I used to think I understood mass-energy equivalence. Well, to the extent that you can understand it without seriously learning modern physics, anyway.
It seemed to be pretty simple: Mass simply is energy. That is to say -- well, there are two things we mean by mass, right? Gravitational mass and inertial mass, though of course relativity demands that the two are equal. Well, both of them are simply equal to energy -- this is only meaningful of course if energy has an "absolute zero", but, well, apparently it does. Energy warps spacetime around it, attracting things to itself; and the more energetic a body, the more its inertia. When something speeds up it gains mass and when it slows down it loses mass. An object on earth has slightly less mass than that same object 1000 miles above the earth. Rest mass of a particle I guess is just how much energy it takes to create that particle ex nihilo -- OK, I imagine there's a better explanation than that if you actually understand QFT, but I don't.
Except I keep reading that the notion of "relativistic mass" (i.e. γm0) is not really right, and it shouldn't be used, because it acts like mass in some ways but not others, and it shouldn't be thought of as mass, and objects shouldn't thought of as gaining mass when they accelerate.
(...on the other hand, there do seem to be number of contexts where I see "mass" being used to mean "relativistic mass". So I dunno.)
This doesn't make sense to me. So you're telling me that if I have, say, 2 protons and 2 neutrons, and I put them together to form an alpha particle, they do lose mass because of the energy difference, but if they're moving really fast and I declerate them, they don't? Huh?
"Energy is mass" I can kind of wrap my head around. "Energy is sometimes mass" is just weird.
-Harry
It seemed to be pretty simple: Mass simply is energy. That is to say -- well, there are two things we mean by mass, right? Gravitational mass and inertial mass, though of course relativity demands that the two are equal. Well, both of them are simply equal to energy -- this is only meaningful of course if energy has an "absolute zero", but, well, apparently it does. Energy warps spacetime around it, attracting things to itself; and the more energetic a body, the more its inertia. When something speeds up it gains mass and when it slows down it loses mass. An object on earth has slightly less mass than that same object 1000 miles above the earth. Rest mass of a particle I guess is just how much energy it takes to create that particle ex nihilo -- OK, I imagine there's a better explanation than that if you actually understand QFT, but I don't.
Except I keep reading that the notion of "relativistic mass" (i.e. γm0) is not really right, and it shouldn't be used, because it acts like mass in some ways but not others, and it shouldn't be thought of as mass, and objects shouldn't thought of as gaining mass when they accelerate.
(...on the other hand, there do seem to be number of contexts where I see "mass" being used to mean "relativistic mass". So I dunno.)
This doesn't make sense to me. So you're telling me that if I have, say, 2 protons and 2 neutrons, and I put them together to form an alpha particle, they do lose mass because of the energy difference, but if they're moving really fast and I declerate them, they don't? Huh?
"Energy is mass" I can kind of wrap my head around. "Energy is sometimes mass" is just weird.
-Harry
