Quantum physics unveils a strange subatomic realm where particles can exhibit counterintuitive behaviors, defying our common sense notions of how the world works. One of the most bizarre quantum effects is quantum tunnelling, the ability of particles to pass through barriers that classical physics would deem impenetrable.
This phenomenon seems to flout basic physical principles, yet it is integral to many quantum processes. By delving into the science behind quantum tunnelling and how it relates to speed limits like the speed of light, we can unravel the mysteries of this quirky effect.
The Uncertainty of the Quantum World
To understand quantum tunnelling, we must first grasp a key principle of quantum mechanics. Unlike a classical tennis ball that has a definite position and trajectory, a quantum particle like an electron is described by a probabilistic wavefunction (Schrödinger equation). There is no way to know the electron’s exact position at any moment, only the probabilities of where it could be.
When a quantum particle encounters a barrier, its wave function interacts with the barrier in complex ways. Some portions of the wavefunction will reflect backwards, while other portions can penetrate the barrier to the other side, either through classical hopping or quantum tunnelling. The probability of tunnelling depends on factors like barrier thickness, height, and the particle’s energy. But the intrinsic uncertainty means the particle’s path remains probabilistic, not precisely predictable. This property sharply distinguishes the quantum world from the classical realm, where particles follow well-defined trajectories.
Visualizing the Quantum Tunnelling Process
Quantum tunnelling defies our intuition about solid barriers, so how can we visualize this process?
Imagine repeatedly throwing balls at a wall. Usually, they bounce back, but very rarely, a ball unpredictably passes through like a ghost. Similarly, electrons usually reflect from barriers but occasionally slip through via tunnelling.
Alternatively, picture the quantum particles as waves washing over a barrier island. Most waves crash into the shore but small ripples may tunnel through gaps. The waves do not speed up when passing – they simply percolate through. This illustrates why tunnelling involves no special speed boost.
Of course, true quantum tunnelling differs fundamentally from metaphorical ghost balls or tunnelling waves. But these analogies highlight key principles, probability, and preservation of speed limits. While tunnelling literally transcends classical physics, relatable imagery provides footholds for comprehending the quantum realm.
Probing the Quantum Tunnelling Timescale
A fascinating question arises, if a quantum particle tunnels through a barrier, how much time does the tunnelling process itself take? This seems difficult to ascertain since we cannot trace the precise path of a probabilistic quantum particle. But modern techniques using ultrafast laser pulses enable tracking tunnelling timescales by controlling and timing a particle’s journey.
In a recent breakthrough experiment, researchers at the Australian Attosecond Science Facility used light pulses to excite electrons in hydrogen atoms into higher energy states. This enabled the triggering and timing of the electrons as they tunnelled out of the atoms into open space. Remarkably, they measured the act of tunnelling to occur essentially instantaneously, taking at most 1.8 attoseconds, or 1.8 billionths of a billionth of a second. This demonstrates tunnelling does not incur any discernible delay beyond the time limitations imposed by relativistic physics.
But what does it mean for tunnelling to be “instantaneous”?
How does this relate to the cosmic speed limit, the speed of light?
Can quantum particles travel faster than light?
Unpacking this paradox requires delving deeper into the quantum world.
Quantum Particles Move in Mysterious Ways
If we fire a stream of quantum particles at a barrier, each particle has a certain likelihood of reflecting backwards and a probability of tunnelling through. Moreover, these processes do not follow a predictable sequence and therefore we cannot forecast exactly when particles will tunnel versus reflect.
Now imagine we create a short pulse of quantum particles and fire it at a barrier. We can measure the pulse’s starting position and time. Assuming some particles do cross via tunnelling, we can also measure when the pulse emerges on the other side. The particles detected on the other side will not form a neat pulse moving at a single speed. Their transit times fluctuate probabilistically, just as their trajectories do.
So while tunnelling itself occurs instantly, the overall pulse transit time conforms to quantum uncertainty and, importantly, its speed does not exceed the light speed limit set by relativity theory.
Therefore, while probabilistic quantum effects allow strange behaviours like tunnelling and instantaneous transit speeds, no process enables FTL (faster-than-light) travel. Quantum and relativistic physics align on this key principle.
Why is Faster-Than-Light Motion Impossible?
Relativity dictates that no object with mass can reach light speed because its mass would become infinite. Massless particles like light photons could travel at light speed. Probabilistic or not, all particles obey this cosmic speed limit.
So when experimenters create the illusion of FTL pulses by timing particles hopping over barriers, the results actually reflect quantum variability in transit times, not true FTL travel. No single particle outpaces light, even if its speed appears higher.
Reality remains confined by relativity’s ultimate speed limit. While the quantum world boasts plenty of quirks, upending this basic principle is beyond physics’ reach. Quantum particles spread unpredictably but never outrun light. Tunnelling sidesteps barriers in strange ways – but without ever outracing Einstein.
Applications Enabled by Quantum Tunnelling
Far from being just an esoteric phenomenon, quantum tunnelling plays vital roles across fields like electronics, computing, astronomy, chemistry, and scanning microscopy. Understanding tunnelling mechanics aids practical applications.
In electronics, tunnelling enables microcircuits and devices by helping particles cross insulating barriers and thin films. Metal-insulator-metal junctions made possible by quantum tunnelling are integral to microelectronics and CMOS technology.
In computing, tunnelling allows electrons to leap between quantum energy states, enabling faster processing speeds. Tunneling also aids electron transfer during DNA mutation and replication critical for computation and data storage. In chemistry, tunnelling drives key reaction mechanisms, like hydrogen atom transfer and enzyme-mediated reactions. Fusion in stars likewise relies on tunnelling to occur under stellar core conditions.
So from the tiniest subatomic scales to the mammoth mechanisms of stars, quantum tunnelling profoundly shapes our cosmos. Mastering its nuances empowers technology leaps. Even if CERN scientist Massimo Angeletti calls the tunnelling phenomenon “a minor scandal in physics” its practical impact proves anything but minor.
Quantum Tunneling – Physics at the Threshold
By peering into the counterintuitive process of quantum tunnelling, we uncover profound insights into our probabilistic reality. Particles crossing barriers instil wonder at the strange quantum frontier, even if FTL travel remains science fiction.
Einstein once said,
“The most beautiful thing we can experience is the mysterious.”
Quantum tunnelling epitomizes this mystery as particles defy classical constraints. Yet deep analyses reveal tunnelling obeys relativistic principles, underscoring the harmonious alliance between quantum and cosmic-scale physics.