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Scientific researchers, especially younger ones dreaming of glory and fame, are generally averse to setting boundaries and claiming absolute limits. This stems both from a hopefulness that Nature will be kind enough to give us plenty of new secrets to unravel in the future as well as an anxiety about ending up later as the villain in a just-so story about how the "establishment" naively criticized a new theory when it first appeared. In light of this, it's always notable when researchers find a new way of stating just how limiting the laws we know are. A very fine example of this kind of work appeared recently on the arXiv, in a paper by Gia Dvali and Cesar Gomez, Self-Completeness of Einstein Gravity.
This paper rains on the parade of those who hope against hope that there will be a fundamental theory of gravity with a regime of validity that pierces the Planck length. The Planck length is the smallest length scale presently contemplated in all of Physics: it is the length scale where gravity becomes as strong as the other fundamental forces. General relativity (GR) predicts that any experiment with enough energy to probe the Planck length must necessarily form a black hole in the process, thwarting the ability of the experiment to send out any results describing what happened on shorter length scales. What we're not sure of is whether GR is right about this.
Dvali and Gomez's paper is a subtle and powerful argument that GR is correct on this point: they pull together a variety of lines of reasoning to this effect, but the central claim and most powerful claim is that there is, in effect, _no_ _difference_ between the super-high energy physics inside the Planck length and well-known black hole physics. The idea is this: the way gravity works is that it wraps a black hole horizon around any region of space that reaches Planckian energy-densities. This black hole is a kind of buffer that absorbs the huge energy density, converting it into the relatively slower moving physics of black holes, where the energy will leak out later in the form of Hawking radiation. In other words: the Planck length also sets a minimum time scale in nature, a Planck time, faster than which nothing can occur. If an event is set to occur more quickly than this time, it get bounced or reflected by gravity into a black hole that flips that fast moving event into a slower-moving black hole horizon. The bigger the energy, the bigger the resulting black hole, and hence the _longer_ it takes for the energy to escape as Hawking radiation.
If true, this means that the real locus of our hope for novel physics is the Planck length itself. A black hole that just barely has enough energy to form evaporates most quickly. If we could see such a thing happen, we could get a window into the ultimate energy scale of physics. However, such a black hole is far beyond the scope of our most powerful Earth-based accelerators: it is fifteen _orders_ _of_ _magnitude_ smaller than the length scales that the Large Hadron Collider's record-breaking collisions will probe, for instance. (Ie, to get to the Planck length, you'd need an accelerator 1,000,000,000,000,000 times more powerful than the LHC!). So it remains a big open question as to whether Dvali and Gomez's ideas will ever be tested. In the meantime, we will have plenty of opportunity to explore whether there are any sneaky ways to get around their arguments; and you can count on it that many researchers are already looking to do just that, if they can!