When Does A Theory Become A Fact?

Does it ever? We jump into the thorny issue feet first to investigate when, if ever, “theories” become “facts” and according to whom.


Dr. Alfredo Carpineti


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master's in Quantum Fields and Fundamental Forces.

Senior Staff Writer & Space Correspondent

Issac Newton's apple faling from the tree showing ravity pulling it down

There may be a linguistic barrier between what the public means when talking about “theories” and “facts” compared to a scientist. Image credit: (C) James Rodrigues/IFLScience

This article first appeared in Issue 2 of our free digital magazine CURIOUS

There are few simple questions out there that can shake the foundations of science like “when does a theory become a fact?” A scientist like Stephen Hawking will tell you that it never does, that all scientific theories are just observations of facts from which we obtain a universal law. A historian of science might tell you that this view is more an aspiration for scientists than a rule, even among the most famous and successful theories, like the Big Bang or evolution. And the person that asked the question is none the wiser. 


The “truth” of a theory is in its usefulness in linking a large number of facts that seek to explain a particular phenomenon and enabling predictions of new ones that may have yet to be observed. Its great strength is that if a theory comes up against a fact that can’t be explained or even disproves it, it can be eventually discarded and replaced by a better one. This does, however, leave it open to attack by anti-science movements due to the dissonance in what we mean by “theory” and “fact” in the first place.

Are Theories And Facts The Same For Everyone? 

There is certainly a linguistic barrier between what the public means when talking about “theories” and “facts” compared to a scientist. Often, in common parlance, “theory” is used as a synonym for a hypothesis. To use the phrase “it’s just a theory” dismissively, meaning it’s just an untested unproven hypothesis, is almost the opposite of what scientists mean when they talk about theory. 

“For everyday people, this is a big tension. We use the word theory much more lightly,” Rachel Ankeny, a professor of philosophy of science at the University of Adelaide, told IFLScience. 

“Theories in science run the spectrum. There may be theories that are just being tested but, in many cases, scientists mean ‘this is a theory that has been shown to be the case’ and it’s going to involve data, it’s going to include all sorts of observations about the world, it’s going to include what they would even call facts.” 


For most scientists, a theory is a robustly tested explanation for a phenomenon or group of facts that take in both scientific hypotheses and scientific “laws”. It’s the best explanation we have to explain these phenomena or facts – until a better one is shown. 

A good example is Isaac Newton and Albert Einstein’s theories of gravity. Newton put forward his theory of gravitation in 1687. It remained the prevailing theory for hundreds of years until Einstein put forward an alternative in 1915. Einstein’s general theory of relativity produced many of the same results as Newton’s, but it could also explain aspects that Newton’s could not. In 1919, Einstein’s theory was proven to work, while Newton’s did not, and though relativity has been tested and challenged many times in the century since, it still holds up as the best explanation we have.

A Theoretical Philosophical Issue

So the definition of a theory is different from the common parlance but, at the same time, how it’s used is not a rigid mold. Some theories have so much experimental evidence that it seems impossible they can ever be refuted. Others offer a really good explanation, and yet they are not perfect and we are aware of their limitations, like Einstein’s relativity or the Standard Model of particle physics. And there are others that are much more speculative still being investigated.

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Depending on the field of study and even on the particular theories, scientists have very different approaches to how you should deal with them. Some use that which Karl Popper, one of the most influential 20th-century philosophers of science, put forward. Popper did not believe that a theory could never be considered true, instead, scientists should prove time and time again that it is not false. 


We know that general relativity and particle physics are extremely good at explaining what we see, but they are also flawed and limited. Their many tests (some of which they pass and some they don’t), under Popper’s view, would be an attempt at falsifying the theories. But many scientists are not working to prove ideas wrong. 

Thomas Kuhn, another influential 20th-century philosopher of science and author of the much-cited The Structure of Scientific Revolutions (1962), disagreed with Popper. Kuhn instead believed that for most periods of time, research is in a “normal science” state and scientists shouldn’t look for anomalies or try to prove the leading theory. The “immature” science is “pre-paradigmatic”. But if anomalies arise then at some point there will be a revolution that will leave the old theory behind, and the science that has matured becomes “paradigmatic”. 

This scenario can be seen at play both historically and in more recent times. There are those actually looking for anomalies in prevailing theories and limitations in our knowledge to push us forward, and there are theories that are thrown out completely from the history of science, but a good part of them continue to be useful even when a better interpretation of the world exists. Newtonian mechanics and Einstein’s relativity are good examples of this. 

While the Newton versus Einstein discourse first occurred in the early 20th century, this debate is still alive and well. The concept of a theory is at the very heart of science and given the role of scientific investigation in society, it is an issue that affects us all. 

The sky is blue is an everyday fact but the scientific fact is that the color of the sky is due to an effect we call Rayleigh scattering.

A scientific theory needs to explain observations and make predictions. Failing to do so may lead to the theory being rejected or to it being refined into a better version. This tweaking is not a way to save a theory from its demise, either, but comes from the fact that nature is extremely complicated, and there is always more for us to know, and to know we have to test.

“Anything that is truly scientific is going to allow for that kind of refinement and even that kind of rejection. But this is not done lightly. Findings build up on each other, so it is not just finding one instance that seems to contradict it and then throwing out the whole theory,” Professor Ankeny explained to IFLScience.

A Matter Of Facts

Similar to the separation in meaning between theory in common parlance and theory in science, an everyday “fact” and a scientific “fact” are also not necessarily the same thing. The sky is blue is an everyday fact (as long as you agree what blue is) but the scientific fact is that the color of the sky is due to an effect we call Rayleigh scattering.

“Scientific facts have credentials that come along with them. For something to be a scientific fact, typically, they have to be a finding that has resulted from careful attention to building empirical evidence. Again, in different fields, these are going to be different but it often involves observations, testing, and measuring through experimentations,” Professor Ankeny told IFLScience. 

Scientific facts can be falsified or improved, they might even change. 

“[The different fields] share this idea that something becomes a fact by being the result of something that can be repeated and something that has robust methods associated with its observation, measurement, or testing.”

Scientific facts can be falsified or improved, they might even change. Scientific theories can explain and situate those scientific facts in a wider context, connecting them to a coherent picture. 

In common parlance, we might say that gravity is a fact, or that microbes are a fact. They require more details to be seen as scientific facts but they have entered the everyday vocabulary following the introduction of revolutionary theories such as Newton’s universal gravitation or Pasteur’s germ theory. The connection between scientific theories and common facts is there, and should not be underestimated. Scientific theory is how they became everyday facts. 

Let’s Start A Conversation 

Criticism of scientific investigation, the scientific method, and “facts” appears to be on the rise in recent years, especially surrounding subjects like COVID-19 and the unfolding climate crisis. Some of the criticisms, especially with regard to the pandemic, focus on uncertainty and how “facts” seem to change. But scientific research is not a snap-your-fingers portal to the ultimate truth. 


“It’s not about any one fact or set of facts, or any one theory. It is about the method. And that the method is something that is agreed to, that scientists in every field know what counts as a reasonable test of a theory,“ Professor Ankeny said. 

“Science can hang its hat on being the best available understanding of the natural world because of all those standardized methods.”

There is an important conversation to be had about the limits of scientific theories and scientific facts, a conversation that needs to be shared across society to enable both better critics and better supporters of science as a discipline. Pinning down what we mean by these terms is just the start.

CURIOUS is a digital magazine from IFLScience featuring interviews, experts, deep dives, fun facts, news, book excerpts, and much more. Issue 7 is OUT NOW.


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