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| Q: | Does light have mass?
Do shadows have mass?
-Natasha (age 11)
mcGraw , Fort Collins,CO |
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| A: | Natasha -
One question at a time. Let’s start with the easy one.
Do shadows have mass?
No.
Shadows are what you get when you don’t have any light
shining on something. Think of it this way: If you’re standing outside
on a sunny day, the sunlight is hitting both you and the ground. But in
some spots, you’re getting in the way of the light. So there are some
spots where the light doesn’t hit the ground as much. These spots are
shadows. Since shadows are basically just spots where there /isn’t/ any
light, shadows don’t have any mass.
Next... Does light have mass?
That depends on which of two definitions of ’mass’ a particular
physicist likes to use, and what type of light you’re talking about.
One definition of mass says that anything which has some
gravitational pull on other objects has mass. By that definition, light
has mass. This definition of mass is the same as the definition you get
if you ask what ’m’ you have to multiply the velocity ’v’ by to get the
’momentum’ of an object. Momentum is a measure of how much stuff is
moving which way. When things bump into each other, the total momentum
doesn’t change even though it might be traded between the objects.
Think of when two balls bounce off each other. Light has momentum,
which means we can actually measure the push it gives to objects it
runs into. This is the definition used by Einstein, for example in the
famous equation E=mc^2.
On the other hand, physicists often find it convenient to think of
mass as something that doesn’t depend on how an object is moving, also
sometimes called the ’rest mass’. They call the mass of some object the
mass that it would have according to somebody who says the object isn’t
moving. Light always is moving, so by this definition (or more careful
versions of it) a light ray has no mass.
Switching between those definitions can lead to a lot of confusion.
If you have a box of light, with the rays going every direction,
the light does contribute to the total mass of the box, by any
definition.
-Tamara (and mike)
(republished on 07/29/06) |
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