Checking a tyre's pressure changes the pressure you are measuring
To read the air pressure in a car tyre, you have to let a little air out, so the act of measuring alters the thing measured. Scientists call this the observer effect. Every observation, human or mechanical, disturbs the world a little, and physics shows that it also increases entropy.
In the natural sciences, observation means actively gathering information from a primary source, noticing phenomena and recording data through direct engagement. The word covers both the act and the information it yields. Living creatures observe with their senses, but science routinely extends perception with instruments. Observations are either qualitative, describing features such as colour, texture or behaviour, or quantitative, assigning numbers by counting or measuring.
The scientific method depends on them at every turn. Researchers pose a question, form a tentative hypothesis, predict consequences not yet checked, test those predictions through experiments, field studies or simulations, then revise and repeat, finally describing their methods and submitting results for peer review. Reproducibility demands that different observers get comparable, consistent results.
That is hard with raw senses. Impressions are subjective and difficult to standardise, and perception is fooled by illusions. Measurement solved part of the problem by comparing a phenomenon with an agreed standard unit that anyone can reproduce, turning observations into numbers; two results with the same value count as equivalent within the method's precision. Instruments extend reach further. Scales, clocks, telescopes, microscopes and thermometers sharpen ordinary perception, while voltmeters, spectrometers, infrared cameras, Geiger counters and radio receivers detect things the senses cannot register at all.
Better instruments can shrink the observer effect to negligible levels in many fields. At a deeper level, any observation involves amplification, making it a thermodynamically irreversible process. And in some domains results depend on the observer's circumstances in ways everyday life never reveals. In relativity, observers moving at different speeds relative to an object can measure different lengths, times and masses, producing apparent paradoxes in which the same event looks different from two viewpoints.
Source: Observation
