Bengaluru: A wide swath of fundamental physics and chemistry is defined by the pursuit of the ground state. Since we began elucidating the structure of the atom in the 1910s, much of what we know about how particles, quasiparticles and molecules behave can be described by a desire among each of these entities to lose all the energy that they can in the given circumstances and simply exist with the bare minimum. This is why the ground state is both an illuminating and fascinating object of study: the former because it is what particles are always tending towards and the latter because it is the ultimate destiny of the ergic constituents of our world, symbolising a kind of particulate amor fati. The ground state is the home to which all matter seeks to return; by studying the home and the forces that keep it, we can explain to a large extent the nature of the things that want to return there.
“The ground state is interesting because small excitations above it are what we effectively mean by (quasi)particles,” says Madhusudhan Raman, a theoretical physicist. “That is, when you find the right variables in which to study small excitations of the ground state, you have understood your physical system perturbatively.”
For example, the electrons around an atomic nucleus are forbidden from occupying the same… the same what? “Might I suggest an analogy?” Raman butts in. “Electrons are like home-owners: they may live in the same town, or even on the same street, but no two electrons live together. That is the exclusion principle.” And all the electrons in an atom are concentric vis-a-vis the atomic nucleus. By asking why they would do this when they could all simply journey around the nucleus in an orbit that affords them the lowest energy possible, we come upon the work of Wolfgang Pauli, Paul Dirac, Enrico Fermi, among others. By wondering if other particles in other systems are subjugated similarly, we come upon the work of Satyendra Nath Bose and Albert Einstein, among others.
Why, fast-forward to 2011, when the Higgs boson was discovered because the unstable amount of energy it embodied ‘decayed’ into clumps of lighter, long-lived and so more observable particles. If particles didn’t behave this way, the Large Hadron Collider would be completely useless – nor would we have had last week’s exciting blazar neutrino discovery.
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A choice example from the realm of physics is the superconductor, which – as we all know – is a material that can conduct an electric current with zero resistance. One way to explain this phenomenon is by taking recourse through BCS theory, which imagines the electrons in a superconductor to have joined up in specific circumstances to form so-called Cooper pairs, effectively getting transformed from being fermions of higher energy to bosons of lower energy. And bosons are exempt from Pauli’s exclusion principle, free to form a phase of matter called the condensate at low temperatures. This condensate sea of electrons is what conducts the electricity. “This, incidentally, is a good example of finding the right ground state,” Raman said.
Superconductors are comparable to time crystals, a hypothetical crystal whose particulate constituents would be in motion in their ground state. The principle difference between them is that time crystals exhibit the spontaneous breaking of time-translation symmetry (as explained here), whereas superconductors don’t. However, superconductors are still cooler – not least for their befuddling variety and their involvement in kooky experiments to uncover anomalous quantum effects and ‘artificial’ particles.
Unfortunately, all these materials and their properties are very difficult to engineer and then observe in action. In most cases, the observation itself consists of watching numbers on a screen or reading pre-recorded data. Compare this to how exciting it would be to observe an object oscillating between either sides of its ground state in a classical setting. Of course, this also would be hard to engineer because the object would have to act against gravity, which takes a lot of work, which in turn takes a lot of energy (think of Newton’s cradle). Perhaps it can be made to work if we went just a little smaller, unto a scale where the object is heavy enough to be affected by gravity but also light enough to be affected by one of the other fundamental forces, preferably in the form of a controllable electrochemical reaction.
This is somewhat the case with the mercury heartbeat experiment. Place a drop of mercury in a small pool of acid with an iron nail at a short distance from the drop. The acid strips off electrons from the mercury atoms it comes in contact with, ionising them and forcing them to repel each other. This causes the mercury drop to flatten out – and make contact with the iron nail. The nail has enough negative electrochemical potential, i.e. functions as an anode, to deionise the mercury atoms and cause them to pull themselves together again thanks to surface tension. As a result, the drop de-flattens into a sphere-like shape, loses contact with the iron nail and starts the cycle all over again.