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andresB
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I think I have never seen a force equation for massless particles, I wonder if such thing exist or if not why not?
andresB said:I think I have never seen a force equation for massless particles, I wonder if such thing exist or if not why not?
All such interactions are best modeled as the incoming photon being absorbed in the interaction while a new photon is created. That way, the photons are always traveling on straight (geodesic) paths and we don't have to think about forces pushing them around and changing their trajectory.andresB said:I'm aware about the geodesic equation in GR, but that is not what I'm wondering for (and to clarify I'm just talking about SR for simplicity).
Massless particles not always follows straight lines in flat space-time, for example photons are allowed to compton scatter electrons in a pure classical way. Are other interactions disallowed for some fundamental reasons?
Nugatory said:The intuitive notion of a photon as a particle of light moving through space, with a trajectory determined by the forces applied to it, is very misleading. It may seem natural to think that a beam of light is a stream of photons passing by, the same way that a river is a stream of water molecules flowing by... But it's not.
andresB said:It just seems curious to me that there is no such equation for massless particles.
Electrons and other massive particles can be handled using the methods of ordinary quantum mechanics, where there is a position operator and we can think in terms of position (actually the expectation value of that position operator), velocity (the time derivative of that expectation value), and acceleration (the time derivative of the expectation value of the velocity). We can also expect by the correspondence principle that these quantities will behave like their classical counterparts under conditions where the quantum effects are can be ignored so that we can apply classical force equations.andresB said:Well yes, but the same apply to electron and all other particles, yet there is a classical force equation for them.
Orodruin said:Can we please drop the talk about photons in the same framework as classical forces? It makes my brain cringe ... The classical concept of a force is not quantum mechanical and photons inherently are.
What might be discussed is whether or not it makes sense to talk about a 4-force acting on a massless particle (as of yet unspecified). However, this really also loses any significance as the 4-force is defined as ##dP/d\tau##, where ##P## is the 4-momentum and ##\tau## the proper time (which is unspecified for a massless particle). You could of course differentiate with respect with some other affine curve parameter if you somehow manage to arrange for a force to act on the massless particle, but it would be inherently different from the 4-force. The geodesic equation is what would drop out when there is no external influence making the particle change direction. After all, this is what we have with normal classical mechanics as well - the force is essentially determines the geodesic deviation in the metric placed by the inertia tensor on the configuration space.
pervect said:In relativistic mechanics, ##p = \gamma m v## , ##\gamma = 1/\sqrt{1-(v/c)^2}##.
Newton's 2nd law for Massless Particles in Special Relativity (SR) states that the force applied on a massless particle is equal to the rate of change of its momentum with respect to time.
This law is different in that it takes into account the effects of Special Relativity, which includes the concepts of time dilation and length contraction. It also applies specifically to massless particles, which have different properties and behaviors compared to particles with mass.
No, this law only applies to massless particles. For particles with mass, the traditional Newton's 2nd law, F=ma, should be used.
Some examples of massless particles include photons, gluons, and gravitons. These particles have no rest mass and travel at the speed of light.
This law helps us better understand the behavior of massless particles, such as their constant speed and their ability to travel at the speed of light. It also helps explain phenomena such as the bending of light by gravity, which is a result of the momentum of photons being affected by the curvature of spacetime.