Lesson 15: Quantum Physics III

So, what have we learned so far?

We learned that Max Plank solved the Ultraviolet Catastrophe by showing energy from electromagnetic waves comes in discreet, undividable chunks of energy called Quanta, the plural of Quantum. We mentioned how a lightning bolt looks like a continuous stream of energy, but it is really chunky.  It is just that the chunks are too small for us to see, so it looks continuous.

Then we saw Einstein introduce photons to the world.  He said light was made of photons which were physical particles, not waves.  He showed this with the photoelectric effect by shining light on metal plates, and the photons knocked off electrons like they were playing a game of pool.

After that, things only got weirder.

The double-slit experiment was performed, and it seemed light could be both a wave and a particle, even though that makes no sense.  When you try to observe photons, they are particles.  When you aren’t looking, they are waves.

We now need to introduce the next major player in Quantum Physics.  He was a Danish physicist, and he helped to revolutionize how we see the atom today.

Let’s go back for a minute and see where we left off with the atom.

If you remember, we said it was JJ Thompson who created the first model of the atom, which was called the plum pudding model.  The model had electrons embedded in a plum pudding gelatinous structure which has a positive charge and cancels the negative charge of the electrons making the atom neutral.

Then Rutherford developed the solar system model of the atom.  He said the electrons are in orbit around the nucleus. He also showed electrons were very far away from the nucleus.  This is how most of us picture an atom.  It was what we were shown in our school books. It is wrong!

But remember, we also said there was a problem with the model.  Using electromagnetic calculations (notice how everything always comes down to electromagnetism), electrons should run out of energy and crash into the nucleus of the atom.  Atoms should not exist.  We should not exist.  Nothing should exist.

It was Niels Bohr who helped save the day.  He moved from Denmark to England to work with JJ Thompson.  Bohr and Thompson did not get along with each other.  The problem probably stemmed from the fact that Bohr kept telling Thompson his model of the atom was wrong.

So, Bohr went to work as an assistant for Rutherford and helped to further develop the concept of the atom.  He quantized the atom.

The first thing he did, unlike with Thompson, agreed with Rutherford.  He said electrons do orbit around the nucleus of the atom just like the moon orbits the earth.  They, too, use angular momentum to stay in orbit and keep from crashing into the nucleus.

Second, Bohr said there are fixed orbits at specific distances from an atom’s nucleus. Bohr said electrons don’t give off energy when in orbit but do give off energy when they “leap” from one orbit to the other.  Hence the familiar term Quantum Leap.  But do not picture the electron physically jumping from one orbit to the other.  Picture it disappearing from one orbit and then suddenly reappearing in another.  So, where does the electron go when it disappears?  It goes nowhere.  It is just in one place and then in another.

When the electron makes this leap, it either absorbs or emits energy. It emits energy (a photon) when it drops to a lower orbital level and absorbs energy (a photon) when it jumps to a higher level. Guess what determines how much energy.  E=hf.  Look familiar.  The electron gives or receives a quanta of energy, and the amount of energy can be determined by Plank’s formula.

Bohr’s work also led him to develop the Principle of Complementarity.  The principle states that you can’t define one thing without referring to its opposite.  You can’t have good without bad or up without down.  We will talk a lot more about Bohr and his views in upcoming lessons.  His interpretation of Quantum Physics was not mainstream, to say the least.

Just about 10 years after Bohr developed his model of the atom, a French prince named Louis de Broglie came up with the crazy but correct conclusion that the electrons orbiting atoms were waves.  He was influenced by Einstein’s relativity and especially his great equation E=mc2.

He was led to the conclusion that is the mother of all weirdness of Quantum Physics.  Are you ready?

Not only does light behave as a particle and a wave, but all matter behaves as a particle and a wave.

No.  Just no.  That can’t be.


Matter and energy are the same. Therefore, if light (which is energy) can be both a particle and wave, then so can matter.

De Broglie even came up with an equation to calculate the wavelength of a particle of matter.

wavelength = h/mv

The wavelength of a particle is determined by its Plank’s Constant (h) divided by the mass (m) of the particle times its velocity (v).  There is Plank’s Constant once again.  No wonder he was called the grandfather of Quantum Physics.

Really think about this.  You are made of matter.  Are you sometimes a particle and sometimes a wave?

The proof finally came by doing a different version of the double-slit experiment. 

You will recall the first experiment involved sending photons of light through two slits, and depending on whether or not the experiment was observed, the photon was either a wave (if not observed) or a particle (if observed).  They could tell it was a wave because it displayed a wave interference pattern.

In the newer experiment, they had the same setup, but this time they sent electrons (matter) through the slit.  Again they sent one electron at a time. 

The results…

Exactly the same.

If the experiment was observed, the electron was a particle.  If the electron was not observed, it was a wave.  The wave interference pattern emerged.

How can you get a wave interference pattern from an electron?  An electron is physical matter.

If you have not heard this before, your life has just been forever changed.

Everything you see in the universe is made of electrons.  So, does that mean if no one is looking, the moon is not there, or if no one is in your kitchen, is it just a bunch of waves?

We have no problem saying there is no sound, only waves if no one is there to listen.  Should it not be the same with sight?

You have no problem with the fact that the sounds and images streaming on your smartphone are just waves until the phone converts the waves into what you see and hear.  Should it not be the same with your brain? Is your brain not way more powerful than a phone?

You are made of electrons.  You have about 1000000000000000000000000 electrons in your body. You see your physical body but are you sometimes a wave? Okay, we are going to stop here.  You need to process this before we move on.  And yes, there is more.  Just wait and see.  Right now, you have a lot to think about.  Seriously give this some consideration.  What is your mind saying to you right now?

I’m curious to hear about reactions from others when you tell them what you just learned.  Let me know how it goes.  Draw that pic too!