Sir

You are either wildly optimistic, or confused, to suggest in your News Feature 'Extreme light' (Nature 446, 16–18; 2007) that extreme laser experiments could “rip apart the fabric of space and time”.

The phenomena of Unruh radiation and Schwinger pair production, which are the motivation for extreme laser experiments, simply probe the behaviour of quantum fields on a fixed, smooth space-time. Even if the experiments are totally successful, the only thing that could be 'ripped apart' is the vacuum state of some quantum fields.

If one considers quantum theories of gravity, there is no formal way to distinguish the space-time manifold from the quantum fields, because space-time itself should be quantized. (Don't ask me how.) But at low momenta and energy densities, relative to the fundamental scale of quantum gravity, usually around the Planck scale of 1019 GeV, such theories should reduce to something like the well-known picture of quantum field theory on a classical, weakly curved manifold described by general relativity, which has been successful in describing particle physics and optics.

Hence it is very difficult to probe quantum gravity experimentally, unless the low-energy limit of the theory gives some (necessarily small) deviation from this established picture. For example, varying 'constants', equivalence principle violations or Lorentz violation could amount to an indirect probe.

There are also special classes of theories in which the fundamental scale of quantum gravity may be much lower, even a few thousand GeV. However, these extra dimensions would not be probed by laser experiments, since photons do not propagate along them, unless the energy density approached the fundamental (TeV4) scale. Whether an exawatt beam (rated by power, rather than energy density) could do so depends on its focusing and pulse length: I highly doubt it.

The main hope at present to probe theories with TeV fundamental scale comes from the Large Hadron Collider (LHC), which is designed to produce such large energy densities in some of its collisions.

It is conceivable that black holes might result from gravitational collapse of dense regions — which would, finally, be a tale of ripping the fabric of space-time. Before the LHC switches on, space-time should be quite safe. Depending on your point of view, this may be a source either of relief or of disappointment.