The rules of quantum mechanics (QM) typically apply to the micro-world of infinitely small things like atoms or electrons and not to larger things such as a baseball. This division leads to a confounding question – can the principles of QM be extrapolated to the macro world, and to what extent? Is there a limit on the size of objects that can be governed by the laws of QM?

Researchers in Kolkata and London1 are now exploring these questions afresh with what they claim to be the simplest method, different from prevalent ones, to probe the macro-limit of the quantum world. Their method tries to figure out whether objects much larger than macro-molecules are not essentially “classical” objects.

In recent years, physicists have extended the dividing line between the micro and macro world to include larger and larger objects. They now know that not only electrons and atoms, but even very large macro-molecules can be regarded as quantum objects. In order to examine the quantum behaviour of larger objects, experimentalists are capturing various nano-scale objects in optical traps and ion traps. However, demonstrating that these trapped objects exhibit quantum behaviour and are different from the "classical", is still a challenge.

"It is in this context that our work offers an alternative and easier way to probe the “non-classicality” of a trapped object," Dipankar Home, physicist at the Bose Institute in Kolkata and one of the authors told Nature India . "Importantly it also proposes to probe Einstein’s intriguing and much debated question: Is the Moon there when nobody looks at it?”.

Everyday objects around us are considered to be “real” in the sense that they exist in a definite state having specific properties, irrespective of any observation or measurement being made. On the other hand, microscopic objects following QM, are known to disobey the above notion of "realism".

For this, the physicists exploit an "inequality" devised in 19852 by Anthony James Leggett and Anupam Garg. "By using correlations between measurements on an object at different times, one can quantify the degree to which the nano-object violates our everyday notion of realism," says Home. Previously, Leggett-Garg "inequality" has been applied to systems which do not have a well-defined classical limit, the report says.

"Engineering quantum behaviour for macro-objects is one of the most important targets in contemporary physics that may have ramifications in practical applications such as quantum sensing," Sougato Bose, physicist at the University College in London and a co-author of the paper told Nature India . "Our experimental proposals show how macro-objects with a well-defined classical description can be made to empirically violate our everyday notion of realism."