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dsaun777
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Is the four current in relativity an invariant quantity? I know the divergence is zero for the four gradient, i.e. the continuity equation. But is the four current a vector in the sense that it has invariant properties?
So it is an invariant? I was under the impression that the four vector referred to the four momentum and the four current was apart of the energy tensor and measured fluxes and therefore densities as well. The four T^ob components of the energy tensor form an invariant?Orodruin said:The 4-current is a 4-vector.
dsaun777 said:I was under the impression that the four vector referred to the four momentum and the four current was apart of the energy tensor and measured fluxes and therefore densities as well.
The four current of momentum.PeterDonis said:What "four-current" are you referring to? Do you have a reference?
Please provide the reference Peter asked for.dsaun777 said:The four current of momentum.
I've seen this in textbooks before but here is Professor Susskind talking about the momentum current. The four components of the fourth component. The T^oa components it stops at about 1:38:50Orodruin said:Please provide the reference Peter asked for.
Momentum current is not the same thing as 4-current. What is typically intended when you just say "4-current" without further specification is the electromagnetic 4-current ##J^\mu = (\rho,\vec j)##, where ##\rho## is the charge density and ##\vec j## the (spatial) current density, which is a 4-vector.dsaun777 said:I've seen this in textbooks before but here is Professor Susskind talking about the momentum current. The four components of the fourth component.
The divergence of ##T^{0\nu}## is an invariant though, correct?. Maybe I'm confusing concepts but I thought the collection of the four components that make up ##T^{0\nu}## represents measured energy flux across all surfaces, and therefore an invariant.Orodruin said:Momentum current is not the same thing as 4-current. What is typically intended when you just say "4-current" without further specification is the electromagnetic 4-current ##J^\mu = (\rho,\vec j)##, where ##\rho## is the charge density and ##\vec j## the (spatial) current density, which is a 4-vector.
The components of the stress energy tensor do not transform like the components of a 4-vector because they are the components of a second rank tensor. Of course, as pointed out in the previous post, each index by itself transforms in the appropriate manner and the stress energy tensor itself is an invariant object. However, your time direction is not invariant under Lorentz transformations and therefore (in general) ##T'^{0\nu} \neq \Lambda^\nu_{\phantom\nu\mu} T^{0\mu}##.
Kappa could be considered an outside four vector acting on some dust cloud type of matter described by the energy tensor?Orodruin said:To expand a little bit on #11, generally ##\kappa^\nu = \partial_\mu T^{\nu\mu}## is a 4-vector. Thus the 0-th component of that 4-vector is invariant only if the 4-vector itself is the zero vector.
Well, to be pedantic: if it supposed to be vector components under general coordinate transformations, the partial derivative should be a covariant one :Pvanhees71 said:##T^{\mu \nu}(x)## are 2nd-rank-tensor-field components. That's why ##\partial_{\mu} T^{\mu \nu}## are vector-field components.
The four-current in relativity is a four-dimensional vector that describes the flow of electric charge and current in space and time. It combines the three-dimensional current density with the time component to account for the effects of special relativity.
The four-current is important in relativity because it allows for the consistent treatment of electric charge and current in both inertial and non-inertial reference frames. It also plays a crucial role in the formulation of Maxwell's equations in the framework of special relativity.
Yes, the four-current is an invariant quantity in relativity. This means that its value remains the same in all inertial reference frames, regardless of their relative motion. This is one of the fundamental principles of special relativity.
The four-current is directly related to the conservation of charge in relativity. In any given region of space and time, the divergence of the four-current must be equal to the negative of the charge density. This ensures that electric charge is conserved in all reference frames.
While the value of the four-current remains the same in all inertial reference frames, its components may appear to change due to the effects of length contraction and time dilation. However, these changes are purely apparent and do not affect the overall conservation of charge.