Quantum Tunneling
About 150 million kilometers away from us, there is a huge sphere
of hot plasma, which we call the Sun. Just like any other star, the Sun makes
its energy by colliding lighter atomic nuclei to form a heavier element. This
process, called nuclear fusion, is crucial for the existence of every single
star in the universe.
However, there is a
problem. The colliding nuclei are all positively charged, which means that they
repel each other electrically. How do the nuclei fuse, then? There is another
force – the strong nuclear force – which brings them together, but only when
they are really close to each other to begin with. Therefore, the nuclei must
have a huge energy (and thus velocity) in order to approach each other to the
point where the attractive nuclear strong force surpasses the repulsive
electrical force for nuclear fusion to occur. But when the temperature of the
Sun was ascertained by its spectrum, it came to light that it does not even
remotely reach the values necessary for nuclear fusion. In other words, the Sun
simply should not shine whatsoever. This conclusion is obviously wrong – the
Sun evidently shines, for which we owe to a peculiar phenomenon of quantum
physics – quantum tunnelling.
Quantum tunnelling is a
phenomenon wherein particles or even whole atoms have a certain probability of
surpassing a barrier, even though they do not have enough energy to surpass it,
which is unambiguously against the principles of classical physics. This
phenomenon may not seem that peculiar at first sight, but the opposite is true.
It would probably be quite strange if a person who run up against a wall
appeared on the other side of the wall or even inside the wall. However
incredible it may sound; this is essentially what happens to objects from the
microworld during quantum tunnelling.
Quantum tunnelling can
be explained using the principle of quantum superposition and the uncertainty
principle. How? According to classical physics, the Sun does not have the
sufficient temperature for atomic nuclei to approach each other enough for
fusion to occur. However, the principle of quantum superposition states that
the nuclei can be in more places at once (due to their wave nature), so
there is a certain probability of them approaching enough and fusing. According
to the Heisenberg uncertainty principle, on the other hand, there is always
some uncertainty regarding the momentum of an object, so from time to time, one
or both nuclei obtain an immense velocity (momentum) and fuse.
Quantum tunnelling is
one of a few phenomenon of quantum mechanics whose consequences we can hugely
feel in the macroworld as well. The structure of our own bodies, for instance,
is determined by the DNA molecule. However, it has been theorized that protons
within this molecule can experience quantum tunnelling and therefore change our
genetic makeup! These random genetic mutations caused by quantum tunnelling may
even be linked to the existence of cancer, but more research is needed.
Tunnelling also occurs during radioactive decay or in flash discs.