One of the reasons we study physics is that we don't yet have all the answers to life, the universe and everything (apologies to Douglas Adams). Ok, I know Douglas always maintained that the answer was forty-two, but I suspect that was an over simplification. Anyway, physicists are constantly formulating new hypotheses about the nature of matter, energy and the universe in general. In fact, some of them think there might be multiple universes. Having a theory is all very well of course, but how do you go about proving it? In the case of the multiple universe theory, I suspect it may be some time before anybody can actually prove anything one way or the other. Most physicists spend their time working in laboratories, observatories or even offices, investigating somewhat more down-to-earth matters. The answers they seek are often found through a process of careful investigation, experimentation, and measurement.
When we measure a quantity of some kind, what we are actually doing is comparing that quantity with some standard unit of quantity. If the quantity being measured is mass for example, we are comparing the amount of mass we have with some standard unit of mass. The result will be a number that represents the ratio of the mass we are measuring to the standard unit of mass. Scientists all over the world today use the International System of Units (usually referred to as SI units), which is described in detail elsewhere in this section. The standard SI unit of mass is the kilogram. Thus, if whatever we are measuring has a mass that has six-and-a-half times the mass of the kilogram, we simply say that it has a mass of six-and-a-half kilograms (6.5 kg). The availability of a standardised international system of units allows scientists working on similar projects, in facilities all over the world, to compare the results of their measurements in a meaningful way.
Measurement is something most people do from time to time, even if they are not physicists. You might measure the size of your windows with a tape measure, for example, before buying a set of blinds. If you were on a diet, you might stand on a set of scales every day to measure your weight. When preparing a meal from a recipe found in a cookbook or magazine, you might use a measuring jug and a set of kitchen scales to make sure you have the correct quantity of each ingredient. You can probably think of hundreds of situations where things need to be measured for one reason or another. Choosing an appropriate measuring instrument for the job in hand is something we often do without even thinking about it.
In physics, the quantities being measured are often either much bigger or much smaller than the sort of things we tend to measure in everyday life. They also tend to be much more difficult to measure, and require more sophisticated measuring apparatus and techniques. If we repeatedly measure the same physical quantity, under exactly the same conditions, we can reasonably expect to get the same result each time. The instruments and devices we use for the measurements should therefore be reliable enough to produce consistent results when used to make repeated measurements of the same quantity, or at least results that vary only within certain predefined limits. Accuracy and precision are also of paramount importance, because physicists often need to be able to detect extremely small variations in length, mass, volume or some other physical property of matter or energy in order to prove or disprove a particular hypothesis.
Because the ability to make accurate, precise and repeatable measurements is of critical importance, the devices used to obtain those measurements must be properly maintained. The routine maintenance of measuring equipment, which includes safety checks, calibration, and cleaning, is often the work of laboratory technicians rather than the physicists themselves. Nevertheless, the physicist must know how to use the equipment correctly. Appropriate training should be provided to ensure that personnel use equipment safely, without causing damage to sensitive (and expensive!) equipment. Training in the correct use of measuring equipment will also help to ensure that the results obtained are accurate. In general, the physicist is responsible for choosing the correct measuring device for the job in hand, for following the correct procedures when using the device, and for ensuring that they comply with general health and safety guidelines.