^{1}

^{*}

^{2}

^{*}

^{2}

^{*}

EPR experiment on system in 1998 [1] strongly hints that one should use operators and for the wavefunction (WF) of antiparticle. Further analysis on Klein-Gordon (KG) equation reveals that there is a discrete symmetry hiding in relativistic quantum mechanics (RQM) that PT=C. Here PT means the (newly defined) combined space-time inversion (with x→-x,t→-t), while C the transformation of WF Ψ between particle and its antiparticle whose definition is just residing in the above symmetry. After combining with Feshbach-Villars (FV) dissociation of KG equation (Ψ=φ+x) [2], this discrete symmetry can be rigorously reformulated by the invariance of coupling equation of φ and x under either the combined space-time inversion PT or the mass inversion (m→-m), which makes the KG equation a self-consistent theory. Dirac equation is also discussed accordingly. Various applications of this discrete symmetry are discussed, including the prediction of antigravity between matter and antimatter as well as the reason why we believe neutrinos are likely the tachyons.

In 1956-1957, the historical discovery of the parity violation [3-6] reveals that both P and C symmetries are violated to maximum in weak interactions. Then in 1964- 1970, both CP and T are experimentally verified to be violated in some cases (though to a tiny degree) [7,8] whereas the product symmetry CPT holds intact to this day [

Regrettably, the counterpart of “strong reflection” at the level of RQM went nearly unnoticed in the past decades. In this paper, we are going to study the RQM thoroughly. Not only a discrete symmetry is found in RQM as the counterpart of “strong reflection” in QFT, it is also evolved into the invariance of space-time inversion or mass inversion , showing that a WF in RQM is always composed of two parts in confrontation inside a particle and then RQM becomes a self-consistent theory. Furthermore, this symmetry can serve as a “theoretical tool” in searching for new applications in today’s physics.

The organization of this paper is as follows: In section II, the EPR paradox [

To our knowledge, beginning from Bohm and Bell [15,16], physicists gradually turned their research of EPR paradox [

Consider two particles in one dimensional space with positions and momentum operators

. Then a commutation relation arises as

According to QM’s principle, there may be a kind of common eigenstate having eigenvalues of these two commutative (i.e., compatible)observables like:

with D being their distance. The existence of such kind of eigenstate described by Equation (2.2) puzzled Guan, he asked: “How can such kind of quantum state be realized?” A discussion between Guan and one of present authors (Ni) in 1998 led to a paper [

Here we are going to discuss further, showing that the correlation experiment on a system (which just realized an entangled state composed of two spinless particles) in 1998 by CPLEAR collaboration [

(with being the time during which the i-th particle is detected). In accordance with Ref. [

Here and correspond to the proper time and in Ref.[

A pair, created in a antisymmetric state, can be described by a two-body WF depending on time as ([

with

where the CP violation has been neglected and

, and being the masses and decay widths, respectively. From Equation (7), the intensities of events with like-strangeness (or) and unlike-strangeness (or) can be evaluated as

where and

or.

Similarly, for created in a or symmetric state as:

the predicted intensities read

(2.12)

The experiment [

What we learn from Ref. [

(a) Because only back-to-back events are involved in the system, we denote three commutative operators as: the “distance” operator

and, Equations (2.1) and (2.3) read

So they may have a kind of common eigenstate during the measurement composed of and projected from the symmetric state shown by Equation (11). It is assigned by a continuous eigenvalue (with continuous index) of operator acting on the WF, , as^{1}

where the lowest eigenvalue of is

, and that of is

respectively. These eigenstates of like-strangeness events predicted by Equation (11) are really observed in the experiment [

(b) The more interesting case occurs for pair created in the antisymmetric state with intensity given by Equation (10) being a function of (not as shown by Equation (12) for symmetric states)

which is proportional to in the S system. In the EPR limit, events dominate whereas likestrangeness events are strongly suppressed as shown by Equation (9) (see

) but are not suitable for antisymmetric states), there are another three operators shown by Equations (4) and (5) being: the operator of “flight-path difference” and

with commutation relations as:

which are just suitable for antisymmetric states. For back-to-back events, assume that one of two particles, say 2, is an antiparticle with its momentum and energy operators being

(the superscript means “antiparticle”) versus that for particle being

For instance, a freely moving particle’s WF reads^{2}:

whereas

for its antiparticle with and being momentum and energy of the antiparticle in accordance with Equation (2.18). If using Equations (2.18)-(2.21), we find

with continuous index referring to continuous eigenvalues. Here, the WF in space-time of this system during measurement reads approximately:

with antiparticle 2 moving opposite to particle 1 and.

Now we use on system, yielding

Similarly, we have and find

Hence we see that once Equations (2.18) and (2.21)

are accepted, the WFs show up in experiments as the only WFs with strongest intensity at the EPR limit corresponding to their three eigenvalues being all zero: and they won’t change even when accelerator’s energies are going up.

If using Equation (2.18), the eigenvalues of and

for the WF are

and respectively, while that of and for the WF

are and

, respectively, those eigenvalues are much higher than zero and going up with the accelerator’s energy.

Something is very interesting here: If we deny Equation (2.18) but insist on unified operators and for both particle and antiparticle, there would be no difference in eigenvalues between like-strangeness events and unlike-strangeness ones. For example, the and would be and too (instead of “0” as in Equations (2.24) and (2.25)). This would mean that three commutative operators and are not enough to distinguish the WF from the WF even they behave so differently as shown by Equations (9) and (10)), especially at the EPR limit.

Equation (2.18) together with the identification of WF

by three zero eigenvalues implies that the difference of a particle from its antiparticle is not something hiding in the “intrinsic space” like opposite charge (for electron and positron) or opposite strangeness (for and) but can be displayed in their WFs evolving in space-time at the level of QM.

In summary, instead of one set of WF with its operators (Equations (2.19) and (2.20)), two sets of WFs with operators separately (shown as Equations (2.18)- (2.21)) are strongly supported by the original EPR paradox and its “solution” provided by the experiment.

To our knowledge, Equation (2.18) can be found at a page note of a paper by Konopinski and Mahmaud in 1953 [

Let us begin with the energy conservation law for a particle in classical mechanics:

Consider the rule promoting observables into operators:

and let Equation (3.1) act on a wavefunction (WF), the Schrödinger equation

follows immediately. In mid 1920’s, considering the kinematical relation for a particle in the theory of special relativity (SR):

and using Equation (3.2) again, the Klein-Gordon (KG) equation was established as:

For a free KG particle, its plane-wave solution reads:

However, two difficulties arose:

(a) The energy E in Equation (6) has two eigenvalues:

In general, , the WFs of KG particle’s energy eigenstates can always be divided into two parts:

where only the original operators Equation (3.2) are used. But what the “negative energy” means?

(b) The continuity equation is derived from Equation (5) as

where

and

are the “probability density” and “probability current density” respectively. While the latter is the same as that derived from Equation (3.3), Equation (3.11) seems not positive definite and dramatically different from in Equation (3.3). Why?

In hindsight, for a linear equation in RQM, either KG or Dirac equation, the emergence of WFs with both positive and negative energy is inevitable and natural. From mathematical point of view, the set of WFs cannot be complete if without taking the negative energy solutions into account. And physicists believe that these negative-energy solutions might be relevant to antiparticles. However, we physicists admit that both a rest particle’s energy and a rest antiparticle’s energy are positive, as verified by numerous experiments like that of pair-creation process . The above contradiction constructs socalled “negative-energy paradox” in RQM. For Dirac particle, majority (not all) of physicists accept the “hole theory” to explain the “paradox”. But for KG particle, no such kind of “hole theory” can be acceptable. It was this “negative-energy paradox” as well as the four “commutation relations”, Equations (2.1)-(2.5), hidden in the two-particle system discussed by EPR [

Once getting rid of the constraint in the above notion and introducing two sets of WFs and operators for particle and antiparticle respectively, we can identify the negative energy solution, Equation (3.9), with the antiparticle’s WF directly

which implies an antiparticle with positive energy by using Equation (2.18). This claim will be proved rigorously in the next subsection.

One may ask: When you assume the negative energy solution being the WF of antiparticle, how about the difficulty of negative probability density? Below we will see how to solve these two difficulties simultaneously and make KG equation a self-consistent theory at the level of RQM.

Let us introduce an operator of (newly defined) combined space-time inversion for KG equation. It should change the space-time coordinates as

then accordingly

Because the antiparticle has opposite charge versus for particle, so

When performing inversion on KG equation, Equation (3.5), from left to right, we meet eventually the WF and define the antiparticle’s WF as

Thus KG particle’s equation, Equation (3.5), is transformed into

or

which is formally the same as Equation (3.5) though we should use for. Hence the KG equation remains invariant under the operation, Equations (3.14)-(3.17). Notice further that Equation (3.18) is just the “quantized” equation of the kinematical relation for an antiparticle in SR

which is the counterpart of Equation (3.4) for a particle. For example, a KG particle’s scattering WF is attracted by an spherically symmetric potential and so has a positive phase-shift (in the, say, state). Then physically, its antiparticle’s WF

is repelled by the potential and has a negative phaseshift.

Note that, however, corresponding to, there is another negative energy particle’s WF

satisfying Equation (3.5)

whose space-time behavior is precisely the same as the antiparticle’s WF with

as shown by Equation (3.18) since

. Thus, for avoiding confusion, we have

and

achieving the proof of the discrete symmetry for KG particle shown by Equation (3.17). In summary, the “negative-energy paradox” for KG equation is solved in a physical way with following advantages:

a) By using two sets of WFs and momentum-energy operators for particle and antiparticle respectively, both particle’s WF and antiparticle’s WF have positive energies and respectively.

b) While satisfying the same KG equation with same potential formally, and are actually subject to opposite “force” for particle and antiparticle respectively.

c) The space-time behavior of can be identified with that of a negative energy particle’s WF

, in a one-to-one correspondence.

Thus from mathematical point of view, all solutions of KG equation form a complete set including both positive and negative energy values of one operator

exactly.

By contrast, usually, aiming at finding an anti-particle’s WF, one performs the CPT transformation on a particle’s WF, yielding [29-32]

whose character can also be summed up as follows:

a’) By using one set of WF and relevant operators for both particle and antiparticle, at the LHS of Equation (3.24), , and at RHS must have opposite energies inevitably.

b’) By design in the C transformation, and in Equation (3.24) satisfy different equations with and respectively. But with opposite energies, they are actually subject to the same (either attractive or repulsive) “force”. So one cannot distinguish particle from antiparticle through what their WFs “feel” after the CPT transformation.

c’) From mathematical point of view, we should keep all negative-energy solutions for one equation. However, even facing WFs in doubled numbers, we still don’t know how to choose half of them for describing particle and its antiparticle separately in physics.

But we haven’t solve the difficulty of negative probability density in KG equation yet, awaiting for another enlightenment which was already there since 1958.

In 1958, dividing the WF into, Feshbach and Villars [^{3}:

where

. Interestingly, the “probability density”, Equation (3.11) can be recast into a difference between two positive-definite densities [18,20]:

while the probability current density contains interference terms between and:

The expression of as shown by Equation (3.27) strongly hints that the symmetry proved in the last subsection may be combined with the FV dissociation of KG equation such that the positive-definite property of can be ensured for both particle and antiparticle.

Indeed, after inspecting Equation (3.25) carefully, we do find a hidden symmetry in the sense that it is invariant (in its form) under the following reformulated space-time inversion, i.e., transformation:

Performing transformation Equation (3.29) on Equation (3.26), we find satisfying the same equation of and satisfying that of. They read

Remember, for, we should use operator Equation (3.15). Accordingly, the probability density for is defined as

Similarly, we have

For simplicity, consider a free KG particle with WF Equation (3.6). Then

But for a free KG antiparticle with WF Equation (2.21), it has

Equations (3.33) and (3.34) satisfy all physical conditions we need. If, as long as for particle or for antiparticle, the situation remains the same. However, once or, some complications would occur. For further discussion, please see the Appendix.

Therefore, we see that the reformulated space-time inversion, Equation (3.29), reflects the underlying symmetry between a particle’s WF and its antiparticle’s WF. As both and in or and in are positive definite, all difficulties in KG equation disappear and the latter becomes a self-consistent theory.

Moreover, instead of Equation (3.29), a “mass inversion” can realize the same symmetry, the invariance under a transformation, via the following operation on Equation (3.25):

Notice that, when, we have and

, i.e., , in contrast to Equation (3.15) [^{4}

The reason why in the space-time inversion Equation (3.29) whereas in the mass inversion Equation (3.35) can be seen from the classical equation: The Lorentz force F on a particle exerted by an external potential reads:. As the acceleration of particle will change to for its antiparticle, there are two alternative explanations: either due to the inversion of charge (i.e., but keeping unchanged) or due to the inversion of mass (but keeping unchanged).

The success of FV’s dissociation of KG equation should be ascribed to their deep insight that a unified WF is composed of two fields and in confrontation. Note that Equation (3.25) reduces into two equations separately for a static KG particle:

with two separated solutions being:

Once the particle (antiparticle) is moving with a velocity, , and (and) couple together and the WF for a free particle (antiparticle) read (in one-dimensional space)

respectively. In Equation (4.3a), dominates. By contrast, in Equation (4.3b) it is who dominates (The status remains the same for cases as discussed in the last section).

Despite and (and) having the “intrinsic tendency” to evolve as

, however, in a WF of particle (antiparticle), must follow to evolve like that shown by Equation (4.3a) (Equation

(4.3b)), as. So it seems suitable to name the “hidden particle field” inside a particle while the “hidden antiparticle field” (rather than the “negative-energy component”) inside the same particle.

Let us try to reinterpret the phenomena displayed in the kinematics of special relativity (SR) via the enhancement of field in a particle [22-25]:

(a) Lorentz transformation Consider a particle’s WF shown by Equation (4.3a) in an inertial frame S (laboratory). Then take another frame resting on the particle, so and . The WF in frame reads:

Here the space-time coordinates are introduced and defined in the frame via the phase of WF as follows: Based on the assertion that “phase remains invariant under the coordinate transformation” which was named the “law of phase harmony” by de Broglie and was regarded by himself as the fundamental achievement all his life [

, one finds

Then, all formulas in the Lorentz transformation can be obtained. In some sense, what used here is a particle’s wave-packet which serves as a microscopic “ruler”, also a “clock” simultaneously.

(b) There is a speed limit c for a massive particle.

For a free KG particle, using Equation (3.33), we may define an “impurity ratio” for the amplitude of hidden field to that of field and calculate it being

When, with the increase of v, increases monotonously. The particle becomes more and more “impure” until as a limit of particle being still a particle. As shown by Equation (4.6), the reason why its velocity has a limiting value c (the speed of light) is because and have opposite evolution tendencies in space-time as shown by Equations (4.1)- (4.3) essentially, strives to hold back from going forward until a balance nearly reached when and.

(c) The “length contraction” (FitzGerald-Lorentz contraction) and “time dilation”

As usual, we will show “length contraction” via a wave-packet of KG particle moving at a high-speed but further ascribe it to the enhancement of field hidden inside the particle.

First, consider a wave-packet of KG particle at rest [25, 35]

Assuming, we have approximately that

If, the diffusion of wave-packet at low speed can be ignored. Then we perform a “boost transformation”

to push the wave-packet to high velocity, yielding

where and

Here is the width of wave-packet measured from its center. Equations (4.7)-(4.10) show the “length contraction”.

Second, we calculate from Equations (4.9) and (3.33)

the values of and the probability density

respectively.^{5} Their peak values all increase with the increase of v (boost effect). However, the “intensity” of or increases even faster than that of while keeping the constraint in the boosting process.

We also calculate the square of “impurity ratio” for this moving wave-packet:

which is the counterpart of Equation (4.6) for a plane WF of KG particle.

With these calculations, we might intuitively understand the length contraction as an effect of coupling (i.e. entanglement) between and fields due to their opposite evolution tendencies in space as discussed in previous point (b).

Let’s turn to the “time dilation” shown by the variation of the mean life

of a particle, say, a pion (or) with its velocity.

To understand it, let’s return back to Equations (4.1)- (4.3) at and view the WF on its complex plane with and (and) as abscissa and ordinate. We may see that the time reading of the “inner clock” for a particle (or an antiparticle) is “clockwise” (or “counter clockwise”). Thus with the increase of particle velocity, though the time reading remains clockwise (due to the dominance of field), it runs slower and slower because of the enhancement of hidden field.

(d) WF’s group velocity versus phase velocity.

In RQM, a particle’s velocity should be identified with its group velocity. Actually, we have

However, the fact that there is an upper bound for particle’s velocity doesn’t mean that no speed can exceed that of light,. Indeed, there is another velocity, the phase velocity in the WF

And the relation implies that

In our opinion, the role of here is crucial to maintain the quantum coherence of WF in the space-time globally, we will further discuss this problem elsewhere. In 1923, de Broglie discovered Equation (4.15) in his relativistic theory. However, in the Schrödinger equation of NRQM, the phase velocity remains undefined. See Ref [

Let us turn to the Dirac equation describing an electron

with and being matrices, the WF is a four-component spinor

Usually, the two-component spinors and are called “positive” and “negative” energy components. In our point of view, they are the hiding “particle” and “antiparticle” fields in a particle (electron) respectively ([

(are Pauli matrices). Equation (3) is invariant under the combined space-time inversion with

showing that in its form of two-component spinors, Dirac equation is in conformity with the underlying symmetry Equation (3.29). Note that under the space-time inversion, the remain unchanged (However, see Equations (9)- (11) below). Alternatively, Equation (3) also remains invariant under a mass inversion as

In either case of Equation (5.4) or (5.5), we have^{6}

For concreteness, we consider a free electron moving along the z axis with momentum and having a helicity, its WF reads:

with. Under a space-time inversion

or mass inversion

, it is transformed into a WF for positron (moving along axis)

with. However, the positron’s helicity becomes. This is because the total angular momentum operator for an electron reads

Under a space-time inversion, the orbital angular momentum operator is transformed as

To get with, we should have

Hence the values of matrix element for positron’s spin operator is just the negative to that for in the same matrix representation.

Notice that Equation (7) describes an electron with positive helicity, i.e., ^{7}. Under a space-time inversion, it is transformed into

in Equation (8), i.e.,

, meaning that Equation (8) describes a positron with negative helicity.

In its form of four-component spinor, Dirac equation, Equation (5.1) with, is usually written in a covariant form as (Pauli metric is used:

, see Ref. [

Under a space-time (or mass) inversion, it turns into an equation for antiparticle:

with an example of shown in Equation (8). Let us perform a representation transformation:

and arrive at

due to. Since and are essentially the same in physics, (this is obviously seen from its resolved form, Equation (5.3)), it is merely a trivial thing to change the position of in the 4-component spinor (lower in Equation (5.14) and upper in Equation (5.8)).

What important is for characterizing an antiparticle versus for a particle. Therefore, if a particle with energy E runs into a potential barrier, its kinetic energy

becomes negative, and its WF’s third component in Equation (5.7) suddenly turns into

, whose absolute magnitude is larger than that of the first component. This means that it is an antiparticle’s WF satisfying Equation (5.15) (with and) and will be crucial for the explanation of Klein paradox in Dirac equation (For detail, please see Appendix). However, we need to discuss the “probability density” and “probability current density” for a Dirac particle versus and for its antiparticle. Different from that in KG equation, now we have

which is positive definite for either particle or antiparticle. On the other hand, we have

(we prefer to keep rather than for antiparticle). For Equations (5.7), (5.8) and (5.14), we find

which means that the probability current is always along the momentum’s direction for either a particle or antiparticle.

Above discussions at RQM level may be summarized as follows: The first symptom for the appearance of an antiparticle is: If we perform an energy operator

on a WF and find a negative energy

or a negative kinetic energy, we’d better doubt the WF being a description of antiparticle and use the operators for antiparticle, Equation (2.18). Then for further confirmation, two more criterions for and are needed (see Appendix).

In QFT, the starting point is the field operator which is constructed for free complex boson field as [

Similarly, the field operator for free Dirac field reads:

(6.2)

In Equation (6.1), the annihilation operator for particle and the creation operator for antiparticle in Fock space are introduced. In Equation (6.2), instead of index (, the spin’s projection along the fixed axis in space), the helicity is used. See Ref. [

Let us return back to the CPT theorem proved by Lüders and Pauli in 1954-1957 [10-12]. The proof of CPT theorem contains a crucial step being the construction of so-called “strong reflection”, consisting in a reflection of space and time about some arbitrarily chosen origin, i.e..

Pauli proposed and explained the strong reflection in Ref. [

What Pauli claimed, in our understanding, means that under the strong reflection for boson field, one has

The mutual transformation, Equation (6.3), in Fock space ensures the field operators, Equation (6.1), invariant under the strong reflection in the sense of (see also [25,26]):

Here let us introduce the notation to represent the strong reflection so that the presentation could be easier and clearer as shown above. Similarly, for Dirac field, under the strong reflection one has

Here it is important to notice that the helicity, , will be reversed before and after the strong reflection for a particle and its antiparticle respectively as discussed in Section V. Because Equation (6.2) is written in 4 component spinor covariant form, the invariance of Dirac field operator under the strong reflection should be expressed rigorously as

which are useful in proving the “spin-statistics connection” by strong reflection invariance.

QFT is a successful theory just because it is established on sound basis with the field operator being one of its cornerstones. Historically, through various trials and checks, Equations (6.1)-(6.2) were eventually found (see Section 3.5 of Ref. [

However, as emphasized by Pauli [

(a) The order of an operator product in Fock space has to be reversed under the strong reflection, e.g.,

. So is the order of a process occurred in a many-particle system.

(b) Another rule is: One should always take the normal ordering when dealing with quadratic forms like etc.

Then Pauli and Lüders were able to prove that the Hamiltonian density for a broad kind of model in relativistic QFT is invariant under an operation of “strong reflection”, i.e.,

The Hamiltonian density is also invariant under a Hermitian conjugation (H.C.) as:

Furthermore, they proved the CPT theorem via the identification of the product of T, C, and P in QFT with the combined operation of the strong reflection and a Hermitian conjugation.

The validity of CPT invariance, i.e. Equations (6.8) and (6.9) has been verified experimentally since the discovery of parity violation ([3-8] etc.) and the establishment (and development) of standard model ([

After restudying the historical contribution of PauliLüders strong reflection invariance, we feel good in understanding that what we claim in RQM (Sections III-V) is essentially the same as or very close to their idea.

In fact, this paper is the direct continuation of our first one in 1974 [

and especially by Pauli’s invention of the strong reflection in 1955 [

Below, we would like to show that WFs for a particle and its antiparticle given in Equations (5.7) and (5.8) are precisely that derived from QFT as expected.

Using Equation (6.2) for Dirac field, we find the WF of an electron being

but the hermitian conjugate of a positron’s WF is given by

which leads to positron’s WF being

Similarly, Equations (2.20) and (2.21) can be derived from Equation (6.1) as expected.

Through analysis in RQM till QFT, we stress the necessity of using helicity to describe a fermion or antifermion. Here is an interesting example. Since 2002, Shi and Ni [39-43] predicted a parity-violation phenomenon as follows:

An unstable (decaying) fermion (e.g., neutron or muon) has different mean lifetimes for being right-handed (RH) or left-handed (LH) polarized during its flight with the same speed

where, the mean lifetime when it is at rest. Similarly, for its antifermion, their lifetimes will be

Hence, the lifetime asymmetry can be defined as

This is not a small effect. For instance, in Fermilab, physicists consider to build a muon collider [

The problem is: How can such a parity-violation phenomenon be overlooked since 1956-1957? One theoretical reason is: in the past, for describing a fermion in flight, instead of helicity states, the “spin-states” assigned by (spin’s projection along the fixed axis in space) were often incorrectly used (see [40-42]). So previous calculations on the lifetime always led to a prediction that without parity-violation in contrast to Equations (7.1)-(7.3).^{8}

The interesting thing is: While Equations (7.1) and (7.2) display the violation of P or C symmetry to its maximum, their “cross-symmetry”, and , reflects the symmetry of shown by Equation (6.5) exactly.

In the standard representation of Dirac equation for free particle

Let us choose

, then

As discussed in section V, Equations (8.1) and (8.2) are invariant under the space-time inversion:

with subscript “c” meaning the antiparticle.

After transforming into the “Weyl representation” (chiral representation) as

we have

If, Equation (8.5) reduces into two Weyl equations describing two kinds of permanently LH and RH polarized massless fermions respectively. So we may name and (which are usually called as chirality states or chiral fields in 4-component covariant form) as the “hidden LH and RH spinning fields” inside a Dirac particle, which can be either LH or RH polarized (with helicity or 1) explicitly. See below.

A new symmetry is hidden in Equation (8.5), which remains invariant under the pure space inversion transformation, i.e., the parity operation as

Here we add “” in the superscript of RHS to stress that the WF after the space inversion may be different from that at the LHS (before the space inversion). We knew that the WF in Dirac representation after a space inversion reads

Using Equation (8.6), the RHS of Equation (8.7) turns out to be

Hence, we understand the reason why a Dirac particle respects the parity symmetry as shown by Equation (8.7) is because it enjoys the symmetry Equation (8.6) hiding in the 2-component spinor form (in Weyl representation).

For concreteness, let’s write down the solution of Equation (8.1)

Furthermore, we choose a simplest “spin state” with

and:

While Equation (8.10) is an eigenfunction of with eigenvalue, its helicity remains unfixed, depending on the value of being positive or negative. Only after is fixed, can we have a “helicity state” describing a RH particle with:

Looking at Equation (8.11) in the Weyl representation, we see that

. So Equation (8.11) describes a RH particle just because the field dominates the field. Now we perform a space inversion on Equation (8.11), according to the rule Equation (8.7), yielding

(8.13)

Hence we see that the reason why becomes a LH WF, i.e.,

is just because of the dominance of field over

field after the P-operation. Before and after the operation, , the dominant (subordinate) field is transformed into dominant (subordinate) field:

, as shown by Equation (8.6).

In summary, Dirac equation is invariant under a space inversion whereas its concrete solution of WF may be not. The latter may change from that for a RH particle to a LH one or vice versa, but with the same mass m, showing the law of parity conservation exactly.

Now a question arises: Can we find an equation which violates the symmetry of pure space inversion?

The answer is “yes”. Let’s introduce a new equation in Weyl representation from Equation (8.5) by erasing the superscript (D), replacing the mass term by and changing its sign from “+” to “−” in the first equation of Equation (8.5) only [

where (real and positive) refers to the mass of a hypothetical particle. We will see immediately that it is a “superluminal particle” or “tachyon”.

Indeed, substituting a plane-wave solution

with the particle’s helicity into Equation (8.15), we find that

Since and, from Equation (8.17), the dispersion-relation of wave reads

As in Section IV, we define the wave’s phase velocity as

while its group velocity

being identical with the particle’s velocity. Equation (8.19) yields a relation between them coinciding with Equation (4.15) exactly:

However, the relations among and are dramatically different

which dictate such that are real and.

Like Equation (8.4), we define:

and find from Equation (8.15) that (in Dirac representation)

. Despite the difference between Equation (8.26) and Dirac equation, Equation (8.2), both of them respect the combined space-time inversion symmetry like Equation (8.3)

with

Similarly, we define the WF in Weyl representation after inversion as:

Based on Equations (8.27)-(8.29), we find

which can also be obtained via the operation on Equation (8.15). Equations (8.15) and (8.31) are better to be compared in the following form:

. Interestingly, Equation

(8.33) can also be reached from Equation (8.32) via a “mass inversion” like that in Sections III and V:

Furthermore, the probability density and probability current density before and after the inversion can be derived as:

and

respectively. It is the sharp contrast between Equation (8.35) and Equation (5.16) for Dirac equation (i.e.,

), that makes Equation (8.15)

so unique as shown below.

Let us look at the example of WF for tachyon, Equations (8.16)-(8.18), with and. It is allowed just because and so. Second choice of Equation (8.16) with

but

should be fobidden due to its ρ < 0. Another two possible WFs with have and respectively, only the last one with

is allowed due to its

and.

Let us turn to the solution of Equation (8.31) for antitachyon with by just performing operation on Equation (8.16) yielding:

Now if, since

, so helicity. Substitution of Equation (8.38) into Equation (8.33) yields:

which is allowed due to. Second choice of Equation (8.38) with but

should be forbidden due to its. In another two possible WFs with, only that with

is allowed due to.

Hence we see that: The tachyon can only exist in a left-handed (LH) polarized state (with helicity) whereas antitachyon only in a right-handed (RH) polarized state (with). We tentatively link this strange feature with that found in neutrinos—only and exists in nature whereas and are strictly forbidden.

Furthermore, at first sight, although Equation (8.15) certainly has no symmetry under the space inversion, it seems to enjoy a pure “timeinversion” symmetry like

We add “” in the superscript of to stress that (being a time reversed WF), though looks like some antitachyon’s WF, is obviously different from gained through the inversion, Equation (8.31). Actually, based on Equations (8.29)-(8.31) and (8.41)-(8.42), we have:

Interestingly, we cannot find from Equation (8.42) the “physical solution” of with (so) and (for) simultaneously. Only makes physical sense, but it is just like that discussed in Equation (8.39). Notice that the sign change in the phase of WF makes a change in the direction of momentum. But a WF is always composed of two fields in confrontation, like versus here. And the explicit helicity is determined by which one of these two hidden fields being in charge. So the change of in these four equalities of Equation (8.43) does reverse the status of versus (or vs), rendering helicity reversed explicitly. The subtlety of tachyon equation, unlike Dirac equation, lies in the fact that only and exist whereas and are strictly forbidden, i.e., the parity symmetry is violated to maximum. Hence, in strict sense, there is also no physically meaningful WF after the operation of pure “time inversion” on Equation (8.15). We will insist on Equation (8.31) rather than Equation (8.42)—there is only one correct way leading from tachyon to antitachyon via the inversion essentially.

In 2000, Equation (8.25) was first proposed by Tsao Chang and then collaborated with Ni in Ref. [

with being an antihermitian matrix.

Usually, for an equation with nonhermitian Hamiltonian, there is no guarantee for the completeness of its mathematical solutions. In other words, the unitarity of its physical states is at risk. Sometimes, however, a nonhermitian Hamiltonian can be accepted in physics. For example, in the optical model for nuclear physics, an imaginary part of potential, , is used to describe the absorption of incident particles successfully. The interesting thing for “tachyonic neutrino” is: Solutions of Equation (8.15) for are coinciding with that for whereas another would-be solutions with but (but) are forbidden, see Equations (8.37) and (8.40). It seems like half of would-be solutions disappear automatically. Equivalently, from physical point of view, only half of states with or are allowed in nature whereas another half with or are not. Hence one unique feature of “tachyon” equation, like Equation (8.15) or (8.26), lies in its strange realization of unitarity violation that half of would-be states (being tentatively identified with and) are absolutely forbidden whereas another half (and) are stabilized. The permanently longitudinal polarization property of neutrino and antineutrino like that analysed above was first predicted by Lee and Yang in 1957 [3-5] and had been verified by GGS experiment in 1958 [

In hindsight, there are two Lorentz invariants in the kinematics of SR:

It seems quite clear that Equation (9.1) is invariant under the space-time inversion and Equation (9.2) remains invariant under the mass inversion We believe that these two discrete symmetries are deeply rooted at the SR’s dynamics via its combination with QM and developing into RQM and QFT—the particle and its antiparticle are treated on equal footing and linked by the symmetry essentially. Hence we can perform a mass inversion on Equation (9.2) in each of two inertial frames with arbitrary relative velocity in the sense of

, yielding:

The invariance of Equation (9.2) under mass inversion as a whole reflects the experimental fact that particle and antiparticle are equally existing in nature even at the level of classical physics.

Example: The motion equation for a charged particle (say, electron with charge) in the external electric and magnetic fields, and, is given by the Lorentz formula:

Then the operation of either or

on Equation (9.4) will realize the transformation from particle into its antiparticle (say, positron with charge) with the acceleration change from as

Based on what we learn from RQM (Sections III-V) as well as Equations (9.1)-(9.5), we may conjecture that for a classical theory being capable of treating matter and antimatter on an equal footing, it must be invariant under a mass inversion.

Notice that, however, Equation (9.4) (Equation (9.5)) is only valid for particle (antiparticle) moving at low speed, it must be modified to adapt to high-speed cases through the invariance of continuous Lorentz transformation. So we need “double checks” for testing a classical theory being really “relativistic” or not.

Let us restudy the theory of general relativity (GR). In a metric, the Einstein field equation (EFE) reads (see, e.g. , Refs. [54-56]),

Of course, Equation (9.6) is covariant with respect to the Lorentz transformation. But could it withstand the test of mass inversion?

On the LHS of Equation (9.6), the Einstein tensor contains no any mass and no charge as well. But on the RHS, the energy-momentum current density tensor is proportional to particle’s mass m and so changes its sign under an operation of. Hence as a whole, Equation (9.6) cannot remain invariant under the mass inversion. The reason seems rather clear that antimatter was not taking into account when GR was established in 1915. To modify EFE such that it can preserve the invariance of mass inversion, in 2004, one of us (Ni) proposed to add another term with for antimatter, yielding [

which remains invariant under a mass inversion since:

In a weak-field (or the post-Newtonian) approximation, this modified EFE, MEFE, Equation (9.7), will lead to modified Newton gravitational law as

where the “” sign means attractive force between and being both matter or antimatter whereas the “+” sign means repulsive force between and (both positive) if one of them is antimatter.

If we define the “gravitational mass” for matter and antimatter separately

Then Equation (9.9) can be recast into one equation

which bears a close resemblance to the Coulomb law in classical electrodynamics (CED)

In 1986, within the framework of classical field theory (CFT) plus some assumptions, Jagannathan and Singh derived the potential energy of two static point sources as [

where and are spin and mass of the mediating field, is the “charge” of the source. For CED, whereas for gravitational field (in both cases). So Equation (9.13) is in conformity with Equations (9.11) and (9.12) for the case of “like sources” (with) [

In 2011, the antigravity between matter and antimatter was also claimed by Villata in Ref.[

1) Being the combination of SR and QM, RQM is capable of dealing with particle and antiparticle on an equal footing. As long as we admit that the antiparticle’s momentum and energy operators should be

and versus and for particle, it can be proved that the “negative-energy” WF of particle corresponds to a “positive-energy” WF of antiparticle precisely.

2) In general, an equation in RQM always has a discrete symmetry which shows up as a transformation between a particle’s WF and its antiparticle’s WF:. For a free particle, it simply means. This is in conformity with the “strong reflection” in QFT invented by Pauli and Lüders, showing that the intrinsic property of a particle cannot be detached from the space-time.

3) Following Feshbach-Villars’ deep insight, we are able to divide each and every WF in RQM into two parts,. Then the above symmetry is further rigorously expressed by an invariance of motion equation in RQM through the transformations and under either the space-time inversion

or a mass inversion. Since in whereas in, we may name as the (dominant) hidden particle field in while the (subordinate) hidden antiparticle field in. In this way, both the “probability density” for a particle and for an antiparticle can be proved to be positive definite. Now we may say that the RQM is ensured to be self-consistent and can be regarded as a sound basis for QFT.

4) All kinematical effects in SR can be ascribed to the enhancement of the magnitude of field in a particle’s WF accompanying with the increase of particle’s velocity.

5) As proved for Dirac particle with spin, the helicity of a particle is just opposite to that of its antiparticle under a space-time (or mass) inversion. Therefore, the experimental tests for the CPT invariance should include not only the equal mass and lifetime of particle versus antiparticle, but also the following fact: A particle and its antiparticle with opposite helicities must coexist in nature with no exception. A prominent example is the neutrino —A neutrino (antineutrino) is permanently lefthanded (right-handed) polarized whereas the fact that no exists in nature must means no as well (as verified by the GGS experiment [

6. Based on the invariance of space-time inversion or mass inversion (at the level of RQM) and the latter’s generalization to the classical physics, we tentatively discuss some interesting problems in today’s physics, including the prediction of antigravity between matter and antimatter, as well as the reason why we believe neutrinos are likely the tachyons.

We thank E. Bodegom, T. Chang, Y. X. Chen, T. P. Cheng, X. X. Dai, G. Tananbaum, V. Dvoeglazov, Y. Q. Gu, F. Han, J. Jiao, A. Kellerbauer, T. C. Kerrigan, A. Khalil, R. Konenkamp, D. X. Kong, J. S. Leung, P. T. Leung, Q. G. Lin, S. Y. Lou, D. Lu, Z. Q. Ma, D. Mitchell, E. J. Sanchez, Z. Y. Shen, Z. Q. Shi, P. Smejtek, X. T. Song, R. K. Su, G. Tananbaum Y. S. Wang, Z. M. Xu, X. Xue, J. Yan, F. J. Yang, J. F. Yang, R. H. Yu, Y. D. Zhang and W. M. Zhou for encouragement, collaborations and helpful discussions.

We will discuss the Klein paradox [

Consider that a KG particle moves along axis in onedimensional space and hits a step potential

Its incident WF with momentum and energy reads

If, we expect that the particle wave will be partly reflected at with WF and another transmitted wave emerged at:

with. See

Two continuity conditions for WFs and their space derivatives at the boundary give two simple equations

The Klein paradox happens when because the momentum is real again and the reflectivity of incident wave reads

(See Ref. [

As discussed in Section III, for a KG particle (or its antiparticle), two criterions must be held: its probability density ρ (or) must be positive and its probability current density (or) must be in the same direction of its momentum (or).

See

. From now on we will replace KG WF by and according to Equation (3.26), if still describes a “particle”, whose probability density should be evaluated by Equation (27) with

yielding:

And its probability current density should be given by Equation (3.12), yielding:

Equation (A.8) is certainly not allowed. So to consider a “particle” with momentum moving to the right makes no sense. Instead, we should consider (which also makes no sense for a particle due to the boundary condition) and regard as an antiparticle’s WF by rewriting it as:

Now using Equation (2.18) we see that Equation (A.10) does describe an antiparticle with momentum

and energy

. In the mean time, from the antiparticle’s point of view (i.e., with), the potential becomes (comparing Equation (2.21) with Equation (A.10) as shown by

It is easy to see from Equations (3.30), (3.31) and (A.10) that

So the reflectivity, Equation (A.6), should be fixed as:

And the transmission coefficient can also be predicted as:

The variation of seems very interesting:

Above equations show us that the incident KG particle triggers a process of “pair creation” occurring at, creating new particles moving to the left side (to join the reflected incident particle) so enhancing the reflectivity and new antiparticles (with equal number of new particles) moving to the right.

To our understanding, this is not a stationary state problem for a single particle, but a nonstationary creation process of many particle-antiparticle system. It is amazing to see the Klein paradox in KG equation being capable of giving some prediction for such kind of process at the level of RQM. Further investigations are needed both theoretically and experimentally.^{10}

Beginning from Klein [

Based on similar picture shown in

(A.17)

where. Unlike Equation (A.8)

for KG equation, the probability density for Dirac WF is positive definite (see Equation (5.16))

Hence we will rely on two criterions: First, the probability current density and momentum must be in the same direction for either a particle or antiparticle. For and, their probability current density are

(A.19)

as expected. However, for, we meet difficulty similar to that in Equation (A.9)

the direction of is always opposite to that of! The second criterion is: while for particle, we must have for antiparticle. Now in (or

), (or), but the situation in

is dramatically changed, the existence of renders!

The above two criterions, together with the experience in KG equation, prompt us to choose and regard as an antiparticle’s WF. So we rewrite:

where (with new normalization constant

replacing) describes an antiparticle with momentum, energy

and. Using Equation (5.17) we find

as expected. Now it is easy to match Dirac WFs at the boundary, (, yielding^{11}

where. The reflectivity and transmission coefficient follow from Equations (A.19) and (A.22) as:

where

and

The variation of bears some resemblance to Equation (A.15) for KG equation but shows striking difference due to sharp contrast between Equations (A.24)- (A.28) and Equations (A.12)-(A.15).

To our understanding, in the above Klein paradox for Dirac equation, there is no “pair creation” process occurring at the boundary. The paradox just amounts to a steady transmission of particle’s wave into a high potential barrier at region where shows up as an antiparticle’s WF propagating to the right. In some sense, the existence of a potential barrier plays a “magic” role of transforming the particle into its antiparticle. Because the probability densities of both particle and antiparticle are positive definite, the total probability can be normalized over the entire space like that for one particle case:

(is the Heaviside function) and the probability current density remains continuous at the boundary. In other words, the continuity equation holds in the whole space just like what happens in a one-particle stationary state.

It is interesting to compare our result with that in Refs. [

However, the author in Ref. [

where

The argument for the validity of his Equations (A.30)- (A.31) is based on the hole theory (see also section 5.2 in Ref. [

Fortunately, we learn from section 10.7 in Ref. [