Comments about "How Einstein Discover Relativity" by Stephan W. Hawking

This document contains comments about the book: "A Brief History of Time" by Stephan W. Hawking. Bantham edition 1989
In the last paragraph I explain my own opinion.


Chapter 1. Our picture of the Universe.

To read this chapter select: Chapter 1. Our picture of the Universe.
At page 11 we read:
Any physical theory is always provisional, in the sense that it is only hypothesis: you can never prove it.
A physical theory is a description of the physical reality in a mathematical form.
Newton's Law is such a theory. It is an equation based on observations. Using Newton's Law you can make predictions about future observations.
No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory.
Generally speaking when you repeat experiments the results will be the same (within the same margin).
On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.
One single observation is not enough.
The fact that Einstein's predictions matched what was seen, while Newton's did not, was one of the crucial confirmations of the new thoery.
This is a typical case, if correct, that to decide which theory is correct, not one but many observations are involved.
(Newton's theory also has the great advantage that it is much simpler to work with than Einstein's!)
The problem is that to use Einstein's GR is almost impossible when many objects are involved.
At page 13 we read:
Now, if you believe that the universe is not arbitrary, but is governed by definte laws, you ultimately have to combine the partial theories into a complete that will describe everything in the universe.
It is impossible to describe everything in the present universe because we can only observe a small part of the universe in the past.
The only answer that I can give to this problem is based on Darwin's principle of natural selection.
Darwin's evolution theory has nothing to do with the evolution of the Universe.

Chapter 2. Space and Time

To read this chapter select: Chapter 2. Space and Time.

Chapter 3. Expanding Universe

To read this chapter select: Chapter 3. Expanding Universe

Chapter 4. The Uncertainty Principle

At page 59 we read:
So the velocity of the particle will be disturbed by a larger amount. In other words, the more accurately you try to measure the position of the particle, the less accurately you can measure the speed, and vice versa.
It is important to note that we are discussing here the concept of measuring.
It is also important that to measure speed, three measurements are involved: position (twice) and duration.
Heisenberg showed that the uncertainty in the position of the particle times the uncertainty in its velocity times the mass of the particle can never be smaller than a certain quantity, which is known as Planck's constant.
The mathematical formula is: dx * m * dv = dx * dp => h
For the derivation see: In principle okay. The problem ofcourse is how do you measure these 3 parameters. A different issue what is the importance of Planck's constant versus the Laws of Nature.
Next we read:
Moreover etc: Heisenberg's uncertainty principle is a fundamental, inescapable propertry of the world.
Heisenberg's uncertainty principle is not a law of physics. In some limited sense it describes our human limitations to describe (measure) the state of the physical reality accurately.
A futher down we read:
The uncertainty principle signaled an end to Laplace's dream of a model of the universe that would be completely deterministic: one certainly cannot predict future events exactly if one cannot even measure the present state of the universe precisely!
The conclusion is correct, but you do not need the uncertainty principle to disgree with the idea (that the universe is deterministic). A further down:
This approach (Occam's razor) led Heisenberg etc to reformulate mechanics into a new theory called Quantum mechanics based on the uncertainty principle.
That is tricky as we will see.
In this theory particles no longer had separate well-defined positions and velocities that could not be observed. In stead they had a quantum state, which was a combination of position and velocity
and this quantum state can also not be observed We continue reading at page 60.
Quantum Mechanics predicts a number of possible outcomes and tells us how likely each of these is.
This sentence is misleading. In fact you perform thousands of identical experiments and you try to identify the mathematics involved. The mathematics uncovered mimics the probability of the results of individual experiments.
Quantum mechanics therefore introduces an avoidable element of unpredicatability or randomness into science.
Quantum mechanics underscores an element of unpredictability when performing actual experiments.
But that does not imply that all physical reactions and experiments are unpredictable. It is important to know that the majority of research in Quantum Mechanics is in the area of elementary particles.
Einstein objected to this very strongly, despite the important role he had played in the development of these ideas.
It is very important to study what Einstein actual said or wrote.
Einstein never accepted that the universe was governed by chance.
The problem of course it that in many reactions we humans do not know why something in particular is happening in seemingly identical experiments. At the same time that does not discharge us of the duty by changing certain aspects of an experiment to learn more.

5. Elementary Particles and the Forces of Nature

At page 70 we read:
Using the wave/particle duality, discussed in the last chapter, everything in the universe, including light and gravity can be described in terms of particles.
IMO the wording "everything in the universe" is a too strong.
These particles have a property called spin.
It is tricky to tread both photons and gravitons at the same footing.
At page 72 we read:
The matter particles obey what is called Pauli's exclusion Principle.
The word obey is wrong.
Pauli's exclusion Principle says that two similar (matter) particles cannot exist in the same state, that is they cannot have both the same position and the same velocity, within the limits given by the uncertainty principle.
That two electrons around the nucleas can not have the same position seems logical.
What is more important that the average distance (radius) towards the nucleas should not be the same.
A different issue is why cannot these two electron's have the same speed. The reason is that if two electrons have the same average distance and almost the same average speed they easily can collide. That is why to improve stability its make sense that all electrons have different distances and speeds.
The uncertainty principle has nothing to do with this.
The exclusion principle is crucial because it explains why matter particles do not collapse to a state of very high density under the forces produced by the particles of spin 0, 1, and 2: if the matter particles have very nearly the same positions, they must have different velocities, which means that they will not stay in the same position for long.
I expect that the last sentence should read:
if the matter particles follow the same trajectories, they must have different velocities, which means that they will not stay within in the same distance for long.
The exclusion principle cannot be used to explain the physical stability of atoms.
If the world had been created without the exclusion principle, quarks would not form separate well-defined protons and neutrons.
The fact that the building blocks of protons and neutrons are quarks has nothing to do with the exclusion principle.
At page 73 we read:
It is an important property of the force-carrying particles that they do not obey the exclusion principle.
A much better rule to follow is that for matter particles the particle characteristic is the most important and for the force-carying particles the wave characteristic.
At the end of page 73 we read:
Particles of spin 0, 1 and 2 do also exist in some circumstances as real particles, when they can be directly detected.
It is much more clear to write that all force carrying particles can only be detected indirectly.
At page 74 we read:
They then appear to us as what a classical physicist would call waves, such as waves of light or gravitational waves.
Again it is tricky to treat both on the same footing. In case of photons you can detect one photon. In the case of gravitons we always speak of gravitational waves (plural).
At page 75 we read:
Real gravitons make up what classical physicists would call gravitational waves, which are weak - and so difficult to detect that they have never yet been observed.
Gravitons make up what we call a gravitational field and the varying gravitational field of two BH's produces what are called gravitational waves. For an object moving in an other wise straight line this shows up as wringles in its path.

6. Blackholes

Almost at the end of page 87 we read:
The idea was this: When the star becomes small, the matter particles get very near each other and so accordingly to the Pauli exclusion principle, they must have very different velocities.
You cannot use the Pauli exclusion principle to explain this phenomena. The Pauli exlusion principle is not the cause of something. What happens inside stars is a much more complex physical problem. Immediate next at page 88 we read:
This makes them move away from each other and so tends to make the star expand.
In some sense strange. next we read:
A star can therefore maintain itself at a constant radius by a balance between the attraction of gravity and the repulsion that arises from the (Pauli) exclusion principle, just as earlier in its life gravity was balanced by the heat.
The repulsion comes because particles (atoms) become more closely packed.
Chandrasekhar realized, however, that there is a limit to the repulsion that the exclusion principle can provide. The theory of relativity limits the maximum difference in the velocities of the matter particles in the star to the speed of light.
My understanding is that when you try to compress something the overall speed (degrees of freedom) will degrees. At page 89 we read:
How would it know that it had to lose weight?
The evolution of stars from birth to dead is a physical process, influenced by external circumstances i.e other stars and gass clouds. It is the challenge to look for similarities and to describe these similarities as carefull as possible. These similarities can be in mass, size and composition.

7. Black Holes Ain't So Black

Reflection 1. Pauli exclusion principle

Reflection 2


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Created: 8 January 2017

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