For a little over a decade, physicists have been studying a strange phenomenon in the quantum world. At very small scales, the temporal order between different events may not always be well defined.
Quantum physics describes the microscopic world with impressive precision. His predictions have never been contradicted by experiments. But it is also renowned for its strangeness.
Indeed, microscopic objects behave in a counter-intuitive way. First, their properties (such as their position and speed) can sometimes only take certain very specific values. To make an analogy with our macroscopic world, everything happens as if, when we move on a straight line, we can only move in “jumps” of one meter, without ever being able to have an intermediate position. Then, two entities seem to be able to influence each other at very great distances, at speeds greater than those of light. Finally, some objects have properties (such as their positron, or velocity) that lie in “quantum superpositions” of several values. What does it mean for an object to be in a “superposition” of several positions? Is the object nowhere? Everywhere at once? These questions have animated physicists and philosophers for decades.
One more strangeness in the quantum world
However, the latter decade has seen the emergence of new discoveries that raise the complexity take the problem up a notch. Work by physicists scattered around the world indicates that when two events occur in the quantum world, the temporal order between these events is sometimes undefined.
At our level, it is always possible to tell if a person sneezed first before apologizing, or the other way around. Quantum physics suggests that on small scales, sometimes neither of these two possibilities is correct.
However, the temporal order between different events is strongly linked to causal relationships. Indeed, a cause must always precede its effect. Thus, if the temporal order between different events is undefined, the same could be true for their causal order.
How can we make sense of a world in which things do not happen in a well-defined order? This question is a challenge to philosophers of science. Bold answers will undoubtedly be proposed, and we may have to accept a profound questioning of our vision of the physical world.
A disturbing experience
We can observe undefined causal orders in the laboratory, for example thanks to the “quantum switch”, a very particular experimental arrangement having been carried out on various occasions. Let us detail one of these experimental achievements. Two experimenters each perform an operation on the same particle of light, called photon. These manipulations consist, for example, of modifying a property of this photon, which we call “spatial mode”. The order between the two operations is determined, not by the scientists themselves, but by the value of another property of the photon, called “polarization”.
When the polarization of the photon is in a “quantum superposition” of two distinct values, and after a third experimenter has measured this polarization at the end of the experiment, this entire experimental arrangement cannot be described, either by the scenario where the particle was first manipulated by the first experimenter before being sent to the second, nor by the opposite scenario.
This intriguing research is still in its early stages. They will make it possible to study the behavior of temporal or causal relationships on a very small scale, in the quantum world. It is important to be able to make sense of the lack of temporal or causal order between events. Indeed, the order of events across time (and space) forms the foundation on which humans build their understanding of everything.
For example, when an object breaks after a fall, we explain it by its impact with the ground, after it has followed a very precise trajectory in the air. Similarly, the history of humanity is told through a continuous succession of events that occurred in various places around the world, at very specific times.
In order to preserve our classical modes of reasoning, we must therefore understand what becomes of the notions of time and space in the quantum world. We must also make sense of their possible absence. To answer these questions, some philosophers and physicists consider, for example, that the future can influence the past. Others contemplate the idea that time and space can only be the “by-product” of more fundamental phenomena, the nature of which is yet to be grasped.
Elias Kauerhof/Unsplash, CC BY
Finally, the discovery of the “quantum switch” and indefinite causal orders could well prove useful in the field of quantum computing, and for the development of future “quantum computers” of a new kind. Indeed, the existence of these phenomena could be exploited in order to carry out new tasks. They could also make it possible to perform certain calculations more efficiently than with more standard quantum computers.
Thus, recent research in quantum physics promises possible revolutions, both philosophical and technological.
By Laurie Letertre – Doctoral student in philosophy of physics, Université Grenoble Alpes (UGA).
Cyril Branciard, researcher at the CNRS, reread this article and the author would like to thank him.
