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.

Down on Earth it’s a bit different. A hammer will hit the ground first.

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, though they are both equally affected by gravity.

The experiment on the Moon was the definitive demonstration of how objects with different ‘masses’ fall towards a center of gravity at the same speed.


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, and it changes depending on acceleration and gravity.

Mass is always consistent, and is how heavy the same thing would feel if it were on Earth.

Physicists always use mass instead of weight, as acceleration and gravity often change.

So no matter where Astronaut Scott stands, his hammer will always have more mass than his feather.

This isn’t just because there are more atoms crammed into a hammer. It’s also because their atoms are different. The hammer is mostly iron, whereas the feather is carbonoxygen and nitrogen.

Iron atoms have way more protons and neutrons inside of them than those other elements.

A carbon atom
A carbon atom
An iron 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, which are 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 their ripples and charges 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
This is the gluon field, rippling away. Image Credit: D. Leinweber

Quarks are held together by particles called ‘gluons’. In the below diagram, gluons are represented by the lines holding the larger particles together.

‘d’ and ‘u’ are types of quarks. Gluons are the lines holding them together.
‘d’ and ‘u’ are types of quarks. Gluons are the lines holding them together.

It’s not that gluons are 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 account for the vast majority of mass, but there is one more component to mass which is part of a larger story.

It is the subject of one of the most recent major discoveries in physics, and it is called the Higgs Field.

Additional notes