There is a brass band though. They are playing something vaguely Pomp and Circumstance from within the barricades which stretch from the church steps, along Hardy street and then dogleg through Montgomery Arcade into the Saturday Market. 

The green-painted Spud Cart, a potential hotbed of IRA sympathisers, is not under any kind of surveillance. Beside the ANZ, there’s a small cluster of people including a child in a glittery wig and another dressed as a tiger. A woman in a peasant skirt sits smoking on a bench outside the museum. On the other side of the street, there’s another woman waiting for something to happen. She’s sitting in a striped folding chair, knees pressed against the barricade with a Fox Terrier on her lap. She’ll have a prime view of the royal entourage as it strolls by, but at the moment it’s a lonely vigil she’s keeping. A couple of women stand chatting outside Cruella’s Natural Fibre Boutique. One of them is clacking away at some knitting like an antipodean Madame Defarge. 

Even with human subjects informed consent is not mandatory. Thought experiments cost absolutely nothing to conduct and so the results are unlikely to be corrupted by pharmaceutical lobby groups or other questionable sources of funding. 

All that is required for a thought experiment is what - if you were telling a joke - would be the set-up, plus some imagination and wild surmising. The set-up can be very simple or mind-bendingly elaborate and aimed at illuminating the farthest shores of philosophical and  scientific thought. Apparently Galileo didn’t actually lob tennis balls and refrigerators off the leaning tower of Pisa in order to conclude that that objects fall at the same speed regardless of their weight. He saved himself all the hard yakker by coming to the same conclusion via a thought experiment.

And of course Herr Schrödinger, in his famous thought experiment, only had to lock an imaginary cat in a box with an imaginary Geiger counter, some completely non-existent hydrocyanic acid and a dab of fictitious radioactive material, in order to elucidate his argument about quantum mechanics. Whatever that was.

I’ve devised a few thought experiments of my own – necessarily constrained by the fact that this column is intended for a general readership. So please, come along with me to the Thought Lab and apply your imagination, intuition and reason to the following three thought experiments. The first couple of experiments are pretty straightforward. The last one though … well, let’s just say it allows plenty of room for thought. ...