Comments about the article in Nature: Quantum puzzle baffles physicists

For the real article select: Open access.
Also read this: Following is a discussion about this article in Nature Vol 561 27 September 2018, by Davide Castelvecchi
In the last paragraph I explain my own opinion.



In the world's most famous thought experiment, physicist Erwin Schrödinger described how a cat in a box could be in an uncertain predicament.
This line immediate raises two questions:
  • How certain are we that we can unravel the laws of physics by performing a thought experiment.
    IMO in general: we can not. We have to perform real expariments.
  • How do we know that know in which state the cat is? That is an easy question: by performing many identical experiments.
THe pecular rules of quantum theory meant that it could be both dead and alive, until the box was opened and the cat's state measured.
At any moment in time the state of the cat is either alive or dead. The state can never be both at the same time. When you put the cat in the box the cat is alive. There after the cat can die as a result of a chemical reaction which causes the cats dead. The chance of this happening can be calculated by performing the same identical experiment many times. The cat is never both alive and dead.
Now two physicists have devised a modern version of the paradox by replacing the cat with a physicist doing experiments.
Also this physicist can never be in two states simultaneous.
Quantum mechanics underlies nearly all modern physics.
That is true if you consider QM the study of the behaviour of all elementary particles i.e. the standard model.
But the answers it provides can be frustratingly limited.

New cats in town

Physicist have devised a variation of the iconic Schrödinger's cat thought experiment that involves several players who understand quantum theory.
See Reflection 1 - Quantum theory and thought experiments
But surprisingly using the standard interpretation of quantum mechanics the observers sometimes seem to come to different conclusions about a particular event - suggesting that the interpretation contradicts itself for complex systems.
This raises the question: What is the standard interpretation of QM.
When the two observers open their boxes, in some situations they can conclude with certainty how the coin landed - but the conclusions are different.
What are these specific situations? The article unfortunate does not give any detail about the possible combinations that exist.

page 447

Its equations cannot predict the exact outcome of a measurement - for example, of the position of an electron - only probabilities that it can yield particular values.
The equations are a result of actual experiments and if identical experiments give a range of outcomes i.e. probabilities than the equations that mathematical describe these results should do the same.
If the equations give exact outcomes than the equations would be wrong.
Quantum objects such as electrons therefore live in a cloud of uncertainty, mathematically encoded in a 'wavefunction' that changes shape smoothly.
This description is rather romantic.
But when a property such as the electron's position is measured, it always yields one precise value (and yields the same value again if measured immediatly after)
More technical detail is required. My understanding that you cannot measure the position of an electron in general. What you can do is detect an electron. The moment of detection and the detection point define the position of the electron at that moment. If the position of detection point is not changed then the next measurement can give the same result.
But it is also reassuring: although quantum objects live in uncertain states, experimental observations happens in the classical realm and give unambiguous results.
Also here the language used is rather romantic.
Again what the writers should do is only discuss the result of actual experiments.
There are clearly two issues: The results of an experiment and the explanation or interpretation of an experiment.
Electrons don't live in uncertain states. They rotate in shells around a nucleus. The radius for a specific electron is more or less fixed, but the angle at any instant not. Every angle has the same certainty/uncertainty.
In that case, the state of the cat was uncertain until the experimenter opened the box and checked it.
The state of the cat is defined the moment the experiment is stopped and depents about the duration of the experiment. If opening of the box also stops the experiment than the experimenter does not have to look inside the box directly. He can also do that the day after.
One of the two friends (Alice) can toss a coin and using her knowledge of quantum physics - prepare a quantum message to send to the other friend (Bob).
What has 'her knowledge' to do with this? This makes the experiment rather unscientific.
Using his knowledge of quantum theory, Bob can detect the Alice's message and guess the result of her coin toss.
Why is the word guess is used?
The problem is that the article does not give enough detail to properly evaluate its value.
What I expect is that Bob also has a coin and turn his coin either head or tail as a result of the message send by Alice. But this is not mentioned.
If there is no message from Alice Bob can guess that the result. If his guess his tail he will turn the coin to tail and in 50% this will be correct.
When the two Wigners (observers) open their boxes, in some situations they can conclude with certainty which side the coin landed on, - but occasionally their conclusions are inconsistent.
Not more information is supplied about the results experiment. That is strange. What you need to know is in how many cases (percentage) the state of the two coins (assuming that Bob also has a coin) is correct.
The article finishes with the sentence:
he says: perhaps the inconsistency arises from Bob not interpreting Alice's message properly. But admits that he has not found a solution yet.
Maybe Alice did not send the message properly.

Reflection 1 - Quantum theory and thought experiments.

It is interesting to see that after so may years the physical community is still strugling with the Schrödinger's cat paradox. Why do we have to come up with a modern version to setle down the issue once and for all?
The whole issue is: how important is an observer i.e. a human being in relation to the outcome of a physical experiment or process. The answer is: It is completely of no importance.
Of course if you are cooking and the recipe tells you to stir the soup firmly and you don't do that the result will be unexpected.

The bottom point is to what extend you can use a thought experiment to unral the laws of nature. In this case specific the laws that describe the movement of elementary particles: You can not

Reflection 2 - Overall

The "New cats in town" experiment involves only human beings. That is interesting because it reflects my own simplest version of the Schrödingers cat experiment:
You place two human beings on stage with a wall and a door in between them. You give them both a red and a white had and you ask them to select one and place the had on their head.
  1. The question is: now are both human beings before they open the door and observe each other in a superposition state (uncertain predicament / both red and white)?
  2. Next they open the door and left person can see the color of the right person (and vice versa). Does that means that the wave function collases at that moment?
  3. Suppose the left person is blindfolded and cannot see the right person. Does the right person stay in a supperposition state until the left person sees him?
  4. And what about the audience? Are they also involved?
IMO the whole lesson to learn that the concept of superposition, implying uncertainty of human knowledge based on the state of any physical process, is of no physical importance for the process itself. Lack of human knowledge (inaccuracy of measurements) is only important in the way we can actual predict the evolution of a process.

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Created: 30 March 2018

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