We have all learnt that mass is the amount of matter contained in a substance. But what is it?
Is it a fundamental property of nature, or is it a consequence of something even more fundamental?
Scientifically speaking, there are 2 different ways to think about mass: inertial mass and gravitational mass. Inertial mass is defined as the resistance to acceleration, that is, how much force is required to change the speed and/or direction of an object. That should make sense; it seems pretty intuitive that the more massive an object, the more difficult it should be to get it to move.
Now give the box a shove. The back wall will move into the incoming photons, and will feel a little more pressure. The front wall on the other hand, will accelerate away from the photons, and will feel a little less pressure than before. It feels as if there is a backward force that resists the movement. The box seems to have inertial mass! But there is still the question of gravitational mass. According to Einstein’s theory of general relativity, the feeling of acceleration in empty space is indistinguishable from being in a gravitational field. So the box has gravitational mass too! Not only does it feel heavy when it is inside Earth’s gravitational field, but it also creates a gravitational field of its own. Can a massless photon box with mirrored walls really represent matter in general? Yes it can! The photon box can be thought of as confined energy (the energy of the light in the box). It is this energy in the box that causes it to have mass, despite the individual components (the photons and the mirrored walls) being massless.
Take the example of protons and neutrons, which make up the nuclei of atoms. They are actually composite particles, made up of even smaller particles called quarks. Both protons and neutrons have 3 quarks each. And yet, the total mass of the individual quarks make up only a tiny fraction of the mass of the proton or neutron; 99% of their mass comes from the energy that binds the quarks together. This is analogous to the photon box; the confined energy that binds the quarks together is indistinguishable from mass, because it is mass. Even the intrinsic mass of the quarks themselves can be thought of as confined energy; in this case the energy is confined by interactions with the Higgs field. The Higgs field is a field that permeates the universe and gives particles inertia by interacting with them. So it seems that even particles like electrons and quarks are intrinsically massless, and gain inertial mass by interacting with the Higgs field. Many of us have heard about the famous equation E=mc^2. But not all of us know that it is in fact a formulaic description of the fact that mass can be thought of as confined energy, and vice versa. It is not only a powerful description of nature, but also has numerous applications. In fact, it is the premise behind nuclear physics. When neutrons collide with uranium nuclei in nuclear power plants, some of the collective mass is converted into energy, which is then converted into electricity. Nuclear fusion powers the Sun, without which life as we know it wouldn’t exist. So while mass may seem fundamental to most of us, it really is an emergent property.