# Is the Four-gradient of a scalar field a four-vector?

Consider a scalar field $$\phi$$ as a function of spacetime coordinates $$x^\mu$$. The four-gradient of $$\phi$$ is given by

$$$$\frac{\partial \phi}{\partial x^\mu} = \left( \frac{\partial \phi}{\partial t}, \frac{\partial \phi}{\partial x}, \frac{\partial \phi}{\partial y}, \frac{\partial \phi}{\partial z}\right)$$$$

I'm studying a little of classical field theory and in the topic os real scalar field, is commom to introduce four-gradients of these fields. My question is: The components of this four-gradient are not the components of a four-vector. For example, take the first component, and considering $$t = t(x',t')$$

\begin{align} \frac{\partial \phi}{\partial t} &= \frac{\partial \phi}{\partial t'}\frac{\partial t'}{\partial t} + \frac{\partial \phi}{\partial x'}\frac{\partial x'}{\partial t}\\ &= \gamma \left( \frac{\partial \phi}{\partial t'} - v \frac{\partial \phi}{\partial x'} \right) \end{align}

EDIT: I'm some books I've read, the four-gradient $$\partial _\mu$$ is defined as a four-vector, and since the D'alambertian operator $$\Box = \partial _\mu \partial ^\mu$$ is a lorentz invariant, the four-gradients must be four-vectors. But my computations give me this unsatisfactory result. Am I doing computational mistake or what? Please help me

## 3 Answers

The $$4$$-gradient is a $$4$$- vector.

Formally, when $$x^\mu\to x'^\mu=\Lambda^\mu{}_\nu x^\nu$$

\begin{align*} \partial'_\mu &=\frac{\partial}{\partial x'^\mu}\\ &=\frac{\partial}{\partial (\Lambda^\mu {}_\nu x^\nu)}\\ \end{align*} $$\therefore$$ \begin{align*} \Lambda^\mu {}_\nu\partial'_\mu&=\partial_\nu\\ \end{align*}

which makes $$\partial_\mu$$ a 4 vector and is precisely what you are getting

which is not how the $$0^{th}$$ component of a four-vector should transform

It is. What you have derived is $$\partial_\nu$$ in terms of $$\partial'_\mu$$ which matches above.

This is equivalent to the transform of a 4 covector $$\partial'_\mu=\Lambda_\mu{}^\nu\partial_\nu$$ So, \begin{align*} \frac{\partial}{\partial t'}&=\gamma\left(\frac{\partial}{\partial t}+v\frac{\partial}{\partial x}\right)\\ \frac{\partial}{\partial x'}&=\gamma\left(\frac{\partial}{\partial x}+v\frac{\partial}{\partial t}\right) \end{align*} which leads to $$\frac{\partial}{\partial t}=\gamma\left(\frac{\partial}{\partial t'}-v\frac{\partial}{\partial x'}\right)$$ same as before

• But my computations leads me to an inconsistence. Can you say where is my mistake? Feb 1 '20 at 23:03
• @Lil'Gravity I have clarified with an edit Feb 2 '20 at 0:02
• Thanks for the explanation. I realize that I made confusion about the distinct elements "4-vector $A^\mu$" and a "4-covector A_\mu". The textbooks presents the 4-vectors as $A^\mu$. Feb 2 '20 at 0:19

The "gradient" you wrote is not a four vector (and that's not what should be called a gradient). What you have written is a four co-vector (i.e. a dual vector, which is a linear functional over the space of four-vectors). What you have written is technically the differential of the field $$\phi$$ which is an exact on-form:

$$d\phi = \frac{\partial\phi}{\partial x^{\mu}} \, dx^{\mu}$$

The gradient depends on an underlying Lorenzian (or Riemannian) metric $$g_{\,\mu \nu}(x)$$ tensor, with inverse tensor $$g^{\,\mu \nu}(x)$$, that establishes isomorphism between the space of vectors and co-vectors. I.e.

$$\big(\,\nabla \phi \,\big)^{\mu} = g^{\mu \nu}\, \frac{\partial \phi}{\partial x^{\nu}}$$ so when you change the coordinates, $$x^{\mu} \mapsto \tilde{x}^{\mu}$$, the inverse Lorenzian or Riemannian tensor changes as $$g^{\,\mu \nu} = \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\, \frac{\partial {x}^{\nu}}{\partial \tilde{x}^{\beta}}$$ and the partial derivatives transform as $$\frac{\partial\phi}{\partial x^{\nu}} =\frac{\partial \tilde{x}^{\gamma}}{\partial {x}^{\nu}}\, \frac{\partial \phi}{\partial \tilde{x}^{\gamma}}$$ Consequently, the gradient transforms as a four-vector as follows: \begin{align} \big(\,\nabla \phi \,\big)^{\mu} &= g^{\mu \nu}\, \frac{\partial \phi}{\partial x^{\nu}} = \left(\frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\, \frac{\partial {x}^{\nu}}{\partial \tilde{x}^{\beta}} \right)\, \left(\frac{\partial \tilde{x}^{\gamma}}{\partial {x}^{\nu}}\, \frac{\partial \phi}{\partial \tilde{x}^{\gamma}}\right)\\ &= \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\, \left(\frac{\partial {x}^{\nu}}{\partial \tilde{x}^{\beta}} \, \frac{\partial \tilde{x}^{\gamma}}{\partial {x}^{\nu}}\right)\, \frac{\partial \phi}{\partial \tilde{x}^{\gamma}}\\ &= \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\, \left(\frac{\partial \tilde{x}^{\gamma}}{\partial {x}^{\nu}} \, \frac{\partial {x}^{\nu}}{\partial \tilde{x}^{\beta}} \right)\, \frac{\partial \phi}{\partial \tilde{x}^{\gamma}}\\ &= \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\, \delta^{\gamma}_{\beta} \frac{\partial \phi}{\partial \tilde{x}^{\gamma}}\\ &= \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \tilde{g}^{\,\alpha \beta}\,\frac{\partial \phi}{\partial \tilde{x}^{\beta}}\\ &= \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \left(\tilde{g}^{\,\alpha \beta}\,\frac{\partial \phi}{\partial \tilde{x}^{\beta}}\right)\\ & = \frac{\partial {x}^{\mu}}{\partial \tilde{x}^{\alpha}}\, \big(\,\tilde{\nabla} \phi \,\big)^{\alpha} \end{align} where again, in the current $$\tilde{x}^{\alpha}$$, the gradient is $$\big(\,\tilde{\nabla} \phi \,\big)^{\alpha} = \tilde{g}^{\,\alpha \beta}\,\frac{\partial \phi}{\partial \tilde{x}^{\beta}}$$

In flat space-time, you have a Minkowskian constant tensor $$\eta_{\mu \nu}$$.

The four gradient is a four vector but it transforms covariantly, rather than contravariantly. This makes it a "covector".

It also has a contravariant form, obtained by multiplying it by the metric, which transforms like 4 four position or four momentum.