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HomogenousCow
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Hello I was reading something the other day and wondered what a two-particle lagrangian would look like in SR. I'm not exactly sure what lorentz scalar we can write down for the two particles.
For a single particle, we can write the action integral in either of two ways:HomogenousCow said:Hello I was reading something the other day and wondered what a two-particle lagrangian would look like in SR. I'm not exactly sure what lorentz scalar we can write down for the two particles.
Bill_K said:For a single particle, we can write the action integral in either of two ways:
I = ∫ L dτ where τ is the particle's proper time, and in this case L is a Lorentz invariant.
Otherwise, in terms of the coordinate time t,
I = ∫L' dt
In the latter case L' is not Lorentz invariant. For a point particle with mass m, an expression that gives the right equations of motion is
L' = mc2√(1 - β2) - V
where V(x) is an external potential. See for example the chapter in Goldstein.
For N particles this form can be easily generalized:
L' = ∑ mic2√(1 - βi2) - V
where V(x1, x2, ...) is the interaction potential.
Most problems with an interaction potential are not Lorentz invariant.HomogenousCow said:Doesn't the potential break the lorentz invariance?
There's just the one integral over coordinate time, which is why I didn't use the first form that involves proper time.Also once we plug the free terms back into the integral, we have an integration over two separate proper times.
Bill_K said:Most problems with an interaction potential are not Lorentz invariant.
For example V = 1/|x1 - x2|
There's just the one integral over coordinate time, which is why I didn't use the first form that involves proper time.
There is just one integral, I = ∫L' dt where L' = ∑ mic2√(1 - βi2) - VHomogenousCow said:Well what exactly do we integrate over with multiple particles?
Do we simply have separate integrals integrating with respect to the different proper times?
This seems to break the formalism where we have the whole system under one integral sign.
As Orodruin says, the more usual formulation is in terms of field theory.Is this perhaps a hint to the breakdown of particles in relavistic physics?
ChrisVer said:I am not able to understand well the question... but the action for N particles interacting through the EM field is written as:
[itex] S= -\sum_{i=1}^{N} \int dτ_{i} (-m_{i}c \sqrt{n_{ab} \dot{x}_{i}^{a}(τ_{i})\dot{x}_{i}^{b}(τ_{i})} + q_{i} A_{a}(x_{i}(τ_{i}))\dot{x}^{a}_{i}(τ_{i})) -\frac{1}{16 \pi} \int d^{4}x F_{ab}(x)F^{ab}(x) [/itex]
where [itex]x_{i}[/itex] is the curves of each particle i (with mass [itex]m_{i}[/itex] and charge [itex]q_{i}[/itex], parametrized by arbitrary parameter [itex]τ_{i}[/itex] each... The [itex]A_{a}[/itex] is the electromagnetic potential and [itex]F_{ab}= \partial_{a} A_{b} - \partial_{b} A_{a}[/itex] the EM field strength tensor (or in SR they call it the antisymmetric EM tensor)...
This action is Lorentz Invariant...The form of the Lagrangian in fact, isn't in general of physical significance... what's important is the equations of motion...
In the case of such an action, the equations of motion give you the same result as having particles interacting with the EM field in a lorentz covariant form... (you get the inhomog. Maxwell equations as well as the Lorentz force law)
HomogenousCow said:I understand this, however what disturbs me is that we cannot write down a lagrangian without breaking lorentz symmetry. Perhaps that's irrelevant.
A Many Particle SR Lagrangian is a mathematical framework used in particle physics to describe the motion and interactions of multiple particles in special relativity. It is based on the Lagrangian formalism, which is a method for finding the equations of motion for a system by minimizing the action of that system.
A Many Particle SR Lagrangian takes into account the interactions between multiple particles, while a single particle Lagrangian only describes the motion of a single particle. In many cases, a Many Particle SR Lagrangian can be reduced to a single particle Lagrangian by treating the other particles as external forces.
Using a Many Particle SR Lagrangian allows for a more elegant and efficient way to describe the dynamics of a system of multiple particles. It also allows for a better understanding of the interactions between particles and can provide insights into the underlying physical laws governing the system.
A Many Particle SR Lagrangian is used in many practical applications, including high energy physics experiments and simulations. It is also used in theoretical calculations and predictions of particle behavior, such as in quantum field theory and cosmology.
While a Many Particle SR Lagrangian is a powerful tool for describing the dynamics of a system of particles, it does have some limitations. It assumes that the particles are point-like, which may not always be the case, and it does not take into account the effects of quantum mechanics. Additionally, it may become more complex and difficult to solve as the number of particles in the system increases.