer. That means realism is not a physical property.
    In a slightly different setting a measurement is nothing more than a physical process. Each collision in the LHC as such is a measurement is a part of the reality.
  2. The change (reaction rate) of any process or chemical reaction is an inherent parameter of the process itself and has nothing to do with the speed of light.
    When part of the process involve photons, because for example when light signals are involved than of course the speed of light is an issue.
The speed of light is relevant because, according to Einstein's theory of relativity, no causal influence can travel faster than light.
In general causal relations propagate at a speed much slower than the speed of light. Only processes which directly involve photons, propagation is limited by the speed of light.
Skip one line
Suppose that etc the outcomes obtained, in a sufficiently short time that the only way the choice of setting by any party could affect the outcome of any party would be through a faster-than-light influence.
The problem with that sentence is not clear.
The problem with any process is that you have to study it in great detail to see how it works and what is involved, specific if entanglement is involved.
Then, by definition, all Bell inequalities will be satisfied by all local-realistic theories.
I think you are closer to the thruth when you claim that all Bell inequalities are satisfied when no entanglement is involved.
The Bell inequalities can be violated when entanglement is involved
If in these cases also faster than light signals are involved is a whole different question.
An experiment violating a Bell inequality therefore implies that either locality or realism is false.
That is too simple.
When you throw dice there is a 50% that you get head and 50% tail. You know that by performing 1000 experiments. When you get 80% head your reply can not be that there are faster than light signals involved. There can be many other reasons.
In reality you have to perform two sets of experiments which are almost the same.
In the first experiment there are no faster than light signals involved. In the second experiment faster than light signals are involved. From both experiments you have to calculate the Bell inequalities. In the first they should agree and in the second not.
Only when that is the case you could use the Bell inequalities as a yardstick to distinquish between faster than lightsignaling or not.

However it is not that simple. When the difference in the two experiments is only without or with entanglement and when the Bell inequalites are agree and not agree, than as a yardstick the Bell inequalities can only distinquish between: with or without entanglement.

An experiment violating a Bell inequality therefore implies that either locality or realism is false.
Any experiment "always" goes through two different phases. My problem is that in experiments with involves entanglement it is not that simple to claim that faster-than-light signals are involved.
Entanglement is a holistic property of a system of quantum particles that can persist even when the particles are far apart.
The issue with entanglement is that the entangled particles are always made by a specific process. These particles have for example have a spin. In the case of entanglement the spin of each is not random but are closely correlated. They are opposite. When in the X direction in one particle it is a 1, than in the other particle it is a -1.
You prove that by performing 1000 experiments (In real it is the other way around. In many cases such a behavior is found by accident)
When you know that entanglement exists you can also see if it holds for larger distance and if what different types of entanglement there exist.
In this three-party approach A and B each prepare an entangled state of a photon and an electron, keep their electrons in a diamond lattice and send their photons to C.
The document nature 29 Oct 2015 at page 683 gives the impression that the two electrons are also entangled. "We generate entanglement between the two spins etc"
A and B then each choose a setting and measure their electrons, which can be done effeciently, while C performs a joint measurement on the two photons.
It is important to see separate results for each individual setting.
A and B outcomes will be purely random unless C gets a rare succesfull result, in which case the outcomes indicate entanglement between A and B electrons.
Part of the problem in understanding lies in the use of a Random Number Generator. For a final test you can do that, but in order to understand what is happening you should not use such an RNG.


One of the most important part of physics is performing experiments. From each experiment we can learn something and get a better idea what "The laws of Nature" are. As such any experiment can never be wrong. What is an issue, is the interpretation of the experiment. To be more specific: the causal relation between inputs and outputs and what happens in between.
Suppose you perform first an experiment with two photons and you get result A and secondly an experiment with entangled photons and you get result B. When A and B are not the same then the primary cause is entanglement.
Typical in these types of experiments you have a source and a beam splitter in order to measure the direction. When the distance between source and beam splitter(s) is identical than the speed of light is no issue between the different experiments.

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Created: 30 March 2014

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