In a recent study spotted by New Atlas, researchers demonstrated the existence of a particularly counterintuitive phenomenon that seems to go against the main principles of physics: a light beam is capable of projecting a shadow.
Traditionally, the presence of a shadow is considered simply the absence of light. We observe it everywhere in our environment as soon as it is blocked by a more or less opaque object, whether solid, liquid or gaseous.
On the other hand, this does not apply to massless particles, such as the photons that make up light. It is generally considered that they cannot block each other. Except in certain very specific circumstances, two light waves can cross without immediately visible consequences; if you cross the beams of two flashlights, for example, the first will not be blocked by the second; they will both illuminate the surfaces they are aimed at without a shadow being produced.
Of course, when we look closer, the reality is actually more nuanced. The principle of wave-particle duality states that photons can be analyzed as both particles and waves depending on the context. And in this second case, we must take into account a large number of related phenomena, such as interference which results from interactions between two waves. But nothing in this model provides for the creation of a shadow from pure light.
The shadow of a laser beam, a counterintuitive concept
However, this is what researchers at Brookhaven National Laboratory in the United States observed. In their latest experiment, they demonstrated that there is not necessarily a need for an obstacle located in the path of the photons; we can also obtain a shaded surface by crossing two laser beams under certain specific conditions.
The team of researchers behind this work initially worked on the interaction of light with so-called “non-linear” materials. These are materials whose response to different types of external stimulation (electrical, mechanical, etc.) is not directly proportional to the intensity of the phenomenon. The best-known example is undoubtedly that of semiconductors, whose capacity to conduct an electric current can vary depending on the intensity of this current. But here, they were interested in optical non-linearity, that is to say materials which react differently to the passage of light depending on its intensity.
To generate the shadow in question, the researchers started with a pair of lasers (one blue and one green) positioned perpendicular to each other, so that their beams crossed. Under normal conditions (or more specifically in a linear medium), the two rays would have no visible influence on each other. But the situation changes completely when we move into a non-linear environment.
To show this, the researchers used a ruby. This gemstone is made of an aluminum oxide crystal that exhibits optical nonlinearity, and this property completely changes the behavior of the two beams. When they placed a ruby cube precisely at the point of intersection, they observed that the green laser changed the way the crystal reacted to passing blue light.
Under these conditions, the green laser no longer behaved like a simple flow of photons, but like a solid and opaque object. Therefore, once illuminated by the blue laser, the researchers were able to observe the projection of a dark spot. And even if it did not result from the illumination of a solid mass, according to the authors, it still met all the criteria for a shadow: it was both visible to the naked eye and precisely followed the contours of the object (in this case the green laser).
A new axis of research in optical physics
This result might seem completely anecdotal at first glance. But according to the authors, it’s actually more important than it seems. From a technical point of view, this experiment shows that it is possible to directly control the intensity of a laser using another beam of the same type. This phenomenon could therefore be exploited to offer an even greater degree of control to many devices that rely on lasers, such as extremely high-performance measuring devices. A potentially very exciting prospect for physicists and structural chemists, for example.
But more broadly, this also forces physicists to reconsider what a shadow is at the most fundamental level. In the medium and long term, this could lead to the emergence of fascinating new optical techniques.
“Laser light casting a shadow was previously thought to be impossible, because light typically passes through other light without interacting. Our demonstration of this very counterintuitive optical effect invites us to reconsider the notion of shadow”, says Raphael Abrahao, a physicist at Brookhaven National Laboratory and lead author of the study. “This discovery expands our understanding of the interactions between matter and light, and it could lead to new ways of using light that we have never considered before.”, he concludes.
The text of the study is available ici.
Source: www.journaldugeek.com
