In this blog, we talked a lot about the Zitterbewegung model of an electron, which is a model which allows us to think of the elementary wavefunction as representing a radius or position vector. We write:
ψ = r = a·e±iθ = a·[cos(±θ) + i · sin(±θ)]
It is just an application of Parson’s ring current or magneton model of an electron. Note we use boldface to denote vectors, and that we think of the sine and cosine here as vectors too! You should note that the sine and cosine are the same function: they differ only because of a 90-degree phase shift: cosθ = sin(θ + π/2). Alternatively, we can use the imaginary unit (i) as a rotation operator and use the vector notation to write: sinθ = i·cosθ.
In one of our introductory papers (on the language of math), we show how and why this all works like a charm: when we take the derivative with respect to time, we get the (orbital or tangential) velocity (dr/dt = v), and the second-order derivative gives us the (centripetal) acceleration vector (d2r/dt2 = a). The plus/minus sign of the argument of the wavefunction gives us the direction of spin, and we may, perhaps, add a plus/minus sign to the wavefunction as a whole to model matter and antimatter, respectively (the latter assertion is very speculative though, so we will not elaborate that here).
One orbital cycle packs Planck’s quantum of (physical) action, which we can write either as the product of the energy (E) and the cycle time (T), or the momentum (p) of the charge times the distance travelled, which is the circumference of the loop λ in the inertial frame of reference (we can always add a classical linear velocity component when considering an electron in motion, and we may want to write Planck’s quantum of action as an angular momentum vector (h or ħ) to explain what the Uncertainty Principle is all about (statistical uncertainty, nothing ontological), but let us keep things simple as for now):
h = E·T = p·λ
It is important to distinguish between the electron and the charge, which we think of being pointlike: the electron is charge in motion. Charge is just charge: it explains everything and its nature is, therefore, quite mysterious: is it really a pointlike thing, or is there some fractal structure? Of these things, we know very little, but the small anomaly in the magnetic moment of an electron suggests its structure might be fractal. Think of the fine-structure constant here, as the factor which distinguishes the classical, Compton and Bohr radii of the electron: we associate the classical electron radius with the radius of the poinlike charge, but perhaps we can drill down further.
We also showed how the physical dimensions work out in Schroedinger’s wave equation. Let us jot it down to appreciate what it might model, and appreciate why complex numbers come in handy:
This is, of course, Schroedinger’s equation in free space, which means there are no other charges around and we, therefore, have no potential energy terms here. The rather enigmatic concept of the effective mass (which is half the total mass of the electron) is just the relativistic mass of the pointlike charge as it whizzes around at lightspeed, so that is the motion which Schroedinger referred to as its Zitterbewegung (Dirac confused it with some motion of the electron itself, further compounding what we think of as de Broglie’s mistaken interpretation of the matter-wave as a linear oscillation: think of it as an orbital oscillation). The 1/2 factor is there in Schroedinger’s wave equation for electron orbitals, but he replaced the effective mass rather subtly (or not-so-subtly, I should say) by the total mass of the electron because the wave equation models the orbitals of an electron pair (two electrons with opposite spin). So we might say he was lucky: the two mistakes together (not accounting for spin, and adding the effective mass of two electrons to get a mass factor) make things come out alright. 🙂
However, we will not say more about Schroedinger’s equation for the time being (we will come back to it): just note the imaginary unit, which does operate like a rotation operator here. Schroedinger’s wave equation, therefore, must model (planar) orbitals. Of course, the plane of the orbital itself may be rotating itself, and most probably is because that is what gives us those wonderful shapes of electron orbitals (subshells). Also note the physical dimension of ħ/m: it is a factor which is expressed in m2/s, but when you combine that with the 1/m2 dimension of the ∇2 operator, then you get the 1/s dimension on both sides of Schroedinger’s equation. [The ∇2 operator is just the generalization of the d2r/dx2 but in three dimensions, so x becomes a vector: x, and we apply the operator to the three spatial coordinates and get another vector, which is why we call ∇2 a vector operator. Let us move on, because we cannot explain each and every detail here, of course!]
We need to talk forces and fields now. This ring current model assumes an electromagnetic field which keeps the pointlike charge in its orbit. This centripetal force must be equal to the Lorentz force (F), which we can write in terms of the electric and magnetic field vectors E and B (fields are just forces per unit charge, so the two concepts are very intimately related):
F = q·(E + v×B) = q·(E + c×E/c) = q·(E + 1×E) = q·(E + j·E) = (1+ j)·q·E
We use a different imaginary unit here (j instead of i) because the plane in which the magnetic field vector B is going round and round is orthogonal to the plane in which E is going round and round, so let us call these planes the xy– and xz-planes respectively. Of course, you will ask: why is the B-plane not the yz-plane? We might be mistaken, but the magnetic field vector lags the electric field vector, so it is either of the two, and so now you can check for yourself of what we wrote above is actually correct. Also note that we write 1 as a vector (1) or a complex number: 1 = 1 + i·0. [It is also possible to write this: 1 = 1 + i·0 or 1 = 1 + i·0. As long as we think of these things as vectors – something with a magnitude and a direction – it is OK.]
You may be lost in math already, so we should visualize this. Unfortunately, that is not easy. You may to google for animations of circularly polarized electromagnetic waves, but these usually show the electric field vector only, and animations which show both E and B are usually linearly polarized waves. Let me reproduce the simplest of images: imagine the electric field vector E going round and round. Now imagine the field vector B being orthogonal to it, but also going round and round (because its phase follows the phase of E). So, yes, it must be going around in the xz– or yz-plane (as mentioned above, we let you figure out how the various right-hand rules work together here).
You should now appreciate that the E and B vectors – taken together – will also form a plane. This plane is not static: it is not the xy-, yz– or xz-plane, nor is it some static combination of two of these. No! We cannot describe it with reference to our classical Cartesian axes because it changes all the time as a result of the rotation of both the E and B vectors. So how we can describe that plane mathematically?
The Irish mathematician William Rowan Hamilton – who is also known for many other mathematical concepts – found a great way to do just that, and we will use his notation. We could say the plane formed by the E and B vectors is the E–B plane but, in line with Hamilton’s quaternion algebra, we will refer to it as the k-plane. How is it related to what we referred to as the i– and j-planes, or the xy– and xz-plane as we used to say? At this point, we should introduce Hamilton’s notation: he did write i and j in boldface (we do not like that, but you may want to think of it as just a minor change in notation because we are using these imaginary units in a new mathematical space: the quaternion number space), and he referred to them as basic quaternions in what you should think of as an extension of the complex number system. More specifically, he wrote this on a now rather famous bridge in Dublin:
i2 = -1
j2 = -1
k2 = -1
i·j = k
The first three rules are the ones you know from complex number math: two successive rotations by 90 degrees will bring you from 1 to -1. The order of multiplication in the other two rules ( i·j = k and j·i = –k ) gives us not only the k-plane but also the spin direction. All other rules in regard to quaternions (we can write, for example, this: i ·j·k = -1), and the other products you will find in the Wikipedia article on quaternions) can be derived from these, but we will not go into them here.
Now, you will say, we do not really need that k, do we? Just distinguishing between i and j should do, right? The answer to that question is: yes, when you are dealing with electromagnetic oscillations only! But it is no when you are trying to model nuclear oscillations! That is, in fact, exactly why we need this quaternion math in quantum physics!
Let us think about this nuclear oscillation. Particle physics experiments – especially high-energy physics experiments – effectively provide evidence for the presence of a nuclear force. To explain the proton radius, one can effectively think of a nuclear oscillation as an orbital oscillation in three rather than just two dimensions. The oscillation is, therefore, driven by two (perpendicular) forces rather than just one, with the frequency of each of the oscillators being equal to ω = E/2ħ = mc2/2ħ.
Each of the two perpendicular oscillations would, therefore, pack one half-unit of ħ only. The ω = E/2ħ formula also incorporates the energy equipartition theorem, according to which each of the two oscillations should pack half of the total energy of the nuclear particle (so that is the proton, in this case). This spherical view of a proton fits nicely with packing models for nucleons and yields the experimentally measured radius of a proton:
Of course, you can immediately see that the 4 factor is the same factor 4 as the one appearing in the formula for the surface area of a sphere (A = 4πr2), as opposed to that for the surface of a disc (A = πr2). And now you should be able to appreciate that we should probably represent a proton by a combination of two wavefunctions. Something like this:
What about a wave equation for nuclear oscillations? Do we need one? We sure do. Perhaps we do not need one to model a neutron as some nuclear dance of a negative and a positive charge. Indeed, think of a combination of a proton and what we will refer to as a deep electron here, just to distinguish it from an electron in Schroedinger’s atomic electron orbitals. But we might need it when we are modeling something more complicated, such as the different energy states of, say, a deuteron nucleus, which combines a proton and a neutron and, therefore, two positive charges and one deep electron.
According to some, the deep electron may also appear in other energy states and may, therefore, give rise to a different kind of hydrogen (they are referred to as hydrinos). What do I think of those? I think these things do not exist and, if they do, they cannot be stable. I also think these researchers need to come up with a wave equation for them in order to be credible and, in light of what we wrote about the complications in regard to the various rotational planes, that wave equation will probably have all of Hamilton’s basic quaternions in it. [But so, as mentioned above, I am waiting for them to come up with something that makes sense and matches what we can actually observe in Nature: those hydrinos should have a specific spectrum, and we do not such see such spectrum from, say, the Sun, where there is so much going on so, if hydrinos exist, the Sun should produce them, right? So, yes, I am rather skeptical here: I do think we know everything now and physics, as a science, is sort of complete and, therefore, dead as a science: all that is left now is engineering!]
But, yes, quaternion algebra is a very necessary part of our toolkit. It completes our description of everything! 🙂
One thought on “Quaternions and the nuclear wave equation”
Very interesting. I think you might like geometric algebra.