Quotes from the book

1 INTRODUCTION

Every theory contains a number of quantities that can be measured by experiments and a few expressions that cannot possibly be observed. The first represent the observables, and the second are the unobservables. The distinction is not always made and many authors claim some data to be observable, according to arbitrary definitions, which do not correspond to any physical experiment. This leads to inconsistencies and paradoxes that should be avoided at all cost. Here I would take the strictest point of view and assume (after Bridgman) that an observable can be selected only if it corresponds to carefully described experimental equipment and method of observation.

2 INTRODUCTION

At this point we may rise a most important question: How much confidence do scientific theories deserve? The answer must be cautious enough: a good deal, but not too much! There are limitations to all our theories; they are good up to a certain limit and within certain boundaries. They do not represent "the truth, nothing but the truth ..". Every theory is based on experiments that have been checked very carefully, but the result can only be stated "within possible errors" between fixed limits according to the best knowledge of the experimenter. There is always a possibility that a new, unpredictable cause of errors might be playing a role in a new experiment, or that the theory has been extrapolated to far from its domain.
Scientific truth should never be taken so seriously, and every scientist must be ready to accept some adjustment and correction of his pet theories. There is no absolute truth in science, and here I must state that I am thinking of experimental science. Mathematics is another story.
Some traditional sciences are a curious mixture of observations, coupled with interpretations based on the best theories, but with an extrapolation so far from actual experiments that one may feel shivering and wondering: How much wishful thinking, how much science fiction. It is splendid to discuss the creation of our world, but never forget that you are dreaming, and do not expect the reader to believe in any model, whether with a sudden atomic explosion or with a story expanding back and forth from - ∞ to + ∞ . All this is too wonderful to be true, too incredible to be believable.

3 INTRODUCTION

Here some reader may say: We must trust some well established principles of symmetry in space and time, the principle of relativity, etc. Let us sketch now the relativity of the principle of relativity! This famous principle was first discovered in classical mechanics: The laws of motion, stated for a frame of reference at rest, remain exactly the same when the motions are observed from a frame of reference moving with a given constant velocity v. The reader may take notice of the fact that I speak of "frames of reference" instead of "sets of coordinates". There is a fundamental distinction to be made between the definitions, as we shall see in Chapter 4. A set of coordinates is a purely geometrical definition; the coordinates have no mass, for the simple reason that geometry completely ignores masses. A frame of reference must have a mass, and this mass must be assumed to be much greater than that of any object moving within the frame.

4 INTRODUCTION

For the moment, let us concentrate on the word " given." What do we mean by a given velocity? Who is giving us this velocity, and how? I become very suspicious whenever I hear the word "given". There is only one occasion when it has a definite meaning; this is in the statement of a problem given by an examiner to some helpless students. In this situation the velocity is supposed to be exactly the given velocity, with no possible error or discussion. But in real life, this never happens. If I observe an unknown moving object in the sky, nobody can give me its velocity. Whether it be a star or a flying saucer, I have to measure the velocity by some experimental device. I may use optical signals, which will be reflected from the unknown object, to measure the delays, the Doppler shifts, etc. From these measurements, I can compute the velocity, but I should always be aware of the fact that these very experiments always perturb the motion. The velocity after observation is not the same as before observation. Every experiment requires some coupling between the observer and the observed object, and the object is not in the same state of motion after the observation has been made and the coupling removed. This is now a well-known fact, supported by many examples of quantum theory. In the measurement of a velocity, we use light signals containing so many photons. When reflected these photons push back the reflecting object (recoil effect) and change its velocity.
The given velocity is just a myth of our imagination. It is a traditional blunder, resulting from the illusion that "looking at something can do no harm". In the physics of the nineteenth century, such an assumption seemed obvious; it was taken for granted, without any discussion. Now we know better. The frame of reference moving with a given constant velocity does not exist and never did. What can be discussed is the problem of a heavy frame of reference, with such a large mass that the perturbation due to the measurement of the velocity is negligible; such a specification leads to many complications, as we shall see in Chapter 5. The old-fashioned principle of relativity is a dream; it represents only a limiting case, but may not, for instance, be used without much care when it comes to moving atoms, electrons, neutrons, photons, neutrinos, and all these new mysterious "particles" (we have no better word to qualify them) of very small masses.

5 INTRODUCTION

Similar remarks apply to many principles recently put forward with most incomplete discussions of how the "symmetry", for instance, can actually be measured.
These are just a few examples to show how scientists' viewpoints have progressively changed, and how many new problems emerged, which even Einstein's genius was unable to foresee fifty or sixty years ago.
We witnessed the invention of atomic clocks of incredible accuracy, whose physical properties differ very much from the clocks Einstein imagined. This will be discussed in some detail in Chapter 3. Let us mention here a real difficulty resulting from internationally adopted definitions. The unit of length is based on the wavelength of a spectral line of krypton-86 under carefully specified conditions with accuracy 10⁸ and the unit of time is based on the frequency of a spectral line of cesium with accuracy 10¹². Hence, the same physical phenomenon, a spectral line, is used for two different definitions: length and time, and the velocity c of light remains undefined and looks arbitrary. It should be stated, once and for all, whether a spectral line should be used to define a frequency or a wavelength, but not both!
The above definitions are supposed to be made on earth, where there is a certain gravity field; Einstein's relativity predicts some change in the units of length and time when measured in regions with different gravity fields. It also predicts a change in the velocity of light c. With the legal definitions of length and time it seems rather difficult to check experimentally such predictions. This raises a very real problem of metrology. The purpose of this monograph is to consider this as well as other questions arising since the formulation of relativity and quantum theories at the turn of the century. In Chapter 1 we will review the historical sequence of events which led to these theories. [ ... ]