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| Q: | I read your statement about how light has momentum despite the fact that it has no mass. My question to you is regarding gravity in black holes. It is said that light can’t escape the enormous gravitational force in black holes; however, is it not true that gravity is directly proportional to the object’s MASS and inversely proportional to the distance between the two objects (Newtonian, I think). If so, light has no mass. So how would light be effected by this phenomenon??? Thanks for your enthusiasm in physics.
Dan Sweeney
-Dan Sweeney (age 16)
Thayer Academy, Braintree MA, USA |
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| A: | The use of words can make a lot of confusion. Unfortunately, the word
"mass" has been used in two different ways in physics. One was the way
Einstein used it in E=mc^2, where mass is really just the same thing as
energy but measured in different units. This is the same "m" that you
multiply velocity by to find momentum. It’s also the mass that provides
the source of gravitational effects. Light has this "m" because it has
energy (E) and momentum (p). So it is indeed affected by gravity- not
just in black holes but in all sorts of less extreme situations too. In
fact, the first important confirmation of General Relativity came in
1919, when it was found that light from stars bends as it goes by the
Sun.
The other way "mass" is often used, especially in
recent years, is to mean "rest mass" or "invariant mass", which is
sqrt(E^2-p^2*c^2)/c^2. This is invariant because it doesn’t change when
you describe an object at rest or from the point of view of someone who
says it’s moving. Obviously that’s a good type of "mass" to give when
you want to make a list of masses of particles. For
light, E=pc, so this "m" is zero. There is no point of view from which the light is standing still!
Mike
(republished on 07/23/06) |
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