“If you think you understand quantum physics, you don’t really understand quantum physics.” We don’t say it. Says Richard Feynman, Nobel Laureate in Physics for his contributions to quantum electrodynamics and one of the most admired scientists of the 20th century. Quantum mechanics studies the laws that govern the world of the very small, of the particles, and the interactions to which the atomic and subatomic structures are exposed. And most of those rules are radically different from the laws we’ve become familiar with in the world we live in. In the macroscopic world.
Feynman and other scientists who have also made important contributions to quantum mechanics have argued vehemently that trying to understand this branch of physics is a futile effort. Its laws are so different from what we are used to observing in the macroscopic world that they are beyond our comprehension. For this reason, it is reasonable to accept them once have been experimentally confirmed. No more. Take them as the laws that describe the behavior of the Universe, and that may not have a purpose. Or maybe yes.
The laws of quantum mechanics are so different from what we are used to observing in the macroscopic world that they are beyond our comprehension. The reasonable thing is to accept them once they have been confirmed experimentally. Without giving them more laps
Accepting the complexity and the ability to permeate our entire world that quantum physics has is possibly the best way to reconcile with this scientific discipline. The three experiments that we are going to talk about in this article perfectly illustrate how unintuitive this field is. And also how exciting it can be if you embrace it by accepting your inability to understand its laws. Possibly in the future we will have to continue to settle for describing them as, in some way, Richard Feynman suggests with another phrase of his that has deservedly passed down to posterity: «You have to be open-minded, but not so much that you let the brain fall to the ground ».
Stern and Gerlach’s experiment
Spin is a quantum quantity. We know this thanks to the experiment carried out by the German physicists Otto Stern and Walther Gerlach in 1922. This research was crucial in consolidating the experimental bases of quantum mechanics and helped us understand that particles have quantum properties. And that, what is even more surprising, when we measure these properties we are altering them by the mere fact of observing them. But we better start at the beginning.
Otto Stern and Walther Gerlach’s experiment was crucial in establishing the experimental bases of quantum mechanics and helped us understand that particles have quantum properties.
What Stern and Gerlach did in their experiment was to launch a beam of silver atoms to make them collide with a screen after they had passed through an inhomogeneous magnetic field generated by a magnet. Silver atoms have a magnetic moment that causes them to interact with the magnetic field, and when observing the screen these physicists realized that some atoms had deviated upwards, and others downwards. But what was really surprising was that the footprint left by the atoms when they hit the screen did not cover all the possible spin values.
There were only two large areas of impact clearly located, so that one of them corresponded to the positive spin, and the other to the negative spin, which clearly reflects that it is a quantum magnitude that it does not have a correspondence in the macroscopic world that we observe in our day to day life. In that case, what is the spin? It is not easy to define it in a way that is easily understandable, but we can imagine it as a characteristic turn of elementary particles on themselves that has a fixed value and that, together with the electric charge, is one of the intrinsic properties of these particles.
The electron, which has spin 1/2, has to go around itself two times to regain its original position. This feature is highly unintuitive, but even less so is the fact that when measuring the spin of a particle on an axis information is automatically destroyed of the measure on any other axis. Why? Simply because the laws of atomic and subatomic systems dictate so. As Feynman reminds us, it is best to assume that nature behaves in this way and not make vain efforts to try to understand what this behavior obeys.
The quantum Zeno effect
The name of this phenomenon is due to Zeno of Elea, a Greek philosopher of the 5th century BC. C. disciple of Parmenides, and was described for the first time by Alan Turing, the English mathematician who established the foundations of algorithm and artificial intelligence, among other achievements for which it has deservedly gone down in history. Turing realized that if you observe a quantum state you delay its evolution in time, so that if you observe it an infinite number of times will remain in that same state indefinitely. Again we are facing an absolutely counterintuitive phenomenon, which, despite how strange it is, has been experimentally tested many times.
Alan Turing established that if you observe a quantum state an infinite number of times it will remain in that same state indefinitely
Interestingly, this phenomenon plays an essential role in the operation of quantum computers. To understand why we need to review the principle of superposition of states, which argues that in a quantum processor of n qubits a concrete state of the machine is a combination of all the possible collections of n ones and zeros. Each of these possible collections has a probability that tells us, in some way, how much of that particular collection there is in the internal state of the machine, which is determined by the combination of all the possible collections in a specific proportion indicated by the probability of each of them.
As you can see, it is a complex subject, but we have not yet reached the most interesting part: the quantum superposition effect only remains until the moment we measure the value of a qubit. When we carry out this operation, the superposition collapses and the qubit adopts a single value, which will be 0 or 1. This is how quantum computers work, without going into even more complicated details. The collapse of the qubit state was described by Alan Turing long before the invention of these machines, reflecting the immense legacy that this colossal scientist has left us. If you want to investigate and know in more detail how quantum computers work, I suggest you take a look at the article that I link here.
Thomas Young’s Double Slit
The double slit experiment was designed by the English scientist Thomas Young in 1801 with the purpose of finding out if light had a wave nature, or if, on the contrary, it was made up of particles. The result he obtained at that time led him to believe that, as Hooke and Huygens had predicted long before, light was made up of waves. What Young could not imagine is that many years later, at the beginning of the 20th century, his experiment would be repeated many times to prove wave-particle duality, which is one of the fundamental principles of quantum mechanics.
The double slit experiment is used to demonstrate the wave nature of light and the wave-particle duality of matter.
This quantum phenomenon has been empirically demonstrated countless times, and reveals that there is no fundamental difference between particles and waves; particles can exhibit the same wave behavior in some experiments, and preserve their discrete nature in others. Throughout the twentieth century Young’s experiment has been refined little by little, and for decades scientists have been so convinced that the wave nature of light as of the wave-particle duality of matter.
In its most sophisticated form, the double-slit experiment consists of launching a succession of electrons (although protons or neutrons can also be used) towards a screen, but in such a way that a sheet is interposed between the source of electrons and the screen in which Two very fine slits have previously been made. By launching the electrons into the slits one by one and subsequently analyzing which area of the screen they impacted, the scientists have verified that each electron passed through both slits simultaneously, which shows, in effect, that they are behaving as if they were waves. In his original experiment, Thomas Young used a beam of light instead of electrons, but the interference pattern he got on the screen was essentially the same as what modern scientists get from using electrons or other particles.
The best comes right at the end. If you have found what we have seen so far throughout this article surprising, get ready. If we place an instrument behind the double slit that allows us to measure which of them each electron passes through, the interference pattern disappears. This means that the moment we decide to measure through which slit an electron passes, it stops behaving like a wave, and starts behaving like a particle. At that time we verify that it passes only through one crack, and not through both. Somehow we have eliminated the quantum effect.
However, the most unlikely thing is that it does not matter when we decide to carry out the measure. If we use the instrument to check through which slit a particle has passed long after it has done so and has impacted on the screen, the quantum effect is also eliminated, so we are altering something that has happened before, and that describes the way the particle has moved towards the screen. You see, Richard Feynman was right. It is preferable that we accept that quantum mechanics works that way because that is what experiments tell us. It is useless to dwell on this matter further.
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