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The textbook definition of friction often describes it as the force that resists relative motion between two bodies in perfect contact at a planar surface — conditions seldom found in nature. In reality, many discrete contacts form at the interface between two imperfect sliding bodies. On page 76, Jay Fineberg and his colleagues at the Hebrew University of Jerusalem in Israel show that frictional strength is governed by the behaviour of this ensemble of microcontacts. Fineberg tells Nature how this curiosity-based physics experiment may aid earthquake modelling.

How did you begin this line of research?

I was investigating the physics of how bodies fracture when I realized that friction is basically a system of fractures between two bodies. The laws of friction simply say that the force you must apply to any body is proportional to the force pressing down on it. But this makes sense only if you consider the behaviour of the contact area that exists, although is often not visible, at the interface between this body and another. As a result, I started experiments aimed at monitoring how the many frictional contacts that form between surfaces break and reattach.

What was the hardest part of the work?

Developing an appropriate experimental system, which took five years. It consisted of two blocks of acrylic glass and a laser-based method for measuring block movement. Using this system, we were able to see how small frictional contacts making up the interface break and reattach over microseconds, drastically changing the behaviour of the contact area.

Does this work apply to the real world?

Our table-top experiment yielded several insights that may be relevant to the real world. In fact, as we did this work, I realized that the same frictional forces control the movement of tectonic plates. Our empirical findings suggest that there is a reduction in contact strength, caused by the breaking of microcontacts, just before the two blocks move relative to one another. The speed at which microcontacts reattach depends on how fast the sliding occurs. Understanding this behaviour may help us to better predict earthquakes and their potential for damage.

Do you expect your results to be controversial?

I would be disappointed if they weren't. The most controversial research is often the most important because it disrupts preconceptions. That said, sceptics may point out that we've only conducted experiments using acrylic glass. Time will tell if our findings are also true for the rocks that make up a fault.