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When you hear a song playing on the radio, you hear it because:

  1. A speaker vibrates,
  2. Which makes the air vibrate,
  3. Which makes your eardrum vibrate.

This vibration is a ‘sound wave’. There is no such thing as a sound wave by itself, as all that it is the movement of a substance like air. ‘Energy’ is a pretty similar concept. There is no such thing as energy by itself. Instead, energy is contained within everything in the universe.

You can’t shoot someone with a gun that fires pure energy. You have to shoot them with a bullet, which is a form of energy.

The Sun doesn’t just shine with pure energy. It emits electromagnetic radiation, which is a form of energy.

All energy originates with the Big Bang, and has been transferred from one form to another ever since. This is called the conservation of energy.

For instance, the light coming out of your screen right now is a form of energy. It can be traced back to the Sun, which gets its energy from the hydrogen forged in the minutes following the Big Bang. The bullet, the Sun itself, same thing. Everything goes back to the Big Bang.

This is because all forms of energy can theoretically be converted into each other.

Chemical energy within a battery can be converted to power a lightbulb, which illuminates a room with electromagnetic radiation. When wood is burnt on a campfire, chemical energy within wood cells are converted to heat and light. Nuclear energy within uranium can be used to turn a nuclear submarine’s propellers, which will push it through water.

Interestingly, mass, as in any object that weighs something, is a form of energy as well. With the right mechanism (like a few ounces of antimatter) a glass of water can be converted into rocket fuel, or a bomb, or fairy lights.

Put in reverse, with the right mechanism (i.e. way beyond us right now), light could be converted into a ham sandwich.

There is nothing in science that says we can’t turn light into a ham sandwich

Why this is possible comes down to quantum mechanics.

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At the most basic level, everything in the universe is made up of particles described by quantum mechanics. Each of these particle exists because it is a vibration in an underlying field. The theory of these fields is called ‘quantum field theory‘.

For example, light interacts with the electromagnetic field. The photon is a ripple in the underlying field.

Quarks are a ripple in the underlying quark fields.

Gluons are ripples in the underlying gluon field.

The ripples in these fields can interact with each other. The ripples can jump from one field to another, or push and pull them.

When light from the Sun hits a car, it heats it up. What’s happening from a quantum perspective is the photon, which is a ripple in the electromagnetic field, collides with the multiple fields that make up the atoms of the car like quark and gluon fields. It makes them vibrate faster, and this vibration is heat. Energy was converted from light to heat.

When a magnet pushes another magnet, …. they are also pushing the quark fields, because the atoms in the magnet are made up of quarks. In other words, the ripple from one field moved to another.

A boulder pushed up a hill [is more under the influence of gravity??] than one at sea level.

A rocket in motion across space is like a trillion ripples (one for every quark, one for every gluon) moving across spacetime.

A car on Earth in movement collides with atoms in the air, which slow the car down.

What happens when you wind up a spring?

All of these vibrations can

These are classical examples that are a higher level of abstraction away from QFT.

energy comes from the interaction of particles, and all interaction comes from four ‘fundamental interactions‘ (or ‘fundamental forces’). They are called electromagnetism, gravity, and the nuclear forces.

These four interactions can be combined in an infinity of different ways to create new ways of carrying energy.

For instance, heat is actually the vibration of atoms. The difference between a hot day and a cold day is how vigorously the air atoms are vibrating.

These forces combined with quarks give rise to larger combination particles like protons and neutrons, which in turn combine to form atoms, which combine to form large things like oranges, people and galaxies.

This includes quarks, electrons, photons, and gluons, which are described by quantum mechanics.

But energy is usually defined in a much broader way.

Just like with the smaller particles, the interaction of these large things carry energy. A lamp of burning oil releases chemical energy. A speeding car contains kinetic energy from the movement of its atoms. We usually take another step here though. We don’t just consider the interaction of particles to be energy, we also consider the potential of these particles to interact to be energy as well. That speeding car hasn’t hit anything yet, but it could. If the oil lamp hasn’t been lit yet, the energy in the oil was sitting there waiting to be released by a flame.

This is where we get to the proper definition of energy. Energy is described as the potential to do work. A boulder precariously balanced on a cliff has the potential to fall to the ground below and move its atoms around. A clump of uranium (or actually any element heavier than iron) has the potential to release heat and radiation if you split its atom, which can move the atoms of a city around. A battery has the potential to release electrons when you connect one side to the other, and these electrons can power a fan or computer. We say that each of these systems contains energy, and we define their energy based on the type of system it is.

Heat and movement are different forms of energy, and mass is another. The Wikipedia page for energy lists 16 different forms:

Type of energy Description
Mechanical The sum of macroscopic translational and rotational kinetic and potential energies
Electric Potential energy due to or stored in electric fields
Magnetic Potential energy due to or stored in magnetic fields
Gravitational Potential energy due to or stored in gravitational fields
Chemical Potential energy due to chemical bonds
Ionization Potential energy that binds an electron to its atom or molecule
Nuclear Potential energy that binds nucleons to form the atomic nucleus (and nuclear reactions)
Chromodynamic Potential energy that binds quarks to form hadrons
Elastic Potential energy due to the deformation of a material (or its container) exhibiting a restorative force
Mechanical wave Kinetic and potential energy in an elastic material due to a propagated deformational wave
Sound wave Kinetic and potential energy in a fluid due to a sound propagated wave (a particular form of mechanical wave)
Radiant Potential energy stored in the fields of propagated by electromagnetic radiation, including light
Rest Potential energy due to an object’s rest mass
Thermal Kinetic energy of the microscopic motion of particles, a form of disordered equivalent of mechanical energy

What we have found interesting and exceptionally useful is that energy is never used up or destroyed, it is only ever transferred from one form to another. This means that it is very quantifiable. We can work out exactly how much electrical energy it takes to heat one cup of water and with the same conditions, it is the same amount every time. This property is called the Conservation of Energy.

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