4 December 2025

2:20 PM – 3:05 PM

Amphi Grand Kursaal

Marc Lachièze-Rey

Marc Lachièze-Rey is an alumnus of the École normale supérieure (rue d’Ulm) and holds a doctorate in physics. He is currently Director of Research at the CNRS and works at the APC (Astroparticle and Cosmology) laboratory. He specializes in fundamental theoretical physics, and is interested in the relationship between this discipline and mathematics and philosophy. He has written numerous articles and books, including Au-delà de l’espace et du temps : la nouvelle physique (Le Pommier, 2008), Les avatars du vide (Le Pommier, 2005) De l’infini (with J.-P. Luminet, Dunod, 2005), Figures du Ciel (with J.- P. Luminet, Le Seuil / Bibliothèque nationale de France, Paris, 1998), Initiation à la cosmologie (4th edition, Dunod, 2004), Voyager dans le temps: la physique moderne et la temporalité, (Seuil Sciences Ouvertes 2013) and Einstein à la plage (Dunod, 2015).

Is there really a limit to speeds?

No speed can exceed the speed of light. Curious law! Where does it come from? In space-time, motion is a direction. Relative motion is a relationship between two directions. As in ordinary geometry, this is expressed by an angle, and it’s this angle that best expresses the notion of relative motion. We call it “velocity”. But the geometry of space-time is “hyperbolic”. This may mean little to non-specialists, but it implies that angles vary from zero to infinity (or rather from minus infinity to plus infinity). Velocities are the right magnitude to characterize relative motion (for example, unlike velocities, they are naturally composed by adding up). Their value can go up to infinity. But for historical reasons, we’ve decided to use another magnitude, velocity. It’s a function of velocity (its “hyperbolic tangent”). But as velocity tends towards infinity, “speed” tends towards 1, i.e. the speed of light expressed in natural space-time units (such as light-seconds per second). The value of an angle cannot “exceed infinity”: the value of a velocity cannot therefore exceed unity!