It’s a saying that’s been quoted by others, usually scientists and science writers, thousands of times by now, yet it retains the element of a profound insight. And that is ‘the Universe is not only far stranger than we imagine, it’s far stranger than we can imagine’. Every time you turn a corner, there’s an unexpected and rather shocking surprise staring you in the face! Mother Nature never seems to run out of curve balls. Anyway, one such imagination could have our Universe’s strangeness level increased a notch or so in that it allows for micro (quantum) time travel and that perhaps can lead to a Universe that consists of way less stuff than that contained in the smallest speck of dust.
Symmetry: The Yin and the Yang:
Symmetry, or two ways of looking at things, or dual solutions or just plain duality is quite familiar in our world and has lots of parallels in mathematics and physics too. I mean we have wave – particle, forward - back, left - right, east - west, top - bottom, north - south, push - pull, either – or, positive charge - negative charge, matter – antimatter, spin-up – spin-down, mass (matter) – energy, clockwise – counter clockwise, for every action there is an equal and opposite reaction, the irresistible force vs. the immovable object, odd – even, zero – infinity, male – female, mind – body, alive – dead, black – white, night – day, dawn – dusk, right down to heads or tails, old vs. new, war and peace, a glass half full vs. a glass half empty, perhaps Coke vs. Pepsi in that glass!
We’re aware that there are mathematical equations that have two solutions. For an easy example, what is the square root of four? There are two solutions: +2 and -2. Of course the concept of -2 is usually considered to be irrelevant or nonsense (can something be minus two feet in height?), so normally only the positive values are considered justified. One tends to always assume and use +2, which, when squared, is four.
Likewise, the general form of a quadratic equation always has two solutions at its core, again, one that is usually a plus value, one that is usually a minus value, but in any event dual solutions.
Symmetry and the Flow of Time:
So, with all this symmetry and dualism in our lives, in physics and in mathematics, why not add in one more – say the flow or arrow of time. But surely time can only flow in one direction, from past to present to future. Or then again, maybe that’s an unwarranted assumption.
At the micro (quantum) level, there is no physical law or principle equivalent to the one-way macro ‘arrow’ of time. Every micro (quantum) action in what we would term the direction towards the future can be equally valid in what we would term the direction towards the past. If time has no significance at the micro (quantum) level, then all particles and all antiparticles can go both backwards and forwards in time, and in doing so, become pseudo-antiparticles or pseudo-particles respectively. The concept that the arrow of time can reverse must be among the most difficult of ideas to wrap your gray matter around if for no other reason than that our grey matter has only ever actually experienced a unidirectional time’s arrow.
Translating what a dual-solution, a minus or a positive value solution, could mean in real physics, Maxwell’s equations predict the existence of two sorts of light waves, a (positive value) ‘retarded’ light wave (the normal kind that travels from past to present to future), and (you guessed it) a (negative or minus value) ‘advanced’ light wave (a decidedly un-normal variety that would in theory travel from future to present to past).
In physics, one normally has something called CPT symmetry, where C = charge, P = parity (left vs. right), and T = time. To conserve symmetry, if a particle is assumed to be travelling backwards in time, then its handedness is reversed, and so is its charge. It takes on the appearance of that particle’s antimatter counterpart.
In other words, you have to reverse the charge, reverse left-right handedness, and reverse the arrow of time to conserve symmetry. When this is done, an electron, going backwards in time, must take on the appearance of, and be interpreted as, a positron – but of course it’s a pseudo-positron in that’s it’s a normal electron wearing a Halloween mask even though it’s indistinguishable from the real positron McCoy.
Feynman’s Idea:
When (future) Nobel Laureate Professor Richard P. Feynman (albeit starting this track while in his graduate days) looked at physicist Paul Dirac’s equation for electrons, he noticed that if one reversed both the electron’s charge and the time element, then there was perfect symmetry. An electron with normal negative charge going forward in time (one solution), seemed to be identical to an electron with positive charge – a positron – going backwards in time (a second solution)!
A positron is termed an electron’s antiparticle, or equally an electron could be termed a positron’s antiparticle, but in common terms, an electron and a positron are examples of matter and antimatter.
Now all matter particles have an antimatter twin. Their charges are opposite. Particles having no overall charge (like a neutron** or a neutrino) are their own anti-particles (in that their charge is the same – zero). Even so, these particles and their antimatter counterparts differ in other respects, have differing quantum numbers, so particle physicists still talk about such things as an antineutron** or antineutrino, and you’ll find references to them in the antimatter literature. Antimatter isn’t an ivory tower fairy tale concept. Positrons have been created; Mother Nature can create antimatter. In fact, gram for gram, antimatter is the most valuable stuff in existence, worth hundreds of times its weight in gold or diamonds because it’s both naturally rare and can only be manufactured in miniscule quantities at exorbitant costs in particle accelerators. And, as we all are aware, when matter meets antimatter it’s annihilation time – ka-boom – electromagnetic energy (photons) is given off***.
The upshot of all this is that Feynman proposed that antimatter was just ordinary matter but ordinary matter going in the opposite time direction! Perhaps at this juncture I’d be better off to quote Feynman directly:
“We begin with a photon and an electron, and we end up with a photon and an electron. One way this event can happen is: a photon is absorbed by an electron, the electron continues on a bit, and a new photon comes out. This process is called the scattering of light. When we make the diagram and calculations for scattering, we must include some peculiar possibilities. For example, the electron could emit a photon before absorbing one. Even more strange is the possibility that the electron emits a photon, then travels backwards in time to absorb a photon, and then proceeds forward in time again. The path of such a “backwards-moving” electron can be so long as to appear real in an actual physical experiment in the laboratory.
The backwards-moving electron when viewed with time moving forwards appears the same as an ordinary electron, except it’s attracted to normal electrons – we say it has “positive charge”. … For this reason it is called a “positron”. The positron is a sister particle to the electron, and is an example of an “anti-particle”.
This phenomenon is general. Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle.”
[Feynman, Richard P.; QED: The Strange Theory of Light and Matter; Penguin Books, London
Or, in the words of British science writer Jenny Randles:
“Indeed, it is now widely accepted that the best way to interpret a positron – which is the antimatter version of the electron with identical mass but opposite charge – is to view it as an electron that moves backwards through time.”
[Randles, Jenny; Breaking the Time Barrier: The Race to Build the First Time Machine; Paraview Pocket Books, New York; 2005; p.116.]
So, here’s my understanding of what happens, and apparently there are two ways of looking at the picture. What if an electron is going forward in time and for some reason emits or absorbs a photon which would cause it to alter direction, perhaps to the extent of reversing direction in time as well thus taking on the appearance of a pseudo-positron (because charge is reversed). At some later time, the pseudo-positron emits or absorbs a photon, and reverses time direction again turning back into your everyday electron. That’s one way of looking at things.
The other way is to abandon time reversals and just assume that all electrons and all positrons go forward in time, full stop. In either scenario, you have no physical contradictions – two solutions to the same equation as it were. But if you accept the time reversal concept, it’s a sort of micro Dr. Jekyll and Mr. Hyde transformation.
Let’s run that by again.
Reality one: A real electron travelling forward in time absorbs a photon, bounces back in time as a pseudo-positron (because charge has to be reversed) and absorbs a photon and reverses time direction again, back to that of an electron moving forward in time.
Equally, a real positron travels forward in time, absorbs a photon, goes back in time as a pseudo-electron (charge goes from plus to minus), absorbs a photon and reverses time direction. In this picture there is no matter-antimatter annihilation.
Reality two: A real electron (or positron) going forward in time meets its opposite number (caused by an earlier forward time travelling photon energy that disintegrated producing a matter-antimatter particle pair) and that annihilation produces (emits) a photon (energy) also travelling forward in time which can then disintegrate back into an electron-positron pair travelling forward in time. In this view there is annihilation (matter meets antimatter), and energy (photon) producing a matter-antimatter particle pair, but no time travel. In either picture, the total number of electrons, positrons and photons remain the same.
To be continued...
**Old learning’s die hard. Of course both the neutron and the proton are no longer considered to be fundamental particles. Rather, both are composed of a trio of quarks, and thus it is proper to have quarks and anti-quarks rather than protons and antiprotons/neutrons and antineutrons.
***I’ve never actually seen an explanation as to why matter and antimatter annihilate on contact giving off pure energy. It just does in the same, but opposite way (symmetry again) as pure energy can create matter and antimatter particle pairs. The annihilation and creation of matter – antimatter, to and from energy, is not the problem since matter (including antimatter) and energy are two sides of the same coin. [It’s interesting that you have matter and antimatter, but not energy and anti-energy.] So, I’ll just accept that when an electron meets a positron – ka-boom. Also ditto that in that a ka-boom can create an electron – positron pair. But presumably an electron and an antiproton, if meeting up, won’t annihilate because the antiproton is not the antimatter equivalent of an electron. Also ditto that in that energy cannot create an electron – antiproton pair of particles.
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