# The Developments in Quantum Mechanics

FROM THE LECTURE SERIES: Redefining Reality: The Intellectual Implications of Modern Science

## The theory of Quantum Mechanics was borne out of the study of very small particles. Quantum Field Theory, its successor, was developed from the study of the interactions of these particles. What were these studies and interactions that would change our matter worldview from that of a deterministic one to a probabilistic one?

### Louis de Broglie and Particle Wave Duality

As a young researcher, French physicist Louis de Broglie came to realize something quite straightforward, yet incredibly deep. In 1924, the air was full of talk about the two major discoveries in science. One was Planck’s quantization of energy from blackbodies, such that for a single photon E = hυ.

This was the basis for the wave-particle duality of light. Light could be considered a particle or a wave and there were physical tests that demonstrated both aspects.

The other was Einstein’s theory of relativity according to which E = mc2. Mass, Einstein famously said, was just another form of energy and as with other forms, there should be ways to convert this energy back and forth, creating and destroying mass, as long as the energy in total is conserved.

De Broglie thought, energy is energy, it’s something that just changes form. We can turn electricity into heat, heat into chemical reactions such as making water into steam. Steam can then be used to power an engine that does mechanical work. Mechanical work can lift an object giving rise to gravitational potential energy, and so on. So, energy is energy, it moves from form to form.

Planck says that E = hυ, and Einstein says that E = mc2. E, the energy, can change from form to form, so then it ought to necessarily follow from E = hυ and E = mc2 that hυ = mc2.

It seems like simple middle school algebra. But the simplicity hides deep ramifications. In the middle of hυ = mc2 is an equal sign, and on the left is υ or frequency. Only waves have frequencies. On the right is m or mass. Only particles have mass.

Again, particles and waves seemed like completely different things. A particle is an independent object, an entity unto itself. It doesn’t need anything else to exist. It is its own reality. A wave, on the other hand, is a disturbance in a medium. When it appears nothing new has been created and when it peters out nothing has been destroyed. Everything that had existed still exists. The contents of reality are unchanged whether there is a wave or not.

What de Broglie did by joining the two iconic equations of the turn of the 20th century was to assert that the strange particle-wave duality we attributed to light would also hold true for matter. Matter, if this were true, would have to be thought of as a wave.

But waves aren’t things with independent reality. A pen similarly would not be a thing, but a wave in some underlying medium. And it’s not just the pen, but our body, the sun, everything would cease to be independent elements of reality.

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

Einstein was impressed with de Broglie’s dissertation. And with Einstein’s backing, this short simple idea not only earned de Broglie his Ph.D., but a few years later it also got him the Nobel Prize and led physics into a completely new direction.

### Quantum Decoherence

This gave rise to what physicists call quantum decoherence. If I have two electrons that bounce off each other, then classically we think of the interaction like a game of billiards. But in quantum mechanics, all we can know is that two electrons went into the interaction and two came out. There is no sense of identity throughout the collision.

If Ted and Carol were the electrons going in, there is no way to say which is Ted and which is Carol at the time of coming out. There is no identity here. Two go in, two come out, and that’s all we can know.

The problem, of course, is that self-identity is the most basic property of thinghood. Something is an entity, is a real object, if it retains its identity over time. If we surrender identity over time, then we give up on the existence of things. And surely reality is made of things. Right?

Following de Broglie’s result, Rutherford showed that the atom was largely open space with virtually all of the mass focused in the central nucleus. Since atoms were electrically neutral and electrons were negatively charged, the nucleus must not only be massive, but also positively charged due to protons.

### Introduction of New Particles and Forces: Neutrinos and Nuclear Forces

There was something else heavy inside the atom. Since the mass of the atom is in the nucleus and all isotopes are electrically neutral, there had to be something in the nucleus adding the extra mass. Something that contributed to mass but not charge, and we called them neutrons.

Eventually, the neutrons were isolated and we observed them decaying. What popped out? An electron and a proton. Electrons are negative, protons are positive, and if they get too close, then the charge ought to bring them together into a single electrically neutral object. The combined mass matched what the proton had to weigh given the atomic masses of the various isotopes, but there was a problem.

We now had conservation of charge, energy, momentum, but not what physicists call angular momentum, a measure of spin, which had to remain constant overall. So, a third component was proposed to account for the discrepancies before and after the decay, a piece with no charge and virtually no mass. The Italian physicist Enrico Fermi called it a tiny neutral thing in Italian neutrino.

The neutrino complicated matters. It suggested that the subatomic realm was more complicated than the heavy-positive/light-negative image of the atomic world.

Here’s where new problems emerged. If the nucleus is as small as we found it to be, and it is populated by protons which share the same positive charge jammed together tightly, how does it remain stable?

Like charges repel and the closer they get, the stronger the repulsion. Nuclei should blow themselves apart because of the electrostatic force between the protons. The only forces we knew of were gravity and electromagnetism, and neither could explain it. The gravitational force was way too small and there should be no electrical attraction. There must be some other force.

Another problem was that the electrical charge explained how the proton and the electrons stick together in making a neutron, but what about the neutrino? There has to be some sort of force that acts on neutrino, and it would have to be different from electrical charges and gravity since it is neutral and virtually has no mass.

So, there must be some other forces active only at minuscule scales, but incredibly powerful. The force that binds protons to protons would have to overcome the electrostatic force. It must be the stronger of the two, and it should be active only in the nucleus since we see it nowhere else. So this was called the strong nuclear force. And the force that exposes itself when the neutron breaks down to spit out a proton, electron, and neutrino, we call it the weak nuclear force.

So, what started out as a simple picture of reality with two forces––gravity and electromagnetism––and two basic components––electrons and protons––suddenly exploded. We needed more forces and more particles. What had been a neat, tidy little universe was getting messier all the time.

### Common Questions About the Developments in Quantum Mechanics

Q: What are the four quantum mechanics?

The four main phenomena of quantum physics are quantum entanglement, principle of uncertainty, quantization of certain physical properties, and wave-particle duality.

Q: What is quantum effect?

The quantum effect in general terms is considered as any consequence that arises due to the wave-particle duality.

Q: Does quantum field exist?

Yes, the quantum field exists and is considered as the main substratum while the particles, according to the quantum field theory, are considered as mere excitations of the field.

Q: What is the difference between quantum mechanics and quantum field theory?

The difference between quantum mechanics and quantum field theory is that quantum mechanics is the study of very small particles, whereas quantum field theory is the study of the interaction of these particles.