# The Observer Effect

In physics, the “observer effect” is a well known principle – the very act of measuring changes what you’re observing.

Generally, the change is negligible, but it’s important to recognize nonetheless.

To be clear, this effect should not be confused with the uncertainty principle, which limits the precision with which related pairs of physical properties can be known. (Position and momentum, for example.)

The observer effect isn’t about uncertainty. It’s about measurement actually causing change. That is to say – the only way to measure something is to change it.

Imagine, for example, a thermometer. A thermometer starts at room temperature. Stick it in a pot of boiling water, and the temperature of both objects equalizes. The thermometer heats up while the boiling water cools down.

This effect seems obvious if you think of equally sized objects – pour a gallon of room temperature water into a gallon of boiling water, and the two waters together end up with a joint temperature in between the two starting temperatures.

In the case of a thermometer, that object is so small compared to the pot of boiling water that that the change in temperature is negligible.

But there is still a change.

The temperature of the thermometer – the temperature of the changed pot of water – is different from the temperature you were trying to measure. You now know the new temperature, but you don’t know the original temperature.

The measurement changed it.

This may seem like an issue of semantics. If the pot of water is still hot enough to be boiling, who cares if it’s not quite the same temperature as it was before you measured it?

And that’s a reasonable point. I’d certainly concede that day to day living doesn’t require a level of precision which would make the change significant.

But it’s still important to understand that there is a change.

Classical physics deals with the every day world. Quantum physics deals with the true world. The complex world. The impossible to measure world.

Objects traveling close to the speed of light don’t behave like your run-of-the-mill, every day objects. That may not matter for every day living, but it still matters.

The beauty of quantum dynamic equations comes when you use them to consider an everyday object. Suddenly, all the complications drop out and you’re left when the familiar, classical equation that explains the life you observe.

But the quantum equation is still more accurate. The quantum equation is still more True.

And so it is with measurement. As long as your thermometer isn’t the same size as the object you’re measuring, the effect of observation won’t effect day to day living.

But it still matters.

The only way to measure something is to change it.

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