Let’s take a piece of rock, like this one:
If we break it down into smaller and smaller pieces we know that at some point we can’t divide it anymore. We have reached the scale of the fundamental particles here.
The standard model of particle physics talks about various properties of fundamental elementary particles and fields from which all the matter and energy content of the universe is created.
To really understand the concept of mass we have to dive into the world of Quantum Field Theory (QFT) and create our own universe step-by-step.
According to QFT, an electron is just a kink in the electron field that permeates all space. Similarly other fields exist for other elementary particles likes Quarks and Neutrinos.
A vibration in one field can set up a vibration in another.
For example, when a photon hits an electron, it’s the photon field which sets up a vibration in the electron field. The electron might absorb that photon and re-emit it later setting up a vibration in the photon field again.
In QFT, the vibrations in the fields of any particle travel at the speed of light or the ‘speed of causality’. But we know that’s not true for all elementary particles because experiments have revealed that several fundamental particles like quarks, electron or neutrinos don’t travel at the speed of light.
This slowing down of particles because of their interaction with the Higgs Field gives rise to the mass of these particles. In other words, something has mass because the Higgs Field stops it from travelling at the speed of light.
Imagine a big pool full of a very thick syrup, something like maple syrup. Now throw a piece of rock in it as fast as possible. The drag forces acting on the piece of rock because of the viscosity of the liquid will cause it to slow down and the rock will eventually stop. In this analogy, the thick viscous liquid represents the Higgs Field and the rock represents a fundamental particle going through the Higgs Field. Every single molecule of the liquid represents a Higgs Boson.
All fundamental particles have their own properties like charge, spin etc. It is because of the difference in the properties of these fundamental particles that some of them, like electron, are slowed down by the Higgs Field whereas others, like photon, are not.
Einstein’s General Theory of Relativity has a very famous result: the clock of an observer travelling at very high speeds will tick slower than that of a stationary observer. As something travels faster and faster its clock slows down till it comes to a complete halt as it reaches the speed of light.
So, as the Higgs Field is responsible for slowing down particles and preventing them from travelling at the speed of light, it gives them a new property called time. In other words, without the Higgs Field, everything would be flying around at the speed of light and therefore no clocks would tick.
As Einstein described in his Theory of General Relativity, something which has mass distorts the fabric of space-time around it. And objects travelling in this distorted space-time feel the force of gravity. For gravity to exist there has to be fundamental massive particles which distort space-time. Without the Higgs Field, nothing would have mass and therefore there won’t be gravity.
A massless particle always travels at the speed of light. Soon after the Big Bang, when the temperature of the universe was very high, even massive fundamental particles were travelling around at relativistic speeds. About 3 minutes after the Big Bang, the universe had cooled enough and the first nuclei of atoms started to form. These nuclei later formed hydrogen and helium gases which make up all the stars and galaxies.
The importance of the Higgs Field in this process is that it provides a system of slowing down a particle as it loses energy. This doesn’t happen with massless particles. For e.g., a less energetic photon travels at the same speed as a more energetic photon.
In 1929, Edwin Hubble discovered that the universe was expanding. The understanding at that time was that the universe started with an initial explosion so an expanding universe made sense. What Hubble discovered was not only that the universe is expanding but also that the expansion is accelerating. This means that something was providing this extra energy and causing the universe to expand. The energy came to be known as Dark Energy. Dark Energy makes up more than 70% of the total energy content in the universe. It works in a way opposite to gravity: it makes things go further and further away from each other.
If there was no mass in the universe formed because of the interaction of the fundamental particles with the Higgs Field, there would be no force decreasing the effect of the Dark Energy and the universe would have been ripped apart in a scenario called the Big Rip.