In the last few days I’ve been wrestling with one of the unsolvable problems of physics, namely that it is impossible to measure something without changing it.
Here’s an example. Suppose I want to measure the temperature of a pot of warm water. I can do it by putting a thermometer in it. Now, since the thermometer has been sitting out in the kitchen, it is colder than the water, so when I put it in, a small amount of heat from the water flows to the thermometer. The thermometer heats up, and the water cools down slightly, until they reach the same temperature. The temperature I record is then the temperature of the water now, but not the temperature of the water before I put the thermometer into it. So the act of measuring it has changed it.
Now, in many practical cases, this effect makes next to no difference and we can ignore it. But in Quantum Mechanics, it is all-encompassing. It’s hard to give examples in everyday language, but the fridge question comes close:
"When the door of the fridge is closed, is the light on or off?".
Well…., let’s open the door and have a look. ….er….no, that won’t work. Let’s close the door and have a look…No, can’t do that either..Hmmm. Tricky one…. (Not tricky at all – who cares so long as it keeps the beer cold…)
In my case, the problem I have is concerning measuring the voltage between the inside and outside of a biological cell. (Biological cells have lots of ions in and around them, so are like little batteries – usually a few millivolts in strength.) To measure the voltage, we use two electrodes – place one inside the cell, and one outside, and connect them to a voltmeter (crudely speaking). But here is the problem. My electrodes interact with the environment – particularly the 50 Hz (cycles per second) radiation from the electrical mains supply. So what I see is a mixture of what the cell does, and what the electrodes do. The two can’t be separated.
So to determine the voltage of the cell on its own, without electrodes stuck in it, we would need to measure it. And to measure it we have to stick electrodes into it. Oh dear. Sounding like the fridge problem to me. Hopefully the latter still has some beer in it.