The idea that the photon is a composite particle composed of a neutrino-antineutrino pair was first suggested by de Broglie. [1][2]
Historically, particles that were once thought to be elementary such as protons, neutrons, pions, and kaons have turned out to have to be composites. The idea that emission and absorption of a photon is the creation and annihilation of a particle-antiparticle pair is attractive. However, the neutrino theory of light has some serious problems, and according to the standard model the photon is an elementary particle and a gauge boson. Although there are some claims[3][4]
of problems with the current photon model, the problems with the composite photon model are much worse.
Furthermore, there is no experiment evidence that the photon has a composite structure. The ideas and methods used in trying to form a composite photon are of historic importance and may be useful in forming some other composite particles. Some of the problems for the neutrino theory of light are the non-existence for massless neutrinos[5]
with both spin parallel and antiparallel to their momentum and the fact that composite photons are not bosons. Attempts to solve some of these problems will be discussed, but the lack of massless neutrinos makes it impossible to form a massless photon with this theory.
De Broglie continued to work on the composite photon theory until 1950
[2]
and there continues to be some interest
in recent years.
[6][7][8]
De Broglie did not address
the problem of statistics for the composite photon. However,
"Jordan considered the essential part of the problem
was to construct Bose-Einstein amplitudes from Fermi-Dirac amplitudes", as Pryce
[9]
noted. Jordan
[10]
"suggested that it is not the interaction
between neutrinos and antineutrinos that binds them
together into photons, but rather the manner in which
they interact with charged particles that leads
to the simplified description of light in terms of photons."
Jordan's hypothesis
eliminated the need for theorizing
an unknown interaction, but his hypothesis that the
neutrino and antineutrino are emitted in exactly
the same direction seems rather artificial as
noted by Fock.
[11]
His strong desire to obtain
exact Bose-Einstein commutation
relations for the composite photon led him to work
with a scalar or longitudinally polarized photon.
The matrix is Hermitian while is antihermitian. They satisfy the anticommutation relation,
where is the Minkowski metric with signature and is the unit matrix.
The neutrino field is given by,
where stands for .
and are the fermion annihilation operators for
and respectively, while and are
the annihilation operators for and .
is a right-handed neutrino and is a left-handed neutrino.
The 's are spinors with the superscripts and subscripts refering to the energy and helicity states respectively. Spinor solutions for the Dirac equation are,
The neutrino spinors for negative momenta are related to those of positive momenta by,
De Broglie [1] and Kronig [12] suggested the use of a local interaction to bind the neutrino-antineutrino pair. (Rosen and Singer
[14]
have used a delta-function interaction in forming a
composite photon.)
Fermi and Yang
[15]
used a local interaction to bind
a fermion-antiferminon pair in attempting to form a pion. A four-vector field can be created from a fermion-antifermion pair,
[16]
Forming the photon field can be done simply by,
where .
The annihilation operators for right-handed and left-handed photons formed of fermion-antifermion pairs are defined as
[17][18][19][20],
Although many choices for gamma matrices can satisfy the Dirac equation, it
is essential that one use the Weyl representation in order to get the correct photon polarization vectors and and that satisfy Maxwell's equations. Kronig [12]
first realized this. In the Weyl representation,
the four-component spinors are describing two sets of two-component neutrinos.
The connection between the photon antisymmetric tensor and the two-component Weyl equation was also noted by Sen.
[22]
One can also produce the above results using a two-component neutrino theory.
[8]
To compute the commutation relations for the photon field,
one needs the equation,
To obtain this equation, Kronig [12]
wrote a relation between the neutrino spinors that was not
rotationally invariant as pointed out by Pryce. [9]
However, as Perkins [13] showed, this equation
follows directly from summing over the polarization vectors,
Eq. (2), that were obtained by
explicitly solving for the neutrino spinors.
If the momentum is along the third axis,
and reduce to the usual polarization vectors
for right and left circularly polarized photons respectively.
It is well-know that the photon is a boson.
[23]
Does the composite photon
satisfy Bose-Einstein commutation relations?
Fermions are defined as the particles whose creation and
annihilation operators adhere to the anticommutation relations
while bosons are defined as the particles
that adhere to the commutation relations,
The creation and
annihilation operators of
composite particles formed of fermion pairs
adhere to the commutation relations of the form,
[17][18][19][20]
with
For Cooper electron pairs
[19],
"a" and "c" represent different spin directions. For
nucleon pairs (the deuteron)
[18][17],
"a" and "c" represent proton and neutron. For
neutrino-antineutrino pairs
[20],
"a" and "c" represent neutrino and antineutrino.
The size of the deviations from pure Bose behavior,
depends on the degree
of overlap of the fermion wave functions
and the constraints of the Pauli exclusion principle.
If the state has the form,
then the expectation value of Eq. (9) vanishes for
, and the expression for
can be approximated by,
Using the fermion number operators and
, this can be written,
showing that it is the average number
of fermions in a particular state averaged
over all states with weighting factors
and
.
In 1928, Jordan noticed that commutation relations for
pairs of fermions were similar to those for bosons.
[24]
Compare Eq. (7) with Eq. (8).
From 1935 till 1937, Jordan, Kronig, and others
[25]
tried to obtain exact Bose-Einstein commutation
relations for the composite photon. Terms were added to the
commutation relations to cancel out the delta term in Eq. (8).
These terms corresponded to "simulated photons."
For example, the absorption of a photon of momentum could
be simulated by a Raman effect in which a neutrino with momentum
is absorbed while another of another with opposite spin and
momentum is emitted. (It is now known that single neutrinos or antineutrinos interact so weakly that they cannot simulate photons.)
This led Jordan to work
with a scalar or longitudinally polarized photons
instead of transversely polarized ones like real photons.
In 1938, Pryce [9] showed that one cannot obtain
both Bose-Einstein statistics and transversely-polarized photons from
neutrino-antineutrino pairs. Construction of transversely-polarized
photons is not the problem.
[26]
As Berezinski
[27]
noted, "The only actual difficulty is that the construction of a transverse
four-vector is incompatible with the requirement of statistics."
In some ways Berezinski gives a clearer picture of the
problem. A simple version of the proof is as follows:
The expectation values of the commutation relations for composite
right and left-handed photons are:
where
The deviation from Bose-Einstein statistics is caused
by and
,
which are functions of the neutrino numbers operators.
Linear polarization
photon operators are defined by,
A particularly interesting commutation relation is,
which follows from (10) and (12).
For the composite photon to obey Bose-Einstein commutation relations, at
the very least,
Pryce noted [9].
From Eq. (11) and Eq. (13) the
requirement is that
gives zero when applied to any
state vector. Thus, all the coefficients of
and ,
etc. must vanish separately. This means ,
and the composite photon does not exist
[9][27],
completing the proof.
Perkins
[13][20]
reasoned that the photon does
not have to obey Bose-Einstein commutation relations, because the non-Bose
terms are small and they may not cause any detectable effects.
Perkins
[4]
noted, "As presented in many quantum mechanics
texts it may appear that Bose statistics follow from basic principles, but it is really from the classical canonical formalism. This is not a reliable procedure as evidenced by the fact that it gives the completely wrong result for spin-1/2 particles." Furthermore,
"most integral spin particles (light mesons, strange mesons, etc.) are composite particles formed of quarks. Because of their underlying fermion structure, these integral spin particles are not fundamental bosons, but composite quasibosons. However, in the asymptotic limit, which generally applies, they are essentially bosons. For these particles, Bose commutation relations are just an approximation, albeit a very good one. There are some differences; bringing two of these composite particles close together will force their identical fermions to jump to excited states because of the Pauli exclusion principle."
Berezinskii in reaffirming Pryce's theorem argues
that commutation relation (14) is necessary for the
photon to be truly neutral. However, Perkins
[20]
has shown that a neutral photon in the usual sense can be
obtained without Bose-Einstein commutation relations.
The number operator for a composite photon is defined as,
Lipkin
[17]
suggested for a rough estimate to assume
that
where is a constant equal
to the number of states used to construct the wave packet.
Perkins
[4]
showed that the effect
of the composite photon’s
number operator acting on a state of composite photons is,
using .
This result differs from the usual
one because of the second term which is small for large .
Normalizing in the
usual manner
[28],
where is the state of
composite photons having momentum which is created
by applying on the vacuum times.
Note that,
which is the same result as obtained
with boson operators. The formulas in Eq. (15)
are similar to the usual ones with correction factors
that approach zero for large .
The main evidence indicating that photons
are bosons comes from the Blackbody radiation experiments which are
in agreement with Planck's distribution.
Perkins [4] calculated the photon distribution
for Blackbody radiation using the
second quantization method [28],
but with a composite photon.
The atoms in the
walls of the cavity are taken to be
a two-level system with photons emitted from
the upper level and absorbed
at the lower level .
The transition probability for
emission of a photon is enhanced when
photons are present,
where the first of (15) has been used.
The absorption is enhanced less
since the second of (15) is used,
Using the equality,
of the transition rates,
Eqs. (16) and (17) are
combined to give,
The probability of finding the system
with energy E is proportional to
according to Boltzmann's distribution law.
Thus, the equilibrium between emission
and absorption requires that,
with the photon energy .
Combining the last two equations results in,
with .
For , this reduces to
This equation differs from Planck’s law because of the
the term.
For the conditions
used in the Blackbody radiation
experiments of Coblentz
[29], Perkins estimates that
,
and the maximum deviation from Planck's law is less than
one part in , which is too small to be detected.
Experimental results show that only left-handed neutrinos
and right-handed antineutrinos exist. Three sets of neutrinos
have been observed
[30][31], one
that is connected with electrons, one
with muons, and one with tau leptons.[32]
In the standard model the pion and muon decay modes are:
To form a photon, which satisfies parity and charge
conjugation, two sets of two-component neutrinos
(i.e., right-handed and left-handed neutrinos) are needed.
Perkins (see Sec. VI of Ref. [13])
attempted to solve this problem by noting that the needed
two sets of two-component neutrinos would exist if the
positive muon is identified as
the particle and the negative muon as the
antiparticle. The reasoning is as follows: let be
the right-handed neutrino and the left-handed neutrino
with their corresponding antineutrinos (with opposite helicity).
The neutrinos involved in beta decay are
and , while those for decay are
and .
With this scheme the pion and muon decay modes are:
There is convincing evidence that neutrinos have mass.
In experiments at the SuperKamiokande researchers [5]
have discovered neutrino oscillations in which one flavor of
neutrino changed into another. This means that neutrinos have
non-zero mass.
Since massless neutrinos are needed to form a massless photon,
a composite photon is not possible.
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