What does E=Mc squared mean? Let's look at the meaning of the terms first. E stands for energy. M stands for mass, C stands for the speed of light. So energy is equal to mass times the speed of light squared.
Everything has mass except for light and some other sub atomic particles. And clearly what this tells us is that mass is a compressed form of energy.
Let's look at a simple example. Mass is measured in kilograms. So say we have 1 kg of mass/matter. Any kind of mass.
The speed of light is measured, for the purpose of the equation in meters per second to the minus one. That's 300,000,000 meters per second or as written for the equation: 3 x 10 to the 8th ms-1.
Then it's all squared.
So do the math and from 1 kg of matter ( 2.2 pounds) we get: 90,000,000,000,000,000 joules of energy. What does it mean in practical terms? If you could release all that energy in a slow controlled manner, you could run a 100 watt light bulb for 28,519,279 years, or your house for thousands of years. I haven't done the math for that..
And what would be left when all the energy is gone? Nothing. No matter. Why? Because all matter has mass, mass is compressed energy, get rid of the mass and the object is no longer.
By current popular definition a thing has mass and takes up space. But then what's a mass-less photon? Nothing? No. It's an energy wave at a particular frequency. So what is matter? Seemingly it's compressed energy.
We know and accept that all things are made of atoms. And we all used to think atoms were solid objects. Hence we had years of experiment showing that atoms could act like solid particles and/or waves. What a mystery.
Then came Quantum Electro Dynamics and showed that electrons, at least, weren't individual objects, but part of a universal electromagnetic field. QED is considered the most precise theory in physics from experiment. Far more so then even QM.
Then we started the big colliders and got a shock. If you smash two glass cups together you expect to get glass debris. But when we started smashing particles together we discovered the debris contained/created particles not found or related to the particles we smashed together. It was like smashing glasses together and getting spoons, a knife, a rabbit and corn flakes from the debris.
How can a solid do that? It can't. But if like in QED all particles are waves that act like solids, then a collision could produce almost any frequencies, and thus metaphorically: rabbits from glass.
Thus Quantum field theory was born in the 1980s, but kept quiet until the 2000s when enough evidence had been found. And what it tells us is: There is no wave particle duality. There are only waves that can act like solid particles. Particles are not solid individuals, but vibrations/waves in their fields. These fields obviously interact creating all this.
So what is a field made of? Whatever it is, it's super energetic. Some have even called it nothing: Lawrence Krause. Why would he say that? Well, it used to be thought space was empty in places. But we found out it's anything but. It's called Quantum Fluctuation. It seems that somehow, out of empty space, sometimes a pair of particles are created out of nothing. They are matter and antimatter. Most of the time they cancel each other out almost instantly, producing photons.
But sometimes they don't and add to our universe. Krause has a theory that says that's how the universe started: from nothing. Why does he call it nothing? Because it's apparently nothing. But it certainly isn't nothing at all, and even he admits that. And now we know the nothing is quantum fields in equilibrium.
And we now know through experiment, that even there the fields are in constant motion. But what is it made of? Nothing? I say we can call it energy.
Energy is defined as a property of things, not a thing in and of itself. Thing, again, defined as having mass and taking up space. Again, a very bad definition.
As we know, energy can't be destroyed. So if it was just a property, why/how would it be conserved when the thing it is a property of is gone? E=Mc squared tells us matter is energy. And when we remove all energy from matter, the matter is gone.
Sure, a property of all things is energy. Besides the energy we produce by running or working, we and even a rock are made of mass which is compressed interacting energy in different forms. Where does mass come from? Our atoms. What are atoms? Essentially, waves of energy.
No thing can do anything without energy, not even quantum fields. So what's the most important nothing in existence? The thing that makes existence possible? Energy.
We know it exists in quantum fields, and we know it can go into equilibrium and become potential. No, it's not a thing we can see as an object. It's all objects. All things are a manifestation of it and couldn't exist without it.
So is it just a property of another nothing we can't see:;quantum fields? Or is it the quantum fields themselves? Is it the quantum fields that can't be created or destroyed, only transformed, and always conserved?
I think we can safely say there's nothing but energy.