This is a common question addressing a popular misconception about how science classifies the knowledge that it has accumulated. The levels of hypothesis to theory to law often get interpreted as classes of confidence. However, this is not really right. The missing piece here is spatial scale!
It isn’t easy to draw the line for what is scientifically known and what is not. A major reason this line is so blurry is that knowledge can be applicable at different spatial scales. The three categories of scientific understanding (hypothesis, theory, and law) are often defined in introductory science courses as describing how thoroughly a concept has been tested. Although theories do need more testing than hypothesis and laws more testing than theories, the amount of testing required is only a consequence of the real definitions. These three classes are actually describing the extent over which we know an idea to be true. The larger the extent, the more testing that will be required to know that the concept is applicable across the entire extent.
Because we encounter phenomena at a very local scale, this is usually the beginning of our ideas about how the world works. Based on these few observations, or anecdotes, we form a hypothesis about what is happening and why. In popular culture, this idea might be called a ‘theory’, but science would require proof that the concept applies in more situations before elevating it to a theory.
As we gather more data and test the idea more, we can start to explain connections between several phenomena. When we have evidence that a concept has greater applicability than to just a few anecdotes, then we can upgrade it to a theory. Although more testing was required to make this change in knowledge classification, the important part is that we now have confidence in a wider applicability of the idea.
At the top of this hierarchy of scales are scientific laws. These are concepts that have proven to be universally true. Clearly, to be confident that something is universal requires a lot of testing, which is why there is sometimes confusion about the meaning of these categories of scientific knowledge. Being universally applicable does not mean that laws cannot have defined conditions under which they apply. In fact, many are limited to certain conditions, but even with those conditions, scientific laws are expected to be reliable no matter where you go in space.
The clarifications I provide here do not contradict popular definitions, but they point out that oversimplified definitions focus on the wrong part of the scientific knowledge classification, which leads to misconceptions. At a single site, one can make millions of observations of the same thing happening the same way. However, the tested explanation for this one location will never be considered a law, despite a high confidence in that explanation being able to predict the same event occurring again. This is key to understanding why some scientific explanations with a high level of confidence will never get promoted to a higher category of scientific understanding.
To give a specific example, we have observed on the planet Earth that the acceleration due to gravity is between 9.76 and 9.84 m/s2. Despite the high level of confidence in the truth and predictability of this fact, it alone could never be promoted to a law. It cannot be considered a law because it lacks universal applicability. Observations of this phenomenon (plus celestial bodies) did lead to Newton’s Law of Universal Gravitation, which identifies a gravitational force between any two masses that is equal in magnitude for each mass, and is aligned to draw the two masses toward each other:
where, m1 and m2 are the two masses, G is the gravitational constant, and r is the distance between the two masses.
This equation is capable of being a scientific law because it has been generalized to be universally applicable. Of course, rates of gravitational acceleration on Earth are still useful and reliable. The same can be said for many other pieces of scientific information that cannot be or have not yet been established for greater extents.
For more, check out: Berkeley’s webpage on “Science at multiple levels”