Comments about the comment in Nature: Witness gravity's quantum side in the lab

Following is a discussion about this article in Nature Vol 547 13 July 2017, by Chiara Marletto and Vlatko Vedral
To read the article select: http://www.nature.com/news/witness-gravity-s-quantum-side-in-the-lab-1.22273

• The text in italics is copied from the article
  
• Immediate followed by some comments
  

In the last paragraph I explain my own opinion.

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Introduction

Richard Feynman proposed a thought experiment to analyse a deep problem: the incompatibility of quantum theory and general relativity.

Thought experiments generally speaking not the correct way to unravel the laws of nature. They are completely inapropiate to discuss issues at elementary particles, because real experiments are extremely difficult.

General relativity is a ‘classical’ theory, in that any quantity that can be observed — such as the gravitational field — has a definite value that can be represented by real numbers.

The gravitational field can not directly be observed, the strength of the field can only be calculated.
It is interesting that is written: by real numbers. Spacetime is in essence based on imaginary numbers i.e. the parameters x,y and z versus the parameter t. As such we get s = x + i*c*t

In quantum theory, by contrast, observables such as position or velocity cannot both have definite values at the same time.

The problem is largely: How do you measure these quantities.
It should be mentioned that GR and the quantum theory are difficult to compare. GR describes the behaviour of large objects. The quantum theory describes the behaviour of elementary particles. The technical issue when you make any measurement in the quantum realm you disturb that particle.
If you want to measure a position you need one measurement. If you want to measure a velocity you at least two measurements, which is "impossible". See also Reflection 1

A particle may exist in a ‘superposition’ of states — being in two places at once, for example.

IMO this is physical not possible. Typical states involved are polarization directions.

When you measure its location you get a certain value, but you cannot predict ahead of the measurement what it will be.

There is nothing wrong or special with this. The problem are in the details. In a double slit experiment, as in the picture of the article, the most probable outcomes are in certain lines parallel to the slit.
To get a better understanding What you should do first is a simpler experiment with no slit but only with one hole through which the large molecules can pass. You should perform now the experiment and monitor 10 hits. When you observe the results the hits will be almost random. You can repeat this same experiment en you will get a different result also with 10 almost random hits, meaning that you cannot predict the outcome in advance. That is all, nothing special.

Hence the notorious story of Schrödinger’s cat.

You do not need Schrödinger's cat to explain this random or unpredictable behaviour.

According to quantum theory, one can set up an experiment where a cat hidden in a box with deadly poison is in a superposition of being alive or dead until someone opens the box and reveals its fate.

This sounds like a make believe story. In reality there is much more involved. The text is too short to evaluate its value.
Common sense indicates that the cat either alive or dead. When the cat is both you need first a clear definition what it means. What the reader (maybe) does not know that the poisson is released as a result of a reaction i.e. radioactive decay. This reaction has taken place yes or no and has a one to one corresponce with the state of the cat. This means that the cat is either alive or dead independent if anyone has observed the cat.

First, he considers a mass in a quantum superposition of two locations, A and B.

Unfortunate the article does not give enough information what he means and how you know that the mass is actual at two locations simultaneous.

General relativity describes how the mass interacts with the gravitational field: the mass falls according to the strength of gravity locally and also changes the field’s value slightly at A and B by its presence.

Its all in the details. A mass and its gravitational field in some sense are one and the same. They are indivisible linked to each other. That means if one oscillates the other also oscillates. IMO

"Testing Predictions"

"Quantum Gravity Test" If gravity follows quantum theory, it should set into a superposition of many states at once when it interacts with a mass that is also behaving in this way. Ofcourse if it is possible to demonstrate that a mass is in superposition (i.e in two states at once) than ofcourse the gravitational field should also follow that behaviour. The issue is how do you demonstrate that a mass is in superposition . In fact you should be able to demonstrate the difference between a mass in superposition versus a mass which is not in superposition. 4) A second mass brought close takes on the superposition of two gravitational states. 5) When its state is measured, it always gives one outcome linked to that superposition. It is easy to make such a claim but you need a clear description how "its state" is measured and how you know that its state is linked to any superposition In short: this sentence is not very clear.

Test Non Classicality

Indirect tests

Broader Thinking

Reflection 1:Quantum Mechanics

The most important experiment to define Quantum Mechanics is the Schrödinger Cat experiment. The tricky part is that it is a thought experiment and not a real one. As such you can not claim that it explains the physical reality. The biggest problem is that in fact the state of the system, i.e. the cat, is decided by an observer. The observer decides that the cat is either alive or is dead i.e. that the radio active decay has happened. Common sense dictates that this is not logical. When you consider it from a pure physical point of view the cat is alive at the beginning of the experiment and dies immediate when the poisson is released. In fact this is completely independent of a observer or any human.

The whole different issue is the uncertainty theory which states that it is impossible to measure both position and velocity of a particle. That is true. However that does not validate the claim that a particle at each instant does not have both a position and a velocity.

Reflection 2:Quantum Gravity Test

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