# Does light have mass/weight?

If I plug in my memory stick/USB into my laptop and load it up with documents etc. will its mass/weight increase?

• Related:Is a hard drive heavier when it is full?. @DavidZ explains the flash case in his blog Jan 2 '14 at 16:49
• I think you should change your question title or content. I fail to see the corelation between the mass of electrons in flash memory and the mass of photons. Jan 2 '14 at 18:35

You confuse between four different concepts:

• First, between mass and weight.

Yes, mass is not weight. They are two different concepts: mass is a physical property of matter which determines the body's resistance to being accelerated by a force, for example: It requires more force to make a car move than a bicycle, it's because the car has more mass than the bicycle.

Weight is the gravitational force that pulls on mass. On the Moon, your weight is reduced compared to your weight on earth because gravity is weaker there, but your mass is still the same.

• Second, between light and electricity.

Light is an electromagnetic wave, example: the light coming from the sun.

Electricity is not due to electromagnetic waves, but because of a negatively charged subatomic particle, namely the electron. Everything around you is made up of different kinds of atoms which are themselves composed of a dense nuclei and of electrons. Electricity is the flow of those electrons, from an atom to another. So when you charge your smartphone via USB, the electrons flow from the computer, to the wire, to the battery of the smartphone.

Light doesn't have mass but electrons do, their mass is: $9.11×10^{−31}\text{kg}$. In fact, when you load a USB with documents or when you charge a phone, its mass increase by a tiny tiny amount, around $10^{-18}\text g$. It's because of changes in the level of energy stored of the electrons on your phone, their physical number stays the same but they possess more energy, hence more mass, $E=mc^2$ as Einstein cleverly putt.

To continue in our discussion about electromagnetic waves: Light has no mass, but it do possess momentum, hence it has weight because gravity is caused by momentum and not mass.

• You might want to add that though the rest-mass of light is zero, it does have relativistic mass in the sense that it has inertia and couples to gravity. Jan 2 '14 at 15:53
• @mcFreid Adding relativistic mass to the debate will only generate confusion to the OP.
– user29727
Jan 2 '14 at 15:54
• Perhaps, but it more closely answers his question because light does have "weight". Jan 2 '14 at 15:56
• @mcFreid Okay, I will add a paragraph discussing weight.
– user29727
Jan 2 '14 at 15:56
• I tried to simplify it as much as possible, so pardon me if it contains some impreciseness.
– user29727
Jan 2 '14 at 16:18

Re. 2nd question:-

"If I plug in my memory stick/USB into my laptop and load it up with documents etc. will its mass/weight increase?"

Its mass will not necessarily increase or decrease. It depends on the prior state of the memory. If there are the same average numbers of binary ones and zeros before and after the weight would be approximately the same. This is due to how the ones and zeros are stored on the memory stick. Placed electrons create charges on the chip gates to store information, but if you are just rearranging them there is no extra weight.

Ref. Wiki:- Floating Gate Transistor

"Because the floating gate (FG) is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years."