In some theories, microscopic black holes may be produced in particle collisions that occur when very-high-energy cosmic rays hit particles in our atmosphere. These mini-black-holes would decay into ordinary particles in a tiny fraction of a second and would be very difficult to observe in our atmosphere.

The ATLAS Experiment offers the exciting possibility to study them in the lab (if they exist). The simulated collision event shown is viewed along the beampipe. The event is one in which a mini-black-hole was produced in the collision of two protons (not shown). The mini-black-hole decayed immediately into many particles. The colors of the tracks show different types of particles emerging from the collision (at the center).

**The RHIC fireball as a dual black hole**

We argue that the fireball observed at RHIC is (the analog of) a dual black hole. In previous works, we have argued that the large $s$ behaviour of the total QCD cross section is due to production of dual black holes, and that in the QCD effective field theory it corresponds to a nonlinear soliton of the pion field. Now we argue that the RHIC fireball is this soliton. We calculate the soliton (black hole) temperature, and get $T=4a/\pi$, with $a$ a nonperturbative constant. For $a=1$, we get $175.76 MeV$, compared to the experimental value of the fireball ``freeze-out'' of about $176 MeV$. The observed $\eta/ s$ for the fireball is close to the dual value of $1/4\pi$. The ``Color Glass Condensate'' (CGC) state at the core of the fireball is the pion field soliton, dual to the interior of the black hole. The main interaction between particles in the CGC is a Coulomb potential, due to short range pion exchange, dual to gravitational interaction inside the black hole, deconfining quarks and gluons. Thus RHIC is in a certain sense a string theory testing machine, analyzing the formation and decay of dual black holes, and giving information about the black hole interior.

**The case for mini black holes**

Geodesics in Kerr space-time, as predicted by the theory of general relativity. Small black holes produced, for example at colliders, are expected to be spinning. Image: Numerical simulation by Max Planck Institute for Gravitational Physics, Albert Einstein Institute (AEI); visualization by W Benger, Zuse Institute, Berlin/AEI

Approaches of the Gauss-Bonnet type, which include quadratic terms in scalar curvature in the Lagrangian, are good candidates for a description beyond general relativity as they can be supported both by theoretical arguments (heterotic strings in particular) and by phenomenological arguments (Taylor expansion in curvature). In such a case, the coupling constant of the Gauss-Bonnet term, namely the quantum character of the gravitational theory used (and the link with the underlying string theory) can also be reconstructed and the LHC would become a very valuable tool for studying speculative gravitation models.

Other promising avenues are also being investigated for new physics. Firstly, the black holes formed may be excellent intermediate states for highlighting new particles. When the collision energy is higher than the Planck scale ED, the cross-section for the creation of black holes is quite large (~500 pbarn) and has no suppression factor. Moreover, when the temperature of the black hole is higher than the mass of a particle, the particle must be emitted during evaporation in proportion to its number of internal degrees of freedom. There is thus a definite potential for the search for the Higgs or for supersymmetric particles in the evaporation products of black holes, possibly with cross-sections much greater than for the direct processes. Finally, taking account of a D-dimensional cosmological constant also modifies the evaporation law. If the constant is sufficiently high - which is possible without contradicting the low value measured in our brane - the temperature and the coupling coefficients with the entities emitted could be the signature of this particular structure of space-time. It would be quite neat and certainly surprising that a measurement of the cosmological constant in the bulk should come from the LHC!

Microscopic black holes are thus a paradigm for convergence. At the intersection of astrophysics and particle physics, cosmology and field theory, quantum mechanics and general relativity, they open up new fields of investigation and could constitute an invaluable pathway towards the joint study of gravitation and high-energy physics. Their possible absence already provides much information about the early universe; their detection would constitute a major advance. The potential existence of extra dimensions opens up new avenues for the production of black holes in colliders, which would become, de facto, even more fascinating tools for penetrating the mysteries of the fundamental structure of nature

**Public Service Announcement: Black Holes @ RHIC**

*by John Steinberg*

Unfortunately, all of this is overstated. At RHIC we don’t make a “real” black hole, in the sense envisioned by Einstein’s General Theory of Relativity. Rather, Nastase’s point of view is that RHIC collisions can be described by a “dual” black hole. But what does “dual” mean in this context? It’s not “two-ness” in any sense, but rather indicates that one can write down a theory which describes the collision as a black hole, but in a completely different world than that we see around us. To make his model work, he (and many other researchers who are exploring this direction) make a calculation of a black hole in 10 dimensions in order to describe difficult (but gravitationally benign) aspects of the strong interaction in 4 dimensions.

**No Black Holes Today, Thanks**

As George Musser remarked to me in an email,Egads, what a mispresented story. Nastase says they might be *dual* to black holes -- a relation of interest in string theory, but hardly the same thing as an honest-to-god black hole.

Exactly. The point of Nastase's paper is not that the RHIC fireball may be a black hole but that it might be described by the same math used for black holes. Such duality is vital in modern physics, because some problems are easier to formulate and solve within one mathematical framework rather than another, although both are applicable.

Now, if you want to know about the real prospects for making microscopic black holes by colliding particles in an accelerator, watch for the May issue of Scientific American, which will, by happy coincidence, have a feature on that very subject.

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