For millennia, humanity has gazed in awe at the sky, hoping to discover its secrets. We have created a sophisticated set of tools and cognitive capacities to forge explanations for daily events, and we have accomplished this with limited success: we comprehend why the stars shine, how our bodies work, and even what the minuscule germs and viruses that cause us diseases are.

However, when we reduce the size at which we observe the universe to a very small scale, we enter the quantum realm: a world that is almost unpredictable, counterintuitive, and bewildering. Light and matter exhibit their exotic, contradictory and unprecedented nature: they are a wave and a particle at the same time.

The contradictory nature of light

Isaac Newton is remembered and recognized for his eminent contributions to physics and modern science. He developed an impressive description of gravity and the mathematical method behind it: calculus. He also spoke several languages, played instruments, and spent his life thinking and coming up with revolutionary ideas in every field of thought, from mathematics and physics to theology and alchemy.

It was Newton himself who, based on his well-known experiments with light, conjectured that it was made up of particles that behaved similarly to a fluid. Meanwhile, the Dutch physicist Christian Huygens, a contemporary of the Englishman, proposed the controversial idea that light was actually a wave propagating through a hypothetical transparent medium: the ether.

Much to his chagrin, Newton lost the battle, and the scientific community accepted Huygens’s wave model since it explained and predicted more phenomena than Newton’s corpuscular theory.

However, almost three centuries of experiments and tests failed to prove the existence of the ether. It was Albert Einstein who proposed at the beginning of the 19th century that light propagates in a vacuum, without the need for a medium such as the elusive ghostly ether.

Newton was wrong, but he was also right

In 1905, Einstein published the scientific work that would earn him the Nobel Prize. In his article, the celebrated German physicist describes what is known as the photoelectric effect: a phenomenon by which light electrically charges metal. To come up with a coherent explanation, Einstein once again proposes that light is made up of particles (photons). So these light particles would hit the metal with a lot of energy, managing to tear off one of its electrons.

However, Einstein’s suggestion differed from Newton’s in that the first suggested a wild idea: light was composed of particles, but it was still composed of waves. It has a dual behavior: it is both things at the same time. The counter-intuitive nature of this notion sparked outrage in the community, but after the photoelectric effect was empirically confirmed, there was nothing more to do than put Einstein at the forefront of science and give him his well-deserved Nobel.

Wait… electrons also?

In 1924, when Einstein’s wave-particle duality was already digested by the scientific community, the French physicist Louis de Broglie was trying to explain an atomic phenomenon proposed by Niels Bohr years ago: the quantum leap of the electron. The enigma was the electron behavior: it seemed to jump from one atomic orbital to the next without traversing the space in between, disappearing from one orbital and appearing in another as if it were teleporting.

It occurred to de Broglie to borrow the dual model of light from his German colleague to apply to electrons. This is how he found the solution to Bohr’s problem: electrons did not “appear and disappear” because they did not have a fixed position. If the electrons were considered as a vibrating loop (a standing wave) and it was assumed that their position is not punctual but that it is distributed around the loop, then it was not necessary to resort to quantum teleportation of the electrons to explain their jumps. Although this idea is not much less crazy than the previous one, it fits perfectly with the equations of Bohr’s atomic model.

In this way, Einstein and de Broglie generalized the wave-particle duality also for electrons. Now the dual nature of light was also present in the fundamental building blocks of matter. So after all it’s not crazy to think that everything made of electrons, including us, is fundamentally made of waves.

Don’t look at it!

The dual approach of light and electrons is extremely useful for experiments and for developing new technologies. However pragmatically sound the model may be, scientists cannot turn a blind eye to the conflicting problem that looms behind its marvelous features: can something be one thing and another thing at the same time? Is duality indeed a physical reality, or does it appear as such as a consequence of our inability to understand phenomena at a fundamental level?

These questions and many others arise for physicists when they do experiments with light and electrons because depending on what they ask, it is what they observe. Newton was looking for particles in light, and that’s what he found. De Broglie needed waves, and that’s what the electrons gave him. Does our interaction with quantum systems change their behavior? Are electrons and photons complacent entities that want to look good to all observers? Can our intention somehow modify the observed results?

Should modern physics intersect with philosophy and metaphysics to answer these unknowns, or will the development of human knowledge and intuition end up with a coherent interpretation of the reality of the quantum world and with it, of all reality?