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Page 1

Dewey B. Larson 1963 Cazul Împotriva Atomului Nuclear

Tradus de SaDAng 1 05.04.2015

CAPITOLUL TREI



Electronii


I
Este destul de improbabil ca acceptarea ipotezei nucleare a lui Rutherford ar fi fost atât de

rapidă și de necritică dacă nu ar fi fost faptul că terenul era deja pregătit pentru o asemenea
ipoteză prin descoperirea electronului și a radioactivității, care au arătat (1) că există particule
mai mici decât atomul, și (2) că asemenea particule sunt eliminate din atom în procesul de
dezintegrare radioactivă. Concluzia că atomul este o structură compozită, realizată din aceste
entități subatomice, urmează în mod natural și logic; prin urmare întrebarea la care Rutherford
și contemporanii acestuia au încercat să răspundă nu a fost o întrebare generală despre
compoziția atomului, răspuns pe care ei îl considerau ca fiind evident, ci întrebarea era despre
cum erau electronii și alte particule subatomice aranjate în atom.


Totuși, concluzia logică și naturală la prima considerare nu rezistă mereu la o analiză mai

deliberată și aprofundată. Argumentul original bazat pe caracteristicile cunoscute ale
radioactivității pot fi rezumate astfel:

(a) În anumite condiții atomii se dezintegrează
(b) Electronii se găsesc printre produsele dezintegrate
(c) Prin urmare, electronii sunt constituenți ai atomului

La prima vedere acest argument pare a fi corect, iar în anii de formare a ipotezelor
nucleare el a fost acceptat fără întrebări. Chiar și astăzi el reprezintă doctrina ortodoxă. Dar
adevărata valoare a argumentului poate fi realizată clar prin folosirea unui argument analog în
cazul fotonului:

(a) În anumite condiții atomii se dezintegrează
(b) Fotonii se găsesc printre produsele dezintegrate
(c) Cu toate acestea, fotonii nu sunt constituenți ai atomului

Aici putem vedea că pe baza exact a acelorași dovezi, fizicienii ajung la concluzii
diametral opuse. Deoarece idei preconcepute despre electron sugerează că el ar putea fi un
constituent al atomului, evidența din dezintegrare este acceptată ca o dovadă că el este, în
timp ce idei preconcepute similare despre foton sugerează că el nu ar putea fi un constituent al
atomului, pe baza a exact acelorași dovezi care sunt prin urmare folosite în a arăta ca fotonul a
fost creat în procesul de dezintegrare. Practic, desigur, evidențele fizice nu fac distincție între
aceste alternative, nici nu exclud posibilitatea ca alte explicații să fie corecte. Ceea ce
evidențele arată este că electronul fie:

- a fost un constituent al atomului, sau
- preexista în, dar nu ca parte a atomului, sau
- provenea din spațiul înconjurător, sau
- a fost creat în procesul de dezintegrare
- provenea din combinații ale celor descrise anterior, sau
- avea alte origini conforme cu evidențele

Page 2

Dewey B. Larson 1963 Cazul Împotriva Atomului Nuclear

Tradus de SaDAng 2 05.04.2015

lu, au fost probabil întrutotul de neconceput
.

într-o mare varietate de procese, începând cu producerea unei singure
perechi electron-

care i-a fos

în
radioactivitatea beta, electronul este creat în actul emisiei”26.

repet

mare
zilelor noastre. Aceasta nu este o particularitate doar a acestui text. Orice alt text modern care

spune “Experimentele de dezintegrare (care au indicat e ) au
”27 .

electron poate fi emis dintr- dar în ciuda

pagina precedent, aici el este respins
grafic a ce s-a vrut a zice în capitolul introductiv, când teoria

a vechilor idei ale
ultimei ju



electronilor în orice
pt, majoritatea dovezilor sunt acum puternic în favoarea

-

atomilor materiei.


material intra într-

28.

Page 8

Dewey B. Larson 1963 Cazul Împotriva Atomului Nuclear

Tradus de SaDAng 8 05.04.2015

REFERINȚE


26 - Kaplan, Irving, Nuclear Physics, Addison-Wesley Publishing Co., Cambridge, Mass.,
1955, page 154.
27 - Jones, Rotblat and Whitrow, op. cit., page 34.
28 - Sisler, Vanderwerf and Davidson, General Chemistry, The Macmillan Company, New
York, 1919, page 316.
29 - Kaplan, Irving , op. cit., page 140.
30 - Dingle, Herbert, A Century of Science, Hutchinson's Publications, London, 1951, page
315.
31 - Margenau, H., Quantum Theory, Vol. I, edited by D. R. Bates, Academic Press, New
York, 1961, page 6.
32 - Schrodinger, Erwin, Science and the Human Temperament, W. W. Norton & Co., New
York, 1935, page 124.

http://hdl.handle.net/2027/mdp.39015063734571?urlappend=%3Bseq=44
https://archive.org/stream/centuryofscience029484mbp?ui=embed#page/n322/mode/1up
https://archive.org/stream/centuryofscience029484mbp?ui=embed#page/n322/mode/1up
https://archive.org/stream/scienceandthehum029246mbp?ui=embed#page/n129/mode/1up
https://archive.org/stream/scienceandthehum029246mbp?ui=embed#page/n129/mode/1up
https://archive.org/stream/AcidsBasesAndTheChemistryOfTheCovalentBond/Vanderwerf-AcidsBasesAndTheChemistryOfTheCovalentBond?ui=embed#page/n9/mode/1up

Page 9

8-41 THE PROTON-NEUTRON HYPOTHESIS 191

1 particle was found to be neutral and to have a mass close to unity, it was
identified with Rutherford's neutron. Later measurements have shown
that the mass of the neutron is 1.00898 amu, so that it is slightly heavier
than the proton, with a mass of 1.00758 amu.

8-4 The proton-neutron hypothesis. The discovery of a particle,
the neutron, with an atomic weight very close to unity and without electric
charge, led to the assumption that every atomic nucleus consists of protons
and neutrons. This hypothesis was used for the first time as the basis of

- a detailed theory of the nucleus by Heisenberg in 1932. Under the proton-
neutron hypothesis, the total number of elementary particles in the
nucleus, protons and neutrons together, is equal to the mass number A of

. the nucleus; the atomic weight is therefore very close to a whole number.
The number of protons is given by the nuclear charge 2, and the number
of neutrons is A - 2.

The new nuclear model avoids the failures of the protonelectron hy-
pothesis. The empirical rule connecting mass number and nuclear angular
momentum can be interpreted as showing that the neutron, as well as the
~jroton, has a half-integral spin; the evidence is now convincing that the

*

spin of the neutron is indeed ) h / 2 ~ . If both proton and neutron have
spin ) then, according to quantum theory, the resultant of the spins of A
elementary particles, neutrons and protons, will be an integral or half- -
integral multiple of h / 2 r according to whether A is even or odd. This con-
clusion is in accord with all the existing observations of nuclear angular
momenta. The value of the magnetic moment of the neutron is close to
-2 nuclear magnetons; it is opposite in sign to that of the proton, but
not very different in magnitude. The values for both the proton and
neutron are consistent with those measured for many different nuclei.
Finally, since the mass of the neutron is very close to that of the proton,
the argument showing that protons can be contained within the nucleus
is also valid for neutrons.

The neutron-proton hypothesis is consistent with the phenomena of
radioactivity. Since there are several reasons why electrons cannot be
present in the nucleus, it must be concluded that in pradioactivity, the
electron is created in the act of emission. This event is regarded arj the
result of the change of a neutron within the nucleus into a proton, an
electron, and a new particle called a neutrino, and both experimental
and theoretical evidence offer strong support for this view. In Pradio-
activity, then, the nucleus is transformed into a different one with one
proton more and one neutron less, and an electron is emitted. An a-
particle can be formed by the combination of two protons and two neutrons.
It may exist as such in the nucleus, or it may be formed at the instant of
emission; the latter possibility is now regarded as more likely.

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Page 16

6 Η . M A R G E N A U

(4) Millikan's celebrated oil drop experiment, which led to a determination
of the electron's charge.

(5) Wilson cloud chamber tracks bespeak the particle nature of electrons.

1.1.2 Evidence for believing that electrons are waves.

(1) Louis de Broglie's epoch making result which assigned a wavelength λ
to a moving electron. His analysis led to the well known formula λ = hjmv,
h being Planck's constant, m the mass and ν the velocity of the electron.

(2) The Davisson-Germer experiment of 1927 which showed by studying
the diffraction pattern of electrons reflected from a nickel surface, that
electrons do have wavelengths in agreement with the de Broglie formula.

(3) The formation of diffraction rings upon the passage of electrons
through metal foils (G. P. Thomson, 1928).

(4) Diffraction of electrons by means of optical gratings (Rupp, 1929).
The wavelengths obtained from all these diffraction phenomena confirmed
de Broglie's formula.

(5) Polarization of electrons. The attribute formerly regarded as the
spin revealed itself as wholly analogous to the polarization of light waves.
Thus it appeared that the electron possesses all the normal attributes of
waves.

Three types of attitudes have been adopted to this so-called "dualism"
of the nature of electrons. The first affirms that the electron has both cor­
puscular and undulatory properties simultaneously, much in the manner of
Newton's photons which were particles guided by a wave-like disturbance.
This "guiding wave" theory of electrons found expression in the early writings
of Schrödinger and de Broglie. A second possible answer, propagated through
the more popular and elementary literature, holds that the electron is some­
times a wave. In different experiments, the electron manifests different
qualities of existence. This view likewise is no longer held. The third answer
is that electrons are neither particles nor waves. They are entities which,
because of their inaccessibility to immediate observation, have properties
which do not allow themselves to be cast into intuit able or visual forms.
According to this third position, which is most widely held today and is
in harmony with the formalism of the quantum mechanical theories to be
developed, an electron is an abstract thing, no longer intuitable in terms of
the familiar aspects of everyday experience, but determinable through formal
procedures such as the assignment of mathematical operators, observables,
states, and so forth. In sum, the physicist, while still fond of mechanical
models wherever they are available and useful, no longer regards them as

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Page 17

Science and the Human Temperament

indeed quite independently of the question whether

or not one considers it possible to obtain such an

exact field registration of the individual atom. Not

only do the orbits themselves not obey "the ordin-

ary laws of electrodynamics", but the field is also

totally different from what might be expected. It

is made up of quite other frequencies than the

frequencies of electron revolutions are supposed to

be. The average effect resulting from the co-opera-
tion ofmany atoms suffices to reveal this discrepancy,
which was admitted in Bohr's theory from the very
start.

Once we have become aware of this state of affairs,
the epistemological question: "Do the electrons

really exist on these orbits within the atom?" is to

be answered with a decisive No, unless we prefer
to say that the putting of the question itself has

absolutely no meaning. Indeed there does not seem

to be much sense in enquiring about the real
existence of something, if one is convinced that the

effect through which the thing would manifest

itself, in case it existed, is certainly not observed.

Despite the immeasurable progress which we owe
to Bohr's theory, I consider it very regrettable that

the long and successful handling of its models has

blunted our theoretical delicacy of feeling with

reference to such questions. We must not hesitate
to sharpen it again, lest we may be in too great
haste to content ourselves with the new theories
which are now supplanting Bohr's theory, and believe
that we have reached the goal which indeed is still
far away.

124

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