Comments about the article in Nature: QBism puts the scientist back into science

Following is a discussion about this article in Nature Vol 507 27 March 2014, by N.David Mermin
In the last paragraph I explain my own opinion.


The article starts with the following text:
Physical science describes the objective external world: particles, waves and fields; how they change in time and how they give rise to the forms of matter, etc, microscopic and macroscopic.
Phyisical science describes: particles and objects small and large. Fields are abstract, mathematical concepts.
This world makes itself known to each of us through our own private internal perceptions.
In principle this is correct, but overall a description of the physical world is a combined effort by millions of people.
Yet physical science has ignored the 'subject' - the scientist etc.
There is nothing wrong with this. Physical science is an art independent of any human influence.
Two such unrelated longstanding problems are both resolved by recognizing that the perceiving subject has important a role to play in understanding the nature of physical science as does the perceived object.
In general the scientist performing science ie experiments, has no relation with the object being studied. He or she should try to minimise to influence what is measured.

Next those two problems are discussed.

"Quantum Mechanics"

Schrodinger wrote that quantum mechanics "deals only with object-subject relation"
This is to short. Next:
Niels Bohr insisted that the purpose of science was not to reveal "the real essence of the phenomena" but only to find "relations between the manifold aspects of our experience"
Both descriptions are to short to honour what both scientists performed.
In the twenty first century Christopher Fuchs and Rudiger Schack put forth an understanding of quantum mechanics that restored the balance between subject and object.
First you have to explain what is wrong.
People argue to this day about whether wavefunctions are real entities, like stones or ripples on a pond, or mathematical abstractions that help us to organize our thinking like the calculus of probabilities.
The wavefunction is primarily a mathematical function and a set of parameters, similar like the friedmann equation. The first step is to calculate these parameters based on actual experiments.
If the experiments result in two states with a certain probability each than the parameters should reflect this.
If the experiments result in four states with a certain probability each than different parameter values should reflect this.

Nothing should reflect the point of view of the experimenter or any human being involved.

Fuchs and Schack adopt the later view.
That is correct. Next:
They take a wavefunction to be associated with a physical system by an agent - me, for example, based on my past experience.
Me and my past experience have nothing to with science ie the outcome of any experiment.
People who believe wavefunctions to be as real as stones etc
They have it wrong.
But according to QBism the change is only in my personal expectations which I revise to accomodate my new experience.
In general science (physics, astronomy) has nothing to do with my personal expectations.
Except if I threw dice and the result is 1,1,1,2,2,2,1,2 then I learn something.

"Action at a distance"

Another celebrated part is 'quantum non-locality' the belief of some physicists that an action in one region of space can instantly alter the real state of affairs in a faraway region. etc this mysterious action at a distance
If you think that that is possible you should perform an experiment which demonstrates this.
 (red) A<--------------------experiment----------------->B (green) 
 10.00   -------------------->   O    <------------------   10.00 
                             figure 1  
In the above experiment the Observer at O sends out two light signals to A and B. What A and B receive is either a red or a green signal.
What the result of 1000 experiments show that in all cases when A receives a red signal than B receives a green signal or vice versa. Conclusion there is a 100% correlation.
To automate this set up you can also send the signals back to the Observer at O. In that case he can cheque what happens him self. To synchronise there is a clock at each side which the observer can see via separate channels. That means when the observer sees a red signal and the green signal both at 10 o'clock he knows they belong together.

However all of the above does not represent action at a distance. The only thing that A knows if there is 100% correlation that when he receives a green signal B will receive a red signal.
There only exists action at a distance between A and B via a special channel, when A sends out a red signal to B. This signal is reflected back from B to O as explained above. The same for the red signal from A to O. The Observer at O will decide that there is action at a distance involved between A and B because the clock readings and the colors received are always identical.
That means when A sends a red signal this is happening but also fror the green light.

 (red)   -----------------------------------------------> 
   A Experiment                                          B  (red)
 10.00   -------------------->   O    <------------------   10.00 
                              figure 2  
In reality IMO such an action at a distance will never be demonstrated.

This is how we can arrive at a common understanding of our external worlds, in spite of the privacy of our individual experience
The only way to do science is by performing experiments. As much as possible in an automatic way by robots. The whole issue is that how smaller the particles are you want to understand the more difficult it becomes to describe their movements accurately. The only way left is to introduce probabilities.

The now

the next issue is:
the problem of the Now, which arises in purely classical (pre-quantum) physics.
Philosopher Rudolph Carnap recalls that the problem of the Now worried Einstein seriously. Einstein told him that the experience of the present moment means something special for mankind, essentiually different from the past and the future and that physics cannot describe such a difference.
The concept of the Now or the present should be discussed independent of our experience. It has nothing to do with mankind.
What is important that the state of the universe is continuously changing. What we humans try to do is to predict the future based on measurements in the past. In order to do that you need an accurate time base or clock in order to synchronize your measurements.
The issue for Einstein was not the famous relevation of relativity that whether or not two events in two different places happen at the same time can depend on your frame of reference.
We all agree that at a certain position a sequence of events can happen identified by the moments: t0,t1,t2 and t3
The issue is how do we establish that at different places different events can happen at those same moments: t0,t1 ,t2 and t3.
The moment t0 is the Now when it happens. The same for t1 etc.
IMO the only way you can do that is to have a "fixed" grid of equally spaced bars with identical synchronized clocks at all the crossing points. The grid should be larger than the space studied.
At page 423 we read:
It is a basic fact of relativity that my personal time - the progress of my present momemt - keeps pace with the reading of my watch.
I would say that is a practical assumption that my personal time is the time on my watch.
But that does not mean that we both travel from A to B via different routes, that after we meet the time on our watches can not be different. (Assuming they are synchronised at A)
Consider two twins A and B. When they are together at home their Nows coincide. A flies off at 80% of the speed of light etc back home to B at the same speed. Relativity requires that if B's watch has advanced ten years A's has advanced six.
But because each of their present moments has advanced in step with the watch each is carrying, the moment of their reunion continues to be Now for them both.
When they meet and shake hands they do that at the same moment ie Now. This has nothing to do with the watch each is carrying.
The first issue is that when they meet that their watches are not identical.
The second issue is to discover the laws that describe this behaviour.
When you use the grid discussed before this understanding becomes easier. What you see is that B stays at a fixed point in the grid and that A moved through the grid. What you can learn from this example is that how faster you moved through the grid how slower your clock ticks. "A" will discover this when she reaches her first crossing point because her clock lags behind the time of clock at cross point. This decrease increases linear when she moves from crossing point to crossing point.
Physics predicts that our experiences of the Now will continue to have the same familiar features in a future world of interstellar travel at speeds near the speed of light, even for the distinct Nows of many different agents.
When you travel in a space ship at whatever speed the "present" inside the space ship is the same as the "present" from the place where you came from. The fact that the time on our watches is different is of "no" importance. It is the chalenge of the physical scientist to describe these changes.


The purpose of the article is to explain QBism and CBism. The overall purpose of both should be to add something extra such that we understand the universe better. My impression is that what both concepts try to highlight is my own personal involment.
IMO when you want to do science you should do that primarily based on experiments and also as objective as possible. You should not take your own point of view into account and you should try to minimise to disturb the environment when you perform your experiments.
The fact that we humans have limited capabilities and that we cannot measure everything at 100% accuracy does not imply that the underlying reality requires descriptions like: "to be in two states at the same time" (which is difficult to prove) where a much more truthfull answer is: We don't know exactly.

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Created: 30 March 2014

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