Timeline for Does Newtonian mechanics predict the bending of the course of light by objects with mass?
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| Jul 13, 2012 at 15:35 | comment | added | Ron Maimon | @AlfredCentauri: Ok, fine, it's all right, so long as the two terms aren't confused by someone searching for "ghost". Regarding zero mass, you have to think of the infinitesimal mass limit, and it has diverging acceleration (for nongravitational forces) and this is the proper way to take a limit, but it's not a big deal. | |
| Jul 13, 2012 at 15:19 | history | edited | Alfred Centauri | CC BY-SA 3.0 |
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| Jul 13, 2012 at 12:52 | history | edited | Alfred Centauri | CC BY-SA 3.0 |
ghost to cipher
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| Jul 13, 2012 at 11:51 | comment | added | Alfred Centauri | @RonMaimon, as a follow up, would you prefer the word "cipher" rather than "ghost"? From Griffith's "Introduction to Elementary Particles": "In classical mechanics, there's no such thing as a massless particle; its momentum ($mv$) and its KE ($\frac{mv^2}{2}$) would be zero, it could sustain no force, since $F=ma$ - it would be a dynamical cipher | |
| Jul 13, 2012 at 11:16 | comment | added | Alfred Centauri | @RonMaimon, (1) this is not a QFT context so "ghost" is fine and makes sense here. (2) This is not a quantum context so "photon" is ambiguous. (3) The OP asked questions about massless particles in general (see final two questions). In Newtonian mechanics, regardless of whether you choose to call it a "photon" or not, if you consider a zero inertial mass particle, either you accept there are actual infinities, or you accept that the particle has zero momentum always, zero KE always. Sure gravity may accelerate it but what is it? | |
| Jul 13, 2012 at 6:46 | comment | added | Ron Maimon | @AlfredCentauri: The word "ghost" has a specialized technical meaning, the photon is never a ghost, a ghost is a particle with negative sign norm states, and it doesn't make sense outside of quantum mechanics. If you want to say a particle that passes unimpeded through everything, say this, but it isn't true that the zero mass limit is the vanishing force limit, rather it's the infinite acceleration limit. In the zero mass limit, the acceleration due to gravity is unchanged, although the speed goes to infinity so the deflection from a point source vanishes. | |
| Jul 11, 2012 at 19:17 | comment | added | Alfred Centauri | It's not impossible as in logically impossible, i.e., its existence implies a contradiction. Rather, it's that an inertially massless particles can't be observed, can't be detected in principle under Newtonian mechanics. Think of Newton's laws of motion; zero inertial mass implies zero momentum, zero force, zero action and reaction. It doesn't interact at all. It's a dynamical "ghost". Massless particles exist in the relativistic context because there is an invariant speed. In Newtonian mechanics, there is no invariant speed or, sloppily, the invariant speed is infinity. | |
| Jul 11, 2012 at 18:54 | comment | added | Aftnix | So does it mean its impossible to have "zero inertial mass" particle in Newtonian mechanics? That means by newtonian mechanics, photon has to have somekind of mass which can be measured by inertial methods. Another thing is, as far as i know, the equivalence of gravitational and inertial mass in newtonian mechanics is not something demanded by the theory. that means the equivalence is merely accidental. in GR its different. | |
| Jul 11, 2012 at 16:31 | history | answered | Alfred Centauri | CC BY-SA 3.0 |