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It is a starting point in string theory to assume that elementary particles are in fact rotating strings, and the final goal of the theory is a complete description of fundamental physics, including general relativity. This paper is instead concerned with the reversed question: starting from general relativity, is there a good way to motivate why rotating strings should be more natural models for elementary particles than, say, spherical particles or point-particles? Also, the purpose here is not to motivate full string theory. For example, no hidden dimensions come into play, only the four usual ones, and strings are defined in a very simple geometric way. Rather, the focus is on investigating an interesting mathematical property, which implies that strings may have special features with respect to rotation which spherically symmetric particles have not. In particular, it turns out that in a certain sense rotating strings are simpler than non-rotating ones. This is a consequence of the indefinite metric, and the main result states that the curvature of a non-rotating string, as measured by the square of the scalar curvature, may be reduced by letting it rotate in an appropriate way. The calculations underlying this theorem are heavy and have partly been car-ried out using Mathematica, although in principle the essential theorem may not require super-human labour.

String Theory is an ambitious project which starts from the assumption (among other things) that elementary particles can be understood as rotating strings, and it aims at a complete description of fundamental physics, including general relativity.

The present paper is not about string theory, but rather in a certain sense investigates an implication in the other direction: starting from general relativity, is there a good way to motivate why rotating strings should be more natural models for elementary particles than, say, spherical particles or point-particles?

The first question is of course: what should “natural” refer to in this context? A possible hint can be given by an analogy with mechanics: In statics, there is an old and well-known principle which asserts that the most “natural” state, and the one which will actually occur, is the one which minimizes the energy. In general relativity, the presence of mass-energy manifests itself through non-zero curvature, thus it could be argued that in the four-dimensional statics of space-time, the most natural states are the ones which minimize curvature.

Although other view-points are possible, I will in the following interpret this as minimizing the integral

where

In any case, minimizing (1.1) leads to interesting and difficult mathematical problems. In particular, it pin-points an unexpected difference between Euclidian geometry and Lorentz geometry.

The starting point for the study in this paper will be an extremely simple model for a particle: let us simply assume that the particle is defined by the region in space which it occupies. But in addition it can be noted that somewhere inside or close to the particle, the Ricci tensor should in general be different from zero if the resulting metric is to agree with a non-trivial Schwarzschild metric far away from the particle. For instance, if we consider the case of a spherically symmetric metric, then an elementary computation shows that no non-trivial, non-singular such metric will have vanishing Ricci tensor.

Exactly what this deviation from the Schwarzschild geometry looks like is something which a complete theory of elementary particles should be able to tell us, but we are not there yet. General relativity can not really help us either; even if we could in principle attempt to solve the field equations inside the particle, this would inevitably have to make use of so far unmotivated assumptions about matter, and also would require the use of the theory of general relativity in a situation where it has very little support.

An alternative approach, one which is close the the spirit of this paper, is to consider the metric inside the particle to be unknown, but nevertheless try to find general properties of all such metrics. Thus, we are led to a purely mathematical problem and, as it seems, to a difficult one.

In this paper, I will only attempt to achieve partial results. Thus, consider a string-shaped region in three-space (for a more precise definition, see Section 2). Far away from this region, one can expect the metric to be close to the Schwarzschild metric, and in particular to have

Given such a string in an otherwise flat three-space, we can ask what will happen if we add a time-coordinate and let the string perform motions. In particular, we may study the case of a rotating string and ask what speed of rotation of the string will minimize the curvature in the sense of (1.1).

It may seem intuitively obvious that the behavior which generates the least four-dimensional curvature should be to assume the string to stay at rest for all times. And indeed this appears to be what happens in Euclidian geometry. What is curious however, is that this does not seem to be what happens in the case of Lorentz geometry: rotating strings may have lower curvature than non-rotating ones.

It should be said right away that from a four-dimensional, relativistic perspective, such rotations must of course involve the time-coordinate in a non-trivial way. And in general, rotation in general relativity is a complicated concept (see e.g. [

After some introductory definitions in Section 2, I proceed in Section 3 to give a few numerical examples computed by Mathematica to give a feeling for what may happen. In this case, I also consider high-speed rotations, even if this means that one should be careful when drawing conclusions. As it seems, the general behavior is rather independent of the exact form of the metric inside the string; all examples indicate a similar behavior where the minimum of the curvature is assumed when the ends of the string rotate with (approximately) the speed of light.

In Section 4, I state and prove the best I can do rigorously: the main Theorem 1 can be expressed by saying that for any metric with certain specific symmetry properties and which is close to the flat one, the non-rotating case is not curvature minimizing in the sense of (1.1). In fact, what is proved is that for the special kind of rotation considered here, the case of zero angular velocity gives a local maximum for (1.1), rather than a minimum. As a contrast, Theorem 2 states that in the case of Euclidian geometry we really do have a local minimum.

As is not uncommon in differential geometry, the computations of curvature involved tend to be very lengthy. Although it may in fact be possible to carry them out by hand, I have used Mathematica for this purpose. The reason is that even if one could do it all without computers, the amount of work is so large that human errors are almost impossible to avoid. However, the use of Mathematica is restricted to the symbolic part, i.e. the computation of

In Section 5 finally, I discuss possible further developments. Let me also again emphasize that I do not in this paper make any claims about actual string theory. String theory is a quantum mechanical theory, and it is not at all clear what a corresponding quantum treatment of this problem would lead to. Nevertheless, I do think that the topic of this paper has got something important to say about Lorentz geometry on the microscopic level and, as a consequence, may contribute to our understanding of the connection between general relativity and quantum mechanics.

The strings in this paper will not be considered to be strictly one-dimensional, but rather to be three-dimensional objects which can however be arbitrarily thin. The precise definition will be to let the string be the convex hull of two balls

To get a model for a string, rotating around an axis perpendicular to the string itself, let us now for definiteness put

Finally, we suppose that whatever it is that generates mass, it is located at the ends of the string, i.e. within

Consider the following metric: let

Next, put

Clearly, this function is now defined on

We note that it coincides with the ordinary Minkowski metric outside the rotating string, in fact outside the balls

Using Mathematica, one may now attempt to compute numerically the integral of

where I have used the notation

In

Of course, the graphs in

In the next section, I will instead turn to rigorous methods for proving general theorems about these phenomena.

In view of the numerics of the previous section, it would be tempting to conjecture that it is a general fact for the kind of classical strings of this paper, that curvature in the sense of (1.1) can be diminished by letting them rotate in such a way that the ends move with (approximately) the speed of light. (In the limit of very thin strings, one would even suspect that the word “approximately” could be replaced by “exactly”). However, one should be careful when making predictions from this kind of numerical computations. In any case, to decide what actually happens may turn out to be a very difficult problem.

In this section I will prove a weaker result, namely that for any function

Theorem 1. For any

This should be compared with the corresponding situation in Euclidian geometry. In fact, if we replace the metric in (3.2) above by

then we have

Theorem 2. For any

The proof of Theorem 2 is very similar to the proof of Theorem 1, hence will be omitted (see however (4.6) below).

Proof: To prove Theorem 1, we need to compute the scalar curvature

Due to the special diagonal form of the metric in (3.2), a comparatively short computation gives that

Inserting the expression for

where

where

Note that the use of the metric (4.1) would instead give

To prove the claim, it is enough to prove that the integral of the coefficient for

is strictly negative (note the extra factor 2 since we only integrate over one of the balls). Making the trivial change of coordinates

Next, observe that

for simple reasons of symmetry, since

Furthermore, we note that, again for obvious symmetry reasons,

which clearly implies that

In fact, since

which is clearly non-positive. Finally, to prove that

and no such function satisfies the conditions

The calculations in this paper are obviously only a first step towards understanding why curvature is diminished by rotations in Lorentz geometry. The simple kind of rotation used here should of course in the future be replaced by a physically better motivated one. However, presently it is not clear how this should be done. In fact, it may be that when we leave the context of Euclidian Geometry and general relativity as defined by Einstein’s traditional field equations, it can be impossible to define rotations in a consistent way. The approach which to my mind seems to be the most promising one is to start from the minimizing principle (1.1) and then show that minimizing solutions will in fact have properties similar to ordinary rotations. However, this will first of all require making this principle more precise, and in addition we will have to deal with mathematical difficulties of a different order of magnitude as compared to the ones encountered in this paper.

Probably, an easier kind of generalization would be to do without the expansion in

Finally, one can try to generalize the ideas of this paper to topologically non-trivial situations, e.g. to rotating worm-holes. For a (non-rigorous) attempt in this direction, see [

Tamm, M. (2017) Rotating Classical Strings. Journal of Modern Physics, 8, 761-769. https://doi.org/10.4236/jmp.2017.85048