Another deeper thought….
The most useful invention humanity can create right now is: an easy 100% foolproof lie detector.
We probably have had the technology for this for quite a while but there NEVER will be such a product. Imagine if politicians, civil servants, lawyers, bankers - even auto mechanics - were forced to make statements with a lie scanner present. Imagine all the deception and corruption gone. What a society that would be.
Yes, it will never happen.
I beg to differ.
Not only do I disagree with your basic premise ("The most useful invention humanity can create right now is: an easy 100% foolproof lie detector") - as I think that there are other, far more important, areas - of life and living, in our world - that inventions - or developments - can (or could) usefully benefit, I am not even sure of the benefits of such an invention.
Moreover, while such an invention may well have some benefits in the public sphere, I cannot see evidence for many such benefits in the private sphere, the personal space, not least where perspectives - each of whom may believe that they are truthful - may differ dramatically.
This is because half-truths, white lies, omissions, or, outright lies, are often simply survival mechanisms when someone is trapped in an unequal and troubled relationship. For that matter, even ordinary, stable, and loving relationships often require the lubrication of the easing oil of the half-truth, or occasional omission, or outright lie, as the motivation behind such actions (ranging from kindness to a desire to forego the dramatics that may result from a negative response to a truthful observation) is one that considers the possible reaction of one's interlocutor. So, motivation matters.
While it is becoming (all too lamentably clear) clear that some people (irrespective of whether they are rulers or ruled) care little whether something is true or not, - and, it would seem that some are - or, appear to be - increasingly unable to tell the difference between truth and lies - equally, in a society where delusion may be preferred to uncomfortable and unsettling truths, a world where fiction is preferred to fact, the wider world environment for truth-telling is a lot more hostile and unwelcoming than it used to be, while the sanctions - the penalties - for mendacity are not only non-existent, but rather, conversely, mendacity can be seen to be rewarded, and, indeed, - worse still - celebrated.
Rather than a device, or invention, I think that the answer to this lies elsewhere, in the values respected and celebrated and practiced in one's culture and society, and in encouraging the development of these characteristics and features in a society, or culture, or place.
Societies and cultures where facts and truths are respected tend to treat public perspectives fuelled by delusion (and emotions, ranging from anger to hatred) with derision and exclusion. However, societies and worlds where truth doesn't matter, tend to have drastically and dramatically different perspectives and priorities.