Comments about the article in Nature: What is Spacetime?

Following is a discussion about this article in Nature Vol 557 10 May 2018 page 84, by George Musser
In the last paragraph I explain my own opinion.



People have always taken space for granted.
That is the question
It is just emptiness, after all - a backdrop to everything else
To understand you need a clear definition of: what means emptiness. Maybe the universe looks empty towards human beings but is not empty after all.
Time, likewise, simply ticks on incessantly.
To understand you need a clear definition of: what means ticks.
A year earlier he (Albert Einstein) had formulated his general theory of relativity, which postulates that gravity is not a force that propagates through space but a feature of spacetime itself.
To understand you need a clear definition of: what means 'a feature of spacetime'.
As a general rule: if you try to explain a concept with a new concept-that-is-not-clear you explain nothing.
When you throw a ball high into the air, it arcs back to the ground because Earth distorts the space time around it, so that the paths of the ball and the ground intersect again.
That may be true, but how do you describe this behaviour based on actual observations.
In a letter, Einstein contemplated the challenge of merging general relativity with his other brainchild, the nascent theory of quantum mechanics.
If you want to do that first you must describe waht general relativity and quantum mechanics is.
The problem is that general relativity describes the external behaviour of independent objects in space. Quantum mechanics describes the internal structure of objects.
That would not merely distort space but dismantle it.
The same problem as before: You need good definitions about what you mean.
Mathematically, he hardly knew where to begin.
You should start physically. Most processes can not be described mathematically.

"Down the Black Hole"

The only tangible evidence that these processes occur at all is the molted pattern of matter in the very early universe - thought to be caused, in part, by quantum fluctuations of the gravitational field.
And what does that mean?
Quantum fluctuations are very small fluctuations in the positions of quantum particles, which in turn cause fluctuations in the gravitational field, which inturn cause fluctuations of the quantum particles etc etc. This is circular reasoning.
General realtivity predicts that matter falling into a black hole becomes compressed without limit as it approaches the center - a mathematical cul-de-sac (dead end) called a singularity.
Matter can physical never be compressed without limit. Density cannot be infite. From a mathematical point you should introduce boundaries to prevent such a condition to happen. Using Newton's Law the distance between the center of two objects can not be zero.
Researches hope that quantum theory could focus a microscope on that point and track what becomes of material that falls in.
It is dead end street type of research. It is a similar problem of trying to hope what exactly happens if a meteor collides with our Sun. The meteor resolves and becomes part of the liquid medium inside the Sun.
Out at the boundary of the hole, matter is not compressed, gravity is weaker and by all rights, the known laws of physics should still hold.
The real evolution is of physical processes is often much more complex as the laws of physics assume. There are often boundary conditions and constraints which have to be taken into account. Objects are considered round and or points with no dimension. Under normal conditions this is okay, but generally speaking not.
Generally speaking if you want to calculate the trajectory of one object you have to take all the objects in the universe into account. From a practical point you can use only the nearest and largest objects in its immediate surrounding.
Thus it is all the more perplexing that they do not.
Perplexing by who?
The descent, into a blackhole, is irreversible. That is a problem because all known laws of fundamental physics, including those of quantum mechanics as generally understood, are reversible.
What does reversible mean?
Which physical processes are reversible? I think none.
I think reversible is typical a mathematical tool. Assume that a train travels with an average speed of 100 km/hour. The you 'know' where the train will be one hour later and where the train 'was' one hour ago. But does that mean that riding a train is a reversible process? No. The coal burnt on the previous hour will be lost forever.
At least in principle, you should be able to reverse the motion of all the particles and recover what you had.
No, definitly no.
The late Stephen Hawking ... took a huge step...when he applied quantum theory to the radiation field around black holes and showed they have a non zero temperature.
This all depents what the definition of zero temperature means.
Why not mention gravitational field instead of radiation field.
As such they can not only absorb but also emit energy.
Black don't absorb energy. They absorb objects.

"Atoms of Spacetime"

Heat is the random motion of microscopic parts, such as the molecules of gas.
Because black holes can warm up and cool down, it stands to reason that they have parts - or, more generally, a microscopic structure.
What is the physical process that causes that the temperature of a blackhole can change? i.e. that the motion of the particles inside a blackhole can change?
And because a black hole is just empty space (according to general relativity, infalling matter passes through the horizon but cannot linger) the parts of the black hole must be parts of space itself.
This sentence is clearly not clear. See Reflection 2 - Understanding
Final section.
Phase transitions are another common theme See Reflection 2 - Understanding

Entangled Webs

The big realization of recent years is that the relevant relations involve quantum entanglement.
I doubt if this is such discovery. The issue is that the results of certain reactions (particle emission) are physical correlated.
An extrapowerful type of correlation, intrinsic to quantum mechanics, entanglement seems to be more primitive than space.
The results of reactions, what they emit and the concept space in general have nothing to do which each other.
IF they are entangled, they remain coordinated no matter how far apart they may be.
Entanglement can only be established by performing 1000 identical experiments. For example if an experiment involves two photons in each case both photons should be measured. The total results for both sides should be mentioned.
The following line shows the results for both sides :
For example: 1*.. 2*+. 1*-. 2*.+ 1*.- 491*+- 493*-+ 3*++ and 4*--
The results indicate correlation, but no coordination.
Traditionally when people talked about "quantum" gravity, they were referring to quantum discreteness, quantum fluctuations and allmost every other quantum effect in the book - but never quantum entanglement.
IMO it is much better to use the words quantum mechanics instead of "quantum" gravity.
The reason why people skipped the concept of quantum entanglement (I expect) is because it is a very specific physical issue and IMO also has roughly speaking nothing to do with gravity.
If entangled particles behave different versus two particles which are not entangled, I don't know.
That changed when black holes forced the issue.
Black holes also have nothing to do with entanglement.
Over the lifetime of a black hole, entangled particles fall in, but after the hole evaporates fully, their partners on the outside are left entangled with - nothing.
This whole line assumes that entangled particles are in some way physical connected. They are not.
When one entangled particle falls into a black hole it ceases to exist in the way it was before. The other particle moves and behaves accordingly to its own local environment 'unaware' of what happened with its counter part. Any correleration (including entanglement) between the two particles ceases to exist.
Even in a vacuum, which no particles around, electromagnetic and other fields are internally entangled.
To call an electromagnetic field entangled, in the sense that an electronic and magnetic field always coexist, as described by the Maxwell equations, is a misnomer.
If you measure a field at two different spots, your readings will jiggle in a random buut coordinated way.
See Reflection 2 - Understanding

page S6

Reflection 1 - The laws of physics.

The laws of physics are mathematical descriptions of the physical reality. The physical reality is generally speaking what we humans observe. This is what we call the observed physical reality. The true physical reality is much more complex than we can directly observe and can often only be assumed. For example the universe is much larger than we can observe.

Reflection 2 - Understanding.

Understanding something is a very difficult subject. It starts with a clear definition of individuals words and verbes. Using these words and verbes you can create sentences. Sentences are the building blocks to create concepts and all types of laws including physical laws. These physical laws can only be clear and unambigious if the words they use are clear.
The problem is that almost 80% of the text in paragraph Atoms of Spacetime and paragraph Entangled Webs is not clear.

The disturbing question is: what is the reason that the text is so unclear?

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Created: 2 January 2019

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