From Philosophical Research
what is mass?
E = mc^2 describes the conversion of mass into energy or vice versa
without going into exactly how this happens physically, physicists have already observed practical examples of these kinds of conversions in particle accelerators: two photons can convert into an electron and a positron, an electron and a positron can collide and convert into two gamma rays (high-energy photons). the number of discrete photons versus (anti)electrons can vary, related to the fact that photons can hold varying levels of energy; the electron and the positron produce at least one photon each, but they could produce three, which in some physical systems may be the more likely outcome. [*e]
a funny thing about this equation:
on one side is energy. on the other side is a mass times the maximum speed anything can usually go.
when you multiply a mass by a speed which may have some direction, you get momentum.
the velocity of any ordinary tennis ball flying through the air times its mass is its momentum — specifically a linear kind of momentum. this brings up the strange question of whether the tennis ball also has its own conversion from a mass moving at a velocity to energy.
obviously it would not make sense to speak of vaporizing the tennis ball into light like an electron becoming a gamma ray. but when the tennis ball is thrown, it has to get momentum from somewhere. whenever any object accelerates from rest there is a particular force required to move it. a force is commonly calculated as a mass undergoing a particular acceleration, producing a quantity in Newtons.
the interesting thing is that there is only a small difference between momentum and force. force required is acceleration opposed by mass. momentum is current speed aided by mass. so depending on the physical system, force aims to stop momentum. if an object is moving, a force can knock it sideways. if an object is at rest, the mass is going nowhere, at least apparently, but when it sits on earth it is going a particular speed: 9.8 meters per second into the ground. strange as it might sound to call gravity a velocity or a speed, the object is accelerating toward the planet, so it is gaining speed in that direction. to move an object sitting on the ground is actually to stop its silent movement into the ground and push it in a different direction. [that was solidly Newtonian, it wasn't really going past force diagrams into Einstein]
so, to move any object resting on the earth, that object must gain momentum, stopping its movement in one direction to gain movement in another direction. any change of direction is an acceleration, because most objects physically moving at some speed must also be going that speed in a particular direction.
and energy is required to stop a mass moving at a velocity, proportional to velocity squared. the tennis ball does not convert directly into energy but it converts into a tennis ball plus kinetic energy that it could have or not have.
```
E = mc^2 energy is mass per the speed of light
p = mv momentum is a mass at a speed (in a direction)
F = ma force is moving a mass to a different speed
```
[I don't quite remember where this entry was going but it had something to do with the approximation of atoms where protons are solid because they're quarks spinning around really really fast - not necessarily strictly correct, but as a mathematical equivalent to a real thing, interesting to think about.]
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=> en.wikipedia.org/wiki/Positronium *e
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=> 4.3x/1010/1671947517 n. Dialectics all the way down ; Dialectics-Whole-Way
=> 4.3x^/1095/1702191166 n. entropy and labor ; entropy
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== research.moraleconomy.au/entry/Philosophical_Research:MDem/5.1r/1112_mass
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