Where does mass come from, really?
And why the whole universe does not travel at light speed.
Objects are heavy. This may be one of the oldest facts that humans have ever known. A apple, and cartwheel, and big bag of chocolate muffins, they are heavy. But what does ‘heavy’ mean? Let’s look at what physics has to say.
The earliest ‘definition’ of mass can be easily attributed to Isaac Newton.
According to Newton, things are heavy because of an internal property all objects possess. He called it ‘inertia’. Inertia is a tendency for objects to not move.
To move an object you have to put an force on it. But, the object still resists. If you try and push a beachball it seems to move quite easily, but actually it does resist the push a bit. That is the beachball’s inertia. An ant on the other hand can conform for you that the beachball is hard to move. The puny force that the ant applies cannot overcome the beachball’s resistance. For the ant, the experience is somewhat similar to you pushing on a three story building.
Newton defined mass as a measure of that resistance. So, if a person weighs 40 kilograms, that is a direct measurement of how much he can resist a force on him.
Why exactly do objects have inertia? What is the problem with being in motion? We will cover that in some other article. Today, we will focus on why objects have this mass.
Everything in our universe is made out of a specific set of fundamental particles.
These particles are subatomic, meaning they are even smaller than atoms. In fact, even atoms are made of fundamental particles. You may know that atoms consist of protons and neutrons. In the above pictures you may see the particles that are shown with the color purple. The purple particles (they are not really purple in color, it is just a representation) are quarks. They make up protons an neutrons.
Similarly, you may see the electron in the green region. The electron is a fundamental particle. Similarly you may see the photon, the particles of light, marked in orange.
The fundamental particles are really fundamental — theoretically they cannot be made up, like protons. They are the true ‘undividable’, particles.
The catch however, is that when we did the math to describe them, these fundamental particles were found to be travelling at the speed of light.
Now, a few decades ago, Albert Einstein had proved that particles that travel at the speed of light cannot have any mass at all. Yet we knew that these particles had mass, and they did not travel at light-speed. so, what was going on?
What happens at the speed of light?
Now is the best time to take a slight detour and talk about what happens to objects when they travel at light speed.
Without delving into the nitty-gritty details, it boils down to two things.
‘Time stops, nothing changes’ and ‘everything appears to have no length.’
This is important to know. You may think it is weird that time can stop, but so is the counter-intuitive topic of special relativity — which governs what happens at the speed of light or speeds to close to it.
The specifics are quite ineterseting, and deserve their own explanations, so for time being, we will leave it here.
The fundamental particles have a property known as ‘chirality;. This comes in two types: left-handed and right-handed. they are just names, and you can think of the chirality as the direction in which the particle is spinning.
Even photons (which are indeed massless and travel at the speed of light) have chirality. But, as they travel at the speed of light, the chirality does not change (see above, ‘nothing can change’).
But in other particles like the quarks and electrons, the chirality constantly keeps on changing. This is a clear indication of change, and suggests that electrons, quarks and other massive particles cannot travel at light-speed.
But, then what is stopping them? We know that anything that does not travel at light speed has mass. But, by this we can deduce that fundamental particles get their mass due to something that is slowing them down from travelling at the speed of light. It is hindering them, sort of. While it is not affecting the photons at all.
The Weak Force
The Weak Force is an interaction between some of the fundamental particles that causes nuclear decay. That is when atomic nuclei release some particles and lose some mass.
But, an interesting thing that we observed is that the weak force is partial. It only affects particles with left-handed chirality.
We now know that left-handed particles have a property called the ‘weak hyper-charge’ which enables them to interact via the weak force, while the right-handed particles do not have this weak hyper-charge.
Now, we run into another problem: when the electron, for example, changes it’s chirality, what happens to the weak hyper charge? When it changes from left to right, it has to let go that property, and when it goes from right to left, it has to somehow gain it. All of these come together with the introduction of one fundamental concept — the Higgs field.
The Higgs Field
If all of this has been seeming very random and unrelated, it won’t now. The Higgs field ties in all of these together and explains why particles actually have mass.
Developed by Peter Higgs, the concept of the Higgs field is actually quite simple and elegant.
The Higgs Field is a scalar field, which can be thought of as a number at every point in space. The Higgs Field stores and gives away the weak hyper charge to particles. It exchanges the weak hyper charge between particles.
Think about it like this: if an electron is moving through space at the speed of light, it interacts with the field. The field, at some point, gives or takes away the weak hyper charge, thus altering the chirality.
I earlier mentioned that particles get mass because they are slowed down (in a vague sense) by something. The Higgs Field does exactly that. Cause change in the particle’s chirality and make sure they don’t travel at light speed. That ensures that particles have mass.
Why doesn’t this work for photons? Because any particle has to interact with the Higgs Field, which the photon doesn’t. Thus, we have put together the concepts of missing mass, the weak hyper charge disparity, and it seems we have saved the universe.
But wait, there’s more
99.9% of the mass of an atom comes from the nucleus. The nucleus contains protons and neutrons, which are made up of quarks. The quarks contribute to about 1% of the mass of the protons and neutrons.
But, then where is the rest of the mass coming from?
Yes. E = mc² tells us that energy can be converted into mass. Specifically speaking, energy can be heavy, it contributes to inertia. And as we know mass is a measure of inertia, so energy does contribute indirectly to mass.
You see, the quarks are held together by something called the strong force. There is a lot of potential energy which is used to keep the quarks together. That is how, most of the mass of the atom comes from. Because of the energy!
That is what surprises most people about the mass of objects. As more and more atoms group together to form molecules, there is energy in the bonds of the molecule, which contribute not as much as above, but little, to the mass.
Similarly larger objects which are made of molecules have some mass which comes from the bonds between molecules, but by now, the contribution becomes very low, and most of the mass comes from the mass of the atoms.
So, that was mass. And where it comes from. In a nutshell, some fundamental particles get mass from the Higgs field. Then most of the mass of the protons and neutrons come from the bonds between quarks. That is how objects are heavy.
This article is one to be published in Einstein’s Cup Of Tea, a physics, math and astronomy publication. If you enjoyed reading this article, consider following. See you next time!