Observer effect

 The observer effect is the idea that the act of observing or measuring something can change the thing being observed.

There are a few meanings depending on context:

In physics (especially quantum mechanics)

The observer effect refers to measurements affecting a system.

A famous example is the double-slit experiment:

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When particles like electrons pass through two slits, they behave like waves and create an interference pattern. But when scientists measure which slit a particle goes through, the pattern changes and the particle behaves differently. This is often simplified as “observation changes reality,” though more precisely, measurement interaction alters the system.

Related concept: Uncertainty Principle — measuring one property precisely can limit how precisely another can be known.

In everyday science

Observation can physically interfere with what you measure:

• Measuring tire pressure releases a tiny amount of air.
• Putting a thermometer in a liquid slightly changes its temperature.
• Watching people can alter behavior (people act differently when observed).

In psychology / social sciences

People may change behavior because they know they’re being watched. This is related to the Hawthorne effect.

Example:
Employees may work harder during a study because they know researchers are observing them, not because of the experimental change itself. Related: Hawthorne effect

In short: observation is not always passive; measuring can influence outcomes.