There is something wonderfully humbling about static electricity. Children discover it by rubbing balloons on their hair. Anyone who has walked across carpet on a dry day has experienced it through the unpleasant spark that jumps from a fingertip to a metal object. The ancient Greeks knew that rubbed amber could attract small objects, and our word "electricity" ultimately derives from the Greek word for amber, ēlektron. Yet more than two thousand years later, physicists still cannot give a complete microscopic account of what is happening in many of the simplest examples.
That is the remarkable message of a recent Science News investigation into triboelectricity, the production of electric charge when materials contact, separate or rub against one another. We know perfectly well that charge separation occurs. We can measure it, rank materials according to their tendency to become positively or negatively charged, build machines exploiting it and protect industrial equipment against its effects. What remains surprisingly uncertain is exactly what is moving between many ordinary insulating materials when they charge. It may be electrons, ions, charged fragments of material, or different mechanisms under different circumstances.
This is precisely the sort of problem that should make us cautious about the popular image of physics as a nearly completed description of reality. We are told about quarks, curved spacetime, quantum fields and the first fractions of a second after the Big Bang, yet rub two ordinary materials together and ask exactly why one becomes positive and the other negative, and matters rapidly become less certain.
There is a triboelectric series that ranks materials according to their charging tendencies. Put two appropriate materials together and it can often predict which becomes positively charged and which negatively charged. The trouble is that different experiments can produce somewhat different rankings, and there is no universally satisfactory underlying theory explaining why every material occupies its particular position. Stranger still, even apparently identical materials can exchange charge when brought into contact.
Metals provide the relatively comfortable case. Their electron behaviour can be related to quantities such as the work function, the energy required to remove an electron from a surface. Put different metals into contact and electron transfer can be described reasonably well using established solid-state physics. Insulators are where the old confidence begins to dissolve.
Part of the difficulty is that the net charge involved can be astonishingly small relative to the amount of matter being examined. The Science News article gives a useful sense of scale: at the atomic level, the imbalance responsible for an observable static charge may amount roughly to an extra or missing electron for every hundred thousand atoms. The experimenter is therefore trying to identify a tiny asymmetric residue hidden within an enormous amount of ordinary material behaviour.
New experimental techniques are making progress. Researchers can levitate tiny glass beads acoustically, drop them onto surfaces and recapture them, reducing unwanted contact and contamination. Other experiments investigate how microscopic patches of positive and negative charge form on apparently simple surfaces. The hope is eventually to separate several questions that were previously bundled together: what actually carries the charge, what drives the transfer and what determines its direction.
The mystery matters far beyond balloons and carpets. Static charging affects pharmaceutical manufacturing, electronics, industrial fires, dust storms, pollination and space exploration. Lunar dust is particularly troublesome because there is virtually no atmospheric humidity to dissipate charge, while electrostatic forces become comparatively important in the Moon's low-gravity environment. Astronauts and equipment can accumulate charged dust, potentially creating serious problems for future lunar operations.
And then there is the largest and most spectacular electrostatic phenomenon most of us will ever see: lightning.
Benjamin Franklin famously established the connection between ordinary electricity and lightning in the eighteenth century. That discovery is so familiar that it is easy to assume the essential problem was solved at the same time. A thundercloud separates charge, the electrical potential becomes sufficiently great, the air breaks down and lightning discharges the accumulated electricity. That elementary description is correct as far as it goes, but it conceals one of atmospheric physics' enduring puzzles.
The difficulty is starting the discharge. Ordinary air is an excellent electrical insulator. To produce conventional electrical breakdown, the electric field must become strong enough for electrons to accelerate, ionise molecules and generate additional electrons. This creates an avalanche that rapidly turns insulating air into a conducting pathway. At sea level, the conventional breakdown threshold for dry air is around 2.6 million volts per metre. Yet measurements inside thunderclouds have historically found electric fields substantially below what the simplest conventional-breakdown picture seems to require.
Somehow nature starts lightning without conveniently obeying the elementary textbook version of how the process ought to begin. This is where cosmic rays entered the story. The Earth is continually bombarded by high-energy particles originating beyond the atmosphere. When sufficiently energetic cosmic rays strike atmospheric nuclei, they produce cascades of secondary particles known as air showers. Among the products are energetic electrons capable of penetrating regions of thundercloud electric fields.
The important insight was that energetic electrons behave differently from slow electrons moving through air. Above certain energies and electric-field strengths, an electron can gain energy from the field faster than collisions remove it. Instead of settling down, it "runs away." That electron produces further ionisation, generating additional energetic electrons, and a relativistic runaway electron avalanche can develop.
This was an attractive solution to the lightning-initiation puzzle because runaway breakdown can occur at considerably weaker fields than conventional breakdown. Calculations put the runaway threshold around 2.84 × 10^5 volts per metre under the relevant sea-level scaling, roughly an order of magnitude below the conventional threshold. Fields in this general range are much closer to those actually observed inside thunderstorms.
The cosmic-ray hypothesis therefore suggested a beautiful connection between the heavens and an ordinary thunderstorm. A particle arriving from deep space strikes Earth's atmosphere, generates an air shower and provides energetic seed electrons. The thundercloud has already accumulated an enormous electric potential but has not managed to initiate conventional breakdown. The cosmic-ray particles provide the microscopic trigger that allows the stored atmospheric energy to escape as lightning.
It is almost irresistible as a physical picture, but unfortunately nature has not yet agreed to make matters that simple. Earlier models proposed that a sufficiently large cosmic-ray air shower could initiate lightning through runaway breakdown. Later work identified serious difficulties. The resulting electron avalanche can be very broad and diffuse, making it difficult to understand how it would produce the concentrated hot channel required for a lightning leader. One scientist consequently proposed a modified mechanism in which the steady background of energetic particles produced by atmospheric cosmic rays, rather than one spectacular individual shower, could participate in runaway breakdown and amplify electric-field irregularities. X-rays and positrons could then provide feedback, enormously increasing the runaway process.
There is real observational evidence that high-energy particle physics occurs inside thunderstorms. Lightning and thunderstorms can produce X-rays and gamma rays, phenomena that were scarcely part of the traditional picture of atmospheric electricity. The sky above us is not simply accumulating charge and producing a giant conventional spark. Under appropriate conditions, thunderstorms become natural particle accelerators.
Yet the strongest version of the cosmic-ray hypothesis, that identifiable high-energy cosmic-ray air showers routinely trigger lightning, has not fared particularly well observationally. A 2017 study compared 74 days of cosmic-ray air-shower measurements in Florida with lightning mapping data. It found no evidence that the detected showers had initiated lightning, placing an upper limit of about 5 per cent on the probability that any of those detected showers could have initiated a flash. Importantly, the researchers did not claim that cosmic-ray initiation was impossible. They found that their observations did not support it as the general mechanism.
That distinction is important. "Cosmic rays cause lightning" is too strong. "Cosmic-ray-generated energetic particles may participate in processes relevant to lightning initiation" remains scientifically interesting. There is a considerable difference between cosmic rays supplying ubiquitous seed particles to an electrically charged atmosphere and a particular cosmic-ray shower functioning like someone pulling the trigger on a gun.
The deeper mystery remains because the electrical fields actually measured inside thunderstorms still have to be reconciled with the enormous transition required to initiate a lightning channel. Local field enhancements around water droplets, ice particles and complex charge structures may be important. Streamers and leaders may develop through processes operating on very different spatial scales. Runaway electrons may contribute under some conditions. The final answer may involve several mechanisms rather than one elegant universal trigger.
This returns us to the mystery of ordinary static electricity. The two problems are enormously different in scale, but they share a curious feature. At the macroscopic level we know what happens. Rub the balloon and it becomes charged. Build the thundercloud and eventually lightning occurs. The difficulty appears when we ask for the complete microscopic chain of events connecting the beginning to the spectacularly obvious end.
That is an important philosophical point about physics. Prediction is not always explanation. A civilisation can become extremely competent at manipulating a phenomenon without possessing a final account of its underlying mechanism. Engineers have dealt successfully with static electricity for generations, just as meteorologists can detect thunderstorms and lightning without possessing a universally accepted theory of every stage of lightning initiation.
Static electricity is especially revealing because it is impossible to dismiss the remaining puzzle as something occurring only at inaccessible scales. We are not talking about what happened 10^-35 seconds after the Big Bang or what exists beyond an event horizon. The mystery can be produced by rubbing your hair with a balloon.
There may also be a broader lesson in the cosmic-ray connection. Nature refuses to respect the disciplinary boxes humans construct for studying it. Atmospheric electricity turns into particle physics. Cosmic radiation becomes relevant to meteorology. Charged dust becomes important to planetary formation and lunar exploration. A phenomenon first noticed in rubbed amber ultimately connects with processes extending from microscopic surfaces to thunderstorms and perhaps to particles arriving from elsewhere in the galaxy.
The cosmic-ray story also demonstrates how science should work when confronted by an attractive hypothesis. The idea that cosmic-ray showers trigger lightning is elegant, physically motivated and capable of explaining an important difficulty with conventional electrical breakdown. That does not make it true. Researchers tested the proposal against observations and found that the simple version did not receive the support one might have hoped for. The hypothesis then had to be constrained, modified or incorporated into more complicated models. There is no embarrassment in that. The embarrassment would lie in pretending that the mystery had been solved merely because the equations were elegant.
More than two centuries after Franklin's experiments, we know vastly more about electricity than he could possibly have imagined. We manipulate individual electrons, construct semiconductor devices containing billions of transistors, accelerate charged particles almost to the speed of light and describe electromagnetism using one of the most successful theoretical structures in science. Yet the child rubbing a balloon against his hair can still ask a question capable of making a specialist hesitate: what exactly moved between these two materials, and why did it move that way?
Look upwards during the next thunderstorm and the same humility applies on a vastly larger scale. We know that charge has separated, we know that enormous electrical energies have accumulated, and we know that the brilliant channel across the sky is an electrical discharge. But ask precisely how nature crossed the gap between a charged cloud and the first self-sustaining lightning channel and the answer becomes considerably more complicated.
Perhaps cosmic rays provide part of the missing bridge. Perhaps their principal role is merely to supply a permanent background of energetic particles rather than to trigger individual flashes. Perhaps local structures inside thunderclouds ultimately provide most of the answer. The investigation continues.
That is what makes static electricity such a delightful problem for anyone sceptical of declarations that fundamental physics has reality neatly packaged. Some of the deepest unanswered questions are not necessarily hidden at the edge of the observable universe. Sometimes they are crackling at the end of your finger.
https://www.sciencenews.org/article/static-electricity-is-a-mystery