Each year, when I have been teaching Ecology & Evolution I wonder if I am misguiding my students during the photosynthesis lessons. To augment the module, I bring in a dense Holly log, hold it up, and ask: “where did the mass of this come from?”. After leaving them hanging for a few days or weeks, we discuss it again and start to rule out some of their answers (water, “dirt”, sun, water…). I help them rule out the sun, because everyone knows that empty space does NOT have mass…so light can NOT have mass either, because the light would have to travel through empty space to get to Earth from the Sun. But what about Black Holes dah dah dah??? Where’s the “truth”?!
That’s where you hit the fact that mass and energy are two sides of the same coin. Energy doesn’t have mass, but it can be converted to mass.
In this case, the log gets its mass mostly from the reaction of CO2 and H2O to create glucose, using energy from the sun, though of course there are other reactions that contribute.
Black holes warp space-time. It’s not exactly as if light has mass, but rather as if space were curved. Earth’s mass curves light by distorting space, so straight lines aren’t straight. Near a black hole, space is so curved back on itself that light can’t escape - not exactly the way we conceive of gravity pulling on an object that has mass, but it’s all the same according to Einstein.
Comment by Justin Baeder — Sunday, September 5, 2004 @ 2:34 pm