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In a superconductor, such as a metal cooled to very near to absolute zero, electrical resistance disappears because electrons are not scattered by the ionic lattice. Instead, the net force between electrons becomes attractive as a result of net attraction by the lattice. Negative charges pair up, forming what’re called Cooper pairs that ‘flow’ through the metal.
When a very thin strip of non-superconducting material is placed between two such superconductors, a Josephson junction is formed. In this scenario, an effect from quantum mechanics called >tunnelling occurs: some Cooper pairs will be able to tunnel through the barrier and move between superconductors.
Over time, as more Cooper pairs tunnel through, the amount of current flowing through the barrier will exceed what’s called the critical current. This point on, a voltage is induced across the barrier. This voltage then reduces the current across the barrier, causing even more current to flow through it, raising the voltage even further.
The 'Josephson model'
In >a paper submitted on August 27 , physics Nobel Laureate Frank Wilczek proposed to use such Josephson junctions to make his idea of ‘time crystals’ come real. According to Wilczek, ‘time crystals’ are hypothetical perpetual motion machines (PMMs) – machines that function indefinitely without any energy supply.
Crystals in the traditional sense are substances that have an atomic pattern repeating in space. Likewise, time crystals are PMMs that display a repeating pattern in time, such as periodic motion. In his paper, Wilczek argued that a Josephson junction could function as a PMM if the contact at the point of the junction was periodically broken, creating voltage fluctuations in the circuit in such a way that they’d satisfy the conditions of a time crystal.
Despite its outlandishness (not unlike >one of his previous ideas ), and that it flies in the face of the laws of physics, Wilczek’s idea of a PMM that invokes quantum mechanics has caught on among some people. It was conceived around February, 2012, when Wilczek submitted >the first paper describing his idea.
Apart from a bunch of experimental physicists taking up the challenge of >testing if such a thing can exist , Wilczek has seen vehement opposition from one particular theoretical physicist.
Finding the ground states
>Patrick Bruno , from the European Synchrotron Radiation Facility, subsequently submitted a mathematical proof he claims not just disproves Wilczek’s idea but also disallows it for a broad class of systems. It was published in Physical Review Letters in August this year.
Bruno’s offensive was centred on Wilczek’s time crystals not being in their ground state, which is the state of the system’s existence when it has the lowest energy possible. If such a system has more than this much energy, it will lose it and return to its ground state – this is the natural order of things. For a PMM, on the other hand, a system would have to have the lowest energy possible and still be in some sort of motion.
When I contacted Bruno about Wilczek’s suggestion, he dismissed it, saying, “Wilczek now proposes to consider a Josephson junction which is biased by a voltage. Well, such a system is not in its ground state either, since the energy of the system could be lowered by transferring Cooper pairs from on side of the barrier to the other.”
However, the group of experimental physicists is still interested in building the time crystals, and if not that, machines that are ‘metastable’. These are not PMMs but can run for a long time with a small energy input because they dissipate very little energy while in motion. Bruno isn’t dismissive of this idea, but believes “it is neither new nor particularly exciting”.
What is new or particularly exciting, on the other hand, is Wilczek’s considering the possibility that perpetual motion is a plausible phenomenon in the realm of quantum mechanics. The history of this notion dates to the medieval era, but until recently, it has only been considered with Newtonian mechanics in mind. For this, the outcome of the ‘Wilczek-Bruno debate’ is one to look out for.
