What is quantum tunneling? Quantum tunneling is a phenomenon where an electron or an atom can pass through an energy barrier without having enough energy to get through it.
To get through barriers, energy is usually required. A brick wall is a barrier. If I want to get through that wall I need to have enough kinetic energy. Kinetic energy is calculated as half mass multiplied by velocity squared. However, the kinetic energy required to get through that brick wall is higher than the kinetic energy any human could muster. Usain Bolt, for example, weighs 94 kg and his top speed in his world record 100 m run was 44.72 km/h, which is 12.422 meters per second. That gives him a kinetic energy of 7252.38 joules. To break through a brick wall, you would need far more kinetic energy than a person could ever produce. A car could get through that wall because it has a much higher mass and a much higher velocity. However, when it does break through that wall, its energy goes into breaking and moving bricks, deforming the car, producing sound and generating heat. This is conventional physics. A particle can sometimes pass through a potential barrier without losing any of its kinetic energy. This is quantum tunneling.
My brick wall was just an easy visual example. In theory, a particle could tunnel through a brick wall, but the probability would be so incredibly small that it would effectively never happen. So, what is quantum tunneling?
We need to remember that a particle, such as an electron, is both a particle and a wave. However, it is not the same as a wave in the water or a sound wave in the air. These types of waves are disturbances in a medium and they have a frequency and a wavelength. An electron’s wave is a probability wave. When scientists measure electrons, they don’t know where it will be until they measure it, but they do know that it must be somewhere and they can give a probability to the likelihood of where it will be when they do measure it. All of these probabilities will add up to 100% because it must be somewhere, but there could be a 72% chance it will be here and a 0.0003% chance it will be somewhere else. Some locations may have a high probability, others an incredibly small probability, and some may have no probability at all.
This is demonstrated with the famous double split experiment. Electrons are fired at a board with two slits cut in it. If the electrons were only particles, you would expect to detect them in two lines on the other side of each slit. However, because they have this probability wave function, they produce several lines, which is what a wave would do. Waves can interfere with each other. Sometimes they reinforce each other, and sometimes they cancel each other out. This produces a pattern of alternating bands, and electrons produce the same kind of pattern.
Because electrons have this probability wave, they can tunnel through solid barriers. When scientists look at the path an electron might take, the wavefunction can extend into the barrier they are firing the electrons at. The probability of finding the electron along this path will decrease with the thickness of the barrier, but if the barrier is not too thick, there will be a non-zero chance of finding an electron on the other side of the barrier. And, when the electron is on the other side of the barrier, it will not have lost any of its kinetic energy. It will have effectively teleported through a solid object.
Quantum tunneling is important in modern electronics. As transistors have become smaller, their insulating barriers have become thinner, allowing electrons to tunnel through them and create unwanted electrical leakage. This can waste power and generate heat. However, quantum tunneling can also be useful. Flash memory uses tunneling to move electrons through thin insulating barriers, allowing information to be stored or erased.
However, quantum tunneling is vital for something like the sun. The core of the sun is incredibly hot, but that is still not enough kinetic energy to force two positively charged protons together because of the electrical repulsion they have. If they can get close enough, the strong nuclear force can bind them, but getting them close enough for the strong force to take over is impossible because the amount of energy required increases as the particles get closer. At least, it would be impossible if it weren’t for quantum tunneling. Very rarely, quantum tunneling allows two protons to get close enough for a nuclear reaction to occur. In the first step of the Sun’s fusion process, one proton changes into a neutron, forming a heavier type of hydrogen called deuterium. Further reactions eventually produce helium and release energy. Each individual reaction is extremely unlikely, but there are so many protons in the Sun that fusion happens continuously. Without quantum tunneling, the Sun could not produce energy in the way it does, and we would not be here. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Quantum_tunnelling
https://www.quera.com/glossary/quantum-tunneling
https://www.calculatorsoup.com/calculators/physics/kinetic.php
Photo by Nicola Narracci: https://www.pexels.com/photo/visual-representation-of-magnetic-field-lines-38032287/