The common definition of matter is anything that has mass and volume (occupies space). For example, a car would be said to be made of matter, as it occupies space, and has mass. This seems hardly to be a sufficient answer from something that has been studied for hundreds of years now.
During the 20th century, the classical idea of particles of matter, possessing properties such as extension, shape, density, location, momentum or impenetrability… started to dissolve. Physicists realized that atoms are composed of even smaller subatomic particles. An atom may be small, a mere billionth of an inch across, but these subatomic particles are a hundred-thousand times smaller still. Imagine the nucleus of an atom magnified to the size of a tennis ball. The electrons would spinning around it in orbits several miles across, making the atom itself the size of London or Manhatten. As the early twentieth-century British physicist Sir Arthur Eddington put it, matter is mostly empty space, 99.9999999999999% empty space to be a little more exact. If you could take away the empty space then all the subatomic particles in all the six billion people on planet earth would pack into a volume only a little larger than a grain of rice.
Then came the theories of special and general relativity. Matter and energy were suddenly brought into a kind of equivalence, famously described by Einstein’s equation E=mc2, which described the fact that rest mass could be converted into massless radiation and vice versa. Physicists began to understand that there was no fundamental ontological division between matter and energy. In addition, mass was no longer the only measure of gravitational agency… now a photon, for instance, despite having zero rest mass, could exert a gravitational force thanks to its kinetic energy. Turned out that both mass and energy could affect space and time, they could cause the local ‘warping’ of a space-time which was no longer Euclidean. Einstein’s equation forced us to redefine the concept of matter. Matter, somehow, had begun to ‘dematerialise’.
With the advent of quantum theory, it was found that electrons, the composite particles that make up protons and neutrons, and the other subatomic particles were themselves far from solid, and far from even being particles. On closer examination they appear to be just waves of energy, with no exact location in space, just a probability of being around at certain point in space and time. Solid matter had, literally, disappeared into empty space. Astonishingly, a particle would not behave as a particle unless a measurement was made. Unobserved properties of particles were described by probability waves which seemed to describe a world of “potentiality”, whereas observing the properties of particles seemed to be the process which was bringing this potentiality into actuality.
And then of course came the ideas of dark matter and dark energy, not to mention the ideas behind string theory, which describe matter as nothing but the vibration of invisible, unmeasurable strings. It seems to me that this process of de-materialisation of matter is exposing not much else than an emptiness behind it all, it is showing that the only things we have left to cling onto in modern science are nothing but our own conceptual, abstract ideas which, for better or worse, are the only tools we have left when it comes to describing the immateriality of that which we cannot directly measure, the immateriality of that which seems to appear out of nowhere.
It is unquestionable that, over the past century or so, the concept of matter has morphed into something that cannot be said to have the properties of matter at all, by any stretch of the imagination. The irony of it all is that our everyday experience of solidity, extension, or locality fools us into assuming that matter is in fact the ultimate reality. Could it be that matter is not made of "matter?"
Undoubtedly, many cultural, philosophical, religious, technological, economical and social events have influenced Western society to continue clinging onto the idea of matter for such a long time, despite the advances of modern physics. If when we look for matter we end up finding nothing but emptiness, could it be that matter is nothing more than our own subjective experience of solidity, extension and locality? What if matter is just a conceptual construction derived from a familiar experience in our mind?
We hear every day how everything is made of matter, how matter is the only a real, tangible, hard fact of our existence; yet, when you actually investigate matter you find out that there is nothing material about it at all. Matter appears to be just a conceptual construction derived from our subjective experience of solidity.