What Is Quantum Physics?

“Anyone who is not shocked by quantum theory has not understood it” ~ Niels Bohr

I have to agree with Bohr there, not that I am implying that I completely understand quantum theory.

What I do know, is that quantum physics is really quite absurd. Especially when you compare it with what we know and see on a daily basis…so what is it? Quantum Physics is the set of scientific rules that govern the behaviour of matter and energy of particles that are very very small.

The particles in question here are generally the size of atoms or smaller.  The theory is complex and would require more words than I will use here to explain. This is simply intended to be an introduction. If this article intrigues you enough to look further, rather than run screaming, I would be delighted.

We could say that quantum physics explains laws for the atomic world, in the way that relatively does for everyday objects. Despite this, the best way to begin to grasp the concept of quantum theory, is to forget everything you know about classical physics and relativity. Don’t try to relate the behaviour of tiny atoms to the behaviour of matter and energy that you see around you. The rules governing the movements of everyday-life-sized matter relating to their size and speed simply don’t work when you go deeper.

Classical physics makes sense to us, it is what most of us learned at school. We know how long to would take for an apple weighing x grams to reach the floor if it falls from the branch of a tree that is x cms high. We know how quickly ‘car a’ would reach ‘car b’ if they are travelling at ‘speed c’ in opposing directions. When it comes to quantum mechanics you can’t be sure that the apple exists on one particular branch and not another, when maybe it could even be an orange.

In the early part of the 20th Century, as technology improved and allowed for more accurate measurement, many great scientific minds began to dabble in the quantum world of atoms and sub-atomic particles (quantum meaning ‘discrete amount’ or ‘portion’). It became clear that classical physics was subject to a number of limitations. What the early quantum physicists found is still the cause of enormous debate, and many headaches!

Who Discovered Quantum Physics?

Quantum physics, also known as quantum mechanics or quantum theory is really the culmination of concepts and hypotheses from many scientists over the course of a few decades. That said, the first quantum theory to be presented was that of ‘black-body radiation’ by Max Planck in 1900.

His theory is fascinating. When you heat up a black metal poker, why does the colour change? It goes through a series of changes, from black, to red, to yellow, white, and blue as the temperature (thermal radiation) increases. Planck constructed a mathematical model in which the thermal radiation was connected to a particular vibrational frequency (harmonic oscillator).

He proposed that each oscillator produced a specific number of units of energy – i.e. that each temperature related to an exact (or quantised) amount of energy that gave off a specific light. This is opposed to a random, arbitrary amount of energy emitted as the object warms up.

So to summarise, the increase in temperature led to increased emittance of energy and a larger proportion of the energy is focused on the violet end of the light spectrum. The most important part of this is the fact that the energy was quantised – i.e. it increased in specific, set, tiny, incremental amounts. This was groundbreaking.

Einstein’s Input

Planck did not realise the full implications of his theory, but his paper sparked many further hypotheses. At around the same time Albert Einstein had developed his Theory of Relativity (which described the behaviour of objects in motion at high speed). Planck’s work inspired Einstein to look in closer depth at the behaviour of light. He discovered that light could also be quantised and he described the specific-particles-of-light as individual photons. But why did this matter? Einstein’s theory that light could behave as a particle was controversial.

It was in opposition to the current idea that light behaved as a wave, which had been established thanks to decades of experiments in refractiondiffraction and interference, which showed that light tended to move in waves and crests like ripples on a lake, it could be bent, could bounce around corners and more. You may remember some of these experiments with light from school.

After much scepticism, Einstein’s theory was eventually accepted as true, along with the other evidence that it behaved as a wave. This is the second most important thing to take on board. Light can behave as both a particle and a wave. shutterstock_140218453

Development of Quantum Theory

So far we have highlighted 2 important keys to understanding quantum physics:

  1. Sometimes, properties, such as position, speed and colour of matter, can only occur in specific, set QUANTISED amounts.
  2. Light can behave as both a particle and a wave.

This brings us to the third essential element to accept (note I didn’t say understand!).
3. Matter (which was always considered to act only as a particle) can also act as a wave.

Thanks to Einstein’s theory of light wave-particle duality scientists started to consider that matter could also behave in both ways. In 1924, Louis de Broglie showed that indeed, particles are able to exhibit wave-like characteristics, and waves could also exhibit particle-like characteristics.

Many scientists contributed to these theories and provided interpretations of the bizarre results that were showing up time and again. And stranger than ever, it appeared that observing the experiments impacted the outcome. This led to the general acceptance of Quantum Entanglement, otherwise known as Werner Heisenberg’s  1927 Uncertainty Principle. His rule states that the more precisely you measure a particle’s position, the less precisely you will be able to determine its momentum, and vice versa. This is one of the hardest aspects for us to understand.

The best way to describe these 3 cornerstones of the quantum theory is to look at the following 3 infamous experiments.

  • The Double Slit Experiment
  • Schrödinger’s Cat
  • Non-Locality

The Double Slit Experiment

We have discussed this ground-breaking experiment in depth here.

What this experiment finds is that each photon starts out as a single particle and arrives at the detector as a particle.

But it then appears to go through both slits at once, interfere with it’s own path and then place itself perfectly on the detector to add to the overall interference pattern.

This is the conclusion because the interference pattern behind the two slits demonstrates what we would expect to see of waves rather than particles. But how can the photon go through both holes at the same time? If indeed that is what it does. Why does each photon behave differently to the others? How is it decided where the photon will end up?

The experiment found that matter can act as both a wave and a particle depending on whether or not it is being observed. When observed, the photon reverts to behaving like a normal particle and the interference pattern is not produced. It begs the question…do electrons know that they are being watched?

The Copenhagen Interpretation, which is the name given to the generally accepted explanation of this weirdness described the observer-conundrum as the ‘collapse of the wave function’. It is said that the particle does not pass through the two slits, rather a wave of probability does. There is no definite location for the particle, but rather a series of probabilities of where it could end up.

So, an electron that is not being observed doesn’t exist as a particle at all, but rather, as a wave-like entity spanning all areas of probability where it could be located. Once the electron is observed, the wave function collapses and the electron becomes a particle.

Does this mean that observing a particle makes it real? So, that before it is observed it doesn’t exist? Does this mean that something must be watching our universe in order to collapse it’s wave function into the reality that we are experiencing? Hmm, I will leave these answers for another time! Let’s move on to Schrödinger’s Cat.

Schrödinger’s Catshutterstock_227038018

Schrödinger, like many others, did not agree with the Copenhagen Interpretation’s explanation of particles existing in a perpetual state of all possibility until the moment they are observed. He set out to devise a thought experiment that showed the Copenhagen Interpretation to be nonsensical. See if you can follow!

Consider that a box with a lid contains a single electron. According to the Copenhagen Interpretation the electron would exist as a wave of possibility while the lid was closed. It could be in any and all of the possible locations inside the box.

If the box was divided in two halves, the electron could still be in any part of the box. However, once the lid was lifted and the electron was observed, the wave function would be collapsed, and the electron would have to appear in one of the two sides. If the lid was returned the electron would have to remain in the half of the box it had been observed in.

Okay – now we will take it a step further. Imagine that this box has a time controlled half-lid release, and is in a container with a (theoretical) cat and a canister of poisonous gas. The gas would automatically be released if an electron was detected in the container.

When half of the lid is opened there will either be an electron inside or not, it should be a probability of 50:50. So the gas would either be released, or not….and as a result the cat would die…or not. So far so good?

What Schrödinger wants to know is, what is the state of the cat before a conscious observer looks into the container? At what point does the electron decide if it is in the sealed half of the box or the open half? At what point does the canister decide if it detects an electron? At what point does the (theoretical) cat get poisoned or not?

The Copenhagen Interpretation does not allow for a cat that is both dead and alive simultaneously, nor for a cat that is neither dead nor alive. It can only contain either a dead cat or a live cat, until someone looks. And that is the moment at which the actual reality of the situation is determined. Crazy! (But not as crazy as what is coming next….)


Yes, this is even weirder than Schrödinger’s Cat. It is a paradox describing the communication between particles – and unbelievably, it proves that (in the quantum world at least) there exists a speed that is faster than the speed of light!

A pair of atoms, configured in what is described as the singlet state will always have a total angular momentum of zero. This means that they will always spin in equal and opposite amounts. Until they are observed this spin can any of a range of possibilities, but at the moment they are observed they will collapse their probability wave and elect to spin a certain way – one spinning one direction and the other instantly adopting the opposite spin.

This is where it gets strange….. When the particles are separated the exact same thing occurs. The instant the spin of one particle is observed and measured, the other particle in the singlet adopts the opposite spin – instantaneously. There is zero time interval despite the atoms being separated. This is thought to be the case no matter the distance between the two.

In Conclusion…

…there can be no conclusion! At least not yet anyway. What does quantum physics teach us? That there is MUCH that we do not know. The fact that Einstein’s two great theories are mutually exclusive (relativity and non-locality) makes no sense and he spent years trying to find the missing link.

“Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secrets of the Old One. I am at all events convinced that He does not play dice” ~ Albert Einstein

Some scientists have suggested that there are ‘Many Worlds’ that all exist simultaneously, like parallel universes. Each of the myriad possibilities that could happen in every moment do indeed happen, and are playing out in many worlds. Others still believe that we are in a holographic universe, each watching only our version of reality that is projected from our brain.

Life is strong and fragile. It’s a paradox… It’s both things, like quantum physics: It’s a particle and a wave at the same time. It all exists all together ~ Joan Jett

What do I think? I love to contemplate the theories occasionally, it provides welcome relief from the mind-numbing stuff we are fed on a daily basis. But there is no denying the resistance from my common-sense-indicator when I start to go further down the rabbit hole. How about you? Have I succeeded in scratching the surface for you? Or is this a bunch of gibberish? I would love to hear from you!

1 thought on “What Is Quantum Physics?”

  1. WOW. Thank you for such a lucid and friendly explanation! I think I’m ready to delve in at the baby level. You certainly whetted my appetite!


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