# Back to the blog

Having had a rather extended half-time break from blogging I have some time to get it kicked-off again. And what better topic that the Football World Cup.  (Or, as it’s referred to everywhere except New Zealand, the World Cup.)   There are some great examples and analogies of physics that we can pull from this, but I thought I’d start with the Video Assistant Referee (VAR).

Generally speaking, I approve of the use of technology-based refereeing aids in professional sport. Since everyone watching on television has almost instant access to close-up replays from all angles, it seems crazy that the referee can’t have a second look at something. So long as it doesn’t interrupt the game too much. I think it’s worked pretty well here, though it does seem that the penalty count is getting rather extreme.

How long will it be before a team’s defence realizes that it will not get away with anything in the box and starts to change the way it defends? Tackles that could be attempted previously may just be deemed too risky now – get it wrong and a defender knows he will be penalized.  In other words, the fact that the technology is watching changes what the players do.

That’s a problem with carrying out measurements in physics. In order to measure something (be it a temperature, an electric current, a pulse of magnetic field or whatever) we need a sensor, and this sensor becomes part of the physical system. To measure current, we need to pass that current through a device that measures it, but using that device changes slightly the amount of current that flows. (There are clever ways of mimizing this effect but, fundamentally, we can’t get rid of it). Or, if we want to take the temperature of something hot, we might do so by putting a thermometer into it, but that thermometer is going to take away a little bit of heat from the system and therefore change its temperature, just a touch.

The effect is extreme when we consider quantum systems. Here, measurement is fundamentally linked to the system itself. Measure where a particle is, and you change markedly how it behaves. The famous quantum 2-slit experiment epitomizes this effect – as explained here by Jim Al-Khalili.

Taking this to the extreme then, Harry Kane’s performance tomorrow will be influenced by whether or not I can bear to watch England – Columbia tomorrow morning. If only I knew just in what way it would be influenced…]