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I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion.

How I interpret it is:

Energy is the dot product between the four momentum of the object and the four-velocity of the observer (the dot product is negative, due to the metric), and we typically add a negative sign in front of dot product to make the energy positive.

Inside the ergoregion, the metric changes. Therefore, the dot product becomes positive, and due to the negative sign we added before, we get negative energy.

I find a problem with this explanation because:

(1) This explanation seems to imply that all dot products become positive, and therefore energies for all objects become negative. This, however, is not the case. One object needs to have negative energy while the other object needs to have positive energy to make the Penrose process work.

(2) Some textbook say that when the object rotates in the same direction as the black hole, it has positive energy. If the object rotates in the opposite direction as the black hole, it has negative energy. This is confusing to me. First of all, I don't understand why an object can rotate in the opposite direction as the black hole in the ergoregion. Beyond the stationary limit, objects with mass cannot stay stationary and must be moving in the same direction as the black hole. If so, it seems impossible for an object to rotate in the opposite direction as the black hole. Secondly, if moving in the opposite direction of the black hole is allowed, why would it make the energy negative? I did not find any equation in textbooks that prove this idea.

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I suppose you are in the context of a rotating black hole, which is described by the Kerr metric, which in Boyer–Lindquist coordinates is ($c = 1$) \begin{equation*} ds^{2} = - \left( 1 - \frac{GMr}{\rho^{2}} \right) dt^{2} - \frac{4GMar \sin^{2}\theta^{2}}{\rho^{2}} dt d\varphi + \frac{\rho^{2}}{\Delta} dr^{2} + \rho^{2} d\theta^{2} + \frac{(r^{2} + a^{2})^{2} - a^{2} \Delta \sin^{2}\theta}{\rho^{2}} \sin^{2}\theta d\varphi^{2}, \end{equation*} where $\rho^{2} = r^{2} + a^{2} \cos^{2}\theta, \Delta = r^{2} - 2GMr - a^{2}$ and $M, a$ are the "mass" and "angular momentum" of the black hole, respectively.

Energy is the dot product between the four momentum of the object and the four-velocity of the observer

The energy you are referring to is the energy of a particle (or a light ray) whose trajectory is given by its four-momentum $p^{\mu}$, measured by an observer whose trajectory is given by its four-velocity $u_{obs}^{\mu}$. That is, \begin{equation*} E_{\text{obs}} = -p_{\mu} u_{\text{obs}}^{\mu}, \end{equation*} where we add the minus sign simply because is the covariant version of the energy measured by an observer in special relativity (SR). This energy is, in principle, different from conserved mathematical invariant due to $\partial_{t}$ being a Killing vector field, which is given by \begin{equation*} E = - g(\partial_{t}, u) = - g_{\mu \nu} \delta^{\mu}_{t} u^{\nu} = - g_{t \nu} u^{\nu} = - u_{t}, \end{equation*} where $u^{\mu}$ is the four-velocity of the particle (or light ray), which can be interpreted as an energy measured by an observer at infinity. Note that we also add the minus sign for the same reason (and ultimately, since $\partial_{t}$ is time-like). In the Penrose process, one must consider the latter energy notion, and the reason it works (theoretically at least) is because of the sign game of this invariant. Precisely, the magnitude $E$ is always positive outside the ergoregion, and can be either positive or negative inside the ergoregion. This is easy to see, since there can't be a particle with $E = 0$ outside the ergoregion (if the particle ever gets far enough, SR should be valid, where $E_{obs}$, which would then coincide with the invariant $E$, must be at least $m$). Conversely, since the hypersurface $\{ g_{tt} = 0 \}$ is time-like, particles can go through it, and since there clearly exist particles which eventually fall to the black hole, the energy $E$ inside the ergoregion can be positive. In order to see that it can also be negative, consider the concrete expression of $E$, \begin{equation*} E = - g_{t \nu} u^{\nu} = - g_{t t} u^{t} - g_{t \varphi} u^{\varphi} = \left( 1 - \frac{GMr}{\rho^{2}} \right) \dot{t} + \frac{2GMar \sin^{2}\theta^{2}}{\rho^{2}} \dot{\varphi}, \end{equation*} and simply note that if a particle ever passes through $\theta = 0$, then its energy $E$ must be negative, since $g_{tt}$ is positive inside the ergosphere. If the notion of negative energy is bugging you (which is good), it is important to realize that $E$ cannot be always interpreted as energy, but solely a mathematical invariant that regains physical value at infinity.

when the object rotates in the same direction as the black hole, it has positive energy. If the object rotates in the opposite direction as the black hole, it has negative energy

Inside the ergoregion, objects cannot rotate in the opposite direction as the black hole, and furthermore, they must rotate in the same direction, so this would not make any difference in the Penrose process.

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