## Is it possible to get rid of exotic matter in warp drive?

12/05/2019

On 1994, Miguel Alcubierre proposed a solution of the Einstein equations (see here) describing a space-time bubble moving at arbitrary speed. It is important to notice that no violation of the light speed limit happens because is the space-time moving and inside the bubble everything goes as expected. This kind of solutions of the Einstein equations have a fundamental drawback: they violate Weak Energy Condition (WEC) and, in order to exist, some exotic matter with negative energy density must exist. Useless to say, nobody has ever seen such kind of matter. There seems to exist some clue in the way Casimir effect works but this just relies on the way one interprets quantum fields rather than an evidence of existence. Besides, since the initial proposal, a great number of studies have been published showing how pathological the Alcubierre’s solution can be, also recurring to quantum field theory (e.g. Hawking radiation). So, we have to turn to dream of a possible interstellar travel hoping that some smart guy will one day come out with a better solution.

Of course, Alcubierre’s solution is rather interesting from a physical point of view as it belongs to a number of older solutions, like wormholes, time machines and like that, yielded by very famous authors as Kip Thorne, that arise when one impose a solution and then check the conditions of its existence. This turns out to be a determination of the energy-momentum tensor and, unavoidably, is negative. Then, they violate whatever energy condition of the Einstein equations granting pathological behaviour. On the other side, they appear the most palatable for science fiction of possible futures of space and time travels. In these times where this kind of technologies are largely employed by the film industry, moving the fantasy of millions, we would hope that such futures should also be possible.

It is interesting to note the procedure to obtain these particular solutions. One engineers it on a desk and then substitute them into the Einstein equations to see when are really a solution. One fixes in this way the energy requirements. On the other side, it is difficult to come out from the blue with a solution of the Einstein equations that provides such a particular behaviour, moving the other way around. It is also possible that such solutions are not possible and imply always a violation of the energy conditions. Some theorems have been proved in the course of time that seem to prohibit them (e.g. see here). Of course, I am convinced that the energy conditions must be respected if we want to have the physics that describes our universe. They cannot be evaded.

So, turning at the question of the title, could we think of a possible warp drive solution of the Einstein equations without exotic matter? The answer can be yes of course provided we are able to recover the York time, or warp factor, in the way Alcubierre obtained it with its pathological solution. At first, this seems an impossible mission. But the space-time bubble we are considering is a very small perturbation and perturbation theory can come to rescue. Particularly, when this perturbation can be locally very strong. On 2005, I proposed such a solution (see here) together with a technique to solve the Einstein equations when the metric is strongly perturbed. My intent at that time was to give a proof of the BKL conjecture. A smart referee suggested to me to give an example of application of the method. The metric I have obtained in this way, perturbing a Schwarzaschild metric, yields a solution that has an identical York time (warp factor) as for the Alcubierre’s metric. Of course, I am respecting energy conditions as I am directly solving the Einstein equations that do.

The identity between the York times can be obtained provided the form factor proposed by Alcubierre is taken to be 1 but this is just the simplest case. Here is an animation of my warp factor.

It seen the bubble moving as expected along the x direction.

My personal hope is that this will go beyond a mathematical curiosity. On the other side, it should be understood how to provide such kind of perturbations to a given metric. I can think to the Einstein-Maxwell equations solved using perturbation theory. There is a lot of literature about and a lot of great contributions on this argument.

Finally, this could give a meaning to the following video by NASA.

## Ted Jacobson’s deep understanding

05/03/2009

A few weeks ago I published a post about Ted Jacobson and his deep understanding of general relativity (see here).  Jacobson proved in 1995 that Einstein equations can be derived from thermodynamic arguments as an equation of state. To get the proof, Jacobson used Raychaudhuri equation and the proportionality relation between area and entropy holding for all local acceleration horizons. This result implies that exist some fundamental quantum degrees of freedom from which Einstein equations are obtained by properly managing the corresponding partition function. To estabilish such a connection is presently not at all a trivial matter and there are a lot of people around the World trying to achieve this goal even if we lack any experimental result that could lead the way.

Today in arxiv appeared a nice paper by Ram Brustein and Merav Hadad that generalize Jacobson’s result to a wider class of gravitational theories having Einstein equations as a particular case (see here). This result appears relevant in view of the fact that a theory exploiting quantum gravity could have as a low-energy limit some kind of modified Einstein equations, containing at least coupling with matter. Anyhow, we see how vacuum of quantum field theory seems to become even more important in our understanding of behavior of space-time.

## Cramer-Rao bound and Ricci flow

04/02/2009

Two dimensional Ricci flow is really easy to manage. In this case the equation takes a very simple form and a wealth of results can be extracted. As you know from my preceding posts, I have been able to prove in a rigorous way that in this case the Ricci flow arises from Brownian motion (see here). So, the equation for  Einstein manifolds in this case takes the very simple form, $R=\Lambda$ being $\Lambda$ a constant, that is also the equation for a Ricci soliton. This equation is rather well-knwon to physicists as is the equation of 2d Einstein gravity. This equation is nothing else than Liouville equation

$\Delta_2\phi+\Lambda e^{\phi}=0$

that admits an exact solution notwithstanding being non-linear. There is an unexpected application of all this machinery of Riemann geometry to the case of statistics. Statistics has a wide body of application fields as radar tracking, digital communications and so on. Then, any new result about can be translated into a wealthy number of applications.

The problem one meets in this case is that of parameter estimation of a given probability distribution. For a sample of measured data the question is to determine the best probability distribution with respect to the spread of the data themselves with a proper choice of the parameters. A known result in this area is the so called Cramer-Rao bound. This inequality gives limit for the optimality of the chosen estimators of the data entering into the distribution. The result I have found is that, for a probability distribution with two parameters, an infinite class of optimal estimators exists that are all efficient. These estimators are given by the solution of Liouville equation! The result can be extended to the n-dimensional case granted the existence of isothermal coordinates that are the conformal ones.

This result arises from the deep link between differential geometry and statistics that was put forward by Calayampudi Radhakrishna Rao. My personal interest in this matter was arisen working in radar tracking but one can think on a large number of other areas. I should say, as a final consideration, that the work of Hamilton and Perelman can have a deep impact in a large body of our knowledge. We are just at the beginning.