Quantum Nugget #8 — Quantum Tunnelling
Term of the Day
Quantum tunnelling
Greek: Κβαντική σήραγγα or φαινόμενο σήραγγας
In classical physics, an object that does not possess enough energy to overcome a barrier turns back.
A ball that cannot reach the top of a hill cannot appear on the other side.
In Quantum Physics, however, a particle may be detected beyond an energy barrier, even when its energy would not be sufficient to pass over it according to classical physics.
This is called quantum tunnelling.
How Does It Happen?
A quantum object is not described only as a tiny grain following a definite path.
It is described by a wavefunction, which allows us to calculate the probabilities of the possible outcomes.
When the wavefunction encounters a barrier, it does not fall abruptly to zero at its surface. It penetrates the barrier and decays.
If the barrier is not excessively high or wide, a non-zero amplitude may remain on the other side.
There is therefore a probability that the particle will be detected there.
The particle does not open a real tunnel through matter. It does not break the barrier, nor does it temporarily borrow energy.
Total energy is conserved.
The probability of transmission depends on the particle and on the barrier’s height and width.
The higher or wider the barrier, the smaller the tunnelling probability generally becomes.
Not a Rare Curiosity
Without quantum tunnelling:
- Certain forms of radioactive decay would not occur as we know them.
- Nuclear reactions in stars would be far more difficult.
- The scanning tunnelling microscope could not image surfaces on the atomic scale.
This strange phenomenon of the invisible world therefore participates in the workings of the visible one.
The image is symbolic: the luminous trace does not represent a real tunnel or a predetermined path followed by the particle.
Keep this in mind:
Quantum tunnelling does not turn the impossible into the inevitable. It turns something classically forbidden into something quantum-mechanically possible.
The Poetic Bridge
Physics does not promise us that every obstacle in life conceals a secret exit.
Nor does it tell us that belief alone will carry us through a wall.
It does, however, offer us a remarkable image: limits that appear absolute within one description may not be absolute within a deeper one.
The particle does not hope.
It does not persevere.
It does not choose.
The human soul, however, may borrow one thought from nature:
When facing a barrier, let us not immediately call impossible what we do not yet understand.
Some ways through do not look like roads.
They reveal themselves only when we learn to read the obstacle differently.
Scientific reference: The actual tunnelling of electrons through a barrier has been experimentally confirmed, among other cases, through measurements of vacuum tunnelling between metallic electrodes conducted by the National Bureau of Standards/NIST .
