Since the very first theories we had about light, thousands of years ago, our view has
changed dramatically.
First, ancient humans thought our eyes shoot beams that hit objects and that this was responsible
for the sense of sight.
Then we thought, hmmm... maybe objects emit light.
But that wasn't the case either.
Later on, while studying light rays, we figured that instead of light being emitted by objects,
light rays came from the Sun and bounced from objects to our eyes.
And we were right.
Light is pretty strange.
It can look like a particle and a wave, depending on how you look at it.
No pun intended.
In the seventeenth century, Isaac Newton decided that light was made up of particles, while
his rival, Robert Hooke, thought of light as a wave.
Both of them based their theories on observations of light's behaviors: reflection and refraction.
During that time, people were familiar with the reflection of light from a mirror for
example, but not many quite understood what refraction meant.
Refraction is when light travels through a medium, like water, leading to the bending
of light.
However, Newton and Hooke were both right.
Kinda.
Well, I told you, it's weird.
In the 19th century, British polymath Thomas Young designed a genius idea, known as the
double-slit experiment, to see whether light behaved as a particle or a wave.
When Young did the experiment, he showed that light that passed through two slits resulted
in an interference pattern on the detector screen.
Thus, he found that light behaved like a wave, and not like a particle as Newton had suggested.
But not just light, any wave, whether it's a sound wave or waves across the water, can
create an interference pattern.
This experiment, however, is not that easy to make.
If you want to see diffraction, the slits should be in the order of the wavelength of
the light used.
For visible light, that means the slits should be on the order of less than 1 micrometer,
or one-millionth of a meter.
So light is a wave, right!?
Not exactly.
And here comes the mind-blowing part of this experiment.
When particles such as photons or electrons go through the two slits, they create the
same interference pattern on the back wall as waves do.
But when they are observed, they cast a two-slit pattern resembling particles.
Isn't that crazy?
The electrons appear to act like waves when you're not observing them, but as particles
when you are.
It's like the electron knows that we are watching it.
How?
No one knows.
And if you find out, please let me know in the comments.
This shows that at a quantum level, light and matter can exhibit both particle-like
and wave-like properties.
Light is made of quanta called photons.
These quantum objects follow the rules of quantum mechanics, so they can behave as particles
and waves.
In the quantum field theory, objects are not particles but fields.
Particles are unit excitations of these fields.
The lowest energy state of a field is when it thas no excitations at all.
This can't happen in the universe we know.
Photons are excitations of the electromagnetic field.
For a photon to exist, it takes a tremendous amount of energy.
If there's no excitation in the field, there's no photon.
Photons constantly vibrate and have momentum.
We are slowly starting to understand that particles don't even exist.
There are no particles, only fields.
And that's when string theory comes in.
The string theory completely changes the standard description of the universe by replacing all
matter and force particles with tiny vibrating strings that twist and turn in complicated
ways.
These strings are what appear to us as particles.
Depending on the way they twist and turn they gain the properties of different elements.
For example, a string vibrating at a certain frequency can manifest itself as a photon,
another string vibrating with a different frequency can gain the properties of a quark,
and so on.
Everything is made up of these tiny vibrating strings.
Okay, take a deep breath... and let's get back to light.
As you see, very tiny objects do not obey the same rules as everyday objects.
One misconception about light is that it is made of photons.
It doesn't necessarily mean that a collection of photons makes up light.
Just one photon can make light.
Light is made of electromagnetic waves in a quantum field, and this is described by
the quantum electrodynamics, a quantum field theory which describes how light and matter
interact.
An electromagnetic wave may contain a group of photons or even just one.
A single energy-step.
As you see, light and quantum particles act in very strange ways, and I am looking forward
to more experiments in the future.
But one thing is for sure, quantum mechanics is poised to take over technologies of
the 21st century.