What is the Observer Effect?

Every measurement changes the object that we want to measure. This is called the Observer Effect. When we stick a thermometer into a glass of water to measure its temperature, then the thermometer will change the temperature of the water as well. [...]

An episode of the TOKTalk.net podcast, hosted by ok, titled "What is the Observer Effect?" was published on December 24, 2007.

December 24, 2007 · TOKTalk.net

Summary

http://www.toktalk.net/wordpress/wp-content/uploads/MP3/003-toktalk-measurements.mp3 Every measurement changes the object that we want to measure. This is called the Observer Effect. When we stick a thermometer into a glass of water to measure its temperature, then the thermometer will change the temperature of the water as well. Similar effects can be observed when measuring voltage or current in electrical circuits. Also in the social sciences we have a similar problem: people will not behave naturally when they feel that they are observed. In this edition of TOK-TALK we will explore if it is in principle possible to measure anything accurately. How does a measurement change the value of that what you want to measure? Listen to find out! Here in front of me, I have a cup of hot water, and over here we have a thermometer. Let’s put the thermometer into the glass, we have to wait a bit for the temperature reading to adjust. For our listeners, it’s a digital thermometer with a metallic probe. You use similar thermometers to measure the inside temperature of a cake to check if it is finished baking. Now the temperature is slowly climbing and leveling off. We have a reading of 76.2C. We can see that it is fluctuating a bit, now we can see 76.1C. Of course we have to be careful here, because this is the temperature reading of the side of the glass. The center is much warmer. Let’s stir the water a bit. OK, we are up to 82.5C. We could agree that we want to measure the average temperature of the water, so we have to stir the water. But this would of course increase the cooling rate. But then there is the problem that we are measuring a value that is continually changing. The water is cooling down. We could try to completely isolate the water from its colder environment. But we first have to bring up the temperature of the container to the same temperature as the water. Otherwise the container will cool down the water. The isolating container will prevent the cooling of the water, but first the container has to assume the same temperature as the water. So we have to determine the temperature of the container as well. And the situation becomes even worse. Of course the thermometer itself also has a certain temperature. By sticking the thermometer into the water, we are already changing the temperature of the water. Every measurement changes the thing that we want to measure. We could of course try to compensate this error mathematically, but for that we have to know the exact temperature of the thermometer that we are using. So we need another thermometer to measure the temperature of the first thermometer. But then this measurement would of course change the temperature of the first thermometer just like the thermometer would change the temperature of the water. We are pushing the problem out. We could make the mass of the water very large compared to the mass of the thermometer. If we use a large amount of water and also a large thermometer, then we do not gain anything. And can you imagine it if we attempt to use a very large thermometer to measure the temperature of a drop of water? The thermometer would influence the temperature of the drop more than the drop the thermometer. This is what we call the „Observer effect“. When we observe something, we end up changing the thing that we observe. Besides temperature measurement, there are other examples as well. In my younger days I used to play around with electronics a lot. I liked to assemble radios and other electronic gadgets. I bought myself a nice voltmeter to measure volage, resistance and current of electrical circuits. The only problem was that the device, being electronic itself, actually influenced the object that I wanted to measure. The ammeter measured the current flowing through a cable, but the device of course consumed some of this current itself. So it changed the current that I wanted to measure. Of course manufacturers try to keep this [...]

Episode Description