Quantum Physics for Dummies
Quantum Mechanics studies the smallest stuff in the universe. You might know these as the parts of the atom: protons, neutrons, and electrons.
When scientists look at the tiniest stuff in the universe, things begin to act really weird. Contradictory things seem to happen at the same time. Things become uncertain. We have to use probability, making weird guesses about the bizarre behavior of particles. Things appear to be in two places at the same time, or teleport from one location to another. At the smallest level of everything, the world seems to turn upside down. All stability seems to break down.
Many people think quantum mechanics doesn't make sense. Is quantum physics a mystery? Can we ultimately make sense of it? Do quantum mechanics violate the laws of physics?
The Theory of Quantum Mechanics Works!
It might surprise you that quantum mechanics is actually one of the strongest scientific theories in history. We actually know a great deal about what's going with the smallest particles in the universe. The problem is, when we investigate things at the quantum level, what we find seems incredibly weird and counter-intuitive. This doesn't mean that we don't know what's going on there. It means that what's going on there doesn't make sense to us.
The History of Quantum Mechanics
Quantum mechanics won't make sense without understanding the history. This history begins with Max Planck, continues with Albert Einstein and Niels Bohr, and finally cumulating in the current theory with scientists like Schrödinger and Heisenberg. This history will help us learn why scientists think that the universe is fundamentally weird. For a more complicated version of this history, the University of Pittsburgh offers an excellent overview on their website.
Max Planck and Black Body Radiation
Have you ever wondered why fire sometimes looks blue? This happens because of something called Black Body Radiation. Physics tells us that colors are caused by different wavelengths of light. Black Body Radiation describes how something changes color as it gets hot. When things get hot, the light tends to change in this pattern:
Now because of the law of conservation of energy, physicists generally thought that the light would keep climbing the hotter something gets. If things change color the hotter they get, you'd think that blue fire would turn into ultraviolet, right?
The Ultraviolet Catastrophe
As it turns out, the wavelengths undergo a huge drop after reaching the blue spectrum. This was called the "ultraviolet catastrophe."
Max Planck could only make sense of this one way. He hypothesized that the energy jumps from one discrete wavelength to the next. So basically, this says that the energy moves from position 1 to position 2 without passing through positions 1.3, 1.4, 1.7 and so on.
Planck's Constant: Energy Jumps from Place to Place
Planck identified a number called Planck's Constant. Whenever the energy "jumps," it does so in multiples of Planck's number. The number is incredibly small, so you would never notice the leaps. But this means that there's nothing in between these different jumps! It jumps from 1 x Planck's Constant to 2 x Planck's Constant with nothing but empty space between the jumps. It took some time, however, before scientists realized how weird things actually are.
Albert Einstein is best known for his theory of relativity. This, however, wasn't what made Einstein famous. It was actually his work on the Photoelectric Effect that won him the Nobel Prize in physics. This has to do with the properties of light.
Maxwell's Light Experiment: Light moves like a wave!
Previously, physicists had thought that light behaved like a wave. Think of a wave like a ripple in a pond. If there are two different ripples next to each other, when the ripples meet, they will cancel each other out. In the 19th century, Maxwell proved that light works the same way.
Einstein's Genius: Light is both Waves and Particles!
What Albert Einstein realized was that light is actually composed of tiny particles, much like Isaac Newton and others had thought in the 18th century. However, Einstein came up with the weird idea that the particles actually behave like waves.
Picturing Light Photons as Waves:
Imagine that you have a light source outside a cardboard box. You drill a hole in the cardboard box, and have a sheet on the other side that detects the light. Now, if you look at the sheet inside the cardboard box, you will see that the light passes through the hole and spreads out. In this way, light behaves a lot like a wave.
But what would happen if you made the light extremely dim? What if instead of a light source, you just had one tiny piece of light: a particle called a photon. The photon, it would seem, would simply travel through the slit and strike the other side opposite the hole, right?
Except this isn't what happens. The photon actually flies in a seemingly random direction each time. This seems to defy the way that Newton said motion should work. However, if you let photons pass through one at a time, observing the random locations that each one lands, after doing this for a while, all of the photons would start to look like the pattern of a wave.
Photons Follow Planck's Constant
At the same time, Einstein saw that photons always fall into discrete units of energy. Guess what units they fall into? That's right! The number Planck discovered called Planck's Constant. For this reason, Einstein concluded that each photon behaves like part of a wave. It's as if each water droplet in a wave in the ocean acted like it was still part of the wave crashing on the shore, and a different part of that wave each time!
Niels Bohr and the Atom
Quantum mechanics is a theory of matter, which means it's all about atoms. The early model for figuring out the structure of an atom was discovered by Niels Bohr. If you have ever seen a picture of an atom with protons, neutrons, and electrons, you've seen the Bohr model. Bohr found out what Einstein already suspected. The electrons orbiting a nucleus can only orbit in discrete positions. They leap from one position to another, skipping the space in between.
It's as if planets could only orbit at the location of Earth or Venus, but it were physically impossible for there to be a planet between the Earth and Venus. Even more crazy: that there was literally nothing between the Earth and Venus!
What were the places where the electrons were allowed to orbit? You guessed it: Planck's Constant. The electron orbits followed the same kind of pattern Planck had found with blackbody radiation.
The New Theory of Quantum Mechanics
So far we have looked at what's called the "Old Theory" of Quantum Mechanics. This theory sees the electrons as only happening at specific positions with no space in between. However, the so-called New Theory of Quantum Mechanics makes things even weirder. Basically, it combines the ideas of Einstein with the ideas of Bohr. Instead of saying that the electrons orbit at an exact position, jumping from one orbit to the next, the New Theory of Quantum Mechanics says that their movements are way crazier.
Electrons move like waves, much like Einstein said about light.
This discovery was made by physicists like Schrödinger, Heisenberg, and de Broglie. They all had different theories, but scientists quickly realized that they were all saying the same things with different math. This means that at the bottom of everything, matter (made of electrons and photons) moves in crazy and unpredictable ways. After a while, the tiny bits of matter start to form a wave like pattern, but if you just look at a single "quanta" (that is, a single electron or proton), their movements can be completely crazy and unpredictable.
So given how random and wacky particles behave, how should we understand our universe? Einstein refused to accept that the universe was random. In a debate with Niels Bohr, Einstein famously said: "God does not play with dice." Niels Bohr replied: "Don't tell God what to do."
Quantum Mechanics: An Incomplete Universe
So then how should we make sense of quantum mechanics? Here's one way to visualize what this means:
Quantum Mechanics: The Glitch in the Universe
Imagine that you are playing a video game. Inside the game, there's a town where you walk around. In the town are houses that make up the village, but you can't actually go into the houses. Because you can't go inside of the houses, you don't know what's inside of them. Who lives there? What kind of furniture is in there? What are the floors like?
But imagine that there's a glitch in the video game that lets you go inside the house. Now, you can learn what's inside the house. However, you discover that the programmers of the game didn't think you would ever go into the house, so they didn't actually put anything in it. You find nothing but empty space in the house. No one lives inside, there's no furniture, and nothing on the floor.
In this case, it's not that we can't know what's inside the house. The glitch lets us go inside. However, there's nothing inside the house, because it wasn't programmed. This means that we know what's in the house, but the stuff inside the house doesn't make sense.
If the universe were a video game, quantum mechanics would be like discovering a glitch in the universe.We can look at what's happening at the smallest level, but what we find there (like the empty house) doesn't make sense.
It's not that we can't make sense of quantum mechanics; it's that quantum mechanics shows that the universe doesn't make sense!