Why the length of a quantum string cannot be Lorentz contracted

We propose a quantum gravity-extended form of the classical length contraction law obtained in Special Relativity. More specifically, the framework of our discussion is the UV self-complete theory of quantum gravity. Against this background, we show how our results are consistent with, i) the generalised form of the Uncertainty Principle (GUP), ii) the so called hoop-conjecture which we interpret, presently, as the saturation of a Lorentz boost by the formation of a black hole in a two-body scattering, and iii) the intriguing notion of"classicalization"of trans-Planckian physics. Pushing these ideas to their logical conclusion, we argue that there is a physical limit to the Lorentz contraction rule in the form of some minimal universal length determined by quantum gravity, say the Planck Length, or any of its current embodiments such as the string length, or the TeV quantum gravity length scale. In the latter case, we determine the \emph{critical boost} that separates the ordinary"particle phase,"characterized by the Compton wavelength, from the"black hole phase", characterized by the effective Schwarzschild radius of the colliding system. Finally, with the"classicalization"of quantum gravity in mind, we comment on the remarkable identity, to our knowledge never noticed before, between three seemingly independent universal quantities, namely, a) the"string tension", b) the"linear energy density,"or \emph{tension} that exists at the core of all Schwarzschild black holes, and c) the"superforce"i.e., the Planckian limit of the static electro-gravitational force and, presumably, the unification point of all fundamental forces.


Introduction and background
High energy particle physics is based on the notion that smaller and smaller distance scales can be investigated by increasing the energy of the probe particle. Elementary projectiles colliding with a target can resolve distances comparable with their quantum mechanical wavelength. The more is the energy, the shorter is the wavelength in agreement with the relativistic rule of length contraction. Quantum mechanics and Special Relativity work together to open a window on the microscopic world. This simple picture becomes less clear when we imagine to approach the Planck scale of distance, or energy, and consider the concomitant quantum gravity effects [1] This problem has long been ignored on the basis that the Planck energy, roughly 10 19 GeV , is so huge that no particle accelerator will ever be able to approach it. However, the picture is completely different when we consider the stringinspired unified models with large extra-dimensions, where the unification scale can be as low as some T eV . In this kind of scenario, quantum gravity effects, including micro black hole production in partonic hard scattering, have been suggested to occur near the LHC peak energy, i.e., 14 T eV [2,3,4,5,6,7,8,9], [10,11]. In this new physics the distinction between "pointlike" elementary particles and "extended" quantum gravity excitations, whatever they are, i.e., black holes, D-branes, string balls, etc., turns out to be fuzzy, so that standard notions, such as the Lorentz-Fitzgerald length contraction, require a substantial revision, at least insofar as its domain of validity is concerned. For instance, a fundamental, quantum string is, presumably, the smallest object in Nature with a linear size given by l s = √ α ′ . Thus, in this string perspective, no distance shorter than l s can be given a physical meaning. Furthermore, by supplying more and more energy, higher and higher vibration modes are excited making the string longer and longer, in conflict with the length-contraction rule, but not unlike the increasing size of a Schwarzschild black hole. To our mind, this signals the end of validity of special relativity and the onset of gravitational effects. How can we account for that? Before trying to answer this question, it is useful to recall the derivation of the fundamental units that define the domain of quantum gravity, as the answer to our question lies in the very definition of those units.
The appropriate standards of length, mass and time were originally introduced by Max Planck on a purely dimensional basis by combining the speed of light c, the Gravitational coupling constant G * 3 and the Planck constanth. In other words, Planck recognized that it is possible to combine Special Relativity, Quantum Mechanics and Gravity in the following dimensional package, Clearly, this dimensional approach defines the Planck units up to a numerical factor providing only an "orders of magnitude" estimate. In the old days, the Planck world was envisaged as the arena of violent quantum gravity fluctuations disrupting the very fabric of space and time [12]. Eventually, the notion of "space-time foam" evolved into a "Planckian phase" with a different description according to String/M-Theory, Loop Quantum gravity, Non-Commutative geometry, Fractal space-time, etc. In order to determine the numerical constants in (1) some extra argument is due. For instance, one may declare that the Planck Mass is defined by the equality between the quantum mechanical wavelength of a particle and its gravitational critical radius: Thus, An alternative, but consistent, definition of (3), which to our knowledge has never been noted before, is the following: L * is the geometric mean of the quantum mechanical wavelength λ C =h/mc of the particle and its critical gravitational radius R s = 2mG * /c 2 Further insight into the physical meaning of L * can be obtained from the Generalized Uncertainty Principle (GUP) [13,14,15,16,17], where L * is often identified with the string length, i.e., By minimizing the uncertainties, one finds From equation (6) we see that L * represents the minimal uncertainty in the particle/string localization. From this point of view, L * is the minimal length which is physically meaningful since, for a shorter one, the uncertainty is larger than the length itself. In contrast to this, it seems worth observing that the Planck mass is neither an absolute minimum nor an absolute maximum. It is, rather, an extremal value, or turning point, in the sense that, as implied by its definition (2), it represents the largest mass that an elementary particle may possess, or the smallest mass attributable to a micro-black hole. Interestingly enough, we shall argue in the following section, as well as in the last section of this article that there exists in nature a universal, unsurpassable linear energy density, or tension, that lies at the core of every black hole, regardless of its mass or size.

Critical boost and minimal length
We have remarked earlier that the existence of a "quantum of length" [19,20] is in conflict with the conventional rule of " length contraction " derived in special relativity. In a nutshell, the quantum of length L * is a new universal constant on the same footing as c andh, and as such it must be observer independent. It follows that L * must act as an unbreakable barrier to the Lorentz-Fitzgerald contraction. We propose to get around this problem by redefining the Lorentz-Fitzgerald contraction law in the presence of a short-distance Planck barrier. This is the crux of the following discussion. In Special Relativity a rod of length L 0 in its rest frame is seen to be contracted in the direction of motion according to the rule: An immediate consequence of (7) is that L can contract to an arbitrarily small length as β → 1. There are, however, at least two types of objections to this conclusion that require a redefinition of the contraction rule: • "Quantum" objection, or, the absence ofh: even though equation (7) is routinely applied to the world of particle physics, it was conceived with macroscopic, i.e. "classical", objects in mind. Stated otherwise, the quantum of actionh seemingly has no effect in the length-contraction rule, but we expect this to change in the ultra-relativistic regime when one approaches distances of the order of the Planck length. • "Gravitational" objection, or, the absence of G N : equation (7) refers to " abstract " lengths ignoring the fact that any physical object produces its own gravitational field, and thus introduces a "critical" gravitational length scale, that is, the Schwarzschild radius R s = 2MG N /c 2 . If L ≤ R s , the rod is not a rod anymore, rather, it will look like a black hole! This is the so-called "hoop-conjecture": any physical object extending along a certain direction less than its Schwarzschild radius, collapses into a black hole [21]. How a black hole appears in a boosted frame is an overlooked problem except in the somewhat ambiguous " shock wave limit " where γ → ∞, M → 0 while the product is kept finite, i.e. 0 < γM < ∞ [22].
By considering both arguments at the same time, one expects that Quantum Gravity imposes intrinsic limits to the relativistic contraction of physical objects. Presently, the most promising candidate for a self-consistent theory of quantum gravitational phenomena is Super-String Theory. From its vantage point, String Theory "solves" the problem from the very beginning by assuming that the building blocks of everything are finite length, vibrating strings. Nothing can be "smaller," in the sense that any distance (length) smaller than the string length √ α ′ does not have physical meaning. As string theory is a quantum theory of gravity, the string length may be identified with the Planck Length L P ≈ 10 −33 cm. Unfortunately, to our knowledge string theory says nothing about the Lorentz-Fitzgerald contraction and how to modify it. Our foregoing discussion, on the other hand, requires that any quantum gravityinspired extension ought to contain both Newton and Planck constant, G N and h, and reproduce (7) when G N orh are "switched-off". According to the hoop-conjecture there must be a critical boost factor γ * ≡ 1/ 1 − β 2 * that characterizes the transition from a gravitationally interacting two-particle system into a black hole. In order to determine γ * , let us tentatively change the contraction formula into the following expression, where, θ H , is the Heaviside step function which guarantees that the extra term does not affect the measure of L at rest 4 . Moreover, since any macroscopic length is tens of orders of magnitude larger than L * , the second term in (8) gives a relevant contribution only in the ultra-relativistic regime β ≈ 1. The minimum of the functionL ( β ) is For γ > γ * the functionL ( β ) " bounces back " and increases as stipulated in our earlier discussion on the basis that a similar behavior is shown by a fundamental string which cannot shrink below its minimal length l s = √ α ′ while increasing its energy excites higher and higher vibration modes forcing the string to elongate. Thus, a natural choice for L * is L * = l s = √ α ′ . For later convenience, it seems also worth recalling again that a highly excited string looks rather like a black hole. With this identification, equation (9) tells that in any inertial reference frame no physical length can be smaller that the string length: and the critical boost representing the turning point between contraction and dilatation turns out to be γ * = 2L 0 / √ α ′ . Now, let us take a closer look at equation (8)  • Take for L 0 the Compton wavelength λ C = 1/m of a particle. In analogy with the string improved GUP, we obtain the following modified de Broglie formulã As λ ( β ) cannot be smaller than √ α ′ it follows that the mass spectrum of an " elementary " particle is bounded from above by the limiting mass • Next, consider the case of a Schwarzschild black hole, L 0 = R s . Equation (8) tells us how the horizon radius will appear from a Lorentz boosted frame The first term shows how the Schwarzschild radius of a moving mass appears contracted as any other physical length. The second term in (13)

takes into
Sometimes, it is conventionally chosen θ H (0) ≡ 1/2. In this case, a β-independent quantity L 2 * /8L 0 must be subtracted in (8) account the existence of a "hard core" characterized by a universal, unsurpassable linear energy density, or tension, that prevents further contraction, or collapse into a point singularity. We shall come back to this essential point in the concluding section of this paper. The critical boost is For γ = γ * the Schwarzschild radius reaches its minimal value R H ( β * ) = √ α ′ . A snapshot of a black hole at this minimal size will show an object with an effective mass M * defined as In a string theoretical formulation of quantum gravity, the Regge slope can be related to the Newton constant through α ′ = 2G N . Thus, we find M * = 1/ √ 2G N = M P and R H ( β * ) = L P . If we formally assign to the black hole a Compton wavelength λ BH ≡ 1/M, we can write equation (13) as follows, Comparison with equation (11) shows that λ BH enters the modified contraction law in the inverse way with respect to λ C , thus suggesting that the de Broglie wavelength of a black hole can be written as Once the critical boost γ * = 2M √ α ′ = M/M P is passed, the first term in (16) is negligible and the Schwarzschild radius expands: It is worth mentioning that, recently, a new family of singularity-free black hole-metrics was reported in [28,29,30,31,32,33], where the existence of a minimal length is assumed at the outset in the Einstein equations. A remarkable properties of these black holes is to admit extremal configurations even in the neutral non-spinning case. Extremality corresponds to the lowest mass state of the system and to a minimal radius of the event horizon which equals few times the minimal length. In some simple models, it is possible to choose the free length scale that regularizes the short distance behavior in such a way that the radius of the extremal configuration is exactly the Planck length [25,10,34]. Without introducing the improved Lorentz law, the very idea of a minimal size object would become observer-dependent.
To sum up, at this point we have : (1) equation (8) for a (semi)classical length L 0 with string corrections; (2) equation (11) for the deBroglie wave length with string correctins; (3) equation (13) for the Schwarzschild radius with string corrections. Now, it is time to consider the hoop conjecture and check the self-consistency of our formulae. Let us start with case (1) and address the central question: Can a boosted object be seen contracted below its Schwarzschild radius? If so, the hoop conjecture would imply the original object is seen as a black hole... ?
We may regard this relation either as an equation for the radius L 0 below which the object is shielded by an horizon, or as an equation for a hypothetical "terminal speed"β that, once surpassed, will make the object to appear inside its own Schwarzschild hoop. In the first case, it is immediate to recognize that L 0 ≤ R s is the β-independent, somewhat " trivial " solution. In order to be seen as a black object, the maximal length, at rest, must be smaller than the Schwarzschild radius R s . On the other hand, if one assumes that L 0 > R s and tries to determineβ from (19), then one finds: which is unphysical. Only by moving at a speed greater than the speed of light can an object turn into a black hole. Thus, even in the presence of quantum corrections, there is no inertial frame where a classical object with linear size L 0 > R s may appear as a black hole. Let us consider now a quantum particle, rather than a classical object.
Once more, the " terminal boost " is unphysical, i.e. β > 1, and the only acceptable solution is Thus, as before, there does not exist an inertial frame where an isolated elementary particle with (invariant) mass m < M P looks like a black hole. However, this conclusion does not prevent the production of a black hole in the final state of a two-body high energy scattering where the hoop conjecture has been validated using numerical/computational techniques [35]. This different case will be discussed in the next section using a more analytical approach.

High energy collisions and black hole production
We are now ready to extend (11) to the case of a two-body system of colliding partons in the framework of higher dimensional quantum gravity. In this case, the gravitational coupling constant is G * with dimensions (in natural units) [ G * ] = l d−1 , much below the Planck energy, and d is the number of space-like dimensions (≥ 3 ). If the two partons have four-momenta p 1 and p 2 , it is useful to introduce the Mandelstam variable s = − ( p 1 + p 2 ) 2 . In terms of s we can define the "effective Schwarzschild radius" of the system as where L * is the higher dimensional minimal length. The hoop-conjecture states that whenever the two partons collide with an impact parameter b ≤ r H ( s ), then a micro-black hole is produced. In our approach we can rephrase this statement as follows: the two-parton system will collapse into a black hole if the de Broglie wavelength (11) is smaller, or equal, to the Schwarzschild radius (13) where we have switched to natural units,h = c = 1 and no step-function is needed as the two particle are by definition in a relative state of motion. Solving for s we find the threshold invariant energy for the creation of a micro black hole. This is a necessary, but not sufficient condition for this event to occur. As it can be expected, the production channel opens up once the quantum gravity energy scale is reached Equation (25) tells us that in a high energy scattering experiment we can probe distances down to L * but not beyond. The would be trans-Planckian region is shielded by the creation of a black hole with linear dimension increasing with s. This argument is the essence of a recent proposal by Dvali end co-workers [23,24] to explain how quantum gravity can self-regularize in the deep ultraviolet region [25] 5 . Thus, the trans-Planckian regime is actually inaccessible, and the deep UV region is dominated by large, "classical" field configurations. This mechanism has been dubbed "classicalization." [26,27].
There is a second important consequence of the relation (24) regarding the final stage of black hole evaporation. Micro black holes are known to be semiclassically unstable because of Hawking radiation. However, the standard description of thermal decay breaks down when the black hole approaches the full quantum gravity regime. Even worse, no semi-classical model can foresee the end-point of the process which remains open to largely unsubstantiated speculations.
Equation (24) on the other hand, shows not only the transition of a twoparticle system → black hole, but the inverse process as well. Start from the black hole region and decrease the invariant mass of the object. Effective models of " quantum gravity-improved " black holes suggest that for M >> M * the semi-classical model is correct and the particle emission is to a good degree of approximation a grey-body thermal radiation. However, as M → M * and the size of the black hole becomes comparable with L * , the mass of the object reveals a discrete spectrum and the decay process goes on through the emission of few quanta while jumping quantum mechanically towards the ground state. In this late stage of decay the black hole behaves like a hadronic resonance, or an unstable nucleus, rather than a hot body. Thus, it is not surprising that after crossing the critical point M = M * one is left with an " ordinary " elementary particle system [36]. old energy where gravitationally interacting point-particles collapse into an extended micro black hole. This threshold energy is determined by the final unification scale where quantum gravity becomes as strong as the other interactions.
Assuming that the " super-unification " scale is the Planck scale, is there any clue as to what the expression of the " super-unified force " might be? This question leads us to confront the notion of maximal tension introduced by Gibbons' in the framework of General Relativity [18].
Gibbons' conjecture is that there exists in nature a limiting force, let's call it a superforce, 6 whose exact expression is given by: With the above expression in hands, we are in a position to add some final remarks that may shed a different light on the whole sequence of arguments presented in this paper. A more comprehensive account of the following points will be presented in a forthcoming article [37].
(1) Note, first, that our definitions of Planck units are consistent with Gibbons' expression of the super-force. In other words, on dimensional grounds alone, the superforce is the " Planck force. " Having established that, it takes an elementary calculation to verify that the Gibbons-Planck force F s = c 4 /4G N is, indeed, the Planckian limit of both the electrostatic Coulomb force and the static gravitational Newton's force! While this is a definite clue that (26) is the unification point of the electro-gravitational force, it remains an open question whether it also represents the superunified value of all fundamental forces. (2) With hindsight, the conspicuous absence ofh from the Gibbons-Planck expression seems to support the classicalization idea as well as the idea of a transition from " contraction " to " dilation " in the modified expression of the Lorentz-Fitzgerald formula. Again, with hindsight, both ideas are inherent in the fundamental relationship (2): As a matter of fact, inspection of the above equation shows that, on the one hand it defines the Planck scale of mass-energy, but, on the other hand, it signals a trade-off, at the Planck scale of energy, between a quantum length (Compton) and a classical one (Schwarzschild) with the concomitant transition from " Lorentz contraction " in the particle phase to " Schwarzschild expansion " in the black hole phase.
(3) The appearance of G N in F s makes one wonder about the specific role of gravity in the unification of fundamental forces. Here we offer an alternative, gravity inspired, interpretation of the superforce: it represents the ultimate linear energy density of a black hole. In order to underscore this point, consider the conventional volume density of a body. In the case of a black hole this leads to the somewhat counterintuitive result that the density is inversely proportional to the square of the mass so that, while mini black holes may possess a nuclear density, galactic black holes can be less dense than water. On the other hand, by considering the linear energy density, one obtains the universal constant In words, there exists in Nature a limiting linear density that is a universal characteristic of all (Schwarzschild) black holes regardless of their mass or size. At first sight this result may seem surprising and hard to understand. In actual fact, the physical explanation rests on the duality between deep UV and far IR domains in quantum gravity. The unique properties of black holes bridge the gap between trans-Planckian and classical physics [39]! (4) Last, but not least, given the background of ideas advanced in this article, it seems natural to identify ρ * with the energy density of a relativistic string and, therefore, we identify the super-force Eq.(29) with the universal string tension Therefore, there are two equivalent ways of writing ρ * : • classical, macroscopic form given by Gibbons' Maximal Tension (29); • quantum, microscopic form which is the String Tension (30) The two definitions are linked through: i) the existence of a universal linear energy density for black holes exposing their "stringy nature". [40]; ii) the "classicalization mechanism" of quantum gravity that identifies trans-Planckian black holes with classical, macroscopic objects. It seems to be a unique property of gravity to bridge the gap between micro and macro worlds.