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When Astronaut David Scott walked on the Moon in 1971, he conducted a famous experiment. He held a hammer in one hand and a feather in the other, and dropped them at the same time.
Intuitively we might expect the hammer to hit the ground first as it’s heavier, but this is not what happened.
In the zero atmosphere environment of the Moon, both hammer and feather hit the ground at the same time. He demonstrated that objects with different ‘masses’ fall towards a center of gravity at the same speed.
Down on Earth if objects fall at different speeds, it’s not because gravity affects them differently, but because of resistance to moving through the atmosphere. A man with a parachute falls slower than a man without one because parachutes create drag as they fall through the air.
But let’s look at that word ‘mass’ for a second. It’s different from the word we normally use to describe heaviness, which is ‘weight’.
Weight is how heavy something actually feels, wherever you are.
Mass is how heavy the same thing would feel if you were on Earth.
Physicists always use mass instead of weight, because weight changes depending on acceleration and gravity. So no matter where Scott stands, his hammer will always have more mass than his feather.
The further along the periodic table you go the heavier atoms get, as the number of protons and neutrons inside them add up.
A carbon atom
An iron atom
But even protons and neutrons are not the origin of mass, not quite.
To find what really makes an object heavy we need to get down into to the fabric of reality, deep in the world of quantum mechanics.
Quantum mechanics tells us that the universe is made up of different energy fields that stretch in every direction. They ripple with waves, which create the fundamental particles like quarks and photons. The ripples can also have something called a ‘charge’, which bends and manipulates other fields.
One example is the electromagnetic field. When it ripples it creates photons, being the particles that carry light, radio waves, and x-rays, and many others. But it can be bent and manipulated by the charge of electrons, which is what makes magnets work.
This is just field of many, and the ripples and manipulations of all of them create the universe that we see around us.
Protons and neutrons are made of quarks, which are ripples in the quark field. They have a charge called a color charge, which lets them manipulate the field called the gluon field.
This is the gluon field, rippling away. Image Credit: D. Leinweber
which are stuck together with other particles called ‘gluons’. In the diagram, gluons are represented by the lines holding the larger particles together.
Gluons are where mass comes from.
It’s not that they’re heavy. By themselves they have no mass.
It’s when they interact that the magic happens. Gluons are like highly energetic bouncy balls, and move around all over the place within the atom, interacting with quarks and with each other. This movement is the physical description of what mass actually is.
This is why mass and energy are equivalent. Mass is movement, i.e. kinetic energy. Large atoms like gold, lead, or uranium have lots of protons and neutrons, containing lots of gluons, which means lots of internal movement, and thus more mass. Mass is therefore not an intrinsic property of an object. It is a behaviour.
Mass comes from the glue that holds protons and neutrons together.
Gluons accound for the majority of mass, at _%. There is one more component to mass which is part of a larger story, and was the subject of one of the most recent major discoveries in physics. It is called the Higgs Field.