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Let me give you something to ponder.
Look around the room that you’re in.
What you are seeing is not matter.
You can’t really see matter.
What you are seeing is the light bouncing off that matter that enters your eyes, that
your brain then interprets as matter, as objects.
So you are not seeing the objects themselves but the light bouncing off of it.
Nearly all the information that you have about your room is carried by tiny quantum objects
that make up that light called photons.
And this information, needless to say, is traveling at the speed of light.
If light did not exist, you would not be able to gather any visual information about the
universe.
The entire universe would be completely dark to you.
But it would be much worse than that because you also would not exist without light. In
fact, no life as we know it would exist.
Light plays a crucial role in the existence universe.
We are going to take a closer look at what that is.
What is the purpose of light, how does it work, and why does it exist?
That’s coming up, right now!
There had been a controversy regarding whether the true nature of light was particles or
waves.
In the 17th century, many Scientists like Dutch astronomer Christiaan Huygens thought
that light was a wave vibrating in some kind of aether. Aether was
presumed to exist because all waves were thought to need some kind of medium.
But around the same time, other scientists, most prominently Isaac Newton, believed that
light was composed of a stream of corpuscles, which are little particles that travel at
a finite speed.
About 100 years later in 1801, Thomas Young’s double slit experiment seemed to show that
light must be a wave because when a beam of light was split through a double slit, the
resulting overlapping beams of light created an interference pattern.
This phenomenon could only be explained if light was a wave.
However, about 100 year after Young’s experiment, and 200 years after Newton’s corpuscular
theory, none other than Albert Einstein showed that wave behavior or not, light must come
in discrete packets of energy which today we call photons, because that’s the only
way to explain the photoelectric effect.
The photoelectric effect is when electrons are ejected from the surface of a metal when
a certain frequency of light is shone on it.
Whether the electron is ejected, is not affected by the intensity of the light, only its frequency.
If light were a classical wave, the energy of the light would depend on its intensity,
not its frequency.
What Einstein showed is that the energy of light comes in discrete quanta defined by
Planck’s equation, Energy equals that Plank's constant multiplied by its frequency.
In other words, light consists of photons with the energy of each determined by its
frequency.
So light has characteristics of both waves and particles.
Today we call this phenomenon wave-particle duality.
But it is a bit unfortunate though that this wave particle duality is taught in schools
today.
It is ok as a stepping stone to get an intuitive understanding, but it is really not the correct
explanation.
It is more a statement of the problem to say that light appears to behave like a particle
and a wave.
The reality is that it is neither, it is a different kind of thing altogether.
It is a type of quantum object.
And it behaves as all quantum objects do – it obeys the Schrodinger equation, which has
something called a wave function in it.
This function does not show quantum objects as waves in the traditional sense of waves,
but that their quantum states such as position, momentum, spin etc. are in flux, similar to
the way that the amplitude of a classical wave is in flux.
So, if anything, a quantum object such as a photon is a wave of probability, the probability
of finding it at any particular location if we look there.
But about 50 years before these developments with quantum mechanics, around 1865, James
Clerk Maxwell came up with his laws of electromagnetism, which revealed that light was an electromagnetic
wave.
And that its speed could be determined precisely by two constants which are part of electromagnetic
theory – the dielectric constant and magnetic constant, which are now also known as the
permittivity and permeability of free space.
This realization that the speed of light is determined by the innate property of empty
space led Albert Einstein to hypothesize that the speed of light in space is the same for
every reference frame.
And this is the essential postulate which led to the theory of Relativity.
So light and the discovery of its properties played an important role in development of
both quantum mechanics and Relativity – which together are the best understanding we have
today of how the universe works.
But we are not done yet, because light is even more profound in the role that it plays
in our universe.
It is an essential way that our universe uses to transfer energy from one thing to
another.
What do I mean by this?
Let’s take the sun.
How does it shine?
Where does the light come from?
In the core of the sun, 4 Hydrogen atoms combine to form a helium nucleus through a process
called fusion.
This process releases a lot of energy because a single Helium nucleus is more stable and
is in a lower energy state than the 4 hydrogen nuclei used to create it.
Much of this energy released is in the form of gamma rays, which are very high frequency
or high energy photons.
These gamma rays however never reach us because their energy is absorbed in the dense plasma
of the sun.
Although these photons are traveling at the speed of light, it still takes them thousands
of years to reach the surface of the sun due to the random motions they experience within
the core of the sun, such that by the time they reach the surface, their energies are
down in the visible light range.
So this describes the origin of light from the sun, but what about artificial sources
of light that we create on earth, such as from a streetlight, or lamps that you turn
on in your home at night, or fire?
How are photons produced in these processes?
Almost all the light created on earth is the result of electrons collapsing to lower orbits
in an atom, after they have been excited by some source of energy into a higher energy
state.
In the atom, electrons are in specific discrete orbits which can be roughly calculated with
the help of the Schrödinger equation.
These electrons can be excited to a higher orbit when absorbing energy, often it is excited
by a photon.
Now you have an atom with one or more electrons in some excited state, thus a higher energy
state than the lowest energy state that it could be in.
But since all systems in nature prefer to be in their lowest state, this excited electron
will eventually move back into a lower orbit.
And in this process, the excess energy is released as a photon.
Almost all artificial light on earth is produced by the process of electrons dropping to a
lower energy state.
The importance of light is not only evident in what we can see, but also what we can feel.
What do I mean by this?
The theory of quantum electrodynamics developed in the 1940s by Richard Feynman and others,
revealed that the electromagnetic force is mediated by photons.
In other words, when like charges repel each other, as in magnets or two electrons, the
particles carrying this force are virtual forms of photons.
The same is true for the attractive force such as that between protons and electrons,
or two opposite poles of magnets.
Virtual photons are carrying that force too.
If you want to know exactly how this mechanism works, I have a video on that up here.
For now, let me just say that a significant number of forces you feel would not be possible
without photons.
So for example, the friction between your shoes and the ground which allows you to walk,
or friction between your car tires and the road is fundamentally a product of electromagnetism.
This is because friction is caused by the electromagnetic interaction between the atoms
of the two contact surfaces.
Without light, you wouldn’t feel anything either, because touch is a result of electromagnetic
interactions between your skin and the surface you are touching.
And since the very atoms in your body are held together due to the electromagnetic interactions
between the positively charged nucleus and negatively charged electrons orbiting them, atoms would
not exist without light either.
In other words, no light means no atoms, no cars, no houses, no you and me. and no universe
as we know it.
Thus, not only would you not see or feel anything, you wouldn’t exist without light, so you
wouldn’t care about the fact that sound waves may still exist, because you wouldn’t
be there to hear it.
Losing light would be a catastrophe, at least for humankind.
The universe could still exist if we took the electromagnetic force and threw it out
the window, but it would not resemble the universe we know.
We would still have gravity, the strong force and the weak force, so we could still form
nuclei and have certain nuclear decays.
Very large nuclei could probably form in such a universe since there would be no electrostatic
repulsion between protons which prevents very large nuclei from forming.
Life as we know it would not exist without light, but could some other form of life exist?
Hard to say because, we know of no other mechanism for life other than what we see in this universe.
But I don’t think we can rule it out.
So let me again ask the question I asked at the beginning, what is the purpose of light?
Now I want to tell you that this is a philosophical question and not a scientific one, so my answer
is going to be speculative.
But it’s so interesting that I’m going to tackle it anyway.
I want to first make clear that light is an inanimate object and has no innate purpose.
It is not sentient and does not exist to do something.
It simply exists and follows, from everything we can tell, the mathematical quantum mechanical
laws of the universe.
Having said that, it appears to me that other than making the universe look the way it does
today, light plays a vital role in almost all processes where energy is exchanged.
Whenever, energy is either expended or absorbed, photons are almost always involved.
From chemical reactions, to thermal radiation, to friction, even to many nuclear processes,
photons are involved.
So for example, the heat released in any chemical reaction, the sunlight absorbed by plants,
the energy used in the biological processes of your body, even the heat released from
nuclear bombs, all these involve photons, in other words, light somewhere in the equation.
In fact, you’ll notice that the speed of light is present in Einstein’s mass/energy
equivalence equation E=mc^2.
The speed of light here is considered a conversion factor that is used to show how energy and
rest mass are really the same thing.
But let’s think about this a little more deeply.
What does this really mean?
For example, what happens to the mass of the fuel in a nuclear bomb when some of it is
converted to energy?
That energy is manifested in two ways.
One is in the form of kinetic energy carried by the materials that made up the bomb that
are ejected at a high velocity.
But most of the energy is released in the form of heat and light.
This heat and light energy is carried mostly by photons.
And if it were possible to convert 100% of the mass of the nuclear fuel to energy, in
the end, it would all be in the form of photons, because photons are the only known particles
that are capable of having energy and momentum without any rest mass, that can also propagate
freely, unlike gluons which are also massless, but are confined to the nuclei of atoms.
Photos are so ubiquitous, that in fact every object in the universe radiates photons.
This is called black body radiation.
And it’s a property of any object above absolute zero.
And I want to point out that in the process of energy exchange via photons, information
is also exchanged.
So when we feel the heat of the sun’s light, that light also carries information from the
sun.
Indeed, we would have very little information about our universe were it not for the various
forms of light that we detect from stars and other objects in our universe.
This is not just visible light but also infrared, ultraviolet, x rays and gamma rays.
And let’s not forget the microwave background which is present everywhere in the universe.
It’s the leftover embers of the early universe and provides strong evidence for the Big Bang.
So, if I were to speculate or assign a purpose to light, I would say it is the fundamental
currency used to exchange energy and information from one part of the universe to another.
That’s a pretty big purpose.
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I’ll see you in the next video my friend.