I believe that some of our most engrained notions about space and causality should be reconsidered. Anyone who takes quantum mechanics seriously will have reached the same conclusion.

What quantum mechanics tells us, I believe, is surprising to say the least. It tells us that the basic components of objects – the particles, electrons, quarks etc. – cannot be thought of as "self-existent". The reality that they, and hence all objects, are components of is merely "empirical reality".

This reality is something that, while not a purely mind-made construct as radical idealism would have it, can be but the picture our mind forces us to form of ... Of what ? The only answer I am able to provide is that underlying this empirical reality is a mysterious, non-conceptualisable "ultimate reality", not embedded in space and (presumably) not in time either.

How did I arrive at this conclusion? My interest in the foundations of quantum physics developed at quite an early stage in my career, but I soon noticed that my elders deliberately brushed aside the problems the theory raised, which they considered not to be part of physics proper. It was only after I attained the status of a fully-fledged physicist that I ventured to take up the question personally.

To put it in a nutshell, in this quest I first found that whatever way you look at it the quantum mechanical formalism, when taken at face value, compels us to consider that two particles that have once interacted always remain bound in a very strange, hardly understandable way even when they are far apart, the connection being independent of distance.

Even though this connection-at-a-distance does not permit us to transmit messages, clearly it is real. In other words space, so essential in classical physics, seems to play a considerably less basic role in quantum physics.

I soon found out, as often happens, that these things had been known for quite a long time. Schrödinger had even given them a name: entanglement, and had claimed entanglement is essential. But strangely enough he had not really been listened to. Indeed he had been unheard to the extent that the very notion of "entanglement" was hardly mentioned in regular courses on quantum physics.

And in fact most physicists felt inclined to consider that, if not entanglement in general, at least the highly puzzling 'entanglement at a distance' was merely an oddity of the formalism, free of physical consequences and doomed to be removed sooner or later, just through improvements on the said formalism. At the time the general view was therefore that if any problems remained in that realm these problems were of a philosophical, not of a physical nature so that physicists had better keep aloof from them.

I was not convinced I must say, and in the early sixties I wrote and published a book and some articles developing physical arguments that focused attention on such problems by showing that entanglement is truly something worth the physicist's attention.

And then a real breakthrough took place in that John Bell, a colleague of mine at Cern, published his famous inequalities, which - for the first time - opened a possibility of testing whether or not entanglement-at-a-distance had experimentally testable consequences.

The outcome confirmed my anticipations. Entanglement-at-a-distance does physically exist, in the sense that it has physically verifiable (and verified) consequences. Which proves beyond a shadow of a doubt that some of our most engrained notions about space and causality should be reconsidered.

Bernard d'Espagnat is a theoretical physicist, philosopher and winner of the Templeton Prize 2009. He is the author of On Physics and Philosophy, Princeton University Press, 2006